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Abstract

Several research groups have observed magnetism in monolayer-protected gold cluster samples, but the results were often contradictory, and thus, a clear understanding of this phenomenon is still missing. We used Au₂₅(SCH₂CH₂Ph)₁₈⁰, which is a paramagnetic cluster that can be prepared with atomic precision and whose structure is known precisely. Previous magnetometry studies only detected paramagnetism. We used samples representing a range of crystallographic orders and studied their magnetic behaviors using electron paramagnetic resonance (EPR). As a film, Au₂₅(SCH₂CH₂Ph)₁₈⁰ exhibits a paramagnetic behavior, but at low temperature, ferromagnetic interactions are detectable. One or few single crystals undergo physical reorientation with the applied field and exhibit ferromagnetism, as detected through hysteresis experiments. A large collection of microcrystals is magnetic even at room temperature and shows distinct paramagnetic, superparamagnetic, and ferromagnetic behaviors. Simulation of the EPR spectra shows that both spin−orbit (SO) coupling and crystal distortion are important to determine the observed magnetic behaviors. Density functional theory calculations carried out on single cluster and periodic models predict the values of SO coupling and crystal-splitting effects in agreement with the EPR-derived quantities. Magnetism in gold nanoclusters is thus demonstrated to be the outcome of a very delicate balance of factors. To obtain reproducible results, the samples must be (i) controlled for composition and thus be monodisperse with atomic precision, (ii) of known charge state, and (iii) well-defined in terms of crystallinity and experimental conditions.

Introduction

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Gold nanoparticles and nanoclusters exhibit distinct optical, electrochemical, charge-transfer, and catalytic properties. (1-6) These properties are particularly distinctive in monolayer-protected clusters (MPCs), that is, small metal structures stabilized by a molecular layer. Over the past years, knowledge of the properties of most MPCs has expanded very significantly. However, an important property still poorly understood is the nanomagnetism of gold, despite the magnetic nanoparticles and nanoclusters being of intrinsic importance and promising value in data storage, spintronics, quantum computing, optomagnetic devices, biomedical applications, and nanocatalysis. (7-12)

Although bulk gold is known to be a typical diamagnetic material, upon decreasing the size down to the nanoscale a magnetic moment appears. (7-9) Since the first report by Hori et al., (13) several papers described magnetic properties of gold nanoparticles, mostly thiolate MPCs prepared according to the two-phase synthesis by Brust et al., (14) but the contradictory outcome of several of these results has been pointed out, reviewed, and discussed. (7-9) The observed magnetic behavior can indeed be very different. For example, Crespo et al. (15) and Donnio et al. (16) observed ferromagnetism in MPCs with diameters of 1.8–2.1 nm, whereas Yamamoto and Hori used clusters with a mean diameter of 1.9 nm and detected both superparamagnetism and Pauli paramagnetism. (17) Pauli paramagnetism, but no ferro- or superparamagnetism, was observed by Lear and his co-workers on nanoparticles of 1.8–1.9 nm. (18-20) Gréget et al. concluded that 1.9 nm large MPCs were diamagnetic, whereas larger particles (4.4 nm) were ferromagnetic. (21) Although ferromagnetism is generally detected when the MPC size decreases, Muñoz-Márquez et al. observed ferromagnetism in 2.1 nm clusters and diamagnetism in smaller clusters (1.4 nm). (22) It has also been observed that, depending on ligands, ∼2 nm clusters may exhibit ferromagnetism, paramagnetism, and diamagnetism. (23) Ferromagnetic behavior was observed for both films formed of bare Au clusters (24) and Au nanoparticles embedded in films of titania. (11) This astonishing variability in behavior is worsened by the observation that even particles of the same batch may exhibit very different magnetizations. Sometimes, different magnetic behaviors were observed even for samples prepared by using the same synthetic procedures and even by the same group. (9) In addition to these confusing results, some intriguing magnetic phenomena were also observed, such as an unusual dependence of the magnetization on temperature and dimensions of the particles and a very high magnetic anisotropy. (16, 25, 26) Recent compilations of the different magnetic results obtained for Au nanoparticles or nanocluster systems, mostly ranging from 1 to 4 nm, are available. (24, 27) From a theoretical viewpoint, magnetism of gold has been related to a surface effect with an important orbital contribution to the magnetic moment. (28, 29)

Nealon et al. discussed all of these topics in particular detail, and their analysis converged to the quite disappointing but nonetheless thought-provoking conclusion that nobody really knows to what extent and why some MPCs exhibit an intrinsic magnetism. (9) Because the experimental results are often strange, discordant, and rarely reproducible, it is not surprising if a general explanation and even a qualitative understanding of these findings are still missing. We believe that this variegated, intriguing, and also confusing scenario is primarily due to the lack of control of the composition, structure, charge state, and, as we will show here, crystallinity and morphology of MPCs. Indeed, with very few exceptions to be discussed later, the majority of measurements were carried out on Au nanoparticles lacking atomic precision (and thus possessing only an average dimension assessed through transmission electron microscopy images) and of undetermined charge state. Both of these properties are closely linked to the magnetism of materials: by changing the dimension and the charge state, it is indeed possible to switch between different forms of magnetism. For example, the redox steps associated with charging of MPCs of hundreds of Au atoms can be so closely spaced (3, 30) that removing or adding electrons can be easily triggered by oxygen or a mild reductant and via intercluster disproportionation–comproportionation equilibria. Depending on the experimental conditions and material preparation, different magnetic states are thus possible. If the cluster stoichiometry is not controlled, further uncertainty will be obviously introduced, as this increases very significantly the number of available redox couples in the whole sample.

MPCs with a gold core size of less than 1.5 nm exhibit the same general features of molecules. (1-3, 30) Importantly, most molecular MPCs can be prepared in a truly atomically monodisperse form. (1, 2) The most well-known and understood example of them is Au₂₅(SR)₁₈. (31) Its structure is formed of a 13-gold-atom icosahedral core stabilized by six -(SR)-Au-(SR)-Au-(SR)- units (SR = thiolate). (32, 33) Whereas anion Au₂₅(SR)₁₈^– and cation Au₂₅(SR)₁₈⁺ are diamagnetic, the neutral form Au₂₅(SR)₁₈⁰ is paramagnetic. (34-36) For this charge state, which can be defined very precisely through electrochemistry or controlled redox reactions, (33, 37, 38) room temperature NMR spectroscopy shows that the spin density spreads onto the first ligand atoms and causes the corresponding resonances to undergo significant chemical shifts relative to the anionic or cationic diamagnetic state. (35) Continuous-wave electron paramagnetic resonance (cw-EPR) experiments on frozen, glassy solutions show a broad peak detectable at temperatures lower than 100 K and exhibiting the typical features of a doublet state. (34, 36) To complete the solution-phase magnetic picture, low-temperature electron–nuclear double resonance (ENDOR) assessed the interactions of the unpaired electron with both gold (39) and hydrogen nuclei. (40) On the other hand, the knowledge of the magnetism of Au₂₅(SR)₁₈⁰ in the solid state, that is, the physical state that most other gold magnetism data refer to, is far less advanced. According to superconducting quantum interference device (SQUID) magnetometry studies, Au₂₅(SCH₂CH₂Ph)₁₈⁰ (hereafter, we will indicate phenylethanethiolate simply as SC2Ph) is as paramagnetic in the solid state (27, 34, 41) as in the frozen solution. (33, 35) It is worth noting that the nature of the capping ligand cannot be ignored. We have recently shown that by using n-butanethiolate ligands the resulting crystals are formed of a linear sequence of Au₂₅(SBu)₁₈⁰ clusters interconnected by Au–Au bonds: overlap of the singly occupied molecular orbitals (SOMOs) of neighboring clusters allows coupling the individual spins with the formation of an antiferromagnetic polymer, as revealed by EPR spectroscopy. (42) This result shows that possible interactions between paramagnets in the solid state should always be taken into consideration and also be understood in terms of the crystallographic structure.

In this work, we describe the magnetic behavior of Au₂₅(SC2Ph)₁₈⁰ in different solid-state forms, as assessed by EPR spectroscopy. In this connection, it is worth noting that SQUID has been the technique of choice for most of the previously quoted studies on molecular and nonmolecular or nonatomically precise Au nanoparticles. This method allows the detection of the susceptibility of the entire sample, which may be the result of different magnetic contributions (ferromagnetic, paramagnetic, diamagnetic, etc.). In EPR, on the other hand, only unpaired electrons are observed, and therefore, the diamagnetic contribution, which may be very significant, is completely removed. Moreover, different contributions to the overall magnetization can be often separated: for instance, ferromagnetic signals can be easily distinguished from most paramagnetic signals because they are characterized by completely different line shape and temperature dependencies. In the past, consistent EPR studies have been carried out to study both molecular Au₂₅(SR)₁₈ or Au₂₅(SR)₁₈ doped with Pt, Pd, or Hg and larger nonmolecular Au nanoparticles. (19, 20, 34, 36, 39, 42-45) It was also employed for studying the magnetism of gold nanorods and nanoparticles, which showed ferromagnetic signals. (46, 47) The potential of the EPR approach has been evidenced particularly well through the observation of size-dependent signals for gold nanorods. (46) In most cases, on the other hand, analogous samples were found to be EPR-silent or showed very weak and hardly interpretable signals. (22, 48) Our study takes advantage of using a cluster, Au₂₅(SC2Ph)₁₈⁰, whose structure in the neutral state has been refined very recently (49) and whose magnetic properties in solution are well-understood. The interactions between Au₂₅(SC2Ph)₁₈⁰ clusters in the solid state were studied by using a combination of experimental and theoretical analyses. By carrying out a comparative study of the magnetic behavior of samples endowed of different morphology and crystallinity, we could detect and rationalize, for the first time, a series of magnetic behaviors. Independent EPR and density functional theory (DFT) calculations concur in pointing to the importance of spin−orbit (SO) coupling effects to explain the observed phenomena.

Results and Discussion

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Au₂₅(SC2Ph)₁₈⁰ was prepared by the oxidation of the diamagnetic anion Au₂₅(SC2Ph)₁₈^–. By using methodologies that we devised and described previously, oxidation was carried out either chromatographically (50) or electrochemically. (49) Each sample was meant to provide a specific example of different crystalline order and physical state: frozen solution, film, single crystal, immobilized single crystal, collection of 10 crystals, immobilized collection of 10 crystals, and microcrystals. Figure S1 shows images of these samples.

Film

The Au₂₅(SC2Ph)₁₈⁰ film was prepared inside of the EPR tube by evaporation of the solvent from a dichloromethane solution. The film corresponds to a virtually amorphous solid and thus represents the lowest crystalline degree of the solid samples investigated herein. Figure 1 shows the EPR spectra (in black) obtained at temperatures ranging from 5 to 160 K, together with the corresponding simulations (in red). To evidence better the weak signals observed at temperatures larger than 60 K, the data in Figure 1a have been multiplied by a factor of 10.

Each spectrum consists of a quite broad anisotropic line that can be well-simulated by considering an ensemble of randomly oriented paramagnetic clusters with S = 1/2 and a Zeeman interaction described by an orthorhombic g-tensor with principal values (at low temperatures) of 2.53 (x), 2.36 (y), and 1.82 (z); these values are very similar to those previously described for Au₂₅(SC2Ph)₁₈⁰ in frozen solutions at similar temperatures, that is, 2.56, 2.36, and 1.82, respectively. (34, 36)Figure 1 shows that an increase in temperature has the main effect of diminishing the intensity of the signal, which becomes barely detectable for temperatures larger than 160 K. This decrease is qualitatively similar to that already observed for the same cluster in the frozen solution. (36) The main difference between these two cases (Figure S2 shows a direct comparison of the two spectra at 10 K) is that inhomogeneous broadening is more severe in the film than that in the frozen solution. Indeed, the spectrum of the film could be simulated only by including some distribution for the y and z components of the g-tensor, which correspond to the two negative peaks at 3000–4000 G; for 5 K, we used fwhm (full width at half-maximum) values of 0.28 (g_y) and 0.15 (g_z) for y and z, respectively, whereas even larger fwhm values were used for higher temperatures. This distribution suggests the presence of weak orientation-dependent interactions in the film.

Insights into this aspect can be obtained from the temperature dependence of the magnetic susceptibility (χ_m). For an ensemble of perfect, noninteracting paramagnets, χ_m is inversely proportional to the temperature, as described by the Curie law (eq 1)

(1)where the Curie constant C is composed of the number of spins N, the Bohr magneton μ_B, the Landé factor g_L, the quantum number of the total magnetic moment J, and the Boltzmann constant k_B. The EPR signal can be integrated to obtain the corresponding EPR absorption spectrum, and further integration yields the so-called double-integrated EPR intensity (I_EPR); details about these integrations are provided in the Supporting Information. I_EPR is proportional to χ_m, and therefore, as long as the Curie law is obeyed, a plot of I_EPR^–1 versus T should be linear.

The best example of noninteracting paramagnetic MPCs is provided by clusters in a diluted frozen solution. Analysis of the data for 2 mM Au₂₅(SC2Ph)₁₈⁰ in frozen dichloromethane (36) shows that in the experimentally accessible temperature range (6–80 K), a plot of I_EPR^–1 versus T is indeed quite linear (r² = 0.997), as shown in Figure S3. In fact, the observation of a nonzero intercept at 4(1) K suggests that this system could be better described by the Curie–Weiss law (eq 2)

(2)where T_C is the intercept or Curie temperature, which marks the onset of magnetic interactions between the paramagnets. Although this T_C value is indeed very close to zero, we note that even at 2 mM concentration in a frozen solution the distance between the individual clusters is not particularly large: Au₂₅(SC2Ph)₁₈⁰ has a radius of 13.2 Å (36) and a core radius of 4.9 Å, (32) and therefore, the mean Au-core edge-to-edge intercluster distance is 8.4 nm, whereas the mean Au-core edge-to-edge distance between the nearest neighbors (51) is only 4.2 nm. At this distance, a nonzero exchange coupling between the spins of neighboring clusters cannot be completely excluded.

Figure 2 shows the I_EPR^–1 versus T plot for the film. Interestingly, whereas in the high temperature range the plot is quite linear (r² = 0.987), a net deviation from linearity occurs for T < 80 K, with an intercept of 63(4) K. This deviation is attributed to a weak ferromagnetic interaction between the spins of the individual clusters. The not-so-small intercept value is thus in keeping with a non-Curie behavior caused by partial parallel alignment of the spins, as described by eq 2. This shows that some interactions are clearly detectable in the solid state, despite the structurally disordered film sample. This is indeed reasonable because the mean Au-core edge-to-edge intercluster distance in Au₂₅(SC2Ph)₁₈ films can be bracketed between 1 and 2 nm. (36, 49, 52)

Figure 2. Dependence of the double-integrated EPR intensity on the reciprocal of temperature. The solid line is the linear regression of the data (black square) at the higher temperatures.

Single Crystals

A perfectly crystalline sample features the opposite case of an amorphous film. We used one single crystal obtained by the electrocrystallization of Au₂₅(SC2Ph)₁₈⁰. (49)Figure 3 shows that the spectrum consists of a narrow signal centered at about 2500 G. As the temperature increases, the signal becomes weaker and virtually disappears for T > 35 K. The monocrystal signal (peaks at 2460–2630 G) and its temperature dependence are thus very different from those described for the amorphous film (peaks at 2700–3800 G).

Figure 3. Effect of temperature (K) on the cw-EPR spectra of one single crystal of Au₂₅(SC2Ph)₁₈⁰.

A single crystal of interacting paramagnets should be anisotropic, and therefore, one would expect to observe relevant spectral changes in both line shape and position upon rotation of the EPR tube with respect to the direction of the applied magnetic field. We recorded a series of EPR spectra after progressively rotating the tube by 90°, and the results are shown in Figure 4a. Surprisingly, however, the spectrum did not change. A simple explanation for having an isotropic system is highly unlikely because the principal g-tensor values optimized for the film and frozen solution EPR spectra already evidenced a high degree of anisotropy. We suspected that the observed apparent isotropic behavior was caused by a physical reorientation of the crystal inside of the tube, as expected for a ferromagnetic crystal that would minimize its magnetic energy by aligning its anisotropy axis along the direction of the applied field, with the result of observing the same spectrum at each orientation. The spectrum at 5 K could be simulated by two Lorentzian lines with g factors of 2.79 and 2.70, which can be attributed to bulk and surface magnetization, respectively; this difference is due to magnetic surface anisotropy (Figure S4). (53) Differently from the film and the frozen solution, the single-crystal signals are characterized by a single g value corresponding to one definite direction. Figure S4 also shows the unsuccessful simulations carried out by assuming either of the above g factors.

Figure 4. Orientation dependence of the cw-EPR spectra of one single crystal of Au₂₅(SC2Ph)₁₈⁰ uncovered (a) or covered (b) by frozen MeCN. Within each graph, the EPR tube was rotated by 0 (blue), 90 (red), and 180° (black) (T = 5 K).

To confirm this hypothesis, we put one single crystal of a similar size in an EPR tube and then added acetonitrile, a solvent in which the cluster is insoluble. Upon cooling, MeCN freezes and blocks the crystal from possible field-induced reorientations of the crystal. As clearly shown in Figure 4b, the spectrum recorded upon a 90° rotation is completely different from the original spectrum (or, similarly, that obtained upon a 180° rotation), in full agreement with our hypothesis. The difference between the initial states of the two samples is attributed to the way by which the crystal gets immobilized by the frozen solvent in comparison with the free crystal, which can optimize its position with respect to the direction of the applied field.

To test the hypothesis of ferromagnetism, we carried out EPR hysteresis experiments. A typical experiment consisted of an upward scan (from low to high fields) followed by a backward scan carried out after a time long enough (in this specific case, 30 min) for the system to reach equilibrium; this procedure makes the upward and downward scans determined by the situations attained at low or high field, respectively. The experiment also included a third upward scan (again, after a 30 min rest period) to check whether the first scan could be reproduced precisely: this was always verified. Differences between the upward and backward spectra are caused by the magnetization of the sample at high field value and provide an important indication of ferromagnetism.

The hysteresis experiments showed that an effect is perceivable for T < 10 K. This effect is particularly evident at 5 K (Figure 5), at which the low-field signals in the upward and downward spectra are substantially different in both intensity and line shape; as aforementioned, the first and the last upward scans are overlapping. The corresponding, though much less pronounced, hysteresis behavior at 10 K is provided in Figure S5. These temperatures point to an apparent (see below) anisotropy energy on the order of 0.4–0.8 meV. No evident hysteresis, on the other hand, was detectable for the single crystal immobilized in frozen MeCN, whether by optimizing the orientation of the sample or after rotation by 90° (Figure S6). This behavior can be rationalized by considering that whereas the free single crystal can modify its orientation and thus optimize the alignment of its anisotropy axis with the applied field (which maximizes hysteresis), in the second experiment the single crystal is blocked in a random orientation, and therefore, any hysteresis effect is significantly reduced.

Figure 5. Hysteresis cw-EPR experiment for a Au₂₅(SC2Ph)₁₈⁰ single crystal at 5 K. The direction and trace color of the three scans are indicated.

The effect of increasing the complexity of the experimental system was studied by using a collection of 10 single crystals with dimensions comparable to those of the previous samples. Indeed, the presence of more than one crystal modifies the spectrum quite significantly, as shown in Figure S7. The same is true for a similar group of crystals trapped in frozen MeCN (Figure S8). As observed for the isolated single crystals, the hysteresis experiments (Figures S9 and S10) show that differences between the upward and downward traces are evident for the free crystals but not for the MeCN frozen sample.

Comparison of the results obtained for the solid samples clearly shows that when the paramagnetic Au₂₅(SC2Ph)₁₈⁰ clusters are in the crystalline state, the spins of the single clusters are no more independent. The observed effects on the EPR spectrum are due to a cooperative ferromagnetic ordering. These results and comparisons, including the behavior of the film, thus provide compelling evidence for the onset of ferromagnetic behavior and show that the magnetic properties are very sensitive to the crystallinity and the physical state of the sample.

Microcrystals

We then studied a sample consisting of a very large ensemble of much smaller crystals, which will be denoted as microcrystals. This was meant to provide a sample that is more similar to those typically used in SQUID measurements. The EPR spectra were recorded in a particularly wide temperature range (Figure 6), also because the temperature dependence of the spectral pattern proved to be quite complex.

For temperatures decreasing from 100 K, the signal initially broadens and shifts to lower fields. This behavior is attributed to the onset of superparamagnetism, which is typical for small magnetically ordered particles. (54-57) In these systems, the exchange interaction and magnetic anisotropy generate a strong temperature-dependent inner field that adds to the applied external field. For uniaxial symmetry, the two opposite directions of the anisotropy axis correspond to the two minima of the anisotropy energy (E_an), which is the energy barrier to invert the direction of the magnetization. When k_BT > E_an, the temperature is high enough for the magnetization to reverse its direction. This superparamagnetic behavior is somewhat similar to paramagnetism, but the coupled spins give rise to higher magnetization. For k_BT < E_an, on the other hand, this magnetization flipping is hampered and the system becomes ferromagnetic; hysteresis is then usually observed, as it will be discussed in detail below. In addition to the superparamagnetic/ferromagnetic signal, the familiar paramagnetic signal becomes perceivable starting from 40 K, at approximately 2750 G, and its intensity progressively increases as the temperature decreases, as already observed for the film and the frozen solution. It is finally worth mentioning that for T > 100 K the superparamagnetic signal is still present but exhibits the peculiar behavior of initially shifting to lower fields and then, for T > 200 K, to higher fields. This behavior is probably associated with the thermal population of higher energy spin states. In the following, however, we will specifically focus on the results obtained in a temperature range comparable to that explored for the other samples.

Figure 7 shows the outcome of the hysteresis experiments. At each temperature, we carried out the same sequence of three scans explained in the previous section. The third scan was always found to overlap precisely with the first scan and thus is not shown for clarity. As the temperature progressively decreases from 40 K (Figure 7a), the signals observed at 2000–2700 G for the upward and downward scans are substantially different. The main effect is that in the downward spectra the signal is stronger and slightly shifted to higher fields. The hysteresis experiments thus show that the microcrystals exhibit a ferromagnetic behavior. Interestingly, hysteresis is observed at a higher temperature than for the large crystals. The magnetic anisotropy energy E_an of the microcrystal sample can be estimated on the order of approximately 3 meV, which is about 1 order of magnitude larger than the energy value observed for the single crystal. Indeed, this result would be quite unusual because both the magnetocrystalline and magnetostatic anisotropy contributions to the overall magnetic anisotropy are expected to decrease as the size decreases. (58) However, whereas for microcrystals we are dealing mostly with single-domain particles with uniform magnetization, for the much larger single crystal the magnetization is not uniform and the form of the anisotropy energy is conceivably more complex, with several local minima. (59) Indeed, the observation of very different spectra upon rotation of the immobilized crystals already indicates that the overall anisotropy of the single crystal (and that of a collection of large crystals) is certainly large. The field reachable in EPR is comparatively low (5000 G, i.e., a value significantly smaller than that in SQUID experiments) and thus can only rotate a part of the magnetization and overcome local anisotropies, with the result of giving rise to the small hysteresis observed. A size-controlled difference in the anisotropy energy, on the other hand, might be explained on a different basis. Au₂₅(SC2Ph)₁₈⁰ has only one spin s = 1/2 but the not-so-small radius of 13.2 Å. (36) This makes the saturation magnetization low, and the magnetostatic effects should not be particularly relevant. Simulation of the spectrum of the single crystal indicates that the surface contribution to magnetization must be taken into account, and therefore, that the surface anisotropy should be important. Any surface effect is clearly even more important for the microcrystalline sample, which includes a significant fraction of tiny crystals. An increase in the magnetic anisotropy due to the surface effect was already observed. (60) Moreover, surface effects on magnetic moment and anisotropy have been inferred to be important also for the nanomagnetism of gold. (61) The study of the connection between the shape and the magnetic properties is also receiving attention in the context of other metal nanoparticles. (62)

Figure 7. Hysteresis cw-EPR experiments for a large collection of Au₂₅(SC2Ph)₁₈⁰ microcrystals. (a) Effect of decreasing the temperature from 60 to 9 K and (b) corresponding temperature increase. The black and the red traces indicate the low-to-high- and high-to-low-field directions, respectively.

The fact that hysteresis apparently becomes less evident at the lowest temperatures is an artifact related to the rest time spent at 5000 G. Because of the high anisotropy value compared to the thermal energy at these temperatures, the magnetization relaxation time (τ) becomes very long. If the rest time is not sufficiently long, at the beginning of the downward scan the magnetization has not yet completely relaxed; that is, the sample is still experiencing a situation that is slightly similar to that of the low-field equilibrium. An example of the effect of the rest time (at 20 K), which results in a slightly larger hysteresis, is provided in Figure S11. At even lower temperatures, increasing the rest time significantly becomes experimentally unfeasible. For example, use of the Neél–Arrhenius equation, τ = τ₀ exp(E_an/k_BT), (63) and the pertinent approximate E_an values show that, at 10 K, τ of the microcrystals is at least 1 order of magnitude longer than that for the single crystal (for which we waited 30 min). Finally, the persistence of the paramagnetic signal of the isolated clusters, partially overlapping with the ferromagnetic signal, is attributed to the finest or the most amorphous fraction of the sample.

An evident new spectral feature emerges upon reaching the lowest temperature explored. Figure 7a shows that as T decreases the ferromagnetic signal broadens and nearly disappears upon reaching 15 K, whereas it sharpens abruptly at 9 K. Such a sudden change is typical of a phase transition or some other physical changes in the sample. To gain insights into its nature, we recorded an additional set of hysteresis experiments by increasing the temperature from 9 K (Figure 7b). A comparison between the plots in Figure 7a,b shows that the spectra of the two sets are remarkably different. This difference could be attributed to a phase transition, (64) but this would reproduce the pattern when the experiment at the given temperature is repeated. Instead, the memory of the phenomenon that takes place at 9 K is evidently maintained in the subsequent experiments, as shown in Figure 7b, which indicates that an irreversible transformation occurred. It is also worth noting that once the temperature is >40 K, virtually the same spectrum is observed, regardless of how that temperature was reached. The spectra become indistinguishable (we checked it up to 290 K by repeating the same patterns of Figure 6), essentially when hysteresis disappears. The same signal shape obtained at 9 K is also observed after the sample is kept for 1 day at room temperature. It disappears only upon physically removing, shaking, and then reinserting the EPR tube into the cavity. The most plausible explanation for the phenomenon occurring at 9 K is thus a physical reorientation of the microcrystals, as similarly observed for the large crystal/s. The microcrystals would thus minimize their magnetic energy by aligning their anisotropy axes with the magnetic field, with the consequence of sharpening the signal. Once the crystals reorganize, the resulting orientation is maintained. This phenomenon takes place only at low temperature because at higher temperatures the sample can minimize its energy by another relaxation mechanism, that is, partial alignment of the magnetization with the field. To do this, it must overcome an energy barrier due to magnetic anisotropy. At low temperature, this barrier is too high compared to the thermal energy, and therefore, the physical rotation mechanism prevails.

The measurements carried out on the ensemble of microcrystals show that also this sample exhibits a ferromagnetic behavior; in this case, however, a paramagnetic component, associated with weakly interacting clusters in less crystalline zones, is also observed. These results further confirm that the observed ferromagnetic behavior is strongly affected by the physical characteristics of the samples. In the following sections, we will address possible explanations for the observed behaviors.

Theoretical Analysis of the EPR Data

The leading factor that controls the magnetization of a ferromagnetic particle in definite directions is the magnetic anisotropy energy. Microscopically, for heavy elements, the anisotropy energy is mainly determined by the SO interaction. (65) The SO coupling constant for the isolated cluster can be estimated from the cw-EPR spectra of the Au₂₅(SC2Ph)₁₈⁰ film, which corresponds to the solid-state situation in which the clusters are comparatively more magnetically isolated. Thus, we developed a model by starting from the superatom concept (66) in which, for the diamagnetic anion, the highest occupied molecular orbitals (HOMOs) are viewed as consisting of three degenerate P-type superatomic orbitals. In fact, we already discussed that this triple degeneracy is not strictly applicable as one orbital is found at a higher energy than the others; (36) this was also found by taking into account the effect of ligands on the frontier orbitals. (67) Even by assuming full degeneracy, it is clear that upon the removal of one electron to form Au₂₅(SC2Ph)₁₈⁰, which has an effective spin s = 1/2, further orbital splitting occurs. Degeneracy can be removed by SO coupling (68) and/or distortions because of the crystal field and the Jahn–Teller effect. (41, 69)

The total Hamiltonian is then given by eq 3

(3)where the four terms are the SO, the crystal field, the spin, and the orbital Zeeman Hamiltonians (caused by the applied magnetic field); S and L are the spin and orbital moment operators, L and L_z are the modulus quantum number and the z component operator of L, g_e is the electronic g factor, λ is the spin–orbit constant, D is the axial distortion parameter, and B is the applied magnetic field. The eigen energies of this Hamiltonian and the corresponding EPR spectrum were calculated by matrix diagonalization.

The red trace in Figure 8 shows the simulation of the experimental spectrum that was carried out by using both the λ and D values as fitting parameters. We find that, collectively, SO coupling and crystal-field distortion make the energy of the three now-nondegenerate HOMOs span an overall difference of 0.26 eV. This is an interesting result indeed because it provides new relevant information regarding the debated problem of the origin of orbital splitting upon the formation of the Au₂₅ SOMO. One view is that this is mainly due to the SO coupling, (68) whereas another assert is that it is a Jahn–Teller-like distortion effect. (41) In fact, according to our analysis of the EPR data, both SO coupling and distortion contribute by comparable amounts to the overall orbital splitting. As a further matter of fact, Figure 8 shows that the simulations carried out by including only the SO effect or the crystal-field term cannot reproduce the experimental spectrum.

Figure 8. Simulations of the cw-EPR spectrum obtained at 5 K for the Au₂₅(SC2Ph)₁₈⁰ film (black). The simulations include SO and distortion (red), only SO (blue), and only distortion (green).

The effect of the SO coupling on the Au₂₅(SC2Ph)₁₈⁰ crystals can also be evaluated from the spectrum of the single crystal. As we described above, for the single crystal, we obtain a mean g value of 2.745. The remarkable deviation from the free electron g_e value of 2.0023 indicates a high orbital moment and a substantial contribution of SO effects. Indeed, only large SO couplings allow for the orbital moment not to be quenched by the crystal field. The ratio between the orbital and spin moments can be calculated, using the Kittel equation (70)

(4)

For our system, this ratio is 0.37. It is worth mentioning that the importance of the orbital contribution to the observed magnetism is a feature that was already observed for large Au nanoparticles. (26)

DFT Calculations

DFT simulations were performed to draw further insights into the magnetic properties of Au₂₅(SR)₁₈ and quantify to what extent magnetism is affected by the interplay of factors including SO coupling, Jahn–Teller symmetry breaking, and crystal assembly (i.e., the difference between the single cluster and its assembly in the crystal). To disentangle these effects, three different structural models were considered for the neutral Au₂₅(SR)₁₈ species: two of them consist of individual Au₂₅(SCH₃)₁₈ clusters, the first one where the Au₂₅(SC)₁₈ core was taken from the experimental crystal data (49) (adding and relaxing H atoms as needed) and a second one where the geometry of anionic Au₂₅(SCH₃)₁₈^– was fully relaxed at the DFT/PBE0 level, and a third periodic solid-state model of four Au₂₅(SC2Ph)₁₈⁰ clusters in the unit cell. Hereafter, these models are denoted as Au₂₅(SCH₃)₁₈⁰ crystal, Au₂₅(SCH₃)₁₈⁰ anion, and Au₂₅(SC2Ph)₁₈⁰ crystal, respectively. Transforming Au₂₅(SC2Ph)₁₈⁰ into Au₂₅(SCH₃)₁₈⁰ is a convenient way of reducing the computational effort, and the comparison between the Au₂₅(SCH₃)₁₈⁰-crystal and Au₂₅(SC2Ph)₁₈⁰-crystal models will assess the effect of this commonly used approximation. The Jahn–Teller symmetry breaking is absent in the anionic Au₂₅(SCH₃)₁₈^–, which is an electronic closed-shell species, and thus, the comparison between the Au₂₅(SCH₃)₁₈⁰-crystal and Au₂₅(SCH₃)₁₈⁰-anion models helps quantify Jahn–Teller effects. The NWChem package (71) was employed to simulate individual MPCs by using the hybrid B3LYP (72) exchange–correlation (xc) DFT functional at the scalar relativistic level or by treating the SO coupling effects within the zeroth-order relativistic approximation (ZORA) (73) and the van Wüllen formalism. (74) To the best of our knowledge, this is the first time that a hybrid xc functional and SO coupling are simultaneously employed to describe an MPC. The OpenMx package (75) using the local density approximation (LDA) (76) was used for the solid-state nonspin-collinear calculations. Further details and a comparison/validation of the present approach with previous literature are provided in the Supporting Information.

The orbital scheme predicted by these simulations is summarized in Figure 9. In the absence of Jahn–Teller symmetry breaking, the geometry of the Au₂₅(SCH₃)₁₈⁰-anion model approximately corresponds to an S6 point symmetry group that presents triply degenerate superatomic 1P orbitals, although in the anion, as already noted, (36, 68) a residual splitting between two higher lying orbitals and one lower lying orbital is present (∼0.04 eV). Switching to the neutral species and introducing cluster deformation due to the Jahn–Teller effect in the Au₂₅(SCH₃)₁₈⁰-crystal model completely lift the degeneracy of the 1P orbitals, leaving a higher lying SOMO, a HOMO – 1 lower in energy by 0.04 eV, and a HOMO – 2 further lower in energy by 0.09 eV. SO coupling further increases the orbital splitting by bringing the first and second energy gaps to 0.12 and 0.15 eV, respectively. Because of SO coupling and Jahn–Teller effects, the three HOMOs are found to span an overall energy difference of 0.27 eV, in an excellent agreement with the EPR-derived value of 0.26 eV.

Figure 9. Diagram of DFT/B3LYP HOMO orbital energies (eV) in Au₂₅(SCH₃)₁₈⁰ systems. From left to right: Au₂₅(SCH₃)₁₈⁰ at the scalar relativistic level in the Au₂₅(SCH₃)₁₈⁰-anion geometry, Au₂₅(SCH₃)₁₈⁰ at the scalar relativistic level in the Au₂₅(SCH₃)₁₈⁰-crystal geometry, which includes Jahn–Teller (J–T) effects, and Au₂₅(SCH₃)₁₈⁰ including SO coupling (SOC) in the Au₂₅(SCH₃)₁₈⁰-crystal geometry.

Nonspin-collinear DFT/LDA calculations were performed on the Au₂₅(SC2Ph)₁₈⁰-crystal model (for details, see the Supporting Information). Calculations in which spin modulus and orientation were relaxed starting from several initial orientations were first conducted to determine the preferential magnetization axis, which turns out to be the z-axis with a total spin component of 3.59μ_B and a total orbital component of 1.25μ_B per unit cell. This corresponds to a predicted ratio μ_L/μ_S of 0.35, in excellent agreement with the value calculated from the single-crystal spectrum simulation, 0.37. The direction and magnitude of atomic spins in the energetically most stable ferromagnetic solution thus derived are schematically depicted in Figure 10. It is worth noting that the spin density is mostly located on Au atoms but also extends on S and on both aliphatic and aromatic carbons. An exponentially decreasing delocalization of the spin moment from the Au/S MPC framework onto both aliphatic and aromatic C atoms has been noted and studied before. (35) Here, we find that spin polarization is induced also on the phenyl groups, as shown in the form of the small arrows displayed on the rings in Figure 10; this long-range effect is likely due to a solid-state proximity effect by adjacent S atoms. This finding would thus rationalize the experimentally observed magnetism in the solid state and its subtle dependence on crystallinity as caused by the presence of oriented spin moments on the neighboring π-stacked phenyl residues. (77) We were not able to locate the barrier for spin reorientation and thus the anisotropy energy. It is, however, worth mentioning that in our calculations we found another spin local minimum in which the magnetic moment is oriented along the x-axis (see Figure 10) with a total spin component per unit cell of 2.02μ_B and a total orbital component per unit cell of 0.37μ_B, nearly degenerate in energy with the spin global minimum.

Figure 10. Schematic depiction of the direction and magnitude of atomic spins (green arrows) in the putative spin global minimum (the spins on the Au atoms are not shown as they would be out of scale). The image shows the unit cell as seen from the direction c; (49) all clusters but the central one are thus incomplete. The color codes are Au = yellow, S = red, and C = gray. Au and S atoms and bonds are rendered as balls and sticks, whereas C is rendered as the stick style. H atoms have been removed for clarity.

Conclusions

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Magnetometry techniques are generally used to study the magnetic properties of materials but have failed to provide coherent results for Au MPCs. The most important cause of the discrepancy in previous studies was undoubtedly the lack of precise control on MPC stoichiometry and charge state. Interestingly, even for a controlled MPC such as paramagnetic Au₂₅(SC2Ph)₁₈⁰, SQUID was unable to detect magnetic behaviors other than simple paramagnetism. Here, we employ the more molecular experimental approach based on EPR spectroscopy, which allows separating different contributions to the magnetic susceptibility by focusing on clearly distinguishable signals and eliminating diamagnetic contributions.

By using samples meant to provide a range of specific examples of crystalline orders and physical states, we could detect paramagnetism, superparamagnetism, and ferromagnetism and evidence physical reorganization of the samples as a function of the applied field. Besides rationalizing the relevant phenomenological aspects, we carried out theoretical analyses. Simulations of the EPR spectra based on the superatom model showed that both SO coupling and crystal-field distortions play a role in determining the EPR properties of Au₂₅(SC2Ph)₁₈⁰ in the solid state. The excellent agreement of the experimentally derived effects brought about by SO and crystal splitting, as well as the ratio between the orbital and spin moments, with the outcome of complex first principles simulations unequivocally supports the soundness of the present analysis. Calculations point to the proximity effects in the solid state as the origin of magnetic interactions and the reason for their crucial dependence upon crystallinity.

We believe that this study provides a key to understand the conflicting magnetic behaviors in solid MPC samples. Together with our previous findings concerning the ligand-induced antiferromagnetic behavior in Au₂₅ clusters, (42) it is now clear that several factors should be considered for effectively controlling the magnetic behavior of MPCs. As also discussed in the Introduction, for larger MPCs of unknown structure and possibly variable charge states, the situation is more complex and probably definable only on a statistical basis. Regardless, the results described here for Au₂₅(SC2Ph)₁₈⁰ could pave the way to enable controlled magnetism-related applications of gold MPCs, especially those based on the use of molecular MPCs.

Experimental Section

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Au₂₅(SC2Ph)₁₈⁰ Synthesis

The synthesis of Au₂₅(SC2Ph)₁₈ was carried out in tetrahydrofuran (THF). The details are as already described, (35) except for the addition of tetra-n-octylammonium (nOct₄N⁺) bromide, before the reduction steps, to the THF solution containing HAuCl₄·3H₂O. The cluster was prepared as [nOct₄N⁺][Au₂₅(SC2Ph)₁₈^–] and purified by dissolving it in a mixture of diethyl ether (to precipitate most of the residual tetraoctylammonium salt) and by washing the product, obtained by the evaporation of diethyl ether, with ice-cold methanol.

Preparation of the Film

A sample of the so-prepared [nOct₄N⁺][Au₂₅(SC2Ph)₁₈^–] was quantitatively oxidized to form Au₂₅(SC2Ph)₁₈⁰ by passing through a silica-gel chromatography column under aerobic conditions. (50) Au₂₅(SC2Ph)₁₈⁰ (4.0 mg) was dissolved in 1 mL of dichloromethane and injected into an EPR tube. The solvent was evaporated with a stream of nitrogen to leave an amorphous colored film covering the bottom wall of the tubing.

Preparation of the Single Crystals

Large single crystals were prepared by electrocrystallization. (49) The experiments were carried out with a CHI 660c electrochemical workstation, under an Ar atmosphere in an air-tight glass electrochemical cell, at room temperature, and using 20 mL of MeCN containing 0.1 M tetra-n-butylammonium hexafluorophosphate as the solvent–electrolyte system. The working electrode was a 0.75 mm diameter, 15 mm long gold wire, and the counter electrode was a Pt plate inserted into a glass holder separated from the analyte solution with a G3 glass frit and a plug of electrolyte-saturated methylcellulose gel. (78) The electrolysis was carried out at a constant current of 200 nA. The one-electron electro-oxidation of 4.82 × 10^–5 M Au₂₅(SC2Ph)₁₈^– was carried out until 8% of the anion was still present in the solution. The electrogenerated Au₂₅(SC2Ph)₁₈⁰ is insoluble in MeCN and deposits well onto the electrode body to form a forest of single crystals. The single crystals were collected from the electrocrystallization experiment that led to the image shown in Figure S1. All pictures were taken using a Firefly GT800 High Precision Video Microscope.

Electron Paramagnetic Resonance

The crystalline Au₂₅(SCPh)₁₈⁰ samples, one single crystal, a few crystals, or many microcrystals, were introduced into 1.9 mm i.d.–3.0 mm o.d. (used for the film and the microcrystals) or 2.9 mm i.d.–3.9 mm o.d. (used for all other samples) quartz tubes. The tubes containing the film or crystals were degassed by several freeze–pump–thaw cycles and sealed off under vacuum (5 × 10^–5 Torr). X-band cw-EPR spectra were recorded using a Bruker Elexsys E580 spectrometer equipped with a dielectric probehead. The temperature was controlled by a helium continuous-flow cryostat (Oxford CF935) and a variable-temperature controller unit (Oxford ITC-4). When the desired temperature was reached, the samples were thermalized before carrying out the actual experiments. All experimental data were collected under nonsaturating microwave conditions (microwave power P_MW = 150 μW or lower). A modulation frequency of 100 kHz and an amplitude (peak to peak) of 1 G were used for all spectra. The field scan rate was 47.68 G s^–1. Simulation of EPR spectra was carried out by using the Matlab 7.12 software platform. The ferromagnetic and paramagnetic signals were simulated with ad hoc written codes based on the models developed in this paper. The standard g-tensor-based simulations were performed using the routines from the EasySpin toolbox. (79)

Supporting Information

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The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsomega.7b00472.

Details on the DFT calculations, EPR simulations, and further figures (PDF)

ao7b00472_si_001.pdf (11.83 MB)

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Most electronic Supporting Information files are available without a subscription to ACS Web Editions. Such files may be downloaded by article for research use (if there is a public use license linked to the relevant article, that license may permit other uses). Permission may be obtained from ACS for other uses through requests via the RightsLink permission system: http://pubs.acs.org/page/copyright/permissions.html.

Author Information

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Corresponding Authors
- Mikhail Agrachev - Department of Chemistry, University of Padova, via Marzolo 1, 35131 Padova, Italy; Email: [email protected]
- Flavio Maran - Department of Chemistry, University of Padova, via Marzolo 1, 35131 Padova, Italy; Department of Chemistry, University of Connecticut, 55 North Eagleville Road, Storrs, Connecticut 06269, United States; http://orcid.org/0000-0002-8627-6491; Email: [email protected]
Authors
- Sabrina Antonello - Department of Chemistry, University of Padova, via Marzolo 1, 35131 Padova, Italy
- Tiziano Dainese - Department of Chemistry, University of Padova, via Marzolo 1, 35131 Padova, Italy
- Marco Ruzzi - Department of Chemistry, University of Padova, via Marzolo 1, 35131 Padova, Italy
- Alfonso Zoleo - Department of Chemistry, University of Padova, via Marzolo 1, 35131 Padova, Italy
- Edoardo Aprà - William R. Wiley Environmental Molecular Sciences Laboratory, Pacific Northwest National Laboratory, Richland, Washington 99352, United States; http://orcid.org/0000-0001-5955-0734
- Niranjan Govind - William R. Wiley Environmental Molecular Sciences Laboratory, Pacific Northwest National Laboratory, Richland, Washington 99352, United States
- Alessandro Fortunelli - CNR-ICCOM & IPCF, Consiglio Nazionale delle Ricerche, 56124 Pisa, Italy; http://orcid.org/0000-0001-5337-4450
- Luca Sementa - CNR-ICCOM & IPCF, Consiglio Nazionale delle Ricerche, 56124 Pisa, Italy
Notes
The authors declare no competing financial interest.

Acknowledgment

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This work was partially supported by AIRC (FM, Project 12214: Innovative tools for cancer risk assessment and early diagnosis—5 per mille). Computational research was performed with resources provided by PNNL Institutional Computing (PIC), EMSL, which is a DOE Office of Science User Facility, sponsored by the Office of Biological and Environmental Research and located at PNNL, and CINECA Supercomputing Center (ISCRA program).

References

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Granja, L. P.; Martínez, E. D.; Troiani, H.; Sanchez, C.; Illia, G. J. A. A. S. Magnetic Gold Confined in Ordered Mesoporous Titania Thin Films: A Noble Approach for Magnetic Devices ACS Appl. Mater. Interfaces 2017, 9, 965– 971 DOI: 10.1021/acsami.6b15189
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12
Hembury, M.; Chiappini, C.; Bertazzo, S.; Kalber, T. L.; Drisko, G. L.; Ogunlade, O.; Walker-Samuel, S.; Krishna, K. S.; Jumeaux, C.; Beard, P.; Kumar, C. S. S. R.; Porter, A. E.; Lythgoe, M. F.; Boissière, C.; Sanchez, C.; Stevens, M. M. Gold–silica quantum rattles for multimodal imaging and therapy Proc. Natl. Acad. Sci. U.S.A. 2015, 112, 1959– 1964 DOI: 10.1073/pnas.1419622112
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12
Gold-silica quantum rattles for multimodal imaging and therapy
Hembury, Mathew; Chiappini, Ciro; Bertazzo, Sergio; Kalber, Tammy L.; Drisko, Glenna L.; Ogunlade, Olumide; Walker-Samuel, Simon; Krishna, Katla Sai; Jumeaux, Coline; Beard, Paul; Kumar, Challa S. S. R.; Porter, Alexandra E.; Lythgoe, Mark F.; Boissiere, Cedric; Sanchez, Clement; Stevens, Molly M.
Proceedings of the National Academy of Sciences of the United States of America (2015), 112 (7), 1959-1964CODEN: PNASA6; ISSN:0027-8424. (National Academy of Sciences)
Gold quantum dots exhibit distinctive optical and magnetic behaviors compared with larger gold nanoparticles. However, their unfavorable interaction with living systems and lack of stability in aq. solvents has so far prevented their adoption in biol. and medicine. Here, a simple synthetic pathway integrates gold quantum dots within a mesoporous silica shell, alongside larger gold nanoparticles within the shell's central cavity. This "quantum rattle" structure is stable in aq. solns., does not elicit cell toxicity, preserves the attractive near-IR photonics and paramagnetism of gold quantum dots, and enhances the drug-carrier performance of the silica shell. In vivo, the quantum rattles reduced tumor burden in a single course of photothermal therapy while coupling three complementary imaging modalities: near-IR fluorescence, photoacoustic, and magnetic resonance imaging. The incorporation of gold within the quantum rattles significantly enhanced the drug-carrier performance of the silica shell. This innovative material design based on the mutually beneficial interaction of gold and silica introduces the use of gold quantum dots for imaging and therapeutic applications.
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Hori, H.; Teranishi, T.; Nakae, Y.; Seino, Y.; Miyake, M.; Yamada, S. Anomalous magnetic polarization effect of Pd and Au nano-particles Phys. Lett. A 1999, 263, 406– 410 DOI: 10.1016/s0375-9601(99)00742-2
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13
Anomalous magnetic polarization effect of Pd and Au nano-particles
Hori, H.; Teranishi, T.; Nakae, Y.; Seino, Y.; Miyake, M.; Yamada, S.
Physics Letters A (1999), 263 (4,5,6), 406-410CODEN: PYLAAG; ISSN:0375-9601. (Elsevier Science B.V.)
The magnetization of Pd and Au nano-particles with a diam. of at ∼3 nm was measured down to 4.2 K. The data of magnetization of both nano-particles show an unexpectedly large magnetic moment of ∼20 spins per particle. This result can not be understood by so-called odd/even electron no. effect and Stoner's enhancement model. The result might suggest the existence of some common and characteristic spin correlation mechanism in the nano-particles. The exptl. result of size dependence on Pd nano-particles shows existence of a crit. size of ∼3 nm from an at. spin correlation region to a metallic spin correlation region.
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14
Brust, M.; Walker, M.; Bethell, D.; Schiffrin, D. J.; Whyman, R. Synthesis of Thiol-derivatised Gold Nanoparticles in a Two-Phase Liquid–Liquid System J. Chem. Soc., Chem. Commun. 1994, 801– 802 DOI: 10.1039/c39940000801
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14
Synthesis of thiol-derivatized gold nanoparticles in a two-phase liquid-liquid system
Brust, Mathias; Walker, Merryl; Bethell, Donald; Schiffrin, David J.; Whyman, Robin
Journal of the Chemical Society, Chemical Communications (1994), (7), 801-2CODEN: JCCCAT; ISSN:0022-4936.
Using two-phase (water-toluene) redn. of AuCl4- by sodium borohydride in the presence of an alkanethiol, solns. of 1-3 nm gold particles bearing a surface coating of thiol have been prepd. and characterized; this novel material can be handled as a simple chem. compd.
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Crespo, P.; García, M. A.; Pinel, E. F.; Multigner, M.; Alcántara, D.; De La Fuente, J. M.; Penadés, S.; Hernando, A. Fe impurities weaken the ferromagnetic behavior in Au nanoparticles Phys. Rev. Lett. 2006, 97, 177203 DOI: 10.1103/PhysRevLett.97.177203
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16
Donnio, B.; García-Vázquez, P.; Gallani, J.-L.; Guillon, D.; Terazzi, E. Dendronized ferromagnetic gold nanoparticles self-organized in a thermotropic cubic phase Adv. Mater. 2007, 19, 3534– 3539 DOI: 10.1002/adma.200701252
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Dendronized ferromagnetic gold nanoparticles self-organized in a thermotropic cubic phase
Donnio, Bertrand; Garcia-Vazquez, Patricia; Gallani, Jean-Louis; Guillon, Daniel; Terazzi, Emmanuel
Advanced Materials (Weinheim, Germany) (2007), 19 (21), 3534-3539CODEN: ADVMEW; ISSN:0935-9648. (Wiley-VCH Verlag GmbH & Co. KGaA)
Gold nanoparticles possessing a positionally ordered 3-dimensional liq. cryst. phase in the bulk were prepd. Induction of the mesophase results from the synergy between the gold core and the nonmesogenic dendrons. These particles are also ferromagnetic up to 400 K and are able to self-organize in a 2-dimensional hexagonal array on solid substrates, opening perspectives in the field of information storage at the mol. level.
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17
Yamamoto, Y.; Hori, H. Direct observation of the ferromagnetic spin polarization in gold nanoparticles: A review Rev. Adv. Mater. Sci. 2006, 12, 23– 32
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18
Cirri, A.; Silakov, A.; Lear, B. J. Ligand Control over the Electronic Properties within the Metallic Core of Gold Nanoparticles Angew. Chem., Int. Ed. 2015, 54, 11750– 11753 DOI: 10.1002/anie.201505933
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18
Ligand Control over the Electronic Properties within the Metallic Core of Gold Nanoparticles
Cirri, Anthony; Silakov, Alexey; Lear, Benjamin J.
Angewandte Chemie, International Edition (2015), 54 (40), 11750-11753CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
The behavior of electrons within the metallic core of gold nanoparticles (AuNPs) can be controlled by the nature of the surface chem. of the AuNPs. Specifically, the conduction ESR (CESR) spectra of AuNPs of diam. 1.8-1.9 nm are sensitive to ligand exchange of hexanethiol for 4-bromothiophenol on the surface of the nanoparticle. Chemisorption of the arom. ligand leads to a shift in the metallic electron's g-factor toward the value expected for pure gold systems, suggesting an increase in metallic character for the electrons within the gold core. Anal. by UV/Vis absorption spectroscopy reveals a concomitant bathochromic shift of the surface plasmon resonance band of the AuNP, indicating that other electronic properties of AuNPs are also affected by the ligand exchange. In total, our results demonstrate that the chem. nature of the ligand controls the valence band structure of AuNPs.
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Cirri, A.; Silakov, A.; Jensen, L.; Lear, B. J. Chain Length and Solvent Control over the Electronic Properties of Alkanethiolate-Protected Gold Nanoparticles at the Molecule-to-Metal Transition J. Am. Chem. Soc. 2016, 138, 15987– 15993 DOI: 10.1021/jacs.6b09586
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19
Chain Length and Solvent Control over the Electronic Properties of Alkanethiolate-Protected Gold Nanoparticles at the Molecule-to-Metal Transition
Cirri, Anthony; Silakov, Alexey; Jensen, Lasse; Lear, Benjamin J.
Journal of the American Chemical Society (2016), 138 (49), 15987-15993CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
Alkanethiolate protected gold nanoparticles are one of the most widely used systems in modern science and technol., where the emergent electronic properties of the gold core are valued for use in applications such as plasmonic solar cells, photocatalysis, and photothermal heating. Though choice in alkane chain length is not often discussed as a way in which to control the electronic properties of these nanoparticles, we show that the chain length of the alkyl tail exerts clear control over the electronic properties of the gold core, as detd. by conduction ESR spectroscopy. The control exerted by chain length is reported on by changes to the g-factor of the metallic electrons, which we can relate to the av. surface potential on the gold core. We propose that the surface potential is modulated by direct charge donation from the ligand to the metal, resulting from the formation of a chem. bond. Furthermore, the degree of charge transfer is controlled by differences between the dielec. const. of the medium and the ligand shell. Together, these observations are used to construct a simple electrostatic model that provides a framework for understanding how surface chem. can be used to modulate the electronic properties of gold nanoparticles.
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Cirri, A.; Silakov, A.; Jensen, L.; Lear, B. J. Probing ligand-induced modulation of metallic states in small gold nanoparticles using conduction electron spin resonance Phys. Chem. Chem. Phys. 2016, 18, 25443– 25451 DOI: 10.1039/c6cp02205g
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21
Gréget, R.; Nealon, G. L.; Vileno, B.; Turek, P.; Mény, C.; Ott, F.; Derory, A.; Voirin, E.; Rivière, E.; Rogalev, A.; Wilhelm, F.; Joly, L.; Knafo, W.; Ballon, G.; Terazzi, E.; Kappler, J.-P.; Donnio, B.; Gallani, J.-L. Magnetic properties of gold nanoparticles: a room-temperature quantum effect ChemPhysChem 2012, 13, 3092– 3097 DOI: 10.1002/cphc.201200394
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21
Magnetic Properties of Gold Nanoparticles: A Room-Temperature Quantum Effect
Greget, Romain; Nealon, Gareth L.; Vileno, Bertrand; Turek, Philippe; Meny, Christian; Ott, Frederic; Derory, Alain; Voirin, Emilie; Riviere, Eric; Rogalev, Andrei; Wilhelm, Fabrice; Joly, Loic; Knafo, William; Ballon, Geraldine; Terazzi, Emmanuel; Kappler, Jean-Paul; Donnio, Bertrand; Gallani, Jean-Louis
ChemPhysChem (2012), 13 (13), 3092-3097, S3092/1-S3092/13CODEN: CPCHFT; ISSN:1439-4235. (Wiley-VCH Verlag GmbH & Co. KGaA)
The authors propose that the magnetism of gold nanopartides is of orbital origin and due to the conduction electrons being driven into persistent currents. The appearance of persistent currents under an applied magnetic field would explain the observation of both dia- and paramagnetic responses of the various samples, the high variability and the lack of thermal dependence of the magnetic properties. As the currents increase with the applied field, the magnetic moment does not sat. Occasionally, these persistent currents could even become self-sustained, giving rise to a soft ferromagnetism. The effect could be reinforced if the nanopartides are locally arranged with the proper geometry. However, obtaining direct proof of the existence of such persistent currents is challenging. The authors suggest that near-field microscopy techniques could be used. One may think of attaching a gold nanoparticle to the cantilever of an AFM. When placed in a magnetic field, the vibration frequency would be sensitive to any extra force generated by the magnetic moment of the nanoparticle. If the moment is indeed field-induced and nonpermanent, no extra force should be perceived in zero field. One may also consider submitting larger Au nanopartides to high magnetic fields and look for Aharonov-Bohm oscillations in their magnetic response. Given the orbital nature of the magnetic moment, it is also worth trying magnetooptical expts. using circularly polarized light to probe the magnetoplasmons. An increase or a decrease of the magnetization might be obsd. upon reversal of the polarization, though the intensity of the local magnetic field may be too small.
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Muñoz-Márquez, M. A.; Guerrero, E.; Fernández, A.; Crespo, P.; Hernando, A.; Lucena, R.; Conesa, J. C. Permanent magnetism in phosphine- and chlorine-capped gold: From clusters to nanoparticles J. Nanopart. Res. 2010, 12, 1307– 1318 DOI: 10.1007/s11051-010-9862-0
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22
Permanent magnetism in phosphine- and chlorine-capped gold: from clusters to nanoparticles
Munoz-Marquez, Miguel A.; Guerrero, Estefania; Fernandez, Asuncion; Crespo, Patricia; Hernando, Antonio; Lucena, Raquel; Conesa, Jose C.
Journal of Nanoparticle Research (2010), 12 (4), 1307-1318CODEN: JNARFA; ISSN:1388-0764. (Springer)
Magnetometry results have shown that gold NPs (∼2 nm in size) protected with phosphine and chlorine ligands exhibit permanent magnetism. When the NPs size decreases down to the subnanometric size range, e.g. undecagold atom clusters, the permanent magnetism disappears. The near edge structure of the X-ray absorption spectroscopy data points out that charge transfer between gold and the capping system occurs in both cases. These results strongly suggest that nearly metallic Au bonds are also required for the induction of a magnetic response. ESR observations indicate that the contribution to magnetism from eventual iron impurities can be disregarded.
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Guerrero, E.; Muñoz-Márquez, M. A.; García, M. A.; Crespo, P.; Fernández-Pinel, E.; Hernando, A.; Fernández, A. Surface plasmon resonance and magnetism of thiol-capped gold nanoparticles Nanotechnology 2008, 19, 175701 DOI: 10.1088/0957-4484/19/17/175701
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23
Surface plasmon resonance and magnetism of thiol-capped gold nanoparticles
Guerrero, E.; Munoz-Marquez, M. A.; Garcia, M. A.; Crespo, P.; Fernandez-Pinel, E.; Hernando, A.; Fernandez, A.
Nanotechnology (2008), 19 (17), 175701/1-175701/6CODEN: NNOTER; ISSN:0957-4484. (Institute of Physics Publishing)
Surface plasmon resonance measurements and magnetic characterization studies have been carried out for two types of thiol-capped gold nanoparticles (NPs) with similar diams. between 2.0 and 2.5 nm and different org. mols. linked to the sulfur atom: dodecanethiol and tiopronin. In addn., Au NPs capped with tetraoctyl ammonium bromide have also been included in the investigation since such capping mols. weakly interact with the gold surface atoms and, therefore, this system can be used as a model for naked gold NPs; such particles presented a bimodal size distribution with diams. around 1.5 and 5 nm. The plasmon resonance is non-existent for tiopronin-capped NPs, whereas a trace of such a feature is obsd. for NPs covered with dodecanethiol mols. and a bulk-like feature is measured for NPs capped with tetralkyl ammonium salts. These differences would indicate that the modification of the surface electronic structure of the Au NPs depends on the geometry and self-assembling capabilities of the capping mols. and on the elec. charge transferred between Au and S atoms. Regarding the magnetization, dodecanethiol-capped NPs have a ferromagnetic-like behavior, while the NPs capped with tiopronin exhibit a paramagnetic behavior and tetralkyl ammonium-protected NPs are diamagnetic across the studied temp. range; straight chains with a well-defined symmetry axis can induce orbital momentum on surface electrons close to the binding atoms. The orbital momentum not only contributes to the magnetization but also to the local anisotropy, giving rise to permanent magnetism. Due to the domain structure of the adsorbed mols., orbital momentum is not induced for tiopronin-capped NPs and the charge transfer only induces a paramagnetic spin component.
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Tuboltsev, V.; Savin, A.; Pirojenko, A.; Räisänen, J. Magnetism in nanocrystalline gold ACS Nano 2013, 7, 6691– 6699 DOI: 10.1021/nn401914b
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Magnetism in Nanocrystalline Gold
Tuboltsev, Vladimir; Savin, Alexander; Pirojenko, Alexandre; Raisanen, Jyrki
ACS Nano (2013), 7 (8), 6691-6699CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
While bulk Au is known to be diamagnetic, there is a growing body of convincing exptl. and theor. work indicating that nanostructured Au can be imparted with unconventional magnetic properties. Bridging the current gap in exptl. study of magnetism in bare Au nanomaterials, the authors report here on magnetism in Au nanocryst. films produced by cluster deposition in the aggregate form that can be considered as a crossover state between a nanocluster and a continuous film. The authors demonstrate ferromagnetic-like hysteretic magnetization with temp. dependence indicative of spin-glass-like behavior and find this to be consistent with theor. predictions, available in the literature, based on 1st-principles calcns.
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Maitra, U.; Das, B.; Kumar, N.; Sundaresan, A.; Rao, C. N. R. Ferromagnetism exhibited by nanoparticles of noble metals ChemPhysChem 2011, 12, 2322– 2327 DOI: 10.1002/cphc.201100121
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25
Ferromagnetism exhibited by nanoparticles of noble metals
Maitra, Urmimala; Das, Barun; Kumar, Nitesh; Sundaresan, Athinarayanan; Rao, C. N. R.
ChemPhysChem (2011), 12 (12), 2322-2327CODEN: CPCHFT; ISSN:1439-4235. (Wiley-VCH Verlag GmbH & Co. KGaA)
Au nanoparticles with av. diams. in the range 2.5-15 nm, prepd. at the org./aq. interface by tetrakis(hydroxymethyl)phosphonium chloride (THPC) as reducing agent, exhibit ferromagnetism whereby the satn. magnetization MS increases with decreasing diam. and varies linearly with the fraction of surface atoms. The value of MS is higher when the particles are present as a film instead of as a sol. Capping with strongly interacting ligands such as alkane thiols results in a higher MS value, which varies with the strength of the metal-S bond. Ferromagnetism is also found in Pt and Ag nanoparticles prepd. as sols, and the MS values vary as Pt > Au > Ag. A careful study of the temp. variation of the magnetization of Au nanoparticles, along with certain other observations, suggests that small bare nanoparticles of noble metals could indeed possess ferromagnetism, albeit weak, which is accentuated in the presence of capping agents, specially alkane thiols which form strong metal-S bonds.
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Hernando, A.; Crespo, P.; García, M. A.; Pinel, E. F.; De La Venta, J.; Fernández, A.; Penadés, S. Giant magnetic anisotropy at the nanoscale: Overcoming the superparamagnetic limit Phys. Rev. B: Condens. Matter Mater. Phys. 2006, 74, 52403 DOI: 10.1103/PhysRevB.74.052403
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27
Krishna, K. S.; Tarakeshwar, P.; Mujica, V.; Kumar, C. S. S. R. Chemically induced magnetism in atomically precise gold clusters Small 2014, 10, 907– 911 DOI: 10.1002/smll.201302393
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27
Chemically Induced Magnetism in Atomically Precise Gold Clusters
Krishna, Katla Sai; Tarakeshwar, Pilarisetty; Mujica, Vladimiro; Kumar, Challa S. S. R.
Small (2014), 10 (5), 907-911CODEN: SMALBC; ISSN:1613-6810. (Wiley-VCH Verlag GmbH & Co. KGaA)
Authors have fabricated atomically precise, ligand-stabilized Au25, Au38 and Au55 clusters and characterized them. They investigated their magnetic properties exptl. using the superconducting quantum interference device (SQUID).
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Hernando, A.; Crespo, P.; García, M. A. Origin of orbital ferromagnetism and giant magnetic anisotropy at the nanoscale Phys. Rev. Lett. 2006, 96, 57206 DOI: 10.1103/PhysRevLett.96.057206
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29
Hernando, A.; Crespo, P.; García, M. A.; Coey, M.; Ayuela, A.; Echenique, P. M. Revisiting magnetism of capped Au and ZnO nanoparticles: Surface band structure and atomic orbital with giant magnetic moment Phys. Status Solidi B 2011, 248, 2352– 2360 DOI: 10.1002/pssb.201147227
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29
Revisiting magnetism of capped Au and ZnO nanoparticles: Surface band structure and atomic orbital with giant magnetic moment
Hernando, Antonio; Crespo, Patricia; Garcia, Miguel Angel; Coey, Michael; Ayuela, Andres; Echenique, Pedro Miguel
Physica Status Solidi B: Basic Solid State Physics (2011), 248 (10), 2352-2360CODEN: PSSBBD; ISSN:0370-1972. (Wiley-VCH Verlag GmbH & Co. KGaA)
In this article we review the exotic magnetism of nanoparticles (NPs) formed by substances that are not magnetic in bulk as described with generality in Section 1. In particular, the intrinsic character of the magnetism obsd. on capped Au and ZnO NPs is analyzed. X-ray magnetic CD (XMCD) anal. has shown that the magnetic moments are intrinsic and lie in the Au and Zn atoms, resp., as analyzed in Section 2, where the general theor. ideas are also revisited. Since impurity atoms bonded to the surface act as donor or acceptor of electrons that occupy the surface states, the anomalous magnetic response is analyzed in terms of the surface band in Section 3. Finally, Section 4 summarizes our last theor. proposal.
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Murray, R. W. Nanoelectrochemistry: Metal Nanoparticles, Nanoelectrodes, and Nanopores Chem. Rev. 2008, 108, 2688– 2720 DOI: 10.1021/cr068077e
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Nanoelectrochemistry: Metal Nanoparticles, Nanoelectrodes, and Nanopores
Murray, Royce W.
Chemical Reviews (Washington, DC, United States) (2008), 108 (7), 2688-2720CODEN: CHREAY; ISSN:0009-2665. (American Chemical Society)
A review. Electrochem. of metal nanoparticles, monolayers of nanoparticles, multilayers of nanoparticles, etc., is described. Nanoelectrode fabrication and properties are discussed.
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Parker, J. F.; Fields-Zinna, C. A.; Murray, R. W. The Story of a Monodisperse Gold Nanoparticle: Au₂₅L₁₈ Acc. Chem. Res. 2010, 43, 1289– 1296 DOI: 10.1021/ar100048c
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The Story of a Monodisperse Gold Nanoparticle: Au25L18
Parker, Joseph F.; Fields-Zinna, Christina A.; Murray, Royce W.
Accounts of Chemical Research (2010), 43 (9), 1289-1296CODEN: ACHRE4; ISSN:0001-4842. (American Chemical Society)
A review. Au nanoparticles (NPs) with protecting organothiolate ligands and core diams. smaller than 2 nm are interesting materials because their size-dependent properties range from metal-like to mol.-like. This Account focuses on the most thoroughly studied of these NPs, Au25L18. Future advances in nanocluster catalysis and electronic miniaturization and biol. applications such as drug delivery will depend on a thorough understanding of nanoscale materials in which mol.-like characteristics appear. This Account tells the story of Au25L18 and its assocd. synthetic, structural, mass spectrometric, electron transfer, optical spectroscopy, and magnetic resonance results. The authors also ref. other Au NP studies to introduce helpful synthetic and measurement tools. Historically, nanoparticle sizes were described by their diams. Recently, researchers have reported actual mol. formulas for very small NPs, which is chem. preferable to solely reporting their size. Au25L18 is a success story in this regard; however, researchers initially mislabeled this NP as Au28L16 and as Au38L24 before correctly identifying it by electrospray-ionization mass spectrometry. Because of its small size, this NP is amenable to theor. studies. Au25L18's accessibility in pure form and mol.-like properties make it an attractive research target. The properties of this NP include a large energy gap readily seen in cyclic voltammetry (related to its HOMO-LUMO gap), a UV-visible absorbance spectrum with step-like fine structure, and NIR fluorescence emission. A single crystal structure and theor. anal. have served as important steps in understanding the chem. of Au25L18. Researchers detd. the single crystal structure of both its native as-prepd. form, a [N((CH2)7CH3)41+][Au25(SCH2CH2Ph)181-] salt, and of the neutral, oxidized form Au25(SCH2CH2Ph)180. A d. functional theory (DFT) anal. correctly predicted essential elements of the structure. The NP is composed of a centered icosahedral Au13 core stabilized by six Au2(SR)3 semirings. These semirings present interesting implications regarding other small Au nanoparticle clusters. Many properties of the Au25 NP result from these semiring structures. This overview of the identification, structure detn., and anal. properties of perhaps the best understood Au nanoparticle provides results that should be useful for further analyses and applications. The authors also hope that the story of this nanoparticle will be useful to those who teach about nanoparticle science.
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Heaven, M. W.; Dass, A.; White, P. S.; Holt, K. M.; Murray, R. W. Crystal Structure of the Gold Nanoparticle [N(C₈H₁₇)₄][Au₂₅(SCH₂CH₂Ph)₁₈] J. Am. Chem. Soc. 2008, 130, 3754– 3755 DOI: 10.1021/ja800561b
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Crystal Structure of the Gold Nanoparticle [N(C8H17)4][Au25(SCH2CH2Ph)18]
Heaven, Michael W.; Dass, Amala; White, Peter S.; Holt, Kennedy M.; Murray, Royce W.
Journal of the American Chemical Society (2008), 130 (12), 3754-3755CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
The authors report the crystal structure of the thiolate Au nanoparticle [TOA+][Au25(SCH2CH2Ph)18-], where TOA+ = N(C8H17)4+. Crystallog. data are given. The crystal structure reveals three types of Au atoms: (a) one central Au atom whose coordination no. is 12 (12 bonds to Au atoms); (b) 12 Au atoms that form the vertexes of an icosahedron around the central atom, whose coordination no. is 6 (five bonds to Au atoms and one to a S atom), and (c) 12 Au atoms that are stellated on 12 of the 20 faces of the Au13 icosahedron. The arrangement of the latter Au atoms may be influenced by aurophilic bonding. Together they form six orthogonal semirings, or staples, of -Au2(SCH2CH2Ph)3- in an octahedral arrangement around the Au13 core.
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Zhu, M.; Aikens, C. M.; Hollander, F. J.; Schatz, G. C.; Jin, R. Correlating the Crystal Structure of a Thiol-Protected Au²⁵ Cluster and Optical Properties J. Am. Chem. Soc. 2008, 130, 5883– 5885 DOI: 10.1021/ja801173r
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Correlating the Crystal Structure of A Thiol-Protected Au25 Cluster and Optical Properties
Zhu, Manzhou; Aikens, Christine M.; Hollander, Frederick J.; Schatz, George C.; Jin, Rongchao
Journal of the American Chemical Society (2008), 130 (18), 5883-5885CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
The total structure detn. of thiol-protected Au clusters has long been a major issue in cluster research. The authors report an unusual single crystal structure of a 25-Au-atom cluster ((Au25(SCH2CH2Ph)18) 1.27 nm diam., surface-to-surface distance) protected by eighteen phenylethanethiol ligands. Crystallog. data and at. coordinates are given. The Au25 cluster features a centered icosahedral Au13 core capped by twelve Au atoms that are situated in six pairs around the three mutually perpendicular 2-fold axes of the icosahedron. The thiolate ligands bind to the Au25 core in an exclusive bridging mode. This highly sym. structure is distinctly different from recent predictions of d. functional theory, and it also violates the empirical golden rule - cluster of clusters, which would predict a biicosahedral structure via vertex sharing of two icosahedral M13 building blocks as previously established in various 25-atom metal clusters protected by phosphine ligands. These results point to the importance of the ligand-Au core interactions. The Au25(SR)18 clusters exhibit multiple mol.-like absorption bands, and the authors find the results are in good correspondence with time-dependent d. functional theory calcns. for the obsd. structure.
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Zhu, M.; Aikens, C. M.; Hendrich, M. P.; Gupta, R.; Qian, H.; Schatz, G. C.; Jin, R. Reversible Switching of Magnetism in Thiolate-Protected Au25 Superatoms J. Am. Chem. Soc. 2009, 131, 2490– 2492 DOI: 10.1021/ja809157f
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Reversible Switching of Magnetism in Thiolate-Protected Au25 Superatoms
Zhu, Manzhou; Aikens, Christine M.; Hendrich, Michael P.; Gupta, Rupal; Qian, Huifeng; Schatz, George C.; Jin, Rongchao
Journal of the American Chemical Society (2009), 131 (7), 2490-2492CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
The authors report reversible switching of paramagnetism in a well-defined gold nanoparticle system consisting of atomically monodisperse nanoparticles contg. 25 gold atoms protected by 18 thiolates [abbreviated as Au25(SR)18]. The magnetism in these nanoparticles can be switched on or off by precisely controlling the charge state of the nanoparticle, i.e., the magnetic state of the Au25(SR)18 nanoparticles is charge-neutral while the nonmagnetic state is an anionic form of the particle. EPR spectroscopy measurements establish that the magnetic state of the Au25(SR)18 nanoparticles possess one unpaired spin per particle. EPR studies also imply an unusual electronic structure of the Au25(SR)18 nanoparticle. D. functional theory calcns. coupled with the expts. successfully explain the origin of the obsd. magnetism in a Au25(SR)18 nanoparticle as arising from one unpaired spin having distinct P-like character and delocalized among the icosahedral Au13 core of the particle in the HOMO. Probably the Au25(SR)18 nanoparticles are best considered as ligand-protected superatoms.
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Venzo, A.; Antonello, S.; Gascón, J. A.; Guryanov, I.; Leapman, R. D.; Perera, N. V.; Sousa, A.; Zamuner, M.; Zanella, A.; Maran, F. Effect of the Charge State (z = −1, 0, +1) on the Nuclear Magnetic Resonance of Monodisperse Au₂₅[S(CH₂)₂Ph]_18z Clusters Anal. Chem. 2011, 83, 6355– 6362 DOI: 10.1021/ac2012653
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Antonello, S.; Perera, N. V.; Ruzzi, M.; Gascón, J. A.; Maran, F. Interplay of Charge State, Lability, and Magnetism in the Molecule-like Au₂₅(SR)₁₈ Cluster J. Am. Chem. Soc. 2013, 135, 15585– 15594 DOI: 10.1021/ja407887d
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Interplay of charge state, lability, and magnetism in the molecule-like Au25(SR)18 c luster
Antonello, Sabrina; Perera, Neranjan V.; Ruzzi, Marco; Gascon, Jose A.; Maran, Flavio
Journal of the American Chemical Society (2013), 135 (41), 15585-15594CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
Au25(SR)18 (R = -CH2-CH2-Ph) is a mol.-like nanocluster displaying distinct electrochem. and optical features. Although it is often taken as an example of a particularly well-understood cluster, very recent literature has provided a quite unclear or even a controversial description of its properties. We prepd. monodisperse Au25(SR)180 and studied by cyclic voltammetry, under particularly controlled conditions, the kinetics of its redn. or oxidn. to a series of charge states, -2, -1, +1, +2, and +3. For each electrode process, we detd. the std. heterogeneous electron-transfer (ET) rate consts. and the reorganization energies. The latter points to a relatively large inner reorganization. Redn. to form Au25(SR)182- and oxidn. to form Au25(SR)182+ and Au25(SR)183+ are chem. irreversible. The corresponding decay rate consts. and lifetimes are incompatible with interpretations of very recent literature reports. The problem of how ET affects the Au25 magnetism was addressed by comparing the continuous-wave ESR (cw-EPR) behaviors of radical Au25(SR)180 and its oxidn. product, Au25(SR)18+. As opposed to recent exptl. and computational results, our study provides compelling evidence that the latter is a diamagnetic species. The DFT-computed optical absorption spectra and d. of states of the -1, 0, and +1 charge states nicely reproduced the exptl. estd. dependence of the HOMO-LUMO energy gap on the actual charge carried by the cluster. The conclusions about the magnetism of the 0 and +1 charge states were also reproduced, stressing that the three HOMOs are not virtually degenerate as routinely assumed: in particular, the splitting of the HOMO manifold in the cation species is severe, suggesting that the usefulness of the superatom interpretation is limited. The electrochem., EPR, and computational results thus provide a self-consistent picture of the properties of Au25(SR)18 as a function of its charge state and may furnish a methodol. blueprint for understanding the redox and magnetic behaviors of similar mol.-like gold nanoclusters.
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Antonello, S.; Dainese, T.; De Nardi, M.; Perotti, L.; Maran, F. Insights into the Interface Between the Electrolytic Solution and the Gold Core in Molecular Au₂₅ Clusters ChemElectroChem 2016, 3, 1237– 1244 DOI: 10.1002/celc.201600276
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Insights into the Interface Between the Electrolytic Solution and the Gold Core in Molecular Au25 Clusters
Antonello, Sabrina; Dainese, Tiziano; De Nardi, Marco; Perotti, Lorena; Maran, Flavio
ChemElectroChem (2016), 3 (8), 1237-1244CODEN: CHEMRA; ISSN:2196-0216. (Wiley-VCH Verlag GmbH & Co. KGaA)
We used a large series of Au25(SCnH2n+1)180 clusters (n=2, 3, 4, 5, 6, 8, 10, 12, 14, 16, 18) to study the effect of n and the presence of electrolytes on the redox potentials. The electrochem. results were analyzed in the framework of concentric-capacitor models that were previously proposed to describe larger monolayer-protected clusters. We found that the av. dielec. const. of the monolayer (.vepsiln.m) is significantly larger than that of the ligand chains. The effective value of .vepsiln.m depends on n and is the result of contributions from ligands, solvent, and electrolyte. As n increased, .vepsiln.m decreased until a virtually const. value was attained for sufficiently long ligands. The electrochem. calcd. HOMO-LUMO energy gap, conversely, does not depend on n and, therefore, .vepsiln.m. This energy gap is, within error, the same as that obtained from the optical spectra of the same clusters.
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Antonello, S.; Hesari, M.; Polo, F.; Maran, F. Electron Transfer Catalysis with Monolayer Protected Au₂₅ Clusters Nanoscale 2012, 4, 5333– 5342 DOI: 10.1039/c2nr31066j
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Electron transfer catalysis with monolayer protected Au25 clusters
Antonello, Sabrina; Hesari, Mahdi; Polo, Federico; Maran, Flavio
Nanoscale (2012), 4 (17), 5333-5342CODEN: NANOHL; ISSN:2040-3372. (Royal Society of Chemistry)
Au25L18 (L = S(CH2)2Ph) clusters were prepd. and characterized. The resulting monodisperse clusters were reacted with bis(pentafluorobenzoyl) peroxide in dichloromethane to form Au25L18+ quant. The kinetics and thermodn. of the corresponding electron transfer (ET) reactions were characterized via electrochem. and thermochem. calcns. Au25L18+ was used in homogeneous redox catalysis expts. with sym-substituted benzoyl peroxides, including the above peroxide, bis(para-cyanobenzoyl) peroxide, dibenzoyl peroxide, and bis(para-methoxybenzoyl) peroxide. Peroxide dissociative ET was catalyzed using both the Au25L18/Au25L18- and the Au25L18+/Au25L18 redox couples as redox mediators. Simulation of the CV curves led to detn. of the ET rate const. (kET) values for concerted dissociative ET to the peroxides. The ET free energy ΔG° could be estd. for all donor-acceptor combinations, leading to observation of a nice activation-driving force (log kETvs. ΔG°) relation. Comparison with the kET obtained using a ferrocene-type donor with a formal potential similar to that of Au25L18/Au25L18- showed that the presence of the capping monolayer affects the ET rate rather significantly, which is attributed to the intrinsic nonadiabaticity of peroxide acceptors.
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Dainese, T.; Antonello, S.; Gascón, J. A.; Pan, F.; Perera, N. V.; Ruzzi, M.; Venzo, A.; Zoleo, A.; Rissanen, K.; Maran, F. Au₂₅(SEt)₁₈, a Nearly Naked Thiolate-Protected Au₂₅ Cluster: Structural Analysis by Single Crystal X-ray Crystallography and Electron Nuclear Double Resonance ACS Nano 2014, 8, 3904– 3912 DOI: 10.1021/nn500805n
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Au25(SEt)18, a Nearly Naked Thiolate-Protected Au25 Cluster: Structural Analysis by Single Crystal X-ray Crystallography and Electron Nuclear Double Resonance
Dainese, Tiziano; Antonello, Sabrina; Gascon, Jose A.; Pan, Fangfang; Perera, Neranjan V.; Ruzzi, Marco; Venzo, Alfonso; Zoleo, Alfonso; Rissanen, Kari; Maran, Flavio
ACS Nano (2014), 8 (4), 3904-3912CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
X-ray crystallog. has been fundamental in discovering fine structural features of ultra-small gold clusters capped by thiolated ligands. For still unknown structures, however, new tools capable of providing relevant structural information are sought. The authors prepd. a 25-gold atom nanocluster protected by the smallest ligand ever used, ethanethiol. This cluster displays the electrochem., mass spectrometry, and UV-vis absorption spectroscopy features of similar Au25 clusters protected by 18 thiolated ligands. The anionic and the neutral form of Au25(SEt)18 were fully characterized by 1H and 13C NMR spectroscopy, which confirmed the monolayer's properties and the paramagnetism of neutral Au25(SEt)180. X-ray crystallog. anal. of the latter provided the first known structure of a gold cluster protected by a simple, linear alkanethiolate. The authors also report the direct observation by electron nuclear double resonance (ENDOR) of hyperfine interactions between a surface-delocalized unpaired electron and the gold atoms of a nanocluster. The advantages of knowing the exact mol. structure and having used such a small ligand allowed the authors to compare the exptl. values of hyperfine couplings with DFT calcns. unaffected by structure's approxns. or omissions.
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Agrachev, M.; Antonello, S.; Dainese, T.; Gascón, J. A.; Pan, F.; Rissanen, K.; Ruzzi, M.; Venzo, A.; Zoleo, A.; Maran, F. A Magnetic Look into the Protecting Layer of Au₂₅ Clusters Chem. Sci. 2016, 7, 6910– 6918 DOI: 10.1039/c6sc03691k
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A magnetic look into the protecting layer of Au25 clusters
Agrachev, Mikhail; Antonello, Sabrina; Dainese, Tiziano; Gascon, Jose A.; Pan, Fangfang; Rissanen, Kari; Ruzzi, Marco; Venzo, Alfonso; Zoleo, Alfonso; Maran, Flavio
Chemical Science (2016), 7 (12), 6910-6918CODEN: CSHCCN; ISSN:2041-6520. (Royal Society of Chemistry)
The field of mol. metal clusters protected by organothiolates is experiencing a very rapid growth. So far, however, a clear understanding of the fine interactions between the cluster core and the capping monolayer has remained elusive, despite the importance of the latter in interfacing the former to the surrounding medium. Here, we describe a very sensitive methodol. that enables comprehensive assessment of these interactions. Pulse electron nuclear double resonance (ENDOR) was employed to study the interaction of the unpaired electron with the protons of the alkanethiolate ligands in four structurally related paramagnetic Au25(SR)018 clusters (R = Et, Pr, Bu, 2-methylpropyl). Whereas some of these structures were known, we present the first structural description of the highly sym. Au25(SPr)018 cluster. Through knowledge of the structural data, the ENDOR signals could be successfully related to the types of ligand and the distance of the relevant protons from the central gold core. We found that orbital distribution affects atoms that can be as far as 6 Å from the icosahedral core. Simulations of the spectra provided the values of the hyperfine coupling consts. The resulting information was compared with that provided by 1H NMR spectroscopy, and mol. dynamics calcns. provided useful hints to understanding differences between the ENDOR and NMR results. It is shown that the unpaired electron can be used as a very precise probe of the main structural features of the interface between the metal core and the capping ligands.
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Tofanelli, M. A.; Salorinne, K.; Ni, T. W.; Malola, S.; Newell, B.; Phillips, B.; Häkkinen, H.; Ackerson, C. J. Jahn–Teller effects in Au₂₅(SR)₁₈ Chem. Sci. 2016, 7, 1882– 1890 DOI: 10.1039/c5sc02134k
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Jahn-Teller effects in Au25(SR)18
Tofanelli, Marcus A.; Salorinne, Kirsi; Ni, Thomas W.; Malola, Sami; Newell, Brian; Phillips, Billy; Hakkinen, Hannu; Ackerson, Christopher J.
Chemical Science (2016), 7 (3), 1882-1890CODEN: CSHCCN; ISSN:2041-6520. (Royal Society of Chemistry)
A review. The relationship between oxidn. state, structure, and magnetism in many mols. is well described by first-order Jahn-Teller distortions. This relationship is not yet well defined for ligated nanoclusters and nanoparticles, esp. the nano-technol. relevant gold-thiolate protected metal clusters. Here we interrogate the relationships between structure, magnetism, and oxidn. state for the three stable oxidn. states, -1, 0 and +1 of the thiolate protected nanocluster Au25(SR)18. We present the single crystal X-ray structures of the previously undetd. charge state Au25(SR)18+1, as well as a higher quality single crystal structure of the neutral compd. Au25(SR)180. Structural data combined with SQUID magnetometry and DFT theory enable a complete description of the optical and magnetic properties of Au25(SR)18 in the three oxidn. states. In aggregate the data suggests a first-order Jahn-Teller distortion in this compd. The high quality single crystal X-ray structure enables an anal. of the ligand-ligand and ligand-cluster packing interactions that underlie single-crystal formation in thiolate protected metal clusters.
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De Nardi, M.; Antonello, S.; Jiang, D.-E.; Pan, F.; Rissanen, K.; Ruzzi, M.; Venzo, A.; Zoleo, A.; Maran, F. Gold Nanowired: A Linear (Au₂₅)_n Polymer from Au₂₅ Molecular Clusters ACS Nano 2014, 8, 8505– 8512 DOI: 10.1021/nn5031143
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Gold Nanowired: A Linear (Au25)n Polymer from Au25 Molecular Clusters
De Nardi, Marco; Antonello, Sabrina; Jiang, De-en; Pan, Fangfang; Rissanen, Kari; Ruzzi, Marco; Venzo, Alfonso; Zoleo, Alfonso; Maran, Flavio
ACS Nano (2014), 8 (8), 8505-8512CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
Au25(SR)18 has provided fundamental insights into the properties of clusters protected by monolayers of thiolated ligands (SR). Because of its ultrasmall core, 1 nm, Au25(SR)18 displays mol. behavior. We prepd. a Au25 cluster capped by n-butanethiolates (SBu), obtained its structure by single-crystal X-ray crystallog., and studied its properties both exptl. and theor. Whereas in soln. Au25(SBu)180 is a paramagnetic mol., in the crystal it becomes a linear polymer of Au25 clusters connected via single Au-Au bonds and stabilized by proper orientation of clusters and interdigitation of ligands. At low temp., [Au25(SBu)180]n has a nonmagnetic ground state and can be described as a one-dimensional antiferromagnetic system. These findings provide a breakthrough into the properties and possible solid-state applications of mol. gold nanowires.
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Liao, L.; Zhou, S.; Dai, Y.; Liu, L.; Yao, C.; Fu, C.; Yang, J.; Wu, Z. Mono-mercury doping of Au₂₅ and the HOMO/LUMO energies evaluation employing differential pulse voltammetry J. Am. Chem. Soc. 2015, 137, 9511– 9514 DOI: 10.1021/jacs.5b03483
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Mono-Mercury Doping of Au25 and the HOMO/LUMO Energies Evaluation Employing Differential Pulse Voltammetry
Liao, Lingwen; Zhou, Shiming; Dai, Yafei; Liu, Liren; Yao, Chuanhao; Fu, Cenfeng; Yang, Jinlong; Wu, Zhikun
Journal of the American Chemical Society (2015), 137 (30), 9511-9514CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
Controlling the bimetal nanoparticle with at. monodispersity is still challenging. Herein, a monodisperse bimetal nanoparticle was synthesized in 25% yield (on gold atom basis) by an unusual replacement method. The formula of the nanoparticle is Au24Hg1(PET)18 (PET: phenylethanethiolate) by high-resoln. ESI-MS spectrometry in conjunction with multiple analyses including XPS and TGA. X-ray single-crystal diffraction reveals that the structure of Au24Hg1(PET)18 remains the structural framework of Au25(PET)18 with one of the outer-shell gold atoms replaced by one Hg atom, which is further supported by theor. calcns. and exptl. results as well. Importantly, differential pulse voltammetry (DPV) is 1st employed to est. the highest occupied mol. orbit (HOMO) and the lowest unoccupied mol. orbit (LUMO) energies of Au24Hg1(PET)18 based on previous calcns.
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Tian, S.; Liao, L.; Yuan, J.; Yao, C.; Chen, J.; Yang, J.; Wu, Z. Structures and magnetism of mono-palladium and mono-platinum doped Au₂₅(PET)₁₈ nanoclusters Chem. Commun. 2016, 52, 9873– 9876 DOI: 10.1039/c6cc02698b
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44
Structures and magnetism of mono-palladium and mono-platinum doped Au25(PET)18 nanoclusters
Tian, Shubo; Liao, Lingwen; Yuan, Jinyun; Yao, Chuanhao; Chen, Jishi; Yang, Jinlong; Wu, Zhikun
Chemical Communications (Cambridge, United Kingdom) (2016), 52 (64), 9873-9876CODEN: CHCOFS; ISSN:1359-7345. (Royal Society of Chemistry)
Herein the authors report three important results of widespread interest, which are (1) the crystal structure of [Au24Pt(PET)18]0, (2) the crystal structure of [Au24Pd(PET)18]0 and (3) the main source of magnetism in [Au25(PET)18]0. The prepd. compds. Au24Pt(PET)18 (1), Au24Pd(PET)18 (2) and magnetic property of Au25(PET)18 (3). These are mono-palladium and mono-platinum doped Au25(PET)18 nanocluster.
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Song, Y.; Jin, S.; Kang, X.; Xiang, J.; Deng, H.; Yu, H.; Zhu, M. How a Single Electron Affects the Properties of the “Non-Superatom” Au₂₅ Nanoclusters Chem. Mater. 2016, 28, 2609– 2617 DOI: 10.1021/acs.chemmater.5b04655
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How a Single Electron Affects the Properties of the "Non-Superatom" Au25 Nanoclusters
Song, Yongbo; Jin, Shan; Kang, Xi; Xiang, Ji; Deng, Huijuan; Yu, Haizhu; Zhu, Manzhou
Chemistry of Materials (2016), 28 (8), 2609-2617CODEN: CMATEX; ISSN:0897-4756. (American Chemical Society)
The authors successfully synthesized the rod-like [Au25(PPh3)10(SePh)5Cl2]q (q = +1 or +2) nanoclusters through kinetic control. The single crystal x-ray crystallog. detd. their formulas to be [Au25(PPh3)10(SePh)5Cl2](SbF6) and [Au25(PPh3)10(SePh)5Cl2](SbF6)(BPh4), resp. Compared to the previously reported Au25 coprotected by phosphine and thiolate ligands (i.e., [Au25(PPh3)10(SR)5Cl2]2+), the two new rod-like Au25 nanoclusters show some interesting structural differences. Nonetheless, each of these three nanoclusters possesses two icosahedral Au13 units (sharing a vertex gold atom) and the bridging Au-Se(S)-Au motifs. The compns. of the two new nanoclusters were characterized with ESI-MS and TGA. The optical properties, electrochem., and magnetism were studied by EPR, NMR, and SQUID. All these results demonstrate that the valence character significantly affects the properties of the nonsuperatom Au25 nanoclusters, and the changes are different from the previously reported superatom Au25 nanoclusters. Theor. calcns. indicate that the extra electron results in the half occupation of the highest occupied MOs in the rod-like Au25+ nanoclusters and, thus, significantly affects the electronic structure of the nonsuperatom Au25 nanoclusters. This work offers new insights into the relation between the properties and the valence of the nonsuperatom gold nanoclusters.
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Inagaki, Y.; Yonemura, H.; Sakai, N.; Makihara, Y.; Kawae, T.; Yamada, S. Magnetism of gold nanorods probed using electron spin resonance Appl. Phys. Lett. 2016, 109, 72404 DOI: 10.1063/1.4961369
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Dutta, P.; Pal, S.; Seehra, M. S.; Anand, M.; Roberts, C. B. Magnetism in dodecanethiol-capped gold nanoparticles: Role of size and capping agent Appl. Phys. Lett. 2007, 90, 213102 DOI: 10.1063/1.2740577
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Magnetism in dodecanethiol-capped gold nanoparticles: Role of size and capping agent
Dutta, P.; Pal, S.; Seehra, M. S.; Anand, M.; Roberts, C. B.
Applied Physics Letters (2007), 90 (21), 213102/1-213102/3CODEN: APPLAB; ISSN:0003-6951. (American Institute of Physics)
In gold nanoparticles (Au NPs) capped with dodecanethiol (DT), the authors report the observation of superparamagnetic blocking temp. TB ≃ 50 K in D ≃ 5 nm NPs but only diamagnetism in 12 nm NPs. For T < TB = 50 K, the strong temp. dependence of coercivity Hc, satn. magnetization Ms, and exchange bias He (in the field-cooled sample) confirm the blocked state resembling ferromagnetism with Hc ≃ 250 Oe, He ≃ -40 Oe, and Ms ≃ 10-2 emu/g at 5 K. The obsd. electron magnetic resonance line shows expected shift, broadening, and reduced intensity below TB. A magnetic moment μ ≃ 0.006μB per Au atom attached to DT is detd. using a model which yields Ms varying as 1/D, with its source being holes in the 5d band of Au produced by charge transfer from Au to S atoms in DT.
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Guerrero, E.; Muñoz-Márquez, M. A.; Fernández, A.; Crespo, P.; Hernando, A.; Lucena, R.; Conesa, J. C. Magnetometry and electron paramagnetic resonance studies of phosphine- and thiol-capped gold nanoparticles J. Appl. Phys. 2010, 107, 64303 DOI: 10.1063/1.3327414
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48
Magnetometry and electron paramagnetic resonance studies of phosphine- and thiol-capped gold nanoparticles
Guerrero, E.; Munoz-Marquez, M. A.; Fernandez, A.; Crespo, P.; Hernando, A.; Lucena, R.; Conesa, J. C.
Journal of Applied Physics (2010), 107 (6), 064303/1-064303/7CODEN: JAPIAU; ISSN:0021-8979. (American Institute of Physics)
In the last years, the no. of studies performed by wholly independent research groups that confirm the permanent magnetism, 1st obsd. in the authors' research lab, for thiol-capped Au nanoparticles (NPs) has rapidly increased. Throughout the years, the initial magnetometry studies were completed with element-specific magnetization measurements based on, for example, the x-ray MCD technique that have allowed the identification of gold as the magnetic moment carrier. In the research work here presented, the authors have focused the authors' efforts in the evaluation of the magnetic behavior and iron impurities content in the synthesized samples by superconducting quantum interference device magnetometry and ESR spectrometry, resp. As a result, hysteresis cycles typical of a ferromagnetic material were measured from nominally iron-free gold NPs protected with thiol, phosphine, and chlorine ligands. Also for samples contg. both, capped gold NPs and highly dild. iron concns., the magnetic behavior of the NPs is not affected by the presence of paramagnetic iron impurities. The hysteresis cycles reported for phosphine-chlorine-capped gold NPs confirm that the magnetic behavior is not exclusively for the metal-thiol system. (c) 2010 American Institute of Physics.
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49
Antonello, S.; Dainese, T.; Pan, F.; Rissanen, K.; Maran, F. Electrocrystallization of Monolayer-Protected Gold Clusters: Opening the Door to Quality, Quantity, and New Structures J. Am. Chem. Soc. 2017, 139, 4168– 4174 DOI: 10.1021/jacs.7b00568
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49
Electrocrystallization of Monolayer-Protected Gold Clusters: Opening the Door to Quality, Quantity, and New Structures
Antonello, Sabrina; Dainese, Tiziano; Pan, Fangfang; Rissanen, Kari; Maran, Flavio
Journal of the American Chemical Society (2017), 139 (11), 4168-4174CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
Thiolate-protected metal clusters are materials of ever-growing importance in fundamental and applied research. Knowledge of their single-crystal x-ray structures was instrumental to enable advanced mol. understanding of their intriguing properties. So far, however, a general, reliable, chem. clean approach to prep. single crystals suitable for accurate crystallog. anal. was missing. Single crystals of thiolate-protected clusters can be grown in large quantity and very high quality by electrocrystn. This method relies on the fact that charged clusters display a higher soly. in polar solvents than their neutral counterparts. Nucleation of the electrogenerated insol. clusters directly onto the electrode surface eventually gives a dense forest of millimeter-long single crystals. Electrocrystn. of three known Au25(SR)180 clusters is described. A new cluster, Au25(S-nC5H11)18, was also prepd. and found to crystallize by forming bundles of millimeter-long Au25 polymers.
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Antonello, S.; Arrigoni, G.; Dainese, T.; De Nardi, M.; Parisio, G.; Perotti, L.; René, A.; Venzo, A.; Maran, F. Electron Transfer through 3D Monolayers on Au₂₅ Clusters ACS Nano 2014, 8, 2788– 2795 DOI: 10.1021/nn406504k
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50
Electron Transfer through 3D Monolayers on Au25 Clusters
Antonello, Sabrina; Arrigoni, Giorgio; Dainese, Tiziano; De Nardi, Marco; Parisio, Giulia; Perotti, Lorena; Rene, Alice; Venzo, Alfonso; Maran, Flavio
ACS Nano (2014), 8 (3), 2788-2795CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
The monolayer protecting small gold nanoparticles (monolayer-protected clusters, MPCs) is generally represented as the 3-dimensional equiv. of 2-dimensional self-assembled monolayers (SAMs) on extended gold surfaces. However, despite the growing relevance of MPCs in important applied areas, such as catalysis and nanomedicine, the authors' knowledge of the structure of 3-dimensional SAMs in soln. is still extremely limited. The authors prepd. a large series of monodisperse Au25(SCnH2n+1)18 clusters (n = 2, 4, 6, 8, 10, 12, 14, 16, 18) and studied how electrons tunnel through these monolayers. Electron transfer results, nicely supported by 1H NMR spectroscopy, IR absorption spectroscopy, and mol. dynamics results, show that there is a crit. ligand length marking the transition between short ligands, which form a quite fluid monolayer structure, and longer alkyl chains, which self-organize into bundles. At variance with the truly protecting 2-dimensional SAMs, efficient electronic communication of the Au25 core with the outer environment is thus possible even for long alkyl chains. These conclusions provide a different picture of how an ultrasmall gold core talks with the environment through/with its protecting but not-so-shielding monolayer.
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51
Chandrasekhar, S. Stochastic problems in physics and astronomy Rev. Mod. Phys. 1943, 15, 1– 89 DOI: 10.1103/revmodphys.15.1
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52
Carducci, T. M.; Murray, R. W. Kinetics and Low Temperature Studies of Electron Transfers in Films of Small (<2 nm) Au Monolayer Protected Clusters J. Am. Chem. Soc. 2013, 135, 11351– 11356 DOI: 10.1021/ja405342r
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52
Kinetics and Low Temperature Studies of Electron Transfers in Films of Small (<2 nm) Au Monolayer Protected Clusters
Carducci, Tessa M.; Murray, Royce W.
Journal of the American Chemical Society (2013), 135 (30), 11351-11356CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
This work examines the temp. dependence of electron transfer (ET) kinetics in solid-state films of mixed-valent states of monodisperse, small (<2 nm) Au monolayer protected clusters (MPCs). The mixed valent MPC films, coated on interdigitated array electrodes, are Au25(SR)180/1-, Au25(SR)181+/0, and Au144(SR)601+/0, where SR = hexanethiolate for Au144 and phenylethanethiolate for Au25. Near room temp. and for ca. 1:1 mol:mol mixed valencies, the bimol. ET rate consts. (assuming a cubic lattice model) are ∼2 × 106 M-1 s-1 for Au25(SR)180/1-, ∼3 × 105 M-1 s-1 for Au25(SR)181+/0, and ∼1 × 108 M-1 s-1 for Au144(SR)601+/0. Their activation energy ET barriers are 0.38, 0.34, and 0.17 eV, resp. At lowered temps. (down to ca. 77 K), the thermally activated (Arrhenius) ET process dissipates revealing a tunneling mechanism in which the ET rates are independent of temp. but, among the different MPCs, fall in the same order of ET rate: Au144+1/0 > Au250/1- > Au251+/0.
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53
Teale, R. W.; Pelegrini, F. Magnetic surface anisotropy and ferromagnetic resonance in single-crystal GdAl₂ J. Phys. F: Met. Phys. 1986, 16, 621– 635 DOI: 10.1088/0305-4608/16/5/011
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53
Magnetic surface anisotropy and ferromagnetic resonance in single-crystal gadolinium-aluminum (GdAl2)
Teale, R. W.; Pelegrini, F.
Journal of Physics F: Metal Physics (1986), 16 (5), 621-35CODEN: JPFMAT; ISSN:0305-4608.
Ferromagnetic resonance in GdAl2 single crystals shows 2 overlapping absorption lines. The variation of this structure with the temp. of the specimen and the crystallog. orientation of the magnetization M0 is described. The line with the lower resonance field is attributed to the normal bulk mode and that at higher field to a surface-induced mode. The latter is shifted to a higher field by magnetic surface anisotropy. At T > 80 K the bulk mode is more intense but as T is reduced the relative strength changes and at <30 K the surface-induced mode is entirely dominant. A model is based on a classical equation of motion for M0. Maxwell's equations and boundary conditions for the magnetization at the sample surface which involve the magnetic surface anisotropy. By interpreting the sepn. of the resonance lines as a function of the orientation of M0 the expression for the magnetic surface anisotropy energy as a function of this orientation is deduced, and values for the necessary 3 magnetic surface anisotropy consts. are deduced as a function of temp. Interpretation of the resonance linewidth and relative intensity of the 2 modes is not satisfactory. It is probably because the measured lines are influenced by inhomogeneous broadening which is absent from the theory.
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54
Raikher, Y. L.; Stepanov, V. I. The effect of thermal fluctuations on the FMR line shape in dispersed ferromagnets Sov. Phys.—JETP 1992, 75, 764– 771
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55
Berger, R.; Kliava, J.; Bissey, J.-C.; Baïetto, V. Magnetic resonance of superparamagnetic iron-containing nanoparticles in annealed glass J. Appl. Phys. 2000, 87, 7389– 7396 DOI: 10.1063/1.372998
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55
Magnetic resonance of superparamagnetic iron-containing nanoparticles in annealed glass
Berger, Rene; Kliava, Janis; Bissey, Jean-Claude; Baietto, Vanessa
Journal of Applied Physics (2000), 87 (10), 7389-7396CODEN: JAPIAU; ISSN:0021-8979. (American Institute of Physics)
The authors study borate glasses doped with a low concn. of iron oxide by X band (9.5 GHz) electron magnetic resonance. These glasses (compn.: 0.63B2O3-0.37Li2O-0.75 × 10-3 Fe2O3 in mole %) were annealed at increasing temps. Ta, starting at the glass transition temp. A new composite resonance at gef ≈ 2.0 arises in the spectra measured at room temp. (300 K). The narrow component of this resonance is predominant in glasses annealed at lower Ta while the broad component increases in intensity as Ta increases. This resonance is ascribed to an assembly of superparamagnetic nanoparticles of a cryst. iron-contg. compd. Numerical simulations assuming a lognormal particle vol. distribution show that the mean particle diam. increases from 5.3 to 8.5 nm as Ta increases from 748 to 823 K The integrated spectra intensity shows that the total no. of spins in the nanoparticles increases rapidly with Ta. At lower anneal temps. Ta, a striking increase occurs in the particle diams., while at higher Ta these diams. reach a limit value. When the measurement temp. is increased, the resonance spectra show a reversible narrowing and an increase in intensity. The temp. dependence of the individual linewidths is attributed to thermal fluctuations of the orientations of the magnetic moments with respect to the magnetic anisotropy axes.
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56
Noginov, M. M.; Noginova, N.; Amponsah, O.; Bah, R.; Rakhimov, R.; Atsarkin, V. A. Magnetic resonance in iron oxide nanoparticles: Quantum features and effect of size J. Magn. Magn. Mater. 2008, 320, 2228– 2232 DOI: 10.1016/j.jmmm.2008.04.154
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56
Magnetic resonance in iron oxide nanoparticles: Quantum features and effect of size
Noginov, Maxim M.; Noginova, N.; Amponsah, O.; Bah, R.; Rakhimov, R.; Atsarkin, V. A.
Journal of Magnetism and Magnetic Materials (2008), 320 (18), 2228-2232CODEN: JMMMDC; ISSN:0304-8853. (Elsevier B.V.)
To better understand the transition from quantum to classical behavior in spin system, electron magnetic resonance (EMR) was studied in suspensions of superparamagnetic magnetite nanoparticles with an av. diam. of ∼9 nm and analyzed in comparison with the results obtained in the maghemite particles of smaller size (∼5 nm). Both types of particles demonstrate common EMR behavior, including special features such as the temp.-dependent narrow spectral component and multiple-quantum transitions. These features are common for small quantum systems and not expected in classical case. The relative intensity of these signals rapidly decreases with cooling or increase of particle size, marking gradual transition to the classical ferromagnetic resonance (FMR) behavior.
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Fittipaldi, M.; Sorace, L.; Barra, A.-L.; Sangregorio, C.; Sessoli, R.; Gatteschi, D. Molecular nanomagnets and magnetic nanoparticles: The EMR contribution to a common approach Phys. Chem. Chem. Phys. 2009, 11, 6555– 6568 DOI: 10.1039/b905880j
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Wei, D. Micromagnetics and Recording Materials; Springer: New York, 2012; pp 21– 52.
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59
Moon, T. S.; Merrill, R. T. Nucleation theory and domain states in multidomain magnetic material Phys. Earth Planet. Inter. 1985, 37, 214– 222 DOI: 10.1016/0031-9201(85)90053-6
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Nucleation theory and domain states in multidomain magnetic material
Moon, T. S.; Merrill, R. T.
Physics of the Earth and Planetary Interiors (1985), 37 (2-3), 214-22CODEN: PEPIAM; ISSN:0031-9201.
Most of the stable remanent magnetization in rocks resides in magnetic grains which should have ≥2 domains at equil. Calcns. were made to det. the no. of domains that can exist in a grain of given size. The nucleation of domain walls at grain boundaries is investigated for materials free of cryst. defects. Owing to the energy required to nucleate domain walls, a much larger range of grain sizes can have the same no. of domains than indicated by considering the lowest energy domain states alone.
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60
Oyarzún, S.; Tamion, A.; Tournus, F.; Dupuis, V.; Hillenkamp, M. Size effects in the magnetic anisotropy of embedded cobalt nanoparticles: From shape to surface Sci. Rep. 2015, 5, 14749 DOI: 10.1038/srep14749
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60
Size effects in the magnetic anisotropy of embedded cobalt nanoparticles: from shape to surface
Oyarzun, Simon; Tamion, Alexandre; Tournus, Florent; Dupuis, Veronique; Hillenkamp, Matthias
Scientific Reports (2015), 5 (), 14749CODEN: SRCEC3; ISSN:2045-2322. (Nature Publishing Group)
Strong size-dependent variations of the magnetic anisotropy of embedded cobalt clusters are evidenced quant. by combining magnetic expts. and advanced data treatment. The obtained values are discussed in the frame of two theor. models that demonstrate the decisive role of the shape in larger nanoparticles and the predominant role of the surface anisotropy in clusters below 3 nm diam.
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61
Raghavender, A. T.; Hong, N. H.; Swain, B. S.; Jung, M.-H.; Lee, K.-J.; Lee, D.-S. Surface-induced magnetism in Au particles/clusters Mater. Lett. 2012, 87, 169– 171 DOI: 10.1016/j.matlet.2012.07.084
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61
Surface-induced magnetism in Au particles/clusters
Raghavender, A. T.; Hong, Nguyen Hoa; Swain, Bhabani S.; Jung, M.-H.; Lee, K.-J.; Lee, D.-S.
Materials Letters (2012), 87 (), 169-171CODEN: MLETDJ; ISSN:0167-577X. (Elsevier B.V.)
Magnetic properties of Au particles/clusters with different sizes and different shapes were investigated. It is found that magnetic behaviors of Au particles/clusters are strongly influenced by their morphol. The 25 nm Au particles annealed at 700 °C for 5 h show a big change in shape with a much greater surface than that of 5 nm Au particles, or 25 nm particles without annealing, and as a result, magnetism was induced. It suggests that surface effect can play a key role in tailoring magnetic properties of Au particles/clusters.
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62
Di Paola, C.; D’Agosta, R.; Baletto, F. Geometrical Effects on the Magnetic Properties of Nanoparticles Nano Lett. 2016, 16, 2885– 2889 DOI: 10.1021/acs.nanolett.6b00916
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63
Neél, L. Théorie du traînage magnétique des ferromagnétiques en grains fins avec applications aux terres cuites Ann. Geophys. 1949, 5, 99– 136
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64
Uhlíř, V.; Arregi, J. A.; Fullerton, E. E. Colossal magnetic phase transition asymmetry in mesoscale FeRh stripes Nat. Commun. 2016, 7, 13113 DOI: 10.1038/ncomms13113
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64
Colossal magnetic phase transition asymmetry in mesoscale FeRh stripes
Uhlir, V.; Arregi, J. A.; Fullerton, E. E.
Nature Communications (2016), 7 (), 13113CODEN: NCAOBW; ISSN:2041-1723. (Nature Publishing Group)
Coupled order parameters in phase-transition materials can be controlled using various driving forces such as temp., magnetic and elec. field, strain, spin-polarized currents and optical pulses. Tuning the material properties to achieve efficient transitions would enable fast and low-power electronic devices. Here we show that the first-order metamagnetic phase transition in FeRh films becomes strongly asym. in mesoscale structures. In patterned FeRh stripes we obsd. pronounced supercooling and an avalanche-like abrupt transition from the ferromagnetic to the antiferromagnetic phase, while the reverse transition remains nearly continuous over a broad temp. range. Although modest asymmetry signatures have been found in FeRh films, the effect is dramatically enhanced at the mesoscale. The activation vol. of the antiferromagnetic phase is more than two orders of magnitude larger than typical magnetic heterogeneities obsd. in films. The collective behavior upon cooling results from the role of long-range ferromagnetic exchange correlations that become important at the mesoscale and should be a general property of first-order metamagnetic phase transitions.
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65
Aharoni, A. Introduction to the Theory of Ferromagnetism, 2nd ed.; Oxford University Press: Oxford, 1996; pp 84– 108.
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66
Walter, M.; Akola, J.; Lopez-Acevedo, O.; Jadzinsky, P. D.; Calero, G.; Ackerson, C. J.; Whetten, R. L.; Gronbeck, H.; Hakkinen, H. A unified view of ligand-protected gold clusters as superatom complexes Proc. Natl. Acad. Sci. U.S.A. 2008, 105, 9157– 9162 DOI: 10.1073/pnas.0801001105
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66
A unified view of ligand-protected gold clusters as superatom complexes
Walter, Michael; Akola, Jaakko; Lopez-Acevedo, Olga; Jadzinsky, Pablo D.; Calero, Guillermo; Ackerson, Christopher J.; Whetten, Robert L.; Gronbeck, Henrik; Hakkinen, Hannu
Proceedings of the National Academy of Sciences of the United States of America (2008), 105 (27), 9157-9162CODEN: PNASA6; ISSN:0027-8424. (National Academy of Sciences)
Synthesis, characterization, and functionalization of self-assembled, ligand-stabilized gold nanoparticles are long-standing issues in the chem. of nanomaterials. Factors driving the thermodn. stability of well documented discrete sizes are largely unknown. Herein, we provide a unified view of principles that underlie the stability of particles protected by thiolate (SR) or phosphine and halide (PR3, X) ligands. The picture has emerged from anal. of large-scale d. functional theory calcns. of structurally characterized compds., namely Au102(SR)44, Au39(PR3)14X6-, Au11(PR3)7X3, and Au13(PR3)10X23+, where X is either a halogen or a thiolate. Attributable to a compact, sym. core and complete steric protection, each compd. has a filled spherical electronic shell and a major energy gap to unoccupied states. Consequently, the exceptional stability is best described by a "noble-gas superatom" analogy. The explanatory power of this concept is shown by its application to many monomeric and oligomeric compds. of precisely known compn. and structure, and its predictive power is indicated through suggestions offered for a series of anomalously stable cluster compns. which are still awaiting a precise structure detn.
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67
Aikens, C. M. Effects of Core Distances, Solvent, Ligand, and Level of Theory on the TDDFT Optical Absorption Spectrum of the Thiolate-Protected Au₂₅ Nanoparticle J. Phys. Chem. A 2009, 113, 10811– 10817 DOI: 10.1021/jp9051853
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67
Effects of Core Distances, Solvent, Ligand, and Level of Theory on the TDDFT Optical Absorption Spectrum of the Thiolate-Protected Au25 Nanoparticle
Aikens, Christine M.
Journal of Physical Chemistry A (2009), 113 (40), 10811-10817CODEN: JPCAFH; ISSN:1089-5639. (American Chemical Society)
D. functional theory calcns. are employed to calc. geometries (R = H, CH3, CH2CH3, CH2CH2Ph) and excitation energies (R = H, CH3, CH2CH3) for the Au25(SR)18- nanoparticle. The splitting between the 1st two peaks in the optical absorption spectrum is known to arise as a result of ligand-field splitting of superatom D orbitals, and the value of this splitting is a very sensitive probe of Au-Au distances in the Au25(SH)18- nanoparticle core. LDA functionals such as Xα with a triple-ζ basis set predict core geometries in good agreement with expt., which suggests that this level of theory may be useful in future structural predictions. Asymptotically correct potentials SAOP and LB94 with triple-ζ basis sets yield excitation energies within 0.15-0.20 eV of exptl. values; LB94 with a frozen-core basis set is an inexpensive alternative to the preferred SAOP potential. The size of the ligand plays a minor role on the optical absorption spectrum and solvent effects on geometries and excitation energies are negligible, which demonstrates that the core geometric and electronic structure is primarily responsible for the discrete optical absorption exhibited by this nanoparticle.
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68
Jiang, D.-e; Kühn, M.; Tang, Q.; Weigend, F. Superatomic Orbitals under Spin–Orbit Coupling J. Phys. Chem. Lett. 2014, 5, 3286– 3289 DOI: 10.1021/jz501745z
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68
Superatomic Orbitals under Spin-Orbit Coupling
Jiang, De-en; Kuhn, Michael; Tang, Qing; Weigend, Florian
Journal of Physical Chemistry Letters (2014), 5 (19), 3286-3289CODEN: JPCLCD; ISSN:1948-7185. (American Chemical Society)
The Au25(SR)18- cluster has been the poster child of success in applying the superatom complex concept and remains the most studied system of all of the monolayer-protected metal clusters. In this Letter, we try to solve a mystery about this cluster: the low-temp. UV-vis absorption spectrum shows double peaks below 2.0 eV while simulation by scalar relativistic time-dependent d. functional theory (TDDFT) shows only one peak in this region. Using a recently implemented two-component TDDFT, we show that spin-orbit coupling (SOC) leads to those two peaks by splitting the 1P superat. HOMO orbitals. This work highlights the importance of SOC in understanding the electronic structure and optical absorption of thiolated gold nanoclusters, which has not been realized previously.
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69
Kwak, K.; Tang, Q.; Kim, M.; Jiang, D.-e.; Lee, D. Interconversion between Superatomic 6-Electron and 8-Electron Configurations of M@Au₂₄(SR)₁₈ Clusters (M = Pd, Pt) J. Am. Chem. Soc. 2015, 137, 10833– 10840 DOI: 10.1021/jacs.5b06946
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69
Interconversion between Superatomic 6-Electron and 8-Electron Configurations of M@Au24(SR)18 Clusters (M = Pd, Pt)
Kwak, Kyuju; Tang, Qing; Kim, Minseok; Jiang, De-en; Lee, Dongil
Journal of the American Chemical Society (2015), 137 (33), 10833-10840CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
The exceptional stability of thiolate-protected Au25 clusters, [Au25(SR)18]-, arises from the closure of superat. electron shells, leading to a noble-gas-like 8-electron configuration (1S21P6). Here the authors present that replacing the core Au atom with Pd or Pt results in stable [MAu24(SR)18]0 clusters (M = Pd, Pt) having a superat. 6-electron configuration (1S21P4). Voltammetric studies of [PdAu24(SR)18]0 and [PtAu24(SR)18]0 reveal that the HOMO-LUMO (HOMO-LUMO) gaps of these clusters are 0.32 and 0.29 eV, resp., indicating their electronic structures are drastically altered upon doping of the foreign metal. D. functional studies confirm that the HOMO-LUMO gaps of these clusters are indeed smaller, resp. 0.33 and 0.32 eV, than that of [Au25(SR)18]- (1.35 eV). Anal. of the optimized geometries for the 6-electron [MAu24(SR)18]0 clusters shows that the MAu12 core is slightly flattened to yield an oblate ellipsoid. The drastically decreased HOMO-LUMO gaps obsd. are therefore the result of Jahn-Teller-like distortion of the 6-electron [MAu24(SR)18]0 clusters, accompanying splitting of the 1P orbitals. These clusters become 8-electron [MAu24(SR)18]2- clusters upon electronic charging, demonstrating reversible interconversion between the 6-electron and 8-electron configurations of MAu24(SR)18.
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70
Kittel, C. On the gyromagnetic ratio and spectroscopic splitting factor of ferromagnetic substances Phys. Rev. 1949, 76, 743– 748 DOI: 10.1103/physrev.76.743
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Valiev, M.; Bylaska, E. J.; Govind, N.; Kowalski, K.; Straatsma, T. P.; Van Dam, H. J. J.; Wang, D.; Nieplocha, J.; Apra, E.; Windus, T. L.; de Jong, W. A. NWChem: A comprehensive and scalable open-source solution for large scale molecular simulations Comput. Phys. Commun. 2010, 181, 1477– 1489 DOI: 10.1016/j.cpc.2010.04.018
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A review. The latest release of NWChem delivers an open-source computational chem. package with extensive capabilities for large scale simulations of chem. and biol. systems. Utilizing a common computational framework, diverse theor. descriptions can be used to provide the best soln. for a given scientific problem. Scalable parallel implementations and modular software design enable efficient utilization of current computational architectures. This paper provides an overview of NWChem focusing primarily on the core theor. modules provided by the code and their parallel performance.
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Journal of Chemical Physics (1993), 98 (7), 5648-52CODEN: JCPSA6; ISSN:0021-9606.
Despite the remarkable thermochem. accuracy of Kohn-Sham d.-functional theories with gradient corrections for exchange-correlation, the author believes that further improvements are unlikely unless exact-exchange information is considered. Arguments to support this view are presented, and a semiempirical exchange-correlation functional (contg. local-spin-d., gradient, and exact-exchange terms) is tested for 56 atomization energies, 42 ionization potentials, 8 proton affinities, and 10 total at. energies of first- and second-row systems. This functional performs better than previous functionals with gradient corrections only, and fits expt. atomization energies with an impressively small av. abs. deviation of 2.4 kcal/mol.
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Nichols, P.; Govind, N.; Bylaska, E. J.; de Jong, W. A. Gaussian Basis Set and Planewave Relativistic Spin–Orbit Methods in NWChem J. Chem. Theory Comput. 2009, 5, 491– 499 DOI: 10.1021/ct8002892
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Gaussian Basis Set and Planewave Relativistic Spin-Orbit Methods in NWChem
Nichols, Patrick; Govind, Niranjan; Bylaska, Eric J.; de Jong, W. A.
Journal of Chemical Theory and Computation (2009), 5 (3), 491-499CODEN: JCTCCE; ISSN:1549-9618. (American Chemical Society)
Relativistic spin-orbit d. functional theory (DFT) methods have been implemented in the mol. Gaussian DFT and pseudopotential planewave DFT modules of the NWChem electronic-structure program. The Gaussian basis set implementation is based upon the zeroth-order regular approxn. (ZORA) while the plane-wave implementation uses spin-orbit pseudopotentials that are directly generated from the at. Dirac-Kohn-Sham wave functions or at. ZORA-Kohn-Sham wave functions. Compared to solving the full Dirac equation these methods are computationally efficient but robust enough for a realistic description of relativistic effects such as spin-orbit splitting, MO hybridization, and core effects. Both methods have been applied to a variety of small mols., including I2, IF, HI, Br2, Bi2, AuH, and Au2, using various exchange-correlation functionals. Our results are in good agreement with expt. and previously reported calcns.
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van Wüllen, C.; Michauk, C. Accurate and efficient treatment of two-electron contributions in quasirelativistic high-order Douglas-Kroll density-functional calculations J. Chem. Phys. 2005, 123, 204113 DOI: 10.1063/1.2133731
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Accurate and efficient treatment of two-electron contributions in quasirelativistic high-order Douglas-Kroll density-functional calculations
van Wullen, Christoph; Michauk, Christine
Journal of Chemical Physics (2005), 123 (20), 204113/1-204113/8CODEN: JCPSA6; ISSN:0021-9606. (American Institute of Physics)
Two-component quasirelativistic approaches are in principle capable of reproducing results from fully relativistic calcns. based on the four-component Dirac equation (with fixed particle no.). For one-electron systems, this also holds in practice, but in many-electron systems one has to transform the two-electron interaction, which is necessary because a picture change occurs when going from the Dirac equation to a two-component method. For one-electron properties, one can take full account of picture change in a manageable way, but for the electron interaction, this would spoil the computational advantages which are the main reason to perform quasirelativistic calcns. Exploiting those picture change effects are largest in the at. cores, which in mol. applications do not differ too much from the cores of isolated neutral atoms, we propose an elegant, efficient, and accurate approxn. to the two-electron picture change problem. The new approach, called the "model potential" approach because it makes use of at. (four- and two-component) data to est. picture change effects in mols., shares with the nuclear-only approach that the Douglas-Kroll operator needs to be constructed only once (not in each self-consistent-field iteration) and that no time-consuming multicenter relativistic two-electron integrals need to be calcd. The new approach correctly describes the screening of both the nearest nucleus and distant nuclei, for the scalar-relativistic as well as the spin-orbit parts of the Hamiltonian. The approach is tested on at. and mol.-orbital energies as well as spectroscopic consts. of the lead dimer.
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Duy, T. V. T.; Ozaki, T. A three-dimensional domain decomposition method for large-scale DFT electronic structure calculations Comput. Phys. Commun. 2014, 185, 777– 789 DOI: 10.1016/j.cpc.2013.11.008
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A three-dimensional domain decomposition method for large-scale DFT electronic structure calculations
Duy, Truong Vinh Truong; Ozaki, Taisuke
Computer Physics Communications (2014), 185 (3), 777-789CODEN: CPHCBZ; ISSN:0010-4655. (Elsevier B.V.)
With tens of petaflops supercomputers already in operation and exaflops machines expected to appear within the next 10 years, efficient parallel computational methods are required to take advantage of such extreme-scale machines. In this paper, we present a three-dimensional domain decompn. scheme for enabling large-scale electronic structure calcns. based on d. functional theory (DFT) on massively parallel computers. It is composed of two methods: (i) the atom decompn. method and (ii) the grid decompn. method. In the former method, we develop a modified recursive bisection method based on the moment of inertia tensor to reorder the atoms along a principal axis so that atoms that are close in real space are also close on the axis to ensure data locality. The atoms are then divided into sub-domains depending on their projections onto the principal axis in a balanced way among the processes. In the latter method, we define four data structures for the partitioning of grid points that are carefully constructed to make data locality consistent with that of the clustered atoms for minimizing data communications between the processes. We also propose a decompn. method for solving the Poisson equation using the three-dimensional FFT in Hartree potential calcn., which is shown to be better in terms of communication efficiency than a previously proposed parallelization method based on a two-dimensional decompn. For evaluation, we perform benchmark calcns. with our open-source DFT code, OpenMX, paying particular attention to the O ( N ) Krylov subspace method. The results show that our scheme exhibits good strong and weak scaling properties, with the parallel efficiency at 131,072 cores being 67.7% compared to the baseline of 16,384 cores with 131,072 atoms of the diamond structure on the K computer.
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Ceperley, D. M.; Alder, B. J. Ground State of the Electron Gas by a Stochastic Method Phys. Rev. Lett. 1980, 45, 566– 569 DOI: 10.1103/physrevlett.45.566
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Physical Review Letters (1980), 45 (7), 566-9CODEN: PRLTAO; ISSN:0031-9007.
An exact stochastic simulation of the Schroedinger equation for charged bosons and fermions was used to calc. the correlation energies, to locate the transitions to their resp. crystal phases at zero temp. within 10%, and to establish the stability at intermediate densities of a ferromagnetic fluid of electrons.
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Nimmala, P. R.; Theivendran, S.; Barcaro, G.; Sementa, L.; Kumara, C.; Jupally, V. R.; Apra, E.; Stener, M.; Fortunelli, A.; Dass, A. Transformation of Au₁₄₄(SCH₂CH₂Ph)₆₀ to Au₁₃₃(SPh-tBu)₅₂ Nanomolecules: Theoretical and Experimental Study J. Phys. Chem. Lett. 2015, 6, 2134– 2139 DOI: 10.1021/acs.jpclett.5b00780
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Transformation of Au144(SCH2CH2Ph)60 to Au133(SPh-tBu)52 Nanomolecules: Theoretical and Experimental Study
Nimmala, Praneeth Reddy; Theivendran, Shevanuja; Barcaro, Giovanni; Sementa, Luca; Kumara, Chanaka; Jupally, Vijay Reddy; Apra, Edoardo; Stener, Mauro; Fortunelli, Alessandro; Dass, Amala
Journal of Physical Chemistry Letters (2015), 6 (11), 2134-2139CODEN: JPCLCD; ISSN:1948-7185. (American Chemical Society)
Ultrastable gold nanomol. Au144(SCH2CH2Ph)60 upon etching with excess tert-butylbenzenethiol undergoes a core-size conversion and compositional change to form an entirely new core of Au133(SPh-tBu)52. This conversion was studied using high-resoln. electrospray mass spectrometry which shows that the core size conversion is initiated after 22 ligand exchanges, suggesting a relatively high stability of the Au144(SCH2CH2Ph)38(SPh-tBu)22 intermediate. The Au144 → Au133 core size conversion is surprisingly different from the Au144 → Au99 core conversion reported in the case of thiophenol, -SPh. Theor. anal. and ab initio mol. dynamics simulations show that rigid p-tBu groups play a crucial role by reducing the cluster structural freedom, and protecting the cluster from adsorption of exogenous and reactive species, thus rationalizing the kinetic factors that stabilize the Au133 core size. This 144-atom to 133-atom nanomol.'s compositional change is reflected in optical spectroscopy and electrochem.
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Farnia, G.; Maran, F.; Sandonà, G.; Severin, M. G. Protonation kinetics of anionic intermediates in the electrochemical reduction of triphenylethylene: A disproportionation mechanism J. Chem. Soc., Perkin Trans. 2 1982, 1153– 1158 DOI: 10.1039/p29820001153
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Protonation kinetics of anionic intermediates in the electrochemical reduction of triphenylethylene: a disproportionation mechanism
Farnia, Giuseppe; Maran, Flavio; Sandona, Giancarlo; Severin, Maria Gabriella
Journal of the Chemical Society, Perkin Transactions 2: Physical Organic Chemistry (1972-1999) (1982), (9), 1153-8CODEN: JCPKBH; ISSN:0300-9580.
A study is reported of the voltammetric behavior of Ph2C:CHPh (I) in DMF contg. traces of H+ donor. I•- is stable, I2- is rapidly protonated, and the resulting carbanion is a relatively strong base. In the presence of added H2O I•- decays by disproportionation followed by protonation of I2-. The rate of decay is 2nd order in I•- and is enhanced by H2O and inhibited by I. The disproportionation step is rate-limiting.
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Stoll, S.; Schweiger, A. EasySpin, a Comprehensive Software Package for Spectral Simulation and Analysis in EPR J. Magn. Reson. 2006, 178, 42– 55 DOI: 10.1016/j.jmr.2005.08.013
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EasySpin, a comprehensive software package for spectral simulation and analysis in EPR
Stoll, Stefan; Schweiger, Arthur
Journal of Magnetic Resonance (2006), 178 (1), 42-55CODEN: JMARF3; ISSN:1090-7807. (Elsevier)
EasySpin, a computational package for spectral simulation and anal. in EPR, is described. It is based on Matlab, a com. tech. computation software. EasySpin provides extensive EPR-related functionality, ranging from elementary spin physics to data anal. It provides routines for the simulation of liq.- and solid-state EPR and ENDOR spectra. These simulation functions are built on novel algorithms that enhance scope, speed and accuracy of spectral simulations. Spin systems with an arbitrary no. of electron and nuclear spins are supported. The structure of the toolbox as well as the theor. background underlying its simulation functionality are presented, and some illustrative examples are given.
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Abstract
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Figure 1
Figure 1. Experimental (black) and calculated (red) cw-EPR spectra of an Au₂₅(SC2Ph)₁₈⁰ amorphous film at different temperatures (K), as indicated. In (a), the data were multiplied by a factor of 10 with respect to those in (b). In (b), the blue trace corresponds to the EPR cavity at 5 K.
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Figure 2
Figure 2. Dependence of the double-integrated EPR intensity on the reciprocal of temperature. The solid line is the linear regression of the data (black square) at the higher temperatures.
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Figure 3
Figure 3. Effect of temperature (K) on the cw-EPR spectra of one single crystal of Au₂₅(SC2Ph)₁₈⁰.
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Figure 4
Figure 4. Orientation dependence of the cw-EPR spectra of one single crystal of Au₂₅(SC2Ph)₁₈⁰ uncovered (a) or covered (b) by frozen MeCN. Within each graph, the EPR tube was rotated by 0 (blue), 90 (red), and 180° (black) (T = 5 K).
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Figure 5
Figure 5. Hysteresis cw-EPR experiment for a Au₂₅(SC2Ph)₁₈⁰ single crystal at 5 K. The direction and trace color of the three scans are indicated.
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Figure 6
Figure 6. Effect of temperature (K) on the cw-EPR spectra of an Au₂₅(SC2Ph)₁₈⁰ collection of microcrystals.
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Figure 7
Figure 7. Hysteresis cw-EPR experiments for a large collection of Au₂₅(SC2Ph)₁₈⁰ microcrystals. (a) Effect of decreasing the temperature from 60 to 9 K and (b) corresponding temperature increase. The black and the red traces indicate the low-to-high- and high-to-low-field directions, respectively.
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Figure 8
Figure 8. Simulations of the cw-EPR spectrum obtained at 5 K for the Au₂₅(SC2Ph)₁₈⁰ film (black). The simulations include SO and distortion (red), only SO (blue), and only distortion (green).
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Figure 9
Figure 9. Diagram of DFT/B3LYP HOMO orbital energies (eV) in Au₂₅(SCH₃)₁₈⁰ systems. From left to right: Au₂₅(SCH₃)₁₈⁰ at the scalar relativistic level in the Au₂₅(SCH₃)₁₈⁰-anion geometry, Au₂₅(SCH₃)₁₈⁰ at the scalar relativistic level in the Au₂₅(SCH₃)₁₈⁰-crystal geometry, which includes Jahn–Teller (J–T) effects, and Au₂₅(SCH₃)₁₈⁰ including SO coupling (SOC) in the Au₂₅(SCH₃)₁₈⁰-crystal geometry.
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Figure 10
Figure 10. Schematic depiction of the direction and magnitude of atomic spins (green arrows) in the putative spin global minimum (the spins on the Au atoms are not shown as they would be out of scale). The image shows the unit cell as seen from the direction c; (49) all clusters but the central one are thus incomplete. The color codes are Au = yellow, S = red, and C = gray. Au and S atoms and bonds are rendered as balls and sticks, whereas C is rendered as the stick style. H atoms have been removed for clarity.
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  Gold quantum dots exhibit distinctive optical and magnetic behaviors compared with larger gold nanoparticles. However, their unfavorable interaction with living systems and lack of stability in aq. solvents has so far prevented their adoption in biol. and medicine. Here, a simple synthetic pathway integrates gold quantum dots within a mesoporous silica shell, alongside larger gold nanoparticles within the shell's central cavity. This "quantum rattle" structure is stable in aq. solns., does not elicit cell toxicity, preserves the attractive near-IR photonics and paramagnetism of gold quantum dots, and enhances the drug-carrier performance of the silica shell. In vivo, the quantum rattles reduced tumor burden in a single course of photothermal therapy while coupling three complementary imaging modalities: near-IR fluorescence, photoacoustic, and magnetic resonance imaging. The incorporation of gold within the quantum rattles significantly enhanced the drug-carrier performance of the silica shell. This innovative material design based on the mutually beneficial interaction of gold and silica introduces the use of gold quantum dots for imaging and therapeutic applications.
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13. 13
  Hori, H.; Teranishi, T.; Nakae, Y.; Seino, Y.; Miyake, M.; Yamada, S. Anomalous magnetic polarization effect of Pd and Au nano-particles Phys. Lett. A 1999, 263, 406– 410 DOI: 10.1016/s0375-9601(99)00742-2
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  13
  Anomalous magnetic polarization effect of Pd and Au nano-particles
  Hori, H.; Teranishi, T.; Nakae, Y.; Seino, Y.; Miyake, M.; Yamada, S.
  Physics Letters A (1999), 263 (4,5,6), 406-410CODEN: PYLAAG; ISSN:0375-9601. (Elsevier Science B.V.)
  The magnetization of Pd and Au nano-particles with a diam. of at ∼3 nm was measured down to 4.2 K. The data of magnetization of both nano-particles show an unexpectedly large magnetic moment of ∼20 spins per particle. This result can not be understood by so-called odd/even electron no. effect and Stoner's enhancement model. The result might suggest the existence of some common and characteristic spin correlation mechanism in the nano-particles. The exptl. result of size dependence on Pd nano-particles shows existence of a crit. size of ∼3 nm from an at. spin correlation region to a metallic spin correlation region.
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14. 14
  Brust, M.; Walker, M.; Bethell, D.; Schiffrin, D. J.; Whyman, R. Synthesis of Thiol-derivatised Gold Nanoparticles in a Two-Phase Liquid–Liquid System J. Chem. Soc., Chem. Commun. 1994, 801– 802 DOI: 10.1039/c39940000801
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  14
  Synthesis of thiol-derivatized gold nanoparticles in a two-phase liquid-liquid system
  Brust, Mathias; Walker, Merryl; Bethell, Donald; Schiffrin, David J.; Whyman, Robin
  Journal of the Chemical Society, Chemical Communications (1994), (7), 801-2CODEN: JCCCAT; ISSN:0022-4936.
  Using two-phase (water-toluene) redn. of AuCl4- by sodium borohydride in the presence of an alkanethiol, solns. of 1-3 nm gold particles bearing a surface coating of thiol have been prepd. and characterized; this novel material can be handled as a simple chem. compd.
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15. 15
  Crespo, P.; García, M. A.; Pinel, E. F.; Multigner, M.; Alcántara, D.; De La Fuente, J. M.; Penadés, S.; Hernando, A. Fe impurities weaken the ferromagnetic behavior in Au nanoparticles Phys. Rev. Lett. 2006, 97, 177203 DOI: 10.1103/PhysRevLett.97.177203
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16. 16
  Donnio, B.; García-Vázquez, P.; Gallani, J.-L.; Guillon, D.; Terazzi, E. Dendronized ferromagnetic gold nanoparticles self-organized in a thermotropic cubic phase Adv. Mater. 2007, 19, 3534– 3539 DOI: 10.1002/adma.200701252
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  16
  Dendronized ferromagnetic gold nanoparticles self-organized in a thermotropic cubic phase
  Donnio, Bertrand; Garcia-Vazquez, Patricia; Gallani, Jean-Louis; Guillon, Daniel; Terazzi, Emmanuel
  Advanced Materials (Weinheim, Germany) (2007), 19 (21), 3534-3539CODEN: ADVMEW; ISSN:0935-9648. (Wiley-VCH Verlag GmbH & Co. KGaA)
  Gold nanoparticles possessing a positionally ordered 3-dimensional liq. cryst. phase in the bulk were prepd. Induction of the mesophase results from the synergy between the gold core and the nonmesogenic dendrons. These particles are also ferromagnetic up to 400 K and are able to self-organize in a 2-dimensional hexagonal array on solid substrates, opening perspectives in the field of information storage at the mol. level.
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17. 17
  Yamamoto, Y.; Hori, H. Direct observation of the ferromagnetic spin polarization in gold nanoparticles: A review Rev. Adv. Mater. Sci. 2006, 12, 23– 32
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18. 18
  Cirri, A.; Silakov, A.; Lear, B. J. Ligand Control over the Electronic Properties within the Metallic Core of Gold Nanoparticles Angew. Chem., Int. Ed. 2015, 54, 11750– 11753 DOI: 10.1002/anie.201505933
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  18
  Ligand Control over the Electronic Properties within the Metallic Core of Gold Nanoparticles
  Cirri, Anthony; Silakov, Alexey; Lear, Benjamin J.
  Angewandte Chemie, International Edition (2015), 54 (40), 11750-11753CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
  The behavior of electrons within the metallic core of gold nanoparticles (AuNPs) can be controlled by the nature of the surface chem. of the AuNPs. Specifically, the conduction ESR (CESR) spectra of AuNPs of diam. 1.8-1.9 nm are sensitive to ligand exchange of hexanethiol for 4-bromothiophenol on the surface of the nanoparticle. Chemisorption of the arom. ligand leads to a shift in the metallic electron's g-factor toward the value expected for pure gold systems, suggesting an increase in metallic character for the electrons within the gold core. Anal. by UV/Vis absorption spectroscopy reveals a concomitant bathochromic shift of the surface plasmon resonance band of the AuNP, indicating that other electronic properties of AuNPs are also affected by the ligand exchange. In total, our results demonstrate that the chem. nature of the ligand controls the valence band structure of AuNPs.
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19. 19
  Cirri, A.; Silakov, A.; Jensen, L.; Lear, B. J. Chain Length and Solvent Control over the Electronic Properties of Alkanethiolate-Protected Gold Nanoparticles at the Molecule-to-Metal Transition J. Am. Chem. Soc. 2016, 138, 15987– 15993 DOI: 10.1021/jacs.6b09586
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  19
  Chain Length and Solvent Control over the Electronic Properties of Alkanethiolate-Protected Gold Nanoparticles at the Molecule-to-Metal Transition
  Cirri, Anthony; Silakov, Alexey; Jensen, Lasse; Lear, Benjamin J.
  Journal of the American Chemical Society (2016), 138 (49), 15987-15993CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
  Alkanethiolate protected gold nanoparticles are one of the most widely used systems in modern science and technol., where the emergent electronic properties of the gold core are valued for use in applications such as plasmonic solar cells, photocatalysis, and photothermal heating. Though choice in alkane chain length is not often discussed as a way in which to control the electronic properties of these nanoparticles, we show that the chain length of the alkyl tail exerts clear control over the electronic properties of the gold core, as detd. by conduction ESR spectroscopy. The control exerted by chain length is reported on by changes to the g-factor of the metallic electrons, which we can relate to the av. surface potential on the gold core. We propose that the surface potential is modulated by direct charge donation from the ligand to the metal, resulting from the formation of a chem. bond. Furthermore, the degree of charge transfer is controlled by differences between the dielec. const. of the medium and the ligand shell. Together, these observations are used to construct a simple electrostatic model that provides a framework for understanding how surface chem. can be used to modulate the electronic properties of gold nanoparticles.
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20. 20
  Cirri, A.; Silakov, A.; Jensen, L.; Lear, B. J. Probing ligand-induced modulation of metallic states in small gold nanoparticles using conduction electron spin resonance Phys. Chem. Chem. Phys. 2016, 18, 25443– 25451 DOI: 10.1039/c6cp02205g
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21. 21
  Gréget, R.; Nealon, G. L.; Vileno, B.; Turek, P.; Mény, C.; Ott, F.; Derory, A.; Voirin, E.; Rivière, E.; Rogalev, A.; Wilhelm, F.; Joly, L.; Knafo, W.; Ballon, G.; Terazzi, E.; Kappler, J.-P.; Donnio, B.; Gallani, J.-L. Magnetic properties of gold nanoparticles: a room-temperature quantum effect ChemPhysChem 2012, 13, 3092– 3097 DOI: 10.1002/cphc.201200394
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  21
  Magnetic Properties of Gold Nanoparticles: A Room-Temperature Quantum Effect
  Greget, Romain; Nealon, Gareth L.; Vileno, Bertrand; Turek, Philippe; Meny, Christian; Ott, Frederic; Derory, Alain; Voirin, Emilie; Riviere, Eric; Rogalev, Andrei; Wilhelm, Fabrice; Joly, Loic; Knafo, William; Ballon, Geraldine; Terazzi, Emmanuel; Kappler, Jean-Paul; Donnio, Bertrand; Gallani, Jean-Louis
  ChemPhysChem (2012), 13 (13), 3092-3097, S3092/1-S3092/13CODEN: CPCHFT; ISSN:1439-4235. (Wiley-VCH Verlag GmbH & Co. KGaA)
  The authors propose that the magnetism of gold nanopartides is of orbital origin and due to the conduction electrons being driven into persistent currents. The appearance of persistent currents under an applied magnetic field would explain the observation of both dia- and paramagnetic responses of the various samples, the high variability and the lack of thermal dependence of the magnetic properties. As the currents increase with the applied field, the magnetic moment does not sat. Occasionally, these persistent currents could even become self-sustained, giving rise to a soft ferromagnetism. The effect could be reinforced if the nanopartides are locally arranged with the proper geometry. However, obtaining direct proof of the existence of such persistent currents is challenging. The authors suggest that near-field microscopy techniques could be used. One may think of attaching a gold nanoparticle to the cantilever of an AFM. When placed in a magnetic field, the vibration frequency would be sensitive to any extra force generated by the magnetic moment of the nanoparticle. If the moment is indeed field-induced and nonpermanent, no extra force should be perceived in zero field. One may also consider submitting larger Au nanopartides to high magnetic fields and look for Aharonov-Bohm oscillations in their magnetic response. Given the orbital nature of the magnetic moment, it is also worth trying magnetooptical expts. using circularly polarized light to probe the magnetoplasmons. An increase or a decrease of the magnetization might be obsd. upon reversal of the polarization, though the intensity of the local magnetic field may be too small.
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22. 22
  Muñoz-Márquez, M. A.; Guerrero, E.; Fernández, A.; Crespo, P.; Hernando, A.; Lucena, R.; Conesa, J. C. Permanent magnetism in phosphine- and chlorine-capped gold: From clusters to nanoparticles J. Nanopart. Res. 2010, 12, 1307– 1318 DOI: 10.1007/s11051-010-9862-0
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  22
  Permanent magnetism in phosphine- and chlorine-capped gold: from clusters to nanoparticles
  Munoz-Marquez, Miguel A.; Guerrero, Estefania; Fernandez, Asuncion; Crespo, Patricia; Hernando, Antonio; Lucena, Raquel; Conesa, Jose C.
  Journal of Nanoparticle Research (2010), 12 (4), 1307-1318CODEN: JNARFA; ISSN:1388-0764. (Springer)
  Magnetometry results have shown that gold NPs (∼2 nm in size) protected with phosphine and chlorine ligands exhibit permanent magnetism. When the NPs size decreases down to the subnanometric size range, e.g. undecagold atom clusters, the permanent magnetism disappears. The near edge structure of the X-ray absorption spectroscopy data points out that charge transfer between gold and the capping system occurs in both cases. These results strongly suggest that nearly metallic Au bonds are also required for the induction of a magnetic response. ESR observations indicate that the contribution to magnetism from eventual iron impurities can be disregarded.
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23. 23
  Guerrero, E.; Muñoz-Márquez, M. A.; García, M. A.; Crespo, P.; Fernández-Pinel, E.; Hernando, A.; Fernández, A. Surface plasmon resonance and magnetism of thiol-capped gold nanoparticles Nanotechnology 2008, 19, 175701 DOI: 10.1088/0957-4484/19/17/175701
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  23
  Surface plasmon resonance and magnetism of thiol-capped gold nanoparticles
  Guerrero, E.; Munoz-Marquez, M. A.; Garcia, M. A.; Crespo, P.; Fernandez-Pinel, E.; Hernando, A.; Fernandez, A.
  Nanotechnology (2008), 19 (17), 175701/1-175701/6CODEN: NNOTER; ISSN:0957-4484. (Institute of Physics Publishing)
  Surface plasmon resonance measurements and magnetic characterization studies have been carried out for two types of thiol-capped gold nanoparticles (NPs) with similar diams. between 2.0 and 2.5 nm and different org. mols. linked to the sulfur atom: dodecanethiol and tiopronin. In addn., Au NPs capped with tetraoctyl ammonium bromide have also been included in the investigation since such capping mols. weakly interact with the gold surface atoms and, therefore, this system can be used as a model for naked gold NPs; such particles presented a bimodal size distribution with diams. around 1.5 and 5 nm. The plasmon resonance is non-existent for tiopronin-capped NPs, whereas a trace of such a feature is obsd. for NPs covered with dodecanethiol mols. and a bulk-like feature is measured for NPs capped with tetralkyl ammonium salts. These differences would indicate that the modification of the surface electronic structure of the Au NPs depends on the geometry and self-assembling capabilities of the capping mols. and on the elec. charge transferred between Au and S atoms. Regarding the magnetization, dodecanethiol-capped NPs have a ferromagnetic-like behavior, while the NPs capped with tiopronin exhibit a paramagnetic behavior and tetralkyl ammonium-protected NPs are diamagnetic across the studied temp. range; straight chains with a well-defined symmetry axis can induce orbital momentum on surface electrons close to the binding atoms. The orbital momentum not only contributes to the magnetization but also to the local anisotropy, giving rise to permanent magnetism. Due to the domain structure of the adsorbed mols., orbital momentum is not induced for tiopronin-capped NPs and the charge transfer only induces a paramagnetic spin component.
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24. 24
  Tuboltsev, V.; Savin, A.; Pirojenko, A.; Räisänen, J. Magnetism in nanocrystalline gold ACS Nano 2013, 7, 6691– 6699 DOI: 10.1021/nn401914b
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  24
  Magnetism in Nanocrystalline Gold
  Tuboltsev, Vladimir; Savin, Alexander; Pirojenko, Alexandre; Raisanen, Jyrki
  ACS Nano (2013), 7 (8), 6691-6699CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
  While bulk Au is known to be diamagnetic, there is a growing body of convincing exptl. and theor. work indicating that nanostructured Au can be imparted with unconventional magnetic properties. Bridging the current gap in exptl. study of magnetism in bare Au nanomaterials, the authors report here on magnetism in Au nanocryst. films produced by cluster deposition in the aggregate form that can be considered as a crossover state between a nanocluster and a continuous film. The authors demonstrate ferromagnetic-like hysteretic magnetization with temp. dependence indicative of spin-glass-like behavior and find this to be consistent with theor. predictions, available in the literature, based on 1st-principles calcns.
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25. 25
  Maitra, U.; Das, B.; Kumar, N.; Sundaresan, A.; Rao, C. N. R. Ferromagnetism exhibited by nanoparticles of noble metals ChemPhysChem 2011, 12, 2322– 2327 DOI: 10.1002/cphc.201100121
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  25
  Ferromagnetism exhibited by nanoparticles of noble metals
  Maitra, Urmimala; Das, Barun; Kumar, Nitesh; Sundaresan, Athinarayanan; Rao, C. N. R.
  ChemPhysChem (2011), 12 (12), 2322-2327CODEN: CPCHFT; ISSN:1439-4235. (Wiley-VCH Verlag GmbH & Co. KGaA)
  Au nanoparticles with av. diams. in the range 2.5-15 nm, prepd. at the org./aq. interface by tetrakis(hydroxymethyl)phosphonium chloride (THPC) as reducing agent, exhibit ferromagnetism whereby the satn. magnetization MS increases with decreasing diam. and varies linearly with the fraction of surface atoms. The value of MS is higher when the particles are present as a film instead of as a sol. Capping with strongly interacting ligands such as alkane thiols results in a higher MS value, which varies with the strength of the metal-S bond. Ferromagnetism is also found in Pt and Ag nanoparticles prepd. as sols, and the MS values vary as Pt > Au > Ag. A careful study of the temp. variation of the magnetization of Au nanoparticles, along with certain other observations, suggests that small bare nanoparticles of noble metals could indeed possess ferromagnetism, albeit weak, which is accentuated in the presence of capping agents, specially alkane thiols which form strong metal-S bonds.
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26. 26
  Hernando, A.; Crespo, P.; García, M. A.; Pinel, E. F.; De La Venta, J.; Fernández, A.; Penadés, S. Giant magnetic anisotropy at the nanoscale: Overcoming the superparamagnetic limit Phys. Rev. B: Condens. Matter Mater. Phys. 2006, 74, 52403 DOI: 10.1103/PhysRevB.74.052403
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27. 27
  Krishna, K. S.; Tarakeshwar, P.; Mujica, V.; Kumar, C. S. S. R. Chemically induced magnetism in atomically precise gold clusters Small 2014, 10, 907– 911 DOI: 10.1002/smll.201302393
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  27
  Chemically Induced Magnetism in Atomically Precise Gold Clusters
  Krishna, Katla Sai; Tarakeshwar, Pilarisetty; Mujica, Vladimiro; Kumar, Challa S. S. R.
  Small (2014), 10 (5), 907-911CODEN: SMALBC; ISSN:1613-6810. (Wiley-VCH Verlag GmbH & Co. KGaA)
  Authors have fabricated atomically precise, ligand-stabilized Au25, Au38 and Au55 clusters and characterized them. They investigated their magnetic properties exptl. using the superconducting quantum interference device (SQUID).
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28. 28
  Hernando, A.; Crespo, P.; García, M. A. Origin of orbital ferromagnetism and giant magnetic anisotropy at the nanoscale Phys. Rev. Lett. 2006, 96, 57206 DOI: 10.1103/PhysRevLett.96.057206
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29. 29
  Hernando, A.; Crespo, P.; García, M. A.; Coey, M.; Ayuela, A.; Echenique, P. M. Revisiting magnetism of capped Au and ZnO nanoparticles: Surface band structure and atomic orbital with giant magnetic moment Phys. Status Solidi B 2011, 248, 2352– 2360 DOI: 10.1002/pssb.201147227
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  29
  Revisiting magnetism of capped Au and ZnO nanoparticles: Surface band structure and atomic orbital with giant magnetic moment
  Hernando, Antonio; Crespo, Patricia; Garcia, Miguel Angel; Coey, Michael; Ayuela, Andres; Echenique, Pedro Miguel
  Physica Status Solidi B: Basic Solid State Physics (2011), 248 (10), 2352-2360CODEN: PSSBBD; ISSN:0370-1972. (Wiley-VCH Verlag GmbH & Co. KGaA)
  In this article we review the exotic magnetism of nanoparticles (NPs) formed by substances that are not magnetic in bulk as described with generality in Section 1. In particular, the intrinsic character of the magnetism obsd. on capped Au and ZnO NPs is analyzed. X-ray magnetic CD (XMCD) anal. has shown that the magnetic moments are intrinsic and lie in the Au and Zn atoms, resp., as analyzed in Section 2, where the general theor. ideas are also revisited. Since impurity atoms bonded to the surface act as donor or acceptor of electrons that occupy the surface states, the anomalous magnetic response is analyzed in terms of the surface band in Section 3. Finally, Section 4 summarizes our last theor. proposal.
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30. 30
  Murray, R. W. Nanoelectrochemistry: Metal Nanoparticles, Nanoelectrodes, and Nanopores Chem. Rev. 2008, 108, 2688– 2720 DOI: 10.1021/cr068077e
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  30
  Nanoelectrochemistry: Metal Nanoparticles, Nanoelectrodes, and Nanopores
  Murray, Royce W.
  Chemical Reviews (Washington, DC, United States) (2008), 108 (7), 2688-2720CODEN: CHREAY; ISSN:0009-2665. (American Chemical Society)
  A review. Electrochem. of metal nanoparticles, monolayers of nanoparticles, multilayers of nanoparticles, etc., is described. Nanoelectrode fabrication and properties are discussed.
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31. 31
  Parker, J. F.; Fields-Zinna, C. A.; Murray, R. W. The Story of a Monodisperse Gold Nanoparticle: Au₂₅L₁₈ Acc. Chem. Res. 2010, 43, 1289– 1296 DOI: 10.1021/ar100048c
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  31
  The Story of a Monodisperse Gold Nanoparticle: Au25L18
  Parker, Joseph F.; Fields-Zinna, Christina A.; Murray, Royce W.
  Accounts of Chemical Research (2010), 43 (9), 1289-1296CODEN: ACHRE4; ISSN:0001-4842. (American Chemical Society)
  A review. Au nanoparticles (NPs) with protecting organothiolate ligands and core diams. smaller than 2 nm are interesting materials because their size-dependent properties range from metal-like to mol.-like. This Account focuses on the most thoroughly studied of these NPs, Au25L18. Future advances in nanocluster catalysis and electronic miniaturization and biol. applications such as drug delivery will depend on a thorough understanding of nanoscale materials in which mol.-like characteristics appear. This Account tells the story of Au25L18 and its assocd. synthetic, structural, mass spectrometric, electron transfer, optical spectroscopy, and magnetic resonance results. The authors also ref. other Au NP studies to introduce helpful synthetic and measurement tools. Historically, nanoparticle sizes were described by their diams. Recently, researchers have reported actual mol. formulas for very small NPs, which is chem. preferable to solely reporting their size. Au25L18 is a success story in this regard; however, researchers initially mislabeled this NP as Au28L16 and as Au38L24 before correctly identifying it by electrospray-ionization mass spectrometry. Because of its small size, this NP is amenable to theor. studies. Au25L18's accessibility in pure form and mol.-like properties make it an attractive research target. The properties of this NP include a large energy gap readily seen in cyclic voltammetry (related to its HOMO-LUMO gap), a UV-visible absorbance spectrum with step-like fine structure, and NIR fluorescence emission. A single crystal structure and theor. anal. have served as important steps in understanding the chem. of Au25L18. Researchers detd. the single crystal structure of both its native as-prepd. form, a [N((CH2)7CH3)41+][Au25(SCH2CH2Ph)181-] salt, and of the neutral, oxidized form Au25(SCH2CH2Ph)180. A d. functional theory (DFT) anal. correctly predicted essential elements of the structure. The NP is composed of a centered icosahedral Au13 core stabilized by six Au2(SR)3 semirings. These semirings present interesting implications regarding other small Au nanoparticle clusters. Many properties of the Au25 NP result from these semiring structures. This overview of the identification, structure detn., and anal. properties of perhaps the best understood Au nanoparticle provides results that should be useful for further analyses and applications. The authors also hope that the story of this nanoparticle will be useful to those who teach about nanoparticle science.
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32. 32
  Heaven, M. W.; Dass, A.; White, P. S.; Holt, K. M.; Murray, R. W. Crystal Structure of the Gold Nanoparticle [N(C₈H₁₇)₄][Au₂₅(SCH₂CH₂Ph)₁₈] J. Am. Chem. Soc. 2008, 130, 3754– 3755 DOI: 10.1021/ja800561b
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  32
  Crystal Structure of the Gold Nanoparticle [N(C8H17)4][Au25(SCH2CH2Ph)18]
  Heaven, Michael W.; Dass, Amala; White, Peter S.; Holt, Kennedy M.; Murray, Royce W.
  Journal of the American Chemical Society (2008), 130 (12), 3754-3755CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
  The authors report the crystal structure of the thiolate Au nanoparticle [TOA+][Au25(SCH2CH2Ph)18-], where TOA+ = N(C8H17)4+. Crystallog. data are given. The crystal structure reveals three types of Au atoms: (a) one central Au atom whose coordination no. is 12 (12 bonds to Au atoms); (b) 12 Au atoms that form the vertexes of an icosahedron around the central atom, whose coordination no. is 6 (five bonds to Au atoms and one to a S atom), and (c) 12 Au atoms that are stellated on 12 of the 20 faces of the Au13 icosahedron. The arrangement of the latter Au atoms may be influenced by aurophilic bonding. Together they form six orthogonal semirings, or staples, of -Au2(SCH2CH2Ph)3- in an octahedral arrangement around the Au13 core.
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33. 33
  Zhu, M.; Aikens, C. M.; Hollander, F. J.; Schatz, G. C.; Jin, R. Correlating the Crystal Structure of a Thiol-Protected Au²⁵ Cluster and Optical Properties J. Am. Chem. Soc. 2008, 130, 5883– 5885 DOI: 10.1021/ja801173r
  [ACS Full Text ], [CAS], Google Scholar
  33
  Correlating the Crystal Structure of A Thiol-Protected Au25 Cluster and Optical Properties
  Zhu, Manzhou; Aikens, Christine M.; Hollander, Frederick J.; Schatz, George C.; Jin, Rongchao
  Journal of the American Chemical Society (2008), 130 (18), 5883-5885CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
  The total structure detn. of thiol-protected Au clusters has long been a major issue in cluster research. The authors report an unusual single crystal structure of a 25-Au-atom cluster ((Au25(SCH2CH2Ph)18) 1.27 nm diam., surface-to-surface distance) protected by eighteen phenylethanethiol ligands. Crystallog. data and at. coordinates are given. The Au25 cluster features a centered icosahedral Au13 core capped by twelve Au atoms that are situated in six pairs around the three mutually perpendicular 2-fold axes of the icosahedron. The thiolate ligands bind to the Au25 core in an exclusive bridging mode. This highly sym. structure is distinctly different from recent predictions of d. functional theory, and it also violates the empirical golden rule - cluster of clusters, which would predict a biicosahedral structure via vertex sharing of two icosahedral M13 building blocks as previously established in various 25-atom metal clusters protected by phosphine ligands. These results point to the importance of the ligand-Au core interactions. The Au25(SR)18 clusters exhibit multiple mol.-like absorption bands, and the authors find the results are in good correspondence with time-dependent d. functional theory calcns. for the obsd. structure.
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34. 34
  Zhu, M.; Aikens, C. M.; Hendrich, M. P.; Gupta, R.; Qian, H.; Schatz, G. C.; Jin, R. Reversible Switching of Magnetism in Thiolate-Protected Au25 Superatoms J. Am. Chem. Soc. 2009, 131, 2490– 2492 DOI: 10.1021/ja809157f
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  34
  Reversible Switching of Magnetism in Thiolate-Protected Au25 Superatoms
  Zhu, Manzhou; Aikens, Christine M.; Hendrich, Michael P.; Gupta, Rupal; Qian, Huifeng; Schatz, George C.; Jin, Rongchao
  Journal of the American Chemical Society (2009), 131 (7), 2490-2492CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
  The authors report reversible switching of paramagnetism in a well-defined gold nanoparticle system consisting of atomically monodisperse nanoparticles contg. 25 gold atoms protected by 18 thiolates [abbreviated as Au25(SR)18]. The magnetism in these nanoparticles can be switched on or off by precisely controlling the charge state of the nanoparticle, i.e., the magnetic state of the Au25(SR)18 nanoparticles is charge-neutral while the nonmagnetic state is an anionic form of the particle. EPR spectroscopy measurements establish that the magnetic state of the Au25(SR)18 nanoparticles possess one unpaired spin per particle. EPR studies also imply an unusual electronic structure of the Au25(SR)18 nanoparticle. D. functional theory calcns. coupled with the expts. successfully explain the origin of the obsd. magnetism in a Au25(SR)18 nanoparticle as arising from one unpaired spin having distinct P-like character and delocalized among the icosahedral Au13 core of the particle in the HOMO. Probably the Au25(SR)18 nanoparticles are best considered as ligand-protected superatoms.
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35. 35
  Venzo, A.; Antonello, S.; Gascón, J. A.; Guryanov, I.; Leapman, R. D.; Perera, N. V.; Sousa, A.; Zamuner, M.; Zanella, A.; Maran, F. Effect of the Charge State (z = −1, 0, +1) on the Nuclear Magnetic Resonance of Monodisperse Au₂₅[S(CH₂)₂Ph]_18z Clusters Anal. Chem. 2011, 83, 6355– 6362 DOI: 10.1021/ac2012653
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36. 36
  Antonello, S.; Perera, N. V.; Ruzzi, M.; Gascón, J. A.; Maran, F. Interplay of Charge State, Lability, and Magnetism in the Molecule-like Au₂₅(SR)₁₈ Cluster J. Am. Chem. Soc. 2013, 135, 15585– 15594 DOI: 10.1021/ja407887d
  [ACS Full Text ], [CAS], Google Scholar
  36
  Interplay of charge state, lability, and magnetism in the molecule-like Au25(SR)18 c luster
  Antonello, Sabrina; Perera, Neranjan V.; Ruzzi, Marco; Gascon, Jose A.; Maran, Flavio
  Journal of the American Chemical Society (2013), 135 (41), 15585-15594CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
  Au25(SR)18 (R = -CH2-CH2-Ph) is a mol.-like nanocluster displaying distinct electrochem. and optical features. Although it is often taken as an example of a particularly well-understood cluster, very recent literature has provided a quite unclear or even a controversial description of its properties. We prepd. monodisperse Au25(SR)180 and studied by cyclic voltammetry, under particularly controlled conditions, the kinetics of its redn. or oxidn. to a series of charge states, -2, -1, +1, +2, and +3. For each electrode process, we detd. the std. heterogeneous electron-transfer (ET) rate consts. and the reorganization energies. The latter points to a relatively large inner reorganization. Redn. to form Au25(SR)182- and oxidn. to form Au25(SR)182+ and Au25(SR)183+ are chem. irreversible. The corresponding decay rate consts. and lifetimes are incompatible with interpretations of very recent literature reports. The problem of how ET affects the Au25 magnetism was addressed by comparing the continuous-wave ESR (cw-EPR) behaviors of radical Au25(SR)180 and its oxidn. product, Au25(SR)18+. As opposed to recent exptl. and computational results, our study provides compelling evidence that the latter is a diamagnetic species. The DFT-computed optical absorption spectra and d. of states of the -1, 0, and +1 charge states nicely reproduced the exptl. estd. dependence of the HOMO-LUMO energy gap on the actual charge carried by the cluster. The conclusions about the magnetism of the 0 and +1 charge states were also reproduced, stressing that the three HOMOs are not virtually degenerate as routinely assumed: in particular, the splitting of the HOMO manifold in the cation species is severe, suggesting that the usefulness of the superatom interpretation is limited. The electrochem., EPR, and computational results thus provide a self-consistent picture of the properties of Au25(SR)18 as a function of its charge state and may furnish a methodol. blueprint for understanding the redox and magnetic behaviors of similar mol.-like gold nanoclusters.
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37. 37
  Antonello, S.; Dainese, T.; De Nardi, M.; Perotti, L.; Maran, F. Insights into the Interface Between the Electrolytic Solution and the Gold Core in Molecular Au₂₅ Clusters ChemElectroChem 2016, 3, 1237– 1244 DOI: 10.1002/celc.201600276
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  37
  Insights into the Interface Between the Electrolytic Solution and the Gold Core in Molecular Au25 Clusters
  Antonello, Sabrina; Dainese, Tiziano; De Nardi, Marco; Perotti, Lorena; Maran, Flavio
  ChemElectroChem (2016), 3 (8), 1237-1244CODEN: CHEMRA; ISSN:2196-0216. (Wiley-VCH Verlag GmbH & Co. KGaA)
  We used a large series of Au25(SCnH2n+1)180 clusters (n=2, 3, 4, 5, 6, 8, 10, 12, 14, 16, 18) to study the effect of n and the presence of electrolytes on the redox potentials. The electrochem. results were analyzed in the framework of concentric-capacitor models that were previously proposed to describe larger monolayer-protected clusters. We found that the av. dielec. const. of the monolayer (.vepsiln.m) is significantly larger than that of the ligand chains. The effective value of .vepsiln.m depends on n and is the result of contributions from ligands, solvent, and electrolyte. As n increased, .vepsiln.m decreased until a virtually const. value was attained for sufficiently long ligands. The electrochem. calcd. HOMO-LUMO energy gap, conversely, does not depend on n and, therefore, .vepsiln.m. This energy gap is, within error, the same as that obtained from the optical spectra of the same clusters.
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38. 38
  Antonello, S.; Hesari, M.; Polo, F.; Maran, F. Electron Transfer Catalysis with Monolayer Protected Au₂₅ Clusters Nanoscale 2012, 4, 5333– 5342 DOI: 10.1039/c2nr31066j
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  38
  Electron transfer catalysis with monolayer protected Au25 clusters
  Antonello, Sabrina; Hesari, Mahdi; Polo, Federico; Maran, Flavio
  Nanoscale (2012), 4 (17), 5333-5342CODEN: NANOHL; ISSN:2040-3372. (Royal Society of Chemistry)
  Au25L18 (L = S(CH2)2Ph) clusters were prepd. and characterized. The resulting monodisperse clusters were reacted with bis(pentafluorobenzoyl) peroxide in dichloromethane to form Au25L18+ quant. The kinetics and thermodn. of the corresponding electron transfer (ET) reactions were characterized via electrochem. and thermochem. calcns. Au25L18+ was used in homogeneous redox catalysis expts. with sym-substituted benzoyl peroxides, including the above peroxide, bis(para-cyanobenzoyl) peroxide, dibenzoyl peroxide, and bis(para-methoxybenzoyl) peroxide. Peroxide dissociative ET was catalyzed using both the Au25L18/Au25L18- and the Au25L18+/Au25L18 redox couples as redox mediators. Simulation of the CV curves led to detn. of the ET rate const. (kET) values for concerted dissociative ET to the peroxides. The ET free energy ΔG° could be estd. for all donor-acceptor combinations, leading to observation of a nice activation-driving force (log kETvs. ΔG°) relation. Comparison with the kET obtained using a ferrocene-type donor with a formal potential similar to that of Au25L18/Au25L18- showed that the presence of the capping monolayer affects the ET rate rather significantly, which is attributed to the intrinsic nonadiabaticity of peroxide acceptors.
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39. 39
  Dainese, T.; Antonello, S.; Gascón, J. A.; Pan, F.; Perera, N. V.; Ruzzi, M.; Venzo, A.; Zoleo, A.; Rissanen, K.; Maran, F. Au₂₅(SEt)₁₈, a Nearly Naked Thiolate-Protected Au₂₅ Cluster: Structural Analysis by Single Crystal X-ray Crystallography and Electron Nuclear Double Resonance ACS Nano 2014, 8, 3904– 3912 DOI: 10.1021/nn500805n
  [ACS Full Text ], [CAS], Google Scholar
  39
  Au25(SEt)18, a Nearly Naked Thiolate-Protected Au25 Cluster: Structural Analysis by Single Crystal X-ray Crystallography and Electron Nuclear Double Resonance
  Dainese, Tiziano; Antonello, Sabrina; Gascon, Jose A.; Pan, Fangfang; Perera, Neranjan V.; Ruzzi, Marco; Venzo, Alfonso; Zoleo, Alfonso; Rissanen, Kari; Maran, Flavio
  ACS Nano (2014), 8 (4), 3904-3912CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
  X-ray crystallog. has been fundamental in discovering fine structural features of ultra-small gold clusters capped by thiolated ligands. For still unknown structures, however, new tools capable of providing relevant structural information are sought. The authors prepd. a 25-gold atom nanocluster protected by the smallest ligand ever used, ethanethiol. This cluster displays the electrochem., mass spectrometry, and UV-vis absorption spectroscopy features of similar Au25 clusters protected by 18 thiolated ligands. The anionic and the neutral form of Au25(SEt)18 were fully characterized by 1H and 13C NMR spectroscopy, which confirmed the monolayer's properties and the paramagnetism of neutral Au25(SEt)180. X-ray crystallog. anal. of the latter provided the first known structure of a gold cluster protected by a simple, linear alkanethiolate. The authors also report the direct observation by electron nuclear double resonance (ENDOR) of hyperfine interactions between a surface-delocalized unpaired electron and the gold atoms of a nanocluster. The advantages of knowing the exact mol. structure and having used such a small ligand allowed the authors to compare the exptl. values of hyperfine couplings with DFT calcns. unaffected by structure's approxns. or omissions.
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40. 40
  Agrachev, M.; Antonello, S.; Dainese, T.; Gascón, J. A.; Pan, F.; Rissanen, K.; Ruzzi, M.; Venzo, A.; Zoleo, A.; Maran, F. A Magnetic Look into the Protecting Layer of Au₂₅ Clusters Chem. Sci. 2016, 7, 6910– 6918 DOI: 10.1039/c6sc03691k
  [Crossref], [PubMed], [CAS], Google Scholar
  40
  A magnetic look into the protecting layer of Au25 clusters
  Agrachev, Mikhail; Antonello, Sabrina; Dainese, Tiziano; Gascon, Jose A.; Pan, Fangfang; Rissanen, Kari; Ruzzi, Marco; Venzo, Alfonso; Zoleo, Alfonso; Maran, Flavio
  Chemical Science (2016), 7 (12), 6910-6918CODEN: CSHCCN; ISSN:2041-6520. (Royal Society of Chemistry)
  The field of mol. metal clusters protected by organothiolates is experiencing a very rapid growth. So far, however, a clear understanding of the fine interactions between the cluster core and the capping monolayer has remained elusive, despite the importance of the latter in interfacing the former to the surrounding medium. Here, we describe a very sensitive methodol. that enables comprehensive assessment of these interactions. Pulse electron nuclear double resonance (ENDOR) was employed to study the interaction of the unpaired electron with the protons of the alkanethiolate ligands in four structurally related paramagnetic Au25(SR)018 clusters (R = Et, Pr, Bu, 2-methylpropyl). Whereas some of these structures were known, we present the first structural description of the highly sym. Au25(SPr)018 cluster. Through knowledge of the structural data, the ENDOR signals could be successfully related to the types of ligand and the distance of the relevant protons from the central gold core. We found that orbital distribution affects atoms that can be as far as 6 Å from the icosahedral core. Simulations of the spectra provided the values of the hyperfine coupling consts. The resulting information was compared with that provided by 1H NMR spectroscopy, and mol. dynamics calcns. provided useful hints to understanding differences between the ENDOR and NMR results. It is shown that the unpaired electron can be used as a very precise probe of the main structural features of the interface between the metal core and the capping ligands.
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41. 41
  Tofanelli, M. A.; Salorinne, K.; Ni, T. W.; Malola, S.; Newell, B.; Phillips, B.; Häkkinen, H.; Ackerson, C. J. Jahn–Teller effects in Au₂₅(SR)₁₈ Chem. Sci. 2016, 7, 1882– 1890 DOI: 10.1039/c5sc02134k
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  41
  Jahn-Teller effects in Au25(SR)18
  Tofanelli, Marcus A.; Salorinne, Kirsi; Ni, Thomas W.; Malola, Sami; Newell, Brian; Phillips, Billy; Hakkinen, Hannu; Ackerson, Christopher J.
  Chemical Science (2016), 7 (3), 1882-1890CODEN: CSHCCN; ISSN:2041-6520. (Royal Society of Chemistry)
  A review. The relationship between oxidn. state, structure, and magnetism in many mols. is well described by first-order Jahn-Teller distortions. This relationship is not yet well defined for ligated nanoclusters and nanoparticles, esp. the nano-technol. relevant gold-thiolate protected metal clusters. Here we interrogate the relationships between structure, magnetism, and oxidn. state for the three stable oxidn. states, -1, 0 and +1 of the thiolate protected nanocluster Au25(SR)18. We present the single crystal X-ray structures of the previously undetd. charge state Au25(SR)18+1, as well as a higher quality single crystal structure of the neutral compd. Au25(SR)180. Structural data combined with SQUID magnetometry and DFT theory enable a complete description of the optical and magnetic properties of Au25(SR)18 in the three oxidn. states. In aggregate the data suggests a first-order Jahn-Teller distortion in this compd. The high quality single crystal X-ray structure enables an anal. of the ligand-ligand and ligand-cluster packing interactions that underlie single-crystal formation in thiolate protected metal clusters.
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42. 42
  De Nardi, M.; Antonello, S.; Jiang, D.-E.; Pan, F.; Rissanen, K.; Ruzzi, M.; Venzo, A.; Zoleo, A.; Maran, F. Gold Nanowired: A Linear (Au₂₅)_n Polymer from Au₂₅ Molecular Clusters ACS Nano 2014, 8, 8505– 8512 DOI: 10.1021/nn5031143
  [ACS Full Text ], [CAS], Google Scholar
  42
  Gold Nanowired: A Linear (Au25)n Polymer from Au25 Molecular Clusters
  De Nardi, Marco; Antonello, Sabrina; Jiang, De-en; Pan, Fangfang; Rissanen, Kari; Ruzzi, Marco; Venzo, Alfonso; Zoleo, Alfonso; Maran, Flavio
  ACS Nano (2014), 8 (8), 8505-8512CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
  Au25(SR)18 has provided fundamental insights into the properties of clusters protected by monolayers of thiolated ligands (SR). Because of its ultrasmall core, 1 nm, Au25(SR)18 displays mol. behavior. We prepd. a Au25 cluster capped by n-butanethiolates (SBu), obtained its structure by single-crystal X-ray crystallog., and studied its properties both exptl. and theor. Whereas in soln. Au25(SBu)180 is a paramagnetic mol., in the crystal it becomes a linear polymer of Au25 clusters connected via single Au-Au bonds and stabilized by proper orientation of clusters and interdigitation of ligands. At low temp., [Au25(SBu)180]n has a nonmagnetic ground state and can be described as a one-dimensional antiferromagnetic system. These findings provide a breakthrough into the properties and possible solid-state applications of mol. gold nanowires.
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43. 43
  Liao, L.; Zhou, S.; Dai, Y.; Liu, L.; Yao, C.; Fu, C.; Yang, J.; Wu, Z. Mono-mercury doping of Au₂₅ and the HOMO/LUMO energies evaluation employing differential pulse voltammetry J. Am. Chem. Soc. 2015, 137, 9511– 9514 DOI: 10.1021/jacs.5b03483
  [ACS Full Text ], [CAS], Google Scholar
  43
  Mono-Mercury Doping of Au25 and the HOMO/LUMO Energies Evaluation Employing Differential Pulse Voltammetry
  Liao, Lingwen; Zhou, Shiming; Dai, Yafei; Liu, Liren; Yao, Chuanhao; Fu, Cenfeng; Yang, Jinlong; Wu, Zhikun
  Journal of the American Chemical Society (2015), 137 (30), 9511-9514CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
  Controlling the bimetal nanoparticle with at. monodispersity is still challenging. Herein, a monodisperse bimetal nanoparticle was synthesized in 25% yield (on gold atom basis) by an unusual replacement method. The formula of the nanoparticle is Au24Hg1(PET)18 (PET: phenylethanethiolate) by high-resoln. ESI-MS spectrometry in conjunction with multiple analyses including XPS and TGA. X-ray single-crystal diffraction reveals that the structure of Au24Hg1(PET)18 remains the structural framework of Au25(PET)18 with one of the outer-shell gold atoms replaced by one Hg atom, which is further supported by theor. calcns. and exptl. results as well. Importantly, differential pulse voltammetry (DPV) is 1st employed to est. the highest occupied mol. orbit (HOMO) and the lowest unoccupied mol. orbit (LUMO) energies of Au24Hg1(PET)18 based on previous calcns.
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44. 44
  Tian, S.; Liao, L.; Yuan, J.; Yao, C.; Chen, J.; Yang, J.; Wu, Z. Structures and magnetism of mono-palladium and mono-platinum doped Au₂₅(PET)₁₈ nanoclusters Chem. Commun. 2016, 52, 9873– 9876 DOI: 10.1039/c6cc02698b
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  44
  Structures and magnetism of mono-palladium and mono-platinum doped Au25(PET)18 nanoclusters
  Tian, Shubo; Liao, Lingwen; Yuan, Jinyun; Yao, Chuanhao; Chen, Jishi; Yang, Jinlong; Wu, Zhikun
  Chemical Communications (Cambridge, United Kingdom) (2016), 52 (64), 9873-9876CODEN: CHCOFS; ISSN:1359-7345. (Royal Society of Chemistry)
  Herein the authors report three important results of widespread interest, which are (1) the crystal structure of [Au24Pt(PET)18]0, (2) the crystal structure of [Au24Pd(PET)18]0 and (3) the main source of magnetism in [Au25(PET)18]0. The prepd. compds. Au24Pt(PET)18 (1), Au24Pd(PET)18 (2) and magnetic property of Au25(PET)18 (3). These are mono-palladium and mono-platinum doped Au25(PET)18 nanocluster.
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45. 45
  Song, Y.; Jin, S.; Kang, X.; Xiang, J.; Deng, H.; Yu, H.; Zhu, M. How a Single Electron Affects the Properties of the “Non-Superatom” Au₂₅ Nanoclusters Chem. Mater. 2016, 28, 2609– 2617 DOI: 10.1021/acs.chemmater.5b04655
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  45
  How a Single Electron Affects the Properties of the "Non-Superatom" Au25 Nanoclusters
  Song, Yongbo; Jin, Shan; Kang, Xi; Xiang, Ji; Deng, Huijuan; Yu, Haizhu; Zhu, Manzhou
  Chemistry of Materials (2016), 28 (8), 2609-2617CODEN: CMATEX; ISSN:0897-4756. (American Chemical Society)
  The authors successfully synthesized the rod-like [Au25(PPh3)10(SePh)5Cl2]q (q = +1 or +2) nanoclusters through kinetic control. The single crystal x-ray crystallog. detd. their formulas to be [Au25(PPh3)10(SePh)5Cl2](SbF6) and [Au25(PPh3)10(SePh)5Cl2](SbF6)(BPh4), resp. Compared to the previously reported Au25 coprotected by phosphine and thiolate ligands (i.e., [Au25(PPh3)10(SR)5Cl2]2+), the two new rod-like Au25 nanoclusters show some interesting structural differences. Nonetheless, each of these three nanoclusters possesses two icosahedral Au13 units (sharing a vertex gold atom) and the bridging Au-Se(S)-Au motifs. The compns. of the two new nanoclusters were characterized with ESI-MS and TGA. The optical properties, electrochem., and magnetism were studied by EPR, NMR, and SQUID. All these results demonstrate that the valence character significantly affects the properties of the nonsuperatom Au25 nanoclusters, and the changes are different from the previously reported superatom Au25 nanoclusters. Theor. calcns. indicate that the extra electron results in the half occupation of the highest occupied MOs in the rod-like Au25+ nanoclusters and, thus, significantly affects the electronic structure of the nonsuperatom Au25 nanoclusters. This work offers new insights into the relation between the properties and the valence of the nonsuperatom gold nanoclusters.
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46. 46
  Inagaki, Y.; Yonemura, H.; Sakai, N.; Makihara, Y.; Kawae, T.; Yamada, S. Magnetism of gold nanorods probed using electron spin resonance Appl. Phys. Lett. 2016, 109, 72404 DOI: 10.1063/1.4961369
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47. 47
  Dutta, P.; Pal, S.; Seehra, M. S.; Anand, M.; Roberts, C. B. Magnetism in dodecanethiol-capped gold nanoparticles: Role of size and capping agent Appl. Phys. Lett. 2007, 90, 213102 DOI: 10.1063/1.2740577
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  47
  Magnetism in dodecanethiol-capped gold nanoparticles: Role of size and capping agent
  Dutta, P.; Pal, S.; Seehra, M. S.; Anand, M.; Roberts, C. B.
  Applied Physics Letters (2007), 90 (21), 213102/1-213102/3CODEN: APPLAB; ISSN:0003-6951. (American Institute of Physics)
  In gold nanoparticles (Au NPs) capped with dodecanethiol (DT), the authors report the observation of superparamagnetic blocking temp. TB ≃ 50 K in D ≃ 5 nm NPs but only diamagnetism in 12 nm NPs. For T < TB = 50 K, the strong temp. dependence of coercivity Hc, satn. magnetization Ms, and exchange bias He (in the field-cooled sample) confirm the blocked state resembling ferromagnetism with Hc ≃ 250 Oe, He ≃ -40 Oe, and Ms ≃ 10-2 emu/g at 5 K. The obsd. electron magnetic resonance line shows expected shift, broadening, and reduced intensity below TB. A magnetic moment μ ≃ 0.006μB per Au atom attached to DT is detd. using a model which yields Ms varying as 1/D, with its source being holes in the 5d band of Au produced by charge transfer from Au to S atoms in DT.
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48. 48
  Guerrero, E.; Muñoz-Márquez, M. A.; Fernández, A.; Crespo, P.; Hernando, A.; Lucena, R.; Conesa, J. C. Magnetometry and electron paramagnetic resonance studies of phosphine- and thiol-capped gold nanoparticles J. Appl. Phys. 2010, 107, 64303 DOI: 10.1063/1.3327414
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  Magnetometry and electron paramagnetic resonance studies of phosphine- and thiol-capped gold nanoparticles
  Guerrero, E.; Munoz-Marquez, M. A.; Fernandez, A.; Crespo, P.; Hernando, A.; Lucena, R.; Conesa, J. C.
  Journal of Applied Physics (2010), 107 (6), 064303/1-064303/7CODEN: JAPIAU; ISSN:0021-8979. (American Institute of Physics)
  In the last years, the no. of studies performed by wholly independent research groups that confirm the permanent magnetism, 1st obsd. in the authors' research lab, for thiol-capped Au nanoparticles (NPs) has rapidly increased. Throughout the years, the initial magnetometry studies were completed with element-specific magnetization measurements based on, for example, the x-ray MCD technique that have allowed the identification of gold as the magnetic moment carrier. In the research work here presented, the authors have focused the authors' efforts in the evaluation of the magnetic behavior and iron impurities content in the synthesized samples by superconducting quantum interference device magnetometry and ESR spectrometry, resp. As a result, hysteresis cycles typical of a ferromagnetic material were measured from nominally iron-free gold NPs protected with thiol, phosphine, and chlorine ligands. Also for samples contg. both, capped gold NPs and highly dild. iron concns., the magnetic behavior of the NPs is not affected by the presence of paramagnetic iron impurities. The hysteresis cycles reported for phosphine-chlorine-capped gold NPs confirm that the magnetic behavior is not exclusively for the metal-thiol system. (c) 2010 American Institute of Physics.
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49. 49
  Antonello, S.; Dainese, T.; Pan, F.; Rissanen, K.; Maran, F. Electrocrystallization of Monolayer-Protected Gold Clusters: Opening the Door to Quality, Quantity, and New Structures J. Am. Chem. Soc. 2017, 139, 4168– 4174 DOI: 10.1021/jacs.7b00568
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  49
  Electrocrystallization of Monolayer-Protected Gold Clusters: Opening the Door to Quality, Quantity, and New Structures
  Antonello, Sabrina; Dainese, Tiziano; Pan, Fangfang; Rissanen, Kari; Maran, Flavio
  Journal of the American Chemical Society (2017), 139 (11), 4168-4174CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
  Thiolate-protected metal clusters are materials of ever-growing importance in fundamental and applied research. Knowledge of their single-crystal x-ray structures was instrumental to enable advanced mol. understanding of their intriguing properties. So far, however, a general, reliable, chem. clean approach to prep. single crystals suitable for accurate crystallog. anal. was missing. Single crystals of thiolate-protected clusters can be grown in large quantity and very high quality by electrocrystn. This method relies on the fact that charged clusters display a higher soly. in polar solvents than their neutral counterparts. Nucleation of the electrogenerated insol. clusters directly onto the electrode surface eventually gives a dense forest of millimeter-long single crystals. Electrocrystn. of three known Au25(SR)180 clusters is described. A new cluster, Au25(S-nC5H11)18, was also prepd. and found to crystallize by forming bundles of millimeter-long Au25 polymers.
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50. 50
  Antonello, S.; Arrigoni, G.; Dainese, T.; De Nardi, M.; Parisio, G.; Perotti, L.; René, A.; Venzo, A.; Maran, F. Electron Transfer through 3D Monolayers on Au₂₅ Clusters ACS Nano 2014, 8, 2788– 2795 DOI: 10.1021/nn406504k
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  50
  Electron Transfer through 3D Monolayers on Au25 Clusters
  Antonello, Sabrina; Arrigoni, Giorgio; Dainese, Tiziano; De Nardi, Marco; Parisio, Giulia; Perotti, Lorena; Rene, Alice; Venzo, Alfonso; Maran, Flavio
  ACS Nano (2014), 8 (3), 2788-2795CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
  The monolayer protecting small gold nanoparticles (monolayer-protected clusters, MPCs) is generally represented as the 3-dimensional equiv. of 2-dimensional self-assembled monolayers (SAMs) on extended gold surfaces. However, despite the growing relevance of MPCs in important applied areas, such as catalysis and nanomedicine, the authors' knowledge of the structure of 3-dimensional SAMs in soln. is still extremely limited. The authors prepd. a large series of monodisperse Au25(SCnH2n+1)18 clusters (n = 2, 4, 6, 8, 10, 12, 14, 16, 18) and studied how electrons tunnel through these monolayers. Electron transfer results, nicely supported by 1H NMR spectroscopy, IR absorption spectroscopy, and mol. dynamics results, show that there is a crit. ligand length marking the transition between short ligands, which form a quite fluid monolayer structure, and longer alkyl chains, which self-organize into bundles. At variance with the truly protecting 2-dimensional SAMs, efficient electronic communication of the Au25 core with the outer environment is thus possible even for long alkyl chains. These conclusions provide a different picture of how an ultrasmall gold core talks with the environment through/with its protecting but not-so-shielding monolayer.
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51. 51
  Chandrasekhar, S. Stochastic problems in physics and astronomy Rev. Mod. Phys. 1943, 15, 1– 89 DOI: 10.1103/revmodphys.15.1
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  Carducci, T. M.; Murray, R. W. Kinetics and Low Temperature Studies of Electron Transfers in Films of Small (<2 nm) Au Monolayer Protected Clusters J. Am. Chem. Soc. 2013, 135, 11351– 11356 DOI: 10.1021/ja405342r
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  Kinetics and Low Temperature Studies of Electron Transfers in Films of Small (<2 nm) Au Monolayer Protected Clusters
  Carducci, Tessa M.; Murray, Royce W.
  Journal of the American Chemical Society (2013), 135 (30), 11351-11356CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
  This work examines the temp. dependence of electron transfer (ET) kinetics in solid-state films of mixed-valent states of monodisperse, small (<2 nm) Au monolayer protected clusters (MPCs). The mixed valent MPC films, coated on interdigitated array electrodes, are Au25(SR)180/1-, Au25(SR)181+/0, and Au144(SR)601+/0, where SR = hexanethiolate for Au144 and phenylethanethiolate for Au25. Near room temp. and for ca. 1:1 mol:mol mixed valencies, the bimol. ET rate consts. (assuming a cubic lattice model) are ∼2 × 106 M-1 s-1 for Au25(SR)180/1-, ∼3 × 105 M-1 s-1 for Au25(SR)181+/0, and ∼1 × 108 M-1 s-1 for Au144(SR)601+/0. Their activation energy ET barriers are 0.38, 0.34, and 0.17 eV, resp. At lowered temps. (down to ca. 77 K), the thermally activated (Arrhenius) ET process dissipates revealing a tunneling mechanism in which the ET rates are independent of temp. but, among the different MPCs, fall in the same order of ET rate: Au144+1/0 > Au250/1- > Au251+/0.
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53. 53
  Teale, R. W.; Pelegrini, F. Magnetic surface anisotropy and ferromagnetic resonance in single-crystal GdAl₂ J. Phys. F: Met. Phys. 1986, 16, 621– 635 DOI: 10.1088/0305-4608/16/5/011
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  Magnetic surface anisotropy and ferromagnetic resonance in single-crystal gadolinium-aluminum (GdAl2)
  Teale, R. W.; Pelegrini, F.
  Journal of Physics F: Metal Physics (1986), 16 (5), 621-35CODEN: JPFMAT; ISSN:0305-4608.
  Ferromagnetic resonance in GdAl2 single crystals shows 2 overlapping absorption lines. The variation of this structure with the temp. of the specimen and the crystallog. orientation of the magnetization M0 is described. The line with the lower resonance field is attributed to the normal bulk mode and that at higher field to a surface-induced mode. The latter is shifted to a higher field by magnetic surface anisotropy. At T > 80 K the bulk mode is more intense but as T is reduced the relative strength changes and at <30 K the surface-induced mode is entirely dominant. A model is based on a classical equation of motion for M0. Maxwell's equations and boundary conditions for the magnetization at the sample surface which involve the magnetic surface anisotropy. By interpreting the sepn. of the resonance lines as a function of the orientation of M0 the expression for the magnetic surface anisotropy energy as a function of this orientation is deduced, and values for the necessary 3 magnetic surface anisotropy consts. are deduced as a function of temp. Interpretation of the resonance linewidth and relative intensity of the 2 modes is not satisfactory. It is probably because the measured lines are influenced by inhomogeneous broadening which is absent from the theory.
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54. 54
  Raikher, Y. L.; Stepanov, V. I. The effect of thermal fluctuations on the FMR line shape in dispersed ferromagnets Sov. Phys.—JETP 1992, 75, 764– 771
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  Berger, R.; Kliava, J.; Bissey, J.-C.; Baïetto, V. Magnetic resonance of superparamagnetic iron-containing nanoparticles in annealed glass J. Appl. Phys. 2000, 87, 7389– 7396 DOI: 10.1063/1.372998
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  Magnetic resonance of superparamagnetic iron-containing nanoparticles in annealed glass
  Berger, Rene; Kliava, Janis; Bissey, Jean-Claude; Baietto, Vanessa
  Journal of Applied Physics (2000), 87 (10), 7389-7396CODEN: JAPIAU; ISSN:0021-8979. (American Institute of Physics)
  The authors study borate glasses doped with a low concn. of iron oxide by X band (9.5 GHz) electron magnetic resonance. These glasses (compn.: 0.63B2O3-0.37Li2O-0.75 × 10-3 Fe2O3 in mole %) were annealed at increasing temps. Ta, starting at the glass transition temp. A new composite resonance at gef ≈ 2.0 arises in the spectra measured at room temp. (300 K). The narrow component of this resonance is predominant in glasses annealed at lower Ta while the broad component increases in intensity as Ta increases. This resonance is ascribed to an assembly of superparamagnetic nanoparticles of a cryst. iron-contg. compd. Numerical simulations assuming a lognormal particle vol. distribution show that the mean particle diam. increases from 5.3 to 8.5 nm as Ta increases from 748 to 823 K The integrated spectra intensity shows that the total no. of spins in the nanoparticles increases rapidly with Ta. At lower anneal temps. Ta, a striking increase occurs in the particle diams., while at higher Ta these diams. reach a limit value. When the measurement temp. is increased, the resonance spectra show a reversible narrowing and an increase in intensity. The temp. dependence of the individual linewidths is attributed to thermal fluctuations of the orientations of the magnetic moments with respect to the magnetic anisotropy axes.
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56. 56
  Noginov, M. M.; Noginova, N.; Amponsah, O.; Bah, R.; Rakhimov, R.; Atsarkin, V. A. Magnetic resonance in iron oxide nanoparticles: Quantum features and effect of size J. Magn. Magn. Mater. 2008, 320, 2228– 2232 DOI: 10.1016/j.jmmm.2008.04.154
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  Magnetic resonance in iron oxide nanoparticles: Quantum features and effect of size
  Noginov, Maxim M.; Noginova, N.; Amponsah, O.; Bah, R.; Rakhimov, R.; Atsarkin, V. A.
  Journal of Magnetism and Magnetic Materials (2008), 320 (18), 2228-2232CODEN: JMMMDC; ISSN:0304-8853. (Elsevier B.V.)
  To better understand the transition from quantum to classical behavior in spin system, electron magnetic resonance (EMR) was studied in suspensions of superparamagnetic magnetite nanoparticles with an av. diam. of ∼9 nm and analyzed in comparison with the results obtained in the maghemite particles of smaller size (∼5 nm). Both types of particles demonstrate common EMR behavior, including special features such as the temp.-dependent narrow spectral component and multiple-quantum transitions. These features are common for small quantum systems and not expected in classical case. The relative intensity of these signals rapidly decreases with cooling or increase of particle size, marking gradual transition to the classical ferromagnetic resonance (FMR) behavior.
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57. 57
  Fittipaldi, M.; Sorace, L.; Barra, A.-L.; Sangregorio, C.; Sessoli, R.; Gatteschi, D. Molecular nanomagnets and magnetic nanoparticles: The EMR contribution to a common approach Phys. Chem. Chem. Phys. 2009, 11, 6555– 6568 DOI: 10.1039/b905880j
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  Wei, D. Micromagnetics and Recording Materials; Springer: New York, 2012; pp 21– 52.
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  Moon, T. S.; Merrill, R. T. Nucleation theory and domain states in multidomain magnetic material Phys. Earth Planet. Inter. 1985, 37, 214– 222 DOI: 10.1016/0031-9201(85)90053-6
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  Nucleation theory and domain states in multidomain magnetic material
  Moon, T. S.; Merrill, R. T.
  Physics of the Earth and Planetary Interiors (1985), 37 (2-3), 214-22CODEN: PEPIAM; ISSN:0031-9201.
  Most of the stable remanent magnetization in rocks resides in magnetic grains which should have ≥2 domains at equil. Calcns. were made to det. the no. of domains that can exist in a grain of given size. The nucleation of domain walls at grain boundaries is investigated for materials free of cryst. defects. Owing to the energy required to nucleate domain walls, a much larger range of grain sizes can have the same no. of domains than indicated by considering the lowest energy domain states alone.
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60. 60
  Oyarzún, S.; Tamion, A.; Tournus, F.; Dupuis, V.; Hillenkamp, M. Size effects in the magnetic anisotropy of embedded cobalt nanoparticles: From shape to surface Sci. Rep. 2015, 5, 14749 DOI: 10.1038/srep14749
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  Size effects in the magnetic anisotropy of embedded cobalt nanoparticles: from shape to surface
  Oyarzun, Simon; Tamion, Alexandre; Tournus, Florent; Dupuis, Veronique; Hillenkamp, Matthias
  Scientific Reports (2015), 5 (), 14749CODEN: SRCEC3; ISSN:2045-2322. (Nature Publishing Group)
  Strong size-dependent variations of the magnetic anisotropy of embedded cobalt clusters are evidenced quant. by combining magnetic expts. and advanced data treatment. The obtained values are discussed in the frame of two theor. models that demonstrate the decisive role of the shape in larger nanoparticles and the predominant role of the surface anisotropy in clusters below 3 nm diam.
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  Raghavender, A. T.; Hong, N. H.; Swain, B. S.; Jung, M.-H.; Lee, K.-J.; Lee, D.-S. Surface-induced magnetism in Au particles/clusters Mater. Lett. 2012, 87, 169– 171 DOI: 10.1016/j.matlet.2012.07.084
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  Surface-induced magnetism in Au particles/clusters
  Raghavender, A. T.; Hong, Nguyen Hoa; Swain, Bhabani S.; Jung, M.-H.; Lee, K.-J.; Lee, D.-S.
  Materials Letters (2012), 87 (), 169-171CODEN: MLETDJ; ISSN:0167-577X. (Elsevier B.V.)
  Magnetic properties of Au particles/clusters with different sizes and different shapes were investigated. It is found that magnetic behaviors of Au particles/clusters are strongly influenced by their morphol. The 25 nm Au particles annealed at 700 °C for 5 h show a big change in shape with a much greater surface than that of 5 nm Au particles, or 25 nm particles without annealing, and as a result, magnetism was induced. It suggests that surface effect can play a key role in tailoring magnetic properties of Au particles/clusters.
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62. 62
  Di Paola, C.; D’Agosta, R.; Baletto, F. Geometrical Effects on the Magnetic Properties of Nanoparticles Nano Lett. 2016, 16, 2885– 2889 DOI: 10.1021/acs.nanolett.6b00916
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  Neél, L. Théorie du traînage magnétique des ferromagnétiques en grains fins avec applications aux terres cuites Ann. Geophys. 1949, 5, 99– 136
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  Uhlíř, V.; Arregi, J. A.; Fullerton, E. E. Colossal magnetic phase transition asymmetry in mesoscale FeRh stripes Nat. Commun. 2016, 7, 13113 DOI: 10.1038/ncomms13113
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  Colossal magnetic phase transition asymmetry in mesoscale FeRh stripes
  Uhlir, V.; Arregi, J. A.; Fullerton, E. E.
  Nature Communications (2016), 7 (), 13113CODEN: NCAOBW; ISSN:2041-1723. (Nature Publishing Group)
  Coupled order parameters in phase-transition materials can be controlled using various driving forces such as temp., magnetic and elec. field, strain, spin-polarized currents and optical pulses. Tuning the material properties to achieve efficient transitions would enable fast and low-power electronic devices. Here we show that the first-order metamagnetic phase transition in FeRh films becomes strongly asym. in mesoscale structures. In patterned FeRh stripes we obsd. pronounced supercooling and an avalanche-like abrupt transition from the ferromagnetic to the antiferromagnetic phase, while the reverse transition remains nearly continuous over a broad temp. range. Although modest asymmetry signatures have been found in FeRh films, the effect is dramatically enhanced at the mesoscale. The activation vol. of the antiferromagnetic phase is more than two orders of magnitude larger than typical magnetic heterogeneities obsd. in films. The collective behavior upon cooling results from the role of long-range ferromagnetic exchange correlations that become important at the mesoscale and should be a general property of first-order metamagnetic phase transitions.
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65. 65
  Aharoni, A. Introduction to the Theory of Ferromagnetism, 2nd ed.; Oxford University Press: Oxford, 1996; pp 84– 108.
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  Walter, M.; Akola, J.; Lopez-Acevedo, O.; Jadzinsky, P. D.; Calero, G.; Ackerson, C. J.; Whetten, R. L.; Gronbeck, H.; Hakkinen, H. A unified view of ligand-protected gold clusters as superatom complexes Proc. Natl. Acad. Sci. U.S.A. 2008, 105, 9157– 9162 DOI: 10.1073/pnas.0801001105
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  A unified view of ligand-protected gold clusters as superatom complexes
  Walter, Michael; Akola, Jaakko; Lopez-Acevedo, Olga; Jadzinsky, Pablo D.; Calero, Guillermo; Ackerson, Christopher J.; Whetten, Robert L.; Gronbeck, Henrik; Hakkinen, Hannu
  Proceedings of the National Academy of Sciences of the United States of America (2008), 105 (27), 9157-9162CODEN: PNASA6; ISSN:0027-8424. (National Academy of Sciences)
  Synthesis, characterization, and functionalization of self-assembled, ligand-stabilized gold nanoparticles are long-standing issues in the chem. of nanomaterials. Factors driving the thermodn. stability of well documented discrete sizes are largely unknown. Herein, we provide a unified view of principles that underlie the stability of particles protected by thiolate (SR) or phosphine and halide (PR3, X) ligands. The picture has emerged from anal. of large-scale d. functional theory calcns. of structurally characterized compds., namely Au102(SR)44, Au39(PR3)14X6-, Au11(PR3)7X3, and Au13(PR3)10X23+, where X is either a halogen or a thiolate. Attributable to a compact, sym. core and complete steric protection, each compd. has a filled spherical electronic shell and a major energy gap to unoccupied states. Consequently, the exceptional stability is best described by a "noble-gas superatom" analogy. The explanatory power of this concept is shown by its application to many monomeric and oligomeric compds. of precisely known compn. and structure, and its predictive power is indicated through suggestions offered for a series of anomalously stable cluster compns. which are still awaiting a precise structure detn.
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67. 67
  Aikens, C. M. Effects of Core Distances, Solvent, Ligand, and Level of Theory on the TDDFT Optical Absorption Spectrum of the Thiolate-Protected Au₂₅ Nanoparticle J. Phys. Chem. A 2009, 113, 10811– 10817 DOI: 10.1021/jp9051853
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  67
  Effects of Core Distances, Solvent, Ligand, and Level of Theory on the TDDFT Optical Absorption Spectrum of the Thiolate-Protected Au25 Nanoparticle
  Aikens, Christine M.
  Journal of Physical Chemistry A (2009), 113 (40), 10811-10817CODEN: JPCAFH; ISSN:1089-5639. (American Chemical Society)
  D. functional theory calcns. are employed to calc. geometries (R = H, CH3, CH2CH3, CH2CH2Ph) and excitation energies (R = H, CH3, CH2CH3) for the Au25(SR)18- nanoparticle. The splitting between the 1st two peaks in the optical absorption spectrum is known to arise as a result of ligand-field splitting of superatom D orbitals, and the value of this splitting is a very sensitive probe of Au-Au distances in the Au25(SH)18- nanoparticle core. LDA functionals such as Xα with a triple-ζ basis set predict core geometries in good agreement with expt., which suggests that this level of theory may be useful in future structural predictions. Asymptotically correct potentials SAOP and LB94 with triple-ζ basis sets yield excitation energies within 0.15-0.20 eV of exptl. values; LB94 with a frozen-core basis set is an inexpensive alternative to the preferred SAOP potential. The size of the ligand plays a minor role on the optical absorption spectrum and solvent effects on geometries and excitation energies are negligible, which demonstrates that the core geometric and electronic structure is primarily responsible for the discrete optical absorption exhibited by this nanoparticle.
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68. 68
  Jiang, D.-e; Kühn, M.; Tang, Q.; Weigend, F. Superatomic Orbitals under Spin–Orbit Coupling J. Phys. Chem. Lett. 2014, 5, 3286– 3289 DOI: 10.1021/jz501745z
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  Superatomic Orbitals under Spin-Orbit Coupling
  Jiang, De-en; Kuhn, Michael; Tang, Qing; Weigend, Florian
  Journal of Physical Chemistry Letters (2014), 5 (19), 3286-3289CODEN: JPCLCD; ISSN:1948-7185. (American Chemical Society)
  The Au25(SR)18- cluster has been the poster child of success in applying the superatom complex concept and remains the most studied system of all of the monolayer-protected metal clusters. In this Letter, we try to solve a mystery about this cluster: the low-temp. UV-vis absorption spectrum shows double peaks below 2.0 eV while simulation by scalar relativistic time-dependent d. functional theory (TDDFT) shows only one peak in this region. Using a recently implemented two-component TDDFT, we show that spin-orbit coupling (SOC) leads to those two peaks by splitting the 1P superat. HOMO orbitals. This work highlights the importance of SOC in understanding the electronic structure and optical absorption of thiolated gold nanoclusters, which has not been realized previously.
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69. 69
  Kwak, K.; Tang, Q.; Kim, M.; Jiang, D.-e.; Lee, D. Interconversion between Superatomic 6-Electron and 8-Electron Configurations of M@Au₂₄(SR)₁₈ Clusters (M = Pd, Pt) J. Am. Chem. Soc. 2015, 137, 10833– 10840 DOI: 10.1021/jacs.5b06946
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  69
  Interconversion between Superatomic 6-Electron and 8-Electron Configurations of M@Au24(SR)18 Clusters (M = Pd, Pt)
  Kwak, Kyuju; Tang, Qing; Kim, Minseok; Jiang, De-en; Lee, Dongil
  Journal of the American Chemical Society (2015), 137 (33), 10833-10840CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
  The exceptional stability of thiolate-protected Au25 clusters, [Au25(SR)18]-, arises from the closure of superat. electron shells, leading to a noble-gas-like 8-electron configuration (1S21P6). Here the authors present that replacing the core Au atom with Pd or Pt results in stable [MAu24(SR)18]0 clusters (M = Pd, Pt) having a superat. 6-electron configuration (1S21P4). Voltammetric studies of [PdAu24(SR)18]0 and [PtAu24(SR)18]0 reveal that the HOMO-LUMO (HOMO-LUMO) gaps of these clusters are 0.32 and 0.29 eV, resp., indicating their electronic structures are drastically altered upon doping of the foreign metal. D. functional studies confirm that the HOMO-LUMO gaps of these clusters are indeed smaller, resp. 0.33 and 0.32 eV, than that of [Au25(SR)18]- (1.35 eV). Anal. of the optimized geometries for the 6-electron [MAu24(SR)18]0 clusters shows that the MAu12 core is slightly flattened to yield an oblate ellipsoid. The drastically decreased HOMO-LUMO gaps obsd. are therefore the result of Jahn-Teller-like distortion of the 6-electron [MAu24(SR)18]0 clusters, accompanying splitting of the 1P orbitals. These clusters become 8-electron [MAu24(SR)18]2- clusters upon electronic charging, demonstrating reversible interconversion between the 6-electron and 8-electron configurations of MAu24(SR)18.
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  Ultrastable gold nanomol. Au144(SCH2CH2Ph)60 upon etching with excess tert-butylbenzenethiol undergoes a core-size conversion and compositional change to form an entirely new core of Au133(SPh-tBu)52. This conversion was studied using high-resoln. electrospray mass spectrometry which shows that the core size conversion is initiated after 22 ligand exchanges, suggesting a relatively high stability of the Au144(SCH2CH2Ph)38(SPh-tBu)22 intermediate. The Au144 → Au133 core size conversion is surprisingly different from the Au144 → Au99 core conversion reported in the case of thiophenol, -SPh. Theor. anal. and ab initio mol. dynamics simulations show that rigid p-tBu groups play a crucial role by reducing the cluster structural freedom, and protecting the cluster from adsorption of exogenous and reactive species, thus rationalizing the kinetic factors that stabilize the Au133 core size. This 144-atom to 133-atom nanomol.'s compositional change is reflected in optical spectroscopy and electrochem.
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Magnetic Ordering in Gold Nanoclusters

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Abstract

Introduction

Results and Discussion

Film

Figure 1

Figure 2

Single Crystals

Figure 3

Figure 4

Figure 5

Microcrystals

Figure 6

Figure 7

Theoretical Analysis of the EPR Data

Figure 8

DFT Calculations

Figure 9

Figure 10

Conclusions

Experimental Section

Au25(SC2Ph)180 Synthesis

Preparation of the Film

Preparation of the Single Crystals

Electron Paramagnetic Resonance

Supporting Information

Terms & Conditions

Author Information

Acknowledgment

References

Cited By

Abstract

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

Figure 7

Figure 8

Figure 9

Figure 10

References

Supporting Information

Supporting Information

Terms & Conditions

STEP 1:

STEP 2:

Au₂₅(SC2Ph)₁₈⁰ Synthesis