The performance of solar cells based on hybrid halide perovskites has seen an unparalleled rate of progress, while our understanding of the underlying physical chemistry of these materials trails behind. Superficially, CH₃NH₃PbI₃ is similar to other thin-film photovoltaic materials: a semiconductor with an optical band gap in the optimal region of the electromagnetic spectrum. Microscopically, the material is more unconventional. Progress in our understanding of the local and long-range chemical bonding of hybrid perovskites is discussed here, drawing from a series of computational studies involving electronic structure, molecular dynamics, and Monte Carlo simulation techniques. The orientational freedom of the dipolar methylammonium ion gives rise to temperature-dependent dielectric screening and the possibility for the formation of polar (ferroelectric) domains. The ability to independently substitute on the A, B, and X lattice sites provides the means to tune the optoelectronic properties. Finally, ten critical challenges and opportunities for physical chemists are highlighted.

Charge carrier generation and drift dynamics have been investigated in two types of dye:fullerene heterojunctions: vacuum-deposited merocyanine:C₆₀ and solution-processed merocyanine:PC₆₁BM blends by combining electric-field-induced fluorescence quenching and ultrafast time-resolved carrier drift measurements. We demonstrate that interfacial charge transfer (CT) states are strongly heterogeneous with energies dependent on the acceptor material and its domain sizes. Interfacial CT states on large C₆₀ domains have low energies, while CT states on PC₆₁BM domains have larger energies, which are weakly dependent on the domain sizes. We distinguish two interfacial CT state dissociation pathways: (i) ultrafast, weakly dependent on the electric field and (ii) slow field-assisted dissociation during entire CT state lifetime. We attribute process i to low-energy, weakly bound CT states on large fullerene domains and process ii to strongly bound CT states on small domains or single fullerene molecules. The electron mobility in films with 50% of C₆₀ is several times higher than in the films with PC₆₁BM and orders of magnitude higher than the hole mobility. We conclude that efficient carrier generation at low electric fields typical for operating solar cells relies on unperturbed motion of highly mobile electrons; thus, fast motion and extraction of electrons are crucial for efficient solar cells.

Chevrel phases, or CPs (Mo₆T₈; T = S, Se), can accommodate cations (Li⁺, Mg²⁺ etc.) within the Mo₆T₈ open framework at room temperature due to their unusually high electronic conductivity and ionic mobility and are hence proposed as positive electrodes for secondary batteries. However, cation insertion into Mo₆T₈ generates strong repulsion between the cation–cation or cation–Mo atoms, leading to partial charge trapping within the Mo₆T₈ structure. The present work examines CPs as positive electrodes for sodium-ion batteries. In this regard, ternary CPs of Cu_xMo₆S₈ and Cu_xMo₆Se₈ phase were prepared by solution chemistry and high energy mechanical milling (HEMM) routes, respectively, followed by acid leaching of copper. X-ray diffraction and scanning electron micrographs revealed the formation of 1–1.5 μm size cuboidal Cu_1.8Mo₆S₈ particles, whereas, HEMM of CuSe, MoSe₂ and Mo powder followed by heating leads to the formation of Cu₂Mo₆Se₈ phase. Results from cyclic voltammetry and galvanostatic cycling of Na/Mo₆S₈ and Na/Mo₆Se₈ cells within 1.2–2.2 V versus sodium revealed that two-step sodiation/desodiation reaction occurs with a gradual capacity fade due to Na-ion trapping within two terminal compositions, Na_xMo₆T₈ (T = S, Se; x ∼ 1 and 3). Electrochemical impedance spectroscopy at ∼0.1 V intervals during the sodiation/desodiation process illustrates that partial Na-ion trapping resulted in an increase in charge transfer resistance, R_e, due to the formation of stable Na_∼1Mo₆S₈ phase after the first charge cycle. However, charge trapping continues to occur during the first and second cycles in the case of Mo₆Se₈ phase. Nevertheless, the ease of fabrication, stable capacity, and high Coulombic efficiency render Mo₆T₈ (T = S, Se) as promising Na-ion positive electrodes for stationary electrical energy storage (EES) applications.

The incorporation and diffusion behaviors of Xe in uranium mononitride (UN) have been studied using first-principles density functional theory calculations. The incorporation and binding energies of Xe located at different sites are calculated. Because of strain relief related to moving Xe atom from highly strained interstitial site into the large steric vacancy site, a stronger binding energy between the incorporated Xe and the large steric vacancy forms. Using ab initio molecular dynamics simulations and climbing-image nudged elastic band calculations, we found that the activation barrier of interstitial Xe in UN in the “kick-out” diffusion mechanism is lower than that in the direct interstitial mechanism, and the net Xe diffusion occurs with vacancies mediated; that is, once an interstitial Xe atom is trapped in a U vacancy site, it will be immobile without other uranium vacancies mediated.

We report herein new pyridine-substituted spirobifluorene (SBF) dyes, i.e., 4-(9,9′-spirobi[fluoren]-4-yl)pyridine (4-4Py-SBF), 3-(9,9′-spirobi[fluoren]-4-yl)pyridine (4-3Py-SBF), and 2-(9,9′-spirobi[fluoren]-4-yl)pyridine (4-2Py-SBF), built on the association of the 4-substituted spirobifluorenyl core and various regioisomers of pyridine. These organic semiconductors possess high triplet energy levels (E_T around 2.7 eV) in accordance with their use as hosts for green and sky-blue phosphorescent organic light-emitting diodes (PhOLEDs). These dyes have been synthesized from the 4-bromo-spirobifluorene (4-Br-SBF) platform, obtained from a new and efficient synthetic approach using the promising building block 4-bromofluorenone as a key intermediate. Synthesis, structural, thermal, electrochemical, and photophysical properties of the three dyes have been investigated in detail and compared to other model compounds, namely, 4-phenyl-SBF (4-Ph-SBF), 2-phenyl-SBF (2-Ph-SBF), and 2,4-pyridyl-SBF (2-4Py-SBF), in order to precisely study the influence of (i) the pyridine unit, (ii) the position of the nitrogen atom within the pyridyl core (in position 4, 3, or 2), and (iii) the substitution at the C4 position of SBF. This rational structure–properties relationship study sheds light on the effect of the substitution in position 4 of the SBF core and may pave the way to the development of such materials in electronics. Finally, the high performance of green PhOLEDs, ca. 63 cd/A, and of sky-blue PhOLED, ca.16 cd/A, clearly evidence the potential of these new SBF derivatives as hosts for phosphorescent dopants.

The photoelectrochemical activity of a mesoporous NiO electrode sensitized by a ruthenium complex was investigated with several rhodium and cobalt H₂-evolving catalysts. Photocurrent as high as 80 μA/cm² was produced by irradiation of such photocathode in the presence of the Rh(III) polypyridyl complexes, while cobalt complexes gave almost no photocurrent. Photolysis experiments led to the two-electron reduced form of the Rh(III) complexes into Rh(I) complexes and demonstrate the occurrence of an electron transfer chain from NiO to the catalyst. Mott–Schottky experiments evidenced the pH dependence of the NiO flat band potential, explaining the dramatic drop of the photocurrent in acidic conditions (cyanoanilinium). By contrast, in weaker acid conditions (formic acid) the photocurrent increases and the key Rh(III) hydride intermediate was efficiently generated. In acetonitrile solution, Rh(III)-H slowly reacts with HCOOH to generate H₂. However, this process was not catalytic, because the reduction potential of the Ru sensitizer is not sufficiently negative to reduce the Rh(III)-H into Rh(II)-H.

LiBH₄–Mg₂NiH₄ reactive hydride composites have been nanoconfined into two types of mesoporous carbons: a templated carbon with ordered small pores of ∼4 nm and a carbon aerogel with pores size of ∼30 nm. In situ synchrotron X-ray diffraction has revealed the formation of the MgNi_2.5B₂ compound during dehydrogenation at 300 °C and 5 bar of H₂ pressure. The hydrogen desorption from nanoconfined LiBH₄–Mg₂NiH₄ shows a single-step reaction at around 300 °C, as observed by mass spectroscopy coupled with thermogravimetric analysis. A synergistic effect is suggested, which facilitates lower hydrogen release than previously reported nanoconfined systems. Effective nanoconfinement provides faster kinetics of hydrogen release. Nevertheless, LiBH₄–Mg₂NiH₄ shows progressive loss of capacity during cycling.

Inspired by the bioadhesion mechanism found in mussel, a catechol derivative, 3-(3,4-dihydroxyphenyl)propionic acid (diHPP), is employed as both linker and reducer of Ag⁺ to synthesize the Ag/TiO₂ nanotube (Ag/TNT) heterojunction under ambient conditions in this study. In the prepared Ag/TNT composite, Ag nanocrystals about 3.8 nm in diameter distribute over the TNT surface uniformly and form the heterojunction structure with TNT. The diHPP first links to the TNT surface through the bidentate chelation of catechol group with Ti⁴⁺ and then acts as both an anchor and a reducer to in situ nucleate and grow Ag nanocrystals on the TNT surface. By adjusting the AgNO₃ concentration, the loading amount of Ag nanocrystals on the TNT surface can be controlled easily, and the visible-light absorption ability of Ag/TNT heterojunctions enhances with increasing the Ag loading amount. Moreover, their photocatalytic activity was evaluated by the degradation capability of Rhodamine B (RhB) under visible light. The Ag/TNT heterojunctions exhibit the high visible-light photocatalytic activity, which can almost degrade 100% RhB within 2 h. This excellent performance can be attributed to the local electric field caused by the surface plasmon resonance (SPR) of Ag nanocrystals and the high adsorption capability of TNTs with large specific surface area.

Hematite (α-Fe₂O₃) is commonly considered for converting solar energy into hydrogen fuel through water splitting. Recent experiments performed in 2013 reached a maximum efficiency in Fe₂O₃ photoelectrochemical cells while using platinum-doped Fe₂O₃. In order to understand how platinum increases efficiency, we use the density functional theory + U (DFT+U) method to model the bulk and the (0001) surface of platinum-doped Fe₂O₃. We also give a unique ligand field theory combined with Bader charge analysis to explain changes resulting from symmetry breaking by the dopant. First, we find that, although platinum has a lower oxidation state than usual n-type dopants, platinum donates electrons. We find a theoretical ideal doping range of 0.64–2.96 atom % for enhanced electron conductivity, which is within the optimal range obtained by previous experiments. Second, we find that the energy gap decreases upon doping, improving solar energy absorption. Third, in agreement with previous experiments, we calculate an unfavorable increase in overpotential for oxidizing water upon platinum doping. Since platinum has both good and bad effects, we recommend bypassing this duality by platinum doping with a gradient-based strategy: high doping in the bulk for enhanced conductivity and low doping at the surface to not interfere with catalysis. We anticipate that experimentally testing our proposed strategy will advance the development of better electrodes for photoelectrochemistry.

Silicon-based lithium-ion battery anodes have brought encouraging results to the current state-of-the-art battery technologies due to their high theoretical capacity, but their large-scale application has been hampered by a large volume change (>300%) of silicon upon lithium insertion and extraction, which leads to severe electrode pulverization and capacity degradation. Polymeric surfactants directing the combination of silicon nanoparticles and reduced graphene oxide have attracted great interest as promising choices for accommodating the huge volume variation of silicon. However, the influence of different polymeric surfactants on improving the electrochemical performance of silicon/reduced graphene oxide (Si/RGO) anodes remains unclear because of the different structural configurations of polymeric surfactants. Here, we systematically study the effect of different polymeric surfactants on enhancing the Si/RGO anode performance. Three of the most well-known polymeric surfactants, poly(sodium 4-styrenesulfonate) (PSS), poly(diallydimethylammonium chloride) (PDDA), and polyvinylpyrrolidone (PVP), were used to direct the combination of silicon nanoparticles and RGO through van der Waals interaction. The Si/RGO anodes made from these composites act as ideal models to investigate and compare how the van der Waals forces between polymeric surfactants and GO affect the final silicon anode performance from both experimental observations and theoretical simulations. We found that the capability of these three surfactants in enhancing long-term cycling stability and high-rate performance of the Si/RGO anodes decreased in the order of PVP > PDDA > PSS.

In this work, we demonstrate the complex excited-state nature of the conjugated polymer, polyspirobifluorene (PSBF), using steady-state and time-resolved spectroscopy techniques to understand the origin of excited charge transfer state (CT) formation and their contribution to the total photoluminescence (PL). The measurements were compared in two solvents with different polarity, for example, methyl cyclohexane (MCH) and 2-methyltetrahydrofuran (2-MeTHF), which allow us to reveal solvent quality and temperature dependent CT state formation arising from “inter/intrachain” interaction phenomena. The inter/intrachain interactions are explained by means of spatial conformational changes of the polymer chain configuration, such as coiling and collapse of the backbone with concomitant side chain reorganization. It has been found that the PL emission at room temperature (RT) demonstrates a mixed state configuration containing contributions from ¹(π, π*) excited states along with the CT states contribution, with the spectra arising from a mixture of the two emissive species. However, with decreasing temperatures to ca. 145 K (prior to the freezing point) in 2-MeTHF, the two emissive species become separated, with the emission from the CT state showing a red-shift with decreasing temperature. At 145 K, we observe the formation of an unstructured, wholly new emission band, which is strongly red-shifted relatively to the ¹(π, π*) excited-state and shows classic Gaussian line shape. This emission is attributed to the formation of “inter/intrachain” CT states. In the case of frozen solutions (∼90 K), the spectra dramatically blue-shifts and loses all contribution from the “inter/intrachain” species, and emission then arises completely from the pure “intrachain” CT excitonic state. The behavior of the polymer is strongly dependent on both solvent quality and temperature effects on the excited state geometry relaxation by means of the local solvent–solute interactions that stabilize the CT states, due to solvation of the new charge distribution, and also changes on the transition states via manipulating energy barriers.

In this paper, nanostructured hematite p-CaFe₂O₄/n-Fe₂O₃ heterojunction photoanodes have been fabricated employing a facile template-less film processing technique by controlling the chemical bath. Anisotropic growth of a β-FeOOH akaganeite film on FTO conductive glass from an aqueous FeCl₃ solution containing CaCl₂ followed by a two-step thermal annealing at 550 and 800 °C induces the formation of a p-CaFe₂O₄/n-Fe₂O₃ heterojunction. The structural, morphological, electronic states, and electrochemical characteristics of the films have been investigated by X-ray diffraction, scanning electron microscopy, X-ray photoelectron spectroscopy, and impedance spectroscopy, respectively. The heterojunction photoanode showed 100% higher photocurrent response than that obtained using a bare hematite electrode under simulated 1-sun illumination (100 mW/cm²). The photocurrent enhancement is attributed to the enhanced charge carrier separation and the reduced resistance in the charge transfer across the electrode and the electrolyte as revealed by electrochemical impedance spectroscopy analysis. The modification of the p-CaFe₂O₄/n-Fe₂O₃ heterojunction photoanode with CoPi cocatalyst further facilitates the electron transfer at the electrode/electrolyte interface and thus enhances the photoelectrochemical water oxidation. Since cheap and abundant materials have been employed for the synthesis of the heterojunction photoanode via a simple route, the current results have great importance, both from a scientific and an economical point of view.

The present work delivers the first assessment of BiFeO₃ (BFO) thin films as an absorber for sustainable all-oxide photovoltaic devices. Films are deposited from a metal–organic precursor complex solution followed by annealing in air at 673 K for 2 h. X-ray diffraction, complemented by quantitative analysis, indicated formation of pure BFO with rhombohedral structure (R₃C). Atomic force microscopy suggests deposition of compact and smooth films with spherical particles of sizes ∼150 nm. A direct band gap of 2.2 eV is ascertained from UV–vis–NIR spectroscopy. Mechanistic aspects of the BFO formation are discussed based on thermograveminetric analysis, differential scanning calorimetry, and infrared spectroscopy of the precursor complex. A proof-of-concept BFO/ZnO heterojunction based solar cell fabricated by solution processing delivered a photoconversion efficiency of 3.98% with open-circuit voltage (V_oc), short-circuit current density, and fill factor of 642 mV, 12.47 mA/cm², and 50.4%, respectively. The device exhibits a maximum external quantum efficiency of nearly 70%. These parameters are among the highest values reported for all oxide PV. Analysis of the V_oc, series resistance, and conversion efficiency as a function of temperature revealed valuable information about recombination processes.

This study examines the formation of previously unreported crystalline phases of N-methoxyethyl-N-methylpyrrolidinium bis(trifluoromethanesulfonyl)imide (Pyr_12O1TFSI). The melting point of pristine Pyr_12O1TFSI, determined by conductivity measurements, is between −20 and −17.5 °C. Formation of this crystalline phase is difficult and only occurs under specific conditions. Pyr_12O1TFSI readily forms 1:1 phases with both NaTFSI and Mg(TFSI)_2. The results of single crystal structure determinations are presented. The Na⁺ crystalline phase provides clear evidence that the Pyr_12O1⁺ cation can coordinate some metal ions, but this coordinative interaction does not occur with all metal cations, e.g., Mg²⁺, and in all states of matter, e.g., Na⁺-IL solutions. The TFSI^– ions are found in two different aggregate solvates in the Pyr_12O1TFSI:NaTFSI 1:1 phase and in contact ion pair and aggregate solvates in the Pyr_12O1TFSI:Mg(TFSI)₂ 1:1 phase. The Pyr_12O1TFSI:Mg(TFSI)₂ crystalline phase gives insight into the local structure of the liquid electrolyte, where it is likely that a maximum of approximately 30% of the total TFSI^– can likely be coordinated in a bridging geometry, and the rest are in a bidentate coordination geometry. This ratio is determined from both the crystal structure and the Raman spectroscopy results.

Theoretical predictions show that depending on the populations of the Fe 3d_xy, 3d_xz, and 3d_yz orbitals two possible quintet states can exist for the high-spin state of the photoswitchable model system [Fe(terpy)₂]²⁺. The differences in the structure and molecular properties of these ⁵B₂ and ⁵E quintets are very small and pose a substantial challenge for experiments to resolve them. Yet for a better understanding of the physics of this system, which can lead to the design of novel molecules with enhanced photoswitching performance, it is vital to determine which high-spin state is reached in the transitions that follow the light excitation. The quintet state can be prepared with a short laser pulse and can be studied with cutting-edge time-resolved X-ray techniques. Here we report on the application of an extended set of X-ray spectroscopy and scattering techniques applied to investigate the quintet state of [Fe(terpy)₂]²⁺ 80 ps after light excitation. High-quality X-ray absorption, nonresonant emission, and resonant emission spectra as well as X-ray diffuse scattering data clearly reflect the formation of the high-spin state of the [Fe(terpy)₂]²⁺ molecule; moreover, extended X-ray absorption fine structure spectroscopy resolves the Fe–ligand bond-length variations with unprecedented bond-length accuracy in time-resolved experiments. With ab initio calculations we determine why, in contrast to most related systems, one configurational mode is insufficient for the description of the low-spin (LS)–high-spin (HS) transition. We identify the electronic structure origin of the differences between the two possible quintet modes, and finally, we unambiguously identify the formed quintet state as ⁵E, in agreement with our theoretical expectations.

Density functional theory was used to investigate the mechanistic aspects of the adsorption of sulfur-containing compounds over Ti–Ce mixed metal oxides. We elucidate the promotional effect of the Ce dopant on TiO₂ and report the importance of oxygen vacancy-bound molecular oxygen as an active site on Ti–Ce mixed metal oxides for adsorption of thiophenic sulfur. The presence of surface-activated molecular oxygen leads to the oxidation of the sulfur, thus providing strongly bound sulfoxide and sulfone species. Ce doping of TiO₂ makes the oxidation process feasible both thermodynamically and kinetically. Surface oxygen vacancy sites act as catalytic sites in an adsorption cycle. DRIFTS results corroborate the presence of vacancy bound molecular oxygen. Our DFT calculations also examine thiophene and methyl-, dimethyl-, benzo-, and dibenzothiophenes adsorbed on TiO₂ and Ce-doped TiO₂ (001), (101), and (100) surfaces.

Using quantum chemical calculations, we investigate surface reactions of copper precursors and diethylzinc as the reducing agent for effective Atomic Layer Deposition (ALD) of Cu. The adsorption of various commonly used Cu^(II) precursors is explored. The precursors vary in the electronegativity and conjugation of the ligands and flexibility of the whole molecule. Our study shows that the overall stereochemistry of the precursor governs the adsorption onto its surface. Formation of different Cu^(II)/Cu^(I)/Cu⁽⁰⁾ intermediate complexes from the respective Cu^(II) compounds on the surface is also explored. The surface model is a (111) facet of a Cu₅₅ cluster. Cu^(I) compounds are found to cover the surface after the precursor pulse, irrespective of the precursor chosen. We provide new information about the surface chemistry of Cu(II) versus Cu(I) compounds. A pair of CuEt intermediates or the dimer Cu₂Et₂ reacts in order to deposit a new Cu atom and release gaseous butane. In this reaction, two electrons from the Et anions are donated to copper for reduction to metallic form. This indicates that a ligand exchange between the Cu and Zn is important for the success of this transmetalation reaction. The effect of the ligands in the precursor on the electron density before and after adsorption onto the surface has also been computed through population analysis. In the Cu^(I) intermediate, charge is delocalized between the Cu precursor and the bare copper surface, indicating metallic bonding as the precursor densifies to the surface.

Two types of biomimetic hydroxyapatite (HA) nanoparticles were prepared by acid–base neutralization reactions, using Ca(OH)₂ or Ca(CH₃COO)₂ as a calcium source, to evaluate the effect of acetate anions on particle formation. High-resolution transmission electron microscopy observations provided evidence that in both cases nanoparticles are elongated along the c-axis, but to a more limited extent when prepared in the presence of acetates, and are mainly limited by {010} facets. IR spectra of nanoparticles containing adsorbed CO revealed that the actual termination of these are both of the {010}_Ca-rich and {010}_P-rich type, the latter being significantly more abundant for HA nanoparticles grown in the medium containing CH₃COO^– species. Moreover, these nanoparticles appeared to be more sensitive toward aggregative stacking by thermal treatment, resulting in a significant decrease in specific surface area, while retaining the size of primary particles.

Recently, our group proposed the catalytic transfer hydrogenation for refining biomass-derived 5-hydroxymethylfurfural and furfural to 2,5-dimethylfuran and 2-methylfuran, respectively. With a metallic Ru/C catalyst, a selectivity of ∼30% was achieved. The promotion of the Ru/C catalyst with the Lewis acid RuO_x resulted in a selectivity of ∼80%. In the current study, we employ density functional theory calculations on the RuO₂ (110) surface in order to elucidate the role of the Lewis acidity in the reduction of the biomass-derived furfuryl alcohol to 2-methylfuran. We identify the rate-limiting step to be the scission of the C–O bond of the side chain. In addition, we find evidence for the activation of the furan ring via an insertion of a hydrogen atom. However, the Lewis basicity of a neighboring bridging oxygen results in the furan ring being deactivated. Finally, the formation of water from the reduction process is facile. However, owing to the strong binding energy between the RuO₂ surface and the water molecule, poisoning of the catalytic surface by adsorbed water is possible. Finally, we show that the RuO₂ (110) carries out the reduction of furfural to furfuryl alcohol via the Meerwin–Ponndorf–Verley (MPV) reaction fairly easily, consistent with published experimental data.

Various methods that aim to improve the photocatalytic activity of TiO₂ have been reported in the literature. Herein, we report that addition of CuWO₄ into the aqueous suspension of TiO₂ can result in significant enhancement in the rate of phenol degradation. As the amount of CuWO₄ increased, the rate of phenol degradation increased and then decreased. A maximum rate of phenol degradation observed with 2 wt % CuWO₄ was about 2.83 times that in the absence of CuWO₄. A similar result was also observed with CuO. However, six consecutive tests showed that CuWO₄/TiO₂ was much more stable than CuO/TiO₂, due to the very high stability of CuWO₄ against photocorrosion. The improved activity of TiO₂ is not due to CuWO₄ and CuO themselves and also does not match their solubility in aqueous solution. Moreover, for the generation of OH radicals, and for the decomposition of H₂O₂ in aqueous solution, CuWO₄/TiO₂ was also more active than TiO₂. Through a (photo) electrochemical measurement, a possible mechanism is proposed, involving electron transfer from the irradiated TiO₂ to CuWO₄ that facilitates the charge separation of TiO₂ and consequently accelerates reactions at interfaces.

Recently, polymer–fullerene based bulk heterojunction (BHJ) solar cells, which contain blends of poly({4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b′]dithiophene-2,6-diyl}{3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno[3,4-b]thiophenediyl}) (PTB7) and [6,6]-phenyl-C₇₁-butyric acid methyl ester (PC₇₁BM), have been widely studied due to exhibiting high power conversion efficiency (PCE) and well-defined nanomorphology. Because of the short exciton diffusion pathway (less than 10 nm) in organic thin films, the optimization of PTB7:PC₇₁BM BHJ with optimized morphology is very important between the donor and acceptor. In order to increase nanoscale phase separation, the chemical additives of 1,8-diiodooctane (DIO) have been used in PTB7:PC₇₁BM blend systems. However, the mechanism studies of DIO in BHJ solar cells and its effectiveness on device stability are unclear. In this study, we fabricated polymer solar cells (PSCs) based on PTB7:PC₇₁BM BHJ with various DIO concentrations to investigate not only correlation between device performances and different morphologies, but also the influence of additives on device stabilities. Positive effects of DIO, which were induced by efficient charge separation in BHJ at optimized blending ratio, are proved by the results of time-resolved photoluminescence (TRPL), and negative effects of DIO on a device stability have been investigated according to the ISOS-D-1 protocol.

Through a combined quantum mechanics/molecular mechanics (QM/MM) approach, we evaluate the photodynamics of the mono- and dithiolated azobenzenes when they are chemisorbed on a gold surface. The analysis of steric effects suggests that the presence of the surface influences the photoisomerization process of the chemisorbed monothiolated azobenzene. In particular, the trans → cis quantum yields decrease, and the nπ* state lifetimes become longer. The approach to the twisted conical intersection needed for the isomerization is hindered when the molecule is attached to a substrate because of the van der Waals interactions with the surface. For the cis isomer, the cis → trans photoisomerization quantum yield is almost unaffected, since this isomer is not flat, and thus the interaction with the surface is less remarkable. Dithiolated azobenzene can photoisomerize both trans → cis and cis → trans, also when doubly linked to the surface, preserving the two bonds with the gold atoms: the flexibility of the central azo-moiety enables the molecule to photoisomerize without any bond breaking. The quantum yields in this case are even higher than in the monothiolated case, probably because of the strained initial conformation, which must adapt to the available distances between the anchoring sites.

Reactive Cu deposition in an O₂ ambience is used to prepare homogeneous copper oxide films on an Au(111) support. X-ray photoelectron spectroscopy reveals a predominant Cu₂O stoichiometry of the ad-layers. Their growth morphology and atomic structure is determined with scanning tunneling microscopy (STM) and low-energy electron diffraction. The films grow in a layer-by-layer fashion and expose different terminations depending on the preparation conditions. While as-prepared films develop a variety of surface reconstructions, the structure of postannealed films is similar to the one of bulk-cut Cu₂O(111). STM conductance spectroscopy is used to probe the local electronic structure and work function of the material. Apart from the ∼2.0 eV band gap of Cu₂O, an unoccupied surface state is identified for annealed films. The high crystallographic quality of the cuprous oxide films renders them suitable for further investigations of the defect landscape and the optical properties of this reference material for photocatalysis and photovoltaics.

Gold-core platinum-shell (Au@Pt) nanoparticles with ultrathin platinum overlayers, ranging from submonolayer to two monolayers of platinum atoms, were prepared at room-temperature using a scalable, wet-chemical synthesis route. The synthesis involved the reduction of chloroauric acid with tannic acid to form 5 nm (nominal dia.) gold nanoparticles followed by addition of desired amount of chloroplatinic acid and hydrazine to form platinum overlayers with bulk Pt/Au atomic ratios (Pt surface coverages) corresponding to 0.19 (half monolayer), 0.39 (monolayer), 0.58 (1.5 monolayer) and 0.88 (2 monolayers). The colloidal particles were coated with octadecanethiol and phase-transferred into chlroform-hexane mixture to facilitate sample preparation for structural characterization. The structure of the resultant nanoparticles were determined to be Au@Pt using HRTEM, SAED, XPS, UV–vis and confirmed by cyclic voltammetry (CV) studies. Monolayers of octadecanethiol coated Au@Pt nanoparticles were self-assembled at an air–water interface and transfer printed twice onto a gold substrate to form bilayer films for electrochemical characterization. Electrochemical activity on such films was observed only after the removal of the octadecanethiol ligand coating the nanoparticles, using a RF plasma etching process. The electrochemical activity (HOR, MOR studies) of Au@Pt nanoparticles was found to be highest for particles having a two atom thick platinum overlayer. These nanoparticles can significantly enhance platinum utilization in electrocatalytic applications as their platinum content based activity was three times higher than pure platinum nanoparticles.

The interfacial electrochemistry of hematite (α-Fe₂O₃) is a key aspect for understanding the behavior of this important mineral phase in photocatalytic water-splitting devices as well as in terrestrial and atmospheric systems. Nano- to microsized particles are often multifaceted and exhibit terminations of varied crystallographic orientations and structures. As structure often controls reactivity, this study was devised to identify the impact of crystallographic orientation on the electrochemical response of hematite electrode surfaces contacted with technologically, geochemically, and environmentally important solutions of inorganic ions (NaCl, NaHCO₃, and NH₄Cl). Electrochemical impedance spectroscopy (EIS) measurements of single hematite crystals oriented along the (001) and (012) faces were used for this purpose. The EIS responses of the electrodes were described in terms of an equivalent electrical circuit that accounts for fast bulk and slower interfacial processes. Capacitance and resistance values for the bulk processes confirmed the anisotropic conductivity attributes of hematite and supported the use of the EIS data for interpreting the crystallographic orientation dependence of interfacial processes. These efforts extracted diffuse (C_dl) and compact (T_ad) layer capacitances and resistance (R_ad), as well as relaxation times pertaining to the re-equilibration of interfacial species during EIS. Capacitance values confirmed the greater charge-storing capability of the (012) face (C_dl = 1–10 μF·cm^–2; T_ad = 3–35 μF·cm^–2·s^–φ) compared to the (001) face (C_dl = 0.2–0.6 μF·cm^–2; T_ad = 0.2–0.6 μF·cm^–2·s^–φ). This was also confirmed through the resistance values pertaining to the transfer of charge carriers across the compact plane, which were lower (R_ad = 0.0–0.8 MΩ·cm^–2) on the (012) face than on the (001) face (R_ad = 1–4 MΩ·cm^–2). Binding of chloride and (bi)carbonate on the (012) face under acidic conditions was associated with an increase in capacitance values and relaxation times. The lowest capacitances and relaxation times occurred in the pH 8–9 region, which correspond to a likely point of zero charge. The capacitance values in NH₄Cl were considerably lower than in NaCl and NaHCO₃, owing to hydrogen bonding between the NH₄⁺/NH₃ species and surface (hydr)oxo groups. Such interactions can block protonation reactions and can be translated to negligible relaxation times for this system. Collectively, these findings underpin the interdependency of the hematite electrode surface orientation on its electrochemical signatures for important inorganic ions of direct relevance to technological and natural systems.

The pre-H₂O treatment and Al₂O₃ film growth under a two-temperature-regime mode in an oxygen-deficient atomic layer deposition (ALD) chamber can induce n-type doping of graphene, with the enhancement of electron mobility and no defect introduction to graphene. The main mechanism of n-type doping is surface charge transfer at graphene/redox interfaces during the ALD procedure. More interestingly, this n-type doping of graphene is reversible and can be recovered by thermal annealing, similar to hydrogenated graphene (graphane). This technique utilizing pre-H₂O treatment and an encapsulated layer of Al₂O₃ achieved in an oxygen-deficient ALD chamber provides a simple and novel route to fabricate n-type doping of graphene.

The electronic structure and nature of the defect states below the conduction band edge of an HfO₂ gate dielectric grown on InP substrate prepared by atomic layer deposition was examined using X-ray photoelectron spectroscopy (XPS), X-ray absorption spectroscopy (XAS), and density functional theory (DFT). When the HfO₂ dielectric was deposited on an InP substrate with an abrupt interface, the resulting HfO₂ develops a tetragonal (t) structure, which minimizes the interfacial lattice mismatch. The O K-edge absorption features and DFT calculations indicated that additional structural distortion occurred by a q = −2 charged O vacancy (VO^–2) in the t-HfO₂. The electronic structure and the charge-transition levels of t-HfO₂ with VO^–2 were assigned based on a second derivative analysis of O K-edge features; 12 distinct pre-edge defect states below the Hf 5d conduction band edge were evident when the degeneracies resulting from Jahn–Teller splitting and crystal field splitting were removed. No changes in the electronic structure near valence band edge by VO^–2 were observed in XPS valence spectra. Moreover, O vacancies in t-HfO₂ lead to substantial midgap states caused by interstitial elemental In or P in the t-HfO₂ due to the enhanced out-diffusion of elemental In or P through O vacancies. We conclude that both the defect states near the CBE and the midgap states could be controlled by the incorporation of nitrogen into the HfO₂ using a thermal NH₃ treatment.

We show that Derjaguin’s theory of adsorption can be used to predict adsorption on bare and modified surfaces using parameters available to simple experiments. Using experiment and molecular simulation of adsorption of various gases on hydroxylated, methylated, and trifluoromethylated silica, this simple parametrization of Derjaguin’s model allows predicting adsorption on any functionalized surface using a minimum set of parameters such as the heat of vaporization of the adsorbate and the Henry constant of the adsorption isotherm. This general yet simple scheme constitutes a powerful tool as it avoids having to carry out tedious and complex adsorption measurements.

The adsorption of diethyl ether (Et₂O) on Si(001) was studied by means of scanning tunneling microscopy (STM) and photoelectron spectroscopy. Et₂O reacts on Si(001) via a datively bonded intermediate, which was isolated at surface temperatures below 100 K. At higher surface temperature, Et₂O converts dissociatively into the final state by cleaving one O–C bond; the resulting −O–C₂H₅ and −C₂H₅ fragments are found to attach on two Si dimers of neighboring dimer rows. Tip-induced hopping of the −C₂H₅ fragment on one dimer was observed at positive sample bias. The results are discussed in the context of recent experiments on the reaction of tetrahydrofuran (THF) on Si(001) (Mette et al. ChemPhysChem 2014, 15, 3725) and allow a more general description of the reaction of ethers on Si(001).

The structure of Ag nanoparticles of different size, supported on the cerium oxide (111) surface, was investigated by X-ray absorption fine structure at the Ag K-edge. The results of the data analysis in the near and extended energy range are interpreted with the help of the results obtained by X-ray photoelectron spectroscopy and scanning tunneling microscopy measurements and allow to obtain a detailed atomic scale description of the model system investigated. The Ag nanoparticles have an average size of a few tens of angstroms, which increases with increasing deposited Ag amount. The nanoparticles show a slight tendency to nucleate at the step edges between different cerium oxide layers and they have a face centered cubic structure with an Ag–Ag interatomic distance contracted by 3–4% with respect to the bulk value. The interatomic distance contraction is mainly ascribed to dimensionality induced effects, while epitaxial effects have a minor role. The presence of Ag–O bonds at the interface between the nanoparticles and the supporting oxide is also detected. The Ag–O interatomic distance decreases with decreasing nanoparticle size.

Photoelectron spectroscopy is a powerful tool for investigating the electronic structure of supported clusters, especially when initial state and final state contributions to the electron binding energy can be separated. We have performed a combined experimental and theoretical study of ultraviolet and X-ray photoelectron spectroscopy (UPS and XPS) using atomically size-selected Pd_n clusters on rutile TiO₂(110). Theoretical investigations allow for the UPS and XPS shifts to be split into initial state and final state contributions. In XPS, the occupation of the 4d orbital of Pd controls the initial state shift offering information about the hybridization of the cluster, while the size and the charging of the cluster controls the final state shift. In UPS, we evaluate two methods for calculating the final state shift in periodic unit cells and find that both methods give reasonable results for pristine TiO₂; however, using a p-type dopant fails when two separate donor–acceptor pairs are present. The observed UPS shifts can be described by combining the surface dipole and the final state shifts. Metallic contacts to the semiconductor surface result in band alignment between the metallic contact and the cluster, shifting the Fermi level to lie just below the conduction band of the TiO₂. Information about the charge state and hybridization of the cluster are revealed by separating the initial and final state effects.

Zn was suggested to be a promising additive to Pt in the catalysis of dehydrogenation reactions. In this work, mixed Pt–Zn clusters deposited on two simple oxides, MgO(100) and TiO₂(110), were investigated. The stability of these systems against cluster sintering, one of the major mechanisms of catalyst deactivation, is simulated using a Metropolis Monte Carlo scheme under the assumption of the Ostwald ripening mechanism. Particle migration, association to and dissociation from clusters, and evaporation and redeposition of monomers were all included in the simulations. Simulations are done at several high temperatures relevant to reactions of catalytic dehydrogenation. The effect of temperature is included via both the Metropolis algorithm and the Boltzmann-weighted populations of the global and thermally accessible local minima on the density functional theory potential energy surfaces of clusters of all sizes and compositions up to tetramers. On both surfaces, clusters are shown to sinter quite rapidly. However, the resultant compositions of the clusters most resistant to sintering are quite different on the two supports. On TiO₂(110), Pt and Zn appear to phase separate, preferentially forming clusters rich in just one or the other metal. On MgO(100), Pt and Zn remain well-mixed and form a range of bimetallic clusters of various compositions that appear relatively stable. However, Zn is more easily lost from MgO through evaporation. These phenomena were rationalized by several means of chemical bonding analysis.

The electronic properties of free-standing and Cu-supported pristine and boron-doped, nitrogen-doped, and codoped graphene have been studied by means of density functional theory (DFT) with the vdW-DF2^C09x functional. The effects of substitutional chemical doping, metal support, lattice parameter strain, and their eventual interplay have been investigated. We find that only boron-doped graphene strongly interacts with the copper substrate, due to chemical bonds between the boron atom and the underlying metal. The binding energy and charge transfer from Cu are also highly enhanced compared to both pristine and nitrogen-doped supported graphene. The BN codoped system behaves similarly to pristine graphene with a weakly physisorbed state and a small charge transfer from Cu. However, the presence of the nonmetal dopants makes the codoped sheet extremely tunable for redox purposes, with the boron site acting as an electron acceptor and the nitrogen site as an electron donor.

Pd/CeO₂–ZrO₂–Pr₂O₃ (CZP) catalysts were synthesized with different Ce/Zr molar ratios and 8.0 wt % Pr₂O₃ doping. Their structures were characterized by various techniques, especially using X-ray diffraction (XRD) in combination with Rietveld refinement and Raman analysis. The XRD pattern of CZP with Ce/Zr molar ratios >1 can be indexed satisfactorily to the fluorite structure with a space group Fm-3m. When the Ce/Zr molar ratio reaches 1/2, the XRD patterns only display diffraction peaks of the tetragonal phase (S.G. P4₂/nmc). The Pr additive in the crystal lattice mainly replaces the position of Ce, which improves the redox reaction activity of catalysts when compared with Pr-free catalysts. Raman results reveal the enrichment of defect sites on the surface. The presence of moderate ZrO₂ can increase both oxygen vacancies and the ZrO₈-type complex in supports, which is in accord with the change regularity of their oxygen storage capacity (OSC) values. Moreover, the addition of moderate Zr and Pr promotes the catalytic activity of NO_x, HC, and CO conversions and thermal stability and obviously widens the operational window due to their larger number of oxygen vacancies and higher OSC value, especially for Pd/CZP catalysts with Ce/Zr molar ratios from 2/1 to 1/2.

We employed a combination of isotopic labeling experiments, density functional theory calculations, and first-principles microkinetic modeling to investigate the mechanism of H/D exchange of furanic platform molecules. Alkylated furans (e.g., 2-methylfuran (2-MF)) exhibit appreciable H/D exchange, but furan and oxygenated furanics (e.g., furfuryl alcohol) do not. Detailed mass fragmentation pattern analysis indicates H/D exchange only occurs at unprotected α-carbons. Simulations show that, in the presence of coadsorbed toluene (solvent), the most likely pathway involves Ru surface mediated scission of the C–O bond in the furan ring at the unsubstituted carbon atom, followed by dehydrogenation, deuteration, and ring-closure steps. The degree of H/D exchange reaction depends mainly on the adsorption strength of exchange intermediates: strongly bound compounds, e.g., furan and furfuryl alcohol, inhibit H/D exchange via site blocking and slow desorption, whereas alkylated furans are sterically repelled by the solvent freeing up catalyst sites for exchange at the unsubstituted α-carbon of the furan ring. The binding strength of exchange intermediates is governed by interaction of the substituent group both with the surface and with the coadsorbed solvent molecules. The proposed H/D exchange mechanism on metal catalysts, which involves the opening of furan ring, is in stark contrast to the Brønsted catalyzed ring activation and suggests a possible pathway for the formation of ring-opening products and for rational selection of solvents.

Ab initio molecular dynamics simulation based on density functional theory was performed to investigate (Ti, Cr, or W)-incorporated diamond-like carbon (DLC) films. The structure models were generated from liquid quench containing 64 atoms. The dependence of the residual compressive stress, bulk modulus and tetra-coordinated C content on the Ti, Cr, and W concentrations in the range of 1.56 to 7.81 atom % was studied. The present simulation results reveal that the residual stress strongly depends on the incorporated Ti, Cr, and W atoms. With the incorporation of Ti at 1.56 atom %, Cr at 4.69 atom %, and W at 3.13 atom % to DLC films, the compressive stress was reduced by 46.9%, 81.4%, and 82.5%, respectively, without obvious deterioration of the mechanical properties. However, at higher Ti, Cr, and W concentrations, the compressive stress increased for each case, which was consistent with the experimental results. Structural analysis using both the bond angle and bond length distributions indicates that the small amount of Ti or W incorporation efficiently relaxes both the highly distorted bond angles and bond lengths, whereas the Cr incorporation only relaxes the distorted bond lengths, which decreases the residual compressive stress and provides theoretical explanations for the experiments.

The anatase TiO₂ single crystal exposed high-surface-energy facets have attracted much attention. However, the evolution mechanism and process of the high surface energy facets are still not clear. Here, based on the analysis of experimental results and theoretical calculation results, the evolution process and balanced coexistence for high-surface-energy TiO₂ facets, such as {001} and {110} facets, was well explicated. Thus, this work will help for better understanding and controlling of the morphology of metal oxide crystals with different facets.

The interaction between zinc(II) tetraphenylporphyrin (ZnTPP) molecules and a Au(111) surface is investigated using scanning tunnel microscopy, from initial adsorption sites to monolayer self-assembly, with a particular emphasis on its relation to the surface atomic structure reorganization. At low coverage, no ZnTPP molecules are observed on terraces, and adsorbates appear to only decorate step edges. At intermediate coverage, ZnTPPs adsorb into self-organized islands of flat-lying macrocycles in quasi-registry with the underlying surface reconstruction, in areas delimited by herringbone reconstruction domain walls. At monolayer coverage, the adsorption geometry of the self-organized molecular layer can be fully characterized with respect to the atomic structure of Au(111) surface atoms. Moreover alteration of the Au(111) surface reconstruction domain size is observed, caused by an adsorbate-induced reduction of the Au(111) surface stress anisotropy. This behavior is not ubiquitous to ZnTPP monolayers as a monolayer prepared from the desorption of a ZnTPP multilayer does not alter the domain size of the Au(111) surface. In this case, the additional thermal energy leads to a complete rearrangement of the self-assembled structure, and the surface stress anisotropy returns to its value for the clean surface.

We study adsorption-induced deformation of microporous carbons in the vicinity of the critical temperature of the adsorbed fluid for a range of subcritical pressures. The thermodynamic model (Kowalczyk, P.; Ciach, A.; Neimark A. Langmuir 2008, 24, 6603) coupled with molecular simulations and experimental dilatometric measurements at T/T_c = 1.623 (T and T_c denote experimental and critical temperature, respectively) is used for constructing dilatometric deformation curve at T/T_c = 1.001. We find that the initial contraction of a microporous carbon sample upon near-critical argon adsorption is ∼1.5 vol % (∼2 orders of magnitude higher than at T/T_c = 1.623). Large initial contraction of microporous carbon is a result of significantly higher solvation pressures in carbon micropores generated by the near-critical argon adsorption. In supermicropores and narrow mesopores (pore size ∼ 0.7–5.0 nm) we observe a crossover between the oscillatory solvation force in thin pores, and the attractive thermodynamic Casimir force in pores of very large thickness. In the crossover regime, the oscillatory decay is superimposed on an attractive background, and the repulsion between the confining surfaces appears only when the pressure exceeds a certain value depending on the pore width. The attractive force in all pores exceeding a certain width can lead to a significant increase of the contraction of the sample containing supermicropores and narrow mesopores when the critical temperature is approached at sufficiently low subcritical pressure (∼0.4 MPa in the case of argon).

Although it has been widely accepted that the crystal phase, morphology, and facet significantly influence the catalytic and photocatalytic activity of TiO₂, establishing the correlation between structure and activity of heterogeneous reactions is very difficult because of the complexity of the structure. Utilizing ultrahigh vacuum (UHV) based temperature-programmed desorption (TPD) and density functional theory (DFT) calculations, we have successfully assessed the photoreactivity of two well characterized rutile surfaces ((011)-(2×1) and (110)-(1×1)) through examining the photocatalyzed oxidation of methanol. The photocatalytic products, such as formaldehyde and methyl formate, are the same on both surfaces under UV illumination. However, the reaction rate on (011)-(2×1) is only 42% of that on (110)-(1×1), which contradicts previous reports in aqueous environments where characterization of TiO₂ structure is difficult. The discrepancy probably comes from the differences of the TiO₂ structure in these studies. Our DFT calculations reveal that the rate-determining step of methanol dissociation on both surfaces is C–H scission,; however, the barrier of this elementary step on (011)-(2×1) is about 0.2 eV higher than that on (110)-(1×1) because of their distinct surface atomic configurations. The present work not only demonstrates the importance of surface structure in the photoreactivity of TiO₂, but also provides an example for building the correlation between structure and activity using surface science techniques and DFT calculations.

Heats of adsorption of methane, ethane, and propane in H-chabazite (Si/Al = 14.4) have been measured and entropies have been derived from adsorption isotherms. For these systems quantum chemical ab initio calculations of Gibbs free energies have been performed. The deviations from the experimental values for methane, ethane, and propane are below 3 kJ/mol for the enthalpy, and the Gibbs free energy. A hybrid high-level (MP2/CBS): low-level (DFT+dispersion) method is used to determine adsorption structures and energies. Vibrational entropies and thermal enthalpy contributions are obtained from vibrational partition functions for the DFT+dispersion potential energy surface. Anharmonic corrections have been evaluated for each normal mode separately. One-dimensional Schrödinger equations are solved for potentials obtained by (curvilinear) distortions of the normal modes using a representation in internal coordinates.

By using experiments and an intuitive model, this study reports on the dependence of different dipole resonances in gold nanoparticles (NPs) with rectangular, elliptical, and diamond-like footprints on the length of the three principal axes and on particle geometry. The length of the two in-plane principal axes of the NPs studied is between 50 and 300 nm, while the particle height is between 10 and 50 nm. The scattering experiments reveal characteristic dependencies of the in-plane dipolar resonances on axis lengths and particle geometry. These experimental findings are discussed according to an intuitive resonance condition based on the electrostatic eigenmode method extended to include terms for retardation (dynamic depolarization) and particle curvature. Focus is laid on the dependence of the retardation term on the relevant depolarization factor, the contributions of static and dynamic depolarization to the resonance wavelength, and to a generalization of the depolarization factors that describe the static depolarization in order to account for the differences in the dipolar surface modes of NPs with different geometries. The result of these studies is a detailed description and improved understanding of the scaling behavior of individual dipolar NP plasmon resonances.

Growth of a metal on nanoparticles has been considered to be a useful synthetic tool in a wide variety of applications ranging from catalysis to nanomedicine. This technique can combine more than two functionalities into a single nanoparticle. Most of the methods for such growth rely on a trial-and-error approach to produce grown nanoparticles with the desired sizes and shapes, which is rather time consuming and difficult to reproduce. Here we systematically studied the equilibrium morphology of metal/dielectric dimeric nanoparticle. A computational model was developed by considering the diffusion and surface energy of a metal and the interface energy between the metal and a dielectric. As a proof-of-concept, the growth of Au on a dimeric nanoparticle consisting of Au and polystyrene (PS) was considered. The effects of the surface and interface energy, the concentration of Au ion over the course of the growth, and the size of PS on the shape (i.e., morphology) of the grown nanoparticle were examined. Interestingly, the effects of the surface and interface energy of Au on its coverage of PS are found to be relatively negligible compared to the other two factors. A diagram for the equilibrium morphology with respect to the concentration of Au ion and the size of PS is proposed, which is qualitatively consistent with the experiment.

In this study, two salicylidene ligands, N,N′-bis(salicylidene)-1,2-phenylenediamine and N,N′-bis(salicylidene)-4,5-diaminopyrimidine, and their respective aquo-zinc(II) coordination compounds were synthesized. Their characterization was performed by FTIR, proton and carbon NMR, elemental analysis, mass spectroscopy, and cyclic voltammetry. Crystal structures of the ligands were determined by monocrystal X-ray diffraction. The photoluminescent properties under photostationary conditions indicate that the ligand emission predominates in both the pristine materials and their zinc(II)complexes. For both ligands, the coordination of a metal atom leads to a redshift of their emission bands in both solvent and solid state. Molecular structures and excitation energies of ligands and complexes were evaluated at the DFT level using PBE0/aug-cc-pVDZ. Their ligand and complex electronic transitions can be assigned mainly to the intraligand π→π* type, mainly involving frontier molecular orbitals, with a small participation of the metal. According to our calculations, there is an increase in the planarity of the ligand structure in the complex, which could explain the redshifting observed in the absorption and emission spectra. The dynamic photoluminescence suggests the occurrence of excited state intramolecular proton transfer from the oxygen to the nitrogen atoms in the coordination site of the sal-4,5-pym. Moreover, they are able to predict the occurrence of the excited state internal proton transfer for the sal-4,5-pym. The dynamic of this proton transfer is demonstrated by both time-resolved emission spectra (TRES) and studies in protic solvent (ethanol).

Conventionally, the core electron density (ED) of atoms in molecules is considered to be virtually unperturbed by chemical bonding effects. Here we report a combined experimental and theoretical investigation of the ED of cubic boron nitride including a detailed modeling of the core ED. By modeling structure factors obtained from very-high-resolution synchrotron powder X-ray diffraction data, it is possible to model not only the valence ED but also the response of the core ED to the effects of chemical bonding. The biggest challenge when studying the core ED is the deconvolution of the thermal motion from the experimental structure factors, since the thermal motion is strongly correlated to core ED deformation. However, atomic displacement parameters could be estimated from a full pattern Rietveld-multipolar refinement, and they are shown to be in good correspondence with ab initio lattice dynamics calculations. The corresponding extended multipole model including both core and valence ED refinement suggests that 2.0 electrons are transferred from the boron atomic basin to the nitrogen atomic basin. The core density was found to deplete upon bonding, which is in line with a significant charge transfer.

The absorption and emission properties of macrocyclic cis-indolenine squaraine dyes are studied experimentally and theoretically. As compared to the monomeric species, large Stokes shifts of the emission are observed for cyclic trimers and tetramers. The theoretical analysis predicts that these shifts are caused by deviations of the excited state geometries from the strictly symmetric configurations. Breaking the symmetry makes fluorescence from lower excited states possible which otherwise is forbidden. The line shapes of the spectra are dominated by vibronic structures which can be identified using a vibronic coupling model. From the results, the important fact emerges that macrocyclic squaraine aggregates can be effectively used as building blocks for near-infrared (NIR) emitters.

We develop a quantum kinetic theory of the dynamic response of typical noble metals. Our approach is based on the density functional theory (DFT) and incorporates new important elements as compared to the conventional time-dependent DFT formalism. The kinetic DFT is derived starting from the master equation of motion for the density matrix, which involves both momentum and energy relaxation processes. Therefore, the quantum system is described by two relaxation parameters, unlike the conventional time-dependent DFT incorporating only one relaxation parameter. This allows us to describe both the absorption of light and the generation of hot plasmonic electrons. Using our kinetic DFT theory, we also observe the transition from the multiple peaks in small size-quantized systems to the intensive plasmonic resonance in large classical systems. Unlike the standard picture of collisional broadening of the plasmon peak in small systems, we observe a very different scenario: the formation of multiple plasmonic-like peaks in small quantized systems. These peaks are the result of a hybridization of the collective plasmon mode and the single-particle transitions in a quantized electron gas. There are a few experimental observations that seem to correlate with such a scenario of the plasmonic broadening in small systems. Our approach also incorporates the interband transitions, which are important for a qualitative description of gold and silver. Although this paper gives an application of our kinetic DFT only to the slab geometry, our theory can be applied to nanocrystals of arbitrary shape. The kinetic DFT formalism developed here can be employed to model and predict a variety of metal and hybrid nanostructures for applications in photocatalysis, sensors, photodetectors, metamaterials, etc.

The aggregated noble metal system, consisting of two or more nanoparticles, possesses unique optical properties. The most important is the ability to generate larger local electric field enhancement than a single particle. In this work, we have modeled the system composed of silver nanoprisms with different geometries. For this purpose, the optical properties of the single silver nanoprism and the aggregated nanoprism dimer with adjacent and coplanar bases configuration have been studied by the finite integration technique. The relationship between the geometrical parameters, in particular the radius of the edges of curvature of a single nanoprism, and the position of the extinction peak has been described in a form of a mathematical equation. By moving and rotating one of the nanoprisms relative to the other in the dimer system, the coupling strength between them has been investigated by analyzing the near-field and far-field properties. On the basis of these results, a hybridization model of the nanoprism dimer has been proposed. Our theoretical considerations presented in this article can be a useful tool for predicting the optical properties of the organized metal nanoparticles and the optimization of the assembly process.

Polarization-sensitive asymmetric metamaterial structures, which consist of both conductive and capacitive interactions between tilted gold nanorods, are investigated both theoretically and experimentally. The near-infrared transmission spectra of the fabricated metamaterial display two transmission dips related to the dipole modes of the nanorods. These dipole modes couple to each other, and the coupled mode is polarization-sensitive and converts between the bonding mode and the antibonding mode. This conversion leads to the polarization-sensitive and continuous resonance wavelength shifts of the dipole modes. This phenomenon is different from those observed in traditional Dolmens or tilted split ring resonators whose resonant modes do not show obvious wavelength shift. In addition, geometry tuning of the structure is also carried out, which greatly affects the polarization-sensitive resonance wavelength shifts shown above. Meanwhile, experimental results demonstrate that the metamaterial structure is able to rotate the polarization of light.

For efficient operation of next-generation fossil fuel technologies, development of sensors capable of withstanding harsh environments is required. Optical waveguide based sensing platforms have become increasingly important, but a need exists for materials that exhibit useful changes in optical properties in response to changing gas atmospheres at high temperatures. In this manuscript, the onset of a near-IR absorption associated with an increase in free carrier density in doped metal oxide films to form so-called conducting metal oxides is discussed in the context of results obtained for undoped and La-doped SrTiO₃ films. Film characterization results are presented along with measured changes in optical absorption resulting from various high temperature treatments in a range of gas atmospheres. Optical property changes are also discussed in the context of a simple model for optical absorption in conducting metal oxide thin films. The combination of experimental results and theoretical modeling presented here suggests that such materials have potential for high temperature optical gas sensing applications. Simulated sensing experiments were performed at 600–800 °C, and a useful, rapid, and reproducible near-IR optical sensing response to H₂ confirms that this class of materials shows great promise for optical gas sensing.

Thiolate-protected gold clusters are promising candidates for imaging applications due to their interesting, size-dependent properties. Their high stability and the ability to functionalize the clusters with biocompatible ligands render the clusters interesting for various imaging techniques such as fluorescence microscopy or second-harmonic generation microscopy. The latter nonlinear optical effect has not yet been observed on this type of ultrasmall nanoparticle. We hereby present second- and third-harmonic generation and multiphoton fluorescence of two thiolate-protected gold clusters: Au₂₅(SCH₂CH₂Ph)₁₈ and Au₃₈(SCH₂CH₂Ph)₂₄. At a fundamental wavelength of 800 nm, the Au₃₈(SCH₂CH₂Ph)₂₄ cluster is active. In contrast, Au₂₅(SCH₂CH₂Ph)₁₈ does not yield significant SHG signal. We ascribe this to the center of inversion in the Au₂₅ cluster. Measurements on chiral Au₂₅(capt)₁₈ (capt: captopril) gave an SHG response, supporting this interpretation. We also observed third-harmonic generation at a fundamental wavelength of 1200 nm. At 800 and 1100 nm, the clusters decompose after short illumination time but are stable at illumination at 1200 nm. This may be exploited in combined deep tissue imaging and photothermal heating for theranostics applications.

Metallic nanostructures can manipulate light-matter interactions to induce absorption, scattering, and local heating through their localized surface plasmon resonances. Recently, plasmonic behavior of semiconductor nanocrystals has been investigated to stretch the boundaries of plasmonics farther into the infrared spectral range and to introduce unprecedented tunability. However, many fundamental questions remain regarding characteristics of plasmons in doped semiconductor nanocrystals. Field enhancement, especially near features with high curvature, is essential in many applications of plasmonic metal nanostructures, yet the potential for plasmonic field enhancement by semiconductor nanocrystals remains unknown. Here, we use the discrete dipole approximation (DDA) to understand the dependence of field enhancement on size, shape, and doping level of plasmonic semiconductor nanocrystals. Indium-doped cadmium oxide is considered as a prototypical material for which faceted cube-octohedral nanocrystals have been experimentally realized; their optical spectra are compared to our computational results. The computed extinction spectra are sensitive to changes in doping level, dielectric environment, and shape and size of the nanocrystals, providing insight for materials design. High-scattering efficiencies and efficient local heat production make 100 nm particles suitable for photothermal therapies and simultaneous bioimaging. Meanwhile, single particles and dimers of nanocrystals demonstrate strong, shape- and wavelength-dependent near-field enhancement, highlighting their potential for applications in infrared sensing, imaging, spectroscopy, and solar conversion.

Electron-beam-induced nonuniform shrinkage of single-walled carbon nanotube (SWCNT) and the passivation effect of catalyst metal nanoparticle on the shrinkage were reported for the first time. It was found that pristine SWCNT shrank in its axial direction preferentially from the most curved, free cap end of the tube whereas the shrinkage of the tube diameter was almost offset by the axial shrinkage. However, once being attached to Fe nanoparticle, the most curved cap end of SWCNT became intriguingly passivated. As a consequence, the axial shrinkage of the SWCNT was retarded and the shrinkage of the tube diameter was revealed. A new mechanism of atom “diffusion” combined with atom “evaporation” as athermally driven by the nanocurvature of SWCNT and electron beam activation was proposed to elucidate the observed new phenomenon, which is different from the existing knock-on mechanism.

Magnetic nanobeads are synthesized by coprecipitation of hollow iron oxide nanoparticles and an amphiphilic polymer. The resulting nanobeads can be tuned in diameter and nanoparticle content. X-ray absorption near-edge structure (XANES) spectroscopy and superconducting quantum interferometer device (SQUID) characterization of the nanobeads reveal that they exhibit an increased effective magnetic anisotropy as compared to the individual nanoparticles, despite that no structural changes of the particles had occurred during the embedding process into the polymer. The spin–spin relaxation times of the pristine hollow nanoparticles and of the final magnetic nanobeads reveal a high R₂ relaxivity of 206 s^–1 mM^–1 for the magnetic nanobeads. This result should enable their application as negative contrast enhancing agents in magnetic resonance imaging.

We investigate possible causes of molecular rectification in electrode–molecule–electrode junctions. By using a simple model and simulated conductance histograms, we show that a molecular bias drop is responsible for rectification; conversely, asymmetric molecule–electrode couplings do not directly result in rectification. Instead, the degree of coupling (a)symmetry can be observed in the line shapes of the conductance histograms used to experimentally assess the current–voltage properties of such molecular junctions. More coupling asymmetry leads to less positively skewed histogram peaks.

The structure and properties of a (CdSe)₆@(CdSe)₃₀ cluster doped with one or two Mn atoms were studied using density functional theory. Geometrical structure optimizations were performed using relativistic effective core potentials for Se and Cd atoms and an all-electron description for Mn atoms. The (CdSe)₆ core corresponds to a bulk-type structure, and each shell atom is connected to three atoms of the opposite type. We considered different Mn substitutions of a single Se and Cd atom, substitutions of two Cd atoms with two Mn atoms, and additions of one and two Mn atoms to the (CdSe)₆@(CdSe)₃₀ cluster. We found that antiferromagnetic singlet and high-spin ferromagnetic states of the Mn₂Cd₄Se₆@(CdSe)₃₀ clusters are nearly degenerate in total energy. In the Cd-substituted clusters, the binding energies of Mn atoms were found to be appreciably larger than the binding energies of Cd atoms in the initial cluster, which explains the high miscibility of Mn in bulk CdSe. Experimental observations of dramatic changes in the magnetic behavior of Mn-doped QDs after annealing can be explained from the results of our calculations of total energy as a function of total spin.

Quantum dot nanocrystals (NQDs) present within organic conducting (polymer) host environments form hybrid organic–inorganic materials that are applied in a range of technologies such as light emitting diodes or solar cells. Understanding hole-transport and exciton dynamics in these hybrid materials is central to device performance and efficiency. Integral to hole-transport is the understanding of multiexciton processes such as charged excitons as well as neighbor–neighbor NQD interactions (on the nano and micrometer length scales). Studied here are the photoluminescence dynamics of single alloyed NQDs in conducting (or insulating) polymer environments. We find that conducting polymers (through hole transport) affect the presence and dynamics of charged excitons relative to insulating environments. The presence of such charged excitons induces a change in blinking dynamics with a corresponding increase in photoluminescence correlation between neighboring NQDs found using spatiotemporal statistical analysis. Understanding such phenomena advances the understanding of photoluminescence processes central to device design.

Structural stability and electronic structure of homogeneously arranged nitrogen-doped graphene have been investigated using first-principles calculations within the density functional theory. The structures of the homogeneously doped graphene are uniquely specified by the chiral index (n,m) inherent in each model and by the doping configurations. While the formation energy increases in proportional to the nitrogen density, there are specific arrangements for which the formation energies become lower compared to the proportional trend. Such an anomalous stabilization has been found in the honeycomb-type configuration with the chiral index (n,m) which satisfies the relation n – m = 3l +2(l = 0, 1, ...). This stabilization is originated from the lowering of the one-electron energy with the band gap formation, which is attributed to the decoupling of the degenerate states.

Theoretical and experimental studies were performed on the structure, optical properties, and growth of silver nanostructures in silver phosphate (Ag₃PO₄). This material was synthesized by the coprecipitation method and processed in a microwave-assisted hydrothermal system at 150 °C for different times. The structural behavior was analyzed by means of X-ray diffraction, Rietveld refinement, and Raman spectroscopy. Field emission gun scanning electron microscopy as well as transmission electron microscopy revealed the presence of irregular spherical-like Ag₃PO₄ microparticles; metallic silver nanostructures were found on their surfaces. The growth processes of Ag nanostructures when irradiated with an electron beam were explained by theoretical calculations. First-principles calculations, within a quantum theory of atoms in molecules framework, have been carried out to provide deeper insight and understanding of the observed nucleation and early evolution of Ag nanoparticles on Ag₃PO₄ crystals, driven by an accelerated electron beam from an electronic microscope under high vacuum. The Ag nucleation and formation is a result of structural and electronic changes of the AgO₄ tetrahedral cluster as a constituent building block of Ag₃PO₄, consistent with Ag metallic formation. The optical properties were investigated by ultraviolet–visible spectroscopy and photoluminescence (PL) measurements at room temperature. PL properties of this phosphate were explained by the recombination phenomenon of electron–hole pairs via cluster-to-cluster charge transfer.

Electronic relaxation dynamics were studied for a series of gold monolayer-protected clusters (MPCs) whose sizes ranged from 1.5 to 2.4 nm. Au₉₆(mMBA)₄₂, Au₁₀₂(pMBA)₄₄, Au₁₁₅(pMBA)₄₉, Au₁₁₇(mMBA)₅₀, Au₁₄₄(pMBA)₆₀, Au₂₅₀(pMBA)₉₈, and Au₄₅₉(pMBA)₁₇₀ (pMBA = para-mercaptobenzoic acid; mMBA = meta-mercaptobenzoic acid) were selected for study because they bridged the expected transition from nonmetallic to metallic electron behavior. Excitation-pulse-energy-dependent measurements confirmed Au₁₄₄(pMBA)₆₀ (1.8 nm) as the smallest MPC to exhibit metallic behavior, with a quantifiable electron–phonon coupling constant of (1.63 ± 0.25) × 10¹⁶ W m^–3 K^–1. Smaller, nonmetallic MPCs exhibited nanocluster-specific transient extinction spectra characteristic of transitions between discrete quantum-confined electronic states. Volume-dependent electronic relaxation dynamics for ≤1.8 nm MPCs were observed and attributed to a combination of large energy differences between electronic states and phonon frequencies and spatial separation of photoexcited electrons and holes. Evidence for the latter was obtained by substituting mMBA for pMBA as a passivating ligand, which resulted in a 4-fold increase in the relaxation rate constant.

This paper reports the effect of the addition of varied concentrations of Zn²⁺ on the photophysical properties of RNA-mediated PbSe nanostructures. An increasing addition of Zn²⁺ results in diminishing of the excitonic feature in the optical spectrum associated with a decrease in red emission with a simultaneous increase in the near-infrared (NIR) region up to 10 × 10^–5 mol dm^–3. It causes the emission lifetime to decrease from 255 to 208 ns at 770 nm and increase from 10.4 to 17.6 ns at 1000 nm. The addition of Zn²⁺ changes the nature of Q-PbSe from direct to indirect band gap semiconductor by creating different surface states within its band gap, inducing additional transitions. It is understood to facilitate the phonon-assisted relaxation populating the deeper traps responsible for enhanced NIR fluorescence. The adsorption capacity of aged Zn^2+/PbSe is enhanced for Nile blue (NB) as compared to its fresh samples due to increased porosity. The excitation of PbSe with energy greater than the bandgap energy in NB-supported PbSe nanostructures results in an energy transfer from excited PbSe to NB involving multiple exciton generation per photon. The porosity, enhanced adsorption capacity with fairly high emission yield, and lifetime in the NIR region give Zn^2+/PbSe significant potential as a synthesized nanosystem for tissue and bioimaging applications.

We use grand canonical Monte Carlo and molecular dynamics simulations to systematically investigate the membrane-based separation performance of four diamond-like frameworks (PAF-1, Diamondyne, TND-1, and TND-2) for CO₂/H₂, CO₂/N₂, CO₂/CH₄ and CH₄/H₂ mixtures. Diamondyne (also named D-Carbon) shows high membrane selectivity for gas mixtures of CO₂/H₂, CO₂/N₂, CO₂/CH₄, and CH₄/H₂ compared to MOF and COF membranes. Comprehensively considering the permeation selectivity and permeability, we find that diamondyne and TND-2 are promising candidates for CO₂/H₂ and CO₂/N₂ separation. Moreover, diamondyne and TND-2 exceed the Robeson’s upper line for CO₂/N₂ mixtures. The separation performance of diamondyne for CO₂/CH₄ mixtures also exceeds the Robeson’s upper limitation, indicating that diamondyne is also a promising candidate for separation of the CO₂/CH₄ mixtures. It is expected that this work can provide guidance and reference for development and design of high selectivity membranes for gas mixtures.

Time-resolved single-molecule spectroscopy of individual colloidal CdTe/ZnS quantum dots (QDs) decorated on single-layer graphene (SLG) demonstrates significantly enhanced probability of multiphoton emission, accompanied by reduced fluorescence intensity and decreased lifetime. A rigorous analysis of time-resolved spectroscopy on individual QDs on SLG reveals a decrease in radiative decay rate and a significant enhancement (12.6 times) in nonradiative decay rate. The enhanced multiphoton emission of QDs on SLG is a consequence of the competition between the enhanced nonradiative decay and fast Auger process, as suggested by the dominant fast decay (<0.8 ns) of QDs on SLG. Our findings provide important information on the emission behavior of QDs coupled with carbon nanomaterials, such as graphene and carbon nanotubes, and will assist in the creation of effective single-photon or multiphoton sources and devices. Furthermore, this is also a possible pathway for suppressing the Auger recombination of QDs while enhancing the generation of multicarriers, which has significant implications for light harvesting.

Transient absorption spectroscopy is a powerful technique for understanding charge carrier dynamics and recombination pathways. Analyzing the results is not trivial due to nonexponential relaxation dynamics away from equilibrium, leading to a disparity in reported charge-transfer rates. An inversion analysis technique is presented that transforms transient signals back into their original rate equation. The technique is demonstrated on two CdSe/TiO₂ heterostructures with different surface states. Auger recombination is identified at higher carrier densities and radiative recombination at lower carrier densities. The heterostructure with additional surface traps exhibits increased trap-state Auger recombination at high carrier densities and changes to radiative recombination at low carrier densities due to a Shockley–Read–Hall process. Carrier-dependent electron-transfer rates are determined and compared to common methods that only capture the magnitude of the charge transfer at specific carrier densities. The presented transient absorption analysis provides direct understanding of the recombination mechanisms with minimal additional analysis or with presumption of decay mechanisms.

Functionalized nanoparticle networks offer a model system for the study of charge transport in low-dimensional systems as well as a potential platform to implement and test electronic functionalities. The electrical response of a nanoparticle network is expected to sensitively depend on the molecular interconnects, i.e., on the linker chemistry. If these linkers have complex charge transport properties, then phenomenological models addressing the large-scale properties of the network need to be complemented with microscopic calculations of the network building blocks. In this study we focus on the interplay between conformational fluctuations and electronic π-stacking in single-molecule junctions and use the obtained microscopic information on their electrical transport properties to parametrize transition rates describing charge diffusion in mesoscopic nanoparticle networks. Our results point out the strong influence of mechanical degrees of freedom on the electronic transport signatures of the studied molecules. This is then reflected in the varying charge diffusion at the network level. The modeling studies are complemented with first charge transport measurements at the single-molecule level of π-stacked molecular dimers using state-of-the-art mechanically controllable break junction techniques in a liquid environment.

A critical limitation that has hampered widespread application of the electrically conducting reduced graphene oxide (r-GO) is its poor aqueous dispersibility. Here we outline a strategy to obtain water-dispersible conducting r-GO sheets, free of any stabilizing agents, by exploiting the fact that the kinetics of the photoreduction of the insulating GO is heterogeneous. We show that by controlling UV exposure times and pH, we can obtain r-GO sheets with the conducting sp²-graphitic domains restored but with the more acidic carboxylic groups, responsible for aqueous dispersibility, intact. The resultant photoreduced r-GO sheets are both conducting and water-dispersible.

This work is concerned with the kinetics of laser-induced reductive sintering of nonstoichiometric crystalline nickel oxide (NiO) nanoparticles (NPs) under ambient conditions. The mechanism of photophysical reductive sintering upon irradiation using a 514.5 nm continuous-wave (CW) laser on NiO NP thin films has been studied through modulating the laser power density and illumination time. Protons produced due to high-temperature decomposition of the solvent present in the NiO NP ink, oxygen vacancies in the NiO NPs, and electronic excitation in the NiO NPs by laser irradiation all affect the early stage of the reductive sintering process. Once NiO NPs are reduced by laser irradiation to Ni, they begin to coalesce, forming a conducting material. In situ optical and electrical measurements during the reductive sintering process take advantage of the distinct differences between the oxide and the metallic phases to monitor the transient evolution of the process. We observe four regimes: oxidation, reduction, sintering, and reoxidation. A characteristic time scale is assigned to each regime.

Single-molecule Raman spectra were studied at high pressure (1–4 GPa) in a diamond-anvil cell (DAC) with an Ar hydrostatic pressure medium, with the intent of resolving the different pressure-induced vibrational blueshifts of individual molecules. The molecules were two isotopologues of the dye rhodamine 6G (R6G and d₄-R6G), adsorbed on colloidal Ag particles immobilized in poly(vinyl alcohol) (PVA). Surface-enhanced Raman (SERS) ensemble measurements were compared to single-molecule surface-enhanced Raman (SMSERS) measurements made in a confocal Raman microscope. Spectra of mixed isotopologues in the 610 cm^–1 region (the “isotope-sensitive” transition) allowed us to identify when the majority of spectra came from single-isotope sites, and were thereby statistically likely to arise from single molecules. There was a dramatic drop in SERS intensity when samples were pressurized in the DAC. SMSERS measurements revealed the intensity drop was caused by a pressure-induced destruction of SMSERS-active hot spots. A hot spot is a site with ultrahigh Raman enhancement containing at least one R6G molecule. The hot spots that were not destroyed had large enhancement factors. The disappearance of hot spots was attributed to deoptimization of the gap junctions between Ag nanoparticles due to pressure-induced strain. Because the isotope-sensitive transition had little pressure-induced blueshift (<5 cm^–1 between 0 and 6 GPa), we also studied a transition near 1650 cm^–1 (the “pressure-sensitive” transition), which had a >30 cm^–1 blueshift and an approximate doubling of line width in the 0–6 GPa pressure range. The single-molecule spectra of this transition did not broaden as pressure was increased to 4.1 GPa. However, there was a variation in the blueshift of different molecules. The fwhm of the blueshift variation was able to account for most or all of the observed pressure-induced spectral broadening. The pressure-induced broadening of this R6G vibrational transition is due to the different blueshifts of different molecules.

Desorption/ionization on silicon (DIOS) is widely used in modern mass spectrometry for obtaining ions of various organic substances. The high efficiency of DIOS suggests that it may be a promising method in ion-mobility spectrometry (IMS) using gas-phase ion separation. The influence of laser wavelength and intensity on DIOS of trinitrotoluene (TNT) molecules under ambient conditions has been studied. If laser with a wavelength of 266 or 355 nm is used, TNT molecules predominantly form (TNT – H)⁻ negative ions. Their formation has been found to result from laser-induced proton transfer from TNT molecules to the porous silicon (pSi) surface, rather than gas-phase ion–molecule reactions. The dependence of the yield of (TNT – H)⁻ ions on the laser intensity has been analyzed. The ion yield curve has been demonstrated to fit the Arrhenius function at laser intensity lower than ∼2.5×10⁷ W/cm². Experiments have shown that the desorption/ionization of TNT molecules is not a purely thermal process. The results demonstrate that DIOS can be widely used in the IMS technology.

The stability of nanoparticles is strongly dependent on the thermodynamics of interfaces. Providing reliable data on surface and grain boundary energies is therefore of key importance for predicting and improving nanostability. In this work, we used a combination of high-temperature oxide melt drop solution calorimetry and water adsorption microcalorimetry to demonstrate the effect of a dopant (manganese) on both surface and grain boundary energies of SnO₂, and discussed the impacts on the average particle size at a given temperature. The results show a significant decrease in the grain boundary energy with increasing manganese content and a concomitant moderate decrease in the surface energy, consistently with segregation enthalpy values acquired from an analytical fitting model. The results explain the measured increase in stability with increasing dopant content (smaller sizes) and suggest the grain boundary energy has a much more important role in defining particle stability than previously supposed.

Experimental work has already demonstrated that Al-doped ZnO nanostructures exhibit a higher response than the pure ZnO sample to CO gas and can detect it at sub-ppm concentrations. In this work, using density functional theory calculations (at B3LYP, M06-L, and B97D levels), we studied the effect of Al-doping on the sensing properties of a ZnO nanocluster. We investigated several doping and adsorption possibilities. This study explains the electrical behavior that has been obtained from the ZnO nanostructures upon the CO adsorption. There is a relationship between the HOMO–LUMO energy gap (E_g) and the resistivity of the ZnO nanostructure. If a Zn atom of the ZnO nanocluster is replaced by an Al atom, a CO molecule can be adsorbed from its C-head on the doped site with ΔG of −5.0 kcal/mol at room temperature. In contrast to the pristine cluster, Al-doped ZnO cluster can detect CO molecules due to a significant decrease in the E_g and thereby in the resistivity. We also found that the E_g decreases by increasing the number of Al atom up to 4, and then it starts to increase by increasing the Al atoms with its trend analogous to the resistivity change in the experimental work.

We have studied the electronic and magnetic structures of MoS₂ nanotubes by using a first-principles method. Various kinds of defects such as substitution and vacancy are examined for triggering spin magnetic moments toward one-dimensional diluted magnetic semiconductors. Our results suggest that the presence of impurity states within the energy gap and its large contribution to the density of states at the Fermi level are the key factors in inducing a magnetic moment. In particular, the nanotube curvature turns out to affect the energy level of impurity states, which can be exploited for tailoring magnetic properties. Also, 3d transition metal impurities (V, Mn, Fe, and Co atoms) on a Mo site can create large magnetic moments.

Conjugated polymers with cyclic structures are interesting because their symmetry leads to unique electronic properties. Recent advances in Vernier templating now allow large shape-persistent fully conjugated porphyrin nanorings to be synthesized, exhibiting unique electronic properties. We examine the impact of different conformations on exciton delocalization and emission depolarization in a range of different porphyrin nanoring topologies with comparable spatial extent. Low photoluminescence anisotropy values are found to occur within the first few hundred femtoseconds after pulsed excitation, suggesting ultrafast delocalization of excitons across the nanoring structures. Molecular dynamics simulations show that further polarization memory loss is caused by out-of-plane distortions associated with twisting and bending of the templated nanoring topologies.

Understanding dynamical characteristics of excited electronic states is crucial for rational design of functional nanomaterials. Using real-time time-dependent density functional theory, we present a fully quantum mechanical study on the transfer and decay of an exciton in an archetypal metal nanostructure. We introduce several approaches to analyze the dipole moment’s time evolution to resolve exciton transfer rates and the pure dephasing times. These approaches are applied to studies of exciton diffusion length in a silver nanowire array. Calculated rates of polarization-induced transfer exhibit neither Förster’s “sixth-power” dependence on donor–acceptor distance nor the perfect exponential separation dependence that typifies the Dexter transfer mechanism, suggesting that the nonperturbative, ab initio quantum dynamics captures intricacies of exciton transfer between quantized nanosystems that are beyond the reach of the canonical models of electronic energy transfer.

Phase change materials are the active compounds in optical disks and in nonvolatile phase change memory devices. These applications rest on the fast and reversible switching between the amorphous and the crystalline phases, which takes place in the nano domain in both the time and the length scales. The fast crystallization is a key feature for the applications of phase change materials. In this work, we have investigated by means of large scale molecular dynamics simulations the crystal growth of the prototypical phase change compound GeTe at the interface between the crystalline and the supercooled liquid reached in the device upon heating the amorphous phase. A neural network interatomic potential, markedly faster with respect to first-principles methods, allowed us to consider high-symmetry crystalline surfaces as well as polycrystalline models that are very close to the actual geometry of the memory devices. We have found that the crystal growth from the interface is dominant at high temperatures while it is competing with homogeneous crystallization in the melt at lower temperatures. The crystal growth velocity markedly depends on the crystallographic plane exposed at the interface, the (100) surface being kinetically dominant with respect to the (111) surface. Polycrystalline interfaces, representative of realistic conditions in phase change memory devices, grow at significantly slower pace because of the presence of grain boundaries.

Iron oxide nanocrystals, synthesized by surfactant-assisted thermal decomposition of Fe(CO)₅, were selectively incorporated into the microseparated PS block phase of polystyrene-block-poly(methyl methacrylate) (PS-b-PMMA) diblock copolymer to design novel nanostructured inorganic/organic materials with magnetic properties. The colloidal synthesis led to oleic acid- and oleylamine- capped magnetic γ-Fe₂O₃ maghemite nanocrystals, which resulted well-dispersible in the block copolymer up to 60 wt % due to the affinity between PS block and organic surfactants. Atomic force and scanning electron microscopy confirmed that nanocomposites with nanocrystal loading lower than 40 wt % maintained cylindrical morphology, even without additional treatment applied to enhance their nanostructuration. Nanocrystals appeared well-dispersed in the nanocomposites at low contents, while nanocrystal clusters, with size depending on their loading, were observed at higher contents. At the highest nanocrystal content, the nanostructured area size decreased and nanocrystals covered the entire surface. Magnetic force microscopy measurements confirmed the magnetic behavior of γ-Fe₂O₃ nanocrystals confined in PS block of PS-b-PMMA block copolymer after applying UV treatment to the samples. The results demonstrated that the incorporation of as-synthesized colloidal iron oxide nanocrystals (up to 40%) in self-assembled PS-b-PMMA block copolymer offers a simple and direct approach to successfully design and fabricate novel nanostructured magnetic composites.

Nanocoiled assemblies of organic π-conjugated molecules have attracted intense attention because of their various practical applications. Herein, the assembly of highly fluorescent monolayer and bilayer nanocoils from asymmetric perylene diimide (PDI) molecules is reported. Through systematic investigation of 21 asymmetric PDI derivatives, some critical molecular structural parameters for the formation of nanocoils, involving the position of methoxy substituents at the phenyl moiety on one side and the appropriate linker that attaches the phenyl moiety to the PDI core, are formulated. The J-aggregate nature of the helical π-stacking geometry within the nanocoil is demonstrated by optical characterization. All of the nanocoils are highly emissive, with a fluorescence quantum yield greater than 25%. Furthermore, all of the nanocoils exhibited a NIR emission with a band maximum greater than 710 nm. This new class of highly NIR fluorescent nanostructures offers promising applications in areas such as optoelectronics, fluorescent sensors, and biological imaging.