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Odd–Even Effect in Electron Beam Irradiation of Hybrid Aromatic–Aliphatic Self-Assembled Monolayers of Fatty Acid
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C: Spectroscopy and Dynamics of Nano, Hybrid, and Low-Dimensional Materials

Odd–Even Effect in Electron Beam Irradiation of Hybrid Aromatic–Aliphatic Self-Assembled Monolayers of Fatty Acid
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The Journal of Physical Chemistry C

Cite this: J. Phys. Chem. C 2021, 125, 17, 9310–9318
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https://doi.org/10.1021/acs.jpcc.1c01857
Published April 21, 2021

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Abstract

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Electron irradiation of aromatic self-assembled monolayers (SAMs) in combination with different lithographic approaches provides an interesting alternative for high-resolution surface patterning. More recently, it has been also demonstrated that this process can be used for carbon nanomembrane (CNM) fabrication, which forms technologically attractive 2D materials, with potential applications in different areas of nanotechnology such as ultrafiltration and nanobiosensing. To better understand the relation between the original SAM structure and the resulting CNM formation, in the current study, we conduct systematic analysis of the electron irradiation process for a model SAM system deposited on Ag substrates and based on a homologue series of biphenyl substituted carboxylic acids [C6H5–C6H4–(CH2)n–COO/Ag, n = 2–6] with different lengths of the aliphatic linker defined by the number n. Our results of X-ray photoelectron spectroscopy of irradiated monolayers show that the process of electron-induced desorption, cross-linking, and elimination of the SAM binding group depend on the parity of the parameter n (the odd–even effect). Our observations indicate a way for controlling thickness and purity of such nanomembranes, which are the key parameters determining the range of CNM applications.

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1. Introduction

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The increased miniaturization of integrated devices used in organic and molecular electronics as well as in biology and medicine demands novel methods for surface and thin-film control at the microscale and nanoscale. Self-assembled monolayers (SAMs) (1,2) provide a simple way for functionalization of solid substrates by well-defined organic monolayers which become particularly attractive in combination with a variety of patterning methods (3) applied for building organic electronic devices (4) and controlling biocompatible interfaces. (5) In particular, electron-beam irradiation provides an effective means for SAM modification from the micrometer scale down to the nanometer scale. Depending on the chemical and structural composition of the monolayer, the interaction of electron beams with SAMs can be used to initiate different processes that include partial desorption, (6−8) cross-linking of aromatic molecules, (9−12) and surface modification of their chemical functionality. (13) As demonstrated in recent years, these fundamental processes in turn can be used for different types of lithography of organic (13−17) and inorganic (18,19) materials, the formation of chemical gradients on the surface, (20) metal substrate work function modification, (21) and transforming SAMs into carbon-based materials such as graphene (22,23) or carbon nanomembranes (CNMs). (24−27)
To date electron-beam irradiation experiments on SAMs were done almost exclusively using different types of thiols chemisorbed on the gold substrate. One of the exceptions are very recent experiments for hybrid aromatic–aliphatic (28) and purely aromatic (29,30) SAMs based on using carboxylic acid as an anchoring group to the silver substrate. It has been demonstrated that these SAMs can be successfully used for CNM formation of similar quality as in the case of analogous SAMs based on thiols. (28,29) Importantly, experiments performed for hybrid aromatic–aliphatic SAMs based on the biphenyl unit indicate that in contrast to SAMs based on thiols, the combination of desorption and the cross-linking process leads to elimination of the anchoring group and thus to the formation of free-standing CNMs without the chemically active group which is important for applications such as ultrafiltration. (28) This effect has not been reported for the purely aromatic carboxylic SAMs based on biphenyl (30) or terphenyl, (29) which indicates the crucial role of the aliphatic linker in the process of head group elimination. To elucidate the role of the aliphatic spacer on electron-induced modification of hybrid aromatic–aliphatic SAMs with the carboxylic head group, we analyze in the current study a series of C6H5–C6H4–(CH2)n–COO/Ag (denoted as BPnCOO/Ag, n = 2–6) SAMs. The structure formed by this homologue series was investigated in detail in former studies, which revealed that depending on the odd or even value of the parameter n, the BPnCOO/Ag monolayers can form either a crystalline (n = even) or 2D disordered, liquid-like (n = odd) structure. (31) It was shown that the crystalline form of even members of the BPnCOO/Ag series is associated with the more upright orientation of the biphenyl unit and the anchoring carboxylic group, both of which result in a higher packing density of molecules on the Ag(111) surface and a higher thickness of the monolayer. (31) Consequently, for the liquid-like structure of odd members, the lower packing density and film thickness are the results of the more canted orientation of both the biphenyl unit and the anchoring group. (31) The observed structural odd–even effect was explained by the simple, qualitative model in which for even members of the series, the three main contributions to the overall energetics of the system such as packing density (density of the COO–Ag bonds), the Ag–O–C bending potential, and the intermolecular interactions act in a cooperative way, that is, they are satisfied by the same configuration of molecules and thus lead to a well-defined and stable monolayer. In contrast, for odd members of the series, the more canted orientation of the biphenyl group leads to the reduction in packing density and intermolecular interactions, both of which can be improved only at the price of changing the Ag–O–C bending potential from its optimal configuration. As a result, the competitive relationship between these three factors results in the 2D disordered liquid-like structure and lower stability of odd-numbered BPnCOO/Ag SAMs. Thus, this strong structural odd–even effect gives us a unique opportunity to test if the modification of the adsorption geometry (the molecular backbone and adsorption group), packing density, and the type of 2D ordering (crystalline vs liquid-like structure) of SAMs can influence their modification by the electron beam and, as a result of this process, the formation of the CNMs. We note at this point that our analysis has much broader context since the odd–even structural effects in SAMs (32) have also been reported for other hybrid aliphatic–aromatic monolayers based on thiols (33−41) and selenols (42−44) on Au or Ag substrates with some of studies devoted to their impact on the electron (45) or ion (46) beam-induced modification of SAMs.

2. Experimental Methods

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2.1. SAM Preparation

The C6H5–C6H4–(CH2)n–COO (BPnCOOH) compounds with n = 2 and n = 3–4 were purchased from Alfa Aesar and Wako, respectively. The synthesis procedure of BPnCOOH compounds with n = 5–6 is described elsewhere. (47) The Ag(111) substrates were prepared by evaporation of ∼100 nm (rate 0.1 nm/s) of silver on mica sheets at 533 K. All SAMs were prepared by immersion (5 min) of Ag(111) substrates in the solution (1 mM) of the respective compound in absolute ethanol (99.8%, POCH-Poland). After incubation, the samples were rinsed with pure ethanol and dried under a nitrogen stream.

2.2. Electron Irradiation

The electron irradiation was performed in the same ultra-high-vacuum (UHV) system as used for the X-ray photoelectron spectroscopy (XPS) analysis with a 50 eV electron beam generated using an NEK-SC 150 (Staib) electron gun.

2.3. X-ray Photoelectron Spectroscopy

XPS was performed in situ using a Multiprobe system (Scienta Omicron) in UHV (base pressure 2 × 10–10 mbar) equipped with a monochromatic X-ray source (Al Kα) and an electron analyzer (Argus CU) with a 0.6 eV spectral energy resolution. The binding energy (BE) scale was calibrated using the Ag 3d5/2 peak as a reference (BE = 368.2 eV). (48) All spectra were fitted by Voigt functions (30:70) after background subtraction using the linear (O 1s) or Shirley (C 1s, Ag 3d) function.

3. Results and Discussion

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The process of electron irradiation was monitored in situ using XPS with respective data and analysis summarized in Figures 1–5. To indicate the main features of the XPS spectra, the data obtained for the BP3COO/Ag sample are presented in Figure 1 as a representative example for the entire homologue series of BPnCOO/Ag (n = 2–6) SAMs. The C 1s spectrum for native BP3COO/Ag (Figure 1a, black) shows the main peak located at a BE of ∼283.7 eV, which is due to the biphenyl moiety and the short aliphatic spacer. It also reveals a smaller component at BE ∼ 287.1 eV due to the COO– group involved in the bidentate Ag–carboxylate bond. (28,31,49) This assignment is more directly confirmed by the O 1s spectrum shown in Figure 1b, which can be described by a single peak located at BE ∼ 530.4 eV. (28,31,49) The general effect of low-energy electron irradiation of BP3COO/Ag is exemplified in Figure 1a–c. As shown in Figure 1a for the main C 1s component, associated with the biphenyl and aliphatic part of the backbone, the increased dose of electron irradiation leads to gradual reduction in the intensity together with the pronounced shift toward higher values of BE. In contrast, the high-energy C 1s component, associated with the carboxylic group, vanishes for high electron doses. This behavior is fully correlated with the O 1s spectra which for high electron doses exhibit almost complete signal loss (Figure 1b), confirming electron-induced removal of the carboxylic bonding group. The drop of the C 1s and O 1s signals is also consistent with the electron irradiation-induced increase of the Ag 3d5/2 signal intensity as a result of the monolayer thickness reduction (Figure 1c). We note that apart from the increase in the Ag 3d5/2 signal intensity, the electron irradiation of BPnCOO/Ag also resulted in certain reduction of the peak full width at half-maximum (fwhm) by ∼50 meV and the shift of the BE toward higher values by ∼40 meV. Considering former synchrotron-based high-resolution XPS studies comparing the Ag 3d5/2 signal for the bar Ag(111) substrate and after adsorption of SAMs, (50) these observations are consistent with the modification of the surface component of the Ag 3d5/2 signal due to the electron-induced elimination of the chemical bonding of BPnCOO molecules with the Ag substrate and partial desorption of the monolayer.

Figure 1

Figure 1. C 1s, O 1s, and Ag 3d XPS spectra for native and electron-beam-irradiated (a–c) BP3COO/Ag samples using an electron energy of 50 eV and a total electron dose of 150 mC/cm2. The black and red lines mark signals for native and irradiated samples, respectively. For better presentation, intensities of the O 1s and Ag 3d5/2 spectra are multiplied by the factor presented in the figure.

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To discuss the electron-beam-induced modification of the BPnCOO/Ag SAMs in more detail, we analyze first the intensity of the C 1s signal. As shown in Figure 1, the electron irradiation of BPnCOO/Ag SAMs leads to a significant drop of the carbon signal. This drop is associated with the partial desorption of the monolayer and reduction of the effective film thickness (d) which can be calculated using a standard procedure based on the C 1s/Ag 3d intensity ratios, (8,51) assuming exponential attenuation of the photoelectron signal with attenuation lengths (λAg3d5/2 = 26.8 Å and λC1s = 28.08 Å) calculated on the basis of previously reported analysis (52) and taking as a reference the film thickness of HDT/Ag (HDT—hexadecane thiol) SAMs measured in the same experiment. The effect of film thickness reduction as a function of the electron irradiation dose is presented in Figure 2a using BP3COO/Ag as a representative example (see Figure S1 in the Supporting Information file for the complete set of data). To calculate the characteristic thickness cross-section (σTH) for this process, we have used the standard saturation function (7,53) defined as
(1)
where d0 and dSAT are the thicknesses for the native and extensively irradiated monolayers, respectively, and Q/S [C/m2] is the primary electron irradiation dose. The values of the d0 and dSAT parameters obtained for the BPnCOO/Ag series are presented in Figure 2b. In agreement with former structural analysis, (31) the increase of the thickness d0 of the native monolayer with the parameter n is overlaid with an additional odd–even modulation as a consequence of the odd–even variation in the tilt of both the biphenyl moiety and the carboxylate bonding group toward the silver substrate. The extensive electron irradiation process leads to reduction of the initial monolayer thickness d0 to the value defined by dSAT. As evident from the data presented in Figure 2b, the parameter dSAT exhibits similar dependence to d0, that is, a general growth with the parameter n with an additional odd–even variation, which has the same phase as for the native monolayer. The calculated values of the respective cross-sections σTH are presented in Figure 2c and show pronounced odd–even dependence with significantly higher values obtained for odd-numbered members of the BPnCOO/Ag series. As shown in Figure 2d, the odd–even variation of σTH is also correlated with the odd–even variation of the percentage of the total thickness reduction defined as (d0 – dSAT)/d0 × 100%. A higher efficiency of the thickness reduction process (higher σTH) observed for odd-numbered monolayers indicates a more effective electron-induced desorption process which has to be correlated with their structural difference compared to even-numbered monolayers. As pointed above, former spectroscopic and microscopic analysis (31) revealed that odd-numbered BPnCOO/Ag SAMs exhibit a lower packing density and a disordered, liquid-like structure, which are associated with a more canted orientation of biphenyl and anchoring groups. These structural features of odd-numbered monolayers indicate lower intermolecular interactions compared to highly ordered, and densely packed, even-numbered monolayers, which can increase the efficiency of the desorption process of molecules, and their fragments, after electron-induced cleavage of a given chemical bond. The higher percentage of the total thickness reduction for odd-numbered monolayers indicates that the cross-linking process, which hampers further electron-induced desorption of the aromatic monolayer, is less effective in this case, and thus, the desorption process is continued longer during the initial steps of electron irradiation. Since the cross-linking process is mainly related to the electron-induced C–H bond scission and the subsequent formation of new C–C bonds between biphenyl units of neighboring molecules in the monolayer, the lower packing density and the disordered structure of these units for odd-numbered BPnCOO/Ag may explain the lower efficiency of this process in this case. The range of σTH values for thickness reduction reported here (σTH ∼ (0.5 – 2.0) × 10–16 cm2) for BPnCOO/Ag is lower compared to recently published analogous analysis (8) obtained for 50 eV electron irradiation of SAMs based on alkanethiols (σTH ∼ (3 – 4) × 10–16 cm2). Such a difference confirms the much higher efficiency of the cross-linking process in the case of BPnCOO/Ag systems which effectively quenches desorption and thus thickness reduction upon electron irradiation. The same mechanism was also reported earlier for purely aromatic SAMs based on biphenyl thiols and selenols where even lower values of cross-section for thickness reduction were reported (∼2 × 10–17 cm2). (54) In fact, we expect that the efficiency of the cross-linking process for BPnCOO/Ag compared to that for alkanethiols must be even higher considering very efficient elimination of the anchoring group in the case of BPnCOO/Ag (vide infra) with at the same time very little efficiency of this process for SAMs based on thiols on the Ag (25) or Au (8,55) substrate.

Figure 2

Figure 2. Electron-induced modification (electron energy 50 eV) of film thickness for BPnCOO/Ag (n = 2–6). (a) Film thickness as a function of the electron irradiation dose for the BP3COO/Ag sample. (b) Film thickness before and after (saturation level) electron irradiation. (c) Cross-section for this process. (d) Thickness reduction as a function of the parameter n.

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As shown in Figure 1a, together with the intensity, the position, that is, the BE of the main component of C 1s, is modified upon electron irradiation. A detailed analysis of this effect is shown in Figure 3a for BP3COO/Ag as a function of the electron dose (see Figure S2 in the Supporting Information file for the complete set of data). Obtained data exhibit gradual transition of BE from the value characteristic for the native monolayer toward a much higher value of BE which saturates at high irradiation doses and can be described by the analogous saturation function as defined by eq 1. While such behavior is common for all members of the BPnCOO/Ag (n = 2–6) series, the data presented in Figure 3b show that the value of the BE for the native, and to some extent also for fully electron-beam-modified samples (i.e., at the dose of 150 mC/cm2), depends on the parameter n. In particular, for the native samples, the respective BE value gradually grows with the increased value of n from ∼283.5 eV for n = 2 up to ∼284.0 eV for n = 6. To explain this effect as a first point, we note that previous analysis comparing full analogue BP2S/Ag and BP2COO/Ag monolayers (28,49) revealed for the former system significantly higher values of the BE for the C 1s main component in the range of ∼284.2–284.5 eV. A similar range of BEs was also recently reported for a variety of aromatic SAMs based on thiols on Au with the oligophenyl, acene, or oligo(phenylene ethynylene) backbone. (55) This shift in the BE for the C 1s main component toward a lower energy (by ∼0.8–0.5 eV) associated with the modification of the bonding group from thiolate to carboxylate can be attributed to the charge rearrangement at the molecule–metal interface (56) which for the COO– based SAMs leads to the formation of an interfacial dipole layer oriented toward the metal surface and most probably originating from the ionic O–Ag bond formation. The normal (perpendicular to the metal surface) component of such a dipole layer lowers the BE of the C 1s level for all carbon atoms located above the dipole layer. Since this effect should decrease with the increased vertical distance of a given carbon atom from the dipole layer, we expect that the resulting shift in the C 1s BE, which is measured as an averaged value over all carbon atoms belonging to the biphenyl moiety and the aliphatic spacer, should be reduced for longer molecules, as indeed observed for the BPnCOO/Ag series in Figure 3b. Moreover, considering that the odd–even modification of the parameter n in BPnCOO/Ag induces reorientation of the biphenyl moiety which is more tilted toward the Ag surface for odd-numbered monolayers, (31) one could expect a lower BE of the C 1s spectra for odd-numbered members of the series (due to a smaller average distance from the surface dipole layer). This additional effect, although much weaker, is overlaid on the length dependence visible in Figure 3b. We note at this point that the odd–even variation of the C 1s BE was also reported (33) earlier for BPnS/Ag SAMs (n = 3–6) with higher BE values for even-numbered members of the series, similar to the current analysis for BPnCOO/Ag series. Considering the same phase of the structural odd–even effect in both systems (31,33,34) as well as the same orientation of the normal component of the surface dipole moment (toward the metal surface) for S–Ag (57,58) and O–Ag (28,31,49) bonds associated with the charge transfer to the anchoring group, this effect is consistent with currently proposed explanation.

Figure 3

Figure 3. Electron-induced BE modification (electron energy 50 eV) of the main C 1s XPS peak for BPnCOO/Ag (n = 2–6). (a) BE as a function of the electron irradiation dose (energy 50 eV) for the BP3COO/Ag sample. (b) BE before and after electron irradiation (energy 50 eV, dose 150 mC/cm2) as a function of the parameter n, (c) cross-section for this process as a function of the parameter n, and (d) change in BE due to electron irradiation as a function of the number n.

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The BE of the main C 1s signal for electron-irradiated BPnCOO/Ag SAMs reached a saturation value of ∼284.4 eV, which is also close to the values reported earlier for other electron-irradiated aromatic SAMs based on thiols (28,45,55) (see Figure 3b). In former studies analyzing electron irradiation of aromatic SAMs, (55) the change in BE toward a higher energy was attributed to chemical modification of the film structure via electron-induced scission of the C–H bonds and the subsequent cross-linking process. However, the electron irradiation-induced change of the BE reported here (Figure 3d) for BPnCOO/Ag SAMs is ∼5–8 times bigger as compared to those reported earlier for thiol-based aromatic SAMs on the Au surface despite the same mechanism of the cross-linking process. (55) Therefore, current results indicate that at least for BPnCOO/Ag SAMs, the pronounced modification of the C 1s BE can be primary attributed to the electron irradiation-induced destruction of the molecule–substrate dipole layer via termination of the Ag–O bonding formed in the case of BPnCOO/Ag SAMs as confirmed by the shift in BE for native monolayers with carboxylate and thiolate anchoring groups and much more effective elimination of the bonding group for these SAMs during electron irradiation (vide infra). Following such interpretation, a weak odd–even variation of the BE saturation level visible in Figure 3b, with slightly higher values for even-numbered members of the series, indicates that the level of the molecule–substrate dipole layer elimination and/or the film thickness of the electron-irradiated monolayer is higher in this case. Since the level of oxygen concentration upon complete electron irradiation (vide infra) shows an appositive odd–even relation (Figure 4b), we attribute this effect to the film thickness measured upon electron irradiation which shows the same phase of the odd–even relation (Figure 2b).

Figure 4

Figure 4. Electron-induced (electron energy 50 eV) fwhm modification of the main C 1s XPS peak for BPnCOO/Ag (n = 2–6). (a) C 1s fwhm as a function of the electron irradiation dose for BP3COO/Ag (electron energy 50 eV). (b) C 1s fwhm before and after (saturation level) electron irradiation as a function of the parameter n, (c) cross-section for this process as a function of the parameter n, and (d) change in fwhm due to electron irradiation as a function of the parameter n.

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The values of the cross-section related to BE modification (σBE) are presented in Figure 3c and show odd–even variation with higher values calculated for odd-numbered members of the series. The phase of this odd–even effect is the same as that for σTH, thus indicating correlation of the O–Ag bond termination and reduction in film thickness.
The final point of the C 1s signal analysis was the fwhm. As shown in Figure 4a for BP3COO/Ag, the electron irradiation leads to an increase of this parameter which follows the saturation function (see Figure S3 in the Supporting Information file for the complete set of data). The data presented in Figure 4b show that this effect is common for all members of the BPnCOO/Ag series analyzed here. A similar increase in fwhm was also reported in former analysis of electron irradiation of both aliphatic (8) and aromatic (55,59) SAMs. The analysis conducted in the current study additionally reveals the odd–even effect related to the fwhm parameter with higher values for even-numbered members of the BPnCOO/Ag series observed for native and fully electron-irradiated samples (Figure 4b). The respective cross-section (σFWHM) calculated for this process also reveals odd–even behavior with exception of data obtained for the BP6COO/Ag sample which does not fit to such a trend (Figure 4c).
A similar level of increase in the fwhm of C 1s upon electron irradiation (55,59) as well as a similar value of the respective cross-sections (σFWHM) calculated in the current study and reported in former analysis for aromatic SAMs (55) appears quite intuitive and related to a common cross-linking process which by the formation of new C–C/C═C bonds and elimination of the C–H bonds increases chemical inhomogeneity of the monolayer and thus broadening of the C 1s peak. However, the same phase and similar amplitude of the odd–even effect in fwhm observed for the native and irradiated BPnCOO/Ag series are more difficult to explain. Considering, however, a former study (55,59) comparing few different types of aromatic SAMs with levels of differences in fwhm values measured for native and irradiated samples similar to those reported here (40,47) as well as those in the calculated cross-sections, (55) we suppose that observed odd–even effects in the fwhm parameter are related to odd–even structural differences in packing density and orientation of molecules which are, apparently, partially preserved upon irradiation.
In the final step of our analysis, we have monitored the relative change in oxygen concentration in the monolayer as a function of the electron irradiation dose. To estimate this parameter, we did not directly use the intensity of the measured O 1s signal (ImO1s) but the corrected intensity (IcO1s), which is proportional to the oxygen concentration considering exponential decay of the O 1s signal during transmission through the monolayer of a given film thickness in the form
(2)
where λO1s = 24.25 Å is the attenuation length for electrons emitted from the O 1s level calculated according to the data in ref (52) and d is the film thickness for the given irradiation dose as presented in Figure 2b. Based on this calculation, the normalized value of the oxygen concentration as a function of the irradiation dose is presented in Figure 5a for BP3COO/Ag as a representative example (see Figure S4 in the Supporting Information file for the complete set of data). Similar to the film thickness, the drop of oxygen concentration, monitored by the IcO1s signal, exhibits behavior which can be described by the saturation function defined by eq 1. The values of this signal obtained for the native film and at the saturation level of the irradiation process are presented in Figure 5b for the entire BPnCOO/Ag series. The native concentration of oxygen (measured in arbitrary units) reveals a pronounced odd–even effect with a higher concentration for even-numbered members of the series in agreement with former analysis (31) indicating a higher packing density for these types of monolayers. This native odd–even relation is also partially preserved at the saturation point of the electron irradiation. The cross-section for oxygen elimination (σO) also exhibits a well-defined odd–even effect with higher values for odd-numbered members of the series (Figure 5c). This odd–even effect has the same phase as the odd–even effects observed in the cross-sections for BE (σBE) and film thickness (σTH) modification. The correlation of the odd–even effect in the efficiency of BE modification (σBE) with the odd–even effect in the efficiency of oxygen elimination (σBE) confirms the above proposed model in which the shift in BE of the C 1s level is associated with the electron-induced destruction of the dipole layer formed at the molecule–metal interface by the polarized O–Ag bonds.

Figure 5

Figure 5. Electron-induced modification (electron energy 50 eV) of the oxygen concentration for BPnCOO/Ag (n = 2–6) based on XPS data. (a) Normalized oxygen concentration as a function of the electron irradiation dose for BP3COO/Ag. (b) Oxygen concentration before and after (saturation level) electron irradiation as a function of the parameter n, (c) cross-section for this process as a function of the parameter n, and (d) oxygen reduction as a function of the number n.

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The correlation of odd–even effects for σTH and σO is more complicate to analyze. First, we note that while the elimination of oxygen during the electron irradiation process analyzed here is almost complete (Figure 5d), the reduction of the corresponding film thickness is on the level of only ∼30% (Figure 2d). This indicates that despite the fact that oxygen is responsible for chemical bonding of individual molecules with the metal substrate, its elimination is still possible without complete desorption of the monolayer. This is a result of the electron-induced cross-linking process which stabilizes the monolayer during oxygen elimination. The elimination of oxygen has two separate stages. The first stage is termination of the respective chemical bonds, that is, Ag–O and O–C, and the second is the desorption process of oxygen or oxygen-containing molecular fragments. Both stages have different controlling factors. The process of bond termination via electron irradiation is controlled by the efficiency of the electron-induced excitation to the dissociative state and by the possible relaxation process from this state which prevents bond termination. The electron-induced excitation will depend on the flux of electrons at the point of excitation. We note that to reach the carboxylic bonding group located at the metal interface, the electron beam has to pass through the monolayer. Considering an attenuation length of ∼6.5 Å for 50 eV electrons during their transmission through SAMs, (52) this flux can be increased by ∼36% by reducing the SAM thickness by just 2 Å. Therefore, the reduced thickness for the odd-numbered BPnCOO/Ag SAMs can amplify the efficiency of Ag–O and O–C bond termination, thus correlating the odd–even effects in cross-sections of film thickness (σTH) and oxygen elimination (σO). Moreover, when excitation of a given bond is located in the molecule chemisorbed to the metal surface, as is the case for the metal bonding carboxylic group in BPnCOO/Ag SAMs, the efficiency of bond termination is strongly controlled by the non-radiative decay process which involves charge transfer between the excited state and the metal substrate. (60) Thus, the efficiency of the electron-induced bond termination in SAMs for chemical bonds located close to the metal surface critically depends on the local electronic structure at the molecule–metal interface. (53,60) Considering that the odd–even structural effect in BPnCOO/Ag SAMs leads to an odd–even modification of the carboxylate bonding group conformation, as is evident from former infrared reflection absorption spectroscopy analysis, (31) we can suppose that this will result in an odd–even efficiency of the electron-induced C–O or Ag–O bond termination. The magnitude and the phase of this odd–even effect remain unknown at the moment considering the unknown impact of the carboxylate bonding conformation on the charge-transfer process. As pointed above, the desorption of oxygen, and oxygen-containing molecular fragments, is a separate process which is controlled by the efficiency of intermolecular interactions, steric limitations, and the thickness of the film through which these species have to be transmitted. These parameters in turn depend on monolayer packing density, the efficiency of the cross-linking process during film irradiation, and the resulting film thickness. The film thickness modification is controlled by the desorption of molecules and the cross-linking process which hampers desorption. The data in Figure 2 indicate that for odd-numbered members of the BPnCOO/Ag series, thickness modification is more pronounced with a lower efficiency of the cross-linking process. Higher desorption of molecules, a lower packing density, a less efficient cross-linking process, and the lower value of the cross-linked film thickness are all consistent with more pronounced elimination of oxygen. Summarizing this part, we come to the conclusion that all main physical processes involved in electron-beam-induced oxygen elimination indicate high sensitivity to the odd–even structural modification of BPnCOO/Ag SAMs. Moreover, in the case of the initial electron excitation efficiency as well as the final desorption probability, the correlation of the odd–even behavior for σO and σTH is well justified.

4. Summary and Conclusions

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Detailed XPS analysis of an electron-beam-irradiated homologue series of hybrid aliphatic–aromatic SAMs of BPnCOO/Ag presented here revealed a significant role of the aliphatic spacer (defined by the parameter n) in the electron-induced desorption/modification of such monolayers and as a result the structure and purity of the CNMs produced by this process as well. Our data show that partial desorption of the monolayer upon electron irradiation, and thus the resulting effective thickness of the CNMs, is more efficient for odd-numbered members of the series. This odd–even effect is correlated with the odd–even differences in the structure of BPnCOO/Ag which for odd-numbered members of the series have a disordered liquid-like structure with a lower packing density and thus less effective molecule–molecule interactions controlling the efficiency of the desorption process upon electron-beam-induced termination of the chemical bonding with the substrate. We show that electron irradiation also leads to almost complete elimination of oxygen from the system, which is confirmed not only directly by the concentration of this element in the monolayer but also indirectly by destruction of the interfacial dipole layer which is formed by the O–Ag bonds. The process of oxygen elimination also shows the odd–even dependence with a higher efficiency for odd-numbered members of the series, that is, it is also correlated with the structural odd–even effect, similar to the case of the thickness reduction process.

Supporting Information

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The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acs.jpcc.1c01857.

  • Electron irradiation data obtained for BPnCOO/Ag SAMs (with n = 2, 4, 5, 6) (PDF)

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Author Information

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  • Corresponding Authors
  • Authors
    • Monika Kruk - Smoluchowski Institute of Physics, Jagiellonian University, Łojasiewicza 11, Krakow 30-348, Poland
    • Christof Neumann - Institute of Physical Chemistry, Friedrich Schiller University Jena, Lessingstraße 10, Jena 07743, Germany
    • Martha Frey - Institute of Physical Chemistry, Friedrich Schiller University Jena, Lessingstraße 10, Jena 07743, Germany
    • Krzysztof Kozieł - Faculty of Chemistry, Jagiellonian University, Krakow 30-387, Poland;  Orcidhttp://orcid.org/0000-0002-3376-0861
  • Notes
    The authors declare no competing financial interest.

Acknowledgments

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C.N. and A.T. thank the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) for support within the research grants TU149/8–2 and 364549901—TRR 234 “CataLight” (B7, Z2) and the research infrastructure grant INST 275/25 7–1 FUGG. P.C. thanks Priority Research Area SciMat under the program Excellence Initiative—Research University at the Jagiellonian University in Kraków. This work was supported financially by the National Science Centre, Poland (grant UMO-2015/19/B/ST5/01636).

References

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This article references 60 other publications.

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    Low-energy electron-induced damage in hexadecanethiolate (HDT) monolayers on gold substrates has been investigated using IR reflection-absorption spectroscopy (IRAS), angle-resolved near-edge x-ray absorption fine structure spectroscopy (NEXAFS), and advancing water contact angle measurements. HDT films were exposed to electrons of energies 10-100 eV and doses between 30 and 14 000 μC/cm2. The induced damage was monitored both "in situ" by NEXAFS measurements interleaved with electron irradiations and "ex-situ" by NEXAFS, IRAS, and contact angle measurements after exposure of the irradiated samples to air. A progressive film damage was obsd. with increasing electron energy and dose of irradn. This damage was found to occur during irradn. in UHV and was not induced by chem. reactions with airborne mols. during subsequent exposure of the irradiated films to air. The damage starts in the region of the terminal Me groups of the HDT films and propagates into the bulk of the film. An anal. of the IRAS and NEXAFS data shows that the conformational and orientational order within the HDT film are most sensitive to low-energy electron irradn. Electron-induced cleavage of C-H and C-C bonds resulting in a partial desorption of the film constituents also occurs and leads to formation of C:C double bonds in the film as inferred from the appearance of a π*-resonance in the C 1s NEXAFS spectra. The obtained results are of importance for both the optimization of self-assembled-monolayers-based lithog. processes and for the general understanding of irradn.-induced changes in org. films.
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    Zharnikov, M.; Frey, S.; Heister, K.; Grunze, M. Modification of Alkanethiolate Monolayers by Low Energy Electron Irradiation: Dependence on the Substrate Material and on the Length and Isotopic Composition of the Alkyl Chains. Langmuir 2000, 16, 2697– 2705,  DOI: 10.1021/la991034r
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    The low energy electron induced damage in self-assembled monolayers of dodecanethiolate, octadecanethiolate, and perdeuterated eicosanethiolate on gold and octadecanethiolate on silver has been investigated in situ by XPS and angle resolved near edge X-ray absorption fine structure spectroscopy. All investigated systems exhibit qual. similar behavior with respect to low energy electron irradn. The most noticeable processes are the loss of orientational and conformational order, partial dehydrogenation with C:C double bond formation, desorption of the layer fragments, redn. of the thiolate species, and the appearance of new sulfur species. The cross sections for the rates of the individual irradn.-induced processes have been detd. For the films on gold all these processes are found to evolve with similar rates, except for the formation of C:C double bonds and desorption of sulfur-contg. fragments. The extent of the latter process is noticeably smaller in the longer-chain films as compared to their shorter-chain counterparts. The response of the alkyl matrix and the S-Au interface to electron irradn. are not directly correlated. Whereas the irradn.-induced processes in the alkyl matrix are found to be essentially independent of the alkyl chain length and the substrate material, the extent and rate of the thiolate species redn. and new sulfur species formation are mainly detd. by the strength and character of the thiolate-substrate bond. No large isotopic effect in the irradn.-induced dehydrogenation process was obsd. Deuterated films are found to be only slightly less sensitive to electron irradn. as compared to their hydrogen-contg. counterparts.
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    Schmid, M.; Wan, X.; Asyuda, A.; Zharnikov, M. Modification of Self-Assembled Monolayers by Electron Irradiation: The Effect of Primary Energy (10–500 eV). J. Phys. Chem. C 2019, 123, 28301– 28309,  DOI: 10.1021/acs.jpcc.9b09125
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    Well-defined alkanethiolate self-assembled monolayers (SAMs) represent a useful model system for a variety of thin org. films on solid supports, in particular in context of their modification by electron irradn. Taking a dodecanethiolate SAM on Au(111) as an example, the authors studied the primary electron energy dependence of the resp. processes at 10-500 eV. Cross sections of major irradn.-induced processes were derived, including thickness redn., progressing chem. inhomogeneity, and damage of the SAM-ambient and SAM-substrate interfaces. All the cross sections showed qual. similar behavior as functions of the primary electron energy, viz. a strong, progressive increase at 10-100 eV and a nearly energy independent character at the higher energies, up to 500 eV. This behavior was primary attributed to the energy dependence of the ionization cross section, with a certain effect of the secondary electron yield. Quant., the cross sections directly related to irradn.-induced bond breaking followed by desorption or relocation of the released fragments turned out to be noticeably higher (by a factor 2-2.5) than those representing the secondary processes in the SAM, such as reorganization of the SAM and SAM-ambient interface. The balance between the film decompn., resulting in material loss, and crosslinking, resulting in the preservation of the material, was found to shift toward the decompn. at lower primary electron energies as compared to higher ones.
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    Cyganik, P.; Vandeweert, E.; Postawa, Z.; Bastiaansen, J.; Vervaecke, F.; Lievens, P.; Silverans, R. E.; Winograd, N. Modification and Stability of Aromatic Self-Assembled Monolayers upon Irradiation with Energetic Particles. J. Phys. Chem. B 2005, 109, 5085– 5094,  DOI: 10.1021/jp0478209
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    Turchanin, A.; Käfer, D.; El-Desawy, M.; Wöll, C.; Witte, G.; Gölzhäuser, A. Molecular Mechanisms of Electron-Induced Cross-Linking in Aromatic SAMs. Langmuir 2009, 25, 7342– 7352,  DOI: 10.1021/la803538z
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    Molecular Mechanisms of Electron-Induced Cross-Linking in Aromatic SAMs
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    Langmuir (2009), 25 (13), 7342-7352CODEN: LANGD5; ISSN:0743-7463. (American Chemical Society)
    When arom. self-assembled monolayers (SAMs) are electron-irradiated, intermol. cross-links are formed and the SAMs transform into carbon nanosheets with mol. thickness. These nanosheets have a very high mech. stability and can withstand temps. >1000 K. The authors study the electron induced crosslinking of 1,1'-biphenyl-4-thiol (BPT) SAMs on gold by combining XPS, x-ray absorption spectroscopy (NEXAFS), thermal desorption spectroscopy (TDS), and UPS. The exptl. data were acquired as a function of electron dose and temp. and compared with quantum chem. calcns. Details of the intermol. crosslinking, the microstructure of cross-linked films, and their structural transformations upon heating were obtained to derive a view of the mechanisms involved. The anal. shows that room-temp. electron irradn. causes a lateral crosslinking via the formation of C-C linked Ph species as well as a new sulfur species. The thermal stability of the BPT films increases with the electron dose and sats. at ∼50 mC/cm2. Nevertheless, nonlinked fragments in the thermal desorption spectra indicate an incomplete crosslinking even at high doses, which can be attributed to steric reasons and quenching due to the reduced band gap of partially linked mols. At temps. >800 K, all sulfur species are thermally desorbed, while the remaining film reveals an onset of carbonization.
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    Eck, W.; Stadler, V.; Geyer, W.; Zharnikov, M.; Gölzhäuser, A.; Grunze, M. Generation of Surface Amino Groups on Aromatic Self-Assembled Monolayers by Low Energy Electron Beams - A First Step Towards Chemical Lithography. Adv. Mater. 2000, 12, 805– 808,  DOI: 10.1002/(sici)1521-4095(200006)12:11<805::aid-adma805>3.0.co;2-0
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    The transformation of surface nitro groups to amino groups was obsd. during electron-beam exposure of biphenylthiol monolayers deposited on gold surface. The hydrogen atoms required for redn. of the nitro groups were generated by the electron-induced dissocn. of the C-H bonds in the biphenyl units. As a consequence, the monolayer was crosslinked and resistant towards chem. etching agents or displacement by alkane thiols. The resulting amino groups could be further chem. modified, as was shown by acetylation with trifluoroacetic acid anhydride. This electron-induced formation of nucleophilic surface groups may serve as a basis for a soft and chem. defined lithog. Covalent coupling of functional entities to the irradiated regions may allow generation of chem. defined nanostructures on amino-terminated patterns within a nitro-terminated surface. Future expts. will examine the attainable size resoln. when electron beam lithog. and ultrasharp field emitters are used for patterning.
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    Turchanin, A.; Schnietz, M.; El-Desawy, M.; Solak, H. H.; David, C.; Gölzhäuser, A. Fabrication of Molecular Nanotemplates in Self-Assembled Monolayers by Extreme-Ultraviolet-Induced Chemical Lithography. Small 2007, 3, 2114– 2119,  DOI: 10.1002/smll.200700516
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    Extreme-UV interference lithog. (EUV-IL) is applied to create chem. nanopatterns in self-assembled monolayers (SAMs) of 4'-nitro-1,1'-biphenyl-4-thiol (NBPT) on gold. XPS shows that EUV irradn. induces both the conversion of the terminal nitro groups of NBPT into amino groups and the lateral crosslinking of the underlying arom. cores. Large-area (≈2 mm2) nitro/amino chem. patterns with periods ranging from 2000 nm to 60 nm can be generated. Regions of pristine NBPT on the exposed samples are exchanged with protein-resistant thiol SAMs of polyethyleneglycol, resulting in the formation of mol. nanotemplates, which can serve as the basis of complex biomimetic surfaces.
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    Electron-induced chem. lithog. combined with self-assembled monolayers and multivalent chelators for high-affinity capturing of His-tagged proteins are used to obtain specific, stable, highly parallel, and functional protein micro- and nanoarrays on solid substrates. The functionality of the generated large-area protein arrays is shown in situ via specific, homogeneous, oriented and reversible immobilization of His6-tagged 20S proteasome and fluorescence labeled His10-tagged maltose binding proteins.
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    The authors show that protein-repelling films, both self-assembled monolayer (SAM)- and polymer-like, can be used as a primary matrix for direct electron-beam lithog. (EBL) of both nonspecific and specific protein patterns. The authors used protein-repelling SAMs of oligoethylene glycol-substituted alkane-thiols on evapd. Au(111) substrates. The required irradn. dose is smaller by two orders of magnitude than for an alternative, multistep EBL approach. The nonspecific template can be easily converted into a specific one by adsorption of a mediator protein with specific binding sites for the secondary target protein. The EBL allows the generation of patterns over a length scale ranging from centimeters to nanometers with no limitations to the pattern shape, including complex, gradient-like assays.
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    Ballav, N.; Schlip, S.; Zharnikov, M. Electron Beam Chemical Lithography with Aliphatic Self-Assembled Monolayers. Angew. Chem., Int. Ed. 2008, 47, 1421– 1424,  DOI: 10.1002/anie.200704105
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    Electron-beam chem. lithog. using aliph. self-assembled monolayer (SAMs) resists is demonstrated. Fabrication of polymer micro- and nanobrushes is demonstrated. Patterning requires a much lower dose than electron-beam chem. lithog. using arom. SAMs resists.
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    Abufager, P. N.; Solano Canchaya, J. G.; Wang, Y.; Alcami, M.; Martin, F.; Alvarez Soria, L.; Martiarena, M. L.; Reuter, K.; Busnengo, H. F.
    Physical Chemistry Chemical Physics (2011), 13 (20), 9353-9362CODEN: PPCPFQ; ISSN:1463-9076. (Royal Society of Chemistry)
    We compare the stability of various structures of high coverage self-assembled monolayers (SAMs) of short alkylthiolates, S(CH2)n-1CH3 (= Cn), on Ag(111) and Au(111). We employ: (i) the ab initio thermodn. approach based on d. functional theory (DFT) calcns., to compare the stability of SAMs of C1 (with coverages Θ = 3/7 and 1/3) on both substrates, and (ii) a set of pairwise interat. potentials derived from second-order Moller-Plesset (MP2) perturbation theory calcns., to est. the role of chain-chain (Ch-Ch) interactions in the structure and stability of SAMs of longer chain alkylthiolates. For C1/Ag(111) (C1/Au(111)) the SAM with Θ = 3/7 is more (less) stable than for Θ = 1/3 in a wide range of temps. and pressures in line with expts. In addn., for the mol. densities of SAMs corresponding to Θ = 3/7 and 1/3, the MP2-based Ch-Ch interaction potential also predicts the different chain orientations obsd. exptl. in SAMs of alkylthiolates on Ag(111) and Au(111). Thus, for short length alkylthiolates, a simple model based on first principles calcns. that sep. accounts for mol.-surface (M-S) and Ch-Ch interactions succeeds in predicting the main structural differences between the full coverage SAMs usually obsd. exptl. on Ag(111) and Au(111).
  20. 20
    Ballav, N.; Shaporenko, A.; Terfort, A.; Zharnikov, M. A Flexible Approach to the Fabrication of Chemical Gradients. Adv. Mater. 2007, 19, 998– 1000,  DOI: 10.1002/adma.200602168
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    A flexible approach to the fabrication of chemical gradients
    Ballav, Nirmalya; Shaporenko, Andrey; Terfort, Andreas; Zharnikov, Michael
    Advanced Materials (Weinheim, Germany) (2007), 19 (7), 998-1000CODEN: ADVMEW; ISSN:0935-9648. (Wiley-VCH Verlag GmbH & Co. KGaA)
    Chem. gradients on different substrates is fabricated by tuning the exchange reaction between a self-assembled monolayer covering the substrate and a potential mol. substituent by electron irradn. A hydrophilicity gradient was created using this method.
  21. 21
    Sauter, E.; Yildirim, C.; Terfort, A.; Zharnikov, M. Adjustment of the Work Function of Pyridine and Pyrimidine Substituted Aromatic Self-Assembled Monolayers by Electron Irradiation. J. Phys. Chem. C 2017, 121, 12834– 12841,  DOI: 10.1021/acs.jpcc.7b03302
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    21
    Adjustment of the Work Function of Pyridine and Pyrimidine Substituted Aromatic Self-Assembled Monolayers by Electron Irradiation
    Sauter, Eric; Yildirim, Can; Terfort, Andreas; Zharnikov, Michael
    Journal of Physical Chemistry C (2017), 121 (23), 12834-12841CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
    Self-assembled monolayers (SAMs) are frequently used to manipulate injection barriers in org. electronics by introduction of a specific dipole moment at the interfaces between the electrodes and adjacent org. layers. This is usually achieved by the selection of a proper dipolar terminal tail group comprising the SAM-ambient interface, which was recently complemented by embedding such a group into the mol. backbone. Here we show that the work function of SAMs can also be adjusted by electron irradn. in a quite broad range and in controlled fashion as far as these films contain pyridine or pyrimidine group. This effect is demonstrated by the example of several representative arom. SAMs with either terminal pyridine group or embedded pyrimidine group. The obsd. behavior is presumably related to specific chem. transformations involving the nitrogen atoms in these moieties. The SAMs with the embedded pyrimidine group are then esp. attractive because this moiety is decoupled from the SAM-ambient interface. The extent of the effect is very large (a work function change of up to ∼0.8 eV) as far as it is monitored in situ but is diminished upon the exposure of the irradiated films to ambient. Practical implications of this effect are discussed, including work function lithog., which is demonstrated by representative patterns.
  22. 22
    Neumann, C.; Kaiser, D.; Mohn, M. J.; Füser, M.; Weber, N.-E.; Reimer, O.; Gölzhäuser, A.; Weimann, T.; Terfort, A.; Kaiser, U. Bottom-Up Synthesis of Graphene Monolayers with Tunable Crystallinity and Porosity. ACS Nano 2019, 13, 7310– 7322,  DOI: 10.1021/acsnano.9b03475
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    Bottom-Up Synthesis of Graphene Monolayers with Tunable Crystallinity and Porosity
    Neumann, Christof; Kaiser, David; Mohn, Michael J.; Fueser, Matthias; Weber, Nils-Eike; Reimer, Oliver; Goelzhaeuser, Armin; Weimann, Thomas; Terfort, Andreas; Kaiser, Ute; Turchanin, Andrey
    ACS Nano (2019), 13 (6), 7310-7322CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
    We present a method for a bottom-up synthesis of atomically thin graphene sheets with tunable crystallinity and porosity using arom. self-assembled monolayers (SAMs) as mol. precursors. To this end, we employ SAMs with pyridine and pyrrole constituents on polycryst. copper foils and convert them initially into mol. nanosheets-carbon nanomembranes (CNMs)-via low-energy electron irradn. induced crosslinking and then into graphene monolayers via pyrolysis. As the nitrogen atoms are leaving the nanosheets during pyrolysis, nanopores are generated in the formed single-layer graphene. We elucidate the structural changes upon the crosslinking and pyrolysis down to the at. scale by complementary spectroscopy and microscopy techniques including X-ray photoelectron and Raman spectroscopy, LEED, at. force, helium ion, and high-resoln. transmission electron microscopy, and elec. transport measurements. We demonstrate that the crystallinity and porosity of the formed graphene can be adjusted via the choice of mol. precursors and pyrolysis temp., and we present a kinetic growth model quant. describing the conversion of mol. CNMs into graphene. The synthesized nanoporous graphene monolayers resemble a percolated network of graphene nanoribbons with a high charge carrier mobility (∼600 cm2/(V s)), making them attractive for implementations in electronic field-effect devices.
  23. 23
    Matei, D. G.; Weber, N.-E.; Kurasch, S.; Wundrack, S.; Woszczyna, M.; Grothe, M.; Weimann, T.; Ahlers, F.; Stosch, R.; Kaiser, U. Functional Single-Layer Graphene Sheets from Aromatic Monolayers. Adv. Mater. 2013, 25, 4146– 4151,  DOI: 10.1002/adma.201300651
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    23
    Functional Single-Layer Graphene Sheets from Aromatic Monolayers
    Matei, Dan G.; Weber, Nils-Eike; Kurasch, Simon; Wundrack, Stefan; Woszczyna, Miroslaw; Grothe, Miriam; Weimann, Thomas; Ahlers, Franz; Stosch, Rainer; Kaiser, Ute; Turchanin, Andrey
    Advanced Materials (Weinheim, Germany) (2013), 25 (30), 4146-4151CODEN: ADVMEW; ISSN:0935-9648. (Wiley-VCH Verlag GmbH & Co. KGaA)
    Self-assembled monolayers of arom. mols. on copper substrates can be converted into high-quality single-layer graphene using low-energy electron irradn. and subsequent annealing. This two-dimensional solid state transformation is characterized on the at. scale and the phys. and chem. properties of the formed graphene sheets are studied by complementary microscopic and spectroscopic techniques and by elec. transport measurements. As substrates, Cu(111) single crystals and the technol. relevant polycryst. copper foils are successfully used.
  24. 24
    Turchanin, A.; Gölzhäuser, A. Carbon Nanomembranes from Self-Assembled Monolayers: Functional Surfaces without Bulk. Prog. Surf. Sci. 2012, 87, 108– 162,  DOI: 10.1016/j.progsurf.2012.05.001
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    Carbon nanomembranes from self-assembled monolayers: Functional surfaces without bulk
    Turchanin, Andrey; Goelzhaeuser, Armin
    Progress in Surface Science (2012), 87 (5-8), 108-162CODEN: PSSFBP; ISSN:0079-6816. (Elsevier B.V.)
    In this topical review we describe the fabrication, characterization and applications of 1 nm thick, mech. stable carbon nanomembranes (CNMs). They represent a new type of functional two-dimensional (2D) materials, which can be concisely described as "surfaces without bulk". Because CNMs are made by electron-induced crosslinking of arom. self-assembled monolayers (SAMs), we start with an overview of SAMs with a special emphasis on arom. SAMs. We describe the chem. modification of SAMs by electron, ion and photon irradn., introduce the concepts of irradn.-induced crosslinking and chem. nanolithog. of arom. SAMs and discuss the underlying phys. and chem. mechanisms. We present examples for applications of these phenomena in the engineering of complex surface architectures, e.g., nanopatterns of proteins, fluorescent dyes or polymer brushes. Then we introduce a transfer procedure to release cross-linked arom. SAMs from their original substrates and to form free-standing CNMs. We discuss mech. and elec. properties of CNMs and demonstrate that they can be converted into graphene upon annealing. This transformation opens an original and flexible mol. route towards the large-scale synthesis of graphene sheets with tunable properties. Finally, we demonstrate the lithog. and chem. tailoring of CNMs to fabricate novel functional 2D carbon materials: supports for high resoln. transmission electron microscopy (HRTEM) and nanolithog., nanosieves, Janus nanomembranes, polymer carpets, complex layered structures. Prospects of combining different types of nanomembranes made of SAMs (CNMs, graphene, nanosieves, Janus nanomembranes) towards the engineering of novel functional nanomaterials for a variety of electronic, optical, lab-on-a-chip and micro-/nanomech. (MEMS/NEMS) devices are discussed.
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    Turchanin, A.; Beyer, A.; Nottbohm, C. T.; Zhang, X.; Stosch, R.; Sologubenko, A.; Mayer, J.; Hinze, P.; Weimann, T.; Gölzhäuser, A. One Nanometer Thin Carbon Nanosheets with Tunable Conductivity and Stiffness. Adv. Mater. 2009, 21, 1233– 1237,  DOI: 10.1002/adma.200803078
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    One nanometer thin carbon nanosheets with tunable conductivity and stiffness
    Turchanin, Andrey; Beyer, Andre; Nottbohm, Christoph T.; Zhang, Xianghui; Stosch, Rainer; Sologubenko, Alla; Mayer, Joachim; Hinze, Peter; Weimann, Thomas; Goelzhaeuser, Armin
    Advanced Materials (Weinheim, Germany) (2009), 21 (12), 1233-1237CODEN: ADVMEW; ISSN:0935-9648. (Wiley-VCH Verlag GmbH & Co. KGaA)
    Atomically thin (∼1 nm) carbon films and membranes whose elec. behavior can be tuned from insulating to conducting are fabricated by a novel route. These films present arbitrary size and shape based on mol. self-assembly, electron irradn., and pyrolysis, and their tech. applicability is demonstrated by their incorporation into a microscopic pressure sensor.
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    Turchanin, A.; Gölzhäuser, A. Carbon Nanomembranes. Adv. Mater. 2016, 28, 6075– 6103,  DOI: 10.1002/adma.201506058
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    Carbon Nanomembranes
    Turchanin, Andrey; Goelzhaeuser, Armin
    Advanced Materials (Weinheim, Germany) (2016), 28 (29), 6075-6103CODEN: ADVMEW; ISSN:0935-9648. (Wiley-VCH Verlag GmbH & Co. KGaA)
    A review. Carbon nanomembranes (CNMs) are synthetic 2D carbon sheets with tailored phys. or chem. properties. These depend on the structure, mol. compn., and surroundings on either side. Due to their mol. thickness, they can be regarded as "interfaces without bulk" sepg. regions of different gaseous, liq., or solid components and controlling the materials exchange between them. Here, a universal scheme for the fabrication of 1 nm-thick, mech. stable, functional CNMs is presented. CNMs can be further modified, for example perforated by ion bombardment or chem. functionalized by the binding of other mols. onto the surfaces. The underlying phys. and chem. mechanisms are described, and examples are presented for the engineering of complex surface architectures, e.g., nanopatterns of proteins, fluorescent dyes, or polymer brushes. A simple transfer procedure allows CNMs to be placed on various support structures, which makes them available for diverse applications: supports for electron and X-ray microscopy, nanolithog., nanosieves, Janus nanomembranes, polymer carpets, complex layered structures, functionalization of graphene, novel nanoelectronic and nanomech. devices. To close, the potential of CNMs in filtration and sensorics is discussed. Based on tests for the sepn. of gas mols., it is argued that ballistic membranes may play a prominent role in future efforts of materials sepn.
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    Turchanin, A. Synthesis of Molecular 2D Materials via Low-Energy Electron Induced Chemical Reactions. Chimia 2019, 73, 473– 479,  DOI: 10.2533/chimia.2019.473
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    27
    Synthesis of molecular 2D materials via low-energy electron induced chemical reactions
    Turchanin, Andrey
    Chimia (2019), 73 (6), 473-479CODEN: CHIMAD ISSN:. (Swiss Chemical Society)
    After the demonstration of a variety of inorg. two-dimensional (2D) materials (graphene, hBN, MoS2, etc.), mol. 2D materials have attracted a significant research interest as well. However, the direct synthesis of these materials is an exceptionally challenging task for chemists. In this review article, a simple and robust phys. method for the synthesis of mol. 2D materials is presented based on low-energy electron induced chem. reactions in arom. mol. layers. In this way, ultrathin (∼1 nm) mol. nanosheets with adjustable chem. and phys. properties called Carbon Nanomembranes (CNM) can be prepd. Moreover, the method enables the synthesis of various other 2D org.-inorg. hybrids (e.g.MoS2-CNM, graphene-CNM lateral heterostructures, etc.) or ∼20 nm thick nanosheets of org. semiconductors. Mechanisms of the reaction and functional properties of these mol. 2D materials including their chem. functionalization and engineering of hybrid hierarchical structures for application in nanoscience and nanotechnol. are discussed in this article.
  28. 28
    Neumann, C.; Szwed, M.; Frey, M.; Tang, Z.; Kozieł, K.; Cyganik, P.; Turchanin, A. Preparation of Carbon Nanomembranes without Chemically Active Groups. ACS Appl. Mater. Interfaces 2019, 11, 31176– 31181,  DOI: 10.1021/acsami.9b09603
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    Preparation of Carbon Nanomembranes without Chemically Active Groups
    Neumann, Christof; Szwed, Monika; Frey, Martha; Tang, Zian; Koziel, Krzysztof; Cyganik, Piotr; Turchanin, Andrey
    ACS Applied Materials & Interfaces (2019), 11 (34), 31176-31181CODEN: AAMICK; ISSN:1944-8244. (American Chemical Society)
    The electron-irradn.-induced synthesis of carbon nanomembranes (CNMs) from arom. thiol-based self-assembled monolayers (SAMs) on gold substrate is a well-established method to form mol. thin nanosheets. These mol. two-dimensional materials can be prepd. with tunable properties; therefore, they find a variety of applications in nanotechnol. ranging from ultrafiltration to nanobiosensors. However, no chem. inert CNM was fabricated up to now, as the reactive thiol group is present on the membrane surface even after transferring it to other substrates. Here, we study the electron irradn. of carboxylic acid-based SAMs on a silver substrate as an alternative route for CNM formation. Our anal., based on a combination of XPS and SEM demonstrates that for this type of SAMs, purely carbonaceous CNMs with tunable porosity can be obtained.
  29. 29
    Dementyev, P.; Naberezhnyi, D.; Westphal, M.; Buck, M.; Gölzhäuser, A. Carbon Nanomembranes from Aromatic Carboxylate Precursors. ChemPhysChem 2020, 21, 1006– 1011,  DOI: 10.1002/cphc.202000150
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    Carbon Nanomembranes from Aromatic Carboxylate Precursors
    Dementyev, Petr; Naberezhnyi, Daniil; Westphal, Michael; Buck, Manfred; Goelzhaeuser, Armin
    ChemPhysChem (2020), 21 (10), 1006-1011CODEN: CPCHFT; ISSN:1439-4235. (Wiley-VCH Verlag GmbH & Co. KGaA)
    Self-assembled monolayers (SAMs) serve as convenient platform for fabricating carbon nanomembranes (CNMs) of extended lateral dimensions. Highly porous CNMs are emerging as interesting materials for membrane technologies as they exhibit selectivity for water permeation and, owing to their reduced dimensionality, promise increased energy efficiency compared to established systems. In the present study terphenylcarboxylate SAMs, prepd. on silver underpotential deposited on Au and irradiated by 100 eV electrons, were successfully converted into free-standing CNMs. IR and XPS reveal pronounced chem. changes both of the anchoring carboxylate moiety and the arom. backbone upon electron irradn. Permeation studies showed high specificity for water as demonstrated by the sepn. from THF. Compared to thiols on gold, the std. CNM precursor system, the carboxylic acid based SAM exhibits equiv. characteristics. This suggests that electron-induced carbonization is insensitive to the particular choice of the anchor moiety and, therefore, the choice of precursor mols. can be extended to the versatile class of arom. carboxylic acids.
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    Asyuda, A.; de la Morena, R. O.; Sauter, E.; Turner, K.; McDonald, K.; Buck, M.; Zharnikov, M. Electron-Induced Modification of Self-Assembled Monolayers of Aromatic Carboxylic Acids. J. Phys. Chem. C 2020, 124, 25107– 25120,  DOI: 10.1021/acs.jpcc.0c07577
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    Electron-Induced Modification of Self-Assembled Monolayers of Aromatic Carboxylic Acids
    Asyuda, Andika; de la Morena, Rodrigo Ortiz; Sauter, Eric; Turner, Kelly; McDonald, Kirsty; Buck, Manfred; Zharnikov, Michael
    Journal of Physical Chemistry C (2020), 124 (45), 25107-25120CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
    The effects of low-energy electrons on arom. self-assembled monolayers (SAMs) with carboxylic acid (CA) docking groups were studied with a focus on the dose range below 5 mC/cm2. The SAMs were prepd. on an underpotentially deposited Ag bilayer and comprised nonsubstituted and CA-substituted monolayers with a rod-like biphenyl backbone and a monolayer of a Y-shaped, CA-substituted mol., 1,3,5-benzenetribenzoic acid (H3BTB), formed either as a single-component film or as a binary one by mixing with adamantane-CA (Ad-CA). X-ray photoelectron and near-edge X-ray absorption fine structure spectra suggest high proneness of the CA groups to electron irradn. at both SAM/substrate and SAM/ambient interfaces. Cleavage of the carboxylate-substrate bond results in substantial mol. desorption at the initial stage of irradn. until electron-induced crosslinking gradually takes over. The CA groups at the outer SAM interface undergo substantial chem. changes, indicating that they participate in the crosslinking chem. The electron-induced processes are accompanied by mol. reorientation. Disordering for the SAMs formed by the rod-like mols. is contrasted by the H3BTB-based systems where changes also occur but some mol. order is preserved as explained by a proposed model invoking conformational changes. In SAMs of H3BTB mixed with Ad-CA, the latter shows higher proneness to irradn.-induced desorption than the former, as well as an influence on the crosslinking chem. The results of the present study suggest that CA-based SAMs on Ag offer addnl. options for crosslinking in SAMs and, as exemplarily demonstrated by the generation of Cu patterns on structured H3BTB templates, can be efficiently used for lithog. and nanofabrication.
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    Krzykawska, A.; Szwed, M.; Ossowski, J.; Cyganik, P. Odd–Even Effect in Molecular Packing of Self-Assembled Monolayers of Biphenyl-Substituted Fatty Acid on Ag(111). J. Phys. Chem. C 2018, 122, 919– 928,  DOI: 10.1021/acs.jpcc.7b10806
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    Odd-Even Effect in Molecular Packing of Self-Assembled Monolayers of Biphenyl-Substituted Fatty Acid on Ag(111)
    Krzykawska, Anna; Szwed, Monika; Ossowski, Jakub; Cyganik, Piotr
    Journal of Physical Chemistry C (2018), 122 (1), 919-928CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
    Self-assembled monolayers (SAMs) of the homologous series of biphenyl-substituted fatty acids on Ag(111) in the form of (C6H4)2-(CH2)n-COO/Ag (BPnCOO/Ag, n = 1-4) were studied using IR reflection absorption spectroscopy (IRRAS), XPS, and scanning tunneling microscopy (STM). The combination of spectroscopic (IRRAS and XPS) and microscopic analyses (STM) revealed that depending on the parity of the parameter n, which defines the length of the short aliph. linker, two types of structures are formed by the BPnCOO/Ag. For n = even, highly ordered and stable SAMs are formed in a very short time. For n = odd, the resp. monolayers have a disordered liquidlike structure with more canted orientation of the mol. backbone and the anchoring carboxylic group, which results in lower packing d. and film thickness compared to the even-numbered SAMs. By comparing obtained results with former odd-even effects reported for analogous SAMs based on thiols and selenols, a common, qual. model relating the odd-even effect to the monolayer stability and structure is discussed. For BPnCOO/Ag, this odd-even effect is particularly strong and fully controls the ability of mols. to form highly ordered structures. This observation seems to be of key importance for the design of SAMs based on the carboxylic group, which, for correctly designed mols., forms much better two-dimensionally ordered structures compared to commonly used thiols.
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    Tao, F.; Bernasek, S. L. Understanding Odd-Even Effects in Organic Self-Assembled Monolayers. Chem. Rev. 2007, 107, 1408– 1453,  DOI: 10.1021/cr050258d
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    Understanding Odd-Even Effects in Organic Self-Assembled Monolayers
    Tao, Feng; Bernasek, Steven L.
    Chemical Reviews (Washington, DC, United States) (2007), 107 (5), 1408-1453CODEN: CHREAY; ISSN:0009-2665. (American Chemical Society)
    A review on the structure and property odd-even effects of various org. monolayers self-assembled on solid surfaces. It is organized by the class of solid substrate on which org. self-assembled monolayers were formed and includes HOPG, Mo disulfide (MoS2), metal substrates including Au (111), Ag (111), Cu, Al, and Hg, and inorg. compd. substrates Al2O3 and SiOx/Si. For each substrate, the presentation is categorized by the different series of org. mols. The odd-even effects seen for the different categories of org. mols. on the same substrate are compared when they are described. Following the description of structural effects, the induced odd-even differences in various properties and functions of these thin films are rationalized. Finally, the origin and features of odd-even effects on different substrates are contrasted and discussed.
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    Zharnikov, M.; Frey, S.; Rong, H.; Yang, Y.-J.; Heister, K.; Buck, M.; Grunze, M. The Effect of the Sulfur-Metal Bond on the Structure of Self-Assembled Monolayers. Phys. Chem. Chem. Phys. 2000, 2, 3359– 3362,  DOI: 10.1039/b004232n
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    The effect of sulfur-metal bonding on the structure of self-assembled monolayers
    Zharnikov, M.; Frey, S.; Rong, H.; Yang, Y.-J.; Heister, K.; Buck, M.; Grunze, M.
    Physical Chemistry Chemical Physics (2000), 2 (15), 3359-3362CODEN: PPCPFQ; ISSN:1463-9076. (Royal Society of Chemistry)
    The equil. structure of alkanethiol monolayers self-assembled on metal substrates is detd. by a delicate interplay of the intermol. chain-chain and chemisorptive substrate-head group interactions. To understand the role of the individual constituents of this interplay, we studied the structure of biphenyl and perfluoroalkyl terminated alkanethiol monolayers self-assembled on Au and Ag. The structural characteristics of the monolayers derived from NEXAFS, FTIRRAS and XPS measurements point to a decisive role of the directional substrate-head group interactions.
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    Rong, H.-T.; Frey, S.; Yang, Y.-J.; Zharnikov, M.; Buck, M.; Wühn, M.; Wöll, C.; Helmchen, G. On the Importance of the Headgroup Substrate Bond in Thiol Monolayers: A Study of Biphenyl-Based Thiols on Gold and Silver. Langmuir 2001, 17, 1582– 1593,  DOI: 10.1021/la0014050
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    On the Importance of the Headgroup Substrate Bond in Thiol Monolayers: A Study of Biphenyl-Based Thiols on Gold and Silver
    Rong, Hai-Tao; Frey, Stefan; Yang, Yong-Jie; Zharnikov, Michael; Buck, Manfred; Wuehn, Mario; Woell, Christof; Helmchen, Guenter
    Langmuir (2001), 17 (5), 1582-1593CODEN: LANGD5; ISSN:0743-7463. (American Chemical Society)
    Self-assembled monolayers of a series of ω-(4'-methyl-biphenyl-4-yl)-alkanethiols (CH3-C6H4-C6H4-(CH2)m-SH, m = 1-6) formed on polycryst. gold and silver surfaces were characterized in detail by contact angle measurements, optical ellipsometry, XPS, reflection absorption IR spectroscopy (IRRAS), and near-edge X-ray absorption fine structure spectroscopy (NEXAFS). The orientation of the biphenyl moiety, detd. by combining the results from IRRAS and NEXAFS, exhibits a pronounced dependence on the no. of methylene groups. Similar to n-alkanethiols an odd-even effect is obsd. which on silver is opposite to that on gold. For m = odd on gold and m = even on silver the arrangement of the arom. moieties agrees well with the bulk structure of biphenyl, and the bonding of the thiols to the substrate is in agreement with an sp3 hybridization of the sulfur on gold and sp on silver, resp. In the opposite case of m = even on gold and m = odd on silver, the biphenyl moieties adopt a significantly more canted orientation which, as a consequence, results in a lower coverage. The odd-even behavior of the coverage is in sharp contrast to that seen for n-alkanethiols. The expts. provide evidence that a significant driving force exists to pertain the sp3 and sp hybridization of sulfur on gold and silver, resp. In the case of gold substrates the exptl. results are in conflict with available bending potentials derived from ab initio calcns.
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    Heister, K.; Rong, H.-T.; Buck, M.; Zharnikov, M.; Grunze, M.; Johansson, L. S. O. Odd-Even Effects at the S-Metal Interface and in the Aromatic Matrix of Biphenyl-Substituted Alkanethiol Self-Assembled Monolayers. J. Phys. Chem. B 2001, 105, 6888– 6894,  DOI: 10.1021/jp010180e
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    Odd-Even Effects at the S-Metal Interface and in the Aromatic Matrix of Biphenyl-Substituted Alkanethiol Self-Assembled Monolayers
    Heister, K.; Rong, H.-T.; Buck, M.; Zharnikov, M.; Grunze, M.; Johansson, L. S. O.
    Journal of Physical Chemistry B (2001), 105 (29), 6888-6894CODEN: JPCBFK; ISSN:1089-5647. (American Chemical Society)
    Synchrotron-based high-resoln. XPS was applied to characterize self-assembled monolayers (SAM) of biphenyl-substituted alkanethiols CH3(C6H4)2(CH2)nSH (BPn, n = 1-4) on Au and Ag substrates. Beyond previously identified odd-even changes in the packing d. and the tilt angle of the biphenyl moieties, the high-resoln. spectra reveal a no. of addnl. odd-even effects upon variation of the no. of methylene groups in the aliph. part in the BPn mol. Their occurrence and mutual correlation suggests that a BPn SAM represents a strongly correlated, highly ordered mol. assembly. In particular, periodical changes of a shake up feature in the C 1s region are obsd., which are related to the differences in the arrangement of the arom. matrix. The width and binding energy position of the S 2p signals also exhibit odd-even changes. The width changes are assocd. with the occupation of either equiv. or nonequivalent adsorption sites on the polycryst. (111) Au and Ag substrates. The comparison of the width values with those for conventional alkanethiols implies that the substrate bonding of alkanethiols on gold cannot be described by a single adsorption site.
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    Azzam, W.; Cyganik, P.; Witte, G.; Buck, M.; Wöll, C. Pronounced Odd-Even Changes in the Molecular Arrangement and Packing Density of Biphenyl-Substituted Alkanethiol SAMs. Langmuir 2003, 19, 8262– 8270,  DOI: 10.1021/la030165w
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    Pronounced Odd-Even Changes in the Molecular Arrangement and Packing Density of Biphenyl-Based Thiol SAMs: A Combined STM and LEED Study
    Azzam, W.; Cyganik, P.; Witte, G.; Buck, M.; Woell, Ch.
    Langmuir (2003), 19 (20), 8262-8270CODEN: LANGD5; ISSN:0743-7463. (American Chemical Society)
    Self-assembled monolayers (SAMs) of ω-(4'-methylbiphenyl-4-yl) alkanethiols CH3(C6H4)2(CH2)nSH (BPn, n = 3 and 4) on Au(111) substrates were studied using scanning tunneling microscopy (STM) and LEED (LEED). Prepn. at elevated temp. results in highly ordered layers with large domains. Whereas the (2√3 × √3) structure of the BP3 SAMs is similar to that reported previously for other arom. thiols, SAMs made from BP4 exhibit a very different structure. A (5√3 × 3)rect unit cell contg. 8 mols. is found which corresponds to a packing d. reduced by 25% compared to that of BP3. The odd-even effect obsd. in the molecularly resolved STM images of BP3 and BP4, therefore, confirms the pronounced influence of the spacer chain on the structure of these biphenyl-based SAMs.
  37. 37
    Cyganik, P.; Buck, M.; Azzam, W.; Wöll, C. Self-Assembled Monolayers of ω-Biphenyl-Alkane Thiols on Au(111): Influence of Spacer Chain on Molecular Packing. J. Phys. Chem. B 2004, 108, 4989– 4996,  DOI: 10.1021/jp037307f
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    Self-Assembled Monolayers of ω-Biphenylalkanethiols on Au(111): Influence of Spacer Chain on Molecular Packing
    Cyganik, Piotr; Buck, Manfred; Azzam, Waleed; Woell, Christof
    Journal of Physical Chemistry B (2004), 108 (16), 4989-4996CODEN: JPCBFK; ISSN:1520-6106. (American Chemical Society)
    Self-assembled monolayers (SAM) of ω-(4'-methylbiphenyl-4-yl)alkanethiols CH3(C6H4)2(CH2)nSH (BPn, n = 1-6) on Au(111) substrates, prepd. at room and elevated temps., were studied using scanning tunneling microscopy (STM). Molecularly resolved images reveal that all BPn SAMs form well-ordered layers over areas easily exceeding 50 × 50 nm2. Only two basic structures are alternatingly adopted with n changing between odd and even. The unit cell of odd-numbered SAMs is described by an oblique (2√3 × √3)R30° structure and contains two mols. In contrast, the even-numbered SAMs are described by a much larger, rectangular (5√3 × 3) structure with eight mols. per unit cell and occupying an area per mol. larger by about 25% compared to n = odd. With the exception of BP1 and BP6 the prepn. at elevated temps. resulted in a significant improvement in structural quality, yielding very large domains. For BP6 prepd. at 343 K a strong domain anisotropy is obsd., which is explained by the influence of the alkane spacer chain. For BP1 prepd. at 343 K formation of gold islands is concluded.
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    Cyganik, P.; Buck, M. Polymorphism in Biphenyl-Based Self-Assembled Monolayers of Thiols. J. Am. Chem. Soc. 2004, 126, 5960– 5961,  DOI: 10.1021/ja049146e
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    Polymorphism in Biphenyl-Based Self-Assembled Monolayers of Thiols
    Cyganik, Piotr; Buck, Manfred
    Journal of the American Chemical Society (2004), 126 (19), 5960-5961CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    Self-assembled monolayers of ω-(4'-methylbiphenyl-4-yl) butanethiol (H3C-C6H4-C6H4-(CH2)n-SH) on Au(111) substrates were investigated with scanning tunneling microscopy and contact angle measurements. A striking polymorphism was obsd. upon annealing, and structural changes were paralleled by a switch in stability against exchange by other thiols from unstable to stable. The phase formed at temps. above 413 K was characterized by a very high structural perfection over areas exceeding 105 nm2. The results suggest an addnl. dimension in the control of structure and properties of thiol monolayers if different factors contributing to the energetics of SAMs enter in a competing rather than a cooperative way.
  39. 39
    Cyganik, P.; Buck, M.; Strunskus, T.; Shaporenko, A.; Wilton-Ely, J. D. E. T.; Zharnikov, M.; Wöll, C. Competition as a Design Concept: Polymorphism in Self-Assembled Monolayers of Biphenyl-Based Thiols. J. Am. Chem. Soc. 2006, 128, 13868– 13878,  DOI: 10.1021/ja0640647
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    Competition as a Design Concept: Polymorphism in Self-Assembled Monolayers of Biphenyl-Based Thiols
    Cyganik, Piotr; Buck, Manfred; Strunskus, Thomas; Shaporenko, Andrei; Wilton-Ely, James D. E. T.; Zharnikov, Michael; Woell, Christof
    Journal of the American Chemical Society (2006), 128 (42), 13868-13878CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    Self-assembled monolayers (SAMs) of two ω-(4'-methylbiphenyl-4-yl)alkanethiols (CH3(C6H4)2(CH2)nSH, BPn, n = 4, 6) on Au(111) substrates, prepd. from soln. at room temp. and subsequently annealed at temps. up to 493 K under a nitrogen atm., were studied using scanning tunneling microscopy (STM), high-resoln. XPS (HRXPS), and near-edge X-ray absorption fine structure spectroscopy (NEXAFS). In striking contrast to BPn SAMs with n = odd, for which only one phase is obsd., the even-numbered BPn SAMs exhibit polymorphism. Irreversible phase transitions occur which involve three phases differing substantially in d. and stability. Upon annealing, BP4 and BP6 transform into a β-phase, which is characterized by an exceptionally high structural quality with virtually defect-free domains exceeding 500 nm in diam. Exchange expts., monitored by contact angle measurement, reveal that the β-phase exhibits a dramatically improved stability. The fundamental differences in the phase behavior of even- and odd-numbered BPn SAMs are discussed in terms of two design strategies based on cooperative and competitive effects.
  40. 40
    Azzam, W.; Bashir, A.; Terfort, A.; Strunskus, T.; Wöll, C. Combined STM and FTIR Characterization of Terphenylalkanethiol Monolayers on Au(111): Effect of Alkyl Chain Length and Deposition Temperature. Langmuir 2006, 22, 3647– 3655,  DOI: 10.1021/la053065u
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    Combined STM and FTIR Characterization of Terphenylalkanethiol Monolayers on Au(111): Effect of Alkyl Chain Length and Deposition Temperature
    Azzam, W.; Bashir, A.; Terfort, A.; Strunskus, T.; Woell, Ch.
    Langmuir (2006), 22 (8), 3647-3655CODEN: LANGD5; ISSN:0743-7463. (American Chemical Society)
    Self-assembled monolayers (SAMs) of 4,4'-terphenyl-substituted alkanethiols C6H5(C6H4)2(CH2)n-SH (TPn, n = 1-6) on Au (111) substrates were studied using scanning tunneling microscopy (STM) and IR reflection absorption spectroscopy (IRRAS). When the SAMs were prepd. at room temp. (RT, 298 K), TPn films (except TP2) exhibit an odd-even effect regarding both mol. orientation and packing d. For all investigated films, STM data reveals the presence of a large degree of lateral order. In the case of odd-numbered TPns, the films revealed a (2√3 × √3)R30° mol. arrangement. For the even-numbered TP4 and TP6 SAMs, a c(5√3 × 3) rectangular unit cell was found. The packing d. for the even-numbered TPn SAMs is 25% lower than that for the odd-numbered TPn SAMs. When the SAMs were prepd. at 333 K, the even-numbered SAMs were found to form structures with a significantly lower packing d. In the case of TP2, instead of the (2√3 × √3)R30° structure formed at room temp., a c(5√3 × 3) structure was obsd. For TP6 SAMs, the room-temp. c(5√3 × 3) structure was replaced by a (6√3 × 2√3)R30° structure.
  41. 41
    Chesneau, F.; Schüpbach, B.; Szelągowska-Kunstman, K.; Ballav, N.; Cyganik, P.; Terfort, A.; Zharnikov, M. Self-Assembled Monolayers of Perfluoroterphenyl-Substituted Alkanethiols: Specific Characteristics and Odd–Even Effects. Phys. Chem. Chem. Phys. 2010, 12, 12123– 12137,  DOI: 10.1039/c0cp00317d
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    Self-assembled monolayers of perfluoroterphenyl-substituted alkanethiols: specific characteristics and odd-even effects
    Chesneau, Frederick; Schuepbach, Bjoern; Szelagowska-Kunstman, Katarzyna; Ballav, Nirmalya; Cyganik, Piotr; Terfort, Andreas; Zharnikov, Michael
    Physical Chemistry Chemical Physics (2010), 12 (38), 12123-12137CODEN: PPCPFQ; ISSN:1463-9076. (Royal Society of Chemistry)
    Self-assembled monolayers (SAMs) formed by perfluoroterphenyl-substituted alkanethiols (C6F5-C6F4-C6F4-(CH2)n-SH, FTPn) with variable length of the aliph. linker (n = 2 and 3) were prepd. on (111) Au and Ag and characterized by a combination of several complementary spectroscopic and microscopic techniques. A specific feature of these systems is the helical conformation of the FTP moieties, which, along with the high electronegativity of fluorine, distinguishes them from the analogous non-fluorinated systems and makes them attractive for different applications. The SAMs were found to be well-defined, highly ordered, and densely packed, which suggests a perfect correlation between the orientations and, in particular, twists of the FTP helixes in the adjacent mols. Significantly, the SAM exhibited pronounced odd-even effects, i.e. a dependence of the mol. orientation and packing d. on the length of the aliph. linker in the target mols., with parity of n being the decisive parameter and the direction of the effects on Au opposite to that on Ag. The presence of the odd-even effects in the FTPn system brings new aspects into the discussion about the origin and mechanism of these phenomena. Specifically, the helical conformation of the FTP moieties in the dense phase excludes a variation of the intramol. torsion and mol. twist as the mechanism behind the odd-even effects.
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    Weidner, T.; Shaporenko, A.; Müller, J.; Schmid, M.; Cyganik, P.; Terfort, A.; Zharnikov, M. The Effect of the Bending Potential on Molecular Arrangement in Alkaneselenolate Self-Assembled Monolayers. J. Phys. Chem. C 2008, 112, 12495– 12506,  DOI: 10.1021/jp8044077
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    Effect of the Bending Potential on Molecular Arrangement in Alkaneselenolate Self-Assembled Monolayers
    Weidner, Tobias; Shaporenko, Andrey; Muller, Jan; Schmid, Martin; Cyganik, Piotr; Terfort, Andreas; Zharnikov, Michael
    Journal of Physical Chemistry C (2008), 112 (32), 12495-12506CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
    Self-assembled monolayers (SAMs) of hybrid 4,4'-biphenyl-substituted alkaneselenolates, CH3(C6H4)2(CH2)nSe- (BPnSe) with a variable length of the aliph. part (n = 1-6, 10, 11) have been prepd. on (111) gold and silver substrates and characterized by a variety of complementary exptl. techniques. The packing d. of the SAM constituents and the orientation of the biphenyl moieties were found to exhibit a pronounced "odd-even" variation with the no. (n) of methylene units in the aliph. linker of BPnSe moieties, which was opposite on silver as compared to gold. In particular, a smaller inclination and a corresponding higher packing d. of the biphenyl moieties was obsd. for odd nos. of the methylene units in BPnSe/Au and for even nos. of these units in BPnSe/Ag. The obsd. odd-even effects were explained by a significant bending potential, favoring definite orientation of the metal-Se-C bond and entering the balance of the structure-building interactions either cooperatively or competitively. The existence of this bending potential is supposed to be closely related to the exact bonding configuration of the headgroup atom, with the optimal substrate-Se-C angles being different for Au and Ag substrates. In view of the analogous behavior of thiolate-based systems, one can assume a common origin of the obsd. phenomena in chalcogen-based SAMs, in both of which the bonding configuration of the headgroup seems to be an important or even deciding factor in the balance of structure-building interactions.
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    Szelagowska-Kunstman, K.; Cyganik, P.; Schüpbach, B.; Terfort, A. Relative Stability of Thiol and Selenol Based SAMs on Au(111)—Exchange Experiments. Phys. Chem. Chem. Phys. 2010, 12, 4400– 4406,  DOI: 10.1039/b923274p
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    Relative stability of thiol and selenol based SAMs on Au(111) - exchange experiments
    Szelagowska-Kunstman, Katarzyna; Cyganik, Piotr; Schuepbach, Bjoern; Terfort, Andreas
    Physical Chemistry Chemical Physics (2010), 12 (17), 4400-4406CODEN: PPCPFQ; ISSN:1463-9076. (Royal Society of Chemistry)
    Two fully analog homolog series of thiol and selenol based arom. self-assembled monolayers (SAMs) on Au(111) as Me(C6H4)2(CH2)n-S-Au(111) (BPnS/Au(111), n = 2-6) and Me(C6H4)2(CH2)n-Se-Au(111) (BPnSe/Au(111), n = 2-6), resp., were used to elucidate the relative stability of the S-Au(111) and Se-Au(111) bonding by monitoring their exchange by alkanethiol and alkaneselenol mols. from their resp. solns. The exchange process was monitored using IR reflection absorption spectroscopy (IRRAS). Two main results obtained by these study are: (1) the selenium-based BPnSe/Au(111) series is significantly more stable than their sulfur analogs; (2) a clear odd-even effect exists for the stability of both BPnS/Au(111) and BPnSe/Au(111) SAMs towards exchange processes with the even-numbered systems being less stable. The results obtained are discussed in view of previously reported microscopic and spectroscopic data of the same SAMs addressing the issue of the relative stability of S-Au(111) and Se-Au(111) bonding, which is an important factor for the rational design of SAMs.
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    Dendzik, M.; Terfort, A.; Cyganik, P. Odd-even Effect in the Polymorphism of Self-Assembled Monolayers of Biphenyl-Substituted Alkaneselenolates on Au(111). J. Phys. Chem. C 2012, 116, 19535– 19542,  DOI: 10.1021/jp3051998
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    Odd-even effect in the polymorphism of self-assembled monolayers of biphenyl-substituted alkaneselenolates on Au(111)
    Dendzik, Maciej; Terfort, Andreas; Cyganik, Piotr
    Journal of Physical Chemistry C (2012), 116 (36), 19535-19542CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
    Self-assembled monolayers (SAMs) of ω-(4'-methylbiphenyl-4-yl) alkaneselenolates CH3(C6H4)2(CH2)nSe- (BPnSe, n = 2-6) on Au(111) substrates, prepd. in soln. at elevated temp. (333 K), were studied using scanning tunneling microscopy (STM). Molecularly resolved images reveal that even-numbered BPnSe SAMs form two or three different coexisting phases, including the one obsd. at room temp. In contrast, odd-numbered species exhibit only a single phase, which is the same as the one obsd. at room temp., i.e. close to a commensurate oblique (2√3×√3)R30° structure with two mols. per unit cell. Importantly, one of the phases obsd. for even-numbered BPnSe/Au(111) at room temp. (α-phase) has a well-defined periodicity only in 1D, whereas the new addnl. phases obsd. at elevated temp. are fundamentally different and have 2D periodic character, exhibiting a commensurate rectangular 5×2√3 lattice with four mols. per unit cell (β-phase) and an incommensurate oblique 2√3×1.2√3 lattice with two mols. per unit cell (γ-phase). For all systems, partial reorientation of the Au(111) step edges was obsd. upon SAM formation, indicating significant mobility of the topmost gold atoms induced by the adsorbates. To elucidate the effect of the S → Se substitution on the SAMs structure on Au(111), present results for BPnSe/Au(111) systems are discussed in view of the previously reported microscopic, spectroscopic, and desorption data obtained for these SAMs and for their thiol analogs, i.e. for BPnS/Au(111).
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    Frey, S.; Rong, H.-T.; Heister, K.; Yang, Y.-J.; Buck, M.; Zharnikov, M. Response of Biphenyl-Substituted Alkanethiol Self-Assembled Monolayers to Electron Irradiation: Damage Suppression and Odd-Even Effects. Langmuir 2002, 18, 3142– 3150,  DOI: 10.1021/la011288o
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    Response of Biphenyl-Substituted Alkanethiol Self-Assembled Monolayers to Electron Irradiation: Damage Suppression and Odd-Even Effects
    Frey, S.; Rong, H. T.; Heister, K.; Yang, Y. J.; Buck, M.; Zharnikov, M.
    Langmuir (2002), 18 (8), 3142-3150CODEN: LANGD5; ISSN:0743-7463. (American Chemical Society)
    The low-energy electron-induced damage in self-assembled monolayers (SAMs) formed from ω-(4'-methylbiphenyl-4-yl)alkanethiols Me(C6H4)2(CH2)nSH (BPn, n = 0, 1, 4, 5, and 12) on Au substrates was studied. The pristine and heavily (8000 μC/cm2) irradiated films were characterized in detail by XPS, near-edge x-ray absorption fine structure spectroscopy, IR reflection absorption spectroscopy, and advancing contact angle measurements. In contrast to SAMs of conventional alkanethiols but similar to pure arom. thiol-derived systems, only minor damage is obsd. for the aliph.-arom. BPn films. In particular, the orientational order and anchoring to the substrate are retained upon the irradn. At the same time, C-H bond scissions in the arom. part occur, leading to a crosslinking between the neighboring biphenyl moieties. Whereas the general behavior of the BPn SAMs with respect to electron irradn. is qual. similar, the extent of the irradn.-induced changes depends on the packing of these systems. The densely packed BP1 and BP5 SAMs are much more stable with respect to electron bombardment than the less densely packed BP4 films. The relation between the packing d. and the extent of the irradn.-induced changes seems to be a general phenomenon in monomol. films, which provides a tool to tailor the reaction of these systems toward ionizing radiation for lithog. applications.
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    Vervaecke, F.; Wyczawska, S.; Cyganik, P.; Bastiaansen, J.; Postawa, Z.; Silverans, R. E.; Vandeweert, E.; Lievens, P. Odd-Even Effects in Ion-Beam-Induced Desorption of Biphenyl-Substituted Alkanethiol Self-Assembled Monolayers. ChemPhysChem 2011, 12, 140– 144,  DOI: 10.1002/cphc.201000610
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    Odd-Even Effects in Ion-Beam-Induced Desorption of Biphenyl-Substituted Alkanethiol Self-Assembled Monolayers
    Vervaecke, Frederik; Wyczawska, Sabina; Cyganik, Piotr; Bastiaansen, Jeroen; Postawa, Zbigniew; Silverans, Roger E.; Vandeweert, Erno; Lievens, Peter
    ChemPhysChem (2011), 12 (1), 140-144CODEN: CPCHFT; ISSN:1439-4235. (Wiley-VCH Verlag GmbH & Co. KGaA)
    This study report on the anal. of ion-induced desorption of biphenyl-substituted (BPnS)/Au(111) with secondary neutral mass spectrometry (SNMS) as a new of approach to probe the strength mol.-substrate interaction in self-assembly monolayers (SAMs). The odd-even effect in the S-C bond scission, obsd. by desorption of the desulfurized fragment, is in antiphase when compared to the Au-S bond scission. The odd-even effect is also visible in the scission efficiency of the C-C bond that links two sep. parts of the mol. backbone, i.e., the alkane spacer and the biphenyl tail, resulting in a higher emission of the m/z = 168 fragment for even numbered BPnS/Au(111) SAMs. Besides a clear odd-even effect in the total desorption signal, it was obsd. that with increasing chain length the desorption efficiency for even-numbered BPnS/Au(111) SAMs gradually approaches the value which is typical for odd-numbered systems. The results obtained directly indicate the nature of these modifications as due to the systematic changes in the stability of Au-S and S-C bonds.
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    Cisneros, J. A.; Björklund, E.; González-Gil, I.; Hu, Y.; Canales, Á.; Medrano, F. J.; Romero, A.; Ortega-Gutiérrez, S.; Fowler, C. J.; López-Rodríguez, M. L. Structure–Activity Relationship of a New Series of Reversible Dual Monoacylglycerol Lipase/Fatty Acid Amide Hydrolase Inhibitors. J. Med. Chem. 2012, 55, 824– 836,  DOI: 10.1021/jm201327p
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    Structure-Activity Relationship of a New Series of Reversible Dual Monoacylglycerol Lipase/Fatty Acid Amide Hydrolase Inhibitors
    Cisneros, Jose A.; Bjorklund, Emmelie; Gonzalez-Gil, Ines; Hu, Yanling; Canales, Angeles; Medrano, Francisco J.; Romero, Antonio; Ortega-Gutierrez, Silvia; Fowler, Christopher J.; Lopez-Rodriguez, Maria L.
    Journal of Medicinal Chemistry (2012), 55 (2), 824-836CODEN: JMCMAR; ISSN:0022-2623. (American Chemical Society)
    The two endocannabinoids, anandamide (AEA) and 2-arachidonoylglycerol (2-AG), play independent and nonredundant roles in the body. This makes the development of both selective and dual inhibitors of their inactivation an important priority. In this work, we report a new series of inhibitors of monoacylglycerol lipase (MAGL) and fatty acid amide hydrolase (FAAH). Among them, (±)-oxiran-2-ylmethyl 6-(1,1'-biphenyl-4-yl)hexanoate (I) and (2R)-(-)-oxiran-2-ylmethyl(4-benzylphenyl)acetate (II) stand out as potent inhibitors of human recombinant MAGL (IC50 (I) = 4.1 μM; IC50 (II) = 2.4 μM), rat brain monoacylglycerol hydrolysis (IC50 (I) = 1.8 μM; IC50 (II) = 0.68 μM), and rat brain FAAH (IC50 (I) = 5.1 μM; IC50 (II) = 0.29 μM). Importantly, and in contrast to the other previously described MAGL inhibitors, these compds. behave as reversible inhibitors either of competitive (I) or noncompetitive nature (II). Hence, they could be useful to explore the therapeutic potential of reversible MAGL inhibitors.
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    Citrin, P. H.; Wertheim, G. K.; Baer, Y. Surface-Atom X-ray Photoemission from Clean Metals: Cu, Ag, and Au. Phys. Rev. B: Condens. Matter Mater. Phys. 1983, 27, 3160– 3175,  DOI: 10.1103/physrevb.27.3160
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    Surface-atom x-ray photoemission from clean metals: copper, silver, and gold
    Citrin, P. H.; Wertheim, G. K.; Baer, Y.
    Physical Review B: Condensed Matter and Materials Physics (1983), 27 (6), 3160-75CODEN: PRBMDO; ISSN:0163-1829.
    Surface-atom core-level shifts from evapd. noble metals are reported using angle-dependent x-ray photoemission with monochromatized Al Kα radiation. The absence of line broadening with increasing takeoff angle for the case of Al, where the surface shift is known to be small, confirms that the shifts obsd. in the noble metals are real surface phenomena. An anal. procedure was developed which establishes that the effect of the vacuum-solid interface in these systems is confined to the 1st at. layer. The asym. (final-state-related) line shapes of the surface- and bulk-atom photopeaks are also identical within exptl. error. The surface core-level shifts are -0.40 ± 0.02 eV for Au, -0.08 ± 0.03 eV for Ag, and -0.24 ± 0.02 eV for Cu, with the surface contributions occurring at lower electron binding energy. The anal. addnl. yields electron mean free paths of 19 ± 3 Å at 1400 eV in Au and 14 ± 3 Å at 550 eV in Cu. It was possible to isolate the contribution of the surface-atom valence electrons. The width of the surface d. of states is narrowed (8 ± 2)% with respect to the bulk d. of states and its center of gravity is shifted by -0.5 ± 0.1 eV. The anal. procedures and conclusions presented here should be applicable to core and valence surface-atom photoemission from a wide variety of other systems.
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    Krzykawska, A.; Ossowski, J.; Żaba, T.; Cyganik, P. Binding groups for highly ordered SAM formation: carboxylic versus thiol. Chem. Commun. 2017, 53, 5748– 5751,  DOI: 10.1039/c7cc01939d
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    Binding groups for highly ordered SAM formation: carboxylic versus thiol
    Krzykawska, A.; Ossowski, J.; Zaba, T.; Cyganik, P.
    Chemical Communications (Cambridge, United Kingdom) (2017), 53 (42), 5748-5751CODEN: CHCOFS; ISSN:1359-7345. (Royal Society of Chemistry)
    The structure of analogous SAMs chemisorbed on an Ag substrate via a -SH or -COOH binding group is reported. COOH binding groups can be considered as a promising alternative for the current std. based on the S binding groups, particularly when 2-dimensional ordering, SAM fabrication time and stability under ambient conditions are of great importance.
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    Heister, K.; Zharnikov, M.; Grunze, M.; Johansson, L. S. O. Adsorption of Alkanethiols and Biphenylthiols on Au and Ag Substrates: A High-Resolution X-ray Photoelectron Spectroscopy Study. J. Phys. Chem. B 2001, 105, 4058– 4061,  DOI: 10.1021/jp010127q
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    Adsorption of alkanethiols and biphenylthiols on Au and Ag substrates: A high-resolution x-ray photoelectron spectroscopy study
    Heister, K.; Zharnikov, M.; Grunze, M.; Johansson, L. S. O.
    Journal of Physical Chemistry B (2001), 105 (19), 4058-4061CODEN: JPCBFK; ISSN:1089-5647. (American Chemical Society)
    Synchrotron-based high-resoln. XPS was applied to monitor the formation of self-assembled monolayers (SAM) of alkanethiols (AT) and biphenylthiols on Au and Ag substrates. Pronounced chem. shifts in the adsorbate- and substrate-related photoemission lines upon SAM formation were obsd. Only 1 sulfur species could be detected in the S 2p spectra of the investigated SAMs, consistent with a thiolate bond. From the FWHM's of the core level photoemission spectra conclusions on the heterogeneity of the adsorption sites and adsorption geometry can be made. The exptl. data imply several (at least 2) slightly different adsorption geometries for the AT moieties in AT/Au. Significant final state effects in the C 1s photoemission were found for both the aliph. and arom. SAMs.
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    Dannenberger, O.; Weiss, K.; Himmel, H.-J.; Jäger, B.; Buck, M.; Wöll, C. An Orientation Analysis of Differently Endgroup-Functionalised Alkanethiols Adsorbed on Au Substrates. Thin Solid Films 1997, 307, 183– 191,  DOI: 10.1016/s0040-6090(97)00280-0
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    An orientation analysis of differently end group-functionalized alkanethiols adsorbed on Au substrates
    Dannenberger, O.; Weiss, K.; Himmel, H.-J.; Jager, B.; Buck, M.; Woll, Ch.
    Thin Solid Films (1997), 307 (1,2), 183-191CODEN: THSFAP; ISSN:0040-6090. (Elsevier Science S.A.)
    The mol. orientation in self-assembled films of methyl-, hydroxyl-, and carboxylic acid-terminated alkanethiols of different chain lengths (n-docosanethiol [DCT], 22-mercaptodocosanol [MDO], 22-mercaptodocosanoic acid [MDC], n-hexadecanethiol [HDT], 16-mercaptohexadecanol [MHO], 16-mercaptohexadecanoic acid [MHC]) on Au substrates has been investigated by near edge X-ray absorption fine structure (NEXAFS) spectroscopy and XPS. Whereas XPS-measurements do not reveal changes in thickness upon replacing the end-groups, the degree of orientation as detd. with NEXAFS exhibits significant differences. The two methyl-terminated thiols show the same alkyl-chain tilt-angle α of 39°. For the short-chain OH-terminated films no significant difference could be detected, but the monolayers formed from the long-chain OH-terminated chains exhibit a slightly stronger anisotropy and thus a smaller tilt-angle α than the corresponding CH3-terminated thiol. NEXAFS-spectra recorded for films from COOH-terminated thiols reveal only a very small anisotropy which indicates the absence of significant mol. orientation resulting from a high degree of disorder.
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    Lamont, C. L. A.; Wilkes, J. Attenuation Length of Electrons in Self-Assembled Monolayers of n-Alkanethiols on Gold. Langmuir 1999, 15, 2037– 2042,  DOI: 10.1021/la981168p
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    Attenuation length of electrons in self-assembled monolayers of n-alkanethiols on gold
    Lamont, Christine L. A.; Wilkes, John
    Langmuir (1999), 15 (6), 2037-2042CODEN: LANGD5; ISSN:0743-7463. (American Chemical Society)
    The interaction of both photoelectrons and X-rays with self-assembled monolayers of n-alkanethiols on gold has been measured using synchrotron radiation as the photon source in the energy range 140-1100 eV. The attenuation length of photoelectrons (λ) was found to vary from a min. of ∼5 Å at an electron kinetic energy (E) of 100 eV up to ∼23 Å at a kinetic energy of 1000 eV and can be described by the expression λ = 0.3E0.64 in the range 300-1000 eV. Exposure of the self-assembled monolayer to X-rays leads to fission of the C-S bond with a cross section of the order of 10-17 cm2 which displays no apparent dependence on the incident photon energy.
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  • Abstract

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    Figure 1

    Figure 1. C 1s, O 1s, and Ag 3d XPS spectra for native and electron-beam-irradiated (a–c) BP3COO/Ag samples using an electron energy of 50 eV and a total electron dose of 150 mC/cm2. The black and red lines mark signals for native and irradiated samples, respectively. For better presentation, intensities of the O 1s and Ag 3d5/2 spectra are multiplied by the factor presented in the figure.

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    Figure 2

    Figure 2. Electron-induced modification (electron energy 50 eV) of film thickness for BPnCOO/Ag (n = 2–6). (a) Film thickness as a function of the electron irradiation dose for the BP3COO/Ag sample. (b) Film thickness before and after (saturation level) electron irradiation. (c) Cross-section for this process. (d) Thickness reduction as a function of the parameter n.

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    Figure 3

    Figure 3. Electron-induced BE modification (electron energy 50 eV) of the main C 1s XPS peak for BPnCOO/Ag (n = 2–6). (a) BE as a function of the electron irradiation dose (energy 50 eV) for the BP3COO/Ag sample. (b) BE before and after electron irradiation (energy 50 eV, dose 150 mC/cm2) as a function of the parameter n, (c) cross-section for this process as a function of the parameter n, and (d) change in BE due to electron irradiation as a function of the number n.

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    Figure 4

    Figure 4. Electron-induced (electron energy 50 eV) fwhm modification of the main C 1s XPS peak for BPnCOO/Ag (n = 2–6). (a) C 1s fwhm as a function of the electron irradiation dose for BP3COO/Ag (electron energy 50 eV). (b) C 1s fwhm before and after (saturation level) electron irradiation as a function of the parameter n, (c) cross-section for this process as a function of the parameter n, and (d) change in fwhm due to electron irradiation as a function of the parameter n.

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    Figure 5

    Figure 5. Electron-induced modification (electron energy 50 eV) of the oxygen concentration for BPnCOO/Ag (n = 2–6) based on XPS data. (a) Normalized oxygen concentration as a function of the electron irradiation dose for BP3COO/Ag. (b) Oxygen concentration before and after (saturation level) electron irradiation as a function of the parameter n, (c) cross-section for this process as a function of the parameter n, and (d) oxygen reduction as a function of the number n.

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  • References

    This article references 60 other publications.

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      Schmid, Martin; Wan, Xianglong; Asyuda, Andika; Zharnikov, Michael
      Journal of Physical Chemistry C (2019), 123 (46), 28301-28309CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
      Well-defined alkanethiolate self-assembled monolayers (SAMs) represent a useful model system for a variety of thin org. films on solid supports, in particular in context of their modification by electron irradn. Taking a dodecanethiolate SAM on Au(111) as an example, the authors studied the primary electron energy dependence of the resp. processes at 10-500 eV. Cross sections of major irradn.-induced processes were derived, including thickness redn., progressing chem. inhomogeneity, and damage of the SAM-ambient and SAM-substrate interfaces. All the cross sections showed qual. similar behavior as functions of the primary electron energy, viz. a strong, progressive increase at 10-100 eV and a nearly energy independent character at the higher energies, up to 500 eV. This behavior was primary attributed to the energy dependence of the ionization cross section, with a certain effect of the secondary electron yield. Quant., the cross sections directly related to irradn.-induced bond breaking followed by desorption or relocation of the released fragments turned out to be noticeably higher (by a factor 2-2.5) than those representing the secondary processes in the SAM, such as reorganization of the SAM and SAM-ambient interface. The balance between the film decompn., resulting in material loss, and crosslinking, resulting in the preservation of the material, was found to shift toward the decompn. at lower primary electron energies as compared to higher ones.
    9. 9
      Geyer, W.; Stadler, V.; Eck, W.; Zharnikov, M.; Gölzhäuser, A.; Grunze, M. Electron-Induced Crosslinking of Aromatic Self-Assembled Monolayers: Negative Resists for Nanolithography. Appl. Phys. Lett. 1999, 75, 2401– 2403,  DOI: 10.1063/1.125027
      9
      Electron-induced crosslinking of aromatic self-assembled monolayers: Negative resists for nanolithography
      Geyer, W.; Stadler, V.; Eck, W.; Zharnikov, M.; Golzhauser, A.; Grunze, M.
      Applied Physics Letters (1999), 75 (16), 2401-2403CODEN: APPLAB; ISSN:0003-6951. (American Institute of Physics)
      We have explored the interaction of self-assembled monolayers of 1,1'-biphenyl-4-thiol (BPT) with low energy electrons. X-ray photoelectron, IR, and near edge x-ray absorption fine structure spectroscopy showed that BPT forms well-ordered monolayers with the Ph rings tilted ∼15° from the surface normal. The films were exposed to 50 eV electrons and changes were monitored in situ. Even after high (∼10 mC/cm2) exposures, the mols. maintain their preferred orientation and remain bonded on the gold substrate. An increased etching resistance and changes in the IR spectra imply a crosslinking between neighboring Ph groups, which suggests that BPT can be utilized as an ultrathin neg. resist. This is demonstrated by the generation of patterns in the underlying gold.
    10. 10
      Zharnikov, M.; Grunze, M. Modification of Thiol-Derived Self-Assembling Monolayers by Electron and X-Ray Irradiation: Scientific and Lithographic Aspects. J. Vac. Sci. Technol., B: Microelectron. Nanometer Struct.--Process., Meas., Phenom. 2002, 20, 1793– 1807,  DOI: 10.1116/1.1514665
      10
      Modification of thiol-derived self-assembling monolayers by electron and x-ray irradiation: scientific and lithographic aspects
      Zharnikov, M.; Grunze, M.
      Journal of Vacuum Science & Technology, B: Microelectronics and Nanometer Structures (2002), 20 (5), 1793-1807CODEN: JVTBD9; ISSN:0734-211X. (American Institute of Physics)
      A review. This article reviews recent expts. on the modification of thiol-derived self-assembling monolayers (SAMs) by electron and x-ray irradn. Several complementary exptl. techniques such as near-edge x-ray absorption fine structure spectroscopy, XPS and microscopy, and IR reflection absorption spectroscopy were applied to gain a detailed knowledge on the nature and extent of irradn.-induced damage in these systems. The reaction of a SAM to electron and x-ray irradn. was found to be detd. by the interplay of the damage/decompn. and crosslinking processes. Ways to adjust the balance between these two opposing effects by mol. engineering of the SAM constituents are demonstrated. The presented data provide the phys.-chem. basis for electron-beam patterning of self-assembled monolayers to extend lithog. down to the nanometer scale. A review with refs.
    11. 11
      Cyganik, P.; Vandeweert, E.; Postawa, Z.; Bastiaansen, J.; Vervaecke, F.; Lievens, P.; Silverans, R. E.; Winograd, N. Modification and Stability of Aromatic Self-Assembled Monolayers upon Irradiation with Energetic Particles. J. Phys. Chem. B 2005, 109, 5085– 5094,  DOI: 10.1021/jp0478209
      11
      Modification and Stability of Aromatic Self-Assembled Monolayers upon Irradiation with Energetic Particles
      Cyganik, P.; Vandeweert, E.; Postawa, Z.; Bastiaansen, J.; Vervaecke, F.; Lievens, P.; Silverans, R. E.; Winograd, N.
      Journal of Physical Chemistry B (2005), 109 (11), 5085-5094CODEN: JPCBFK; ISSN:1520-6106. (American Chemical Society)
      We have studied ion and electron irradn. of self-assembled monolayers (SAMs) of 2-(4'-methyl-biphenyl-4yl)-ethanethiol (BP2, CH3-C6H4C6H4CH2CH2-SH), Ph mercaptan (PEM, C6H5CH2CH2-SH), and 4'-methyl-biphenyl-4-thiol (BP0, CH3-C6H4C6H4-SH) deposited on Au(111) substrates. Desorption of neutral particles from PEM/Au and BP2/Au was investigated using laser ionization in combination with mass spectrometry. The ion-induced damage of both BP2 and PEM SAMs is very efficient and interaction with a single ion leads to the modification of tens of mols. This feature is the result of a desorption process caused by a chem. reaction initiated by an ion impact. Both for ions and electrons, expts. indicate that the possibility for scission of the Au-S bond strongly depends on the chem. nature of the SAM system. We attribute the possible origin of this effect to the orientation of the Au-S-C angle or adsorption sites of mols. The anal. of electron-irradiated PEM/Au and BP2/Au, using ion-initiated laser probing, enabled measurements of the cross section for the electron-induced damage of the intact mol. or specific fragment. Anal. of electron-irradiated BP0/Au by using time-of-flight secondary ion mass spectrometry (TOF-SIMS) provides direct evidence for the quasi-polymn. process induced by electron irradn.
    12. 12
      Turchanin, A.; Käfer, D.; El-Desawy, M.; Wöll, C.; Witte, G.; Gölzhäuser, A. Molecular Mechanisms of Electron-Induced Cross-Linking in Aromatic SAMs. Langmuir 2009, 25, 7342– 7352,  DOI: 10.1021/la803538z
      12
      Molecular Mechanisms of Electron-Induced Cross-Linking in Aromatic SAMs
      Turchanin, Andrey; Kaefer, Daniel; El-Desawy, Mohamed; Woell, Christof; Witte, Gregor; Goelzhaeuser, Armin
      Langmuir (2009), 25 (13), 7342-7352CODEN: LANGD5; ISSN:0743-7463. (American Chemical Society)
      When arom. self-assembled monolayers (SAMs) are electron-irradiated, intermol. cross-links are formed and the SAMs transform into carbon nanosheets with mol. thickness. These nanosheets have a very high mech. stability and can withstand temps. >1000 K. The authors study the electron induced crosslinking of 1,1'-biphenyl-4-thiol (BPT) SAMs on gold by combining XPS, x-ray absorption spectroscopy (NEXAFS), thermal desorption spectroscopy (TDS), and UPS. The exptl. data were acquired as a function of electron dose and temp. and compared with quantum chem. calcns. Details of the intermol. crosslinking, the microstructure of cross-linked films, and their structural transformations upon heating were obtained to derive a view of the mechanisms involved. The anal. shows that room-temp. electron irradn. causes a lateral crosslinking via the formation of C-C linked Ph species as well as a new sulfur species. The thermal stability of the BPT films increases with the electron dose and sats. at ∼50 mC/cm2. Nevertheless, nonlinked fragments in the thermal desorption spectra indicate an incomplete crosslinking even at high doses, which can be attributed to steric reasons and quenching due to the reduced band gap of partially linked mols. At temps. >800 K, all sulfur species are thermally desorbed, while the remaining film reveals an onset of carbonization.
    13. 13
      Eck, W.; Stadler, V.; Geyer, W.; Zharnikov, M.; Gölzhäuser, A.; Grunze, M. Generation of Surface Amino Groups on Aromatic Self-Assembled Monolayers by Low Energy Electron Beams - A First Step Towards Chemical Lithography. Adv. Mater. 2000, 12, 805– 808,  DOI: 10.1002/(sici)1521-4095(200006)12:11<805::aid-adma805>3.0.co;2-0
      13
      Generation of surface amino groups on aromatic self-assembled monolayers by low energy electron beams - a first step towards chemical lithography
      Eck, Wolfgang; Stadler, Volker; Geyer, Wolfgang; Zharnikov, Michael; Golzhauser, Armin; Grunze, Michael
      Advanced Materials (Weinheim, Germany) (2000), 12 (11), 805-808CODEN: ADVMEW; ISSN:0935-9648. (Wiley-VCH Verlag GmbH)
      The transformation of surface nitro groups to amino groups was obsd. during electron-beam exposure of biphenylthiol monolayers deposited on gold surface. The hydrogen atoms required for redn. of the nitro groups were generated by the electron-induced dissocn. of the C-H bonds in the biphenyl units. As a consequence, the monolayer was crosslinked and resistant towards chem. etching agents or displacement by alkane thiols. The resulting amino groups could be further chem. modified, as was shown by acetylation with trifluoroacetic acid anhydride. This electron-induced formation of nucleophilic surface groups may serve as a basis for a soft and chem. defined lithog. Covalent coupling of functional entities to the irradiated regions may allow generation of chem. defined nanostructures on amino-terminated patterns within a nitro-terminated surface. Future expts. will examine the attainable size resoln. when electron beam lithog. and ultrasharp field emitters are used for patterning.
    14. 14
      Turchanin, A.; Schnietz, M.; El-Desawy, M.; Solak, H. H.; David, C.; Gölzhäuser, A. Fabrication of Molecular Nanotemplates in Self-Assembled Monolayers by Extreme-Ultraviolet-Induced Chemical Lithography. Small 2007, 3, 2114– 2119,  DOI: 10.1002/smll.200700516
      14
      Fabrication of molecular nanotemplates in self-assembled monolayers by extreme-ultraviolet-induced chemical lithography
      Turchanin, Andrey; Schnietz, Mark; El-Desawy, Mohamed; Solak, Harun H.; David, Christian; Goelzhaeuser, Armin
      Small (2007), 3 (12), 2114-2119CODEN: SMALBC; ISSN:1613-6810. (Wiley-VCH Verlag GmbH & Co. KGaA)
      Extreme-UV interference lithog. (EUV-IL) is applied to create chem. nanopatterns in self-assembled monolayers (SAMs) of 4'-nitro-1,1'-biphenyl-4-thiol (NBPT) on gold. XPS shows that EUV irradn. induces both the conversion of the terminal nitro groups of NBPT into amino groups and the lateral crosslinking of the underlying arom. cores. Large-area (≈2 mm2) nitro/amino chem. patterns with periods ranging from 2000 nm to 60 nm can be generated. Regions of pristine NBPT on the exposed samples are exchanged with protein-resistant thiol SAMs of polyethyleneglycol, resulting in the formation of mol. nanotemplates, which can serve as the basis of complex biomimetic surfaces.
    15. 15
      Turchanin, A.; Tinazli, A.; El-Desawy, M.; Großmann, H.; Schnietz, M.; Solak, H. H.; Tampé, R.; Gölzhäuser, A. Molecular Self-Assembly, Chemical Lithography, and Biochemical Tweezers: A Path for the Fabrication of Functional Nanometer-Scale Protein Arrays. Adv. Mater. 2008, 20, 471– 477,  DOI: 10.1002/adma.200702189
      15
      Molecular self-assembly, chemical lithography, and biochemical tweezers: a path for the fabrication of functional nanometer-scale protein arrays
      Turchanin, Andrey; Tinazli, Ali; El-Desawy, Mohamed; Grossmann, Helge; Schnietz, Mark; Solak, Harun H.; Tampe, Robert; Goelzhaeuser, Armin
      Advanced Materials (Weinheim, Germany) (2008), 20 (3), 471-477CODEN: ADVMEW; ISSN:0935-9648. (Wiley-VCH Verlag GmbH & Co. KGaA)
      Electron-induced chem. lithog. combined with self-assembled monolayers and multivalent chelators for high-affinity capturing of His-tagged proteins are used to obtain specific, stable, highly parallel, and functional protein micro- and nanoarrays on solid substrates. The functionality of the generated large-area protein arrays is shown in situ via specific, homogeneous, oriented and reversible immobilization of His6-tagged 20S proteasome and fluorescence labeled His10-tagged maltose binding proteins.
    16. 16
      Ballav, N.; Thomas, H.; Winkler, T.; Terfort, A.; Zharnikov, M. Making Protein Patterns by Writing in a Protein-Repelling Matrix. Angew. Chem., Int. Ed. 2009, 48, 5833– 5836,  DOI: 10.1002/anie.200900950
      16
      Making Protein Patterns by Writing in a Protein-Repelling Matrix
      Ballav, Nirmalya; Thomas, Heidi; Winkler, Tobias; Terfort, Andreas; Zharnikov, Michael
      Angewandte Chemie, International Edition (2009), 48 (32), 5833-5836, S5833/1-S5833/8CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
      The authors show that protein-repelling films, both self-assembled monolayer (SAM)- and polymer-like, can be used as a primary matrix for direct electron-beam lithog. (EBL) of both nonspecific and specific protein patterns. The authors used protein-repelling SAMs of oligoethylene glycol-substituted alkane-thiols on evapd. Au(111) substrates. The required irradn. dose is smaller by two orders of magnitude than for an alternative, multistep EBL approach. The nonspecific template can be easily converted into a specific one by adsorption of a mediator protein with specific binding sites for the secondary target protein. The EBL allows the generation of patterns over a length scale ranging from centimeters to nanometers with no limitations to the pattern shape, including complex, gradient-like assays.
    17. 17
      Ballav, N.; Schlip, S.; Zharnikov, M. Electron Beam Chemical Lithography with Aliphatic Self-Assembled Monolayers. Angew. Chem., Int. Ed. 2008, 47, 1421– 1424,  DOI: 10.1002/anie.200704105
      17
      Electron-beam chemical lithography with aliphatic self-assembled monolayers
      Ballav, Nirmalya; Schilp, Soeren; Zharnikov, Michael
      Angewandte Chemie, International Edition (2008), 47 (8), 1421-1424CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
      Electron-beam chem. lithog. using aliph. self-assembled monolayer (SAMs) resists is demonstrated. Fabrication of polymer micro- and nanobrushes is demonstrated. Patterning requires a much lower dose than electron-beam chem. lithog. using arom. SAMs resists.
    18. 18
      Küller, A.; Eck, W.; Stadler, V.; Geyer, W.; Gölzhäuser, A. Nanostructuring of Silicon by Electron-Beam Lithography of Self-Assembled Hydroxybiphenyl Monolayers. Appl. Phys. Lett. 2003, 82, 3776– 3778,  DOI: 10.1063/1.1578537
      There is no corresponding record for this reference.
    19. 19
      Abufager, P. N.; Solano Canchaya, J. G.; Wang, Y.; Alcamí, M.; Martín, F.; Alvarez Soria, L.; Martiarena, M. L.; Reuter, K.; Busnengo, H. F. Theoretical Study of the Structure of Self-Assembled Monolayers of Short Alkylthiolates on Au(111) and Ag(111): The Role of Induced Substrate Reconstruction and Chain–Chain Interactions. Phys. Chem. Chem. Phys. 2011, 13, 9353– 9362,  DOI: 10.1039/c0cp02490b
      19
      Theoretical study of the structure of self-assembled monolayers of short alkylthiolates on Au(111) and Ag(111): the role of induced substrate reconstruction and chain-chain interactions
      Abufager, P. N.; Solano Canchaya, J. G.; Wang, Y.; Alcami, M.; Martin, F.; Alvarez Soria, L.; Martiarena, M. L.; Reuter, K.; Busnengo, H. F.
      Physical Chemistry Chemical Physics (2011), 13 (20), 9353-9362CODEN: PPCPFQ; ISSN:1463-9076. (Royal Society of Chemistry)
      We compare the stability of various structures of high coverage self-assembled monolayers (SAMs) of short alkylthiolates, S(CH2)n-1CH3 (= Cn), on Ag(111) and Au(111). We employ: (i) the ab initio thermodn. approach based on d. functional theory (DFT) calcns., to compare the stability of SAMs of C1 (with coverages Θ = 3/7 and 1/3) on both substrates, and (ii) a set of pairwise interat. potentials derived from second-order Moller-Plesset (MP2) perturbation theory calcns., to est. the role of chain-chain (Ch-Ch) interactions in the structure and stability of SAMs of longer chain alkylthiolates. For C1/Ag(111) (C1/Au(111)) the SAM with Θ = 3/7 is more (less) stable than for Θ = 1/3 in a wide range of temps. and pressures in line with expts. In addn., for the mol. densities of SAMs corresponding to Θ = 3/7 and 1/3, the MP2-based Ch-Ch interaction potential also predicts the different chain orientations obsd. exptl. in SAMs of alkylthiolates on Ag(111) and Au(111). Thus, for short length alkylthiolates, a simple model based on first principles calcns. that sep. accounts for mol.-surface (M-S) and Ch-Ch interactions succeeds in predicting the main structural differences between the full coverage SAMs usually obsd. exptl. on Ag(111) and Au(111).
    20. 20
      Ballav, N.; Shaporenko, A.; Terfort, A.; Zharnikov, M. A Flexible Approach to the Fabrication of Chemical Gradients. Adv. Mater. 2007, 19, 998– 1000,  DOI: 10.1002/adma.200602168
      20
      A flexible approach to the fabrication of chemical gradients
      Ballav, Nirmalya; Shaporenko, Andrey; Terfort, Andreas; Zharnikov, Michael
      Advanced Materials (Weinheim, Germany) (2007), 19 (7), 998-1000CODEN: ADVMEW; ISSN:0935-9648. (Wiley-VCH Verlag GmbH & Co. KGaA)
      Chem. gradients on different substrates is fabricated by tuning the exchange reaction between a self-assembled monolayer covering the substrate and a potential mol. substituent by electron irradn. A hydrophilicity gradient was created using this method.
    21. 21
      Sauter, E.; Yildirim, C.; Terfort, A.; Zharnikov, M. Adjustment of the Work Function of Pyridine and Pyrimidine Substituted Aromatic Self-Assembled Monolayers by Electron Irradiation. J. Phys. Chem. C 2017, 121, 12834– 12841,  DOI: 10.1021/acs.jpcc.7b03302
      21
      Adjustment of the Work Function of Pyridine and Pyrimidine Substituted Aromatic Self-Assembled Monolayers by Electron Irradiation
      Sauter, Eric; Yildirim, Can; Terfort, Andreas; Zharnikov, Michael
      Journal of Physical Chemistry C (2017), 121 (23), 12834-12841CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
      Self-assembled monolayers (SAMs) are frequently used to manipulate injection barriers in org. electronics by introduction of a specific dipole moment at the interfaces between the electrodes and adjacent org. layers. This is usually achieved by the selection of a proper dipolar terminal tail group comprising the SAM-ambient interface, which was recently complemented by embedding such a group into the mol. backbone. Here we show that the work function of SAMs can also be adjusted by electron irradn. in a quite broad range and in controlled fashion as far as these films contain pyridine or pyrimidine group. This effect is demonstrated by the example of several representative arom. SAMs with either terminal pyridine group or embedded pyrimidine group. The obsd. behavior is presumably related to specific chem. transformations involving the nitrogen atoms in these moieties. The SAMs with the embedded pyrimidine group are then esp. attractive because this moiety is decoupled from the SAM-ambient interface. The extent of the effect is very large (a work function change of up to ∼0.8 eV) as far as it is monitored in situ but is diminished upon the exposure of the irradiated films to ambient. Practical implications of this effect are discussed, including work function lithog., which is demonstrated by representative patterns.
    22. 22
      Neumann, C.; Kaiser, D.; Mohn, M. J.; Füser, M.; Weber, N.-E.; Reimer, O.; Gölzhäuser, A.; Weimann, T.; Terfort, A.; Kaiser, U. Bottom-Up Synthesis of Graphene Monolayers with Tunable Crystallinity and Porosity. ACS Nano 2019, 13, 7310– 7322,  DOI: 10.1021/acsnano.9b03475
      22
      Bottom-Up Synthesis of Graphene Monolayers with Tunable Crystallinity and Porosity
      Neumann, Christof; Kaiser, David; Mohn, Michael J.; Fueser, Matthias; Weber, Nils-Eike; Reimer, Oliver; Goelzhaeuser, Armin; Weimann, Thomas; Terfort, Andreas; Kaiser, Ute; Turchanin, Andrey
      ACS Nano (2019), 13 (6), 7310-7322CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
      We present a method for a bottom-up synthesis of atomically thin graphene sheets with tunable crystallinity and porosity using arom. self-assembled monolayers (SAMs) as mol. precursors. To this end, we employ SAMs with pyridine and pyrrole constituents on polycryst. copper foils and convert them initially into mol. nanosheets-carbon nanomembranes (CNMs)-via low-energy electron irradn. induced crosslinking and then into graphene monolayers via pyrolysis. As the nitrogen atoms are leaving the nanosheets during pyrolysis, nanopores are generated in the formed single-layer graphene. We elucidate the structural changes upon the crosslinking and pyrolysis down to the at. scale by complementary spectroscopy and microscopy techniques including X-ray photoelectron and Raman spectroscopy, LEED, at. force, helium ion, and high-resoln. transmission electron microscopy, and elec. transport measurements. We demonstrate that the crystallinity and porosity of the formed graphene can be adjusted via the choice of mol. precursors and pyrolysis temp., and we present a kinetic growth model quant. describing the conversion of mol. CNMs into graphene. The synthesized nanoporous graphene monolayers resemble a percolated network of graphene nanoribbons with a high charge carrier mobility (∼600 cm2/(V s)), making them attractive for implementations in electronic field-effect devices.
    23. 23
      Matei, D. G.; Weber, N.-E.; Kurasch, S.; Wundrack, S.; Woszczyna, M.; Grothe, M.; Weimann, T.; Ahlers, F.; Stosch, R.; Kaiser, U. Functional Single-Layer Graphene Sheets from Aromatic Monolayers. Adv. Mater. 2013, 25, 4146– 4151,  DOI: 10.1002/adma.201300651
      23
      Functional Single-Layer Graphene Sheets from Aromatic Monolayers
      Matei, Dan G.; Weber, Nils-Eike; Kurasch, Simon; Wundrack, Stefan; Woszczyna, Miroslaw; Grothe, Miriam; Weimann, Thomas; Ahlers, Franz; Stosch, Rainer; Kaiser, Ute; Turchanin, Andrey
      Advanced Materials (Weinheim, Germany) (2013), 25 (30), 4146-4151CODEN: ADVMEW; ISSN:0935-9648. (Wiley-VCH Verlag GmbH & Co. KGaA)
      Self-assembled monolayers of arom. mols. on copper substrates can be converted into high-quality single-layer graphene using low-energy electron irradn. and subsequent annealing. This two-dimensional solid state transformation is characterized on the at. scale and the phys. and chem. properties of the formed graphene sheets are studied by complementary microscopic and spectroscopic techniques and by elec. transport measurements. As substrates, Cu(111) single crystals and the technol. relevant polycryst. copper foils are successfully used.
    24. 24
      Turchanin, A.; Gölzhäuser, A. Carbon Nanomembranes from Self-Assembled Monolayers: Functional Surfaces without Bulk. Prog. Surf. Sci. 2012, 87, 108– 162,  DOI: 10.1016/j.progsurf.2012.05.001
      24
      Carbon nanomembranes from self-assembled monolayers: Functional surfaces without bulk
      Turchanin, Andrey; Goelzhaeuser, Armin
      Progress in Surface Science (2012), 87 (5-8), 108-162CODEN: PSSFBP; ISSN:0079-6816. (Elsevier B.V.)
      In this topical review we describe the fabrication, characterization and applications of 1 nm thick, mech. stable carbon nanomembranes (CNMs). They represent a new type of functional two-dimensional (2D) materials, which can be concisely described as "surfaces without bulk". Because CNMs are made by electron-induced crosslinking of arom. self-assembled monolayers (SAMs), we start with an overview of SAMs with a special emphasis on arom. SAMs. We describe the chem. modification of SAMs by electron, ion and photon irradn., introduce the concepts of irradn.-induced crosslinking and chem. nanolithog. of arom. SAMs and discuss the underlying phys. and chem. mechanisms. We present examples for applications of these phenomena in the engineering of complex surface architectures, e.g., nanopatterns of proteins, fluorescent dyes or polymer brushes. Then we introduce a transfer procedure to release cross-linked arom. SAMs from their original substrates and to form free-standing CNMs. We discuss mech. and elec. properties of CNMs and demonstrate that they can be converted into graphene upon annealing. This transformation opens an original and flexible mol. route towards the large-scale synthesis of graphene sheets with tunable properties. Finally, we demonstrate the lithog. and chem. tailoring of CNMs to fabricate novel functional 2D carbon materials: supports for high resoln. transmission electron microscopy (HRTEM) and nanolithog., nanosieves, Janus nanomembranes, polymer carpets, complex layered structures. Prospects of combining different types of nanomembranes made of SAMs (CNMs, graphene, nanosieves, Janus nanomembranes) towards the engineering of novel functional nanomaterials for a variety of electronic, optical, lab-on-a-chip and micro-/nanomech. (MEMS/NEMS) devices are discussed.
    25. 25
      Turchanin, A.; Beyer, A.; Nottbohm, C. T.; Zhang, X.; Stosch, R.; Sologubenko, A.; Mayer, J.; Hinze, P.; Weimann, T.; Gölzhäuser, A. One Nanometer Thin Carbon Nanosheets with Tunable Conductivity and Stiffness. Adv. Mater. 2009, 21, 1233– 1237,  DOI: 10.1002/adma.200803078
      25
      One nanometer thin carbon nanosheets with tunable conductivity and stiffness
      Turchanin, Andrey; Beyer, Andre; Nottbohm, Christoph T.; Zhang, Xianghui; Stosch, Rainer; Sologubenko, Alla; Mayer, Joachim; Hinze, Peter; Weimann, Thomas; Goelzhaeuser, Armin
      Advanced Materials (Weinheim, Germany) (2009), 21 (12), 1233-1237CODEN: ADVMEW; ISSN:0935-9648. (Wiley-VCH Verlag GmbH & Co. KGaA)
      Atomically thin (∼1 nm) carbon films and membranes whose elec. behavior can be tuned from insulating to conducting are fabricated by a novel route. These films present arbitrary size and shape based on mol. self-assembly, electron irradn., and pyrolysis, and their tech. applicability is demonstrated by their incorporation into a microscopic pressure sensor.
    26. 26
      Turchanin, A.; Gölzhäuser, A. Carbon Nanomembranes. Adv. Mater. 2016, 28, 6075– 6103,  DOI: 10.1002/adma.201506058
      26
      Carbon Nanomembranes
      Turchanin, Andrey; Goelzhaeuser, Armin
      Advanced Materials (Weinheim, Germany) (2016), 28 (29), 6075-6103CODEN: ADVMEW; ISSN:0935-9648. (Wiley-VCH Verlag GmbH & Co. KGaA)
      A review. Carbon nanomembranes (CNMs) are synthetic 2D carbon sheets with tailored phys. or chem. properties. These depend on the structure, mol. compn., and surroundings on either side. Due to their mol. thickness, they can be regarded as "interfaces without bulk" sepg. regions of different gaseous, liq., or solid components and controlling the materials exchange between them. Here, a universal scheme for the fabrication of 1 nm-thick, mech. stable, functional CNMs is presented. CNMs can be further modified, for example perforated by ion bombardment or chem. functionalized by the binding of other mols. onto the surfaces. The underlying phys. and chem. mechanisms are described, and examples are presented for the engineering of complex surface architectures, e.g., nanopatterns of proteins, fluorescent dyes, or polymer brushes. A simple transfer procedure allows CNMs to be placed on various support structures, which makes them available for diverse applications: supports for electron and X-ray microscopy, nanolithog., nanosieves, Janus nanomembranes, polymer carpets, complex layered structures, functionalization of graphene, novel nanoelectronic and nanomech. devices. To close, the potential of CNMs in filtration and sensorics is discussed. Based on tests for the sepn. of gas mols., it is argued that ballistic membranes may play a prominent role in future efforts of materials sepn.
    27. 27
      Turchanin, A. Synthesis of Molecular 2D Materials via Low-Energy Electron Induced Chemical Reactions. Chimia 2019, 73, 473– 479,  DOI: 10.2533/chimia.2019.473
      27
      Synthesis of molecular 2D materials via low-energy electron induced chemical reactions
      Turchanin, Andrey
      Chimia (2019), 73 (6), 473-479CODEN: CHIMAD ISSN:. (Swiss Chemical Society)
      After the demonstration of a variety of inorg. two-dimensional (2D) materials (graphene, hBN, MoS2, etc.), mol. 2D materials have attracted a significant research interest as well. However, the direct synthesis of these materials is an exceptionally challenging task for chemists. In this review article, a simple and robust phys. method for the synthesis of mol. 2D materials is presented based on low-energy electron induced chem. reactions in arom. mol. layers. In this way, ultrathin (∼1 nm) mol. nanosheets with adjustable chem. and phys. properties called Carbon Nanomembranes (CNM) can be prepd. Moreover, the method enables the synthesis of various other 2D org.-inorg. hybrids (e.g.MoS2-CNM, graphene-CNM lateral heterostructures, etc.) or ∼20 nm thick nanosheets of org. semiconductors. Mechanisms of the reaction and functional properties of these mol. 2D materials including their chem. functionalization and engineering of hybrid hierarchical structures for application in nanoscience and nanotechnol. are discussed in this article.
    28. 28
      Neumann, C.; Szwed, M.; Frey, M.; Tang, Z.; Kozieł, K.; Cyganik, P.; Turchanin, A. Preparation of Carbon Nanomembranes without Chemically Active Groups. ACS Appl. Mater. Interfaces 2019, 11, 31176– 31181,  DOI: 10.1021/acsami.9b09603
      28
      Preparation of Carbon Nanomembranes without Chemically Active Groups
      Neumann, Christof; Szwed, Monika; Frey, Martha; Tang, Zian; Koziel, Krzysztof; Cyganik, Piotr; Turchanin, Andrey
      ACS Applied Materials & Interfaces (2019), 11 (34), 31176-31181CODEN: AAMICK; ISSN:1944-8244. (American Chemical Society)
      The electron-irradn.-induced synthesis of carbon nanomembranes (CNMs) from arom. thiol-based self-assembled monolayers (SAMs) on gold substrate is a well-established method to form mol. thin nanosheets. These mol. two-dimensional materials can be prepd. with tunable properties; therefore, they find a variety of applications in nanotechnol. ranging from ultrafiltration to nanobiosensors. However, no chem. inert CNM was fabricated up to now, as the reactive thiol group is present on the membrane surface even after transferring it to other substrates. Here, we study the electron irradn. of carboxylic acid-based SAMs on a silver substrate as an alternative route for CNM formation. Our anal., based on a combination of XPS and SEM demonstrates that for this type of SAMs, purely carbonaceous CNMs with tunable porosity can be obtained.
    29. 29
      Dementyev, P.; Naberezhnyi, D.; Westphal, M.; Buck, M.; Gölzhäuser, A. Carbon Nanomembranes from Aromatic Carboxylate Precursors. ChemPhysChem 2020, 21, 1006– 1011,  DOI: 10.1002/cphc.202000150
      29
      Carbon Nanomembranes from Aromatic Carboxylate Precursors
      Dementyev, Petr; Naberezhnyi, Daniil; Westphal, Michael; Buck, Manfred; Goelzhaeuser, Armin
      ChemPhysChem (2020), 21 (10), 1006-1011CODEN: CPCHFT; ISSN:1439-4235. (Wiley-VCH Verlag GmbH & Co. KGaA)
      Self-assembled monolayers (SAMs) serve as convenient platform for fabricating carbon nanomembranes (CNMs) of extended lateral dimensions. Highly porous CNMs are emerging as interesting materials for membrane technologies as they exhibit selectivity for water permeation and, owing to their reduced dimensionality, promise increased energy efficiency compared to established systems. In the present study terphenylcarboxylate SAMs, prepd. on silver underpotential deposited on Au and irradiated by 100 eV electrons, were successfully converted into free-standing CNMs. IR and XPS reveal pronounced chem. changes both of the anchoring carboxylate moiety and the arom. backbone upon electron irradn. Permeation studies showed high specificity for water as demonstrated by the sepn. from THF. Compared to thiols on gold, the std. CNM precursor system, the carboxylic acid based SAM exhibits equiv. characteristics. This suggests that electron-induced carbonization is insensitive to the particular choice of the anchor moiety and, therefore, the choice of precursor mols. can be extended to the versatile class of arom. carboxylic acids.
    30. 30
      Asyuda, A.; de la Morena, R. O.; Sauter, E.; Turner, K.; McDonald, K.; Buck, M.; Zharnikov, M. Electron-Induced Modification of Self-Assembled Monolayers of Aromatic Carboxylic Acids. J. Phys. Chem. C 2020, 124, 25107– 25120,  DOI: 10.1021/acs.jpcc.0c07577
      30
      Electron-Induced Modification of Self-Assembled Monolayers of Aromatic Carboxylic Acids
      Asyuda, Andika; de la Morena, Rodrigo Ortiz; Sauter, Eric; Turner, Kelly; McDonald, Kirsty; Buck, Manfred; Zharnikov, Michael
      Journal of Physical Chemistry C (2020), 124 (45), 25107-25120CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
      The effects of low-energy electrons on arom. self-assembled monolayers (SAMs) with carboxylic acid (CA) docking groups were studied with a focus on the dose range below 5 mC/cm2. The SAMs were prepd. on an underpotentially deposited Ag bilayer and comprised nonsubstituted and CA-substituted monolayers with a rod-like biphenyl backbone and a monolayer of a Y-shaped, CA-substituted mol., 1,3,5-benzenetribenzoic acid (H3BTB), formed either as a single-component film or as a binary one by mixing with adamantane-CA (Ad-CA). X-ray photoelectron and near-edge X-ray absorption fine structure spectra suggest high proneness of the CA groups to electron irradn. at both SAM/substrate and SAM/ambient interfaces. Cleavage of the carboxylate-substrate bond results in substantial mol. desorption at the initial stage of irradn. until electron-induced crosslinking gradually takes over. The CA groups at the outer SAM interface undergo substantial chem. changes, indicating that they participate in the crosslinking chem. The electron-induced processes are accompanied by mol. reorientation. Disordering for the SAMs formed by the rod-like mols. is contrasted by the H3BTB-based systems where changes also occur but some mol. order is preserved as explained by a proposed model invoking conformational changes. In SAMs of H3BTB mixed with Ad-CA, the latter shows higher proneness to irradn.-induced desorption than the former, as well as an influence on the crosslinking chem. The results of the present study suggest that CA-based SAMs on Ag offer addnl. options for crosslinking in SAMs and, as exemplarily demonstrated by the generation of Cu patterns on structured H3BTB templates, can be efficiently used for lithog. and nanofabrication.
    31. 31
      Krzykawska, A.; Szwed, M.; Ossowski, J.; Cyganik, P. Odd–Even Effect in Molecular Packing of Self-Assembled Monolayers of Biphenyl-Substituted Fatty Acid on Ag(111). J. Phys. Chem. C 2018, 122, 919– 928,  DOI: 10.1021/acs.jpcc.7b10806
      31
      Odd-Even Effect in Molecular Packing of Self-Assembled Monolayers of Biphenyl-Substituted Fatty Acid on Ag(111)
      Krzykawska, Anna; Szwed, Monika; Ossowski, Jakub; Cyganik, Piotr
      Journal of Physical Chemistry C (2018), 122 (1), 919-928CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
      Self-assembled monolayers (SAMs) of the homologous series of biphenyl-substituted fatty acids on Ag(111) in the form of (C6H4)2-(CH2)n-COO/Ag (BPnCOO/Ag, n = 1-4) were studied using IR reflection absorption spectroscopy (IRRAS), XPS, and scanning tunneling microscopy (STM). The combination of spectroscopic (IRRAS and XPS) and microscopic analyses (STM) revealed that depending on the parity of the parameter n, which defines the length of the short aliph. linker, two types of structures are formed by the BPnCOO/Ag. For n = even, highly ordered and stable SAMs are formed in a very short time. For n = odd, the resp. monolayers have a disordered liquidlike structure with more canted orientation of the mol. backbone and the anchoring carboxylic group, which results in lower packing d. and film thickness compared to the even-numbered SAMs. By comparing obtained results with former odd-even effects reported for analogous SAMs based on thiols and selenols, a common, qual. model relating the odd-even effect to the monolayer stability and structure is discussed. For BPnCOO/Ag, this odd-even effect is particularly strong and fully controls the ability of mols. to form highly ordered structures. This observation seems to be of key importance for the design of SAMs based on the carboxylic group, which, for correctly designed mols., forms much better two-dimensionally ordered structures compared to commonly used thiols.
    32. 32
      Tao, F.; Bernasek, S. L. Understanding Odd-Even Effects in Organic Self-Assembled Monolayers. Chem. Rev. 2007, 107, 1408– 1453,  DOI: 10.1021/cr050258d
      32
      Understanding Odd-Even Effects in Organic Self-Assembled Monolayers
      Tao, Feng; Bernasek, Steven L.
      Chemical Reviews (Washington, DC, United States) (2007), 107 (5), 1408-1453CODEN: CHREAY; ISSN:0009-2665. (American Chemical Society)
      A review on the structure and property odd-even effects of various org. monolayers self-assembled on solid surfaces. It is organized by the class of solid substrate on which org. self-assembled monolayers were formed and includes HOPG, Mo disulfide (MoS2), metal substrates including Au (111), Ag (111), Cu, Al, and Hg, and inorg. compd. substrates Al2O3 and SiOx/Si. For each substrate, the presentation is categorized by the different series of org. mols. The odd-even effects seen for the different categories of org. mols. on the same substrate are compared when they are described. Following the description of structural effects, the induced odd-even differences in various properties and functions of these thin films are rationalized. Finally, the origin and features of odd-even effects on different substrates are contrasted and discussed.
    33. 33
      Zharnikov, M.; Frey, S.; Rong, H.; Yang, Y.-J.; Heister, K.; Buck, M.; Grunze, M. The Effect of the Sulfur-Metal Bond on the Structure of Self-Assembled Monolayers. Phys. Chem. Chem. Phys. 2000, 2, 3359– 3362,  DOI: 10.1039/b004232n
      33
      The effect of sulfur-metal bonding on the structure of self-assembled monolayers
      Zharnikov, M.; Frey, S.; Rong, H.; Yang, Y.-J.; Heister, K.; Buck, M.; Grunze, M.
      Physical Chemistry Chemical Physics (2000), 2 (15), 3359-3362CODEN: PPCPFQ; ISSN:1463-9076. (Royal Society of Chemistry)
      The equil. structure of alkanethiol monolayers self-assembled on metal substrates is detd. by a delicate interplay of the intermol. chain-chain and chemisorptive substrate-head group interactions. To understand the role of the individual constituents of this interplay, we studied the structure of biphenyl and perfluoroalkyl terminated alkanethiol monolayers self-assembled on Au and Ag. The structural characteristics of the monolayers derived from NEXAFS, FTIRRAS and XPS measurements point to a decisive role of the directional substrate-head group interactions.
    34. 34
      Rong, H.-T.; Frey, S.; Yang, Y.-J.; Zharnikov, M.; Buck, M.; Wühn, M.; Wöll, C.; Helmchen, G. On the Importance of the Headgroup Substrate Bond in Thiol Monolayers: A Study of Biphenyl-Based Thiols on Gold and Silver. Langmuir 2001, 17, 1582– 1593,  DOI: 10.1021/la0014050
      34
      On the Importance of the Headgroup Substrate Bond in Thiol Monolayers: A Study of Biphenyl-Based Thiols on Gold and Silver
      Rong, Hai-Tao; Frey, Stefan; Yang, Yong-Jie; Zharnikov, Michael; Buck, Manfred; Wuehn, Mario; Woell, Christof; Helmchen, Guenter
      Langmuir (2001), 17 (5), 1582-1593CODEN: LANGD5; ISSN:0743-7463. (American Chemical Society)
      Self-assembled monolayers of a series of ω-(4'-methyl-biphenyl-4-yl)-alkanethiols (CH3-C6H4-C6H4-(CH2)m-SH, m = 1-6) formed on polycryst. gold and silver surfaces were characterized in detail by contact angle measurements, optical ellipsometry, XPS, reflection absorption IR spectroscopy (IRRAS), and near-edge X-ray absorption fine structure spectroscopy (NEXAFS). The orientation of the biphenyl moiety, detd. by combining the results from IRRAS and NEXAFS, exhibits a pronounced dependence on the no. of methylene groups. Similar to n-alkanethiols an odd-even effect is obsd. which on silver is opposite to that on gold. For m = odd on gold and m = even on silver the arrangement of the arom. moieties agrees well with the bulk structure of biphenyl, and the bonding of the thiols to the substrate is in agreement with an sp3 hybridization of the sulfur on gold and sp on silver, resp. In the opposite case of m = even on gold and m = odd on silver, the biphenyl moieties adopt a significantly more canted orientation which, as a consequence, results in a lower coverage. The odd-even behavior of the coverage is in sharp contrast to that seen for n-alkanethiols. The expts. provide evidence that a significant driving force exists to pertain the sp3 and sp hybridization of sulfur on gold and silver, resp. In the case of gold substrates the exptl. results are in conflict with available bending potentials derived from ab initio calcns.
    35. 35
      Heister, K.; Rong, H.-T.; Buck, M.; Zharnikov, M.; Grunze, M.; Johansson, L. S. O. Odd-Even Effects at the S-Metal Interface and in the Aromatic Matrix of Biphenyl-Substituted Alkanethiol Self-Assembled Monolayers. J. Phys. Chem. B 2001, 105, 6888– 6894,  DOI: 10.1021/jp010180e
      35
      Odd-Even Effects at the S-Metal Interface and in the Aromatic Matrix of Biphenyl-Substituted Alkanethiol Self-Assembled Monolayers
      Heister, K.; Rong, H.-T.; Buck, M.; Zharnikov, M.; Grunze, M.; Johansson, L. S. O.
      Journal of Physical Chemistry B (2001), 105 (29), 6888-6894CODEN: JPCBFK; ISSN:1089-5647. (American Chemical Society)
      Synchrotron-based high-resoln. XPS was applied to characterize self-assembled monolayers (SAM) of biphenyl-substituted alkanethiols CH3(C6H4)2(CH2)nSH (BPn, n = 1-4) on Au and Ag substrates. Beyond previously identified odd-even changes in the packing d. and the tilt angle of the biphenyl moieties, the high-resoln. spectra reveal a no. of addnl. odd-even effects upon variation of the no. of methylene groups in the aliph. part in the BPn mol. Their occurrence and mutual correlation suggests that a BPn SAM represents a strongly correlated, highly ordered mol. assembly. In particular, periodical changes of a shake up feature in the C 1s region are obsd., which are related to the differences in the arrangement of the arom. matrix. The width and binding energy position of the S 2p signals also exhibit odd-even changes. The width changes are assocd. with the occupation of either equiv. or nonequivalent adsorption sites on the polycryst. (111) Au and Ag substrates. The comparison of the width values with those for conventional alkanethiols implies that the substrate bonding of alkanethiols on gold cannot be described by a single adsorption site.
    36. 36
      Azzam, W.; Cyganik, P.; Witte, G.; Buck, M.; Wöll, C. Pronounced Odd-Even Changes in the Molecular Arrangement and Packing Density of Biphenyl-Substituted Alkanethiol SAMs. Langmuir 2003, 19, 8262– 8270,  DOI: 10.1021/la030165w
      36
      Pronounced Odd-Even Changes in the Molecular Arrangement and Packing Density of Biphenyl-Based Thiol SAMs: A Combined STM and LEED Study
      Azzam, W.; Cyganik, P.; Witte, G.; Buck, M.; Woell, Ch.
      Langmuir (2003), 19 (20), 8262-8270CODEN: LANGD5; ISSN:0743-7463. (American Chemical Society)
      Self-assembled monolayers (SAMs) of ω-(4'-methylbiphenyl-4-yl) alkanethiols CH3(C6H4)2(CH2)nSH (BPn, n = 3 and 4) on Au(111) substrates were studied using scanning tunneling microscopy (STM) and LEED (LEED). Prepn. at elevated temp. results in highly ordered layers with large domains. Whereas the (2√3 × √3) structure of the BP3 SAMs is similar to that reported previously for other arom. thiols, SAMs made from BP4 exhibit a very different structure. A (5√3 × 3)rect unit cell contg. 8 mols. is found which corresponds to a packing d. reduced by 25% compared to that of BP3. The odd-even effect obsd. in the molecularly resolved STM images of BP3 and BP4, therefore, confirms the pronounced influence of the spacer chain on the structure of these biphenyl-based SAMs.
    37. 37
      Cyganik, P.; Buck, M.; Azzam, W.; Wöll, C. Self-Assembled Monolayers of ω-Biphenyl-Alkane Thiols on Au(111): Influence of Spacer Chain on Molecular Packing. J. Phys. Chem. B 2004, 108, 4989– 4996,  DOI: 10.1021/jp037307f
      37
      Self-Assembled Monolayers of ω-Biphenylalkanethiols on Au(111): Influence of Spacer Chain on Molecular Packing
      Cyganik, Piotr; Buck, Manfred; Azzam, Waleed; Woell, Christof
      Journal of Physical Chemistry B (2004), 108 (16), 4989-4996CODEN: JPCBFK; ISSN:1520-6106. (American Chemical Society)
      Self-assembled monolayers (SAM) of ω-(4'-methylbiphenyl-4-yl)alkanethiols CH3(C6H4)2(CH2)nSH (BPn, n = 1-6) on Au(111) substrates, prepd. at room and elevated temps., were studied using scanning tunneling microscopy (STM). Molecularly resolved images reveal that all BPn SAMs form well-ordered layers over areas easily exceeding 50 × 50 nm2. Only two basic structures are alternatingly adopted with n changing between odd and even. The unit cell of odd-numbered SAMs is described by an oblique (2√3 × √3)R30° structure and contains two mols. In contrast, the even-numbered SAMs are described by a much larger, rectangular (5√3 × 3) structure with eight mols. per unit cell and occupying an area per mol. larger by about 25% compared to n = odd. With the exception of BP1 and BP6 the prepn. at elevated temps. resulted in a significant improvement in structural quality, yielding very large domains. For BP6 prepd. at 343 K a strong domain anisotropy is obsd., which is explained by the influence of the alkane spacer chain. For BP1 prepd. at 343 K formation of gold islands is concluded.
    38. 38
      Cyganik, P.; Buck, M. Polymorphism in Biphenyl-Based Self-Assembled Monolayers of Thiols. J. Am. Chem. Soc. 2004, 126, 5960– 5961,  DOI: 10.1021/ja049146e
      38
      Polymorphism in Biphenyl-Based Self-Assembled Monolayers of Thiols
      Cyganik, Piotr; Buck, Manfred
      Journal of the American Chemical Society (2004), 126 (19), 5960-5961CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      Self-assembled monolayers of ω-(4'-methylbiphenyl-4-yl) butanethiol (H3C-C6H4-C6H4-(CH2)n-SH) on Au(111) substrates were investigated with scanning tunneling microscopy and contact angle measurements. A striking polymorphism was obsd. upon annealing, and structural changes were paralleled by a switch in stability against exchange by other thiols from unstable to stable. The phase formed at temps. above 413 K was characterized by a very high structural perfection over areas exceeding 105 nm2. The results suggest an addnl. dimension in the control of structure and properties of thiol monolayers if different factors contributing to the energetics of SAMs enter in a competing rather than a cooperative way.
    39. 39
      Cyganik, P.; Buck, M.; Strunskus, T.; Shaporenko, A.; Wilton-Ely, J. D. E. T.; Zharnikov, M.; Wöll, C. Competition as a Design Concept: Polymorphism in Self-Assembled Monolayers of Biphenyl-Based Thiols. J. Am. Chem. Soc. 2006, 128, 13868– 13878,  DOI: 10.1021/ja0640647
      39
      Competition as a Design Concept: Polymorphism in Self-Assembled Monolayers of Biphenyl-Based Thiols
      Cyganik, Piotr; Buck, Manfred; Strunskus, Thomas; Shaporenko, Andrei; Wilton-Ely, James D. E. T.; Zharnikov, Michael; Woell, Christof
      Journal of the American Chemical Society (2006), 128 (42), 13868-13878CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      Self-assembled monolayers (SAMs) of two ω-(4'-methylbiphenyl-4-yl)alkanethiols (CH3(C6H4)2(CH2)nSH, BPn, n = 4, 6) on Au(111) substrates, prepd. from soln. at room temp. and subsequently annealed at temps. up to 493 K under a nitrogen atm., were studied using scanning tunneling microscopy (STM), high-resoln. XPS (HRXPS), and near-edge X-ray absorption fine structure spectroscopy (NEXAFS). In striking contrast to BPn SAMs with n = odd, for which only one phase is obsd., the even-numbered BPn SAMs exhibit polymorphism. Irreversible phase transitions occur which involve three phases differing substantially in d. and stability. Upon annealing, BP4 and BP6 transform into a β-phase, which is characterized by an exceptionally high structural quality with virtually defect-free domains exceeding 500 nm in diam. Exchange expts., monitored by contact angle measurement, reveal that the β-phase exhibits a dramatically improved stability. The fundamental differences in the phase behavior of even- and odd-numbered BPn SAMs are discussed in terms of two design strategies based on cooperative and competitive effects.
    40. 40
      Azzam, W.; Bashir, A.; Terfort, A.; Strunskus, T.; Wöll, C. Combined STM and FTIR Characterization of Terphenylalkanethiol Monolayers on Au(111): Effect of Alkyl Chain Length and Deposition Temperature. Langmuir 2006, 22, 3647– 3655,  DOI: 10.1021/la053065u
      40
      Combined STM and FTIR Characterization of Terphenylalkanethiol Monolayers on Au(111): Effect of Alkyl Chain Length and Deposition Temperature
      Azzam, W.; Bashir, A.; Terfort, A.; Strunskus, T.; Woell, Ch.
      Langmuir (2006), 22 (8), 3647-3655CODEN: LANGD5; ISSN:0743-7463. (American Chemical Society)
      Self-assembled monolayers (SAMs) of 4,4'-terphenyl-substituted alkanethiols C6H5(C6H4)2(CH2)n-SH (TPn, n = 1-6) on Au (111) substrates were studied using scanning tunneling microscopy (STM) and IR reflection absorption spectroscopy (IRRAS). When the SAMs were prepd. at room temp. (RT, 298 K), TPn films (except TP2) exhibit an odd-even effect regarding both mol. orientation and packing d. For all investigated films, STM data reveals the presence of a large degree of lateral order. In the case of odd-numbered TPns, the films revealed a (2√3 × √3)R30° mol. arrangement. For the even-numbered TP4 and TP6 SAMs, a c(5√3 × 3) rectangular unit cell was found. The packing d. for the even-numbered TPn SAMs is 25% lower than that for the odd-numbered TPn SAMs. When the SAMs were prepd. at 333 K, the even-numbered SAMs were found to form structures with a significantly lower packing d. In the case of TP2, instead of the (2√3 × √3)R30° structure formed at room temp., a c(5√3 × 3) structure was obsd. For TP6 SAMs, the room-temp. c(5√3 × 3) structure was replaced by a (6√3 × 2√3)R30° structure.
    41. 41
      Chesneau, F.; Schüpbach, B.; Szelągowska-Kunstman, K.; Ballav, N.; Cyganik, P.; Terfort, A.; Zharnikov, M. Self-Assembled Monolayers of Perfluoroterphenyl-Substituted Alkanethiols: Specific Characteristics and Odd–Even Effects. Phys. Chem. Chem. Phys. 2010, 12, 12123– 12137,  DOI: 10.1039/c0cp00317d
      41
      Self-assembled monolayers of perfluoroterphenyl-substituted alkanethiols: specific characteristics and odd-even effects
      Chesneau, Frederick; Schuepbach, Bjoern; Szelagowska-Kunstman, Katarzyna; Ballav, Nirmalya; Cyganik, Piotr; Terfort, Andreas; Zharnikov, Michael
      Physical Chemistry Chemical Physics (2010), 12 (38), 12123-12137CODEN: PPCPFQ; ISSN:1463-9076. (Royal Society of Chemistry)
      Self-assembled monolayers (SAMs) formed by perfluoroterphenyl-substituted alkanethiols (C6F5-C6F4-C6F4-(CH2)n-SH, FTPn) with variable length of the aliph. linker (n = 2 and 3) were prepd. on (111) Au and Ag and characterized by a combination of several complementary spectroscopic and microscopic techniques. A specific feature of these systems is the helical conformation of the FTP moieties, which, along with the high electronegativity of fluorine, distinguishes them from the analogous non-fluorinated systems and makes them attractive for different applications. The SAMs were found to be well-defined, highly ordered, and densely packed, which suggests a perfect correlation between the orientations and, in particular, twists of the FTP helixes in the adjacent mols. Significantly, the SAM exhibited pronounced odd-even effects, i.e. a dependence of the mol. orientation and packing d. on the length of the aliph. linker in the target mols., with parity of n being the decisive parameter and the direction of the effects on Au opposite to that on Ag. The presence of the odd-even effects in the FTPn system brings new aspects into the discussion about the origin and mechanism of these phenomena. Specifically, the helical conformation of the FTP moieties in the dense phase excludes a variation of the intramol. torsion and mol. twist as the mechanism behind the odd-even effects.
    42. 42
      Weidner, T.; Shaporenko, A.; Müller, J.; Schmid, M.; Cyganik, P.; Terfort, A.; Zharnikov, M. The Effect of the Bending Potential on Molecular Arrangement in Alkaneselenolate Self-Assembled Monolayers. J. Phys. Chem. C 2008, 112, 12495– 12506,  DOI: 10.1021/jp8044077
      42
      Effect of the Bending Potential on Molecular Arrangement in Alkaneselenolate Self-Assembled Monolayers
      Weidner, Tobias; Shaporenko, Andrey; Muller, Jan; Schmid, Martin; Cyganik, Piotr; Terfort, Andreas; Zharnikov, Michael
      Journal of Physical Chemistry C (2008), 112 (32), 12495-12506CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
      Self-assembled monolayers (SAMs) of hybrid 4,4'-biphenyl-substituted alkaneselenolates, CH3(C6H4)2(CH2)nSe- (BPnSe) with a variable length of the aliph. part (n = 1-6, 10, 11) have been prepd. on (111) gold and silver substrates and characterized by a variety of complementary exptl. techniques. The packing d. of the SAM constituents and the orientation of the biphenyl moieties were found to exhibit a pronounced "odd-even" variation with the no. (n) of methylene units in the aliph. linker of BPnSe moieties, which was opposite on silver as compared to gold. In particular, a smaller inclination and a corresponding higher packing d. of the biphenyl moieties was obsd. for odd nos. of the methylene units in BPnSe/Au and for even nos. of these units in BPnSe/Ag. The obsd. odd-even effects were explained by a significant bending potential, favoring definite orientation of the metal-Se-C bond and entering the balance of the structure-building interactions either cooperatively or competitively. The existence of this bending potential is supposed to be closely related to the exact bonding configuration of the headgroup atom, with the optimal substrate-Se-C angles being different for Au and Ag substrates. In view of the analogous behavior of thiolate-based systems, one can assume a common origin of the obsd. phenomena in chalcogen-based SAMs, in both of which the bonding configuration of the headgroup seems to be an important or even deciding factor in the balance of structure-building interactions.
    43. 43
      Szelagowska-Kunstman, K.; Cyganik, P.; Schüpbach, B.; Terfort, A. Relative Stability of Thiol and Selenol Based SAMs on Au(111)—Exchange Experiments. Phys. Chem. Chem. Phys. 2010, 12, 4400– 4406,  DOI: 10.1039/b923274p
      43
      Relative stability of thiol and selenol based SAMs on Au(111) - exchange experiments
      Szelagowska-Kunstman, Katarzyna; Cyganik, Piotr; Schuepbach, Bjoern; Terfort, Andreas
      Physical Chemistry Chemical Physics (2010), 12 (17), 4400-4406CODEN: PPCPFQ; ISSN:1463-9076. (Royal Society of Chemistry)
      Two fully analog homolog series of thiol and selenol based arom. self-assembled monolayers (SAMs) on Au(111) as Me(C6H4)2(CH2)n-S-Au(111) (BPnS/Au(111), n = 2-6) and Me(C6H4)2(CH2)n-Se-Au(111) (BPnSe/Au(111), n = 2-6), resp., were used to elucidate the relative stability of the S-Au(111) and Se-Au(111) bonding by monitoring their exchange by alkanethiol and alkaneselenol mols. from their resp. solns. The exchange process was monitored using IR reflection absorption spectroscopy (IRRAS). Two main results obtained by these study are: (1) the selenium-based BPnSe/Au(111) series is significantly more stable than their sulfur analogs; (2) a clear odd-even effect exists for the stability of both BPnS/Au(111) and BPnSe/Au(111) SAMs towards exchange processes with the even-numbered systems being less stable. The results obtained are discussed in view of previously reported microscopic and spectroscopic data of the same SAMs addressing the issue of the relative stability of S-Au(111) and Se-Au(111) bonding, which is an important factor for the rational design of SAMs.
    44. 44
      Dendzik, M.; Terfort, A.; Cyganik, P. Odd-even Effect in the Polymorphism of Self-Assembled Monolayers of Biphenyl-Substituted Alkaneselenolates on Au(111). J. Phys. Chem. C 2012, 116, 19535– 19542,  DOI: 10.1021/jp3051998
      44
      Odd-even effect in the polymorphism of self-assembled monolayers of biphenyl-substituted alkaneselenolates on Au(111)
      Dendzik, Maciej; Terfort, Andreas; Cyganik, Piotr
      Journal of Physical Chemistry C (2012), 116 (36), 19535-19542CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
      Self-assembled monolayers (SAMs) of ω-(4'-methylbiphenyl-4-yl) alkaneselenolates CH3(C6H4)2(CH2)nSe- (BPnSe, n = 2-6) on Au(111) substrates, prepd. in soln. at elevated temp. (333 K), were studied using scanning tunneling microscopy (STM). Molecularly resolved images reveal that even-numbered BPnSe SAMs form two or three different coexisting phases, including the one obsd. at room temp. In contrast, odd-numbered species exhibit only a single phase, which is the same as the one obsd. at room temp., i.e. close to a commensurate oblique (2√3×√3)R30° structure with two mols. per unit cell. Importantly, one of the phases obsd. for even-numbered BPnSe/Au(111) at room temp. (α-phase) has a well-defined periodicity only in 1D, whereas the new addnl. phases obsd. at elevated temp. are fundamentally different and have 2D periodic character, exhibiting a commensurate rectangular 5×2√3 lattice with four mols. per unit cell (β-phase) and an incommensurate oblique 2√3×1.2√3 lattice with two mols. per unit cell (γ-phase). For all systems, partial reorientation of the Au(111) step edges was obsd. upon SAM formation, indicating significant mobility of the topmost gold atoms induced by the adsorbates. To elucidate the effect of the S → Se substitution on the SAMs structure on Au(111), present results for BPnSe/Au(111) systems are discussed in view of the previously reported microscopic, spectroscopic, and desorption data obtained for these SAMs and for their thiol analogs, i.e. for BPnS/Au(111).
    45. 45
      Frey, S.; Rong, H.-T.; Heister, K.; Yang, Y.-J.; Buck, M.; Zharnikov, M. Response of Biphenyl-Substituted Alkanethiol Self-Assembled Monolayers to Electron Irradiation: Damage Suppression and Odd-Even Effects. Langmuir 2002, 18, 3142– 3150,  DOI: 10.1021/la011288o
      45
      Response of Biphenyl-Substituted Alkanethiol Self-Assembled Monolayers to Electron Irradiation: Damage Suppression and Odd-Even Effects
      Frey, S.; Rong, H. T.; Heister, K.; Yang, Y. J.; Buck, M.; Zharnikov, M.
      Langmuir (2002), 18 (8), 3142-3150CODEN: LANGD5; ISSN:0743-7463. (American Chemical Society)
      The low-energy electron-induced damage in self-assembled monolayers (SAMs) formed from ω-(4'-methylbiphenyl-4-yl)alkanethiols Me(C6H4)2(CH2)nSH (BPn, n = 0, 1, 4, 5, and 12) on Au substrates was studied. The pristine and heavily (8000 μC/cm2) irradiated films were characterized in detail by XPS, near-edge x-ray absorption fine structure spectroscopy, IR reflection absorption spectroscopy, and advancing contact angle measurements. In contrast to SAMs of conventional alkanethiols but similar to pure arom. thiol-derived systems, only minor damage is obsd. for the aliph.-arom. BPn films. In particular, the orientational order and anchoring to the substrate are retained upon the irradn. At the same time, C-H bond scissions in the arom. part occur, leading to a crosslinking between the neighboring biphenyl moieties. Whereas the general behavior of the BPn SAMs with respect to electron irradn. is qual. similar, the extent of the irradn.-induced changes depends on the packing of these systems. The densely packed BP1 and BP5 SAMs are much more stable with respect to electron bombardment than the less densely packed BP4 films. The relation between the packing d. and the extent of the irradn.-induced changes seems to be a general phenomenon in monomol. films, which provides a tool to tailor the reaction of these systems toward ionizing radiation for lithog. applications.
    46. 46
      Vervaecke, F.; Wyczawska, S.; Cyganik, P.; Bastiaansen, J.; Postawa, Z.; Silverans, R. E.; Vandeweert, E.; Lievens, P. Odd-Even Effects in Ion-Beam-Induced Desorption of Biphenyl-Substituted Alkanethiol Self-Assembled Monolayers. ChemPhysChem 2011, 12, 140– 144,  DOI: 10.1002/cphc.201000610
      46
      Odd-Even Effects in Ion-Beam-Induced Desorption of Biphenyl-Substituted Alkanethiol Self-Assembled Monolayers
      Vervaecke, Frederik; Wyczawska, Sabina; Cyganik, Piotr; Bastiaansen, Jeroen; Postawa, Zbigniew; Silverans, Roger E.; Vandeweert, Erno; Lievens, Peter
      ChemPhysChem (2011), 12 (1), 140-144CODEN: CPCHFT; ISSN:1439-4235. (Wiley-VCH Verlag GmbH & Co. KGaA)
      This study report on the anal. of ion-induced desorption of biphenyl-substituted (BPnS)/Au(111) with secondary neutral mass spectrometry (SNMS) as a new of approach to probe the strength mol.-substrate interaction in self-assembly monolayers (SAMs). The odd-even effect in the S-C bond scission, obsd. by desorption of the desulfurized fragment, is in antiphase when compared to the Au-S bond scission. The odd-even effect is also visible in the scission efficiency of the C-C bond that links two sep. parts of the mol. backbone, i.e., the alkane spacer and the biphenyl tail, resulting in a higher emission of the m/z = 168 fragment for even numbered BPnS/Au(111) SAMs. Besides a clear odd-even effect in the total desorption signal, it was obsd. that with increasing chain length the desorption efficiency for even-numbered BPnS/Au(111) SAMs gradually approaches the value which is typical for odd-numbered systems. The results obtained directly indicate the nature of these modifications as due to the systematic changes in the stability of Au-S and S-C bonds.
    47. 47
      Cisneros, J. A.; Björklund, E.; González-Gil, I.; Hu, Y.; Canales, Á.; Medrano, F. J.; Romero, A.; Ortega-Gutiérrez, S.; Fowler, C. J.; López-Rodríguez, M. L. Structure–Activity Relationship of a New Series of Reversible Dual Monoacylglycerol Lipase/Fatty Acid Amide Hydrolase Inhibitors. J. Med. Chem. 2012, 55, 824– 836,  DOI: 10.1021/jm201327p
      47
      Structure-Activity Relationship of a New Series of Reversible Dual Monoacylglycerol Lipase/Fatty Acid Amide Hydrolase Inhibitors
      Cisneros, Jose A.; Bjorklund, Emmelie; Gonzalez-Gil, Ines; Hu, Yanling; Canales, Angeles; Medrano, Francisco J.; Romero, Antonio; Ortega-Gutierrez, Silvia; Fowler, Christopher J.; Lopez-Rodriguez, Maria L.
      Journal of Medicinal Chemistry (2012), 55 (2), 824-836CODEN: JMCMAR; ISSN:0022-2623. (American Chemical Society)
      The two endocannabinoids, anandamide (AEA) and 2-arachidonoylglycerol (2-AG), play independent and nonredundant roles in the body. This makes the development of both selective and dual inhibitors of their inactivation an important priority. In this work, we report a new series of inhibitors of monoacylglycerol lipase (MAGL) and fatty acid amide hydrolase (FAAH). Among them, (±)-oxiran-2-ylmethyl 6-(1,1'-biphenyl-4-yl)hexanoate (I) and (2R)-(-)-oxiran-2-ylmethyl(4-benzylphenyl)acetate (II) stand out as potent inhibitors of human recombinant MAGL (IC50 (I) = 4.1 μM; IC50 (II) = 2.4 μM), rat brain monoacylglycerol hydrolysis (IC50 (I) = 1.8 μM; IC50 (II) = 0.68 μM), and rat brain FAAH (IC50 (I) = 5.1 μM; IC50 (II) = 0.29 μM). Importantly, and in contrast to the other previously described MAGL inhibitors, these compds. behave as reversible inhibitors either of competitive (I) or noncompetitive nature (II). Hence, they could be useful to explore the therapeutic potential of reversible MAGL inhibitors.
    48. 48
      Citrin, P. H.; Wertheim, G. K.; Baer, Y. Surface-Atom X-ray Photoemission from Clean Metals: Cu, Ag, and Au. Phys. Rev. B: Condens. Matter Mater. Phys. 1983, 27, 3160– 3175,  DOI: 10.1103/physrevb.27.3160
      48
      Surface-atom x-ray photoemission from clean metals: copper, silver, and gold
      Citrin, P. H.; Wertheim, G. K.; Baer, Y.
      Physical Review B: Condensed Matter and Materials Physics (1983), 27 (6), 3160-75CODEN: PRBMDO; ISSN:0163-1829.
      Surface-atom core-level shifts from evapd. noble metals are reported using angle-dependent x-ray photoemission with monochromatized Al Kα radiation. The absence of line broadening with increasing takeoff angle for the case of Al, where the surface shift is known to be small, confirms that the shifts obsd. in the noble metals are real surface phenomena. An anal. procedure was developed which establishes that the effect of the vacuum-solid interface in these systems is confined to the 1st at. layer. The asym. (final-state-related) line shapes of the surface- and bulk-atom photopeaks are also identical within exptl. error. The surface core-level shifts are -0.40 ± 0.02 eV for Au, -0.08 ± 0.03 eV for Ag, and -0.24 ± 0.02 eV for Cu, with the surface contributions occurring at lower electron binding energy. The anal. addnl. yields electron mean free paths of 19 ± 3 Å at 1400 eV in Au and 14 ± 3 Å at 550 eV in Cu. It was possible to isolate the contribution of the surface-atom valence electrons. The width of the surface d. of states is narrowed (8 ± 2)% with respect to the bulk d. of states and its center of gravity is shifted by -0.5 ± 0.1 eV. The anal. procedures and conclusions presented here should be applicable to core and valence surface-atom photoemission from a wide variety of other systems.
    49. 49
      Krzykawska, A.; Ossowski, J.; Żaba, T.; Cyganik, P. Binding groups for highly ordered SAM formation: carboxylic versus thiol. Chem. Commun. 2017, 53, 5748– 5751,  DOI: 10.1039/c7cc01939d
      49
      Binding groups for highly ordered SAM formation: carboxylic versus thiol
      Krzykawska, A.; Ossowski, J.; Zaba, T.; Cyganik, P.
      Chemical Communications (Cambridge, United Kingdom) (2017), 53 (42), 5748-5751CODEN: CHCOFS; ISSN:1359-7345. (Royal Society of Chemistry)
      The structure of analogous SAMs chemisorbed on an Ag substrate via a -SH or -COOH binding group is reported. COOH binding groups can be considered as a promising alternative for the current std. based on the S binding groups, particularly when 2-dimensional ordering, SAM fabrication time and stability under ambient conditions are of great importance.
    50. 50
      Heister, K.; Zharnikov, M.; Grunze, M.; Johansson, L. S. O. Adsorption of Alkanethiols and Biphenylthiols on Au and Ag Substrates: A High-Resolution X-ray Photoelectron Spectroscopy Study. J. Phys. Chem. B 2001, 105, 4058– 4061,  DOI: 10.1021/jp010127q
      50
      Adsorption of alkanethiols and biphenylthiols on Au and Ag substrates: A high-resolution x-ray photoelectron spectroscopy study
      Heister, K.; Zharnikov, M.; Grunze, M.; Johansson, L. S. O.
      Journal of Physical Chemistry B (2001), 105 (19), 4058-4061CODEN: JPCBFK; ISSN:1089-5647. (American Chemical Society)
      Synchrotron-based high-resoln. XPS was applied to monitor the formation of self-assembled monolayers (SAM) of alkanethiols (AT) and biphenylthiols on Au and Ag substrates. Pronounced chem. shifts in the adsorbate- and substrate-related photoemission lines upon SAM formation were obsd. Only 1 sulfur species could be detected in the S 2p spectra of the investigated SAMs, consistent with a thiolate bond. From the FWHM's of the core level photoemission spectra conclusions on the heterogeneity of the adsorption sites and adsorption geometry can be made. The exptl. data imply several (at least 2) slightly different adsorption geometries for the AT moieties in AT/Au. Significant final state effects in the C 1s photoemission were found for both the aliph. and arom. SAMs.
    51. 51
      Dannenberger, O.; Weiss, K.; Himmel, H.-J.; Jäger, B.; Buck, M.; Wöll, C. An Orientation Analysis of Differently Endgroup-Functionalised Alkanethiols Adsorbed on Au Substrates. Thin Solid Films 1997, 307, 183– 191,  DOI: 10.1016/s0040-6090(97)00280-0
      51
      An orientation analysis of differently end group-functionalized alkanethiols adsorbed on Au substrates
      Dannenberger, O.; Weiss, K.; Himmel, H.-J.; Jager, B.; Buck, M.; Woll, Ch.
      Thin Solid Films (1997), 307 (1,2), 183-191CODEN: THSFAP; ISSN:0040-6090. (Elsevier Science S.A.)
      The mol. orientation in self-assembled films of methyl-, hydroxyl-, and carboxylic acid-terminated alkanethiols of different chain lengths (n-docosanethiol [DCT], 22-mercaptodocosanol [MDO], 22-mercaptodocosanoic acid [MDC], n-hexadecanethiol [HDT], 16-mercaptohexadecanol [MHO], 16-mercaptohexadecanoic acid [MHC]) on Au substrates has been investigated by near edge X-ray absorption fine structure (NEXAFS) spectroscopy and XPS. Whereas XPS-measurements do not reveal changes in thickness upon replacing the end-groups, the degree of orientation as detd. with NEXAFS exhibits significant differences. The two methyl-terminated thiols show the same alkyl-chain tilt-angle α of 39°. For the short-chain OH-terminated films no significant difference could be detected, but the monolayers formed from the long-chain OH-terminated chains exhibit a slightly stronger anisotropy and thus a smaller tilt-angle α than the corresponding CH3-terminated thiol. NEXAFS-spectra recorded for films from COOH-terminated thiols reveal only a very small anisotropy which indicates the absence of significant mol. orientation resulting from a high degree of disorder.
    52. 52
      Lamont, C. L. A.; Wilkes, J. Attenuation Length of Electrons in Self-Assembled Monolayers of n-Alkanethiols on Gold. Langmuir 1999, 15, 2037– 2042,  DOI: 10.1021/la981168p
      52
      Attenuation length of electrons in self-assembled monolayers of n-alkanethiols on gold
      Lamont, Christine L. A.; Wilkes, John
      Langmuir (1999), 15 (6), 2037-2042CODEN: LANGD5; ISSN:0743-7463. (American Chemical Society)
      The interaction of both photoelectrons and X-rays with self-assembled monolayers of n-alkanethiols on gold has been measured using synchrotron radiation as the photon source in the energy range 140-1100 eV. The attenuation length of photoelectrons (λ) was found to vary from a min. of ∼5 Å at an electron kinetic energy (E) of 100 eV up to ∼23 Å at a kinetic energy of 1000 eV and can be described by the expression λ = 0.3E0.64 in the range 300-1000 eV. Exposure of the self-assembled monolayer to X-rays leads to fission of the C-S bond with a cross section of the order of 10-17 cm2 which displays no apparent dependence on the incident photon energy.
    53. 53
      Olsen, C.; Rowntree, P. A. Bond-Selective Dissociation of Alkanethiol Based Self-Assembled Monolayers Adsorbed on Gold Substrates, Using Low-Energy Electron Beams. J. Chem. Phys. 1998, 108, 3750– 3764,  DOI: 10.1063/1.475780
      53
      Bond-selective dissociation of alkanethiol based self-assembled monolayers adsorbed on gold substrates, using low-energy electron beams
      Olsen, C.; Rowntree, P. A.
      Journal of Chemical Physics (1998), 108 (9), 3750-3764CODEN: JCPSA6; ISSN:0021-9606. (American Institute of Physics)
      We have conducted a study of electron-stimulated reactions in butanethiol, octanethiol, dodecanethiol, and hexadecanethiol monolayers adsorbed onto Au/mica substrates, using in situ IR spectroscopy to quantify the processes; the electron dose dependence of the depletion of various C-H stretching modes has permitted the detn. of the first dissocn. cross sections for electron stimulated reactions in self-assembled org. monolayers. Electron-induced dehydrogenation of alkanethiol/Au/mica films in the 0-15 eV regime is shown to proceed principally via dissociative electron attachment, thus confirming previous work that directly measured H2 desorption yields during irradn. The dissocn. probabilities exhibit a well-resolved max. at 10 eV, with a full-width at half-max. of ∼4 eV. Unlike previous studies, our spectroscopic investigation shows that the dehydrogenation is not uniformly distributed throughout the org. film, but is strongly localized near the Me terminations of the film. The dissocn. cross sections at this interface increase rapidly with increasing chain length. We have shown that these increases are not due to the interaction of the dissociative anionic state with the film via charge-induced dipole forces, nor are they due to interactions with the metal substrate via charge-image charge forces. Our results are consistent with a dipole-image dipole quenching model, whereby the excited state lifetimes are reduced from an estd. ∼26 fs (for a gas-phase electron-alkane collision) to ∼2-10 fs, depending on the chain length. These distance-dependent lifetimes cause the dissocn. yields for short-chain systems to be significantly lower than long-chain systems, and it is predicted that the electron-induced dissocn. cross sections for alkanethiol monolayers should show much stronger isotopic dependencies than found with the gas-phase alkane species.
    54. 54
      Weidner, T.; Ballav, N.; Grunze, M.; Terfort, A.; Zharnikov, M. Modification of Biphenylselenolate Monolayers by Low-Energy Electrons. Phys. Status Solidi B 2009, 246, 1519– 1528,  DOI: 10.1002/pssb.200945078
      54
      Modification of biphenylselenolate monolayers by low-energy electrons
      Weidner, T.; Ballav, N.; Grunze, M.; Terfort, A.; Zharnikov, M.
      Physica Status Solidi B: Basic Solid State Physics (2009), 246 (7), 1519-1528CODEN: PSSBBD; ISSN:0370-1972. (Wiley-VCH Verlag GmbH & Co. KGaA)
      The effect of low-energy (50 eV) electron irradn. on self-assembled monolayers (SAMs) of arom. selenolates on polycryst. Au{111} was studied by synchrotron-based XPS and near-edge X-ray absorption fine structure spectroscopy. As a test system, SAMs of biphenylselenolate (BPSe) were used, and analogous biphenylthiolate (BPT) SAMs were taken as a ref. The BPSe films were found to exhibit the expected high stability of arom. SAMs toward electron irradn., primarily mediated by the dominance of crosslinking between the individual mol. species. The cross-sections of the most prominent irradn.-induced processes in the BPSe SAMs were obtained and found to be in a range of 0.07-0.2 × 10-16 cm2. Comparison of the BPSe and BPT SAMs showed a higher stability of the former system toward electron irradn. This phenomenon was explained by a combined effect of a higher degree of crystallinity of the BPSe SAMs and a higher strength of the selenolate-gold as compared to thiolate-gold bond. The latter aspect is an important result of the present study, which may help to rationalize the current controversy regarding the relative strength of these two anchor groups.
    55. 55
      Yildirim, C.; Füser, M.; Terfort, A.; Zharnikov, M. Modification of Aromatic Self-Assembled Monolayers by Electron Irradiation: Basic Processes and Related Applications. J. Phys. Chem. C 2016, 121, 567– 576,  DOI: 10.1021/acs.jpcc.6b11269
      There is no corresponding record for this reference.
    56. 56
      Taucher, T. C.; Hehn, I.; Hofmann, O. T.; Zharnikov, M.; Zojer, E. Understanding Chemical versus Electrostatic Shifts in X-ray. Photoelectron Spectra of Organic Self-Assembled Monolayers. J. Phys. Chem. C 2016, 120, 3428– 3437,  DOI: 10.1021/acs.jpcc.5b12387
      56
      Understanding Chemical versus Electrostatic Shifts in X-ray Photoelectron Spectra of Organic Self-Assembled Monolayers
      Taucher, Thomas C.; Hehn, Iris; Hofmann, Oliver T.; Zharnikov, Michael; Zojer, Egbert
      Journal of Physical Chemistry C (2016), 120 (6), 3428-3437CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
      The focus of the present article is on understanding the insight that XPS measurements can provide when studying self-assembled monolayers. Comparing d. functional theory calcns. to exptl. data on deliberately chosen model systems, we show that both the chem. environment and electrostatic effects arising from a superposition of mol. dipoles influence the measured core-level binding energies to a significant degree. The crucial role of the often overlooked electrostatic effects in polar self-assembled monolayers (SAMs) is unambiguously demonstrated by changing the dipole d. through varying the SAM coverage. As a consequence of this effect, care has to be taken when extg. chem. information from the XP spectra of ordered org. adsorbate layers. Our results, furthermore, imply that XPS is a powerful tool for probing local variations in the electrostatic energy in nanoscopic systems, esp. in SAMs.
    57. 57
      Otálvaro, D.; Veening, T.; Brocks, G. Self-Assembled Monolayer Induced Au(111) and Ag(111) Reconstructions: Work Functions and Interface Dipole Formation. J. Phys. Chem. C 2012, 116, 7826– 7837,  DOI: 10.1021/jp300512k
      57
      Self-Assembled Monolayer Induced Au(111) and Ag(111) Reconstructions: Work Functions and Interface Dipole Formation
      Otalvaro, Diana; Veening, Thijs; Brocks, Geert
      Journal of Physical Chemistry C (2012), 116 (14), 7826-7837CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
      Adsorption of self-assembled monolayers (SAMs) on metal surfaces leads to interface dipole layers that strongly modify the metal work functions. Recently, alkanethiolate SAMs have been shown to give rise to substantial reconstructions of the Au(111) and Ag(111) surfaces. The authors studied by means of first-principles calcns. how such reconstructed alkanethiolate SAMs on Au and Ag interfaces modify the interface dipole layer and the work function. The impact of SAM-induced reconstructions is remarkably moderate in the Au case, giving rise to work function changes of .ltorsim. 0.25 eV as compared with the unreconstructed case. Neither the Au work function is altered much by reconstructions nor the orientation of the mol. dipoles in the SAMs. In contrast, in the Ag case, a SAM-induced reconstruction alters the work function by ⪆ 0.4 eV. The different behavior of Au and Ag substrates is partly explained by the participation of the Au 5d states in the surface electronic structure, moderating the impact of a reconstruction, whereas there is no such participation of the Ag 4d states.
    58. 58
      de Boer, B.; Hadipour, A.; Mandoc, M. M.; van Woudenbergh, T.; Blom, P. W. M. Tuning of Metal Work Functions with Self-Assembled Monolayers. Adv. Mater. 2005, 17, 621– 625,  DOI: 10.1002/adma.200401216
      58
      Tuning of metal work functions with self-assembled monolayers
      De Boer, Bert; Hadipour, Afshin; Mandoc, M. Magdalena; Van Woudenbergh, Teunis; Blom, Paul W. M.
      Advanced Materials (Weinheim, Germany) (2005), 17 (5), 621-625CODEN: ADVMEW; ISSN:0935-9648. (Wiley-VCH Verlag GmbH & Co. KGaA)
      Work functions of Au and Ag are varied by over 1.4 and 1.7 eV, resp., by using self-assembled monolayers. Using these modified electrodes, the hole current in a poly(2-methoxy-5-(2'-ethylhexyloxy)-1,4-phenylene vinylene) light-emitting diode is tuned by more than six orders of magnitude (see Figure). Suppression of the hole current enables measurement of the electron current in a polymer/polymer blend photovoltaic cell.
    59. 59
      Angelova, P.; Vieker, H.; Weber, N.-E.; Matei, D.; Reimer, O.; Meier, I.; Kurasch, S.; Biskupek, J.; Lorbach, D.; Wunderlich, K. A Universal Scheme to Convert Aromatic Molecular Monolayers into Functional Carbon Nanomembranes. ACS Nano 2013, 7, 6489– 6497,  DOI: 10.1021/nn402652f
      59
      A Universal Scheme to Convert Aromatic Molecular Monolayers into Functional Carbon Nanomembranes
      Angelova, Polina; Vieker, Henning; Weber, Nils-Eike; Matei, Dan; Reimer, Oliver; Meier, Isabella; Kurasch, Simon; Biskupek, Johannes; Lorbach, Dominik; Wunderlich, Katrin; Chen, Long; Terfort, Andreas; Klapper, Markus; Muellen, Klaus; Kaiser, Ute; Goelzhaeuser, Armin; Turchanin, Andrey
      ACS Nano (2013), 7 (8), 6489-6497CODEN: ANCAC3; ISSN:1936-0851. (American Chemical Society)
      Free-standing nanomembranes with mol. or at. thickness are currently explored for sepn. technologies, electronics, and sensing. Their engineering with well-defined structural and functional properties is a challenge for materials research. The authors present a broadly applicable scheme to create mech. stable carbon nanomembranes (CNMs) with a thickness of ∼0.5 to ∼3 nm. Monolayers of polyarom. mols. (oligophenyls, hexaphenylbenzene, and polycyclic arom. hydrocarbons) were assembled and exposed to electrons that cross-link them into CNMs; subsequent pyrolysis converts the CNMs into graphene sheets. In this transformation the thickness, porosity, and surface functionality of the nanomembranes are detd. by the monolayers, and structural and functional features are passed on from the mols. through their monolayers to the CNMs and finally on to the graphene. This procedure is scalable to large areas and allows the engineering of ultrathin nanomembranes by controlling the compn. and structure of precursor mols. and their monolayers.
    60. 60
      Avouris, P.; Persson, B. N. J. Excited States at Metal Surfaces and Their Nonradiative Relaxation. J. Phys. Chem. 1984, 88, 837– 848,  DOI: 10.1021/j150649a004
      60
      Excited states at metal surfaces and their non-radiative relaxation
      Avouris, Phaedon; Persson, Bo N. J.
      Journal of Physical Chemistry (1984), 88 (5), 837-48CODEN: JPCHAX; ISSN:0022-3654.
      A review with 74 refs.

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    • Electron irradiation data obtained for BPnCOO/Ag SAMs (with n = 2, 4, 5, 6) (PDF)


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