Effect of Electron Irradiation on Electric Transport Properties of

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The Effect of Electron Irradiation on Electric Transport Properties of Aromatic Self-Assembled Monolayers Can Yildirim, Eric Sauter, Andreas Terfort, and Michael Zharnikov J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b01243 • Publication Date (Web): 16 Mar 2017 Downloaded from http://pubs.acs.org on March 17, 2017

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The Journal of Physical Chemistry

The Effect of Electron Irradiation on Electric Transport Properties of Aromatic SelfAssembled Monolayers

Can Yildirim,1 Eric Sauter,1 Andreas Terfort,2 and Michael Zharnikov1* 1 2

Applied Physical Chemistry, Heidelberg University, 69120 Heidelberg, Germany

Institut für Anorganische und Analytische Chemie, Universität Frankfurt, Max-von-LaueStraße 7, 60438 Frankfurt, Germany

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Abstract

We studied the effect of electron irradiation on electric transport properties of aromatic thiolate self-assembled monolayers (SAMs) with oligophenyl (OPh), acene, and oligo(phenylene ethynylene) (OPE) backbones of variable length. The resistance of these SAM was found to increase progressively, by 2-3 orders of magnitude upon irradiation with doses up to 40 mC/cm2. The electric transport properties of the irradiated SAMs could be well described by the simplified Simmons' equation, with the attenuation factor (β) and contact resistance (R0) as parameters. The β value decreased moderately for the OPh and OPE films but increased slightly for the acene monolayers upon irradiation. The R0 value increased significantly upon irradiation for all three types of SAMs and was considered as the major reason for the observed increase in the total resistance of these films. In its turn, the increase in R0 was attributed to the combined effect of a partly deteriorated anchoring to the substrate at the SAM-substrate interface and progressive irradiation-promoted adsorption of airborne molecules at the SAM-ambient interface. The effect of the work function, studied in the given context as well and exhibiting a small change only upon irradiation, is believed to be of negligible influence on R0. Finally, for the three-rings systems, the transition voltage (Vtrans) could be monitored. It showed progressive decrease upon irradiation for all three kinds of the aromatic backbone. This behavior was considered as a fingerprint for the decrease in the width of the HOMO-LUMO gap upon irradiation, which can be potentially useful for bandgap engineering.


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1. Introduction Charge transport (CT) at the nanoscale is an important issue of fundamental and practical significance. Particular important systems in this context are self-assembled monolayers. Apart from their direct relevance for molecular and organic electronics,1-4 they represent highly useful model systems in the context of CT, relying on their well defined character and chemical composition, which can be flexibly designed and adjusted to a particular experiment or an application.5,6 Among other aspects, this allows to monitor the electric conductance properties of "molecular wires", serving as backbones of the SAM constituents, and also to study effectiveness of molecular anchoring (in terms of CT),7,8 effects of the level alignment at the organic-inorganic interface,8 influence of molecular dipole,9 effect of molecular conformation,10,11 etc. In the sense of electric conductance, SAMs represent generally nanoscale insulators. Accordingly, their electric transport properties are well described by a simplified version of Simmons’ equation,12 J = J0 exp(− ßl), where J is the current through a metal-molecule-metal junction, J0 is a parameter closely related to the molecular contact resistance R0, ß is a tunneling attenuation factor, and l is the length of the molecular backbone. The value of ß depends predominantly on the identity of the SAM backbone and is usually lower for unsaturated hydrocarbon chains such as alkenes, oligoacenes, oligophenyls, and oligo(phenylene ethynylenes) as compared to saturated hydrocarbon chains such as alkanes. The value of R0 depends predominantly on the character of the molecular anchor (contact to the conductive substrate) and quality of so-called top contact in the standard, two-terminal junction geometry. Apart from the chemical design of the SAM constituents, the identity and properties of a SAM can be modified by physical means. A particular important tool in this regard is electron irradiation, which, at small doses, gives additional possibilities for SAM design,13,14 and, at large doses, results in their severe modification.15 Such a modification is especially useful for aromatic SAMs, for which electron irradiation leads to extensive cross-linking of the SAM constituents, transforming an assembly of weakly interacting molecules into a quasihomogeneous hydrocarbon matrix.15-19 The respective cross-linked films can even be separated from the substrate in form of carbon nanomembranes (CNMs)20-27 and either used as such or serve as precursors for graphene-like nanosheets, obtained by CNM pyrolysis.25,26,28 Among other properties, electron irradiation affects the electric conductive properties of the aromatic SAMs. The first results in this regard were obtained for SAMs of [1,1';4',1''terphenyl]-4,4''-dimethanethiol (TPDMT) on Au.29 According to the impedance spectroscopy 3 ACS Paragon Plus Environment


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data, the resistance of the strongly irradiated TPDMT films increased by a factor of 2 upon extensive electron irradiation (10 eV; 40-45 mC/cm2). This behavior was tentatively explained by the modification of the electronic structure, viz. a decrease in the HOMOLUMO gap upon the irradiation, based on the experimental spectroscopy data and theoretical calculations.30,31 This conclusion was indirectly supported recently by molecular junction experiments on a series of SAMs with oligophenyl (OPh) backbone as well as respective CNMs (100 eV; 50 mC/cm2).32 The CNMs exhibited much lower (by 1 order of magnitude) conductance as compared to the parent SAMs which was predominantly associated with a change in the contact resistance. Significantly, so-called transition voltage, manifesting a change in the conductance regime and considered as a measure of a tunneling barrier at the substrate-SAM interface,8,33-36 was found to decrease upon the irradiation.32 The height of the tunneling barrier is defined by the energy offset between the Fermi energy, EF, and the nearest molecular orbital. For thiol-terminated molecules, the nearest level is commonly the highest occupied molecular orbital (HOMO, with energy EHOMO),33,34,37 so that the charge transport is mostly mediated by hole injection and the barrier height depends directly on the EHOMO positioning with respect to EF. Consequently Vtrans can also be considered as a measure of the HOMO-LUMO gap, so that the lower value of this parameter in the CNMs suggested a decrease in the width of the HOMO-LUMO gap upon the irradiation,32 in agreement with the previous results for the strongly irradiated terphenyl-based SAMs.30,31 Considering that all above experiments related to the effect of electron irradiation on the electric transport properties of the aromatic SAMs were performed with high doses only, it is interesting to look at the respective behavior in more detail, monitoring the changes of electric transport properties in the course of irradiation. In addition, it is important to study SAMs with other aromatic backbones, to look at the generality of the behavior observed exclusively for the monolayers with the OPh backbone. Finally, it is reasonable to consider and to study additional aspects, beyond the positions of the frontier orbitals, which can be of importance for this behavior. In this context, in the given study, we investigated the effect of electron irradiation on electric transport properties of SAMs with rod-like OPh, acene, and oligo(phenylene ethynylene) (OPE) backbones of variable length (see Figure 1). These SAMs were prepared on conductive Au(111) substrates serving as bottom electrodes in the twoterminal junction setup used in our experiments. Whereas the OPh films represent the most basic aromatic SAMs studied extensively in context of electron irradiation, acene and OPE monolayers, which are capable of irradiation-induced cross-linking as well,25,27 have distinctly 4 ACS Paragon Plus Environment

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different backbones because of their fused and strongly conjugated molecular chains, respectively.

2. Experimental part All solvents and chemicals were purchased from Sigma-Aldrich. The SAM precursors used in this study are shown in Figure 1, along with their abbreviations. They represent three different series, as mentioned in the figure caption. The precursors were either purchased from SigmaAldrich (PT, BPT, TPT, NphT, and OPE3) or custom-synthesized (OPE227 and AnthT38) following the literature recipes. The gold substrates were provided by Georg Albert PVD-Beschichtungen, Silz, Germany. They were prepared by thermal evaporation of 70-75 nm of gold (99.99% purity) onto polished single-crystal silicon (100) wafers (Silicon Sense) that had been precoated with a 5 nm titanium adhesion layer. The films were polycrystalline, exposing preferably (111) orientated surfaces of individual crystallites. The SAMs were prepared by immersion of the freshly fabricated substrates into 0.5-1 mmol solutions of the respective precursors in absolute ethanol (NphT and AnthT) or DMF (PT, BPT, TPT, OPE2, and OPE3) for 24 h at room temperature. For deprotection of the OPE2 and OPE3 precursors, triethylamine was added. After immersion, the films were rinsed with ethanol or DMF and blown dry with argon. Extensive spectroscopic characterization showed no evidence of impurities or oxidative degradation products.27 In addition, reference SAMs of hexadecanethiolate (HDT) were prepared on the same kind of gold substrates using a standard procedure.39 The SAMs were homogeneously irradiated with a flood gun (FG20, Specs, Germany), at a base pressure better than 1×10-8 mbar. The electron energy was set to 50 eV and the dose, set at 10, 20, and 40 mC/cm2, was calibrated by an array of the Faraday cups. The electric conductance measurements were performed ex situ, after exposure of the samples to ambience. The experiments were performed at room temperature and under ambient conditions, so that possible environment effects40 could not be completely excluded. A custom-designed two-terminal junction setup, based on a Keithley 2635A source meter, was used,41 with the conductive Au substrate functioning as the bottom electrode and the liquid metal eutectic GaIn (EGaIn) tip serving as the top contact. A schematic of this setup is shown in Figure 2. The EGaIn tips were prepared according to the literature procedure;36 their preparation is illustrated in Figure S1 in the Supporting Information. Alkanthiolate passivated 5 ACS Paragon Plus Environment


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Hg electrode was tried as the top electrode as well but found less suitable for the given systems. The measurements were performed under ambient conditions (a relative humidity of 35-45%) and room temperature. About 10 measurements at different places were performed for each sample, with a small deviation for most of the curves and the average values being calculated. The contact area was estimated by analyzing the image taken by the video camera (Imaging Source DMK22AUC03 1/3 in. Micron with MR 8/O). The current through the junction

was recorded as a function of the applied voltage by using the Keithley source meter. Data points were collected using a voltage ramp with a bias step of ~45 mV and an interval of at least 5 s between individual steps. The voltage was swept from −0.01 V to −0.5 V and 0.01 V to 0.5 V. In addition to the above electric conductance experiments, work function (WF) measurements were carried out using a UHV Kelvin Probe 2001 system (KP Technology Ltd., UK). The measurements were performed in situ, without the exposure of the irradiated samples to ambience. The pressure during the measurements was ~5×10-8 mbar. The setup was calibrated to the work function of sputtered Au(111) at 5.2 eV. In addition, during the experiments, the WF values of the pristine and irradiated OPh, acene, and OPE SAMs were directly calibrated to the WF of the HDT functionalized Au(111) at 4.3 eV. The measurements were carried out at room temperature. The effective thickness of the pristine and irradiated OPh, acene, and OPE SAMs was measured by X-ray photoelectron spectroscopy. The measurements were performed in situ, immediately after the irradiation and without the exposure of the irradiated films to ambient. The details of the experiments and evaluation procedure can be found elsewhere.27

3. Results Semilogarithmic plots of the current density versus voltage for the two-terminal junctions comprising the pristine SAMs of this study are presented in Figure 3a. The current density decreases with the molecular length as can be expected. More detailed information is provided by the semilogarithmic plots of the current density versus molecular length shown for the pristine SAMs in Figure 3b for a bias voltage of −0.5 eV. These plots can be well fitted by straight lines, mimicking the equation J = J0 exp(− ßl). The slope of these lines gives the values of the attenuation factor, β. For the pristine OPh, acene, and OPE monolayers these values were estimated at 0.52, 0.49, and 0.33 A-1, respectively. They are in good agreement with our previous results, viz. 0.53 A-1 for OPh and 0.3 A-1 for OPE,41 as well as with the data


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by the others, viz. 0.41-0.7 Å-1 for OPh,5,42-44 ~0.5 Å-1, for acenes,7,8 and 0.23-0.27 Å-1 for OPE40,45. The electric transport properties of all SAMs studied changed strongly upon irradiation. Semilogarithmic plots of the average tunneling resistance of these SAMs versus irradiation dose are presented in Figure 4. According to these plots, the resistance of all SAM studied increases progressively in the course of irradiation. The increase is more rapid at low doses, showing a close-to-exponential rise, and slower at high doses, exhibiting a transition to a leveling-off behavior. The plots for the one-, two-, and three-rings systems within the OPh, acene, and OPE series are very similar to each other, apart from the vertical offsets, reflecting the difference in the resistance of the pristine SAMs. The data shown in Figure 4 could also be presented in different fashion, checking the validity of the simplified Simmons’ equation for the irradiated SAMs. The respective semilogarithmic plots of the tunneling resistance versus effective film thickness for the two-terminal junctions comprising the irradiated OPh, acene, and OPE SAMs are presented in Figure 5; the data for the pristine SAMs are presented as well, for comparison. The selection of the effective film thickness as the relevant parameter for the irradiated SAMs is intentional. Whereas for the pristine SAMs the tunneling through the molecular chain ("through-bond") is preferable,46 tunneling through the SAM matrix ("through-space"), with the SAM thickness as parameter, is presumably the case for the irradiated monolayers since, due to extensive cross-linking, individual molecules do not exist anymore and the SAM matrix represents a quasihomogeneous material.17-19 The data for the irradiated SAMs could be fitted well by straight lines, following the "inverse" Simmons’ equation, R = R0 exp(ßd), where R is the tunneling resistance of the SAM-containing junction and d is the thickness of the film studied (the meanings of R0 and ß are explained in Section 1), suggesting its validity for these systems as well. The values of ß and R0 could be derived as the slope of the fitting lines and their crossing points with the Y axis, respectively. The values of ß exhibited certain dependence on irradiation dose and an additional dependence on the bias voltage, as demonstrated by the example of the OPh SAMs in Figure 6a, representative of the acene and OPE monolayers as well. The dependence on the bias voltage is rather weak for the pristine and weakly irradiated (10 mC/cm2) films but is quite strong for the strongly irradiated ones (20 and 40 mC/cm2), showing a noticeable decrease in the β values with increasing bias, in accordance with the literature data for the CNMs derived from the OPh SAMs.32 The representative values of β for all aromatic SAMs of this study, averaged over the entire bias range, are compiled in Figure 7 ACS Paragon Plus Environment


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6b. Whereas these values remain more or less independent of the irradiation dose for the acene SAMs (a slight increase), a decrease in the β values is observed for the OPE and OPh monolayers. The latter result disagrees in part with the previously reported data for the CNMs derived from the OPh SAMs, where the β values for the CNMs were at the average somewhat smaller than those for the pristine SAMs but the difference was not as large as in Figure 6b.32 The major effect of the irradiation is however not the difference in the β values but the changes in the contact resistance, as can be directly traced in Figure 5 (crossing points of the fitting lines with the Y axis). This is additionally visualized by Figure 7 where the contact resistance of the pristine and irradiated OPh, acene, and OPE SAMs is depicted versus irradiation dose in the semilogarithmic fashion. As shown in this plot, the contact resistance of the monolayers increases progressively in the course of irradiation. The increase is more rapid at low doses and slower at high doses, exhibiting a transition to a leveling-off behavior. Along with the standard form of the I/V curves as shown in Figure 3a, their representation in form of Fowler-Nordheim (FN) plots was performed as depicted in Figure 8 for the pristine and irradiated OPE2 (a) and AnthT (b) SAMs, representative of all other monolayers of this study. Such plots are quite useful to trace the transition between the tunneling within the "classical" Simmons model (step-like tunneling barrier) occurring at a small bias and FN tunneling or field emission (triangular barrier) taking place at a large bias. The voltage corresponding to this transition, Vtrans, is then determined from the minimum of the graph. Because of the limited range of the bias voltage, such a minimum could only be observed for few selected SAMs of this study, viz. the TPT, AnthS, OPE2, and OPE3 films, suggesting that Vtrans for the other monolayers exceed the bias range used (in agreement with the literature data)8,32,36. With the exception of OPE2, all these films are three-ring systems. This agrees with the previous observations that the HOMO-LUMO gap of the aromatic backbone decreases substantially with increasing number of rings7 and, in line with this, that Vtrans in aromatic SAMs exhibits the same tendency.8,36 But even for some of these selected SAMs, the minimum was not observed in the pristine state but only upon the irradiation treatment as illustrated by Figure 8. This suggests that the value of Vtrans decreases in the course of irradiation, as additionally visualized by Figure 9 where the Vtrans values for the pristine and irradiated AnthT, TPT, OPE2, and OPE3 SAMs are plotted versus irradiation dose for both negative and positive bias directions. A decrease in the of Vtrans with increasing dose is obvious for all four SAMs suggesting that this behavior is typical of all types of aromatic SAMs, being thus most likely related to irradiation-induced 8 ACS Paragon Plus Environment

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cross-linking - a common process in all these systems. This finding agrees well with the previous results for the CNMs fabricated from the OPh SAMs, where the analogous decrease in Vtrans was observed for all systems studied (a large bias range was applied).32 Interestingly, whereas the Vtrans values for the pristine and moderately irradiated (10 mC/cm2) SAMs are different for the different systems, they become quite similar for the strongly irradiated films, which is understandable since these films lose their initial identity and represent just extensively cross-linked networks of individual and annulated phenyl rings as well as their residuals. The absolute values of Vtrans for all three-ring systems were estimated at 0.21-0.25 V, with slightly lower absolute value for the negative bias. Both this value and the bias related difference are in excellent agreement with the previous results for the CNM fabricated from the TPT SAM (Vtrans(+) = 0.21; Vtrans(-) = −0.17 V).32 The absolute values of Vtrans for the OPE2 film, which was the only two-rings system addressed by the Vtrans spectroscopy in our study, is somewhat higher, viz. 0.25-0.3 eV, once again in good agreement with the previous results for the CNM fabricated from the BPT SAM (Vtrans(+) = 0.24; Vtrans(-) = −0.23 V).32 An additional parameter, studied in context of irradiation-induced changes, was the work function. The dependence of this parameter on irradiation dose for the OPh, acene, and OPE SAMs is presented in Figure 10. With the exception of PT, a small progressive increase in the work function was observed for all SAMs studied. The largest increase of 0.18-0.25 eV was observed for the acene SAMs, followed by the OPh films (0.18-0.2 eV), and OPE monolayers (~0.12 eV). No significant difference in the work function behavior between the two-rings and three-rings systems of the same type was observed. In contrast, the only one-ring system studied, PT/Au, exhibited a rapid decrease in the work function at low irradiation doses (up to 10 mC/cm2) and a leveling-off behavior at higher doses. The reason for this special behavior is not clear at the moment but it is presumably related to both a limited quality of the pristine PT SAM as compared to the two- and three-rings systems47 and a limited ability of this onering monolayer to form an extensive, 2D cross-linking network upon electron irradiation.27

4. Discussion Electron irradiation was found to affect significantly the electric conductance properties of all three basic types of aromatic SAMs studied in the present work. The resistance of these films increases progressively in the course of irradiation, with a more rapid, exponentially-like increase at low doses and a slower increase at high doses, exhibiting a transition to a levelingoff behavior. The respective plots for the one-, two-, and three-rings systems within the OPh, 9 ACS Paragon Plus Environment


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acene, and OPE series are very similar to each other, apart from the vertical offsets, reflecting the difference in the resistance of the pristine SAMs. The electric conductance properties of the irradiated SAMs could be described well by the "inverse" Simmons' equation, R = R0 exp(ßd), suggesting its validity for these systems, considered not as molecular assemblies but quasi-homogeneous hydrocarbon films with extensively cross-linked and annulated phenyl rings. The relevant parameters, viz. β and R0, could be derived and compared to those of the pristine monolayers. The tunneling attenuation factor, β, decreases progressively, and somewhat moderately, in the case of the OPh and OPE SAMs, viz. from 0.52 A-1 to 0.29 A-1 (40 mC/Cm2) for OPh/Au and from 0.33 A-1 to 0.19 A-1 (40 mC/Cm2) for OPE/Au (Figure 6b). It contrast, it increases slightly, viz. from 0.49 A-1 to 0.59 A-1 (40 mC/Cm2) for the acene monolayers. This difference is presumably related to the annulated character of the initial acene moiety, so that the effect of irradiation-induced cross-linking, playing the major role in the case of aromatic SAMs,27 is less productive for the acene films as compared to the OPh and OPE ones. Note also that a noticeable decrease in β was not observed for the OPh-stemming CNMs in the previous study;32 however, the β value in ref 32 was based on the tunneling resistance values for two-, three, and four-ring systems, with the exclusion of PT/Au. Consideration of the one-, two-, and three-rings systems, as in the present work, would also give a lower β value for these CNMs as compared to the original SAMs. But the strongest effect was observed for the contact resistance (R0), which increased significantly for all three types of aromatic SAMs studied (Figure 7). Note that such a strong increase was observed previously for the CNM fabricated from the OPh SAMs, confirming, thus, our results for this particular type.32 Interestingly, the behavior of R0 as a function of irradiation dose is very similar to that of the average total resistance (Figure 4), showing a more rapid increase at low doses and slower increase at high doses, with a transition to a leveling-off behavior. Also the range of the R0 change, viz. 2-3 orders of the magnitude upon irradiation with a dose of 40 mC/cm2 corresponds coarsely to the range of the R change. Note that due to the validity of the simplified Simmons' equation for the irradiated SAMs, it is not the absolute value of R0 but its logarithm which is of importance for the total resistance. This suggests that the change in R0 is the major reason for the change in the total resistance of the aromatic SAMs upon electron irradiation. In this context the behavior of R0 becomes of a particular importance. Several reasons for this behavior can be considered. 10 ACS Paragon Plus Environment

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The first factor, which certainly makes an impact, is a damage of the SAM-substrate interface, progressing in an exponential fashion in the course of irradiation.27 However, according to the spectroscopic data (see ref 27 and Figures S2-S5 in the Supporting Information), this process occurs with a higher rate as compared to the change of R0 and about 50% of the pristine thiolate groups, providing the anchoring to the substrate, still persist even at high doses (40 mC/cm2), as shown schematically in Figure 11. Such a reduction, by only a factor of 2, cannot completely explain the observed increase in R0, by several orders of magnitude. The second possible reason is a change in the tunneling barriers for the charge carriers at the SAM-substrate and SAM-EGaIn interfaces. Such a change occurs indeed upon electron irradiation, as monitored by the behavior of Vtrans, assuming its widely accepted meaning as a fingerprint parameter for the height of this barrier.8,33-36 A noticeable decrease in the Vtrans value is observed for all SAMs of this study where this parameter could be traced. However, a decrease in the Vtrans value means presumably a decrease in the tunneling barrier at the interfaces to the electrodes, which can be hardly associated with the exponentially increasing R0. The third possible reason is the change in the work function (WF), which generally affects the height of the tunneling barrier at the SAM-electrode interfaces. However, the behavior of WF for the PT SAM and the two-/three-rings systems upon irradiation is distinctly different, whereas a similar increase in resistance is observed for all these films. Also, a higher WF, as observed in the given case for the irradiated SAMs (two- and three-rings systems), is generally associated with lower contact resistance,8 which is opposite to the observed tendency. Note that apart from the possible effect on R0, the change in the WF of the SAMs studied correlates with the change in Vtrans: the increase in WF is accompanied by the decrease in Vtrans, similar to the case of the pyrimidine-substituted aromatic SAMs.9 Finally, there is a further additional factor, which is frequently ignored in context of electron irradiation of SAMs, viz. adsorption of airborne molecules on their surface upon the exposure of the irradiated SAMs to ambient, triggered by their enhanced (as compared to the pristine films) chemical reactivity.48,49 As a result of such an adsorption, an additional overlayer of presumably weakly conducting molecules will be formed (as shown schematically in Figure 11), resulting in an increase of R0. Significantly, as demonstrated by the dedicated experiments on the BPT, TPT, OPE2, and OPE3 SAMs, the thickness of such an additional overlayer increases progressively in the course of irradiation achieving, e.g., ~0.35 nm at 10 mC/cm2 and ~0.8 nm at 40 mC/cm2 for the OPE2 monolayer. Such an increase in thickness 11 ACS Paragon Plus Environment


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should lead to an exponential increase in resistance associated with the additional overlayer, exactly as we observe for R0. Following the above discussion, we believe that the most important factors in context of the observed, exponential increase in R0 in the course of irradiation are the partial damage of the original thiolate anchors at the SAM-substrate interface and progressive adsorption of airborne molecules on the SAM-ambient interface. A further factor which can also be relevant to some extent for the electric conductance of the irradiated aromatic SAMs is quantum interference. As shown by the experiments on model arylethynylene thiolates, the electrical conductance of aromatic molecules with linear conjugation is higher than that of analogous molecules with cross-conjugation and brokenconjugation.50,51 The electron irradiation of the aromatic SAMs of this study results in creation of different local structural motifs within the electron-induced cross-linking network,17-19 included those with cross-conjugation and broken-conjugation, which, generally, should result in a decrease of electrical conductance. However, such a decrease should occur progressively with increasing length of the molecular backbone, predominantly affecting the attenuation factor and much less the contact resistance. Thus the effect of quantum interference on R0 is presumably small but is probably of importance for β, in particular in the case of acenes (model units in refs 50 and 51) where an increase in β upon irradiation was observed (Figure 6b). The final parameter describing the electric conductance properties of the irradiated aromatic SAMs is Vtrans, which was already partly discussed above, in context of the R0 behavior. As mentioned above Vtrans decreases progressively in the course of irradiation, achieving a similar absolute value of 0.21-0.25 V for all three-rings SAMs studied and only a slightly higher value of 0.25-0.3 eV for the OPE2 film, the only two-rings system of this study where Vtrans could be monitored. The most important conclusion in this regard is the progressive decrease in the width of the HOMO-LUMO gap in the course of irradiation, in agreement with the published spectroscopic data for a SAM with a terphenyl-based backbone.30,31

5. Conclusions We studied the effect of electron irradiation on electric conductance properties of most basic aromatic SAMs with OPh, acene, and OPE backbones. The energy of electrons was set to 50 eV. The length of the backbone was varied from one to three phenyl rings. Whereas the standard "through-bond" charge transport pathway was assumed for the pristine SAMs, 12 ACS Paragon Plus Environment

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"through-space" conductance was believed to be more appropriate for the irradiated ones, since, due to the extensive cross-linking, they are better described as quasi-homogeneous materials than as assemblies of weakly interacting molecules. Electron irradiation was found to affect significantly the electric conductance properties of the SAMs studied, with mostly similar behavior for all three basic types of the aromatic backbone. The resistance of the two-terminal junctions containing these SAMs increased progressively over 2-3 orders of the magnitude at a dose variation up to 40 mC/cm2. For the irradiated SAMs, the resistance could still be described by the standard equation, R = R0 exp(ßd), but the values of ß and especially R0 changed noticeably as compared to those for the pristine SAMs. The ß values decreased to some extent in the course of irradiation for the OPh and OPE films but increased slightly for the acene layers, ascribed to a lesser change in the degree of annulation. The most dramatic variation was however observed for R0 which exhibited a significant growth with increasing dose. Since the behavior of R0 was very similar to that of the total resistance, it was considered as the major reason for the observed increase in the total resistance upon irradiation. In its turn, the behavior of R0 was analyzed in detail and explained by the partial damage of the anchoring to the substrate at the SAM-substrate interface and progressive adsorption of airborne molecules at the SAM-ambient interface of the irradiated SAMs. The work function, which was also considered as possible factor for the R0 change, was found to increase slightly in the course of irradiation for the two- and threerings systems, and to decrease rapidly for PT/Au, underlying once more a special behavior of this one-ring system in terms of irradiation-induced cross-linking. Following the Fowler-Nordheim representation of the experimental data, behavior of Vtrans, characteristic of the transition between the "classical" tunneling and the field emission regime, could be monitored for all three-rings systems and the OPE2 film as well. The absolute value of Vtrans for both bias directions was found to decrease continuously in the course of irradiation, achieving the same value of 0.21-0.25 V for all three-rings films and 0.25-0.3 eV for the OPE2 film. This behavior suggests a progressive decrease in the onset of the HOMO level with respect to the Fermi level and, consequently, analogous decrease in the width of the HOMO-LUMO gap upon irradiation. Considering the above results in context of applications, the band gap engineering of aromatic SAMs by electron irradiation can be probably useful for energy level alignments at the interfaces where these SAMs are used as intermediate layers. The related changes in the work function are comparably small. Significantly, the irradiated aromatic SAMs remain insulators, 13 ACS Paragon Plus Environment


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with their β changing only moderately and their contact resistance increasing significantly as compared to the pristine monolayers.

Associatted content Supporting Information The Supporting Information is available free of charge on the ACS Publication website at DOI: Schematic of the EGaIn tip fabrication; representative spectroscopic data for the TPT, AnthT, and OPE3 SAMs (PDF).

Author information Corresponding Author *Phone: +49 6221 54 4921. Fax: +49 6221 54 6199. E-mail: [email protected]

Notes The authors declare no competing financial interest.

Acknowledgement We thank Matthias Füser for the synthesis of the OPE2 compound and Patrick Antoni for the assistance during the exposure-to-ambient experiments. M.Z appreciates a financial support of the German Research Society (Deutsche Forschungsgemeinschaft; DFG). We thank Ryan Chiechi (Groningen) and the members of his group for useful advices regarding the fabrication of the EGaIn electrodes.


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Figure Captions Figure 1. The structures of the precursors for the SAM studied, along with their abbreviations. The precursors build three different series, with oligophenyl (OPh), acene, and OPE backbones, respectively. The length of the backbone is varied from one to three phenyl rings. PT serves as the first member of all three series. Figure 2. Schematic of the two-terminal tunneling junction setup with the top electrode provided by an EGaIn tip. BPT SAM is taken as a representative example. Figure 3. (a) Semilogarithmic plots of the current density versus voltage for the two-terminal junctions comprising the pristine aromatic SAMs of this study. (b) Semilogarithmic plots of the current density (at a bias of −0.5 eV) versus molecular length for the two-terminal junctions comprising these SAMs. The legends are given in the panels. Figure 4. Semilogarithmic plots of the average tunneling resistance versus irradiation dose for the OPh (a), acene (b), and OPE (c) SAMs. The legends are given in the panels. Figure 5. Semilogarithmic plots of the average tunneling resistance versus molecular length (pristine SAMs) or effective film thickness (irradiated SAMs) for the two-terminal junctions comprising the pristine and irradiated OPh (a), acene (b), and OPE SAMs. The data for the pristine SAMs and those irradiated with 10, 20, and 40 mC/cm2 are shown by black squares, green triangles, red points, and blue diamonds, respectively. The linear fits to the experimental points are drawn by the dashed lines shown in the respective colors. Figure 6. (a) Tunneling decay constant versus bias voltage for the two-terminal junctions comprising the pristine and irradiated aromatic OPh SAMs of this study, representative of the acene and OPE monolayers as well. These constant are based on the current values measured for every bias. The data for pristine SAMs and those irradiated with 10, 20, and 40 mC/cm2 are shown by black squares, green triangles, red points, and blue diamonds, respectively. (b) Tunneling decay constant versus irradiation dose for the two-terminal junctions comprising the pristine and irradiated OPh (dark yellow circles), acene (purple squares), and OPE (navy triangles) SAMs. These constants are based on the average resistance values, shown in Figure 5. Figure 7. Semilogarithmic plots of the contact resistance versus irradiation dose for the twoterminal junctions comprising the pristine and irradiated OPh (dark yellow circles), acene (purple squares), and OPE (dark blue triangles) SAMs. The dashed curves, shown in the respective colors, are exponential fits to the experimental points. 20 ACS Paragon Plus Environment

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Figure 8. Fowler-Nordheim (FN) plots for the two-terminal junctions comprising the pristine and irradiated OPE2 (a) and AnthT (b) SAMs. The data for the pristine SAMs and those irradiated with 10, 20, and 40 mC/cm2 are shown by black squares, green triangles, red circles, and blue diamonds, respectively. Figure 9. Transition voltage at the positive (black squares) and negative (blue circles) bias versus irradiation dose for the two-terminal junctions comprising the pristine and irradiated AnthT (a), TPT (b), OPE2 (c), and OPE3 (d) SAMs. Figure 10. Work function of the OPh (a), acene (b), and OPE SAMs versus irradiation dose. The systems with one, two, and three phenyl rings are shown by black squares, red circles, and blue triangles, respectively. The legends are given in the panels. Figure 11. Schematic of the changes induced in aromatic SAMs by electron irradiation and subsequent exposure of these films to ambient. BPT is taken as a representative example.



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Figure 1


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Figure 2



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logJ (A/cm2)

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PT BPT TPT NphT AnthT OPE2 OPE3

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OPh acenes OPE

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10

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14

16

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22

Molecular length (Å)

Figure 3


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log R (Ω.cm2)

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Figure 4

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b

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1 0 0

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15

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pristine 10 mC/cm2 20 mC/cm2 40 mC/cm2

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Figure 5



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0.1 0.0 -0.6

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Figure 6


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Figure 7



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a

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Figure 9


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Work function (meV)


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Effect of Electron Irradiation on Electric Transport Properties of

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