Charge Transport through Carbon Nanomembranes - The Journal of

Article pubs.acs.org/JPCC

Charge Transport through Carbon Nanomembranes Paul Penner,† Xianghui Zhang,*,† Emanuel Marschewski,† Florian Behler,‡ Polina Angelova,† André Beyer,† Jens Christoffers,‡ and Armin Gölzhaü ser† †

Fakultät für Physik, Universität Bielefeld, 33615 Bielefeld, Germany Institut für Chemie, Carl von Ossietzky Universität Oldenburg, 26111 Oldenburg, Germany

‡

S Supporting Information *

ABSTRACT: Molecular junctions incorporating pristine and cross-linked aromatic self-assembled monolayers (SAMs) are fabricated and investigated. A two-terminal setup composed of a eutectic Ga−In (EGaIn) top electrode and the gold substrate on which SAMs are prepared as a bottom electrode was used to characterize the charge transport. SAMs of phenylthiol (PT), biphenylthiol (BPT), p-terphenylthiol (TPT), and pquaterphenylthiol (QPT) are then irradiated with low-energy electrons and converted into carbon nanomembranes (CNMs). A comparison of charge transport through SAMs and CNMs reveals a decrease of conductance of CNM-based junctions by 1 order of magnitude, as well as a conversion of asymmetric junctions with SAMs into symmetric junctions with CNMs, which could be attributed to the decoupling of CNMs from the Au substrate and the partial loss of aromaticity of CNMs after irradiation. Transition voltage spectroscopy (TVS) was also employed to investigate both types of junctions. We observe the length-dependent behavior of transition voltages in both systems and a reduction of transition voltages of CNM-based junctions in comparison to SAM-based junctions. junctions.19−21 The nature of molecule-contact interactions, rather than the change in molecular structures, seems to play a dominating role in the junction conductance of aromatic molecules strongly coupled to the electrode.22 Therefore, one prospect of molecular electronics is to create a flexible yet stable molecular system that is readily adapted to other substrates and less affected by the contact. To this end, we propose that carbon nanomembranes (CNMs) could be considered as novel building blocks for molecular electronics. CNMs are a type of quasi-two-dimensional (2D) material that are prepared via cross-linking of highly ordered aromatic SAMs (we will use the term “CNMs” and “cross-linked SAMs” interchangeable in this report).23 The cross-linking can be done via radiation with electrons,24 photons,25 and ions.26 The lateral cross-linking gives rise to enhanced thermal stability27 and mechanical strength,28 which allows the CNM being released from the initial substrate and transferred onto arbitrary substrates to form suspended membranes with macroscopic lateral size.29 We recently demonstrated that a variety of polyaromatic molecules can be used to fabricate freestanding CNMs with a thickness between 0.5 and 3 nm and with tunable mechanical stiffness, as well as with and without nanopores.30,31 While a molecular junction incorporating a SAM is an ensemble of many single-molecular junctions, a molecular junction comprising a CNM should be considered as a single

1. INTRODUCTION Highly ordered self-assembled monolayers (SAMs) provide functional building blocks for large-area molecular electronics applications and may offer a promising route to the fabrication of reliable and stable devices.1−3 For small-area molecular junctions incorporating SAMs, different methods have been used to characterize the electronic transport properties, such as scanning tunneling microscopy (STM),4 conducting-probe atomic force microscopy (CP-AFM),5 nanopore devices,6 and cross-wire junctions.7 To characterize large-area molecular junctions incorporating SAMs, a variety of approaches have been applied, such as using protective interlayer for lithographically defined interconnects,8,9 or directly using liquid metal top electrodes like Hg 10,11 and eutectic Ga−In (EGaIn).12 Among these methods, using EGaIn top-electrodes appears less destructive to SAMs and thus allows the collection of large numbers of data out of many working junctions. Despite the inconsistency in values of current density for EGaIn-based junctions in comparison with other types of junctions, the fact that the values of tunneling decay constants fall within the consensus range makes it a reliable technique for studying charge transport through SAMs.13 Whereas molecular junctions comprising aliphatic SAMs have been intensively investigated by various techniques, aromatic molecular junctions exhibit more interesting features and functions, e.g. charge transfer dynamics,14,15 quantum interference effect,16,17 and photoswitchable molecular cargo lifter.18 Different contact materials have been investigated in order to obtain high-yield and reproducible molecular © 2014 American Chemical Society

Received: July 4, 2014 Revised: August 25, 2014 Published: August 25, 2014 21687

dx.doi.org/10.1021/jp506689n | J. Phys. Chem. C 2014, 118, 21687−21694

The Journal of Physical Chemistry C

Article

Figure 1. (a) Schematic diagrams of molecular junctions: Au-BPT−SAM//Ga2O3/EGaIn (left), Au|BPT−CNM//Ga2O3/EGaIn (middle), and Au//annealed BPT−CNM//Ga2O3/EGaIn (right). (b) Characteristics of current density of BPT−SAM-based junctions with the comparison between the histogram of log|J| versus voltage and the Gaussian mean (dash dot) of log|J|. (c, d) Characteristics of current density of BPT−CNM and annealed BPT−CNM-based junctions, respectively.

150 LAB FHR) for 5 min, rinsed with ethanol, and then blown dry under a nitrogen stream. Afterward the substrates were immersed into a ∼10 mM solution of PT or BPT in dry and degassed dimethylformamide (DMF) for 72 h in a sealed flask under nitrogen atmosphere at room temperature. After the SAM formation, the samples were rinsed thoroughly with DMF and ethanol, and then blown dry under a nitrogen stream. For the preparation of p-terphenylthiol and p-quaterphenylthiol SAMs, the same procedure was applied with the exception of SAM formation for 24 h in a sealed flask at 70 °C. Cross-linking of SAMs was achieved in high vacuum (5 × 10−8 mbar) with an electron flood gun at an electron energy of 100 eV and with an electron dose of 50 mC/cm2 being applied. Annealing of crosslinked SAMs was performed in an electron beam heating stage in ultrahigh vacuum (base pressure: 5 × 10−10 mbar), where a molybdenum sample holder was placed ∼3 mm from a hot filament that acts as an electron source. 2.2. Charge Transport Measurements of Molecular Junctions. The two-terminal setup consists of a grounded Au substrate mounted on an X−Y positioning stage and a syringe with a flat needle connected to a Z axis micrometer positioning stage. The setup was placed inside a home-built aluminum Faraday cage to prevent electromagnetic influences. Furthermore, the cage can be flushed with gases. A certain amount of Ga−In eutectic (75.5 wt % Ga and 24.5 wt % In) was stored in a syringe with a flat-profiled metallic needle that was electrically connected to the measurement unit. The tip was prepared by generating a small droplet of EGaIn and bringing it into contact with a sacrificial Au substrate on which the EGaIn adheres, and then retracting the syringe slowly until the EGaIn tip on the needle separates from the remaining EGaIn tip on the Au substrate. The tip formation process was observed by a microscope camera and the cage was under ambient conditions during this process. After the tip formation, the cage remained open for 15 min to form an oxide layer (Ga2O3) around the EGaIn tip. During the measurements, the Faraday cage was

junction composed of a two-dimensional (2D) system consisting of covalently linked molecules. Relevant studies have shown that cross-linking is accompanied by changes in the electronic structures of a terphenyldimethanethiol (TPDMT) SAM where a reduction in the gap between the highest occupied molecular orbitals (HOMO) and the lowest unoccupied molecular orbitals (LUMO) was observed.32,33 Pristine CNMs are insulating and structurally amorphous but can be converted into well-conducting nanocrystalline graphene upon thermal treatment.34,35 The fabrication of all-carbon vertical heterostructures of amino-terminated CNMs and single-layer graphene indicates that CNMs can be used as a 2D dielectric in graphene-based electronics.36 Therefore, understanding the charge transport through CNMs along the vertical direction is of fundamental importance and we also expect some insights into potential applications like incorporating CNMs as ultrathin dielectric layers in molecular electronics. The present study aims at comparing the electronic transport characteristics of molecular junctions comprising pristine and cross-linked SAMs of aromatic molecules of different lengths. We studied SAMs and CNMs from phenylthiol homologues, i.e. phenylthiol (PT), biphenylthiol (BPT), p-terphenylthiol (TPT), and p-quaterphenylthiol (QPT). Transition voltage spectroscopy (TVS) was also used to compare both types of molecular junctions.

2. EXPERIMENTAL SECTION 2.1. Sample Preparation. Precursor molecules used in this study were bought from Sigma-Aldrich (phenylthiol and biphenylthiol) or specially synthesized (p-terphenylthiol and p-quaterphenylthiol). The synthesis of p-quaterphenylthiol is described in the Supporting Information. For the preparation of phenylthiol (PT) and biphenylthiol (BPT) SAMs, we used a 100 nm polycrystalline Au layer thermally evaporated on 9 nm Ti primed Si(100) substrates (Georg Albert PVD, Germany). The substrate was cleaned with a UV/ozone cleaner (UVOH 21688



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Figure 2. (a) Comparison of log|J| versus voltage for molecular junctions composed of pristine and cross-linked SAMs. All the standard deviations of log|J| are ∼1 (see Figure 1b,c). (b) The representative J−V curves for BPT−SAM and BPT−CNM as a comparison. Notice that there is a 10-fold difference in the scales of current density.

closed and flushed with nitrogen to maintain a low level of humidity (RH = 12%). Molecular junctions were formed by bringing the soft Ga2O3/EGaIn tip into contact with SAMs or CNMs formed on Au substrates. No residues were observed on the surfaces after retracting the Ga2O3/EGaIn tip from any of the samples under investigation. For statistical analysis, I−V curves were measured on each sample with five different tips, where each tip was used to form up to five junctions. The contact area was estimated by analyzing the image taken by the camera. The current through the junction was recorded as a function of the applied voltage by using a source measurement unit (Keithley 236).

CNM//Ga2O3/EGaIn, in which a further decrease in the current density was observed, due to the lack of chemical bonds between the CNM and the Au substrate, as shown in Figure 1d. Figure 2a shows the plot of Gaussian means of current densities versus voltage for molecular junctions incorporating SAMs and CNMs of the whole series of phenylthiol homologues studied. On the whole, the current density J decreases with increasing molecular length and the J values of junctions incorporating SAMs are approximately 1 order of magnitude higher than those of junctions incorporating CNMs for all four precursor molecules. Taking a closer look at the difference in the J−V curves between junctions with BPT− SAM and BPT−CNM, as shown in Figure 2b, we found two additional changes: first, a slightly asymmetric behavior for SAMs becomes more symmetric for CNMs; second, both J−V curves show different bias-dependent nonlinear characteristics. For a densely packed BPT−SAM, the charge rearrangements are mainly localized at the immediate interface and decay rapidly to the gold substrate and the first phenyl ring.39,40 A bond dipole is often used to picture the potential energy step across the Au/SAM interface, which shifts the electrostatic potential of the SAM relative to EF and thus leads to a final level alignment. On the contrary, the internal electronic structure of the SAM is perturbed by the bond formation between metal and molecules.39,40 After electron irradiation, the lateral linking of molecules leads to a modified internal electronic structure. It was reported that the cross-linking resulted in a reduction in the HOMO−LUMO gap of BPT−SAM.38 As previously mentioned, the cross-linked TPDMT−SAM was also demonstrated to exhibit a decrease in the HOMO−LUMO gap.32,33 A partial loss of aromaticity in the cross-linked SAMs of BPT and TPT was identified from the near-edge X-ray absorption spectroscopy (NEXAFS) and the high resolution electron energy loss spectra (HREELS), respectively.38,41 This phenomenon is likely related to the formation of isolated graphitic species which are electrically isolated by disordered aliphatic carbon, as a consequence of the reorientation of molecules and the disintegration of structural orderings. Nevertheless, the internal molecular structure of the CNM is still not fully known. It has been described by a disordered metastable state that is formed in the energetic initial process of electron irradiation and depends on the process of relaxation into the sheet phase.42 After electron irradiation, the lateral linking of molecules also leads to a modified interfacial electronic structure at the CNM/ Au interface: First, the electron irradiation induces changes

3. RESULTS AND DISCUSSION Two terminal I−V measurements were performed on the prepared molecular junctions under a nitrogen atmosphere to minimize the influence of adsorbed water at interfaces. Figure 1a illustrates three molecular junctions comprising BPT molecules. To highlight the different couplings between molecules and the two electrodes, as an example, we label molecular junctions with the pristine BPT−SAM as Au-BPT− SAM//Ga2O3/EGaIn and the BPT−CNM as Au|BPT− CNM//Ga2O3/EGaIn, respectively, where “|” denotes a reduced density of chemical bonds at the interface after electron irradiation and “//” denotes the lack of chemical bonds at the interface. Figure 1b,c shows representative 2D histograms of log|J| as a function of applied voltages for molecular junctions based on BPT−SAM and BPT−CNM, where J is the current density. The color gradient depicts the number of J−V measurement points falling into the intervals [Vi − 0.005,Vi + 0.005] and [log|Ji| − 0.05,log|Ji| + 0.05] with (Vi,log|Ji|) being the midpoint. The measured current densities spread over around 2 orders of magnitude, which makes it necessary to treat the data with statistical methods. To this end, we adopted a statistical model for data analysis, i.e. fitting a Gaussian function to histograms of log|J| and extracting the Gaussian mean and its standard deviation as fitting parameters, as recommended by Reus et al.37 The Gaussian mean coincides with the apparent peak of the histogram, indicating a normally distributed current. We also studied junctions incorporating BPT−CNM that have been annealed at a low temperature (320 °C) in ultrahigh vacuum (UHV), where the annealing treatment does not cause any structural transformation of CNMs, but only the cleavage of all C−S bonds at the interface.38 We label such junctions as Au//annealed BPT− 21689



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Table 1. Summary of Charge Transport Measurements through SAMs and CNMs

Figure 3. (a) Plot of the tunneling decay constant β versus positive (upper part) and negative (lower part) bias voltages (the bottom Au electrode is grounded). The gray bars mark the ranges of transition voltages of CNMs. (b) A semilog plot of low-bias (±0.1 V) tunneling resistance R as a function of thickness for two types of junctions. Notice that the partially cross-linked PT−SAM was excluded from the curve fitting.

To quantify the length dependence, we adopted the simplified approximation of the Simmons model, J ∝ J0 exp(−βd), in which the current density follows an exponential decay with molecular length.45 From the least-squares fitting of the current density J with respect to the effective thickness d of the molecular junctions we could obtain the tunneling decay constant β of these systems. In comparison to SAMs, the thicknesses of the BPT, TPT, and QPT CNM are slightly reduced by ∼10%, as obtained from the X-ray photoelectron spectroscopy (XPS) and shown in Table 1. A significant reduction in the thickness of the irradiated PT−SAM, i.e. from 6 to 3.4 Å, can be attributed to the desorption of PT molecules from the surface and the formation of some partially crosslinked PT molecules. The inefficient cross-linking is likely due to the fact that PT molecules lack the torsional degrees of freedom and the carbon density of the PT−SAM is smaller than that of graphene. We label an irradiated PT−SAM as cPT−SAM to describe the partially cross-linked SAM. As a consequence, the c-PT−SAM is excluded in the determination of the tunneling decay constants of CNMs. Figure 3a shows the tunneling decay constant in the bias range between −1 V and 1 V, where the β values at positive and negative bias were separately plotted. The obtained βSAM values

such as the dissociation of Au−S bonds (∼35%) and the formation of physisorbed organosulfur species;38 second, there is a remarkable change in the morphology of underlying substrate, as observed in the irradiated nitrobiphenylthiol (NBPT) SAM on Cu with scanning tunneling microscopy (STM).43 Moreover, the interfacial electronic structure at the Au|CNM interface also could be perturbed by the aforementioned modified internal electronic structure of the CNM. As shown in Figure 2b, a slightly asymmetric behavior for SAMs became more symmetric for CNMs. We used the rectification ratio (r ≡ |J(1 V)/J(−1 V)| to quantify this asymmetry.44 Table 1 shows a rectification ratio of ∼2 for SAMs and a rectification ratio of ∼1 for CNMs of BPT, TPT, and QPT (see Figure S3 in the Supporting Information). The unity values for the rectification ratios of CNMs suggest a similar barrier at Au| CNM and CNM//Ga2O3/EGaIn interfaces. Relating those previous studies with our results, we could attribute the remarkable decrease in conductance of CNM-based junctions to an enhanced barrier emerging at the Au|CNM interface and a partial loss of aromaticity of the CNM as well as a modified alignment of molecular orbitals with respect to the Fermi levels of the electrodes. 21690



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Figure 4. (a) Fowler−Nordheim (FN) plots for molecular junctions of Au-SAM//Ga2O3/EGaIn and Au|CNM//Ga2O3/EGaIn. (b) The corresponding transition voltages for both bias polarities versus the number of phenyl rings of four precursor molecules.

decrease from 0.50 ± 0.07 Å−1 to 0.40 ± 0.05 Å−1 with increasing the bias voltage from 0.05 V to 1 V. For negative bias voltages, the βSAM values decrease from 0.51 ± 0.05 Å−1 to 0.37 ± 0.03 Å−1 with decreasing the bias voltage from −0.05 V to −1 V. This slight bias-dependent behavior is attributed to the barrier lowering effect due to the applied bias, which was observed for alkanethiol SAMs with use of nanopore devices.46 The differences in the βSAM values for both bias polarities might originate from the asymmetric tunneling path. As a whole, the βSAM values we obtained are generally in agreement with the β values reported for oligophenylene SAMs. For example, Holmlin et al. reported β values of 0.61 Å−1 for oligophenylene thiols.47 β values of molecular junctions composed of SAMs from benzylic derivatives of oligophenylene thiols, i.e. HSCH2(C6H4)m−1C6H5 (m = 1, 2, 3), were obtained by different methods, such as CP-AFM (β = 0.35−0.5 Å−1),48,49 STM (β = 0.55 Å−1),50 and Hg top electrode methods (β = 0.67 Å−1).47 Surprisingly, the βCNM values decrease more dramatically with increasing voltages until around ±0.2 V and then appear constant with an average value of 0.42 ± 0.01 Å−1 for positive bias and 0.45 ± 0.01 Å−1 for negative bias, respectively. It is apparent that characteristics of charge transport through CNMs are different from those of SAMs, especially at low bias, which will be discussed by considering the transition voltages in the following paragraphs. To understand the drop in βCNM for small bias voltages, we determined the tunneling resistance R of junctions directly calculated from the linear fittings of J−V curves at small voltages. As the resistance increases exponentially with increasing the molecular length, using the relation R ∝ R0 exp(βRd), the decay constant βR (we use this symbol to distinguish it from the aforementioned β derived from J) and the contact resistance R0 of junctions can be extracted as slope and intercept from the R−d curves (see Figure 3b). To avoid ambiguity, the partially cross-linked PT−SAM was excluded from the curve fitting. The βRCNM values also show biasdependent behavior as a result of the nonlinearity of J−V curves even at small bias, which could be associated with the emerging tunneling barrier at the Au|CNM interface. Within the bias voltage range of −0.1 V to 0.1 V, the βR values are estimated to be 0.50 ± 0.05 Å−1 for SAMs and 0.53 ± 0.05 Å−1 for CNMs, respectively (see Figure 3b). This is in good agreement with the β values determined from current densities at voltages of ±0.1 V. The R0 values are estimated to be 6 Ω·cm2 for SAMs and 164 Ω·cm2 for CNMs, respectively. As Au-SAM//Ga2O3/

EGaIn and Au|CNM//Ga2O3/EGaIn junctions have the identical van der Waals top interface as well as the same specific resistance RGa2O3 of the Ga2O3 layer, the enhancement of the tunneling resistance R can only be accounted for by the modifications at the bottom interface and changes in the resistance RCNM of the CNM. The former has also been verified by the further decrease of the conductance of Au//annealed BPT−CNM//Ga2O3/EGaIn junctions (see Figure S2 in the Supporting Information), which is associated with the further enhanced tunneling barrier that suppresses the electron transmission across the Au//annealed CNM interface. The latter is associated with the modified internal electronic structure of the CNMs. The transition voltage refers to the minimum in a Fowler− Nordheim (FN) plot. Beebe et al. interpreted it as a transition from deep-tunneling into field emission transport.51 Later, transition voltage was interpreted within a coherent transport model based on a single electronic level.52 Since then transition voltage spectroscopy (TVS) has been considered as a spectroscopic tool to investigate the position of the frontier molecular orbital relative to the electrode Fermi level.53,54 However, most experimental and theoretical studies on TVS are based on identical electrodes. It is not well understood whether molecular orbitals can be detected with a Ga2O3/ EGaIn top electrode. Instead of molecular orbitals, the oxide state of the Ga2O3/EGaIn electrode was considered to be the closest state to the Fermi level of the electrode for alkanethiol SAMs.55 Surprisingly, Fracasso et al. demonstrated that using EGaIn as a top electrode is sufficiently sensitive to resolve the difference between two series of SAMs (phenyl- and pyridylterminated SAMs) with different values of EF − EHOMO.56 Therefore, we expect that TVS could be used to distinguish two systems and help us understand more about our molecular junctions. To perform TVS, all the J−V curves in Figure 2a were replotted in the form of the above-mentioned FN plot. The Gaussian means were taken to depict the characteristic FN plots of junctions from both SAMs and cross-linked SAMs, as shown in Figure 4a. Taking into consideration that the asymmetry of the junction could have an impact on the transition voltage, we determined both Vtrans(+) for positive bias and Vtrans(−) for negative bias voltages from FN curves. The positive bias corresponds to the case where the Au substrate is grounded and a positive voltage is applied to the Ga2O3/EGaIn electrode. To have a more detailed understanding of transition voltages, we performed a statistical analysis of the measured J− 21691



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4. CONCLUSIONS In summary, we have compared the charge transport through pristine and cross-linked self-assembled monolayers (SAMs) of oligophenyl derivatives that form on Au substrates using EGaIn as a top electrode. The current densities of CNM-based junctions in the investigated bias range are approximately 1 order of magnitude lower than those of the corresponding SAM-based junctions, which could be accounted for by the enhanced tunneling barrier at the Au|CNM interface and the partial loss of aromaticity of CNMs in comparison to pristine SAMs. The slightly asymmetric SAM-based current−voltage characteristics become more symmetric after cross-linking due to the decoupling of CNMs from the Au substrate after irradiation. The tunneling decay constant βSAM values decrease with increasing voltages from both bias polarities, whereas the βCNM values drop fast until a certain voltage and then appear constant. The range of this turning point can be related to the transition voltages. Transition voltages of both types of molecular junctions show length-dependent behavior, implying the participation of molecular orbitals in the junction conductance. Transition voltages of CNM-based junctions are relatively lower than those of their SAM-based counterparts, which could be attributed to the decreased HOMO−LUMO gap and the broadening of molecular orbitals after electron irradiation.

V curves. Due to the lack of informative features for the determination of transition voltages, almost half of the J−V curves were excluded from the statistics. The Gaussian means and their standard deviations of the transition voltages are shown in Figure 4b (also in Table 1) and all histograms are displayed in Figure S4 in the Supporting Information. For both types of junctions, there is a tendency that |Vtrans(±)| values decrease with increasing molecular length, except for the QPT-based junctions whose |Vtrans(±)| are comparable with those for TPT. This indicates that TVS is very likely related to the molecules in the junction. It was suggested elsewhere that the HOMO is the closest molecular level in aromatic molecular junctions (PT, BPT, and TPT), based on the measured positive thermopower of the junctions.57,58 If we assume that the obtained Vtrans(+) values are related to the energy separation between the metal Fermi level and the HOMO (Δ = EF − EHOMO), it must be pointed out that the Vtrans(+) values of AuSAM//Ga2O3/EGaIn junctions are 0.2−0.3 V smaller than the transition voltages of Au-SAM//Au junctions.57,59 This finding supports the statement that Vtrans values are related to the energetic separation Δ between the metal Fermi level and the closest molecular orbital, but not identical to the Δ value. Similar results were reported on alkanethiol SAMs, where Vtrans values determined by using the Ga2O3/EGaIn top electrode are about 0.8 V smaller than those determined with CP-AFM using Au coated tips.55 This offset could be attributed to the different work functions of two electrodes (ΦAu = ∼5.3 eV, ΦEGaln = ∼4.3 eV). Surprisingly, our |Vtrans(−)| values are bigger and have a much wider distribution than Vtrans(+) values (see Figure S4, Supporting Information), and they are very close to the aforementioned Vtrans(+) values determined by using CP-AFM.57 A simple interpretation would be that the enhanced barrier at the SAM//Ga2O3/EGaIn interface accounts for higher |Vtrans(−)| values and their scatter for negative bias voltages. In comparison to Au-SAM//Ga2O3/EGaIn junctions, the | Vtrans(±)| values of Au|CNM//Ga2O3/EGaIn junctions become systematically smaller, which could be due to the decreased HOMO−LUMO gap of CNMs after electron irradiation. Taking into account the decrease in aromaticity of CNMs after electron irradiation, we also expect more broadened molecular orbitals of CNMs. We notice that Vtrans(+) and |Vtrans(−)| values are very close to each other for junctions with BPT−CNMs, TPT−CNMs, and QPT−CNMs, most likely arising from similar barriers at both interfaces. It must be pointed out that modifications of energy level alignment may occur after crosslinking, which implies that HOMO is not necessarily the closest molecular orbital in the case of CNMs. To correlate the | Vtrans(±)| with the β values that were obtained from the current density, we marked the range of transition voltages of CNMs (see gray regions in Figure 3a), which is surprisingly associated with the transition of βCNM value from decaying to constant. This indicates an absence of barrier lowering effect and probably a stable participation of molecular orbitals in the charge transport for CNMs at an applied bias voltage above transition voltages. Our findings suggest that TVS can be used to analyze the alignment of molecular orbitals to the Fermi level of the electrode, but could be readily influenced by slight changes in the couplings of molecules and the electrodes. A more detailed understanding of interfaces is of significance in proper interpretations of transition voltages.

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ASSOCIATED CONTENT

S Supporting Information *

Organic synthesis of p-quaterphenylthiol; XPS measurements; comparison of junctions comprising BPT−CNM and annealed BPT−CNM; and statistical analysis of rectification ratios and transition voltages. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: +49-521-106 5351. Fax: +49-521-106 6002. Author Contributions

The paper was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank A. Terfort for providing us with TPT molecules and B. Völkel for technical support. The authors thank the Volkswagenstiftung and the Bundesministerium für Bildung und Forschung (BMBF) for financial support.

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REFERENCES

(1) Akkerman, H. B.; Blom, P. W. M.; de Leeuw, D. M.; de Boer, B. Towards Molecular Electronics with Large-Area Molecular Junctions. Nature 2006, 441, 69−72. (2) DiBenedetto, S. A.; Facchetti, A.; Ratner, M. A.; Marks, T. J. Molecular Self-Assembled Monolayers and Multilayers for Organic and Unconventional Inorganic Thin-Film Transistor Applications. Adv. Mater. 2009, 21, 1407−1433. (3) Zhang, Y.; Zhao, Z.; Fracasso, D.; Chiechi, R. C. Bottom-up Molecular Tunneling Junctions Formed by Self-Assembly. Isr. J. Chem. 2014, 54, 513−533.

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