Letter Cite This: J. Phys. Chem. Lett. 2018, 9, 4578−4583
pubs.acs.org/JPCL
Deconvolution of Tunneling Current in Large-Area Junctions Formed with Mixed Self-Assembled Monolayers Junji Jin, Gyu Don Kong, and Hyo Jae Yoon* Department of Chemistry, Korea University, Seoul 02841, Korea
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S Supporting Information *
ABSTRACT: Whereas single-component self-assembled monolayers (SAMs) have served widely as organic components in molecular and organic electronics, how the performance of the device is influenced by the heterogeneity of monolayers has been little understood. This paper describes charge transport by quantum tunneling across mixed SAMs of n-alkanethiolates of different lengths formed on ultraflat template-stripped gold substrate. Electrical characterization using liquid metal comprising eutectic gallium−indium alloy reveals that the surface topography of monolayer largely depends on the difference in length between the thiolates and is translated into distribution of tunneling current density. As the length difference is more significant, more phase segregation takes place, leading to an increase in the modality of Gaussian fitting curves. Consequently, statistical analysis permits access to deconvolution of tunneling currents, mirroring the phase-segregated surface. Our work provides an insight into the role of surface topography in the performance of molecular-scale electronic devices.
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self-passivating Ga2O3 layer (Figure 1b). Junction measurements as a function of different length of SCn diluent and its mole fraction reveal that the distribution of tunneling current density (J, A/cm2) mirrors the phase-segregated monolayers. Deconvolution of histograms of log-scale current density (log| J|) leads us to reach the conclusion that the influence of phase separation on tunneling data largely depends on the length difference between the n-alkanethiols deposited on a substrate. Our work provides insight into the role of the surface topography of organic components in the electronic function of molecular-scale electronic devices. For this study we focus on four different sets of mixed SAMs based on n-alkanethiolates (SC16 SAM diluted with SC14, SC12, SC10, or SC8) for the following reasons. Phase segregation could be controlled by varying the difference in length between molecules used to form monolayers (Figure 1c). Chen et al.22 previously reported that for mixed SAMs of n-alkanethiolates deposited on Au(111), when the length difference is less than four carbons, homogeneous mixing is favorable, whereas phase segregation becomes significant when the length difference is equal to or greater than four carbons. All of the molecules we use have the same methyl terminal group and methylene backbone. This molecular design eliminates complexities arising from different natures of organic−electrode contacts and lateral interactions within monolayers. For junction measurements, we use the EGaIn technique.21 The EGaIn technique enables noninvasive junction formation over delicate organic surfaces under ambient conditions and guarantees
elf-assembly of more than one type of molecule or nanoscale component frequently undergoes spontaneous phase segregation on nano- and micrometer scales. Understanding how such a phase segregation influences the performances of nanomaterials is important for a variety of research fields including electronics,1,2 plasmonics,3,4 photonics,5 catalysis,6 and biomimetics.7−9 One of the widely utilized self-assembled nanomaterials is self-assembled monolayer (SAM),10 which is the product resulting from vertical chemisorption of molecules on surface. Synthetic accessibility and structural tailorability at the atomic level have led SAMs to receive significant attention as active organic component in organic and molecular electronics.11−13 In particular, SAMbased large-area tunnel junctions have enabled one to relate their electronic function to the structure of molecules comprising the SAM.14−16 Most of the previous studies focus in particular on single-component SAMs, and the heterogeneity of SAMs has usually been avoided for honing performance of tunneling devices. Hence, little has been investigated about how charge tunneling behaves in heterogeneous monolayersthose composed of at least two kinds of moleculesand whether the magnitude and distribution of tunneling current can be fine-tuned by surface topology dominated by phase segregation.17−19 Here we show a physical−organic study to solve this fundamentally interesting and important problem through systematic dilution of single-component, long n-alkanethiolate (n-hexadecane-thiolate; denoted as SC16) SAM with another short n-alkanethiol (SCn where n = 14, 12, 10, 8) (Figure 1a). We focus on large-area molecular junctions of form AuTS/ mixed SAM//Ga2O3/EGaIn, where AuTS is the ultrasmooth template-stripped gold substrate20 and Ga2O3/EGaIn is the liquid eutectic gallium−indium (EGaIn)21 alloy covered with a © XXXX American Chemical Society
Received: June 25, 2018 Accepted: July 27, 2018
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DOI: 10.1021/acs.jpclett.8b01997 J. Phys. Chem. Lett. 2018, 9, 4578−4583
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The Journal of Physical Chemistry Letters
Figure 1. Schematic illustrations of (a) mixing two n-alkanethiolates on template-stripped gold (AuTS); (b) large-area junction of form, AuTS/mixed SAM//Ga2O3/EGaIn; and (c) phase segregation in mixed SAMs formed with n-alkanethiols of similar (homogeneous mixing) and different (heterogeneous mixing) lengths. The molecular length was estimated with ChemDraw 3D software assuming the molecules are in fully extended trans structure. The tilt angle of molecules on gold is ∼30°.10
convenience in operation and fabrication.13,16 Moreover, this technique shows a high yield of working junctions and makes collection of statistically significant sets of tunneling data possible. In a typical experiment, we prepared solutions containing two different thiols by mixing 3 mM anhydrous and N2degassed ethyl alcohol solutions of two thiols at a desired ratio; the total concentrations were kept at 3 mM. Freshly prepared AuTS chips were immersed in the mixed thiol solutions for 3 h under a N2 atmosphere at room temperature, removed from the solution, and then rinsed with pure ethyl alcohol. Mixed SAMs were characterized by contact-angle measurements with static water and hexadecane droplets and X-ray photoelectron spectroscopy (XPS). Values of water contact angle for all of the SAMs were indistinguishable (θ = 105−108°) regardless of length of diluent. (See Figure S8 and Table S6 in the Supporting Information for detailed data.) In contrast, we observed changes of contact angles when SAMs were analyzed with a nonpolar solvent, hexadecane (Figure 2). The SC16 SAM diluted with the short diluent (SCn, n = 12, 10, 8) exhibited nearly identical contact angle trends, a sharp decrease in contact angle at χsoln HSCn = 0.9 and thereafter a gradual increase with decreasing degree of dilution (see green and blue regions pointed out in Figure 2), whereas the SC16 SAM diluted with the long diluent (SCn, n = 14) exhibited insignificant changes of contact angle with dilution. The trends of the contact angle of hexadecane could be rationalized by the ratio of surfaceexposed methyl and methylene groups: Because a monolayer has disordered alkanes, surface-exposed methylene groups lead to a decrease in contact angle, whereas an ordered monolayer mostly has surface-exposed methyl groups and thus a high contact angle.23 With the interpretation of contact angle, we were able to describe the surface structure of our mixed SAMs. For the combination of SC16 and the short SCn diluents, at
Figure 2. (a−d) Plots of static contact angles measured with hexadecane droplets as a function of the degree of dilution for HSCn (n = 14, 12, 10, 8). (e) Schematic describing the exemplary structure of homogeneously mixed monolayers diluted with the long diluent, HSC14. (f,g) Schematics describing the exemplary structures of mixed monolayers diluted with the short diluents, HSCn (n = 12, 10, 8), at low (the green regions in the contact angle plots, b−d) and high (the blue regions) degrees of dilution, respectively. To emphasize the supramolecular structure of SC16, SCn domains were underestimated relative to the actual ones.
χsoln HSCn = 0.9, the surfaces mostly have domains of SCn and a few disordered SC16 molecules between these, leading to highly disordered SC16 molecules. Thereafter, as the mole fraction of the SC16 increased, SC16 domains, in addition to SCn domains, formed, leading to relatively ordered, methyl-exposed surfaces. These were clearly reflected into the contact angle trends. For the combination of SC16 and the long SCn diluent (SC14), the fact that there was no significant change in the contact angle with dilution confirms the homogeneously mixed monolayers. XPS survey spectra for all of the SAMs showed the presence of carbon and sulfur as expected (Figures S10 and S11 in the Supporting Information). All of the SAMs exhibited the identical S 2p peaks at ∼162.5 and ∼163.7 eV. We were unable to determine quantitatively the surface mole fraction of each molecule with high-resolution XPS spectra of C 1s due to indistinguishable carbon species in the n-alkanethiolates. For 4579
DOI: 10.1021/acs.jpclett.8b01997 J. Phys. Chem. Lett. 2018, 9, 4578−4583
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The Journal of Physical Chemistry Letters the sake of simplicity, we assume that the mole fraction in solution is not significantly different from that on surface.24,25 We plotted the ratio of C 1s to Au 4f XPS intensities ([C 1s]/ [Au 4f]) against the degree of dilution (Figure S12 in the Supporting Information). The trends of [C 1s]/[Au 4f] were dominated by [Au 4f] intensities, which were overall one order of magnitude higher than [C 1s] intensities. In the plot, we found the overall regression of [C 1s]/[Au 4f] with increasing the mole fraction of SCn diluent. This finding indicates that the average thickness of SAM decreased with increasing the mole fraction of short diluent SCn due to the dependence of kinetic energy of emitted electron on the thickness of monolayer,26 and confirms the formation of mixed SAMs. We formed EGaIn-based large-area junctions and measured J values in the voltage range from 0 to ±0.5 V. The data were collected from several samples and junctions. Tables S1−S4 in the Supporting Information summarize the data. Interestingly, all log|J(±0.5 V)| histograms of SC16−SC14 mixed SAMs exhibited single-modal Gaussian distributions regardless of degree of dilution (mole fraction of short diluent SCn in solution, χsoln HSCn) like the single-component SAMs (Figure 3a). In contrast, those of SC16−SCn where 8 ≤ n < 14 exhibited multimodal Gaussian distributions (those having two or more peaks). The multimodality predominantly depended on the difference in length for SCn and SC16: The larger length difference led to log|J| histogram that was split into higher multimodal distributions upon dilution (Figure 3b−d). Many of histograms with multimodal distributions contained mean values of log|J| (log|J| mean ) identical to those of the corresponding pure (single-component) SAMs, as indicated with dotted lines in Figure 3b−d. Values of log|J|mean (blue data points in Figure 3) except them were mostly smaller than log|J| of diluent molecules, suggesting that the disordered structure of the longer n-alkanethiolate (SC16) resulted in the blue data points. We also observed a blue data point (χsoln HSC12 = 0.9 in Figure 3b) higher than the log|J| of pure SAM of the short molecule (SC12 diluent), which was attributed to disorder of the diluent or the SC16 upon dilution. It has been established that mixed monolayers formed with molecules of significantly different lengths have nanoscopic pits and gauche defects,27,28 which supports the observation. Figure 4 summarizes the trend of relative areas of Gaussian fitting curves as a function of the degree of dilution (χsoln HSCn). We observed a decrease in log|J| bin count corresponding to pure SC16 SAM with increasing degree of dilution. Because the SCn diluent was shorter, the log|J| value of the pure diluent SAM appeared in a wider range of χsoln HSCn. We further estimated the resistance of SAMs at low biases to make sure that there was no significant cross-effect with rectification (see later for the discussion on rectification) in the plots of log|J| at ±0.5 V (as Figure 3). The slope of the J−V curve in the low bias (ohmic region) was obtained and further converted into the apparent specific resistance (Ω· cm2). The trends of apparent specific resistance (Figure S14 in the Supporting Information) were nearly identical to the corresponding blue data points in Figure 3. This finding indicates that there is no or little cross-effect by the small rectification. The transition from single-modal to multimodal distributions of log|J| could be explained by the deconvolution of tunneling current density mirroring the phase segregation of nalkanethiolates on the surface. As we previously hypothesized, mixed SAMs formed with n-alkanethiolates of similar lengths
Figure 3. Plots of log|J(+0.5 V)| against the degree of dilution, mole fraction of short diluent SCn in solution (χsoln HSCn), and the exemplary histograms for some data points for mixed SAMs of SC16 with (a) SC14, (b) SC12, (c) SC10, and (d) SC8.
would result in homogeneously mixed monolayers and hence single-modal Gaussian distributions. On the contrary, the deposition of n-alkanethiols of significantly different lengths on a substrate induces phase segregation and heterogeneous surface. Our junction data indicate that the topological heterogeneity in the surface of mixed SAMs is translated into 4580
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(Figure 5a). The frequency of shorting junctions in mixed monolayers gradually increased with decreasing length of SCn diluent (Figure 5b). These findings manifest the absence and presence of significant defects in mixed SAMs diluted with SC14 and the other short SCn molecules, respectively. Further study using cyclic voltammetry supports this conclusion. Wetelectrochemical measurements of reduction currents across SAMs allow one to quantify the degree of structural disorder in monolayers.31 Reduction current density across the identical SAMs we used for junction measurements was measured in 0.1 KCl solution containing 1 mM Ru(NH3)63+ as a redox probe, and the percent of electrochemically active surface area (% EAS; the ratio of peak reduction currents for a SAM-bound AuTS to the corresponding bare AuTS) was estimated. As shown in Figure 5c, values of %EAS were fluctuated more (evidenced by not only the averaged values but also the error bars) as the length difference between SC16 and SCn increased. This %EAS result confirms that the SC16−SC14 mixed SAMs are less defective than the other mixed SAMs. Whereas log|J(V)| was deconvoluted with the molecular dilution, all of the histograms of the rectification ratio, the quotient of current densities at +V and −V (r = |J(+V)|/| J(−V)|), were well fitted with single modal Gaussian curves regardless of the length of SCn diluent and the degree of dilution (Figure 6). This finding confirmed the self-referencing nature of rectification: |J(+V)| and |J(−V)| are measured on a single junction having identical SAMs, electrodes, and their interfaces. We found a slightly increased standard deviation of log|r| (σlog|r|) upon dilution, as compared with that of pure SAMs, for all of the diluents, which was attributed to defective nature of mixed SAMs (Figure 6). Interestingly, the polarity of rectification (defined as positive when |J(+V)| > |J(−V)| and negative when |J(+V)| < |J(−V)|) was inversed upon dilution for all of the diluents. (See the blue data points in Figure 6.) Given that the small rectification ratios (0.6 to 1.8) in AuTS/ pure n-alkanethiolate SAM//Ga2O3/EGaIn junctions occurs in a pure tunneling regime,32 the inversion of rectification ratio is perhaps due to a change in the amount of electrode−molecule coupling at the top interface resulting from a change in distance between the electrode and the surface-exposed moiety of SAM, which, in turn, results in a change of asymmetric potential profile along the molecule or a bias drop across the molecule;33−35 Or, given the transmission function-based understanding of molecular conduction, the dilution-induced defect might change the energy state closest to the Fermi level of electrode from LUMO to HOMO. Further study is needed to understand in detail the nature of the observed inversion in rectification polarity, but statistically significant and reliable
Figure 4. Trends of relative areas of Gaussian fitting curves in the log| J(+0.5 V)| histograms as a function of the degree of dilution (χsoln HSCn) for mixed SAMs of SC16 with (a) SC14, (b) SC12, (c) SC10, and (d) SC8.
the junction data in the context of the degree of modality in the log|J| distribution, as shown in Figure 3. This finding further suggests that top-contacts between Ga2O3/EGaIn and mixed SAMs are conformal over the heterogeneous surfaces. The deconvolution of tunneling current data we observed implies that the size of the formed molecular domain in the mixed SAMs is probably equivalent to or larger than the electrical contact area of the top-electrode. Typical contact areas we used in junction measurements were ∼700−1000 μm2, geometrical contact areas estimated by optical microscopy, and Simeone et al.29 have shown that the effective contact area of EGaIn conical tip with a template-stripped metal substrate (those we used here) is ∼104 times smaller than the corresponding geometrical contact area. Consequently, our junction measurements were based on effective contact areas of ∼0.07 to 0.1 μm2. Provided that the size of the molecular domain in the SAM is limited by the size of silver grains in the AgTS substrate whose average size of grain is ∼0.8 to 1.0 μm2,30 the effective contact area in our junction is smaller than the size of the molecular domain in the SAM by an order or magnitude and hence accounts for our observation. Whereas the homogeneously mixed SC16−SC14 SAMs showed narrow dispersion of data and little change of log|J| standard deviation (expressed as sum of σlog|J|, ∑σlog|J|) upon dilution, σlog|J| values were increased with dilution for heterogeneously mixed SC16−SCn SAMs (n = 12, 10, 8)
Figure 5. Plots of (a) sum of σlog|J| values (∑σlog|J|) in the log|J(+0.5 V)| histograms, (b) yield of working junctions (%), and (c) percent of electrochemically active surface area (%EAS) as a function of the degree of dilution (χsoln HSCn). 4581
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Letter
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Hyo Jae Yoon: 0000-0002-2501-0251 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This research was supported by the NRF of Korea (NRF2017M3A7B8064518). H.J.Y. also acknowledges the support from the Future Research Grant by Korea University.
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Figure 6. Plots of log-scale rectification ratio (log|r|) as a function of the degree of dilution (χsoln HSCn) for mixed SAMs of SC16 with (a) SC14, (b) SC12, (c) SC10, and (d) SC8. (e,f) Exemplary histograms of log|r| before and after dilution.
data over a wide range of mixed SAMs we measured will make the inversion study possible. Herein, we have fabricated and electrically characterized large-area tunnel junctions with mixed SAMs composed of nalkanethiolates of different lengths on AuTS, where we demonstrate that with systematic molecular-scale dilution of monolayers we can translate the heterogeneity of SAM’s surface topography into the degree of multimodality in the distribution of tunneling current density. Indeed, we have found the deconvolution of tunneling currents depending on not only the degree of dilution but also the difference in length between n-alkanethiolates deposited onto a substrate. Importantly, our results demonstrate the power of harnessing molecular dilution strategy to engineer nanoscale surface topography on organic thin films. This approach will enable systematic exploration of organic−electrode interfacial characteristics, most notably conformality of contact and probably the degree of electrode−molecule coupling. We envisage that the modulatable interfacial characteristics could be utilized for not only the control of other molecular and organic electronic devices’ performance but also the development of quantumencoding nanomaterials based on SAMs.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpclett.8b01997. Experimental details, data of junction measurements, wet electrochemical analysis, contact-angle measurements, X-ray photoelectron spectroscopy, and minor discussions. (PDF) 4582
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