Rectification Ratio and Tunneling Decay Coefficient Depend on the

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The Rectification Ratio and Tunneling Decay Coefficient Depend on the Contact Geometry Revealed by In Situ Imaging of the Formation of EGaIn Junctions Xiaoping Chen, Hongting Hu, Jorge Trasobares, and Christian A. Nijhuis ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 22 May 2019 Downloaded from http://pubs.acs.org on May 28, 2019

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ACS Applied Materials & Interfaces

The Rectification Ratio and Tunneling Decay Coefficient Depend on the Contact Geometry Revealed by In Situ Imaging of the Formation of EGaIn Junctions Xiaoping Chen1, Hongting Hu1, Jorge Trasobares1, and Christian A. Nijhuis1,2* 1Department

of Chemistry, National University of Singapore, 3 Science Drive 3, Singapore

117543, Singapore 2Centre

for Advanced 2D Materials and Graphene Research Centre, National University of

Singapore, 6 Science Drive 2, Singapore 117546, Singapore

*Author to whom correspondence should be addressed: [email protected]

Keywords: Molecular diodes, Tunneling Decay Coefficient, EGaIn, Molecular Junctions, Contact Area

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ABSTRACT: This paper describes how the intensive (tunneling decay coefficient β and rectification ratio R) and extensive (current density J) properties of Ag-S(CH2)n-1CH3//GaOx/EGaIn junctions (n = 10, 14, 18) and molecular diodes of the form of Ag-S(CH2)11Fc//GaOx/EGaIn depend on Ageo, the contact area between the self-assembled monolayer and the cone-shaped EGaIn tip. Large junctions with Ageo ≥ 1000 µm2 are unreliable and defects, such as pinholes, dominate the charge transport characteristics. For S(CH2)11Fc-based SAMs, R decreases from 130 to unity with increasing Ageo due to an increase in the leakage current (the current flowing across the junction at reverse bias when the diodes block current flow). The value of β decreased from 1.00 ± 0.06 n-1 to 0.70 ± 0.03 n-1 with increasing Ageo which also indicates that large junctions suffer from defects. Small junctions with Ageo ≤ 300 µm2 are not stable due to the high surface tension of the bulk EGaIn resulting in unstable EGaIn tips. In addition, the contact area for such small junctions is dominated by the rough tip apex reducing the effective contact area and reproducibility significantly. The contact area of very large junctions is dominated by the relatively smooth side walls of the tips. Our findings show that there is an optimum range for the value of Ageo between 300 – 500 µm2 where the electrical properties of the junctions are dominated by molecular effects. In this range of Ageo, the value of J (defined by I/Ageo where I is the measured current) increases with Ageo until it plateaus for junctions with Ageo > 1000 µm2 in agreement with recently reported findings by the Whitesides group. In this regime reproducible measurements of J can be obtained provided Ageo is kept constant.


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INTRODUCTION One of the major reasons that hampers molecular electronics to develop into a mature technology is that the reproducibility between laboratories and different junction platforms is notoriously poor: the measured tunneling rates across the same type of molecules (e.g., alkanthiols) differ by a factor of 108.1 The reasons behind this huge spread in currents are unclear, but how conformal the contact area between the top electrode and the self-assembled monolayer (SAM), or monolayers of covalently linked molecules, is, and the role of defects and how they scale with junction area, are usually unknown. A large variety of methods have been reported to form the top-contacts without altering the (chemical) structure of the monolayer,1-5 but each technique has a certain set of ad- and disadvantages; most reports, however, emphasize on the advantages of a particular technique while the limitations and causes of junction failure are usually not documented. For example, the role of contact pressure has been investigated only for a few techniques (e.g., conductive probe based techniques6, and, recently, for the EGaIn technique7. Each type of large area junction suffers from defects, but it is not always clear how the junction characteristics scale with junction area (see for examples refs 8-13). Junction failure is on its own an ill-defined term. Although catastrophic junction failure (e.g., a short) can be easily recognized and removed from a data set, junctions may be altered during the fabrication process in subtle ways resulting in leakage currents that are difficult to recognize.2,8,12,14-20 For these reasons, it is important to investigate how the fabrication process affects the junction properties and to elucidate for which fabrication parameters the junctions are dominated by molecular effects and not by leakage current across, for example, pinholes or other types of defects2,8,14,15,21 (which is a well-known problem in other types of tunneling junctions22-25. The “EGaIn -technique” (EGaIn is a eutectic alloy of Ga and In) is one of the techniques that has produced consistent results between different research groups to measure currents



across a large variety of systems which relies on molding a non-Newtonian liquid-metal alloy of Ga and in which spontaneously forms in air a surface layer of about 0.7 nm GaOx.26,27 This GaOx layer is inhomogeneous and amorphous in nature resulting in large uncertainties in the effective contact area (see below).13,26-31 The GaOx layer reduces the surface tension of the bulk GaIn alloy and gives it non-Newtonian properties32,33 so that it can be shaped into coneshape tips,13,32 or stabilized in through-holes in microfluidic networks,34,35 which then are used to from electrical contacts to SAMs. The GaOx layer itself has low resistivity and does not contribute significantly to the measured junction resistance.36,37 In a typical experiment, cone-shaped EGaIn tips are made by simply pulling out a micro-needle from a drop of EGaIn involving wrinkling and rupturing of the GaOx surface layer.13,32 Naturally, this process involves a certain level of “art and skill” causing variations in the measured junction properties for the same type of SAMs between different groups. Consequently, there is a general consensus between the various EGaIn techniques on the measured intensive quantities (e.g., the tunneling decay coefficient β or the rectification ratio R), but not on the extensive quantities (e.g., current density J, in A/cm2) that depend on the details of the contact area.13,28 For instance, within the various EGaIn techniques, the values of J across S(CH2)n1CH3

SAMs reported by different groups vary by 2 orders of magnitude but the value of β is

always close to 1.0 n-1.8,13,14,17,29,36,38 In broader context, there is consensus on the intensive quantities but not on the extensive quantities across different types of molecular junctions: the measured values of J measured across alkanethiols vary by a factor of 108 yet β is around 1.0 n-1 across a large number of different types of junctions including single molecule and large-area junctions (see ref 1 for a review). Therefore, it is important to identify the reasons for the observed discrepancies in the measured value of J. There are several reasons why the measured values of J vary across different platforms so greatly, but the major culprits seem to be differences in the nature of the molecule-electrode


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contacts and associated contact resistances, effective contact areas, and defects. For example, Frisbie et al. have shown that a chemisorbed contact (i.e., Au-S(CH2)nS-Au junctions) can be 10-102 times more conductive than physisorbed (i.e., Au-S(CH2)n-1CH3//Au junctions) contacts.39 Whitesides et al. reported that for EGaIn junctions the effective contact area (Aeff) is 104 smaller than the geometrical contact area (Ageo) resulting in a gross underestimation of the value of J since J and the measured current I are interconverted using eq 1.29 Later, we showed that the Aeff / Ageo ratio in our EGaIn junctions is close to 10-6.27 The effective contact areas for other type of junctions are unknown. J = I / Ageo

(1)

Each type of large area junction contains defects caused by, for instance, the roughness of the electrodes, defects in the SAMs, or contaminations that hamper the SAM formation process.8,14,15,17,21,40-42 These defects may decrease the separation between the electrodes (i.e., a pin-hole) resulting in large local currents which dominate the device properties.2,8,15,16,41,43 Junctions with large values of Ageo are more likely to probe defects than junctions with small values of Ageo.2,5,8,16-18,41,43 For this reason, it is desirable to minimize Ageo. On the other hand, increasingly small junctions are difficult to fabricate and thus the minimum value of Ageo is limited for practical reasons. In case of EGaIn junctions, the minimum value of Ageo is limited by the high surface tension of the bulk EGaIn (624 mN/m) resulting in stable tips only for tip curvatures of >10 µm.44,45 Whiteside and co-workers argued for this reason that cone-shape tip EGaIn junctions are not stable for junctions with Ageo < 1000 µm2 and therefore they recommend to use junctions with Ageo > 1000 µm2.13 They reached this conclusion based on the following three observations recorded from Ag-S(CH2)9CH3//GaOx/EGaIn junctions. i) The value of J increases with increasing Ageo but then plateaus for Ageo > 1000 µm2. ii) The contact pressure of the EGaIn tip with the SAM does not increase by “pushing” the EGaIn tip against the



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SAM surface to increase Ageo. iii) The reproducibility (in terms of the log-standard deviation σlog, see below) was higher (i.e., σlog was lower) in the plateau region for large junctions than for small junctions. Although these seem to be valid justifications to promote the use of large junctions, this recommendation goes against the general wisdom to use small junctions to minimize the probability to probe defects induced by grain boundaries, step-edges, and other types of defects.2,5,8,15-18,22,41 The values of β (in n-1) and R are given by eq 2 and 3 wherein the d is usually taken as 𝐽 = 𝐽0𝑒 ―𝛽𝑑 = 𝐽010 ―𝛽𝑑/2.303 |𝐽(𝑉)|for

𝑅 = |𝐽|(V)rev

(2) (3)

the molecular length (or the number of CH2 units in case of aliphatic SAMs, n), J0 is a preexponential factor, and |𝐽(𝑉)|for and |𝐽(𝑉)|rev are the absolute current densities for a given voltage at opposite bias when the junction allows the current to flow (forward bias) or blocks the current (reverse bias). We have elucidated how defects (mainly pinholes induced by exposed grain boundaries at which SAMs cannot pack well) in EGaIn junctions with S(CH2)n-1CH3 SAMs resulted in measured values of β ranging between 0.4 to 1.2 n-1 and a change of factor of 103 in the values of J0.8,25,40 For molecular diodes with S(CH2)11Fc SAMs, the value of R dropped from 100 to unity due to defects15,17,42. These studies, in agreement with others,19,46 have shown that exposed grain boundaries (at which SAMs cannot pack well) are the major source of defects. Therefore, one would ideally aim for values of Aeff that are smaller than the average area of a grain to avoid sampling of exposed grain boundaries. We note that in the study by Whitesides and co-workers, only Ag-S(CH2)n1CH3//GaOx/EGaIn

junctions with n = 10 were evaluated which are rather insensitive to

defects.13 The S(CH2)10CH3 SAM inside these junction, however, is liquid-like in character and therefore capable of self-repair41 masking the presence of defects inside the tunnelling junction as we have shown previously14. Molecular dynamics simulations revealed that in 6 ACS Paragon Plus Environment

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liquid-like SAMs the molecules can readily rotate and compensate for defects that would have otherwise resulted in thin-area defects. In contrast, thick SAMs are more crystalline in nature with high packing energies and therefore the molecules in such SAMs have a lower degree of freedom and cannot rotate into defective regions which may result in thin area defects and associated leakage currents. By investigating the tunnelling behaviour across AgS(CH2)n-1CH3//GaOx/EGaIn junctions with n = 2-18 (only even n) on rough and ultra-smooth Ag electrodes, we showed that thin SAMs (n < 10) yield better tunneling junctions than thick SAMs (n > 10) on rough electrodes.14 For these reasons, we report here how Ageo of cone-shaped EGaIn junctions affects the measured values of J0 and β for junctions with SAMs of S(CH2)n-1CH3 with n = 10, 14, and 18, and R for junctions with SAMs of S(CH2)11Fc. By using semi-transparent Ag electrodes and fabricating the junctions on an inverted microscope platform, we were able to measure the Aeff as a function of Ageo in situ with optical resolution. These measurements revealed that small cone-shaped tip junctions with Ageo < 300 µm2 are not stable, and that the values of R, J0, and β, all vary with Ageo for large junctions with Ageo of > 500 µm2. Therefore, the best trade-off between junction stability and quality is a foot print of 300 µm2 < Ageo < 500 µm2. Our experiments are important to understand the origins and justifications of the variations in the junction fabrication practices, and the differences in the measured results between different laboratories that use EGaIn based junctions. In extension, the major findings reported here also apply, at least to a certain degree, to other types of large area junctions since the effective contact areas or how the top-electrode interacts, or forms contacts, with the monolayer during the fabrication process are usually unknown.

EXPERIMENTAL SECTION



Materials. All regents were used directly as supplied unless specially stated. Di-ionized water was purified from Elga Purelab option-Q system. Silica gel (high-purity grade, pore size 60Å, 40-63 µm particle size) was obtained from Sigma-Aldrich. HS(CH2)11Fc was synthesized using the same method as reported elsewhere,47 the structure and purity were determined using 1H NMR and 13C NMR spectroscopy using a Bruker Avance 300 MHz spectrometer and gas chromatography–mass spectrometry (GC-MS). The n-alkanethiols were purchased from Sigma-Aldrich (with original purities of 96-98 %) and purified using column chromatography followed by recrystallization from ethanol using previously reported methods40. The SAM precursors were stored under an atmosphere of N2 at -50 ℃. The purity of the precursor was monitored once every two weeks using TLC (thin layer chromatography). The Ag (purity: 99.99 %) was purchased from MOS Group Pte Ltd (Singapore). The ethanol (assay: 99.94 % in V/V) was purchased from VWR Chemicals (France) and distilled for SAM incubation. Ag Substrates and SAMs Formation. We used template-stripped Ag substrates that were obtained via previously reported method.27 The Ag surfaces used in this study had a rootmean-square surface roughness of 0.6 ± 0.1 nm measured over an area of 5.0 × 5.0 μm2. The SAMs were formed by immersing an Ag surface immediately after template-stripping) in freshly distilled ethanolic solution of 3 mM of the corresponding thiol for 3 h under N2. The Ag substrates were then taken out from the solution, rinsed with ethanol, and dried in a stream of N2. J(V) Data Collection and Analysis Method. The EGaIn setup to fabricate junctions with cone-shaped tips has been previously described.48 We used two methods for recorded J(V) data as a function of Ageo. Method 1: using a single tip by gradually increasing Ageo by adjusting the manipulator. We first formed a small area junctions and subsequently increased Ageo by adjusting the micromanipulator on which the syringe was mounted from which the 8 ACS Paragon Plus Environment

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EGaIn tip was suspended. Thus, a single junction was formed of which Ageo was increased from 1.2 ×102 to 4.0×103 μm2. For each junction area, we measured 20 J(V) traces. This procedure was repeated three times and the averaged values of log10|J| and log10R with logstandard deviations (σlog) were plotted vs. Ageo in the main text. In the supporting information, we show one representative data set. Method 2: using 20 separate junctions (each junction 20 traces) with different values of Ageo. Here, we formed junctions with the desired contact area directly, and each Ageo was obtained using a different tip. For each contact area, we recorded and analyzed the J(V) traces to determine G, G, and log-standard deviations σlog, following previously reported methods.48 Briefly, all values of log10|J| for each measured bias were plotted in histograms to which a Gaussian (denoted by G) was fitted to obtain the Gaussian values of G and σlog, which, in turn, were used to reconstruct the logaverage J(V) curves. Optical Images and Videos. The optical images and videos were recorded using an inverted optical microscope (Nikon Eclipse Ti-E) with NIS-Elements BR 4.10.00 software (this setup has been described before30). The SAMs were formed on 20 nm thin, semi-transparent template-stripped Ag substrates using an optically transparent adhesive (Norland 61). These thin Ag surfaces typically had a root-mean-square surface roughness of 1.2 ± 0.2 nm measured over an area of 1.0 × 1.0 µm2. The images and video (at 8× of their original speed) of the junction area and the junction formation process were recorded through the bottomelectrode using a 60 × oil immersed objective with a numerical aperture of 1.49 and a halogen lamp source to illuminate the junctions through the objective. On the microscope platform, we placed a small micromanipulator (Thorlab, 25 mm XYZ-translation stage) equipped with the Hamilton syringe (10 µL, Model 1701RN) loaded with EGaIn from which the EGaIn tips were suspended. A second micromanipulator (Crest innovation, S-725PLM) with a tungsten probe was used to ground the Ag surface. An USB camera (Edmund optics,



EO-3112C color USB camera) equipped with a zoom ratio 4:1 and 2.5-10× magnification zoom imaging lens (Edmund optics, VZM100DI) was mounted on a stand and used to image the junctions from the side and to monitor the movement of the tip. The junctions were illuminated with a LED light source (Shenzhen Selectech Electronics Co. Ltd.; SE-DM500X). A Keithley (Model 6340) was used to measure the J(V) and J(t) characteristics as described in the main text. Usually, we prepare the EGaIn tips using a micromanipulator (Leica, Germany, Cat. No. 11520137) as this manipulator allows for precise fine-adjustment of tens of nm in the z-direction needed to fabricate the tips. This micromanipulator is too heavy (~2 kg) to mount directly on the microscope sample holder. For this reason, the tips were fabricated using the Leica micromanipulator and then transferred to the micromanipulator placed on the inverted microscope. A photo of the entire setup is provided in Figure S2. Scanning Electron Microscope Images of EGaIn Tips on Ag. A FEI Verios 460 Field Emission Scanning Electron Microscope (SEM) was used to record the images of an inverted EGaIn tip on a Ag substrate. We made a new EGaIn tip on a clean template-stripped Ag surface, the “inverted tip” standing on the Ag surface was used for imaging. The SEM images were recorded at different magnifications and the SEM was operated at 2.00 kV and 0.10 nA in secondary electron mode. We note that it is important to handle the inverted tips carefully and avoid vibrations which could cause the tips to collapse, and to make the tips small enough to ensure the whole structure is in the non-Newtonian regime and thus stable during handling and imaging. We note that these results are similar to those obtained by Barber et al.49

RESULTS AND DISCUSSION


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In-situ Imaging of the EGaIn Junction Formation Process. Figure 1a shows the experimental platform that was used to image the formation of the contact between a SAM and a cone-shaped EGaIn top electrode. Normally, EGaIn junctions are only imaged from the side (camera 1 as indicated in Figure 1a) to record an image which is then used to determine the diameter of the junction from which then Ageo is calculated by assuming the footprint has a circular geometry. Figure 1b and c shows two images with different values of Ageo of 1.2 × 102 and 4.0 × 103 µm2 by adjusting the diameter of contact point by simply lowering the syringe from which the EGaIn tip was suspended (see Figure S1 and Videos S1 and 2). Considering the high roughness of the EGaIn surface and the large discrepancy between the values Ageo and Aeff, it is important to directly measure the footprint of the EGaIn contact with the SAM. Therefore, we used an inverted microscope platform to image the junctions through the bottom electrode using an objective (camera 2) as indicated in Figure 1a (Video S3). We used 20 nm Ag and optically transparent adhesive for template-stripping to ensure the bottom-electrode was transparent enough (20 nm Ag has a 34.5% transparency50) to allow us to image the junction area.



Figure 1. (a) Schematic illustration of the experiment to image the junction formation process, and the foot print the EGaIn tip with the SAM in situ while measuring the electrical characteristics of the junctions. Camera 1 images the junction from the side, camera 2 images the junction through the objective. The contact area of the junction can be adjusted by lowering the tip using a micromanipulator as indicated by the red dashed arrow and light grey cone. The figure is not drawn to scale. The bottom electrode was grounded and the top EGaIn electrode was biased. (b-c) Optical photographs of the junctions with two different diameters taken from the side of the junction (camera 1). The reflection of the tip in the Ag surface and an obvious wrinkle are indicated by white arrows. (d) The footprint of the EGaIn tip with a Ag-S(CH2)9CH3 SAM taken through the objective (camera 2). The dashed black ellipse indicates the tip apex. (e-g) SEM images of an inverted EGaIn tip with different magnifications, sample was 30° tilted around the x-axis (see supporting information Section 3). In panel e, an obvious wrinkle is indicated by the white arrow. Figure 1d shows an example of an image of a cone-shaped EGaIn junction with a S(CH2)9CH3 SAM recorded through the bottom electrode with a Ageo of 4.65 × 103 µm2. As can be readily seen, the SAM//GaOx/EGaIn interface is inhomogeneous due to wrinkles formed during the tip fabrication process. The roughness is higher in the centre part of the junction (dashed black circle) than in the outer parts as that is the location where the EGaIn ruptured resulting in a dark area in the image which indicates an area of non-contact. These non-contact areas are macroscopic, i.e., (sub)micrometer scale, as these images are diffraction limited. Figure 1e shows a wide field SEM image of the EGaIn tip which shows that the tip apex is indeed very rough due to the rupture event corroborating the optical image of the SAM//EGaIn interface (Figure 1d) showing that this part of the tip does not form a good contact with the SAM. Figure 1d also shows that the EGaIn seems to form a more homogeneous contact in the area surrounding the dark feature, here the side walls of the cone-shaped tip are in contact with the SAM as the EGaIn-tip is forced onto the SAM. The (broken) concentric rings in Figure 1d mirror the wrinkles on the sides of the EGaIn-tip that can been seen in Figure 1e resulting in a large reduction of the Aeff with the SAM. The magnified area of the tip (Figure 1f) shows that the roughness at the apex of the tip is indeed more than one order (~33 times, see Section S3 for more details) larger than the sides of the cones which explains why the tips form more efficient contacts with the SAM when 12 ACS Paragon Plus Environment

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the EGaIn tip is forced onto the SAM. In other words, this result explains why Whitesides and co-workers found that J increases with Ageo up to about Ageo = 1000 µm2 after which J scales with Ageo and why they recommend to use large area junctions:13 by using large area junctions, the uncertainties in the contact area with the SAMs caused by the large roughness of the tip apex are compensated by the relatively smooth contacts of the side walls of the EGaIn tip. Although these observations explain the behaviour of the junctions at the (sub)micrometer length scale, the SEM image shown in Figure 1g shows that the EGaIn tips also contains microscopic roughness that is present everywhere in the junction. This observation explains why we reported a lower effective contact area of 10-6 (ref 27) than the value of 10-4 by Whitesides and co-workers29. It is well-known that the Aeff between two solid surfaces with microscopic roughness this ratio about 10-2-4,28,29,36,51,52small area coneshaped EGaIn junctions also have macroscopic roughness causing an additional reduction of the correction factor of Aeff of 10-2 in our previous studies.

Performance of Molecular Diodes vs. Contact Area. As mentioned in the Introduction, the rectification behavior of S(CH2)11Fc SAMs inside EGaIn junctions is very sensitive to defects caused by, e.g., the roughness of the bottom electrode,17 purity of the SAM precursor,53 or disorder in the supramolecular structure of the SAM15,48. As a group, these studies show that primarily the leakage currents (i.e., Jrev; eq 3) are affected by defects and the current at forward bias (Jfor; eq 3) is rather insensitive to defects.15 The advantage of exploring how R changes as a function of Ageo is that by examining R, Jrev, and Jfor, as a function of Ageo we can determine whether leakage currents are important in large junctions. We used two different methods to increase Ageo by first by making a small contact and subsequently increasing Ageo by simply lowering the cone-shaped EGaIn tip suspended from the syringe using a micromanipulator (method 1), i.e., here the effect of Ageo on a single



junction is captured, or by making separate junctions with different Ageo (method 2). Figure 2a shows the values of Jrev and Jfor vs. Ageo obtained by method 1 (Figure S4 shows the J(V) curves and Table S1 summarizes the junction properties). Junctions with small Ageo of 1.2 ×102 µm2 are not stable due to the high surface tension of the bulk EGaIn resulting in large current fluctuations due to deformation of the tip, we were not able to make smaller junctions than Ageo < 1.2 ×102. For junctions with Ageo > 1.2 ×102 µm2, Jfor is constant. In sharp contrast, the value of Jrev is small for junctions Ageo < 1000 µm2, but rapidly increases for larger junctions by two orders magnitude. Figure 2b reflects this behavior and shows that the diodes perform well for small junctions (Ageo < 1000 µm2) with values of R of 200, but not for large junctions where R dropped to unity. Hence, the leakage current is sensitive to Ageo, indicating that defects play a prominent role in large cone-shaped tip junctions. Figure 2c and d show the same results but recorded with method 2 (all J(V) curves, histograms, and statistics are shown Figures S5-6 and Table S2). Table S2 shows that the fabrication yield and σlog are rather insensitive of Ageo. These observations reinforce our previous findings that the yield and σlog are not per se good indicators of the quality of junctions.8,14 Based on these results, we conclude that for junctions with 2.5 × 102 < Ageo < 1.0 × 103 µm2, the electrical characteristics of the molecular diodes are reproducible, stable, and do not suffer from leakage currents resulting in large rectification ratios.


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Figure 2. (a) The values of log10|J| at ±1.0 V (i.e., Jfor and Jrev) and (b) log10R recorded from Ag-S(CH2)11Fc//GaOx/EGaIn junctions vs. Ageo using method 1. The error bars represent σlog measured from 3 different junctions. (c) The values of G at ±1.0 V and (d) G recorded from Ag-S(CH2)11Fc//GaOx/EGaIn junctions using method 2. The error bars represent 95% confidence levels from 20 different junctions. Performance of S(CH2)n-1CH3 SAMs Junctions vs. Contact Area. To determine whether the values of J0 and β are sensitive to the value of Ageo, we recorded J(V) curves for junctions with S(CH2)n-1CH3 with n = 10, 14, and 18, as a function of Ageo using methods 1 and 2 described above. The J(V) curves and statistics are shown in Figures S7-12, and Tables S3-8. Figure 3 shows the results for the three junctions using both methods and that both methods yield indistinguishable results. For all junctions, the value of J increases with increasing Ageo, but J plateaus for Ageo > 1.0 ×103 µm2; this observation agrees with the results reported in ref 13. We notice that the data in Figure 3 shows a clear transition opposite to the data in Figure 2 which shows a more gradually change. The reason for this difference in behavior may be caused by a difference in wetting of the SAMs by the top-electrode or the more liquid-like 15 ACS Paragon Plus Environment


nature of the S(CH2)11Fc SAMs caused by the mismatch in diameter of the Fc head groups and the alkyl chains. Tables S6-8 show that small junctions with Ageo ~1.2 ×102 µm2 were not stable resulting in large σlog. Thus, very small junctions with Ageo < 2.5 ×102 µm2 yield irreproducible junctions. As mentioned earlier, thin SAMs are less sensitive to defects than thick SAMs inside molecular junctions because thin SAMs are more dynamic and readily can self-repair defects than thick SAMs.14 This property of thin SAMs is also reflected in the data shown in Figure 3 since the values of J only change by 1.2 ± 0.1 order of magnitude for n = 10, but 1.6 ± 0.1 and 1.9 ± 0.1 orders of magnitude for n = 14 and 18, respectively. Therefore, it is important to examine how β and J0 depend on Ageo.

Figure 3. The value of log10|J| (determined with eq 1) of Ag-S(CH2)n-1CH3//GaOx/EGaIn junctions with n = 10 (a), n = 14 (b), and n = 18 (c) as a function of Ageo using method 1 (a-c), and method 2 (d-f). n=10 corresponds to panels a and d, n=14 with b and e, and n = 18 with c and f. The error bar for panels a-c is the log standard deviation (σlog) from three separate 16 ACS Paragon Plus Environment

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datasets using method 1. For panels d-f the error bars represent the 95% confidence levels of log10|J| from 20 different junctions using method 2. Figure 4a shows how the values of β and G vary with Ageo. Here, the values of G at +0.5 V along with 95% of confidence levels were obtained from method 2 for each value of Ageo (the results for -0.5 V are shown in Figure S13 and the values of β and J0 as function of Ageo are summarized in Table S9). The solid lines are fits to eq 2. The value of β is constant around 1.0 n-1 for values of Ageo from 1.2 ×102 µm2 to 5.0 ×102 µm2, while log10|J0| increases from 1.57 ± 0.05 to 2.23 ± 0.34 A/cm2. For junctions with Ageo > 5.0 ×102 µm2, β gradually decreases to 0.70 ± 0.03 n-1, and log10|J0| to 1.48 ± 0.19 A/cm2, with Ageo. These small β values for large junctions indicate the presence of defects induced by exposed grain boundaries.8,40 Based on these observations we conclude that the safe region for charge transport measurements is 2.5 < Ageo < 5.0 × 102 µm2 to ensure optimal balance between junction stability while minimizing the role of defects and associated leakage currents. This range is very similar to the one suggested above for the molecular diodes. The upper limit also agrees with ref 8 with EGaIn stabilized in through-holes in PDMS; this upper limit value can be explained as follows. The template-stripped Ag electrodes have grains with an area Agr of 0.8-1.0 μm2.40 For Aeff / Ageo = 10-4, junctions with Ageo < 1000 μm2 have an Aeff value of 1000 μm2, the value of Aeff approaches Agr, and Aeff ≈ Agr applies resulting in a sharp decrease in the junction performance. Factoring in that small junctions can have Aeff / Ageo values as low as 10-6, Aeff 1000 µm2. For these reasons, they promote to use large areas as J is more reproducible than small area junctions and there is no risk in damaging the SAMs as no additional pressure is involved. The reasoning is perfectly valid, however, they did not report reasons to limit the upper boundary of Ageo contradicting our previous findings8. The work reported here reaches the following 6 conclusions and recommendations. 1) By studying both intensive and extensive charge transport parameters (J0, β, and R) as a function of Ageo, we found that J0, β, and R, all decrease with Ageo when large junctions are used (Ageo > 500 µm2). Low values of β indicate the presence of defects in the junctions, and the value of R decreased because the currents at reverse bias (the leakage current) increased. Based on these observations, we conclude that the upper



limit for Ageo is 500 µm2 to minimize the role of defects in junctions based on coneshaped EGaIn tips. 2) In agreement with Whitesides and co-workers, J depends on Ageo for small junctions. Therefore, it is important to ensure that from junction-to-junction, and experiment-toexperiment, Ageo is kept constant and its value should always be reported. Provided the user keeps Ageo constant, small values of σlog of 0.2-0.5 can be readily obtained as has been demonstrated in various papers before14,56,57,59; these small values of σlog are similar to those reported for large area junctions13,60. Thus, both “small area” and “large area” junctions have good reproducibilities albeit that “small area” junctions require careful attention to the junction area. 3) The origin of the variation of J with Ageo is caused by the rough tip apex which dominates the contact area in small junctions resulting in a correction factor for Ageo of 10-6 due to macroscopic scale roughness, while the contact area in large junctions is dominated by the sides of the EGaIn tip resulting in a correction factor of 10-4. By imaging the SAM//EGaIn contact, the effective contact area (Aeff,O) could be determined and J is independent of Ageo,O for both small and large area junctions. Based on this observation we conclude that differences in reported J0 value are caused by the macroscopic roughness of the junctions related to the rough tip apex. Both small and large area junctions, however, suffer from the same microscopic roughness of the GaOx/EGaIn surface which we quantified. The non-contact area caused by the tip apex (A0) is (3.0 ± 0.9) × 102 μm2, and the non-contact zone is about 10% (k = 0.89 ± 0.06) in large junctions. 4) The mechanism of contact formation involves deformation of the EGaIn tip. The EGaIn-tip “remembers” its shape due to its non-Newtonian properties when contacts are made and subsequently broken. For this reason, EGaIn-tips can be used to make 5-


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7 junctions with very similar SAM//EGaIn contact geometries (i.e., similar Aeff,O) resulting in an insignificantly small spread in J values (provided Ageo is kept constant when small junctions are used). 5) Molecular effects dominate in small area junctions, leakage currents dominate large area junctions. The Whitesides group claimed in a series of papers that a large number of functional groups in junctions with aliphatic SAMs did not result in measurable effect in the electrical characteristics of the junctions.54,60-63 In sharp contrast, other groups, including Thuo, Chiechi, or Yoon, and us, reported clear molecular effects for a wide range of functional groups including (embedded) dipoles,56,64 polarizable groups,59,65 aromatic groups,18 donor-acceptor systems,66 and redox moieties27,48. By investigating the rectifying properties of a well-established molecular diode as a function of Ageo, we found that mainly the currents at reverse bias increase diminishing the rectifying properties of the junctions. This experiment indicates that large area junctions risk to be dominated by defects resulting in large leakage currents masking molecular effects. This finding could explain the lack of molecular features in the large-area junction experiments reported by the Whitesides group. 6) Too small junctions are unstable due to the high surface tension of EGaIn. The results reported here indicate that junctions with Ageo < 3.0 × 102 µm2 are not stable enough resulting in a large junction-to-junction variation of the measured tunneling currents. The values of J0, β, and R, all decrease substantially for junctions with values of Ageo of > 5.0 × 102 µm2, therefore, we recommend to use values of Ageo between 3.0 × 102 to 5.0 × 102 µm2 for cone-shaped EGaIn junctions to ensure an optimal balance between stability while keeping the role of defects to minimum so that the mechanism of charge transport is dominated by molecular effects.



In conclusion, this work reconciles the differences in reported tunneling rates across molecular junctions with EGaIn top electrodes and gives new insights in how, and why, EGaIn junctions work. In other types of large area junctions (e.g., with top contacts made of conductive polymer,1,11,20 printed metal,2 graphene/graphene oxides,3,67 or nanoparticles coated with stabilizing polymers or SAMs9,68) the effective contact area, conformity of the top electrode with the SAM, role of macroscopic and microscopic roughness, role of wettability of the top electrode and the SAM, phase transitions18,69 and odd-even effects in SAMs55,70, or “safe” Ageo values to minimize the role of defects, are often unknown, complicating the direct comparison of measured tunneling rates across different types of test beds. We hope that our efforts will encourage others to undertake similar investigations to elucidate the origins of the differences observed in both the in- and extrinsic variables between different experimental platforms. We hope that our efforts will encourage others to undertake similar investigations to elucidate the origins of the differences observed in both the in- and extrinsic variables between different experimental platforms.

ASSOCIATED CONTENT Supplemental Information The Supporting Information is available free of charge on the ACS Publications website at DOI:. Supporting Information contains optical images of EGaIn junctions with different contact diameters, EGaIn setup on inverted optical microscope, summary of the J(V) curves, raw optical images of the EGaIn junctions, and Videos recorded (8× faster than normal speed) from the junctions from the sides and through the bottom-electrode.

AUTHOR INFORMATION Corresponding Author


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*[email protected] ORCID Christian A. Nijhuis: 0000-0003-3435-4600 Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS We acknowledge the Ministry of Education (MOE) for supporting this research under award No. MOE2015-T2-2-134. Prime Minister’s Office, Singapore under its Medium sized center program is also acknowledged for supporting this research. We thank Hong Zhang for extracting the effective contact area using “Image J”. The Laboratory is a National Research Infrastructure under the National Research Foundation Singapore.

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Rectification Ratio and Tunneling Decay Coefficient Depend on the

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