Supporting information for: The Influence of an. Comprising Self-Assembled Monolayers

Transcription

1 Supporting information for: The Influence of an Atom in EGaIn/Ga 2 O 3 Tunneling Junctions Comprising Self-Assembled Monolayers Davide Fracasso, Mutlu Iskender Muglali, Michael Rohwerder, Andreas Terfort, and Ryan C. Chiechi, Stratingh Institute for Chemistry and Zernike Institute for Advanced Materials, University of Groningen, Nijenborgh 4, 9747 AG Groningen, the Netherlands, Max-Planck-Institut für Einsenforschung GmbH, Max-Planck-Straße 1, Düsseldorf, Germany, and Institut für Anorganische und Analytische Chemie, Goethe-Universität Frankfurt, Max-von-Laue-Straße 7, Frankfurt, Germany r.c.chiechi@rug.nl To whom correspondence should be addressed University of Groningen Max-Planck-Institut University of Frankfurt S1

2 Sample Preparation The Au substrates used in this work are made by mechanic Template Stripping (mts) described in detail elsewhere. 1 In our case, we deposited 200 nm of Au (99.99%) by thermal vacuum deposition onto a 3" Silicon wafer (with no-adhesion layer). Using UV-curable Optical Adhesive (OA) Norland 61, we glued 1 cm 2 glass chips on the metal surfaces. The mts procedure provides ultra flat smooth surfaces, which allows self-assembly process to achieve higher yields of working junctions. All samples were made by incubation of freshly cleaved gold slides in 20 µm solution in ethanol at room temperature for 24h. Data Acquisition Data were acquired in a home-built setup that is described in detail elsewhere. 2 Each SAM was measured by placing a tip of EGaIn in contact and acquiring at least 420 scans in 40 different positions across 3 substrates for an average of 5 scans per junction. For longer and more stable SAMs we record up to 1460 scans. Traces in which the instrument reached compliance were considered shorts and were discarded (they reflect only the compliance limit of the instrument and have no physical meaning). Histograms of the values of J at each value of V were fit to Gaussian distributions and the log-averages were determined. The results of these two methods of determining J are shown in Figure S1. The two methods produce very similar values. The difference in P1, PP1, and PyP1 are due to bi-model Gaussian (or incomplete) distributions; rather than arbitrarily selecting one distribution to fit, we used the geometric average values for J and the variance for the error throughout this paper. Table S1 is a summary of the number of traces and the percentage of junctions that shorted for each SAM. We calculated this percentage from the number of junctions that shorted during a measurement, however, since we collected an average of five scans per junction, these yields can be described as the percentage of junctions that shorted within approximately five scans. That is why two SAMs, PyPP1 and PyPP2, exhibit 0% shorts. S2

3 Table S1: Number of traces acquired and the % of shorted junctions for the Ph-SAM and Py-SAM series SAMs number of traces % of junctions that shorted P % PP % PPP % PPP % PPP % Py % PyP % PyPP % PyPP % PyPP % 0 Py1 PyP1 PyPP1 PyPP2 PyPP3 Log mv Ph-SAM Geometric Ph-SAM Gaussian Py-SAM Gaussian Py-SAM Geometric P1 PP1 PPP1 PPP2 PPP3 SAM Figure S1: Plots of the values of J at 100 mv for the Ph-SAM (red, bottom axis) and Py-SAM (blue, top axis) series from Gaussian fits (squares) and geometric means (circles). S3

4 Calculations All calculations were performed using Gaussian 03 on a 12-CPU GNU/Linux cluster. Structures were first minimized by AM1. The torsional angles, q, were then fixed to known values in SAMs and then the point-energies calculated DFT using the B3LYP/6-311+g(2d,2p) basis set. In accordance to literature procedures, the hydrogen atoms were removed from the thiol groups before calculating the dipole moments, µ. 3 The net perpendicular dipole moment, µ?, was calculated by summing the contributions along the X and Z axes (Y lies completely parallel to the substrate) using the formula µ? = µ x,z cosa where a is the tilt angle along the respective axis. 4 Figure S2 shows the densities of the HOMO orbitals for the Ph-SAM and Py-SAM series and Table S2 is a summary of the parameters for and results of the DFT calculations. Figure S3 is a plot of the calculated energies of the HOMO levels as a function of XPS-derived SAM thickness along with the linear fits of J = J 0 e bd from Figure 2 showing that the HOMO energies track very closely with the length of the molecules and that the presence of the nitrogen atom in the Py-SAM series shifts the values more negative, but does not substantially affect the overall trend. While we recognize that DFT calculations done on molecules in the gas phase do not perfectly capture their properties in SAMs, by calculating the Ph-SAM and Py-SAM series in the geometries that they adopt in SAMs, we compensate for many of the discrepancies. There are, potentially, issues regarding the partial coupling of the producing gold orbitals with the HOMOs of the molecules, but accurately calculating these effects is beyond the scope of this paper and is not necessary to accurately evaluate the the dipole moments or the trend in HOMO energies within a series of molecules. Kelvin Probe The measurements were performed as described elsewhere. 4 The Kelvin probe was calibrated with freshly cleaved highly oriented pyrolytic graphite (HOPG) in a nitrogen atmosphere (glove box). Freshly cleaved HOPG is known to have a stable work function of 4.48 ev. 12 The Au probe was bring close to the surface at narrow fixed distance and the surface potential once stabilized was S4

5 P1 Py1 PP1 PyP1 PPP1 PyPP1 PPP2 PyPP2 PPP3 PyPP3 Figure S2: DFT calculations of the Ph-SAM series (left) and Py-SAM series (right) showing the even distribution of the density of the HOMO orbitals. Table S2: Parameters for and results of B3LYP/6-311+g(2d,2p) DFT calculations of the Ph- SAM and Py-SAM series. Molecule E HOMO (ev) a µ X (D) b µ Z (D) b µ? (D) b a x ( ) c a z ( ) c q ( ) d N (m 2 ) c P PP PPP PPP PPP Py PyP PyPP PyPP PyPP a Calculated with the thiol hydrogen intact b Calculated with the thiol hydrogen removed 3 c Experimental values from the literature 5 7 d Combined theoretical and experimental values from the literature 5 11 S5

6 HOMO (ev) Ph-SAM HOMO Py-SAM HOMO Log J Length (Å) Figure S3: Plots of the calculated energies of the HOMOs (left axis) of the Ph-SAM (red squares) and Py-SAM (blue circles) versus XPS-derived molecular lengths. The dotted and dashed lines (right axis) are the fits to J = J 0 e bd from Figure 2 in the main text. reported. By measuring the surface potential of HOPG freshly cleaved we were able to calculate the work function for the Py- and Ph-SAMs. Transition Voltages The values of V trans (+) in this work were calculated from the minima in Fowler-Nordheim plots. We found the minimum from each trace and fit the resulting histograms with a Gaussian function as shown in Figure S5. In the main text we report only the positive values as these are more straightforward to compare to existing literature, however, for most of the molecules analyzed in this work we also observed a V trans ( ). In a recent report 13 Bâldea introduced two fairly simple equations to describe the offset between the Fermi energy and the closest molecular orbital e h, and g which is defined as voltage division factor or also called degree of symmetry h in other theoretical work. 14 Bâldea in his report utilized experimental values of V trans (±) from Beebe et al. and derived e h and g. In Table S4 we report the values obtained for our two series: Ph-SAM and Py-SAM by using equation (6) and (7) reported elsewhere. 13 S6

7 Table S3: Measured V trans (+) for comparison with reported value by Beebe et al. SAM on Au TS V trans V trans Literature Hexadecanethiol ± OPE3 disac a ± a Beebe et al. measured OPE3-SH Table S4: Experimental values of V trans (±), calculated values of e h and g according to equations reported elsewhere. 13 SAMs V trans (+) V trans ( ) e h g P PP PPP PPP PPP Py PyP PyPP PyPP PyPP S7

8 Figure S4: Schematic representation of the energy level diagram of the Au TS -SAM//Ga 2 O 3 /EGaIn, showing the energy Fermi level of Au and EGaIn, the approximate vacuum level shift induced by the collective dipole moment, and the upper and lower relative position of the HOMOs for the two series Ph-SAM and Py-SAM S8

9 Figure S5: Representative histograms of V trans (+), for SAMs of PPP3 and PyPP3. The log-normal distribution of V trans allows fitting with a Gaussian function. We found this method to be superior to deriving V trans (+) plots of mean values of J vs V, which obfuscates the true distribution of V trans. S9

10 Figure S6: Representative Fowler-Nordheim plots of 10 traces for SAMs of PyPP3. The arrow shows the minima in these plots (V trans (+) ) which we used to construct the histograms in Figure S5. 2 ln J (A/cm 2 ) Py-SAM Ph-SAM Length ( ) Figure S7: The same data as in Figure 2 from the main text, plotted with 99% confidence bands, showing that the two vales of b are within the confidence limit and are therefore statistically indistinguishable. S10

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