Ion-assisted Resonant Injection and Charge Storage in Carbon-based Molecular Junctions

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Supporting Information

Ion-assisted Resonant Injection and Charge Storage in

Carbon-based Molecular Junctions

Mustafa Supur, Shailendra K. Saxena, and Richard L. McCreery*

Department of Chemistry, University of Alberta, Edmonton, Alberta T6G 2R3, Canada

* Corresponding Author Email: mccreery@ualberta.ca

Contents:

1. Fabrication of Large Area Molecular Junctions and Thickness Determination 2. Electrical Measurements on Molecular Junctions in Vacuum and in ACN Vapor 3. UV-Vis and Raman Spectroscopy

4. Comparison to TPP/AlOx Molecular Junctions 5. Multiple Cycles in Vacuum and ACN Vapor

6. Additional Integration Results with Varying Scan Limits 7. Reproducibility and Statistics of ACN effect

8. Thickness dependence of TPP only MJs

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1. Fabrication of Large Area Molecular Junctions and Thickness Determination

Deposition of bottom and top contacts of molecular junctions (Au and Carbon) by electron-beam deposition was described in several previous reports.1-4 A previously described5-6 in-situ procedure was used for grafting of tetraphenylporphyrin (TPP oligomers) on bottom electrodes. In this procedure, the amino precursor of TPP, 5-aminophenyl)-10,15,20-triphenylporphine (4-AminoTPP, Frontier Scientific), is treated with excess tertiary butyl nitrate7-8 in solution to convert the amino group (TPP-NH2) to diazonium salt (TPP-N2+), which is then electrochemically reduced to aryl radical (Scheme S1). Aryl radicals attach to the available sites on the electrode surface by a carbon-carbon covalent bond, which enables strong electronic coupling. Continuous electrochemical reduction leads to covalent growth of oligomers on contact stripes. Molecular layer thicknesses (d) were confirmed by atomic force microscopy (AFM) “scratching” method9 on a DI Multimode SPM instrument. Grafting of molecular layer is followed by electron-beam deposition top LiF film and top contact (carbon/Au) with thicknesses indicated in Figure 1. Active junction area is 0.00125 cm2.

Scheme S1. Grafting of TPP layer on C surfaces by diazonium reduction

Deposition of LiF layer on TPP layer was achieved by electron-beam deposition in a Kurt Lesker PVD 75 vacuum system. A high energy (7-8 kV) electron beam is directed onto LiF pellets by a

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magnetic field in vacuum of less than 10−5 Torr. Thickness of LiF was followed by quartz crystal microbalance (QCM) inside the deposition chamber. New LiF pellets were used for each deposition process. Thickness (d) of LiF layer was measured by AFM through the sharp edges of the LiF layer. Edges were formed by depositing the LiF on photoresist stripes obtained by photolithography on Si/SiOx chip. After deposition of LiF, photoresist film was removed by brief rinsing with acetone (Figure S1).

Figure S1. Line profiles across the trench (1×1 mm) in the inset AFM image showing the profile

thickness of 20 nm TPP layer on electron-beam deposited carbon(eC)/Au bottom contact (A) and across the edge of LiF layer on Si/SiOx (B). Histograms for the bottom area of the trench and the surrounding top area of TPP layer (C) and Si/SiOx surface and LiF stripe (D), which were fit by two different Gaussian functions to estimate the depth of trench and LiF layer. The trench was obtained by AFM “scratching” method.

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2. Electrical Measurements on Molecular Junctions in Vacuum and in ACN Vapor

A Janis Research ST-500-1 micromanipulated, vacuum probe station was used to collect current vs bias (IV) curves at room temperature, with the junctions kept in a vacuum of <10−5 Torr during measurements (Figure S2). IV curves were acquired with a 2-wire geometry using a Labview-based data acquisition system utilizing a Stanford Research Systems Low-Noise Current Preamplifier (Model SR570). In all cases, a positive bias indicates that the bottom contact adjacent to TPP is more positive than the top contact above the LiF film. Positive current indicates electron transport from the bottom Au/eC contact to the top in the external circuit. ACN vapour was added through a solvent vessel connected to the Janis chamber with a valve (Figure S2). Under vacuum conditions, the solvent vessel valve is closed. After vacuum (<10−5 Torr) was reached in the chamber, the pump valve was closed and then solvent vessel is opened to let ACN vapour flow into the chamber of probe station. I–V measurements are performed usually after a ca. five-minute rest time for ACN vapour to penetrate into junction.

Density functional theory (DFT) calculations were performed to optimize the model structures and to predict the energy levels frontier orbitals of 18-atom LiF cluster in Figure 4 by using the B3LYP functional and 6-31G(d) basis set, as implemented in the Gaussian 09 program. Graphical outputs of the computational results were generated with the GaussView program (version 5.0) developed by Semichem, Inc.

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Figure S2. Experimental setup for electrical measurements on molecular junctions in the airtight

Janis probe station in vacuum (A) and ACN vapor (B), in which Li and F ions are assumed to mobilize.

3. UV-Vis and Raman Spectroscopy

UV-Vis spectroscopic measurements were performed on an Agilent 8453 spectrophotometer. Bottom electrode (Au/eC) was deposited on fused quartz chips for UV-Vis measurements, prepared as described previously.6 Deposition of TPP was achieved by diazonium reduction as described above for molecular layers on eC surfaces. Absorption background of bottom Au/eC electrode was subtracted from UV-Vis spectra of molecular and LiF layers (Figure S3). Oxidation of TPP was achieved by NOBF4 in benzonitrile (Figure S3) as described in the reaction below: TTP + NO+ →TPP•+ + NO(g)

Raman spectra were acquired by a custom-built Raman spectrometer, equipped with 532-nm laser source and a detector, a backthinned CCD (Andor DU940N) cooled to –80 °C.10 The Raman shift

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axis was calibrated with polystyrene. Raman features of Si and bottom electrode are subtracted from spectra. TPP monomer powder was manually smeared on a Si chip to obtain Raman spectrum in Figure 1E.

Figure S3. (A) Absorption spectra of TPP on eC/Au before and after subtraction of eC/Au

background, eC/Au bottom contact, and LiF layer (20 nm). (B) Absorption spectra of TPP and its radical cation in benzonitrile.

Figure S4. Raman spectra of a complete Au/eC/TPP/eC/Au junction (A), TPP grafted on eC

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4. Comparison to TPP/AlOx Molecular Junctions

Figure S5. JV curves of illustrated TPP/AlOx MJ compared to TPP/LiF MJ in vacuum and ACN

vapour (scan rate: 1000 V/s). Note large difference in Y-axis scales.

5. Multiple Cycles in Vacuum and ACN Vapor

Figure S6. First and 1000th IV cycles of TPP/LiF junction in vacuum (<10–5 torr). Rest time between the cycles is 20 s. Scan rate: 1000 V/s.

Integrity of the TPP/LiF junction in ACN vapor was tested by running repetitive IV cycles (Figure S7A and S7B). Faradaic peaks are mainly preserved after 500 cycles upon ± 1.5 V cycling in ACN vapor.

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Figure S7. IV cycles of TPP/LiF junction in ACN vapour (left) and plot showing charge stored

relative to first cycle vs. number of cycles. Rest time between the cycles was 20 s. Scan rate: 1000 V/s.

5. Additional Integration Results with Varying Scan Limits

Integrated charge was evaluated for four scan ranges with applied bias limits of ± 1, 1.5, and 2 V. In each case, scan was initiated at V=0 at 1000 V/s to the positive limit, then to the negative limit and back to zero. Integrated charge was divided into four quadrants (Q1 to Q4) shown in Figure 2 with varying bias limits indicated in Table S1.

Table S1. Integration results for varying scan limits, all in nanoCoulombs.

charge, nC ACN 1V ACN 1.5V ACN 2V VAC 1V

VAC 1.5V VAC 2V Bias limits: ± 1.0 ± 1.5 ± 2.0 ± 1.0 ± 1.5 ± 2.0 Q1 9.69 18.72 22.69 0.10 0.15 0.20 Q2 -4.54 -9.16 -10.94 -0.10 -0.14 -0.18 Q3 -5.88 -12.89 -19.53 -0.11 -0.16 -0.21 Q4 3.11 4.70 5.87 0.09 0.13 0.17 total +Q 12.80 23.42 28.56 0.20 0.28 0.37 total –Q -10.41 -22.05 -30.47 -0.20 -0.30 -0.40 recovery 81% 94% 107% 104% 108% 108% ACN/VAC (+Q) 65.6 83.3 77.7 ACN/VAC (–Q) 51.1 72.7 77.1

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7. Reproducibility and Statistics of ACN Effect

Seven Au30/eC10/TPP20/LiF9/eC3/Au25 molecular junctions (subscripts are in nm) were first evaluated in vacuum, then exposed to ACN vapor as described in Figure 2 of the main text. Figure S8 shows that all MJs exhibited a large increase in charge storage relative to vacuum, ranging from a factor of 19.5 to 77.7, with an average of 48.2. The ratio of negative to positive total charge ranged from 0.91 to 1.28 indicating an average “recovery” of 109%.

Figure S8 Initial positive scans of seven different TPP/LiF MJs after exposure to ACN vapor, as

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Table S2 Integration results for Seven TPP/LiF MJs in ACN vapor

a. (Q2+Q3)/(Q1+Q4) b. (Q1+Q4) - (Q2+Q3)

c. the ratio of Qtotal for each MJ in ACN divided by that for #1 in vacuum.

d. Vacuum responses for all junctions were similar to that for MJ#1, but not all were recorded.

8. Thickness dependence of TPP only MJs

Figure S9 shows JV scans for TPP-only MJs having four different thicknesses. A least squares determination of the attenuation slope yields β=0.383. This value was used to adjust the 18.8 nm IV curve to a thickness equal to that used in the main text for the TPP/LiF MJs.

Junction Q1 (nC) Q2 (nC) Q3 (nC) Q4 (nC)

Positive

Q (nC) Negative Q (nC) Recoverya Qtotal (nC)b QACN/QVACc

#1 ACN 22.69 -10.94 -19.53 5.92 28.61 -30.47 107% 59.08 77.73 #2 ACN 19.82 -9.30 -17.85 5.43 25.25 -27.15 108% 52.40 68.95 #3 ACN 13.25 -5.39 -12.46 3.63 16.88 -17.85 106% 34.73 45.70 #4 ACN 21.25 -10.17 -18.64 5.67 26.92 -28.81 107% 55.73 73.33 #5 ACN 6.91 -2.31 -9.37 3.24 10.15 -11.68 115% 21.83 28.73 #6 ACN 7.26 -2.48 -11.86 3.97 11.23 -14.34 128% 25.58 33.65 #7 ACN 12.15 -9.24 -3.45 1.81 13.96 -12.69 91% 26.65 35.07 Average 14.76 -7.12 -13.31 4.24 19.00 -20.43 109% 39.43 51.88 StDev 6.07 3.40 5.39 1.39 7.20 7.52 14.81 19.49 mean increase 48.28 junction #1 VACd 0.20 -0.18 -0.21 0.17 0.37 -0.39 105% 0.76

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Figure S9. (A) JV curves of Au30/eC10/TPPxx/ eC10/Au25 junctions with different molecular layer thicknesses. (B) Attenuation (β) plot (lnJ vs thickness, nm–1) derived from JV curves of TPP junctions at at +0.5 V.

9. Photocurrent Measurements on Molecular Junctions in Vacuum and in ACN Vapor

MJs used for photocurrent (PC) measurements were fabricated on fused quartz chips.6 A home-built vacuum chamber with glass viewports and feed-through connectors was used to obtain PC spectra in vacuum and in ACN vapour. PC spectra were obtained using a 150 W Xenon arc source coupled with a monochromator (bandpass = 13 nm) through an optical beam chopper and focused onto the MJ through the top contact.6, 11 A lock-in amplifier was employed for PC detection referenced to the optical beam chopper. The detailed procedure for measurements and verification of PC direction had been previously reported.

References:

1. Morteza Najarian, A.; Szeto, B.; Tefashe, U. M.; McCreery, R. L., ACS Nano 2016, 10, 8918-8928.

2. Najarian, A. M.; Bayat, A.; McCreery, R. L., J. Am. Chem. Soc. 2018, 140, 1900−1909. 3. Smith, S. R.; McCreery, R. L., Adv. Electron. Mater. 2018, 4, 1800093.

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5. Najarian, A. M.; Supur, M.; McCreery, R. L., J. Phys. Chem. C 2020, 124, 1739−1748. 6. Saxena, S. K.; Smith, S. R.; Supur, M.; McCreery, R. L., Adv. Optical Mater. 2019, 7,

1901053.

7. Baranton, S.; Bélanger, D., Electrochimica Acta 2008, 53, 6961-6967.

8. Bayat, A.; Lacroix, J.-C.; McCreery, R. L., J. Am. Chem. Soc. 2016, 138, 12287-12296. 9. Anariba, F.; DuVall, S. H.; McCreery, R. L., Analytical Chemistry 2003, 75, 3837-3844. 10. Ramsey, J. D.; Ranganathan, S.; Zhao, J.; McCreery, R. L., Applied Spectroscopy 2001,

55, 767-773.

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