Switching of Current Rectification Ratios within a Single Nanocrystal by Facet-Resolved Electrical Wiring

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1 Supporting Information for Switching of Current Rectification Ratios within a Single Nanocrystal by Facet-Resolved Electrical Wiring Yan B. Vogel, Jinyang Zhang, Nadim Darwish* and Simone Ciampi* * simone.ciampi@curtin.edu.au, nadim.darwish@curtin.edu.au S1. Band Diagram for the Pt/Cu 2O/a-Si Interface Figure S1. Band diagram for the Pt/Cu2O/a-Si interface. qx = semiconductor electron affinity, qφ = work function, E c = conduction band energy, E v = valence band energy, E F = Fermi level, E g = band gap energy, Φ = built-in potential. The band diagram was drawn based on the following energy considerations: the Pt/Cu 2O contact is taken as ohmic 2 and the Cu 2O/a-Si p-n junction is rectifying. For the a-si, the Fermi level used is positioned 0.2 ev below the conduction band for a resistivity of 10 4 Ω cm, 3 an electron affinity of 3.9 ev 4 and a band gap of 1.7 ev. 5 For Cu 2O, the electron affinity is 3.2 ev, 6 a band gap of 2.2 ev and the Fermi level 0.25 ev over the valence band. 7 The relatives energies for the Cu 2O 111 and 100 are given according to 8. 1

2 S2. Extracted Cu 2O Series Resistance as a Function of Contact Heights Estimates of the changes to the series resistance values as a function of the Cu 2O particle heights are shown below. These values were obtained by fitting the experimental I V curves using the piecewise linear approximation. The resistance of the Cu 2O particles is in the order of 10 2 GΩ suggesting a resistivity in the order of 40 MΩ cm, which is in agreement with literature values for Cu 2O. 1 Figure S2. Resistance values as a function of the contact height (AFM tip-to-silicon substrate distance) for (a) polyhedral Cu 2O particles (Figure 2b, main text) and (b) octahedral Cu 2O particles (Figure 4, main text). The values of resistance were extracted by fitting the respective current potential curves using the piecewise linear approximation. S3. X-Ray Photoelectron Spectroscopy (XPS) XPS measurements were performed on a Kratos Axis Ultra DLD spectrometer using a monochromatic Al-Kα ( ev) irradiation source operating at 150 W. Spectra were taken in normal emission at or below Torr. Data files were processed using CasaXPS software and the reported XPS energies are binding energies expressed in ev. Analysis of the XPS spectra of the copper oxide nanoparticles followed the procedures of Koberstein and co-workers, 9 and Cuenya and co-workers. 10 Data analysis involved background subtraction using the Shirley routine and a subsequent nonlinear least-squares fitting to mix Gaussian Lorentzian functions. A variable weighting of these functions was used depending on the specific envelope. The p-type particles of this work are grown by means of discharging Cu 2+ ions present in solution onto a 1,8-nonadiyne-modified amorphous silicon photoelectrode. Due to the dark electrical insulating nature of these substrate the calibration of the XPS binding energies becomes important. Binding energies of all high-resolution scans were calibrated by means of applying a rigid shift to bring the main C1s emission of the carbonaceous diyne-derived film to ev (e.g. Figure S3f). The Auger parameter for copper in the samples, as derived from the sum of kinetic energy of the LMM Auger line ( 917 ev, e.g. Figure S3b) and binding energy of the 2

3 Figure S3. Light intensity, degree of cubicity and spectral features. XPS spectrographs for amorphous silicon photoelectrodes onto which copper oxide particles are electro-deposited by projecting on all-on pixels image at a variable light intensity (high = 1030 lx; low = 170 lx). (a c) XPS data for octahedral particles (high light intensity), and (d f) spectral data for cubic particles (low light intensity). (a) and (d) XPS narrow scans of the Cu 2p 3/2 region (~933 ev) and Cu 2p 1/2 (~952 ev). In panels (b) and (e) the Cu LMM peaks confirm the main oxidation state of copper in the particles is +1. XPS spot size is ca. 1.6 mm 2. Samples are prepared by applying a single-step bias to the working a-si electrode which is stopped when 9 mc of charge has passed though the interface. The height of the potential step was -0.1 V. The electrolyte solution was 0.5 M K 2SO 4 and 50 mm CuSO 4 with a trace amount of KCl (concentration below detection limit of 15 M as determined by conductivity after ion chromatography separation using a Dionex DX-500 system). Bias is given against the Ag AgCl KCl (sat. KCl) reference electrode. Representative high-resolution C 1s region (nonadiyne monolayer chemically passivating the a-si surface) in (f) and representative Cl 2p region in (c). photoelectron signal (Cu 2p3/2, ev, FWHMs of 1.1 ev) is in average 1850 ev which turns out to be consistent with a predominant Cu(I) population. 11 This parameter is independent of reference energies and charging effects and plotting this value in a Wagner diagram suggest our experimental data are in line with those reported for Cu2O. The high binding energy shoulder in the XPS Cu 2p3/2 signal and the presence of shake-up satellites at a ca. 9 e V higher binding energies than those of the main 2p3/2 and 2p1/2 peaks highlight however a minor presence of Cu(II) species. These shake-ups are generally more evident for CuO than for either bulk Cu2O or metallic Cu (this four-peak set is the typical fingerprint of Cu(II) salts 12 ), 13 and presumably result from a thin CuO surface layer (ca. 934 ev) over the Cu2O core (ca. 933 ev) of the nanoparticles. 9 Further, mild reduction conditions can produce a thin layer of Cu metal on Cu2O, but the possible presence of metallic copper cannot be excluded by analyzing the 2p3/2 peak alone as the Cu 0 XPS emission (metallic) cannot be resolved from the Cu(I) signal; both are falling around ev. However, chemical shifts in X-ray excited Auger emissions are normally larger than those of the photoelectron lines, hence to test for Cu 0 we looked at Cu LMM Auger peaks. The Cu LMM auger peak after the deposition of these nanoparticles (Figure S3 and S4) are generally found around 570 ev (917 ev of kinetic energy), reinforcing on a major Cu2O population 3

4 Figure S4. Cuboctahedrons. XPS spectrographs for amorphous silicon photoelectrodes onto which copper oxide octahedral particles are electro-deposited by projecting on all-on pixels image at a high light intensity (1030 lx). (a) XPS narrow scans of the Cu 2p 3/2 (~933 ev) and region and Cu 2p 1/2 (~952 ev). (b) Cu LMM peaks confirming the main oxidation state of copper in the particles is +1. A single step bias is applied to the working electrode and stopped when 9 mc of charge had passed though the interface. The height of the potential step was -0.1 V. The electrolyte solution was 0.5 M K 2SO 4 and 50 mm CuSO 4 with a trace amount of KCl (concentration below 15 M as determined by conductivity after ion chromatography separation using a Dionex DX-500 system). Bias is given against the Ag AgCl KCl (sat.) reference electrode. (reported LMM binding energy of Cu(I) is ev) with absence of the characteristic signals from Cu 0 (reported LMM binding energy of Cu 0 is ev) and suggesting metallic copper impurities are most likely absent. We note however that the reduction of Cu 2+ species under X-ray irradiation in UHV has been reported, and it is therefore possible that the observed amount of CuO is an underestimate because of its reduction during XPS acquisition. All the samples presents an XPS Cl 2p spin orbit-split peak at binding energies of ca. 200 ev, indicating a minor presence of inorganic chloride on the samples regardless of the deposition bias or illumination (generally ~0.8% of the total atomic composition), which is in agreement with our hypothesis of halide anions adsorption being a central factor in defining the degree of cubicity of the particles. XPS data cannot point unambiguously towards CuCl or CuxOyClz traces. The Si 2p binding energy is generally observed at 99.4 ev (average of the Si 2p1/2 and Si 2p3/2 signals, the area ratio for the two spin orbit-split peaks (2p1/2 to 2p3/2) is 1:2 and full width half maximum values, FWHMs of 0.7 ev) which is unchanged when compared with the value of 99.4 ev obtained prior to copper deposition, suggesting a large-scale XPS measurement (spot size ca. 1.6 mm 2 ) does not contain information on local changes to band bending after the growth of a sub-layer of Cu2O particles. 4

5 S4. Shape-Tuning of Cu 2O Nanocrystals (further considerations) Figure S5. SEM images of Cu2O particles grown on a-si by varying the electrodeposition bias in the presence of 0.1 mm KCl. The isotherm-type curve (Figure 3d) is displaced towards more negative potentials by adding excess chloride, i.e. the transition from cubic to octahedral particles requires less positive biasing. The electrolyte is 0.5 M K 2SO 4, 50 mm of CuSO 4 and 0.1 mm of KCl and the Cu 2O crystal where deposited over an amorphous silicon photoelectrode under constant light illumination (1030 lx). The deposition potential is specified above each image. Figure S6. SEM images of Cu2O particles electrodeposited at constant potential ( 0.3 V) where shapes are varying in response to changes to the working electrode illumination. From left to right, the light intensities are 390 lx, 240 lx and 100 lx. The electrolyte is 0.5 M K 2SO 4, 50 mm of CuSO 4 and <15 µm of KCl. The Cu 2O crystals where deposited over an amorphous silicon photoelectrode. 5

6 Figure S7. SEM images of Cu2O particles electrodeposited varying the chloride bulk concentration at a constant potential of 1.0 V. The putative isotherm curve for the electro-adsorption of chloride (e.g. Figure 3d) is displaced to the left by adding chloride (i.e. towards more negative potentials). The transition to from cubic to octahedral particles shifts towards very negative potentials and becomes effectively bias-independent. The electrolyte is 0.5 M K 2SO 4, 50 mm of CuSO 4 and the Cu 2O crystal where prepared at a constant light intensity (1030 lx). The chloride bulk concentration is indicated above of each image. S5. Inter-Particle Distances and Particle Sizes for Cu 2O Cubic crystals Figure S8. SEM images showing changes in the particle size for Cu 2O cubes organized in separate discrete arrays grown within the same macroscopic a-si substrate by projecting a gradient of density of ON pixels (Figure S17a). The particles were electrodeposited at 0.1 V, with 16 mc cm 2 of charged passed during the electrodeposition and the density of ON (bright) pixels across the sample being set to 20% (a), 30% (b) and 40% (c). 6

7 Figure S9. SEM images for the Cu 2O cubes analysed electrically in Fig. 5 of the main text. Cubes are organized in separate areas within the same macroscopic a-si substrate. The profile of the light stimulus used during coper discharge is achieved by means of by projecting a gradient of ON pixels (Figure S17a). The particles were electrodeposited at 0.1 V, with 32 mc cm 2 of charged passed during the electrodeposition and the density of ON (bright) pixels across the sample set to 20% (a), 30% (b) and 40% (c). S6. Additional Electrical AFM Data Figure S10. (a-c)tunneling current images taken at 2 V with an inverted current scale to the one used in Figure 5 (panels d f, respectively) of the main text. 7

8 Figure S11. Size-dependent degree of rectification for Cu2O cubic crystals. Representative current potential traces curves for the data shown in Figure 5 of the main text. The density of ON (bright) pixels across the sample was set to 20% (a), 30% (b) and 40% (c). Figure S12. Current-potential relationship for cubic Cu2O crystals as a function of the array interparticle distance. Representative current potential traces curves for the data shown in Figure 6 of the main text. (a) Low density system and (b) densely-packed particles. 8

9 Figure S13. Current-potential curves taken on the a-si substrate at different particle density. Mapping of the a-si substrate for the sample presented in Figure 5 of the main text. Red arrows in (a c) are only a guide to the eye, i.e. indicative of the current sampling position, and the current potential plots in (d f) represent an average of curves that were sampled at a constant force and at different locations on the a-si substrate (20-50 locations), with the standard deviation shown as semi-transparent colored contours. Schematic depiction of the putative changes to degree of space-charge overlap in the silicon. Figure S14. SEM images of Cu2O particles of similar size but grown at different inter-particle spacing. The particles were electrodeposited on the a-si photoelecrtrodes at a bias of 0.0 V (vs. reference) using either (a) 40% pixel density in the FLCoS light pattern and passing 16 mc cm 2 of charge, or (b) 20% pixel density and 3.6 mc cm 2. 9

10 Figure S15. Current-potential relationship of cubic Cu2O crystals as a function of the particle height. (ab) AFM height images of cubic particles of similar particle density and height of ca. 700 nm and 250 nm, respectively. (c d) Comparison of the current-potential relationship as a function of the particle height. The black line is for the particles shown in (a) and the red line is for the particles shown in (b). The solid lines represent the average of 50 independent I V spectra taken on different locations of particles and the standard deviation is shown as a semi-transparent contour plot. Rectification ratios (RR) are calculated for 4.5 to 4.5 V and are RR = 3 and RR = 13 for particles of panel (a) and (b) respectively. Particles were electrodeposited at 0.0 V, using a 20% pixel density and (a) 39 mc cm 2 and (b) 7 mc cm 2 (see Figure S17b for details on the illumination patterns). 10

11 S7. Electrochemical Cell Design Figure S16. Electrochemical cell used in conjunction of the FLCoS projector. Details on the optics and electronics design of the FLCoS spatial modulator used to drive the electrodeposition of the Cu 2O particles on a-si are reported elsewhere. 14 (a) Blown-up view scheme of the electrochemical cell used for the electrodeposition of the Cu 2O particles. The working electrode is illuminated by the FLCoS modulated (time and space) beam of light that is focused by a 50 mm objective before passing through the FTO glass counter electrode. (b) Pictures of the components and fully-assembled electrochemical cell. 11

12 S8. Large-Scale Arrays Figure S17. Large millimeter-scale arrays. (a-b) Matlab-generated bitmap images loaded in the FLCoS and used to build the large scale Cu 2O arrays on silicon discussed in the main text (Figures 5 and 6, respectively). (a) Progressive increase in the density of ON pixels from 0 to 100% in steps of 10% every 100 pixels on the x- axis direction, corresponding to steps of ca. 460 µm on the projected image. Particles were electrodeposited at 0.0 V in the presence of 3 mm of potassium chloride until reaching a transferred total charge of 9 mc. (b) density of pixels at 20% (bottom) and 40% (top). S9. References 1. Han, X.; Han, K.; Tao, M. N-Type Cu 2O by Electrochemical Doping with Cl. Electrochem. Solid-State Lett. 2009, 12, H89 H Yang, W.-Y.; Rhee, S.-W. Effect of Electrode Material on the Resistance Switching of Cu 2O Film. Appl. Phys. Lett. 2007, 91, Chittick, R. C.; Alexander, J. H.; Sterling, H. F. The Preparation and Properties of Amorphous Silicon. J. Electrochem. Soc. 1969, 116, Matsuura, H.; Okuno, T.; Okushi, H.; Tanaka, K. Electrical Properties of N Amorphous/P Crystalline Silicon Heterojunctions. J. Appl. Phys. 1984, 55, Wronski, C. R.; Carlson, D. E.; Daniel, R. E. Schottky Barrier Characteristics of Metal Amorphous Silicon Diodes. App. Phys. Lett. 1976, 29, Olsen, L. C.; Addis, F. W.; Miller, W. Experimental and Theoretical Studies of Cu 2O Solar Cells. Sol. Cells 1982, 7, Siol, S.; Hellmann, J. C.; Tilley, S. D.; Graetzel, M.; Morasch, J.; Deuermeier, J.; Jaegermann, W.; Klein, A. Band Alignment Engineering at Cu 2O/ZnO Heterointerfaces. ACS Appl. Mater. Interfaces 2016, 8,

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