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High Quantum Efficiency Hot Electron Electrochemistry Hyun Uk Chae, Ragib Ahsan, Qingfeng Lin, Debarghya Sarkar, Fatemeh Rezaeifar, Stephen B. Cronin, and Rehan Kapadia Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.9b02289 • Publication Date (Web): 21 Aug 2019 Downloaded from pubs.acs.org on August 22, 2019

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High Quantum Efficiency Hot Electron Electrochemistry Hyun Uk Chae†,‡, Ragib Ahsan†,‡, Qingfeng Lin†, Debarghya Sarkar†, Fatemeh Rezaeifar†, Stephen B. Cronin†, Rehan Kapadia*,† †Department

of Electrical and Computer Engineering, University of Southern California, Los Angeles, California 90089, USA

KEYWORDS: MIS devices, hot electron, hydrogen evolution reaction, quantum efficiency, Monte Carlo simulation, scattering mechanism.

ABSTRACT

Using hot electrons to drive electrochemical reactions has drawn considerable interest in driving high-barrier reactions and enabling efficient solar to fuel conversion. However, the conversion efficiency from hot electrons to electrochemical products is typically low due to high hot electron

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scattering rates. Here, it is shown that the hydrogen evolution reaction (HER) in an acidic solution can be efficiently modulated by hot electrons injected into a thin gold film by an Au-Al2O3-Si metal-insulator-semiconductor (MIS) junction. Despite the large scattering rates in gold, it is shown that the hot electron driven HER can reach quantum efficiencies as high as ~85% with a shift in the onset of hydrogen evolution by ~0.6 V. By simultaneously measuring the currents from the solution, gold, and silicon terminals during the experiments, we find that the HER rate can be decomposed into three components: (i) thermal electron, corresponding to the thermal electron distribution in gold (ii) hot electron, corresponding to electrons injected from silicon into gold which drive the HER before fully thermalizing and (iii) silicon direct injection, corresponding to electrons injected from Si into gold that drive the HER before electron-electron scattering occurs. Through a series of control experiments, we eliminate the possibility of the observed HER rate modulation coming from lateral resistivity of the thin gold film, pinholes in the gold, oxidation of the MIS device, and measurement circuit artifacts. Next, we theoretically evaluate the feasibility of hot electron injection modifying the available supply of electrons. Considering electron-electron and electron-phonon scattering, we track how hot electrons injected at different energies interact with the gold-solution interface as they scatter and thermalize. The simulator is first used to reproduce other published experimental pump-probe hot electron measurements, and then simulate the experimental conditions used here. These simulations predict that hot electron injection first increases the supply of electrons to the gold-solution interface at higher energies by several orders of magnitude and causes a peaked electron interaction with the gold-solution interface at the electron injection energy. The first prediction corresponds to the observed hot electron electrochemical current, while the second prediction corresponds to the observed silicon direct


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injection current. These results indicate that MIS devices offer a versatile platform for hot electron sources that can efficiently drive electrochemical reactions.

Efficiently using hot electrons before thermalization has been an aim of fields such as hotelectron transistors1-4, solar cells5-7, plasmonics8-14, photoemission15, 16, memory17, 18, and solar-tofuel19 devices. However, non-equilibrium electrons exhibit lifetimes of ~1-100 fs due to electronelectron and electron-phonon interactions20-22. These ultra-short hot carrier lifetimes drive the thermalization process to dominate over most other technologically relevant processes, causing devices to generally have low hot-electron quantum efficiencies. Multiple strategies have been explored to overcome these challenges, including engineering systems with low-electron densities and weak electron-phonon coupling, such as quantum dots23, minimizing the transit length of hot electrons, by creating devices using 2-D materials1, 24, 25, and the search for new materials with naturally favorable scattering rates, such as perovskites 26. However, the overall efficiency27, 28 of these hot electron devices have mainly precluded their practical use, with photoemitters 29, 30 and Flash memory31 as the two exceptions. In this work, we use a metal-insulator-semiconductor junction to controllably create a hot electron population via tunneling of electrons from the semiconductor conduction band into the metal and then use that hot electron population to modulate the electrochemical reaction rate at a metal-electrolyte junction. Figure 1a shows the basic device structure which consists of an n-type silicon wafer, an aluminum oxide insulator layer, and a thin gold layer. The entire device is encapsulated in an epoxy, leaving only the top gold layer exposed, and is immersed in a 0.5 M H2SO4 solution. A cartoon schematic of the band diagram is shown in Figure 1b. MIS junctions enable injection of the highest energy hot electrons when compared to both metal-insulator-metal


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Figure 1. Device schematics and hot electron injection process. (a) Schematic of the structure of metal insulator semiconductor device used here. (b) Band diagram of the device and two different paths for injected hot electrons into the Au region. (c) Current density vs applied voltage for the Au/Al2O3/Si diode used here in both linear and log scale. (d) TCAD Sentaurus simulations of surface electron concentration as a function of applied bias in the Au/Al2O3/Si device.

tunnel (MIM) junctions, or metal-semiconductor (MS) junctions. In MIM junctions, the large density of states around the Fermi levels of the metals cause large currents to flow with relatively smaller applied biases, limiting the energy at which hot electrons can be injected. For MS junctions, the offset between the semiconductor conduction band and metal Fermi level are pinned at the interface, and this causes the injected energy of the hot-electrons to be fixed by the Schottky


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barrier height. In this device, as the voltage across the MIS junction increases, there will be an increase in both current and the energy at which the hot electrons are injected. This behavior occurs due to the insulator layer depinning the semiconductor conduction band from metal Fermi level at the junction. Thus the MIS structure is expected to generate the hottest electrons in the metal when compared to MS or MIM structures. Figure 1c shows the current density plotted as a function of applied bias for an Au/Al2O3/Si device with a 12 nm thick gold layer, 6 nm thick Al2O3 layer, and a moderately doped n-type (5×1016 cm-3) Si wafer. From the device measurements, we see that the current increases exponentially until VAu-Si~0.4V, and then increases linearly. To understand this behavior, we simulate the device using a 2-D Technology computer-aided design (TCAD) Sentaurus simulation, which self-consistently solves the drift-diffusion and Poisson equations. From this simulation, we extract the silicon surface electron density, plotted in Figure 1d on both linear and log scales. Importantly, the sheet charge density also shows a clear exponential and linear regime. These two regimes occur depending on where the applied voltage is dropped. In the exponential regime (0 V < VAu-Si < 0.4 V), the majority of the applied voltage is dropped across the semiconductor depletion region, changing the semiconductor band bending and therefore surface charge density, but without causing any significant change in the band offset between the metal Fermi level and semiconductor conduction band. This occurs due to the oxide capacitance being much greater than the semiconductor capacitance. Thus, although more electrons are injected from the semiconductor into the metal in this regime, the relative energy at which they are injected does not significantly change. Next, the linear charge density regime (0.4 V < V Au-Si) occurs when the majority of the applied voltage is dropped across the oxide. In this regime there will be a nearly 1:1 ratio between the applied bias and the change in the semiconductor conduction band edge position with respect


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c

30 25 20 15 10

14 12 10 8 6 4

Hot electron shift

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-1 -2 -3 -4 -5

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10 10 10 10 10 10

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VAu-Si (V)

b 10

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2.4

VAu-Si = 1.0V VAu-Si = 1.5V VAu-Si = 2.0V

ii) 190mV/dec

2.0 1.6

i) 175mV/dec

1.2 0.8 0.4 0.0 -5.0

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log j (mA/cm )

Figure 2. Linear sweep voltammetry (LSV) curves of 12nm Au MIS device. (a) Linear scale solution current density vs applied Au-Solution voltage for varying Au-Si diode voltages and (b) log scale of (a). (c) Solution current density vs applied Au-Si voltage for varying Au-Solution voltages and (d) log scale of (c). (e) Tafel relation for the low V Au-Si and (f) high V Au-Si. to the metal Fermi level. This regime occurs due to the semiconductor capacitance in accumulation being much greater than the oxide capacitance. From these measurements and simulations, we estimate that the initial 0.4 V applied bias does not change the offset between the silicon conduction band and metal, but that for voltages above 0.5 V, the voltage is primarily dropped across the oxide until the series resistance limits the current. The electrochemical measurement was carried out using a potentiostat with two working electrodes to apply independent bias voltages between the gold and solution terminals (V Au-Solution)


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and the gold and silicon terminals (V Au-Si). We use a platinum wire as a counter electrode in the solution and an Ag/AgCl reference electrode. A schematic diagram of the measurement setup and configuration is shown in Figure S1. Unlike traditional electrochemical cells, where the working electrode (WE) and the counter electrode (CE) are the primary current components (ignoring current flow into the reference electrode), and are by definition equal in magnitude, in this device there are three currents, the current leaving the silicon, 𝐼𝑆𝑖 , the current leaving the gold, 𝐼𝐴𝑢 , and the current leaving the counter electrode, 𝐼𝑆𝑜𝑙𝑢𝑡𝑖𝑜𝑛 . Supplementary Figure S1a shows the three measured current components (𝐼𝑆𝑖 , 𝐼𝐴𝑢 , 𝐼𝑆𝑜𝑙𝑢𝑡𝑖𝑜𝑛 ) and the internal current components which will flow in the device. In our MIS device, the current flow is due primarily to electrons, and thus the electron flux directions are opposite to the current arrows. Supplementary section S1 also shows the mathematical relationships between current components in this measurement setup. To study the redox behavior of these devices, we carried out two types of ISolution measurements. First, we sweep the voltage of the Au-Solution (V Au-Solution) junction and simultaneously step the voltage of the Au-Silicon (V Au-Si) junction. Figures 2a,b plot the linear and log scale results of these measurements, respectively. From the linear scale plot (Fig. 2a), as the voltage between the Au-Silicon junction increases, the turn-on voltage for hydrogen reduction is reduced, and current density increases. For an applied V Au-Solution = -1.5 V, the current increases from ~13 mA/cm2 to ~42 mA/cm2 when V Au-Si = 2 V. From the log-scale current plots (Fig 2b), we can more clearly see the reduction curves shift as the V Au-Si voltage is increased. This shift to lower voltages between the gold and solution is attributed to the increased flux of electrons impinging on the gold solution surface, caused by the injection of hot electrons into the gold from the Si. To understand the effect of the Au-Si junction voltage, we have swept VAu-Si while stepping VAu-Sol. Figure 2c shows the results on a linear scale. In all cases of current vs V Au-Si, there is a turn-on voltage, which becomes


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smaller as the V Au-Solution voltage is increased. This should occur as the higher voltage will change both the concentration of [H+] at the gold/solution interface, and modify the energy barrier. Figure 2d shows the same graphs on a log scale. At low diode voltages, the solution current is nearly constant, limited by the thermal electrons in gold. Once V Au-Si becomes sufficiently large, we see an exponential increase in the current until the linear regime shown in Figure 2c. The initial exponential increase is attributed to the increase in energy of the electrons injected into gold. Finally, there is a clear difference in the current levels at which the crossover from exponential to linear occurs. These data show the electrochemical reaction rate on the gold surface dramatically shifting due to the Au-Si junction. To analyze the electrocatalytic activity and to elucidate the reaction mechanism of hot electron devices, a Tafel analysis is introduced. In conventional electrochemistry, the Tafel equation is well defined as: 𝜂 = 𝑎 + 𝑏 𝑙𝑜𝑔10 𝐽𝑠𝑜𝑙𝑢𝑡𝑖𝑜𝑛

(1)

where η is the overpotential, which is the difference between the electrode potential and the standard electrode potential, 𝑎 =

2.303𝑅𝑇 𝛼𝐹

𝑙𝑜𝑔10 (𝐽0 ) ,where R=8.314 Jmol-1K-1 is the universal gas

constant, F= 96485.3 Cmol-1 is the Faraday constant, 𝐽0 is the exchange current density, α is the phenomenological charge transfer coefficient and 𝑏 =

2.303𝑅𝑇 𝛼𝐹

is called the Tafel slope. For a single

electron transfer process, α is often found to be ~0.5 which leads the Tafel slope to be ~120 mV/dec32-34. It is noteworthy that the Tafel equation originates from the Butler-Volmer equation: (2)

αFη

𝐽𝑆𝑜𝑙𝑢𝑡𝑖𝑜𝑛 = 𝐽0 ∗ (𝑒 − RT − 𝑒

(1−α)Fη RT

)

where the second exponential term becomes negligible at large overpotential and reduces to the simplified Tafel equation32, 33. Figure 2e,f shows the Tafel slopes of different regions of the solution current density at different V Au-Si conditions. As shown in the Figure 2e, when VAu-Si = 0


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and 0.5V, which generates no or less hot electrons, the Tafel slope is ~127 mV/dec, which is close to the often observed 120 mV/dec. This Tafel slope indicates that the hydrogen evolution reaction happening at the electrode is predominantly limited the by single electron transfer step which is popularly known as the Volmer reaction step (H + + e- = Hads)32-34. With increasingly negative overpotential, Tafel slope starts increasing and the solution current density starts getting saturated. This saturation can be attributed to a number of factors: (i) as the current increases, the reaction gets limited by the mass transport to and from electrode 32-34, (ii) adsorption of reduced hydrogen atoms at the electrode32-34, and (iii) deviation from the conventional Tafel equation at higher overpotentials. The charge transfer coefficient, α=0.5 is generally not applicable at higher overpotentials when the change in the activation energy of the redox reaction with overpotential starts becoming non-linear34. Since our MIS device does not show any Tafel slope that is below 100 mV/dec, the reaction mechanism is not being limited by either the Heyrovsky step (H + + Hads + e- = H2, Tafel slope of 40 mV/dec) or the Tafel step (H ads + Hads = H2, Tafel slope of 30 mV/dec)3234

. When there is a large positive V Au-Si, as shown in the Figure 2f, we can see that there are two

different regions with different Tafel slopes. We observe a Tafel slope of i) ~175 mV/dec, and ii) ~190 mV/dec at lower and higher overpotentials respectively. While the Tafel equation works reasonably for the lower VAu-Si case, it deviates from the ideal form for hot electron cases. This deviation stems from the fact that the derivation of Butler-Volmer equation considers electron flow from the Fermi level of the electrode to the redox states 34. Since the hot electrons have considerably higher energy than the Fermi level, the conventional Tafel slopes do not manifest themselves in the higher VAu-Si cases. For high VAu-Si, we attribute the Tafel slope (~175 mV/dec) at the lower overpotential to the hot electrons being transmitted to the redox states with considerably larger transmission probability than the thermal electrons in gold. With the increasing overpotential, the


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transmission probability of the hot electrons does not increase considerably while the supply of the hot electrons remains constant which leads to a saturation of the current followed by the first exponential increase. As the overpotential increases further, the thermal electrons of gold also acquire a considerably large transmission probability and we can see the second exponential increase in current with a different Tafel slope (~190 mV/dec). While apparently it seems that the charge transfer efficiency (α~0.31) has decreased compared to the V Au-Si = 0 case (α~0.47), it is noteworthy that the magnitude of current density increased considerably. At this higher current density, the mass transport limitation, ohmic losses and adsorption will also be higher which may collectively manifest as a larger Tafel slope. To make sure that the observed currents are not resulting from any experimental artifact, we have carried out a set of control experiments. First, we have systematically modified the voltage sources, to ensure that the result was not an artifact from the potentiostat (Supplementary Section S2). The key results show that if Au-Si junction bias is carried out by an independent voltage source, the results are identical to the two working electrodes based potentiostat measurement setup (Figure S2, S3, S4). After eliminating the possibility of a measurement artifact, we studied whether the effect could be attributed to the lateral resistivity of the gold film or pinholes in the gold film using control samples (Figure S5), with details in Supplementary Section S3. First, Au film resistivity measurement (Figure S6) were carried out, with the measured resistivity used as an input to a 3-D TCAD Sentaurus simulation which allowed us to accurately simulate the expected current density and voltage drop across the gold films. The simulation results (Figure S7) show that the lateral resistance of the gold films only cause a maximum voltage drop of ~6 mV for a current density of 10 mA/cm2, which is negligible with respect to the current and voltage shifts here. Once lateral resistivity was eliminated as a potential source of the observed current shift, we


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studied the effect of pinholes in the gold films on the current. It should be noted that from atomic microscope force (AFM) measurements, the root mean square (RMS) roughness of the thin gold films are ~0.71 nm (Figure S8). To controllably test the effect of these holes, we fabricated a thick gold film (100 nm) with lithographically defined holes (Figure S5b). By then carrying out the same VAu-Si and VAu-Solution sweeps, it can be determined if pinholes in the film could explain the results. However, Figure S9 shows that even with engineered holes, there is a small change in ISolution as a function of bias between the silicon and gold, dramatically smaller than the experimental data. We have also studied metal-semiconductor (MS) junctions, discussed in Supplementary Section S4. The key results show that an MS junction with 12 nm gold and moderately doped Si (5 × 10 16 cm3)

give similar overall behavior, but with much lower current and voltage shifts (Figure S10a,d).

As the thickness of the gold is increased to 100 nm, the effect becomes negligible (Figure S10b,e). Furthermore, if a thin gold layer is used with heavily doped Si, then the Schottky barrier becomes thin and turns into a tunnel barrier, which causes electrons to be injected into the gold near the Fermi level, which also eliminates the observed behavior (Figure S10 c,f). These results also validate the idea that an MIS junction will provide hotter electrons than an equivalent MS junction. To determine if the oxide or silicon is degrading, causing current due to the dissolution of the electrode, we carried out stability tests (Supplementary Section S5). If the sample was attacked during the electrochemical measurements, we expect the diode characteristics to change dramatically. Figures S11a,b show the I-V curve of the MIS diode when it was first fabricated, before any electrochemical measurements were carried out, and after all the measurements in this paper were carried out, with minimal difference. To highlight this measurement, the before and after curves shown in Figure S11a,b were separated by about one year, highlighting the stability of the Au/Al2O3/Si devices used here under our experimental conditions. Figure S11c shows the


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current vs time curve for the MIS device, highlighting the stability. Figure S12 and S13 shows the three current components for our MIS device vs time, to highlight the fact that the observed currents are stable, and not time dependent or due to any kind of capacitive effects. These control experiments shed light on the mechanism behind the observed behavior, and eliminate measurement error, lateral resistivity, pinholes, or oxidation of the substrate itself as the possible cause of the observed current behavior. Thus, we conclude that hot electrons injected into the gold are responsible for the modulation of the reaction rate at gold/solution interface. Next, we study how each of the three measured current components (ISolution, IAu, ISi) change as a function of applied voltage. Figure 3a shows the measured current components, and the internal current components which comprise them. The source of hot electrons in the gold is the electron injection from the silicon conduction band. The electrochemical reduction current on the gold is separated into two components, reduction due to the thermal electron population, IThermal Electron, and reduction due to the hot-electron population, IHot

Electron.

Finally, there is a direct

electrochemical reduction component from the silicon to the solution, IDirect

Injection.

These

correspond to the measured components from the following relationships. (3)

ISolution = -(IThermal Electron + IHot Electron+ IDirect Injection)

(4)

IAu = IThermal Electron + IHot Electron – IAu-Si

(5)

ISi = IAu-Si + IDirect Injection From these relationships, we can see that when V Au-Si = 0 V, IAu-Si = 0 A, this becomes the

traditional 3-electrode measurement where the gold is the working electrode, and ISolution = -IAu. Figure 3b shows the three measured current components during a V Au-Solution voltage sweep for VAu-Si = 0 V. The observed behavior is as expected, with the IAu = -ISolution. Figure 3c,d show the currents for V Au-Si = 0.5 V and V Au-Si = 1.5 V, respectively. Surprisingly, ISi increases with the


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voltage applied between the solution and gold. Since the voltage between the gold and silicon is fixed, and it was previously shown that the silicon does not directly inject current into the solution (Figure S9), the current injected from the silicon into the gold should be constant with respect to VAu-Si. However, as seen in Figure 3c,d, an increase in the measured current, ISi exists for both applied voltages. This increase in ISi can be explained by four mechanisms, (i) a change in the gold electrode voltage due to the lateral currents, (ii) holes in the gold which allow direct reduction of hydrogen due to the potential of the silicon with respect to the solution, (iii) oxidation of the silicon/Al2O3 substrate, or (iv) hydrogen reduction by injected elect a gold/hydrogen complex which directly accepts electrons from the silicon, driving reduction without the need for a multistep electron transfer to the ‘bulk’ gold and then to the solution. Our control experiments analyzing the lateral gold potential drop (Figure S7), with lithographically defined holes (Figure S9), and stability measurements (Figure S11) eliminate mechanisms (i), (ii), and (iii). From this, we conclude that the increase in ISi is due to direct injection of electrons into a species that is complexed with the gold. This is observable due to the independent voltage control and current measurement of the gold and silicon terminals.


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Figure 3. Current flow mechanism and measurement result. (a) Schematic diagram of main current components. Major currents are composed with several minor current components. (b)-(d) Three major current measurements of the closed system, (i.e. ISi, IAu, and ISolution) under different Au-Si voltages.

We note that since Fig. 3b-d are steady state measurements, for all cases, ISolution + IAu + ISi = 0.


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Most importantly, from these graphs, we can quantify the three components of the electrochemical reduction current, which are thermal electrons from the gold, hot electrons from the gold, and direct injection from the silicon. Using Figure 3b, we can identify the contribution of the thermal electrons in gold as a function of applied bias. At any given V Au-Solution, the thermal contribution from the gold should stay constant. Thus, we can define the net change in solution current as: (6)

ISolution = ISolution - ISolution(VAu-Si=0) = ISi + IHot Electron,

where ISolution(VAu-Si=0) refers to the current composed only with Au thermal electrons at nonbiased condition, ISi represents the change in ISi due to direct injection of electrons from the silicon to the solution, and IHot Electron is composed with hot electrons in the Au region. So, the net change of solution current shown in the equation (6) can be explained by analyzing the individual components which are presented in the equation (3) – (5).


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Figure 4. Hot electron measurement and characterization. (a) Current component ratio map along the increase of Au-Si voltage. Portion of the hot electrons in total current keep increase as Au-Si voltage increase. (b) Current density from direct injected electrons from Si to electrolyte at different fixed V AuSolution.

(c) Quantum efficiency of hot electron device at fixed 2.0V Au-Si voltage and (d) at fixed -

0.8V Au-Solution voltage. From these curves, we can then extract the thermal, hot electron, and direct injection components as a function of applied bias. Figure 4a shows the solution current density J Solution vs VAu-Solution sweeps for five diode bias potentials (V Au-Si=0, 0.5, 1, 1.5, 2 V), with the total solution current density separated into thermal electron, hot electron, and direct injection current density components. When VAu-Si=0 V, all the measured current is due to the thermal electrons in the gold. However, as the applied bias across the diode increases, both the direct injection, and hot electron


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components increase. However, while the hot electron current increases monotonically, the direct

Figure 5. Hot electron simulations. (a) electron-electron and electron phonon scattering rates in gold plotted as a function of energy above Fermi level. (b) Energy loss rate per fs, obtained by multiplying scattering rate at a given energy by average energy loss per scattering event. (c) Log scale attempt rate of electrons tunneling into gold/solution interface plotted as a function of energy with and without hot electron injection. (d) Linear scale attempt rate plot.


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injection current appears to have a peak. Figure 4b plots the direct injection current density as a function of Au-Si bias. At higher V Au-Solution, a clear peak is observed. We attribute this behavior to the direct injection of carriers from the silicon into hydrogen ions on the surface of the gold, with a well-defined energy state. Figure 4c shows the quantum efficiency of the hot electron induced hydrogen reduction, defined as JHot Electron /JSi, where JHot Electron is the current density of hot electrons as a function of VAuSolution,

while JSi is the diode current density. Critically, it is seen that the hot electron efficiency

increases to ~85% before saturating at high solution potentials. Harvesting the hot electrons in the Platinum based MS device35 and transferring plasmon induced hot electrons from Au nanoparticles36 have shown ~5.6%-8.5% and ~40% efficiency, respectively. This work presents the highest quantum efficiency reported to date for a hot-electron mediated electrochemical process. Furthermore, we also show the current density efficiency as a function of V Au-Si, and demonstrate that at high diode biases the current efficiency is >50%. Figure S14 shows the overall quantum efficiency in different biased conditions characterized in Fig 2. These efficiencies suggest there is a clear path towards using MIS structures as efficient sources for hot electron devices. To gain further insight into the hot electron dynamics in gold, we have carried out detailed simulations using a modified 2D Monte Carlo simulation package for semiconductor transport, Archimedes37. We have modified the simulator by implementing the gold density of states, electron-electron (e-e), and electron-phonon (e-p) scattering. We describe the details of the implementation in Supplementary section S11. Figure 5a shows the scattering rates for the e-e, ep, and total scattering as a function of energy above the Fermi level in gold. To verify the scattering rates used here, we show that the electron-electron scattering rates used here match those published in literature38 (Supplementary Figure S19). Furthermore, we show that the simulator used here


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can accurately reproduce experimental electron temperature vs time profiles as shown in Figure S20. While the e-p scattering rate is high compared to the e-e scattering rate, due to the relatively small energy of acoustic phonons, at high energy, the energy loss per fs for hot electrons is dominated by e-e scattering events (Figure 5b), due to the relatively large average energy loss per scattering event for high energy electrons interacting with electrons near the Fermi surface. Next, we carry out a simulation where we inject electrons at varying energies above the gold Fermi level and track the decay. From these results, the rate of attempts at the gold surface due to hot electrons is obtained as a function of energy. Using the attempt rate enables us to account for the interactions of the electrons in the gold with the surface. Essentially, this approach is analogous to the attempt frequency approach used when calculating tunneling rates out of quantum wells, where the tunneling rate is proportional to the rate at which electrons reflect off the quantum well barriers. Here we use the attempt rate as an analog for the attempt rate for tunneling in quantum wells. This approach simultaneously allows us to capture the rate of interactions between the hot electrons and the gold/solution interface and normalize this rate accurately with the interaction rate between the thermal electrons and the gold/solution interface. The rate of attempts at the gold surface due to the thermal electrons in gold is also obtained from the simulation. By then normalizing these two rates of attempts as described in supplementary section S11, we plot the attempt rate for gold with no hot electron injection, and gold with 26 mA/cm2 of hot electrons injected 2 eV above the Fermi level. This is the current density of our MIS device at V Au-Si = 2V. While many injection energies were simulated (Figure S20a), we show the 2 eV injection result as it closely approximates the expected offset given by considering the initial band offset between our n-doped Si and a gold electrode with a workfunction of 5.1 eV, which is ~1eV, and then adding in the expected increase


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in energy due to the applied voltage. While the applied voltage is 2V, the initial ~0.5 V can be assumed to drop over the silicon depletion region, and the final ~0.5 V are expected to be limited by series resistances (Figure S15), giving us an approximate injection energy of 2 V. Figure 5c shows the results with the energy with respect to the gold Fermi-Level on the y-axis and attempt rate on the x-axis in log scale, and Figure 5d shows a zoomed in view in linear scale. We immediately see that for high energies, there is significant increase in the hot-electron attempt rate. Note that these results are in units of 1/cm2-s-eV, thus we can see that the injection of hotelectrons at a rate of 26 mA/cm2 creates an attempt rate on the order of ~1018 /cm2-s-eV. While these may seem high, they can be understood by considering the velocity of hot electrons in gold to be vHE~108 cm/s, leading to a transit time across 10 nm thick gold of Au ~10-14 s. Considering an energy loss rate of PHE~10meV/fs, an injected particle would lose 1 eV in HE~100 fs, leading to nrefl ~10 attempts/injected particle. A current level of 10 mA/cm2 leads to a hot electron flux of FHE~1017 /cm2-s. Taking FHE× nrefl we get an interaction rate of ~1018 /cm2-s-eV. This is a significant interaction rate which allows us to explain why these devices exhibit such high efficiencies with respect to hot electron driven electrochemistry—despite the high scattering rates, there is still a significant number of interactions between the hot electron and the gold/solution interface. Finally, we also see a peak in the attempt rate (Fig. 5c) at the hot electron injection energy. This occurs when electrons have been injected into the gold, but have not yet undergone e-e scattering, as e-e scattering for electrons at 2 eV above the Fermi level would cause the loss of an average of ~700 meV per scattering event, which is obtained by dividing the values in Figure 5a and 5b. Before e-e scattering occurs, the injected electrons still have significant energy, and if they are backscattered due to acoustic phonons, will have a non-zero chance of being injected back to the


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Si. However, if the injected electrons are immediately transferred to the solution before e-e scattering no current will flow in the gold due to this injected electron, and, simultaneously, the net flux across the Si/Au interface will increase due to suppression of backscattering from the Au into the Si. For our observed devices, we attribute the observed ISi, shown in Figure 3c,d to this mechanism, and call it ‘direct injection current’ here. Furthermore, the peaked shape shown in Figure 4b indicated the silicon direct injection current is related to a sharper feature in the electron distribution, and not the hot electron tail which we see in Figure 5d. In conclusion, we demonstrate that a MIS tunnel diode can act as a source of hot electrons for efficiently driving electrochemical reductions, with efficiencies reaching ~85% for high biases. This approach is general, and not limited to the Si/Al2O3/Au device with hydrogen reduction shown here. Future experiments could explore hot holes for high-energy reduction reactions, other redox reactions for carbon-to-fuel reactions, and other materials such as graphene and other 2-D materials that transport electrons more efficiently to see if high efficiencies can be achieved at lower voltages.

METHODS Sample Preparation. Moderately phosphorous doped (Nd = 5×1016 cm-3) (100) and heavily phosphorous doped (Nd = 1×1019 cm-3) (100) silicon wafer (MTI Corporation) were used as the substrate. Native SiO2 was removed with 1:10 ratio of HF:H2O (Sigma Aldrich, 49% CMOS grade) etching for 1 minute. After oxide etching, 1nm of titanium and 100nm of silver back contact metals were evaporated in an electron beam evaporator (Temescal, SL1800). To prevent front side damage, a blank Si handle wafer was used after an acetone, IPA, D.I water rinse. The metal insulator semiconductor (MIS) structure was fabricated by depositing an aluminum oxide insulator


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layer with Atomic Layer Deposition (Ultratech/Cambridge Savannah ALD) using Trimethyl aluminum (Aldrich, 1001278062) and water (Aldrich, W4502) precursors. Au films (10-100nm thick) were evaporated under two different conditions. Room temperature Au film was deposited with Electron beam evaporator (Temescal). Cryo (90K) temperature Au films were evaporated with thermal evaporator (Denton Vacuum Inc, DV-502A). For the device reported in the main text, cryo evaporated Au films were used. Image reversal photolithography was done for control device with holes. (Karl Suss, 100UV030). For contact wire attachment, copper wire wrapped with aluminum foil at the one end was used. Two wires are connected to front and back side of the devices each with fast drying silver paint (Ted Pella Inc, 16040-30). A ring contact was drawn in the front side of device in Au film region. For device encapsulation, a glass slide (VMR Micro slides) was used as a back holder. Fabricated devices with contacts were placed on the glass with epoxy (Gorilla Epoxy clear) to encapsulate the device while leaving the Au electrode surface exposed. Electrical Measurements. All the electrochemical I-V measurements were done by using a potentiostat (Admiral Instrument, Squidstat Prime). Two different channels were used to control separate voltages applied to the system. Each channel has its own working, reference, and counter electrode. The first channel was connected to the Au as the working electrode, a platinum wire as the counter electrode, and an Ag/AgCl reference electrode; the second channel consists of Si emitter as the working electrode, a platinum wire as the counter electrode, and Au as reference electrode to bias the Au-Si junction. 0.5M H2SO4 was used as the electrolyte solution. Schottky, Ohmic and four-probe I-V measurement of devices were characterized by Semiconductor Parameter Analyzer (Keysight B1500a). The roughness of the both Au film evaporated at 300 K and 90K substrate temperature we characterized using an Atomic Force Microscope.


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ASSOCIATED CONTENT Supporting Information. Additional figures and explanation of simulations. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. Author Contributions H.U.C., Q.L., and R.K. designed the experiments. H.U.C., and Q.L., carried out the sample fabrication and measurements. R.A., D.S., F.R., and R.K carried out the simulations. H.U.C., R.A., Q.L., D.S., S.C., and R.K. contributed to analyzing the data. H.U.C., R.A., and R.K. wrote the paper while all authors provided feedback. ‡ H.U.C and R.A. contributed equally to this work.

Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work was supported by an ACS-PRF grant #55993-ND5 (R.K.), AFOSR Grant No. FA9550-16-1-0306 (R.K.), National Science Foundation Award No. 1610604 (R.K), Army Research Office ARO Award No. W911NF-17-1-0325 (S.C.), and the Molecular Foundry at Lawrence Berkeley National Laboratory, a user facility supported by the Office of Science, Office


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of Basic Energy Sciences of the U.S. Department of Energy (DOE) under Contract No. DEAC0205CH11231. R.A acknowledges a USC Provost Graduate Fellowship. REFERENCES 1. Yan, J.; Kim, M. H.; Elle, J. A.; Sushkov, A. B.; Jenkins, G. S.; Milchberg, H. M.; Fuhrer, M. S.; Drew, H. Nature nanotechnology 2012, 7, 472. 2. Vicarelli, L.; Vitiello, M.; Coquillat, D.; Lombardo, A.; Ferrari, A.; Knap, W.; Polini, M.; Pellegrini, V.; Tredicucci, A. Nature materials 2012, 11, 865. 3. Yajima, T.; Hikita, Y.; Hwang, H. Y. Nature Materials 2011, 10, 198. 4. Brongersma, M. L.; Halas, N. J.; Nordlander, P. Nature Nanotechnology 2015, 10, 25. 5. Koppens, F.; Mueller, T.; Avouris, P.; Ferrari, A.; Vitiello, M.; Polini, M. Nature nanotechnology 2014, 9, 780. 6. Würfel, P. Solar Energy Materials and Solar Cells 1997, 46, 43-52. 7. Reineck, P.; Lee, G. P.; Brick, D.; Karg, M.; Mulvaney, P.; Bach, U. Advanced Materials 2012, 24, 4750-4755. 8. Wu, K.; Chen, J.; McBride, J. R.; Lian, T. Science 2015, 349, 632-635. 9. Mukherjee, S.; Libisch, F.; Large, N.; Neumann, O.; Brown, L. V.; Cheng, J.; Lassiter, J. B.; Carter, E. A.; Nordlander, P.; Halas, N. J. Nano letters 2012, 13, 240-247. 10. Brongersma, M. L.; Halas, N. J.; Nordlander, P. Nature nanotechnology 2015, 10, (1), 25. 11. Clavero, C. Nature Photonics 2014, 8, 95-103. 12. Sobhani, A.; Knight, M. W.; Wang, Y.; Zheng, B.; King, N. S.; Brown, L. V.; Fang, Z.; Nordlander, P.; Halas, N. J. Nature communications 2013, 4, 1643. 13. Kale, M. J.; Avanesian, T.; Christopher, P. Acs Catalysis 2013, 4, 116-128. 14. Mohammadparast, F.; Dadgar, A. P.; Addanki Tirumala, R. T.; Mohammad, S. A.; Topal, C. O.; Kalkan, A. K.; Andiappan, M. The Journal of Physical Chemistry C 2019,123, 11539-11545. 15. Plummer, E.; Eberhardt, W. Advances in Chemical Physics 1982, 533-656. 16. Knight, M. W.; Sobhani, H.; Nordlander, P.; Halas, N. J. Science 2011, 332, 702-704. 17. Tomiye, H.; Terano, T.; Nomoto, K.; Kobayashi, T. In A novel 2-bit/cell MONOS memory device with a wrapped-control-gate structure that applies source-side hot-electron injection, VLSI Technology, 2002. Digest of Technical Papers. 2002 Symposium on, 2002; IEEE: pp 206-207. 18. Ohnakado, T.; Mitsunaga, K.; Nunoshita, M.; Onoda, H.; Sakakibara, K.; Tsuji, N.; Ajika, N.; Hatanaka, M.; Miyoshi, H. In Novel electron injection method using band-to-band tunneling induced hot electrons (BBHE) for flash memory with a P-channel cell, Electron Devices Meeting, 1995. IEDM'95., International, 1995; IEEE: pp 279-282. 19. Linic, S.; Christopher, P.; Ingram, D. B. Nature materials 2011, 10, 911. 20. Sun, C.-K.; Vallée, F.; Acioli, L.; Ippen, E.; Fujimoto, J. Physical Review B 1994, 50, 15337. 21. Guo, C.; Rodriguez, G.; Taylor, A. J. Physical review letters 2001, 86, 1638. 22. Brandt, N. C.; Keller, E. L.; Frontiera, R. R. The journal of physical chemistry letters 2016, 7, 3179-3185. 23. Zibik, E.; Grange, T.; Carpenter, B.; Porter, N.; Ferreira, R.; Bastard, G.; Stehr, D.; Winnerl, S.; Helm, M.; Liu, H. Nature materials 2009, 8, 803. 24. Vaziri, S.; Lupina, G.; Henkel, C.; Smith, A. D.; Östling, M.; Dabrowski, J.; Lippert, G.; Mehr, W.; Lemme, M. C. Nano letters 2013, 13, 1435-1439.


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