Kinetic photovoltage along semiconductor-water interfaces

External photo-stimuli on heterojunctions commonly induce an electric potential gradient across the interface therein, such as photovoltaic effect, giving rise to various present-day technical devices. In contrast, in-plane potential gradient along the interface has been rarely observed. Here we show that scanning a light beam can induce a persistent in-plane photoelectric voltage along, instead of across, silicon-water interfaces. It is attributed to the following movement of a charge packet in the vicinity of the silicon surface, whose formation is driven by the light-induced potential change across the capacitive interface and a high permittivity of water with large polarity. Other polar liquids and hydrogel on silicon also allow the generation of the in-plane photovoltage, which is, however, negligible for nonpolar liquids. Based on the finding, a portable silicon-hydrogel array has been constructed for detecting the shadow path of a moving Cubaris. Our study opens a window for silicon-based photoelectronics through introducing semiconductor-water interfaces.


Supplementary Note 1: Light sources used for illumination.
Except for simulated solar light, narrow-spectrum light (FWHM ≤ 20 nm) was another light beam source that has been used in our work in Supplementary Fig. 3,5,6,8,10 and 12. It was provided by LEDs for wavelength below 1000 nm and by a xenon lamp with a monochromator for wavelength above 1000 nm. An optical cable was mounted on the step motor to control its movement and guided the narrow-spectrum light onto the silicon surface with a circular spot of ~1 cm diameter.

Supplementary Note 2: Characterization of the Si/SiOx/water interface.
KPFM was performed on the SmartSPM (AIST-NT Inc) integrated with optical spectroscopies. Au coated probe (Multi75GB-G, Budgetsensors) with a spring constant of about 3 N/m was used for surface potential measurements. The topography was scanned and then the tip was lifted 40 nm above the surface to map the surface potential in a tapping mode. A laser spot of 532 nm irradiated on the tip-scanning region and was switched on and off during the mapping.
To study the Si/SiOx/water interface behavior, electrochemical impedance spectroscopy (EIS) and cyclic voltammetry (CV) measurements were performed on a Biologic VMP-300 electrochemical workstation. Three-electrode configuration was employed, consisting of silicon sample (~1 cm 2 ) as working electrode, platinum as counter electrode and Ag/AgCl as reference electrode. The oscillating amplitude for EIS measurements was fixed at 20 mV, and the scanning frequency ranges from 7 MHz to 0.1Hz. CV experiments were performed by scanning a range from −1.0 to 1.0 V with a scan rate of 5 mV/s.

Supplementary Note 3: Numerical simulation of light induced signal response.
The simulation is based on the circuit model as shown in in Supplementary Fig. 9. Upon light illumination, the circuit responses with time according to the Kirchhoff laws as where W is the width of the silicon strip. The above equations need to be simplified for numerical calculation. Let = − and = + , there is = The boundary condition is = 0, when x=xL or x=xR. Correspondingly, there is no horizontal electric current across both boundaries. We use the finite difference method to solve Eq. (4) numerically and adopt an implicit form as , where ∆ and are step sizes for x and t. The simulation was implemented with Matlab.
(2) yields = − + ( ) + ℎ( ) . Thus, = − = + − ( ) − ℎ( ). We get the potential distribution of silicon as a function of f, The in-plane voltage between two terminals of silicon can be calculated via the solved f The simulation was conducted with ∆≤ 0.0002 m and ≤ 0.001 . To fit the experimental results in Fig. 3a, ρs, W and s are set to 1000 Ω·cm -1 , 1 cm and 1 cm, respectively. In addition, c and V0 take 30 μF·cm -2 and 0.2 V according to previous report 1,2 .
Parameters are slightly adjusted for the numerical simulation in Fig. 2, where ρs, c and V0 are 500 Ω·cm -1 , 20 μF·cm -2 and 0.08V, respectively. The circuit model omitted bulk water capacitance and nonlinear effects rising from ion desorption, adsorption and diffusion, which may specially influence its accuracy of high frequency response.  (c) show the equivalent circuit of the impedance spectrum corresponding to the blue fitted curve. R0 is the internal resistance, including the resistance from both Si sample and its electrical contact. R1 and C1 (small capacitance) are attributed to the resistance and geometric capacitance of bulk water 3 . R2 and C2 are contributed to the chemical reaction between Si and water. W and Ci represent Warburg impedance and interface double layer capacitor (large capacitance), respectively.