Observation and control of potential-dependent surface state formation at a semiconductor-electrolyte interface via the optical anisotropy

TL;DR

The paper addresses how potential-dependent surface states form at semiconductor–electrolyte interfaces and how to observe them operando with high temporal resolution. It introduces electrochemical reflection anisotropy spectroscopy (EC-RAS) applied to InP(100) in aqueous electrolyte, employing the linear electro-optic effect $ abla\epsilon_{ij} = Z_{ijk}F_k$ so that the interfacial electric field modulates the RA signal via $\ abla r_F/r = \sin(2\theta)\left[g_0(E)|F(V_{bb})| + g_1(E)|F(V_{appl})|\right]$, capturing both band-bending and Helmholtz-layer contributions. The results show that the formation of in-gap surface states pins the Fermi level and shifts the potential drop from the semiconductor into the Helmholtz layer, i.e. changes in $e\Delta\Psi_H$ while $V_{bb}$ is fixed, with RA transients revealing fast and slow processes and distinct surface-reconstruction–dependent regimes (e.g., $(2\times2)$-6H vs $(2\times2)$-mixed-4H) corroborated by EIS. The study proposes an electrochemical variant of the LE-OE effect to quantify these changes and demonstrates potential-switchable activation/passivation of surface states, offering a new operando route to understand and control semiconductor–electrolyte interfaces.

Abstract

The interface between semiconductors and ion-conducting electrolytes is characterised by charge distributions and potential drops that vary substantially with the evolution of surface states. These surface states at the very interface to the liquid can form or be passivated, depending on the applied potential between electrode and electrolyte, and hereby fundamentally impact properties such as charge transfer. Characterisation and understanding of such potential-dependent surface states with high spatial and temporal resolution is a significant challenge for the understanding and control of semiconductor-electrolyte interfaces. Here, we show that the optical anisotropy of InP(100) can be used to detect the potential-dependent formation of highly ordered surface states under operating conditions. Upon formation of a surface state in the bandgap of the semiconductor, the potential drop and hence the electric field is shifted away from the semiconductor to the Helmholtz-layer of the electrolyte. This modifies the instantaneous response of the optical anisotropy to disturbances of the applied potential. We propose an electrochemical variant of the linear electro-optical effect and our findings open a novel route for understanding these interfaces. The results show how surface states from surface reconstructions at this reactive interface can be switched on or off with the applied potential.

Figures (3)

• Figure 1: Band diagrams of a p-type semiconductor in contact with an electrolyte. (a) Semiconductor in contact with an electrolyte and no surface states. (b) Same situation as (a), but with a high density of surface states (vertical box, striped for occupied), shifting the potential drop from the solid to the Helmholtz-layer, $e\Delta\Psi_H$. A variation of the applied potential leads to a system response (blue arrows) in (a) by a change in band-bending and (b) a change in the Helmholtz-layer potential drop.
• Figure 2: Optical response of the system as a function of the applied potential. (a) Reflection anisotropy spectra in the electrolyte during and after chronoamperometry (CA) at -0.9 V vs. the Ag/AgCl reference electrode (RE) as well as before and after conditioning by potential cycling. The vertical line marks the photon energy selected for the transients. (b) Response of the system in the potential range without surface states. Upper panel: Current density (red) and RA-transient (blue) at 2.25 eV photon energy following the applied potential steps (lower panel) of 50 mV/5 min between OCP and -0.95 V vs. RE. (c) Response of the system in the extended potential range.
• Figure 3: Evolution of surface states and optical anisotropy in the different potential regimes. (a) Nyquist plots from the three electrochemical impedance measurements (EIS) conducted in one experiment sequence in which the following potential steps of 18 min each: first -0.75 V, second -1.25 V, and finally back to -0.75 V. The changed half-cycles show the increased density of surface states from a semiconducting behaviour (1) to metal-like properties (2), and -- in a partially reversible manner -- back to semiconducting characteristics (3). (b) The RA transient at 2.25 eV (blue) is plotted together with the applied potential (black). The aforementioned potential regimes are highlighted in orange: (I) the RA is decreasing, (II) it is constant, (III) rising with fast response, (IV) the RA is increasing without steps due to the constant band bending, (V) the RAS is decreasing without steps, (VI) the RA is decreasing with subdued steps, and (VII) finally without steps.