Band alignment at semiconductor-water interfaces using explicit and implicit descriptions for liquid water

TL;DR

This study compares implicit and explicit water models in simulating semiconductor-water interfaces, revealing that explicit first solvation shells are crucial for accurate band alignment and providing insights into potential offsets.

Contribution

It demonstrates that explicit first water layers are essential for consistent band alignment results and clarifies the origin of potential offsets between models.

Findings

Explicit water layers yield consistent band alignments across materials.

Absolute alignments agree within 0.1-0.2 V when accounting for potential offsets.

Absence of water surface dipole explains the potential offset in implicit models.

Abstract

In this work we study and contrast implicit solvation models against explicit atomistic, quantum mechanical models in the description of the band alignment of semiconductors in aqueous environment, using simulations based on density functional theory. We find consistent results for both methods for 9 different terminations across 6 different materials whenever the first solvation shell is treated explicitly, quantum mechanically. Interestingly this first layer of explicit water is more relevant when water is adsorbed but not dissociated, hinting at the importance of saturating the surface with quantum mechanical bonds. Furthermore, we provide absolute alignments by determining the position of the averaged electrostatic reference potential in the bulk region of explicit and implicit water with respect to vacuum. It is found that the absolute level alignments in explicit and implicit simulations agree within $\sim 0.1-0.2$ V if the implicit potential is assumed to lie 0.33 V below the vacuum reference level. By studying the interface between implicit and explicit water we are able to trace back the origin of this offset to the absence of a water surface dipole in the implicit model, as well as a small additional inherent polarization across the implicit-explicit interface.