Photochemical Anisotropy and Direction-dependent Optical Absorption in Semiconductors

TL;DR

This study uses hybrid DFT to map optical transition probabilities in semiconductors, revealing how anisotropic photochemical reactions depend on surface orientation and transition dipole moments, aiding the design of better photocatalysts.

Contribution

It introduces a spherical heat map approach to identify reactive surfaces in semiconductors, emphasizing the importance of transition dipole moments over traditional band-structure analysis.

Findings

Heat maps predict dominant photo-generated carrier directions.

Conventional band-structure plots are insufficient alone.

Transition dipole moments are crucial for accurate predictions.

Abstract

Photochemical reactions on semiconductors are anisotropic, since they occur with different rates on surfaces of different orientation. Understanding the origin of this anisotropy is crucial to engineering more efficient photocatalysts. In this work, we use hybrid density functional theory (DFT) to identify the surfaces associated with the largest number of photo-generated carriers in different semiconductors. For each material we create a spherical heat map of the probability of optical transitions at different wave vectors. These maps allow to identify the directions associated with the majority of the photo-generated carriers and can thus be used to make predictions about the most reactive surfaces for photochemical applications. Results indicate that it is generally possible to correlate the heat maps with the anisotropy of the bands observed in conventional band-structure plots, as previously suggested. However, we also demonstrate that conventional bands-structure plots do not always provide all the informations and that taking into account the contribution of all possible transitions weighted by their transition dipole moments is crucial to obtain a complete picture.