Photoexcited Hole States at the SrTiO3(001) Surface Imaged with Noncontact AFM

TL;DR

This study demonstrates that SrTiO3(001) surfaces can accumulate and retain photoexcited charges for days at cryogenic temperatures, with atomic-scale imaging revealing localized hole states and trapped charges.

Contribution

It introduces a combined STM/AFM and KPFM approach to image and localize photoexcited charges at the atomic level on SrTiO3 surfaces.

Findings

Photoexcited charges persist for days at cryogenic temperatures.

Atomic-scale imaging localizes holes at oxygen 2p orbitals near Sr vacancies.

DFT calculations confirm long-lived trapped hole states.

Abstract

The behaviour of excess charges in ionic lattices, such as the formation of polarons and charge trapping at defect sites, influences the physical and chemical properties of materials and translates into applications in electronics, optics, photovoltaics, and catalysis. Here we show that the bulk-terminated SrTiO3(001) surface accumulates photoexcited charges and keeps the associated photovoltage for many days at cryogenic temperatures. A combination of scanning tunneling microscopy, atomic force microscopy (STM/AFM) and Kelvin probe force microscopy (KPFM) was used to measure this photovoltage and to localize the photoexcited charges with atomic precision down to the single-quasiparticle limit. Density functional theory (DFT) shows that holes favor localization at oxygen 2p orbitals adjacent to Sr vacancies, creating long-lived trapped states. The methodology presented here provides guidelines for imaging of charges trapped in the crystal lattice using noncontact AFM.