Highly localized charge transfer excitons in metal oxide semiconductors

Significance 

The chemical properties of the transition metal oxides, especially those represented in the first row of the periodic table have distinct chemical and physical properties that enable them to be widely used as photocatalysts. For instance, they are electrochemically stable, have the desired surface kinetics and are readily available in abundance. With the continuing discoveries of more applications of such metal oxides, there is a need to enhance their performance through a comprehensive understanding of the catalytic energy conversion and the electronic structure of these materials.

As a result, different scholars have conducted studies to understand the charge localization in the valence band of the metal oxide semiconductors. This also includes the factors that affect the charge carrier localization such as crystal structure or dopants..

Ohio State University researchers led by Dr. Robert Baker developed femtosecond XUV Reflection-Absorption spectroscopy for investigating charge transfer dynamics, especially in metal oxide semiconductors with element and oxidation state specificity, ultrafast time resolution, and surface sensitivity. They also examined the dynamics of the photoexcited electrons and holes, and the relationship between the electron-hole pairs. Their research is currently published in the journal Nano Letters.

The research team commenced their experiments by first making an observation of the excited state signatures of holes and electrons in the metal oxidesby probing the O L1 and metal M2,3– absorption edges, respectively. These oxides included Co3O4, Fe2O3, and NiO. They were also able to differentiate electronic excitations from thermal effects by measuring excited state dynamics of the metal oxides within the first 3 ps.

From the experimental analysis, the research team observed that in all the materials, charge localization to the metal 3d conduction band and O 2p valence band occurred within the 120 fs instrument response.. The relative position of the O L1 edge transition changes with each metal oxide which enables   an in-depth understanding and comparison of the metal-oxygen bond covalency. Understanding the metal-oxygen bond covalency provides new insights into the rational design of efficient energy conversion materials.

As a significant contribution of their study, the authors for the first time successfully observed that it was possible to directly probe a  photogenerated hole in the valence band of the O 2p states. Additionally, by performing pump fluence dependent experiments the authors were able to determine that the photogenerated excitons were localized across a single metal-oxygen bond length.

By directly measuring the excited state dynamics of the photoexcited electrons and holes independently, the authors provide significant understanding of the metal-oxygen bond covalency as well as the highly localized nature of the excitons in these materials. Their study will help advance our understanding of the various systems that support photochemical energy conversion processes and reactions.

“It’s really exciting to be able to follow excited state dynamics in these relevant materials not only in real time but also with element specificity and surface sensitivity. Providing detailed understanding of the photoinduced chemical dynamics provides important design parameters for next generation high efficiency catalysts.” said Jakub Husek, a co-author on the manuscript

Reference

Biswas, S., Husek, J., Londo, S., & Baker, L. (2018). Highly Localized Charge Transfer Excitons in Metal Oxide Semiconductors. Nano Letters, 18(2), 1228-1233.

 

Go To Nano Letters

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