By Josh Perry, Editor
[email protected]

Chemists from the Institute of Physical Chemistry of the Polish Academy of Sciences (IPC PAS) and the Warsaw University of Technology (WUT) discovered that in the thermal decomposition of zinc alkoxide compounds, which are precursors to semiconducting zinc oxide, there were previously unreported intermediate clusters with gapless electronic states.

The exotic transformations causes that one of the precursors of zinc oxide, initially an insulator, at approx. 300°C goes to a state with electrical properties typical of metals, and at ~400°C it becomes a semiconductor. (IPC PAS)

According to a report from IPC PAS, the compound begins as an insulator but changes to a conductor-like material when heated rapidly to around 300°C. Further heating to around 400°C coverts it into a semiconductor.

Zinc oxide is a semiconductor with a wide range of applications in the electronics industry, but there remain challenges in creating well-defined nanomaterials and the single precursor method has been a process, where heating the material causes the organic parts to degrade and the inorganic parts to self-assemble into the desired material.

“The tested precursor had the properties of an insulator, with an energy gap of about five electronvolts,” the article said. “When heated, it eventually transformed into a semiconductor with an energy gap of approximately 3 eV.”

The research was recently published in Materials Horizon. The abstract read:

“Zinc oxide (ZnO) is one of the most versatile semiconductor materials with multifarious potential applications. Easily accessible alkylzinc alkoxides have been widely exploited as single-source precursors of ZnO-based nanomaterials but their multi-step decomposition pathways have not been understood in detail.

“Herein, the formation mechanism of ZnO nanocrystals via solid-state thermal decomposition of a model pre-organised alkylzinc alkoxide precursor, i.e. [tBuZn(μ₃-OtBu)]₄, is elucidated using in situ valence-to-core X-ray emission (v2c-XES) and high energy resolution off-resonant spectroscopy (HEROS) in conjunction with theoretical calculations.

“Combination of in situ spectroscopic measurements and theoretical simulations indicates that the precursor structural evolution is initiated by the homolytic cleavage of the R–Zn bond, which leads to the formation of a transient radical ([?Zn(μ₃-OR)][RZn(μ₃-OR)]₃) species, which is responsible for the initial decomposition process. The ensuing multistep transformations involve the formation of intermediate radical zinc oxo-alkoxide clusters with gapless electronic states.

“Hitherto, the formation of clusters of this type has not been considered either as intermediate structures en route to a semiconductor ZnO phase or as potential species accounting for various defect states of ZnO NCs, particularly the singly charged oxygen vacancy, V_o⁺.”