Atomic-Scale Observation of Monoclinic Nanodomain in VO₂ with Ultra-Fast and Energy Efficient Metal-Insulator Transition

Nanodomains have been rigorously studied as an emerging source for improving switching performance of multi-phase electronic devices. For example, the embedment of ferroelectric nanodomains in non-ferroelectric matrix shows enhanced energy density and efficiency during switching, compared to the sole ferroelectric system, despite the low volume fraction of ferroelectrics [1].<br/><br/>Vanadium dioxide (VO₂), a typical correlated oxide, is a potential material for ultra-fast electronic switching device [2]. It exhibits abrupt insulator-metal transition (IMT) with monoclinic-rutile structural phase transition (SPT), which can be triggered by thermal, electrical, or optical excitation. A promising route to design high-performance IMT switching has been to introduce cation doping on VO₂. However, most studies on doped VO₂ have been conducted using microscopic analyses, which lack the spatial resolution to identify the few unit-cell scale local structural phase changes [3].<br/><br/>In this study, using aberration-corrected scanning transmission electron microscopy (STEM), we discovered few unit-cell scale monoclinic nanodomains in titanium (Ti)-doped VO₂ thin film below transition temperature, resulting in ultra-fast switching and low energy consumption. To distinguish the subtle difference between rutile and monoclinic in unit-cell scale, we adopted automated peak analysis technique on STEM images: (i) peak position extraction and (ii) calculation of relations between peak positions. Even though a STEM image contains about a thousand of atomic columns, we could obtain a distribution map of two structural phases within a few minutes. We believe that this fast and direct characterization of phase coexistence with nanodomain is essential to understanding and engineering more efficient and ultra-fast IMT switching devices.<br/><br/>[1] Pan et al., <i>Science</i> <b>365</b>, 578-582 (2019)<br/>[2] Z. Yang et al., <i>Annu. Rev. Mater. Res.</i> <b>41</b>, 337-367 (2011)<br/>[3] E. Strelcov et al., <i>Nano Lett.</i> <b>12</b>, 6198-6205 (2012)