Image courtesy of Markus Raschke, University of Colorado
Ultrafast pump-probe microscopy on individual vanadium dioxide microcrystals measures the spatial and temporal variability of ultrafast dynamics of the insulator-to-metal transition. Surprisingly, nominally identical crystals show dramatically different transition times. Additional results from nano-imaging (lower right) reveal a complex structure suggesting high sensitivity to heterogeneous nanoscale features.
More energy-efficient computers, cell phones, and other electronic devices often begin with new materials. One promising material is vanadium dioxide (VO2) due to its ability to rapidly transform from an insulator into a conductor in femtosecond (10−15 or one millionth of one billionth of a second) timescales. Recently, scientists discovered that VO2 responds non-uniformly on the nanoscale, contrary to prior assumptions, even in well-defined single crystals.
This study identified a non-uniform response in the insulator-to-metal transition, which suggests that the role of defects, strain, and doping (the strategic replacement of a portion of certain elements that make up the material with other elements) has been underestimated. Understanding the factors that influence the transition could inform the design of new materials where electron-electron interactions play a major role in determining proprieties—called “strongly correlated” materials. These materials could lead to more efficient technologies that can reduce energy costs for applications such as optical communications and data storage. Further, the precise characterization approach used in this research could further our understanding of quantum materials for novel approaches to next generation electronics.
Vanadium dioxide (VO2) shows significant promise for technological applications, ranging from advanced optical materials to data storage, and has encouraged extensive research because laser light can transform it abruptly from an electrical insulator to an electrical conductor on femtosecond timescales. For the past five decades, VO2 has been studied using a wide range of optical and electronic techniques, yet no researcher has been able to determine the exact mechanism that drives the oxide’s transition. Further, most experimental observations have been inconsistent, and implicitly assumed that the structural transition (associated with the change from insulator into metal) is uniform across the sample. Recently, researchers investigated high-quality microcrystals of VO2 with well-defined transition temperature and controlled strain states. Using a combination of femtosecond laser pump-probe microscopy and infrared scanning-probe nano-imaging (see image above for more information), researchers observed a high variability of both the femtosecond dynamics and the nanoscale domain behavior of the insulator-to-metal transition. The results suggest a high sensitivity of the insulator-to-metal transitions to static and dynamic effects that may be associated with atomic defects, doping, or local strain and favor an electronic mechanism dominating the insulator-to-metal transition. The results of this study could embolden more efficient technologies that can reduce energy costs, and provide a benchmark for future studies.
Markus B. Raschke
University of Colorado at Boulder
U.S. Department of Energy, Office of Science, Basic Energy Sciences.
B. T. O’Callahan, A.C. Jones, J.H. Park, D.H. Cobden, J.M. Atkin, and M.B. Raschke, “Inhomogeneity of the ultrafast insulator-to-metal transition dynamics of VO2.” Nature Communications 6, 6849 (2015). [DOI: 10.1038/ncomms7849]