Electron Injection Transforms a Thin Film

Simply applying a small voltage dramatically changes the atomic structure, vital to creating materials for advanced computer memory.

Image courtesy of Lawrence Berkeley National Laboratory

The Science

If you want to change how a material handles electrons, re-arrange the atoms. Scientists found a reversible way to alter the atom’s arrangement. Recently, researchers predicted that injecting electrons into a thin film could build up enough energy to move atoms (altering the crystal structure) and change the material’s properties. Now, a team proved this prediction correct. Simple electron injections reversibly change the crystal structure of atomically thin films of a common semiconductor. This method to change the crystal structure uses far less energy than heating or chemical processes.

The Impact

To create atomically thin computer memory, scientists are studying certain ultra-thin semiconductors. These thin films are based on a class of materials called transition-metal dichalcogenides. The films have unique properties. The team’s discovery about electron injections opens up options for creating designer films. The films are of interest for advanced computers and solar panels.

Summary

Monolayer thin films of 2-D, transition-metal dichalcogenides have emerged as important materials with unique properties, drawing great attention in science and technology. Of particular interest are their electronic and optical properties. The electrical properties can vary between no electrical resistance to unique spin-related electrical transport properties. Validating theoretical predictions, researchers reported an experimental demonstration which suggested that the simple application of a small voltage could be used to induce a structural phase transformation in an atomically thin film of molybdenum ditelluride (MoTe₂). The semiconductor film was coated with an ionic liquid that stores electric charges and controls the charge injection to the atomically thin film. Electrons accumulated in the film through a process called electrostatic doping, similar to traditional chemical doping, where the addition of a small amount of dopant changes the electronic properties of a material. But here, electrons’ injection is dynamic and clean, without creating defects. In response to the electrostatic doping, the atoms rearranged themselves in the film. The rearrangement formed a different crystal structure, which was metallic. The electrostatic doping made the crystal structure more slanted. While varying the gate voltage, the researchers used spectroscopic techniques (Raman scattering and second-harmonic generation) to monitor changes in the film. When the dopant electrons were removed from the film, by lowering the voltage, the original crystal structure returned. The structural transformations were reversible and uniform across the whole sample. This simple method controls the crystal structure and alters the electronic properties between 2-D semiconductors and semi-metals. It opens up exciting possibilities in the development of new, phase-change devices.

Contact

Xiang Zhang
University of California, Berkeley
Lawrence Berkeley National Laboratory
[email protected]

Funding

This work was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences (transport studies) including research at the Light-Material Interactions in Energy Conversion (optical measurements) Energy Frontier Research Center; National Science Foundation (device design and fabrication); and Tsinghua University (reference material). Researchers at Stanford University were supported by Army Research Office; Office of Naval Research; National Science Foundation; and a Stanford Graduate Fellowship.

Publications

Y. Wang, J. Xiao, H. Zhu, Y. Li, Y. Alsaid, K.Y. Fong, Y. Zhou, S. Wang, W. Shi, Y. Wang, A. Zettl, E.J. Reed, and X. Zhang, “Structural phase transition in monolayer MoTe2 driven by electrostatic doping.” Nature 550, 487-491 (2017). [DOI: 10.1038/nature24043]