Phase-change materials could make chemical sensors

July 26, 2017 // By Peter Clarke
A team of scientists at the U.S. Naval Research Laboratory (NRL) has shown that monolayer 2D transition metal dichalcogenides (TMDs) undergo a change from a semiconductor to a metallic phase when exposed to airborne chemical vapors.

This finding could be used to create a new class of chemical vapor sensors that are potentially more sensitive than current state-of-the-art sensors. Military interest comes from the possibility that sensors could be made selective to specific nerve agents and explosive compounds.

Chemically abbreviated as MX2, where M is a transition metal and X is a chalcogen, the monolayer TMDs include insulators, semiconductors, metals and other types of materials, and include a variety of properties not observed in their bulk material equivalents. Certain films respond selectively through a charge transfer process to a class of analytes that includes nerve agents, such as venomous agent X (VX). A microscopic quantity of analyte lying on the surface of the TMD acts as an electron donor and local reducing agent, which measurably affects the conductance of the film.

The NRL team hypothesized that certain strong electron donor chemical analytes, like those relevant for sensing certain nerve agents and explosives, can also provide enough charge transfer to the TMD to achieve a phase change.

Both electrical and optical observation has established that is indeed the case. Previous studies of similar diffusion-less phase changes in materials have shown speeds in the nanosecond range.

The Office of Naval Research (ONR) provided funding for the research and the results are reported in the June 2017 issue of Nature Publishing Group’s Scientific Reports ( DOI:10.1038/s41598-017-04224-4) and in a provisional patent.

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