Semiconductor-to-Metallic Phase Transition Could Lead to Entirely New Class of Chemical Vapor Sensors

A team of Scientists at the U.S. Naval Research Laboratory (NRL) proved that monolayer 2D Transition Metal Dichalcogenides (TMDs), referred to as atomically thin semiconductors, go through a transformation from semiconductor-to-metallic phase when they are exposed to airborne chemical vapors.

This interdisciplinary team validated electronic and optical evidence of the phase transition and also studied how the behavior can be used for producing a totally new class of chemical vapor sensors. These new instruments are considered to be potentially more sensitive than existing modern models and selective to particular nerve agents and explosive compounds, which are of immense concern on the existing battlefields.

These materials have continued to expose remarkable and new properties and behaviors since the discovery in 2004-2005 that single monolayer films of TMDs can be isolated from bulk materials because of the weak interlayer bonding of atoms called van der Waals bonding.

These materials are extremely promising for chemical vapor sensing applications because the inherent few-atom-thickness of the material greatly enhances their sensitivity to even the smallest surface disturbance. Apart from the immediate interest to basic research, as this particular method of creating of phase transition in TMDs has never been observed or explored before, it has great potential application in a new type of phase-based, multifunctional chemical vapor sensor.

Dr. Adam L. Friedman, Research Physicist, Material Science and Technology Division

Monolayer TMDs offer potential improvements in technology over existing material models, which make room for flexible, cost-effective, high-performance devices capable of exploiting their unique surface-dominated functionality.

The monolayer TMDs, chemically abbreviated as MX2, where M is a transition metal and X is a chalocogen, comprise of semiconductors, metals, insulators and other types of materials, and a wide range of properties not found in their bulk material equivalents. Specific films respond particularly via a charge transfer process to a class of analytes that comprise of nerve agents, such as such as venomous agent X (VX). A microscopic quantity of analyte existing on the surface of the TMD behaves as an electron donor and local reducing agent, which significantly affects the conductance of the film.

The NRL team assumed that specific strong electron donor chemical analytes, such as those applicable for sensing particular nerve agents and explosives, can also offer adequate charge transfer to the TMD in order to attain a phase change. The Researchers tested their hypothesis by exposing monolayer TMD films to powerful electron donor chemical vapor analytes and then monitoring them for their optical response and conductance. They discovered that the conductance response of their devices stopped after adequate exposure and the total magnitude of the conductance abruptly increased significantly at that moment, which signaled a phase change. The optical response also confirmed a phase change.

We assembled an exceptionally large data set that included multiple methods of measuring these types of films and concluded that the behavior that we observed is not due to doping and is most likely due to partial, localized phase changes in the areas of the TMD film where weakly adsorbed analyte transfers charge to the lattice.

Dr. Adam L. Friedman, Research Physicist, Material Science and Technology Division

This newly discovered behavior makes way for a totally new possibility for flexible, low-power, versatile chemical vapor sensor devices. The possibility of harnessing the phase transition to directly sense strong electron donor analytes will result in it producing a totally new chemical vapor-sensing model. It will allow the merging of passive-type optical measurements with, or used separately from, active conductance measurements in order to detect analyte vapors all with the same device and be employed as the operating mechanism for a new method to detect chemical compounds and the existence of  hazardous vapor.

Earlier studies of similar phase changes that were diffusion free have demonstrated speeds in the nanosecond range, and the anticipated devices will also be fast, which will go beyond the state-of-the art in detection speed. A suite of concurrently sensing TMD materials will permit different strength electron donors/acceptors to be detected and also identified with the necessary redundancy in order to minimize error since the amount of charge needed for inducing a phase change in each TMD material is different. These sensors, because of their low space requirements and expense, can also effortlessly be incorporated with current sensors in order to develop an even more versatile instrument for the existing Department of Defense (DoD) platforms.