Giant auxetic behavior in remote-plasma synthesized few-layer tungsten semicarbide

Auxetic crystals exhibiting highly positive lateral expansivity when stretched are an experimentally elusive class of two-dimensional (2D) materials with tremendous potential, for example in the direct transduction of electric signals and the compensation of thermal expansion at the nanoscale. 2D tungsten semi-carbide (W₂C) was theoretically predicted to exhibit giant auxetic behavior, but has yet to be synthesized, as the corresponding full carbide (WC) is energetically favored under thermodynamic equilibrium synthesis processes such as furnace-based chemical vapor deposition. Here, we report on an ad hoc designed dual-zone remote plasma deposition system specially conceived to grow tungsten carbides out of thermodynamic equilibrium with well-tuned ratios of W and C precursors. We report on the specific conditions under which this system allowed for the synthesis of flakes of few-layer tungsten semicarbide (FL-W₂C) which are 2D in nature due to retained periodicity at the mesoscopic level in a Stranski-Krastanov growth process. Under applied strain, FL-W₂C 2D crystals exhibit the strongest auxetic behavior observed to date. This result suggests that the theoretically predicted high negative Poisson's ratio of single-layer W₂C, also extends to thicker FL-W₂C flakes that are retaining the periodicity of the 2D crystal at the mesoscopic level.