There has been a recent surge of interest in nanostructured materials for vacuum nanoelectronic devices and optoelectronic applications. Specifically, the high electric fields surrounding a nanostructure can create highly nonlinear tunnelling currents across few-nanometer free-space gaps. Such nanostructures can be used for creating low-power, high-speed nonlinear electronic circuit elements for operation in harsh environments, or novel optoelectronic devices exhibiting petahertz-level bandwidths. While noble metals exhibit high field enhancement, especially in the optical regime due to their plasmonic behavior, they are prone to damage and performance degradation because of their low melting point. Refractory plasmonic materials are promising for applications in harsh environments due to their high bulk melting point.