Sensing of phosgene by a porous-like nanocrystalline diamond layer with buried metallic electrodes

Nanocrystalline diamond with a porous-like morphology was used as the functional part of a semiconductor gas sensor. The device function is based on the two-dimensional p-type surface conductivity of intrinsic diamond with a H-terminated surface. Metallic electrodes are buried beneath the diamond film. Therefore, these electrodes are protected from harmful substances, and the electronic connection is facilitated by grain boundaries. The gas sensing properties of the sensor structure were examined using oxidising gases (i.e., phosgene, humid air) at various operating temperatures. A pronounced and selective increase by two orders of magnitude was found in the surface conductivity after sensor exposure to phosgene gas (20 ppm) at 140 °C. Density functional theory calculations indicated no direct charge transfer between the phosgene molecule and diamond. We present a model in which phosgene indirectly yet efficiently increases the H3O+ concentration, which consequently leads to multiplied electron transfer and a pronounced sensor response.