The present paper compares three different kinds of semiconductor gas sensor materials: metal oxides (MOX), hydrogen-terminated
diamond (HD), and hydrogenated amorphous silicon (a-Si:H).
Whereas in MOX materials oxygen is the chemically reactive
surface species, HD and a-Si:H are covalently bonded semiconductors with
hydrogenterminated surfaces. We demonstrate that these dissimilar semiconductor
materials exhibit the same kind of low-temperature gas response.
This low-temperature response-mechanism is mediated by a
thin layer of adsorbed water with the semiconductor materials themselves acting
as pH sensors. In this adsorbate-limited state the gas sensitivity is limited
to molecular species that can easily dissolve in and subsequently undergo
electrolytic dissociation.
At higher temperatures, where a closed layer of adsorbed
water can no longer exist, the gas response is limited by direct
molecule-semiconductor interactions. In this latter mode of operation, MOX gas
sensors respond to adsorbed gases according to their different oxidising or
reducing properties. Hydrogenated amorphous silicon (a-Si:H), on the other
hand, exhibits a significantly different cross sensitivity profile, revealing that
gas-surface interactions may largely be restricted to analyte molecules with
lone-pair and electron-deficient three-centre orbitals.
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