Abstract
Surface ionization (SI) provides a simple, sensitive, and selective method for the detection of high-proton affinity substances, such as organic decay products, medical and illicit drugs as well as a range of other hazardous materials. Tests on different kinds of SI sensors showed that the sensitivity and selectivity of such devices is not only dependent on the stoichiometry and nanomorphology of the emitter materials, but also on the shape of the electrode configurations that are used to read out the SI signals. Whereas, in parallel-plate capacitor devices, different kinds of emitter materials exhibit a high level of amine-selectivity, MEMS (micro-electro-mechanical-systems) and NEMS (nanowire) versions of SI sensors employing the same kinds of emitter materials provide significantly higher sensitivity, however, at the expense of a reduced chemical selectivity. In this paper, it is argued that such sensitivity-selectivity trade-offs arise from unselective physical ionization phenomena that occur in the high-field regions immediately adjacent to the surfaces of sharply curved MEMS (NEMS) emitter and collector electrodes.
Highlights
Surface ionization (SI) is a form of gas detection, which relies on the chemisorption of analyte molecules on heated solid surfaces and on the extraction of the formed analyte ions from the adsorbent surface towards an oppositely biased counter electrode
Tests on different kinds of SI sensors showed that the sensitivity and selectivity of such devices is dependent on the stoichiometry and nanomorphology of the emitter materials, and on the shape of the electrode configurations that are used to read out the SI signals
In parallel-plate capacitor devices, different kinds of emitter materials exhibit a high level of amine-selectivity, MEMS and NEMS versions of SI sensors employing the same kinds of emitter materials provide significantly higher sensitivity, at the expense of a reduced chemical selectivity
Summary
Surface ionization (SI) is a form of gas detection, which relies on the chemisorption of analyte molecules on heated solid surfaces and on the extraction of the formed analyte ions from the adsorbent surface towards an oppositely biased counter electrode. A very interesting feature of the SI process is that it is largely insensitive to small, high-ionization-energy molecules, like N2, O2, H2O, CO2, and methane, but very sensitive towards low-ionization-energy analytes, such as secondary and tertiary amines [1,2,3,4,5,6,7] This kind of selectivity makes SI detection interesting for the detection of organic decay products, illicit drugs, and a range of other hazardous materials [8,9,10]. While much of the pioneering work on SI gas detection had been performed on macroscopic versions of noble and refractory metal ion emitters [1,2,3,4], more recent work concentrated on miniaturized kinds of SI devices, including MEMS and NEMS versions [11,12,13]. Our discussion shows that there is a wealth of high-field processes, which can modify the primary SI processes occurring at the surfaces of MEMS and NEMS versions of SI detectors, and that these call for further systematic experimental assessment and verification
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