Microcrystal sensors

The most obvious way to raise the sensitivity of sensors to RGMAs is by activating their surface with additives that actively interact with metastable atoms and have some electron coupling with semiconductor. These additives can be microcrystals of metals. As previously shown, the de-excitation of RGMAs on a metallic surface truly proceeds at high efficiency and is accompanied by electron emission. Microcrystals of the metal being applied to a semiconductor surface have some electron coupling with the carrier [159]. These two circumstances allow one to suppose that the activation of metals by microcrystals adds to the sensitivity of semiconductor films to metastable atoms. 

Another methodical trait of the Au/ZnO sensor application to detect metastable atoms of rare gases is the limitation of the range of operating temperatures. When heated to above 500 K, these sensors irreversibly loose their sensitivity to RGMAs. The loss of sensitivity is associated with the coalescence of Au microcrystals applied to a ZnO surface. The causes of this will be discussed later. 

An important role in the mechanism is plaid by the applied particles of metal. One can conclude so from the experiments in studying heterogeneous deactivation of RGMAs on a pure and Au microcrystal-activated surfaces of glass and zinc oxide [162], The experiments were conducted by the Smith method, Au/ZnO sensors being used as RGMA detectors. The results of these investigations are tabulated in Tables 5.3. 

The coalescence of Au microcrystal in the islet film occurs during thermal annealing, as well as when the metal concentration is increased. It is probably the coalescence that does determine the above-mentioned loss of Au/ZnO-sensor sensitivity during high-temperature heating operations. 

We heated the substrate of zinc oxide containing 10 cm 2 of silver atoms (in this case there was already no emission after completion of deposition) at 300 C. Such thermal treatment results in formation of microcrystals, rather than evaporation adatoms on the surface of the substrate made of zinc oxide. In paper [34] it was shown that microcrystals with diameter 100 A deposited on the zinc oxide surface are acceptors of electrons, therefore the formation of microcrystals results in increase of resistivity of a sensor substrate above the initial value (prior to silver deposition). In this case the initial value of the resistance of sensor-substrate was 2.1 MOhm, after adsorption of silver atoms it became 700 kOhm, and as a result of heating at 300°C and formation of microcrystals - acceptors of electrons it in increased up to 12 MOhm. If such a substrate is subject to deposition of 3-10 5 cjjj-2 silver again, then emission of silver atoms gets detected. From the change of resistivity of sensor-detector due to deposition of silver atoms one can conclude that in this case the emission of atoms is 4 times as low than in experiment with pure substrate made of zinc oxide, which confirms the supposition made on the mechanism of emission of adatoms. 

A few other types of anion sensors have been mentioned recently in the literature. Tetrathiofulvalene microcrystals immobilised at a platinum electrode displayed electrochemical properties that were affected by the presence of anions in solution, with some selectivity for anions such as bromide [ 145]. A flow-injection analysis system using anion-exchange columns for separation and polyaniline electrodes as detectors could detect dichromate down to 0.004 ppb and could be used for seawater samples [146]. 

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