The emissions of Taylor cones from a wide range of ionic liquids (ILs) have been tested in vacuo in an attempt to identify what physical properties favor the purely ionic regime (PIR). This regime is well known in the case of Taylor cones of liquid metals. For nonmetallic liquids, it has been previously observed in conventional (capillary tube) electrospray sources at room temperature only for the room temperature molten salt (ionic liquid) EMI–BF4 (EMI=1-ethyl-3-methylimidazolium). A large number of other ILs and their mixtures have been studied here, most of which (but not all) are unable to reach the PIR at room temperature. Based on these results and additional theoretical considerations, strong support is assembled for the notion that the PIR is favored by ILs not only of high electrical conductivity but also of high surface tension. This hypothesis is confirmed by tests with three recently synthesized ILs, EMI–GaCl4, EMI–C(CN)3, and EMI–N(CN)2, all of which combine exceptional surface tension and electrical conductivity, and all of which reach the PIR at room temperature far more readily than EMI–BF4.
The decay of photo-induced conductivity is measured for GaAs/SnO 2 heterostructure, after illumination with appropriate wavelength. The top oxide layer is deposited by sol-gel-dip-coating and doped with Eu 3+ , and the GaAs bottom layer is deposited by resistive evaporation. It shows quite unusual behavior since the decay rate gets slower as the temperature is raised. The trapping by intrabandgap defects in the SnO 2 top layer is expected, but a GaAs/SnO 2 interface arrest becomes also evident, mainly for temperatures below 100 K. Concerning the SnO 2 layer, trapping by different defects is possible, due to the observed distinct capture time range. Besides Eu 3+ centers and oxygen vacancies, this sort of heterostructure also leads to Eu 3+ agglomerate areas in the SnO 2 top layer surface, which may contribute for electron scattering. The electrical behavior reported here aims to contribute for the understanding of the electrical transport mechanisms which, combined with emission from Eu 3+ ions from the top layer of the heterostructure, opens new possibilities for optoelectronic devices because samples in the form of films are desirable for circuit integration. The modeling of the photo-induced decay data yields the capture barrier in the range 620-660 meV, and contributes for the defect rules on the electrical properties of this heterostructure.
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