Ordered hybrid nanoneedle arrays for gas sensing
Date of Issue2014
School of Materials Science and Engineering
This thesis presents ordered hybrid nanoneedle arrays for gas sensing application. The ion milling process on the metal/silicon material system is investigated in depth, in order to fabricate iron/silicon and platinum nanocrystallites/silicon hybrid nanoneedle array. The field ionization gas sensors using those unique highly ordered nanoarrays are prototyped and studied, which achieve the high gas sensitivity under an ultralow operation voltage. A hybrid nanostructure is a material system consisting of two or more components, which brings about the significant improvement of the property of the individuals. More recently, the capability of hybrid nanostructures has drawn the growing attention worldwide. However, the control during their assembly is rather challenging, especially for the one-dimensional metal/semiconductor hybrid nanomaterials. In the present thesis, a novel ordered metal/silicon hybrid nanoneedle array is fabricated using an ion beam that patterns the metal/silicon substrate producing a pillar array followed by the self-organization of hybrid nanoneedles. Each nanoneedle consists of a nanoneedle with a nanodot sitting on its top. The nanodot is an alloy of Ga and initially deposited metal. However, such ordered hybrid nanoneedles are not formed on the pure silicon substrate. Nor are the hybrid nanoneedles produced on the silicon surface coated with a thin metal that has a very high sputtering rate, such as Au and Ag. A mechanism of nanoneedle formation is proposed considering the alloying process during incident gallium ion irradiation to the silicon substrate coated with a thin layer of a sufficient low sputter rate. Iron, is a metal which has a pretty low sputtering rate and allows the formation of a perfectly ordered hybrid nanoneedle array on the surface of Fe/Si substrate. This unique nanostructure has been demonstrated to be highly sensitive to various types of gases, including air, nitrogen, hydrogen and carbon monoxide. And it also has ability to differentiate the same gas with various concentrations. Moreover, the sensitivity can be significantly enhanced when Fe metal is replaced by Pt nanocrystallites. In this case, a lateral electric field is applied across the nanoneedle array rather than a vertical electric field that is engaged during the evaluation of the Fe/Si gas sensing device. The breakdown voltage of nitrogen for Pt/Si comes up with only ~7 V in contrast to ~ 30V, which is required to N2 discharge for Fe/Si hybrid nanoneedle array. It is also found that the homogeneous silicon nanoneedles failed to generate any gas sensing signal. Based on the insightful analysis of gas ionization currents, such anomalous high field ionization efficiency has been attributed to the superimposed effects to the one-dimensional nanoneedles. For example, iron/silicon nanoarray provides abundant surface states for efficient gaseous electron tunneling, and polarized platinum nanocrystallites enhance the local electric field sufficient to direct ionization, respectively. Added by these experimental results and theoretic analysis, the previous physical scenarios of field ionization in a gaseous electronics system have been extended at nanoscale.