Surface Properties of Nanowire-based Inorganic-organic Hybrid Gas Sensors

The functionalization of semiconductor nanowires with self-assembled monolayers (SAMs) provides a viable route for the development of gas sensors with high molecular selectivity. Recently, selective and sensitive detection of nitrogen dioxide (NO2) has been demonstrated with an inorganic-organic hybrid gas sensor based on SnO2 nanowires and N-[3-(Trimethoxysilyl)propyl]ethylenediamine (en-APTAS 1) self-assembled on the surfaces of the nanowires (M. W. G. Hoffmann et al., Adv. Funct. Mater. 2014, 24, 595-602). Toxic air pollution by nitrogen oxides (NOx), an increasing problem in cities with strong emissions from combustion processes in car engines, affects the human health already in concentrations at the ppb level. NO2 is responsible for respiratory ailments and is also believed to cause cancer due to its high reactivity with genetic material and organic solvents forming nitrosamines.

In this project, surface chemical and physical properties of inorganic-organic hybrid NO2 gas sensors based on ZnO- and GaN nanowires will be characterized at the nanometer scale. It is the aim of this project to (i) correlate efficiency and selectivity of such sensors with chemical and structural surface properties and (ii) provide insights into those structural and chemical parameters relevant for improving sensitivity and selectivity of the sensor. Research in this project will be performed in close cooperation with the Wasisto and Waag groups at the Institute of Semiconductor Technology (IHT) of TU Braunschweig where the hybrid sensors will be prepared and characterized with respect to their selectivity and sensitivity to various gaseous pollutants. The surface properties of the sensor materials will also be characterized at the Institute of Energy Research and Physical Technologies of TU Clausthal. The chemical composition of the nanowire surfaces and of the SAM will be studied with high lateral resolution using scanning Auger electron spectroscopy. In addition, X-ray photoelectron spectroscopy (XPS) will be applied for a characterization of binding states, while structural aspects will be mainly addressed by scanning electron microscopy (SEM). We also aim to apply optical surface vibrational spectroscopy, in particular infrared-visible sum-frequency generation (SFG), for a characterization of molecular properties of the SAMs. This way, information from vibrational spectroscopy with high sensitivity to conformational changes in the SAMs will complement microscopic structural and chemical information on the hybrid sensors to yield a more complete picture at the molecular level.

PhD Student