Synthesis, characterization and chemical functionalization of germanium nanowires

Functionalised nanostructures are a viable approach to realise detectors with high sensitivity and very limited weight and electrical consumption, thanks to their high surface/volume ratio.Germanium nanowires (NWs) are proposed here as a fundamental building block of a sensor to detect traces of explosives molecules (in particular TNT), down to a detection limit of ~1x10- 6 ppt.In this communication we will report on the growth, structural and electrical characterization of these nanostructures. The first experiments on chemical functionalization tailored to bind the aromatic ring of TNT to the nanostructure will be presented. Germanium NWs were grown by Vapor Phase Epitaxy using iso buthyl germanium as a novel Ge source, at temperatures ranging from 340 to 460°C on (001) and (111) silicon and germanium substrates. Gold nanoparticles of different size (20-80 nm) were used as catalyst. The optimization of the growth procedure resulted in Ge NWs to 20 m in length. Details of the growth process and the growth conditions will be discussed, with particular emphasis on the control of the length of the NWs and their tapering. Different NW density and orientations are obtained, depending on the precursor flow and on the substrate orientation. The NW crystalline structure was observed by Transmission Electron Microscopy (TEM), evidencing their high structural quality. The NWs were detached from the substrates with ultrasonication and dispersed on a carrier substrate with Au interdigitated electrodes. Single NWs were observed with Scanning Electron Microscopy (SEM) and contacted to Au electrodes with Pt deposited by Focused Ion Beam (FIB). Pt contacts were deposited with 50 pA and 30 kV ion beam current and acceleration voltage,respectively, at each ends of the NW. Linear I-V characteristics showed the ohmic nature of Pt-NW contacts. From RT resistance measurements, using two-terminal configuration, we obtained NW electrical resistivity values in the 0.05 - 0.5 ohm cm range. We propose the unctionalization of the NWs surface with long chain alkanethiols terminating with an amino group, in order to provide the explosives recognition elements. Besides providing passivation of the NWs surface, the electronrich amino groups will bind the electron-deficient explosive molecules of TNT through chargetransfer donor-acceptor interactions, thus causing sharp changes in the conductance of the electrical-sensing anoelements.