Synthesis of germanium nanowires and carbon nanotubes tailored to explosive detection

Functionalised nanostructures are an interesting approach to realise detectors with high sensitivity and very limited weight and electrical consumption. The favourable surface/volume ratio typical of nanostructure can be exploited to realise sensitive, selective and small detectors suitable to be carried by drones. In our project we aim to use two types of nanostructures, germanium nanowires and carbon nanotubes, to detect traces of explosives molecules (in particular TNT) down to a detection limit of ~1x10-6 ppt. Surface functionalization with electron-rich amino groups will allow the binding to the electron-deficient explosive molecules of TNT through charge-transfer donor-acceptor interactions, thus causing sharp changes in the conductance of the electrical-sensing nanoelements. In this work we present the synthesis of germanium nanowires (NW) and carbon nanotubes (CNTs), that will be the basic building blocks of the proposed sensors. Germanium NW are grown with by Vapor Phase Epitaxy at 400 °C on silicon substrate with gold as catalyst, using a novel iso buthyl germanium source. Details of the growth process and the growth conditions will be discussed. Scanning Electron Microscopy (SEM) was used to asses Ge NW diameters, ranging from 20 to about 80 nm. Different NW density and orientations are obtained, depending on the precursor flow and on the substrate orientation. The crystalline structure of the NW was observed by transmission electron microscopy (TEM). Two types of CNTs (powder form and direct deposition on the sapphire and silicon substrates) were grown by aerosol-chemical vapor deposition (A-CVD) technique [1] and analyzed by x-Ray diffraction, SEM, TEM and Raman spectroscopy. Very smooth and long CNTs with good electrical conductive properties are obtained.