Amir A. Yasseri

Surface charge density of unpassivated and passivated metal-catalyzed silicon nanowires

Metal-catalyzed nanowires have previously been proposed as the active elements of field-effect devices, such as metal oxide field effect transistors and sensors. For these applications, the nanowire surface properties must be understood and controlled. Initial measurements of nanowire surface-charge density are presented here. The surface-charge density was obtained by measuring nanowires of different diameters grown between electrodes. The surface-charge density of a nanowire covered with native oxide is about 2 X 10 12 cm -2 ; the density appears to decrease by a factor of two to four when the native oxide is replaced by a high-quality thermally grown oxide, with further improvement possible.

Metal-catalysed, bridging nanowires as vapour sensors and concept for their use in a sensor system

Metal-catalysed silicon nanowires were grown between silicon electrodes and exposed to vapours containing HCl or NH3 at reduced pressure. Charge from adsorbed vapour modulated the conductance of the nanowires by changing the number of mobile carriers. Exposing the nanowires to HCl vapour increased the conductance while exposure to NH3 vapour decreased the conductance. The observed results suggest the use of an array of nanowire sensors integrated with silicon electronics. Preliminary area estimates indicate that integrated amplification and signal processing is feasible for an array of 1000 sensors.

Alkylsiloxane self-assembled monolayer formation guided by nanoimprinted Si and SiO2 templates

Alkylsiloxane self-assembled monolayers (SAMs) were formed on surface relief created by nanoimprinting and etching recesses into Si and SiO2. Rather than exhibiting the isolated domains seen on unpatterned surfaces after limited formation time, the SAM on nanoimprinted surfaces became continuous and uniform after only a short (∼5–10min) reaction time. The hydrocarbon chains are densely packed and nearly vertically oriented. X-ray photoemission spectroscopy indicates complete hydrolysis and elimination of Cl from the octadecyltrichlorosilane precursor. The results suggest that the pattern edges enhance the nucleation rate, leading to the rapid coverage, and provide in-plane order within the SAM.

Electroless Deposition of Au Nanocrystals on Si ( 111 ) Surfaces as Catalysts for Epitaxial Growth of Si Nanowires

Au nanocrystals were prepared by reacting solutions containing NaAuCl 4 2H 2 0 with hydrogen-terminated silicon (111) surfaces. The nanocrystals catalyze the subsequent growth of silicon nanowires by chemical vapor deposition using a mixture of SiH 4 and HCl in a H 2 ambient. Depositing the metal nanocrystals from nonaqueous solutions, such as ethanol, minimizes the oxidation of Si that can occur in an aqueous solution and allows epitaxially aligned growth of the nanowires. The Au nanocrystals preferentially deposit onto hydrogen-terminated Si( 111) compared to silicon dioxide. This selective deposition allows positioning of nanocrystals on patterns formed using either conventional lithography or nanoimprint lithography.