Utilizing Dielectropherisis for controlled positioning of III-V nanowires to realize integrated sensors

Semiconductor nanowires have become an increasingly important class of materials for novel photonic and nano electronic devices. The field has matured to a stage where the growth of a large number of material systems has been demonstrated. III-V based semiconductors are of particular interest because of properties such as high carrier mobility and narrow bandgaps. There is also a growing interest in utilizing nanowire based structures and devices to interact with biological material for bio-sensing applications. This interest stems from the large surface area to volume ratio offered by nanowires resulting in changes on the surface having an easily measurable effect on the total electrical and optical properties of the nanowire.

Nanowire based sensors have been demonstrated by integrating nanowires with microfluidic structures for bio-sensing application. When the molecules of interest interact with the surface of the nanowire (either directly or via evanescent coupling) there is a resultant change in the conductivity of the nanowire allowing specific molecules to be detected selectively. Much of this work has focused on the use of silicon nanowires or carbon nanotubes, however III-V based semiconductors are also of interest due to their potentially higher carrier motilities and the presence of a surface charge accumulation layer which may make the wires more sensitive to their ionic environment. For this technique the nanowires (either individually or in arrays) need to be carefully aligned with the underlying microfluidic structures to enable the molecules to interact with the surface states of the nanowires, this is currently achieved using atomic force microscopes to position accurately position the nanowires, which is an expensive and time prohibitive process. As such although III-V semiconductor based nanowire structures and devices offer a number of theoretical advantages over more traditional bulk semiconductor structures, the difficulties and complexities in fabricating reliable devices on a large has greatly limited their use and exploitation.

During a dielectrophertic process (DEP), a polarizable nano-object is subjected to a non-uniform alternating electric field, the intrinsic charges separate and accumulate at the surface to form a dipole, which then experiences a force dependent on the gradient of the electric field. This results in the self-assembly of the dipoles across the electrode gap. The technique has been previously utilized to form arrays of nanowires of semiconductor materials such as Si, ZnO and carbon nanotubes (CNTs), allowing diodes and field effect transistors to be demonstrated as well as allowing for the accurate positioning of individual nanowires. While DEP has been used for some time in the fabrication of devices from CNTs and Si nanowires it has not been investigated as a fabrication technique for III-V semiconductor based nanowire devices. The potential to extend the use of DEP for use on III-V based nanowires has a number of distinct advantages leading to a greater understanding of underlying semiconductor physics as well as providing a route for novel devices to be realized. As such DEP offers an attractive route to integrate nanowire devices and microfluidic systems, allowing the wires to be accurately positioned and orientated to maximize the bio sensors efficiency in a simple and potentially mass producible technique.

The proposed work in this project will develop fabrication techniques to enable electrodes for DEP of nanowires to be realized on microfluidic systems. The project will involve the fabrication and experimental characterization of nanowire devices realized via DEP and the simulation, design and characterization of resultant bio-sensors.

Applications should be sent to Dr Ian Sandall ([Email Address Removed]) and should include a copy of curriculum vitae, academic transcripts, statement of research interests, and list of publications (if applicable).