Designing Nanostructures for Sensor Applications

Journal of Electronic Materials, May 2006 by Zhao, Y-P, Li, S-H, Chaney, S B, Shanmukh, S, Et al

Nanostructured materials have shown great potential in improving the sensitivity and reliability of chemical and biological sensors. The ability to control the geometric shape (size, separation, orientation, alignment, etc.) of nanostructures and to integrate nanostructures from different materials becomes one of the great challenges for sensor fabrication. Glancing angle deposition techniques can fabricate well-aligned three-dimensional nanostructures through computer programming. By rotating the substrate in both polar and azimuthal directions, one can fabricate desired nanostructures, such as nanorod arrays with different shapes, nanospring arrays, and even multilayer nanostructures. This method offers full three-dimensional control of the nanostructure with the additional capability of self-alignment and can be easily integrated into microdevices and optical devices. With the high surface area and high aspect ratio of those nanostructures, different sensors such as enzyme-based biosensors and optical sensors with higher sensitivity have been demonstrated.

Key words: Glancing angle deposition, nanorod, enzyme-based sensors, surface enhance Raman scattering

INTRODUCTION

Recently, nanostructured substrates, especially one-dimensional (1-D) nanostructures such as nanorods and nanowires, have been used extensively to improve the sensitivity and reliability of conventional chemical and biological sensors. In order to make practical devices, the nanofabrication technique should have the ability to fabricate the desired 1-D nanorod structures with specific size, shape, alignment, and architecture. In particular, the challenges for the nanostructure fabrication method are (1) the ability to control the size, aspect ratio, and shape of the nanostructures; (2) the ability to grow the desired nanostructure at low temperature and onto a particular substrate geometry, e.g., flat, cylindrical, or tapered; (3) the ability to fabricate metallic and dielectric nanostructures with multilayer structures; and (4) the ability to seamlessly integrate the fabrication process with other conventional microfabrication techniques.

Four general approaches have been employed to date for the fabrication of nanowire/nanorod structures: nanolithography-based methods, solution-based approaches, vapor-based methods, and templatebased methods.

The nanolithography-based method is a widely used technique in the fabrication of 1-D nanostructures.1-3 It employs advanced lithographic techniques, such as electron beam lithography, x-ray lithography, and proximal probe lithography4 with deposition and plasma etching processes. Most 1-D nanostructures fabricated by nanolithography are planar structures. The procedure is expensive, and the aspect ratio of the vertical nanorods is limited by the etching process. Currently, this method is not suitable for large-scale fabrication of 1-D nanostructures.

The solution-based approach employs controlled wet chemical reactions to synthesize nanostructures.5-8 For example, in solvothermal chemical synthesis,6,8 nanocrystals are produced by mixing a certain metal precursor with a possible crystal growth regulating or template agent under the right reaction conditions. A drawback of this process is that it is very complicated and requires a detailed understanding of the chemical reaction and crystal growth mechanisms. However, this method has been used to produce Ag nanorod structures for surface-enhanced Raman scattering (SERS) applications.9

Vapor-based methods generally consist of vapor transport or vapor reactions at suitable temperature and pressure.5,10 One popular vapor-based method is vapor-liquid-solid (VLS) growth, which has been applied for whisker growth since the 1960s.11 This method employs a catalyst to promote the anisotropic crystal growth, and a large number of materials have been grown into nanowire/rod form based upon this process. In general, a specific catalyst must be chosen for each material, and usually the growth temperature is relatively high.

Template-based methods use anisotropic nanoporous materials, such as anodized alumina, tracketched polycarbonate membranes, block copolymer membranes, etc., to serve as hosts. The nanochannels in the hosts may be filled using solutions, sol-gel, or vapor to generate the desired 1-D nanostructures. The products may be released from the templates by selectively removing the host matrix.12-14

Using these four major approaches to nanostructure fabrication, many pure elements-especially metals and semiconductors-have already been fabricated into 1-D nanowire/nanorod structures. These methods have also been successfully employed to fabricate some binary materials such as compound semiconductors and oxides into nanostructures. However, none of these fabrication methods adequately address the challenges identified above involving the fabrication of nanostructured substrates. We believe that a nanostructure fabrication technique called glancing angle deposition (GLAD) can solve those challenges and can be used to fabricate sophisticated nanostructured substrates for different sensor applications.