SERS Behavior of Ag Nanorod Array

  Recently, nanostructured substrates, especially one dimensional 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 one-dimensional nanorod structures with specific size, shape, alignment, and architectures. In particular, the challenges for the nanostructure fabrication method are: (1) the ability to control the size, aspect ratio, 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.

We are particularly interested in a surface enhanced Raman spectroscopy (SERS) based sensor. Our preliminary SERS experiments on Ag nanorod samples have shown very promising results. A stepped nanorod sample was deposited onto a Ag-coated glass slide resulting in six different regions, denoted as A, B, C, D, E, and F (Fig. 1). These six regions have randomly aligned Ag nanorod arrays with lengths of l = 0, 190, 218, 300, 366, and 508 nm, respectively. The SEM images in Fig. 1 show representative morphologies of several of the structures. The diameter of the Ag nanorods is about 80-90 nm.

Using these Ag nanorod samples, surface enhanced Raman spectra were acquired using a near-IR confocal Raman microscope at an excitation wavelength of 785 nm. The molecular probe used in this study was trans-1,2-bis(4-pyridyl)ethene (BPE). A representative SERS spectrum of BPE on an Ag nanorod array with a rod length of 366 nm is presented as an insert in Fig. 1. From these spectra, the SERS Surface Enhancement Factor (SEF) was calculated for BPE on the stepped Ag nanorod samples.

Figure 1 illustrates the calculated SERS SEF plotted as a function of the length of Ag nanorod. The SEF increased from almost zero for Region A to over 10 5 for a very short Ag nanorod in Region B (l = 190 nm), and then increased another three orders of magnitude ( 108 ) for a nanorod in Region F ( l = 508 nm).

Fig. 1. The Ag nanorod samples for SERS experiments and the enhancement factor as a function of the nanorod length.