Large Area Fabrication of Leaning Silicon Nanopillars for Surface Enhanced Raman Spectroscopy

Abstract

Surface enhanced Raman spectroscopy (SERS) [1] was discovered in 1974 and is now a powerful analysis technique which is becoming more and more widespread due to the development of cheaper lasers and spectrometer systems. SERS derives information about the type of chemical bonds in trace amounts of analyte molecules when these are adsorbed onto, or placed adjacent to, a metal surface or structure: a so-called SERS substrate. As it is vibrations in the chemical bonds of the analyte that cause a Raman shift, most chemical species can in theory be identified. For this reason SERS is used as a versatile analytical tool for both chemical and biochemical sensing in liquid and gas phases. [2][3][4][5][6] It is widely recognized that the SERS phenomena is based on laser excitation inducing an electromagnetic field at the surface of noble metals. Areas with particularly large electromagnetic fields, also called "hot spots", are found in between adjacent metal nanostructures if these are located sufficiently close to each other, i.e., on the order of a few nanometers. If an analyte molecule is located inside this hot spot, it will result in a comparably large Raman signal from the analyte. Recent studies have shown that hot spots account for a disproportionally large contribution to the total Raman signal from a SERS substrate. [7,8] There is generally an emphasis on reproducibility and uniformity to ensure consistent chemical detection sensitivity across the surface of a SERS substrate. A trade-off between reproducibility and enhancement is often mentioned. The current dilemma is that the more uniform the enhancement is across a substrate, i.e. the better the reproducibility, which is often achieved using a top down lithographic approach, the lower the enhancement tends to be and vice versa.[9] Currently, the inability to mass produce large areas of cost effective nanostructured SERS substrates with large numbers of hot spots and hence suitable Raman enhancement is impeding the use of SERS sensors in both laboratories and mobile applications.Ideally, a SERS substrate should enhance the Raman effect sufficiently to enable suitable chemical detection levels while at the same time being practical to use in a sensor system. Broadly speaking, SERS substrates can be classified into two categories, i) metallic nanoparticles in colloidal solution and ii) roughened metallic surfaces. Colloidal suspensions of metallic nanoparticles have so far been reported to have the largest enhancement factors and even single molecule detection has been demonstrated. [10][11][12] However, with colloidal solutions it is a challenge to bring the metal nanoparticles sufficiently close to form hot spots around the analyte, while at the same time preventing the formation of large conglomerations of nanoparticles before the analyte is introduced.Rough silver and gold surfaces can be fabricated by a number of methods including chemical etching, mechanical deformation, electroplating and oblique angle deposition. [13][14][15] Processes where well define...

Wafer-Scale Leaning Silver Nanopillars for Molecular Detection at Ultra-Low Concentrations

“…Upon solvent drying, capillary forces pull Ag NPs together causing NP clustering that increases the LSPR coupling between Ag NPs. 41 The results in Figure 3(b) show that the SERS signal from BPE gradually decreases as the O 2 -plasma exposure time is increased. A 4 min exposure to O 2 -plasma reduces the SERS intensity by a factor of ~4, thus the Ag NP EF decreases dramatically.…”

Electrospray surface-enhanced Raman spectroscopy (ES-SERS) for probing surface chemical compositions of atmospherically relevant particles

“…Furthermore, a red shift from 980 to 967 cm −1 occurred. A commercially available Ag-coated SERS substrate (SERStrate, Silmeco) (Schmidt et al, 2012) was also used to investigate the Raman enhancement of the AS / sucrose particles (Fig. 4).…”

Passivation of all-angle black surfaces for silicon solar cells

“…The nanowires can be seen to bunch, an effect more prominent with the alumina coating. The bunching occurs as MACE is a water-based etching process, so during the drying step the NWs agglomerate in order to reduce surface tension, and are held together with Van der Waals forces [35]. Conformal coverage of the NWs with alumina using ALD was achieved by inserting a precursor diffusion time of 10 s for every Å of deposition, in order to achieve good exposure of the high surface area structures.…”