Research Of Surface Enhanced Raman Scattering On Several Nanoscale Semiconductor Substrates

Surface enhanced Raman scattering (SERS) can provide important information of surface molecular structure and kinetic process directly. SERS technique is becoming a strong hand in surface science and electrochemistry ,and widely used in trace analysis and even single molecule detecting(SM-SERS), medical chemistry, environmental sciences, biologic and medical systems, nano-materials and sensors. For many yeas SERS is still restricted primarily for analytes adsorbed onto coinage (Au, Ag, and Cu) or alkali (Li, Na, K) rough metallic surface. Our group have successfully gain high signal-noise SERS spectra from semiconductor substrates Since semiconductors are commonly considered to be non-SERS active, this work could be an important contribution, indicating a possible extension of the SERS effect from metal to semiconductor in order to widen the range of SERS substrate materials.We also obtained the SERS effect for noble metal to semiconductor. Boosted by the long-range effect of the enhanced electromagnetic (EM) field generated by the highly SERS-active Ag, the originally low surface enhancement of the semicondutor can be substantially improved. Our study is outlined as follows:1. Pb3O4 was prepared by sol-gel method, high signal-to-noise ratio (S/N) SERS spectra from low concentration of 4-Mpy molecules on Pb3O4 substrates were obtained. Compared with Raman spectra from Pb3O4 and Ag substrates, the particular features of semiconductor substrates from metal substrates have been revealed. The enhancement factor was estimated about 102. The enhancement mechanism: for most semiconductors, the plasmon resonance is typically in the infrared. This value is too far away from the 514.5 nm incident light thus plasmon resonances may be ruled out. The chemical enhancement mechanism should be responsible for the observed enhancement in Pb3O4. The Raman spectra under different exaction lines also demonstrate very good results.2. Further, micro-nanostructure Zn/ZnO film was prepared by a very simple chemical etching method. By analyzing XPS and Raman spectra, it is concluded 4-Mpy has chemisorptions on the ZnO film. The resulting Zn/ZnO film can serve as good surface enhanced Raman spectroscopy (SERS) substrates, exhibit about 104~105 enhancement factors. The results of the electrochemical roughening also confirm the SERS signal was from the interaction between 4-Mpy and ZnO. Similar enhancement was obtained from 2, 2-bipridine adsorbed Zn/ZnO film.3. We obtained the SERS signal from 4-mercaptopyridine (4-MPy) on nanoscale CuO thin film coated with a discontinuous layer of 5, 15 and 30nm Ag island film over CuO thin film substrates. We were not able to detect SERS signal of 4-MPy from CuO surface without the deposited Ag film. After coating with 5 nm, CuO thin film substrate exhibit a better SERS effect than the pure 5 nm Ag film. When the thickness of Ag island film reaches 30 nm, all the CuO thin film was almost covered Ag island films .Raman spectra of 4-Mpy adsorbed these two substrates are alike in most part. It is further confirmed the SERS signal (Ag island film<30 nm) was from the CuO thin film.By changing the thickness of Ag island films, we can gain a proper local electromagnetic field enhancement on CuO thin films to reach the best SERS effect. It is to be noted that the SERS effect of discontinuous Ag island films on which SERS activity degrade soon after preparation. To the contrast, SERS activity maintains about a week on CuO thin film.4. Cu2O/Ag composite was prepared by using a very simple chemical reduction method. First, The Cu2O nanooctahedron was sensitized by Sn2+ and then deoxidizes by the Ag (NH3)2+ with the reducing agent formaldehyde in aqueous solution on the surface. We fabricate Cu2O/Ag composites and gain comparable enhancement on SERS-inactive substrates as that before only gained on traditional SERS-active substrates.The enhancement factor was estimated 2.1×104. Therefore, one can detect adsorbed molecules on other semiconductor surfaces by borrowing enhancement generated by SERS active material and have a further understanding of structural information about adsorbed molecules. The difference spectrum was obtained between the Raman spectrum of Cu2O/Ag composites and pure Ag substrates. The bands and the relative intense of that spectrum were similar as the spectrum on the Cu nanoparticles. This phenomenon further confirms the chemisorptions between 4-Mpy and Cu2O were bonded by Cu-S.5. These researches will benefit to understand the mechanism of SERS: There are two main SERS enhancement mechanism: electromagnetic (EM) and chemical (CHEM) enhancement mechanisms. The EM enhancement is due to a large increase of the electric field caused by surface plasmon resonances induced by the laser light in nano-sized metal clusters on the surface. This effect is usually considered the most important, contributing about 104–106 of the total observed enhancement and is often called the physical effect, since allthat is required is that the molecules are physisorbed at or near the surface. In metals such as silver, the plasmon resonance is in the visible or near UV, making it suitable for enhancement of the Raman spectrum excited in that region. For most semiconductors, the plasmon resonance is typically in the infrared. This value is too far away from the 514.5 nm incident light to be responsible for the observed enhancement, thus plasmon resonances may be ruled out. The CHEM enhancement results from the chemisorption interaction and the photon-driven charge transfer, which leads to an increase in the polarizability of the molecule. The chemical effect is usually considered to contribute about 102 of the total observed enhancement. The exact chemical mechanism of the enhancement effect of SERS is still a matter of debate in the literature. We fabricate metal/semiconductor system and gain comparable enhancement on SERS-inactive substrates as that before only gained on traditional SERS-active substrates. Therefore, one can detect adsorbed molecules on other semiconductor surfaces by borrowing enhancement generated by SERS active material and have a further understanding of structural information about adsorbed molecules. With this method which borrowing strong electromagnetic from SERS active materials it is possible to observe the enhanced Raman scattering on other SERS-inactive substrate surfaces.