Study On Enhancement Theory And Applied Technique Of Raman Spectroscopy

As a technique with multiple disciplines intercrossing and syncretizing each other, Raman spectroscopy is on its developing time now, and studies on its theories and applications are being on the way to advance. In the study of Raman spectroscopy, enhancement theory, analysis method and data process are very hot. Studies on the three subjects are carried out in this dissertation.Raman spectra are usually very weak. Therefore, the signal enhancement becomes crucial to develop Raman spectroscopy as a widely used analytical tool. Surface-Enhanced Raman Scattering (SERS) observed in 1974 can enhance Raman signal by 10⁶ even up to 10¹⁴, and has made much progress in analytical application. However, this enhancement is dependent on transitional metal substrates which are rough and SERS-active, so SERS cannot be applied to analyze materials whose surface are smooth, or not SERS-active. Today SERS mechanisms are still unknown due to the baroque substrate. Based on SERS technique, Tip-Enhanced Raman Spectroscopy (TERS) has been studying from 2000. The electromagnetic enhancement of TERS is achieved by a metal tip, which make that TERS can be used to analyze any material, and studies on the enhancement mechanisms become easy. In addition to the high enhancement factor, TERS still has very high spatial resolution about several nm. Recently, studies on theory and application have become important forward subjects in Raman spectroscopy field.Employing theoretics of dielectric constant, this dissertation investigated mechanisms of TERS. The studies were focused on the affections of the dielectric constants and shapes of the transitional metal (Au, Ag, Cu, Fe, Co, Ni) tips on the enhancement factor and the frequency of surface plasma resonance (SPR). The results are as follows:1. Ag tip is best for TERS. The enhancement factor on Ag tip surface is larger than those on the surface of Au, Cu, Fe, Co and Ni tip, when SPR is excited. And the characteristics of SPR on Fe, Co and Ni tip surface are not obvious.2. The enhancement factor is also affected by the shape of the tip. The enhancement on ellipsoidal tip surface is stronger than on spherical surface. The frequency of SPR shifts to low frequency, and the enhancement factor increases quickly, when the ratio of major axis to minor axis increases.3. With the distance from the tip to the specimen increasing, the enhancement of located electromagnetic field decreases.4. The enhancements of scattering lights of different frequencies are different. With the increase of Raman shift of scattering light, the variety of enhancement factor is down.Qualitative analysis using Raman spectroscopy requests the assignments of Raman peaks. Assigning Raman peak is usally very difficult because the vibration modes of molecules are rather complicated. The Raman peaks of toluene, ethylbenzene, o-xylene, m-xylene and p-xylen were assigned, and then the five organic components were identified from aqueous solution of mixture in this dissertation.Although the Raman intensity has proved theoretically proportional to the concentration of the specimen, it is often affected by other factors, such as the temperature, the laser intensity, and the background. It is difficult even inefficient to calculate the concentration directly using the Raman intensity in practice. The ratio method was employed to reduce or eliminate the influences of these factors and acquire the concentration of the specimen in this dissertation. The result demonstrates that the ratio of the reference's Raman intensity I_A and the specimen's Raman intensity I_b is affected by the reference's concentration C_a and the specimen's concentration C_B, and there is an excellent linear relationship between I_A/I_B and C_A/C_B.Because Raman spectrum is usually noisy, removing noise is vital for qualitative and quantitative analysis by Raman spectroscopy. Wavelet transform (WT) technique was employed to de-noise. Based on analyzing shot noise and fluorescence background in Raman spectrum, a new thresholding selection method-interval thresholding method was used to remove these noises from the simulated and experimental Raman spectra. The de-noising performance of the interval thresholding method was compared with that of the global thresholding method. The results indicate that the interval thresholding method is more efficient in removing shot noise and fluorescence background.