The Preliminary Simulation Of Electromagnetic Enhancement Of Gold SERS Substrates By Three Dimensional Finite Difference Time Domain Method

Surface-Enhanced Raman Scattering (SERS) is able to obtain remarkably rich in-situ vibrational information from surface species due to its extremely high detection sensitivity and has been developed into one of important techniques in surface science. It has found wide applications in chemistry, physics, biology, nanoscience, and etc. However, up to now the SERS enhancement mechanisms are still not completely illustrated. Moreover, a stumbling block in further expanding SERS application is the lack of standard and routine methods for preparing the SERS substrates with the optimal SERS activity and good reproducibility. All these prevent the further development and applications of SERS. In this thesis, we employed mainly the three dimensional finite difference time domain (3D-FDTD) method to simulate the electromagnetic enhancement of gold nanostructured substrates with high SERS activity obtained in our experiments, on considering the influence of the particle size, near-field coupling, and excitation wavelength.The thesis is divided into four chapters. In the first chapter, the aims and tasks of this work were presented after reviewing recent progress of SERS and substrate preparation. In chapter two, we introduced concisely 3D-FDTD method and a modified Debye function of metal dielectric constant. In chapter three, we gave emphasis on the size dependent SERS activity of gold nanoparticles. By synthesizing Au nanoparticles with controllable size from about 16 to 160 run and measuring their SERS activity, We found that Au nanoparticles film with a size in the range of 120-135 nm showed the highest SERS activity with the 632.8 nm laser excitation. The 3D-FDTD method was employed to simulate the electromagnetic distribution of gold substrate, and analyzed the size dependent SERS activity. In chapter four, we quantitatively studied the correlations between near-field coupling, excitation wavelength and SERS activity of gold nanoparticles. The result can well explain that why we could not observe UV-SERS under the 325 nm laser excitation. The main progresses of this work are listed as follows:1. We systematically investigated the size dependent SERS activity of gold nanoparticles by 3D-FDTD method. The result agrees well with our experimental data. For Au nanoparticles with larger size, such as 220 nm, the multipolar effect leads to the appearance of the second maximum enhancement with the increasing particles size. This will give some instructions for future experiments in synthesizing gold nanoparticles with strong enhancement.2. We quantitatively studied the correlations between the coupling effect, excitation wavelength and SERS activity of gold nanoparticles. The results showed that near field coupling between nanoparticles plays a critical role in the electromagnetic enhancement of gold nanostructure. When nanoparticle gap is narrowed to below several nanometers, significantly enhanced local electromagnetic enhancement will be produced leading to the formation of SERS hot spot. Further decreasing the gap up to the touch of the two nanoparticles, the surface plasmons resonance enhancement will reduce sharply due to the exchange of charges between them. If the gap is at the dimension of the size of the nanoparticles, the electromagnetic coupling effect can be negligible. The excitation wavelength for producing the maximal enhancement will redshift with the increase of the size of the Au nanoparticles.3. We further analyzed the SERS activity of Au nanoparticles in the ultra violet region. The surface plamons resonance can not be effectively excited by the UV excitation due to the interband transition of gold nanoparticles. Furthermore, the detection sensitivity of present Raman instruments is still far lower than that in the visible region. It becomes reasonable that we could not observe UV-SERS in the UV region.A good agreement between 3D-FDTD simulation and experimental results clearly demonstrates that surface enhancement of gold nanoparticles was mainly contributed by the electromagnetic enhancement. This result will be beneficial to further understanding the SERS mechanism. Meanwhile, the results will be of great help in instructing the preparation of SERS substrates with high enhancement and high stability under the desired excitation wavelength.