Application Of Tip Enhanced Raman Scattering For Nanostructure Identification

Exploration of the microscopic world is always a tireless pursuit of mankind. Nanotechnology is coming of age. Scientists have invented a variety of techniques to observe and understand the microscopic world, aiming to design and control the structures and properties of matter at the nanoscale. However, conventional analysis techniques at the nanoscale mostly tend to have sole functions. For example, scanning tunneling microscope (STM) has ultrahigh spatial resolution but is limited in chemical identification capability. On the other hand, optical spectroscopic technique is advantegeous in chemical identification and ultrafast time resolution, but its spatial resolution is limited by the optical diffraction limit. The desire to push forward the development of science and technology has posed an urgent demand for a combined technique that is capable of sensitive chemical identification with nanoscale spatial resolution.Tip enhanced Raman scattering (TERS) is an emerging surface analysis technology at the nanoscale. In our previous research, the spatial resolution of TERS has been driven down to sub-nanometer scale for an isolated molecule on Raman-silent metal surfaces, which indicates its broad application perspects in surface analysis. In this dissertation, we shall demonstrate the application of sub-nanometer resolved TERS imaging to chemically identify a variety of complex nanostnictures that are formed on surfaces in an ultrahigh vacuum and low temperature STM. Our results indicate the power of TERS in the analysis of complex structures and lay the foundation for the exploration of nano materials, nano devices, surface chemistry and bio-science. The dissertation is composed of the following five chapters.In Chapter one, following a brief introduction on various technologies for chemical analysis with high spatial resolution, we present an overview of TERS. By exploiting the broadband, confinement and enhancement feature of the nanocavity plasmonic field, TERS has been developed into a new surface analysis technique with the ability of sub-nanometer chemical identification.In Chapter two, we investigate the chemical distinguishing of two adjacent different molecules by STM-controlled nonlinear TERS in real space. We present the first demonstration of resolving chemically two adjacent different porphyrin molecules that are within van der Waals contact and of very similar structural skeletons on Ag(111). The detected TERS spetra show the molecule-specific vibrational fingerprints, which can be used to identify different molecular species. By combining with theoretical simulations, we can also probe the adsorption configuration of molecules on the surface.In Chapter three, we carry out a panoramic analysis on different porphyrin nanostructures by performing the TERS imaging and full-spectrum analysis. After a brief introduction of the multivariate full-spectrum analysis method, we start with the mode analysis that is based on single peaks in TERS spectra. We analyze the spatial distribution of Raman bands on different complex nanostructures (ranging from homo-dimer, hetero-molecular chain, and molecular islands), and point out the limitation of mode analysis for identifying similar species. Then, we use the full-spectrum analysis to make component identification on the same TERS imaging dataset for the above-mentioned complex nanostructures. Different components can be easily resolved without ambiguities at the sub-nanometer scale, which suggests that the combination of TERS imaging with the full-spectrum analysis can help to promote TERS as a routine analytical technique.In Chapter four, we perform the TERS study on the second-layer porphyrin molecules with high spatial resolution. Comparing with the TERS spectra from the first layer of molecules which is adorbed directly on the metal surface, the TERS spectra from the second layer of molecules are very different, showing different virbratioal modes and higher Raman activity. Since the far-field Raman signals become detectable for a molecular sample with full second-layer coverage, the TERS enhancement factor can be estimated as high as～1010. In addition, weak molecular fluorescence can be observed in the TERS background on the second layer of molecules, which indicates the involvement of different processes other than TERS. There findings open up new opportunities for probing three-dimesional nanoscale structures.In Chapter five, we report the TERS imaging on single carbon nanotubes (CNTs).The spatial resolution of TERS on CNTs is driven down to about 0.7 nm. With such a high spatial resolution, we visualize the spatial distribution of defects in a CNT and related relaxation length of the D-band scattering in real space. The strain-induced spectral evolution and structural variations can also be tracked at the nanometer scale. Our results indicate that TERS can go beyond chemical identification and serve as a powerful tool to investigate the defect and strain in low dimensional nanostructures and materials, all at the subnanometer scale. Such ability can help to understand, design, and control the performance of nanoscale materials and devices.