| Exploring the relationship between the structures and the physicochemical properties of materials at the molecular or even atomic scale is the basis for understanding their applications in various fields.With the advent of scanning tunneling microscopy(STM),human being’s ability to detect the microscopic world has entered the atomic scale.As an extension of STM for optical measurements,tip-enhanced Raman spectroscopy(TERS)can not only break the optical diffraction limit,but also realize in-situ chemical identification and characterization of samples in real space.At present,TERS has reached single-molecule sensitivity,with spatial resolution even down to single chemical bonds at the Angstrom level.However,such an Angstrom-scale resolution in single-molecule TERS was mainly realized in limited planar molecular systems such as porphyrins or phthalocyanines.There are still questions as to whether the ?ngstr?m-resolved TERS technique can be more widely applied in a variety of different material systems,and fulfil the structural characterization and analysis at the sub-nanometer or even atomic scale.In this thesis,first,by exquisitely tuning the local field enhancement properties of the probing tip,we further explore the fabrication and characterization methods of TERS-active tips.Then,we carry out high-resolution TERS studies on three representative nano-systems in terms of ever-improved requirements for spatial resolution:from the chemical distinguishing of intermolecular systems to the specification of intramolecular functional groups and local configurations,and further to the atomic-scale characterization of the defects in two-dimensional materials.Through these TERS studies,we demonstrate the powerful applications of Angstrom-resolved TERS techniques in nanostructure analyses,which provides a new method for studying the composition and structure of molecules and other materials at the atomic scale.Specifically,this thesis is mainly divided into the following four chapters.In chapter one,we briefly introduce the research background,experimental equipment and data processing methods related to this thesis.First,we present a brief introduction on the principle and major function of STM as well as the basic concept of surface plasmon and its role in the enhancement of Raman signals.Then,we briefly overview the TERS technique used in this work,paying attention to the enhancement mechanism and the latest developments in the field.This is followed by an introduction on the multivariate analysis method,which we develop for the spectral analysis of the hyperspectral TERS imaging data in this thesis.Finally,we briefly describe the experimental equipment used in this work and the research motivations as well as the organization and main research contents of this thesis.In chapter two,we explore the ability of TERS in chemically distinguishing different adjacent DNA base molecules.We select the complementary DNA base pair(adenine A and thymine T)as an example to explore the feasibility of TERS technique to resolve single DNA bases in real space.First,for each type of base molecules,we prepare self-assembled samples separately and obtain their own characteristic TERS spectra.Then,we investigate mixed hydrogen-bonded molecular networks composed of these two different DNA bases by TERS.Spectral distinguishing between adjacent different bases with single-base accuracy has been realized,with a spatial resolution of about 7.5 A.We also demonstrate the automatic recognition and classification of DNA base spectra by using the multivariable analysis technique.These results can extend the potential application of TERS to the high spatial-resolution structural analysis of biomolecules,offering a promise for the base recognition and sequencing of DNA/RNA molecules in real space by Raman spectroscopy.In chapter three,we take a weakly decoupled porphyrin(H2TBPP)as a model molecule to explore the capability of the TERS in monitoring the configurational changes of local functional groups within a single molecule during tip approach.This molecule is modified by attaching four bulky di-tert-butylphenyl groups to the porphyrin core and thus the core is somewhat lifted from the underneath substrate.We find that H2TBPP molecules exhibit different STM morphologies under different tip heights.By changing the tip height(i.e.,gap distance)above the topographically higher lobe,the local configuration of the higher lobe in the molecule exhibits reversible changes during tip approach and retraction.From DFT calculations and the TERS imaging patterns of different vibrational modes,we find that during the tip approaching process,the di-tert-butylphenyl group as a whole would rotate towards to the porphyrin ring plane.At the molecular center,we find that when the tip is approached down to a certain height(～5?),the TERS spectra would jump in terms of both intensities and spectral features,which corresponds to an abrupt change of the molecular structure from the normal adsorption configuration before contact to the "bowl-like" configuration after contact,likely accompanied by the contact of the surrounding tert-butyl groups to the side of the tip apex.Our results show that the TERS technique with Angstrom spatial resolution can be used to track the continuous subtle changes of the local structures within a single molecule,which lays a foundation for the analysis of the microscopic mechanism of surface chemical reactions and catalytic processes.In chapter four,we further extend the research system to two-dimensional materials and investigate the phonon spectra of different layers of sodium chloride(NaCl)films on the silver surface as well as the Raman spectra and images of different surface defects at the atomic scale.First,considering that the phonon modes of crystal materials are mainly located in the low wavenumber region,we built up a low-wavenumber TERS measurement system,which can collect the Raman signals with Raman shifts as low as 20 cm-1.Then,we study the influence of the metal substrate on the structure and phonon property of NaCl films at different layer thickness,from 1 monolayer(ML)to 4 ML.The TERS imaging patterns on both the 4ML-and 3ML-NaCl films show a spatial resolution down to the single-atom scale.Particularly on the 4ML-NaCl film,there is a significant difference in the TERS intensity between Na+cations and Cl-anions,which may be related to the different phonon densities of states of these two surface atoms.These observations at 4ML-NaCl suggest that the surface atoms of the 4th-layer NaCl appear less affected by the metal substrate.By contrast,from the 3ML-NaCl TERS imaging,we can extract a reconstructed 3X3 Moire pattern upon multivariate analysis.On the other hand,the 2ML-NaCl film is greatly affected by the substrate,and the reconstructed structure of the NaCl thin layer can be clearly and directly observed from the TERS imaging even without using multivariable analysis.The energy of the transverse phonon mode at different positions on the 2ML-NaCl film changes periodically,which indicates that the lateral spacing of neighboring atoms would change periodically.For the 1ML-NaCl film,due to the direct influence of the substrate,the heights of surface atoms fluctuate and the corresponding TERS spectra are generally irregular over the surface,indicating an irregular distribution for the distances between surface atoms.Finally,we use STM manipulation to generate point defects on three-layer NaCl films by applying voltage pulses.The TERS imaging on the defects and their surroundings indicates the emergence of local vibrational modes associated with the defects and the influence region of the defects to the surrounding lattice.The TERS imaging resolution of the defect is as small as 3.8 ?.We also characterize the line defect structures at the edges of NaCl films with different layers by TERS,and study the evolution of phonon spectra correspondingly.These results are believed to be instructive for future studies of two-dimensional materials and their defect structures with atomic spatial resolution by TERS. |
PDF Full Text Request