Angstr(?)m-Resolved Single-Molecule Tip-Enhanced Raman And Photoluminescence Imaging

Aspirations for achieving atomic resolution with light have been a major force in shaping nanophotonics,although this goal was once considered to be unreachable due to the diffraction limit.The advent of tip enhanced photoluminescence(TEPL)and Raman spectroscopy(TERS)based on scanning probe microscopy ignited great hopes for reaching atomic resolution.The spatial resolution is no longer limited by the diffraction limit,but depends on the ability to spatially confine the local field underneath the probe,which becomes quickly localized as the probe's size and its separation to the sample are decreased.Recently,sub-nanometer spatial resolution was successfully demonstrated in the TERS imaging of a single molecule on metal surfaces,so the next challenge is to explore what is the ultimate limit of spatial resolution and how this technique can be best utilized.On the other hand,unlike the light-scattering process in TERS,TEPL imaging with angstrom resolution has not been realized to date because fluorescence is usually expected to be strongly quenched in the very immediate vicinity of metals.In this thesis,through the precise control of the confinement and enhancement of nanocavity plasmons,we successfully push the spatial resolution of single-molecule Raman imaging further down to 1.5 A at the single-chemical-bond level and further propose a new methodology for structural determination,which we have called "scanning Raman picoscopy".By further combining with molecular electronic decoupling,we also realize sub-nanometer-resolved(8 A)single-molecule photoluminescence imaging.Our findings provide new routes to reveal the structure of matter and the nature of light-matter interactions at the atomic scale.The dissertation is composed of the following four chapters.In chapter one,we briefly introduce the research background and experimental basis of this thesis.Firstly,the system components and working principle of STM as well as its basic application are briefly introduced,and then a broad overview of plasmon enhanced photoluminescence and Raman scattering including its basic principle and current status is presented,especially surface and tip enhanced spectroscopy.Finally,we describe the experimental setup and the main content of this dissertation.In chapter two,we probe the ultimate limit of spatial resolution on TERS imaging by selecting a magnesium porphine(MgP)molecule as the model system.By developing a TERS system operating at liquid helium temperatures,we not only have better control over the structure of the plasmonic nanocavity,but also can make the tip closer to the molecule,thus making the local field stronger and more confined.Such technical advancements allow us to push the spatial resolution of single-molecule Raman imaging further down to 1.5 A,enabling vibrational imaging at the single-chemical-bond level.Furthermore,we go on to propose a new methodology for structural determination,which we have called "scanning Raman picoscopy(SRP)",that can be utilized for visually constructing the chemical structure of a single molecule.It is achieved by taking advantage of three key elements.First,the full mapping of individual vibrational modes with angstrom-level resolution allows the placements of atoms or chemical bonds to be visually determined.Second,the position-dependent interference effect for local symmetric and antisymmetric vibrations enables the connectivity of the chemical groups involved to be identified.The third element is the combination of spectromicroscopic images and Raman fingerprints for different chemical groups that conclusively ensures the definite arrangement of constituent components of a single molecule.The terminology scanning Raman picoscopy(SRP)highlights the power of both Raman-based scanning technique and delicate structural determination for single molecules in real space.We demonstrate that the construction of a single MgP model molecule requires only a few vibrational images through a simple Lego-like building process.The protocol established here can be generalized to other molecular systems,particularly with the aid of artificial intelligence and machine learning techniques.The SRP protocol not only provides new means for determining the molecular structures and studying the process of surface physics and chemistry as well as catalysis reaction at the single-chemical-bond level,but also offers new ways for high resolution imaging and structural determination of biomolecules.In chapter three,we explore the spatial resolution of TEPL imaging by selecting a zinc phthalocyanine(ZnPc)molecule as a model system.Because the photonic density of state(PDOS)affects directly the radiative properties of a molecular emitter in the nanocavity and the modification of PDOS is expected to be dictated by atomic-scale features of tip apex,the key to overcome the fluorescence quenching and realize high-resolution TEPL imaging is to have exquisite control over both the tip-apex structure and the electronic state of the molecule in the nanocavity.To address these challenges,the structure of plasmonic nanocavity is further precisely modified,particularly regarding the atomistic structure of tip apex.Through tip-apex engineering with an atomistic protrusion and the spectral matching of the nanocavity plasmon mode to both the laser excitation and the PL emission,as well as by further combining with proper electronic decoupling from the metal substrate via three-monolayer-thick sodium chloride dielectric spacer,we succeeded in suppressing both fluorescence quenching and disturbance from far-field background noise and realize for the first time sub-nanometer resolution(8 A)in single-molecule TEPL imaging.With such a junction design and control,the molecular fluorescence is found to exhibit a monotonically enhancing behavior rather than becoming quenched even for a tip-molecule distance of less than 1 nm,as the tip approaches the molecule.In combination with theoretical analyses and simulation,highly localized and enhanced electromagnetic field is generated in the nanocavity formed by the tip with atomistic protrusion and metal substrate,as a result of both the nanocavity plasmon resonance and the atomic-scale lightning rod effect.The local mode volume is thus squeezed down to less than 1 nm3.The significantly increased PDOS in the plasmonic nanocavity leads to mass enhancement on both excitation and emission rates.These effects not only beat fluorescence quenching at very close tip-molecule distance,but also enable to realize the sub-nanometer-resolved TEPL imaging.In other words,to achieve a resolution down to sub-nanometer,both the size of the tip and its distance from the target must be also on the same scale.In addition,we also realize TEPL spectroscopic imaging with sub-molecular resolution and reveal the subtle influence of plasmon-exciton coupling on the fluorescence intensity and peak position as well as linewidth.These findings achieve a longstanding goal of resolving the inner structure of a single molecule using photons in the field of the scanning near-field optical microscopy and provide new routes to probe and manipulate local photonic environment and light-matter interaction at sub-nanometer scale,which are important in both aspects of basic science and possible applications for near-field spectroscopy and microscopy.In chapter four,we go on to explore chemical enhancement by studying the TERS spectra and imaging of a decoupled molecule using a ZnPc molecule on the three-monolayer-thick NaCl as a model system.The presence of the NaCl decoupling layer prevents charge transfer from the metal substrate to the molecule,the TERS spectra thus obtained from ZnPc on the NaCl are very similar to the powder Raman fingerprint.TERS images of a single ZnPc molecule on NaCl also reach sub-nanometer resolution and present different patterns depending on the symmetry of the vibrational mode.Furthermore,we measure the evolution of TERS spectra as a function of tip-molecule distance.When the tip approaches to the molecule until making a contact,the TERS signal is dominated by the electromagnetic enhancement.However,the TERS signal becomes suddenly enhanced when the tip is making a contact to the molecule,due to additional chemical enhancement arising from charge transfer.In this situation,the TERS intensities enhance by 15 to 25 times and Raman peak positions for some vibrational modes get slightly blue-shifted.In addition,we also observe the chemical enhancement on the first and second overtone and combination Raman bands.Surprisingly,the intensity of the second overtone and combination Raman peak is comparable to that of the first overtone and combination Raman peak probably due to the enhancement of the broad electronic Raman scattering underneath the second overtone and combination Raman peaks.These findings provide new means for studying the charge transfer process at the interface.