STM-based Spectroscopy At The Single-molecule Scale:from Raman Scattering To Electroluminescence

It is challenging to study the optoelectronic properties at the single molecule level due to optical diffraction limit in conventional spectroscopy.Thanks to the invention of scanning tunneling microscope(STM,highly localized excitations by tunneling electrons enable us to obtain sub-nanometer resolution in both topographic and spectroscopic measurements.We can selectively excite an isolated single molecule to realize electrically driven single molecule fluorescence.On the other hand,the collective electron oscillations in subwavelength metallic structures can trigger the localized surface plasmon resonance with subwavelength confinement,which could allow strong interaction with other nearby photonic elements such as single molecule quantum emitters.The amplification role played by the local plasmonic field enhancement is important not only for achieving single molecule electroluminescence,but also for detecting Raman scattering signals at the single molecule scale.The knowledge and ability to characterize these two optoelectronic phenomena with high spatial,temporal,and spectral resolutions will strongly benefit the development of promising optoelectronic applications.In the previous studies of our group,tip-enhanced Raman spectroscopy(TERS)and STM-induced luminescence(STML)techniques have been used to investigate the chemical structural heterogeneity and electroluminescence property of many molecular systems.However,most of the molecules studied are planar ?-conjugated systems that are lying flat on the surface.What remains less explored is the capability of TERS to probe the structural information of standing-up ?-conjugated systems and more importantly,the electroluminescence behavior of such systems under the STML setup.With such a motivation in mind,in the first part of this thesis,we select[n]cycloparaphylene([n]CPP)molecules as our research object since they are a group of hoop-shaped ?-conjugated macrocycle molecules that are believed to stand up on surfaces.However,while TERS studies reveal detailed structural information,the molecules turn out to be absent for STML.Therefore,in order to systematically investigate the molecular excitation mechanisms in STML,in the second part of the thesis,we turn to well-studied optoelectronic molecules and select platinum phthalocyanine(PtPc)molecules as the research object.The PtPc is found to be stable and show molecule-specific electroluminescence at both bias polarities,thus allowing to investigate the excitation mechanism in a systematic way.This dissertation is composed of the following four chapters:In Chapter One,we start by briefly introducing the basic concepts and principles regarding the molecular spectroscopy and surface plasmons.Then,we provide an overview of the different features of STM,e.g.high spatial resolution,manipulation,and scanning tunneling spectroscopy(STS).This is followed by an extended background introduction on STM-induced.luminescence(STML).In addition to a brief historic description of the STML studies on the metal,semiconductor,and molecular systems,we pay special attention to the role of the localized surface plasmons in the STM junction and excitation mechanisms.The chapter is concluded with a brief introduction of tip-enhanced Raman spectroscopy(TERS),particularly regarding its enhancement mechanisms and spatial resolution.In Chapter Two,we describe the experimental methods used for the study of the present thesis work.We first describe the techniques for the sample,preparation including molecular deposition and substrate cleaning.Then,we describe the techniques for tip preparation,since the tip quality is crucial for successful TERS and STML measurements.This is followed by a detailed description of the characterization techniques by STM,TERS,and STML,particularly on the optical setups and optical alignment procedures.In Chapter Three,we utilize the TERS and STML techniques to probe the structural information and electroluminescence properties of[n]CPP molecules with standing-up ?-conjugated systems on surfaces.We first use TERS to study the adsorption configurations of CPP molecules on different metal substrates.TERS spectra for[12]CPP on Cu(100)are found to be always similar over the hoop.Their vibrational fingerprints also resemble well with the powder Raman pattern in terms of peak positions and thus indicate that the hoop structure is not destroyed upon deposition on the metal surface.However,distinctive spectral features around the 3000 cm-1 region are observed when the tip is located at the molecular center.Additional low-energy peaks associated with C-H stretching vibrations show up when the tip is brought very close to the molecule,suggesting that TERS is capable of detecting Raman signals from the bottom molecular components through tip height manipulation.On the other hand,for[12]CPP on Ag(110),the molecular shape is found distorted because of the lower symmetry of the substrate,showing two-bright and two-dark lobes in morphology,which can be visualized more clearly by using a CO-functionalized tip.Such changes in adsorption configurations can also be verified by the TERS spectra and spatial mappings,which can even resolve the inner structures of the distorted CPP molecule.The last part of this chapter is devoted to the study of the fluorescence property of CPP molecules by STML.Nevertheless,no electroluminescence was observed from such standing-up ?-conjugated systems,even when the molecules are well-decoupled from the underneath metal substrate by a dielectric NaCl spacer.Possible mechanisms are discussed regarding the absence of the electroluminescence by STML,e.g.,dipole-forbidden symmetry or fast internal conversion,vibrational relaxation,or intersystem crossing.In Chapter Four,we investigate the electroluminescence behavior of a single platinum phthalocyanine(PtPc)molecule with the STML technique.The molecule is found to show a bipolar emission behavior on NaCl/Ag(100),which enables us to investigate the excitation mechanisms at both bias polarities systematically.At negative excitation bias,the fluorescence occurs only from neutral molecular states judging from the peak energy observed.By combining voltage-dependent STML measurements with differential conductance data related to the energy diagram in the junction,the fluorescence behavior can be categorized into three voltage regions with three different excitation mechanisms.Strong fluorescence in region(I)is caused by the carrier-injection(CI)mechanism,moderate fluorescence in region(II)by the inelastic electron scattering(IES)mechanism,and weak fluorescence associated with an up-conversion process by a combined(IES+CI)mechanism involving the spin-triplet as a relay state.At positive excitation bias,the sharp molecule-specific emission peak at-911 nm is attributed to the anionic emission originated from the LUMO+1?LUMO transition,whose excitation is dominated by a CI mechanism with electrons firstly injected into the LUMO+1 or higher-lying empty orbitals.These results provide comprehensive fundamental understanding on the excitation mechanism of single-molecule electroluminescence and may shed new light for the development of novel molecular optoelectronic devices.This thesis intends to be primer on TERS and STML techniques in molecular nano science research,covering both the background of physics and practical experimental approach.By including the most useful tips and caveats of the trade,I hope to provide basic background knowledge that is immediately useful in the laboratory.Last but not least,this thesis is a stepping stone to tackle more complex phenomena of STML and TERS in the future.