Probing Optoelectronic Properties Of Molecules With Different Charging States By Tip-Enhanced Spectroscopy

Charge transfer is ubiquitous in nature and plays an important role in photosynthesis,electrochemistry,and photovoltaic devices.The charge transfer process is inevitably accompanied by the change of charged state of the system,and the optoelectronic properties such as the molecular chemical reaction rates,catalytic activity and luminescent characteristics of materials show different behaviors under different charging states.Therefore,it is particularly important to control and probe the charging states of system.Usually,researchers combine the optical and electronic techniques with the electrochemical system to regulate and control the charging states of the system and probe their optoelectronic properties simultaneously.However,previous studies focus mainly on ensemble systems involving a large amount of molecules,and it is thus difficult to reveal the optoelectronic properties of the system and microscopic mechanisms behind at the single molecule level.Scanning tunneling microscope(STM)is a powerful tool for studying the surface science and single-molecule science at the atomic scale thanks to its ability to achieve ultra-high spatial resolution.Combining STM with optical excitation and detection system can break the diffraction limit of traditional optics and provide an excellent platform for us to probe and analyze the optoelectronic behaviors of the system in different charging states at the single-molecule scale.In this thesis,by adopting STM induced luminescence(STML),tip enhanced Raman scattering(TERS)and tip enhanced fluorescence(TEF)technique,we characterize the optoelectronic properties of single perylene-3,4,9,10-tetraca rboxylic dianhydride(PTCDA)molecules at neutral,singly or doubly negatively charged states systematically from different aspects,such as electronic states,vibronic states,and excitonic states,etc.Based on these results,by further combining with STM manipulation,we investigate the charge transfer and energy transfer processes for constructed molecular homo-dimer,hetero-dimer and trimer systems.Moreover,we also examine the STML and excitation mechanism of Q band and B band electroluminescence of single zinc-porphyrin(ZnTPP)molecule,which demonstrates that the intermediate cationic state plays an important role in the Q band emission for porphyrin molecules.The dissertation is composed of the following six chapters.In chapter one,we introduce the experimental techniques and research background of this thesis.Firstly,we briefly introduce the principles and application of STM,and then present the working principle and research progress of the tip enhanced spectroscopy technique,including STML,TERS and TEF techniques.Next,we present an overview of the research background in controlling and probing the electronic properties of different charging states.Finally,we introduce the instruments used in experiments and the major contents of this dissertation.In chapter two,the vibrational characteristics of a single PTCDA molecule adsorbed on 4 ML-NaCl/Ag(100)substrate at neutral,singly negatively and doubly negatively charged states are studied by using the TERS technique.We first measure the bias-dependent Raman and photoluminescence spectra,which are compared with the dI/dV spectra that reflect the characteristics of molecular electronic states.It is found that the PTCDA molecules adsorbed on NaCl are singly negatively charged(i.e.at-1 valence state),and strong fluorescence and Raman intensity could be detected only when the electrons could be injected into the singly unoccupied molecular orbital(SUMO)of the PTCDA molecule.Considering that the PTCDA molecule is already singly negatively charged itself upon adsorption on the 4-ML-NaCl/Ag(100)surface,as well as the good matching between the TERS spectra and the DFT calculation or the STML experimental results,it can be concluded that the TERS signal is derived from the resonance Raman scattering of the doubly negatively charged PTCDA molecule,while the fluorescence is derived from the radiative decay of Si?So transition for PTCDA2-molecule.Therefore,the vibrational characteristics of the transient doubly negatively charged state of PTCDA molecule are obtained by means of steady-state resonance Raman spectroscopy.By using the same experimental and analytical method,the vibrational characteristics of the neutral or singly negatively charged PTCDA molecule can be also obtained by adjusting the bias voltage to the singly occupied molecular orbital(SOMO)and SUMO-SOMO gap positions.By comparing the TERS spectra of different charging states of the PTCDA molecule,it was found that their Raman signals show significantly different intensities.Our results provide a method to study the charge transfer process at the single-molecule scale.In chapter three,we investigate the optoelectronic properties of PTCDA molecules at neutral and doubly negatively charged states by using STML and TEF techniques.Firstly,fluorescence and phosphorescence emission of neutral PTCDA molecules are achieved using STML by injecting holes into the highest occupied molecular orbital(HOMO),and the fluorescence and phosphorescence emission of doubly negatively charged PTCDA molecules are achieved by injecting electrons into different unoccupied orbitals of PTCDA-anionic molecules on the 4-ML-NaCl/Ag(100)substrate.So the electron spin inside the PTCDA molecule can be manipulated by the electron transport process between the tip and molecule,accompanying the formation of excitons with different excited states and spin multiplicities.Then,we study the vibronic coupling phenomenon in the doubly negatively charged PTCDA molecule by spatial spectral imaging and group theory analysis.Next,we investigate the photoluminescence of doubly negatively charged PTCDA molecules using the TEF technique,and we find that only when the radial plasmonic field satisfies both excitation and emission dipole condition,can the photoluminescence be detected,which reflects the direction-selective property of the nanocavity plasmon enhancement effect.No phosphorescence signal is detected in the TEF spectra through optical excitation.However,by constructing PTCDA dimers with different relative orientations,we find that phosphorescence by TEF can be achieved when the two adjacent PTCDA molecules are perpendicular to each other,which might be related to the influence of external carbonyl groups promoting the intersystem crossing,based on the analyses from the differential conductance spectra,position-dependent STML spectra and distance-dependent STML spectra of the PTCDA dimer.In chapter four,we further invesgate the energy transfer process in the PTCDA-ZnPc(zinc-phthalocyanine)hetero-dimer and between a PTCDA and a ZnPc-dimer using TEF technique.We find that the energy can be transferred one-way from high-lying excited state of PTCDA2-to ZnPc.Through precise STM manipulation,we study the energy transfer properties of PTCDA-ZnPc hetero-dimers with different relative orientations and find that the orientation of PTCDA molecule has a great influence on the interaction between them.When the short axis of PTCDA is aligned with the ZnPc lobe,the interaction is relatively weak and the energy levels of ZnPc is almost unchanged whether PTCDA is at-1 or-2 valence states.However,when the long axis of PTCDA is aligned with the ZnPc lobe,it is found that different valence states of PTCDA have different influence on the energy levels of ZnPc molecule,and the PTCDA at-2 valence state causes splitting of the energy levels of nearby ZnPc.Further research on energy transfer process between a PTCDA and a ZnPc dimer also supports these results.In chapter five,we demonstrate the realization of the B band emission of a single zinc-porphyrin(ZnTPP)molecule by using an aluminum tip with weak absorbance at short wavelengths to tune the plasmon mode to the blue light region,which provides effective plasmon enhancement for the S2? S0 transition that arises from the radiative decay of high-lying excited state and is regarded as the "azulene anomaly" fluorescence By combining the bias-dependent STML spectra and differential conductance spectra,we reveal different excitation mechanisms for the Q-band and B-band electroluminescence.The former for the Q-band electroluminescence undergoes a two-step carrier injection process,which indicates the involvement of an intermediate positively charged cationic state in the excitation process.The latter for the B-band electroluminescence is an inelastic electron scattering process,and the molecule stays in the neutral state during both the excitation and emission processes.Our work provides a route to understand the photophysical properties and the dynamic behaviors of isolated molecules in their excited states.In chapter six,we summarize the major work presented in this thesis and provide a prospect for future research.The optoelectronic properties of molecules in different charging states have been characterized systematacially using different tip-enhanced spectroscopy techniques.Our results provide a new route for studying the charging and energy transfer processes in molecular systems at the single-molecule scale,and also offer important information for understanding the microscopic mechanisms of organic optoelectronic devices as well as for their performance optimization.