Study On Intermolecular Energy Transfer With Ultrahigh Spatial Resolution

Energy transfer is one of the core topics in photophysics and photochemistry,and plays a critical role in many natural and artificial systems.Intermolecular energy transfer refers to the energy migration and transfer processes between a donor molecule and an acceptor molecule.It can occur through a radiative process,when a photon emitted from the donor is subsequently absorbed by the acceptor,or through a non-radiative process,when the energy transfer happens due to the interaction between the molecules.Research on the nonradiative energy transfer has attracted great attension for a long time.According to the types of intermolecular of interactions,non-radiative energy transfer processes could be roughly divided into two categories,the Forster energy transfer due to the Coulomb interaction and the Dexter energy transfer due to the electron-exchange interaction.In terms of the interaction strength,it could also be classified into the hopping mode when the coupling strength is weak and the coherent excitonic coupling mode when the coupling strength is strong.Given its central role in artificial light-harvesting systems,fluorescence resonance energy transfer?FRET?technology,organic light emitting diodes?OLED?and other emerging industries,non-radiative energy transfer has been widely studied by optical spectral analysis and ultrafast exciton dynamics analysis techniques.However,owing to the diffraction limit in conventional optics,it is challenging to reveal in real space the energy transfer processes at the nanoscale.Scanning tunnelling microscope?STM?is a powerful and widely-adpoted tool in surface science and single-molecule science thanks to its ability to achieve atomic resolution.STM-induced luminescence?STML?technique,combining STM with an optical dection system,enables nano-imaging beyond the diffraction limit.In this thesis,by adopting a combined strategy of electronic decoupling with nanocavity plasmon enhancement,we obtain sub-nanometre-resolved single-molecule electroluminescence through the excitation of highly localized tunneling electrons in STM.By further combining with STM manipulation,we investigate the coherent dipole-dipole coupling in a constructed molecular homo-dimer as well as the emission property from the collective states in molecular oligomers.In addition,we also study preliminarily the energy transfer phenomenon between two different types of molecules.This dissertation is composed of the following four chapters.In chapter one,we introduce the research background of the thesis and the instruments for experimental studies.Firstly,we briefly present the basic knowledge of the de-excitation processes of a molecular excited state,the classification of intermolecular energy transfer and the practical significance of energy transfer.Then,the principle of STM and STML is introduced,and a broad review of the history and current status of STML is also presented.At the end of the chapter,we describe the instruments used in experiments and the main content of this dissertation.In chapter two,we begin with the realization of single-molecule electroluminescence in the STM.In our strategy,an ultrathin sodium chloride?NaCl?spacer is used as the decoupling layer to screen the interaction between the molecule and the metal surface,a single zinc-phthalocyanine?ZnPc?molecule is excited by the charge carriers from the STM tip.A silver?Ag?tip and the Ag?100?surface are used to generate localized nanocavity plasmonic fields with high intensity and appropriate energy.Finally,the emission from the molecule is coupled to the nanocavity plasmons and extracted from the cavity at the far field.Owing to the highly localized excitation by the tunneling electrons,it is possible to obtain sub-nanometre-resolved spectroscopic images of a single molecule,which reflects the local optical response from the transition dipoles of the molecules.In addition,we also analyze the excitation mechanism of single-molecule electroluminescence on the basis of bias-dependent STML spectra.The molecule is supposed to be excited by the scattering of the tunneling electrons at low bias and by carrier injection?or sequential tunneling?at high bias.The realization of single-molecule STML makes it possible to study the intermolecular interaction and the interaction between plasmons and excitons.Then,we artificially construct a ZnPc dimer by STM manipulation and investigate the electroluminescence spectra from the dimer.From an isolated ZnPc monomer to a coustructed dimer,the spectral features change dramatically with the single emission peak from a monomer split into five emission modes and the site-dependent intensities of these modes.In combination with theoretical analyses and simulations,we find that the exciton splitting can be attributed to the coherent dipole-dipole coupling of the transition dipoles of the individual molecules in the dimer.The luminescence patterns obtained for the excitons reveal the local optical response of the system that is dependent on the relative orientation and phase of the transition dipoles.These finding provide detailed spatial information about coherent dipole-dipole coupling in molecular systems,which enable a deeper understanding and rational engineering of light-harvesting structures and quantum light sources.In chapter three,we go on to explore the optical properties of chains of chromophores.With STM manipulation,ZnPc oligomers of up to 12 sub-units are constructed.Firstly,we focus on the emission from the in-line in-phase superradiance modes.The sub-nanometre-resolved photon maps of the superradiance modes suggest that coherence is well maintained in these multi-molecule systems and the exciton is delocalized over each sub-unit molecule.The photon correlation measurement was carried out with a home-built Hanbury Brown and Twiss?HBT?setup.The antibunching dips were observed in the second-order correlation functions,which reveals the single-photon behavior of the superradiance emission.This is the first demonstration of single-photon superradiance from a single molecular chain.Further analyses of the linewidth and the peak position with increasing number of sub-units show linewidth narrowing and peak red-shift expected on the basis of the exciton delocalization and the specificity of STML.We also study the emission behavior of the other collective modes of the oligomers apart from the superradiance mode.The luminescence patterns of these collective modes also reveal the spatial distribution of the coupling of the dynamic transition dipoles.Our results provide useful information to the design of molecular quantum systems.In chapter four,we study the energy transfer processes between heterogeneous molecules with single-molecule STML technique.Through analyses of site-dependent STML spectral features for a heterogeneous dimer composed of a Zinc tetraphenylporphyrin?ZnTPP?and a free-base phthalocyanine?H₂PC?,we find that the energy could be transferred from a ZnTPP molecule to a H₂PC molecule.Then we focus on the energy transfer between H₂Pc molecules and ZnPc molecules,a system with better configuration control.The spectral analyses provide clear evidence of energy transfer processes in the sequence H₂Pc-Qy state?ZnPc-Q state?H₂Pc-Qx state.Both time-dependent energy transfer behavior and the spatially resolved peak shifts indicate that energy transfer is associated with the dipole-dipole interaction.Furthermore,we construct a heterogeneous trimer composed of two ZnPc molecules and a H₂Pc molecule:ZnPc-ZnPc-H₂Pc.When we excite the trimer at the center of the first ZnPc molecule,where the tip is far away from the H₂Pc molecule,a clear evidence of energy transfer is observed,but further analysis is needed to reveal whether coherent excitonic coupling exists in such heterogeneous structures.The ability to study the energy transfer process at the single-molecule level in real space opens up a new route to the design and development of molecular optoelectronic devices.