Multi-Property Characterization Of Single Molecules And Their Reactions Using Multifunctional Tip-Based Microscopy

A polyatomic molecule is composed of two or more atoms connected by chemical bonds and represents one of the smallest units that constitute a substance.Even at the level of single molecules,many parameters are needed to describe the structural and chemical properties,not only recogonition of different atoms and bond orientations in molecular skeletons,but also electronic,vibrational,and spin states.Realization of globally characterizing such intrinsic parameters is of fundamental importance in comprehensive understanding of the reaction mechanisms and the functional properties of commonly complicated molecules,and also has profound implication in various applications.Since the inventions of scanning tunneling micrsocopy(STM)and atomic force microscopy(AFM)in 1980’s,STM and AFM have been powerful tools in atomistic measurements of structural and chemical properties of single molecules.However,STM and AFM lack the chemical resolution,which sometimes lead to biased interpretation of the results.As an additional tip-based technique,recently developed tip-enhanced Raman spectroscopy(TERS)have shown its capability in chemical resolution because it provides the finger-print information of vibrations at chemical bond-resolution.The dreams to combining and even integrating these techniques have triggered world-wide study interests,for the integrated techniques of STM,AFM,and TERS can be an effective approach to characterize specific chemical bonds by providing possibly global parameters for interpreting the physical properties on the basis of the chemical recognition of molecules.However,it is still a challenging task to develop the integrated techniques,so far.Aim to this goal,we have focused on the development of integrated techniques of STM,AFM,and TERS,and have applied the techniques to systematically measure global parameters including structural,electronic,and vibrational properties with chemical-bond-resolved information,which have provided indepth understanding of C-H related reactions,intramolecular isotope effects,and recogonition of specific nonbenzenoid rings.The main achievements of this PhD dissertation are summarized below.1.We have designed and developed a "Three in One" probe sensor,realizing the integration of STM-AFM-TERS techquies.Beyond the switchable tips on the basis of joint STM-AFM-TERS strategy,we successfully developed the "Three in One" probe sensor that highly integrated the qPlus force sensor with a plasmonic tip to characterize specific molecular structures by collecting electronic,vibrational signals,as well as molecular skeleton.In order to achieve comprehensive measurements of multiple intrinsic properties of a single molecule at the single chemical-bond resolution,two breakthroughs were achieved in the technology:(1)We redesigned qPlus force sensor based on the E158 quartz tuning fork,which exhibit a high-quality factor(Q>40000)and high stability under ultrahigh vacuum and liquid helium conditions.(2)Two optical collection systems were developed for TERS measurements,including a spectrometer based on a charge-coupled device(CCD)and a single-photon high-speed counter based on an avalanche photodiode(APD).The spectrometer based on CCD allows for the energy resolution of about 2.7 cm-1,while the single-photon high-speed detector based on APD enables a spectral acquisition rate of about 25 ms/pixel.Thanks to the perfect performance of the STM-AFM-TERS integrated system,we successfully achieved multi-domain measurements of various intrinsic parameters such as the skeleton,electronic states,and vibrational states of a single pentacene molecules adsorbed on Ag(110)surface with a spatial resolution of～1.5? and a temporal resolution of a millisecond level.The realization in an inetrated manner of different tip-based technics provides a powerful tool in the field of molecular science and with possibly extended applications in catalysis.2.Using the joint STM-AFM-TERS method,we revealed the microscopic mechanisms of C-H bond activation of pentacene(C22H14)molecule adsorbed on Ag(110)surface.Theoretical calculation has predicted that the activation energy of CH bond is usually higher than 3.0 eV.Therefore,it is of vital important to explore C-H activation mechanisms.We studied the mechanisms of a single C-H bond breaking under different external fields.We compared the dehydrogenation processes of C-H bond breaking of single pentacene activated by tunneling electrons,visible light,and thermal annealing.We found that the breaking of the C-H bond can be triggered by tunneling electrons only when the electron energy higher than 1.5～1.6 eV,where the energy threshold is independent on the the polarity of bias voltages.By measuring the tip-induced light emission from tunnel junction under different biases,we found that tunneling electrons-induced dehydrogenation of pentacene is related to the plasmon excited by inelastic tunneling electrons.Furthermore,it was found that only when the STM tip was involved,visible light could induce the dehydrogenation reaction of pentacene,confirming the crucial role of plasmons in the C-H bond breaking.By controlling the gap distance,polarization and power of incident light,we confirmed that light induced C-H bond breaking was also involved in the excitation of the nanocavity plasmon.Our results show that the dehydrogenation process depend on the light intensity,and quantitatively resolved that the two photons participate the nonlinear process,which is consistent with the theoretically-predicted activation energy of about 3.0 eV for C-H bond.The experimental results reveal that the localized plasmons excited in gap contribute to C-H activation even under the inelastic tunneling electrons.Different from the reaction mechanism of a single molecule,it is found that sufficient events of C-H bond breaking happen in the thermal annealing experiment of the pentacene on the Ag(110)surface even after annealing at only 410 K.Based on the experimental observations,we propose two possible reaction mechanisms involved with silver adatom and super-hydrogenation of pentacene molecule,which are supported by our theoretical calculations.3.Using the joint STM-AFM-TERS method,we revealed intramolecular isotope effects with chemical-bond precision.After removal of two hydrogen atoms from the middle benzene ring of pentacene molecule C22H14 on the Ag(110)surface,a relatively stable species C22H12 was formed.STM-TERS was employed to compare the isotope effects of seven vibrational modes of C22H14 and its fully deuterated form C22D14 in the region of energy and real space.The results show that even within a single molecule,different vibrational modes exhibit totally different isotope effects in energy(Isotope effect IE=υH/υD=1.02～1.33).Combining potential energy distribution(PED)calculations,we found that the difference in contribution of H/D atoms in different vibrational modes determines their isotope effects in energy.Based on analysis of the PED spatial distribution,we also revealed the influence of isotope substitution on the spatial distribution of vibrational modes.4.In addition,we extended our research by combining surface synthesis with the joint STM-AFM-TERS method to investigate the dimer structures formed by pentacene on the Ag(110)surface.These dimer structures exhibit pentagonal rings,and we aimed to uncover the underlying chemical reaction mechanism,structure-properties relationships,as well as characterize the specific vibrational modes and spatial distribution characteristics of these pentagonal rings with chemical bond resolution.Intact pentacene molecules served as precursors,and through annealing at approximately 460 K on the Ag(110)surface,they underwent reactions leading to the formation of four distinct dimer structures exhibiting T-shapes.Using the joint STMAFM-TERS method,we conducted a systematic characterization of the skeleton,local vibrational modes,and electronic states of these dimer structures.Our findings revealed that the coupling site of the dimer molecule formed pentagonal rings.These rings cause not only significant spatial distortion of the molecular skeleton but also charge redistribution in molecules.Our experimental results show that the electronic states near the Fermi level in T-shaped molecules exhibit the characteristic that belongs to the lowest unoccupied molecular orbital(LUMO)of intact pentacene,indicating a strong charge transfer between the substrate and the T-shaped molecules.Notably,we revealed the Raman fingerprints related to the pentagonal rings in these T-shaped structures,with Raman shifts ranging from 586 to 599 cm-1 with spatial distribution highly relevant to the pentagonal ring.Our findings demonstrate the potential application of our method in detecting single non-benzenoid structural defects in graphene-based nanostructures.The dissertation consists of five chapters.Chapter one provides a concise introduction to the technical backgrounds of STM,AFM,and TERS,as well as the measurement of global parameters for a single pentacene molecule adsorbed on the Ag(110)surface using the joint STM-AFM-TERS method.Chapter two describes three technical breakthroughs aimed at integrating STM,AFM and TERS techniques.These include the construction of TERS optical systems,the design of the qPlus force sensor and the development of a "Three in One" probe sensor.Chapter three explores the three reaction mechanisms responsible for single C-H bond breaking,namely inelastic tunneling electron,plasmon,and thermal effects.Chapter four delves into the intramolecular isotope effect of pentacene-derivative species(C22H12)revealed with chemical-bond precision.Chapter five presents our extended research on carbon-based nanostructure,which combines surface synthesis with the integrated STM-AFM-TERS method,to systematically measure global parameters of dimer structures formed by pentacene on the Ag(110)surface.Finally,the dissertation concludes with the main findings and conclusions,followed by a brief outlook on the future prospects of the integrated STM-AFM-TERS method.