Probing Bond Breaking And Making In Single Molecules By Tip-Enhanced Raman Spectroscopy

Bond breaking and making are central in chemical reactions.Probing the bond breaking and making process and related structural changes at the single-molecule level can provide deep insights into the reaction pathways and mechanisms.Tip-enhanced Raman spectroscopy(TERS)is a combined technique of scanning probe microscopy and Raman spectroscopy,which can significantly enhance the Raman signal of target molecules in a nano-cavity due to strongly localized plasmonic fields,thus exhibiting high spatial resolution and sensitive chemical identification simultaneously.With the rapid developments of TERS in recent years,its spatial resolution has reached Angstrom level,enabling to identify the vibration mode of a single chemical bond within a molecule and even to reconstruct its molecular chemical structure.In this dissertation,we use the latest developed TERS technique to make a proof-of-principle demonstration on probing the bond breaking and making process as well as corresponding structural changes of horizontally and vertically adsorbed single molecules at the single-chemical-bond level.For vertically adsorbed molecular systems,we also investigate the vertical resolving ability of TERS.Furthermore,we use tip manipulation to induce the vertically adsorbed molecule to transform into unknown species and demonstrate the capability of sub-nanometer resolved TERS to structurally reconstruct the possible adsorption configurations for these unknown species.All these results will provide scientific basis for the application of TERS in the fields of surface reaction,surface catalysis,chemical adsorption,and molecular structural recognition.The dissertation is composed of four chapters:Chapter one is the introduction of this dissertation,mainly describing the research background,experimental equipment and so on.In this chapter,we first briefly introduce the basis of scanning probe microscopy,including scanning tunneling microscopy and atomic force microscopy.Then we briefly introduce the basic concepts of plasmonics and the developments of plasmon enhanced Raman spectroscopy,including surface enhanced Raman spectroscopy and tip enhanced Raman spectroscopy.The chapter is concluded with an introduction on the experimental equipment used in this dissertation and the main research content of this dissertation.In chapter two,we select the horizontally adsorbed magnesium porphine(MgP)molecules on the Ag(100)substrate as a model system for exploring the bond-breaking and making process within a single molecule.By precisely positioning the tip at the specific site of the molecule,we can induce the dehydrogenation of MgP by applying a+1.0 V bias within the tunneling junction under laser illumination.It is worth noting that the dehydrogenation process is reversible.By applying the same experimental conditions,the structure of a single MgP molecule can be switched from free-base to single,double,triple,and quadruple dehydrogenated configurations in a controlled manner.The above dehydrogenation and hydrogenation reactions are characterized by TERS.We find that the MgP molecules with different dehydrogenation configurations always show different TERS spectral characteristics.Especially,the Raman peaks related with the carbon-hydrogen(C-H)vibrational modes disappear due to the breaking of corresponding C-H bonds at each dehydrogenation site.In this chapter,we demonstrate the ability of TERS in studying the on-surface reactions of a horizontally absorbed single molecule by identifying the bond breaking and making process at the single-bond level.In chapter three,we select the melamine molecule on the Cu(100)surface as a model system for TERS studies of vertical up-standing molecules.Up to date,most studies on sub-nanometer resolved TERS are mainly performed on planar molecules lying flat on surfaces.However,when a molecule is chemisorbed on a substrate,its structure is likely to change due to the chemical bonding to the surface atoms,and the molecule may even stand up through the bond breaking and making processes.In this chapter,we investigate the vertical resolving ability of TERS by using upstanding single melamine model molecule that chemisorbed on the Cu(100)surface,and demonstrate the ability of TERS in tracking in situ bond breaking and making as well as structural changes.By combining TERS spectra and mapping images with theoretical calculations,the up-standing configuration can be validated for the chemisorbed single melamine molecule involving a dehydrogenation process at the bottom.By tracking spectral evolution,we can even monitor the photochemical reaction of the dehydrogenated molecule under laser illumination,specifying the breaking of one N-H bond during the transformation into its tautomer through hydrogen transfer.Laterally,the spatial distribution of different vibrational modes are obtained through TERS mapping.Vertically,we can resolve the N-H stretching vibrations as well as other vibrational modes at different heights,and also estimate the TERS detection depth of about 4 A by probing the bottom N-H bond.The ability to access the structural information of an up-standing molecule on surfaces in three dimensions,as demonstrated here both laterally and vertically,offers the promise of TERS applications to non-planar stereo molecules.Our research results show again that TERS is powerful for investigating surface chemical reactions.In chapter four,we further demonstrate the ability of TERS in identifying the structure of unknown species or molecules.The characterization of the chemical structure for single molecules has always been an important research field.For a single molecule whose structure is unknown,whether TERS can resolve its molecular structure still remains unexplored.In this chapter,we further induce the reaction of dehydrogenated melamine molecules under laser illumination by applying positive bias,transforming them into two unknown molecular structures named as S1 and S2 molecules.For these two unknown molecules,we can always observe Raman peaks slightly larger than 2000 cm-1 in their TERS spectra.Combining with theoretical calculations,the Raman peaks in this region can be attributed to the C≡N stretching vibration mode,indicating the existence of C≡N bond in the unknown species.Through TERS mapping images,we can resolve the spatial distribution of C≡N and N-H bonds in both molecules.Further combination with theoretical simulations allows us to roughly determine the molecular structure of these two unknown molecules.These results shows the structural identification ability of TERS for unknown molecules,which may have potential applications in surface reactions as well as in resolving biological molecular structures.