Development Of Multifunctional Tip-Based Microscopy And Its Applications

The structural and chemical heterogeneities and functionals of molecules originate from various intrinsic properties,such as electronic states,vibrational states,and spin states of chemical bonds.These states are commonly sensitive to the interactions with environments and external fields,like electric field,magnetic field,and light,accompanying subtle variations and even chemical transformations.Measuring these intrinsic parameters in multi-domains globally with high precision is of fundamental importance for the comprehensive understanding of the elementary reactions in synthesis of functional materials and their applications in molecular electronics and quantum devices.Owing to the complexity,it is still a great challenge to realize the global and precise characterizations of these parameters in multi-domains,in particular,the dynamic processes of molecules before and during chemical transformations.Such measurements with a high precision for these essential quantum states are also important and at the frontiers for the potential applications in quantum technology.In my thesis,it describes our studies on the development of the instrument by combining techniques of scanning tunneling microscopy(STM),atomic force microscopy(AFM),and tip-enhanced Raman spectroscopy(TERS),that is,STM-AFM-TERS instrument,and present the investigation using this state-of-the-art technique.Our results show the powerful capability of the technique that enables us to characterize globally the structural and chemical heterogeneities of pentacene molecules and derivatives on Ag(110)surface,including the properties in electronic states,chemical structures,and chemical transformations with high spatial and energy resolutions.Our results also highlight the possibility to trace chemical transformation in a time scale of about 25 ms.The extension of this technique to explore the widely concerned system of oxides is also introduced.Chapter 1 briefly introduces the tip-based techniques and their applications,including the advantages and limitations of STM,AFM,and TERS.After a short review of the progress in precise measurements of intrinsic parameters in multi-domains and the demanding instrument in development of combined STM,AFM,and TERS techniques,we show the necessity,urgency,technical challenges,and possibilities to develop the instrument in a more comprehensive way.Motivated to this goal,our analysis shows that in principle the tip-based techniques of STM,AFM,and TERS are at the cutting-edge to be combined together to capture the intrinsic parameters of molecules in multi-domains,which may break through the limitation of a single technique.Chapter 2 briefly introduces the development of the first instrument by combining STM-AFM-TERS techniques during my Ph.D program.The technical specifications of the instrument were tested and finely adjusted.The overall parameters are given below:the spatial resolution of all STM,AFM,and TERS is about 1.5 A,and the realspace imaging speed of Raman spectroscopy reaches 25 ms/pixel.These parameters of our developed instrument,in accordance with our designed specifications,may represent the highest level in the world wide nowadays.This instrument provides us a powerful tool to characterize molecules nearly simultaneously for their intrinsic properties in different domains,like electronic states,skeleton of molecular structures,chemical recognition of different chemical species.It should be highlighted that the use of single-photon avalanche photodiode(APD)with an edge-tunable band-pass filter to record the TERS signals allows us to achieve a time resolution of about 25ms/pixel,which shortens the acquisition time of TERS by 2 orders of magnitude in comparison with the commonly used charge-coupled device(CCD)spectrometer.Chapter 3 shows the measurements using the function of STM unit in characterizing the structural and the electronic structures of pentacene molecule and its derivatives on Ag(110)surface.The derivatives of pentacene were produced by applying voltage pulses of about 2 V.The STM measurements reveal that the electronic structures of the three species are quite different.The high-resolution spatial distributions of their molecular orbitals were also measured using a CO-decorated STM tip.These tests show that the STM function in the combined instrument reaches the designed parameters,including their electronic and spatial resolution.However,it is seen that the single technique of STM is insufficient to determine the exact chemical structures of the derivative species.Chapter 4 shows the measurements using the AFM unit of the chemical structures of pentacene and its derivatives.In the measurements,the CO-decorated qPlus-AFM tips were used.Our AFM results clearly show the molecular chemical structure at the single-bond limit,which reveals the A-level changes in of the benzene rings of pentacene and its derivative,as well as the differences in brightness of protrusions located around the C-H bonds of the central benzene ring.These results allow us to propose two conjectures:C-H bond breaking at the central benzene ring with skeleton deformation or the opening of the central benzene ring with atom rearrangements.Although these results indicate the AFM function in the combined instrument work well with our designed parameters,the AFM results even joint with the STM results cannot give decisive conclusions about the exact structures of the different species,which certainly demand for the chemical resolution.Chapter 5 presents the characterizations of the combined functions of TERS with STM and AFM to investigate the pentacene and its derivatives by measuring the parameters in multi-domains.Our results show that the use of the state-of-the-art technique by combination of STM-AFM-TERS allows us to obtain global parameters,including highly structural resolution(STM-AFM-TERS)in real space,energy resolution in electronic states(STM)and vibrational states(TERS)in energy,as well as the bond-resolved skeletons(AFM).The combined STM-AFM-TERS measurements enable us to unambiguously determine that with the dehydrogenation process of a single C-H at the central benzene ring there is a lateral shift of 0.89 A for the left C-H stretching mode on the other side.The experimental results provide a better reference for the parameter selection,such as the voltage window in the simulated STM images and the effective charge at the CO decorated AFM tip apex in the simulated AFM images.These key parameters are also crucial for modelling the systems in the theoretical calculations and simulations.Our results reveals that the C-H bond breaking at the central benzene ring and the intact of conjugation of benzene ring involves two quanta during light illumination,in agreement with the observed voltage threshold of about 1.5 eV for the C-H bond breaking during voltage-pulses applying.These quantitative parameters indicate an activation energy is higher than the 3.0 eV,in excellent agreement with the theoretically predicted activation energy of 3.01 eV.The involvement of two quanta of plasmons could be reasonable to provide enough energy for the transformations of pentacene,where one may expect that the tip-induced nanocavity plasmons are responsible for the transformations of pentacene.Chapter 6 demonstrates the extended application of the combined STM-AFMTERS technique in a much complex iron oxides.Three different iron oxide islands were grown on Ag(100)surface using a MBE method.The electronic structures and local chemical bonds of these three iron oxide islands were measured by STM and TERS.It is preliminarily determined that two of them are wüstite with different surface terminals,while the other is magnetite with surface rearrangement.By increasing the oxygen pressure and the substrate temperature,the wüstite islands with three types of defects on the surface were highly reproducibly obtained.The localized chemical bond vibration modes of the defects were measured by TERS to correlate two different defects measured by STM.Our results indicate that the combined STM-AFM-TERS technique could be applicable to investigate the oxide surfaces.We expect that this combined techniques can be further used to characterize molecular species and their reactions in such oxide surfaces.Chapter 7 gives the main conclusions and a brief outlook.The combined STMAFM-TERS technique in this thesis can provide precise measurement of intrinsic parameters of molecules in multi-domains,and can be further applied in characterization of much complicated oxide systems that are widely concerned in oxide catalysis.Finally,the outlook of the STM-AFM-TERS about the technically developing trends and its possible applications in surface/interface catalysis and on-surface chemistry is given.