Characterization Of 2D Materials By Combining Scanning Tunneling Microscopy And Raman Spectroscopy

The discovery of graphene and its novel properties has inspired broad interest in the research on two-dimensional（2D）materials.Tremendous advances have been reported on the development of novel electronic devices and optoelectronic sensors based on 2D materials such as transition metal dichalcogenides（TMDs）,few-layer phosphorene and so on.In order to obtain in-depth understanding on the physical and chemical properties of 2D materials,cutting-edge experimental techniques have been developed,especially those with atomic scale spatial resolution.Scanning tunneling microscopy（STM）is a powerful tool for studying the structure and properties of 2D materials.However,it is usually difficult for STM to directly provide chemical information of nanostructures on surfaces.If we want to characterize and analyze 2D materials in more comprehensive and accurate ways,additional methods are necessary.Raman spectroscopy is an excellent alternative because its capability to identify the chemical information of molecules and nanostructures by detecting their characteristic vibrational fingerprints.Furthermore,the tip-enhanced Raman spectroscopy（TERS）,which combines STM and Raman spectroscopy,is capable to achieve a sub-nanometer Raman spatial resolution,and it has been proved a powerful tool to characterize the local vibrational properties of 2D materials.In this thesis work,we devoted our effort to upgrade an existing home-made low-temperature ultrahigh vacuum（LT-UHV）STM-TERS system before doing experiments.First,as originally the system has only one optical path with a 532 nm laser for Raman excitation,which limits the applicable plasmonic substrate and tip to Ag substrate and Ag tip.In this work we added a new 633 nm laser and upgrade the optical system to a dual-wavelength configuration,thus one can also use Au/Cu substrates and Au tip for TERS experiments.Second,we designed a vacuum transfer chamber,which can be used to transfer samples between our TERS system and other sample preparation systems such as molecular beam epitaxy（MBE）.With the upgraded system,we studied a serious of 2D materials by combining STM,Raman spectroscopy and TERS.Our work is mainly composed of the following three parts:First,we clarified the growth dynamics of antimony on noble metal surfaces including Ag（111）and Cu（111）.As reported recently,antimonene can form on these two substrates in a buckled honeycombβphase with highly strain.However,it is hard to be judge whether there is alloying between antimony and Ag/Cu by STM images alone.In this work,by scanning tunneling spectroscopy（STS）and in-situ Raman spectroscopy measurements,we provide compelling evidences that these highly strained"β-antimonene"reported in previous studies are both surface alloys,namely Ag₂Sb and Cu₂Sb.Instead,we found thatα-antimonene can form on both alloys surfaces,which were not reported previously in these two systems.Theα-antimonene exhibits different strain induced by Ag₂Sb and Cu₂Sb substrates.Second,we used TERS to characterize theαphase borophene on Ag（111）,which usually co-exist in small patches with other borophene phases on Ag（111）and it is impossible to measure its Raman spectrum by conventional Raman spectroscopy.Remarkably,a series of Raman peaks were discovered in the TERS spectra ofαphase borophene,exhibiting obvious difference with other flat phases of borophene,especially for those apparent TERS peaks at high frequency.Combining with DFT calculation,we identified theαphase as an asymmetric buckled structure,and explained the origin of those high frequency peaks in TERS.Third,we studied melamine molecule adsorption and self-assembly structures on Ag（111）surface by STM combined with TERS and calculations.We proved that the previously reportedαandβphases of melamine self-assemblies on Ag（111）have flat-lying and mixed up-standing/tilted configurations,respectively.Moreover,we found that dehydrogenation and standing up of melamine molecules can be induced not only by the thermal annealing,but also by the STM probe tip with a controllable photo-assist dehydrogenation process.