Local Vibrational Properties of Low-dimensional Materials Studied by Tip-enhanced Raman Spectroscopy

Author:Sheng Shao Xiang

Supervisor:wu ke hui


Degree Year:2018





With the development of science and technology,we have entered the nano era.Various new low-dimensional materials are emerging.Two-dimensional materials,represented by graphene,have achieved great breakthrough and progress in recent years.They possess many excellent optical,electronic,and magnetic properties,which are expected to shine in the next generation of electronic and optoelectronic devices.While the characterization of their properties at nanoscale puts up new challenges for the experiment instruments at the same time.Tip-enhanced Raman spectroscopy(TERS),which combined with the scanning probe microscopy(SPM)and Raman spectroscopy,processes the spatial resolution and the capability to access chemical information with ultrahigh sensitivity.It is a powerful tool to study the photon,phonon,electron,plasmon and their interactions,to characterize the nanostructures and nanoptical properties at atomic scale.In this thesis,we mainly focus on building a low-temperature ultrahigh vacuum(LT-UHV)TERS system,which combined with the molecular beam epitaxy(MBE)for in-situ growing low-dimensional materials,and using TERS to study the local structures and vibrational properties of the materials.First,we built one high performance LT-UHV-TERS system,which based on a low-temperature scanning tunneling microscopy(STM)and MBE union system.In order to achieve the TERS function,we modified the STM scanner design,which was with hard contact to the cryostat.On the one hand,considerable space can be reserve for installing the aspherical lens closer to the scanner.On the other hand,it can improve the efficiency of heat exchange between the scanner and the cryostat,thus stabilizes the scanner temperature.We can drive the len for focusing the laser onto the junction by a homemade three-dimensional(3D)adjustable piezoelectric motor.The laser is illuminated at 30°with the sample surface,and the scattered light is collected with a backscattering configuration.The Raman optical is fixed on the STM platform,connecting with the laser and spectrograph through fibers.To compensate for the insufficient noise isolation of the STM scanner,we designed multi-level damping system outside the STM chamber.Then we used the homebuilt TERS system to study the vibrational properties of silicene on Ag(111)surface.TERS shows a different enhancement behavior on different Raman modes.It can largely enhance the out-of-plane vibration modes only,in which the?zz component of the Raman tensor is nonzero.Combined with the first principle calculations,we analyzed every Raman modes,and their origination.Using silicene as the model,we got 109 TERS enhancement factor,and subnanometer spatial resolution.The ultrahigh sensitivity of TERS allowed us to identify vibrational properties of different silicene phases,which differ only in the bucking direction of the Si-Si bonds.Local vibrational features from defects,domain boundaries and strains in silicene were also distinguished by TERS.We studied the Raman spectroscopy of boropheneβ12 sheet andχ3 sheet for the first time,and assigned the Raman modes combined with TERS and calculations.The light-element boron has strong electron phonon coupling,which causes large Raman peaks width.We calculated the electron phonon coupling strength in borophene,about 0.5,which located between AlB2 and the high temperature superconductor MgB2.The calculated superconducting transition temperatures ofβ12 andχ3 sheet are about 3.1 K and 4.1 K,respectively,which may decreased due to the substrate interaction.We also got 109 TERS enhancement factor and studied the substract effect on the local structures and properties of borophene by TERS.The rarely appeared?sheet also could be identified.Combined with TERS and non-contact atomic force microscopy(nc-AFM),we proved the pentagonal nature of the self-assembled silicon nanoribbons and magic clusters on Ag(110)substrate.The same TERS spectra of Si single-strand and double-strand nanoribbons(SNR,DNR)indicated that they have the same atomic structures.The pentagon-ring structures were visualized directly by nc-AFM.Moreover,the vibrational fingerprints of individual Si cluster was retrieved for the first time by subnanometer resolution TERS,and the pentagon structure was located on an Ag di-vacancy.In addition,when more Ag atoms were removed underneath the cluster,it became unstable and could transfer into the NR structure spontaneously.Thus,this model can explain the dynamics transition from cluster precursor to nanoribbon very well.