Raman Spectroscopy Study On Interlayer Coupling And Defects Of Two-Dimensional Materials

The physics of two-dimensional(2D)materials and heterostructures based on such crystals has been developing extremely fast in recent years.With these new materials,truly 2D physics has begun to appear.Novel heterostructure devices—such as tunneling transistors,resonant tunneling diodes,and light-emitting diodes—are also emerging.Composed from individual 2D crystals,such devices use the properties of those materials to create functionalities that are not accessible in other heterostructures.The coupling between layers is the key factor determining the physical properties of two-dimensional material electronic structure,band gap,adjustable quantum Hall phase,etc.In addition,various defects can have a large impact on the electrical,magnetic,and quantum transport properties of two-dimensional materials.In this paper,several typical two-dimensional material systems,including type ? Weyl semimetal(transition metal chalcogenide)WTe2,topological nodal line semimetal ZrSiTe,and Graphene/MoS2 heterostructure will be discussed.We mainly use Raman spectroscopy to investigate the physical properties such as interlayer interaction and defects.In chapter three,we have performed a comprehensive and systematic investigation of few layers WTe2 flakes by combining Raman scattering and density functional theory(DFT)calculations.We observed Raman signatures corresponding to the structure transition from noncentrosymetric to centrosymetric phases occurring when the thickness is scaled down from bilayer to monolayer Furthermore,we showed that the polarized Raman scattering features strong angular dependence on the WTe2 film thickness.DFT calculations reproduce well the experimental results,including the presence of the A9(A1)and Bg(A2)modes located between 75-95 cm-1 in few-layer samples from 1L to 4L.Our results show that high-quality Raman spectra can be used not only as a reliable fingerprint for the determination of the thickness and the crystallographic orientation,but also as probes for the relatively strong directional interlayer interactions in few layers of WTe2.In chapter four,defects usually have an important role in tailoring various properties of two-dimensional(2D)materials.However,optical detection of defects,especially single-atom point defects,is very challenging for 2D layered materials.Here,we report our systematic studies on the Raman-activated defect vibrational modes in 2D semimetallic material by combining Raman spectroscopy,density functional theory(DFT)calculation and scanning tunneling microscopy(STM).We observed three common Raman-active vibrational modes located at 95(A1g2,228(A1g1)and 304 cm-1(B1g1)in ZrSiTe few-layers,consistent with our theoretical calculations.Moreover,a pronounced mode sitting at 131.7 cm-1 was found in the ZrSiTe monolayer.This mode fades out quickly in the bilayer(2L)and eventually disappears in 4L.The high-resolution STM images and DFT calculations suggest this mode can be attributed to a combined signal of an intralayer shear mode at the Brillouin zone boundary and several localized modes which is activated by atomic point defects,and STM-based inelastic tunneling spectrum further confirms the existence of such a defect mode.In chapter five,although the Graphene/MoS2 2D stack achieved a lot of attention in recent 2D semiconductor device research,the interface interaction between MoS2 and graphene,and its effects on the electronic properties are rarely studied.Here,we report our recent research on Raman spectroscopy and photoluminescence studies of the Graphene/MoS2 heterostructure.A nearly perfect interface between these two materials was achieved with a dry transfer and forming gas annealing.Raman spectroscopy of the heterostmcture reveals the strong interface interaction leads to the blue shifts of the MoS2 and graphene Raman modes.Furthermore,the photoluminescene of the heterostructure indicates that the photon excitation and electronic dynamics of MoS2 are significantly influenced by additional graphene layer on top.Our results reveal that the interface of heterostructures plays an important role to determine the physical properties.