Development Of A Low-temperature Optical Scanning Probe Microscopy System And Investigation Of Several 2D Materials

Two-dimensional(2D)atomic/molecular crystal stimulate research interests in different research fields due to their fascinating physical properties.Molecular beam epitaxy is one way to grow high-quality 2D materials,while scanning probe microscope can image the atomic structures of the material,therefore the combination of scanning probe microscope and molecular beam epitaxy makes an ideal system for the growth and characterization of 2D materials.Moreover,to investigate the optical properties of 2D atomic/molecular crystal,it is desirable to make the scanning probe microscope system compatible with light path.This dissertation focuses on the investigation of 2D atomic/molecular crystal,and three main parts are included.In the first part,the development of a low-temperature optical scanning probe microscope system is introduced.In the second part,the fabrication and characterization of 2D supramolecular lattice using molecular beam epitaxy and scanning probe microscope is presented.The theoretical calculations of electronic properties of silicene-based heterostructures are introduced in the third part.The first part mainy introduces the development of instruments: a low-temperature optical scanning probe microscope system,a scanning probe microscope scanner head,a cryostat,and the performance of the system.This system incorporates four main chambers: the scanning probe microscope characterization chamber,the molecular beam epitaxy growth chamber,the preparation chamber,and the loadlock chamber.The scanning probe microscope scanner head adopts a modified Pan-type stepper motor,and is constructed based on a unibody frame,resulting in an improved intrinsic frequency and high rigidity.The scanning probe microscope scanner head takes a modular design,making the assembly and repair of it easy and convenient.It has been tested at both room temperature and low temperature,and the atomic resolution images can be easily obtained.The scanning tunneling spectroscopy of Au(?)surface is successfully obtained under low temperature.We also present the design of a homemade liquid helium cryostat,which can hold 3.7 L liquid helium and 16 L liquid nitrogen.Using a liquid helium cryostat,the lowest temperature of the sample can reach below 5 K.The testing results of the cryostat shows that the low temperature holding time is over 60 h.The second part mainly introduces the experimental investigation of 2D molecular cyrstal: the fabrication and characterization of 2D supramolecular structure.FePc and Te are grown on Au(?)surface using MBE.Two phases are observed when FePc is deposited on Te-covered Au(?)surface: the molecular-array phase and the honeycomb phase.Moreover,the eptaxial growth of FePc on Se-covered Au(?)surface was investigated.The Se layer can reduce the interaction between FePc and the substrate.As a result,the molecular orbitals can be clearly resolved using scanning probe microscope.The third part introduces the theoretical investigation of 2D atomic crystal heterostructures: the theoretical calculations of electronic properties of silicene in 2D van der Waals heterostructures.The intrinsic properties of silicene are preserved in BN/silicene/BN heterostructures,where the Dirac cones of silicene are left intact at the Fermi energy.In multilayer BN/silicene heterostructures,there is interlayer coupling between silicene layers.By increasing the number of BN layers between silicene layers,the coupling decreases and becomes negligible at three BN layers.Moreover,in BN/silicene superlattice,a conversion between Dirac points and Dirac lines can be achieved by changing the number of BN layers.