Epitaxial Growth And Structural Transition Of Molecular/Atomic Thin Films On Cd(0001) Surface

Molecular/atomic thin films on metal surfaces have received extensive attention due to their fundamental scientific significance and potential application values.In recent years,with the development of experimental techniques and computational theory,it is now possible to investigate the growth process of thin films at the molecular or atomic scale.In this thesis,two typical materials,pentacene monolayer films on Cd（0001）surface and stanene films on Cd（0001）surface,are selected as the research objects.The nucleation,growth,and structural transition of molecular and atomic thin films were studied by the scanning tunneling microscope（STM）and density functional theory（DFT）.Organic semiconductor thin films have important application prospects in organic（opto）electronics.High-quality organic epitaxial thin films are the key to the preparation of high-performance organic semiconductor devices.In particular,the monolayer at the interface plays a crucial role in the subsequent film structure,morphology,and electronic properties.Therefore,it is of great fundamental significance to study the monolayer structure on solid surfaces.Pentacene(C₂₂H₁₄)is a typical planar aromatic molecule,whose majority carrier is hole,and its mobility is as high as 2.5 cm²/Vs.It is recognized as the best p-type organic semiconductor molecule.It has been found in the past that pentacene molecules form highly ordered two-dimensional self-assembled structures on metal substrates,with adjacent molecules aligned in parallel.At lower coverage,however,pentacene molecules usually form gas-like disordered structures.With the increase of coverage,the pentacene molecules undergo a disorder-order phase transition.Nevertheless,at present,the physical mechanism of this gas-like disordered structure is not clear,and there is no unified explanation for the disorder-ordered phase transition.Furthermore,the pentacene molecule is a highly symmetrical achiral molecule.Can its self-assembled structure continue to maintain this symmetry?Will there be chiral features caused by symmetry breaking in pentacene self-assembled structures on metal surfaces?Two-dimensional atomic crystal materials are the current research hotspot in condensed matter physics.The reduction of the system dimension means the shrinking of the phase space and the weakening of the shielding effect,which enhances the quantum confinement effect,electronic correlation effect,and surface/interface effect in the system,thus resulting in some novel electrical,optical,and magnetic properties.In recent years,Considerable efforts have been devoted for finding and developing new two-dimensional atomic crystal materials and nanostructures beyond graphene.On one hand,it is expected to make up for the lack of graphene without energy gap.On the other hand,it is possible to discover some more novel physical and chemical properties.It is predicted that the new two-dimensional atomic crystal material stanene is not only a two-dimensional topological insulator with large energy gap,but also a two-dimensional topological superconductor.However,the stanene films prepared in experiments often exhibit metallic characteristics due to the coupling with the substrate,and the topological energy gap caused by the spin-orbit coupling is very small.Therefore,it is necessary to find suitable substrates and new growth methods to prepare stanene films with different structures.In view of the above problems,in this thesis,pentacene monolayer films and single-layer stanene films were prepared on metal Cd（0001）surface,respectively,using molecular beam epitaxy.The nucleation and growth process of pentacene monolayer films,as well as the structural transition between different self-assembled structures were investigated using low-temperature STM.Of particular interest,we discovered a new chiral pinwheel structure in the pentacene monolayer.The electronic structure of pentacene monolayer was studied by scanning tunneling spectroscopy（STS）and first-principles calculations.The stanene with hexagonal structure is prepared by low temperature growth method.The arrangement of Sn atoms is exactly the same as that of the substrate（1×1 reconstruction）.DFT calculations show that Sn atoms tend to occupy the hollow sites of the substrate.The charge transfer effect between stanene and the substrate is very weak,and the van der Waals effect is dominant.（1）Structural transition of pentacene monolayerWe mainly investigated the 2D self-assemblies and structural transitions of pentacene on Cd（0001）surface with a low temperature scanning tunneling microscopy.It is found with the coverage increasing,pentacene molecules show a structural evolution from the initial disordered gas-like phase,porous network structure,herringbone structure,and finally to the brick-wall structure at the full monolayer.Orientational frustration and cooperative rotation of pentacene molecules take place in the herringbone structure.Furthermore,successive STM scanning leads to the structural interconversions between the porous network structure,herringbone structure,and brick-wall structure,indicating the metastability of the 2D assembled structures of pentacene on Cd（0001）.These structural transitions and interconversion can be attributed to the interplay between the repulsive electrostatic forces resulted from the charge transfer from substrate to pentacene and the attractive effects originating from dipole-dipole interactions and intermolecular van der Waals forces.（2）Chiral self-assembly of pentacene and chirality switchingPentacene molecules on Cd（0001）surface form the porous network structure with building blocks of hexamer pinwheels.Driven by the electric field from a STM tip,the supramolecular chirality of pentacene pinwheels and the organizational chirality of entire chiral pinwheel domains can be simultaneously switched from one enantiomer to another.Furthermore,such chiral switching is reversible and repeatable under successive voltage pulses.Combined with the first-principles calculations,we come up with the Coulomb expansion mechanism caused by charge transfer,which can well explain the experimental observation.First-principles calculations demonstrate that electric field significantly modulates the interfacial charge transfer and induce the Coulomb expansion of pentacene layers,and the subsequent reaggregation leads to the chiral flipping of the supramolecular pinwheels and 2D domains.These results provide a new idea for regulating chirality inversion and structural transition with electric field.（3）A new two-dimensional atomic crystal:staneneFew layer stanene films with hexagonal structure was epitaxially grown on Cd（0001）surface by molecular beam epitaxy.It was found that both the first and second layer of Sn reveal the epitaxial growth with a 1×1 commensurate lattice.Scanning tunneling microscopy measurements indicate the Sn monolayer exhibits a metallic behavior.DFT calculations indicate that all the Sn atoms in the first Sn layer occupy the energetically preferable HCP-hollow sites.Very small amount of charge is transferred from Cd（0001）to the Sn monolayer,indicating the interface of Sn/Cd（0001）is governed by the weak van der Waals interaction.Synthesizing large-scale and high-quality two-dimensional nanomaterials is the key to new semiconductor nanodevices.In view of the importance of the diversity of interfacial structures in enhancing and optimizing device performance,we believe that our findings on structural transition of pentacene on the Cd（0001）surface can provide a basis for exploring new functional nanodevices.In addition,our proposed electric-field-induced coulomb expansion mechanism provides a new strategy for dynamic control of the two-dimensional chiral structures and help to steer the supramolecular assembly toward homochirality.On the other hand,the 1×1 epitaxial Sn films prepared on the Cd（0001）surface provide scientific data for the fabrication of novel ultrathin two-dimensional Sn sheets.