Stm Investigation On The Epitaxial Growth And Structural Phase Transition Of C₆₀ And Bi Thin Films On Cd（0001） Surface

C₆₀ is one kind of highly symmetrical spherical cage-like molecule that is considered to be the basic unit of carbon-based nanomaterials.Due to its unique geometry and novel electronic properties,C₆₀ has important applications in the fields of molecular electronics and photovoltaic cells.In the fields of basic research,C₆₀ molecules exhibit a wealth of physical properties such as superconductivity,magnetism and quantum transport phenomena because of the strong electron-electron correlation and electron-phonon coupling.In molecular electronics devices,C₆₀ molecules need to be in contact with metal electrodes.Therefore,it is of great practical importance to study the interactions between C₆₀ and metal surfaces,such as molecular adsorption and self-assembly,molecular orientation ordering,interfacial charge transfer,epitaxial growth of thin films,strain relief and domain wall formation.The strong electron affinity of C₆₀ molecules can cause a charge transfer effect occuring at the C₆₀/metal interface,and thus the C₆₀ molecules near the interface will carry a small negative charge.This charge transfer effect can be seen as’self-doping’of the interfacial C₆₀molecules.If the work function of the metal substrate is relatively small,the charge transfer effect will be significantly enhanced,which is expected to achieve a metal-insulator phase transition or superconducting phase transition similar to that induced by’alkali metal doping’,while avoiding the structural disorder associated with alkali metal doping.Therefore,Cd（0001）,a metal with a small work function（and electron affinity energy）,is chosen as the substrate in this dissertation to produce the charge transfer effect from the substrate surface to the C₆₀ molecule,which further enhances the interaction between the molecule and the substrate,and thus enables the modulation of the electronic properties of the C₆₀ film.In addition,we will study the epitaxial growth,strain relief and orientation ordering of C₆₀ films on Cd（0001）substrates.Bismuth（Bi）is a typical semi-metal with a large Fermi wavelength and very strong spin-orbit coupling.In recent years,it has been found that epitaxially grown Bi（111）atomic bilayers and Bi（110）atomic monolayers are both two-dimensional topological insulators with dissipation-free quantum edge states at the edges of the two-dimensional islands,and the latter has potential applications in low-energy quantum transport and quantum computing.In this dissertation,we will investigate the epitaxial growth of Bi（110）films with orthogonal lattice on hexagonal symmetric Cd（0001）substrates and explore the lattice matching mechanism between two atomic layers with different symmetries.The main findings of this thesis are summarised in the following three areas:（1）Chiral superstructure and orientation ordering of C₆₀ moleculesSelf-assembled monolayers of C₆₀ molecules were prepared on Cd（0001）substrates using a room-temperature deposition method.STM in situ observations reveal that the alignment of C₆₀ molecules in the self-assembled molecular monolayers have different off-angles from the substrate lattice orientation,and thus constitute C₆₀ domains with different periodic structures.What is of great importance is that different levels of orientation ordering occur in the different domain regions.（i）In the 2√3×2√3 domains with a 30 degrees declination angle,the C₆₀ molecules show a random disorder in their orientation.Using high-resolution STM images combined with density function theory calculations,we find that the C₆₀ molecules are oriented in three ways:pentagon facing up,hexagon facing up and the prongs between the two hexagons facing up（6-6 bond）.（ii）In two domains with 4 and 8degrees of declination,we find a chiral superstructure:six C₆₀ molecules with 6-6 bonds facing up around a hexagon facing-up C₆₀ molecule form a short-range ordered seven-molecule cluster.Interestingly,the seven-molecule cluster has a chiral character and can be aligned either in a clockwise or anticlockwise direction.（iii）In the domain with a 26degrees declination,all C₆₀ molecules adopt a 6-6 bond facing up orientation,showing a long-range ordering of molecular orientation.（2）Phase transition and strain relief of C₆₀filmsBy changing the temperature of the substrate,such as low-temperature deposition or annealing,we have discovered several novel self-assembled structures in C₆₀ molecular monolayers,such as wavy structures,high-order commensurate phases with 10×10reconstruction,incommensurate phases composed of heptameric arrays,and Kagome structure with?13×?13 reconstruction.（i）When the deposition temperature is～100 K,the C₆₀molecules align in a straight line along one?3 direction of the substrate,but bend in the other?3 direction,forming a one-dimensional wavy structure.When the wavy structures in the two different directions converge,the arrangement of C₆₀ molecules appear as an edge dislocation phenomenon.（ii）When the deposition temperature is～200 K,the C₆₀ molecules form a higher order commensurate phase with a 10×10 reconstruction.This structure is very similar to the surface structure of Si（111）-7×7 with a corner hole at each of the four top corners of the unitcell.（iii）After annealing at room temperature,the 10×10 higher-order commensurate phase transforms into an incommensurate phase consisting of an array of C₆₀heptamers.In addition,we also find anti-phase domains and Kagome structures with?13×?13 reconstruction formed by room temperature deposition.（3）Epitaxial growth of Bi（110）thin films on Cd（0001）substrateThe epitaxial growth of Bi films on Cd（0001）substrate and their structural evolution were investigated using low-temperature deposition and post-annealing methods.When the coverage of Bi atoms is 0.15 ML,we find stripe structures on the substrate surface.The high-resolution STM image and subsequent annealing result show that the stripe structure is a composite structure consisting of a Bi（110）atomic layer together with Cd atoms adsorbed on the buckling sites.As the coverage increases,the Bi atoms form a highly ordered Bi（110）crystalline film which appears as a nanoribbon on the surface of the substrate.Interestingly,one-dimensional superstructure with a period of 3.9 nm is present in the Bi nanoribbons of the first and second layers.Further analysis shows that the formation of the one-dimensional superstructure can be attributed to the Nishiyama-Wassermann epitaxial relationship between the pseudo-tetragonal lattice of Bi（110）and the hexagonal lattice of the Cd（0001）substratewhich results in the Bi（110）film being commensurate to the substrate in one lattice direction and incommensurate to the substrate in the other lattice direction.The above findings will provide experimental basis for the preparation of high-quality Bi（110）epitaxial films.