Optimization Of The Ultrafast Laser-Molecular Beam Epitaxy-Scanning Tunneling Microscope System And The Construction Of Several Kinds Of Two-Dimensional Crystals

The Scanning Tunneling Microscopy?STM?has become a powerful and useful instrument in surface science since its invention in 1981,as a result of its high spatial resolution.In the past three decades,photo-assisted STM developed rapidly.On the one hand,surface photophysical or photochemical process can be measured by combining the STM and optical tools.On the other hand,the information carried by the tunneling current,which is what traditional STM generally measured,is determined and limited by the local density orbital states?LDOS?.Nevertheless,we can acquire more information by collecting the optical signal from the STM tunneling junction.For example,Tip Enhanced Electro-photoluminescence and Tip Enhanced Raman Spectroscopy?TERS?,have succeed in expanding the capability of STM.Moreover,it is a key issue to understand the ultrafast quantum dynamics in nanoscience,which may be solved by an in-situ experimental measurement of ultrafast process with the spatial resolution of molecular or atomic scale.The temporal resolution of STM is limited?<100 kHz?because of the circuit bandwidth of its preamplifier.The recombination of STM and ultrafast pump–probe technique is a viable approach to get high temporal resolution at the atomic-scale.This technique is very meaningful to analysis ultrafast process on surface.This article mainly includes the following works based on a home-built UHV-STM.1.We established a tip-enhanced electron photoluminescence photon-collection system on our STM and successfully detected the electron photoluminescence from the Ag tip on Ag?111?.Based on this system,we may preform tip enhanced electron photoluminescence and tip enhanced Raman Spectroscopy in the future.Moreover,we built a femtosecond time-resolved system,which is the foundation of our future ultrafast time-resolved STM.What's more,we designed a multi-order damping system to improve the S/N ratio of the signal of the time-resolved STM.Based on the numerical calculation,this system is proved to gain great benefits to damp the mechanical vibration from the external sources.2.We have constructed a new type of two-dimensional?2D?atomic crystal material–AgTe–on a Ag?111?surface.The Ag₂Te nanocrystals have been demonstrated to be a promising candidate for thermoelectric material and infrared detection.Also,?-Ag₂Te is theoretically predicted to be topologically nontrivial and an electronic topological transition in Ag₂Te is observed under high pressure.Although silver telluride film has been experimentally achieved,monolayer silver telluride has not been reported yet.We prepared the sample in ultrahigh vacuum by deposition of Te on Ag?111?.Using a scanning tunneling microscope?STM?and low electron energy diffraction?LEED?,we investigated the atomic structure of the samples.The STM images LEED pattern showed that monolayer AgTe Crystal is formed on Ag?111?.Four kinds of atomic structure of AgTe and Ag?111?were observed,1)flat honeycomb structure,2)bulked honeycomb,3)stripe structure,4)hexagonal structure.Structural analysis indicates that the formation of the different atomic structures is due to the crystalline direction and the lattice distortion of the AgTe.Our results provided a simple and convenient method to produce AgTe monolayer on Ag?111?and a template for study of the novel physical properties and for the future quantum devices.Moreover,under the control of deposition rate of the Te atoms,we studied the process of AgTe growth from sub-monolayer to multilayers and found several kinds of defects in sub-monolayer and multi-layer AgTe.These defects originate from the abundance of both Ag and Te atoms.This work provides details of the growth and defects in single layer AgTe,and will be a template for study of related 2D materials.3.The self-assembly of two-dimensional?2D?organic molecules on various substrates has been investigated extensively in the past two decades.We have studied the self-assembly behavior of 4,4?-diamino-p-terphenyl?DAT?and pentacene molecules on Au?110?.By combining the STM experiments with DFT calculations,we systematically explored the thermo-controlled configuration transformations and formation mechanism of the self-assembled single-layer DAT molecules on Au?110?surface.At a low annealing temperature of 100°C,DAT molecules only form one ladder-like structure in the?1×5?surface reconstructed channels.At a high annealing temperature of 200°C,the amino groups of the molecules are partially dehydrogenated.The–2H-DAT and–4H-DAT molecules can be found in both the?1×5?and?1×3?surface reconstructions,respectively.At a higher annealing temperature of 350°C,there are only–4H-DAT molecules in the?1×3?surface reconstructions.The investigation on these novel self-assembled structures,which is combined with surface reconstruction and dehydrogenation process,enriches our knowledge of the self-assembly behavior of organic molecules on metal surfaces.On the other hand,at about 470 K,pentacene molecules can induce both Au?110?-?1×3?surface reconstruction with the end-to-end molecular configuration and Au?110?-?1×6?surface reconstruction with the side-by-side configuration with coverage ranging from submonolayer to monolayer.Importantly,we used the special lattice feature of the anisotropic Au?110?substrate to successfully construct one-dimensional metal–organic molecular wires consisting of pentacene molecules with Au adatoms at about 520 K.These metal–organic molecular wires are not only straight and free of defects but also retain the conjugation property of pentacene molecules.In a second layer,pentacene molecules adopt only the side-by-side configuration on top of pentacene molecules in the?1×3?surface reconstruction at about 470 K,which reveals detailed interface information about the initial growth of the pentacene crystal system on metals.