Preparation And Quantum Effects Of Organic/Inorganic Two-dimentional Materials

Two-dimentional?2D?material is an important frontier in modern science and technology.With the continuous development of the technologies in material preparation and the analysis methods,the thin film materials in atomic scale and nanoscale can be prepared.The structures of these 2D materials and the novel properties,different from the bulk materials,can be expected.It is very important to control the growing process of the 2D materials and study the physical mechanisms,because the novel properties of 2D materials are closely related to its growing process.Scanning tunneling microscopy?STM?is videly used to reveal the structures of 2D materials.It can not only characterize the morphologies of nanomaterials,but also the electronic structures.Metal phthalocyanines?MPcs?film,as a member of the self-assembled organic 2D material,has great application prospect in gas sensors,organic field-effect transistors,organic light emitting devices and solar cells etc.,due to the stable structure and good symmetry.More and more surface physical researchers at home and abroad carry out the research enthusiastically.Since the Almaden Research Center obtained the high resolution STM images of MPcs,the geometric adsorption and electronic structures of MPcs adsorbed on metal surfaces?e.g.gold,silver,copper,etc.?and insulator surface?e.g.sodium chloride?have been extensively studied.However,the researches of MPcs adsorbed on semi-metallic surface?e.g.bismuth?and its transition from sub-monolayer to multilayer are rarely reported.The growth of metal thin films on the surface of semi-conductor silicon is a traditional and vital research in the thin film science.Ga on the Si?111?-7×7 surface,as a typical group?metal adsorbed on silicon surfaces,has already been widely studied.When the Ga coverage reaches to 1/3 monolayer?ML?and the annealing temperature reaches to 550?,the???×???-Ga R30°surface structure appears.Further increasing the Ga coverage up to 1 ML,various reconstructions were observed,such as the phases of 6.3×6.3-Ga,11×11-Ga,???×???-Ga R30°and 1×1-Ga.The first three phases have been widely studied by low energy electron diffraction?LEED?and STM.However,the 1×1 phase was only observed from LEED measurements,and the STM observation in real space needs further study.The interface structure usually plays a key role in the growth of 2D materials.Understanding the interface structure provides the basis for our investigation of the mechanism of epitaxial growth films.In the past few decades,many experimental techniques have been used to investigate the interface structure of epitaxial growth films,and the remarkable results have been obtained.It is found that the metal-semicondutor interface structures deep buried can be observed by STM.Investigating the Pb-Si?111?system by STM,the Pb film looks like transparent,and the interface structure can be displayed directly.In this paper,the epitaxial growth of cobalt phthalocyanine?CoPc?2D material on Bi surface and the 2D metal materials?Ga and Cd?on Si?111?surface are studied by the low temperature scanning tunnel microscopy?LT-STM?in ultrahigh vacuum.Meantime,the growth mechanism and the physical properties are discussed.The main research contents and experimental results are as follows.1.Structural transition of CoPc molecule self-assembled on Bi?111?At the low coverage,the isolated CoPc molecules deposited on the Bi?111?substrate rotate around the Co²⁺located at the center of molecules,indicating that the interaction between CoPc molecules and Bi?111?substrate is weak.When the molecules meet to form clusters,they no longer rotated due to the Van der Walls interactions among the molecules.They are absorbed on the substrate in the form of"flat-lying".Further increasing the molecular coverage,CoPc molecules self-assemble and form monolayer.The structure of monolayer transforms from"zigzag chains"?0.75 ML?to"linear chains"?0.85 ML?,and then to"linear dimerized chains"?1 ML?with the increase of coverage.In the monolayer,both the molecular orientations and the chain orientations follow the orientations of Bi?111?substrate,and the molecules in the same chain have the same orientation.In the"zigzag chains",the molecules have the same orientation aligned at one of the three principal axes of the Bi?111?substrate,and the chains are along the other two orientations.While in the"linear chains"and"linear dimerized chains",the chains have the same orientation along one of the Bi?111?orientations,and the molecules in different chains are along the other two orientations.The"linear chains"is the intermediate state of"zigzag chains"and"linear dimerized chains".The"linear dimerized chains"has a 12.5%increase in the packing density compared with the"zigzag chains".When the coverage reaches to 2 ML,the orientation of CoPc molecules gradually change into"standing-up".Meanwhile,we have found a"mixed domain"with"flat-lying chain"and"standing-up chain"alternately arranging.It is a transition domain from"flat-lying domain"to"standing-up domain".With the increase of CoPc molecular coverage,the"mixed domain"changes into"standing-up domain".The transition of molecular orientation from"flat-lying domain"to"standing-up domain"implies that the"molecule-molecule interaction"becomes dominantinthecompetitionbetween"molecule-substrateinteraction"and"molecule-molecule interaction"due to the shielding of the first molecular layer.2.The research on growth and structure of Ga films on Si?111????×???-Ga substrateGa monolayer and Ga bilayer are grown on Si?111?-???×???-Ga substrate,and in-situ detected by LT-STM.The Ga monolayer has the rarely reported stucture 4×???-Ga,and the thickness is 0.25 nm.The STM topography changes significantly with the different bias.When the bias is large,the Ga film is striped and the directions are oriented along the Si?111?orientations.When the bias decreases,each cell presents three highlights,and the direction of the line formed by two highlights rotates 74owith the orientation of Si?111?.Based on generalized-gradient approximation,using the density functional theory as implemented in the Vienna ab initio Simulation Package,Ga atoms adsorbed on Si?111?-???×???-Ga surface are simulated.The structure of 4×???-Ga is determined by comparing the simulated STM images with the experimental STM images.It consists of 28 Ga atoms combined with Ga-Ga bonds,and forms"gallenene".The second Ga layer is 1×1-Ga,and the thickness is 0.4 nm.After annealing at 120o,it is changed from 1×1-Ga phase to 1×1-Ga,6.3×6.3-Ga and 5×5-Ga phases coexisting.The first two are known to us,while the third stucture 5×5-Ga is a new phase that has not been reported.The STM image of 5×5-Ga is very beautiful.Each cell consists of seven bright spots?one spot acts as a center,and the other six are uniformly around it?,and looks like a"flower".Furthermore,we find the distance between adjacent Ga atoms is??? times of the lattice constant of the 1×1-Ga.Thus the 5×5-Ga can be regarded as a modified superstructure of Si?111?-???×???-Ga surface.The structures of 4×???-Ga,1×1-Ga and 5×5-Ga are novel,but they have low thermal stability.When the Ga film coexisting three phases and the film with 4×???-Ga phase are annealed at 150°,the structures all change to the 6.3×6.3-Ga.The emergence of multiple structures in Ga films is the result of a delicate balance of competition between"Si?111?-Ga"interaction and"Ga-Ga"interaction.3.The growth and interface imaging of Cd?0001?films on Si?111?substrateHighly smooth Cd?0001?films are grown on Si?111?-7×7 substrate,and the Si-Cd?0001?interface is imaged at atomic scale by LT-STM.The resolution of the interface imaging is not only dependent on bias,but also depends on the thickness of the Cd films.A series of STS spectra obtained on the films with different thicknesses show that there are peaks with equal energy spacing on each spectra.With the increase of the thickness,the positions of peaks gradually move,and the energy spacing gradually decreases.The peaks changed with the film thickness indicate that a one-dimensional quantum well is formed in the direction of perpendicular to the film.When the bias is located at the discrete energy levels?peaks in STS?,the Si-Cd interface imaging has the best resolution.Not only the Si?111?-7×7 structure is clear,but also 12 adatom atoms are visible.When the bias is located at the middle positions of two discrete levels,the resolution of Si-Cd interface imaging is the lowest,and the atomic resolution of Cd?0001?surface can be obtained.According to the period of STS,we can calculate the electronic Fermi wavelength of Cd?0001?film is 2.2×10⁸ cm/s.By the analysis of STS with a small range,we found that there is a gap about 10mV near the Fermi level of Cd?0001?film.The gaps in the spectra obtained at the center of Cd clusters are slightly larger than that obtained at other locations,indicating the interface scattering at the center of clusters is stronger than other locations.At the same time,the interface scattering has induced the Anderson localization.The perfect transparency of Cd films can be attributed to the anisotropic electron motion with large lateral effective mass,due to the Anderson localization.