Theoretical Study Of The Transport And Interface Properties Of Several Low-dimentional Materials

How will the information technology revolution be extended?How will the world’s energy demands of future generations be met?What new discoveries await us in the nanoworld?These are three of six challenges proposed by the United States National Academy of Sciences in condensed matter physics and materials physics and chemistry in 2010.Recently,with the help of the rapid development of computer science and the progress of theory,predicting and designing materials theoretically become possible.Unfortunately,at present,most of the simulation software in materials science used in domestic institutes and universities are monopolized by foreign countries,technology is not autonomous and controllable,meaning that we will face technological blockade at any time.In this dissertation for Ph.D degree,based on density functional theory combined with non-equilibrium Green’s function method,we develop a quantum transport software(Tetrans)and examine the spin-dependent transport,thermoelectric transport,inelastic electron tunneling properties of several low-dimensional materials and molecules,the peak positions and image features of the experimentally observed inelastic electron tunneling spectra are explained successfully.Besides,two types of graphene nanoribbons with high thermoelectric conversion properties are predicted.By using independently developed software AISP,we screen out a novel two-dimensional graphene-like anode material for potassium ion batteries with high HER/OER electrocatalytic performance.In addition,we cooperate with the experimental group to study the interfacial effect on the magnetic and photocatalytic properties of two low-dimensional nano-materials.This dissertation includes the following seven chapters.In Chapter 1,we briefly review molecular electronics,density functional theory(DFT),elastic and inelastic transport theories,and thermoelectric transport theory as well as several DFT-based computational packages adopted in this dissertation.In Chapter 2,based on the framework of DFT combined with non-equilibrium Green’s function method,we develop a cross-platform program(Tetrans),which can be used to calculate the dynamic matrix,phonon transmission spectrum,phonon thermal conductance,phonon spectrum,phonon local density of states,Seebeck coefficient,power factor,electrical conductance,electronic thermal conductance,and thermoelectric ZT value.By using the Tetrans package,we predict a rhombic porous carbon nitride nanoribbon(rPCNNR)with a high thermoelectric figure of merit(ZT)value.According to its structure,the constructed vertical van der Waal junctions exhibit excellent thermoelectric properties.The high thermoelectric performance of the vertical junction is achieved by the synergy effect between the large Seebeck coefficients,the good electron conductivity provided by the strong overlapping of delocalized valence band-and conduction band-derived states in the scattering region,and the dramatically suppressed thermal conductance in the in-plane direction due to the weak van der Waals interaction,built-in porous structure,and rhombic edge.In the first part of Chapter 3,we systematically investigate the electronic structures,spin-resolved transport,and thermoelectric transport properties of the experimentally synthesized graphene nanoribbon connected by four-membered rings.It is clear that the introduction of four-membered rings can effectively achieve 100%spin polarization of graphene nanoribbon near Fermi level,and the system has high thermoelectric ZT values(0.66～1.36)maintaining over a wide temperature range(77 to 800 K).In the second part of this chapter,a novel metallic 2D carbon allotrope thgraphene containing four-membered rings is predicted and generated by using the structure search software AISP,which offers structural generation with symmetry constraints combined with the first-principles genetic algorithm(GA)for searching stable and metastable structures.Theoretical results show that thgraphene has excellent HER/OER electrocatalytic performance and it is also a promising anode material for potassium-ion batteries due to the excellent ductility(Poisson’s ratio～0.39),ultra-low K-ion diffusion barrier(0.06 eV),high theoretical specific capacity(744 mA·h·g-1),and moderate average open-circuit voltage(0.30 V).As fingerprint information,inelastic electron tunneling spectroscopy(IETS)can not only be used to identify molecules,but also provide useful information such as the interface configuration between molecules and electrodes.In the first part of Chapter 4,we simulate the IETS and spatial distribution of the inelastic electronic signal of four DNA bases sandwiched between copper electrodes and find that these IETS results can be used to identify different DNA bases.In the second part of this chapter,cooperating with the scanning tunneling microscopy research group,based on the spatial distribution of the vibrational wavefunction and the Fermi’s gold rule,we simulate the IETS of RePc2 on Au(111)and successfully explain these observed IETS results in experiments.In Chapter 5,we examine the transport properties of stilbene molecular junctions with the para-and meta-configurations by using different connection positions and find that the quantum interference effect can effectively tune the electron transport properties.Moreover,the high on/off current ratios can be achieved by applying gate voltages.It is well known that the interfacial effect has a significant impact on the electronic structures,magnetic properties,and photocatalytic properties of low-dimensional materials.In collaboration with the experimental group,in the first part of Chapter 6,we investigate the interfacial effects between the graphene and Fe layer on the magnetic properties.According to the calculated spin density,spin-resolved partial density of states,and band structure,we find that the magnetic reduction of Fe layer overlaid on graphene is attributed to the strong hybridization between the 3dz2 orbitals of Fe and the C 2pz orbitals.The C atoms in the underlying graphene are slightly magnetized,implying that spin injection into graphene is achieved in this ferromagnetic transition metal/graphene composite structure.In the second part of this chapter,we propose a novel multi-phase facet junction including TiO2{101}/{001} co-exposed surfaces.The DFT calculations reveal the existence of type-Ⅱ band alignment between the co-exposed{001} and {101} facets and the well-separated photogenerated carrier can improve the efficiency of photocatalytic hydrogen production.We present a short summary and a brief outlook of this Ph.D dissertation in the last chapter.