First-principles Study On The Thermoelectric Transport And Molecule-electrode Interaction In Molecular Junctions

Molecular electronics,which aims at designing and assembling complex electronic devices with single or multiple molecules as basic units,is a promising complementary scheme for silicon-based electronic devices to make the Moore's law continue.The single molecular junction is the most basic unit of molecular electronics.Applying different driving force between two electrodes,we can study the electrical,thermal or optical transport properties of single molecular junctions.When studying transport properties at the molecular scale,quantum effect of electrons cannot be ignored.Comparing with the traditional electronic device,the introduction of quantum effect makes a single molecular device possesses unique properties,such as quantum interference effect.We can construct single molecular devices with different functions,including molecular thermoelectric devices,molecular switches,etc.Due to the development of new experimental techniques,such as mechanically controllable break junctions(MCBJ)and scanning tunneling microscope(STM),substantial progress has been made in the experimental fabrication and characterization of single molecular junction.At the same time,theoretical study of single molecular junction has also developed rapidly.Especially,the nonequilibrium Green's function(NEGF)method combined with density functional theory(DFT)has been widely used in the calculation of the transport properties of single molecular junctions.Experimental technology and theoretical method complement each other,which makes molecular electronics an interdisciplinary research field spanning physics,chemistry,electronics and other disciplines.Although some progress has been made,there are still many problems to be solved.For example,the effect of quantum interference on the thermoelectric efficiency of single molecular junctions,the control of molecule-electrode coupling,nonlinear interaction,the influence of molecules on electronic states of metal electrodes.Based on the above problems,combining the first-principles calculation and transport theory,we have carried out the following works:(1)We systematically investigated thermoelectric properties of a single molecular junc-tion which is formed by contacting a porphyrin molecule to graphene electrodes via acety-lene anchors.The roles of several factors,including contact geometry,molecular length,and twisting angle between molecule and graphene electrodes,on its performance as a heat engine are considered.The main findings are as following: Firstly,certain contact geometry gives rise to destructive quantum interference effect.Although the quantum interference reduces electrical conductance of the molecular heat engine,its performance is enhanced after considering the electronic thermal conductance and Seebeck coefficient.Secondly,the electrical conductance,maximum output power,and the efficiency at maximum power all increase with molecular length.Finally,when the twisting angle ? between the molecule and electrode plane changes from 0°to 90°the electrical conductance decreases but the figure of merit is enhanced.These results show that single molecular junctions using graphene electrodes are potential candidates for highly efficient,compact heat engines.And their performance is controllable via different ways,such as altering contact geometry,molecular length,or changing the twisting angle between the molecule and electrodes.(2)When covalently bonding to triazatriangulene(TATA)molecule adsorbed on metal surface,the photochromic molecule diarylethene(DAE)decouples from the surface and‘stand'perpendicularly to the metal surface.DAE and TATA form synthetic molecule (DAE-TATA).At the same time,the distance between different DAE-TATA molecules can be controlled by changing the length of the alkane chain connected to TATA.This is a significant advantage for single molecular junction.We firstly investigate adsorption configuration of DAE-TATA on Au(111)surface.We find that self-assembled monolayer of DAE-TATA tends to lock to a fixed relative angle between different molecules and form ordered structures.By calculating the project density of state(PDOS)and STM images,we show that it is possible to observe the closed and open configuration of DAE-TATA in the case of positive bias applied to the sample in STM experiment.Next,we study the thermoelectric transport properties of a single molecular junction composed of a closed and an open DAE-TATA molecule connected with gold electrodes.According to the computed results,the conductance of closed DAE-TATA single molecular junction is 2 order of magnitude higher than the open form.The DAE-TATA still have the potential to perform as an electronic switch.(3)Carbon monoxide(CO)molecules adsorbed on the surface of Cu(111)repel the surface electrons of copper.Using this repulsive interaction,several molecules can be used to confine electrons and form artificial atoms in real space.Different kinds of two-dimensional lattice structures can be realized on copper surface by using the periodic arrangement of the artificial atoms.We have analyzed the orbital characteristics of the energy bands formed by square and Lieb lattice in detail,and compared them with the relevant experimental studies.We find that,the high-energy electrons with special spatial distribution observed in STM experiments are derived from the p orbital of artificial atoms.Our study gives a clear theoretical explanation for these experimental results.This may be relevant in further study the molecule-electrode interface in single molecular junctions.