Theoretical Study On The Electronic Structures And Rectifying Properties Of The Metal-organic Semiconductor Interfaces

With continuous miniaturization of the devices,limits of conventional silicon-based electronics are close to be reached.When the size of the device is reduced to nanometer scale,pure quantum effects will appear which would change the device structures and functional design.One possible route of eliminating silicon in these devices is by replacement with organic components,which comparatively have the advantages of small size,high stability,flexible bending and high tunability of their properties.Numerous experimental and theoretical results show that the charge injection barrier(Schottky barrier)at the metal-organic interface has a large impact on the rectification performance,which controls the electrical and optical properties of the devices.However,due to the complicated interfacial interactions,including covalent bond,van der Waals(vd W)force,Pauli repulsion,hydrogen bond and charge transfer,the interaction mechanism of the interfacial electronic structure on the Schottky barrier and rectification property has become extremely complex,which still lacks in-depth exploration and systematic research.In this dissertation,by employing the vd W-inclusive density functional theory(DFT)combined with non-equilibrium Green’s function method(NEGF),we systematically studied the interfacial electronic structure and its effect on the rectifying properties of a series of organic molecules(benzene,naphthalene,anthracene,tetracene,anthradithiophene,anthradiselenophene and tetrachloropyrazine)on different metal surface(Cu,Ag,Au,Pt).We studied the adsorption structure and electronic property of monostable metal-organic interface,proposed the method to regulate the Schottky barrier by using the bistable interfacial electronic structures,and improved the rectifying performance of the bistable interface with the asymmetric contact.For the key scientific problems in this field that the large forward current and small reverse leakage current are hard to obtain in single contact,we proposed a novel method to simultaneously enhance the forward and reverse current by transforming the bistable adsorption states of the metalorganic interface,revealed the electronic interactions mechanism of the typical physisorbed and chemisorbed interfaces,established the inner relationship between the work function changes,molecular dipole,interface dipole and the Schottky barrier height,and designed an asymmetric metal-organic-metal interface with high rectification ratio based on the bistable system.The major conclusions can be summarized as follows:(1)For the complex metal-organic interface,we revealed the electronic interactions mechanism of the typical physisorbed and chemisorbed interfaces,established the inner relationship between the work function changes,molecular dipole,interface dipole and the Schottky barrier of the metal-organic interface.We systematically studied the adsorption behavior of benzene,naphthalene,anthracene and tetracene on the transition metal surfaces by using the vd W-inclusive DFT method.The adsorption of organic molecules on metal surfaces can be characterized by two typical adsorption categories: physisorption and chemisorption.The adsorption of organic molecules would lead to an obvious decrease of the metal work function for the two states,which is caused by the molecular dipole and interfacial dipole.This is also the main factor resulting in the failure of the Schottky-Mott limit.Notably,the physisorption and chemisorption behave distinct characteristics: the physisorbed system shows smaller molecular deformation,smaller interface dipole and larger Schottky barrier,while the chemisorbed system has larger interface dipole and smaller Schottky barrier.The electronic interactions of the interface are general to different types of organic molecules on metals.Moreover,with the size of the organic molecule increasing,the lowest unoccupied molecular orbital become lower,leading to a stronger interaction with the metal substrate.This makes an increased interfacial dipole and a decreased Schottky barrier height.This study provides a new idea to improve the transport performance of the metal-organic interface.(2)To simultaneously increase the forward output current and reduce the reverse leakage current at a single metal-organic interface,we proposed a novel method to regulate the Schottky barrier and output current by transforming the bistable interfaces.Anthradithiophene exhibits two stable adsorption configurations on the Cu(111)surface:physisorption and chemisorption.The adsorption configurations and electronic structures of the two states are obvious different: the adsorption height is 2.88 (?) and the transferred charge of the physisorbed state is 0.39 e;while for the chemisorbed state they change to2.31 (?) and 0.43 e,respectively.The Schottky barrier difference(0.82 e V vs 0.67 e V)between the two states is caused by the interfacial dipole.Moreover,the Schottky barrier is negatively correlated with the interface dipole.Combined with the NEGF calculations,both the physisorbed and chemisorbed systems show obvious rectifying characteristics.The forward output current of the physisorption is about 10 times larger than that of the chemisorption due to the lower Schottky barrier under 0.12 e V.While the reverse leakage current of the interface gets significantly decreased with the transition from the physisorption to the chemisorption accomplished under-0.12 e V.Hence,high output current and low leakage current can be achieved in a single contact via switching between the two states,which further improves the rectifying property of the interface.Our method is general to system with bistable states,and is expected to bring new design idea to the function of the nano-electronic devices.(3)In order to improve the rectifying performance of the metal-organic interface in contact with the electrode,we designed an asymmetric metal-organic-metal interface with high rectification ratio based on the bistable adsorption system.Compared with the anthradithiophene/Cu(111)interface,anthradiselenophene also exhibits bistable behaviors on the Cu(111)surface.The difference in the electronic structures,Schottky barriers(1.57 e V vs 0.70 e V)and output current between the physisorption and chemisorption is more obvious,resulting in a better rectifying performance.When the bistable interface contacts with the asymmetric electrodes(including Ag,Au and Pt),the rectification ratio of the device gets decreased due to the small increase rate of the transmission coefficient near the fermi level in the physisorption.Moreover,by further putting the symmetric Cu electrode at the asymmetric site(the top site of the carbon atom in the middle benzene ring of the organic molecule),the interactions between the electrode and molecule in the chemisorbed state get significantly weakened,which finally improved the rectification ratio by nearly three times for the system.Our study provides a new strategy for improving the rectifying performance of the bistable metal-organic interface.