Spectroscopic interrogation and charge transport properties of molecular transistors

ABSTRACT

Molecular transistors have been extensively investigated as the building blocks for the ultimate miniaturization of electronic devices. They are assembled from single molecules and molecular monolayers serving as a current-carrying channel in a conventional field-effect transistor configuration, in which gate electrodes have been electrically or electrochemically implemented in several types of test beds such as electromigration junctions, mechanically controllable break junctions, and devices with carbon-based electrodes. The energy level alignments of the component molecules incorporated into the transistor can be tuned using molecular orbital gating and it can ultimately control the flow of charge carriers. Herein, we review recent progress in studying spectroscopic characterization techniques and charge transport properties of molecular transistors.     

Electronic properties of organic monolayers and molecular devices

We review some of our recent experimental results on charge transport in organic nanostructures such as self-assembled monolayer and monolayers of organic semiconductors. We describe a molecular rectifying junction made from a sequential self-assembly on silicon. These devices exhibit a marked current-voltage rectification behavior due to resonant transport between the Si conduction band and the π molecule highest occupied molecular orbital of the π molecule. We discuss the role of metal Fermi level pinning in the current-voltage behavior of these molecular junctions. We also discuss some recent insights on the inelastic electron tunneling behavior of Si/alkyl chain/metal junctions.     

Effect of end groups on contact resistance of alkanethiol based metal-molecule-metal junctions using current sensing AFM

Tuning the charge transport through a metal-molecule-metal junction by changing the interface properties is widely studied and is of paramount importance for applications in molecular electronic devices. We used current sensing atomic force microscopy (CSAFM) as a tool to study the contact resistance of metal-molecule-metal (MmM) junctions formed by sandwiching self-assembled monolayers (SAMs) of alkanethiols with various end groups (-CH₃, -OH and -NH₂) between Au(1 1 1) substrates and Au coated AFM tips. The effect of interface chemistry on charge transport through such SAMs with varying end groups was studied in an inert, non-polar liquid (hexadecane) environment. We find that the contact resistances of these MmM junctions vary significantly based on the end group chemistry of the molecules.     

电场对分子线电子结构的影响

从第一性原理出发,利用密度泛函理论计算了分子2-氨基-5-硝基-1,4-二乙炔基-4-苯硫醇基苯与金原子团形成的分子线的电子结构,从轨道、能级及吸附电子三个方面讨论了电场对分子线电子结构的影响.该工作将有利于未来纳米电子学器件的设计.     

分子的位置取向对分子器件电输运特性的影响

以1,4-二硫酚(DTB)分子为研究对象，利用第一性原理计算方法和非平衡格林函数理论，研究了分子的位置取向对分子电子结构以及分子结电输运性质的影响.计算结果表明，分子位置取向的改变会影响分子的电子结构，从而影响分子体系的电输运特性，扩展分子的平衡态不是电子输运的最佳状态，适当调节分子的位置取向可以提高分子的电输运特性. 关键词： 位置取向 电子输运 分子电子学     

分子线电子输运特性的第一性原理研究

从第一性原理出发 ,利用密度泛函理论研究了SH -C8H16-SH分子和金表面的相互作用 ,并利用分子前线轨道理论和微扰理论定量地确定了该相互作用能常数 ,然后 ,利用弹性散射格林函数方法研究了该分子与金表面形成的分子线的伏 安特性 .研究结果表明 ,当含有硫氢官能团的有机分子化学吸附于金表面时 ,硫原子将与金原子形成以共价键为主的混和键 ,此时 ,扩展的分子轨道使分子线的电导呈现出欧姆特性 ,而对于局域的分子轨道 ,电子的输运只能通过隧道效应来实现 .对分子线伏 安特性的计算结果显示 ,在零偏压附近 ,存在一个电流禁区 ,随着偏压的增加 ,分子线的电导呈现出平台特征 .     

Electrical characterization of Al/AlOx/molecule/Ti/Al devices

We report experimental electrical characterization of Al/AlO_x/molecule/Ti/Al planar crossbar devices incorporating Langmuir–Blodgett organic monolayers of eicosanoic acid, fast blue, or chlorophyll-B. Current–voltage and capacitance–voltage measurements on all three molecular device structures exhibited controllable switching hysteresis. Control devices containing no molecules showed no evidence of switching. A model of interface trapped charge mediating electronic transport appears consistent with all of the data. This data illustrates the importance of considering the complete device system (consisting of the molecules, the electrodes, and the interfaces) when analyzing its electrical behavior. PACS 85.65.+h; 73.40.Rw; 73.50.Gr     

Self-assembled and electrochemically deposited mono/multilayers for molecular electronics applications

For the development of molecular electronics, it is desirable to investigate characteristics of organic molecules with electronic device functionalities. In near future, such molecular devices could be integrated with silicon to prepare hybrid nanoelectronic devices. In this paper, we review work done in our laboratory on study of characteristics of some functional molecules. For these studies molecular mono and multilayers have been deposited on silicon surface by self-assembly and electrochemical deposition techniques. Both commercially available and specially designed and synthesized molecules have been utilized for these investigations. We demonstrate dielectric layers, memory, switching, rectifier and negative differential resistance devices based on molecular mono and multilayers.     

1,8-二巯基芘分子电学特性的理论研究

在第一性原理的基础上 ,对 1,8 二巯基芘分子的电学特性进行了理论研究 .采用了 3个Au原子构成的团簇来模拟Au表面 .首先利用密度泛函理论计算了 1,8 二巯基芘分子的电子结构及其和Au表面的相互作用 ,再利用前线轨道理论和微扰理论定量地确定了该分子和Au表面的相互作用能常数 .最后利用弹性散射格林函数法研究了该分子结的伏 安特性 .计算结果表明 ,分子中的硫原子和Au原子形成很强的共价键 .当外加偏压小于 1V时分子结存在电流禁区 ,随着偏压升高 ,分子结的电导出现平台结构 .分子结的电导特性和其电子结构密切相关 ,扩展分子轨道为电荷的迁移提供了通道 ,而局域轨道对电流贡献很小     

Negative differential resistance in polymer molecular devices modulated with molecular length

We study the electronic transport properties of polymer molecular devices by applying first-principles method. The results show that the electronic transport properties depend on molecular length. Negative differential resistance can be observed and can be modulated with molecular length.     

以石墨烯为电极的有机噻吩分子整流器的设计及电输运特性研究

传统硅基半导体器件受到了量子尺寸效应的限制,发展分子电子学器件有可能解决这一难题.本文提出了由石墨烯电极和有机噻吩分子相结合构造分子器件的思想,建构了"石墨烯-噻吩分子-石墨烯"结构的分子器件,并运用非平衡态格林函数结合密度泛函理论的方法研究了其电输运特性.系统地分析了电子给体"氨基"和电子受体"硝基"两种取代基的位置对有机噻吩分子电输运的影响.计算表明,有机噻吩二聚物被"氨基"和"硝基"取代后会产生明显的负微分电阻效应和整流效应.进一步对产生这些效应的物理机制进行分析,发现氨基的位置可以调整负微分电阻的强弱,硝基的位置可以改变整流的方向.     

Design of perylene-diimides-based small-molecules semiconductors for organic solar cells

A series of perylene-diimide-based small molecules have been designed to explore their optical, electronic and charge transport properties as organic solar cell materials. The frontier molecular orbitals analysis has turned out that the vertical electronic transitions of absorption are characterised as intramolecular charge transfer between perylene diimide moieties and substituent aromatic groups. Our results suggest that the optical and electronic properties and reorganisation energies are affected by the introduction of different aromatic groups to these molecules. The calculation results showed that the designed molecules own the large longest wavelength of absorption spectrum, the oscillator strength and absorption region values. On the basis of the investigated results, the designed molecules could be used as solar cell material with intense broad absorption spectra. Furthermore, they are expected to be the promising candidates for hole and/or electron transport materials.     

Device design based on the covalent homocoupling of porphine molecules

Porphine has a great potential application in molecular electronic devices. In this work, based on the density functional theory(DFT) and combining with nonequilibrium Green's function(NEGF), we study the transport properties of the molecular devices constructed by the covalent homocoupling of porphine molecules conjunction with zigzag graphene nanoribbons electrodes. We find that different couple phases bring remarkable differences in the transport properties. Different coupling phases have different application prospects. We analyze and discuss the differences in transport properties through the molecular energy spectrum, electrostatic difference potential, local density of states(LDOS), and transmission pathway. The results are of great significance for the design of porphine molecular devices in the future.     

Organic Surface Science: Creating Order And Complexity Using Self-Assembly

Surfaces of organic materials are receiving an increased attention since their physical and chemical properties can be tailored very specifically by the choice of an appropriate organic molecule. The fabrication of well-defined organic surfaces with a high degree of structural order, however, is not straightforward. In many cases the preferred route is to deposit organic molecules on a solid, inorganic substrate. The growth of soft matter, molecules, on hard matter, metals, semiconductors or insulators, however, requires a detailed understanding of the substrate-adlayer interaction on a molecular level. Here we will discuss typical problems encountered in the epitaxy of organic molecules on inorganic substrates. Some basic concepts are outlined and illustrated, with particular emphasis on the epitaxial growth of organic semiconductors relevant for making molecular electronics devices and on the formation of selfassembled organothiolate monolayers on metal surfaces.     

Nanoelectronic devices from self-organized molecular switches

The development of molecular electronic switching devices for memory and computing applications presents one of the most exciting contemporary challenges in nanoscience and nanotechnology. One basis for such a device is a two-terminal molecular-switch tunnel junction that can be electrically switched between high- and low-conductance states. Towards this end, the concepts of self-assembly and molecular recognition have been pursued actively for synthesizing two families of redox-controllable mechanically interlocked molecules – bistable catenanes and bistable rotaxanes – as potential candidates for solid-state molecular-switch tunnel junctions. This article reviews logically the development and understanding of Langmuir, Langmuir–Blodgett and self-assembled monolayers of amphiphilic bistable and functionalized bistable rotaxanes and their catenanes counterparts. Our increased understanding of the superstructures of these monolayers has guided our recent efforts to incorporate these self-organized molecular switches into devices. The methodologies that are being employed are in their early stages of development. Certain characteristics of the molecules, monolayers, electrodes and devices are emerging that serve as lessons to be consider in responding to the ample opportunities for further research and process development in the field of nanoelectronics. PACS 81.07.-b; 81.07.Nb; 85.65.+h     

基于石墨烯电极的蒽醌分子器件开关特性

研究了基于石墨烯电极的蒽醌分子器件的开关特性.分别选取了锯齿型和扶手椅型的石墨烯纳米带作为电极,考虑蒽醌基团在氧化还原反应下的两种构型,即氢醌(HQ)分子和蒽醌(AQ)分子,构建了双电极分子结,讨论了氧化还原反应和不同的电极结构对蒽醌分子器件开关特性的影响.研究发现,无论是锯齿型石墨烯电极还是扶手椅型石墨烯电极,HQ构型的电流都明显大于AQ构型的电流,即在氧化还原反应下蒽醌分子呈现出显著的开关特性.同时,当选用锯齿型石墨烯电极时其开关比最高能达到3125,选用扶手椅型石墨烯电极时开关比最高能达到1538.此外,当HQ构型以扶手椅型石墨烯为电极时,在0.7-0.75 V之间表现出明显的负微分电阻效应.因此该系统在未来分子开关器件领域具有潜在的应用价值.     

噻吩低聚物分子器件电输运特性的理论研究

利用从头计算方法和弹性散射格林函数理论,对不同噻吩低聚物分子的电输运性质进行理论研究．结果显示,由于分子几何结构对称性的不同使得末端基团跟金电极的连接方式不同,导致了分子与电极间耦合常数以及分子轨道的扩展性不同．出现了同系列的噻吩低聚物分子中较长的分子比较短的分子导电性更好的反常现象.     

Scanning tunneling microscopy of adsorbates on insulating films. From the imaging of individual molecular orbitals to the manipulation of the charge state

Ultrathin insulating films on metal substrates are unique systems for using a scanning tunneling microscope to study the electronic properties of single atoms and molecules that are electronically decoupled from the metallic substrate. Individual gold atoms on an ultrathin insulating sodium chloride film supported by a copper surface exhibit two different charge states, which are stabilized by the large ionic polarizability of the film. The charge state and associated physical and chemical properties such as diffusion can be controlled by adding or removing a single electron to or from the adatom with a scanning tunneling microscope tip. The simple physical mechanism behind the charge bistability in this case suggests that this is a common phenomenon for adsorbates on polar insulating films. In the case of molecules on ultrathin NaCl films, the electronic decoupling allows the direct imaging of the unperturbed molecular orbitals, as will be shown in the case of individual pentacene molecules. PACS 68.37.Ef; 73.61.Ng; 73.20.Hb     

Study of the transport mechanism in molecular self-assembling devices

This paper focuses on the intrinsic charge transport in self-assembled monolayers (SAMs) and on the nature of transport in organic systems, in which surface and bulk properties are undistinguishable due to scale of consistent materials. Developed SAM-OFETs and photovoltaic (SAM-PVC) devices are characterized independently to study a role of charge delocalization both in electrical and optical manifold. The dynamics of charge transport are determined and used to clarify a transport mechanism. Taken together, these SAM devices provide a unique tool to study the fundamentals of polaronic transport on organic surfaces and to discuss the SAM-OFET and SAM PVC performance. Vapor phase molecular self-assembly of 1, 4, 5, 8-naphthalene-tetracarboxylic diphenylimide (NTCDI) having a rich π-stacking charge delivery system is used to enhance the performance of SAM-OFET and SAM PVC devices. Charge mobility in SAM-OFET could achieve values of more than 30 cm² V⁻¹ s⁻¹. The dynamics of charge transport in NTCDI-derived SAM-OFETs were probed using time-resolved measurements in an NTCDI-derived photovoltaic cell device. Time-resolved photovoltaic studies allow us to separate the charge annihilation kinetics in the conductive NTCDI channel from the overall charge kinetic in a SAM-OFET device. It has been demonstrated that tuning of the type of conductivity in NTCDI SAM-OFET devices is possible by changing Si substrate doping. In addition, the possibility of measuring transport in highly ordered SAM structures shines light on the polaron charge transfer in organic materials. Our study proposes that a cation-radical exchange (redox) mechanism is the major transport mechanism in SAM nanodevices. The role and contribution of the transport through delocalized states of redox active surface molecular aggregates of NTCDI are exposed and investigated in this report.