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Quantum Interference Controlled Molecular Electronics  [PDF]
San-Huang Ke,Weitao Yang,Harold U. Baranger
Physics , 2008, DOI: 10.1021/nl8016175
Abstract: Quantum interference in coherent transport through single molecular rings may provide a mechanism to control current in molecular electronics. We investigate its applicability by using a single-particle Green function method combined with ab initio electronic structure calculations. We find that the quantum interference effect (QIE) depends strongly on the interaction between molecular pi states and contact sigma states. It is absent in small molecular rings with Au leads, such as benzene, due to strong pi-sigma hybridization, while it is preserved in large rings, such as [18]annulene, which then could be used to realize QIE transistors.
Modelling of inelastic effects in molecular electronics  [PDF]
A. P. Jauho
Physics , 2005, DOI: 10.1088/1742-6596/35/1/029
Abstract: Ab initio modeling of molecular electronics is nowadays routinely performed by combining the Density Functional Theory (DFT) and Nonequilibrium Green function (NEGF) techniques. This method has its roots in the current formula given by Meir and Wingreen, and we discuss some applications and accompanying pitfalls and restrictions of this approach. Quite recently papers have begun to appear where inelastic effects are considered, and we illustrate these new developments by describing our own work on transport in atomic gold wires.
Silicon-based molecular electronics  [PDF]
T. Rakshit,G-C. Liang,A. W. Ghosh,S. Datta
Physics , 2003, DOI: 10.1021/nl049436t
Abstract: Molecular electronics on silicon has distinct advantages over its metallic counterpart. We describe a theoretical formalism for transport through semiconductor-molecule heterostructures, combining a semi-empirical treatment of the bulk silicon bandstructure with a first-principles description of the molecular chemistry and its bonding with silicon. Using this method, we demonstrate that the presence of a semiconducting band-edge can lead to a novel molecular resonant tunneling diode (RTD) that shows negative differential resistance (NDR) when the molecular levels are driven by an STM potential into the semiconducting band-gap. The peaks appear for positive bias on a p-doped and negative for an n-doped substrate. Charging in these devices is compromised by the RTD action, allowing possible identification of several molecular highest occupied (HOMO) and lowest unoccupied (LUMO) levels. Recent experiments by Hersam et al. [1] support our theoretical predictions.
Molecular electronics and first-principles methods  [PDF]
J. J. Palacios,A. J. Perez-Jimenez,E. Louis,J. A. Verges,E. SanFabian
Physics , 2003,
Abstract: We discuss the key steps that have to be followed to calculate coherent quantum transport in molecular and atomic-scale systems, making emphasis on the ab-initio Gaussian Embedded Cluster Method recently developed by the authors. We present various results on a simple system such as a clean Au nanocontact and the same nanocontact in the presence of hydrogen that illustrate the applicability of this method in the study and interpretation of a large range of experiments in the field of molecular electronics.
Spin Electronics and Spin Computation  [PDF]
S. Das Sarma,Jaroslav Fabian,Xuedong Hu,Igor Zutic
Physics , 2001, DOI: 10.1016/S0038-1098(01)00111-9
Abstract: We review several proposed spintronic devices that can provide new functionality or improve available functions of electronic devices. In particular, we discuss a high mobility field effect spin transistor, an all-metal spin transistor, and our recent proposal of an all-semiconductor spin transistor and a spin battery. We also address some key issues in spin-polarized transport, which are relevant to the feasibility and operation of hybrid semiconductor devices. Finally, we discuss a more radical aspect of spintronic research--the spin-based quantum computation and quantum information processing.
Models of Electrodes and Contacts in Molecular Electronics  [PDF]
San-Huang Ke,Harold U. Baranger,Weitao Yang
Physics , 2005, DOI: 10.1063/1.1993558
Abstract: Bridging the difference in atomic structure between experiments and theoretical calculations and exploring quantum confinement effects in thin electrodes (leads) are both important issues in molecular electronics. To address these issues, we report here, by using Au-benzenedithiol-Au as a model system, systematic investigations of different models for the leads and the lead-molecule contacts: leads with different cross-sections, leads consisting of infinite surfaces, and surface leads with a local nanowire or atomic chain of different lengths. The method adopted is a non-equilibrium Green function approach combined with density functional theory calculations for the electronic structure and transport, in which the leads and molecule are treated on the same footing. It is shown that leads with a small cross-section will lead to large oscillations in the transmission function, T(E), which depend significantly on the lead structure (orientation) because of quantum waveguide effects. This oscillation slowly decays as the lead width increases, with the average approaching the limit given by infinite surface leads. Local nanowire structures around the contacts induce moderate fluctuations in T(E), while a Au atomic chain (including a single Au apex atom) at each contact leads to a significant conductance resonance.
The role of contacts in molecular electronics  [PDF]
G. Cuniberti,F. Grossmann,R. Gutierrez
Physics , 2002,
Abstract: Molecular electronic devices are the upmost destiny of the miniaturization trend of electronic components. Although not yet reproducible on large scale, molecular devices are since recently subject of intense studies both experimentally and theoretically, which agree in pointing out the extreme sensitivity of such devices on the nature and quality of the contacts. This chapter intends to provide a general theoretical framework for modelling electronic transport at the molecular scale by describing the implementation of a hybrid method based on Green function theory and density functional algorithms. In order to show the presence of contact-dependent features in the molecular conductance, we discuss three archetypal molecular devices, which are intended to focus on the importance of the different sub-parts of a molecular two-terminal setup.
Fabrication of multilayer edge molecular electronics and spintronics devices  [PDF]
Pawan Tyagi
Physics , 2011,
Abstract: Advancement of molecular devices will critically depend on the approach to establish electrical connections to the functional molecule(s). We produced a molecular device strategy which is based on chemically attaching of molecules between the two magnetic/nonmagnetic metallic electrodes along the multilayer edge(s) of a prefabricated tunnel junction. Here, we present the fabrication methodology for producing these multilayer edge molecular electronics/spintronics devices (MEMEDs/MEMSDs) and details of the associated challenges and their solutions. The key highlight of our MEMED/MEMSD approach is the method of producing exposed side edge(s) of a tunnel junction for hosting molecular conduction channels by a simple liftoff method. The liftoff method ensured that along the tunnel junction edges, the minimum gap between the two metal electrodes equaled the thickness of the tunnel barrier. All of the tunnel junction test beds used a ~2 nm alumina (AlOx) tunnel barrier. We successfully bridged the magnetic organometallic molecular clusters and non-magnetic alkane molecules across the AlOx insulator along the exposed edges, to transform the prefabricated tunnel junction into the molecular electronics or spintronics devices. Tunnel junction test beds were fabricated with a variety of metal electrodes, such as NiFe, Co, Ni, Au, Ta, Cu and Si. Stability of ultrathin thin AlOx varied with the type of bottom metal electrodes used for making MEMED/MEMSD. Additionally, molecular solution used for bridging molecular channels in a MEMED was not compatible with all the metal electrodes; molecular solution resistant ferromagnetic electrodes were developed for the fabrication of MEMSDs. MEMSD approach offers an open platform to test virtually any combination of magnetic electrodes and magnetic molecules, including single molecular magnets.
Thermoelectric effect in molecular electronics  [PDF]
M. Paulsson,S. Datta
Physics , 2003, DOI: 10.1103/PhysRevB.67.241403
Abstract: We provide a theoretical estimate of the thermoelectric current and voltage over a Phenyldithiol molecule. We also show that the thermoelectric voltage is (1) easy to analyze, (2) insensitive to the detailed coupling to the contacts, (3) large enough to be measured and (4) give valuable information, which is not readily accessible through other experiments, on the location of the Fermi energy relative to the molecular levels. The location of the Fermi-energy is poorly understood and controversial even though it is a central factor in determining the nature of conduction (n- or p-type). We also note that the thermoelectric voltage measured over Guanine molecules with an STM by Poler et al., indicate conduction through the HOMO level, i.e., p-type conduction.
Intermolecular Effect in Molecular Electronics  [PDF]
Rui Liu,San-Huang Ke,Harold U. Baranger,Weitao Yang
Physics , 2004, DOI: 10.1063/1.1825377
Abstract: We investigate the effects of lateral interactions on the conductance of two molecules connected in parallel to semi-infinite leads. The method we use combines a Green function approach to quantum transport with density functional theory for the electronic properties. The system, modeled after a self-assembled monolayer, consists of benzylmercaptane molecules sandwiched between gold electrodes. We find that the conductance increases when intermolecular interaction comes into play. The source of this increase is the indirect interaction through the gold substrate rather than direct molecule-molecule interaction. A striking resonance is produced only 0.3 eV above the Fermi energy.
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