Gabrijel Posted July 2, 2010 Report Share Posted July 2, 2010 NANOTECHNOLOGY Current Research - Molecular Electronics and Self Assembled Monolayers This project is focused on the computational simulation of the chemical assembly, structural properties, and conductance characteristics of self-assembled monolayers (SAMs) of organic molecules that may be used in emerging revolutionary computational devices. The necessary science for a wall-to-wall simulation over multiple scales requires a vertically integrated and interdisciplinary effort encompassing molecular dynamics simulations, molecular structure computation (such as Density Functional Theory (DFT)), and conductance calculations based on both DFT and approaches with full dynamic electron correlation. These simulations and the corresponding code developments will allow us to predict device properties for molecular-scale electronics. Background Figure 1. Snapshot from MD simulation of 144 benzenedithiol molecules adsorbed on Au(111). Color code: sulfur (red); carbon (grey); yellow (gold), anchor point on gold surface (white). (Click image for larger view) The design of novel molecular electronics devices such as diodes, transistors, and logic gates will greatly benefit from the demonstrated ability to accurately predict properties such as electron conduction by molecules and their physical response to applied source and control voltages on the basis of first-principles transport calculations. Our interdisciplinary research collaboration is pursuing an integrated effort that assembles the SAMs, characterizes the structural properties of the individual molecular components, calculates the impact of correlations beyond DFT, and calculates conductance properties both within DFT and correlated approaches. Since the self-assembly studies require intermolecular potentials, and surfaces affect these potentials, a natural project integration has occurred. Feedback between the surface and the molecular layers significantly affects conductance properties but has never been taken into account before. One major goal of this work is to develop computational approaches and a suite of tools that allows us to perform computer simulations coupled over multiple scales describing the assembly and physical, chemical, and electronic properties of SAMs of organic molecules. These include: molecular and Brownian dynamics (MD and BD) simulations to study the formation process of the SAM and the occurrence of defects; DFT and DFT-MD to describe the adsorption process of the alkane headgroup to the surface structure and the chemical functionality of the monolayer; DFT, many-body perturbation theory, and quantum Monte Carlo approaches to predict the current-voltage characteristics of these molecular assemblies. Thus this effort constitutes a world-class, Terascale computational study of molecular self-assembly and molecular electronics. Technical Accomplishments We performed preliminary molecular simulations of self-assembled monolayers composed of benzenethiol (BT) and benzenedithiol (BDT) molecules on the 111 surface of gold. A literature search was performed to obtain state-of-the-art force fields; additional information useful for details of the force field are being generated through the electronic structure calculations. In addition, Collin Wick, a DOE computational science graduate fellow at the University of Minnesota, spent the summer of 1991 at ORNL working on this project. Wick brings to the project expertise in modeling benzene molecules. Both molecular dynamics (MD) and Monte Carlo (MC) simulation are being used. Gibbs ensemble MC is being used to establish the equilibrium between adsorbed monolayers of BT and BDT with a low-density solution. MD is then being used to equilibrate the structures thus found. The structure of the adsorbed monolayers appears to be consistent with available experimental results, and we expect to write a publication on this during the next few months. During FY 2002, are working to increase the sophistication of the MD and MC models using results from electronic structure calculations, and link the atomistic simulations with ES calculations to produce a multiscale iterative simulation of molecular electronics functionality in BDT monolayers adsorbed between two gold surfaces. We developed a set of tools that utilize high-performance parallel- architecture quantum structure codes to calculate the conductance and current voltage characteristics of a chosen prototype organic molecule. For electronic conductance, we calculated the Green function for the molecule, the self-energies of the semi-infinite leads and of the coupling between them. These were on an accurate description of their respective electronic structures. An important advantage of the current molecular electronic research in comparison to previous work is the possibility provided by highly parallelized molecular structure codes (NWCHEM) to utilize more realistic models of the lead-molecule system by including a large number of lead atoms and a large number of basis states. Figure 2 DFT calculation of the structure of two benzenedithiol molecules with gold leads. (Click image for larger view) The calculations were performed on the benzene-1,4 dithiolate molecule, successively increasing the size of the cluster used to represent the gold leads from 3 to 118 atoms. The uncorrelated Hartree-Fock (HF) method, as well as the correlated Gaussian Density Functional Theory (DFT) and the MP2 or Moller-Plesset multibody perturbation theory were used to treat these systems. We showed that the closed system boundary conditions, used in molecular structure methods, leads to incorrect electrical behavior of the leads. We employed a generalized tight-binding model to calculate the lead self-energies, which provided the correct description of this open-system electron transport problem. Although electron correlation effects were visible in the electronic structure of the system, the magnitude of their effect on the electrical characteristics is still under investigation. Figure 3. Molecular model of the type of conducting polymer chain than can be scaled in length to span electrode gaps that may be lithographically defined. (Click image for larger view) The transmission function of a single Fermi level electron through the molecule, defining the system conductance for zero temperature and zero electric field bias was obtained using a suite of codes developed under the LDRD project. These constitute a solid foundation for a realistic finite temperature-finite bias multielectron transport calculation to be executed in the future phase of the project. We have developed a code for the Auxiliary Field Monte Carlo evaluation of the energy for the fully correlated ground state of a benzene molecule. We are using for two-body interaction matrix elements the electron-electron correlation functions calculated at the Hartree-Fock level in NWChem. An example of ongoing research is calculations of the electrical conductance in long-chain conducting molecules sufficiently long to span fabricated electrode gaps of size of approximately 5 nm, such as the molecule shown in Fig. 3. Authors: J. C. Wells(1), P.T. Cummings(2,4), D.J. Dean(3), P. Krstic(3), D. Keffer(4), X. Zhang1, K.K. Likharev(5) Center for Engineering Science Advanced Research (1)Computer Science and Mathematics Division, Oak Ridge National Laboratory (2)Chemistry Division, Oak Ridge National Laboratory (3)Physics Division, Oak Ridge National Laboratory (4)Department of Chemical Engineering, University of Tennessee, Knoxville (5)Department of Physics, State University of New York, Stony Brook Research Support Current research support for this activity includes: Thank you very much indeed, Please I will like you to accept this token with good faith as this is from the bottom of my heart. Thanks and God bless you and your family. Hope to hear from you soon. Your' s Faithfully, firstname.lastname@example.org Link to comment Share on other sites More sharing options...
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