Alessio GagliardiPaderborn, Univ., Diss., 200

Gagliardi, Alessio

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Alessio Gagliardi, Theoretical Modeling and Simulation of Electron-Phonon Scattering Processesin Molecular Electronic Devices. PhD Thesis (in English), Department of Physics, Faculty ofScience, University of Paderborn, Germany (2007).AbstractThe entire field of molecular electronics relies on a very simple concept: a single organic molecule,or a thin layer of organic molecules, are used as a bridge between two metallic contacts. Thebias applied induces a current from one contact to the other. The basic idea is that the current isdominated by the chemical properties of the molecule between the contacts and that this electriccharacteristic can be easily tuned by an external perturbation in order to get a switch. However, de-spite the intense investigation many issues are still waiting to be fully addressed. One of them is thedissipation induced in the molecule by the coupling between the vibrational modes and the chargecarriers. Due to the size of the device this interaction must be described quantum mechanically viaan electron-phonon scattering process. The quantitative evaluation of dissipation in molecular elec-tronics is fundamental for the entire field, especially for future introduction of such kind of devicesin the electronic industry.The author of this work and Dr. Alessandro Pecchia of the University of Rome “Tor Vergata”have implemented a code based on non-equilibrium Green’s functions formalism to investigateelectron-phonon scattering in molecular devices. The first application presented is the evaluationof the dissipation in octane-thiols sandwiched between two gold contacts showing which are themost relevant modes in scattering electrons. The electron-phonon scattering is much more than thestudy of the “quantum resistance”. The inelastically scattered electrons in fact can be directly usedas probes to investigate the geometry and the electronic structure of the device. This spectroscopytechnique is called inelastic electron tunneling spectroscopy (IETS). Despite the IETS is an oldspectroscopy technique, its application in the molecular electronic field is recent and many featuresneed to be fully understood.The electron-phonon code has been used to investigate the properties of IETS, first simulatingdirectly IETS in octane-thiol systems, later on addressing the problem of selection rules in thisspectroscopy technique. It is well known in fact that the selectivity of IETS is rather different fromthe one of other more conventional techniques, like infrared or Raman. However the lack of aprecise knowledge of selection rules in IETS renders a clear assignment of IET spectra difficult.The definition of selection rules requires two steps. First, the definition of the point group ofthe system. It is obvious that, also if the molecule in the gas phase shows high symmetry, thiswill be reduced by the presence of the electrodes and the distortions of the structure. Despite that,the molecule remains the dominant feature of IETS measurements. We can univocally identify thepoint group of the device just taking the point group of the unperturbed molecule and reducing it toa subgroup due to the effects of the electrodes. The second issue is to define a basis set to which thepoint group is applied to. It is well known that canonical molecular orbitals are not the best choicedue to the strong interference effects between them during transport. However, the current canbe decomposed in a sum of independent “channels” obtained diagonalizing directly the operatorinvolved in the transmission and not the Hamiltonian. The choice of this set of channels is notunique, but all of them permit to avoid interference.These tools have been applied to investigate IETS in a benzenedithiol molecule between modelgold contacts. The interpretation of the current in terms of independent channels provides not onlyinformation about selection rules, but also in which region electrons are scattered, leading to a deepcomprehension of where the dissipation occurs and which chemical substitutions in the device canimprove its performances.

Theoretical modeling and simulation of electron-phonon scattering processes in molecular electronic devices

Page 1. Theoretical modeling and simulation of electron-phonon scattering processes in molecular
 electronic devices Alessio Gagliardi Universität Bremen-BCCMS PaSCO Universität Paderborn
 Page 2. Molecular Electronics Reed, APL, 2002 ▪ A single molecule or a single layer of molecules
 used as electronic switch ▪ Self-Assembling properties ▪ New electronic features Reed, Chem. Phys.,
 2002 Tautz group, Jacobs Bremen University Page 3. Molecular Electronics: Basic Concepts eV
 Energy Left electrode Right electrode Left electrode Right ..

Theoretical modeling and simulation of electron-phonon scattering processes in molecular electronic devices

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