Opportunities and limitations of transition voltage spectroscopy: a theoretical analysis

In molecular charge transport, transition voltage spectroscopy (TVS) holds the promise that molecular energy levels can be explored at bias voltages lower than required for resonant tunneling. We investigate the theoretical basis of this novel tool, using a generic model. In particular, we study the length dependence of the conducting frontier orbital and of the 'transition voltage' as a function of length. We show that this dependence is influenced by the amount of screening of the electrons in the molecule, which determines the voltage drop to be located at the contacts or across the entire molecule. We observe that the transition voltage depends significantly on the length, but that the ratio between the transition voltage and the conducting frontier orbital is approximately constant only in strongly screening (conjugated) molecules. Uncertainty about the screening within a molecule thus limits the predictive power of TVS. We furthermore argue that the relative length independence of the transition voltage for non-conjugated chains is due to strong localization of the frontier orbitals on the end groups ensuring binding of the rods to the metallic contacts. Finally, we investigate the characteristics of TVS in asymmetric molecular junctions. If a single level dominates the transport properties, TVS can provide a good estimate for both the level position and the degree of junction asymmetry. If more levels are involved the applicability of TVS becomes limited.Comment: 8 pages, 12 figure

Molecule-Electrode Interface Energetics in Molecular Junction: a Transition Voltage Spectroscopy Study

We assess the performances of the transition voltage spectroscopy (TVS) method to determine the energies of the molecular orbitals involved in the electronic transport though molecular junctions. A large number of various molecular junctions made with alkyl chains but with different chemical structure of the electrode-molecule interfaces are studied. In the case of molecular junctions with clean, unoxidized electrode-molecule interfaces, i.e. alkylthiols and alkenes directly grafted on Au and hydrogenated Si, respectively, we measure transition voltages in the range 0.9 - 1.4 V. We conclude that the TVS method allows estimating the onset of the tail of the LUMO density of states, at energy located 1.0 - 1.2 eV above the electrode Fermi energy. For oxidized interfaces (e.g. the same monolayer measured with Hg or eGaIn drops, or monolayers formed on a slightly oxidized silicon substrate), lower transition voltages (0.1 - 0.6 V) are systematically measured. These values are explained by the presence of oxide-related density of states at energies lower than the HOMO-LUMO of the molecules. As such, the TVS method is a useful technique to assess the quality of the molecule-electrode interfaces in molecular junctions.Comment: Accepted for publication in J. Phys. Chem C. One pdf file including manuscript, figures and supporting informatio

Characterizing the Metal–SAM Interface in Tunneling Junctions

his paper investigates the influence of the interface between a gold or silver metal electrode and an n-alkyl SAM (supported on that electrode) on the rate of charge transport across junctions with structure Met(Au or Ag)TS/A(CH2)nH//Ga2O3/EGaIn by comparing measurements of current density, J(V), for Met/AR = Au/thiolate (Au/SR), Ag/thiolate (Ag/SR), Ag/carboxylate (Ag/O2CR), and Au/acetylene (Au/C≡CR), where R is an n-alkyl group. Values of J0 and β (from the Simmons equation) were indistinguishable for these four interfaces. Since the anchoring groups, A, have large differences in their physical and electronic properties, the observation that they are indistinguishable in their influence on the injection current, J0 (V = 0.5) indicates that these four Met/A interfaces do not contribute to the shape of the tunneling barrier in a way that influences J(V).Chemistry and Chemical Biolog

Modeling Molecular Junctions: Weak and Strong Coupling Regimes

Electron transport through single molecule connected to the electrodes is an interesting problem from a fundamental point of view, and because of possible applications. From the theoretical point of view, the hope is that understanding the transport phenomena in such systems enables us to explain measurements and develop devices with new functionalities. In this thesis, different theoretical approaches are presented to address the characteristics of the molecular devices with electrical and optical probes. We have combined the non-equilibrium Green's function formalism with density functional theory (DFT) to address molecular junctions in which Coulomb correlations play a major role. An important issue in the field is the determination of the molecular levels, which contribute to transport. We have investigated the opportunities and limitations of Transition Voltage Spectroscopy (TVS) which has been advocated as a method to determine these molecular level positions without applying large voltages. We also studied a series of molecules, used recently in a self-assembled monolayer experiment, to rationalize the effects of the molecular structure on transport. Finally, we have analyzed the Raman response of several molecules in different charge states and suggested experiments in which these states could be identified using the Raman technique.Quantum NanoScienceApplied Science

Density functional theory based many-body analysis of electron transport through molecules

We present a method which uses density functional theory (DFT) to treat transport through a single molecule connected to two conducting leads for the weak and intermediate coupling. This case is not accessible to standard nonequilibrium Green’s function calculations. Our method is based on a mapping of the Hamiltonian on the molecule to a limited set of many-body eigenstates. This generates a many-body Hamiltonian with parameters obtained from ground-state local (spin) density approximation-DFT calculations. We then calculate the transport using many-body Green’s function theory. We compare our results with existing density matrix renormalization group calculations for spinless and for spin-1/2 fermion chains and find good agreement.QN/Quantum NanoscienceApplied Science