IJERT-Length-Dependant Tunneling And Hopping Mechanism In Molecular Wires (original) (raw)
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Length-Dependant Tunneling And Hopping Mechanism In Molecular Wires
International journal of engineering research and technology, 2013
The temperature-dependant electron transport characteristics of three molecular wires of different molecular lengths belonging to the family of conjugated benzene molecules were studied. In this article, the conductance values for three molecular wires consisting of different number of benzene rings and amounting to different lengths at different temperature were calculated. The percentage change in conductance values were plotted with respect to temperature for each molecular wire in this research work. We concluded that the longest molecular wire of benzene having molecular length of 17.055A 0 showed the most pronounced effect of temperature on conductance, even though this value was much smaller than the value exhibited by shortest molecular wire of length 5.003A 0 . The results demonstrated that the shorter wires showed highly length dependence and temperature invariant conductance, whereas the longest wire exhibited weak length dependant and temperature variant behaviour. This ...
Molecular Wires: Charge Transport, Mechanisms, and Control
Annals of the New York Academy of Sciences, 1998
By molecular wires, one generally means molecular structures that transmit a signal between two termini. We discuss some theoretical models and analysis for electronically conductive molecular wires in which a single molecule conducts charge between two electrodes. This situation resembles both intramolecular non-adiabatic electron transfer, in which electronic tunneling between donor and acceptor is seen, and mesoscopic quantum transport.
Journal of the American Chemical Society, 2010
The charge transport characteristics of a family of long conjugated molecular wires have been studied using the scanning tunneling microscope break junction technique. The family consists of four wires ranging from 3.1 to 9.4 nm in length. The two shortest wires show highly length dependent and temperature invariant conductance behavior, whereas the longer two wires show weakly length dependent and temperature variant behavior. This trend is consistent with a model whereby conduction occurs by two different mechanisms in the family of wires: by a coherent tunneling mechanism in the shorter two and by an incoherent charge hopping process in the longer wires. The temperature dependence of the two conduction mechanisms gives rise to a phenomenon whereby at elevated temperatures longer molecules that conduct via charge hopping can yield a higher conductance than shorter wires that conduct via tunneling. The evolution of molecular junctions as the tip retracts has been studied and explained in context of the characteristics of individual transient current decay curves.
Length dependence of the electronic transparence (conductance) of a molecular wire
Europhysics Letters (EPL), 1996
The electronic transparence of a single molecular wire connecting the two electrodes of a metal-insulating-metal nanojunction decreases exponentially with its length. The transparence attenuation can be quite small depending of the homo-lumo gap of the molecule and of electronic interaction of the wire ends with the electrodes. For a 10 nm long polyene connecting two nano-electrodes, a 100 mV bias voltage will lead to a tunnelling current intensity in the 10 pA range.
Physical Review B, 2010
We report a first-principles study of quantum transport in a prototype two-terminal device consisting of a molecular nanowire acting as an interconnect between two gold electrodes. The wire is composed of a series of bicyclo͓1.1.1͔pentane ͑BCP͒ cage-units. The length of the wire ͑L͒ is increased by sequentially increasing the number of BCP cage units in the wire from 1 to 3. A two terminal model device is made out of each of the three wires. A parameter free, nonequilibrium Green's function approach, in which the bias effect is explicitly included within a many body framework, is used to calculate the current-voltage characteristics of each of the devices. In the low bias regime that is considered in our study, the molecular devices are found to exhibit Ohmic behavior with resistances of 0.12, 1.4, and 6.5 ⍀ for the wires containing one, two, and three cages respectively. Thus the conductance value, G c , which is the reciprocal of resistance, decreases as e −L with a decay constant ͑͒ of 0.59 Å −1. This observed variation of conductance with the length of the wire is in excellent agreement with the earlier reported exponential decay feature of the electron transfer rate predicted from the electron transfer coupling matrix values obtained using the two-state Marcus-Hush model and the Koopman's theorem approximation. The downright suppression of the computed electrical current for a bias up to 0.4 V in the longest wire can be exploited in designing a three terminal molecular transistor; this molecular wire could potentially be used as a throttle to avoid leakage gate current.
Charge Hopping in Molecular Wires as a Sequence of Electron-Transfer Reactions
Journal of Physical Chemistry A, 2003
Charge transport in molecular wires is investigated theoretically within the framework of a simple hopping model. The model suggests that each elementary hopping step can be treated as an electron-transfer reaction between ionic and neutral states of π-conjugated structural units coupled through σ-bonded spacers. Within this mechanistic picture, the ability of wire to transport a charge depends crucially on the internal reorganization energy, λ. Using unrestricted Hartree-Fock and density functional theory methods, we evaluate λ for benzene, 3-methylbiphenyl, 2,6-dimethyl-1-phenyl-pyridinium (DMPP), and 4-(p-suflhydrylphenylpyridinium-1′-yl)-2,6-dimethylpyridinium, selected as representative examples of structures used for chemical attachment to σ-bonded structural spacers in real molecular wires. The results are exploited to estimate the upper and lower limits of hole and electron mobility in wires that consist of aromatic ring units linked to the antipodal bridgeheads of σ-bonded molecular "cages", bicyclo[1.1.1]pentane (BCP), cubane (CUB), and bicyclo[2.2.2]octane (BCO). Our calculations show that the highest mobility of holes is expected for coplanar alignment of aromatic rings at the end of molecular cages as, in this configuration, the electron coupling is most efficient. We also analyze the situation in which thermally induced twisting motion destroys coplanarity of aromatic rings. The obtained results suggest that, for wires with the BCO spacer, hopping transitions are slower than twisting motion and, therefore, the mean hole mobility is determined by the equilibrium average twist angles. In the opposite case, relevant to the benzene/BCP and benzene/CUB systems, large deviations of the twist angles from the equilibrium value represent a bottleneck for the transport process.
Nanotechnology, 2007
This article describes arylene-ethynylene molecular wires with 7 nm long backbones and thiolated termini. Cyclic voltammetric studies in solution reveal that the reduction waves of the fluorene, 9-[(4-pyridyl)methylene]fluorene and 9-[di(4-pyridyl)methylene]fluorene units which are embedded in the conjugated π -systems endow these wires with n-doping characteristics. An x-ray crystal structure investigation of 2,7-diiodo-9-[bis(4-pyridinium)methylene]fluorene bis(tetrafluoroborate) 8 established that protonation occurs on both nitrogens of this unit. Self-assembled monolayers of the 7 nm wire 2 on gold substrates exhibit symmetrical current-voltage (I -V ) characteristics when contacted by a gold scanning transmission microscope (STM) tip. The dipyridyl functionality of 2 served to obtain a rectifying junction in which the diprotonated cationic wire is the electron accepting component in combination with an adjacent anionic phthalocyanine as the electron-donating layer. This ionic Au-2H 2+ 2 [CuPc(SO − 3 ) 4 (Na + ) n ] 2/(4−n) bilayer assembly exhibits rectification with current ratios of 15-50 at ±1 V. This dramatic change in I -V characteristics upon simple chemical manipulation proves that the conductivity is a property of the wire molecules 2 in the junction. Ab initio calculations suggest that the molecular wires possess useful structural features which allow the conductance of the molecule to be altered by changing the properties of the side groups attached to the fluorene units.
Bending of conjugated molecular wires and its effect on electron conduction properties
Nanotechnology, 2010
The electronic structure and electron transport properties of simple conjugated molecular wires like oligophenylene ethynylene (OPE) and oligophenylene vinylene (OPV) are studied under compression. If artificially confined to a given shorter length, the oligomers tend to bend and bending causes a loss in the overlap of the conjugated molecular orbitals. Theoretical modeling of electronic transport has been carried out for all undistorted and compressed OPE/OPV oligomers. OPV exists in step-like or V-like conformations and they have the same stability with very similar frontier molecular orbitals. The conductances of these molecular wires are calculated when inserted between two gold probes and the conductances for OPV are found to be comparable to OPE when the interfaces are same. The conductance decreases with bending due to the gradual loss in overlap of the molecular orbitals. It is also found that the conductances of the molecular wires decrease very strongly if the terminal sulfur atom is simultaneously bonded to hydrogen and a gold surface, thus reflecting the importance of the interface in determining the conductance in two-probe systems. From the conductance studies it may be concluded that if one or more benzene rings of OPE are rotated from coplanar conditions, the orthogonal molecular orbitals may completely block the electronic transport, rendering the molecule insulating.
Electrical transport through a mechanically gated molecular wire
Physical Review B, 2011
A surface-adsorbed molecule is contacted with the tip of a scanning tunneling microscope (STM) at a pre-defined atom. On tip retraction, the molecule is peeled off the surface. During this experiment, a two-dimensional differential conductance map is measured on the plane spanned by the bias voltage and the tip-surface distance. The conductance map demonstrates that tip retraction leads to mechanical gating of the molecular wire in the STM junction. The experiments are compared with a detailed ab initio simulation. We find that density functional theory (DFT) in the local density approximation (LDA) describes the tipmolecule contact formation and the geometry of the molecular junction throughout the peeling process with predictive power. However, a DFT-LDA-based transport simulation following the non-equilibrium Green's functions (NEGF) formalism fails to describe the behavior of the differential conductance as found in experiment. Further analysis reveals that this failure is due to the mean-field description of electron correlation in the local density approximation. The results presented here are expected to be of general validity and show that, for a wide range of common wire configurations, simulations which go beyond the mean-field level are required to accurately describe current conduction through molecules. Finally, the results of the present study illustrate that well-controlled experiments and concurrent ab initio transport simulations that systematically sample a large configuration space of molecule-electrode couplings allow the unambiguous identification of correlation signatures in experiment.