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Electron Transport Properties Of The Conjugated Polymer Molecule

Posted on:2016-07-03Degree:MasterType:Thesis
Country:ChinaCandidate:X L YangFull Text:PDF
GTID:2180330461477229Subject:Condensed matter physics
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For organic conjugated polymers, the influence of electron-phonon interaction on the electron transport properties is still an important research topic currently. When the bias voltage is low, the conduction mechanism of some conjugated polymer moleculars transition from the coherent nonresonant tunneling to incoherent thermally activated hopping with increasing molecular length; If the bias voltage is large enough, so that the conduction electron and the molecular level will be in significant resonance situations, then the conjugated polymer molecules show rich transport phenomena. The trans-polyacetylene as an example of a class of simple flexible conjugate polymer molecules will be studied in this thesis.The trans-polyacetylene molecule is described with the Su-Schrieffer-Heeger(SSH) model Hamiltonian and the source and drain are described by non-interacting electron reservoirs. The equations of motion for the molecular density matrix, which are derived from Liouville equation of motion by the hierarchical equations of motion(HEOM) method, are combined with the equation of motion for the molecule lattice and the Poisson equation, in order to simulate the time-dependent evolution of the trans-polyacetylene molecule under conduction conditions. We have analyzed the impact of the lattice vibrations, potential distribution and gate voltage on the electron transport properties of a single trans- polyacetylene molecule. As for the potential distribution, two cases are treated: non-ohmic contact is between the electrodes and molecular, in which case the potential fully drops at interface between the electrodes and molecular, and there is no intramolecular potential fluctuations; ohmic contact is between the electrodes and molecular, in which case the potential drops in the molecular, and changes with the electron distribution in the molecule.This article mainly focuses on the electron transport properties of the trans-polyacetylene molecule in the absence of a gate voltage, in which case the π band center is at the center of the bias window.In the trans-polyacetylene molecule an electron or hole tends to form a soliton coupling with the nearby lattice distortion. The solitons excited in the trans-polyacetylene molecule in a conduction state tend to be joined together, so that their electronic bound states can interact to form more delocalized electronic states, which allows the molecular to obtain much larger conductance. The conductance of molecule with solitons excited is larger than that of the ground state at a bias lower than the bias threshold of soliton excitation from the ground state, which is greater than the bias at witch all the solitons disappear and thereby the molecule recovers the ground state, so the molecule has a conductance bistable nature.When these soliton electronic states are the conduction channels, the lattice vibrations make electric current through the molecule oscillate over time, and generally have a bad effect on the conduction. Under the non-ohmic contact conditions, the lattice vibration amplitude is positively related to the degree of the lattice change caused by bias change, so different initial states probably lead to different conductance values under a same bias. However, under the ohmic contact conditions, owing to the electric field force acting on molecular lattice points, the lattice vibrations lost the memory for the special initial lattice change and tend towards the same situation over time under the same bias, and thus the conductance difference under the same bias caused by different initial state is relatively small.Under the ohmic contact conditions, the potential drop in the molecular causes localization of the electronic states of the conduction and valence band and solitons. The coupling of localized electronic states to the source and drain is asymmetric, and even the coupling to one is reduced to zero. Therefore, the localized electronic states in the bias window have two effects, one of which is low conduction efficiency, and the second is a large deviation from half occupation.There are two harmful factors for conduction of soliton electronic states, one of which is molecular lattice vibrations, and the second is the potential drop in the molecule, and these two factors are positively correlated. Under the ohmic contact conditions, the electric field force acting on lattice points make lattice vibrations more purely dependent on the bias and the relative position of molecular levels with respect to the Fermi levels of the two electrodes, so lattice vibrations may be weak at some bias but increase significantly when the bias is slightly increased, and as a result, the negative differential resistance phenomenon occurs.At a gate voltage, the center of the π energy band of the trans-polyacetylene molecule will deviate from the center of the bias window. When this deviation is large enough, solitons will be periodically excited and charged at one end of the molecule, then annihilated and discharged at the other end, so pulsed current appears.
Keywords/Search Tags:electron transport, trans-polyacetylene, electron-phonon interaction, (non) ohmic contact
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