Quantum Interference Effects In Single-molecule Devices

Molecular electronic is focused on the charge transport properties and mechanism at single-molecule scale,which is of great scientific significance for understanding various quantum effects in organic molecules and designing future organic optoelectronic devices.The quantum interference(QI)effect describes a phenomenon produced by the interaction of the electron wave functions along different paths of molecules during propagation.According to the phase of the wave function,superposition state can be generated as constructive QI or destructive QI effects.Due to the QI effects,the small adjustment of the molecular device structure may bring about orders of magnitude change to the conductance.The understanding and regulating of the QI effect in organic molecules is of great significance to the design of new molecular devices and nanocircuits.The main topic of this dissertation is to investigate the influence of QI effects on device conductance in molecular devices.We have designed and synthesized a series of organic molecules with various conjugated geometries to investigate the effect of molecular structure on molecular conductance and explore the regulation mechanism of QI effects on charge transport.The designed model systems had been taken into account of many structural factors such as multi-channel transport,molecular lengths and anchor positions,isomers,the electronic coupling strength of donor-acceptor,and the bending conjugated system.Two techniques,including Scanning Tunneling Microscope Break-Junction(STM-BJ)and Mechanically Controllable Break Junction(MCBJ),were utilized to investigate the charge transport characteristics.Moreover,aided by theoretical simulations,the mechanism of QI effects in the charge transport manipulation of single-molecule devices were further discussed.The main research contents and achievements are shown as follows:1.Two series of molecules with multiple anchors were designed and synthesized,and the QI effects between the channels were studied with MCBJ and theoretical similation.As stated in the classic Kirchhoff's circuit laws,the total conductance of two parallel channels in an electronic circuit is the sum of the individual conductance(G_total=G₁+G₂).However,in molecular circuits,the QI between the individual channels may lead to apparent invalidity of Kirchhoff's laws.Such effect could be very significant in single-molecule circuits consisting of partially overlapped multiple transport channels.Herein,we conducted an investigation on how the molecular circuit conductance correlates to the individual channels in the presence of QI.We found that the conductance of multi-channel circuit consisting of both constructive and destructive QI is significantly smaller than the addition of individual ones due to the interference between channels.In contrast,the circuit consisting of destructive QI channels exhibits an additive transport.This investigation provides a new cognition of transport mechanism and manipulation of transport in multi-channel molecular circuits.2.We have designed and synthesized a series of bipyridine molecules with different lengths and substituent positions and investigated the charge transport properties of the single-molecule devices.Due to the different QI effects in para-para,meta-para,and meta-meta substituted molecules,the single-molecule conductance exhibits different molecular length-conductance dependence.The conductances of para-para and meta-para substituted ones show an exponential decay trend with the increasing molecular length,while the conductance of meta-meta coupled ones exhibits an abnormal phenomenon with the increasing molecular length.Theoretical simulations point out that the abnormal conductance changes of meta-meta coupled molecules are caused by the conformational isomerization of the anchoring groups in the presence of destructive QI.This study provides new understanding of the influence of QI effects and conformational isomerism on the single-molecule charge transport.3.With the investigation of the reversible stimuli-responsive QI effects of diketopyrrolopyrrole isomers(SDPP and SPPO)employing the MCBJ technique,we realized the recognition of organic molecular isomers at the single-molecule level.How to distinguish structural isomers at the single-molecule level remains a challenge.The single-molecule conductances of the two isomers are indistinguishable under the comparison of transport capacities.The reversible protonation and deprotonation of the SPPO molecular junction leads to more than 17 times conductance change,however,the conductance of SDPP show little difference before and after protonation.The dramatically enhanced conductance of protonated SPPO leads to the conductance difference between the two isomers and realizes the recognition of the two isomers.Theoretical study reveals that the dramatic conductance switching is due to the reversible QI effect.It is suggested that the combinated stimuli-response and QI could be an efficient strategy to enhance isomer recognition and conductance switching at the single molecule level.4.A series of D-A-D type thiophene-fluorene-thiophene molecules with different donor-acceptor coupling strength were synthesized,and their photoelectric response properties of the molecules were studied.Conductance measurements showed that the electrical transport capacity of the wire is in the order T-FCN-T>T-FO-T>T-DMF-T=T-F-T with the increase of electron-withdrawing ability of the 9-position substituent on fluorene(hydrogen,TFT;dimethyl,TDMFT;carbonyl,T-FO-T;dicyano,T-FCN-T).Meanwhile,the changes of the charge transport properties of molecular wires under light stimulation were also conducted.The conductances of T-DMF-T,and T-F-T show no changes.On the contrary,the transport capacity of T-FO-T under light stimulation increased by 70%and the conductance of T-FCN-T decreased.These experimental results show that the interaction between light and molecules could be imployed to regulate the charge transport through single-molecule devices,providing a new vision for the design of single-molecule optoelectronic devices.5.A series of new bowl-shaped molecules containing sulfur,selenium and tellurium were designed and their single-molecule conductances were investigated.The single-molecule device experiments show that the charge transport capabilities of the most bowl-shaped molecules are around the 10^-55 G₀.Combined with theoretical calculations,the special odd-symmetric configuration of the bowl-shaped molecules causes the degeneration of molecular orbitals near the Fermi level,which in turn gives rise to the destructive QI phenomena and low charge transport capacity.This special molecular configuration provides new ideas for the design of single-molecule devices and high-performance thermoelectric molecules.