Research On The Conduction Mechanism Of Monomolecular And Self-assembled Monolayers

As biomedical,aerospace,new energy and other fields have become more demanding in terms of integration of electronic devices,the size of electronic devices has also developed in a small direction.For example,transistors with a size of less than 10 nm have been developed to improve the performance of computer processors.However,the continued reduction in the size of electronic devices is hindered by technical limitations(the ever-decreasing feature sizes of electronic devices,the quantum effects,size effects,and short-channel effects appearing in devices,which may cause the performance of electronic devices to decline or even invalid)or the lack of a basic understanding of electronic device transmission mechanisms.In this sense,it is remarkable that chemically identical molecules,with sizes on the order of 1 nm,can be synthesized in bulk while accomplishing a variety of electronic tasks,thus,they have the potential to partly replace traditional solid-state device counterparts in the future.Comprehensive experimental findings in electron transport through individual molecules introduce the idea that beyond traditional complementary metal oxide semiconductor(CMOS)technology,the ultimate goal for shrinking electrical circuits is the realization of molecular-scale/single-molecule electronics because single molecules constitute the smallest stable structures imaginable.Molecular-scale electronics,which is the concept of creating functional electrical circuits on the basis of properties inherent in individual or ensemble molecules,have several unparalleled advantages as compared to silicon-based electronic devices.First,the extremely reduced size of the molecules in order of 1 nm may enable heightened capacities and faster performance.Moreover,such small size of the molecule provides the ability to surpass the limit of conventional silicon circuit integration.Second,the abundant diversity in the molecular structures,which can be changed via flexible chemical designs,may lead to a direct observation of novel effects as well as the fundamental discovery of physical phenomena that are not accessible using traditional materials or approaches.Third,another attractive feature of this approach is the universal availability of molecules due to the ease of bulk synthesis,thus potentially leading to low-cost manufacturing.Therefore,the development of promising single-molecules or molecular layers to partially replace traditional solid-state electronic devices in the future is the main research goal of researchers.As a matter of fact,molecular-scale electronics is currently a research area of focus because it not only meets the increasing technical demands of the miniaturization of traditional silicon-based electronic devices but also provides an ideal window of exploring the intrinsic properties of materials at the molecular level.However,numerous challenges need to be addressed before single-molecule devices can be used as commercial products.The basic challenge involves determining the structurefunction relationships for the electronic transport(intra-or intermolecular)through a junction containing one or a few molecules,and developing a technique for low-cost,mass-produced single-molecule devices.Based on these challenges,this dissertation uses the scanning tunneling microscope break junction(STM-BJ)technology,electrochemical experiment,density functional theory(DFT)method and molecular docking simulation method to investgate the effects of hydrogen bonding on the electron transport of single-molecule junctions of imidazole compounds;explore the conductivity of polyoxometalates(POM)molecules in ionic liquids;study the regulation of protein electron transport(ETp)band gap by molecular doping;study the relationship between protein secondary structure and protein ETp performance.The specific research contents and results are as follows:1.This section investigated the effect of hydrogen bonding on the charge transport of imidazole compounds in single-molecule junctions based on STM-BJ technology.For one thing,The STM-BJ technique was used to fabricate the junctions by repeatedly crashing the STM tip into the substrate and then withdrawing it at constant sample bias(200 m V).During the withdrawal process,1H-imidazole formed a single-molecule junction in the gap between the tip and the substrate due to its formation of imidazolatemetal bonds.As the junctions were pulled apart,the length of a single-imidazole junction greatly surpassed the single-molecule N?N distance assemble between the two electrodes,thus suggested the formation of supramolecular entities as the singleentity junction was stretched further.For other,DFT and transport calculations were performed for the interpretation of the data obtained from STM-BJ experiments.The results shown that the calculated values of the conductance of monomer,dimer and trimer and the length of the junction were in good agreement with the measured values.Combining experimental and theoretical results,the connection of the two electrodes via the formation of oligomeric allowed for an effective charge transport,we believed that the peculiar behaviour of imidazole can be attributed to the formation of hydrogenbonded networks.2.This section studied the electrical conductivity and charge transport mechanism of a functionalised Anderson-Evans POM in a three-state electrochemical transistor single-molecule device.Firstly,following a published procedure to prepare the pyridylcapped compound Anderson-Evans POM.To confirm the structure of the AndersonEvans POM,the crystals was grown and the samples suitable for SCXRD was obtained.Secondly,cyclic voltammetry was performed on Anderson-Evans POM,and two quasireversible redox couples can be observed.Finally,the electrochemical STM-BJ(ECSTM-BJ)experiments were performed,that is,using a bipotentiostat and a 4-electrode cell(with the Au substrate as working electrode,a platinum counter electrode and a platinum reference electrode),the junctions showed a clear OFF-ON-OFF behaviour under electrochemical control,with a difference in conductance of more than one order of magnitude between two adjacent charge states.Combining the Nernstian model analysis and the two-terminal bias modulation experiments,the charge transport of single molecule junctions happened through a non-resonant,one-step tunnelling mechanism and junctions were robust even under high bias.3.This section reported a facile method to modulate the ETp band gaps of a protein(bovine serum albumin,BSA)via binding with foreign molecule(hemin).The formation of the hemin-BSA complex was first confirmed by theoretical simulation(molecular docking)and experimental characterization(fluorescence and absorption spectra),which shown that the hemin was positioned inside a hydrophobic cavity formed by hydrophobic amino acid residues and near to the Trp213 at subdomain IIA of BSA,and no significant effect on the structure of BSA.Circular dichroism(CD)spectra indicated that BSA conformation remained essentially invariant after formed the hemin-BSA complex because the helicity of free BSA(not binding)and hemin-BSA complex was estimated to be 66 and 65%,respectively.Moreover,this conformation structure was still preserved after hemin-BSA was immobilized on the Au substrate surface.The hemin-BSA complex was immobilized on an Au substrate surface with the same orientation via the –SH group of Cys34 on the protein surface.The current-voltage(I-V)responses were measured using the eutectic gallium-indium(EGa In)as a top electrode and Au film as a substrate electrode,and shown that the ETp processes of BSA and the hemin-BSA on the Au surface have obvious semiconductor characteristics.The CB and VB,which were estimated from the analysis of the differential conductance spectra,of the free BSA was ?0.75 ± 0.04 and ??0.75 ± 0.08 e V,respectively,and was equally distributed around Fermi level(0 e V),with a band gap of ?1.50 ± 0.05 e V.After binding with hemin,the protein CB(?0.64 ± 0.06 e V)and VB(?–0.29 ± 0.07 e V)were closer to the Fermi levels,resulting in a band gap of ?0.93 ± 0.05 e V,and demonstrating that hemin molecules can effectively regulate the ETp characteristics and the transport band gap of BSA.4.This section studied the transformation of protein secondary structure and the effects of different secondary structures on protein ETp performance.Firstly,four kinds of protein to Apo-Myoglobin(without heme,Apo-Mb),?-Chymotrypsin(?-Chy),Lysozyme and Lipase B were selected for electrochemical tests,their mainly secondary structure have ?-helix,?-sheet,?+? and ?/?,respectively.From their I-V curves,the result of ETp performance were: ?-Chy> Lipase B> Lysozyme> Apo-Mb.In view of this result,it can be assumed that a protein with more ?-sheet structure has higher ETp performance.Then,TFE was used to convert the ?-sheet structure of ?-Chy protein to ?-helix and the ETp performance before and after protein conversion were measured.The results also shown that with the decrease of ?-sheet structure,the ETp performance of ?-Chy protein decreased,which further explained that ?-sheet is beneficial to the protein's ETp performance.