Electronic Property Of Large Conjugated Carbon System And Its Application In Molecular Device

Large conjugated materials based on carbon,such as carbon nanotubes and graphene nanoribbons,has broad application prospects from microscopic nanodevices to macroscopic energy materials,owing to their distinct geometrical and electronic structures.The electronic properties of the large conjugated carbon materials include the electon transport between large conjugated carbon systems,the electron transport in single large conjugated carbon system and the interfacial electron transport between large conjugated carbon system and functional materials,which influence the electronic properties of nanodevices and energy materials.So far,these three kinds of electron transport behavior has not been studied systematically.This thesis studied them systematically for the first time using density functional theory(DFT)combined with non-equilibrium Green function(NEGF)methods.The static properties of the large conjugated carbon materials,such as geometrical and electronic structures are investigated with Gaussian software and the electron transport behavior of the carbon materials is dynamically analyzed using Atomistix Toolkit software.By designing the graphene nanoribbon with finite scale,we studied the electron transport behavior between large conjucated carbon systems and the behavior in single large conjucated carbon system.In terms of the interfacial electron transport behavior between large conjucated carbon system and functional materials,we designed and studied a series of molecular devices and energy materials.The systematical investigation on these three kinds of electron transport behavior contributes to understanding the general electronic properties of molecular devices and energy materials based on large conjucated carbon.The thesis includes the following sections:1.In order to understand the electron transport behavior between large conjugated carbon systems,we investigated the electronic properties of parallel oligophenyls with ?-stacked units.With the increasing number of ?-stacked units,the?-? stacking interaction between parallel oligophenyl chains is enhanced whereas the coupling between the central molecules and the electrodes is weakened.These two effects reach a balance in the parallel oligophenyls with 5 ?-stacked units.The ?-?stacking interaction is weakened exponentially with the separating distance.Large separating distance impedes electrons to pass through parallel molecules.The result shows that the electron transport between parallel oligophenyl chains is negligible when the parallel distance is larger than 3.5 A,which matches the conductance anisotropy in bulk graphite.For studying the electron transport behavior in single large conjugated carbon system,we designed a series of graphene nanoribbons composed of oligoacenes with different overlapping degrees.In the Au-graphene nanoribbon-Au single molecular junction,the alternating single-double bonds compose the most probable electron transport pathway.The effciency of electron transport in this pathway is irrelevant to the width of graphene nanoribbon,however,is strongly dependent on its length.The exponential conductance attenuation with length indicates the electron transport in the graphene nanoribbon follows the tunnelling mechanism,which provides a theoretical basis for the investigation on the electron transport behavior between large conjugated carbon system and functional materials.2.The electronic properties of organic functional molecules are the key factor on the interfacial electron transport behavior between large conjugated carbon system and functional materials.To this end,we studied the electronic properties of a series of photochromic molecules with Au electrodes.By analyzing the optical switching properties caused by different configuration transformations,we found some common reasons that affect the efficiency of electron transport in photochromic molecules.The electron can transport through the molecule efficiently if a pathway composed of alternating single-double bonds exists in the molecular structure,so the configuration is in the current-on state,and vice versa.The same configuration transformation usually leads to the similar optical switching properties of different molecules with specified units.So as to research on the interfical electron transport behavior between large conjugated system and functional molecule,we used a series of optical molecular switches by using perfluorocyclopentene(DT)as the functional molecule.Graphene(GN),single-wall carbon nanotube(SWCNT)and Au are applied as the electrode materials in optical molecular switch,respectively.The large conjugated system and DT molecule is connected with C-C covalent bond,which is part of the pathway composed of alternating single-double bonds in closed-DT.The electron can transport efficiently on the pathway,hence closed-DT is in current-on state.However,the configuration transformation upon photoexitation not only weakens the ?-coupling in DT molecule,but also weakens the coupling between large conjugated system and DT.As a result,the current of opened-DT with large conjugated electrodes is greatly reduced compared to that with Au electrodes.The coupling between GN and DT is much more sensitive to the configuration transformations than that between SWCNTs and DT.In the same bias range,the DT molecular switch with GN electrodes can reach the largest current on-off ratio 270.3.To investigate the influence of the chiralities of large conjugated carbon system and the connection configuration of functional molecule on the interfacial electron transport between large conjugated carbon system and functional molecule,we designed a kind of CO sensor that is composed of Fe(?)porphyrin(PP-Fe)and two single-wall carbon nanotube electrodes(SWCNTs).We studied the influence of connection conjugations and SWCNT chiralities on the sensitivity of PP-Fe to CO.When PP-Fe is connected to SWCNT electrodes in diagonal configuration,it shows higher CO sensitivity than that in para connection mode,owing to the fact that electron passes through the Fe center,Meanwhile,the PP-Fe sensor with armchair SWCNTs is more sensitive to CO than that in zigzag junction,since the coupling interaction between central molecule and armchair SWCNT electrodes is stronger.The PP-Fe sensor anchored on armchair SWCNTs in diagonal configuration can reach the current on-ofd ratio of 2.1×104 in response to the chemisorption of CO,showing better CO sensitive characteristics than that with zigzag SWCNTs and metallic electrodes.With the development of experimental techniques,it is completely possible to fabricate this kind of CO sensor by experimental means.4.In order to clarify the interfacial electron transport behavior between large conjugated carbon system and inorganic materials without direct connections,we modified graphene substrate with oxidized defects and then absorbed Ni(OH)2 with the same number of oxidized defects on reduced graphene oxide(rGNO).By analysing the interaction between graphene substrate and oxidized defects with different reduction degrees and the absorption energy between Ni(OH)2 molecule and rGNO,we found that the interaction between hydroxyl group and graphene is stronger than that with less reduction degrees and the adsorption energy between Ni(OH)2 and hydroxyl-graphene oxide is also the strongest.As a result,the effciency of electron transport between hydroxyl-graphene oxide and Ni(OH)2 is the highest,which contribute to the preparation of Ni(OH)2 supercapacitor based on rGNO,Meanwhile,the strong coordination between Ni atom and hydroxyl group on the neighboring Ni(OH)2 molecules leads to that the diameter of Ni(OH)2 particles reaches nanoscale level,which can improve the utilization ratio of Ni(OH)2 in supercapacitor.On the other hand,the interaction between oxided defects and graphene substrate results in the asymmetrical geometrical configuration of graphene substrate.When bias is applied,rGNO shows remarkable negative differential resistance(NDR)phenomenon.The NDR effect is much more significant when the reduction degree of oxided defect is improved,because hydroxyl group can make graphene sheet more asymmetrical.Similarly,after absorbing Ni(OH)2,the NDR effect is intensified under the influence of interaction between Ni(OH)2 and the oxided defects on rGNO.This NDR effect is promising in the application of molecular switch,logic circuit and memory device.The research in this part is fundational to the wide application of large conjugated carbon systems in integrated circuit.