Theoretical Study Of The Mechanical Properties And Electronic Transport Behavior Of The Small Contact

The small contact can be defined as the contact that formed by a molecule, an atomic wire, or a nanostructure contacting with two micro electrodes. Because of its interesting mechanical and electronic transport properties, the small contact has become a focus of both theory and experiment recently.The study of the mechanical and electronic transport properties of small contact is of special importance of both theory and application. On one hand, when rapture takes place during the elongation, there can be only a single molecule or atom in the break point of the contact. Thus, the study of the deformation of small contact can give information on the mechanical and electric properties of a single molecule or an atom. On the other hand, the contacts formed by different metal, molecule, and nanostructure can have very different conductance, the study of its transport properties can do help to the design of the micro device. Therefore, the study of the small contact has become the focus of both condensed matter physics and molecular electronics.In this work, the density functional theory (DFT) and non-equilibrium Green's function method that based on DFT is used to study the mechanical and transport properties of the small contact during the deformation, some interesting and valuable results are obtained.I Effect of the gas atmosphere on the deformation behavior of small metal contactThe variation of the mechanical and electronic transport properties of the small Ag contact during deformation at an O₂ atmosphere is studied. The result shows that theO₂ molecule can move into the Ag contact during its elongation, and the molecular contact can be formed during this process. The mechanism of the O₂ molecule moving into the metal contact is investigated and it is found that the force to stretch the contact plays an important role in this process. A molecule-inserted-contact shows very different mechanical and transport properties from a pure metal one. The force to stretch the molecular contact and the length just before its break are all significantly different from pure metal one. The current and conductance at 100mV are calculated. The transmission spectrum and the projected density of state are used to analysis the result. It is found that when the O₂ molecule moves into the Ag contact, the transmission channels that based on the Ag atom are displaced by the one that based on the O₂ molecule. The break conductance of the molecular contact is determined by the O₂ molecule. Thus, there are two kinds of transmission channel in the contact, one is directly through the Ag molecule, and one is through the O₂ molecule. Two processes of the break of the bond which mainly determined characteristic of the conductance evolution are found during the elongation, one is the break of the Ag-Ag break, and one is the break of the Ag-0 bond.The evolution of the Pt contact at a H₂ atmosphere is studied. It is found that when being positioned perpetual to the stretch direction of the system, the H₂ molecule will abruptly moves into the Pt contact during the elongation. The whole process takes place in one elongation step, which is very different from the evolution of an Ag contact at an O₂ atmosphere. When being poisoned along the stretch direction of the contact, the H₂ molecule will detach during the elongation, with only one atom move into the contact, the other one will be absorbed by the base Ag atom of the pyramid. When H₂ moves into the contact, the two H atoms are first perpetual to the stretch direction of the system, and then turn an angle of Ï€/2 abruptly when the system is elongated continually. The system will at last break at the H-H bond which demonstrated that the H-Pt has a stronger interaction than the H-H bond. The current-voltage characteristic of two type of Pt-H₂ contact is calculated and the result is compared with the experiment. It is found that conductance of the Pt-H₂ contact in which the H₂ is along the stretch direction of the system is consistent very well with that of the experiment. The Feynman path method is used to elucidate the electronic conduct in the contact and relate the transmit channel with the localized states. It is shown that the anti-bonding state of the H₂ molecule takes part in the electronic conduct at low bias. According to their effect in the electronic transportation, the electronic state of the extended molecule in the scattering region is divided into type: that acted as the conducting state and that acted as reservoir state. The transport properties of CO molecule between two Au electrodes are investigated. We find that the interactions between the CO molecule and the two electrodes are very different. The C-Au interaction is very strong, while the O-Au interaction is very weak. The conductance spectrum of the system show strong asymmetry. The density of state projected on the CO molecule shows good resemblance in shape with the transmission spectrum. This demonstrates that the electron conduct is mainly by the CO molecule. The interaction between the molecule and the electrode significantly changes the electronic structure of the CO molecule, as a result, the eigenstate of CO molecule can not align with the transmission peak very well. II Transport properties of the Di-thiol-benzene (DTB) moleculeThe transport properties of the DBT molecule touched by two Au electrodes are studied. The effects of the coupling geometries between the molecule and the two electrodes, the H atoms at the end of the molecule, the bias voltage, and the interaction between the molecules on the electronic transport of the system are investigated. The transmission channels are related to the localized orbitals of the atoms in the scattering region, the results show that the coupling geometries between the molecule and the two electrodes can significantly change the transport properties of the system. The end H atoms of the Di-thiol-benzene decreases the system conduct. The current of the system changes with the position of the molecule at a no zero bias, the maximum value of current doesn't correspond to a symmetry coupling geometry. The two transmit peaks below and beyond the Fermi level are formed by the HOMO and LUMO of the molecule. When a molecule is chemically absorbed by one electrode at one side but physically absorbed by another electrode at another side, the symmetry of the conductance spectrum to the negative and positive bias will disappear and the conductance at low bias will remarkably decreased. The electronic transport properties of two molecules are studied and it is found that the interactions between them significantly change the system conductance.III Effects of adsorption, electrode and bias on the transport properties of the carbon nanotube. The effect of the adsorption of small gas molecule, like NH3, NO2, H2O, on the transport properties of the carbon nanotube is studied. It is found that the binding energies of the carbon nanotube and the gas molecules are all very small. The small gas molecules can not change the conductance of the carbon nanotube significantly. The binding energy and charge transfer between the H₂O molecule and the carbon nanotube are investigated, and the variation of the charge transfer with the distance between the molecule and carbon nanotube is checked. It is demonstrates that the conductance decrease after the adsorption of H₂O molecule found in the experiment is not an intrinsic properties of carbon nanotube but due to the doping of the nanotube. The change transfer between the carbon nanotube and the H₂O molecule can change the density of the carrier. A new point is put forward to elucidate the mechanism of the effect of the charge transfer on the transport properties of the carbon nanotube: there exists a region around the nanotube in which all the molecule can attribute to the charge transfer. Based on this point of view, an equation about the relation between the conductance of the carbon nanotube and the concentration of the surrounding H₂O gas is given, a good consistence with that of the experiment is achieved. By the comparisons of the I-V characteristics, transmission spectrum, and PDOS of the carbon nanotube that using the Au electrodes and that using the ideal carbon nanotube electrodes, the effect of the Au electrode on the electronic structure and transport properties of the carbon nanotube is investigated. The binding energy varision with the bias is calculated and it is found the bias can change the atomic structure of the system. The Stark effect due to the bias can significantly change the transmission spectrum of the carbon nanotube. IV Transport properties of two one dimensional carbon chainsThe transport properties of two one-dimensional-carbon chains are studied. It is found that according to the different distance between the two chains, there are three different interactions between the two chains as follows:(1) The atomic orbitals interaction. When the distance of the two chains is less than 3.5A, the current and conductance of the two chains is significantly different from two time of a single chain. In this case, the interaction between the orbitals of the two chains can change their electronic structure each others, and the current is very sensitive to the distance between the two chains.(2) The interference of the transmit mode. As the distance between the two chains is lager than 4.5A, the transmission spectrum of the two-chain-system shows high relevance to the adsorption site on the Al surface. We divide the sites in the electrode surface into two different types, and hence the coupling geometry of the two chains with the electrodes into two types: those coupling to the same sites and different type. The systems with the same coupling type show almost the same transmission spectra which independent of the distance of two chains, while the transmission spectra of systems with different coupling type are very different. A father investigation shows that this effect has nothing to do with the charge transfer and the orbitals interaction between the two chains. We attribute this effect to the interference between the electronic waves through the two chains (3) The Classical transportation. When the distance between the two chains is bigger than the coherent length of the electron, the electron through the two chains will transport independently and the current of the two-chain-system will equal to the two times of a one chain system. This is a classical effect and the system shows the same properties with a macroscopic conductor.