Electron Transport And Spin Manipulation Of The Organic Molecules Junction Sandwiched Between The Graphene Nanoribbon Electrodes

The preparation of multifunctional and high-performance molecular electronic devices by using the electrical properties of molecules has become one of the research hotspots of the molecular electronics.Based on the special electrical properties of molecules,various molecular electronic devices with different functions have been constructed,such as,molecular switches,molecular memories,molecular rectifiers,spin valves,and molecular wires.Organic conjugated small molecules and their oligomers are often used to construct molecular devices,because of its wide range of sources,chemically modification and adjustable structures.Based on the first-principles method of the density functional theory(DFT)combined with the non-equilibrium Green's function(NEGF),the thesis investigates the unique electronic transport properties such as double spin diode,spin filter,negative differential resistance,switching and molecular rectification effect of the isomeric quinoline molecular junctions and benzoquinone molecular junctions sandwiched between the nonmagnetic and/or magnetized graphene nanoribbon electrodes.The main research contents are following.The first chapter introduces the definition of electronic devices and briefly analyzes the development of electronic devices from the silicon-based semiconductor materials to microscopic molecules,except for the research progress of molecular electronic devices and the phenomenon of negative differential resistance,spin filtering,molecular rectification and molecular switching effect.Finally,the molecular electronic device materials and the research content and scientific significance of this thesis are introduced.The second chapter introduces the investigation method for the transport theory of the molecular electronic device.The transport properties,such as the transmission coefficient,current and conductance are derived from the dual-probe molecular electronic device,basing on the density functional theory(DFT),the non-equilibrium Green's function(NEGF)method,and the method of combining the DFT and NEGF.In addition for the brief introduction of the scientific research software used in the thesis,and the calculation steps for transport properties of the molecular electronic devices.In the third chapter,based on the first-principles calculation method combinding the DFT and NEGF,the electronic transport properties of isomeric quinoline molecular junctions sandwiched between the non-magnetic and magnetization graphene nanoribbon electrodes are studied.In the case of non-magnetic electrodes,the current of the quinoline molecular junction demonstrates a linear change in the bias range of [-0.3V,+0.3V],while in the bias range of [-0.4V,-0.9V] and [+0.5V,+0.8V],it decreases with the increase of the bias,demonstrating an obvious negative differential resistance(NDR)effect.In addition,when there is a certain angle between the central quinoline molecule and graphene nanoribbon electrodes,the current should exhibit a significant NDR effect,and the NDR effect is independent of the rotation direction of the central quinoline molecule.When the central quinoline molecule is perpendicular to the graphene nanoribbon electrodes,the current is forbidden.The position of the nitrogen atom in the quinoline molecule has been demonstrated to have no qualitative effect on the current-voltage(I-V)curve of the device.In the case of magnetized electrodes,the coplanar orientation configuration between the quinoline molecule,isoquinoline molecule and ZGNR,the up current(P-up)with parallel spin orientation of electrodes(PSOE)illustrates a linear and nonlinear I-V characteristics at the low(<0.2V)and high(>0.2V)bias regime,respectively.Moreover,the spin-parallel(P-down)current of the coplanar orientation configuration of the quinoline molecule tends to zero in the whole bias range,and the device exhibits a perfect spin filtering effect(spin polarization rate is 100%).Interestingly,a good dual spin diode effect appears in the case of anti-PSOE(APSOE).The up/down current with APSOE(AP-up/AP-down)under a negative bias is completely opposite to that under a positive bias.The electron of AP-up/AP-down substantially flows only in the negative/positive bias range,whereas it blocks in the positive/negative bias range.Therefore,the unidirectional spin-polarized current can be selectively generated and controlled by the bias voltage.However,when the quinoline molecular plane,isoquinoline molecular plane and graphene nanoribbon electrode plane are perpendicular,the current is less than 0.075?A.The switching effect can be achieved by changing the orientation between the quinoline molecule,the isoquinoline molecule and the electrode plane.Based on the first-principles method of the DFT and NEGF,the fourth chapter invesitigates the negative differential resistance and rectification effect of benzoquinone-based molecular electronic devices sandwiched between the graphene nanoribbon electrodes.The device currents with the central p-benzoquinone and o-benzoquinone molecular behaves negative differential resistance effect in the bias voltage range of [±0.9V,± 1.5V] and [±0.6V,±1.1V],respectively.In addition,the interesting NDR effect of the device with the central carbon and nitrogen atom connected o-and p-benzoquinone molecules has been observed in the bias voltage range of [0.9V,1.2V] and [-0.8V,-1.0V],respectively.The current of the device with the central sulfur and oxygen atom connected o-and p-benzoquinone molecules should decrease with the increase of the bias voltage at the regime of [0.8V,1.0V],showing a negative differential resistance effect.While that should be forbidden when a negative bias is applied,showing an interesting molecular rectification effect.The maximum rectification ratio of the device with the central sulfur and oxygen atom connected o-benzoquinone and p-benzoquinone molecules is observed to be up to 57.85 and 55.85,respectively.The fifth chapter briefly summarizes and prospects the work of the full text.