Please use this identifier to cite or link to this item: http://scholarbank.nus.edu.sg/handle/10635/30731
Title: First-principles simulations of nanomaterials for nanoelectronics and spintronics
Authors: CAI YONGQING
Keywords: Density functional theory; Nonequilibrium Green's function theory; Molecular electronics; Spintronics; Nanotubes; Graphene
Issue Date: 17-Aug-2011
Source: CAI YONGQING (2011-08-17). First-principles simulations of nanomaterials for nanoelectronics and spintronics. ScholarBank@NUS Repository.
Abstract: Rapid developments of nanoelectronics and spintronics call for the design of new materials for building blocks of nanoscale electronic devices. In this thesis, first principles calculations were carried out to study the physical properties of various kinds of nanomaterials and investigate their potential applications in nanoelectronics and spintronics. Firstly, we studied coherent electronic transport through a single light sensitive diarylethene molecule sandwiched between two graphene nanoribbons (GNRs). The "open" and "closed" isomers of the diarylethene molecule that can be converted between each other upon photo-excitation were found to have drastically different current-voltage characteristics. More importantly, when one GNR is metallic and another one is semiconducting, strong rectification behavior of the "closed" diarylethene isomer with the rectification ratio >10^3 was observed. The results open possibilities for the design of a new class of molecular electronic devices. Secondly, electronic and/or transport properties of gold nanotubes and carbon nanotubes (CNTs) were studied. For gold nanotubes, effects of adsorbates (CO molecule and O atom) and defects on the electronic and transport properties of Au (5,3) and Au (5,5) nanotubes were investigated. After CO adsorption, the conductance of Au (5,3) decreases by 0.9 G0, and the conductance of Au (5,5) decreases by approximately 0.5 G0. For O adsorbed Au tubes, O atoms strongly interact with Au tubes, leading to around 2 G0 of drop of conductance for both Au tubes. When a monovacancy defect is present, the conductance decreases by around 1 G0 for both tubes. For CNT, strain dependence of work functions of both pristine and potassium doped CNTs was calculated. We found that for pristine cases, the uniaxial strain has strong effects on the work functions of CNTs, and the responses of work functions of CNT (5,5) and (9,0) to the strain are distinctly different. When coated with potassium, for both CNTs, work functions can be reduced by more than 2.0 eV, and the strain dependence of work functions changes drastically. Finally, effects of strain on transport properties of Co2CrAl/NaNbO3/Co2CrAl magnetic tunneling junction (MTJ) were studied. Both spin polarization and tunnel magnetoresistance (TMR) of the MTJ were found to respond to positive (tensile) and negative (compressive) strains asymmetrically. While a compressive strain up to 4% causes slight increases in the spin polarization and small fluctuations in TMR, a tensile strain of a few percent significantly reduces the TMR. This study provides a theoretical understanding on relationship between transport properties through a MTJ and interface atomic structural changes induced by an external strain.
URI: http://scholarbank.nus.edu.sg/handle/10635/30731
Appears in Collections:Ph.D Theses (Open)

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