Theoretical studies of germanium nanowires and single-walled carbon nanotubes.
SK, Mahasin Alam.
Date of Issue2012
School of Chemical and Biomedical Engineering
In this thesis we study the electronic properties of OH passivated germanium nanowires (OH-GeNWs) using density functional theory (DFT) calculations to investigate the cause of experimentally observed electrical hysteresis in GeNWs. We reveal that water molecule is initially physisorbed on the GeNW surface which is a reversible adsorption and over time, water molecule would dissociate into OH˙ and H˙ radicals leading to irreversible adsorption and formation of OH-GeNWs. In addition, we also show that the amount of water adsorbed is a main factor that affects stability and electronic properties of GeNWs. Based on our study, we attribute the causes of electrical hysteresis observed in GeNWs to (a) different amount of water being adsorbed, (b) alignment of the OH groups on the surface of GeNWs; and (c) presence of trap state defects on the surface of the OH-GeNWs. Surface dangling bond (SDB) defect induced electronic and magnetic properties of GeNWs are also investigated using DFT method. We show that single SDB defected GeNWs remain semiconducting as its non-defected form while double or multiple SDB defects result either semiconducting or metallic, depending on the defects’ locations on the surface. More importantly, we show that the electronic properties of surface defected GeNWs can also be fine-tuned by applying tensile and compressive strains. Upon the right loading, the surface defected GeNWs become half-metallic. In addition, surface defected GeNWs can be classify into: (1) GeNWs with zero magnetic moment are either metallic or semiconducting; (2) GeNWs with net magnetic moments equal to the number of SDBs are semiconducting with distinct spin-up and spin-down configurations; and (3) GeNWs with net magnetic moments significantly lower than the number of SDBs. We also found that defected GeNWs that fall under (3) are potentially half-metallic. Our results predict that half-metallic GeNWs can be obtained via engineering of the surface defects and the structures without the presence of impurity dopants. The interactions of different molecules/ radicals with single-walled carbon nanotubes (SWNTs) using DFT method are also investigated to obtain the fundamental understanding about the binding properties of molecules/ radicals and the mechanisms of separation of SWNTs which are essential for various applications of CNTs.