First-principles Study Of Defects In Carbon And Silicon Based Nanostructures

Many types of defects exist in or can be introduced into carbon and silicon based hon-eycomb structures. Line defects arise when the atom network of a nanostructure changes.Foreign atoms or molecules can bind to surfaces of nanostructures. And substitution occurswhen foreign atoms replace atoms of a nanostructure or bind to vacancies in the nanos-tructure. The properties of nanostructures with defects undergo changes with respect to thepristine structures. The presence of a particular defect can also make the system exhibitnovel properties. We have performed first-principles calculations to investigate the struc-tural, energetic, electronic and magnetic properties of various types of defects in carbonand silicon based nanostructures: line defects, adsorption or substitution of metal atoms aswell as adsorption of small polar molecules.A topological line defect in graphene can be obtained with one octagon and a pair ofpentagons periodically repeated along the zigzag direction. For zigzag graphene nanorib-bons with topological line defects, there are transitions from an antiferromagnetic (AFM)semiconductor to an AFM half-metal, and then to a ferromagnetic (FM) metal when chang-ing the ribbon width and the position of the line defect. For systems with AFM groundstates, the nanoribbons show half-metallicity as the line defect is close to one edge. Thishalf-metallicity is the intrinsic property of a particular topological network of carbon atom-s, and no external transverse electric field or impurities are required for realization of half-metallicity in this system. For a grain boundary with large vacancies in graphene, ourcalculations show that the vacancies undergo great reconstruction and dangling bonds inthe grain boundary result in flat bands just above the Fermi energy.Metal atoms can be adsorbed on the surfaces of a nanostructure or be embedded intothe nanostructure. In contrast to metal adatoms on graphene which tend to form clusters,some alkali and alkali-earth as well as transition metal adatoms on silicene obtain a largerbinding energy than the cohesive energy in bulk metals, indicating stable adsorption ofthese metal atoms on silicene. For the alkali metal adatoms on silicene, the bonding isapproximately ideal ionic. For the Ca adatom, hybridization between the Ca3d states andsilicene states occurs around the Fermi energy besides charge transfer from Ca to silicene.For groups III and IV and transition metal adatoms on silicene, the adatom states arestrongly hybridized with silicene states with electrons, holes or no charge carries doped into silicene. In addition, several metal adatoms on silicene are magnetic. For substitutionof metal atoms in semiconducting nanostructures, the electronic and magnetic propertiesare related to the size of the band gap. Monolayer BC3has a mediate band gap. For Fe,Co, and Ni embedded in BC3, substitutions of Fe for B and Co for C have a magneticmoment of1μB. For substitution of Fe for B, the system has only impurity levels ofminority-spin in the band gap. The doping of Ni introduces electrons into the conductionbands of BC3and there are no impurity levels in the band gap. Compared with BC3,monolayer MoS2has a rather wide band gap, and substitutions of Mn, Fe, and Co for Moall have impurity levels in the band gap. The occupation of impurity levels in the band gapas well as the magnetic moments of the dopants can be tuned by changing the chemicalpotential of electron in monolayer MoS2. The three dopants all have a maximal magneticmoment of3μB. In comparison with metal adatoms on graphene, our calculations withdiferent density functionals show that small polar molecules bind to graphene throughweak physical adsorption. We have also calculated adsorption capacity of these moleculeson graphene for diferent concentrations and temperatures. Chemical adsorption of smallpolar molecules can be realized at graphene edges with terminating oxygen atoms.