SnO2 is a prototype "transparent conductor," exhibiting the contradictory properties of high metallic conductivity due to massive structural nonstoichiometry with nearly complete, insulator-like transparency in the visible range. We found, via first-principles calculations, that the tin interstitial and oxygen vacancy have surprisingly low formation energies and strong mutual attraction, explaining the natural nonstoichiometry of this system. The stability of these intrinsic defects is traced back to the multivalence of tin. These defects donate electrons to the conduction band without increasing optical interband absorption, explaining coexistence of conductivity with transparency.
We present a systematic analysis of the effect of radial deformation on the atomic and electronic structure of zigzag and armchair single wall carbon nanotubes using the first-principle plane wave method. The nanotubes were deformed by applying a radial strain, which distorts the circular cross section to an elliptical one. The atomic structure of the nanotubes under this strain are fully optimized, and the electronic structure is calculated self-consistently to determine the response of individual bands to the radial deformation. The band gap of the insulating tube is closed and eventually an insulator-metal transition sets in by the radial strain which is in the elastic range. Using this property a multiple quantum well structure with tunable and reversible electronic structure is formed on an individual nanotube and its band lineup is determined from first principles. The elastic energy due to the radial deformation and elastic constants are calculated and compared with classical theories.
whose conduction band minimum ͑CBM͒ lie below this level ͑i.e., electron affinityϾ3.0Ϯ0.4 eV͒ will become conductive once hydrogen is incorporated into the lattice, without reducing the host. Conversely, materials such as BaO, NiO, SrO, HfO 2 , and Al 2 O 3 , whose CBM lie above this level ͑i.e., electron affinityϽ3.0Ϯ0.4 eV͒ will remain nonconductive since hydrogen forms a deep impurity.
We predict new forms of carbon consisting of one and two dimensional networks of interlinked single wall carbon nanotubes, some of which are energetically more stable than van der Waals packing of the nanotubes on a hexagonal lattice. These interlinked nanotubes are further transformed with higher applied external pressures to more dense and complicated stable structures, in which curvature-induced carbon sp 3 re-hybridizations are formed. We also discuss the energetics of the bond formation between nanotubes and the electronic properties of these predicted novel structures.PACS numbers: 61.48.+c,61.46.+w,62.50.+p,61.50.Ah,71.20.Tx Carbon nanotubes, originally discovered as by-products of fullerene synthesis 1,2 , are now considered to be the building blocks of future nanoscale electronic and mechanical devices. It is therefore desirable to have a good understanding of their electronic and mechanical properties and the interrelations between them. In particular, single wall carbon nanotubes (SWNT) provide a system where the electronic properties can be controlled by the structure of the nanotubes and by various deformations of their geometries 3-5 . The physical properties can also be altered by intertube interactions between nanotubes packed in hexagonal lattices, as so called "nanoropes".The intertube interactions in nanoropes can be probed by applying external pressure to vary the intertube distance 6-8 . For fullerenes, such high pressure studies have yielded many interesting results including new compounds such as the pressure-induced polymeric phases of C 60 9 . It is, therefore, of interest to inquire if similar covalent-bonding can occur between the nanotubes in a rope. This could have important consequences for nanoscale device applications and composite materials which require strong mechanical properties since nanoropes consisting of inter-linked SWNT will be significantly stronger than nanoropes composed of van der Waals packed nanotubes 10 .A recent Raman study on SWNT ropes carried out up to 25.9 GPa 7 showed that the mode intensities and energies are not completely reversible upon pressure cycling, suggesting irreversible pressure-induced changes in the structure. In another high pressure study Chesnokov et al. 8 observed a very large volume reduction and high compressibility, signaling the presence of a microscopic volume-reducing deformation other than van der Waals compression. Some of these pressure-induced effects are tentatively attributed to possible crushing or flattening the nanotube cross section from circular to elliptical or hexagonal 8 . Motivated by these reports, we investigated possible new pressureinduced ground state structures for (n,0) nanotube ropes 11 from first-principles total energy calculations using the pseudopotential method within the generalized gradient approximation (GGA) 12 . We observed an elliptical distortion of the nanotubes under pressure and subsequent curvature-induced carbon re-hybridization, giving rise to one or two dimensional interlinked networks of ...
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