Atomic Vacancy Defects Research Articles

Theoretical study of the stability of defects in single-walled carbon nanotubes as a function of their distance from the nanotube end

Point defects, including atom vacancy, add atom and Stone-Wale defect, close to a (5, 5) single-walled carbon nanotube (SWNT) end were studied by DFT, semi-empirical PM3 method and empirical Brenner Potential. It is found that closer to the SWNT opening end more stable these defects are. Based on these calculations, a model about fixing defects on a growing SWNT by defects diffusing to the SWNT opening end was proposed. Furthermore, calculations also show that, similar to the DFT calculation, semi-empirical PM3 method is accurate enough for studying defect formation in SWNT. In contrast, the results based on the empirical Brenner potential show large errors and sometimes even not correct in quality.

In situ nanoscale wet imaging of the heterogeneous catalyzationof nitriles in a solution phase: novel hydrogenation chemistry through nanocatalysts on nanosupports

Wet-environmental transmission electron microscopy studies of heterogeneous hydrogenation of complex nitriles in a liquid phase over new mesoporous cobalt-promoted ruthenium nanocatalysts on reducible nanotitania supports are presented. The desorbed organic products in the dynamic liquid phase hydrogenation are imaged situ on the nanoscale. The direct studies on the “nanocomposite” catalysts are correlated with parallel reaction chemistry measurements. They demonstrate high hydrogenation activity at low operating temperatures in the presence of atomic scale anion vacancy defects associated with Lewis acid sites at the nanosupport surface and an electronic and synergistic contribution to the promoter mechanism. The combined synergistic effect between the two metals and the interaction with the reduced nanosupport leading to an electronic modification lead to highly reactive site for the hydrogenation catalysis. The results illustrate novel selective hydrogenation chemistry with mesoporous nanocatalyst systems on nanosupports.

The structural defects and UV–VIS spectral characterization of TiO 2 particles doped in the lattice with Cr 3+ cations

Titania nanoparticles doped with Cr 3+ (2% relative to molar quantity of titania) were prepared and examined by EDS, HRTEM, XRD, and UV–VIS analysis. HRTEM images showed the detailed atomic arrays and vacancy defects of the doped Titania nanocrystals and revealed that the implanted Cr element existed in titania mainly as Cr 3+ ions which located at the lattice positions of Ti 4+ ions. Compared with pure titania, the UV–VIS spectra of the Cr 3+ doped titania show significantly increased absorbance in visible light region. This indicated that the presence of the Cr 3+ ions affected the lattice structure of titania nanocrystals and plays an reformative role in spectral feature of titania.

STM/STS investigation of the interaction of Si with atomic scale vacancies on cleaved GaAs

Cross-sectional scanning tunnelling microscopy (STM) images on a freshly cleaved III–V semiconductor laser facet regularly indicate atomic scale defects, such as steps and vacancies on the non-polar (1 1 0) surface. In this work, studies were made when sub-monolayer quantities of Si were deposited at 280 °C on clean cleaved GaAs(1 1 0). Selective adsorption of Si on atomic defects has been observed with STM imaging of the surface. The simultaneous use of STM and scanning tunnelling spectroscopy (STS) allows the quantification of the Fermi shifts present at atomic steps and vacancies before and after Si adsorption. Localised current–voltage measurements, performed using STS, on the atomic steps and vacancy defect clusters of p-type GaAs(1 1 0) surfaces showed Fermi-level shifts of ∼0.8 and 0.7 eV towards mid-gap position, respectively. However, measurements taken from silicon-coated atomic steps and defect clusters showed that the Fermi-level reverted back towards the ‘ideal’ flat band position by ∼0.4 eV. This behaviour was distinctively different to silicon adsorbed at defect-free surface of GaAs(1 1 0). These results indicated the passivating potential of silicon on the defect sites of GaAs(1 1 0) surface when deposited under these conditions. The ability to measure and control surface properties on the atomic scale is crucial to the success of nano-technology and the STM/STS studies demonstrate the effectiveness of the technique for this role.

Optimization of Xe adsorption kinetics in single walled carbon nanotubes

Closed end (10, 10) single walled carbon nanotubes (SWNTs) have been opened by oxidation at their ends and at wall defect sites, using ozone. Oxidation with ozone, followed by heating to 973 K to liberate CO and CO2, causes etching of the nanotube surface at carbon atom vacancy defect sites. The rate of adsorption of Xe has been carefully measured as a function of the degree of nanotube etching by ozone. It is found that a level of etching corresponding to wall openings of about 5–7 Å radius is optimal for maximizing the rate of Xe adsorption. Beyond this level of etching, the rate of Xe adsorption decreases as the surface area of the SWNTs decreases due to further carbon atom removal. Both experiment and modeling show that the presence of polar oxidized groups, such as –COOH or –COR groups, with dipole moments in the range 1.5–3.0 D at the perimeter of the defect sites, causes a retardation of the rate of Xe adsorption due to dipole-induced dipole interactions. This effect is larger for smaller radius defect sites and decreases as the defect sites increase in size beyond about 7 Å radius. At large defect radii, the energetic profile of the adsorption pathway controls the physisorption rate. Modeling shows that after Xe adsorption has been completed inside the nanotubes, then Xe clusters begin to form on the outer surface of the nanotubes at the defect sites where polar groups are present. The Xe clustering effect also occurs to a smaller degree when the defect sites are not decorated by polar groups. The experiments and modeling demonstrate how one may optimize the rate of adsorption of a gas into nanotubes by the adjustment of the size and polar character of the vacancy-site entry ports in the walls of the nanotubes.

THE EQUILIBRIUM DIAGRAM OF THE Fe-Ga BINARY SYSTEM

The equilibrium diagram of the Fe-Ga system below 1000℃ has been determined mainly by X-ray investigation incorporated with differential thermal analysis. The system consists at room temperature of three intermediate phases ε, χ, and ψ. The ε phase with a very narrow homogeneous range has an ordered face-centred structure corresponding to the ideal formula Fe3Ga. At about 550℃, it transforms into another phase ζ which separates at higher temperature directly from the solid solution of Ga in Fe. The structure of the high temperature phase ζ has not been determined. The homogeneous range of the χ phase extends at room temperature from 55 at. % Ga to 60 at. % Ga. The structure of this phase appears to be very complicated, presumably corresponding to the structural formula Fe4Ga5, Fe3Ga4 or Fe2Ga3. It is formed at about 960℃ by a peritectic reaction. The ψ phase is formed by another peritectic reaction at 820℃. The structure is tetragonal, with a=6.2628? and c=6.5559? at 20℃. The space group is D4h14-P42/mnm, and each unit cell contains four formula units corresponding to FeGa3. No noticeable solubility of Fe in Ga has been observed. There exists an eutectic isothermal ranging from FeGa3 to Ga, the eutectic point being very near to the pure component. There exists also an eutectoid isothermal at about 590℃, the eutectoid point being approximately at 50 at. % Ga, at which the solid solution of Ga in Fe decomposes simultaneously into ζ and χ. The most remarkable feature of this system is the primary solid solution region of Ga in Fe. The solubility at room temperature is 15.2 at. %. It gradually increases with temperature, and above 700℃, it increases suddenly up even to 50 at. %. The structure below 625℃ is body-centred cubic, designated as a in the phase diagram. But above 625℃, it changes into two other structures with their respective phase regions, designated as α1 and α2. The structures of α1 and α2 have not been elucidated yet, but most probably they are derived from a by stacking together the fundamental unit cells with atomic rearrangement or vacancy defect.

The Photographic Latent Image. III. Dimensional Changes in Silver Bromide Following Exposure

Using previously tested equations and constants, calculations are made of the dimensional changes to be expected in AgBr crystals as a result of the presence of varying proportions of uncharged bromine atom and silver-ion vacancy defects. The results depend to a considerable extent on the unknown degree of ordering of these defects. With reasonable assumptions as to this degree of ordering and as to the magnitudes of the other unknown and approximately known quantities involved, the experimentally observed behavior—a lattice constant decrease, followed by a return to approximately the original value on further exposure—is quantitatively accounted for.