Ab initio calculations of structure and stability of small boron nitride clusters

Clusters of boron nitride B_xN_x (x = 1–4, 12, 15, 30) were investigated by the Hartree-Fock and density functional methods using the 6-31G* basis. It was found that linear, cyclic, and shell structures are stable against minor deformations of the B_xN_x cluster. Inclusion of electron correlation in calculation markedly changes the electron density distribution and the structure of the clusters.     

The structure of small carbon clusters

New non linear isomers of C _n forn=4, 5, 6 have been observed using the combination of the laser photodetachment technique and the Coulomb Explosion Imaging method. Electron affinities of these isomers were found to be lower than the corresponding known linear isomers. The structure of low electron affinity C₄ isomer was found to be rhombic in accordance with recent theory. Indications for the non linearity of the low electron affinity isomers of C₅ and C₆ are also presented.     

The electronic structure of small sodium clusters

Ab initio molecular orbital calculations using the STO3-21G basis set has been carried out for the cluster series Na _n ⁺ , Na _n , and Na _n ^– (wheren=2–7). The basis set is shown to be reliable compared with more extensive basis sets at the Hartree-Fock level. Thirty-one optimized structures are reported and discussed, many of which (especially for the anions) have not been considered. The STO3-21G//STO3-21G calculations suggest that for most of the species the optimum geometries are planar. In particular, the optimized structures for the anionic species should provide a starting point for more sophisticated configuration interaction calculations.     

The electronic structure of small lithium clusters

Ab initio molecular orbital calculations using the STO-6G and STO6-21G basis sets have been performed for the cluster series Li _n ⁺ , Li _n , and Li _n ^– (wheren=2–7). Thirty-two optimized structures are discussed and reported, many of which (especially for the anionic structures) have not yet been considered. The calculations suggest that for all three species the optimum geometries are planar. Of the two levels of theories that were investigated, STO-6G//STO-6G and STO6-21G//STO-6G, the latter hybrid theory was found to be less reliable. In particular, for the anionic structures these calculations should provide a platform from which more sophisticated, i.e., configuration interaction, geometry optimization can be performed.     

Bonding and energetics in small clusters of gallium and arsenic

Density functional theory is used to study the electronic, geometric, and bonding properties of small Ga_n, As_m, and Ga_nAs_m clusters with up to eight atoms. New ground state structures for many of these clusters are reported. Electronic states, stabilities, and charges are also reported and discussed in terms of the bonding in the clusters. © 2003 Wiley Periodicals, Inc. Heteroatom Chem 14:189–196, 2003; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/hc.10127     

Electronic structure of small copper clusters

An all-electron ab initio LCAO -MO SCF calculation has been carried out for the electronic structure of small copper clusters (Cu_n, n = 2–6). The basis set superposition error occurring in the calculation, the equilibrium configuration of Cu₃, the bond energy in the clusters, and the localized d-hole in excited and ionized states of Cu₂ are closely examined.     

Controllable electrochemical-magnetorheological finishing of single-crystal gallium nitride wafers

Journal of Solid State Electrochemistry - Electrochemical magnetorheological finishing (EC–MRF) is an ultrasmooth planarization method that etches a workpiece surface with a controllable...     

Template assisted design of microporous gallium nitride materials

For the first time the synthesis of a porous gallium nitride material (GaN1.15H1.18O0.06C0.04) with narrow pore size distribution in the micropore regime using a nonylamine assisted sol-gel route is reported. The yellow solid has a very high micropore volume of 0.176 cm3 g(-1) comparable to that of zeolitic solids.     

Catalytic synthesis and photoluminescence of gallium nitride nanowires

Single crystalline wurzite GaN nanowires were successfully synthesized on the NiO catalyzed alumina substrate through a simple thermal chemical vapor deposition method. The mixture of Ga and GaN powder was used as the source material of Ga for synthesizing GaN nanowires. The diameter of nanowires ranged 50–60 nm and the length was about hundreds of micrometers. The nanowires were single crystal with hexagonal wurzite structure. The peaks of Raman spectra of GaN nanowires appeared broadened and asymmetric compared with those of bulk GaN. PL spectra under excitation at 325 nm showed a strong emission at 3.315 eV from near band-edge transition and a broad weak emission at 2.695 eV related to deep level defects.     

Oligonuclear gallium nitrogen cage compounds: molecular intermediates on the way from gallium hydrazides to gallium nitride

Gallium hydrazides are potentially applicable as facile starting compounds for the generation of GaN by thermolysis. The decomposition pathways are, however, complicated and depend strongly on the substituents attached to the gallium atoms and the hydrazido groups. This paper describes some systematic investigations into the thermolysis of the gallium hydrazine adduct Bu(t)(3)Ga←NH(2)-NHMe (1a) and the dimeric gallium hydrazides [R(2)Ga(N(2)H(2)R')](2) (2b, R = Bu(t), R' = Bu(t); 2c, R = Pr(i), R' = Ph; 2d, R = Me, R' = Bu(t)) which have four- or five-membered heterocycles in their molecular cores. Heating of the adduct 1a to 170 °C gave the heterocyclic compound Bu(t)(2)Ga(μ-NH(2))[μ-N(Me)-N(=CH(2))]GaBu(t)(2) (3) by cleavage of N-N bonds and rearrangement. 3 was further converted at 400 °C into the tetrameric gallium cyanide (Bu(t)(2)GaCN)(4) (4). The thermolysis of the hydrazide (Bu(t)(2)Ga)(2)(NH-NHBu(t))(2) (2b) at temperatures between 270 and 420 °C resulted in cleavage of all N-N bonds and the formation of an octanuclear gallium imide, (Bu(t)GaNH)(8) (6). The trimeric dialkylgallium amide (Bu(t)(2)GaNH(2))(3) (5) was isolated as an intermediate. Thermolysis of the hydrazides (Pr(i)(2)Ga)(2)(NH-NHPh)(NH(2)-NPh) (2c) and (Me(2)Ga)(2)(NH-NHBu(t))(2) (2d) proceeded in contrast with retention of the N-N bonds and afforded a variety of novel gallium hydrazido cage compounds with four gallium atoms and up to four hydrazido groups in a single molecule: (Pr(i)Ga)(4)(NH-NPh)(3)NH (7), (MeGa)(4)(NH-NBu(t))(4) (8), (MeGa)(4)(NH-NBu(t))(3)NBu(t) (9), and (MeGa)(4)(NHNBu(t))(3)NH (10). Partial hydrolysis gave reproducibly the unique octanuclear mixed hydrazido oxo compound (MeGa)(8)(NHNBu(t))(4)O(4) (11).     

The isomers of small carbon clusters

Absolute cross-sections for photodetachment of negative carbon clusters are reported for C _n ^? (n=3,...,8). These measurements are made using different types of ion sources, which create different isomers. These new results indicate that various negative and neutral isomers exist, some with electron affinities as low as ～1 eV.     

Polarized Raman confocal microscopy of single gallium nitride nanowires

Polarized Raman spectra and corresponding Raman scattering intensity images of an isolated gallium nitride nanowire with a diameter of 170 nm are presented. The sensitivity of the confocal microscope combined with a high-resolution piezoelectric stage enables analysis of the crystalline phase and crystallographic orientation of an individual nanowire with an excellent spatial and spectral resolution in a short acquisition time.     

Electronic structure and magnetic properties of small manganese oxide clusters

To investigate the electronic structure and magnetic properties of manganese oxide clusters, we carried out first-principles electronic structure calculations for small MnO clusters. Among various structural and magnetic configurations of the clusters, the bulklike [111]-antiferromagnetic ordering is found to be favored energetically, while the surface atoms of the clusters exhibit interesting electronic and magnetic characteristics which are different from their bulk ones. The distinct features of the surface atoms are mainly attributed to the reduction of Mn coordination numbers and the bond-length contractions in the clusters, which may serve as a key factor for the understanding of physical and chemical properties of magnetic oxide nanoparticles.     

Second-order phase transitions in amorphous gallium clusters

Ion mobility and calorimetry measurements have been used to probe the nature of the phase transitions in gallium clusters with 29-55 atoms. While most clusters appear to undergo a first-order transition between solidlike and liquidlike phases, a few show the signature of melting without a significant latent heat. These transitions appear to be the finite size analogue of a second-order phase transition, and they presumably occur for some cluster sizes because their solidlike phase is amorphous.     

Catalytic growth and characterization of gallium nitride nanowires.

The preparation of high-purity and -quality gallium nitride nanowires is accomplished by a catalytic growth using gallium and ammonium. A series of catalysts and different reaction parameters were applied to systematically optimize and control the vapor-liquid-solid (VLS) growth of the nanowires. The resulting nanowires show predominantly wurtzite phase; they were up to several micrometers in length, typically with diameters of 10-50 nm. A minimum nanowire diameter of 6 nm has been achieved. Temperature dependence of photoluminescence spectra of the nanowires revealed that the emission mainly comes from wurtzite GaN with little contribution from the cubic phase. Moreover, the thermal quenching of photoluminescence was much reduced in the GaN nanowires. The Raman spectra showed five first-order phonon modes. The frequencies of these peaks were close to those of the bulk GaN, but the modes were significantly broadened, which is indicative of the phonon confinement effects associated with the nanoscale dimensions of the system. Additional Raman modes, not observed in the bulk GaN, were found in the nanowires. The field emission study showing notable emission current with low turn-on field suggests potential of the GaN nanowires in field emission applications. This work opens a wide route toward detailed studies of the fundamental properties and potential applications of semiconductor nanowires.     

The Hubbard model for small clusters

The Hubbard model with nearest neighbor hopping, and one type of orbital is applied to clusters of 4 and 5 sites. Results are reported for the ground state as a function of the number of electrons. A rather complicated dependence of the spin of the ground state on occupation number, geometry, and model parameters is found. Ground state energies are compared. The excitation spectrum is illustrated in a particular example.