High-resolution transmission electron microscopy investigation of nanostructures in SnO2 thin films prepared by pulsed laser deposition

Pulsed laser deposition (PLD) was used to grow nanocrystalline SnO₂ thin films onto glass substrates. The nanocrystallites and microstructures in SnO₂ thin films grown by PLD techniques have been investigated in detail by using X-ray diffraction and high-resolution transmission electron microscopy (HRTEM). The PLD process was carried out at room temperature under a working pressure of about 2×10⁻⁶ mbar. Experimental results indicate that thin films are composed of a polycrystalline SnO₂ and an amorphous SnO phase. In particular, the presence of such an amorphous SnO phase in the thin films greatly limits their practical use as gas-sensing devices. HRTEM observations revealed that SnO₂ nanocrystallites with tetragonal rutile structure embed in an amorphous SnO matrix, which are approximatively equiaxed. These approximatively equiaxed SnO₂ nanocrystallites contain a high density of defects, such as twin boundaries and edge dislocations. The grain growth of SnO₂ thin films may be discussed in terms of the coalescent particle growth mechanism.     

Characterization of vacancy-related defects introduced into silicon during heat treatment by deep-level transient spectroscopy and gamma-ray diffraction techniques

Vacancy-related defects introduced into n-Si during annealing or aluminium diffusion at high temperature (1000–1250°C) have been studied. Different ambients (argon, nitrogen, vacuum and chlorine-containing atmosphere) were used to create a vacancy supersaturation during heat treatments. Three deep-level centers whose formation is governed by the presence of vacancies have been identified. They were characterized by the following temperature dependences of the thermal emission rate:e3 = 7.92 × 10⁷ T ² × exp(– 0.455/kT),e ₅ = 2.64 × 10⁶ T ² × exp( – 0.266/kT),e ₇ = 7.26 × 10⁶ T ² × exp (– 0.192/kT). The influence of different factors, such as heat-treatment conditions, concentration of oxygen and doping level in initial crystals, on center formation was studied. An asymmetric diffuse-ray scattering was observed near the surface of a crystal irradiated by thermal neutrons and annealed in a chlorine-containing atmosphere. This scattering is related to the formation of structural defects of the vacancy type. In the same region of the crystal, the concentration of the E7 center was one order of magnitude higher than that of other deep-level centers. Comparison of the-ray diffraction and deeplevel transient spectroscopy (DLTS) data suggests that the formation of the center occurs under the conditions of Si supersaturation with vacancies.     

Formation of μ-dinitrogen (salen)osmium complexes via ligand-induced N···N coupling of (salen)osmium(VI) nitrides

Treatment of [N(n)Bu(4)][Os(VI)(N)Cl(4)] with a stoichiometric amount of H(2)L (L = N,N'-bis(salicylidene)-o-cyclohexylenediamine dianion) in the presence of PF(6)(-) or ClO(4)(-) in MeOH affords [Os(VI)(N)(L)(OH(2))](PF(6)) 1a and [Os(VI)(N)(L)(CH(3)OH)](ClO(4)) 1b, respectively. The structure of 1b has been determined by X-ray crystallography and the Os≡N bond distance is 1.627(3) ?. In the presence of a N-donor heterocyclic ligand in CH(3)CN, 1a reacts at room temperature to afford the mixed-valence μ-N(2) (salen)osmium species [(X)(L)Os(III)-N≡N-Os(II)(L)(X)](PF(6)), 2-14 (X = py 2; 4-Mepy 3; 4-(t)Bupy 4; pz 5; 3-Mepz 6; 3,5-Me(2)pz 7; Im 8; 1-MeIm 9; 2-MeIm 10; 4-MeIm 11; 1,2-Me(2)Im 12; 2-Meozl 13; 4-MeTz 14). These complexes are formed by ligand-induced N···N coupling of two [Os(VI)≡N](+) to give initially [Os(III)-N(2)-Os(III)](2+), which is then reduced to give the more stable mixed-valence species [Os(III)-N(2)-Os(II)](+). Cyclic voltammograms (CVs) of 2-14 show two reversible couples, attributed to Os(III,III)/Os(III,II) and Os(III,II)/Os(II,II). The large comproportionation constants (K(com)) of (5.36-82.3) × 10(13) indicate charge delocalization in these complexes. The structures of 3 and 14 have been determined by X-ray crystallography, the salen ligands are in uncommon cis-β configuration. Oxidations of 4 and 14 by [Cp(2)Fe](PF(6)) afford the symmetrical species [(X)(L)Os(III)-N≡N-Os(III)(L)(X)](PF(6))(2) (X = 4-(t)Bupy 15; 4-MeTz 16). These are the first stable μ-N(2) diosmium(III,III) complexes that have been characterized by X-ray crystallography.     

MBE-grown Si: Er light-emitting structures: Effect of implantation and annealing on the luminescence properties

Features appearing in the photo-and electroluminescence spectra of light-emitting structures based on MBE-grown Si: Er layers are studied. The luminescence properties of Si layers implanted by Er and O ions were used as a reference. The temperature quenching of the photoluminescence intensity of Er-containing centers in MBE-grown and implanted layers can be approximated adequately by the same functional relationships with equal activation energies but with preexponential factors differing by more than two orders of magnitude. It is shown that the electroluminescence of Er³⁺ ions can be increased by additional coimplantation of erbium and oxygen ions into MBE-grown light-emitting diode structures and subsequent annealing. After this treatment, the Er-containing centers continue to dominate the luminescence spectrum.     

Silicon LEDs emitting in the band-to-band transition region: Effect of temperature and current strength

The parameters of silicon light-emitting diodes (LEDs) prepared through boron implantation into n-Si, followed by annealing at 700–1200°C, were studied. The maximum room-temperature internal quantum efficiency of electroluminescence (EL) in the region of band-to-band transitions was estimated as 0.4% and reached at an annealing temperature of 1100°C. This value did not vary more than twofold within the operating temperature range 80–500 K. The EL growth and decay kinetics was studied at various currents. Following an initial current range of nonlinear dependence, the EL intensity scaled linearly with the current. It is shown that interpretation of this result will apparently require a revision of some present-day physical concepts concerning carrier recombination in silicon diodes.

     

A local relaxation method for the cardinality constrained portfolio optimization problem

The NP-hard nature of cardinality constrained mean-variance portfolio optimization problems has led to a number of different algorithms with varying degrees of success in reaching optimality given limited computational resources and under the presence of strict time constraints present in practice. The proposed local relaxation algorithm explores the inherent structure of the objective function. It solves a sequence of small, local, quadratic-programs by first projecting asset returns onto a reduced metric space, followed by clustering in this space to identify sub-groups of assets that best accentuate a suitable measure of similarity amongst different assets. The algorithm can either be cold started using a suitable heuristic method such as the centroids of initial clusters or be warm started based on the last output. Results, using a basket of up to 3,000 stocks and with different cardinality constraints, indicates that the proposed algorithm can lead to significant performance gain over popular branch-and-cut methods. One key application of this algorithm is in dealing with large scale cardinality constrained portfolio optimization under tight time constraint, such as for the purpose of index tracking or index arbitrage at high frequency.     

Nonequilibrium dynamical phase transition of 3D kinetic Ising／Heisenberg spin system

We have studied the nonequilibrium dynamic phase transitions of both three-dimensional (3D) kinetic Ising and Heisenberg spin systems in the presence of a perturbative magnetic field by Monte Carlo simulation. The feature of the phase transition is characterized by studying the distribution of the dynamical order parameter. In the case of anisotropic Ising spin system (ISS), the dynamic transition is discontinuous and continuous under low and high temperatures respectively, which indicates the existence of a tri-critical point (TCP) on the phase boundary separating low-temperature order phase and high-temperature disorder phase. The TCP shifts towards the higher temperature region with the decrease of frequency, i.e. T_{TCP}=1.33×exp(-ω/30.7). In the case of the isotropic Heisenberg spin system (HSS), however, the situation on dynamic phase transition of HSS is quite different from that of ISS in that no stable dynamical phase transition was observed in kinetic HSS after a threshold time. The evolution of magnetization in the HSS driven by a symmetrical external field after a certain duration always tends asymptotically to a disorder state no matter what an initial state the system starts with. The threshold time τ depends upon the amplitude H_{0}, reduced temperature T/T_C and the frequency ω as τ=C·ω^α·H_0^{-β}·(T/T_C)^{-γ}.     

A comparative study of donor formation in dysprosium, holmium, and erbium implanted silicon

Formation of donor centers in Czochralski grown silicon doped with dysprosium, holmium, and erbium is discussed. Donor states of three kinds are introduced in the implanted layers after annealing at T=700°C. Shallow donor states with ionization energies between 20 and 40 meV are attributed to oxygen -related thermal donors. Other donor centers in the energy range of E_C−(60…70) meV and E_C−(100…120) meV appear to be dependent on dopants. After a 900°C anneal strong changes in the donor formation are observed only in silicon doped with erbium. Instead of donors at E_C−(118±5) meV, new donor centres at E_C−(145±5) meV are formed. Reportedly, the latter ones are involved in the excitation process of the Er³⁺ ions with a characteristic luminescence line at ≈1.54 μm.     

Co chemisorption on PtCu surfaces II. (1 × 1)-CO/Pt0.98Cu0.02(110)

The chemisorption of CO on the Pt atoms of an initially (1 × 3) reconstructed Pt_0.98Cu_0.02(110) surface at ~ 373 K can lead to the formation of a (1 × 1) surface. Comparisons are made with (1 × 3)-CO surfaces formed by CO exposures at 293 or 155 K. Thermal desorption shows that the (1 × 1)-CO surface has an enhanced population of high temperature CO peak ( ~ 543 K) from Pt sites. The CO-induced structural conversion also leads to a decrease in the subsequent CO uptake on the low temperature Pt sites and on the Pt-Cu “mixed” sites, with a concomitant increase in adsorption on the Cu-like sites. Such a reduction in the number of the Pt-Cu “ mixed” sites is also reflected in the CO-induced changes of the Cu 3d-derived states and the $\text{Cu 2p}{}_{\mathrm{<mtext>3</mtext><mtext>2</mtext>}}$ core levels. A dynamic interplay between chemisorption and surface structure is thus demonstrated.