Showing papers on "Bismuth published in 2000"

Bismuth-coated carbon electrodes for anodic stripping voltammetry

Abstract: Bismuth-coated carbon electrodes display an attractive stripping voltammetric performance which compares favorably with that of common mercury-film electrodes. These bismuth-film electrodes are prepared by adding 400 μg/L (ppb) bismuth(III) directly to the sample solution and simultanously depositing the bismuth and target metals on the glassy-carbon or carbon-fiber substrate. Stripping voltammetric measurements of microgram per liter levels of cadmium, lead, thallium, and zinc in nondeaerated solutions yielded well-defined peaks, along with a low background, following short deposition periods. Detection limit of 1.1 and 0.3 ppb lead are obtained following 2- and 10-min deposition, respectively. Changes in the peak potentials (compared to those observed at mercury electrodes) offer new selectivity dimensions. Scanning electron microscopy sheds useful insights into the different morphologies of the bismuth deposits on the carbon substrates. The in situ bismuth-plated electrodes exhibit a wide accessible po...

Transport properties of Bi nanowire arrays

Abstract: To explain various temperature-dependent resistivity measurements [R(T)] on bismuth (Bi) nanowires as a function of wire diameter down to 7 nm, a semiclassical transport model is developed, which explicitly considers anisotropic and nonparabolic carriers in cylindrical wires, and the relative importance of various scattering processes. R(T) of 40 nm Bi nanowires with various Te dopant concentrations is measured and interpreted within this theoretical framework.

Solvation of the bismuth(III) ion by water, dimethyl sulfoxide, N,N'-dimethylpropyleneurea, and N,N-dimethylthioformamide. An EXAFS, large-angle X-ray scattering, and crystallographic structural study.

Abstract: The structure of the solvated bismuth(III) ion in aqueous, dimethyl sulfoxide, N,N‘-dimethylpropyleneurea, and N,N-dimethylthioformamide solution has been studied by means of EXAFS and large-angle X-ray scattering (LAXS). The crystal structures of the solid compounds octakis(dimethyl sulfoxide)bismuth(III) perchlorate, [Bi(OS(CH3)2)8](ClO4)3, hexakis(N,N‘-dimethylpropyleneurea)bismuth(III) perchlorate, [Bi(OCN2(CH2)3(CH3)2)6](ClO4)3, and nonaaquabismuth(III) trifluoromethanesulfonate, [Bi(H2O)9](CF3SO3)3 (redetermination), have been determined. The aqueous solutions must be strongly acidic, since the hydrated bismuth(III) ion starts to hydrolyze into Bi6O4(OH)46+ complexes already at an excess of strong acid at 1.0 mol·dm-3. For very acidic aqueous perchlorate solutions, the LAXS and EXAFS data gave a satisfactory fit for eight-coordination of the bismuth(III) ion, with a mean Bi−O bond distance of 2.41(1) A. The crystal structure of octakis(dimethyl sulfoxide)bismuth(III) perchlorate shows that the bismu...

Hydrothermal synthesis of sodium and potassium bismuth titanates

Abstract: A thermodynamic model of electrolyte solutions has been used as a tool for equilibrium calculations in the Na−Bi−Ti−H2O and K−Bi−Ti−H2O systems. To perform the calculations, unknown standard-state properties for Na0.5Bi0.5TiO3 (NBT), K0.5Bi0.5TiO3 (KBT) and other solid compounds that exist in these systems were established. As a result of the calculations, stability and yield diagrams of the investigated systems were computed. Thus, the optimum conditions (i.e., temperature, synthesis precursor concentrations, solution pH) for the hydrothermal synthesis of phase-pure KBT and NBT are presented in a graphical form. They show the concentrations of precursors and pH-adjusting agents required for the formation of phase-pure alkaline bismuth titanates. The computed diagrams show that the formation of phase-pure sodium and potassium bismuth titanates takes place in highly alkaline solutions (e.g., at pH above 11.4 and 12.2, respectively, at 473 K). Results of our experiments validated the newly developed yield d...

Determination of trace impurities in some nickel compounds by flame atomic absorption spectrometry after solid phase extraction using Amberlite XAD-16 resin

Abstract: A simple flame atomic absorption spectrometric (FAAS) procedure for the determination of lead, bismuth, gold, palladium and cadmium as impurities in Raney nickel and nickel oxide was developed using a preconcentration step on an Amberlite XAD-16 resin packed column. Lead, bismuth, gold, palladium and cadmium were quantitatively recovered and separated from a solution containing 1 M HCl and 0.3 M NaI by the column system. Effects of the various parameters such as reagent concentrations, sample volume, matrix effects, etc. have been investigated. Under optimized conditions, the relative standard deviation of the combined method of sample treatment, preconcentration and determination with FAAS (n = 7) is generally lower than 12%. The limit of detection (3s, n = 20) was between 10–270 ng/g. The results were used for separation and preconcentration of five trace elements from nickel matrices.

A Naked Diatomic Molecule of Bismuth, [Bi2]2-, with a Short Bi−Bi Bond: Synthesis and Structure

Abstract: We have made a naked diatomic dianion of bismuth with a short Bi-Bi bond of 2.8377(7) Å that suggests possible multiple bonding. While diatomic molecules with multiple bonds are known among the lighter elements and are abundant in nature (O2 and N2 account for 99% of the atmosphere, C 2 in CaC2), they are rare or nonexistent among the heavier main-group elements (only S 2, Se2, and P2 at high temperatures). Substituted diatomic species with multiple homoatomic bonds, on the other hand, have become quite common. Numerous examples of RP d PR, RAsdAsR, R2SidSiR2, and R2GedGeR2 are known. 1 In addition to these, a few examples of the heaviest pnictogens, Sb and Bi,2 and group 14 congeners, Sn and Pb, 3 have been made more recently although the multiplicity of the bonds in the latter two is questionable. A triply bonded gallium molecule was also reported, 4 but the multiplicity of its bond has been also disputed ever since. 5 An important feature common for all of these substituted species is the extremely bulky organic substituents that stabilize the species but at the same time makes them inaccessible for reactions. The multiple bonds are the result of the abnormally low coordination that is sterically forced by the organic groups. There is also a number of substituted diatomic species with single bonds. 1,6,7Singly bonded naked molecules are also known but all in insoluble neat solids. Examples are CaP, CaAs, Ca11Sb10, and Ca11Bi10 with P2, As2, Sb2, and Bi2, respectively, and Ca 5Si3, Ca5Ge3, Ba5Sn3, and Ba5Pb3 which contain Si 2, Ge2, Sn2, and Pb2, respectively. 8 All of these are isoelectronic with the well-known soluble O 2, S2, and the neutral diatomic molecules of the halogens. This collection, until now, lacked a naked diatomic molecule with a double bond. We report here the synthesis and structure of the soluble (K-crypt) 2Bi2 (1)9 which contains Bi 2 with a very short bond distance comparable to the distances in the two known RBi d R compounds with a double Bi dBi bond.2a,b Compound1 was initially detected as a small fraction, ∼10%, of a multiphase product crystallized from ethylenediamine solution of a precursor compound K 5In2Bi4, the latter made by heating a stoichiometric mixture of the elements. 10 The green-blue solution of the precursor in ethylenediamine and crypt is filtered, and the filtrate is carefully layered with toluene. 11 The reaction vessel was left undisturbed for a week in the initial synthesis. During that time crystals grow on the walls as a result of the toluene’s diffusion into the ethylenediamine. The synthesis is reproducible and was successfully carried out repeatedly. The product usually contains two other phases as well, (K-crypt) 2(InBi3) (2) with tetrahedra of [InBi 3] ( ∼80%) and (K-crypt) 6(In4Bi5)2 (3) with monocapped square antiprisms of [In 4Bi5] ( ∼10%). The structures of2 and 3 were also determined from single-crystal X-ray diffraction12 and were found to be isostructural with previously reported compounds. Compound 2 is isostructural and isoelectronic with (K-crypt) 2(Sn2Bi2) and (K-crypt)2(Pb2Sb2), while compound 3 is isostructural but not isoelectronic with (Kcrypt)6(Sn9)2 and (K-crypt)6(Pb9)2. The initial goal was to investigate whether it was possible to make deltahedral heteroatomic clusters of elements differing by two or more groups, i.e., In and Bi from groups 13 and 15, respectively. Thus, the existence of compound3 proves that this is possible. The yield of 1 was later improved greatly by increasing the amount of toluene and prolonging the time for its diffusion into the ethylenediamine. Apparently, the solubility of 1 in ethylenediamine is very high, and quantitative extraction (crystallization) is achieved by lowering that solubility with additional toluene and longer time for the diffusion. The product of such process carried out for over a month contains nearly 80% of 1 and the size of the crystals is in the order of 3 to 4 mm. The crystals of1 are hexagonal red-brown plates (greenish when thin). Several of them were selected and sealed in glass capillaries, and the structure was determined from X-ray diffraction data collected from one of them. 15 The compound crystallizes in a very high-symmetry space group, rhombohedral R3 h, perhaps

Growth of InxGa1−xAs/GaAs heterostructures using Bi as a surfactant

Abstract: The effects of a bismuth surfactant layer on the molecular beam epitaxy of GaAs and InxGa1−xAs layers on GaAs (001) were studied. The InxGa1−xAs surface reconstruction changed from arsenic stabilized 2×4 to bismuth stabilized 1×3 for high enough bismuth fluxes and low enough substrate temperatures. Maintaining a bismuth stabilized surface during InxGa1−xAs growth resulted in a larger number of reflection high-energy electron diffraction (RHEED) oscillations. RHEED patterns were also streakier after InxGa1−xAs growth with Bi. Roughness measurements using atomic force microscopy showed reduced root mean square roughness with Bi, e.g., from 3.8 to 2.8 nm, for 4 nm thick In0.3Ga0.7As layers. Simulations of x-ray diffraction results from 10 period In0.5Ga0.5As/GaAs superlattices showed that Bi reduced interface roughness from 1.1 to 0.5 nm and reduced interfacial broadening from 2.8 to 2.1 nm. The latter was attributed to reduced In segregation. InxGa1−xAs/GaAs (x=0.2–0.4) multiple quantum wells grown with Bi exhibited photoluminescence peaks that were more intense than those grown without Bi.

Ferroelectric and Dielectric Properties of Strontium Bismuth Niobate Vanadates

Abstract: Strontium bismuth niobate vanadates, SrBi2 (VxNb1-x)2O9 (with 0 ≤ x ≤ 0.1), were prepared by reaction sintering of powder mixtures of constituent oxides. With partial substitution of niobium by vanadium cations (up to 10 at.%), the single-phase layered perovskite structure was preserved, and the sintering temperature of the system was significantly lowered (∼200 °C). The incorporation of vanadium into the layered perovskite structure resulted in a shift of the Curie point to higher temperatures from 435 to 457 °C, with 10 at.% vanadium doping, and an increase in dielectric constant from ∼700 to ∼1100, with 10 at.% vanadium doping, at their respective Curie points. The remanent polarization increased from ∼2.4 to ∼8 µC/cm2, while the coercive field decreased from ∼63 to ∼45 kV/cm with 10 at.% V5+ doping.

The effect of oxygen stoichiometry on the humidity sensing characteristics of bismuth iron molybdate

Abstract: The sensitivity of the four probe electrical conductance of compressed bismuth iron molybdate (Bi3FeO4(MoO4)2) powder to humid air was measured under conditions of varying degrees of chemical reduction at 24°C, 40°C and 60°C. It is expected that the oxygen vacancies created will alter the water adsorption sites and have a big effect on a conductance dominated by proton hopping, hydronium ion diffusion, or vacancy donor traps releasing electrons into the conduction band. The data was analyzed as the logarithm of the conductance change due to humid air vs. the logarithm of the humidity. Slopes were calculated as limiting values at the two ends of the graphs and were interpreted in terms of the Fleming model of surface conductance, where they represent the average number of physisorbed molecules per cation site. This analysis implies that a Grotthuss chain reaction through multi-layers of surface water is the correct mechanism for the oxidized surface at both low and high humidity. A maximum slope of four argues against significant micropore condensation and a minimum slope of one argues against electronic effects. But for the chemically reduced surfaces, electronic conduction in the semiconductor is increasingly important at low humidity, as seen by slopes less than one. If this is the case, chemical reduction may increase the material's sensitivity to fermi level shifts due to adsorbed water emptying vacancy surface traps, as has been claimed for other materials.

Synthesis and characterization of basic bismuth(III) nitrates

Abstract: The syntheses of the basic bismuth(III) nitrates [Bi6O5(OH)3](NO3)5·3H2O, A, [Bi6O6(OH)3](NO3)3·15H2O, B, and [Bi6O5(OH)3](NO3)5·2H2O, X, were investigated A is readily obtained in hydrolysis of bismuth(III) nitrate solutions with sodium hydroxide solutions and in homogeneous hydrolysis with dilute solutions of urea [Bi6O6(OH)3](NO3)3·15H2O is obtained in hydrothermal synthesis at 190 °C from Bi(NO3)3·5H2O or from [Bi6O5(OH)3](NO3)5·3H2O [Bi6O5(OH)3](NO3)5·2H2O was obtained at 100 °C from an aqueous solution which was 074 M with respect to bismuth(III) nitrate and 150 M with respect to urea The compositions of B and X were derived from thermogravimetric analysis Indexed powder patterns are reported for B: a = 38175(5), c = 17149(4) A for a tetragonal cell, and for X: a = 15185(1), c = 15834(2) A for a rhombohedral cell using hexagonal setting

Coordination complexes of bismuth(III) involving organic ligands with pnictogen or chalcogen donors

Abstract: Publisher Summary The chapter presents an overview of the structural diversity demonstrated by complexes of bismuth(III) with organic-based ligands involving the elements of Groups 15 (pnictogens) and 16 (chalcogens) as donor sites, recognizing the potential for tailoring structure and reactivity by manipulation of the organic moiety. Although most complexes of bismuth obey standard valence guidelines, the high and variable coordination numbers accessible to bismuth are responsible for interesting and unpredictable structural arrangements. The formulas of well-defined bismuth compounds are presented to outline the type and extent of characterization data available for each, enabling assessment of the significance and relevance of the observations regarding synthesis and reactivity. Some of the structural features are described in detail to illustrate the novelty or generality of a particular arrangement. A diverse coordination chemistry is emerging for bismuth(III) made possible by a spacious and flexible coordination environment, allowing for coordination numbers in excess of 9.

Synthesis and Characterization of A3Bi2 (A = K, Rb, Cs) with Isolated Diatomic Dianion of Bismuth, [Bi2]2-, and an Extra Delocalized Electron

Abstract: Homonuclear diatomic molecules are the simplest chemical species to manifest the power of atomic attraction, the chemical bond. Yet, despite their simplicity, there are still diatomic species in the yet-to-be-found category. The existence of such species, especially with multiple bonds, among the heavier elements brings always wonder and a degree of disbelief and the challenge to find the answers to the usual questions, why, how, and where else. Recently, we reported the synthesis of the diatomic doublebonded naked dianion [Bi dBi] 2of the heaviest main-group element of the periodic system. 1 This diatomic molecule with distance of 2.8377(7) Å was characterized in the compound (Kcrypt)2Bi2 crystallized from ethylenediamine and crypt (crypt ) 4,7,13,16,21,24-hexaoxa-1,10-diazabicyclo[8.8.8]hexacosane) solution of the intermetallic precursor K 5In2Bi4. The structure of the precursor, chains of edge-sharing indium-centered tetrahedra of bismuth, and the presence of indium were initially thought to be essential for the formation of the molecule. Nevertheless, it was found later that the molecule can be made in much better yields from a more rational precursor without indium, K 3Bi2. The structure, bonding, and properties of this precursor with unexpected diatomic bismuth molecules are the subject of this contribution. Remarkably, large areas of such simple binary phase diagrams as alkali-metal -bismuth are structurally blank, and the stoichiometries of many proposed compounds are estimated only from thermal analysis. Thus, in the systems with K, Rb, or Cs known are only the Laves phases ABi 2 (MgCu2-type) and the simple octet compounds with isolated bismuth atoms A 3Bi (Li 3Bior Na3Astypes). Proposed but structurally unknown are A 3Bi2 and A5Bi4. Their existence, but not exact stoichiometries, has been confirmed by powder X-ray diffraction 40 years ago, and nothing further has been reported since. 2 The structures of liquid melts, on the other hand, have been studied more extensively lately. 3 A maximum in the electrical resistivity of such melts at bismuth fraction of 0.4 (corresponds to A 3Bi2) suggests a compound formation, and neutron diffraction results have indicated short chains of bismuth atoms in the melts. Initially, we studied the equimolar regions of the phase diagrams in order to explore for rational precursors for the solution molecule [Bi 2], and the compounds K 3Bi2, Rb3Bi2, and Cs3Bi2 were found in the products. Later, they were made in high yields from the corresponding stoichiometric mixtures heated at 650 °C for a day and slowly cooled to room temperature (5 °C/h).4 The compounds are dark gray and brittle but shiny with metallic luster. They form large lumps of irregular crystals that are easy to separate from the usual second phase of powderish ABi 2. Singlecrystal studies of Cs 3Bi2 showed a very simple structure of isolated dimers of bismuth with long interdimer distances of 5.102(1) Å surrounded by alkali-metal cations (Figure 1). 5 Crystallographically there is only one bismuth atom, and a nearby 2-fold axis of the monoclinic cell generates the dimer. The dimers are all aligned along the direction of the long diagonal of the a, c face, the [1 h01] direction (Figure 1).

Oligomerization and Oxide Formation in Bismuth Aryl Alkoxides: Synthesis and Characterization of Bi4(μ4-O)(μ-OC6F5)6{μ3-OBi(μ-OC6F5)3}2(C6H5CH3), Bi8(μ4-O)2(μ3-O)2(μ-OC6F5)16, Bi6(μ3-O)4(μ3-OC6F5){μ3-OBi(OC6F5)4}3, NaBi4(μ3-O)2(OC6F5)9(THF)2, and Na2Bi4(μ3-O)2(OC6F5)10(THF)2

Abstract: Exposing [Bi(OR)3(toluene)]2 (1, R = OC6F5) to different solvents leads to the formation of larger polymetallic bismuth oxo alkoxides via ether elimination/oligomerization reactions. Three differen...

Atomic Layer CVD in the Bi–Ti–O System

Abstract: Thin films in the Bi-Ti-O system have, for the first time, been deposited by atomic layer CVD (ALCVD) using triphenyl bismuth, titanium isopropoxide, and water as precursors. The influence of the Bi:Ti precursor pulsing ratio at the deposition temperature of 260 °C and the influence of deposition temperature in the interval 200-325 °C were investigated. The growth rates of the films were about 0.2 A/metal cycle, and the carbon content was less than 1.5 at.-%. The as-deposited films contained metallic bismuth but became amorphous upon annealing at 500 °C in air, and further annealing resulted in the formation of the dielectric Bi 2 Ti 2 O 7 phase. The films annealed at 500 °C had permittivity values of around 60.

New Cathode Materials for Solid Oxide Fuel Cells Ruthenium Pyrochlores and Perovskites

Abstract: The ruthenium pyrochlores, A{sub 2}Ru{sub 2}O{sub 7{minus}{delta}} (A = Pb, Bi), and ruthenium perovskites, ARuO{sub 3} (A = Ca, Sr) were characterized as new electrode materials for solid oxide fuel cells. The electrical conductivity, cathodic polarization, thermal expansion, and reactivity with yttria-stabilized zirconia were examined. The pyrochlores showed low cathodic overpotential even at 800 C, metallic behavior with high electrical conductivity, and no reaction with yttria-stabilized zirconia at 900 C. The thermal expansion coefficient of the bismuth pyrochlore was comparable to yttria-stabilized (8 mol % Y{sub 2}O{sub 3} content) zirconia. The pyrochlores are very attractive for application as cathode materials in a solid oxide fuel cell with low operating temperature.

Metal oxide pattern forming method

Abstract: In a pattern forming method for selectively forming an oxide layer on a substrate surface, the substrate surface is selectively coated with a coating layer. On the coating layer and an exposed part of the substrate surface, an oxide layer is formed by the use of a predetermined solution. Subsequently, the oxide layer on the coating layer is removed together with the coating layer to selectively leave the oxide layer on the substrate surface. Thus, a pattern is formed. The coating layer is removed in a liquid phase or optically together with the oxide layer on the coating layer. The oxide layer is formed by the use of, as the predetermined solution, an aqueous solution or a hydrofluoric acid solution of a fluoro metal complex compound and/or metal fluoride of at least one element selected from the group consisting of alkaline earth metal, transition metal, gallium, indium, silicon, tin, lead, antimony, and bismuth in the presence of an fluoride ion capturing agent.

Method for producing acrylonitrile, catalyst for use therein and the method for preparing the same

Abstract: A method for producing acrylonitrile by ammoxidation of propylene, which comprises using a fluidized bed catalyst comprising molybdenum, bismuth, iron, potassium, M-component, N-component and silica as essential components, containing iron antimonate being present as a crystalline phase, and having a number of Mo/Me of 0.8 to 1, the number being obtained through dividing the product of the valency number of molybdenum as molybdic acid and the atomic ratio of molybdenum in the empirical formula of the catalyst, that is 20, by the sum of the products of respective valency numbers and atomic ratios of the metal elements capable of forming a metal molybdenate other than iron antimonate, that is, bismuth, iron, potassium and M-, N- and T-component elements, and preferably further comprises carrying out the reaction while adding a molybdenum-containing material in an appropriate manner. The method can be used for achieving a high yield and maintaining it for a long period of time in producing acrylonitrile by ammoxidation of propylene.

Bismuth quantum-wires arrays fabricated by electrodeposition in nanoporous anodic aluminum oxide and its structural properties

Abstract: Ultrafine bismuth nanowire arrays have been fabricated by electrodeposition into nanoporous anodic aluminum templates in a composite electrolyte solution, in which the diameter of nanowire as low as 3 nm can be obtained. The micrographs and crystal structures of Bi nanowires are studied by transmission electron microscopy (TEM), selected-area electron diffraction (SAED), and X-ray diffraction (XRD). It is found that each nanowire is essentially a single crystal and has a different orientation in an array. The optical properties of the Bi nanowire arrays are studied by UV-VIS spectra. The experimental results exhibit blue-shifted phenomena with decreasing wire diameter, which is qualitatively consistent with the effective medium theory. © 2000 Elsevier Science S.A. All rights reserved.

Preparation and transport properties of polycrystalline Bi and Bi–SiO2 nanocomposites

Abstract: Bismuth–silica nanocomposites and polycrystalline bismuth were prepared via powder metallurgy in order to study the influence of silica inclusions on the thermoelectric properties of bismuth. Bi–SiO2 powders containing from 0.5 to 15 vol. % of silica and pure Bi powders were produced by an arc-plasma processing. Transmission electron microscopy investigations revealed the presence of a nanometric silica shell around the Bi grains. The powders were cold pressed and sintered close to the melting temperature of bismuth. The bulk microstructures are very different for the bismuth and the Bi–SiO2 nanocomposites because silica, which is primarily dispersed at grain boundaries, inhibits the grain growth during sintering. The electrical resistivity was measured from 5 to 300 K, while the thermoelectric power and the thermal conductivity were measured from 65 to 300 K on both polycrystalline bismuth and Bi–SiO2 samples containing 0.5, 4, and 15 vol. % of silica, respectively. The transport properties are mainly discussed with regard to the microstructures. In spite of a strong reduction of the thermal conductivity for the nanocomposites, the thermoelectric figure of merit is not improved compared to bismuth due to a dominating concurrent increase of the electrical resistivity resulting from a finite-size effect.

Surfactant effect of bismuth in the MOVPE growth of the InAs quantum dots on GaAs

Abstract: We report on the role of bismuth as a surfactant in the growth of InAs quantum dots (QDs) on GaAs (001) by metal organic vapour phase epitaxy. Atomic force microscopy investigations have shown that bismuth suppresses coalescence of the InAs QDs and advances a more uniform size distribution. The photoluminescence spectra of the Bi-assisted grown QDs present several narrow peaks from the ground and the excited state transitions with full width at half maximum (FWHM) as narrow as 25 meV (both at 77 and 300 K). Due to such low values of the FWHM we were able to observe up to two well resolved excited state transitions in the photovoltage spectra measured by an electrolyte cell technique. The lowest ground transition energies observed were 0.93 eV at 77 K and 0.875 eV at 300 K (emission wavelength 1.46 µm). So using Bi-assisted growth it is possible to cover the 1.3 µm band, which is important for optoelectronic applications in the InAs/GaAs material system. Formation of such `deep' QDs without misfit dislocations was explained by the formation of a graded-composition transient InGaAs alloy layer at the GaAs/InAs hetero-interface as a result of diffusion intermixing of the components. The proposed mechanism for the effect of Bi on the QDs' morphology is that Bi decreases the surface mobility of the In atoms on the growing surface, preventing the coalescence of the QDs. Because of its rather large covalent radius (compared with that of As), Bi is not incorporated into the QDs' material segregating on the growing surface.