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30 Jul 2007

Volume 91, Issue 5, Articles (05xxxx)

Issue Cover Spotlight Figure

Appl. Phys. Lett. 91, 052901 (2007); http://dx.doi.org/10.1063/1.2767146 (3 pages)

Wei-Feng Rao and Yu U. Wang
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Sink-effect of nanocavities: Thermodynamic and kinetic approach

G. Ouyang, X. L. Li, and G. W. Yang

Appl. Phys. Lett. 91, 051901 (2007); http://dx.doi.org/10.1063/1.2767231 (3 pages) | Cited 4 times

Online Publication Date: 30 July 2007

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Sink-effect of nanocavities in a host crystal to metallic impurity atoms is proposed on the basis of the established thermodynamic and kinetic approach at the nanometer scale. The trapping impurity mechanisms of nanocavities are attributed to the thermodynamic contact epitaxy of impurity atoms on the inner surface of nanocavities and the kinetic diffusion flux of impurity atoms pointing to nanocavities. These theoretical results exhibit that the nanocavity in a host crystal as a functional unit could be used to fabricate nanodevices.
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61.46.-w Structure of nanoscale materials
65.80.-g Thermal properties of small particles, nanocrystals, nanotubes, and other related systems

Nonlinear optical properties of thiol-capped CdTe quantum dots in nonresonant region

Lingyun Pan, Naoto Tamai, Kenji Kamada, and Shigehito Deki

Appl. Phys. Lett. 91, 051902 (2007); http://dx.doi.org/10.1063/1.2761494 (3 pages) | Cited 10 times

Online Publication Date: 31 July 2007

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Size-dependent χ(3) of water-soluble thiol-capped CdTe quantum dots (QDs) in nonresonant region has been examined by Z-scan and degenerate four-wave mixing experiments. Two-photon absorption cross section as high as 104 GM was observed for 4.9 nm sized CdTe QDs. Both imaginary and real parts of χ(3) were found to increase with QD size. Compared with the bulk CdTe semiconductor, 30 times enhancement of nonlinear figures of merit defined by Re χQD(3)/Im χQD(3) was found for 3.1 nm CdTe QDs in nonresonant excitation.
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78.67.Hc Quantum dots
78.20.Ci Optical constants (including refractive index, complex dielectric constant, absorption, reflection and transmission coefficients, emissivity)
42.65.Jx Beam trapping, self-focusing and defocusing; self-phase modulation

Cooperative downconversion in GdAl3(BO3)4:RE3+,Yb3+ (RE = Pr, Tb, and Tm)

Q. Y. Zhang, G. F. Yang, and Z. H. Jiang

Appl. Phys. Lett. 91, 051903 (2007); http://dx.doi.org/10.1063/1.2757595 (3 pages) | Cited 86 times

Online Publication Date: 31 July 2007

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An efficient near-infrared (NIR) quantum cutting (QC) in GdAl3(BO3)4:RE3+,Yb3+ (RE = Pr, Tb, and Tm) phosphors has been demonstrated, which involves the conversion of the visible photon into the NIR emission with an optimal quantum efficiency approaching 200%, by exploring the cooperative downconversion mechanism from RE3+ (RE = Pr, Tb, and Tm) excitons to the two activator ions, Yb3+. The development of NIR QC phosphors could open up a new approach in achieving high efficiency silicon-based solar cells by means of downconversion in the visible part of the solar spectrum to ∼ 1000 nm photons with a twofold increase in the photon number.
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78.55.Hx Other solid inorganic materials

Influence of lateral electric fields on multiexcitonic transitions and fine structure of single quantum dots

M. M. Vogel, S. M. Ulrich, R. Hafenbrak, P. Michler, L. Wang, A. Rastelli, and O. G. Schmidt

Appl. Phys. Lett. 91, 051904 (2007); http://dx.doi.org/10.1063/1.2761522 (3 pages) | Cited 30 times

Online Publication Date: 31 July 2007

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The quantum-confined Stark effect of excitonic states in self-assembled (In,Ga)As/GaAs quantum dots was studied by microphotoluminescence spectroscopy. A similar Stark-shift behavior for excitons, biexcitons, and a charged state was observed. Investigations suggest the absence of a permanent dipole moment in the lateral quantum dot plane. Values of the polarizability could be derived for all the investigated states. Furthermore, high-resolution Fabry-Pérot interferometry was applied to resolve the excitonic fine structure splitting and to investigate the influence of a lateral electric field. For a single dot, the splitting could be tuned to zero, thus affording the possibility to create electrically controlled entangled photon pairs.
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73.21.La Quantum dots
78.67.Hc Quantum dots
73.63.Kv Quantum dots
78.55.Cr III-V semiconductors
78.20.Jq Electro-optical effects
73.20.Mf Collective excitations (including excitons, polarons, plasmons and other charge-density excitations)

Enhanced thermal stability of nickel germanide on thin epitaxial germanium by adding an ultrathin titanium layer

Shiyang Zhu, M. B. Yu, G. Q. Lo, and D. L. Kwong

Appl. Phys. Lett. 91, 051905 (2007); http://dx.doi.org/10.1063/1.2768203 (3 pages) | Cited 7 times

Online Publication Date: 31 July 2007

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The thermal stability of NiGe films formed on epitaxial Ge on Si substrates was improved from 450 to 550 °C by simply adding an ultrathin ( ∼ 1 nm) Ti layer during Ni deposition, either as an intermediate layer between Ni and Ge or as a capping layer on Ni. The improvement was attributed to the formation of ternary Ni1−xTixGe phase near the NiGe surface, which acts as a capping layer to suppress agglomeration of the underlying NiGe film at an elevated temperature, as well as modification of the NiGe grain boundaries. The resistivity of NiGe is also slightly reduced by the Ti incorporation, making this method very promising for the germanium technology.
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68.60.Dv Thermal stability; thermal effects
61.72.Mm Grain and twin boundaries
68.55.-a Thin film structure and morphology
73.61.At Metal and metallic alloys

Ductile ultrafine-grained Ti-based alloys with high yield strength

Lai-Chang Zhang, Hai-Bo Lu, Christine Mickel, and Jürgen Eckert

Appl. Phys. Lett. 91, 051906 (2007); http://dx.doi.org/10.1063/1.2766861 (3 pages) | Cited 16 times

Online Publication Date: 31 July 2007

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The authors report on ductile ultrafine-grained (Ti0.72Fe0.28)100−xTax (0 ⩽ x ⩽ 4) alloys with not only high fracture strength but simultaneously high yield strength exceeding 2000 MPa along with distinct plasticity, which are superior to high-strength Ti-based bulk metallic glasses and bimodal composites. All alloys mainly consist of β-Ti and FeTi solid solutions but display different microstructures. The alloys exhibit a high fracture strength >2500 MPa and a high yield strength >2000 MPa as well as large plasticity of ∼ 5%–7.5%. The microstructure-property correlation of these ultrafine-grained alloys is discussed.
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81.40.Lm Deformation, plasticity, and creep
81.40.Np Fatigue, corrosion fatigue, embrittlement, cracking, fracture, and failure
62.20.F- Deformation and plasticity
62.20.M- Structural failure of materials

Dramatic morphological change of scallop-type Cu6Sn5 formed on (001) single crystal copper in reaction between molten SnPb solder and Cu

J. O. Suh, K. N. Tu, and N. Tamura

Appl. Phys. Lett. 91, 051907 (2007); http://dx.doi.org/10.1063/1.2761840 (3 pages) | Cited 20 times

Online Publication Date: 31 July 2007

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Wetting reaction between molten Sn-based solders and Cu produces scallop-type Cu6Sn5. In the present wetting study, a (001) single crystal Cu is used as substrate and a dramatic change in the morphology of Cu6Sn5 is observed: instead of scallop type, the authors observed a rooftop-type Cu6Sn5 grains, elongated along two preferred orientation directions. This was confirmed by electron beam backscattered diffraction and white beam synchrotron x-ray microdiffraction. The results indicate that the nucleation, growth, and ripening behavior of Cu6Sn5 on single crystal substrate can be quite different from the conventional case of wetting on randomly oriented polycrystalline Cu substrates.
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61.50.-f Structure of bulk crystals
68.65.-k Low-dimensional, mesoscopic, nanoscale and other related systems: structure and nonelectronic properties

Aluminum buffer layer for high durability of all-solid-state switchable mirror based on magnesium-nickel thin film

Kazuki Tajima, Yasusei Yamada, Shanhu Bao, Masahisa Okada, and Kazuki Yoshimura

Appl. Phys. Lett. 91, 051908 (2007); http://dx.doi.org/10.1063/1.2761842 (3 pages) | Cited 23 times

Online Publication Date: 31 July 2007

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Aluminum buffer layer was inserted in all-solid-state switchable mirror based on Mg–Ni thin film to improve its durability. Optical switching speed of the device has been improved because the buffer layer might work to transport electrons and protons promptly as an electrode. Durability of the device was evaluated by cyclic switching test. The device with buffer layer showed three times higher stable durability than that of the device without buffer layer. The authors found that the aluminum buffer layer effectively works to suppress the diffusion of nickel and palladium atoms into the Ta2O5 thin film during cyclic switching test, resulted in high durability.
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42.79.Ta Optical computers, logic elements, interconnects, switches; neural networks
42.79.Bh Lenses, prisms and mirrors
42.79.Wc Optical coatings
42.87.-d Optical testing techniques

Valence band anticrossing in GaBixAs1−x

K. Alberi, O. D. Dubon, W. Walukiewicz, K. M. Yu, K. Bertulis, and A. Krotkus

Appl. Phys. Lett. 91, 051909 (2007); http://dx.doi.org/10.1063/1.2768312 (3 pages) | Cited 35 times

Online Publication Date: 31 July 2007

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The optical properties of GaBixAs1−x(0.04<x<0.08) grown by molecular beam epitaxy have been studied by photomodulated reflectance spectroscopy. The alloys exhibit a strong reduction in the band gap as well as an increase in the spin-orbit splitting energy with increasing Bi concentration. These observations are explained by a valence band anticrossing model, which shows that a restructuring of the valence band occurs as the result of an anticrossing interaction between the extended states of the GaAs valence band and the resonant T2 states of the Bi atoms.
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71.20.Nr Semiconductor compounds
71.55.Eq III-V semiconductors
71.70.Ej Spin-orbit coupling, Zeeman and Stark splitting, Jahn-Teller effect
78.66.Fd III-V semiconductors
68.55.-a Thin film structure and morphology

Combined Raman study of InGaAsN from the N-impurity and InGaAs-matrix sides

A. Chafi, O. Pagès, A. V. Postnikov, J. Gleize, V. Sallet, E. Rzepka, L. H. Li, B. Jusserand, and J. C. Harmand

Appl. Phys. Lett. 91, 051910 (2007); http://dx.doi.org/10.1063/1.2767244 (3 pages) | Cited 4 times

Online Publication Date: 1 August 2007

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The authors report a Raman study of In0.08Ga0.92As0.98N0.02 covering both N- and InGaAs-related spectral ranges, supported by ab initio and phenomenological modeling of the spectra. The authors argue that the crystal self-organizes at different scales to reduce the N-induced strain. Locally, this is achieved by In-N-Ga3 configurations, as is well known. Moreover, a long scale organization occurs in the InGaAs matrix, away from N. This is apparent in the over representation of the “short” Ga–As bonds from the In-rich region, as compared with random InGaAs. Quantitative insight is derived at the mesoscopic scale from the 1-bond→2-phonon “percolation”-type behavior of the dominant Ga–As species.
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78.30.Fs III-V and II-VI semiconductors
63.20.D- Phonon states and bands, normal modes, and phonon dispersion

Spontaneous emission lifetime distribution of infinite line antennas in two-dimensional photonic crystals with finite size

Xingsheng Xu and Yiquan Wang

Appl. Phys. Lett. 91, 051911 (2007); http://dx.doi.org/10.1063/1.2764439 (3 pages) | Cited 1 time

Online Publication Date: 1 August 2007

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The authors calculate the lifetime distribution functions of spontaneous emission from infinite line antennas embedded in two-dimensional disordered photonic crystals with finite size. The calculations indicate the coexistence of both accelerated and inhibited decay processes in disordered photonic crystals with finite size. The decay behavior of the spontaneous emission from infinite line antennas changes significantly by varying factors such as the line antennas’ positions in the disordered photonic crystal, the shape of the crystal, the filling fraction, and the dielectric constant. Moreover, the authors analyze the effect of the degree of disorder on spontaneous emission.
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42.70.Qs Photonic bandgap materials
78.20.Ci Optical constants (including refractive index, complex dielectric constant, absorption, reflection and transmission coefficients, emissivity)
77.22.Ch Permittivity (dielectric function)

Underwater pressure amplification of laser-induced plasma shock waves for particle removal applications

Thomas J. Dunbar and Cetin Cetinkaya

Appl. Phys. Lett. 91, 051912 (2007); http://dx.doi.org/10.1063/1.2754359 (3 pages) | Cited 3 times

Online Publication Date: 1 August 2007

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Underwater amplification of laser-induced plasma (LIP)-generated transient pressure waves using shock tubes is introduced and demonstrated. Previously, it has been shown that LIP for noncontact particle removal is possible on the sub-100-nm level. This is now enhanced through shock tube utilization in a medium such as water by substantially increasing shock wave pressure for the same pulse energy. A shock tube constrains the volume and changes the propagation direction of the expanding plasma core by focusing a pulsed-laser beam inside a tube with a blind end, thus increasing the wave front pressure generated. Current amplification approach can reduce radiation exposure of the substrate from the shock wave because of the increased distance from the LIP core to the substrate provided by the increased pressure per unit pulse energy. For the same pulsed laser, with the aid of a shock tube, substantial levels of pressure amplitude amplification (8.95) and maximum pressure (6.48 MPa) are observed and reported.
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52.77.Bn Etching and cleaning
81.05.Cy Elemental semiconductors
81.65.Cf Surface cleaning, etching, patterning
62.50.-p High-pressure effects in solids and liquids

Quantification of free volume differences in a Zr44Ti11Ni10Cu10Be25 bulk amorphous alloy

M. E. Launey, J. J. Kruzic, C. Li, and R. Busch

Appl. Phys. Lett. 91, 051913 (2007); http://dx.doi.org/10.1063/1.2766659 (3 pages) | Cited 19 times

Online Publication Date: 1 August 2007

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Deformation of metallic glasses requires the existence of free volume to allow atomic movement under mechanical loading. Accordingly, quantification of the free volume state of the alloy is crucial to understand its mechanical behavior. By annealing below the glass transition temperature, the free volume of a Zr44Ti11Ni10Cu10Be25 bulk metallic glass was varied via structural relaxation. Differential scanning calorimetry was used to quantify enthalpy differences between the relaxed and as-cast materials, which were then quantitatively related to average free volume differences. These results can be used to characterize the average free volume in this alloy for future mechanical property studies.
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64.70.P- Glass transitions of specific systems
64.70.Q- Theory and modeling of the glass transition
62.20.F- Deformation and plasticity
81.40.Lm Deformation, plasticity, and creep
61.43.Fs Glasses

Nitrogen related shallow thermal donors in silicon

N. Fujita, R. Jones, S. Öberg, and P. R. Briddon

Appl. Phys. Lett. 91, 051914 (2007); http://dx.doi.org/10.1063/1.2767989 (3 pages) | Cited 6 times

Online Publication Date: 1 August 2007

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In this letter, the authors investigate the electrical properties of nitrogen related shallow thermal donor (STD) candidates and their concentrations under different doping conditions by means of density functional theory. Experimentally, the existence of STDs containing one nitrogen atom and both even and odd numbers of oxygen atoms has been proposed. However, so far first principles studies have not presented a candidate for the latter. Here, they show that the NO defect possesses a shallow donor level. Adding one or two more oxygen atoms results in the donor level to become shallower. The fraction of shallow nitrogen related donors to N dimers increases in material with low concentration of nitrogen.
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71.55.Cn Elemental semiconductors
61.72.uf Ge and Si
71.15.Mb Density functional theory, local density approximation, gradient and other corrections
61.72.S- Impurities in crystals

Low resistivity Ga-doped ZnO thin films of less than 100 nm thickness prepared by ion plating with direct current arc discharge

Takahiro Yamada, Aki Miyake, Seiichi Kishimoto, Hisao Makino, Naoki Yamamoto, and Tetsuya Yamamoto

Appl. Phys. Lett. 91, 051915 (2007); http://dx.doi.org/10.1063/1.2767213 (3 pages) | Cited 19 times

Online Publication Date: 1 August 2007

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Low resistivity Ga-doped ZnO films were prepared on a glass substrate by ion plating with direct current arc discharge. Thickness dependent changes in the electrical properties of the films are reported, focusing on the thin films of less than 100 nm thickness. Structural analyses showed that the thinnest film of 30 nm thickness consists of well-oriented columnar grains normal to the substrate, and the resistivity was as low as 4.4×10−4 Ω cm. The changes in lattice strain and c-axis fluctuation with the growth of grains are also shown to be associated with the electrical properties.
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73.61.Ga II-VI semiconductors
81.15.Jj Ion and electron beam-assisted deposition; ion plating
68.55.-a Thin film structure and morphology

Record of the lowest frequency shot noise measurement below 10 Hz

B. Cho, T. Itagaki, and C. Oshima

Appl. Phys. Lett. 91, 051916 (2007); http://dx.doi.org/10.1063/1.2767234 (3 pages) | Cited 4 times

Online Publication Date: 1 August 2007

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The noise of field emission (FE) currents ranging from 10 pA to 100 μA was measured under ∼ 7×10−10 Pa. Stepwise features were routinely observed in damping curves of the FE current, indicating remarkable sensitivity of the FE current measuring system. The lowest frequency measurement of shot noise was recorded below 10 Hz. Space-charge suppression of noise was not observed for FE currents up to 100 μA. Adsorption of residual gas increased the intensity of noise and made the noise of field emission current be dominated by the flicker noise.
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05.40.Ca Noise

Type-I optical emissions in Ge/Si quantum dots

P. F. Gomes, F Iikawa, F. Cerdeira, M. Larsson, A. Elfving, G. V. Hansson, W.-X. Ni, and P.-O. Holtz

Appl. Phys. Lett. 91, 051917 (2007); http://dx.doi.org/10.1063/1.2764113 (2 pages) | Cited 3 times

Online Publication Date: 2 August 2007

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The authors studied the optical emission of Ge/Si quantum dots under externally applied biaxial stress using samples grown with different temperatures varying from 430 to 700 °C. The optical emission energy of samples grown at low temperatures is rather insensitive to the applied external stress, consistent with the type-II band alignment. However, for samples grown at high temperatures we observed a large blueshift, which suggests type-I alignment. The result implies that recombination strength can be controlled by the growth temperature, which can be useful for optical device applications.
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81.07.Ta Quantum dots
81.05.Cy Elemental semiconductors
78.67.Hc Quantum dots
78.55.Ap Elemental semiconductors
81.15.Hi Molecular, atomic, ion, and chemical beam epitaxy
68.65.Hb Quantum dots (patterned in quantum wells)

Effects of energetic ion particles on friction of diamond-like carbon

K. Wang, H. Liang, J. M. Martin, and T. Le Mogne

Appl. Phys. Lett. 91, 051918 (2007); http://dx.doi.org/10.1063/1.2767781 (3 pages) | Cited 6 times

Online Publication Date: 2 August 2007

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The authors demonstrate that energetic argon ions introduce phase transformation of a diamond- like carbon film. Inside an ultrahigh vacuum chamber, energetic Ar ions of 5 keV were generated using an x-ray photon spectroscopy. After ion bombardment, x-ray photon spectroscopy (XPS) and friction tests were conducted in situ. The ex situ surface morphological analysis using an atomic force microscope and the multiple-peaks deconvolution of the C 1s XPS peak indicate that the changing ratio of sp2 and sp3 hybridization of carbon dominates the film friction. The authors conclude that the friction of the diamon dlike carbon film against itself depends on the carbon phase, not on the film surface roughness.
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81.40.Pq Friction, lubrication, and wear
61.80.Jh Ion radiation effects
61.82.Ms Insulators
62.20.Qp Friction, tribology, and hardness
64.70.K- Solid-solid transitions
68.60.Bs Mechanical and acoustical properties

Impact of low 6,13-pentacenequinone concentration on pentacene thin film growth

Ingo Salzmann, Steffen Duhm, Ricarda Opitz, Jürgen P. Rabe, and Norbert Koch

Appl. Phys. Lett. 91, 051919 (2007); http://dx.doi.org/10.1063/1.2767972 (3 pages) | Cited 10 times

Online Publication Date: 2 August 2007

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Thin films of pentacene (P) have been vacuum codeposited on SiO2 with low concentrations of 6,13-pentacenequinone (PQ) in order to investigate the impact on the pentacene thin film structure. Within a range of 2%–20% PQ concentration no intercalation of the compounds can be observed by means of x-ray diffraction and infrared absorption spectroscopy. The crystalline quality of the P films stays unchanged by the presence of PQ, whereas P bulk phase contributions are being suppressed at PQ concentrations ≥ 5%, which could be confirmed by means of atomic force microscopy. From the results the authors suggest phase-separated PQ growth as well as PQ nucleation at P grain boundaries.
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68.55.A- Nucleation and growth
81.15.-z Methods of deposition of films and coatings; film growth and epitaxy
68.65.-k Low-dimensional, mesoscopic, nanoscale and other related systems: structure and nonelectronic properties

Intense ultraviolet upconversion luminescence from Tm3+/Yb3+:β-YF3 nanocrystals embedded glass ceramic

Daqin Chen, Yuansheng Wang, Yunlong Yu, and Ping Huang

Appl. Phys. Lett. 91, 051920 (2007); http://dx.doi.org/10.1063/1.2767988 (3 pages) | Cited 67 times

Online Publication Date: 2 August 2007

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The infrared to ultraviolet upconversion emissions of Tm3+ 1I6→3F4 (346 nm) and 1D2→3H6 (362 nm) transitions, originating from the five- and four-photon upconversion processes, respectively, were observed in the Tm3+/Yb3+ codoped precursor glass and glass ceramic containing β-YF3 nanocrystals. The ultraviolet luminescence of the glass ceramic is 30 times stronger than that of the precursor glass, which could be attributed to the decreased probability of the 3F2→3F4 transition and the increased cross relaxation of 3F2+3H4→3H6+1D2 resulted from the partition of rare earth ions into nanocrystals.
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78.55.Hx Other solid inorganic materials

Highly sensitive optochemical gas detection by luminescent marine diatoms

A. Setaro, S. Lettieri, P. Maddalena, and L. De Stefano

Appl. Phys. Lett. 91, 051921 (2007); http://dx.doi.org/10.1063/1.2768027 (3 pages) | Cited 15 times

Online Publication Date: 2 August 2007

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The modifications of photoluminescence properties of silica frustules of different marine diatoms induced by adsorption of nitrogen dioxide, methane, and carbon monoxide are reported. Different species of diatoms were found to exhibit different relative responses and different gas concentration ranges of sensitivity, depending on the morphology and porosity of their frustules. The photoluminescence quenching shows surface signature, exhibiting a coverage-limited kinetics according to a Langmuir mechanism. Due to the large variety of dimensions, porosities, and surface morphologies available in nature, these materials appear to be promising to improve the selectivity of gas sensing based on photoluminescence optochemical transduction.
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07.07.Df Sensors (chemical, optical, electrical, movement, gas, etc.); remote sensing
78.55.Hx Other solid inorganic materials

Plastic flow in bulk metallic glasses: Effect of strain rate

Fuqian Yang

Appl. Phys. Lett. 91, 051922 (2007); http://dx.doi.org/10.1063/1.2768029 (3 pages) | Cited 5 times

Online Publication Date: 3 August 2007

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An analysis is presented for the onset of plastic deformation in bulk metallic glasses. It is based on the hyperbolic sine flow law and the introduction of a parameter describing the density of flow units over shear plane. The resistance to the onset of the plastic flow in bulk metallic glasses is found to follow the Mohr-Coulomb-like dependence on effective normal stress and depend on strain rate. A semianalytical relation is established between the nominal serrated size and the serrated stress for serrated flow present in uniaxial tests when subjected to constant strain rates. The analysis is in good accord with experimental results reported in literature.
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81.05.Kf Glasses (including metallic glasses)
81.40.Lm Deformation, plasticity, and creep
62.20.F- Deformation and plasticity
61.43.Fs Glasses

Mechanisms behind blue, green, and red photoluminescence emissions in CaWO4 and CaMoO4 powders

A. B. Campos, A. Z. Simões, E. Longo, J. A. Varela, V. M. Longo, A. T. de Figueiredo, F. S. De Vicente, and A. C. Hernandes

Appl. Phys. Lett. 91, 051923 (2007); http://dx.doi.org/10.1063/1.2766856 (3 pages) | Cited 13 times

Online Publication Date: 3 August 2007

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A combined experimental and theoretical study was conducted to analyze the photoluminescence (PL) properties of ordered and disordered CaWO4 (CW) and CaMoO4 (CM) powders. Two mechanisms were found to be responsible for photoluminescence emission in CW and CM powders. The first one, in the disordered powders, was caused by oxygen complex vacancies [MO3∙VOx], [MO3∙VO•] and [MO3∙VO••], where M = W or Mo, which leads to additional levels in the band gap. The second mechanism, in ordered powders, was caused by an intrinsic slight distortion of the [WO4] or [MoO4] tetrahedral in the short range.
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78.55.Hx Other solid inorganic materials
61.43.Gt Powders, porous materials
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