Showing papers on "Lattice constant published in 2003"

Alloyed ZnxCd1-xS Nanocrystals with Highly Narrow Luminescence Spectral Width

Abstract: High-quality alloyed ZnxCd1-xS nanocrystals have been synthesized at high temperature by the reaction of a mixture of CdO- and ZnO-oleic acid complexes with sulfur in the noncoordinating solvent octadecene system. A series of monodisperse wurtzite ZnxCd1-xS (x = 0.10, 0.25, 0.36, 0.53) nanocrystals were obtained with corresponding particle radii of 4.0, 3.2, 2.9, and 2.4 nm, respectively. With the increase of the Zn content, their photoluminescence (PL) spectra blue-shift systematically across the visible spectrum from 474 to 391 nm, indicating the formation of the alloyed nanocrystals. The alloy structure is also supported by the characteristic X-ray diffraction (XRD) patterns of these nanoalloys with different Zn mole fractions, in which their diffraction peaks systematically shift to larger angles as the Zn content increases. The lattice parameter c measured from XRD patterns decreases linearly with the increase of Zn content. This trend is consistent with Vegard's law, which further confirms the forma...

Nanoparticle Optics: The Importance of Radiative Dipole Coupling in Two-Dimensional Nanoparticle Arrays †

Abstract: In this paper, the electromagnetic interactions between noble metal nanoparticles are studied by measuring the extinction spectra of two-dimensional arrays of Au and Ag cylinders and trigonal prisms that have been fabricated with electron beam lithography. The nanoparticles are typically 200 nm in diameter and 35 nm in height; both hexagonal and square array patterns have been considered with lattice spacings that vary from 230 to 500 nm. The extinction spectra typically have a maximum in the 700−800 nm region of the spectrum, and this maximum blue shifts as lattice spacing is reduced, having typically a 40 nm decrease in λmax for a 100 nm decrease in lattice spacing. The results are similar for the different noble metals, array patterns, and nanoparticle shapes. The extinction spectra have been modeled using coupled dipole calculations, and the observed spectral variations are in good qualitative agreement with experimental data. Moreover, the computational analysis indicates that the blue shifts are due...

Domain epitaxy: A unified paradigm for thin film growth

Abstract: We present a unified model for thin film epitaxy where single crystal films with small and large lattice misfits are grown by domain matching epitaxy (DME). The DME involves matching of lattice planes between the film and the substrate having similar crystal symmetry. In this framework, the conventional lattice matching epitaxy becomes a special case where a matching of lattice constants or the same planes is involved with a small misfit of less than 7%–8%. In large lattice mismatch systems, we show that epitaxial growth of thin films is possible by matching of domains where integral multiples of major lattice planes match across the interface. We illustrate this concept with atomic-level details in the TiN/Si(100) with 3/4 matching, the AlN/Si(100)with 4/5 matching, and the ZnO/α−Al2O3(0001) with 6/7 matching of major planes across the film/substrate interface. By varying the domain size, which is equal to intregral multiple of lattice planes, in a periodic fashion, it is possible to accommodate addition...

Zinc-blende compounds of transition elements with N, P, As, Sb, S, Se, and Te as half-metallic systems

Abstract: We report systematic first-principles calculations for ordered zinc-blende compounds of the transition metal elements V, Cr, and Mn with the $\mathrm{sp}$ elements N, P, As, Sb, S, Se, and Te, motivated by a recent fabrication of zinc-blende CrAs, CrSb, and MnAs. They show a ferromagnetic half-metallic behavior for a wide range of lattice constants. We discuss the origin and trends of half-metallicity, present the calculated equilibrium lattice constants, and examine the half-metallic behavior of their transition element terminated (001) surfaces.

Colloidal Crystals with Tunable Colors and Their Use as Photonic Papers

Abstract: This article describes the fabrication and characterization of colloidal crystals whose stop bands could be varied through the application of a liquid. Such a colloidal crystal was generated by infiltrating the voids within an opaline lattice of polystyrene beads with a liquid prepolymer to poly(dimethylsiloxane), followed by thermal curing. When a liquid (e.g., a silicone fluid, hexane, or octane) capable of swelling the elastomer matrix was applied to the surface of this crystal, the lattice constant and thus the wavelength of Bragg-diffracted light was increased. For instance, the color of light diffracted from a colloidal crystal made of 175 nm polystyrene beads could be varied from violet to green, orange, and red simply by swelling it with different solvents. On the basis of this mechanism, we further demonstrated a photonic paper/ink system where color patterns could be conveniently generated on the surface of a thin film of colloidal crystal by writing with a Pilot pen, by screen printing, or by m...

Theory of polarization enhancement in epitaxial BaTiO3/SrTiO3 superlattices

Abstract: The spontaneous polarization of epitaxial BaTiO3/SrTiO3 superlattices is studied as a function of composition using first-principles density-functional theory within the local density approximation. With the in-plane lattice parameter fixed to that of bulk SrTiO3, the computed superlattice polarization is enhanced above that of bulk BaTiO3 for superlattices with BaTiO3 fraction larger than 40%. In contrast to their bulk paraelectric character, the SrTiO3 layers are found to be tetragonal and polar, possessing nearly the same polarization as the BaTiO3 layers. General electrostatic arguments elucidate the origin of the polarization in the SrTiO3 layers, with important implications for other nanostructured ferroelectrics.

Phase transitions in ternary caesium lead bromide

Abstract: Phase transitions in ternary caesium lead bromide (CsPbBr3) were studied by means of DSC, TMA and high temperature X-ray diffraction. The samples were prepared from the solution by water evaporation and from the melt. on the DSC curves as well as on the temperature dependence of the lattice constants of CsPbBr3 only two effects were found belonging to the earlier published phase transitions at 88 and 130°C and no further effects. Linear thermal expansion coefficient α of individual CsPbBr3 modifications were calculated from both TMA and high temperature X-ray diffraction. The structural parameters of the room temperature orthorhombic phase were refined and the results are presented. CsPbBr3 prepared from the solution contained about 10% of CsPb2Br5 and so the DSC curve of pure CsPb2Br5 was also measured and an effect at a temperature of 68.5°C was found.

Carbon-substituted MgB2 single crystals

Abstract: Carbon-substituted MgB 2 single crystals, Mg(B 1− x C x ) 2 of 0.3–1.0 mm size were grown for x =0.02–0.15 by a high-pressure technique. The doping dependence of lattice constants studied in a range of x =0.0–0.2 shows a monotonic decrease in a , while the c parameter remains almost invariant. Using X-ray diffraction and Auger electron spectroscopy, the solubility limit of C in MgB 2 was estimated to be about 15 ± 1%, which is substantially larger than that reported for the polycrystalline samples synthesized by encapsulation techniques. Measurements of the temperature dependence of magnetization and resistivity showed a dramatic decrease in T c with C-substitution, followed by complete suppression of superconductivity for x >0.125. Resistivity measurements in magnetic fields parallel and perpendicular to the basal plane of the crystals showed a nearly isotropic state in the heavily doped crystals ( x >0.1).

Pores in III–V Semiconductors

Abstract: The paper reviews electrochemically etched pores in III-V compound semiconductors (GaP, InP, GaAs) with emphasis on nucleation and formation mechanisms, pore geometries and morphologies, and to several instances of self-organization. Self-organization issues include the formation of single-crystalline two-dimensional hexagonal arrays of pores with lattice constants as small as 100 nm found in InP, synchronized and unsynchronized diameter oscillations coupled to current and voltage oscillations, and pore domain formation. The findings are discussed in relation to pores observed in silicon. Some novel porperties of the porous layers obtained in III-V compounds are briefly described.

Terahertz plasmonic high pass filter

Abstract: Metamaterials, which contain engineered subwavelength microstructures, can be designed to have positive or negative e and μ at desired frequencies. In this letter, we demonstrate a metamaterial which has a “plasmonic” response to electromagnetic waves in the terahertz (THz) range. The sharp change of reflection and transmission at this plasma frequency makes the structure a high pass filter. The reflection response is characterized by Fourier transform infrared spectroscopy, and a plasma frequency at 0.7 THz is observed, which agrees with the theoretical calculation. The metamaterial is a two-dimensional cubic lattice consisting of thin metal wires, having wire diameter of 30 μm, lattice constant of 120 μm, and wire length of 1 mm. The microstereolithography technique is employed to fabricate the high-aspect-ratio lattice.

Adaptive ferroelectric states in systems with low domain wall energy: Tetragonal microdomains

Abstract: Ferroelectric and ferroelastic phases with very low domain wall energies have been shown to form miniaturized microdomain structures. A theory of an adaptive ferroelectric phase has been developed to predict the microdomain-averaged crystal lattice parameters of this structurally inhomogeneous state. The theory is an extension of conventional martensite theory, applied to ferroelectric systems with very low domain wall energies. The case of ferroelectric microdomains of tetragonal symmetry is considered. It is shown for such a case that a nanoscale coherent mixture of microdomains can be interpreted as an adaptive ferroelectric phase, whose microdomain-averaged crystal lattice is monoclinic. The crystal lattice parameters of this monoclinic phase are self-adjusting parameters, which minimize the transformation stress. Self-adjustment is achieved by application of the invariant plane strain to the parent cubic lattice, and the value of the self-adjusted parameters is a linear superposition of the lattice constants of the parent and product phases. Experimental investigations of Pb(Mg1/3Nb2/3)O3–PbTiO3 and Pb(Zn1/3Nb2/3)O3–PbTiO3 single crystals confirm many of the predictions of this theory.

Effect of carbon nanotube doping on critical current density of MgB2 superconductor

Abstract: The effect of doping MgB2 with carbon nanotubes on transition temperature, lattice parameters, critical current density and flux pinning was studied for MgB2−xCx with x=0, 0.05, 0.1, 0.2, and 0.3. The carbon substitution for B was found to enhance Jc in magnetic fields but depress Tc. The depression of Tc, which is caused by the carbon substitution for B, increases with an increasing doping level, sintering temperature, and duration. By controlling the extent of the substitution and addition of carbon nanotubes we can achieve the optimal improvement on critical current density and flux pinning in magnetic fields while maintaining the minimum reduction in Tc. Under these conditions, Jc was enhanced by two orders of magnitude at 8 T and 5 K and 7 T and 10 K. Jc was more than 10 000 A/cm2 at 20 K and 4 T and 5 K and 8.5 T, respectively.

Six low-strain zinc-blende half metals: An ab initio investigation

Abstract: these findings, we further propose substrates on which the growth may be accomplished with minimum strain. Our findings are supported by the recent successful synthesis of ZB CrAs on GaAs and ZB CrSb on GaSb, where our predicted equilibrium lattice constants are within 0.5% of the lattice constants of the substrates on which the growth was accomplished. We confirm previous theoretical results for ZB MnAs, but find ZB MnSb to be half metallic at its equilibrium lattice constant, whereas previous work has found it to be only nearly so. We report here two low-strain half metallic ZB compounds, CrP and MnC, and suggest appropriate substrates for each. Unlike the other five compounds, we predict ZB MnC to become/remain half metallic with compression rather than expansion, and to exhibit metallicity in the minority- rather than majority-spin channel. These fundamentally different properties of MnC can be connected to substantially greater p-d hybridization and d-d overlap, and correspondingly larger bonding-antibonding splitting and smaller exchange splitting. We examine the relative stability of each of the six ZB compounds against NiAs and MnP structures, and find stabilities for the compounds not yet grown comparable to those already grown.

Vacancy hardening in single-crystal TiNx(001) layers

Abstract: We investigate the effect of N vacancies on the mechanical properties of epitaxial δ-TiNx(001) layers with x=0.67–1.0. The relaxed lattice parameter increases linearly with x in good agreement with ab initio density functional calculations, indicating that deviations from stoichiometry are entirely due to anion vacancies. Hardness values increase continuously, while the elastic modulus decreases with increasing N-vacancy concentration. We attribute the observed vacancy hardening to a reduced dislocation mobility arising from an increase in the rate-limiting activation energy for cation migration.

Formation of face-centered-cubic titanium by mechanical attrition

Abstract: This study reports a metastable hcp→fcc polymorphic transformation in elemental titanium induced by high-energy mechanical attrition in a planetary ball mill. The transformation is monitored and verified by x-ray and electron diffraction and high-resolution transmission electron microscopy. The grain size decreases and lattice parameter increases with continued milling. The phase change is gradual and accompanied by about 16% increase in volume per atom. The milling intensity and deformation mode seem crucial for the completion of the change in crystal structure. The extent and influence of both substitutional and interstitial impurities in this transformation have been assessed. It is suggested that structural instability due to negative (from core to boundary) hydrostatic pressure arising out of nanocrystallization or grain refinement, increasing lattice expansion, and plastic strain/strain rate is responsible for this hcp→fcc polymorphic transformation in titanium. Thus, the present transformation is similar in nature and genesis to those in elemental niobium and zirconium earlier reported by us.

Structural and electronic properties of lithium intercalated graphite LiC 6

Abstract: We calculate the lattice properties and electronic structure of graphite and ${\mathrm{LiC}}_{6}$ within the most widely used density-functional theory implementation, the local density approximation (LDA). Improvements to the LDA in the form of a generalized gradient approximation (GGA) are explored. Structural parameters predicted by the LDA, as expected, underestimate experiment within a 1%--2% margin of accuracy. The GGA does not give a good account in the prediction of lattice parameter c, especially in graphite, although it does give a reliable description of ${\mathrm{LiC}}_{6}.$ The effect on intercalating lithium into graphite, where charge transfer from lithium to carbon layers (graphenes) is expected, is discussed from the valence charge density, partial density of states, and energy band structure plots. The latter plot is also compared with inelastic neutron scattering results and low-energy electron diffraction results. We extend this work by calculating the elastic constants and bulk modulus for both graphite and ${\mathrm{LiC}}_{6}$ structures. These results are in excellent agreement with the available experimental data. The calculated hydrostatic pressure dependence of the crystal structures is also found to be in good agreement with the results of high-resolution x-ray structural studies and with other experimental data as well as with other calculations. The analysis of electronic structure at 0 GPa (ambient pressure) is used to resolve inconsistencies between previous LDA calculations.

Strained-channel transistor structure with lattice-mismatched zone

Abstract: A strained-channel transistor structure with lattice-mismatched zone and fabrication method thereof. The transistor structure includes a substrate having a strained channel region, comprising a first semiconductor material with a first natural lattice constant, in a surface, a gate dielectric layer overlying the strained channel region, a gate electrode overlying the gate dielectric layer, and a source region and drain region oppositely adjacent to the strained channel region, with one or both of the source region and drain region comprising a lattice-mismatched zone comprising a second semiconductor material with a second natural lattice constant different from the first natural lattice constant.

Determination of the Avogadro constant via the silicon route

Abstract: A value for the Avogadro constant, NA, was derived from new measurements of the lattice parameter, the density and the molar mass of a silicon single crystal The result NA = 6022 135 3 × 1023 mol−1 has a relative measurement uncertainty and is in excellent agreement with other published data based on the x-ray crystal density molar mass method, indicating the high repeatability of these experiments The value differs significantly from the Committee on Data for Science and Technology's most recent recommended value of 6022 141 99 × 1023 mol−1 by more than 1 × 10−6 NA

Effect of Mn substitution on magnetoelectric properties of bismuth ferrite system

Abstract: The effect of Mn substitution on ferroelectric and antiferromagnetic properties of the Bi09La01Fe1−xMnxO3 system has been studied It is seen that, with the increase in Mn content, there is a contraction in unit cell volume Since the cell volume contraction is isotropic, lattice distortion (c/a) remains the same and so is the ferroelectric transition temperature TC Further, it is observed that the presence of Mn does not affect the loss tangent (tan δ) of the samples On the contrary, with increase in Mn content, a small but linear enhancement in the magnetization is observed

First-principles investigation of lattice constants and bowing parameters in wurtzite AlxGa1−xN, InxGa1−xN and InxAl1−xN alloys

Abstract: First-principles calculations, by means of the full-potential augmented plane wave method using the local density approximation, were carried out for the structural and electronic properties of the AlxGa1−xN, InxGa1−xN and InxAl1−xN alloys in the wurtzite structure. We have investigated the lattice parameters and band gap energies. The lattice constants a and c are found to change linearly for the AlxGa1−xN alloy, while for both InxGa1−xN and InxAl1−xN alloys the lattice parameters, a, exhibit an upward bowing. The calculated band gap variation for the three alloys exhibit a downward bowing of 0.71 eV, 1.7 eV and 4.09 eV for AlxGa1−xN, InxGa1−xN and InxAl1−xN, respectively.

Effect of nano-carbon particle doping on the flux pinning properties of MgB2 superconductor

Abstract: Polycrystalline MgB 2− x C x samples with x =005, 01, 02, 03 and 04 nano-particle carbon powder were prepared using an in situ reaction method under well-controlled conditions to limit the extent of C substitution The phases, lattice parameters, microstructures, superconductivity and flux pinning were characterized by XRD, TEM, and magnetic measurements It was found that both the a -axis lattice parameter and the T c decreased monotonically with increasing doping level For the sample doped with the highest nominal composition of x =04 the T c dropped only 27 K The nano-C-doped samples showed an improved field dependence of the J c compared with the undoped sample over a wide temperature range The enhancement by C doping is similar to that of Si doping but not as strong as for nano-SiC-doped MgB 2 X-ray diffraction results indicate that C reacted with Mg to form nano-size Mg 2 C 3 and MgB 2 C 2 particles Nano-particle inclusions and substitution, both observed by transmission electron microscopy, are proposed to be responsible for the enhancement of flux pinning in high fields