Photoluminescence properties of silica-based mesoporous materials similar to those of nanoscale silicon

Photoluminescence (PL) from composites of 7- and 15-nm sized silica nanoparticles (SNs) and mesoporous silicas (MSs) induced by 266- (4.66-) and 532-nm (2.33-eV) laser light has been studied at room temperature. The multiband PL from MSs in the range of 1.0-2.1 eV is evidenced to originate from isolated bulk and surface non-bridging oxygens (NBOs) and from NBOs combined with variously placed 1-nm sized pore wall oxygen vacancies (OVs). The nature and diversity of NBO light-emitters are confirmed by ab initio calculations. The PL from SNs exhibits only a short wavelength part of the bands (1.5-2.1 eV) originated from isolated bulk and surface NBOs. This fact indicates that the highly OV-bearing structures occur only in extremely thin (∼ 1 nm) silica layers. The similarity of spectroscopic properties of silica-based nanoscale materials to those of surface-oxidized silicon nanocrystals and porous silicon, containing silica-passivating layers of the same width, is discussed. Received 20 November 2000     

First-principles investigation of structural,electronic and optical properties of IVA group spinel nitrides

The Si₃N₄ and Ge₃N₄ are important structural ceramics with many applications because of their attractive high temperature and oxidation resistant properties. The high-pressure and high-temperature spinel phases of these two materials were noticed to have wide, direct electronic band gaps. Other single and double spinel nitrides formed from IVA and IVB group elements have also attracted much attention. Present research focuses on selecting a special substance with promising optical properties and stability besides the attractive electronic properties. The formation energies of spinel nitrides are calculated and stabilities of a group of spinel nitrides are discussed, the structural and electronic properties of them are investigated in detail. By random phase approximation (RPA), the optical properties of spinel nitrides are researched. We obtain that γ-SiGe₂N₄ has some promising properties with potential technological applications from various aspects. The band transitions which contribute most to the peak of ε₂ have been identified. An assumption is proposed to raise the peak of ε₂.     

Band structure of tetragonal BaTiO <Emphasis Type="Bold">3</Emphasis>

The electronic structure, total density of states DOS and electronic density in ferroelectric tetragonal crystal BaTiO₃ are studied using WIEN2k package. This employs the full potential-linearized augmented plan wave FP-LAPW method in the framework of the density functional theory DFT with the generalized gradient approximation (GGA). The results show an indirect band gap of 2.30 eV at the Γ point in the Brillouin zone. The calculated band structure and density of states of BaTiO₃ agree with the previous experimental and theoretical results, as do the charge distribution and the prediction of the nature of the chemical bonding. Received 11 December 2002 / Received in final form 3 February 2003 Published online 1st April 2003 RID="a" ID="a"e-mail: salehihamid@yahoo.com     

Optical properties of the quasi-two-dimensional dichalcogenides 2H-TaSe and 2H-NbSe

We present a comprehensive analysis of the optical constants of the two-dimensional dichalcogenide materials 2 H - TaSe ₂ and 2 H - NbSe ₂ , in an attempt to address the physics of two-dimensional correlated systems. The title compounds were studied over several decades in frequency, from the far-infrared to the ultraviolet. Measurements with linearly polarized light have allowed us to obtain both the in-plane and out-of-plane components of the conductivity tensor. Although the electromagnetic response of dichalcogenides is strongly anisotropic, both the in-plane and out-of-plane components of the conductivity tensor share many common features, including the presence of a well-defined metallic component, as well as a “mid-infrared band”. We discuss the implications of these results in the context of the spectroscopic results of other classes of low-dimensional conductors such as the high-temperature superconducting cuprates. In particular, the analysis of the redistribution of the spectral weight as a function of temperature, as well as the behavior of the quasiparticles relaxation rate, points to significant distinctions between the charge dynamics of dichalcogenides and other classes of low dimensional conductors. Received 28 October 2002 / Received in final form 10 March 2003 Published online 23 May 2003 RID="a" ID="a"e-mail: degiorgi@solid.phys.ethz.ch     

An all-electron LCGTO study of square and hexagonal plutonium monolayers

The linear combinations of Gaussian type orbitals fitting function (LCGTO-FF) method is used to study the electronic and geometrical properties of plutonium monolayers with square and hexagonal symmetry. The effects of several common approximations are examined: (1) scalar-relativity vs. full-relativity (i.e., with spin-orbit coupling included); (2) paramagnetic vs. spin-polarized; and (3) local-density approximation (LDA) vs. generalized- gradient approximation (GGA). The results indicate that spin-orbit coupling has a much stronger effect on the monolayer properties compared to the effects of spin-polarization. In general, the GGA is found to predict a larger lattice constant and a smaller cohesive energy compared to LDA predictions. We also find a significant compression of the monolayers compared to the bulk, contradicting the only other published result on a Pu monolayer. The current result supports the existence of a δ-like surface on α-Pu. Received 17 October 2001 Published online 6 June 2002     

Electron-phonon interaction in carbon schwarzites

The recent synthesis of random schwarzites has stimulated the present ab initio calculation of the electronic structure and electron-phonon interaction in two different periodic D-type schwarzites, fcc-(C₂₈)₂ (made of 24 seven-membered rings per unit cell) and fcc-(C₆₄)₂ (made of 12 eight membered and 48 six-membered rings per unit cell). Like in fullerenes, also in schwarzites the electron-phonon interaction potential is found to increase with the absolute Gauss curvature, though it remains smaller than for doped fullerenes. Received 19 December 2002 Published online 1st April 2003 RID="a" ID="a"e-mail: marco.bernasconi@unimib.it     

Studies on CdS nanoparticles dispersed in silica matrix prepared by sol-gel technique

SiO₂/CdS-nanoparticle composite films (SiO₂:CdS=85:15, 80:20, 75:25 and 70:30) were prepared by the sol-gel route. The films were characterized by studying microstructural (XRD and TEM) and optical (transmittance and photoluminescence) properties. Band gaps of these films annealed at different temperatures (373-473 K) for different times (10-120 min) indicated that the signature of nanocrystallinity is retained throughout the range of our experimental conditions. A thermal diffusion process controlled growth in the crystallite size with increasing annealing time and temperature. The average radii of the nanoparticles varied as the cube root of the annealing time but showed exponential dependence on the inverse of annealing temperature. Photoluminescence (PL) studies of the composite films indicated excitonic transitions. Theoretical analysis of the line shapes of the PL peaks recorded at 300 K and 80 K could be accounted for by the combined effects of size distribution and phonon broadening. It was observed that the deformation potential (E _d) effectively controlled the line shapes of the PL measurements. Received 24 May 2002 Published online 27 January 2003 RID="a" ID="a"e-mail: msakp@mahendra.iacs.res.in     

Effect of interfacial alloying on the surface plasmon resonance of nanocrystalline Au-Ag multilayer thin films

Nanocrystalline Au and Ag in multilayer thin film form with Au/Ag/Au structure were prepared by high pressure (∼40 Pa) d.c. sputtering techniques. The Ag concentrations in Ag_xAu_1-x films were changed from x = 0 to 1. These multilayer films with varying Ag concentration showed significant changes in microstructures obtained from TEM and XRD analyses. The optical absorption spectra of these multilayer films showed a single plasmon band confirming the formation of Au-Ag alloy. We ascribe this alloying to the interfacial reactions in nanophase limited at the Au-Ag interface. The red-shift and broadening of the plasmon bands with the increase in silver concentration could be associated to the increase in size of the nanoparticles and its distribution. The observed red shift in the plasmon band may be associated with the change in electronic structure at the Au-Ag interface due to configuration mixing of the atomic energy levels of Au and Ag. Received 17 October 2002 / Received in final form 26 February 2003 Published online 23 May 2003 RID="a" ID="a"e-mail: msakp@mahendra.iacs.res.in     

Optical conductivity of the Bechgaard salts: the sum rules revisited

The validity of the optical sum rules has been addressed eversince and was always matter of debate. Particularly controversial is the proof that the partial sum rules can be extended to both optical conductivity and energy loss function. We show in this paper that for both transverse (optical conductivity) and longitudinal (energy loss function) absorption processes the corresponding sum rule can be theoretically established and through appropriate conditions for the integration limits exactly verified. We also focus our attention on the one-dimensional case within the microscopic Hubbard model. An application of these concepts to the quasi one-dimensional systems, for which we have chosen the organic (TMTSF)₂PF₆ material, will also be presented. Received: 19 December 1997 / Received in final form: 9 March 1998 / Accepted: 23 March 1998     

Charge excitations in heavy electron metals

We show that the optical response of metals with strong electron-electron correlation consists of two excitations, a renormalized Drude response at zero energy and a mid-infrared peak occurring at frequencies around 2000 cm^-1. The latter originates from a dynamical, correlation-induced gap, as evinced from a many body theoretical approach based on the periodic Anderson model. At very low temperatures, it can be viewed as optical gap between two renormalized quasi-particle bands. The gap size is proportional to the geometric mean of the characteristic lattice Kondo temperature of the material and its bandwidth. Received 28 August 2000 and Received in final form 31 October 2000     

First-principles investigation of electronic and optical properties in wurtzite Zn<Subscript>1-x</Subscript>Mg<Subscript>x</Subscript>O

A first-principles study has been performed to evaluate the electronic and optical properties of wurtzite Zn_1-xMg_xO. Substitutional doping is considered with Mg concentrations of x = 0, 0.0625, 0.125, 0.1875 and 0.25, respectively. Mg incorporation can induce band gap widening due to the decrease of Zn 4s states. The imaginary part of the dielectric function shows that the optical transition from band edge emission decreases slightly with increasing Mg contents. The optical band gap also increases from 3.2 to 3.7 eV with increasing Mg contents from 0.0625 to 0.25. The calculated results suggest that relatively high Mg concentration is necessary for effective band gap engineering of wurtzite Zn_1-xMg_xO.     

A spectroscopic investigation of Y 3 Al 5 O 12 :Pr 3 + in translucent ceramic form: Crystal field analysis assisted by configuration-interaction

This paper presents an investigation of Pr³⁺ doped in the D₂ site of Y₃Al₅O₁₂ (YAG), for the first time on a translucent ceramic sample free of spurious phases, impurity or pair sites. The optical study is carried out by optical absorption, excitation, and emission by selective excitation into ¹D₂ and ³P₀, at different temperatures between 20 K and 60 K, in the 4 300-23 000 cm^-1 range. A detailed account of the line assignments is given. 67 over 91 levels of the 4f² configuration are determined. Several crystal field calculations within the ground configuration 4f² and the larger matrix 4f^2+4f6p are carried out. The energy level fit is slightly improved by configuration interaction. The ³P₂ and ¹I₆ levels are strongly mixed together by the large 6th order crystal field parameters. In sintered samples with different Pr³⁺ concentrations, satellite lines with intensities increasing quadratically with the concentration are observed. A few weak lines forbidden in D₂ site symmetry are observed. Received 9 November 2001 and Received in final form 8 February 2002     

Far-infrared optical properties of YVO4 single crystal

Near-normal incident infrared reflectivity spectra of a (001) YVO₄ single crystal have been measured at different temperatures in the frequency region between 100 and 6000 cm^-1. The reflectivity spectra are analyzed with the factorized form of the dielectric function, and the dielectric properties and optical conductivity of the YVO₄ crystal are obtained. From the TO/LO splitting, effective charges at different temperatures are calculated to study the ionicity of YVO₄. The internal modes of the VO₄^3- ion and the external modes of the Y(VO₄) lattice are compared with SiO₄^4- in zircon and with other rare-earth vanadates.     

Optical and electrical properties of hydrogenated yttrium nanoparticles

We studied the effect of hydrogen in yttrium nanoparticles on a quartz substrate, using optical spectroscopy and electrical resistance measurements. Pulsed laser deposition is used to obtain the Y clusters in an UHV environment. We show, that these clusters are highly sensitive to monoatomic H₁ produced from ambient hydrogen gas pressures, ranging from 10^-5 to 50 mbar with our experimental arrangement. The changes of optical and electrical properties due to the chemical reaction within the particles are sufficient to consider this material as a possible sensor for low concentrations of hydrogen. Received 29 November 2000     

First principles calculations of structural,electronic and optical properties of various phases of CaS

First principles calculations have been performed within the framework of density functional theory to investigate the structural, electronic and optical properties of all four possible B1, B2, B3 and B4 phases of CaS. Apart from the standard local density approximation (LDA) and GGA (PBE), a more accurate nonempirical density functional generalized gradient approximation (GGA), as proposed by Wu and Cohen [Phys. Rev. B 73, 235116 (2006)] for the exchange-correlation energy, E_XC, has been attempted in these calculations. Calculated electronic structure and the density of states are analyzed in terms of the contribution of Ca d states and S s and p states in determining the nature of the fundamental band gap in various phases. Reflectivity, R (ω), the real and imaginary part of the dielectric functions, ε(ω), have been calculated for all the phases and the results have been discussed and compared with the existing experimental data.     

Study of optical properties and molecular structure of plasma polymerized diethylene glycol dimethyl ether

This paper deals with plasma polymerization processes of diethylene glycol dimethyl ether. Plasmas were produced at 150 mtorr in the range of 10 W to 40 W of RF power. Films were grown on silicon and quartz substrates. Molecular structure of plasma polymerized films and their optical properties were analyzed by Fourier transform infrared spectroscopy (FTIR) and ultraviolet-visible spectroscopy. The IR spectra show C–H stretching at 3000–2900 cm^-1, C=O stretching at 1730–1650 cm^-1, C–H bending at 1440–1380 cm^-1, C–O and C–O–C stretching at 1200–1000 cm^-1. The concentrations of C–H, C–O and C–O–C were investigated for different values of RF power. It can be seen that the C–H concentration increases from 0.55 to 1.0 au (arbitrary unit) with the increase of RF power from 10 to 40 W. The concentration of C–O and C–O–C decreases from 1.0 to 0.5 au in the same range of RF power. The refraction index increased from 1.47 to 1.61 with the increase of RF power. The optical gap calculated from absorption coefficient decreased from 5.15 to 3.35 eV with the increase of power. Due to its optical and hydrophilic characteristics these films can be applied, for instance, as glass lens coatings for ophthalmic applications.     

Elastic and brittle properties of the B2-MgRE (RE = Sc,Y, Ce,Pr, Nd,Gd, Tb,Dy, Ho,Er) intermetallics

The brittle and elastic properties of the B2-MgRE (RE = Sc, Y, Ce, Pr, Nd, Gd, Tb, Dy, Ho, Er) intermetallics have been investigated using first-principles density functional calculations. The calculated equilibrium lattice constants and enthalpies of formation are in overall agreement with the available experiment and theoretical results. The related physical properties of those compounds are compared with that of ductile YCu. The Fermi energy occurs above a peak in the DOS for B2-MgRE intermetallics, whereas for ductile YCu the Fermi energy occurs near a minimum in the DOS. For B2-YCu, the partial density of states of d-states at the Fermi energy is low, while for B2-MgRE the RE d-states are partially occupied, indicating their important roles in the directional bonding for this material. The Cauchy pressure (C₁₂-C₄₄) and the ratio of bulk to shear modulus B/G are used to assess the brittle/ductile behavior of B2-MgRE and YCu compounds. It can be concluded that the B2-MgRE alloys have brittle behavior. MgSc is the most brittle, and MgHo is the least brittle amongst those alloys.     

Nonlinear optical characterization of cluster dynamic in water in oil microemulsion by a pump probe laser beam technique

We present a new pump probe laser beams configuration for the nonlinear optical characterization of microemulsions. We detect the variation of the on-axis optical intensity of the probe beam as generated by the concentration profile induced in an optically thin film of microemulsion by the pump beam. A mathematical model has been introduced to describe the phenomenon. The technique allows the determination of both Kerr-like optical nonlinearity and time constants and, therefore, it gives information both on cluster dimension and their shape. We discuss its application to WAD (water/AOT/decane, where AOT denotes sodium-bis-di-ethyl-sulfosuccinate) with the application of a strong electric field of optical source. Comparison between theoretical predictions and experimental results confirms the presence of giant optical nonlinearity in the absence of turbidity divergence. Chainlike shape of clusters, of the kind already reported with the application of strong electric field, could justify this result. Received 26 October 2002 RID="a" ID="a"e-mail: vicari@na.infn.it     

Optical properties of mixed clusters: comparative study of Ni/Ag and Pt/Ag clusters

The optical properties of mixed (Ni_0.5Ag_0.5)_n and Pt/Ag clusters are investigated in the size range 2-5 nm. Low Energy Ion Spectroscopy (LEIS) experiments show that the cluster surface is entirely covered by silver atoms for the two systems. The optical spectra of Ni/Ag clusters exhibit a large Surface Plasmon Resonance (SPR), damped and widened when the cluster size decreases, in agreement with a classical model assuming a core-shell geometry and including the reduction of the conduction electron mean-free path in the silver shell. For Pt/Ag clusters, no SPR emerges in the size range 2-5 nm, although it is predicted within a classical model, a pronounced SPR appearing only for clusters larger than 10 nm in diameter.     

Carrier-induced changes in the phase resolved reflection of GaAs quantum wells

The influence of thermalized non-coherent carriers on the dielectric function of GaAs/AlAs quantum wells is investigated by reflection spectroscopy. Experiments are performed using the method of spectral interferometry, where both amplitude and phase of reflected pulses can be determined. For low excitation density the complex coefficient of reflection can be described using as dielectric function a broadened Elliot formula. With increasing carrier density pronounced nonlinearities appear in both amplitude and phase due to many-body effects between excited carriers. The nonlinear behavior fits very well to the results of a many-body theory based on the Semiconductor Bloch equations including memory effects in the scattering processes between carriers and the polarization induced by the probe pulse. Received 29 May 2002 / Received in final form 23 September 2002 Published online 19 December 2002 RID="a" ID="a"e-mail: manzke@physik3.uni-rostock.de