Growth, EPR and optical absorption spectra of l-threonine single crystals doped with Cu ions

We investigated the crystal growth, electron paramagnetic resonance (EPR) and optical absorption spectra of l-threonine doped with Cu²⁺. The quality, size and habit of the single crystals grown from aqueous solution by the slow solvent evaporation and by the cooling methods vary when the impurities are introduced during the growth process. The variations with the magnetic field orientation of the EPR spectra of single-crystal samples at room temperature and 9.77 GHz in three crystal planes (ab, bc and ac) show the presence of copper impurities in four symmetry-related sites of the unit cell. These spectra display well resolved hyperfine couplings of the of Cu²⁺ with the I_Cu= of the copper nuclei. Additional hyperfine splittings, well-resolved only for specific orientations of the magnetic field, indicate that the copper impurity ions in the interstitial sites have two N ligands with similar hyperfine couplings. The principal values of the g and A_Cu tensors calculated from the EPR data are g₁=2.051(1), g₂=2.062(2), g₃=2.260(2), A_Cu,1=16.9(5)×10⁻⁴ cm⁻¹, A_Cu,2=21.8(6)×10⁻⁴ cm⁻¹, A_Cu,3=180.0(5)×10⁻⁴ cm⁻¹. The principal directions corresponding to g₃ and to A_Cu,3 are coincident within the experimental errors, reflecting the orientation of the bonding planes of the copper ions in the crystal. The values of the crystal field energies are evaluated from the optical absorption spectrum, and the crystal field and bonding parameters of the Cu impurities in the crystal are calculated and analyzed. The EPR and optical absorption results are discussed in terms of the crystal structure of l-threonine and the electronic structure of the Cu²⁺ ions, and compared with data reported for other systems. The effects of the impurities in the growth and habit of the crystals are also discussed.     

Theoretical interpretation of optical and EPR spectra and the substitutional site of ferroelectric LiNbO3:Ni crystal

The optical absorption spectrum, zero-field splitting (ZFS) and EPR g factor of LiNbO₃:Ni²⁺ are explained uniformly on the basis of complete energy matrix diagonalization procedure (CDP) and Zhao's self-consistent field (SCF) d-orbit of free Ni²⁺ ions. The agreement between the calculated results and the experimental data shows quantitatively that impurities Ni²⁺ replace the Nb⁵⁺ rather than Li⁺ sites in LiNbO₃:Ni²⁺.     

Theoretical studies of the optical and EPR spectra for V in Y2O3 crystal

In this paper, we give an alternative suggestion that both the observed optical and electron paramagnetic resonance (EPR) spectra of Yttrium oxide (Y₂O₃):V³⁺ are attributed to V³⁺ ions at the S₆ site of Y₂O₃. This suggestion is different from the opinion in the previous paper that the optical and EPR spectra are attributed to V³⁺ ions at the C₂ and S₆ sites, respectively. From the suggestion, the optical band positions and spin-Hamiltonian parameters are calculated by diagonalizing the complete energy matrix for 3d² ions in trigonal symmetry. The results are in good agreement with the experimental values, suggesting that both the observed optical and EPR spectra in Y₂O₃:V³⁺ may be due to V³⁺ at S₆ site of Y₂O₃ crystal.     

Study of optical absorption and EPR spectra of Mn<Superscript>2+</Superscript> ion in cadmium maleate dihydrate

The optical absorption and EPR spectra of Mn²⁺ ion doped in cadmium maleate dihydrate have been theoretically investigated by diagonalizing the complete energy matrices for a d⁵ configuration ion in a trigonal ligand-field. According to the suggestion of the optical absorption studies, we assume that the Mn²⁺ ion enters the host lattice interstitially and the distorted octahedral symmetry for the impurity ion is trigonal. Moreover, the local lattice structure parameters of the system are determined. The results show that the six oxygen ions around the Mn²⁺ ion are at the same distance R=2.115 ?, and there are three Mn-O bonds forming an angle θ₁ of 66.26° with the C₃-axis and three others forming an angle θ₂ of 43.40°.     

EPR and optical absorption studies of V O ions in L-asparagine monohydrate

Electron paramagnetic resonance (EPR) studies of V O²⁺ ions in L-asparagine monohydrate single crystals are reported at room temperature. It is found that the V O²⁺ ion takes up an interstitial site. The angular variations of the EPR spectra in three mutually perpendicular planes are used to determine the principal g and A values and their direction cosines. The values of g and A parameters are: g_x=1.9011, g_y=2.1008, g_z=1.9891 and A_x=100, A_y=78, A_z=126 (×10⁻⁴) cm⁻¹. The optical absorption spectrum of V O²⁺ ions in L-asparagine monohydrate is also studied at room temperature. The band positions are calculated using the energy expressions and compared with the observed band positions to confirm the transitions. The best-fit values of the crystal field (Dq) and tetragonal (Ds and Dt) parameters are evaluated from the observed band positions.     

The effects of crystal structure on optical absorption/photoluminescence of bis(8-hydroxyquinoline)zinc

In this paper, the transformation processes of two types of bis(8-hydroxyquinoline)zinc: Znq₂ dihydrate and anhydrous (Znq₂)₄ were investigated by X-ray diffraction (XRD), infrared spectra (IR), scanning electron microscope (SEM), differential scanning calorimetry (DSC) and thermogravimetry (TG). The effects of crystal structure on optical properties of bis(8-hydroxyquinoiline)zinc were analyzed. Znq₂ dihydrate can be transformed into anhydrous (Znq₂)₄ during heating under vacuum. Reversal transformation occurs by the interaction between chloroform and (Znq₂)₄. But (Znq₂)₄ was partially transformed into Znq₂ dihydrate by the interaction between ethanol and (Znq₂)₄. The different molecular structure results in different crystal stacking and electronic structure, thereby affect its optical properties.     

EPR theoretical study of local lattice structure in YAG:Mn system

By diagonalizing a set of complete energy matrices constructed for a d⁵ configuration ion in a trigonal ligand field, a reasonable interpretation is obtained for the EPR zero-field splitting of Mn²⁺ ions located at octahedral sites in yttrium aluminum garnet. It is shown that the local lattice structure around an octahedrally coordinated Mn²⁺ center has an expansion distortion, which may be attributed to the fact that the radius of Mn²⁺ ion is larger than that of Al³⁺ ion, and the Mn²⁺ ion will push the oxygen ligands outwards. Simultaneously, the local lattice structure distortion parameters ΔR=0.1825-0.2158 A and Δθ=1.220°-1.315° for the octahedral Mn²⁺ center in the crystal are determined, respectively. Meanwhile, we also demonstrated that the empirical impurity-ligand distance is not suitable for the YAG:Mn²⁺ system which has been approximately taken in previous works.     

Investigations of the optical and EPR spectra for (NiX6) (X=Cl, Br, I) clusters

In this paper, the formulae of optical spectral levels and electron paramagnetic resonance (EPR) spectra in trigonal symmetry of 3d⁸ ions are established on the basis of strong field mechanism and a two spin-coupling (SO) parameters model. Unlike the classical crystal-field approach which has only taken the SO coupling of the central metal ions into account, the contribution of the SO coupling of the ligand ions to the optical and EPR spectra has been included in these formulae. When the optical and EPR spectra of the strong covalent crystals are calculated, the reasonable results can be obtained if the two SO parameters model has been put into action. As an application, the optical and EPR spectra of the (NiX₆)⁴⁻ clusters in CsMgX₃:Ni²⁺ (X=Cl, Br, I) crystals have been studied by the complete diagonalization (of energy matrix) method (CDM). The calculated results agree well with experimental findings. From the investigations, a more valid method to calculate the optical and EPR spectra for 3d⁸ ions clusters is provided.     

Dependence of the anomalous fading of the TL and blue-OSL of fluorapatite on the occupancy of the tunnelling recombination sites

The anomalous fading (AF) of thermoluminescence (TL) and optically stimulated luminescence (OSL) signals in Durango apatite is attributed to tunnelling effects. Electrons from the TL and OSL traps in this material are transferred, via a tunnelling effect, to the recombination sites. The availability of recombination sites for tunnelled electrons is of major importance for the degree of AF rate observed in this material. It is expected that a variation of the number of the electron recombination sites will be reflected in the experimentally measured AF rate. In the present work an investigation of the recombination sites for the tunnelled electrons is attempted by studying the AF effect using a special technique, in which the anomalously faded TL (OSL) is replaced by an equal amount of TL (OSL) induced by a beta dose.     

Optical spectrum, local lattice structure and EPR g factors for Cr impurity ions in MgTiO3 and LiTaO3

On the basis of the 120×120 complete energy matrices for a d³ configuration ion in a trigonal ligand field, for Cr³⁺ ions doped in MgTiO₃ and LiTaO₃, the local structures and EPR g factors of the octahedral (CrO₆)⁹⁻ clusters have been studied, respectively. By simulating the calculated optical spectra and the EPR spectra data to the experimental results, local structure parameters are obtained. The calculated results show that although the local lattice structures around the M (M=Mg²⁺, Ta⁵⁺) ions are obviously different, after Cr³⁺ replacing the M, the local lattice structures around the Cr³⁺ ions are quite similar and close to those of the Cr₂O₃. This may be ascribed to the fact that the octahedral Cr³⁺ center in MgTiO₃:Cr³⁺ and LiTaO₃:Cr³⁺ systems and that in Cr₂O₃ exhibit similar octahedral (CrO₆)⁹⁻ clusters. Moreover, the corresponding theoretical values of the optical spectra have been reported. It is also found that the orbital reduction factor k is very important to understand the EPR g factors for Cr³⁺ ions doped in MgTiO₃ and LiTaO₃.     

First principles analysis of the MgAl2O4:Ni absorption spectrum

Analysis of the energy-level scheme and absorption spectrum of the Ni²⁺ ion in MgAl₂O₄ was performed. The recently developed first-principles approach to the analysis of the absorption spectra of impurity ions in crystals based on the discrete variational multi-electron (DV-ME) method [K. Ogasawara, et al., Phys. Rev. B 64 (2001) 115413) was used in the calculations. The method is based on the numerical solution of the Dirac equation; no phenomenological parameters are used in the calculations. As a result, complete energy-level scheme of Ni²⁺ and its absorption spectra were calculated, assigned and compared with experimental data on the ground and excited state absorption spectra. Numerical contributions of all possible electron configurations into the calculated energy states were determined. By performing analysis of the molecular orbitals population, numerical contributions of the oxygen 2p- and 2s-orbitals into the 3d molecular orbitals were determined.     

Many body effects and internal transitions of confined excitons in GaAs and CdTe quantum wells

Many body effects contribute significantly to the energy states of electron-hole pairs confined in quantum wells in the presence of excess electrons. We present results of optically detected resonance spectroscopy of the internal transitions of photo-excited electron-hole pairs in the presence of excess electrons for GaAs QWs and CdTe QWs. Compared to the case of isolated negatively charged excitons, excess electrons produce a large blue shift of the internal transitions in modulation-doped GaAs quantum wells (QWs) for filling factor <2, and similar effects are found in CdTe QWs. For filling factor >2 no internal transitions are observed. These measurements demonstrate the strong effects of electron-electron correlations on the internal transitions of charged excitons in these quasi-2D systems and the importance of magnetic translation invariance. In the presence of excess electrons, the observed internal transitions are those of a magnetoplasmon bound to a mobile valence band hole.     

Synthesis, effect of capping agents, structural, optical and photoluminescence properties of ZnO nanoparticles

Zinc oxide nanoparticles were synthesized using chemical method in alcohol base. During synthesis three capping agents, i.e. triethanolamine (TEA), oleic acid and thioglycerol, were used and the effect of concentrations was analyzed for their effectiveness in limiting the particle growth. Thermal stability of ZnO nanoparticles prepared using TEA, oleic acid and thioglycerol capping agents, was studied using thermogravimetric analyzer (TGA). ZnO nanoparticles capped with TEA showed maximum weight loss. X-ray diffraction (XRD) and scanning electron microscopy (SEM) were used for structural and morphological characterization of ZnO nanoparticles. Particle size was evaluated using effective mass approximation method from UV-vis spectroscopy and Scherrer's formula from XRD patterns. XRD analysis revealed single crystal ZnO nanoparticles of size 12-20 nm in case of TEA capping. TEA, oleic acid and thioglycerol capped synthesized ZnO nanoparticles were investigated at room temperature photoluminescence for three excitation wavelengths i.e. 304, 322 and 325 nm, showing strong peaks at about 471 nm when excited at 322 and 325 nm whereas strong peak was observed at 411 for 304 nm excitation.     

EPR spectra of Fe centers in layered TlGaSe2 single crystal

A single-crystal TlGaSe₂ doped by paramagnetic Fe ions has been studied at room temperature by electron paramagnetic resonance (EPR) technique. The fine structure of EPR spectra of paramagnetic Fe³⁺ ions was observed. The spectra were interpreted to correspond to the transitions among spin multiplet (S=5/2, L=0) of Fe³⁺ ion, which are splitted by the local ligand crystal field (CF) of orthorhombic symmetry. Four equivalent Fe³⁺ centers have been observed in the EPR spectra and the local symmetry of crystal field at the Fe³⁺ site and CF parameters were determined. Experimental results indicate that the Fe ions substitute Ga at the center of GaSe₄ tetrahedrons, and the rhombic distortion of the CF is caused by the Tl ions located in the trigonal cavities between the tetrahedral complexes.     

Twinned EPR spectra of Fe centers in ternary layered TlGaS2 crystal

TlGaS₂ single crystal doped by paramagnetic Fe³⁺ ions has been studied by electron paramagnetic resonance (EPR) technique. The fine structure of EPR spectra of paramagnetic Fe³⁺ ions was observed. The spectra reveal a nearly orthorhombic symmetry of the crystal field (CF) on the Fe³⁺ ions. Two groups each consisting of four equivalent Fe³⁺ centers were observed in the EPR spectra. The local symmetry of the crystal field on the Fe³⁺ centers and CF parameters were determined. Experimental results indicate that the Fe ions substitute Ga at the center of the GaS₄ tetrahedrons. The rhombic distortion of the sulfur ligand CF is attributed to the effect of Tl ions located in the trigonal cavities between the tetrahedral complexes. The observed twinning of the resonance lines indicates a presence of two non-equivalent positions of Tl ions that confirms their zigzag alignment in the TlGaS₂ crystal structure.     

Excitonic absorption spectra and ultrafast dephasing dynamics in arbitrary carbon nanotubes

We illustrate the potential of the density matrix theory for investigation of optical properties of arbitrary single‐walled carbon nanotubes (CNTs). We have performed microscopic calculations of excitonic absorption spectra for CNTs of different chiral angles and diameters. The obtained results are in good agreement with experiments, in particular the excitonic binding energies match well both experiments and ab initio calculations. Furthermore, we show the strength of our approach by presenting calculations of the ultrafast Coulomb driven non‐equilibrium dynamics in CNTs. We find excitation induced dephasing on the picosecond time scale depending on the excitation strength. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Anisotropy of photoconductivity and nonlinear effects in GaSe monocrystals at high optical excitation

In this work, the photoelectric properties of gallium selenide (GaSe) monocrystals in the edge absorption region, with various configurations of current contacts, at low and high optical excitation levels are investigated. The photoconductivity spectrum behavior is determined by localized electronic and excitonic states along c-axis. It is shown that the localization of electronic and excitonic states in one-dimensional fluctuation potential along c-axis results to an anisotropy in photoconductivity spectrum at various current contacts configurations. At E⊥c the photoconductivity is observed in the hν < E_g and hν > E_g regions. In the case of hv < E_g, the maximum photoconductivity, in the impurity and exciton absorption region are observed at 1.975 eV and 2.102 eV, respectively. With rising of excitation energy level, suppression of photoconductivity in the exciton absorption region and increases in impurity absorption region is observed. At E||c contact configuration, the considerable photoconductivity is observed only in the impurity absorption region, which also increases with rising of excitation level. It is supposed that, suppression of photoconductivity in the exciton absorption region at high excitation levels is connected with exciton-exciton interaction, which results to a nonlinear light absorption. The results are compared with the absorption and photoluminescence measurements.     

Giant thermodynamical optical effect near ferroelastic phase transition point in Cs3Bi2I9 layered crystal

For the first time we have found a new giant thermodynamical optical effect near the ferroelastic phase transition point in Cs₃Bi₂I₉ layered crystal. The effect is appeared as periodical oscillations in time of the reflection coefficient. This phenomenon is caused by the small temperature deviations in thermodynamical system the appearance of which in the reflection spectra is strongly amplified in the ferroelastic phase transition point. The optical oscillations are explained on the base of a model that takes into account the temperature dependence of the refractive index through the order parameter (spontaneous strain) of the crystal.     

Transient behaviour of the mechanoluminescence induced by impulsive deformation of fluorescent and phosphorescent crystals

When a crystal is fractured impulsively by the impact of a moving piston, then initially the mechanoluminescence (ML) intensity increases quadratically with time, attains a peak value and later on it decreases with time. Considering that the solid state ML and gas discharge ML are excited due to the charging and subsequent production of electric field near the tip of moving cracks, expressions are derived for the transient ML intensity I, time t_m and intensity I_m corresponding to the peak of ML intensity versus time curve, respectively, the total ML intensity I_T, and for fast and slow decays of the ML intensity. It is shown that the decay time for the fast decrease of the ML intensity after t_m, is related to the decay time of the strain rate of crystals, and the decay time of slow decay of ML, only observed in phosphorescent crystals, is equal to the decay time of phosphorescence. The value of t_m decreases with the increasing impact velocity, I_m increases with the increasing impact velocity, and I_T initially increases and then it tends to attain a saturation value for higher values of the impact velocity. The values of t_m, I_m and I_T increase linearly with the thickness, area of cross-section and volume of the crystals, respectively. So far as the rise, attainment of ML peak, and fast decay of ML are concerned, there is no any significant difference in the time-evolution of solid state ML, gas discharge ML, and the ML emission consisting of both the solid state ML and gas discharge ML. From the time-dependence of ML, the values of the time-constant for decrease of the surface area created by the movement of a single crack, the time-constant for the decrease of strain rate of crystals, and the decay time of phosphorescence of crystals can be determined. A good agreement is found between the theoretical and experimental results. The importance of fracto ML induced by impulsive deformation of crystals is discussed.