Spectroscopic, and thermal studies of some new binuclear transition metal(II) complexes with hydrazone ligands containing acetoacetanilide and isoxazole

A new chelating ligand, 2-(2-(5-tert-butylisoxazol-3-yl)hydrazono)-N-(2,4-dimethylphenyl)-3-oxobutanamide (HL), and its four binuclear transition metal complexes, M-2(L)(2) (mu-OCH3)(2) [M = Ni(II), Co(II), Cu(II), Zn(II)], were synthesized using the procedure of diazotization, coupling and metallization. Their structures were postulated based on elemental analysis, H-1 NMR, MALDI-MS, FT-IR spectra and UV-vis electronic absorption spectra. Smooth films of these complexes on K9 glass substrates were prepared using the spin-coating method and their absorption properties were evaluated. The thermal properties of the metal(II) complexes were investigated by thermogravimetry (TG) and differential scanning calorimetry (DSC. Different thermodynamic and kinetic parameters namely activation energy (E

Correlations between electronic energy bands spin splitting and magnetic profile symmetry of magnetic superlattice

We have theoretically investigated the energy band structures of two typical magnetic superlattices formed by perpendicular or parallel magnetization ferromagnetic stripes periodically deposited on a two-dimensional electron gas (2DEG), where the magnetic profile in the perpendicular magnetization is of inversion anti-symmetry, but of inversion symmetry in parallel magnetization, respectively. We have shown that the energy bands of perpendicular magnetization display the spin-splitting and transverse wave-vector symmetry, while the energy bands of the parallel magnetization exhibit spin degeneration and transverse wave-vector asymmetry. These distinguishing spin-dependent and transverse wave-vector asymmetry features are essential for future spintronics devices applications. (c) 2008 Elsevier B.V. All rights reserved.

Electronic energy levels in an asymmetric quantum-dots-in-a-well structure for infrared photodetectors

Theoretical calculation of electronic energy levels of an asymmetric InAs/InGaAS/GaAS quantum-dots-in-a-well (DWELL) structure for infrared photodetectors is performed in the framework of effective-mass envelope-function theory. Our calculated results show that the electronic energy levels in quantum dots (QDs) increase when the asymmetry increases and the ground state energy increases faster than the excited state energies. Furthermore, the results also show that the electronic energy levels in QDs decrease as the size of QDs and the width of quantum well (QW) in the asymmetric DWELL structure increase. Additionally, the effects of asymmetry, the size of QDs and the width of QW on the response peak of asymmetry DWELL photodetectors are also discussed.

First-principles study of transition metal impurities in Si

By using ab initio electronic structure calculations within density functional theory, we study the structural, electronic, and magnetic properties of Si doped with a transition metal impurity. We consider the transition metals of the 3d series V, Cr, Mn, Fe, Co, and Ni. To get insight into the level filling mechanism and the magnetization saturation, we first investigate the transition metal-Si alloys in the zinc-blende structure. Next, we investigate the doping of bulk Si with a transition metal atom, in which it occupies the substitutional site, the interstitial site with tetrahedral symmetry, and the interstitial site with hexagonal symmetry. It is found that all of these transition metal impurities prefer an interstitial position in Si. Furthermore, we show that it is possible to interpret the electronic and magnetic properties by using a simple level filling picture and a comparison is made to Ge doped with the same transition metal atoms. In order to get insight into the effect of a strained environment, we calculate the formation energy as a function of an applied homogeneous pressure and we show that an applied pressure can stabilize the substitutional position of transition metal impurities in Si. Finally, the energies of the ferromagnetic states are compared to those of the antiferromagnetic states. It is shown that the interstitial site of the Mn dopant helps us to stabilize the nearest neighbor substitutional site to realize the ferromagnetic state. For doping of Si with Cr, a ferrimagnetic behavior is predicted.

Electronic structure and optical gain saturation of InAs1-xNx/GaAs quantum dots

The electronic band structures and optical gains of InAs1-xNx/GaAs pyramid quantum dots (QDs) are calculated using the ten-band k . p model and the valence force field method. The optical gains are calculated using the zero-dimensional optical gain formula with taking into consideration of both homogeneous and inhomogeneous broadenings due to the size fluctuation of quantum dots which follows a normal distribution. With the variation of QD sizes and nitrogen composition, it can be shown that the nitrogen composition and the strains can significantly affect the energy levels especially the conduction band which has repulsion interaction with nitrogen resonant state due to the band anticrossing interaction. It facilitates to achieve emission of longer wavelength (1.33 or 1.55 mu m) lasers for optical fiber communication system. For QD with higher nitrogen composition, it has longer emission wavelength and less detrimental effect of higher excited state transition, but nitrogen composition can affect the maximum gain depending on the factors of transition matrix element and the Fermi-Dirac distributions for electrons in the conduction bands and holes in the valence bands respectively. For larger QD, its maximum optical gain is greater at lower carrier density, but it is slowly surpassed by smaller QD as carrier concentration increases. Larger QD can reach its saturation gain faster, but this saturation gain is smaller than that of smaller QD. So the trade-off between longer wavelength, maximum optical, saturation gain, and differential gain must be considered to select the appropriate QD size according to the specific application requirement. (C) 2009 American Institute of Physics. [DOI 10.1063/1.3143025]

Investigation of Electronic Energy Levels in InAs Quantum Dot with Shape of Lens by Using B-Spline Technique

The dependence of the electronic energy levels on the size of quantum dots (QDs) with the shape of spherical lens is studied by using the B-spline technique for the first time. Within the framework of the effective-mass theory, the values of electronic energy levels are obtained as a function of the height, radius and volume of QDs, respectively. When the height or radius of QDs increases, all the electronic energy levels lower, and the separations between the energy levels decrease. For lens-shape QDs, height is the key factor in dominating the energy levels comparing with the effect of radius, especially in dominating the ground-state level. These computational results are compared with that of other theoretical calculation ways. The B-spline technique is proved to be an effective way in calculating the electronic structure in QDs with the shape of spherical lens.

Effects of magnetic field on the electronic structure of wurtzite quantum dots: Calculations using effective-mass envelope function theory

The Hamiltonian of the wurtzite quantum dots in the presence of an external homogeneous magnetic field is given. The electronic structure and optical properties are studied in the framework of effective-mass envelope function theory. The energy levels have new characteristics, such as parabolic property, antisymmtric splitting, and so on, different from the Zeeman splitting. With the crystal field splitting energy Delta(c)=25 meV, the dark excitons appear when the radius is smaller than 25.85 A in the absence of external magnetic field. This result is more consistent with the experimental results reported by Efros [Phys. Rev. B 54, 4843 (1996)]. It is found that dark excitons become bright under appropriate magnetic field depending on the radius of dots. The circular polarization factors of the optical transitions of randomly oriented dots are zero in the absence of external magnetic field and increase with the increase of magnetic field, in agreement with the experimental results. The circular polarization factors of single dots change from nearly 0 to about 1 as the orientation of the magnetic field changes from the x axis of the crystal structure to the z axis, which can be used to determine the orientation of the z axis of the crystal structure of individual dots. The antisymmetric Hamiltonian is very important to the effects of magnetic field on the circular polarization of the optical transition of quantum dots.

Electronic structure and binding energy of a hydrogenic impurity in a hierarchically self-assembled GaAs/AlxGa1-xAs quantum dot

We calculate the electronic structures and binding energy of a hydrogenic impurity in a hierarchically self-assembled GaAs/AlxGa1-xAs quantum dot (QD) in the framework of effective-mass envelope-function theory. The variation of the electronic structures and binding energy with the QD structure parameters and the position of the impurity are studied in detail. We find that (1) acceptor impurity energy levels depend more sensitively on the size of the QD than those of a donor impurity; (2) all impurity energy levels strongly depend on the GaAs quantum well (QW) width; (3) a donor impurity in the QD has only one binding energy level except when the GaAs QW is large; (4) an acceptor impurity in the QD has two binding energy levels, which correspond to heavy- and light-hole quantum states; (5) the binding energy has a maximum value when the impurity is located below the symmetry axis along the growth direction; and (6) the binding energy has a minimum value when the impurity is located at the top corner of the QD. (c) 2006 American Institute of Physics.

Effects of electric field on the electronic structure and optical properties of quantum rods with wurtzite structure

The Hamiltonian of wurtzite quantum rods with an ellipsoidal boundary under electric field is given after a coordinate transformation. The electronic structure and optical properties are studied in the framework of the effective-mass envelope-function theory. The quantum-confined Stark effect is illustrated by studying the change of the electronic structures under electric field. The transition probabilities between the electron and hole states decrease sharply with the increase of the electric field. The polarization factor increases with the increase of the electric field. Effects of the electric field and the shape of the rods on the exciton effect are also investigated. The exciton binding energy decreases with the increase of both the electric field and the aspect ratio. In the end, considering the exciton binding energy, we calculated the band gap variation of size- and shape-controlled colloidal CdSe quantum rods, which is in good agreement with experimental results.

Electronic structure and optical properties of quantum rods with wurtzite structure

The Hamiltonian of the wurtzite quantum rods with an ellipsoidal boundary is given after a coordinate transformation. The energies, wave functions, and transition possibilities are obtained as functions of the aspect ratio e with the same method we used on spherical dots. With an overall consideration of both the transition matrix element and the Boltzmann distribution we explained why the polarization factor increases with increasing e and approaches a saturation value, which tallies quite well with the experimental result. When e increases more and more S-z states are mixed into the ground, second, and third states of J(z)=1/2, resulting in an increase of the emission of z polarization. It is just the linear terms of the momentum operator in the hole Hamiltonian that cause the mixing of S and P states in the hole ground state. The effects of the crystal field splitting energy, temperature, and transverse radius to the polarization are also considered. We also calculated the band gap variation with the size and shape of the quantum rods.

Quantum-confinement-effect-driven type-I-type-II transition in inhomogeneous quantum dot structures

We investigate the electronic structures of the inhomogeneous quantum dots within the framework of the effective mass theory. The results show that the energies of electron and hole states depend sensitively on the relative magnitude 77 of the core radius to the capped quantum dot radius. The spatial distribution of the electrons and holes vary significantly when the ratio eta changes. A quantum-confinement-driven type-II-type-I transition is found in GaAs/AlxGa1-xAs-capped quantum dot structures. The phase diagram is obtained for different capped quantum dot radii. The ground-state exciton binding energy shows a highly nonlinear dependence on the innner structures of inhomogeneous quantum dots, which originates from the redistribution of the electron and hole wave functions.

Electronic structures of cubic lattice quantum dots

In this article, we give the electronic structure and optical transition matrix elements of coupled quantum dots (QDs) arranged as different cubic lattices: simple cubic (sc), body-centered cubic (bcc), and face-centered cubic (fcc) superlattices. The results indicate that electron and hole energies of bcc, sc, and fcc superlattices are the lowest, the highest, and the middle, respectively, for the same subband under the same QD density or under the same superlattice constant. For a fixed QD density, the confinement effects in sc, fcc, and bcc superlattices are the strongest, the middle, and the weakest, respectively. There are only one, two, and four confined energy bands, with energies lower than the potential barrier for sc, bcc, and fcc QD superlattices, respectively. The results have great significance for researching and making semiconductor quantum dot devices. (C) 1998 American Institute of Physics. [S0021-8979(98)02119-7]

The effects of electric field on the electronic structure of a semiconductor quantum dot

The effect of electric field on the electronic structure of a spherical quantum dot is studied in the framework of the effective-mass envelope-function theory. The dependence of the energy of electron states and hole states on the applied electric field and on the quantum dot size is investigated; the mixing of heavy holes and light holes is taken into account. The selection rule for the optical transition between the conduction band and valence band states is obtained. The exciton binding energies are calculated as functions of the quantum dot radius and the strength of the electric field. (C) 1998 American Institute of Physics.

Electronic structures of GaAs/AlAs lateral superlattices

The electronic energy subbands and minigaps in lateral superlattices (LSLs) have been calculated by the plane-wave expansion method. The effect of the lateral modulation on the critical well width at which an indirect-direct (X-Gamma) optical transition occurs in the LSLs is investigated. Our theoretical results are in agreement with the available experimental data. Totally at variance with the previous variation calculational results, the minigaps between the first two subbands in LSLs, as functions of the modulation period, exhibit a maximum value at a specific length and disappear on decreasing the modulation period further. The modulations of several types of lateral potential are also evaluated; the indication is that the out-of-phase modulation on either side of the wells is the strongest while the in-phase modulation is the weakest. Our calculations also show that the effect of the difference between the effective masses of the electrons in the different materials on the subband structures is significant.

Electronic, structural, and optical properties of crystalline yttria

The electronic structure of crystalline Y2O3 is investigated by first-principles calculations within the local-density approximation (LDA) of the density-functional theory. Results are presented for the band structure, the total density of states (DOS), the atom-and orbital-resolved partial DOS. effective charges, bond order, and charge-density distributions. Partial covalent character in the Y-O bonding is shown, and the nonequivalency of the two Y sites is demonstrated. The calculated electronic structure is compared with a variety of available experimental data. The total energy of the crystal is calculated as a function of crystal volume. A bulk modulus B of 183 Gpa and a pressure coefficient B' of 4.01 are obtained, which are in good agreement with compression data. An LDA band gap of 4.54 eV at Gamma is obtained which increases with pressure at a rate of dE(g)/dP = 0.012 eV/Gpa at the equilibrium volume. Also investigated are the optical properties of Y2O3 up to a photon energy of 20 eV. The calculated complex dielectric function and electron-energy-loss function are in good agreement with experimental data. A static dielectric constant of epsilon(O)= 3.20 is obtained. It is also found that the bottom of the conduction band consists of a single band, and direct optical transition at Gamma between the top of the valence band and the bottom of the conduction band may be symmetry forbidden.