Type-II core/shell CdS/ZnSe nanocrystals: synthesis, electronic structures, and spectroscopic properties

We report a two-step synthesis of highly luminescent CdS/ZnSe core/shell nanocrystals (emission quantum yields up to 50%) that can produce efficient spatial separation of electrons and holes between the core and the shell (type-II localization regime). Our synthesis involves fabrication of cubic-singony CdS core particles that are subsequently overcoated with a layer of ZnSe in the presence of surfactant-ligands in a noncoordinating solvent. Studies of different growth regime of the ZnSe shell indicate that one approach to obtaining high emission efficiencies is through alloying the CdS/ZnSe interface with CdSe, which leads to the formation of an intermediate ZnCdSe layer with a graded composition. We perform theoretical modeling of these core/shell nanocrystals using effective mass approximation and applying first-order perturbation theory for treating both direct electron-hole coupling and the core/shell interface-polarization effects. Using this model we determine the range of geometrical parameters of the core/shell structures that result in a type-II localization regime. We further applied this model to evaluate the degree of electron-hole spatial separation (quantified in terms of the electron-hole overlap integral) based on measured emission wavelengths. We also discuss the potential applicability of these nanocrystals in lasing technologies and specifically the possibility of single-exciton optical gain in type-II nanostructures.     

Bremsstrahlung emitted during (n;t)-reactions in light atomic systems

Bremsstrahlung emitted during (n;t)-nuclear reactions in light atoms and ions is discussed by using both the classical and quantum-mechanical approaches. In particular, the spatial and angular distributions of the emitted radiation and its spectrum are considered in detail. It is shown that the total radiation I(t) rapidly decreases with time I(t)∼t⁻⁴ at large t. The spectral density of the emitted bremsstrahlung is also studied. In general, bremsstrahlung emitted during (n;t)-nuclear reactions in the fast ions and atoms can be used to study the electron–electron position correlations in these systems. The approach developed in our present study can also be applied to describe the emission of bremsstrahlung during (n;p)- and (n;α)-nuclear reactions in light atoms, spontaneous and neutron-stimulated nuclear fission in heavy atoms.     

Geometry of n-dimensional Euclidean space Gaussian enfoldments

In this study the geometric features and relationships of the points contained into a Gaussian enfoldment of n-dimensional Euclidean space are analyzed. Euclidean distances and angles are described by means of a simple formulation, which demonstrates the topological change underwent by n-dimensional Euclidean spaces upon Gaussian enfoldment, transforming the Euclidean points into enfoldment points lying in a closed sphere of unit radius. This property relates Gaussian enfoldments with the holographic electronic density theorem.     

The einstein a-coefficient of spontaneous emission: a relativistic calculation in the Heisenberg representation

We present a simple approach to the relativistic calculation of the rates of spontaneous emission starting from the Heisenberg picture formula for the power radiated by a charged particle undergoing acceleration, and evaluate atomic decay rates using relativistic Dirac-Coulomb wavefunctions. The spin of the electron, embedded in its relativistic wavefunction, is shown to correctly provide the two polarization states of the emitted radiation. We discuss selection rules and calculate the Hydrogen 2P → 1S transition rate, among others, to be Γ=(6.2650±0.0007)×10⁸ s^?1 in good agreement with the full field theory calculation as well as with experiment.     

Second-order perturbation theory for the angular pair correlation function in molecular fluids

Second-order perturbation theory is used to calculate spherical harmonic coefficients of the angular pair correlation function g(rω₁ω₂) for a liquid in which the molecules interact with a pair potential that is the sum of Lennard-Jones and quadrupole-quadrupole parts. The theory is compared with both molecular dynamics results and with the predictions of the GMF ≡ LHNC, QHNC and first-order perturbation theories. Second-order perturbation theory gives excellent results for the harmonic coefficient g(224,r), but is poorer for g(222,r) and g(202,r).     

Scandium 2-mercaptobenzothiazolate: Synthesis,structure and electroluminescent properties

The 2-mercaptobenzothiazolate scandium complex Sc(mbt)₃(THF) was obtained by the reaction of Sc[N(SiMe₃)₂]₃ with 2-mercaptobenzothiazole in THF. The X-ray diffraction analysis has shown that three flat mbt ligands coordinated to the metal center in η² fashion via N and exocyclic S atoms. To investigate the electronic structure and to interpret its electronic absorption and emission spectra, quantum chemical calculations of the complex at the B3LYP/TZVP DFT level have been performed with use of the Gaussian03 package. It was found that three higher occupied MO’s of the complex are ligand-localized while lower vacant MO’s contain a significant metal d contribution. Sc(III) complex was used as the emitter in the two-layer device with the configuration of indium tin oxide/N,N′-bis(3-methylphenyl)-N,N′-diphenylbenzidine/Sc(mbt)₃/Yb, which exhibits EL emission at 550 nm.     

Excited-state energy eigenvalue and wave-function evaluation of the Gaussian asymmetric double-well potential problem via numerical shooting method 2

This project aims at computation excited-state energy eigenvalues and wave-function of a particle under Gaussian asymmetric double-well potential using numerical shooting method and perturbation theory a method to deal with discrete-eigenvalue problems. We also compare the energy eigenvalue and wave-function with those obtained from other typical means popular among physics students, namely the numerical shooting method and perturbation theory. Show that the idea of program of the numerical shooting method and perturbation theory of this problem (see Sects. 2.1 and 4) The numerical shooting method is generally regarded as one of the most efficient methods that give very accurate results because it integrates the Schr?dinger equation directly, though in the numerical sense. The n = even case is shown in Figs. 4, 5 and 6. In this case, the wave-function has split up on asymmetric nodes under Gaussian asymmetric double-well potential. The n = odd case is shown in Fig. 7. In this case, the wave-function has not split up on asymmetric nodes under Gaussian asymmetric double-well potential.     

IR absorption,reflection, and emission spectra of aqueous disperse systems interacting with nitric oxide

IR absorption, reflection, and emission spectra of aqueous disperse systems that absorbed molecules of nitric oxide are calculated. In order to reveal the effect of the absorption of NO molecules on the dielectric properties of water clusters with different sizes, clusters are divided into two groups. The first group consists of clusters containing two to ten water molecules, while the second group contains from 11 to 20 H₂O molecules. Six systems of clusters are studied, e.g., (H₂O) _n , and (NO)₂(H₂O) _n with 2 ≤ n ≤ 10 and 11 ≤ n ≤ 20 ranges. An increase in the cluster size in each group leads to the amplification of absorption, reflection, and the power of emission of IR radiation. The doubling of the NO concentration in the disperse system results in weak changes in the absorption of IR radiation, reduces the reflection and decreases the number of electrons participating in the interaction with external IR radiation, as well as significantly lowers the power of thermal radiation emitted by the system.     

Memory effects on energy transport in substitutionally disordered molecular crystals

In this paper we consider the manifestations of coherence effects in electronic energy transport (EET) between randomly distributed donors. We have extended previous theoretical schemes for EET in an impurity band to incorporate a finite memory time for the EET. The short-time behaviour of the mean square displacement of the excitation. [x²(t)], and the initial-site survival probability, P₀(t), exhibit two distinct transport regimes: (i) A coherent regime at ultrashort times, where [x²(t)] ∝ t², and (ii) a partially coherent regime, which is characterized by [x²(t)] ∝ t^10/μ, where μ is the order of the multipolar transition rate, which is intermediate between coherent and conventional diffusive behaviour. Coherence effects also result in the retardation of the short-time decay of P₀(t). The short-time partially coherent transport regime may be amenable to experimental interrogation by utilizing sub-ps and fs laser excitation. On the time scale exceeding the memory time, the conventional dispersive diffusive behaviour and the subsequent onset of classical diffusion for EET are recovered.     

Adiabatic perturbation theory for the degenerate case. III. Adiabatic formalism by Gell-Mann and low with an arbitrary switching function

An adiabatic formalism in the degenerate or quasidegenerate subspaces, which does not depend on the particular form of the switching function g(α, t), is outlined. A general factorization theorem for the dynamic operator S_α(t, t₀ | g) is proved. This theorem enables one to formulate the perturbation expansion for the effective Hamiltonian and the wave operator which is free from the adiabatic divergencies.     

Time-dependent perturbation: a recursive generation of the transition amplitudes between bound states

We present a new expansion of the solution to the time-dependent Schrödinger equation it ∂U/∂t = {H₀ + V(t)}U. A complete set of eigenvectors of H₀ spanning the Hilbert space in which H₀ and V operate is partitioned in two subsets. Transition amplitudes from the first subspace to the second one are calculated by building an adequate series of intermediate representations with respect to the couplings which produce the transition from the model space into its orthogonal complement. These expansions yield a coupling matrix series V⁽ⁿ⁾ for which an iterative solution V⁽ⁿ⁾ → V⁽ⁿ⁺¹⁾ is derived. This solution leads to a recursive numerical treatment of the calculation of transition amplitudes. A simple example, concerning a harmonic oscillator under an intense laser field, is considered using a Fourier analysis of the perturbation.     

Interferometric measurements of spectral line widths emitted by an inductively coupled plasma

Line widths of fifteen spectral lines of ten elements introduced into an inductively coupled plasma (ICP) have been measured with a Fabry-Pcrot interferometer. Instrumental broadening is estimated by measuring the line widths emitted by hollow-cathode lamps and assuming their actual profiles as purely Gaussian with Doppler widths at 400 K. For correction of the experimental widths of lines emitted by the ICP, actual profiles of the lines are assumed to be purely Gaussian as a first approximation. The calculated actual widths range from 0.0015 to 0.0051 nm. Values of the a-paramctcr of the Voigt profile are estimated by assuming the Doppler temperature of the plasma to be 5000 K. Most of the lines emitted by the ICP have a-values in a range between 0.2 and 0.7. The fact reported previously that ICP lines suffer from less collisional broadening than lines excited in flames is supported.     

EPR studies on the high spin FeIII tetraphenylporphine with rhombic character,determination of zero-field splitting parameters from the middle kramers transition

Electron paramagnetic resonance transitions (EPR) in the middle Kramers doublet is observed for the first time in the ⁶A₁ ground state of Fe^(III) complex of tetraphenylporphine (TPP) diluted in the polycrystalline free base tetraphenylporphine (THHP₂). The system shows the EPR spectra and resulting from several distinct species of high spin (Fe^(III) TPP with various degress of rhombicity. The middle Kramers as absorptions are in general of much weaker intensity than the corresponding lowest Kramers absorptions, and their g values show marked systematic deviations from the calculated first order prediction. The deviation can be explained by carrying the Zeeman perturbation to the third order. The analysis also rendered it possible to determine the zero-field parameters D and E separately. The results indicate the similarity in the crystal field of the present system with that of cyctochrome P450.     

Reversible Humidity-Driven Transformation of a Bimetallic {EuCo} Molecular Material: Structural,Sorption, and Photoluminescence Studies

Functional molecule-based solids built of metal complexes can reveal a great impact of external stimuli upon their optical, magnetic, electric, and mechanical properties. We report a novel molecular material, {[Eu^III(H₂O)₃(pyrone)₄][Co^III(CN)₆]}·nH₂O (1, n = 2; 2, n = 1), which was obtained by the self-assembly of Eu³⁺ and [Co(CN)₆]³⁻ ions in the presence of a small 2-pyrrolidinone (pyrone) ligand in an aqueous medium. The as-synthesized material, 1, consists of dinuclear cyanido-bridged {EuCo} molecules accompanied by two H-bonded water molecules. By lowering the relative humidity (RH) below 30% at room temperature, 1 undergoes a single-crystal-to-single-crystal transformation related to the partial removal of crystallization water molecules which results in the new crystalline phase, 2. Both 1 and 2 solvates exhibit pronounced Eu^III-centered visible photoluminescence. However, they differ in the energy splitting of the main emission band of a ⁵D₀ → ⁷F₂ origin, and the emission lifetime, which is longer in the partially dehydrated 2. As the 1 ↔ 2 structural transformation can be repeatedly reversed by changing the RH value, the reported material shows a room-temperature switching of detailed luminescent features including the ratio between emission components and the emission lifetime values.     

Studying on the steady-state and time-resolved fluorescence characteristics of melamine

Electronic emission spectral characteristics of melamine solution have been investigated by processing its steady-state emission spectrum with the second derivative method. It was found that the emission spectrum can be fitted into three separated Gaussian curves. The decay-associated emission spectra of lifetime τ₁ can be fitted to be one Gaussian curve, and that of lifetime τ₂ can be fitted into the other two Gaussian curves. These three Gaussian curves correspond to the steady-state emission spectral, showing that melamine has two fluorescent structures. The density functional theory (DFT) method was used to optimize the structures of the weak interaction clusters C₃H₆N₆–(H₂O)_n (n = 1–6). The structures of those clusters were obtained, and the energies of HOMO and LUMO were analyzed. It is found that the theoretical model C₃H₆N₆–(H₂O)₄ matches well with the experiment when the bonding number n is 4.     

Problems in electron propagator calculations of the correlation energy

Approximations to the one-electron propagator, G(ω), are discussed asa basis for correlation energy calculations. The random-phase approximation (RPA) and second-order perturbation theory estimates of the self-energy are used to determine G(ω). Correlation energy expressions, resulting from contour integration, are compared with the standard perturbation expansion. We suggest that some of the simpler approximations to the electron propagator may be unsuited to calculations of the correlation energy.     

Synthesis,crystal structure,and luminescence properties of [TbGd(NAA)6(phen)2] and [Tb2(NNA)6(phen)2] · 2C3H7NO

Lanthanide(III) heteronuclear and binuclear complexes [TbGd(NAA)₆(phen)₂] (1) and [Tb₂(NAA)₆(phen)₂] · 2C₃H₇NO (2) (NAA = 1-naphthylacetic acid, phen = 1,10-phenanthroline) were prepared and their crystal structures were determined. In 1 and 2, each lanthanide is nine-coordinate by two bidentate-bridging and two tridentate chelating-bridging carboxylate groups, one bidentate chelating carboxylate and one phen molecule in a distorted monocapped square antiprism. The solid-state luminescence behavior and the antibacterial activities were studied. Complexes 1 and 2 exhibited characteristic emission of Tb(III) ion ⁵D₄ → ⁷F_J (J = 6–0) under UV radiation at room temperature. A main excitation peak (359 nm) of 2 appears under red emission of 615 nm. By contrast, all emission peak intensities of 1 were enhanced by addition of gadolinium(III), and the 545 nm band is much stronger than the 615 nm band, attributed to, under perturbation of the ligand field, the probability of ⁵D₄ → ⁷F₃ transition of Tb(III) was greatly enhanced in 2. Because of perturbation of the ligand field by addition of gadolinium(III), the probability of ⁵D₄ → ⁷F₅ transition of Tb(III) was greatly enhanced in 1 and green fluorescence was observed. The antibacterial activity showed that the two complexes were active against Escherichia coli, Staphylococcus aureus and Bacillus subtilis.     

Exactly soluble model of a quantum system in external field with periodic time dependence

A model is proposed, in which all possible transitions occuring in a quantum system exposed for a finite time to an external field of the formV(t)=V cos (Ωt+δ) can be calculated exactly. The basic assumption making the model soluble is the separable form of the operatorV. The dynamical equation is transformed fromt (time) to ω (energy) variable and the resulting finite difference equation is solved for ac-number functionf(ω), in terms of which all the transition amplitudes can be expressed. In the considered numerical example we studied the dependence of the spectrum of the emitted particles on the field strength and pulse length.     

The effect of fluctuations on emission spectra: practical distinctions between raman and fluorescence spectra

By the use of stochastic theory it is clearly shown that two types of emission should to fluctuations such as would occur during interactions with their environment are resonantly driven by light. In our approach the electromagnetic field is treated in a classical manner and decay is introduced by a Wigner-Weisskopfansatz. The two types of emission should occur for a three-level system and relaxation to other levels is not involved in the mechanism. Also included is a discussion of operational definitions of fluorescence (and other spontaneous emissions) and of the relationship between absorption and scattering in terms of dephasing or T₂ processes, especially as this relates to conventional emission phenomena.     

Enhancement of Field Emission Characteristics for Multi-Walled Carbon Nanotubes Treated with a Mixed Solution of Chromic Trioxide and Nitric Acid

A simple acid treatment method was applied to functionalize the surface and to modify the structures of multi-walled carbon nanotubes (CNTs) grown on silicon substrates using a mixed solution of chromic trioxide (CrO₃) and nitric acid (HNO₃). Scanning electron microscopy (SEM), transmission electron microscopy (TEM), Raman spectroscopy, and energy dispersive spectrometer (EDS) were employed to investigate the mechanism causing the modified field emission (FE) properties of the CNT films. After 20 min of CrO₃+HNO₃ treatment, the emitted currents were enhanced by more than one order of magnitude compared with those of the untreated CNTs. This large increase in emitted current can be attributed to the favorable surface morphologies, open-ended structures, and highly curved CNT surfaces in the CNT films. These factors altogether caused an increase in the field enhancement factors of CNTs. We also demonstrated that using a mixed solution of CrO₃+HNO₃ post-treatment exhibited a higher emission current and a lower turn-on electric field than in the CNTs treated with HNO₃. The method provides a simple, economical, and effective way to enhance the CNT field emission properties.