Synthesis and photoluminescent properties of ZnS nanocrystals doped with copper and halogen

A novel wet-chemical precipitation method is optimized for the synthesis of ZnS nanocrystals doped with Cu+ and halogen. The nanoparticles were stabilized by capping with polyvinyl pyrrolidone (PVP). XRD studies show the phase singularity of ZnS particles having zinc-blende (cubic) structure. TEM as well as XRD line broadening indicate that the average crystallite size of undoped samples is similar to2 nm. The effects of change in stoichiometry and doping with Cu+ and halogen on the photoluminescence properties of ZnS nanophosphors have been investigated. Sulfur vacancy (Vs) related emission with peak maximum at 434 nm has been dominant in undoped ZnS nanoparticles. Unlike in the case of microcrystalline ZnS phosphor, incorporation of halogens in nanoparticles did not result V-Zn related self-activated emission. However, emission characteristics of nanophosphors have been changed with Cu+ activation due to energy transfer from vacancy centers to dopant centers. The use of halogen as co-activator helps to increase the solubility of Cu+ ions in ZnS lattice and also enhances the donor-acceptor type emission efficiency. With increase in Cu+ doping, Cu-Blue centers (CuZn-Cui+), which were dominant at low Cu+ concentrations, has been transformed into Cu-Green (Cu-Zn(-)) centers and the later is found to be situated near the surface regions of nanoparticles. From these studies we have shown that, by controlling the defect chemistry and suitable doping, photoluminescence emission tunability over a wide wavelength range, i.e., from 434 to 514 nm, can be achieved in ZnS nanophosphors. (C) 2003 Elsevier B.V. All rights reserved.

Transient analysis in Al-doped barium strontium titanate thin films grown by pulsed laser deposition

Thin films of (Ba0.5Sr0.5)TiO3 (BST) with different concentrations of Al doping were grown using a pulsed laser deposition technique. dc leakage properties were studied as a function of Al doping level and compared to that of undoped BST films. With an initial Al doping level of 0.1 at. % which substitutes Ti in the lattice site, the films showed a decrease in the leakage current, however, for 1 at. % Al doping level the leakage current was found to be relatively higher. Current time measurements at elevated temperatures on 1 at. % Al doped BST films revealed space-charge transient type characteristics. A complete analysis of the transient characteristics was carried out to identify the charge transport process through variation of applied electric field and ambient temperature. The result revealed a very low mobility process comparable to ionic motion, and was found responsible for the observed feature. Calculation from ionic diffusivity and charge transport revealed a conduction process associated with an activation energy of around 1 eV. The low mobility charge carriers were identified as oxygen vacancies in motion under the application of electric field. Thus a comprehensive understanding of the charge transport process in highly acceptor doped BST was developed and it was conclusive that the excess of oxygen vacancies created by intentional Al doping give rise to space-charge transient type characteristics. © 2001 American Institute of Physics.

Optical conductivity of perovskite manganites

We calculate the optical conductivity sigma(omega) for doped rare-earth manganites based on the recently proposed microscopic ``two fluid'' l-b model. We study the temperature dependence of sigma(omega) for La(0.825)Sr(0.175)MnO(3), which has a metallic ground state. At low temperatures, the calculated sigma(omega) shows a ``two-peak'' structure consisting of a far-infrared coherent Drude peak and a broad mid-infrared ``polaron'' peak, as observed in experiments. Upon heating, the Drude peak rapidly loses spectral weight, and sigma(omega) crosses over to having just a single broad mid-infrared peak. The temperature dependence of the mid-infrared peak and the spectral weight transfer between the two peaks are also in agreement with experimental findings. We also study the doping dependence of sigma(omega) for the same compound. The integrated spectral weight under the Drude peak increases rapidly as the doping level is increased from an underdoped, insulating state (x = 0.1) to a highly doped, metallic state (x = 0.3), again in agreement with trends seen experimentally.

Self-organization of multiple components in a steroidal hydrogel matrix: design, construction and studies on novel tunable luminescent gels and xerogels

The present work combines two rapidly growing research areas-functional supramolecular gels and lanthanide based hybrid materials. Facile hydrogel formation from several lanthanide(III) cholates has been demonstrated. The morphological and mechanical properties of these cholate gels were investigated by TEM and rheology. The hydrogel matrix was subsequently utilized for the sensitization of Tb(III) by doping a non-coordinating chromophore, 2,3-dihydroxynaphthalene (DHN), at micromolar concentrations. In the mixed gels of Tb(III)-Eu(III), an energy transfer pathway was found to operate from Tb(III) to Eu(III) and by utilizing this energy transfer, tunable multiple-color luminescent hydrogels were obtained. The emissive properties of the hydrogels were also retained in the xerogels and their suspensions in n-hexane were used for making luminescent coating on glass surface.

Interacting magnetic sublattices and ferrimagnetism in Sr-doped DyMnO3

We present the magnetic properties of polycrystalline Dy1−xSrxMnO3 (0.1 ≤ x ≤ 0.4) with an orthorhombic (o) crystal structure. The parent compound, o-DyMnO3, undergoes an incommensurate antiferromagnetic ordering of the Mn spins at 39 K, followed by a spiral order at 18 K. A further antiferromagnetic transition at 5 K marks an ordering of the Dy-sublattice. Doping of divalent Sr ions results in diverse magnetization phenomena. The zero-field cooled (ZFC) and field cooled (FC) magnetization curves display the presence of strongly interacting magnetic sublattices. For x = 0.1 and 0.2, a bifurcation between the ZFC and FC magnetization sets in at around 30 and 32 K, respectively. The ZFC magnetization peaks at about 5 K, indicating antiferromagnetic Dy-couplings similar to the case of o-DyMnO3. For x = 0.3, clear signatures of ferrimagnetism and strong anisotropy are found, including negative magnetization. The compound with x = 0.4 behaves as a spin glass, similar to Dy0.5Sr0.5MnO3.

Imidazole Functionalized Polyaniline: Synthesis, Characterization, and Cu (II) Coordination Studies

Polyaniline functionalized with imidazole as strategically designed receptor group in its backbone was synthesized for copper binding. The synthesized polymer has been characterized using FTIR, NMR, and UV-Vis spectroscopic techniques. The addition of copper (II) to the polymer distinctly changes the properties such as crystallinity, molecular weight, aggregation, and electronic properties. XRD, DLS, SEM, and four-point probe techniques have been used for study of these changes. It is observed that the secondary ion generated as a result of copper coordination results in the doping of the polyaniline backbone, which enhances the conductivity by one order of magnitude. (C) 2011 Wiley Periodicals, Inc. J Appl Polym Sci 123: 526-534, 2012

Advances in Light-Emitting Doped Semiconductor Nanocrystals

Insertion of just a few impurity atoms in a host semiconductor nanocrystal can drastically alter its phase, shape, and physical properties. Such doped nanomaterials now constitute an important class of optical materials that can provide efficient, stable, and tunable dopant emission in visible and NIR spectral windows. Selecting proper dopants and inserting them in appropriate hosts can generate many new series of such doped nanocrystals with several unique and attractive properties in order to meet current challenges in the versatile field of luminescent materials. However, the synthesis of such doped nanomaterials with a specific dopant in a predetermined host at a desired site leading to targeted optical properties requires fundamental understanding of both the doping process as well as the resulting photophysical properties. Summarizing up to date literature reports, in this Perspective we discuss important advances in synthesis methods and in-depth understanding of the optical properties, with an emphasis on the most widely investigated Mn-doped semiconductor nanocrystals.

Effect of pairing fluctuations on low-energy electronic spectra in cuprate superconductors

We describe here a minimal theory of tight-binding electrons moving on the square planar Cu lattice of the hole-doped cuprates and mixed quantum mechanically with their own Cooper pairs. The superconductivity occurring at the transition temperature T(c) is the long-range, d-wave symmetry phase coherence of these Cooper pairs. Fluctuations, necessarily associated with incipient long-range superconducting order, have a generic large-distance behavior near T(c). We calculate the spectral density of electrons coupled to such Cooper-pair fluctuations and show that features observed in angle resolved photoemission spectroscopy (ARPES) experiments on different cuprates above T(c) as a function of doping and temperature emerge naturally in this description. These include ``Fermi arcs'' with temperature-dependent length and an antinodal pseudogap, which fills up linearly as the temperature increases toward the pseudogap temperature. Our results agree quantitatively with experiment. Below T(c), the effects of nonzero superfluid density and thermal fluctuations are calculated and compared successfully with some recent ARPES experiments, especially the observed bending or deviation of the superconducting gap from the canonical d-wave form.

SOS films and interfaces: chemical aspects

The possible chemical reactions that take place during the growth of single crystal films of silicon on sapphire (SOS) are analyzed thermodynamically. The temperature for the growth of good quality epitaxial films is dependent on the extent of water vapor present in the carrier gas. The higher the water vapor content the higher the temperature needed to grow SOS films. Due to the interaction of silicon with sapphire at elevated temperatures, SOS films are doped with aluminum. The extent of doping is dependent on the conditions of film growth. The doping by aluminum from the substrate increases with increasing growth temperatures and decreasing growth rates. The equilibrium concentrations of aluminum at the silicon-sapphire interface are calculated as a function of deposition temperature, assuming that SiO2 or Al6Si2O13 are the products of reaction. It is most likely that the product could be a solid solutio n of Al2O3 in SiO2. The total amount of aluminum released due to the interaction between silicon and sapphire will account only for the formation of not more than one monolayer of reaction product unless the films are annealed long enough at elevated temperatures. This value is in good agreement with the recently reported observations employing high resolution transmission electron microscopy.

Growth, magnetotransport, and magnetic properties of ferromagnetic (In,Mn)Sb crystals

In1−xMnxSb crystals are grown with different Mn doping concentrations (x = 0.006, 0.01, 0.02, and 0.04) beyond the equilibrium solubility limit by the horizontal Bridgman technique. Structural, magnetic, and magnetotransport properties of the grown crystals are studied in the temperature range 1.4–300 K. Negative magnetoresistance and anomalous Hall effect are observed below 10 K. The anomalous Hall coefficient is found to be negative. The temperature dependence of the magnetization measurement shows a magnetic ordering below 10 K, which could arise from InMnSb alloy formation. Also, the saturation in magnetization observed even at room temperature suggests the existence of ferromagnetic MnSb clusters in the crystals, which has been verified by scanning electron microscopy studies. The carrier concentration increases with Mn doping, and this results in a decrease of resistivity. The carrier concentration and mobility at room temperature for the doped crystals are ∼ 2×1019 cm−3 and ∼ 200 cm2/V s, respectively. The observed anomalous Hall effect suggests the carrier mediated ferromagnetism below 10 K in In1−xMnxSb crystals.

Investigations on multimagnetron sputtered Zn1-xMgxO thin films through metal-ferroelectric-semiconductor configuration

The effect of Mg doping in ZnO is investigated through structural, electrical, and optical properties. Zn1−xMgxO (0<×<0.3) thin films were deposited on Si (100) and corning glass substrates using multimagnetron sputtering. Investigations on the structural properties of the films revealed that the increase in Mg concentration resulted in phase evolution from hexagonal to cubic phase. The temperature dependent study of dielectric constant at different frequencies exhibited a dielectric anomaly at 110 °C. The Zn0.7Mg0.3O thin films exhibited a well-defined polarization hysteresis loop with a remnant polarization of 0.2 μC/cm2 and coercive field of 8 kV/cm at room temperature. An increase in the band gap with an increase in Mg content was observed in the range of 3.3–3.8 eV for x = 0–0.3. The average transmittance of the films was higher than 90% in the wavelength region λ = 400–900 nm.

Dielectric properties of (110) oriented PbZrO3 and La-modified PbZrO3 thin films grown by sol-gel process on Pt(111)/Ti/SiO2/Si substrate

Highly (110) preferred orientated antiferroelectric PbZrO3 (PZ) and La-modified PZ thin films have been fabricated on Pt/Ti/SiO2/Si substrates using sol-gel process. Dielectric properties, electric field induced ferroelectric polarization, and the temperature dependence of the dielectric response have been explored as a function of composition. The Tc has been observed to decrease by ∼ 17 °C per 1 mol % of La doping. Double hysteresis loops were seen with zero remnant polarization and with coercive fields in between 176 and 193 kV/cm at 80 °C for antiferroelectric to ferroelectric phase transformation. These slim loops have been explained by the high orientation of the films along the polar direction of the antiparallel dipoles of a tetragonal primitive cell and by the strong electrostatic interaction between La ions and oxygen ions in an ABO3 perovskite unit cell. High quality films exhibited very low loss factor less than 0.015 at room temperature and pure PZ; 1 and 2 mol % La doped PZs have shown the room temperature dielectric constant of 135, 219, and 142 at the frequency of 10 kHz. The passive layer effects in these films have been explained by Curie constants and Curie temperatures. The ac conductivity and the corresponding Arrhenius plots have been shown and explained in terms of doping effect and electrode resistance.

Relaxor characteristics of layered Ba1-(3/2)xLaxBi2Nb2O9 ceramics

Barium lanthanum bismuth niobate Ba1−(3/2)xLaxBi2Nb2O9 (x = 0, 0.05, 0.1, and 0.15) powders have been prepared via solid state reaction route. The monophasic layered perovskite nature of each composition of these was confirmed by x-ray diffraction studies. A continuous decrease in the lattice parameter c of parent BaBi2Nb2O9 with increase in La3+ doping level was noteworthy. A decrease in dielectric constant maximum (εm), a shift in dielectric anomaly to lower temperatures (from 488 to 382 K), and an increase in the diffuseness (γ) (from 1.58 to 1.84) of dielectric anomaly were encountered on increasing x from 0 to 0.15. Vogel-Fulcher analyses showed a decrease in freezing temperature (Tf) (from 157 to 40 K) and an increase in the activation energy (0.53 to 1.12 eV) for frequency dispersion with increase in La3+ content. A downward shift in the peak position of the pyroelectric coefficient with increasing La3+ doping level was observed. The observed changes in the above physical properties were attributed to the increase in A-site chemical heterogeneity as a result of aliovalent La3+ doping on Ba2+ sites and associated A-site vacancy formation.

Repulsive fermions in optical lattices: Phase separation versus coexistence of antiferromagnetism and d-wave superfluidity

We investigate a system of fermions on a two-dimensional optical square lattice in the strongly repulsive coupling regime. In this case, the interactions can be controlled by laser intensity as well as by Feshbach resonance. We compare the energetics of states with resonating valence bond d-wave superfluidity, antiferromagnetic long-range order, and a homogeneous state with coexistence of superfluidity and antiferromagnetism. Using a variational formalism, we show that the energy density of a hole e(hole)(x) has a minimum at doping x = x(c) that signals phase separation between the antiferromagnetic and d-wave paired superfluid phases. The energy of the phase-separated ground state is, however, found to be very close to that of a homogeneous state with coexisting antiferromagnetic and superfluid orders. We explore the dependence of the energy on the interaction strength and on the three-site hopping terms and compare with the nearest-neighbor hopping t-J model.

The interaction of halogen molecules with SWNTs and graphene

The interaction of halogen molecules of varying electron affinity, such as iodine monochloride (ICl), bromine (Br(2)), iodine monobromide (IBr) and iodine (I(2)) with single-walled carbon nanotubes (SWNTs) and graphene has been investigated in detail. Halogen doping of the two nanocarbons has been examined using Raman spectroscopy in conjunction with electronic absorption spectroscopy and extensive theoretical calculations. The halogen molecules, being electron withdrawing in nature, induce distinct changes in the electronic states of both the SWNTs and graphene, which manifests with a change in the spectroscopic signatures. Stiffening of the Raman G-bands of the nanocarbons upon treatment with the different halogen molecules and the emergence of new bands in the electronic absorption spectra, both point to the fact that the halogen molecules are involved in molecular charge-transfer with the nanocarbons. The experimental findings have been explained through density functional theory (DFT) calculations, which suggest that the extent of charge-transfer depends on the electron affinities of the different halogens, which determines the overall spectroscopic properties. The magnitude of the molecular charge-transfer between the halogens and the nanocarbons generally varies in the order ICl > Br(2) > IBr > I(2), which is consistent with the expected order of electron affinities.