Influence of surface modification on the luminescence of colloidal ZnO nanocrystals

The influence of surface modification on the luminescence of colloidal ZnO nanocrystals is described, with particular emphasis given to factors increasing excitonic emission quantum yields. Changes in nanocrystal size, shape, and luminescence intensities have been measured for nanocrystals capped by dodecylamine (DDA) and trioctylphosphine oxide after different growth times. Green trap emission intensities show a direct correlation with surface hydroxide concentrations. Contrary to expectations, there is no direct correlation between excitonic emission quenching and surface hydroxide concentrations. The nearly pure excitonic emission observed after heating in DDA is attributed to the removal of surface defects from the ZnO nanocrystal surfaces and to the relatively high packing density of DDA on the ZnO surfaces. Rapid, nondispersive ripening of ZnO nanocrystals upon heating in DDA is observed and explained using a colloidal growth model.     

The effect of surface properties on visible luminescence of nanosized colloidal ZnO membranes

Luminescence properties of nanosized zinc oxide (ZnO) colloids depend greatly on their surface properties, which are in turn largely determined by the method of preparation. ZnO nanoparticles in the size range from 3 to 9 nm were prepared by addition of tetramethylammonium hydroxide ((CH3)4NOH) to an ethanolic zinc acetate solution. X-ray diffraction (XRD) indicates nanocrystalline ZnO membranes with polycrystalline hexagonal wurtzite structure. The ZnO membranes have a strong visible-emission intensity and the intensity depends upon hydrolysis time. The infrared spectra imply a variety of forms of zinc acetate complexes present on the surface of ZnO particles. The effect of the ZnO membrane surface properties on photoluminescence is discussed.     

Novel dumbbell-like LaVO4:Eu3+ nanocrystals and effect of Ba2+ codoping on luminescence properties of LaVO4:Eu3+ nanocrystals

Journal of Sol-Gel Science and Technology - LaVO4:Eu3+ nanocrystals with different morphologies were synthesized via a simple hydrothermal method in a wide pH range. The as-synthesized samples were...     

The study on the effect and mechanism of the second ligands on the luminescence properties of terbium complexes

The binary complex of Tb(III) with N-phenylanthranilic acid (N-HPA) was synthesized, and the ternary complexes were synthesized by introducing 1,10-phenanthroline (Phen), 2,2'-dipyridyl (Bipy), trioctylphosphine oxide (TPPO) as the second ligand, respectively. These complexes were characterized by infrared spectra, UV spectra and fluorescence spectra. The effect and mechanism of different second ligands on the fluorescent intensity of the terbium N-phenylanthranilic acid complexes was discussed. It showed that all the complexes exhibited ligand-sensitized green emission. The luminescence intensity increased in the sequence of Tb(N-PA)(3)Phen相似文献   

Clean and doped surface electronic structure in angle-resolved and resonant photoemission study

The paper presents the experimental results of the electronic band structure study of the semiconductor crystal clean surface and this surface doped by transition metal or rare earth metal atoms. For clean surfaces of the CdTe crystal the two-dimensional electronic band structure E(k) dependence was determined for surface states located in the valence band energy region. The doping of the clean surface of CdTe was performed by the controlled, sequential deposition of metal atoms (Fe, Gd or Yb) on the surface. After each deposition the synchrotron radiation was used to measure the resonant photoemission spectra (energy distribution curve – EDC, constant initial states – CIS and constant finale states – CFS). The results of the study showed that in the first stages of the metal atoms' deposition in the range of thickness equal to about 0.5 ML the effect of the crystal clean surface doping occurs. For bigger metal layer deposition the metallic islands electronic structure gave the contribution to the measured spectra. Heating of the sample surface covered by metal atoms leads to the diffusion of the atoms into the sample and results in an increase of the crystal doping and decrease of the metallic islands contribution to the measured spectra.     

Role of the interaction between forming nanocrystals and glass surface on the structure and properties of ZnO-based films

The interaction between glass surface and forming nanocrystals plays the important role in the formation of thin ZnO coatings crystal structure. The comparative study of the crystal structure of thin ZnO-based films and powders having similar chemical compositions was performed with the use of SEM, XRD analysis, optical, and luminescent spectroscopy. The influence of different coatings parameters (chemical composition, thickness) on the spectroscopic and morphological properties of thin films and powders reveals the structural features of the interaction between forming ZnO nanocrystals and glass surface. ZnO–SnO₂ coatings and powders were prepared by liquid polymer-salt technique. This method provides the close contact between the coatings’ precursors with a surface of the glass during both the nucleation and the initial growth stage of forming oxide crystals. The interaction of nanocrystals and substrate surface is responsible for the texture formation in the ZnO films and determines some features of their optical properties.     

The effect of the surface of SnO2 nanoribbons on their luminescence using x-ray absorption and luminescence spectroscopy

X-ray excited optical luminescence (XEOL) and x-ray absorption near-edge structure in total electron, x-ray fluorescence, and photoluminescence yields at Sn M5,4-, O K-, and Sn K-edges have been used to study the luminescence from SnO2 nanoribbons. The effect of the surface on the luminescence from SnO2 nanoribbons was studied by preferential excitation of the ions in the near-surface region and at the normal lattice positions, respectively. No noticeable change of luminescence from SnO2 nanoribbons was observed if the Sn ions in the near-surface region were excited selectively, while the luminescence intensity changes markedly when Sn or O ions at the normal lattice positions were excited across the corresponding edges. Based on the experimental results, we show that the luminescence from SnO2 nanoribbons is dominated by energy transfer from the excitation of the whole SnO2 lattice to the surface states. Surface site specificity is not observable due to its low concentration and weak absorption coefficient although the surface plays an important role in the emission as a luminescence center. The energy transfer and site specificity of the XEOL or the lack of the site specificity from a single-phase sample is discussed.     

Laser‐induced periodic surface structure on a polymer surface: The effect of a prerubbing treatment on the surface structure

The influence of the mechanical rubbing of a polyimide (PI) film on the laser‐induced periodic structure (LIPS) was demonstrated. The periodicity and amplitude of LIPS were greater when the rubbing direction was parallel to the laser polarization direction. The amplitude became small and the periodicity of LIPS did not show an obvious change when the rubbing direction was perpendicular to the laser polarization direction. The effect of the rubbing pretreatment on LIPS was explained on the basis of the wave‐guide effect of rubbing‐induced microgrooves on LIPS formation. The orientation of PI chains induced by mechanical rubbing was relaxed after laser irradiation, and a new orientation of PI chains was formed during the LIPS formation. When the rubbing direction was perpendicular to the laser polarization direction, the orientation of PI chains remained in the rubbing direction. The laser‐irradiated, perpendicularly rubbed PI surface could be used to verify the effects of surface morphologies and intermolecular interactions on liquid‐crystal alignment. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 1273–1280, 2003     

Two-photon photoemission study of the coverage-dependent electronic structure of chemisorbed alkali atoms on a Ag(111) surface

We report a systematic investigation of the electronic structure of chemisorbed alkali atoms (Li-Cs) on a Ag(111) surface by two-photon photoemission spectroscopy. Angle-resolved two-photon photoemission spectra are obtained for 0-0.1 monolayer coverage of alkali atoms. The interfacial electronic structure as a function of periodic properties and the coverage of alkali atoms is observed and interpreted assuming ionic adsorbate/substrate interaction. The energy of the alkali atom σ-resonance at the limit of zero coverage is primarily determined by the image charge interaction, whereas at finite alkali atom coverages, it follows the formation of a dipolar surface field. The coverage- and angle-dependent two-photon photoemission spectra provide information on the photoinduced charge-transfer excitation of adsorbates on metal surfaces. This work complements the previous work on alkali/Cu(111) chemisorption [Phys. Rev. B 2008, 78, 085419].     

The effect of different substrate temperatures on the structure and luminescence properties of Y2O3:Bi3+ thin films

Y₂O₃:Bi³⁺ phosphor thin films were prepared by pulsed laser deposition in the presence of oxygen (O₂) gas. The microstructure and photoluminescence (PL) of these films were found to be highly dependent on the substrate temperature. X-ray diffraction analysis showed that the Y₂O₃:Bi³⁺ films transformed from amorphous to cubic and monoclinic phases when the substrate temperature was increased up to 600 °C. At the higher substrate temperature of 600 °C, the cubic phase became dominant. The crystallinity of the thin films, therefore, increased with increasing substrate temperatures. Surface morphology results obtained by atomic force microscopy showed a decrease in the surface roughness with an increase in substrate temperature. The increase in the PL intensities was attributed to the crystallinity improvement and surface roughness decrease. The main PL emission peak position of the thin films prepared at substrate temperatures of 450 °C and 600 °C showed a shift to shorter wavelengths of 460 and 480 nm respectively, if compared to the main PL peak position of the powder at 495 nm. The shift was attributed to a different Bi³⁺ ion environment in the monoclinic and cubic phases.     

In situ 1H NMR study on the trioctylphosphine oxide capping of colloidal InP nanocrystals.

We used trioctylphosphine oxide (TOPO) capped colloidal InP nanocrystals (Q-InP|TOPO) to explore the potential of solution 1H NMR spectroscopy in studying in situ the capping and capping exchange of sterically stabilized colloidal nanocrystals. The spectrum of Q-InP|TOPO shows resonances of free TOPO, superimposed on broadened spectral features. The latter were assigned to TOPO adsorbed at Q-InP by means of pulsed field gradient diffusion NMR and 1H-13C HSQC spectroscopy. The diffusion coefficient of Q-InP|TOPO nanocrystals was inferred from the decay of the adsorbed TOPO NMR signal. The corresponding hydrodynamic diameter correlates well with the diameter of Q-InP. By using the resolved methyl resonance of adsorbed TOPO, the packing density of TOPO at the InP surface can be estimated. Spectral hole burning was used to demonstrate explicitly that the adsorbed TOPO resonances are heterogeneously broadened. Exchange of the TOPO capping by pyridine was demonstrated by the disappearance of the resonances for adsorbed TOPO and the appearance of pyridine resonances in the 1H NMR spectrum. These results show that solution NMR spectroscopy should be considered a powerful technique for the in situ study of the capping of sterically stabilized colloidal nanocrystals.     

Controlled crystalline structure and surface stability of cobalt nanocrystals

The synthesis of monodispersed 10 nm cobalt nanocrystals with controlled crystal morphology and investigation of the surface stability of these nanocrystals are described. Depending on the surfactants used, single crystalline or multiple grain nanocrystals can be reproducibly produced. The relative surface stability of these nanocrystals is analyzed using the temperature dependences of the dc magnetic susceptibility. The novel method, which allows sensitive monitoring of the surface stability, is based on the observation that, with particle oxidation, an anomalous peak appears at 8 K in zero-field-cooled magnetization measurements. It is found that the surfactant protective layer is more important for long-term stability at room temperature, while the high-temperature oxidation rate is controlled by the crystal morphology of the nanoparticles.     

The structure and luminescence properties of three complexes based on bifunctional imidazole-dicarboxylate connector

To study the diverse coordination modes of the H₂btib,three new coordination architectures,namely, [Ag（Hbtib）CH₃CN]_n（1）,)[Ag（Hbtib）]_n（2） and[Pb（btib）（CH₃OH）₂]_n（3）（where H₂btib=4-（（2-butyl-5-（2- carboxy-3-（thiophen-2-yl）prop-1-enyl）-1H-imidazol-1-yl）methyl）benzoic acid） have been synthesized and characterized by elemental analysis,IR spectrum and single-crystal X-ray diffraction.Complex 1 exhibits 1D single chain structure,while 2 shows 1D double metal chain motif,and 3 features a 2D layer topology.Also,1,2 and 3 display strong emission band in the solid state at room temperature.     

The effect of plasma treatment on structure and properties of poly(1-butene) surface

This paper is focused on the chemical and morphology changes in the surface of poly(1-butene) (PB-1) generated by plasma treatment. The radio frequency capacitively coupled plasma (air, argon, argon then allylamine, argon containing ammonia and argon with octafluorocyclobutane) was used. Modified surface of PB-1 was characterized by contact angle measurements, X-ray photoelectron spectroscopy, and atomic force microscopy. The surface hydrophilization by air and argon with ammonia plasmas was evaluated as most sufficient. Oppositely, a high level of hydrophobicity of PB-1 surface was reached by combination of argon with octafluorocyclobutane plasma. Upon plasma modification, hydrophilicity/hydrophobicity of treated surfaces remained stable within three days under air atmosphere and then values of contact angle slowly recovered to those of unmodified PB-1. However, morphology and surface chemical composition of plasma-modified samples remained generally unchanged during observed time. Changes in surface hydrophilicity/hydrophobicity of plasma-treated PB-1 were attributed to variance of conformation of the surface molecules.     

Fluorinated comblike homopolymers: The effect of spacer lengths on surface properties

A series of polyacrylate monomers with F‐alkylalkyl [F(CF₂)_n(CH₂)_n′] side groups were prepared by free‐radical polymerization. The effect of the chemical structure on the surface properties of poly(ethylene terephthalate)s was evaluated by variations in the relative length of the fluorocarbon and hydrocarbon units in the side group. The resulting polymers were quite surface‐active in the solid state. The surface and bulk organization was investigated by X‐ray photoelectron spectroscopy analysis. A strong correlation between the bulk organization and surface properties of the polymers was established. The outmost layer, formed from trifluoromethyl groups and some ester functions, suggests that the side chain is arranged irregularly in the polymer–air interface. The length of the lateral chain governs this organization: long fluorinated chains and short hydrocarbon spacers are essential elements of the molecular design for such low‐surface‐energy materials. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 3737–3747, 2005     

Synthesis and upconversion luminescence properties of YF3:Yb/Tm octahedral nanocrystals

YF₃:Yb³⁺(20%)/Tm³⁺(2%) octahedral nanocrystals were synthesized by a microemulsion method with NH₄HF₂. Pumped with a 980-nm diode laser, the nanocrystals emitted weak blue and intense ultraviolet light. Especially, unusual ³P₂ → ³H₆ (∼265 nm) and ³P₂ → ³F₄ (∼309 nm) emissions, coming from a five-photon excitation process, were observed. The emissions from ¹D₂ and ¹I₆ were much stronger than those from ¹G₄ and ³H₄. The upconversion mechanism was discussed in detail.     

Chemical modification of nanocrystalline metal oxides: effect of the real structure and surface chemistry on the sensor properties

The relationships between the composition, structure, chemistry of the surface, and sensor properties of nanocomposites SnO₂-M _n O _m (M _n O _m = Fe₂O₃, MoO₃, V₂O₅) obtained by chemical precipitation from solutions were analyzed. The relationships between the elemental and phase composition of the nanocomposites and the effect of the composition on the nanostructure and the acidic and oxidation properties of the nanocomposite surface were considered. The modification of the SnO₂ surface by other oxides makes it possible to control the type and density of the acid sites and the oxidation properties of the surface and to enhance the selectivity of the materials in the detection of various gases. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 6, pp. 1086–1105, June, 2008.     

Optical surface properties and morphology of MgO and CaO nanocrystals

Optical absorption and photoluminescence emission properties of dehydroxylated MgO and CaO nanocrystals are discussed with respect to particle morphology and size. On MgO nanocubes with pronounced corner and edge features two emission bands at 3.4 and 3.3 eV result from the excitation of 4-coordinated surface O(4C)(2-) anions in edges at 5.4 eV and of regular oxygen-terminated corners at 4.6 eV, respectively. Morphologically ill-defined CaO particles are a factor of 5 larger, do not display regular corner features, and show only one photoluminescence emission band at 3.0 eV. The associated excitation spectrum indicates electronic excitations above the energy required to excite regular oxygen-terminated CaO corners. It is concluded that in the case of morphologically well-defined MgO nanocubes variations in the next coordination of oxygen-terminated corners can effectively be probed by photoluminescence spectroscopy and thus allows for discrimination between 3-coordinated surface O(2-) in regular corner sites and kinks.