High-quality alloyed CdSxSe1-x whiskers as waveguides with tunable stimulated emission

High-quality ultra-long alloyed CdS(x)Se(1)(-)(x) (0 < or = x < or = 1) whiskers were obtained by a simple thermal evaporation route. The near band-edge emission in these whiskers can be effectively guided at the sub-millimeter scale under continuous-wave laser excitation. As good optical waveguide cavities, the whiskers exhibit stimulated emissions under pulsed light excitation at room temperature for all compositions 0 < or = x < or = 1. The spectral positions of the sharp emission lines of the whiskers were tuned by their compositions, covering the spectral range from green to red.     

Ternary CdSxSe 1-x deposited on Ag(111) by ECALE: synthesis and characterization

Morphology and electronic properties of CdS, CdSe, and the ternary compounds of formula CdSxSe(1-x) deposited on Ag(111) by ECALE have been characterized as a function of the composition. The number of the attainable x values is limited by the necessity of using well-defined CdS/CdSe deposition sequences. However, the quantitative analysis carried out by XPS and electrochemical stripping experiments indicates that the ECALE method has a good control on composition. The AFM images together with the electrochemical characterization indicate both two-dimensional and three-dimensional growth contributions. The photospectra recorded at CdS film electrodes in liquid junction with an alkaline (poly)sulfide electrolyte show good efficiency of photoconversion and band gap typical of the single crystal. Lower photoconversion efficiency and the presence of subband gap response are observed for CdSe; a possible reason is some crystalline disorder due to lower control of the layer-by-layer deposition in the case of Se. The dependence of band gap on composition of ternary CdSxSe(1-x) ECALE films is monotonic and in agreement with literature data reported for bulk materials.     

Ag/ZnO heterostructure nanocrystals: synthesis, characterization, and photocatalysis

A high yield of the dimer-type heterostructure of Ag/ZnO nanocrystals with different Ag contents is successfully prepared through a simple solvothermal method in the absence of surfactants. The samples are characterized by X-ray diffraction, transmission electron microscopy, X-ray photoelectron spectroscopy, UV-vis spectroscopy, and IR spectroscopy. The results show that all samples are composed of metallic Ag and ZnO; Ag nanoparticles locate on the surface of ZnO nanorods; the binding energy of Ag 3d(5/2) for the Ag/ZnO sample with a Ag content of 5.0 atom % shifts remarkably to the lower binding energy compared with the corresponding value of pure metallic Ag because of the interaction between Ag and ZnO nanocrystals; the concentration of oxygen vacancy for the as-synthesized samples varies with the increasing Ag content, and the Ag/ZnO sample with a Ag content of 5.0 atom % has the largest density of oxygen vacancy. In addition, the relationship between their structure and photocatalytic property is investigated in detail. It is found that the photocatalytic property is closely related to its structure, such as heterostructure, oxygen defect, and crystallinity. The presence of metallic Ag nanoparticles and oxygen vacancy on the surface of ZnO nanorods promotes the separation of photogenerated electron-hole pairs and thus enhances the photocatalytic activity.     

Continuous synthesis of CdSe(x)Te(1-x) nanocrystals: chemical composition gradient and single-step capping

Thermosensitive poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) (PEO-PPO-PEO) triblock copolymer, Pluronic F68, containing a hydrophobic unit, oligo-(lactic acid)(oligo-LA) or oligo-caprolactone (oligo-CL), 2-META and RGD as side groups was successfully synthesized and characterized by (1)H NMR, FTIR, and elemental analysis. Their aqueous solution displayed special gel-sol-gel phase transition behavior with increasing temperature from 10 to 70°C, when the polymer concentration was above critical micelle concentration (CMC). The gel-sol phase diagram was investigated using tube inversion method, rheological measurement, and dynamic light scattering. Based on these results, the gelation properties of modified F68 were affected by several factors such as the composition of the substituents, chain length of oligo L-LA or oligo ε-CL, and the concentration of the polymer solutions. The unique phase transition behavior with temperature was observed by modified F68 triblock copolymer, composed of the PPO blocks core and the PEO blocks shell in aqueous solution. This phenomenon was elucidated using (1)H NMR data; the alteration of hydrophobic interaction and chain mobility led to the formation of transparent gel, coexistence of gel-sol, and opaque gel. These hydrogels may be useful in drug delivery and tissue engineering.     

Solution-based synthesis of quaternary Cu-In-Zn-S nanobelts with tunable composition and band gap

We have demonstrated that quaternary Cu(x)In(x)Zn(2(1-x))S(2) nanobelts could be synthesized through a facile solution-based method. The composition and band gap of quaternary nanobelts can be tuned within the broad range by changing the relative ratio of precursors. The growth mechanism of quaternary nanobelts was deduced to be catalyst-assisted growth.     

Synthesis of shape-controlled monodisperse wurtzite CuIn(x)Ga(1-x)S2 semiconductor nanocrystals with tunable band gap

Monodisperse wurtzite CuIn(x)Ga(1-x)S(2) nanocrystals have been synthesized over the entire composition range using a facile solution-based method. Depending on the chemical composition and synthesis conditions, the morphology of the nanocrystals can be controlled in the form of bullet-like, rod-like, and tadpole-like shapes. The band gap of the nanocrystals increases linearly with increasing Ga concentration, with band gap values for the end members being close to those observed in the bulk. Colloidal suspensions of the nanocrystals are attractive for use as inks for low-cost fabrication of thin film solar cells by spin or spray coating.     

Fluoride-assisted synthesis of anatase TiO2 nanocrystals with tunable shape and band gap via a solvothermal approach

A simple solvothermal approach employing oleic acid has been developed to prepare anatase TiO₂ nanocrystals with different shapes, which were tuned from nanorods to nano-ellipsoids by increasing the amount of NaF from 0 to 0.5 mmol, and the optical band gap decreased from 3.47 eV to 3.29 eV accordingly. However, when the fluoride was changed to NH₄F, the resultant TiO₂ nanocrystals possessed an anatase phase but weremade up of smaller-sized nanocrystals and nanorods, and the band gap was increased to 3.53 eV. The X-ray photoelectron spectroscopy (XPS) results illustrated an increase of fluorine content with an increasing amount of NaF could account for the variation of the shape and optical band gap of TiO₂ nanocrystals. Moreover, the absence of fluorine content brought about less change of shape and increase of optical band gap of the product synthesized in the presence of NH₄F. This result may offer another way to alter the shape and band gap of metal oxide nanocrystals with the assistance of fluoride.     

Controlled synthesis and size-dependent polarization domain structure of colloidal germanium telluride nanocrystals

Germanium telluride (GeTe) exhibits interesting materials properties, including a reversible amorphous-to-crystalline phase transition and a room-temperature ferroelectric distortion, and has demonstrated potential for nonvolatile memory applications. Here, a colloidal approach to the synthesis of GeTe nanocrystals over a wide range of sizes is demonstrated. These nanocrystals have size distributions of 10-20% and exist in the rhombohedral structure characteristic of the low-temperature polar phase. The production of nanocrystals of widely varying sizes is facilitated by the use of Ge(II) precursors with different reactivities. A transition from a monodomain state to a state with multiple polarization domains is observed with increasing size, leading to the formation of richly faceted nanostructures. These results provide a starting point for deeper investigation into the size-scaling and fundamental nature of polar-ordering and phase-change processes in nanoscale systems.     

PbTe colloidal nanocrystals: synthesis, characterization, and multiple exciton generation

We report an alternative synthesis and the first optical characterization of colloidal PbTe nanocrystals (NCs). We have synthesized spherical PbTe NCs having a size distribution as low as 7%, ranging in diameter from 2.6 to 8.3 nm, with first exciton transitions tuned from 1009 to 2054 nm. The syntheses of colloidal cubic-like PbSe and PbTe NCs using a PbO "one-pot" approach are also reported. The photoluminescence quantum yield of PbTe spherical NCs was measured to be as high as 52 +/- 2%. We also report the first known observation of efficient multiple exciton generation (MEG) from single photons absorbed in PbTe NCs. Finally, we report calculated longitudinal and transverse Bohr radii for PbS, PbSe, and PbTe NCs to account for electronic band anisotropy. This is followed by a comparison of the differences in the electronic band structure and optical properties of these lead salts.     

Crystal phase-controlled synthesis of Cu2FeSnS4 nanocrystals with a band gap of around 1.5 eV

Cu(2)FeSnS(4) (CFTS) nanocrystals with tunable crystal phase have been synthesized using a solution-based method. As-synthesized CFTS nanocrystals in the shape of oblate spheroid and triangular plate with band gaps of 1.54 ± 0.04 and 1.46 ± 0.03 eV, respectively, appear attractive as a low-cost substitute for thin film solar cells.     

PVP-coated iron nanocrystals: Anhydrous synthesis, characterization, and electrocatalysis for two species

We report the synthesis of Fe nanocrystals (approximately 9 nm) in an anhydrous media, formamide, using poly(N-vinyl-2-pyrrolidone) (PVP) as a protecting agent. The morphology, structure, and composition of the PVP-coated Fe nanocrystals are studied by transmission electron microscopy (TEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and FT-Raman spectroscopy. The surface properties of the PVP-coated Fe nanocrystals are studied by electrochemistry and micro-surface-enhanced Raman scattering (mSERS) using pyridine as a probing molecule. The PVP-coated Fe nanocrystals, when immobilized on an electrode substrate, display very good electrocatalytic activities in the selective reduction of H2O2 in the presence of oxygen and in the oxidation of NO.     

The first synthesis of Pb(1-x)Mn(x)Se nanocrystals

In this work, Mn-doped PbSe nanocrystals (NCs) have been, for the first time, prepared through a high-temperature organic solution approach. To ensure that all the Mn2+ ions are indeed incorporated into the NCs and not only physically presented at the surface, Mn-Se prebonded precursor was selected, and a ligand-exchange process was also conducted before and after the synthesis, respectively. Various analyses including EDS, ICP, XRD, SQUID, and EPR confirm that the Mn2+ ions have been successfully doped into PbSe NCs.     

MnO octahedral nanocrystals and MnO@C core-shell composites: synthesis, characterization, and electrocatalytic properties

We present a simple and facile synthesis of MnO octahedral nanocrystals and MnO@C core-shell composite nanoparticles. The synthesis is accomplished by a single-step direct pyrolysis of cetyltrimethylammonium permanganate in specially made Let-lock union cells. The products are characterized by HRSEM, HRTEM, Raman spectroscopy, and cyclic voltammetry (CV). The product consists mainly of octahedral MnO nanocrystals and MnO coated with carbon (MnO@C). The core-shell particles are observed only when the core size is smaller than 150 nm. The shape of the nanocrystals can be controlled by varying parameters such as reaction temperature and duration. As the temperature increases from 600 to 800 degrees C, the octahedral MnO crystals observed are without any carbon shell. The effect of time and temperature on the octahedral MnO nanocrystal formation is described. The electrocatalytic activities of the products are studied for oxygen reduction reaction in aqueous basic medium and are compared with bulk MnO. The MnO nanocrystals and core-shell composites exhibit higher activity than that of bulk MnO.     

Alloyed (ZnS)(x)(Cu2SnS3)(1-x) and (CuInS2)(x)(Cu2SnS3)(1-x) nanocrystals with arbitrary composition and broad tunable band gaps

Cu(2)SnS(3) nanocrystals with metastable zincblende and wurtzite structures have been successfully synthesized for the first time. Alloyed (ZnS)(x)(Cu(2)SnS(3))(1-x) and (CuInS(2))(x)(Cu(2)SnS(3))(1-x) nanocrystals with arbitrary composition (0 ≤x≤ 1) and ultra-broad tunable band gaps (3.63 to 0.94 eV) were obtained.     

Size-controlled chalcopyrite CuInS2 nanocrystals: One-pot synthesis and optical characterization

Chalcopyrite ternary CuInS2 semiconductor nanocrystals have been synthesized via a facile one-pot chemical approach by using oleylamine and oleic acid as solvents.The as-prepared CuInS2 nanocrystals have been characterized by instrumental analyses such as X-ray diffraction(XRD),X-ray photoelectron spectroscopy(XPS),transmission electron microscopy(TEM)/high-resolution TEM(HRTEM),energy-dispersive X-ray spectroscopy(EDS),UV-vis absorption spectroscopy(UV-vis) and photoluminescence(PL) spectroscopy.The particle sizes of the CuInS2 nanocrystals could be tuned from 2 to 10 nm by simply varying reaction conditions.Oleylamine,which acted as both a reductant and an effective capping agent,plays an important role in the size-controlled synthesis of CuInS2 nanocrystals.Based on a series of comparative experiments under different reaction conditions,the probable formation mechanism of CuInS2 nanocrystals has been proposed.Furthermore,the UV-vis absorption and PL emission spectra of the chalcopyrite CuInS2 nanocrystals have been found to be adjustable in the range of 527-815 nm and 625-800 nm,respectively,indicating their potential application in photovoltaic devices.     

Colloidal HgTe nanocrystals with widely tunable narrow band gap energies: from telecommunications to molecular vibrations

A convenient, aqueous-based synthesis of stable HgTe nanocrystals with widely size-tunable room temperature emission between wavelengths of 1.2 to 3.7 mum is demonstrated. By the choice of the thiols, applied as stabilizers, we optimized the growth dynamics, the luminescence quantum yields (up to 40%), and a ligand-exchange procedure, required to transfer the nanocrystals from water to nonpolar organic solvents. The latter is greatly improved and facilitated by the use of mercaptoethylamine as initial stabilizer. The possibility to tune the HgTe nanocrystal sizes from 3 to 12 nm and to control their surface functionalities (hydrophobic and hydrophilic) makes them very promising for the development of infrared optical devices, emitting in the wavelength region between the telecommunications and the molecular vibrations.     

Homogeneously distributed CdS nanoparticles in nafion membranes: preparation, characterization, and photocatalytic properties

Stable crystalline CdS nanoparticles were synthesized in Nafion ionomer membranes by using thioacetamide (TAA) as a nonionic precursor. Unlike the ionic precursors such as Na(2)S, TAA could diffuse into the cationic-exchangeable ionomer membranes much more uniformly. This led to the formation of homogeneously distributed CdS nanoparticles in the Nafion membranes, which was confirmed by elemental mapping with energy-dispersive X-ray (EDAX) analysis. Results from the characterizations on the physical properties, the chemical stability, and the photocatalytic properties of these CdS nanoparticles embedded in Nafion membranes are presented and discussed. The parallel data from the CdS nanoparticles in Nafion membranes prepared from the ionic Na(2)S precursor are also shown for comparison.     

Selective synthesis and characterization of CdSe nanorods and fractal nanocrystals

CdSe nanorods and dendritic fractals were synthesized through a novel controllable solution-phase hydrothermal method. Soluble selenite was employed to provide a highly reactive Se source in the synthesis. Both morphologies and phases of the CdSe products could be successfully controlled by choosing appropriate complexing agents to adjust the dynamics of the reaction process. Reaction temperature and Cd/Se ratio in raw materials were also important parameters influencing the morphologies and phases of the products. The phase structures, morphologies, and optical properties of the CdSe products were investigated by XRD, TEM, HRTEM, and UV-vis and photoluminescence spectroscopies. The formation mechanisms of the nanorods and fractals were investigated and discussed on the basis of the experimental results.     

Multigram scale synthesis and characterization of monodisperse tetragonal zirconia nanocrystals

A new and simple method has been developed to synthesize large quantities of highly monodisperse tetragonal zirconia nanocrystals. In this synthesis, a nonhydrolytic sol-gel reaction between zirconium(IV) isopropoxide and zirconium(IV) chloride at 340 degrees C generated 4 nm sized zirconia nanoparticles. A high-resolution transmission electron microscopic (HRTEM) image showed that the particles have a uniform particle size distribution and that they are highly crystalline. These monodisperse nanoparticles were synthesized without any size selection process. X-ray diffraction studies combined with Rietveld refinement revealed that the ZrO(2) nanocrystals are the high-temperature tetragonal phase, and very close to a cubic phase. When zirconium(IV) bromide is used as a precursor instead of zirconium chloride, zirconia nanoparticles with an average size of 2.9 nm were obtained. The UV-visible absorption spectrum of 4 nm sized zirconia nanoparticles exhibited a strong absorption starting at around 270 nm. A fluorescence spectrum with excitation at 300 nm showed a broad fluorescence band centered around 370 nm. FTIR spectra showed indication of TOPO binding on the ZrO(2) nanoparticle surface. These optical studies also suggest that the nanoparticles are of high quality in terms of narrow particle size distribution and relatively low density of surface trap states.