Hexagonal Nanocrystals Research Articles

Synthesis and tailoring of GaN nanocrystals at room temperature by RF magnetron sputtering

We report here the synthesis of hexagonal GaN nanocrystals on p-silicon substrates by Radio-Frequency (RF) magnetron sputtering without substrate heating or by post-deposition annealing treatment. GaN nanocrystals are synthesized and tailored as a function of RF power at constant Ar and N2 flow rates (working pressure). The observed reduction in grain sizes as a function of RF power has been correlated with an increase in compressive strain. The effect of RF power on crystallite orientation has been determined by diffraction intensities using the degree of c-axis orientation. Atomic force microscopy shows uniform lateral nanocrystal sizes with spherical in shape by insignificant difference in Root-Mean-Square (RMS) roughness values for all the deposited thin films. Transmission electron microscope and field emission-scanning electron microscope studies have been performed to understand the surface morphologies and grain sizes. Thus, tailoring the size of the nanocrystals has been discussed by correlating RF powers, working pressures and cathode voltages on lattice vibrations.

Effect of infrared irradiation on immobilization of ZnO nanocrystals on multiwalled carbon nanotubes

The effect of infrared (IR) irradiation on the immobilization of ZnO nanocrystals on the surface of functionalized carbon nanotubes (CNTs) is investigated. The hybrids are characterized by X-ray diffraction, infrared spectroscopy, X-ray photoelectron spectroscopy, and microscopy. The hexagonal wurtzite phase ZnO nanocrystals are formed on CNTs surface on calcination of the Zn(OH)2–CNT hybrids at 350 °C in a continuous flow of nitrogen. Microscopic measurements show that ZnO nanocrystals in size the range of 8 nm are formed when IR irradiation is employed; however, in the absence of IR irradiation, ZnO aggregates of the size of more than 100 nm are formed.

Hexagonal polytype of CuCrO2 nanocrystals obtained by hydrothermal method

We had first synthesized hexagonal 2H-CuCrO2 nanocrystals by hydrothermal method. The stability diagram for Cu–Cr–Na–H2O system allows preparing of nanocrystalline CuCrO2, either 2H or 3R pure phase, by a single step, low-temperature hydrothermal synthesis. 2H- and 3R-CuCrO2 quasi-hexagonal shape nanocrystals have shown a wide range of size distribution varying from about 20 to 40 nm. An unexpected result is that 2H-CuCrO2 phase was a low temperature phase in the hydrothermal synthesis. Also, a formation mechanism for the synthesis of CuCrO2 nanocrystal, 2H- and 3R-polytypes was proposed.

Metal Ions Induce Growth and Magnetism Alternation of α‐Fe2O3 Crystals Bound by High‐Index Facets

In this study, quasi-cubic and hexagonal bipyramid α-Fe(2)O(3) polyhedrons with high-index facets exposed were controllably synthesized by applying metal ions Zn(2+) or Cu(2+) as structure-directing agents. The growth of the α-Fe(2)O(3) nanostructures with high-index facets were induced by metal ions without the addition of any other surfactants. The quasi-cubic form controlled by Zn(2+) looks like a cube but has an angle of approximately 86° bound by (012), (10-2), and (1-12) facets, whereas the hexagonal bipyramid form controlled by Cu(2+) has a sixfold axis bound by {012} facets. Magnetic measurements confirm that these two kinds of nanocrystals display shape- and surface-dependent magnetic behaviors. The hexagonal bipyramid iron oxide nanocrystals show a lower Morin transition temperature of 240 K and might be spin-canted ferromagnetically controlled at room temperature, and the ferromagnetism disappears at low temperature. The quasi-cubic nanocrystals have a splitting between FC curve and ZFC curve from the highest experimental temperature and no Morin transformation occurs; this indicates that they would be defect ferromagnetically controlled at low temperature. The reported metal-ion-directing technique could provide a universal method for shape- and surface-controlled synthesis of nanocrystals with high-index facets exposed.

(Invited) Designing, Synthesis and Optical Properties of Ultrasmall β-NaYF4 Nanocrystals Doped by Lanthanides Ions for Bio-Medical Applications

In this work, structural and optical properties of ultrasmall hexagonal NaYF4 nanocrystals will be discussed as a function of different lanthanide ions combinations.

Up-conversion luminescence of Er3 +-doped glass ceramics containing β-NaGdF4 nanocrystals for silicon solar cells

A new Er3+ doped oxyfluoride borosilicate glass ceramics was developed by controlling crystallization of the quenched glass. The hexagonal NaGdF4 nanocrystals were precipitated in the glass matrix, which was confirmed by the X-ray diffraction and TEM results. Notable enhancement of upconversion emission was observed in the Er3+ doped glass ceramics and attributed to the enrichment of Er3+ into NaGdF4 nanocrystals. With the increasing of Er3+ content, the upconversion emission density ration from green to red light greatly changed in the glass ceramics. This material may be used as a potential material for the silicon solar cells.

Cubic sub-20 nm NaLuF4-based upconversion nanophosphors for high-contrast bioimaging in different animal species

A new upconversion luminescence (UCL) nanophosphors based on host matrix of cubic NaLuF4 with bright luminescence have been synthesized by a solvothermal method, facilitate the nanocrystals potential candidates for imaging in vivo, especially large-animals. The sub-20 nm NaLuF4 co-doped Yb3+ and Er3+ (Tm3+) showed about 10-fold stronger UCL emission than that of corresponding hexagonal NaYF4-based nanocrystals with a 20 nm diameter. Near-infrared to near-infrared (NIR-to-NIR) UCL emission of PAA-coated NaLuF4:20%Yb,1%Tm (PAA-LuTm) can penetrate >1.5 cm tissue of pork with high contrast. Based on super-strong UCL emission and deep penetration, PAA-LuTm as optical bioprobe has been demonstrated by in vivo UCL imaging of a normal black mouse, even rabbit with excellent signal-to-noise ratio. Furthermore, such cubic NaLuF4-based nanophosphor was applied in lymph node imaging of live Kunming mouse with rich white fur.

Interaction Between Y3+ and Oleate Ions for the Cubic-to-Hexagonal Phase Transformation of NaYF4 Nanocrystals

Understanding the phase transformation of NaYF4 nanocrystals from cubic to hexagonal is of great importance for both scientific interests and applications. Herein, based on the density functional theory, we found that the dominant difference between cubic and hexagonal phase NaYF4 is the location of Y3+, and oleate ions are particularly favorable to bonding with Y3+. The results indicated that oleate salts as ligand possibly induce the orderly arrangement of Y3+ and lower the energy barrier for the formation of hexagonal phase. The experiments of using different amount of oleate ions as ligand were designed to elucidate their influence on the phase transformation. We found that the approach with relatively high amount of sodium oleate results in dramatic shortening of reaction time (down to 5 min) on cubic-to-hexagonal phase transition for preparing ultrasmall (∼13 nm) hexagonal phase NaYF4 nanocrystals. To exclude the influence of Na+ increasing in using sodium oleate, potassium oleate was also used as l...

Low phonon energy BaCl2 nanocrystals in Nd3+-doped fluorozirconate glasses and their influence on the photoluminescence properties

ABSTRACTNd3+-doped fluorozirconate glasses, which were additionally doped with chlorine ions, were investigated for their photoluminescence (PL) properties. Upon heat treatment of the as-made glass, hexagonal phase BaCl2 nanocrystals are formed within the material, which undergo a phase transformation to orthorhombic BaCl2 upon annealing at a higher temperature. The glasses with hexagonal phase BaCl2 nanocrystals show an enhanced Nd3+ PL in the visible spectral range. Time-resolved spectroscopy on the 4G5/2 / 2G7/2 → 4I9/2 transition shows that the existence of hexagonal BaCl2 nanocrystals results in a significantly longer decay time. The temperature dependence of the lifetime yielded that the enhanced PL is due to a reduced multi-phonon relaxation rate.

Lanthanide dopant-induced phase transition and luminescent enhancement of EuF3 nanocrystals

A novel strategy is proposed to prepare hexagonal EuF3 nanocrystals by doping lanthanide ions (Ln3+) with large ionic radius (such as La3+, Ce3+, Pr3+, Nd3+ and Sm3+). Complete phase transition from orthorhombic to hexagonal phase can be achieved with the increase of ionic radius or doping concentration of Ln3+ ions. Meanwhile, the size and self-assembly degree of the product can be reduced dramatically. The possible growth and phase transformation mechanism of EuF3 nanocrystals was proposed. Finally, Eu3+ ion was used as the structural probe to investigate the effect of phase transition on the luminescence. It was found that the emissions of the 5D0 level of Eu3+ are greatly enhanced with the phase transition from orthorhombic to hexagonal phase. The luminescent enhancement is attributed to the larger radiative transition rate of the 5D0 level in the environment with lower symmetry, which can be confirmed by Judd–Ofelt (J–O) theory.

Hexagonal and cubic Ni nanocrystals grown on graphene: phase-controlled synthesis, characterization and their enhanced microwave absorption properties

Hexagonal close-packed Ni (h-Ni) nanocrystals and face-centered cubic Ni (c-Ni) nanoflowers with uniform size and high dispersion have been successfully assembled on graphene nanosheets (GN) via a facile one-step solution-phase strategy under different reaction conditions, where the reduction process of graphite oxide (GO) sheets into GN was accompanied by the generation of Ni nanocrystals. The reduction of GO by this method is effective, which was confirmed by X-ray diffraction (XRD), Fourier transform infrared (FTIR) and Raman spectroscopy and is comparable to conventional methods. The phase and morphology of nickel can be easily tuned by varying the reaction temperature and solvent. It was shown that the as-formed h-Ni nanocrystals with a diameter as small as 3 nm are grown densely and uniformly on the graphene sheets, and as a result the aggregation of the h-Ni nanocrystals was effectively prevented. In addition, c-Ni nanospheres assembled by c-Ni nanocrystals with a size of 15 nm were also uniformly deposited on the graphene sheets. The investigation of the microwave absorbability reveals that the three Ni/GN nanocomposites exhibit excellent microwave absorbability, which is stronger than the corresponding Ni nanostructures.

Facile Synthesis of Monodisperse, Size-Controlled Hexagonal-Phase NaYF4 Up-Conversion Luminescent Nanorods

A facile strategy to prepare hexagonal-phase, monodisperse, size-controlled NaYF4 nanorods has been demonstrated using mixed solvothermal route. Doping of Gd3+ dramatic shorten the reaction time on cubic-to-hexagonal phase transition for preparing hexagonal phase NaYF4 nanocrystals. The crystalline phase, size, morphology, and luminescence properties were characterized using X-ray diffraction (XRD), field-emission scanning electron microscopy (FE-SEM), and photoluminescence (PL) spectra.

Colloidal Cu2−x(SySe1−y) alloy nanocrystals with controllable crystal phase: synthesis, plasmonic properties, cation exchange and electrochemical lithiation

We report synthetic routes to both cubic and hexagonal phase Cu2−x(SySe1−y) alloy nanocrystals exhibiting a well-defined near-infrared valence band plasmon resonance, the spectral position of which is dependent mainly on x, i.e. on Cu stoichiometry, and to a lesser extent on the crystal phase of the NCs. For cubic Cu2−x(SySe1−y) nanocrystals y could be varied in the 0.4–0.6 range, while for hexagonal nanocrystals y could be varied in the 0.3–0.7 range. Furthermore, the Cu2−x(SySe1−y) nanocrystals could be transformed into the corresponding Cd-based alloy nanocrystals with comparable SySe1−y stoichiometry, by cation exchange. The crystal phase of the resulting Cd(SySe1−y) nanocrystals was either cubic or hexagonal, depending on the phase of the starting nanocrystals. One sample of cubic Cu2−x(SySe1−y) nanocrystals, with S0.5Se0.5 chalcogenide stoichiometry, was then evaluated as the anode material in Li-ion batteries. The nanocrystals were capable of undergoing lithiation/delithiation via a displacement/conversion reaction (Cu to Li and vice versa) in a partially reversible manner.

Hydrothermal synthesis and optical properties of hexagonal tungsten oxide nanocrystals assisted by ammonium tartrate

AbstractCrystals of hexagonal tungsten oxides (hex‐WO3) have been synthesized using hydrothermal method at 150 °C, assisted by the capping reagent of ammonium tartrate (AT). The XRD and EDX results reveal that the lattice distortion exists in all the samples, possibly due to the defects and the intercalation of the residual sodium ions. Different crystal shapes including plate‐like, urchin‐like, and particle structures were obtained by varying concentration of AT and pH values in the precursor solution. Beside the absorption action of the ${\rm NH}_{4}^{ + } $ and Na+ ions, the capping effect can be reinforced by the hydrogen bonding from the tartrate groups in the crystallization process. The bandgap energies were modulated by the size of the nanostructured hex‐WO3 crystals due to quantum confinement effect, which increases from 2.74 to 3.04 eV. Based on the analysis of the photoluminescence and X‐ray photoelectron spectroscopy, the enhancement of the blue emission of the nanocrystals is assigned as a result of a complex of the local intercalation of the residual sodium ions and the oxygen vacancies or defects.

Sonochemical Synthesis of 0D, 1D, and 2D Zinc Oxide Nanostructures in Ionic Liquids and Their Photocatalytic Activity

Ultrasound synthesis of zinc oxide from zinc acetate and sodium hydroxide in ionic liquids (ILs) is a fast, facile, and effective, yet highly morphology- and size-selective route to zinc oxide nanostructures of various dimensionalities. No additional organic solvents, water, surfactants, or templating agents are required. Depending on the synthetic conditions, the selective manufacturing of 0D, 1D, and 2D ZnO nanostructures is possible: Whereas the formation of rodlike structures is typically favored, ZnO nanoparticles can be obtained either under strongly basic conditions or by use of ILs with a long alkyl chain, such as 1-n-alkyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide ([C(n)mim][Tf(2)N]; n>8). A short ultrasound irradiation time favors the formation of ZnO nanosheets. Prolonged irradiation leads to the conversion of the ZnO nanosheets into nanorods. In contrast, ionothermal synthesis (conventional heating) does not allow for morphology tuning by variation of the IL or other synthesis conditions, as the longer reaction times required lead always to the formation of well-developed hexagonal nanocrystals with prismatic tips. The ZnO nanostructures synthesized by using ultrasound were efficient photocatalysts in the photodegradation of methyl orange. The photoactivity was observed to be as high as 95 % for ZnO nanoparticles obtained in [C(10)mim][Tf(2)N].

Anisotropic molecular packing of soluble C 60 fullerenes in hexagonal nanocrystals obtained by solvent vapor annealing

We describe the self-assembly of soluble, chemically modified fullerene [6,6]-phenyl C 61 butyric acid methyl ester (PCBM) into a new crystalline phase where the C 60 moieties are arranged in parallel layers. Minimum C 60 center-to-center distance is 10 Å within the layers, and up to 15 Å perpendicular to the layers. Highly anisotropic, mesoscopic hexagonal crystals of this material, with a lateral size of many microns and a thickness below 1 μm, are obtained from chloroform solution by solvent vapor annealing, and characterized by optical microscopy and X-ray diffraction. The crystalline structure is deduced combining experimental data with molecular modeling and ab initio calculations. The large difference in C 60–C 60 spacing indicates a high anisotropy in electrical and charge transport properties of this new phase.

Synthesis and Characterization of Upconverting NaYF4:Er3+, Yb3+ Nanocrystals via Thermal Decomposition of Stearate Precursor

Er3+-Yb3+ codoped hexagonal NaYF4 nanocrystals were prepared via a method of thermal decomposition of stearate precursor. Their crystal structure, morphologies and photoluminescence (PL) properties were characterized by XRD, SEM, and fluorescence spectra. The hexagonal NaYF4:Er3+, Yb3+ nanocrystals could be well dispersed in cyclohexane to form a clear solution. Under 980 nm excitation, the solution of Er3+-Yb3+ codoped NaYF4 nanocrystals emits bright green upconversion fluorescence.

Sequential Cation Exchange in Nanocrystals: Preservation of Crystal Phase and Formation of Metastable Phases

We demonstrate that it is possible to convert CdSe nanocrystals of a given size, shape (either spherical or rod shaped), and crystal structure (either hexagonal wurtzite, i.e., hexagonal close packed (hcp), or cubic sphalerite, i.e., face-centered cubic (fcc)), into ZnSe nanocrystals that preserve all these characteristics of the starting particles (i.e., size, shape, and crystal structure), via a sequence of two cation exchange reactions, namely, Cd(2+) ⇒Cu(+) ⇒Zn(2+). When starting from hexagonal wurtzite CdSe nanocrystals, the exchange of Cd(2+) with Cu(+) yields Cu(2)Se nanocrystals in a metastable hexagonal phase, of which we could follow the transformation to the more stable fcc phase for a single nanorod, under the electron microscope. Remarkably, these metastable hcp Cu(2)Se nanocrystals can be converted in solution into ZnSe nanocrystals, which yields ZnSe nanocrystals in a pure hcp phase.

Ionic liquid-assisted synthesis, structural characterization, and photocatalytic performance of CdS nanocrystals

With the assistance of the ionic liquid trihexyl(tetradecyl)phosphonium bis(2,4,4-trimethylpentylphosphinate), we have successfully synthesized short nanorods, quasi-nanospheres and faceted CdS nanoparticles via thermal decomposition of cadmium diethyldithiocarbamate complexes. It was shown that the shape, size and crystallinity of the products could be controlled through delicate regulation of the reaction temperature, monomer concentration, reaction time, and ionic liquid ratio. We found that higher temperature was beneficial to the good crystallinity, while the lower temperature and higher monomer concentration were in favor of anisotropic structures. The used ionic liquid contributed to the formation of hexagonal phase CdS nanocrystals, and its ratio played an important role in determining the ultimate morphology of products. The possible mechanism for the formation CdS nanocrystals was proposed. Furthermore, the as-prepared CdS samples demonstrated a highly photocatalytic activity in the degradation of methyl orange under visible light irradiation.

Fabrication of hollow mesoporous NiO hexagonal microspheres via hydrothermal process in ionic liquid

The novel NiO hexagonal hollow microspheres have been successfully prepared by annealing Ni(OH) 2, which was synthesized via an ionic liquid-assisted hydrothermal method. The samples were characterized by X-ray diffraction (XRD), field emission scanning electron microscope (FE-SEM), transmission electron microscopy (TEM), N 2 adsorption–desorption and Fourier transform infrared spectrometer (FTIR). The results show that the hollow NiO microstructures are self-organized by mesoporous cubic and hexagonal nanocrystals. The mesoporous structure possessed good thermal stability and high specific surface area ( ca. 83 m 2/g). The ionic liquid 1-butyl-3methylimidazolium tetrafluoroborate ([Bmim][BF 4]) was found to play a key role in controlling the morphology of NiO microstructures during the hydrothermal process. The special hollow mesoporous architectures will have potential applications in many fields, such as catalysts, absorbents, sensors, drug-delivery carriers, acoustic insulators and supercapacitors.