Hexagonal Nanocrystals Research Articles

Carbon-coated hexagonal magnetite nanoflakes production by spray CVD of alcohols in mixture with water

In this study, we report a successful technique for synthesizing magnetite hexagonal nanoflakes coated with carbon layers using spray thermal decomposition, which is a reproducible method that is easy to scale up. We investigated the effects of mixing different volumes of deionized (DI) water with alcohol on the population and quality of single-crystalline Fe3O4 hexagonal nanoflakes. Methanol and ethanol were used as the carbon and oxygen source, while ferrocene was mainly used as the Fe source. To obtain a large quantity of hexagonal structures, a strongly oxidative atmosphere was required. The DI water was used to enhance the oxidative environment during the reaction and was an important component for obtaining well-shaped hexagonal magnetite crystalline nanoflakes. The use of alcohols, water and the spray chemical vapor deposition (CVD) method make this procedure easy to use. In addition, this method provides a one-step process for synthesizing carbon-coated hexagonal Fe3O4 nanocrystals.

Structure and optical properties of Dy3+/Tm3+ co-doped oxyfluoride glass ceramics containing β-NaGdF4 nanocrystals

Dy3+/Tm3+ co-doped oxyfluoride glass ceramics containing hexagonal β-NaGdF4 nanocrystals were prepared by the melt-quenching method and subsequent heat-treatment procedure. During the crystallization process, the structural evolution of glass network was systematically investigated using XRD, TEM and FTIR spectra. Experimental results provided new evidences that a silica-enriched layer around the precipitated fluoride nanocrystals was formed during the crystallization process. Strong white light emission was obtained in the oxyfluoride glass ceramics by modifying the relative concentration ratio of Tm3+ and Dy3+ ions.

Polarity-dependant Performance of p-Cu2O/n-ZnO Heterojunction Solar Cells

This work presents a simple electroplating route for tuning the degree of extended growth of ZnO nanostructures along the (0001) axis, subsequently producing regular hexagonal disk-like nanocrystals at one extreme and hexagonal rod of various lengths at the other. Heterostructural p-Cu2O/n-ZnO solar cells are fabricated by the consecutive electrochemical deposition of n-ZnO nanodisks or nanorods, followed by deposition of the p-Cu2O layer on a transparent conductive substrate. The two shapes of ZnO nanodisks and nanorods are synthesized in the electrolytes of NO3− and Cl-, respectively, which are attributed to the specific ion adsorption. Their XRD patterns reveal that the relative intensities of (100) and (002) peaks from the nanodisks and nanorods vary with the degree of polarity in the ZnO nanostructures. The cell performance measurements clearly indicate that the polar facets of nanodisks yields a conversion efficiency twice higher than that of ZnO nanorods with nonpolar-facets owing to the efficient charge separation in the polar surfaces that a possess strong spontaneous polarization. Importantly, the proposed strategy enhances the solar cell performance as achieved by electrochemical deposition is highly promising for use in producing inexpensive and environmentally friendly solar devices.

Greatly enhanced Raman scattering and upconversion luminescence of Au–NaYF4 nanocomposites

Novel dual function Au–NaYF4 nanocomposites were prepared by a facile wet chemical method. Hexagonal NaYF4 nanocrystals (NCs) were first produced by a hydrothermal method. Then, these NaYF4 NCs were decorated with gold nanoparticles (NPs) to form hybrid nanostructures. In this dual mode probe, surface enhanced Raman scattering (SERS) and field enhanced fluorescence can be generated independently by using different excitation wavelengths. It was found that the attached gold NPs on the rough surfaces of NaYF4 NCs might generate high density localized electric fields, which could lead to both efficient Raman scattering signal and upconversion (UC) luminescence. The enhancement factors of SERS signals from Au–NaYF4 nanocomposites were investigated using 4-mercaptobenzoic acid. The mechanism of enhanced UC luminescence from the nanocomposites was also discussed based on the population and photoluminescence processes of doped trivalent lanthanide ions. These dual mode nanocomposites may find potential applications in biological detection, imaging, and sensing.

Phase-controllable synthesis, shape evolution and optical performances of CePO4nanocrystals via a simple oil-bath route

Rare-earth inorganic orthophosphates have been attracting considerable research interest because of their significant luminescent properties. In the present work, we employed a facile oil-bath route to successfully realize phase-controlled synthesis of CePO4 nanocrystals. The reactions were carried out at 110 °C for 30 min. The phases of the as-obtained products were determined by X-ray powder diffraction (XRD) analyses. Experiments showed that a mixed phase of hexagonal and monoclinic CePO4 was prepared at the EG/H2O volume ratio of 14/5, keeping the introduced EG/H2O total volume of 19 mL unchanged. Hexagonal CePO4 nanocrystals were obtained at the EG/H2O volume ratio below 14/5; while at a volume ratio above 14/5, monoclinic ones were produced. Interestingly, the morphology of the final product underwent a change from long nanorods, to short nanorods, to nanorod bundles, to shuttle-like nanocrystals, and finally to spheroid nanocrystals with the increase of the EG/H2O volume ratio. Furthermore, the optical performances of the final products with different forms were also investigated.

Yb3+ and Tm3+ concentration-dependent structural and optical properties of hexagonal NaGdF4 nanocrystals

We performed a detailed investigation of the influence of Yb3+ and Tm3+ ions concentration on the structural and optical properties of NaGdF4 nanocrystals. XRD and TEM allowed us to identify a crystal phase and measure the diameter of synthesized nanocrystals. The results have shown that crystal phase and size were conserved for various Tm3+ doping. However, for different Yb3+ ions concentration, NCs diameter varied from 20 to 100 nm. Based on the optical investigations (both in visible and NIR range), we determined the optimum doping conditions for efficient emission from 1G4 and 1D2 states, which correspond to green/red and blue emission, respectively.

Selective synthesis of cubic and hexagonal phase of CuInS2nanocrystals by microwave irradiation

Metastable cubic zincblende and hexagonal wurtzite CuInS2 nanocrystals were successfully synthesized by a facile microwave radiation method. The morphology, structure and phase composition of the as-prepared products were examined, and the results demonstrated that high-purity and uniform CuInS2 nanocrystals were obtained. Further investigation revealed a structural evolution process from cubic zincblende to hexagonal wurtzite with increasing volume ratio of ethylenediamine and ethanol from 1 : 30 to 1 : 1 at 160 °C. In addition, the optical absorption properties of samples were also studied. It was found that the as-synthesized cubic and hexagonal CuInS2 nanocrystals had band gaps of 1.503 and 1.470 eV, respectively. Thanks to the superior optical absorption properties for visible light, CuInS2 nanocrystals may have potential applications in various nanostructured optoelectronic devices. The possible formation mechanism of the product, namely “phase transformation”, was systematically studied.

Structural phase and morphology control of tetragonal and hexagonal YPO4:Eu nanoflakes for tunable luminescence properties

Eu3+-doped tetragonal and hexagonal YPO4 nanocrystals with different phase structures and morphologies were successfully synthesized by a solvothermal method. It was found that the pH value is a crucial factor in determining the phase structures and their morphologies of YPO4:Eu nanocrystals. The crystal structure and the properties of the Eu3+-doped YPO4 nanocrystals were characterized by XRD, SEM, TEM, UV–Vis spectroscopy.

Self-Assembly of Colloidal Hexagonal Bipyramid- and Bifrustum-Shaped ZnS Nanocrystals into Two-Dimensional Superstructures

We present a combined experimental, theoretical, and simulation study on the self-assembly of colloidal hexagonal bipyramid- and hexagonal bifrustum-shaped ZnS nanocrystals (NCs) into two-dimensional superlattices. The simulated NC superstructures are in good agreement with the experimental ones. This shows that the self-assembly process is primarily driven by minimization of the interfacial free-energies and maximization of the packing density. Our study shows that a small truncation of the hexagonal bipyramids is sufficient to change the symmetry of the resulting superlattice from hexagonal to tetragonal, highlighting the crucial importance of precise shape control in the fabrication of functional metamaterials by self-assembly of colloidal NCs.

Monodispersed LaF3 nanocrystals: shape-controllable synthesis, excitation-power-dependent multi-color tuning and intense near-infrared upconversion emission

In this study, monodispersed and high-quality hexagonal phase LaF3 nanocrystals with different shapes and sizes were synthesized by a solvothermal method using oleic acid as the stabilizing agent. The as-prepared LaF3 nanocrystals were characterized by transmission electron microscopy (TEM), x-ray diffraction (XRD), and analysis of the upconversion spectra. The TEM results reveal that the samples present high uniformity and monodispersity and are self-assembled into a two-dimensional ordered array. Moreover, the shape, size and structure of the nanocrystals can be readily tuned by adjusting the NaF content. With increasing content of NaF, the shape of the LaF3 nanocrystals changed from particle to rod and the size gradually increased. More importantly, high NaF content favors the formation of one-dimensional nanorods. High Y b3+ and Er3+ content is beneficial to synthesizing the hexagonal phase of NaLaF4 nanocrystals. Furthermore, the TEM results show that the shape and size of the LaF3 nanocrystals can also be tuned by doping lanthanide ions, which provides a new route for size and shape control of nanocrystals. In addition, LaF3 nanocrystals co-doped with Y b3+/Tm3+ present efficient near-infrared (NIR)–NIR upconversion luminescence. More importantly, the upconversion luminescent colors can be readily tuned from blue-white to blue by adjusting the excitation power. Therefore, it is expected that these LaF3 nanocrystals with well-controlled shape, size and NIR–NIR upconversion emission have potential applications in biomedical imaging fields.

Growth of hexagonal NaGdF4 nanocrystals based on cubic Ln3+: CaF2 precursors and the multi-color upconversion emissions

Lanthanide-doped upconversion nanomaterials have attracted great attention recently for their potential applications in the fields of bio-label, three-dimensional display, solar cell and so on. In this article, we report a new strategy to prepare hexagonal Ln3+:NaGdF4 upconversion nanocrystals. Unlike the routine way of synthesizing NaGdF4 nanocrystals through nucleation and growth, the formation of hexagonal NaGdF4 nanocrystals herein is realized based on the Ln3+-doped cubic CaF2 precursors, following a hetero-valence cation exchange process between Gd3+/Na+ and Ca2+. Evidently, Ln3+ dopants in the CaF2 precursors are retained in the finally formed hexagonal NaGdF4 nanocrystals and, subsequently, multi-color upconversion emissions are easily realized by simply adjusting the Ln3+ dopant species and contents in the CaF2 precursors. This novel hetero-valence cation exchange route may open up a new pathway to synthesize nanomaterials that cannot be fabricated directly.

Role of synthesis variables on controlled nucleation and growth of hexagonal molybdenum oxide nanocrystals: investigation on thermal and optical properties

Solution based chemical precipitation route has been utilized for the controlled synthesis of novel and single phase 3Dhierarchical flower-like microspheres of hexagonal molybdenum oxide nanocrystals.

Synthesis of NaLuF4-based nanocrystals and large enhancement of upconversion luminescence of NaLuF4:Gd, Yb, Er by coating an active shell for bioimaging.

A series of NaLuF4-based hexagonal phase upconversion nanocrystals (UCNs) were synthesized by a facile solvothermal method and the properties of the UCNs were investigated. The results show that the as-prepared nanocrystals exhibit pure hexagonal lattice structures, uniform morphologies, high monodispersities and excellent upconversion luminescence. The upconversion luminescence (UCL) intensities of the UCNs can be enhanced by coating with a shell of NaLuF4. More interestingly, the UCL intensities of active-shell coated nanocrystals (NaLuF4:Gd, Yb, Er@NaLuF4:Yb, Ho and NaLuF4:Gd, Yb, Er@NaLuF4:Yb) are remarkably higher than that of inert-shell coated nanocrystals (NaLuF4:Gd, Yb, Er@NaLuF4), and NaLuF4:Gd, Yb, Er@NaLuF4:Yb, Ho is higher than NaLuF4:Gd, Yb, Er@NaLuF4:Yb. The mechanisms of upconversion luminescence enhancement are discussed in detail. The bioimaging application of the nanocrystals showed that bright upconversion luminescence was observed when UCNs-labeled HeLa cells were excited with 980 nm light. This study presents a facile method for the synthesis of NaLuF4-based upconversion nanocrystals with intense luminescence that can be used as potential fluorescent probes for sensitive bioimaging, and the suggested mechanism could provide new insights into fabrication of upconversion materials with high upconversion fluorescence.

Ag2S nanocaps from AgBr nanoplates: Template symmetry breaking synthesis induced by the polar surfaces

Hexagonal cap-like nanocrystals (nanocaps) of Ag2S are synthesized through a simple ion-exchange reaction by using polar {111} faceted, hexagonally shaped AgBr nanoplates as sacrificial templates, in which the bottom/top surfaces presented distinct reactivity with S2− ions due to their different atomic compositions and arrangements. Surprisingly, the Ag2S nanocaps presented strong surface-enhanced Raman scattering activity.

Separation-dependence evolution of inter-particle interaction in the oriented-attachment growth of nanorods: a case of hexagonal nanocrystals.

Previous reports only evaluated the correlation between oriented-attachment growth and part of the interaction, i.e., either van der Waals interaction or Coulombic interaction. By focusing on hexagonal nanorods, a relatively complex form of 1D nanocrystals, this article takes into account both dominant interactions and systematically investigates their countering effects on the separation dependence of the inter-particle interaction in the oriented-attachment growth of 1D nanocrystals. As elucidated by the Arrhenius equation, the growth kinetics and thermodynamics of oriented-attachment nanocrystals can thus be evaluated with such quantitatively resolved inter-particle interactions between the attaching objects.

Salt-assisted rapid transformation of NaYF4:Yb3+,Er3+ nanocrystals from cubic to hexagonal

High-quality hexagonal nanocrystals (β-NaYF4:Yb3+,Er3+) were prepared through one-pot mild solvothermal synthesis. The crystal structure of NaYF4:Yb3+,Er3+ can be controlled by changing the molar ratio of phosphate to Ln3+ (Ln3+ represents the total amount of Y3+ and the doped rare earth elements such as Yb3+, Er3+).

Multiphonon relaxation in rare-earth doped fluorozirconate-based glasses containing BaCl2 nanocrystals

Nd3+-doped fluorozirconate-based glasses which contain hexagonal BaCl2 nanocrystals are analyzed for their photoluminescence and multiphonon relaxation (MPR) properties. The MPR rates of various Nd3+ levels are obtained from time-resolved spectroscopy using selective laser pulse excitation. The nonradiative decay rates are estimated from the difference between measured and calculated radiative decay rates as well as from the analysis of luminescence rise times. The MPR rates display an exponential dependence on the energy gap. Temperature-dependent studies of the decay indicate that phonons of the BaCl2 nanocrystals are involved in the MPR processes leading to extremely low MPR rates which are orders of magnitude lower than in conventional oxide and halide glasses. Photoluminescence emissions, which are usually quenched by MPR, and enhanced radiative quantum efficiencies are found.

ChemInform Abstract: Synthesis of Uniform Disk‐Shaped Copper Telluride Nanocrystals and Cation Exchange to Cadmium Telluride Quantum Disks with Stable Red Emission.

AbstractStable colloids of disk‐shaped hexagonal Cu2Te nanocrystals are prepared by injecting a trioctylphosphine/Te (3:1) solution into solution of Cu(acac)2 1‐octanethiol, and oleic acid (1.

Enhancing the Electromagnetic Performance of Co through the Phase-Controlled Synthesis of Hexagonal and Cubic Co Nanocrystals Grown on Graphene

Cobalt is a promising soft metallic magnetic material used for important applications in the field of absorbing stealth technology, especially for absorbing centimeter waves. However, it frequently presents a weak dielectric property because of its instability, aggregation, and crystallographic form. A method for enhancing the electromagnetic property of metal Co via phase-controlled synthesis of Co nanostructures grown on graphene (GN) networks has been developed. Hexagonal close-packed cobalt (α-Co) nanocrystals and face-centered cubic cobalt (β-Co) nanospheres with uniform size and high dispersion have been successfully assembled on GN nanosheets via a facile one-step solution-phase strategy under different reaction conditions in which the exfoliated graphite oxide (graphene oxide, GO) nanosheets were reduced along with the formation of Co nanocrystals. The as-synthesized Co/GN nanocomposites showed excellent microwave absorbability in comparison with the corresponding Co nanocrystals or GN, especially for the nanocomposites of GN and α-Co nanocrystals (the reflection loss is -47.5 dB at 11.9 GHz), which was probably because of the special electrical properties of the cross-linked GN nanosheets and the perfect electromagnetic match in their microstructure as well as the small particle size of Co nanocrystals. The approach is convenient and effective. Some magnetic metal or alloy materials can also be prepared via this route because of its versatility.

Membrane Separation Assisted One-Pot Synthesis of Hydroxyapatite Nanorods

In the current paper, hydroxyapatite (HAP) nanorods have been synthesized via a precipitation method with membrane separation technique assisted. The reactants were Ca(NO3)2 ·4H2O, NH4H2PO4 and ammonia water. The synthesized HAP nanocrystals were characterized by X-ray diffraction (XRD) and transmission electron microscopy (TEM) measurements. Patterns of XRD exhibit that the products are indexed high-quality hexagonal crystal system nanocrystals. Though the TEM pictures, large quantities of HAP nanorods could be observed with a length of about 50-80nm and a diameter of about 10-20 nm. Preliminary study on the membrane based purifing process is also discussed by comparion the membrane of molecular weight cut-off 6000 to 10000. The resulting HAP nanorods have extensive use in biomedical area such as the bone graft substitute, drug delivery system, carrier for gene transfection, and etc.