Narrow Results

Niobium carbide nanocrystals of ~1:1 stoichiometry have recently been observed for particle sizes ranging from Nb₄C₄ to Nb₅₀C₅₀. Infrared (IR) spectroscopic measurements show that a new band of IR vibrational modes appears with increasing particle size at Nb₉C₉. Using density-functional theory, we show that the vibrational modes in the new band involve structural features present only in nanocrystals with three or more atomic layers in every direction. The Nb₉C₉ nanocrystal is right at this structural threshold.

Microwave hydrothermal method is employed to synthesize Co₃O₄ nanocrystals. Co(OH)₂ from reaction of C₄H₆CoO₄·4H₂O with NaOH aqueous solution is used as starting material. With a fixed hold time of an hour, the results show that high quality Co₃O₄ nanocrystals can be synthesized at 200 °C. Decrease of the hold temperature leads to deteriorated crystallinity and incomplete reaction.

In this letter we report on successful preparation and characterization of anisotropic gold nanocrystals bio-synthesized by reduction of aqueous chloroaurate ions in pelargonium plant extract. The nanocrystals have been characterized by means of Transmission Electron Microscopy (TEM), UV-VIS absorption spectroscopy and tapping mode atomic force microscopy (TM-AFM). Using these investigation techniques, the successful formation of anisotropic single nanocrystals with the preferential growth direction along the gold (111) plane has been confirmed. The high detail phase images could give us an explanation concerning the growth mechanism of the nanocrystals.

The work is dedicated to the development of the electron-lattice energy exchange theory in metal particles, including dependence on particle size and electron temperature. We compared, for infinite metal, the expressions for the constants of the electron-lattice energy exchange by using the quantum-kinetic approach, with the classical kinetic approach. Both methods give the same result. But as we show, a quantum-kinetic approach in its standard form, for the metal particles smaller than the mean free path of electrons, cannot be applied, while the classical approach can be easily adjusted for such particle.

We found a new peculiarity of the electron-lattice energy exchange. This exchange essentially depends on temperature if a particle size is in the range near a certain "magic size". The change of the electron temperature can alter (around the "magic size") the number of the acoustic modes, which are involved in energy transfer. As a result, the constant of the electron–phonon interaction in nanoparticles can be either much higher or much lower than the corresponding constant for infinite metal.

Low threshold power density cw laser-induced heat has been observed in ${\mathrm{\text{Mn}}}^{2+}$ and ${\mathrm{\text{Er}}}^{3+}$ codoped ${\mathrm{\text{Y}}}_2{\mathrm{\text{O}}}_3$ nanocrystals under excitation by a 980 nm IR laser. Codoped ${\mathrm{\text{Mn}}}^{2+}$ remarkably reduces the power density threshold of laser-induced heat compared with ${\mathrm{\text{Er}}}^{3+}$ doped ${\mathrm{\text{Y}}}_2{\mathrm{\text{O}}}_3$ nanocrystals. When the excitation power density exceed $0.8{\mathrm{\text{W/mm}}}^2$, ${\mathrm{\text{Er}}}^{3+}∕{\mathrm{\text{Mn}}}^{2+}$ codoped ${\mathrm{\text{Y}}}_2{\mathrm{\text{O}}}_3$ nanocrystals emit strong blackbody radiation. The thermal emission of ${\mathrm{\text{Er}}}^{3+}$ should originate from the multiphonon relaxation between neighboring energy levels. One additional UC-PL enhancement is observed. The UC-PL intensity can be enhanced by an order of magnitude through high temperature calcination caused by light into heat.

Energetically stable Ge diamondoids are used as building blocks to investigate Ge nanocrystal properties using density functional theory (DFT). Energy gap, HOMO and LUMO of Ge diamondoids vary according to confinement theory with shape fluctuations. Ge diamondoid vibrational force constants reach 2.18 mDyne/Å which is between that of bulk silicon and tin. Ge–Ge vibrational frequencies and reduced masses reach 357.47 cm${}^{-1}$ and 41.93 amu, respectively, which are higher than the values of bulk Ge. Size variation of UV–Vis shows that the maximum optical peak moves from 163 nm to nearly 290 nm as the size of Ge diamondoids and molecules increases. The higher peak value approaches that of experimental Ge quantum dots at 300 nm. NMR spectra of Ge diamondoids are analyzed as a function of diamondoid sizes. Our results show that all investigated spectroscopic tools are sensitive to molecular or nanocrystals size. NMR is particularly good size indicator. Natural bond orbital (NBO) population analysis shows present diamondoid bondings differ from ideal $s{p}^3$ bonding. The bonding for Ge valence orbitals is in the range $(4s^{1.24}4p^{2.60})-(4s^{1.27}4p^{2.73})$ depending on distance between Ge atom and diamondoid surface. Highest Ge diamondoids vibrational longitudinal optical (LO) mode is blue shifted with respect to experimental bulk value which is the opposite case for C and Si. H surface atom effects on electronic and vibrational properties are discussed.

A model is developed to account for the size- and coherence-dependent cohesive energy, melting temperature, melting enthalpy, vacancy formation energy and vacancy concentration of nanowires and nanofilms. It is found that the variation direction (increasing or decreasing) of the thermodynamic properties is determined by the coherent interface, and the quantity of variation depends on the crystal size. The calculated results on Pb nanowires and nanofilms are consistent with the available experimental values.

A template-free solution method based on solid-liquid interface reaction has been developed for fabricating metal chalcogenide dendritic nanostructures. Well-defined PbSe dendrites with four branches can be large-scale synthesized with metal lead foil as Pb²⁺ source and Na₂SeO₃ as Se²⁺ source in solvothermal conditions. The nanorods in each branch are parallel to each other in the same plane and are perpendicular to the trunk. It was found that PbSe cubes first formed on the lead substrate through a direct surface reaction and that subsequent crystal growth preferentially took place on the vertex of these PbSe cubes, leading to the formation of final branched dendrites. The current solid-liquid interface reaction has been extended to the synthesis of CuS micro-pine dendrites with copper foil and sulfur powder as reaction reagents.

Amorphous films Zr_0.8Si_0.2O₂ were deposited on quartz substrates at room temperature, by using pulsed laser deposition technique. After annealing at high temperature, Zr_0.8Si_0.2O₂ films suffer a phase separation to precipitate tetragonal ZrO₂ nanocrystals in ZrSiO₄ amorphous matrix. Their crystal structure, microstructure, optical absorbance and photoluminescence have been systematically investigated. Tetragonal ZrO₂ nanocrystals show intense UV emission around 392 nm under photonic excitation with a wavelength of 250 nm. The corresponding models of electronic excitation and transition related to UV emission also have been proposed.

A heteroepitaxial system of h-BN/monolayer graphene on Ni(111) has been investigated by means of vibrational spectroscopy, accompanied by low energy electron diffraction (LEED) and Auger electron spectroscopy (AES). The system was prepared in an epitaxial manner on a Ni(111) surface by chemical vapor deposition (CVD). We found that phonon peaks in observed spectra showed typical features of Fuchs–Kliewer (FK) phonons. The vibrational spectra of h-BN films ranging in thickness from 0 to 8.7 monolayers have been compared with theoretical spectra based on a dielectric theory. Detailed analysis has revealed new types of phonons, of which the vibrational amplitudes are localized at edges of h-BN nanocrystals. In addition, we have observed subsidence phenomena of the graphene and h-BN layers into Ni substrate at elevated temperatures.

In this work, a convenient and efficient procedure to control the morphologies of Au nanocrystals has been reported. The Au nanospheres are generated in polyvinyl-pyrrolidone/ethanol glycol system through a thermal chemical method. However, by adding a small amount of K₂CO₃ which influences the growth rate of various facets during the crystal growth, Au nanoplates with broad surface plasmon resonance band in 500–800 nm can be obtained.

Laser cladding (LC) of the Stellite6-BN-Cu-Y₂O₃ pre-placed powders on the Ti-6Al-4V alloy substrate formed the bulk TiN and needle-like Ti-B compounds reinforced composites, which improved the wear resistance of the substrate surface. The results showed that lots of the amorphous-nanocrystalline phases were produced in such composites; the proper content of Cu was used to refine microstructures, also a large number of the nanocrystals can be formed; the elements such as Co, Ni and Fe in Stellite6 favored the amorphous phase to be formed. The addition of Y₂O₃ improved the fluidity of the liquid metal, also retarding the growth of the dendrite, leading the fine microstructures to be formed, improving the property of the LC composites.

The high-temperature and high-pressure liquid (HTPL) crystallization method was proved to be effective to fabricate nanocrystals of quinacridone and its dimethyl derivative. The quinacridone nanocrystals were spherical shape and monodispersed with the average size of around 50 nm, respectively. It was found that quinacridone nanocrystal in different crystal form could be obtained by changing experimental conditions in HTPL method.

It has been observed by experiments and computer simulations that deformation twinning and grown-in twins play important roles during the plastic deformation in nanocrystalline (nc) metals. In this study, molecular dynamics simulation is employed to investigate the role of grown-in twins in the tensile deformation of hexagonal close-packed nc cobalt. Plastic deformation processes in three types of nc samples, namely nc-cobalt without twins, with (0002) twin faults and with (1121) twins, are compared. The results show that the pre-existing twins significantly lower the mechanical strength and do not enhance the ductility of nc cobalt. Instead deformation induced hcp-to-fcc allotropic transformation is found to result in remarkable strain hardening in the modeled nc cobalt.

Soft chemical routes are employed effectively for the synthesis of nanocrystals of semiconductor materials. Several methods have been developed for the synthesis of multi-walled carbon nanotubes (MWNTs) and single-walled carbon nanotubes (SWNTs) and specially noteworthy are the precursor route and nebulized spray pyrolysis for the synthesis of MWNTs and junction nanotubes. Nanotubes of inorganic layered materials are obtained by ingenious chemical methods. Nanowires of inorganic materials can be synthesized not only by high-temperature methods such as the carbon-assisted route but also by soft chemical routes.

We report a quantitatively accurate description of the electronic structure of semiconductor nanocrystals using the sp³d⁵ orbital basis with the nearest neighbor and the next nearest neighbor interactions. The use of this model for II–VI and III–V semiconductors is reviewed in article. The excellent agreement of the theoretical predictions with the experimental results establishes the feasibility of using this model for semiconductor nanocrystals.

The formation of nanocrystals in Zr-based alloys through three different routes, viz by rapid solidification of alloys, by crystallization of rapidly solidified metallic glasses and by crystallization of bulk metallic glasses has been described. The nanocrystal forming behaviors of rapidly solidified metallic glasses and bulk metallic glasses have been compared and contrasted. The rapidly solidified alloys, which have been examined for this purpose, are Zr₇₆Fe_24-xNi_x (x = 0,4,8,12,16,24) and Zr_69.5Cu₁₂Ni₁₁Al_7.5. In the Zr_69.5Cu₁₂Ni₁₁Al_7.5 alloy, formation of a quasicrystalline phase was observed on crystallization. Bulk glass having the composition Zr₅₂Ti₆Al₁₀Cu₁₈Ni₁₄ has been produced by copper mould casting. This has been crystallized in order to obtain nanocrystalline phases having Zr₂Ni and Zr₂Cu structures. The nanocrystalline and the nanoquasicrystalline microstructures have been examined in considerable detail in order to find out the nature of the various types of interfaces in them. Particularly the nanograin boundaries were examined by high-resolution transmission electron microscope (HREM) and their structure has been compared with that of the grain boundary in large grained material. The change in nature of these interfaces and their number with coarsening of the nanocrystal is also investigated.

Highly luminescent monodispersed CdSe nanocrystals were prepared from CdO and Se powder in a three-component hexadecylamine-trioctylphosphine oxide-trioctylphosphine (HAD-TOPO-TOP) mixture under high temperature and nitrogen protection. CdSe/ZnO core/shell nanocrystals were produced by adding Zn(Ac)₂ and LiOH into CdSe nanocrystals solution under ultrasonic at low temperature. ZnO shell grows heterogeneously on the CdSe substrate. The structure and fluorescence have been characterized by TEM (transmission electron micrograph), XRD (X-ray diffraction), UV-Vis and PL spectra. It can be found that after the surface passivation with ZnO shell, electrons were confined in the CdSe core due to the wide band-gap effect of ZnO, which resulted in the obvious increasing of the fluorescent intensity.

Nanocrystalline ZnO of various shapes was obtained without the use of catalysts by dry oxidation of metallic zinc. Oxidation treatments performed below the melting point of Zn(T_m = 419°C) led to the formation of flakes-type structure. At 500°C, long nanofibers of circular and uniform cross section covering the surface of the oxidized samples were observed. At 600°C, large density of cone-shape needles covered the oxidized surface. High quality nanocrystals of hexagonal shape were obtained by vapor deposition on Pt substrate. XRD results showed that the lattice parameters of the needles are 1% smaller than those of microcrystalline ZnO.

The recombination processes leading to tunneling afterglow and photostimulated luminescence were studied in systems based on ionic host crystals with low-dimensional structures (semiconductor quantum dots) formed as a result of the self-organized growth. The systems of AgBr nanocrystals embedded into the KBr crystal lattice and CsPbBr₃ nanocrystals embedded into the CsBr crystal lattice were investigated. The energy of electron–hole recombination in matrices was shown to transfer to quantum dots. To identify the structure of recombining electron and hole centers, magnetic resonance detected optically by monitoring the tunneling afterglow was used.