Nanocrystals Of Different Sizes Research Articles

Influence of crystal size on the electron–phonon coupling in ZnO nanocrystals investigated by Raman spectroscopy

We have synthesized ZnO nanocrystals of different sizes (25–41nm) using the sol–gel method and characterized them using different techniques such as: transmission electron microscopy (TEM) and X-ray diffraction (XRD). Raman spectra of different sizes of ZnO nanocrystals were recorded at two excitation wavelengths, 514 and 647nm, in the spectral range 300–1200cm−1. The vibrational modes were assigned on the basis of group theory analysis. The influence of mean crystallite size on the strength of the electron–phonon coupling is experimentally estimated by the variation of relative intensities of second order Raman band and the first order Raman band for ZnO nanocrystals of different sizes. We found that the intensity ratio of the 2E2 and 1E2 Raman bands decreases almost linearly for both excitation wavelengths with decreasing crystallite size, which reveals that the Fröhlich interaction plays a dominant role in the electron–phonon coupling of the ZnO nanocrystals.

Atomistic Simulation of Shear Mode Deformation of Nanocrystalline Copper with Different Grain Sizes

Mechanical deformation of nanocrystalline copper has been simulated by means of molecular dynamics. The embedded atom method (EAM) potential was adopted for calculating the interatomic interaction. Samples with different grain sizes, 2.7 and 4.2nm, were prepared by sintering spherical nanocrystals of different sizes. About 38000 atoms were contained in the sample, and a heterogeneous structure composed of crystalline and amorphous regions was realized. A shear mode strain was applied to the sample by sliding its upper and lower parts. The simulation was performed under free boundary condition for the surfaces perpendicular to the shear plane, and the stress­strain relation was obtained. The determined flow stress was lager for larger grain sample, namely, the inverse Hall­Petch effect was observed. Simulations for temperature and strain-rate dependences of the flow stress were also performed. [doi:10.2320/matertrans.MA201315]

Dual-Color Electroluminescence from Dot-in-Bulk Nanocrystals

The emission color from colloidal semiconductor nanocrystals (NCs) is usually tuned through control of particle size, while multicolor emission is obtained by mixing NCs of different sizes within an emissive layer. Here, we demonstrate that recently introduced "dot-in-bulk" (DiB) nanocrystals can emit two-color light under both optical excitation and electrical injection. We show that the effective emission color can be controlled by adjusting the relative amplitudes of the core and shell emission bands via the intensity of optical excitation or applied bias in the cases of photoluminescence (PL) and electroluminescence (EL), respectively. To investigate the role of nonradiative carrier losses due to trapping at intragap states, we incorporate DiB NCs into functional light-emitting diodes and study their PL as a function of applied bias below the EL excitation threshold. We show that voltage-dependent changes in core and shell emissions are not due to the applied electric field but rather arise from the transfer of charges between the anode and the NC intragap trap sites. The changes in the occupancy of trap states can be described in terms of the raising (lowering) of the Fermi level for reverse (direct) bias. We find that the applied voltage affects the overall PL intensity primarily via the electron-trapping channel while bias-induced changes in hole-trapping play a less significant role, limited to a weak effect on core emission.

Luminescent materials based on nanoparticles of silicon dioxide for solar cells

The report considers luminescent mesoporous silica dioxide particles of activated RE ions and lead sulfide nanocrystals of different sizes. For the purpose of assessing the possibility of converting by lead sulfide nanoclusters of solar radiation into the red-orange light, the following methods will be drawn to the attention: mesoporous silica particles sol-gel synthesis, additional light scattering method as an additional approach for the determination of particle size. The element composition of a particle size of 300 ± 40 nm was identified using the scanning electron microscope with the system of energy disperse analysis. Particle images were obtained using the field scanning electron microscope. Zeta Potential Analyzer was used to measure the surface charge of the particles. Particles in solution were excited by near ultraviolet and visible light. The intensity of intrinsic emission was registered in the range of 400–550 nm. The diagnosis made visible the electron induced energy transmission from the matrix to the Eu3+ and Tb3+. Emission of Eu3+ and Tb3+ has been registered at 300K. It is assumed that the identical matrixes could work as converters of solar emission spectrum, to which some solar cells are sensitive.

Ab-initio phasing using nanocrystal shape transforms with incomplete unit cells.

X-ray free electron lasers are used in measuring diffraction patterns from nanocrystals in the 'diffract-before-destroy' mode by outrunning radiation damage. The finite-sized nanocrystals provide an opportunity to recover intensity between Bragg spots by removing the modulating function that depends on crystal shape, i.e. the transform of the crystal shape. This shape-transform dividing-out scheme for solving the phase problem has been tested using simulated examples with cubic crystals. It provides a phasing method which does not require atomic resolution data, chemical modification to the sample, or modelling based on the protein databases. It is common to find multiple structural units (e.g. molecules, in symmetry-related positions) within a single unit cell, therefore incomplete unit cells (e.g. one additional molecule) can be observed at surface layers of crystals. In this work, the effects of such incomplete unit cells on the 'dividing-out' phasing algorithm are investigated using 2D crystals within the projection approximation. It is found that the incomplete unit cells do not hinder the recovery of the scattering pattern from a single unit cell (after dividing out the shape transforms from data merged from many nanocrystals of different sizes), assuming that certain unit-cell types are preferred. The results also suggest that the dynamic range of the data is a critical issue to be resolved in order to apply the shape transform method practically.

Influence of nanoparticle size on the nonlinear optical properties of magnetite ferrofluids

The nonlinear index of refraction (n_{2}) and the two-photon absorption coefficient (β) of water-based ferrofluids made of magnetite nanocrystals of different sizes and with different coatings have been measured through the Z-scan technique, with ultrashort (femtoseconds) laser pulses. Their third-order susceptibility is calculated from the values of n_{2} and β. The influence of different particles' coatings and sizes on these nonlinear optical properties are investigated. The values of n_{2} and β depend more significantly on the nanoparticles' size than on the particular coating. We observe a decrease of β as the nanoparticles' diameters decrease, although the optical gap is found to be the same for all samples. The results are interpreted considering modifications in the electronic orbital shape due to the particles' nanosize effect.

Mechanistic Study of Monodisperse Iron Oxide Nanocrystals Formation by Solution Phase Thermal Decomposition of Iron (III) Oleate Complex

To gain better insight into the formation of iron oxide nanocrystals from the solution phase thermal decomposition of iron (III) oleate complex, different reaction conditions including time, heating ramp, as well as concentrations of iron oleate precursor and oleic acid ligand were systematically varied and the resulting nanocrystals were characterized by transmission electron microscopy (TEM) and photon correlation spectroscopy (PCS). Iron oxide nanocrystals of different sizes and shapes were obtained at different reaction conditions. Moreover, thermal analysis of the iron oleate complex was performed to explain the heating up process. Based on the findings, theoretical arguments were constructed to mechanistically describe the thermal decomposition process.Keywords: iron (III) oleate, thermal decomposition, iron oxide nanocrystal, Ostwald ripening, magnetic resonance imaging

Size and shape influence of luminescent orthovanadate nanoparticles on their accumulation in nuclear compartments of rat hepatocytes

In this paper the process of nonfunctionalized negatively charged orthovanadate nanoparticle accumulation and redistribution in cells dependent on their shape and size was investigated. Aqueous colloidal solutions of nReVO4:Eu3+ (Re=Gd, Y, La) luminescent nanocrystals of different sizes and shapes have been synthesized. The average sizes of spherical particles were 2, 20, and 300nm, of spindle-like particles – 22×6.3nm, and of rod-like particles – 57×4.4nm. Luminescence of nReVO4:Eu3+ nanocrystals was effectively excited by UV and visible irradiation. By means of luminescence microscopy and luminescence microspectroscopy, it has been revealed that spherical nanocrystals with an average diameter of 2nm tend to accumulate mainly in the rat hepatocyte nuclei in situ and also in the isolated nuclei of these cells. An additional experiment has shown that nanoparticles reveal tropism to nuclear structural components. The penetration into nuclei does not require any modifications of the surface of nanoparticle and is governed by the shape and size of nanoparticle and also is determined by the cellular type.

Synthesis and Optical Characterization of Silicon Nanoparticles

Various reaction conditions, such as quantity of reducing agent and reaction time were investigated with the aim of finding a simple, optimized synthetic route for the synthesis of luminescent silicon nanoparticles (SiNPs). Si NPs were synthesized from the reaction of ethylenediammonium chloride and magnesium silicide via a low temperature solution route. Optical characterizations of silicon nanoparticles were achieved by using ultraviolet-visible (UV-Vis) and photoluminescence (PL) spectroscopy. As the reaction time longer, silicon nanoparticles grew and their emission wavelength shifted to the longer wavelength. The monotonic shift of the photoluminescence as a function of excitation wavelength resulted in the excitation of different sizes of nanocrystals that had different optical transition energies.

Photoluminescence and time-resolved carrier dynamics in thiol-capped CdTe nanocrystals under high pressure

The application of static high pressure provides a method for precisely controlling and investigating many fundamental and unique properties of semiconductor nanocrystals (NCs). This study systematically investigates the high-pressure photoluminescence (PL) and time-resolved carrier dynamics of thiol-capped CdTe NCs of different sizes, at different concentrations, and in various stress environments. The zincblende-to-rocksalt phase transition in thiol-capped CdTe NCs is observed at a pressure far in excess of the bulk phase transition pressure. Additionally, the process of transformation depends strongly on NC size, and the phase transition pressure increases with NC size. These peculiar phenomena are attributed to the distinctive bonding of thiols to the NC surface. In a nonhydrostatic environment, considerable flattening of the PL energy of CdTe NC powder is observed above 3.0 GPa. Furthermore, asymmetric and double-peak PL emissions are obtained from a concentrated solution of CdTe NCs under hydrostatic pressure, implying the feasibility of pressure-induced interparticle coupling.

Facile preparation and size-dependent photocatalytic activity of Cu2O nanocrystals modified titania for hydrogen evolution

Degussa TiO2 powder (P25) with Cu2O nanocrystals of different sizes deposited on the surface were prepared by a facile ethanol-induced deposition followed with a calcination process at 350 °C and characterized by XRD, XPS, HRTEM and UV–Vis diffuse reflectance spectra, respectively. In our protocol, Cu2O nanocrystals is formed by reducing Cu(II) with the ethanol adsorbed on the surface of P25. The absorption edge as well as photocatalytic activity for H2 evolution are closely related to the size of the Cu2O nanocrystals depending on the Cu content in the as-prepared composites. The 0.9 mol% Cu2O/TiO2 composite with ca. 4 nm Cu2O nanocrystals exhibits the highest photocatalytic H2 evolution rate of 318 μmol h−1 under the illumination of simulant solar light and an apparent quantum efficiency of 28.6% at 365 nm, 397 times higher than that of P25. This enhancement is mainly attributed to the quantum confinement effect of quantum-sized Cu2O nanocrystals (q-Cu2O), which can upgrade the conduction band bottom of q-Cu2O, and thus facilitates the effective separation of photogenerated charge carriers to enhance the photocatalytic activity. The results provided here not only offers a green and simple way for depositing q-Cu2O on the surfaces of P25 to get the photocatalyst with an excellent photocatalytic activity for H2 evolution, but also sheds a new insight for the fabrication of efficient p–n heterojunctions for the separation of electron-hole pairs by quantum confinement effect.

Intense white luminescence in ZnTe embedded porous silicon

Porous silicon layers were embedded with ZnTe using the isothermal close space sublimation technique. The presence of ZnTe was demonstrated using cross-sectional energy dispersive spectroscopy maps. ZnTe embedded samples present intense room temperature photoluminescence along the whole visible range. We ascribe this PL to ZnTe nanocrystals of different sizes grown on the internal pore surface. Such crystals, with different orientations and sizes, were observed in transmission electron microscopy images, while transmission electron diffraction images of the same regions reveal ZnTe characteristic patterns.

L-Histidine-modified biocompatible zinc oxide nanocrystals

In this article, we report a new method of preparing size-controlled and well-dispersed ZnO nanocrystals using L-histidine as the capping agent. The synthesis and properties of ZnO nanocrystals capped with L-histidine prepared by the wet chemical route are presented. The structural properties of L-histidine-capped nanocrystals are investigated by XRD, TEM, EDAX and FTIR spectroscopy. The optical characterisation of the nanocrystals is carried out on the basis of UV-Vis absorption and photoluminescence spectroscopy. The size of the L-histidine-capped ZnO nanocrystals is less than 10 nm as confirmed from TEM images. The elemental composition detected from EDAX establishes the presence of the capping agent in the samples. For L-histidine-modified ZnO nanocrystals, the UV absorption edge shifts towards the blue region compared to pure ZnO, suggesting the formation of nanocrystals of smaller size. The intense photoluminescence emission observed around 527 nm for L-histidine-capped ZnO offers high prospects in bio-imaging applications. The colloidal stability of the capped nanocrystals is very good as it remains stable without settling down for more than three weeks. Nanocrystals of different sizes have also been synthesised and their properties studied. The present synthesis route is of low cost and easy to control offering high purity to the products, using only bio-compatible materials. The functionalised nanoparticles are suitable for biological applications.

Growth kinetic on the optical properties of the Pb1−xMnxSe nanocrystals embedded in a glass matrix: thermal annealing and Mn2+ concentration

Semimagnetic Pb(1-x)Mn(x)Se nanocrystals were synthesized by a fusion method in a glass matrix and characterized by optical absorption (OA), atomic/magnetic force microscopy (AFM/MFM), and photoluminescence techniques. MFM images strongly indicated the formation of Pb(1-x)Mn(x)Se magnetic phases in the glass system. Quantum dot size was manipulated by tuning annealing time. It was shown that Mn(2+) impurity affects nucleation, where Mn(2+)-doped samples present a redshift of the OA peak after a short annealing time and a blueshift after long annealing time compared to undoped PbSe NCs. This behavior was linked to the dependence of band-gap energy and the absorption selection rule on Mn(2+) concentration. Photoluminescence in the Pb(1-x)Mn(x)Se nanocrystals increases as the temperature rises up to a point and then decreases at higher temperatures. Anomalous increases in emission efficiency were analyzed by considering temperature induced carrier-transfer in semimagnetic Pb(1-x)Mn(x)Se quantum dots nanocrystals of different sizes.

Lateral electronic transport in 2D arrays of oxidized Si nanocrystals on quartz: Coulomb blockade effect and role of hydrogen passivation

We report on the lateral transport in a single two-dimensional (2D) array of Si nanocrystals of different sizes grown by low pressure chemical vapor deposition (LPCVD) of silicon on a quartz substrate and subsequent oxidation at high temperature. The initial nanocrystal size in the z-direction was 5 nm, while it was reduced to ∼3 nm after oxidation. The nanocrystals in the x-y plane were connected by grain boundaries and/or by very thin silicon oxide barriers, while a thin oxide layer was formed on their surface. The electrical measurements showed that current in the film is mainly governed by thermionic emission over the barriers (grain boundaries or dielectric barriers) at high temperatures and by tunneling at lower temperatures. Charge traps at the interfaces of the silicon nanocrystals with the oxide and at the grain boundaries cause considerable hysteresis in the current-voltage characteristics. Hydrogen passivation of the charge traps reduces considerably the hysteresis effect and the activation energy of the thermionic emission, while revealing a clear Coulomb gap.

Phasing of coherent femtosecond X-ray diffraction from size-varying nanocrystals

The scattering between Bragg reflections from nanocrystals is used to aid solution of the phase problem. We describe a method for reconstructing the charge density of a typical molecule within a single unit cell, if sufficiently finely-sampled "snap-shot" diffraction data (as provided a free-electron X-ray laser) are available from many nanocrystals of different sizes lying in random orientations. By using information on the particle-size distribution within the patterns, this digital method succeeds, using all the data, without knowledge of the distribution of particle size or requiring atomic-resolution data.

Peculiarities of crystallization at high undercooling: Analysis of the simulation data for aluminum

Kinetics of crystallization in the molecular dynamics model of aluminum has been studied. Data obtained concerning nucleation rates, growth velocities and structure of nanocrystals of different sizes are discussed. Local measurements of energies of atom interaction are fulfilled. The melting heat dependences on temperature and crystal size are determined. The differences in entropy and chemical potentials of coexistent phases are calculated. The question concerning mechanism of crystal growth from an amorphous phase is also discussed. A model for the analytical description of growth of very small crystals is proposed.

Application of the Mi potential to modelling a PbTiO3 crystal

A molecular-dynamic model of a PbTiO3 crystal with a perovskite-like structure that includes the dense packing of soft ions is developed. The effect of the Mi potential parameters on the structure of a model nanocrystal is shown. The model can be applied to describing the structure of isolated perovskite nanocrystals of different sizes.

Discontinuous Growth of II−VI Semiconductor Nanocrystals from Different Materials

We report about the synthesis of II−VI semiconductor nanocrystals (NCs) that grow in discrete steps. CdS, CdTe, ZnSe, and ZnO NCs were synthesized in analogy to a method we recently developed for the discontinuous growth of CdSe NCs. Because of the similar growth conditions, the NCs of these different materials can be directly compared. Also, CdS NCs with Hg impurities showed discontinuous growth. From all materials, we could isolate fractions of NCs of different sizes via size-selective precipitation and characterize their optical and structural properties.

Water-soluble magnetic glyconanoparticles based on metal-doped ferrites coated with gold: Synthesis and characterization

Novel bimetallic superparamagnetic XFe2O4@Au (X = Fe, Mn and Co) nanocrystals of different sizes were synthesized in high-boiling point ether solvent in two steps. Reduction of gold salts on pre-made seeds in the presence of surfactants was performed resulting in the uniform coverage of the seeds. HAADF- and EDXS-STEM elemental mapping indicate the presence of two metals in the same nanocrystal. The apolar solvent-soluble nanoparticles were transferred into water using an amphiphilic linker and different neoglycoconjugates. The as prepared glyco-ferrites are superparamagnetic at room temperature and present comparable or higher transverse relaxivities (T2) than commercial contrast agents like Endorem, Resovist or Sinerem. The carbohydrate coating provides high stability under physiological conditions, low cytotoxicity and lack of immunogenicity. All glyco-ferrites prepared show comparable relaxation times in phantom imaging.