Size Distribution Of Nanocrystals Research Articles

Analytical and computational modeling of fluctuation electron microscopy from a nanocrystal/amorphous composite

A nanocrystal/amorphous composite is an idealized model of the medium-range order structure in a variety of amorphous materials. We have investigated the fluctuation electron microscopy (FEM) signal from such a model analytically and with computer simulations. In the analytical modeling, we improved the previous model by Stratton and Voyles (Ultramicroscopy 108, 727 (2008)) by introducing the partial occupancy of nanocrystals in a column, the effect of the deviation parameter on the diffracted intensity, and a distribution of nanocrystal sizes. The improved model no longer has a maximum in the FEM signal as a function of the volume fraction of nanocrystals. In the computer modeling, we compared the variance calculated using kinematic scattering and dynamical scattering and investigated the effects of disorder and strain inside the nanocrystals on the variance. The variance based on dynamical scattering is about 15% smaller than that based on kinematic scattering. Disorder introduced in the nanocrystal reduces the variance peaks at all scattering vectors, with a much larger reduction at longer scattering vector.

Size characterisation of noble-metal nano-crystals formed in sapphire by ion irradiation and subsequent thermal annealing

Metallic nano-particles embedded in transparent dielectrics are very important for new technological applications because of their unique optical properties. These properties depend strongly on the size and shape of the nano-particles. In order to achieve the synthesis of metallic nano-particles it has been used the technique of ion implantation. This is a very common technique because it allows the control of the depth and concentration of the metallic ions inside the sample, limited mostly by straggling, without introducing other contaminant agents. The purpose of this work was to measure the size of the nano-particles grown under different conditions in Sapphire and its size evolution during the growth process. To achieve this goal, α-Al2O3 single crystals were implanted with Ag or Au ions at room temperature with different fluences (from 2×1016ions/cm2 to 8×1016ions/cm2). Afterwards, the samples were annealed at different temperatures (from 600°C to 1100°C) in oxidising, reducing, Ar or N2 atmospheres. We measured the ion depth profile by Rutherford Backscattering Spectroscopy (RBS) and the nano-crystals size distribution by using two methods, the surface plasmon resonance in the optical extinction spectrum and the Transmission Electron Microscopy (TEM).

Seed-mediated growth method for high-quality noble metal nanocrystals

This review highlights work from the authors’ laboratory on the recent development of seed-mediated growth method for noble metal nanocrystals. The seed-mediated growth method has become one of the most efficient and versatile methods for synthesizing high-quality noble metal nanocrystals. The seed-mediated growth method can separate the nucleation and growth stages of metal nanocrystals, and thus provide better control over the size, size distribution, and crystallographic evolution of metal nanocrystals. Because of its high controllability, the seed-mediated growth method is especially promising in providing mechanistic insights into the growth mechanisms of noble metal nanocrystals. In this review, the thermodynamic and kinetic parameters for the nucleation and growth of noble metal nanocrystals are systematically summarized. Mechanistic understanding of these parameters is provided. These studies provide useful guidelines for the rational design and synthesis of novel noble metal nanocrystals with high quality.

Sol–gel preparation and structural characterization of Ba2TiSi2O8 powder

Fresnoite (Ba2TiSi2O8, BTS) nano-particles were successfully synthesized at a low temperature of 900 °C by a modified Pechini sol–gel process with starting materials of Ba(NO3)2, Si(OC2H5)4 and Ti(OC3H7)4. The structural characterization of the precursors and derived synthesized oxide powders is done by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), thermal analysis (TG-DTG) and electron microscopy (SEM, EDX and TEM) studies. The effect of heat-treating temperature on the crystallinity of the fresnoite was investigated. Combined the XRD data and the strong FTIR peaks assigned to Ti–O and Si–O vibration, indicate the formation of fresnoite phase at a temperature of 900 °C. Also the nano-crystals size distribution was studied and the main diameter of nanoparticles was about 47 nm.

Bimodal Size Distribution of Nanocrystals at Their Early Stage of Growth: The Case Study of ZnO

Bimodal Size Distribution of Nanocrystals at Their Early Stage of Growth: The Case Study of ZnO

Controlled synthesis of high quality PbSe and PbTe nanocrystals with one-pot method and their self-assemblies

High quality PbSe and PbTe nanocrystals were synthesized with non-injection one-pot method which could be scale up easily. The use of traditional dangerous pyrophoric trioctylphosphine (TOP) and tributylphosphine (TBP) reagents was avoided. The crystal size was controlled by the reaction time and temperature, and the shape (sphere and cube) was controlled by the use of different combinations of surfactants. X-ray diffraction (XRD) and selective area electron diffraction (SAED) characterizations demonstrated the rock salt cubic structures; the transition electron microscopy (TEM) images show narrow size distributions of as-synthesized PbSe and PbTe nanocrystals. The uniform size of as-synthesized nanocrystals promoted the self-assemble of PbSe and PbTe nanocrystals into ordered superstructures.

Ge nanocrystals with highly uniform size distribution deposited on alumina at room temperature by pulsed laser deposition: structural, morphological, and charge trapping properties

In this work, we report on the synthesis of Ge nanocrystals (NCs) by pulsed laser deposition (PLD) at room temperature (RT) in an argon atmosphere without any further annealing process. Our results show that functional thin films of crystalline Ge nanoparticles with spherical shapes can be obtained by PLD directly on alumina layers deposited on n-doped Si (100) substrates. In addition, we also demonstrate that a uniform size distribution of NCs with an average diameter of about 3 nm and a density of 2.3 × 1011 cm−2 can be obtained by optimizing a shadow mask set-up, where a solid disk is introduced between the target and the substrate. Charge/discharge effects in Ge NCs deposited on a high-k amorphous alumina layer are also evidenced by conductive atomic force microscopy, which makes them suitable for memory applications.

Synthesis of CuInxGa1-xSe2 nanocrystals for potential thin film photovoltaic application under air condition

A one-step process has been proposed and used for the preparation of CuIn1-xGaxSe2 nanocrystals. In the proposed method, acetylacetonate salts and Se powder were simply refluxed in triethylenetetramine under air condition for 6h. The as-prepared CuIn1-xGaxSe2 nanocrystals were characterized by means of scanning electron microscopy (SEM), transmission election microscopy (TEM), X-ray diffraction (XRD), energy dispersive spectrometry (EDS) and UV–vis spectroscopy. The size distributions of CuIn1-xGaxSe2 nanocrystals were in the ranges of 15–25nm and could be obtained by adjusting the reaction time and/or the acetylacetonate salts concentrations. The synthesis strategy developed in this work may be used as a general process for the synthesis of CuIn1-xGaxSe2 nanocrystals, and may have great potential to be applied in high efficiency, yet low cost photovoltaic areas.

Kinetic transition in the growth of Al nanocrystals in Al-Sm alloys

The formation of Al nanocrystals from an amorphous Al92Sm8 alloy involves kinetic phenomena with very different characteristic length and timescales, including initial nucleation and later growth and coarsening. Insight into these processes can be derived from the evolution of the sizes of nanocrystals as a function of time. Synchrotron small angle x-ray scattering (SAXS) experiments provide information about the evolution of the nanocrystal size distribution, particularly at times after nucleation has reached saturation. Accurately interpreting the distribution of intensity measured using SAXS requires a nanoparticle model consisting of nanocrystalline core of pure Al surrounded by a shell enriched in Sm. With this approach, statistical parameters derived from SAXS are independent of detailed assumptions regarding the distribution of Sm around the nanocrystals and allow the maximum radius of nanocrystals within the distribution to be determined unambiguously. Sizes determined independently using transmission electron microcopy are in excellent agreement with the SAXS results. The maximum radius obtained from SAXS is proportional to the cube root of time at large sizes and long times, consistent with a coarsening model. The diffusivity of Al within the Al-Sm alloy is obtained from a quantitative analysis of the coarsening process. Further analysis with this diffusivity and a particle growth model provides a satisfactory account for the particle size evolution at early times before the kinetic transition to coarsening.

Narrow size distribution of Au nanocrystals formed in sapphire by utilizing Ar ion irradiation and thermal annealing

We describe a method to form narrow size distribution of Au nanoparticles in Al2O3 substrate. A layer of Au about 30nm thick was deposited on surface of single crystalline Al2O3, and then was brought into the matrix by Ar ion irradiation to form Au nanoparticles. The size distribution of the nanoparticles was subsequently modified by annealing treatment. Rutherford Backscattering Spectrometry, X-ray Diffraction and Transmission Electron Microscopy were used to study the structure and the size distribution of the nanoparticles. The results show that a narrow size distribution of nanoparticles formed in single crystalline alumina with an average diameter of 3.9nm and a standard deviation of 0.5nm.

Theoretical and experimental analysis of drug release from an ensemble of polymeric particles containing amorphous and nano-crystalline drug

The analysis of the release kinetics from a delivery system containing amorphous and/or nano-crystalline drug requires the evaluation of many parameters among which the water solubility of the amorphous/nano-crystalline drug is one of the most important. This, in turn, needs the determination of the drug melting enthalpy/temperature dependence on the curvature radius of the crystalline phase (thought, for the sake of simplicity, of spherical shape). Accordingly, this paper is aimed to develop a general theoretical approach devoted to model the melting enthalpy/temperature variation with nano-crystals radius, the solubility dependence on nano-crystals radius and the release process from an ensemble of polymeric particles (crosslinked polyvinylpyrrolidone) containing a poorly water soluble drug (nimesulide) in its amorphous and nano-crystalline status. Drug loading was achieved by means of drug and polymer co-grinding. This study allowed the determination of the nanocrystals fraction and size distribution in the co-ground systems, the drug solubility increase of nano-crystals and the estimation of amorphous drug solubility. This approach yielded to the conclusion that, in the case of nimesulide, a significant increase of solubility occurs for the amorphous form and for nanocrystals characterised by radii lower than 4nm.

Controlled SnO2 nanocrystal growth in SiO2–SnO2 glass‐ceramic monoliths

Crack‐free (100–x) SiO2–x SnO2 glass‐ceramic monoliths have been prepared by the sol–gel method obtaining for the first time SnO2 concentrations of 20% with annealing at 1100 °C. Heat‐treatment resulted in the formation and growth of SnO2 nanocrystals within the silica matrices. Combined use of Fourier transform–Raman spectroscopy and in situ high‐temperature X‐Ray diffraction shows that SnO2 particles begin to crystallize in the cassiterite‐type phase at 80 °C and that their average apparent size remains around 7 nm, even after annealing at 1100 °C. Nanocrystal sizes and size distributions determined by low‐wavenumber Raman are in good agreement with those obtained from transmission electron microscopy measurements. Results indicate that the formation and the growth of SnO2 nanocrystals impose a residual porosity in the silica matrix. Copyright © 2011 John Wiley & Sons, Ltd.

Size- and Temperature-Dependent Charge Transport in PbSe Nanocrystal Thin Films

We report the size- and temperature-dependence of electron transport in thin films of PbSe nanocrystals. Upon increasing temperature over the range 28-200 K, the electron transport underwent a transition in mechanism from Efros-Shklovskii-variable-range-hopping (ES-VRH) to nearest-neighbor-hopping (NNH). The transition occurred at higher temperatures for films with smaller particles. The electron localization length, estimated from the ES-VRH model, was comparable to the nanocrystal size and scaled systematically with nanocrystal diameter. The activation energy from the NNH regime was also size-dependent, which is attributed both to size-dependent Coulomb effects and the size-distribution of nanocrystals.

Size distributions of chemically synthesized Ag nanocrystals

Silver nanocrystals made by a chemical reduction of silver salts (AgNO3) by sodium borohydride (NaBH4) were studied using transmission electron microscopy and light scattering simulations. For various AgNO3/NaBH4 molar ratios, the size distributions of the nanocrystals were found to be approximately log-normal. In addition, a linear relation was found between the mean nanocrystal size and the molar ratio. In order to relate the size distribution of Ag nanocrystals of the various molar ratios to the scattering properties of Ag nanocrystals in solar cell devices, light scattering simulations of Ag nanocrystals in Si, SiO2, SiN, and Al2O3 matrices were carried out using MiePlot. These light scattering spectra for the individual nanocrystal sizes were combined into light scattering spectra for the fitted size distributions. The evolution of these scattering spectra with respect to an increasing mean nanocrystal size was then studied. From these findings, it is possible to find the molar ratio for which the corresponding nanocrystal size distribution has maximum scattering at a particular wavelength in the desired matrix.

Diverging Geometric and Magnetic Size Distributions of Iron Oxide Nanocrystals

An important reason to prepare magnetic nanoparticles of uniform size and shape is to ensure uniform magnetic properties. However, here, we demonstrate that magnetic iron oxide crystals of 20 nm or less with a low polydispersity of the geometric size can nevertheless have a strikingly broad distribution of the magnetic dipole moment. A comparative study was performed on nanoparticles with near-perfect crystallinity, twinning defects, or a high density of dislocations. Size, shape, and crystal defects were characterized with electron microscopy and X-ray diffraction, and magnetic dipole moments were determined from magnetization curves of dilute colloidal dispersions. The largest divergence was found for spherical particles with 3.5% geometric size polydispersity and 35% magnetic size polydispersity due to crystal lattice defects that disrupt single-domain magnetic spin coupling. This is in stark contrast with the usual implicit assumption that uniform size and shape guarantee well-defined magnetic properties of the individual particles.

Focusing Nanocrystal Size Distributions via Production Control

We present a theoretical description of how continuous monomer production affects the focusing of nanocrystal size distributions in solution. We show that sufficiently high monomer production rates can drive a decrease in the polydispersity even as the average nanocrystal size increases. This is in sharp contrast to Ostwald ripening, where polydispersity increases with mean crystal size. We interpret several experimental nanocrystal studies through our model and show how production-controlled growth promises exquisite control over the size and polydispersity of functional nanocrystals.

Optical Properties of Nanocrystal Interfaces in Compressed MgO Nanopowders

The optical properties and charge trapping phenomena observed on oxide nanocrystal ensembles can be strongly influenced by the presence of nanocrystal interfaces. MgO powders represent a convenient system to study these effects due to the well-defined shape and controllable size distributions of MgO nanocrystals. The spectroscopic properties of nanocrystal interfaces are investigated by monitoring the dependence of absorption characteristics on the concentration of the interfaces in the nanopowders. The presence of interfaces is found to affect the absorption spectra of nanopowders more significantly than changing the size of the constituent nanocrystals and, thus, leading to the variation of the relative abundance of light-absorbing surface structures. We find a strong absorption band in the 4.0−5.5 eV energy range, which was previously attributed to surface features of individual nanocrystals, such as corners and edges. These findings are supported by complementary first-principles calculations. The possibility to directly address such interfaces by tuning the energy of excitation may provide new means for functionalization and chemical activation of nanostructures and can help improve performance and reliability for many nanopowder applications.

Nonaqueous synthesis and magnetic properties of ZnFe2O4 nanocrystals with narrow size distributions

We report the nonaqueous synthesis and magnetic properties of narrowly size distributed ZnFe2O4 nanocrystals of two different particle sizes by nanoemulsion, of which structure and properties were investigated by transmission electron microscopy, x-ray diffraction, vibrating sample magnetometry, and physical property measurement system. The morphological and structural characterization show the high crystallinity and excellent particle size distribution of the ZnFe2O4 nanocrystals in the diameters of 5.37 ± 0.68 nm and 6.44 ± 0.95 nm, separately. Meantime, the corresponding magnetic measurements reveal the strong size dependence and well-defined nanomagnetism of the nanocrystals.

Study on phase separation in a-SiO x for Si nanocrystal formation through the correlation of photoluminescence with structural and optical properties

The phase separation in amorphous silicon suboxide (a-SiO x ) films upon thermal annealing for the formation of light emitting silicon nanocrystals (Si-NCs) was studied through the correlation of photoluminescence (PL) and photoluminescence excitation (PLE) with structural and optical properties. The PL and PLE features and the structural and optical properties show a strong dependence on the annealing process and reveal that the precipitation of the excess Si in a-SiO x and the formation of Si-NCs from the precipitated Si are two separate processes which should be distinguished in the phase separation in a-SiO x . They proceed at different temperatures and the formation of Si-NCs is a slow process compared with the precipitation of the excess Si. The nanocrystal size and size distribution evolve with annealing time at the initial stages and are mainly dependent on annealing temperature for a certain O content in the initial a-SiO x with the density of the formed Si-NCs increasing with longer annealing duration.

White light emission from semiconductor nanocrystals by in situ colour tuning in an alternating thermodynamic–kinetic fashion

White light emission from semiconductor nanocrystals was directly obtained by the in situ control of the size and size distribution of nanocrystals in a one-pot hot-injection synthesis. By directing the crystal growth from a thermodynamics-based regime into a kinetics-based one, nanocrystals with a multimodal size distribution showing two or three primary emission colours were simply obtained without any post-mixing processes. The colour axis and colour rendering index of the nanocrystals were flexibly adjusted by changing the experimental parameters of the synthetic process. The as-prepared nanocrystals showed a pure white light and a high colour rendering index so they could be directly used as phosphors for near-UV white light emitting diodes (WLEDs). With the capability of direct tuning the colour composition, the manufacturing process has been significantly simplified and is expected to be a general approach for nanocrystals-based WLEDs.