Size Distribution Of Nanocrystals Research Articles

Crystallization features in Al90Y10 amorphous alloy under a various external influence

The effect of high stress and high temperature on the structure of binary Al90Y10 amorphous alloy has been investigated. Deformation of the alloy was carried out by high pressure torsion at room temperature; heating was produced up to 220°C.It was revealed that both treatments leaded to the primary crystallization of the alloy. Produced Al nanocrystals were randomly distributed over the amorphous matrix. For the deformed sample no correlation between the location of nanocrystals and the positions of shear bands was observed. The nanocrystals formed due to the deformation were more euhedral and smaller in average size (~9nm) than those produced by heating (~15nm). We have shown that the size distributions for two states of the alloy differ from each other substantially. Based on the obtained data for Al nanocrystals size distribution we have concluded that the deformation-induced nanocrystallization in Al90Y10 occurs by heterogeneous mechanism and the crystallization at the heating takes place by the mixed (heterogeneous and homogeneous) one.

Quantification of phosphorus diffusion and incorporation in silicon nanocrystals embedded in silicon oxide

The interest toward the doping of semiconductor nanostructures has been increasing steadily, but a clear understanding must still be reached, due to challenging characterization issues. In this work, we focused on Si nanocrystals (NCs) embedded in SiO2, which represent the most scalable and one of the most interesting and studied systems, but where an accurate quantification of the evolution of dopant profiles is still lacking. High depth resolution time‐of‐flight SIMS with charge compensation has been used to extract secondary ion depth profiles relative to P and Si elements. The relative sensitivity factors of P in SiO2 and of P in a layer containing Si NCs approximately 4.2 nm in diameter, as well as non‐linearity of P intensity at high P concentrations, were determined by comparison with P dose data extracted by Rutherford backscattering spectrometry analysis. Transmission electron microscopy analyses were performed to characterize the NC size distribution and stability upon thermal annealing. As a final result, we obtained a measurement protocol able to extract with high accuracy fully calibrated P concentration profiles in the SiO2 matrix with embedded Si NCs. Copyright © 2014 John Wiley & Sons, Ltd.

Enhancement in visible luminescence from nanocomposite ZnO-SiOx thin films due to annealing

The annealing induced enhancement in visible photoluminescence (PL) from nanocomposite (nc) ZnO – SiO x thin films was investigated. Nc ZnO – SiO x thin films consisting of ZnO nanocrystals in silica matrix were grown by depositing the films using radio frequency (rf) reactive co-sputtering and post-annealing them at temperatures of 350°C and 500°C in high vacuum and air. These films were characterized by Fourier transform infrared (FTIR), (PL) spectroscopy and UV–Vis spectrophotometry measurements. Thin films were also deposited on transmission electron microscopy (TEM) grids in almost identical conditions. The TEM measurement of the thin film deposited on TEM grid shows the formation of ZnO nanocrystals with a size distribution from 3.0 nm to 6.8 nm (+/-0.2 nm) in silica matrix. The UV–Vis spectra of the films show absorption features of ZnO and Zn 2 SiO 4 phases in the films. The visible PL emission intensity and peak width increased in the annealed films. The results suggest increase in the number and size distribution of the ZnO nanocrystals in silica matrix due to the annealing resulting in increase in visible PL emission. The results of vacuum annealed films indicate that these films can be useful in the development of wide band visible light emitting devices using this material.

ChemInform Abstract: Kinetics and Mechanisms of Aggregative Nanocrystal Growth

AbstractReview: 206 refs.

Determination of the energetics of formation of semiconductor/dendrimer nanohybrid materials: implications on the size and size distribution of nanocrystals

The synthesis of inorganic–organic hybrid nanomaterials has attracted considerable interest in recent years because of their multifaceted applications, such as in optoelectronics, cellular imaging, drug delivery, etc. The maneuvering of controlling parameters is key to the successful fabrication of good quality materials. However, the fundamental aspects pertaining to the thermodynamics of growth of such nano hybrid materials has so far not been unraveled. Here, we have investigated the energetics behind the formation of semiconductor–dendrimer nanohybrid materials using isothermal calorimetry. It is apparent from the observed energy release profile that the heat change for the formation of nanoclusters in phase II saturates faster with an increase in starting materials or monomer concentrations. We have also shown variation of the thermodynamic parameters with changes in the synthesis conditions, such as temperature, dendrimer generation and dendrimer core or surface groups. Based on a bi-phasic thermogram and the dependence of thermodynamic parameters on the dendrimer core and surface functionalities, it is suggested that nanoparticles are formed inside dendrimer molecules in the initial time period and on the outer surface at a longer time scale. Furthermore, it is observed that the formation of quantum dot–dendrimer hybrid materials is an exothermic, spontaneous and enthalpy driven process. Also, a lower temperature thermodynamically favors formation in the core of dendrimer molecules leading to smaller particles with a narrower distribution. The observed results suggest that higher values of formation constant and enthalpy are likely to make dendrimers of higher generation better templates for the synthesis of nanoparticles. The dependence of the ratio of concentrations of reacting metal ions (Cd or Zn) to sulfide ions shows a differential size pattern for CdS and ZnS nanoparticles, which has been interpreted in terms of binding constants determined calorimetrically. It is shown that enthalpy–entropy compensation takes place in the synthesis process affording favorable free energy. Such investigation can provide useful guidelines for the synthesis of better quality semiconductor–dendrimer hybrid nanomaterials.

Surfactant Assisted Synthesis of Copper Oxide (CuO) Leaf-like Nanostructures for Electrochemical Applications

Three different copper oxide (CuO) leaf-like nanostructures have been synthesised by micelles micro emulsion method using a surfactant of copper dodecyl sulphate (Cu(DS)2) by varying the concentration of sodium hydroxide (NaOH). This study was carried out to investigate the effect of NaOH concentration on the stability, crystalline domain and pseudocapacitance behaviour of the leaf-like nanostructures. The samples were characterized by X-ray diffraction (XRD), thermogravimetry analysis (TGA), Raman spectroscopy, Fourier-Transform Infrared (FTIR), scanning electron microscopy (SEM) and Transmission electron microscopy (TEM). It was observed that the crystalline domain size (12 nm-18 nm) and size distribution of the as-synthesized nanocrystals decreases with increasing concentration of NaOH. The interactions mechanism and formation of the leaf-like structure have been elucidated and correlated with various analytical techniques. The domain size and NaOH concentration tend to influence the charge transfer resistance.

Monodisperse SnS Nanocrystals: In Just 5 Seconds

As per the classical growth mechanism, tuning the reaction parameters in the growth stage remains pivotal to control the shape, size, dispersity, and size distribution of the colloidal nanocrystals, but what would be the case when the growth is very fast and the nanomaterials are formed instantaneously? Certainly, it needs a different chemical protocol. We investigate here one of such cases: the formation of different shapes of SnS nanostructures. With proper programming of chemical reaction, highly monodisperse α-SnS nanocubes and nanotetrahedrons are obtained within 5 s of the reaction. Furthermore, tuning the density of nucleation, the size of the nanostructures is tuned in a wide window. These two shapes of SnS are also explored for the study of photocatalytic dye degradation, and the facet-dependent rate for this photocatalytic activity has been compared.

Direct characterization of nanocrystal size distribution using Raman spectroscopy

We report a rigorous analytical approach based on one-particle phonon confinement model to realize direct detection of nanocrystal size distribution and volume fraction by using Raman spectroscopy. For the analysis, we first project the analytical confinement model onto a generic distribution function, and then use this as a fitting function to extract the required parameters from the Raman spectra, i.e., mean size and skewness, to plot the nanocrystal size distribution. Size distributions for silicon nanocrystals are determined by using the analytical confinement model agree well with the one-particle phonon confinement model, and with the results obtained from electron microscopy and photoluminescence spectroscopy. The approach we propose is generally applicable to all nanocrystal systems, which exhibit size-dependent shifts in the Raman spectrum as a result of phonon confinement.

Kinetics and Mechanisms of Aggregative Nanocrystal Growth

The aggregative growth and oriented attachment of nanocrystals and nanoparticles are reviewed, and they are contrasted to classical LaMer nucleation and growth, and to Ostwald ripening. Kinetic and mechanistic models are presented, and experiments directly observing aggregative growth and oriented attachment are summarized. Aggregative growth is described as a nonclassical nucleation and growth process. The concept of a nucleation function is introduced, and approximated with a Gaussian form. The height (Γmax) and width (Δtn) of the nucleation function are systematically varied by conditions that influence the colloidal stability of the small, primary nanocrystals participating in aggregative growth. The nucleation parameters Γmax and Δtn correlate with the final nanocrystal mean size and size distribution, affording a potential means of achieving nucleation control in nanocrystal synthesis.

Effect of pHs on dispersity of maize starch nanocrystals in aqueous medium

The effect of different dispersion pHs on zeta potentials, size distribution, and aggregation behavior of starch nanocrystals (SNC) prepared by sulfuric acid hydrolysis was examined in this study. The results showed that zeta potential of starch nanocrystals decreased from −6.7 mV to −34.5 mV as the dispersion pH increased from 2.07 to 11.96. Smaller starch nanocrystals and wider distribution peaks were observed with the increase of dispersion pH. The data obtained from field emission scanning electron microscopy (FE-SEM) also indicated that the aggregated parallelepiped nanoplatelets (1.5 μm) changed to monodispersed spherical-like nanoparticles (50 nm) with increasing of dispersion pH from 2.07 to 11.96. Considering these findings, the stable SNC suspension could be obtained by adjusting the dispersion pH to the range of 7.44–9.45.

Nanosized rare-earth hexaborides: Low-temperature preparation and microstructural analysis

A versatile, rapid and easy synthesis of pure rare-earth-(RE) hexaboride powders was developed, without resorting to hazardous precursors or generating undesired, ineliminable, side products. To this purpose, we employed a metathesis reaction, typically starting from a mixture of a hydrated rare earth trichloride and MgB2, kept at 650 °C for 1 h under vacuum. This methodology affords nanosized RE hexaborides, with average crystallite (domain) sizes down to a few nanometers, useful for tailoring the functional performances of the MgB2 superconducting phase produced by the reactive liquid infiltration method. For the powders showing the lowest average domain sizes (YbB6 and EuB6), an unconventional microstructural analysis, based on Total Scattering methods and on the Debye Function Approach, was also performed, which provided the complete nanocrystal size distributions.

Infrared emitting property and spherical symmetry of colloidal PbS quantum dots

Infrared emitting properties and spherical symmetry of colloidal PbS quantum dots (QDs) chemically synthesized in organic solvent oleylamin as a ligand were investigated in terms of synthesis temperature, time, and precursor concentrations. The progress of the nucleation process and crystal growth of PbS nanocrystals can be interpreted using the LaMer mechanism. By decreasing the temperature or increasing the concentration of precursors, the degree of supersaturation becomes higher, resulting in precipitation of small nuclei. Meanwhile, in the case of the lower supersaturation, the particle size and the size distribution of nanocrystals become larger and narrower. Direct evaluations of the spherical symmetry of the synthesized QDs were carried out using symmetry indexes derived from the transmission electron microscopy images. These results clearly indicated that lower reaction temperature, higher supersaturation, and longer growing time are favorable for the synthesis of spherical QDs emitting at the wavelength of the 1.3μm region.

Size limit on the phosphorous doped silicon nanocrystals for dopant activation

We studied the photoluminescence spectra of silicon nanocrystals doped with and without phosphorus as a function of isothermal annealing time. Silicon nanocrystals were prepared by the implantation of 80keV Si+ into a 500nm SiO2 film to an areal density of 8×1016 at/cm2. Half of the samples were co-implanted with P+ at 80keV to 5×1015 at/cm2. The photoluminescence of the annealed samples were photo-excited at wavelength of 405nm. For short anneal times, when the nanocrystal size distribution has a relatively small mean diameter, formation in the presence of phosphorus yields an increase in the luminescence intensity and a blue shift in the emission peak compared with intrinsic nanocrystals. As the mean size increases with annealing time, this enhancement rapidly diminishes and the peak energy shifts to the red. Our results indicate the donor electron generation depends strongly on the nanocrystal size. We also found a critical limit above which it allows dopant activation.

Donor ionization in size controlled silicon nanocrystals: The transition from defect passivation to free electron generation

We studied the photoluminescence spectra of silicon and phosphorus co-implanted silica thin films on (100) silicon substrates as a function of isothermal annealing time. The rapid phase segregation, formation, and growth dynamics of intrinsic silicon nanocrystals are observed, in the first 600 s of rapid thermal processing, using dark field mode X-TEM. For short annealing times, when the nanocrystal size distribution exhibits a relatively small mean diameter, formation in the presence of phosphorus yields an increase in the luminescence intensity and a blue shift in the emission peak compared with intrinsic nanocrystals. As the mean size increases with annealing time, this enhancement rapidly diminishes and the peak energy shifts further to the red than the intrinsic nanocrystals. These results indicate the existence of competing pathways for the donor electron, which depends strongly on the nanocrystal size. In samples containing a large density of relatively small nanocrystals, the tendency of phosphorus to accumulate at the nanocrystal-oxide interface means that ionization results in a passivation of dangling bond (Pb-centre) type defects, through a charge compensation mechanism. As the size distribution evolves with isothermal annealing, the density of large nanocrystals increases at the expense of smaller nanocrystals, through an Ostwald ripening mechanism, and the majority of phosphorus atoms occupy substitutional lattice sites within the nanocrystals. As a consequence of the smaller band-gap, ionization of phosphorus donors at these sites increases the free carrier concentration and opens up an efficient, non-radiative de-excitation route for photo-generated electrons via Auger recombination. This effect is exacerbated by an enhanced diffusion in phosphorus doped glasses, which accelerates silicon nanocrystal growth.

Controlled growth of Ni nanocrystals on SrTiO3 and their application in the catalytic synthesis of carbon nanotubes

Truncated pyramid-shaped Ni nanocrystals were epitaxially grown on SrTiO(3)(001) surfaces and characterised by scanning tunneling microscopy (STM). These nanocrystals were shown to be catalytically active for the synthesis of carbon nanotubes (CNTs). The narrow size distribution of the Ni nanocrystals results in a similar narrow distribution of CNT diameters.

Efficiency enhancement of dye-sensitized solar cells (DSSCs) using ligand exchanged CuInS2 NCs as counter electrode materials

Exploring inorganic ligand capped nanocrystals (NCs) as counter electrode (CE) materials for dye-sensitized solar cells (DSSCs) with high conversion efficiency. Here, we reported the preparation of inorganic ligand capped CuInS2 NCs through a ligand exchange strategy. After ligand exchange, the organic ligand capped on the surface of CuInS2 NCs were exchanged to inorganic ligand S2− without changing the shape, size distribution and crystal structure of CuInS2 NCs. When used as CE materials for dye-sensitized solar cells, the inorganic ligand capped CuInS2 NCs films showed higher electrocatalytic activity and electrical conductivity, thus can significantly improve the efficiency of DSSCs. It should be noted that the conversion efficiency of the DSSC increased by 17 times after the organic ligand capped CuInS2 NCs were exchanged with the inorganic ligand. The study in this work may pave a new pathway to explore highly electrical conductivity materials for DSSCs.

Size-Dependent Optical Properties of Si Nanocrystals Embedded in a Silica Matrix

We report on a numerical investigation of the size distribution effect on the optical properties of Si nanocrystals embedded in a silica matrix. Firstly, the dielectric function of the composite film is accurately evaluated. Next, a modified version of the Maxwell-Garnett theory is introduced. This model allows the consideration of the size distribution of the Si nanocrystals. We show that a small change in the size dispersion value may induce a sensitive modification in the line shape of the dielectric function. Our results can be used to explain the divergences observed in the literature regarding the dielectric function of Si nanocrystals.

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.

Crystallinity Segregation upon Selective Self-Assembling of Gold Colloidal Single Nanocrystals

Spontaneous separation of single from polycrystalline 5 nm gold nanocrystals (NCs) is observed in colloidal solution. This segregation takes place upon self-assembling of single crystalline NCs at the air-solvent interface and in precipitated superlattices. Polycrystalline NCs are observed to remain in the suspension. Transmission electron microscopy analysis of the size distribution of NCs issued from the different populations indicates that the NC size does not change from each other, excluding therefore any size segregation in this process. Using both low-frequency Raman scattering and X-ray diffraction provides reliable characterization of nanocrystallinity for each population of NCs, thus confirming the crystallinity segregation. The single crystalline NCs are found by electron diffraction to self-assemble into close-packed superlattices with long-range translational and orientational ordering, while polycrystalline NCs behave like spheres with no preferential orientation. The face-to-face orientational ordering, which is only observed for single crystalline NCs, supports the relevance of the specific crystallinity-related morphologies of these NCs in their better ability to self-assemble. Exploiting this spontaneous segregation would open up a simple alternative to other demanding routes for controlling crystallinity of nanocrystals and optimizing their properties for potential applications.

Electronic properties probed by scanning tunneling spectroscopy: From isolated gold nanocrystal to well-defined supracrystals

Scanning tunneling microscopy and spectroscopy at 5 K have been used to determine the electronic properties of 7-nm dodecanethiol-passivated Au nanocrystals in three different configurations: isolated nanocrystal, self-organized thin films (few nanocrystal layers), and large three-dimensional well-defined thick films (over 30 nanocrystal layers) called supracrystals. The electronic properties of both thin and thick well-ordered supracrystals are analyzed in scanning tunneling spectroscopy geometry through $dI/dV$ curves and conductance mapping at different bias voltages. The single particles exhibit a typical $dI/dV$ curve with a Coulomb gap of \ensuremath{\sim}360 meV and a Coulomb staircase. The $dI/dV$ curve of the thin supracrystals presents a Coulomb blockade feature \ensuremath{\sim}100 meV narrower in width than that of the single nanocrystal but without well-defined staircase. On the contrary, the thick supracrystals exhibit a $dI/dV$ curve showing a large Coulomb gap with a Coulomb-staircase-like structure. Generally, the conductance mapping is found to be very homogeneous for both supracrystals. Nevertheless, for some bias voltages, inhomogeneities across individual nanocrystals appear. Additionally, some of these inhomogeneities seem to be related to the supracrystal surface morphology. Finally, these slight variations in the conductance mapping across individual nanocrystals embedded in the supracrystal are discussed in terms of high degree of nanocrystal ordering, low nanocrystal size distribution, and nanocrystal crystallinity.