Growth Mechanism Of Nanocrystals Research Articles

Low temperature studies of the photoluminescence from colloidal CdSe nanocrystals prepared by the hot injection method in liquid paraffin

This article describes the results from low temperature studies of the photoluminescence (PL) from colloidal CdSe nanocrystals prepared by the hot injection method in liquid paraffin. The synthesized nanocrystals were taken from the reaction mixture at different time intervals, their sizes were all within the quantum confinement range, and showed the typical size-dependent UV–vis absorption, PL and Raman spectral characteristics. The PL spectra and their temperature dependences were measured from room temperature (295K) down to temperatures as low as 15K, demonstrating that the exciton PL transitions followed the Varshni equation. The thermal quenching activation energy below 100K was relatively low, while the luminescence was strongly quenched as the temperature was increased above 100K, probably pointing at change in the degree of carrier localization. The smallest (2.6nm in size) CdSe nanocrystals isolated in the beginning of the nanocrystal growth had a relatively symmetric size distribution and showed a single exciton emission band, while the samples isolated at later stages of the growth process showed negatively skewed asymmetric size distribution, indicating that the growth mechanism of nanocrystals could be explained by the model of Ostwald ripening. The obtained results are expected to be helpful for better understanding of the PL properties of quantum dots and their formation and growth in non-polar organic solvents.

Nucleation of silicon nanocrystals in a remote plasma without subsequent coagulation

We report on the growth mechanism of spherical silicon nanocrystals in a remote expanding Ar plasma using a time-modulated SiH4 gas injection in the microsecond time range. Under identical time-modulation parameters, we varied the local density of the SiH4 gas by changing its stagnation pressure on the injection line over the range of 0.1–2.0 bar. We observed that nanocrystals were synthesized in a size range from ∼2 to ∼50 nm with monocrystalline morphology. Smaller nanocrystals (∼2–6 nm) with narrower size distributions and with higher number densities were synthesized with an increase of the SiH4 gas-phase density. We related this observation to the rapid depletion of the number density of the molecules, ions, and radicals in the plasma during nanocrystal growth, which can primarily occur via nucleation with no significant subsequent coagulation. In addition, in our remote plasma environment, rapid cooling of the gas in the particle growth zone from ∼1500 to ∼400 K significantly reduces the coalescence rate of the nanoparticles, which makes the coagulation process highly unlikely. Our observations on nanocrystal formation via nucleation indicated that subsequent coagulation for further growth is not always an essential step on nanoparticle formation.

Crystallization of ZrTiO4 Nanocrystals in Lithium-Alumino-Silicate Glass Ceramics: Anomalous Small-Angle X-ray Scattering Investigation

We report on anomalous small-angle X-ray scattering (ASAXS) investigations of the formation and structure of nanosized ZrTiO4 crystals in lithium-alumino-silicate (LAS) glass during heat treatment at 750 °C for different periods of time. For a sample annealed for 30 min, ASAXS measurements near the X-ray absorption edge of Ti and Zr reveal the formation of particles (ZrTiO4) surrounded by a shell-like region. The sample annealed for 240 min shows the formation of two different types of particles (spherical core–shell: ZrTiO4 and large spherical particles: LiAlSi2O6). Additionally, ASAXS results allow quantitative determination of the average composition and volume fraction of the nanocrystals, the surrounding region (shell), and the remaining glass matrix. Data analysis reveals the formation of an alumina enriched region around ZrTiO4. This alumina enriched layer makes the glass network rigid and hinders further growth of ZrTiO4 nanoparticles. For a prolonged annealing time (240 min), a new phase (LiAlSi2O6) is formed additionally, and the (Al/Si) ratio in the shell surrounding the ZrTiO4 crystals decreases. Moreover, the unannealed sample also shows the formation of a Ti enriched phase during cooling of the samples. The obtained quantitative information helps to understand the crystallization and growth mechanisms of ZrTiO4 nanocrystals in LAS glass ceramics.

ChemInform Abstract: Nanocrystals from Solutions: Catalysts

AbstractReview: 50 refs.

Growth mechanisms of MgO nanocrystals via a sol-gel synthesis using different complexing agents

In the preparation of nanostructured materials, it is important to optimize synthesis parameters in order to obtain the desired material. This work investigates the role of complexing agents, oxalic acid and tartaric acid, in the production of MgO nanocrystals. Results from simultaneous thermogravimetric analysis (STA) show that the two different synthesis routes yield precursors with different thermal profiles. It is found that the thermal profiles of the precursors can reveal the effects of crystal growth during thermal annealing. X-ray diffraction confirms that the final products are pure, single phase and of cubic shape. It is also found that complexing agents can affect the rate of crystal growth. The structures of the oxalic acid and tartaric acid as well as the complexation sites play very important roles in the formation of the nanocrystals. The complexing agents influence the rate of growth which affects the final crystallite size of the materials. Surprisingly, it is also found that oxalic acid and tartaric acid act as surfactants inhibiting crystal growth even at a high temperature of 950°C and a long annealing time of 36 h. The crystallite formation routes are proposed to be via linear and branched polymer networks due to the different structures of the complexing agents.

ChemInform Abstract: Observation of Growth of Metal Nanoparticles

AbstractReview: 110 refs.

Nanocrystals from solutions: catalysts

The ultimate goal of nanoscale science and technology is to manipulate single atoms, assemble atoms in a controllable way and design nanostructured materials with the desired physical and chemical properties (e.g. catalytic properties). In order to achieve this goal, the nucleation and growth mechanism of nanocrystals (NCs) as well as the relationship between the macroscopic properties and microscopic structures of nanocrystals should be fully understood. In this tutorial review, we firstly summarize the latest research developments and experimental methods to exploit the nucleation and growth process of nanocrystals, then discuss the essence of nanocrystal catalysis, and finally present our personal perspectives on the opportunities and challenges of this promising field.

Synergistically controlled nano-templated growth of tunable gold bud-to-blossom nanostructures: a pragmatic growth mechanism

Nano-templating method with a pragmatic nanocrystal growth mechanism that explains the production of variable-size, intrinsically monodispersed, tunable bud-to-blossom plasmonic (580–1300 nm) gold nano-flowers.

Growth mechanism and optical property of ZnO nanocrystals synthesized by corrosion of Cu–Zn alloy

ZnO nanostructures are usually employed to modify the electrode for sensor construction in terms of its biocompatibility and high electron mobility. A simple corrosion of Cu–Zn alloy method for the production of ZnO nanostructures, which was used as the electrode-modified material, is presented in this paper. The growth morphology of ZnO on the Cu electrode could be controlled by the sputtering power of zinc. The possible growth mechanism of ZnO nanocrystals formation by the method has been tried to discuss as well as the optical properties have been demonstrated. Given the simplicity of the procedure and without the use of any organic reagent, the method described here is likely to be of interest in biosystem applications.

Shape-controllable synthesis of hydrophilic NaLuF4:Yb,Er nanocrystals by a surfactant-assistant two-phase system

Water-soluble upconversion nanoparticles (UCNPs) were prepared by a one-pot procedure in a two-phase reacting system. Four kinds of surfactants were tested in the synthesis process as capping agent to tune size and morphology of nanocrystals. Nanoparticles (approximately 70 nm) and rods (400 nm and 2.5 μm) were synthesized, respectively. Then, Fourier transform infrared spectroscopy analysis confirmed the successful linking between UCNP surface and surfactant. Ionic liquids (ILs) and surfactants participated in synthesis process together, competing with each other to cap on UCNPs. ILs still led the competition of capping, while surfactants worked as cooperative assistants to develop functional surface. Further characterizations such as high-resolution transmission electron microscopy and X-ray diffraction indicated the changes in crystallization and phase transformation under the influence of surfactants. In addition, the growth mechanism of nanocrystals and upconversion fluorescence luminance was also investigated in detail. At last, the cytotoxicity of UCNPs was evaluated, which highly suggest that these surface-functionalized UCNPs are promising candidates for biomedical engineering.

ChemInform Abstract: Conversion Reactions of Cadmium Chalcogenide Nanocrystal Precursors.

AbstractReview: [259 refs.]

Conversion Reactions of Cadmium Chalcogenide Nanocrystal Precursors

We survey the chemical reactions between common precursors used in the synthesis of metal chalcogenide nanocrystals and outline how they affect the mechanism and kinetics of nanocrystal growth. We emphasize syntheses of cadmium selenide and cadmium sulfide where a variety of metal and chalcogenide precursors have been explored, though this is supplemented by studies of zinc and lead chalcogenide formation where appropriate. This review is organized into three sections, highlighting kinetics, metal precursors, and chalcogenide precursors, respectively. Section I is dedicated to the role of precursor conversion as a source of monomers and the importance of the supply rate on nanocrystal nucleation and growth. Section II describes the structure and reactivity of cadmium carboxylates, phosphonates, and chalcogenolates. Section III describes the reaction chemistry of commonly employed chalcogenide precursors and the mechanisms by which they react with metal precursors.

Study of the wurtzite zinc-blende mixed-structured GaAs nanocrystals grown on Si (111) substrates

The structure and growth mechanism of GaAs nanocrystals grown on Si (111) substrates by using the molecular beam epitaxy method have been studied using transmission electron microscopy. The isolated nanocrystals had hexangular shapes, with aspect ratio ∼1 and high symmetry. The crystal structure of the GaAs nanocrystals contains a mixture of a stable state of zinc-blende and a metastable state of wurtzite. A number of thin wurtzite layers parallel to the Si (111) plane are introduced into the zinc-blende GaAs nanocrystals as stacking faults. Formation of partial dislocations near the GaAs/Si interface and the small difference in the Gibbs free energy between the zinc-blende and wurtzite structures could cause formation of wurtzite as stacking faults in the zinc-blende structure

Observation of growth of metal nanoparticles

An understanding of nanocrystal growth mechanisms is of significant importance for the design of novel materials. The development of liquid cells for transmission electron microscopy (TEM) has enabled direct observation of nanoparticle growth in a liquid phase. By tracking single particle growth trajectories with high spatial resolution, novel growth mechanisms have been revealed. In recent years, there has been an increasing interest in liquid cell TEM and its applications include real time imaging of nanoparticles, biological materials, liquids, and so on. This paper reviews the development of liquid cell TEM and the progress made in using such a wonderful tool to study the growth of nanoparticles (mostly metal nanoparticles). Achievements in the understanding of coalescence, shape control mechanisms, surfactant effects, etc. are highlighted. Other studies relevant to metal precipitation in liquids, such as electrochemical deposition, nanoparticle motion and electron beam effects, are also included. At the end, our perspectives on future challenges and opportunities in liquid cell TEM are provided.

Shape Control from Thermodynamic Growth Conditions: The Case of hcp Ruthenium Hourglass Nanocrystals

Recent successes in forming different shaped face centered cubic (fcc) metal nanostructures has enabled a greater understanding of nanocrystal growth mechanisms. Here we extend this understanding to the synthesis of hexagonally close packed (hcp) metal nanostructures, to form uniquely faceted ruthenium nanocrystals with a well-defined hourglass shape. The hourglass nanocrystals are formed in a three-step thermodynamic growth process with dodecylamine as the organic stabilizer. The hourglass nanocrystals are then shown to readily self-assemble to form a new type of nanocrystal superlattice.

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.

Crystal growth behaviour in Au-ZnO nanocomposite under different annealing environments and photoswitchability

The growth of gold nanoparticles and ZnO nanorods in atom beam co-sputtered Au-ZnO nanocomposite (NC) system by annealing at two different ambient conditions is demonstrated in this work. Annealing in a furnace at 600 °C (air environment) confirmed the formation of ZnO nanorods surrounded with Au nanoparticles. In-situ annealing inside a transmission electron microscope (TEM) led to the formation of gold nanocrystals with different polygonal shapes. TEM micrographs were obtained in real time at intermediate temperatures of 300 °C, 420 °C, and 600 °C under vacuum. The growth mechanisms of Au nanocrystals and ZnO nanorods are discussed in the framework of Au-Zn eutectic and Zn-melting temperatures in vacuum and air, respectively. Current-voltage responses of Au-ZnO NC nanorods in dark as well as under light illumination have been investigated and photoswitching in Au-ZnO NC system is reported. The photoswitching has been discussed in terms of Au-ZnO band-diagram.

Direct in situ determination of the mechanisms controlling nanoparticle nucleation and growth.

Although nanocrystal morphology is controllable using conventional colloidal synthesis, multiple characterization techniques are typically needed to determine key properties like the nucleation rate, induction time, growth rate, and the resulting morphology. Recently, researchers have demonstrated growth of nanocrystals by in situ electron beam reduction, offering direct observations of single nanocrystals and eliminating the need for multiple characterization techniques; however, they found nanocrystal morphologies consistent with two different growth mechanisms for the same electron beam parameters. Here we show that the electron beam current plays a role analogous to the concentration of reducing agent in conventional synthesis, by controlling the growth mechanism and final morphology of silver nanocrystals grown via in situ electron beam reduction. We demonstrate that low beam currents encourage reaction limited growth that yield nanocrystals with faceted structures, while higher beam currents encourage diffusion limited growth that yield spherical nanocrystals. By isolating these two growth regimes, we demonstrate a new level of control over nanocrystal morphology, regulated by the fundamental growth mechanism. We find that the induction threshold dose for nucleation is independent of the beam current, pixel dwell time, and magnification being used. Our results indicate that in situ electron microscopy data can be interpreted by classical models and that systematic dose experiments should be performed for all future in situ liquid studies to confirm the exact mechanisms underlying observations of nucleation and growth.

Growth Mechanism of Hydrophilic CdTe Nanocrystals with Green to Dark Red Emission

A modified method has been developed to fabricate high-quality CdTe nanocrystals (NCs) with a tunable photoluminescence (PL) range between green and dark red using thioglycolic acid (TGA). Namely, CdTe NCs with dark red emission (PL peak wavelength of 672 nm) and a PL quantum yield (QY) of 46% were created by controlling the NC growth kinetics. The molar ratio of TGA/Cd2+ in solutions and the concentration of starting materials play important roles in creating the NCs with a large size for long PL peak wavelength and decreased surface defects for a target of a high PLQY. Lifetime measurement indicated that the average lifetime of the NCs decreased at the beginning of a reflux and then converted to increase with time. This means that long reflux time resulted in a CdS-like shell formed on CdTe NCs. The X-ray diffraction analysis of the NCs illustrated that CdS-like layer were created on CdTe NCs, in which S(2-) ions generated by the decomposition of TGA during a long reflux period.

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.