Growth Of Semiconductor Nanocrystals Research Articles

Kinetic analysis of the growth of semiconductor nanocrystals from the peak wavelength of photoluminescence

Understanding the rate processes controlling the growth of semiconductor nanocrystals in liquid solutions is of great importance in tailoring the sizes of semiconductor nanocrystals for the applications in optoelectronics, bioimaging and biosensing. In this work, we establish a simple relationship between the photoluminescence (PL) peak wavelength and the growth time of semiconductor nanocrystals under the condition that the contribution of electrostatic interaction to the quantum confinement is negligible. Using this relationship and the data available in the literature for CdSe and CdSe/ZnS nanocrystals, we demonstrate the feasibility of using the PL peak wavelength to analyze the growth behavior of the CdSe and CdSe/ZnS nanocrystals in liquid solutions. The results reveal that the diffusion of monomers in the liquid solution is the dominant rate process for the growth of CdSe/ZnS nanocrystals, and the activation energy for the growth of CdSe nanocrystals in the liquid solution is ∼9 kJ/mol. The feasibility to use this approach in the analysis of the thickness growth of core–shell nanocrystals with and without mechanical stress is also discussed. Such an approach opens a new avenue to in-situ monitor/examine the growth of semiconductor nanocrystals in liquid solutions.

Direct Observations of Twin Formation Dynamics in Binary Semiconductors.

With the increased demand for controlled deterministic growth of III-V semiconductors at the nanoscale, the impact and interest of understanding defect formation and crystal structure switching becomes increasingly important. Vapor-liquid-solid (VLS) growth of semiconductor nanocrystals is an important mechanism for controlling and studying the formation of individual crystal layers and stacking defects. Using in situ studies, combining atomic resolution of transmission electron microscopy and controlled VLS crystal growth using metal organic chemical vapor deposition, we investigate the simplest achievable change in atomic layer stacking-single twinned layers formed in GaAs. Using Au-assisted GaAs nanowires of various diameters, we study the formation of individual layers with atomic resolution to reveal the growth difference in forming a twin defect. We determine that the formation of a twinned layer occurs significantly more slowly than that of a normal crystal layer. To understand this, we conduct thermodynamic modeling and determine that the propagation of a twin is limited by the energy cost of forming the twin interface. Finally, we determine that the slower propagation of twinned layers increases the probability of additional layers nucleating, such that multiple layers grow simultaneously. This observation challenges the current understanding that continuous uniform epitaxial growth, especially in the case of liquid-metal assisted nanowires, proceeds one single layer at a time and that its progression is limited by the nucleation rate.

The Future of Colloidal Semiconductor Magic-Size Clusters.

Atomically defined, zero-dimensional magic-size clusters play pivotal roles in the nucleation and growth of semiconductor nanocrystals. Thus, they provide new opportunities to understand and to control nucleation and growth reactions beyond classical nucleation theory and to employ these reactions in the colloidal synthesis of increasingly complex and anisotropic nanomaterials with atomic level monodispersity. Both challenges require reliable determination of the exact structure and size of these ultrasmall and metastable nanoclusters. In this Perspective, we review and discuss the current challenges in analytics of magic-size clusters, in elucidating their formation mechanism, and in using them as next-generation reagents in colloidal chemistry.

ChemInform Abstract: Doped or Not Doped: Ionic Impurities for Influencing the Phase and Growth of Semiconductor Nanocrystals

AbstractReview: 140 refs.

Metal Oleate Induced Etching and Growth of Semiconductor Nanocrystals, Nanorods, and Their Heterostructures.

Unexpected etching of nanocrystals, nanorods, and their heterostructures by one of the most commonly used metal precursors, metal oleates, is reported. Zn oleate is shown to etch CdS nanorods anisotropically, where the length decreases without a significant change in the diameter. Sodium oleate enhances the etch rate, whereas oleic acid alone does not cause etching, indicating the importance of the countercation on the rate of oleate induced etching. Subsequent addition of Se precursors to the partially etched nanorods in Zn oleate solution can lead to epitaxial growth of CdSe particles rather than the expected ZnSe growth, despite an excess amount of Zn precursors being present. The composition of this epitaxial growth can be varied from CdSe to ZnSe, depending on the amount of excess oleic acid or the reaction temperature. Similar tuning of composition can be observed when starting with collinear CdSe/CdS/CdSe rod/rod/rod heterostructures and spherical CdS (or CdSe/CdS core/shell) nanocrystals. Conversion of collinear rod/rod/rod structures to barbells and interesting rod growth from nearly spherical particles among other structures can also result due to the initial etching effect of metal oleates. These observations have important implications on our understanding of nanocrystal heterostructure synthesis and open up new routes to varying the composition and morphology of these materials.

Doped or Not Doped: Ionic Impurities for Influencing the Phase and Growth of Semiconductor Nanocrystals

Dimension, phase, and shape tunable architectures of nanostructures are of interest due to morphology-dependent modulation of the properties of materials. However, the manipulation of specific structures of nanomaterials, in order to achieve unique physical and chemical properties by modulating the synthesis parameters, remains challenging. Wet chemistry has proven to be quite efficient for such shape- and size-controlled synthesis. Recent architectural progress with different nanostructures suggests that dopants or impurities present in the reaction system can affect the crystal growth and tune the shapes of crystals in ways that go beyond the classical approach where the nature of the precursors, ligands, and reaction conditions dependent facets controlling crystal growth mostly determined the shape. These foreign atoms or ions may or may not be retained in the crystal lattice and may or may not become incorporated into the crystal. This is a new era of current research on the architecture of nanostruct...

The Role of Passivants on the Stoichiometry of CdSe and GaAs Nanocrystals

Passivating molecules are introduced during the growth of semiconductor nanocrystals (NCs) to arrest their growth leading to nanoparticles of a desired size. In order to carry out this function, they bind to the surface atoms of the NC, removing any band gap states in the process. While they have been observed to play a role in modifying various other physical properties of materials, in this work we consider examples of two well-known binary semiconductors, GaAs and CdSe, and find within our calculations that the passivating ligands can play an important role in stabilizing highly nonstoichiometric semiconductors. This has important implications in the observed optical and electronic properties of binary semiconductor NCs where usually no suitable passivant exists for the anion.

A Controlled Growth Process To Design Relatively Larger Size Semiconductor Nanocrystals

The growth of semiconductor nanocrystals in solution is mostly governed by the kinetic and thermal modes of control of the reaction process. In most of the cases, the size of the particles is limited within 5–6 nm, and further annealing mostly defocuses the particles size distribution. But, herein, we report a self-driven growth protocol which supplies the monomer continuously to significant extent and delays the thermal diffusion-controlled ripening process. This has been achieved by choosing appropriate sulfur precursor in the synthesis of metal sulfide nanocrystals which controls the sulfide ion concentration in the reaction medium via establishing an appropriate chemical equilibrium. As a consequence, the monomer concentration retains above their critical limit and it delays the ripening process. Finally, the nanocrystals can grow even larger than 10 nm, which are difficult to obtain from different established synthetic approaches. This has been observed for several semiconductor nanocrystals such as ...

Controlled synthesis of monodisperse nanocrystals by a two-phase approach without the separation of nucleation and growth processes

The synthesis of monodisperse nanocrystals is an important topic in the field of nanomaterials not only for practical applications, but also for scientific interest in fundamental research. In this feature article, we mainly focus on synthesis of monodisperse nanocrystals by a two-phase approach without the separation of nucleation and growth processes, and report some progress made recently in the observation and understanding of nucleation and growth of semiconductor nanocrystals. Firstly, a novel two-phase approach to monodisperse nanocrystals, which is different from the well-established synthesis models, is discussed. We demonstrate that the two-phase approach has a quite lengthy nucleation process, and can be applied to the synthesis of many kinds of binary monodisperse nanocrystals. Then, we provide a summary of recent research progress in the observation and understanding of nucleation and growth of semiconductor nanocrystals in one-phase and two-phase systems, and compare the nucleation and growth mechanisms of the two kinds of reaction systems. Although the lengthy nucleation process of the two-phase approach enables researchers to capture some useful information about the nucleation and growth processes, and the nucleation and growth processes have been studied by optical absorption spectra and transmission electron microscopy, there is little or no experimental data available on the atomic configurations of critical nuclei. Finally, the current difficulties and future challenges in synthesis and structural characterization of critical nuclei are presented.

A facile and green preparation of high-quality CdTe semiconductor nanocrystals at roomtemperature

One chemical reagent, hydrazine hydrate, was discovered to accelerate the growth ofsemiconductor nanocrystals (cadmium telluride) instead of additional energy, which wasapplied to the synthesis of high-quality CdTe nanocrystals at room temperature andambient conditions within several hours. Under this mild condition the mercapto stabilizerswere not destroyed, and they guaranteed CdTe nanocrystal particle sizes with narrowand uniform distribution over the largest possible range. The CdTe nanocrystals(photoluminescence emission range of 530–660 nm) synthesized in this way hadvery good spectral properties; for instance, they showed high photoluminescencequantum yield of up to 60%. Furthermore, we have succeeded in detecting the livingBorrelia burgdorferi of Lyme disease by its photoluminescence image using CdTenanocrystals.

Synthesis of nanocrystals via microreaction with temperature gradient: towards separation of nucleation and growth

By utilizing the symmetrical temperature distribution in a tube furnace chamber, a capillary microreactor was designed with the microchannel passing two well-controlled, stable temperatures in steep temperature gradients. The two-temperature microreator, first developed and implemented by this research team, provides an opportunity to separate the nucleation and growth of semiconductor nanocrystals, leading to better control of nucleation and growth kinetics. For the synthesis of CdSe nanocrystals as a model system, we demonstrated the improved size uniformity achieved by the two-temperature approach, confirming the success of the use of high temperature to burst nucleation and low temperature to promote growth.

Control of photoluminescence properties of CdSe nanocrystals in growth.

The photoluminescence (PL) quantum yield (QY) of CdSe nanocrystals during their growth under a given set of initial conditions increases monotonically to a certain maximum value and then decreases gradually. Such a maximum is denoted as a PL "bright point", which does not always overlap with the minimum point of the PL peak width for the same reaction. The experimental results suggest that the existence of the PL bright point is a general phenomenon during the growth of semiconductor nanocrystals and likely is a signature of an optimal surface structure/reconstruction of the nanocrystals grown under a given set of initial conditions. The position of the bright point, the highest PL QY, the types of the bright points (sharp or flat), the sharpness of the PL peak, etc., were all strongly dependent on the initial Cd:Se ratio of the precursors in the solution. A large excess of the selenium precursor, with 5-10 times more selenium precursor than the amount of the cadmium precursor, was found necessary to achieve a high PL QY value and a narrow emission profile. The existence of the PL bright point and the sensitive temporal variation of the PL QY during the growth of semiconductor nanocrystals can explain the unpredictable nature and poor reproducibility of the PL properties of the as-prepared semiconductor nanocrystals observed previously. Furthermore, the knowledge gained in this study enabled us to reproducibly synthesize highly luminescent CdSe nanocrystals through a relatively simple and safe synthetic scheme. In a traditionally weak emission window for CdSe nanocrystals, the orange-red optical window, the PL QY of the as-prepared CdSe nanocrystals reached as high as 85% at room temperature, and the full width at half-maximum of the corresponding PL peak was as narrow as 23 nm, about 65-80 meV depending on the emitting position. The PL properties of the as-prepared CdSe nanocrystals are stable upon aging for at least several months. These as-prepared nanocrystals represent a series of best emitters that are highly efficient, highly pure in emission color, stable, and continuously tunable by simply varying the size of the nanocrystals.

Formation and growth of semiconductor nanocrystals in phosphate glass matrix

Phase decomposition of the oversaturated solid solution of semiconductors in glass and the evolution of the ensembles of nanocrystals formed in the media are studied with optical absorption spectroscopy and modeled theoretically. Observed in the experiments difference of temporal behavior of the mean size of the nanocrystals for different levels of the oversaturation coincides with the results of numerical calculations within the frames of the model developed.