Nucleation Of Dots Research Articles

Room-temperature solution-phase epitaxial nucleation of PbS quantum dots on rutile TiO2 (100).

Owing to its simplicity and versatility, the successive ionic layer adsorption and reaction (SILAR) method is increasingly being employed to develop low-cost hetero-nanostructured sensitized oxide systems for solar energy conversion, such as solar cells and solar fuels schemes. Understanding the nature of the SILAR quantum dot (QD) nucleation and growth on an insulating oxide is then critical as it will determine the QD density and spatial distribution, as well as the optoelectronic properties of the QD/oxide interfaces (e.g. QD bandgap onset). Here, we demonstrate epitaxial nucleation of lead sulfide (PbS) QDs onto a planar rutile titanium dioxide (100) surface employing the SILAR method. The QDs nucleated by SILAR are crystalline structures characterized by a truncated pyramidal shape, with nucleation occurring preferentially along the rutile (010) and (001) crystal orientations. The PbS QD size distribution is constrained by lattice mismatch causing strain in the lead sulfide. These results highlight the potential of SILAR for the facile growth of high-quality epitaxial nanostructures in liquid phase, under ambient conditions and at room temperature.

Controlling the nucleation process of InP/ZnS quantum dots using zeolite as a nucleation site

A novel synthesis strategy to adjust the emission wavelength of InP/ZnS quantum dots, using zeolite as a quantum dot nucleation template.

Temperature Activated Dimensionality Crossover in the Nucleation of Quantum Dots by Droplet Epitaxy on GaAs(111)A Vicinal Substrates

A temperature activated crossover between two nucleation regimes is observed in the behavior of Ga droplet nucleation on vicinal GaAs(111)A substrates with a miscut of 2° towards (bar{1}bar{1}2). At low temperature (<400 °C) the droplet density dependence on temperature and flux is compatible with droplet nucleation by two-dimensional diffusion. Increasing the temperature, a different regime is observed, whose scaling behavior is compatible with a reduction of the dimensionality of the nucleation regime from two to one dimension. We attribute such behavior to a presence of finite width terraces and a sizeable Ehrlich-Schwöbel barrier at the terrace edge, which hinders adatom diffusion in the direction perpendicular to the steps.

Environmentally friendly approach to the synthesis of monodisperse and bright blue emitting Cd0.15Zn0.85S quantum dots

Nucleation and growth of quantum dots (QDs) in solution are mainly controlled by the kinetic and thermal modes of the reaction process. By influencing any of them, the properties of the final nanocrystals can be tuned. The influences of temperature and chemical nature of starting materials on nucleation and, as a consequence, on the structure and optical properties of Cd0.15Zn0.85S QDs have been systematically investigated by one-pot and hot-injection methods. All reactions were performed in organic disperse medium using N,N′-disubstituted and N,N′,N'-trisubstituted thioureas (TU) as a new sources of sulfur. In addition to environmental friendliness, each of them has a number of unique chemical properties. The effect of the substituents’ nature in thioureas on the morphology, size and optical properties of synthesized QDs has been studied. The strong correlation between the metal ratios taken to the reaction and elemental analysis (EDS) results for all obtained Cd0.15Zn0.85S QDs was found. However, prepared nanomaterials have different size (2.9–4.6 nm) and morphology. The optical features have also changed under the various thermal conditions. Especially, the changes in the photoluminescence spectra of QDs synthesized with different thioureas are noticeable. Highly photoluminescent (photoluminescence quantum yield PL QY up to 67%), morphologically and structurally homogeneous blue-emitting Cd0.15Zn0.85S QDs were obtained.

Four Types of CdTe Magic-Size Clusters from One Prenucleation Stage Sample at Room Temperature.

Four types of colloidal semiconductor CdTe magic-size clusters (MSCs), each of which is in a single-ensemble form, have been obtained at room temperature from a single induction period (IP) sample in dispersion. The induction period is the prenucleation stage that occurs prior to nucleation and growth of colloidal quantum dots (QDs). Three types display sharp optical absorption peaking at either 371, 417, or 448 nm, and the fourth type exhibits a sharp absorption doublet with peaks at 350 and 371 nm. These MSCs are respectively denoted as sMSC-371, sMSC-417, sMSC-448, and dMSC-371. We show that the evolution of the various MSCs is affected by the nature of their dispersions. We hypothesize that the evolution of MSCs involves their precursor compounds (PCs), which are transparent in optical absorption. The present study explores new avenues for the exclusive synthesis of four types of CdTe MSCs (with each in a single-ensemble form) and provides an improved understanding for their formation.

Efficient White LEDs Using Liquid-state Magic-sized CdSe Quantum Dots

Magic clusters have attracted significant interest to explore the dynamics of quantum dot (QD) nucleation and growth. At the same time, CdSe magic-sized QDs reveal broadband emission in the visible wavelength region, which advantageously offer simple integration of a single-type of nanomaterial and high color rendering ability for white light-emitting diodes (LEDs). Here, we optimized the quantum yield of magic-sized CdSe QDs up to 22% via controlling the synthesis parameters without any shelling or post-treatment process and integrated them in liquid-state on blue LED to prevent the efficiency drop due to host-material effect. The fabricated white LEDs showed color-rendering index and luminous efficiency up to 89 and 11.7 lm/W, respectively.

Precursor Self-Assembly Identified as a General Pathway for Colloidal Semiconductor Magic-Size Clusters.

Little is known about the formation pathway of colloidal semiconductor magic‐size clusters (MSCs). Here, the synthesis of the first single‐ensemble ZnSe MSCs, which exhibit a sharp optical absorption singlet peaking at 299 nm, is reported; their formation is independent of Zn and Se precursors used. It is proposed that the formation of MSCs starts with precursor self‐assembly followed by Zn and Se covalent bond formation to result in immediate precursors (IPs) which can transform into the MSCs. It is demonstrated that the IPs in cyclohexane appear transparent in optical absorption, and become visible as MSCs exhibiting one sharp optical absorption peak when a primary amine is added at room temperature. It is shown that when the preparation of the IP is controlled to be within the induction period, which occurs prior to nucleation and growth of conventional quantum dots (QDs), the resulting MSCs can be produced without the complication of the simultaneous coproduction of conventional QDs. The present study reveals the existence of precursor self‐assembly which leads to the formation of colloidal semiconductor MSCs and provides insights into a multistep nucleation process in cluster science.

Carbon Quantum Dots: Elegant Surface of CoNi Alloys toward Efficient Magnetorheological Performances Realized with Carbon Quantum Dots (Adv. Mater. Interfaces 15/2018)

A series of CoNi based hierarchical microspheres with different surface morphologies, including litchi-like, seed of plane tree-like and waxberry-like shapes, are obtained via surface nucleation of carbon quantum dots (CQDs). Taking both the synergistic benefits of paramagnetic CoNi microspheres and surface architecture, the resulting CoNi microspheres exhibit enhanced interparticle friction for enhancing their MR effects compared with smooth surface microspheres. This work is reported by Wen Ling Zhang, Jingquan Liu and co-workers in article number 1800164.

Elegant Surface of CoNi Alloys toward Efficient Magnetorheological Performances Realized with Carbon Quantum Dots

AbstractDesigning and fabricating a rational surface architecture is one of the critical factors to achieve desirable physical and chemical properties and still remains an immense challenge. Herein, a series of CoNi‐based hierarchical microspheres with different surface morphologies, including litchi‐like, seed‐of‐plane‐tree‐like, and waxberry‐like shapes, are obtained via surface nucleation of carbon quantum dots (CQDs) with different concentrations in liquid polyol. Microstructure investigations suggest that all the CoNi microspheres are uniformly with enhanced surface roughness with the increasing concentration of CQDs. This study provides insight into the surface dominant magnetorheological (MR) behaviors of these CoNi‐based suspensions. Taking both the synergistic benefits of paramagnetic CoNi microspheres and surface architecture, the resulting CoNi microspheres exhibit enhanced interparticle friction for enhancing their MR effects compared with smooth surface microspheres, opening up the possible mechanism for revealing the MR effect. Moreover, this facile strategy can be extended as an effective path to achieve high‐performance MR behaviors for engineering applications.

In-situ laser nano-patterning for ordered InAs/GaAs(001) quantum dot growth

A study of in-situ laser interference nano-patterning on InGaAs wetting layers was carried out during InAs/GaAs (001) quantum dot molecular beam epitaxy growth. Periodic nano-islands with heights of a few atomic layers were obtained via four-beam laser interference irradiation on the InGaAs wetting layer at an InAs coverage of 0.9 monolayer. The quantum dots nucleated preferentially at edges of nano-islands upon subsequent deposition of InAs on the patterned surface. When the nano-islands are sufficiently small, the patterned substrate could be spontaneously re-flattened and an ordered quantum dot array could be produced on the smooth surface. This letter discusses the mechanisms of nano-patterning and ordered quantum dot nucleation in detail. This study provides a potential technique leading to site-controlled, high-quality quantum dot fabrication.

Uncovering the reaction mechanism initiating the nucleation of lead sulfide quantum dots in a hines synthesis

Using computational methods we discover a favorable synthesis pathway towards better control and understanding of quantum dot nucleation for photovoltaics.

Selective antimony reduction initiating the nucleation and growth of InSb quantum dots.

Indium antimonide (InSb) quantum dots (QDs) have unique and interesting photophysical properties, but widespread experimentation with InSb QDs is lacking due to the difficulty in synthesizing this material. The key experimental challenge in fabricating InSb QDs is preparing a suitable Sb-precursor in the correct oxidation state that reacts with the In-precursor in a controllable manner. Here, we review and discuss the synthetic strategies for making colloidal InSb QDs and present a new reaction scheme yielding small (∼1 nm diameter) InSb QDs. This was accomplished by employing Sb(NMe2)3 as the antimony precursor and by screening different reducing agents that can selectively reduce it to stibine in situ. The released SbH3, subsequently, reacts with In carboxylate to form small InSb clusters. The absorption features are moderately tunable (from 400 nm to 660 nm) by the amount and rate of reductant addition as well as the temperature of injection and subsequent annealing. Optical properties were probed with transient absorption spectroscopy and show complex time and spectral dependencies.

Effect of CdS Growth Time on the Optical Properties of One-Pot Preparation of CdS-Ag2S Binary Compounds

CdS-Ag2S binary nanoparticles were synthesized using a facile one-pot microwave irradiation method. The effect of initial nucleation of CdS quantum dots (QDs) using 3 min, 5 min, and 7 min of microwave irradiation on the optical properties of the final compound was studied. The composition and crystal structure of the compounds were verified using energy dispersive x-ray spectroscopy and x-ray diffraction. They revealed that existence of Ag and Cd elements with an atomic ratio of 0.19 crystalizes in the form of monoclinic Ag2S and hexagonal CdS. Scanning electron microscope images showed a spherical morphology of the resultant compound, and transmission electron microscope images showed the formation of fine particles of CdS-Ag2S composites with an average size of 5–7 nm and 10–14 nm for CdS and Ag2S, respectively. Photoluminescence spectroscopy revealed that the initial growth time of CdS has a crucial effect on the emission of binary compounds such that for 3 min and 5 min of irradiation of CdS solution, the binary compound obtains strong red and considerable near-IR emission (850 nm), but for longer time, it rapidly quenches. The results indicate that the strong red emission can be tuned from 600 nm up to 700 nm with prolonging nucleation time of CdS. This study also emphasized that the origin of red emission strongly depends on the size and defects created in the CdS QDs.

Strain-Engineered Arrays of InAs Quantum Dots on GaAs(001): Epitaxial Growth and Modeling

Working under critical conditions for dot nucleation in a Molecular Beam Epitaxy chamber, we were able to drive the formation of InAs dot chains to precise locations in multilayered samples grown on a rippled GaAs(001) surface. We discussed the role of the elastic field and the surface curvature in determining the dot arrangement at each stacked layer, proving a new mechanism of self-organization of the dots. In particular, we succeeded in controlling the interplay between elastic and curvature effects and we showed how a selection process is achievable in the chain formation. The role of the stress field was also studied by means of Finite Element Method simulations, and we gained a valuable understanding of the interlayer dot correlations for dot arrays with variable cap thicknesses. We proved the existence of an anisotropy in the cap formation of isolated dots, which appeared to be directly related to our peculiar growth geometry and experimental set-up.

Stranski–Krastanov mechanism of growth and the effect of misfit sign on quantum dots nucleation

The thermodynamics of the Stranski–Krastanov mode of epitaxial growth and the effect of the sign of the lattice misfit are discussed. The Stranski–Krastanov mode of growth represents a sequence of layer-by-layer or Frank-van der Merwe growth followed by the formation of three-dimensional (3D) islands or Volmer–Weber growth. The occurrence of both growth modes mentioned above is in compliance with the wettability criterion of Bauer. The positive wetting function required for the occurrence of the Volmer–Weber growth is originated by the vertical displacements of the atoms close to the edges of the two-dimensional (2D) islands as a result of the relaxation of the lattice misfit. The monolayer high islands become unstable against bilayer islands, bilayer islands in turn become unstable against trilayer islands, etc. beyond some critical islands sizes. Monolayer islands appear as necessary precursors of three-dimensional (3D) islands. The critical island size for mono-bilayer transformation increases steeply with decreasing lattice misfit and diverges at a critical value of the misfit. This value divides the regions of Frank-van der Merwe and Stranski–Krastanov modes in a phase diagram of coordinates wetting-misfit. The transformation of monolayer to multilayer islands takes place either by consecutive nucleation and growth of 2D islands (layer-by-layer transformation), or by nucleation and lateral (2D) growth of multilayer islands (multilayer 2D transformation). The former occurs in the case of “stiff” overlayer materials and mostly in compressed overlayers. The latter takes place in the case of “soft” materials like Pb and In, mostly in tensile overlayers. Tensile films show non-nucleation transformation compared with the nucleation-like behavior of compressed films.

Probing intermediates of the induction period prior to nucleation and growth of semiconductor quantum dots

Little is known about the induction period before the nucleation and growth of colloidal semiconductor quantum dots. Here, we introduce an approach that allows us to probe intermediates present in the induction period. We show that this induction period itself exhibits distinct stages with the evolution of the intermediates, first without and then with the formation of covalent bonds between metal cations and chalcogenide anions. The intermediates are optically invisible in toluene, while the covalent-bonded intermediates become visible as magic-size clusters when a primary amine is added. Such evolution of magic-size clusters provides indirect but compelling evidence for the presence of the intermediates in the induction period and supports the multi-step nucleation model. Our study reveals that magic-size clusters could be readily engineered in a single-size form, and suggests that the existence of the intermediates during the growth of conventional quantum dots results in low product yield.

InAs-GaAs(001)表面再構成における組成ゆらぎと量子ドットの非古典的核形成( 半導体結晶成長機構のその場観察)

InAs-GaAs(001)表面再構成における組成ゆらぎと量子ドットの非古典的核形成( 半導体結晶成長機構のその場観察)

Understanding the nucleation and growth of colloidal quantum dots

Understanding the nucleation and growth of colloidal quantum dots

Role of Nd3+ ions on the nucleation and growth of PbS quantum dots (QDs) in silicate glasses

AbstractThe influence of Nd2O3 addition on the precipitation kinetics of lead chalcogenide (PbS) quantum dots (QDs) in silicate glasses was investigated. Energy dispersive X‐ray spectroscopy (EDS) indicated that the Nd3+ ions are preferentially located inside the PbS QDs rather than in the glass matrix. Changes in diameter (D) of PbS QDs exhibited smaller time dependencies (i.e., D≈t0.270‐0.286) than that predicted by the classical Lifshitz–Slyozov–Wagner (LSW) theory. This is due to the limited concentrations of Pb2+ and S2− ions and the large diffusion distance inside the glass matrix. In addition, extended X‐ray absorption fine structure (EXAFS) results indicated that the formation of PbS QDs was retarded due to the presence of Nd2O3 in the glasses, as the large NdOx polyhedra interrupt the diffusion of Pb2+ and S2− ions. We believe that these Nd3+ ions are primarily located in PbS QDs in the form of Nd–O clusters, and that the PbS QDs are built on top of these clusters.

The effect of differential temperatures on the latent heat in the nucleation of CdSe quantum dots**Project supported by the National Natural Science Fund of China (No. 11204046), the International Science and Technology Cooperation Project of China (No. 2014DFA00670) and the Guizhou Province International Science and Technology Cooperation Project of China (No. QKHG[2011]7001)

This work studied the effect of differential temperatures on the latent heat in the nucleation of CdSe quantum dots (QDs). The result showed that, by the formula of phase change, with increasing the reaction temperature, the latent heat in the nucleation of QDs reduced. CdSe QDs with the size-dispersion from 2.7 to 3.6 nm were synthesized via oleic acid-paraffin liquid system by controlling the reaction temperature from 180 to 220 °C. Synthesized QDs were characterized by UV–vis absorption spectra and X-ray diffraction (XRD). The result of UV–vis absorption spectra showed that with increasing of reaction temperature, the first absorption peak was red-shifted and the size of QD increased. The result of XRD showed that the synthesized QDs were zinc-blende structure.