Hot-injection Synthesis Research Articles

Steric effects of carboxylic capping ligands on the growth of the CdSe quantum dots

Cadmium selenide quantum dots were grown using different carboxylic capping ligands in order to study the steric effects of the ligand chain length and chain branching. Straight chain and branched saturated carboxylic acids (fatty acids) were used as capping ligands. Hot injection synthesis was performed in 1-octadecene via the reaction between Cd carboxylates and trioctylphosphine selenide. At specific times, samples were withdrawn from reaction mixtures. For straight chain carboxylic acids, namely stearic, hexadecanoic, dodecanoic, decanoic, and octanoic, the decrease of the chain length leads to the increase of the CdSe particle sizes due to the faster transport of the monomers through the capping layer. For decanoic and octanoic acids, fast diffusion through the capping layer leads to the reaction-controlled kinetics, and the particles grow as tetrapods instead of dots. α-Branched double chain acids, 2-butyloctanoic and especially 2-hexyldecanoic, create more steric hindrance compared to the straight chain acids of comparable length and produce smaller quantum dots. α-Branched triple chain acids, neodecanoic and 1-adamantanecarboxylic, were expected to show superior steric hindrance during the synthesis. Neodecanoic acid, compared to 2-butyloctanoic acid, the closest to it in length, created less hindrance for the monomer diffusion, probably because two of its three branches are very short. 1-Adamantanecarboxylic acid performed comparable to 2-butyloctanoic acid, despite a much shorter length. This is attributed to the bulky rigid adamantane cages forming a dense capping layer. Straight chain and branched chain ligands lead to different photoluminescence patterns. When capped with straight chain carboxylates, CdSe quantum dots exhibit only sharp bandgap fluorescence, consistent with a high degree of surface site coverage. All of the α-branched ligands enabled intense surface trap emission, likely because of incomplete passivation of the surface atoms due to the nearest neighbor steric repulsion.

Doping by Diffusion

Materials Science The electronic properties of bulk semiconductors can be improved by doping, the deliberate introduction of impurity atoms that increase the number or mobility of charge carriers. Usually, just doping the surface of a bulk semiconductor is sufficient, but for semiconductor nanocrystals, the entire particle needs to be doped to achieve the desired properties. Vlaskin et al. report on the doping of CdSe nanocrystals with Mn ions, a process that has proven especially difficult at the high levels desired to impart magnetic properties. Hot-injection synthesis of Mn-doped CdSe nanocrystals (where the particles rapidly form in solution from precursors) results in particles that are depleted in Cd2+ at the surface. The authors were able to incorporate Mn2+ ions uniformly by starting with CdSe seed nanocrystals, which were injected as a solution along with selenium into a solution of manganese acetate at ∼300°C for times varying from seconds to several hours. This thermodynamically controlled process, which uniformly increased the size of the seeds, resulted in diffusion doping of the entire crystal without requiring Cd2+ ion ejection and allowed doping levels of Mn2+ as high as 20% to be achieved. J. Am. Chem. Soc. 135 , 10.1021/ja4072207 (2013).

Hot-injection synthesis and characterization of monodispersed ternary Cu2SnSe3 nanocrystals for thermoelectric applications

Cu2SnSe3 (copper tin selenide, CTSe) is one of promising copper-based selenides materials in the use of acousto-optic and photovoltaic fields. In this communication, the CTSe nanocrystals was successfully synthesized by a “hot-injection” protocol in the presence of oleylamine (OLA). Selenium powder dissolved in the mixed solution of OLA and dodecanethiol (DT) were injected into a hot solution of OLA, including copper chloride, tin chloride, at a given reaction temperature. The structure, composition and morphology of the synthesized samples were characterized via using powder X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM) and field-emission scanning electron microscopy (SEM). The results revealed that the as-synthesized nanocrystals were single phase polycrystalline, cubic crystal structure, and high monodispersity with an average diameter of 5nm. The thermoelectric properties of these dense materials compacted from nanocrystals with the temperature range from 300K to 598K were studied. The results exhibit a promising figure-of-merit ZT at 598K, and it demonstrates that the prepared CTSe could potentially be useful for efficient thermoelectric materials.

Hot-injection synthesis of Co0.85Se nanocrystals for photo-electrical application

Co0.85Se nanocrystals were synthesized via the hot-injection method using oleylamine (OLA) as solvent. The experimental results indicate that the reaction temperature plays an important role in the formation of Co0.85Se nanocrystals. Meanwhile, it is also found that the source of cobalt (CoCl2) should be excessive to obtain pure Co0.85Se nanocrystals. The obtained nanocrystals exhibit particulate shape and pure hexagonal Co0.85Se phase based on transmission electron microscopy (TEM), X-ray diffraction (XRD) and energy-dispersive X-ray spectroscopy (EDS) analyses. Optical and optoelectronic properties of the film drop-coated by Co0.85Se nanocrystals ink were also characterized. The obtained sample shows an optical band gap value of 1.16eV and remarkable photo-electrical response, suggesting that the prepared Co0.85Se nanocrystals have the great potential in photo-electrical conversion application.

Role of Acid in Precursor Conversion During InP Quantum Dot Synthesis

We have studied the speciation of P(SiMe3)3 during the synthesis of colloidal InP quantum dots in the presence of proton sources. Using 31P NMR spectroscopy, we show H3-nP(SiMe3)n formation on exposure of P(SiMe3)3 to a variety of protic reagents including water, methanol, and carboxylic acid, corroborating observations of P(SiMe3)3 speciation during the hot injection synthesis of InP QDs. Quantitative UV–vis comparisons between InP growth from P(SiMe3)3 and HP(SiMe3)2 show unambiguously that when total H+-content is accounted for, particle size, size dispersity, and concentration are indistinguishable for these two reagents. The dual role of myristic acid in P–Si bond cleavage and as a source of the myristate anion, an essential component of the quantum dot surface, is interrogated using tetrabutylammonium myristate, confirming that it is the protons that are responsible for increased quantum dot polydispersity. Together these data support the existence of a competing acid-catalyzed pathway in the conver...

Reaction Chemistry/Nanocrystal Property Relations in the Hot Injection Synthesis, the Role of the Solute Solubility

Various literature studies show that increasing the concentration of free acid in the hot injection synthesis of colloidal nanocrystals raises the diameter of the resulting nanocrystals. We analyze this reaction chemistry/nanocrystal property relation by combining reaction simulations with an experimental study on a particular CdSe nanocrystal synthesis. We find that increasing the free acid concentration has the same effect on a real synthesis as raising the solute solubility in the simulations. Both lead to larger sizes and a deterioration of the size dispersion at constant reaction rate. Since free acids are used to coordinate the cation precursors in these syntheses, this leads to a meaningful link between a parameter in reaction simulations and the composition of an experimental reaction mixture. We thus explain the increase of the nanocrystal size with the acid concentration as resulting from an enhanced consumption of the solute by nanocrystal growth, which reduces the number of nanocrystals formed. This link between a simulation parameter and the composition of the reaction mixture provides a rational basis to further explore and understand reaction chemistry/nanocrystal property relations in the hot injection synthesis.

A totally phosphine-free synthesis of metal telluride nanocrystals by employing alkylamides to replace alkylphosphines for preparing highly reactive tellurium precursors

Despite the developments in the wet chemical synthesis of high-quality semiconductor nanocrystals (NCs) with diverse elemental compositions, telluride NCs are still irreplaceable materials owing to their excellent photovoltaic and thermoelectric performances. Herein we demonstrate the dissolution of elemental tellurium (Te) in a series of alkylamides by sodium borohydride (NaBH4) reduction at relatively low temperature to produce highly reactive precursors for hot-injection synthesis of telluride NCs. The capability to tune the reactivity of Te precursors by selecting injection temperature permits control of NC size over a broad range. The current preparation of Te precursors is simple, economical, and totally phosphine-free, which will promote the commercial synthesis and applications of telluride NCs.

Facile synthesis of hierarchical ZnIn2S4 submicrospheres composed of ultrathin mesoporous nanosheets as a highly efficient visible-light-driven photocatalyst for H2 production

Flower-like spherical ZnIn2S4 superstructures about 500 nm in diameter composed of ultrathin 2–3 nm thick mesoporous nanosheets were synthesized in several minutes by facile polyol-mediated hot-injection. Due to their higher specific surface area and constituent ultrathin mesoporous nanosheets, the hierarchical ZnIn2S4 superstructures exhibited highly efficient visible-light-driven photocatalytic H2 production activity compared with the typical flower-like ZnIn2S4 microspheres prepared by the conventional hydrothermal method. Furthermore, this polyol-mediated hot-injection synthesis strategy is also applicable for the facile synthesis of other metal sulphide nanomaterials with different morphologies, such as CdS quantum dots, Bi2S3 nanorods, and hierarchical flower-like In2S3 and SnS spheres.

Hot-Injection Synthesis of Bactericidal Sn-Doped TiO2 Nanospheres for Visible-Light Photocatalysis

Hot-Injection Synthesis of Bactericidal Sn-Doped TiO₂ Nanospheres for Visible-Light Photocatalysis

Pt/Sn Intermetallic, Core/Shell and Alloy Nanoparticles: Colloidal Synthesis and Structural Control

For the first time, shape-controlled Pt3Sn, PtSn, and PtSn2 intermetallic nanocrystals were synthesized in octadecene (ODE) by a versatile hot-injection method with 1,2-hexadecanediol (HDD) as the reducing agent. Transmission electron microscopy (TEM) measurements reveal that the metal composition has an influence on the particle morphology: with the increase in the Sn content, the Pt/Sn nanoparticles obtained by the hot-injection synthesis show flower-like, irregular faceted, cubic/tetrahedral, hexagonal, and spherical/nanowire structures. A facile phase-transfer preparative procedure for the synthesis of Pt/Sn core/shell nanoparticles was also developed, in which ligand-free Pt nanoparticles were used as precursors. X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) measurements confirm a Pt-core/Sn-shell structure. The surface characteristic of the Pt/Sn core/shell nanoparticles was also investigated by IR spectroscopy of CO adsorption experiments (i.e., with a highly surface sensitive ...

Alkylthiol-Enabled Se Powder Dissolution in Oleylamine at Room Temperature for the Phosphine-Free Synthesis of Copper-Based Quaternary Selenide Nanocrystals

Enhancement of Se solubility in organic solvents without the use of alkylphosphine ligands is the key for phosphine-free synthesis of selenide semiconductor nanocrystals (NCs). In this communication, we demonstrate the dissolution of elemental Se in oleylamine by alkylthiol reduction at room temperature, which generates soluble alkylammonium selenide. This Se precursor is highly reactive for hot-injection synthesis of selenide semiconductor NCs, such as Cu(2)ZnSnSe(4), Cu(InGa)Se(2), and CdSe. In the case of Cu(2)ZnSnSe(4), for example, the as-synthesized NCs possessed small size, high size monodispersity, strong absorbance in the visible region, and in particular a promising increase in photocurrent under AM1.5 illumination. The current preparation of the Se precursor is simple and convenient, which will promote the synthesis and practical applications of selenide NCs.

Tuning the Postfocused Size of Colloidal Nanocrystals by the Reaction Rate: From Theory to Application

We show that adjusting the reaction rate in a hot injection synthesis is a viable strategy to tune the diameter of colloidal nanocrystals at the end of the size distribution focusing, i.e., the postfocused diameter. The approach is introduced by synthesis simulations, which describe nucleation and growth of colloidal nanocrystals from a solute or monomer that is formed in situ out of the injected precursors. These simulations indicate that the postfocused diameter is reached at almost full yield and that it can be adjusted by the rate of monomer formation. We implement this size-tuning strategy using a particular CdSe quantum dot synthesis that shows excellent agreement with the model synthesis. After demonstrating that the reaction rate depends in first order on the Cd and Se precursor concentration, the proposed strategy of size control is explored by varying the precursor concentration. This enables the synthesis of colloidal nanocrystals with a predefined size at almost full yield and sharp size distributions. In addition, we demonstrate that the same tuning strategy applies to the synthesis of CdS quantum dots. This result is highly relevant especially in the context of reaction upscaling and automation. Moreover, the results obtained challenge the traditional interpretation of the hot injection synthesis, in particular the link between hot injection, burst nucleation, and sharp size distributions.

Heteroepitaxial Growth of Colloidal Nanocrystals onto Substrate Films via Hot-Injection Routes

Hot-injection synthesis of colloidal nanocrystals (NCs) in a substrate-bound form is demonstrated. We show that polycrystalline films submerged into hot organic solvents can nucleate the heteroepitaxial growth of semiconductor NCs, for which the ensuing lattice quality and size distribution are on the par with those of isolated colloidal nanoparticles. This strategy is demonstrated by growing lead chalcogenide NCs directly onto solvent-submerged TiO(2) substrates. The resulting PbX/TiO(2) (X = S, Se, Te) nanocomposites exhibit heteroepitaxial interfaces between lead chalcogenide and oxide domains and show an efficient separation of photoinduced charges, deployable for light-harvesting applications. The extendibility of the present method to other material systems was demonstrated through the synthesis of CdS/TiO(2) and Cu(2)S/TiO(2) heterostructures, fabricated from PbS/TiO(2) composites via cation exchange. The photovoltaic performance of nanocrystal/substrate composites comprising PbS NCs was evaluated by incorporating PbS/TiO(2) films into prototype solar cells.

Infrared Emitting and Photoconducting Colloidal Silver Chalcogenide Nanocrystal Quantum Dots from a Silylamide-Promoted Synthesis

Here, we present a hot injection synthesis of colloidal Ag chalcogenide nanocrystals (Ag(2)Se, Ag(2)Te, and Ag(2)S) that resulted in exceptionally small nanocrystal sizes in the range between 2 and 4 nm. Ag chalcogenide nanocrystals exhibit band gap energies within the near-infrared spectral region, making these materials promising as environmentally benign alternatives to established infrared active nanocrystals containing toxic metals such as Hg, Cd, and Pb. We present Ag(2)Se nanocrystals in detail, giving size-tunable luminescence with quantum yields above 1.7%. The luminescence, with a decay time on the order of 130 ns, was shown to improve due to the growth of a monolayer thick ZnSe shell. Photoconductivity with a quantum efficiency of 27% was achieved by blending the Ag(2)Se nanocrystals with a soluble fullerene derivative. The co-injection of lithium silylamide was found to be crucial to the synthesis of Ag chalcogenide nanocrystals, which drastically increased their nucleation rate even at relatively low growth temperatures. Because the same observation was made for the nucleation of Cd chalcogenide nanocrystals, we conclude that the addition of lithium silylamide might generally promote wet-chemical synthesis of metal chalcogenide nanocrystals, including in as-yet unexplored materials.

From Hot‐Injection Synthesis to Heating‐Up Synthesis of Cobalt Nanoparticles: Observation of Kinetically Controllable Nucleation

Almost equal: Particle nucleation in the widely studied hot-injection synthesis of magnetic cobalt nanoparticles (see TEM image) was found to take place well after the injection and to be temperature-controlled. This insight into particle nucleation led to a technologically relevant heating-up synthesis of nearly monodisperse cobalt nanoparticles on a multigram scale. The particle size could be tuned simply by adjusting the heating rate.

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.

Hot-injection synthesis of iron/iron oxide core/shell nanoparticles for T2 contrast enhancement in magnetic resonance imaging

Here we report a new, bench-top synthesis for iron/iron oxide core/shell nanoparticles via the thermal decomposition of Fe(η(5)-C(6)H(3)Me(4))(2). The iron/iron oxide core/shell nanoparticles are superparamagnetic at room temperature and show improved negative contrast in T(2)-weighted MR imaging compared to pure iron oxides nanoparticles, and have a transverse relaxivity (r(2)) of 332 mM(-1) s(-1).

Hot-injection synthesis and characterization of quaternary Cu 2ZnSnSe 4 nanocrystals

Cu 2ZnSnSe 4 (CZTSe) is one of promising materials in the use of absorber layers of solar cells. It contains earth-abundant elements of zinc and tin, a near-optimal direct band gap of ∼ 1.5 eV, as well as a large absorption coefficient ∼ 10 4 cm-1. The CZTSe nanocrystals in oleylamine (OLA) was successfully prepared via hot-injection method. The characterization of its structure, composition, morphology and absorption spectra were done using powder X-ray diffraction (XRD), energy-dispersive X-ray spectroscopy (EDS), transmission electron microscopy (TEM) and UV–vis absorption spectra. The results revealed that the monodispersed nanocrystals were single phase polycrystalline within the range of 15–20 nm. Optical measurements showed a direct band gap of 1.52 eV, which was optimal for low cost solar cells. The capping property of OLA was also demonstrated by examining Fourier Transform Infrared Spectroscopy (FTIR) feature peaks of CZTSe and OLA, respectively.

Hot-injection synthesis of highly luminescent and monodisperse CdS nanocrystals using thioacetamide and cadmium source with proper reactivity

Thioacetamide (TAA) has served as an excellent sulfur source to react with cadmium stearate to controllably produce highly luminescent and monodisperse CdS nanocrystals through the hot-injection method in dodecylamine solvent. The kinetics and thermodynamics of nucleation/growth of CdS nanocrystals, as well as their optical properties are controlled by changing synthesis conditions such as reaction time, injection/growth temperatures, TAA concentration and cadmium source with different reactivity. Temperature-dependent release of reactive sulfur species from TAA, together with proper reactivity of cadmium source, facilitates the better separation of nucleation and growth stage, the formation of highly monodisperse CdS nanocrystals with tunable size and further self-assembly into ordered superlattices. When cadmium carboxylates such as cadmium stearate and cadmium oleate are used as cadmium sources, surface trap emission of CdS nanocrystals can be gradually removed to obtain bright pure-blue emission with increasing reaction time. The highest quantum efficiency of up to 33.6% is achieved when using cadmium stearate as cadmium source at the injection/growth temperatures of 230/210 °C for 90 min.

Synthesis of Extremely Small CdSe and Bright Blue Luminescent CdSe/ZnS Nanoparticles by a Prefocused Hot-Injection Approach

Although cadmium selenide is a well-explored material in aqueous and hot-injection synthesis of semiconductor quantum dots, only a few syntheses are reported for the preparation of extremely small cadmium selenide nanocrystallites. We present a hot-injection approach based on the enhancement of the nucleation rate to make cadmium selenide quantum dots with a diameter tunable between 1.5 and 2.2 nm. Furthermore, we present a procedure on how to coat these particles with zinc sulfide, leading to highly efficient fluorophors in the blue region of the visible spectrum. The work was performed without the use of highly reactive (pyrophoric) precursors and represents a basic concept for the preparation of small nanoparticles in general.