III-V Semiconductor Nanocrystals Research Articles

Safe Synthesis of InP Quantum Dots for Biological Applications

Semiconductor nanocrystals (also known as quantum dots) have been extensively studied because of their unique optical properties which makes them promising candidates for bio imaging and bio sensing. II-VI semiconductor quantum dots are commonly used in biomedical applications because of their near unit quantum yields and narrow emission profiles. Despite these advantages, II-VI semiconductor nanocrystals contain toxic metals. As a result, III-V semiconductor nanocrystals such as InP are promising replacements for cadmium-based semiconductor nanocrystals. However, the synthesis of bright InP quantum dots with narrow size distribution is challenging mainly due to the use of overly reactive phosphorus precursors like tris(trimethylsilyl) phosphine (TMS)3P. This reagent is also highly flammable. We employed less pyrophoric sterically encumbered tris(triethylsilyl) phosphine and tris(tributylsilyl) phosphine for the synthesis and observed that quantum yield and the emission color saturation (fwhm) of InP nanocrystals are improved compared to those prepared with (TMS)3P, but not up to the levels realized with CdSe. HF treatment is often employed as a post-synthetic treatment to improve the optical properties of quantum dots. We demonstrate that ammonium bifluoride is a safer alternative to extremely hazardous HF. Ammonium bifluoride etched InP core dots were further passivated to create InP/ZnSeS core-shell quantum dots that were later water-solubilized and functionalized for biological applications. Figure 1

Triggering Cation Exchange Reactions by Doping.

Cation exchange (CE) reactions have emerged as a technologically important route, complementary to the colloidal synthesis, to produce nanostructures of different geometries and compositions for a variety of applications. Here it is demonstrated with first-principles simulations that an interstitial impurity cation in CdSe nanocrystals weakens nearby bonds and reduces the CE barrier in the prototypical exchange of Cd2+ ions by Ag+ ions. A Wannier function-based tight binding model is employed to quantify microscopic mechanisms that influence this behavior. To support our model, we also tested our findings in a CE experiment: both CdSe and interstitially Ag-doped CdSe nanocrystals (containing 4% of Ag+ ions per nanocrystal on average) were exposed to Pb2+ ions at room temperature and it was observed that the exchange reaction proceeds further in doped nanocrystals. The findings suggest doping as a possible route to promote CE reactions that hardly undergo exchange otherwise, for example, those in III-V semiconductor nanocrystals.

Lead halide perovskites and other metal halide complexes as inorganic capping ligands for colloidal nanocrystals.

Lead halide perovskites (CH3NH3PbX3, where X = I, Br) and other metal halide complexes (MXn, where M = Pb, Cd, In, Zn, Fe, Bi, Sb) have been studied as inorganic capping ligands for colloidal nanocrystals. We present the methodology for the surface functionalization via ligand-exchange reactions and the effect on the optical properties of IV–VI, II–VI, and III–V semiconductor nanocrystals. In particular, we show that the Lewis acid–base properties of the solvents, in addition to the solvent dielectric constant, must be properly adjusted for successful ligand exchange and colloidal stability. High luminescence quantum efficiencies of 20–30% for near-infrared emitting CH3NH3PbI3-functionalized PbS nanocrystals and 50–65% for red-emitting CH3NH3CdBr3- and (NH4)2ZnCl4-capped CdSe/CdS nanocrystals point to highly efficient electronic passivation of the nanocrystal surface.

Doping efficiency inn-type InP nanowires

We examine two factors that could limit the doping of $n$-type III-V semiconductor nanowires, namely, the solubility of the dopants and the possible formation of $DX$-like defect centers. We find that it is preferable to dope zinc-blende nanowires via anion substitution as opposed to cation substitution. The comparison with previous work on $n$-type III-V semiconductor nanocrystals also allows us to determine the role of dimensionality and quantum confinement on doping characteristics of materials. Our work is based on quantum calculations of InP nanowires using real-space pseudopotentials constructed within density functional theory.

Facile synthesis of uniform large-sized InP nanocrystal quantum dots using tris(tert-butyldimethylsilyl)phosphine

Colloidal III-V semiconductor nanocrystal quantum dots [NQDs] have attracted interest because they have reduced toxicity compared with II-VI compounds. However, the study and application of III-V semiconductor nanocrystals are limited by difficulties in their synthesis. In particular, it is difficult to control nucleation because the molecular bonds in III-V semiconductors are highly covalent. A synthetic approach of InP NQDs was presented using newly synthesized organometallic phosphorus [P] precursors with different functional moieties while preserving the P-Si bond. Introducing bulky side chains in our study improved the stability while facilitating InP formation with strong confinement at a readily low temperature regime (210°C to 300°C). Further shell coating with ZnS resulted in highly luminescent core-shell materials. The design and synthesis of P precursors for high-quality InP NQDs were conducted for the first time, and we were able to control the nucleation by varying the reactivity of P precursors, therefore achieving uniform large-sized InP NQDs. This opens the way for the large-scale production of high-quality Cd-free nanocrystal quantum dots.

Synthesis and characterization of hybrid CdS/MEH-PPV nanocomposites for photovoltaic applications

ABSTRACTInorganic-organic nanocomposites, with II-VI or III-V semiconductor nanocrystals (NCs) embedded in semiconducting polymer matrix, are very promising materials for photovoltaic applications.Here, we present an effective and easy synthesis procedure to obtain a hybrid nanocomposite with CdS NCs dispersed in poly[2-methoxy-5-(2-(2’-ethyl-hexyloxy)-1,4-phenylene vinylene] (MEH-PPV) conjugated polymer. CdS NCs are synthesized directly within the matrix through the decomposition of a suitable unimolecular precursor dispersed homogeneously in the polymer.We show that CdS NCs are formed at low annealing temperature avoiding structural damages and without affecting the functional properties of the MEH-PPV polymer. The NCs diameter ranges between 1.5nm and 4nm depending on the annealing temperature. In addition, no coalescence phenomena of CdS NCs were noticed in TEM observations even at very high particle density (40 wt %).

N-type doping via avoiding the stabilization ofDXcenters in InP quantum dots

We demonstrate that it is preferable to dope III-V semiconductor nanocrystals by $n$-type anion substitution as opposed to cation substitution. Specifically, we show the dopability of zinc-blende nanocrystals is more efficient when the dopants are placed at the anion site as quantified by formation energies and the stabilization of $DX$-like defect centers. Our results are based on first-principles calculations of InP quantum dots by using a real-space implementation of density-functional theory and pseudopotentials.

A developed Ullmann reaction to III–V semiconductor nanocrystals in sealed vacuum tubes

Group III-V (13-15, III = Ga, In, and V = P, As) semiconductor nanocrystals were effectively obtained via a developed Ullmann reaction route through the reactions of preformed nanoscale metallic indium or commercial gallium with triphenylphosphine (PPh(3)) and triphenylarsine (AsPh(3)) in sealed vacuum quartz tubes under moderate conditions at 320-400 degrees C for 8-24 h. The developed synthetic strategy in sealed vacuum tubes extends the synthesis of III-V semiconductor materials, and the air-stable PPh(3) and AsPh(3) with low toxicity provide good alternative pnicogen precursors for the synthesis of III-V nanocrystals. The analysis of XRD, ED and HRTEM established the production of one-dimensional (1D) metastable wurtzite (W) InP, InAs and GaP nanostructures in the zinc blende (ZB) products. Further investigations showed that 1D W nanostructures resulted from kinetic effects under the moderate synthetic conditions employed and the steric effect of PPh(3) and AsPh(3), and that the tendency for the synthesis of III-V nanocrystals was in the orders of IIIP > IIIAs and GaV > InV on the basis of experiments and thermodynamic calculations. Meanwhile, the microstructures and growth mechanism of the III-V nanocrystals were investigated.

Synthesis of InAs/CdSe/ZnSe Core/Shell1/Shell2 Structures with Bright and Stable Near-Infrared Fluorescence

A complex InAs/CdSe/ZnSe core/shell1/shell2 (CSS) structure is synthesized, where the intermediate CdSe buffer layer decreases strain between the InAs core and the ZnSe outer shell. This structure leads to significantly improved fluorescence quantum yield as compared to previously prepared core/shell structures and enables growth of much thicker shells. The shell growth is done using a layer-by-layer method in which the shell cation and anion precursors are added sequentially allowing for excellent control, and a good size distribution is maintained throughout the entire growth process. The CSS structure is characterized using transmission electron microscopy, as well as by X-ray diffraction and X-ray photoelectron spectroscopy which provide evidence for shell growth. The quantum yield for CSS with small InAs cores reaches over 70%-exceptional photoluminescence intensity for III-V semiconductor nanocrystals. In larger InAs cores there is a systematic decrease in the quantum yield, with a yield of approximately 40% for intermediate size cores down to a few percent in large cores. The CSS structures also exhibit very good photostability, vastly improved over those of organically coated cores, and transformation into water environment via ligand exchange is performed without significant decrease of the quantum yield. These new InAs/CdSe/ZnSe CSS nanocrystals are therefore promising near-IR chromophores for biological fluorescence tagging and optoelectronic devices.

Time-resolved intraband relaxation of strongly confined electrons and holes in colloidal PbSe nanocrystals

The relaxation of strongly-confined electrons and holes between 1P and 1S levels in colloidal PbSe nanocrystals has been time-resolved using femtosecond transient absorption spectroscopy. In contrast to II-VI and III-V semiconductor nanocrystals, both electrons and holes are strongly confined in PbSe nanocrystals. Despite the large electron and hole energy level spacings (at least 12 times the optical phonon energy), we consistently observe picosecond time-scale relaxation. Existing theories of carrier relaxation cannot account for these experimental results. Mechanisms that could possibly circumvent the phonon bottleneck in IV-VI quantum dots are discussed.

Sol–Gel Synthesis of Luminescent InP Nanocrystals Embedded in Silica Glasses

III-V semiconductor nanocrystals rarely exist as spherical inclusions inside glasses, due to difficulties during their preparation, such as high toxic reagents or fast oxidation under usual glass technology temperatures. In this letter a sol-gel method for synthesis of InP nanocrystals embedded in silica glasses was described. Gels were synthesized by hydrolysis of a complex solution of Si(OC2H5)4, InCl3.4H2O, and PO(OC2H5)3. Then, the gels were heated at 600 degrees C in the presence of H2 gas to form fine cubic InP crystallites. Raman spectrum showed an InP longitudinal-optic mode (342 cm(-1)) and a transverse-optic mode (303 cm(-1)). The size of InP nanocrystals was found to be from 2 to 8 nm in diameter by transmission electron microscopy. A strong photoluminescence with a peak at 856 nm was observed from InP nanocrystals embedded in silica glasses. The results suggest that it might be possible to synthesize other III-V semiconductor nanocrystals embedded in silica glasses through the sol-gel process.

Sol–Gel Synthesis and Photoluminescence of III-V Semiconductor InAs Nanocrystals Embedded in Silica Glasses

III-V semiconductor nanocrystals rarely exist as spherical inclusions inside glasses, due to the difficulties during their preparation, such as high toxic reagents or fast oxidation under usual glass technology temperatures. In this paper a sol-gel method for synthesis of InAs nanocrystals embedded in silica glasses was described. Gels were synthesized by the hydrolysis of a complex solution of Si(OC2H5)4, InCl3 x 4H2O, and As2O3. The gels were then heated at 200-450 degrees C in the presence of H2 gas to form fine cubic InAs crystallites. The X-ray diffraction patterns showed four strong peaks from InAs. The Raman spectrum showed InAs longitudinal-optic (233 cm(-1)) and transverseoptic modes (215 cm-(-1)). The size of InAs nanocrystals was found to be from 5 to 30 nm in diameter by transmission electron microscopy. A strong room temperature photoluminescence with peaks at 601 and 697 nm was observed from InAs nanocrystals embedded in silica glasses. The results suggest that it might be possible to synthesize other III-V semiconductor nanocrystals embedded in silica glasses through the sol-gel process.

Optically Detected Magnetic Resonance Studies of Colloidal Semiconductor Nanocrystals

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Optically detected magnetic resonance studies of colloidal semiconductor nanocrystals.

The review describes the studies of the magneto-optical properties of II-VI and III-V semiconductor nanocrystals (NCs) capped with organic or inorganic epitaxial shells. The investigations focused on the chemical identification of localization sites (core, shell, or interface) of photogenerated carriers in spherical NCs and elucidated the influence of the surface/interface quality on the optical properties of the materials. Optically detected magnetic resonance (ODMR) spectroscopy was used for the study of the proposed physical properties. The ODMR method provides the means to identify the surface/interface sites and correlate them with specific optical transition. In addition, this method reveals information about the spin multiplicity of band edge and trapped states and the electron-hole exchange interaction, determines the spectroscopic g-factors, distinguishes between the radiative and nonradiative characteristic of a trapping site, and evaluates the spin-lattice relaxation times.

Shape control of III-V semiconductor nanocrystals: synthesis and properties of InAs quantum rods.

A novel approach for synthesis of soluble semiconductor quantum rods using metal nanoparticles to direct and catalyze one-dimensional growth is developed. The method is useful in particular for III-V semiconductors with cubic lattice, where the utilization of surfactant-controlled rod-growth is not easily realized. The growth takes place via the solution-liquid-solid (SLS) mechanism where proper precursors are injected into a coordinating solvent. Centrifugation is used for separation of rod-fractions with different lengths. The reaction is demonstrated for InAs, InP and GaAs. Focusing on InAs rods as a model system, we examined the effects of the type of metal catalyst, and the tuning of reaction conditions with respect to temperature, concentration, catalyst content and reaction time. Within the three types of metal catalysts used--Au, Ag and In, Au was found to provide the best control for achieving rod-growth even though the melting point of bulk gold is significantly higher then the reaction temperature. The structural properties of the rods were characterized by transmission electron microscopy, X-ray diffraction and energy dispersive X-ray spectroscopy. Rods have a cubic lattice and grow mainly along the [111] direction. The relative gold content decreases in shorter rods suggesting Au depletion as a cause for limiting the growth. Room and low temperature absorption and photoluminescence measurements show that the band-gap shifts to the red upon increasing rod length revealing strong quantum confinement along the long axis in InAs rods, providing spectral coverage of the near-IR range relevant for telecommunication applications. Emission intensity also decreases with increased rod-length. These length dependent properties manifest the transition from 0D to 1D quantum confined systems.

Dynamic distribution of growth rates within the ensembles of colloidal II-VI and III-V semiconductor nanocrystals as a factor governing their photoluminescence efficiency.

The distribution of properties within ensembles of colloidally grown II-VI and III-V semiconductor nanocrystals was studied. A drastic difference in the photoluminescence efficiencies of size-selected fractions was observed for both organometallically prepared CdSe and InAs colloids and for CdTe nanocrystals synthesized in aqueous medium, indicating a general character of the phenomenon observed. The difference in the photoluminescence efficiencies is attributed to different averaged surface disorder of the nanocrystals originating from the Ostwald ripening growth mechanism when larger particles in the ensemble grow at the expense of dissolving smaller particles. At any stage of growth, only a fraction of particles within the ensemble of growing colloidal nanocrystals has the most perfect surface and, thus, shows the most efficient photoluminescence. This is explained by a theoretical model describing the evolution of an ensemble of nanocrystals in a colloidal solution. In an ensemble of growing nanocrystals, the fraction of particles with the highest photoluminescence corresponds to the particle size having nearly zero average growth rate. The small average growth rate leads to the lowest possible degree of surface disorder at any given reaction conditions.

A Low Temperature, Solution Phase Synthesis of III-V Semiconductor Nanocrystals

ABSTRACTNanocrystalline materials have been intensely investigated in the recent past due to the novel properties associated with size-quantized particles. We have developed a new method for high yield, solution phase synthesis of nanocrystalline III-V semiconductors which eliminates the use of substituted or unsubstituted Group V hydrides and Group III alkyls. Our approach consists of in situ syntheses of (Na/K)3E (E = P, As, Sb) in aromatic solvents and subsequent reactions of these pnictides with Group III halide solutions in coordinating solvents. The nanocrystalline III-V semiconductors GaP, GaAs, GaSb, InP, InAs and InSb are readily prepared in a wide range of particle sizes (4–36 nm) and in high yields. The resultant Ill-V materials have been characterized by XRD, EDXA, TEM and elemental analyses.