Indium Phosphide Quantum Dots Research Articles

Specifics of the spectral and luminescent properties of ensembles of colloidal quantum dots

Using colloidal solutions of ZnS-shell indium phosphide quantum dots with two average sizes of 2.1 and 3.0 nm and a size distribution variance of 10%, it has been shown that the luminescence and the luminescence excitation spectra of the colloidal quantum dots substantially depend on the wavelength of exciting light and the detection wavelength, respectively, with both the relationships being nonlinear in character, which may indicate the bimodal type of the size distribution function. Similar measurements for CdSe colloidal quantum dots with an average particle size of dav = 2.5 nm and a variance of 6% have shown that the effect of dependence of the luminescence peak position on the excitation wavelength is manifested to a much lesser extent.

Direct Observation of Early-Stage Quantum Dot Growth Mechanisms with High-Temperature Ab Initio Molecular Dynamics

Colloidal quantum dots (QDs) exhibit highly desirable size- and shape-dependent properties for applications from electronic devices to imaging. Indium phosphide QDs have emerged as a primary candidate to replace the more toxic CdSe QDs, but production of InP QDs with the desired properties lags behind other QD materials due to a poor understanding of how to tune the growth process. Using high-temperature ab initio molecular dynamics (AIMD) simulations, we report the first direct observation of the early stage intermediates and subsequent formation of an InP cluster from separated indium and phosphorus precursors. In our simulations, indium agglomeration precedes formation of In-P bonds. We observe a predominantly intercomplex pathway in which In-P bonds form between one set of precursor copies while the carboxylate ligand of a second indium precursor in the agglomerated indium abstracts a ligand from the phosphorus precursor. This process produces an indium-rich cluster with structural properties comparable to those in bulk zinc-blende InP crystals. Minimum energy pathway characterization of the AIMD-sampled reaction events confirms these observations and identifies that In-carboxylate dissociation energetics solely determine the barrier along the In-P bond formation pathway, which is lower for intercomplex (13 kcal/mol) than intracomplex (21 kcal/mol) mechanisms. The phosphorus precursor chemistry, on the other hand, controls the thermodynamics of the reaction. Our observations of the differing roles of precursors in controlling QD formation strongly suggests that the challenges thus far encountered in InP QD synthesis optimization may be attributed to an overlooked need for a cooperative tuning strategy that simultaneously addresses the chemistry of both indium and phosphorus precursors.

Improving the ensemble optical properties of InP quantum dots by indium precursor modification

Indium phosphide quantum dots typically exhibit broad and poorly defined ensemble optical properties due to a highly pronounced nucleation period that consumes virtually all phosphorus precursors, leading to subsequent growth by Ostwald ripening and poor final size distributions. Previous attempts to reduce the reactivity of the phosphorus precursor, and thereby limit its consumption during the nucleation phase, have not led to appreciably better optical properties due to an unwanted increase in the duration of nucleation. Here we present an alternative approach to reducing initial precursor consumption using a simple modification of the indium precursor, in which the widely used carboxylate ligand is replaced by a phosphine. The change of ligand leaves residual precursor available for size-focusing growth after nucleation, leading to significantly improved spectral features. Band-edge emission peaks are typically 30% narrower than for the standard method.

Effect of Trace Water on the Growth of Indium Phosphide Quantum Dots

We report that trace amounts of water impurities in indium myristate precursors can negatively impact indium phosphide nanoparticle growth by limiting its size tunability. Without water, the growth can be effectively tuned by growth temperature and time with the first absorption peak reaching 620 nm; with water, the growth presents a “focused” behavior with the first absorption peak remaining around 550 nm. The results imply that water impurities, either from indium acetate derived indium precursors or generated in situ during nanoparticle growth, may be the cause of the currently observed inhibited growth behavior of indium phosphide quantum dots. We use multistage microfluidic reactors to show that this inhibiting effect occurs at the late stage of particle growth, following precursor depletion. We extend our study by showing that trace amounts of free hydroxide can also inhibit nanoparticle growth. We attribute the inhibited growth behavior to the hydroxylation effect of water or free hydroxide.

Synthesis and properties of colloidal indium phosphide quantum dots

Semiconductor colloidal quantum dots possessing unique spectral-luminescent characteristics are of great interest for the development of novel materials and devices, which are promising to be used in various fields. However, the contemporary high-quality quantum dots contain Cd, Pb, Hg, Se, and Te ions and are toxic, with this circumstance limiting their wide practical application. Therefore, the attention of researchers has recently been focused on AIIIBV semiconductor quantum dots—in particular, InP ones, which are markedly more environmentally safe. InP quantum dots have spectral characteristics similar to those of well-studied CdSe quantum dots and can, in principle, replace the latter in many cases, especially, taking into account their more pronounced size effect caused by the narrow energy gap and the large Bohr radius of the exciton. Moreover, the stronger InP covalent bond than the ionic one in AIIBVI semiconductors makes it possible to expect that they have a higher photostability and stability in active media. This work presents a review of the modern literature devoted to the synthesis and properties of colloidal indium phosphide quantum dots, which are very promising for use as substitutes of toxic cadmium-based quantum dots.

Color tuning of indium phosphide quantum dots for cadmium‐free quantum dot light‐emitting devices with high efficiency and color saturation

AbstractSemiconductor quantum dots (QDs) promise facile color tuning and high color saturation in quantum‐dot light‐emitting devices (QD‐LEDs) by controlling nanoparticle size and size distribution. Here, we demonstrate how this promise can be practically realized for the cadmium‐free InP/ZnSe/ZnS multishell quantum dots. We developed a set of synthesis conditions and core/shell compositions that result in QDs with green, yellow, and red emission color. The QD‐LEDs employing these QDs show efficient electroluminescence (EL) with luminance up to 1800 cd/m2 and efficiency up to 5.1 cd/A. The color coordinates calculated from the EL spectra clearly demonstrate the outstanding color saturation as an outcome of the narrow particle size distribution. These results prove that the performance gap between cadmium‐free and cadmium‐based QDs in QD‐LEDs is shrinking rapidly.

Two-Step Nucleation and Growth of InP Quantum Dots via Magic-Sized Cluster Intermediates

We report on the role of magic-sized clusters (MSCs) as key intermediates in the synthesis of indium phosphide quantum dots (InP QDs) from molecular precursors. Heterogeneous growth from the MSCs directly to InP QDs was observed without intermediate sized particles. These observations suggest that previous efforts to control nucleation and growth by tuning precursor reactivity have been undermined by formation of these kinetically persistent MSCs prior to QD formation. The thermal stability of InP MSCs is influenced by the presence of exogenous bases as well as choice of the anionic ligand set. Addition of a primary amine, a common additive in previous InP QD syntheses, to carboxylate-terminated MSCs was found to bypass the formation of MSCs, allowing for homogeneous growth of InP QDs through a continuum of isolable sizes. Substitution of the carboxylate ligand set for a phosphonate ligand set increased the thermal stability of one particular InP MSC to 400 °C. The structure and optical properties of the MSCs with both carboxylate and phosphonate ligand sets were studied by UV–vis absorption spectroscopy, powder XRD analysis, and solution 31P{1H} and 1H NMR spectroscopy. Finally, the carboxylate-terminated MSCs were identified as effective single-source precursors (SSPs) for the synthesis of high-quality InP QDs. Employing InP MSCs as SSPs for QDs effectively decouples the formation of MSCs from the subsequent second nucleation event and growth of InP QDs. The concentration dependence of this SSP reaction, as well as the shape uniformity of particles observed by TEM suggests that the stepwise growth from MSCs directly to QDs proceeds via a second nucleation event rather than an aggregative growth mechanism.

Large-scale synthesis of high quality InP quantum dots in a continuous flow-reactor under supercritical conditions

The synthesis of indium phosphide quantum dots (QDs) in toluene under supercritical conditions was carried out in a macroscopic continuous flow reaction system. The results of first experiments are reported in comparison with analogous reactions in octadecene. The reaction system is described and details are provided about special procedures that are enabled by the continuous flow system for the screening of reaction conditions. The produced QDs show very narrow emission peaks with full width at half maximum down to 45 nm and reasonable photoluminescence quantum yields. The subsequent purification process is facilitated by the ease of removal of toluene, and the productivity of the system is increased by high temperature and high pressure conditions.

InP Quantum Dots: An Environmentally Friendly Material with Resonance Energy Transfer Requisites

Growing demand for clean energy has intensified the interest in understanding the properties of environmentally friendly materials for future energy devices. Indium phosphide (InP) is relatively nontoxic as compared to cadmium chalcogenides, and herein we demonstrate the successful use of this material for resonance energy transfer applications. Three chromophoric dyes, namely, lissamine rhodamine B ethylene diamine (LiRh), Texas red cadavarine C5 (TxRed), and rhodamine 101 (Rh101), possessing free anchoring groups were used as acceptors in InP quantum dot (QD)-based donor–acceptor pairs. The energy transfer process was monitored by steady-state and time-resolved emission spectroscopic techniques. Large values of quenching constant (kq), in the range of 1013–1014 M–1 s–1, observed on addition of chromophoric dyes to InP overcoated with zinc sulphide (InP/ZnS), confirm that the interaction is predominantly static in nature. Selective excitation of the QD component at 405 nm showed a rapid decay of InP/ZnS emission and a concomitant growth of the acceptor emission (rise time of ∼200 ps), indicating that all these systems follow a nonradiative energy transfer mechanism. Time-resolved emission studies confirm that the photoexcited InP/ZnS QDs decays to the ground state by transferring the excitation energy to the chromophoric dyes leading to the formation of its excited state. The high efficiency of energy transfer observed in these systems further confirms that InP is an excellent energy harvester with potential use in biomedical and photovoltaic applications.

Investigation of Indium Phosphide Quantum Dot Nucleation and Growth Utilizing Triarylsilylphosphine Precursors

We have developed a two-phosphine strategy to independently tune nucleation and growth kinetics based on the relative reactivity of each precursor in the synthesis of indium phosphide (InP) quantum dots (QDs). This approach was allowed by the exploration of the synthesis and reactivity of a series of sterically encumbered triarylsilylphosphines substituted at the para position of the aryl group, P(Si(C6H4-X)3)3 (X = H, Me, CF3, or Cl), as a contrast to P(SiMe3)3, the P3– source commonly employed in such syntheses. UV–vis absorption spectroscopy of aliquots taken during InP QD growth revealed a stark contrast between triarylsilylphosphines with electron-donating and electron-withdrawing groups in both the rate of InP formation and the final particle size. 31P{1H} nuclear magnetic resonance spectroscopy confirmed that precursor conversion remains rate-limiting throughout the nanocrystal synthesis when P(SiPh3)3 is incorporated as the sole phosphorus precursor; however, this is insufficient for effective separation of nucleation and growth in this system because of the slow nucleation rates that result. In all cases, syntheses that employ a single chemical species as the P3– source were found to suffer from a poor match in reactivity with In(O2C(CH2)12CH3)3 as they either fail to separate nucleation from growth because of slow precursor conversion rates [P(SiPh3)3 and P(Si(C6H4-Me)3)3] or preclude size selective growth from rapid precursor conversion [P(SiMe3)3, P(Si(C6H4-Cl)3)3, and P(Si(C6H4-CF3)3)3]. To balance these two extreme cases, we developed a novel approach in which two different P3– sources were introduced to segregate nucleation and growth based on the relative reactivity of each precursor.

Facile dental resin composites with tunable fluorescence by tailoring Cd-free quantum dots

Non-toxic indium phosphide (InP) quantum dots (QDs) were synthesized by the solvothermal method and then embedded into dental resin to tune the emission color of the resin. Cell viability was investigated with a 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assay and a reactive oxygen species (ROS) assay to evaluate biological applications. The results clearly indicate that employing InP QDs in creating dental resin composites allows for the fabrication of restorative materials with biocompatible fluorescence properties.

Single-photon emission from electrically driven InP quantum dots epitaxially grown on CMOS-compatible Si(001)

The heteroepitaxy of III–V semiconductors on silicon is a promising approach for making silicon a photonic platform. Mismatches in material properties, however, present a major challenge, leading to high defect densities in the epitaxial layers and adversely affecting radiative recombination processes. However, nanostructures, such as quantum dots, have been found to grow defect-free even in a suboptimal environment. Here we present the first realization of indium phosphide quantum dots on exactly oriented Si(001), grown by metal–organic vapour-phase epitaxy. We report electrically driven single-photon emission in the red spectral region, meeting the wavelength range of silicon avalanche photodiodes’ highest detection efficiency.

InP/ZnSe/ZnS: A Novel Multishell System for InP Quantum Dots for Improved Luminescence Efficiency and Its application in a Light-Emitting Device

Indium phosphide (InP) quantum dots (QDs) are considered alternatives to Cd-containing QDs for application in light-emitting devices. The multishell coating with ZnSe/ZnS was shown to improve the photoluminescence quantum yield (QY) of InP QDs more strongly than the conventional ZnS shell coating. Structural proof for this system was provided by X-ray diffraction and transmission electron microscopy. QY values in the range of 50–70% along with peak widths of 45–50 nm can be routinely achieved, making the optical performance of InP/ZnSe/ZnS QDs comparable to that of Cd-based QDs. The fabrication of a working electroluminescent light-emitting device employing the reported material demonstrated the feasibility of the desired application.

Cytotoxicity of InP/ZnS quantum dots related to reactive oxygen species generation

Indium phosphide (InP) quantum dots (QDs) have emerged as a presumably less hazardous alternative to cadmium-based particles, but their cytotoxicity has not been well examined. Although their constituent elements are of very low toxicity to cells in culture, they nonetheless exhibit phototoxicity related to generation of reactive oxygen species by excited electrons and/or holes interacting with water and molecular oxygen. Using spin-trap electron paramagnetic resonance (EPR) spectroscopy and reporter assays, we find a considerable amount of superoxide and a small amount of hydroxyl radical formed under visible illumination of biocompatible InP QDs with a single ZnS shell, comparable to what is seen with CdTe. A double thickness shell reduces the reactive oxygen species concentration approximately two-fold. Survival assays in five cell lines correspondingly indicate a distinct reduction in toxicity with the double-shell InP QDs. Toxicity varies significantly across cell lines according to the efficiency of uptake, being overall significantly less than what is seen with CdTe or CdSe/ZnS. This indicates that InP QDs are a useful alternative to cadmium-containing QDs, while remaining capable of electron-transfer processes that may be undesirable or which may be exploited for photosensitization applications.

Surface Chemistry of InP Quantum Dots: A Comprehensive Study

Advanced (1)H, (13)C, and (31)P solution and solid-state NMR studies combined with IR spectroscopy were used to probe, at the molecular scale, the composition and the surface chemistry of indium phosphide (InP) quantum dots (QDs) prepared via a non-coordinating solvent strategy. This nanomaterial can be described as a core-multishell object: an InP core, with a zinc blende bulk structure, is surrounded first by a partially oxidized surface shell, which is itself surrounded by an organic coating. This organic passivating layer is composed, in the first coordination sphere, of tightly bound palmitate ligands which display two different bonding modes. A second coordination sphere includes an unexpected dialkyl ketone and residual long-chain non-coordinating solvents (ODE and its isomers) which interact through weak intermolecular bonds with the alkyl chains of the carboxylate ligands. We show that this ketone is formed during the synthesis process via a decarboxylative coupling route and provides oxidative conditions which are responsible for the oxidation of the InP core surface. This phenomenon has a significant impact on the photoluminescence properties of the as-synthesized QDs and probably accounts for the failure of further growth of the InP core.

Water Splitting by Visible Light: A Nanophotocathode for Hydrogen Production

Efficient production of solar fuels is an imperative for meeting future fossil-fuel-free energy demands. Hydrogen that is derived from the splitting of water by solar energy is clearly attractive as a clean energy vector, and there have been many attempts to construct viable molecular and biomolecular devices for photohydrogen production. A common approach in the construction of such devices is the utilization of tris(bipyridine)ruthenium, zinc porphyrin, or related molecular materials as photosensitizers in conjunction with a tethered or free electrocatalyst or enzymic system. Apart from cost, such systems suffer from having limited lifetimes, which may be attributed at least in part to the intrinsic reactivity of the organic N-donor ligands in the radical anion form of the photoexcited state and photodegradation pathways.

Electron and Hole Transfer from Indium Phosphide Quantum Dots

Electron- and hole-transfer reactions are studied in colloidal InP quantum dots (QDs). Photoluminescence quenching and time-resolved transient absorption (TA) measurements are utilized to examine hole transfer from photoexcited InP QDs to the hole acceptor N,N,N',N'-tetramethyl-p-phenylenediamine (TMPD) and electron transfer to nanocrystalline titanium dioxide (TiO2) films. Core-confined holes are effectively quenched by TMPD, resulting in a new approximately 4-ps component in the TA decay. It is found that electron transfer to TiO2 is primarily mediated through surface-localized states on the InP QDs.

Experimental and theoretical investigation of electronic structure in colloidal indium phosphide quantum dots

We present detailed electronic structure calculations, based on an atomistic pseudopotential approach, for a 42 Å InP QD; theoretical results are correlated to results of experimental measurements. To better understand energy loss dynamics and mechanisms for hot carriers in QDs, we investigate relaxation between electronic levels in InP QDs with diameters significantly smaller than twice the Bohr exciton radius. Energy level spacings exceeding the optical phonon energy present the opportunity for slowed charge carrier cooling (phonon bottleneck effect). However, interaction between confined electrons and holes provides an alternate, efficient relaxation route. Therefore, we expect sufficiently fast charge separation to weaken the Coulomb interaction and decrease the electronic energy loss rate. We study electronic relaxation in InP QDs using variations of subpicosecond transient absorption spectroscopy.

Theoretical and experimental investigation of electronic structure and relaxation of colloidal nanocrystalline indium phosphide quantum dots

We present results of theoretical studies of the electronic structure, and experimental studies of electronic relaxation dynamics, for colloidally synthesized InP quantum dots (QD's). Detailed theoretical calculations of the electronic structure of a 41.8 \AA{} diameter InP QD, based on an atomistic pseudopotiential approach, are presented and discussed in the context of experimental measurements. Using femtosecond transient absorption (TA) spectroscopy, we find that the rate of relaxation of photogenerated excitons to the lowest-energy exciton level varies depending upon excitation energy and surface chemistry. Etching the QD's passivates surface electron traps and yields enhanced carrier cooling, which we ascribe to improved confinement of charge carriers to the QD core. When exciting near or slightly above the first exciton state, we observe a sub-picosecond decay of the band edge TA bleach signal which we attribute to a thermalization process. We also present size-selective transient absorption measurements providing experimental evidence which confirms the existence of two s-like exciton states spaced by \ensuremath{\sim}100 meV.

Anomalies in the linear absorption, transient absorption, photoluminescence and photoluminescence excitation spectroscopies of colloidal InP quantum dots

We report photoluminescence (PL), linear absorption and femtosecond, transient bleaching spectra for a colloidal solution of indium phosphide (InP) quantum dots (QDs) at ambient temperature. The PL quantum yield is shown to depend significantly upon the excitation wavelength and the PL excitation spectrum deviates markedly from the absorption spectrum. The cooling of electrons and holes to the lowest energy excited state is determined, by transient bleaching spectroscopy, to occur with an efficiency that is independent of the excitation wavelength. These results are discussed in terms of a threshold for a non-radiative decay process that resides above the bandgap and a PL quantum yield that depends upon the size of QD.