Indium Phosphide Quantum Dots Research Articles

An Integrated Multilevel Analysis Profiling Biosafety and Toxicity Induced by Indium- and Cadmium-Based Quantum Dots in Vivo.

Indium phosphide quantum dots (QDs) have emerged as a new class of fluorescent nanocrystals for manifold applications, from biophotonics to nanomedicine. Recent efforts in improving the photoluminescence quantum yield, the chemical stability and the biocompatibility turned them into a valid alternative to well established Cd-based nanocrystals. In vitro studies provided first evidence for the lower toxicity of In-based QDs. Nonetheless, an urgent need exists for further assessment of the potential toxic effects in vivo. Here we use the freshwater polyp Hydra vulgaris, a well-established model previously adopted to assess the toxicity of CdSe/CdS nanorods and CdTe QDs. A systematic multilevel analysis was carried out in vivo, ex vivo, and in vitro comparing toxicity end points of CdSe- and InP-based QDs, passivated by ZnSe/ZnS shells and surface functionalized with penicillamine. Final results demonstrate that both the chemical composition of the QD core (InP vs CdSe) and the shell play a crucial role for final outcomes. Remarkably, in absence of in vivo alterations, cell and molecular alterations revealed hidden toxicity aspects, highlighting the biosafety of InP-based nanocrystals and outlining the importance of integrated multilevel analyses for proper QDs risk assessment.

An In Vitro Investigation of Cytotoxic Effects of InP/Zns Quantum Dots with Different Surface Chemistries.

Indium phosphide quantum dots (QDs) passivated with zinc sulphide in a core/shell architecture (InP/ZnS) with different surface chemistries were introduced to RAW 264.7 murine “macrophage-like” cells to understand their potential toxicities. The InP/ZnS quantum dots were conjugated with an oligonucleotide, a carboxylic acid, or an amino-polyethylene glycol ligand, and cell viability and cell proliferation were investigated via a metabolic assay. Membrane integrity was measured through the production of lactate dehydrogenase. Fluorescence microscopy showed cellular uptake. All quantum dots exhibited cytotoxic behaviour less than that observed from cadmium- or lead-based quantum dots; however, this behaviour was sensitive to the ligands used. In particular, the amino-polyethylene glycol conjugated quantum dots proved to possess the highest cytotoxicity examined here. This provides quantitative evidence that aqueous InP/ZnS quantum dots can offer a safer alternative for bioimaging or in therapeutic applications.

Cadmium-Free Quantum Dots as Fluorescent Labels for Exosomes.

Quantum dots are attractive alternatives to organic fluorophores for the purposes of fluorescent labeling and the detection of biomarkers. They can also be made to specifically target a protein of interest by conjugating biomolecules, such as antibodies. However, the majority of the fluorescent labeling using quantum dots is done using toxic materials such as cadmium or lead due to the well-established synthetic processes for these quantum dots. Here, we demonstrate the use of indium phosphide quantum dots with a zinc sulfide shell for the purposes of labeling and the detection of exosomes derived from the THP-1 cell line (monocyte cell line). Exosomes are nano-sized vesicles that have the potential to be used as biomarkers due to their involvement in complex cell processes. However, the lack of standardized methodology around the detection and analysis of exosomes has made it difficult to detect these membrane-containing vesicles. We targeted a protein that is known to exist on the surface of the exosomes (CD63) using a CD63 antibody. The antibody was conjugated to the quantum dots that were first made water-soluble using a ligand-exchange method. The conjugation was done using carbodiimide coupling, and was confirmed using a range of different methods such as dynamic light scattering, surface plasmon resonance, fluorescent microscopy, and Fourier transform infrared spectroscopy. The conjugation of the quantum dot antibody to the exosomes was further confirmed using similar methods. This demonstrates the potential for the use of a non-toxic conjugate to target nano-sized biomarkers that could be further used for the detection of different diseases.

Electronic Structure Origins of Surface-Dependent Growth in III–V Quantum Dots

Indium phosphide quantum dots (QDs) have emerged as a promising candidate to replace more toxic II–VI CdSe QDs, but production of high-quality III–V InP QDs with targeted properties requires a better understanding of their growth. We develop a first-principles-derived model that unifies InP QD formation from isolated precursor and early stage cluster reactions to 1.3 nm magic sized clusters and rationalize experimentally observed properties of full sized >3 nm QDs. Our first-principles study on realistic QD models reveals large surface-dependent reactivity for all elementary growth process steps, including In-ligand bond cleavage and P precursor addition. These thermodynamic trends correlate well to kinetic properties at all stages of growth, indicating the presence of labile and stable spots on cluster and QD surfaces. Correlation of electronic or geometric properties to energetics identifies surprising sources for these variations: short In···In separation on the surface produces the most reactive sites, at odds with conventional understanding of strain (i.e., separation) in bulk metallic surfaces increasing reactivity and models for ionic II–VI QD growth. These differences are rationalized by the covalent, directional nature of bonding in III–V QDs and explained by bond order metrics derived directly from the In–O bond density. The unique constraints of carboxylate and P precursor bonding to In atoms rationalize why all sizes of InP clusters and QDs are In-rich but become less so as QDs mature. These observations support the development of alternate growth recipes that take into account strong surface-dependence of kinetics as well as the shapes of both In and P precursors to control both kinetics and surface morphology in III–V QDs.

How Trap States Affect Charge Carrier Dynamics of CdSe and InP Quantum Dots: Visualization through Complexation with Viologen

The depth of surface trap states in semiconductor quantum dots (QDs) is influenced by the degree of covalency, which in turn affects the charge recombination process in hybrid donor–acceptor systems. By taking relatively ionic cadmium selenide (CdSe) QDs with shallow trap states and covalent indium phosphide (InP) QDs having deep trap states as examples, the charge-transfer dynamics are explored using viologen derivative as an electron acceptor. Light-induced electron transfer in a 1:1 stoichiometric complex of both the donor–acceptor systems occurs in a picosecond time scale. The presence of deep hole trap states in InP QDs retards the charge recombination to a submillisecond time scale, which is 7 orders of magnitude lower than that in CdSe QDs in homogeneous solutions. The immobile quenchers in the quenching sphere of InP further stabilize the electron-transfer products to seconds through charge hopping, which extends the potential of these systems for charge-transfer and transport applications in phot...

Probing Surface Defects of InP Quantum Dots Using Phosphorus Kα and Kβ X-ray Emission Spectroscopy

Synthetic efforts to prepare indium phosphide (InP) quantum dots (QDs) have historically generated emissive materials with lower than unity quantum yields. This property has been attributed to structural and electronic defects associated with the InP core as well as the chemistry of the shell materials used to overcoat and passivate the InP surface. Consequently, the uniformity of the core–shell interface plays a critical role. Using X-ray emission spectroscopy (XES) performed with a recently developed benchtop spectrometer, we studied the evolution of oxidized phosphorus species arising across a series of common, but chemically distinct, synthetic methods for InP QD particle growth and subsequent ZnE (E = S or Se) shell deposition. XES afforded us the ability to measure the speciation of phosphorus reliably, quantitatively, and more efficiently (with respect to both the quantity of material required and the speed of the measurement) than with traditional techniques, i.e., X-ray photoelectron spectroscopy...

Broadband Sensitization of Lanthanide Emission with Indium Phosphide Quantum Dots for Visible to Near-Infrared Downshifting.

Semiconductor quantum dot (QD)-sensitized lanthanide ions hold great promise in producing a broadly absorbing and sharply emitting luminophore, but their synthesis has proven to be difficult. We report the first synthesis of core/shell/shell InP/Ln xY1- xF3/ShF3 (Ln = Yb, Nd; Sh = Lu, Y) nanocrystals that exhibit a broad visible absorption coupled to a sharp near-infrared emission. Additionally, this is the first report of Nd being coupled to a QD absorber. We characterize the system with a variety of electron microscopy and X-ray techniques that prove this unique structure. Optical measurements confirm the correlation of the Ln3+ emission to the QD absorption, while the presence of a trap-state emission gives a clue as to the mechanism of energy transfer between the dot and the lanthanide.

Two-Step “Seed-Mediated” Synthetic Approach to Colloidal Indium Phosphide Quantum Dots with High-Purity Photo- and Electroluminescence

Synthesis of size-tunable indium phosphide (InP) quantum dots (QDs) with narrow size distribution has been long known to be more challenging due to the difficulties in controlling QDs nucleation and growth processes via precursor conversion kinetics. Here we present a “seed-mediated” two-step synthesis method for highly monodisperse InP QDs with sizes ranging from 1.9 to 4.5 nm. We further show the synthesis highly luminescent InP-based core-shell-shell QDs with tunable emission across the vis-ible spectrum (515 - 640 nm), having a full width at half-maximum (FWHM) of less than 40 nm for all the wavelengths. We then demonstrate the fabrication of green and red QDs light emitting diodes (QLEDs), and show that our devices exhibit supe-rior color purity than that of InP QDs-based LEDs reported previously. This study advances the chemical research community toward the goal of realizing high color pure emissions from cadmium-free QDs for display applications.

4‐1: Invited Paper: Role of Phosphorus Oxidation in Controlling the Luminescent Properties of Indium Phosphide Quantum Dots

Indium phosphide (InP) is the leading Cd‐free quantum dot material for application in photoluminescence downconversion display and lighting technologies. To date the performance of InP quantum dots (QDs) has lagged behind cadmium selenide in terms of both luminescence line width and quantum yield. Extensive studies in our lab have implicated kinetically persistent magic‐sized cluster intermediates as a leading contributor to polydispersity in these samples, providing new opportunities for achieving high color purity. Now, using a combination of solid‐state NMR spectroscopy and recently developed benchtop X‐ray emission spectroscopy, we have studied the evolution of oxidized phosphorus species during both synthesis and shell growth.

Enhanced luminescence efficiency of wet chemical route synthesized InP-based quantum dots by a novel method: Probing the humidity sensing properties

In indium phosphide quantum dots (InP QDs), the presence of surface states due to unbonded indium or phosphorus atoms quenches its photoluminescence (PL) efficiencies. Hence, it is imperative to passivate these surface states to achieve high photoluminescence efficiencies of InP nanocrystals. In this work, a novel post-synthesis nascent H chemical treatment to enhance the quantum yield of single-pot, wet chemical route synthesized InP QD's is reported for the first time. The main advantages of this post-synthesis treatment are: (i) it is easy, inexpensive and reproducible and (ii) it does not involve harsh chemical treatment viz. dipping of InP QDs in HF-based solutions, nor it requires annealing at high temperatures which may be detrimental to the fragile structure of InP. The significant increment in PL intensity upon aforementioned hydrogen treatment is due to the passivation of surface states and structural recrystallization mechanism that promotes radiative recombination of electrons and holes and hence higher lifetime values. An enhancement in PL intensity of as-synthesized InP QDs upon nascent hydrogen treatment is quite remarkable and is even better than that accomplished by InP-ZnS core-shell QDs with similar size-distribution. The hydrophobicity of H-treated InP QD's is found to be more than untreated InP thus implying higher compactness and structural rigidity similar to as achieved by InP-ZnS core-shell QDs. Mechanisms related to nascent hydrogen treatment, photo-oxidation and PL enhancement and its effect on the surface stoichiometry of InP QDs are discussed via structural, morphological, compositional and optical studies corroborated by various complementary techniques viz. SEM, HRTEM, EDAX,NMR, DLS, Zeta Potential, Contact Angle, PL and Transient Absorption respectively. The application of InP films as a humidity sensor has been demonstrated and untreated InP film in particular shows higher sensitivity values upon exposure to humid treatment as compared with H-treated InP and InP-ZnS core-shell QD's respectively.

Tunable, Bright, and Narrow-Band Luminescence from Colloidal Indium Phosphide Quantum Dots

Synthesis of cadmium (Cd)-free quantum dots (QDs) with tunable emission and high color purity has been a big challenge for the academic and industrial research community. Among various Cd-free QDs, indium phosphide (InP) QDs exhibit reasonably good color purity with emission full width at half-maximum (fwhm) values between 45 and 50 nm for green and over 50 nm for red emission, which is not good enough, as values less than 35 nm are favorable in commercial display products. In this work, we present the synthesis of highly luminescent In(Zn)P/ZnSe/ZnS QDs with tunable emission from 488 to 641 nm and high color purity. We found that the addition of zinc during the conventional SILAR growth of shell (ZnSe or ZnS) deteriorated the absorption features of core InP QDs and resulted in broader emission line widths. We solved this issue by synthesizing Zn carboxylate covered In(Zn)P QDs in a single step and dramatically decreased the emission fwhm to as low as 36 nm with quantum yields (QYs) up to 67% for the gree...

Zinc–Phosphorus Complex Working as an Atomic Valve for Colloidal Growth of Monodisperse Indium Phosphide Quantum Dots

Growth of monodisperse indium phosphide (InP) quantum dots (QDs) represents a pressing demand in display applications, as size uniformity is related to color purity in display products. Here, we report the colloidal synthesis of InP QDs in the presence of Zn precursors in which size uniformity is markedly enhanced as compared to the case of InP QDs synthesized without Zn precursors. Nuclear magnetic resonance spectroscopy, X-ray photoelectron spectroscopy, and mass spectrometry analyses on aliquots taken during the synthesis allow us to monitor the appearance of metal–phosphorus complex intermediates in the growth of InP QDs. In the presence of zinc carboxylate, intermediate species containing Zn–P bonding appears. The Zn–P intermediate complex with P(SiMe3)3 exhibits lower reactivity than that of the In–P complex, which is corroborated by our prediction based on density functional theory and electrostatic potential charge analysis. The formation of a stable Zn–P intermediate complex results in lower reac...

Harnessing Organic Ligand Libraries for First-Principles Inorganic Discovery: Indium Phosphide Quantum Dot Precursor Design Strategies

Indium phosphide quantum dots (QDs) represent promising replacements for more toxic QDs, but InP QD production lags behind other QD materials due to limited understanding of how to tune InP QD growth. We carry out a first-principles, computational screen of the tuning of In carboxylate precursor chemistry to alter the kinetics of elementary steps in InP QD growth. We employ a large database normally used for discovery of therapeutic drug-like molecules to discover design rules for these inorganic complexes while maintaining realism (i.e., stable, synthetically accessible substituents) and providing diversity in a 210-molecule test set. We show the In–O bond cleavage energy, which is tuned through ligand functionalization, to be a useful proxy for In–P bond formation energetics in InP QD synthesis. Energy decomposition analysis on a 32-molecule subset reveals that lower activation energies correlate to later transition states, due to stabilization from greater In–P bond formation and more favorable reactio...

Effective Integration of Targeted Tumor Imaging and Therapy Using Functionalized InP QDs with VEGFR2 Monoclonal Antibody and miR-92a Inhibitor.

Rapid diagnosis and targeted drug treatment require agents that possess multiple functions. Nanomaterials that facilitate optical imaging and direct drug delivery have shown great promise for effective cancer treatment. In this study, we first modified near-infrared fluorescent indium phosphide quantum dots (InP QDs) with a vascular endothelial growth factor receptor 2 (VEGFR2) monoclonal antibody to afford targeted drug delivery function. Then, a miR-92a inhibitor, an antisense microRNA that enhances the expression of tumor suppressor p63, was attached to the VEGFR2-InP QDs via electrostatic interactions. The functionalized InP nanocomposite (IMAN) selectively targets tumor sites and allows for infrared imaging in vivo. We further explored the mechanism of this active targeting. The IMAN was endocytosed and delivered in the form of microvesicles via VEGFR2-CD63 signaling. Moreover, the IMAN induced apoptosis of human myelogenous leukemia cells through the p63 pathway in vitro and in vivo. These results indicate that the IMAN may provide a new and promising chemotherapy strategy against cancer cells, particularly by its active targeting function and utility in noninvasive three-dimensional tumor imaging.

Tapping the potential of trioctylphosphine (TOP) in the realization of highly luminescent blue-emitting colloidal indium phosphide (InP) quantum dots

In this work, extremely small blue emitting colloidal InP-based quantum dots (size ~ 2–5nm) have been synthesized using trioctylphosphine (TOP) as a source of phosphorus. The method reported here is unconventional, quite rapid (~90min), more viable, less expensive and relatively greener as compared to other conventional methods that employ tristrimethylsilyylphosphine(P(SiMe3)3) which is scarce, expensive, flammable, highly toxic and even banned in a few countries. Highly luminescent InP QDs having bluish-green emission (λ~ 490nm) can be synthesized using this method without resorting to any post-synthesis etching to tune the emission to the blue region. Besides being the source of phosphorus and the particle size regulating agent, the efficacy of TOP is further realized during synthesis via its reduction of indium salt, which aids in the formation of indium metal and then subsequently in the development of InP QDs. The PL intensity of as-synthesized InP QDs is further enhanced by growing a shell of wide band gap material, i.e. ZnS resulting in a concurrent increment in quantum yield from ~25% to ~38% respectively.

Electrostatically driven resonance energy transfer in "cationic" biocompatible indium phosphide quantum dots.

Indium Phosphide Quantum Dots (InP QDs) have emerged as an alternative to toxic metal ion based QDs in nanobiotechnology. The ability to generate cationic surface charge, without compromising stability and biocompatibility, is essential in realizing the full potential of InP QDs in biological applications. We have addressed this challenge by developing a place exchange protocol for the preparation of cationic InP/ZnS QDs. The quaternary ammonium group provides the much required permanent positive charge and stability to InP/ZnS QDs in biofluids. The two important properties of QDs, namely bioimaging and light induced resonance energy transfer, are successfully demonstrated in cationic InP/ZnS QDs. The low cytotoxicity and stable photoluminescence of cationic InP/ZnS QDs inside cells make them ideal candidates as optical probes for cellular imaging. An efficient resonance energy transfer (E ∼ 60%) is observed, under physiological conditions, between the cationic InP/ZnS QD donor and anionic dye acceptor. A large bimolecular quenching constant along with a linear Stern-Volmer plot confirms the formation of a strong ground state complex between the cationic InP/ZnS QDs and the anionic dye. Control experiments prove the role of electrostatic attraction in driving the light induced interactions, which can rightfully form the basis for future nano-bio studies between cationic InP/ZnS QDs and anionic biomolecules.

Characterization of Indium Phosphide Quantum Dot Growth Intermediates Using MALDI-TOF Mass Spectrometry.

Clusters have been identified as important growth intermediates during group III-V quantum dot (QD) formation. Here we report a one-solvent protocol that integrates synthesis, purification, and mass characterization of indium phosphide (InP) QD growth mixtures. The use of matrix-assisted laser desorption/ionization (MALDI) mass spectrometry (MS) successfully tracks the evolution of clusters and the formation of QDs throughout the synthesis. Similar clusters are observed during the formation of large particles, suggesting that these clusters serve as a reservoir for QD formation. Combining MALDI and NMR techniques further enables us to extract extinction coefficients and construct sizing curves for cluster-free InP QDs. The use of MALDI MS opens new opportunities for characterization and mechanistic studies of small-sized air-sensitive clusters or QDs.

Shining Light on Indium Phosphide Quantum Dots: Understanding the Interplay among Precursor Conversion, Nucleation, and Growth

InP quantum dots have emerged as an exciting class of phosphors for displays and energy-efficient solid state lighting. Unfortunately, the synthesis of these materials has lagged behind that of related II-VI and IV-VI materials. It is becoming increasingly apparent that this may be due, in many cases, to the inability to control quantum dot nucleation and crystallization using precursor conversion kinetics. In this perspective, recent work on understanding the nucleation and growth of InP from the perspective of nonclassical nucleation models is discussed. In particular, the recent discovery that kinetically persistent magic-size nanoclusters build up during the high temperature synthesis of this material will be highlighted. Isolation and complete structural characterization of one such InP nanocluster has offered unprecedented insight into the structure and surface chemistry of InP and its deviation from how we think about quantum dot structure and composition based on models built for II-VI materials. ...

Mechanistic Insight and Optimization of InP Nanocrystals Synthesized with Aminophosphines

We study the synthesis of indium phosphide quantum dots using InCl3, tris(dimethylamino)phosphine (P(NMe2)3), and oleylamine. We optimized the reaction conditions to reach high chemical yield (∼70%) and size control of the quantum dots with absorption maxima over all the visible range. Kinetic studies of the formation of the quantum dots show that, under certain conditions, the growth of nanoparticles seems to approach a LaMer type growth. We have used 31P NMR, mass spectroscopy, and DFT calculations to decipher the reaction mechanisms of InP formation at the molecular level. The mechanistic investigation is in good agreement with the conclusions drawn from the optimization of the synthetic conditions.

Towards quantum dot and device implementation with InP–GaAs–InP nanostructure

Nanomaterial-based ultra-low-energy device design is one of the prime research areas in nanoscale computation. In ‘more-than-Moore’ technological trends, new device designs with quantum dot nanostructure have become an emerging domain of research. Various quantum dot-based devices, especially quantum dot cellular automata, have been designed both experimentally and by simulation. These quantum devices operate solely on the principle of charge localisation, which accounts for their high speed and ultra-low energy consumption. Quantum dots have an innate ability to confine electrons. As a result, quantum dot formation in a device is highly desirable in order to design quantum devices. Quantum dot implementation with interfacing indium phosphide (InP)–gallium arsenide (GaAs)–indium phosphide nanostructure and quantum dot formation within the redox centres of an allyl molecule are reported in this paper. The formation of the quantum dot is confirmed by the density-of-state studies conducted by the density functional theory calculation on the proposed nanostructures. The possibility of a new device design with two polarised states, namely ‘state x’ and ‘state x −1’, is also explored. Further, outlined in this paper is the design of quantum devices using Python coding, which makes it easier for a programmer to design any kind of complex nanostructure for nanoscale computation.