InP Core Research Articles

Bright and Uniform Green Light Emitting InP/ZnSe/ZnS Quantum Dots for Wide Color Gamut Displays

There is an urgent demand to improve the efficiency and the color purity of the environment-friendly quantum dots (QDs), which can be used in wide color gamut (WCG) displays. In this study, we optimized the reaction conditions for the InP core synthesis and the ZnSe/ZnS multishell growth on the core. As a result, remarkable improvements were achieved in the photoluminescence quantum yield (PL QY, 95%) and the full width at half-maximum (fwhm, 36 nm), with perfectly matched wavelength (528 nm) for the green color in WCG displays. Injection of the phosphorus precursor at a mild temperature during the InP core synthesis reduced the size distribution of the core to 12%, and the shell growth performed at a high temperature significantly enhanced the crystallinity of the thick passivating layer. We also investigated the photophysical properties, particularly the energy trap distributions and trap state emissions of the InP-based QDs with different shell structures. The time-resolved and temperature-dependent PL...

Coulomb blockade from the shell of an InP-InAs core-shell nanowire with a triangular cross section

We report on growth of InP-InAs core-shell nanowires and demonstration of the formation of single quantum structures, which show the Coulomb blockade effect, over entire lengths of the nanowires. The core-shell nanowires are grown by a selective area growth technique via metal-organic vapor phase epitaxy. The as-grown core-shell nanowires are found to be of wurtzite crystals. The InP cores have a hexagonal cross section, while the InAs shells are grown preferentially on specific {11¯00} facets, leading to the formation of the core-shell nanowires with an overall triangular cross section. The grown core-shell nanowires are transferred onto a Si/SiO2 substrate and then contacted by several narrow metal electrodes. Low-temperature transport measurements show the Coulomb-blockade effect. We analyze the measured gate capacitance and single electron charging energy of the devices and demonstrate that a quantum structure which shows the Coulomb blockade effect of a many-electron quantum dot is formed over the full length of a single core-shell nanowire and consists of the entire InAs shell in the nanowire.

Vertical InGaAs Nanowire Array Photodiodes on Si

We demonstrated vertical InGaAs nanowire (NW) array photodiodes on Si that were optically responsive to visible light (635 nm) and near-infrared light (∼1.55 μm). The vertical NWs were directly grown on Si by selective-area growth. Implementation of a heavily Sn-doped contact layer in the InGaAs NWs improved the diode characteristic because of a lower series resistance, and as a result, a photoresponsivity of 0.25 A/W was obtained at 635 nm, which was twice that before the improvement. Moreover, the photocurrent density of InGaAs–InP core–shell NWs was about 20-fold higher under illumination with light in the 1.55 μm wavelength band as a result of suppression of surface recombination. These findings are expected to be useful for NW-based photovoltaic applications for optical interconnection and Si platforms.

Influence of interface defects on the optical properties of InP/ZnS quantum dots by low temperature synthesis of InP core

We present a series of investigations focused on the effect of synthesis temperature on the optical characteristics of InP/ZnS QDs. Absorption spectra confirmed the successful synthesis of InP QDs under a temperature as low as 140 °C. Because of the poor optical characteristics possibly caused by the inherently high defect states on the surface of InP QDs, a modified ZnS shell overcoating was applied on the InP QDs. An InP core synthesized at a relatively low temperature of 160 °C or less is presumed to possess fewer surface defects; this was confirmed as a reduction in the red-shift due to the shell coating. A satisfactory quantum yield of 52% was obtained from the InP/ZnS QDs synthesized at a reaction temperature and reaction time of 140 °C and 20 h, respectively, with the ZnS shell coating. Thus, we successfully synthesized InP/ZnS QDs using an InP core synthesized at a low temperature of 140 °C through our suggested shell coating process.

Influence of Dithiols on Fluorescence Blinking of Colloidal Quantum Dots InP@ZnS

The influence of 1,6-hexanedithiol as an additional short-chain stabilizing ligand on fluorescence blinking of semiconductor colloidal quantum dots InP@ZnS has been studied using the analysis of the fluorescence decay curves. The optimum proportion of 1,6-hexanedithiol in the mixture has been determined. The influence of the thickness of the ZnS shells and the diameter of the InP core has been also studied. Correlation of these dependences with the analogous dependences for the luminescence quantum yield is observed.

Bandgap Engineering of Indium Phosphide-Based Core/Shell Heterostructures Through Shell Composition and Thickness.

The large bulk bandgap (1.35 eV) and Bohr radius (~10 nm) of InP semiconductor nanocrystals provides bandgap tunability over a wide spectral range, providing superior color tuning compared to that of CdSe quantum dots. In this paper, the dependence of the bandgap, photoluminescence emission, and exciton radiative lifetime of core/shell quantum dot heterostructures has been investigated using colloidal InP core nanocrystals with multiple diameters (1.5, 2.5, and 3.7 nm). The shell thickness and composition dependence of the bandgap for type-I and type-II heterostructures was observed by coating the InP core with ZnS, ZnSe, CdS, or CdSe through one to ten iterations of a successive ion layer adsorption and reaction (SILAR)-based shell deposition. The empirical results are compared to bandgap energy predictions made with effective mass modeling. Photoluminescence emission colors have been successfully tuned throughout the visible and into the near infrared (NIR) wavelength ranges for type-I and type-II heterostructures, respectively. Based on sizing data from transmission electron microscopy (TEM), it is observed that at the same particle diameter, average radiative lifetimes can differ as much as 20-fold across different shell compositions due to the relative positions of valence and conduction bands. In this direct comparison of InP/ZnS, InP/ZnSe, InP/CdS, and InP/CdSe core/shell heterostructures, we clearly delineate the impact of core size, shell composition, and shell thickness on the resulting optical properties. Specifically, Zn-based shells yield type-I structures that are color tuned through core size, while the Cd-based shells yield type-II particles that emit in the NIR regardless of the starting core size if several layers of CdS(e) have been successfully deposited. Particles with thicker CdS(e) shells exhibit longer photoluminescence lifetimes, while little shell-thickness dependence is observed for the Zn-based shells. Taken together, these InP-based heterostructures demonstrate the extent to which we are able to precisely tailor the material properties of core/shell particles using core/shell dimensions and composition as variables.

A Layer-by-Layer Growth Strategy for Large-Size InP/ZnSe/ZnS Core–Shell Quantum Dots Enabling High-Efficiency Light-Emitting Diodes

Shell is of great significance to the enhancement in the photoluminescence quantum yield (PLQY) and stability of core–shell-type quantum dots (QDs). InP/ZnS core–shell QDs without intrinsic toxicity have shown huge potential as a replacement for the widely used cadmium-containing QDs; however, it is still challenging to control the growth of InP-based core–shell QDs due to the lattice mismatch between the InP core and ZnS shell. Here, we report on the synthesis of ∼15-nm-size InP/ZnSe/ZnS QDs with a thick ZnS outer shell by a layer-by-layer shell growth strategy. The ZnS shell was prepared by a circularly gradient temperature rise and long reaction procedure in each step, which not only ensures relatively low precursor concentration preventing the anisotropic growth of QDs but also allows the low-reactivity source to be decomposed sufficiently to achieve layer-by-layer growth of a thick ZnS shell. The resulting QDs show the highest PLQY of 73%, narrow emission line width of up to 40 nm, wide spectrum tuna...

Exciton Fine Structure and Lattice Dynamics in InP/ZnSe Core/Shell Quantum Dots.

Nanocrystalline InP quantum dots (QDs) hold promise for heavy-metal-free optoelectronic applications due to their bright and size-tunable emission in the visible range. Photochemical stability and high photoluminescence (PL) quantum yield are obtained by a diversity of epitaxial shells around the InP core. To understand and optimize the emission line shapes, the exciton fine structure of InP core/shell QD systems needs be investigated. Here, we study the exciton fine structure of InP/ZnSe core/shell QDs with core diameters ranging from 2.9 to 3.6 nm (PL peak from 2.3 to 1.95 eV at 4 K). PL decay measurements as a function of temperature in the 10 mK to 300 K range show that the lowest exciton fine structure state is a dark state, from which radiative recombination is assisted by coupling to confined acoustic phonons with energies ranging from 4 to 7 meV, depending on the core diameter. Circularly polarized fluorescence line-narrowing (FLN) spectroscopy at 4 K under high magnetic fields (up to 30 T) demonstrates that radiative recombination from the dark F = ±2 state involves acoustic and optical phonons, from both the InP core and the ZnSe shell. Our data indicate that the highest intensity FLN peak is an acoustic phonon replica rather than a zero-phonon line, implying that the energy separation observed between the F = ±1 state and the highest intensity peak in the FLN spectra (6 to 16 meV, depending on the InP core size) is larger than the splitting between the dark and bright fine structure exciton states.

Structural control of InP/ZnS core/shell quantum dots enables high-quality white LEDs

Herein, we demonstrate that the structural and optical control of InP-based quantum dots (QDs) can lead to high-performance light-emitting diodes (LEDs). Zinc sulphide (ZnS) shells passivate the InP QD core and increase the quantum yield in green-emitting QDs by 13-fold and red-emitting QDs by 8-fold. The optimised QDs are integrated in the liquid state to eliminate aggregation-induced emission quenching and we fabricated white LEDs with a warm, neutral and cool-white appearance by the down-conversion mechanism. The QD-functionalized white LEDs achieve luminous efficiency (LE) up to 14.7 lm W−1 and colour-rendering index up to 80. The structural and optical control of InP/ZnS core/shell QDs enable 23-fold enhancement in LE of white LEDs compared to ones containing only QDs of InP core.

Stokes-Shift-Engineered Indium Phosphide Quantum Dots for Efficient Luminescent Solar Concentrators.

Luminescent solar concentrators (LSCs) show promise because of their potential for low-cost, large-area, and high-efficiency energy harvesting. Stokes shift engineering of luminescent quantum dots (QDs) is a favorable approach to suppress reabsorption losses in LSCs; however, the use of highly toxic heavy metals in QDs constitutes a serious concern for environmental sustainability. Here, we report LSCs based on cadmium-free InP/ZnO core/shell QDs with type-II band alignment that allow for the suppression of reabsorption by Stokes shift engineering. The spectral emission and absorption overlap was controlled by the growth of a ZnO shell on an InP core. At the same time, the ZnO layer also facilitates the photostability of the QDs within the host matrix. We analyzed the optical performance of indium-based LSCs and identified the optical efficiency as 1.45%. The transparency, flexibility, and cadmium-free content of the LSCs hold promise for solar window applications.

Photoresponse and Field Effect Transport Studies in InAsP–InP Core–Shell Nanowires

A ternary InAsyP1−y alloy is suitable for an application to near-infrared (NIR) optical devices as their direct bandgap energy covers the entire NIR band. A nanowire (NW) system allows an epitaxial integration of InAsyP1−y alloy on any type of substrate since the lattice mismatch strain can be relieved through the NW sidewall. Nevertheless, the very large surface to volume ratio feature of the NWs leads to enormous surface states which are susceptible to surface recombination of free carriers. Here, ternary InAs0.75P0.25 NWs are grown with InP passivation layer (i.e., core–shell structure) to minimize the influence of the surface states, thus increasing their optical and electrical properties. A photoresponse study was achieved through the modeled band structure of the grown NWs. The model and experimental results suggest that 5-nm-thick InP shell efficiently passivates the surface states of the InAs0.75P0.25 NWs. The fabricated core–shell photodetectors and field-effect transistors exhibit improved photoresponse and transport properties compared to its counterpart core-only structure.

Synthesis temperature-dependent optical properties of ZnS-shell formation on InP nanoparticles

In this work reports the controlled synthesis and physical property characterization of nanoparticles of InP/ZnS type with core/shell structure obtained by the colloid chemistry method called "single-step procedure without precursor injection" at different synthesis temperatures. Varying the reaction temperature from 100 to 320 °C was possible to control the formation of the ZnS-shell on InP nanoparticles. Colour changes in the colloid dispersions were clearly observed according to the reaction temperature. The InP/ZnS nanoparticles chemical stoichiometry was determined by Energy-dispersive X-ray spectroscopy, demonstrating that with the increase of the reaction temperature the particles are obtained with a better stoichiometric ratio. The X-ray-diffraction analysis reveals that InP/ZnS nanoparticles showed zinc blende crystalline phase that was confirmed by High-resolution transmission electron microscopy. The average particle sizes (2–10 nm) were estimated by the Wang equation, which are confirmed analysing the grain average diameter by transmission electron microscopy measurements. The particles sizes indicate a high quantum confinement because of they are lower than the Bohr excitonic radius of InP. The obtained semiconductor nanoparticles presented crystalline structure InP-core/ZnS-shell, uniformity in size and exhibit a dependence of emission in the range of 450–650 nm measured by UV–Vis spectroscopy, which allowed obtaining the band gap of the nanostructures. The value of the bandgap could be tuned from 3.73 to 2.38 eV through the variation of the synthesis temperature. The emission peak in InP core quantum dots varied as a function of quantum dots size, ranging in the 2.61–2.17 eV region. The InP/ZnS nanoparticles present a high passivation for samples synthetized at 300 °C.

Improvement in luminance of light-emitting diode using InP/ZnS quantum dot with 1-dodecanethiol ligand

We present the synthesis protocol of a red emissive InP/ZnS quantum dot with a 1-dodecanthiol ligand and its application to a quantum dot light-emitting diode. The ligand change from oleylamine to 1-dodecanthiol, which were connected around the InP/ZnS quantum dot, was confirmed by Fourier-transform infrared spectroscopy and thermal analysis. The absorption peak was blue-shifted by changing 1-dodecanthiol ligands from oleylamine ligands to prevent the unexpected nucleation of the InP core. In addition, the luminance of the light-emitting device was improved by using the InP/ZnS quantum dot with 1-dodecanthiol ligands, and the maximum current efficiency of 7.2 × 10−3 cd/A was achieved. The 1-dodecanthiol ligand is often used for capping to reduce the number of surface defects and/or prevent unexpected core growth, resulting in reduced Auger recombination. This result indicates that 1-dodecanthiol ligands prevent the deactivation of excitons while injecting carriers by applying a voltage, resulting in a high luminance efficiency.

Chemical Structure, Ensemble and Single-Particle Spectroscopy of Thick-Shell InP-ZnSe Quantum Dots.

Thick-shell (>5 nm) InP-ZnSe colloidal quantum dots (QDs) grown by a continuous-injection shell growth process are reported. The growth of a thick crystalline shell is attributed to the high temperature of the growth process and the relatively low lattice mismatch between the InP core and ZnSe shell. In addition to a narrow ensemble photoluminescence (PL) line-width (∼40 nm), ensemble and single-particle emission dynamics measurements indicate that blinking and Auger recombination are reduced in these heterostructures. More specifically, high single-dot ON-times (>95%) were obtained for the core-shell QDs, and measured ensemble biexciton lifetimes, τ2x ∼ 540 ps, represent a 7-fold increase compared to InP-ZnS QDs. Further, high-resolution energy dispersive X-ray (EDX) chemical maps directly show for the first time significant incorporation of indium into the shell of the InP-ZnSe QDs. Examination of the atomic structure of the thick-shell QDs by high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) reveals structural defects in subpopulations of particles that may mitigate PL efficiencies (∼40% in ensemble), providing insight toward further synthetic refinement. These InP-ZnSe heterostructures represent progress toward fully cadmium-free QDs with superior photophysical properties important in biological labeling and other emission-based technologies.

Optical properties and structure of Ag-doped InP quantum dots prepared by a phosphine synthetic route.

We report a novel technique for doping InP quantum dots (QDs) with silver that makes use of a AgCl·TOP complex as a source of silver and PH3 as a phosphorus precursor. Formation of two types of nanoparticles in the reaction mixture is observed: Ag-doped InP QDs form at lower silver concentrations and novel Ag/InP core-shell nanostructures form at high Ag-precursor concentrations. Ag-doped QDs possess two luminescent bands: an excitonic band at approx. 670 nm and an IR-band, indicative of dopant luminescence, at approx. 900 nm. Post-synthetic covering of the quantum dots with ZnS leads to a significant increase in both types of luminescence. The core-shell Ag/InP nanoparticles did not emit any luminescence in the visible and near-IR spectral regions.

Indium Phosphide-Based Quantum Dots with Shell-Enhanced Absorption for Luminescent Down-Conversion.

It is shown that admixing small amounts of cadmium into the shell of InP/ZnSe core/shell quantum dots results in an increased absorption of blue light and a limited redshift of the band-edge emission. These effects reflect the reduced bandgap of (Zn,Cd)Se alloys and their smaller conduction-band offset with InP. Nevertheless, adjusting the InP core size enables InP/ZnSe and InP/(Zn,Cd)Se quantum dots with identical emission characteristics to be made. Processing both materials into remote phosphor disks, it is demonstrated that the shell-enhanced absorbance of InP/(Zn,Cd)Se has the double benefit of suppressing self-absorption and reducing the amount of quantum dots by weight needed to attain a given blue-to-red color conversion.

Temperature dependence of the optical absorption spectra of InP/ZnS quantum dots

The optical-absorption spectra of InP/ZnS (core/shell) quantum dots have been studied in a broad temperature range of T = 6.5–296 K. Using the second-order derivative spectrophotometry technique, the energies of optical transitions at room temperature were found to be E 1 = 2.60 ± 0.02 eV (for the first peak of excitonic absorption in the InP core) and E 2 = 4.70 ± 0.02 eV (for processes in the ZnS shell). The experimental curve of E 1(T) has been approximated for the first time in the framework of a linear model and in terms of the Fan’s formula. It is established that the temperature dependence of E 1 is determined by the interaction of excitons and longitudinal acoustic phonons with hω = 15 meV.

Growth mechanism of InP and InP/ZnS synthesis using colloidal synthesis

This study investigates the main growth mechanism of InP during InP/ZnS reaction of quantum dots (QDs). The size of the InP core, considering a synthesis time of 1-30 min, increased from the initial 2.56 nm to 3.97 nm. As a result of applying the proposed particle growth model, the migration mechani...

Temperature-induced shift of the exciton absorption band in InP/ZnS quantum dots

Using the second-order derivative spectrophotometry method, we have determined the energy of optical transitions of colloidal InP/ZnS core-shell quantum dots at room temperature: E1 = 2.60 eV corresponds to a first exciton absorption peak of the InP core, E2 = 4.70 eV appears to meet the processes in the ZnS shell. We have investigated E1(T) temperature dependence within 6.5 – 296 K range for the first time. The obtained experimental data have been approximated by means of a linear model and Fan’s expression. Also, it has been shown that the interaction between excitons and longitudinal acoustic phonons causes the energy E1 to change with temperature. In this case, the peak’s FWHM remains invariant and amounts to 0.39 eV, which indicates the inhomogeneous broadening character and a high degree of static disorder in the ensemble of the quantum dots studied.

Growth and characterization of wurtzite InP/AlGaP core–multishell nanowires with AlGaP quantum well structures

We report on the selective-area growth and characterization of wurtzite (WZ) InP/AlGaP core–multishell nanowires. Quantum well (QW) structures were fabricated in AlGaP multishells by changing the alloy composition. Transmission electron microscopy revealed that the AlGaP multishells were grown with a WZ structure on the side of the WZ InP core. The lattice constants of the WZ InP core and WZ AlGaP shell were determined by X-ray diffraction. Cathodoluminescence studies showed that the WZ AlGaP QW with an Al composition of 20% exhibited green emissions at 2.37 eV. These results open the possibility of fabricating green light-emitting diodes using WZ AlGaP-based materials.