InP Core Research Articles

InP/ZnS/ZnS core quantum dots for InP luminescence and photoelectrochemical improvement

Indium phosphide quantum dots (InP QDs) is a promising alternative to traditional Quantum Dots (QDs), improving photoluminescence and other properties. We present the synthesis of ZnS/ZnS double shell QDs by the hot injection method. The diameter of QDs was increased from 3.56 nm for the InP core, 4.7 nm for InP/ZnSQDs (with one-time shelling) and 5.53 nm for InP/ZnS/ZnS (double shelling). This experience revealed that the surface morphology of the QDs has a more regular spherical form and is well dispersed. Moreover, the shell's growth results in a strong improvement of the luminescence. Which is due to the passivation of the surface states by shell comparing with ZnSQDs mono shell coating, with an emission peak of 680 nm. Additionally, to improve the photocurrent efficiency, we report the synthesis of nanostructured films by depositing InPQDs, InP/ZnSQDs, and InP/ZnS/ZnSQDs. Spin coating method was used on the surface of Titanium dioxide nanotubes TiO2 NTAs prepared by the electrochemical anodization process. Photoelectrochemical analyses show significant properties improvement of InP/ZnS/TiO2 and InP/ZnS/ZnS/TiO2 in comparison of InP/TiO2. The measurement shows a photocurrent density around 4×10−6mAcm−2 for the photoanode InP/ZnS/ZnS/TiO2. Which is almost six times higher than that measured on TiO2/InPQDs. This result is attributed to a good compromise between Quantum Dots and TiO2NTAs.

Interface engineering of InP/ZnS core/shell quantum dots by the buffer monolayer for exceptional photocatalytic H2 evolution

Remarkable enhancement of photocatalytic hydrogen evolution from quantum dots by introducing buffer layer between the InP core and ZnS shell.

Nonblinking Core–Multishell InP/ZnSe/ZnS Quantum Dot Bioconjugates for Super-resolution Imaging

Colloidal quantum dots (QDs) are vital fluorescent probes with several optical features superior to those found in organic dyes for microscopy imaging. However, the emission blinking behavior of a single QD is an intrinsic drawback for stimulated emission depletion microscopy (STED) or structured illumination microscopy techniques. Herein, we present a rational strategy for the synthesis of a type of deterministic, environmentally friendly, small nonblinking core–multishell InP/ZnSe/ZnS QD using a hydrofluoric acid (HF) etching passivation. The strategy includes four steps: synthesis of the InP core, synthesis of oil-phase core–multishell InP/ZnSe/ZnS QDs using hydrogen fluoride and trioctylphosphine etching passivation, promotion of the water solubility of QDs modified by 3-mercaptopropionic acid (MPA), and enhanced biotargeting of core–multishell InP/ZnSe/ZnS QDs by surface coupling technology. The suppressed blinking mechanism was primarily attributed to surface wrapping shells and HF etching passivation of surface dangling bonds, which reduced surface nonradiative recombination. Nonblinking QDs exhibit high fluorescence efficiency (∼90%), good photostability (>20 min), a large Stokes shift (>135 nm), biocompatibility of the cell viability (>90%), and high targeting. Furthermore, super-resolution multifocal structured illumination microscopy imaging of subcellular structures labeled using these QDs confirmed a resolution improvement of greater than 2.6-fold compared to wide-field microscopy. The results indicate that the nonblinking QDs can function as excellent probes for use in the long-term super-resolution visualization of subcellular dynamics.

Engineering the Interfacial Structure of Heavy Metal‐Free Colloidal Heterostructured Quantum Dots for High‐Efficiency Photoelectrochemical Water Oxidation without Co‐Catalyst

Solar‐driven photoelectrochemical (PEC) water splitting cell fabricated using colloidal quantum dots (QDs) is deemed as low‐cost and high‐efficiency solar‐to‐fuel conversion systems for future carbon neutrality. However, current QDs‐based PEC technologies are still hindered by several critical restrictions including the sluggish water oxidation kinetics and the frequent use of highly toxic QDs (e.g., Pb, Cd‐chalcogenides) as well as co‐catalysts, thus limiting their prospective commercial developments. Herein, the optoelectronic properties of heavy metal‐free InP/ZnSe core/shell QDs are tailored by introducing the interfacial GaP layers with variable thicknesses. As‐prepared InP/GaP/ZnSe core/shell QDs are used to sensitize TiO2 film as photoanodes for PEC water oxidation, showing an unprecedented photocurrent density of 4.1 mA cm−2 at 1.23 V versus reversible hydrogen electrode with excellent durability under one sun AM 1.5 G illumination. It is demonstrated that engineering the thickness of the interfacial GaP layer enables optimized optical properties and can introduce appropriate intermediate energy levels to promote the charge extraction from the InP core to the ZnSe shell for enhanced PEC water oxidation efficiency. This study paves the way for interfacial engineering of “green” core/shell QDs to realize cost‐effective, environment‐friendly, high‐performance, and co‐catalyst‐free QDs‐based PEC water splitting system.

Green InP-based quantum dots and electroluminescent light-emitting diodes

With the advancement toward commercialization of quantum dots (QDs) in the field of lighting and display, improving the performance of Cd-free QDs and related quantum dot light-emitting diodes (QLEDs) becomes necessary. Thus far, the performance of ZnTeSe- and InP-based blue and red QLEDs has been significantly improved by optimizing QDs emitting materials and device structure. However, as one of the three primary color sources, the performance of green InP-based QLEDs still lags behind that of blue and red Cd-free QLEDs. Herein, this review discusses the latest progress of green InP-based emitting materials and corresponding QLEDs, covering the engineering of InP core, the optimization of nanostructure and surface ligands of core/shell QDs, as well as the majorization of device architecture and carrier transport materials. Finally, some challenges and possible development directions of green InP-based QDs and related QLEDs are also identified, which may speed up the commercialization process of Cd-free QDs and corresponding QLEDs.

Inkjet-printed blue InP/ZnS/ZnS quantum dot light-emitting diodes

Inkjet-printed quantum dot light-emitting diodes (QLEDs) are considered to be ideal candidates for use in full-color displays. However, preparation of eco-friendly blue InP QLEDs using inkjet printing techniques remains rather challenging due to the inferior optical properties of QDs. Here, we present the first efficient method for generating inkjet-printed QLEDs with high color purity using blue InP/ZnS/ZnS QDs as the emitting layer. A two-step heat-up and thick shell strategy was employed for the preparation of blue InP/ZnS/ZnS QDs with a photoluminescence (PL) peak at 465 nm, full width at half-maximum of 38 nm, and PL quantum yield (QY) of ∼ 96 %, which are the records for blue InP QDs. We found that the use of a zinc iodide precursor and anhydrous oxygen reaction environment is critical to determining InP core size, and the size distribution of InP cores can be improved by separating formation of InP magic size clusters and cores via a heat-up step at low temperature. We used two kind of zinc precursors to facilitate shell growth, and the engineered shell thickness suppresses energy transfer, enabling InP/ZnS/ZnS QDs film with high PLQYs and contributing to efficient inkjet printing of QLEDs. These results can facilitate the development of blue InP QLEDs for large-area electroluminescent full-color displays.

Full-Spectrum InP-Based Quantum Dots with Near-Unity Photoluminescence Quantum Efficiency

Photoluminescent color conversion by quantum dots (QDs) makes possible the formation of spectrum-on-demand light sources by combining blue LEDs with the light generated by a specific blend of QDs. Such applications, however, require a near-unity photoluminescence quantum efficiency since self-absorption magnifies disproportionally the impact of photon losses on the overall conversion efficiency. Here, we present a synthesis protocol for forming InP-based QDs with +90% quantum efficiency across the full visible spectrum from blue/cyan to red. The central features of our approach are as follows: (1) the formation of InP core QDs through one-batch-one-size reactions based on aminophosphine as the phosphorus precursor, (2) the introduction of a core/shell/shell InP/Zn(Se,S)/ZnS structure, and (3) the use of specific interfacial treatments, most notably the saturation of the ZnSe surface with zinc acetate prior to ZnS shell growth. Moreover, we adapted the composition of the Zn(Se,S) inner shell to attain the intended emission color while minimizing line broadening induced by the InP/ZnS lattice mismatch. The protocol is established by analysis of the QD composition and structure using multiple techniques, including solid-state nuclear magnetic resonance spectroscopy and Raman spectroscopy, and verified for reproducibility by having different researchers execute the same protocol. The realization of full-spectrum, +90% quantum efficiency will strongly facilitate research into light-matter interaction in general and luminescent color conversion in particular through InP-based QDs.

Inorganic ligands-mediated hole attraction and surface structural reorganization in InP/ZnS QD photocatalysts studied via ultrafast visible and midinfrared spectroscopies

Photoinduced carrier dynamical processes dominate the optical excitation properties of photocatalysts and further determine the photocatalytic performance. In addition, as the electrons generally possess a faster transfer rate than holes, hole transfer and accumulation are critical, and they play the key efficiency-limiting step during the photocatalytic process. Therefore, a comprehensive understanding of the dynamics of photogenerated holes and their determining factors in the photocatalytic system is highly essential to rationalize the full catalytic mechanism and develop highly efficient photocatalysts, which have not yet been revealed. In this work, the photoinduced charge carrier dynamics in InP/ZnS quantum dots (QDs) capped with long-chain L-typed ligands (oleylamine) and inorganic ligands (sulfide ion (S2−)) were explored. Time-resolved photoluminescence and femtosecond transient-absorption spectroscopy unambiguously confirmed the ultrafast hole transfer from the InP core to S2− ligands. Moreover, by probing the bleach of vibrational stretching of the ligands with transient midinfrared absorption spectroscopy, the hole transfer time was determined to be 4.2 ps. The injected holes are long-lived at the S2− ligands (>4.5 ns), and they can remove electrostatically attached surfactants to compensate for the spatial charge redistribution. Finally, compared with other inorganic ligands such as Cl− and PO43−, S2− balances the ionic radii and net charge to ensure the optimal condition for charge transfer. Such observation rationalizes the excellent photocatalytic H2 evolution (213.6 µmol mg−1 within 10 h) in InP/ZnS QDs capped with S2− compared with those capped with other ligands and elucidates the role of surface ligands in the photocatalytic activity of colloidal QDs.

Engineering Core Size of InP Quantum Dot with Incipient ZnS for Blue Emission

AbstractBlue indium phosphide quantum dot (InP QD) is an emerging colloidal semiconductor nanocrystal, considered as a promising next‐generation photoactive material for light‐emitting purposes. Despite the tremendous progress in blue InP QDs, the synthetic method for tailoring InP core size to realize the blue‐emissive QDs still lags behind. This work suggests a synthetic method for blue‐emitting InP QDs by engineering the core size with an incipient ZnS (i‐ZnS) shell. The formation of i‐ZnS complexes, before the tris(trimethylsilyl)phosphine injection (e.g., before core growth process), restrains the overgrowth of InP nuclei by rapidly forming a ZnS shell on its surface, thereby resulting in further dwarfed InP cores. With additional ZnS shell coating, the blue QDs exhibit a photoluminescence quantum yield of ≈52% at 483 nm. The origin of bandgap diminution with the increase of shell thickness, or with the utilization of ZnSe shell is unraveled via the first‐principles density functional theory simulations. Simulational evidence on InP‐core densification with the shell coating, along with accompanying changes in chemical and structural properties, is presented. The blue‐emitting InP QD device shows a maximum luminance of 1162 cd m−2 and external quantum efficiency of 1.4%.

Probing strain in wurtzite InP-InAs core-shell nanowires with Raman spectroscopy

We used Raman spectroscopy to investigate phonon modes of single InP-InAs core-shell nanowire with a pure wurtzite phase to reveal the embedded strained state. Raman spectra show that the InP core exhibits tensile strain and the InAs shell exhibits compressive strain due to the radial heterostructure growth. Using different polarization excitation configurations, we identified distinct ${A}_{1}$ (TO) and ${E}_{1}$ (TO) modes, atomic vibrations along and perpendicular to the [0001] direction, respectively. We also identified ${E}_{2h}$ phonon modes in wurtzite InP and InAs, lattice vibration along with the [0001] direction, a fingerprint to distinguish a wurtzite phase from a zinc-blende phase. The Raman spectra changes reveal the resonance effects of InP and InAs between the ${E}_{0}+{\mathrm{\ensuremath{\Delta}}}_{0}$ energy level and the ${E}_{1}$ gap by varying excitation energy, which indicates ${\mathrm{\ensuremath{\Gamma}}}_{7\mathrm{c}}\text{\ensuremath{-}}{\mathrm{\ensuremath{\Gamma}}}_{6\mathrm{v}}$ transition consists of a split-off valence band and a band-gap transition along [0001] directions of the Brillouin zone. The polar patterns show the Raman ${A}_{1}$ (TO) mode of InP only predominates in the $x(yy)\overline{x}$ configuration, indicating the InP ${A}_{1}$ (TO) phonon is sensitive to excitation polarization, suggesting the resonant Raman spectroscopy with specific polarization offers a direct way for characterizing wurtzite InP. Our findings deepen the understanding of strain in wurtzite-phase core-shell nanowire, providing a significant reference for engineering strain in core-shell nanowire to control optical and electric properties.

Radiative dynamics and delayed emission in pure and doped InP/ZnSe/ZnS quantum dots.

We have used time-correlated single photon counting to elucidate the radiative dynamics of InP/ZnSe/ZnS core/shell/shell quantum dots (QDs) that differ in the amount and distribution of excess indium. Stoichiometric QDs having an In:P atom ratio very near unity exhibit simple luminescence kinetics. The photoluminescence (PL) rises with the 40ps instrument response function and exhibits a decay that is close to a single exponential with a time constant that decreases from 32 to 28ns with increasing shell thickness. QDs having excess indium (In:P ratio of 1.15-1.63) show a significant component of a slower rise time assigned to transient population of indium-based hole traps in the ZnSe shell. They also have a slower PL decay, attributed to an equilibrium between these traps, which are optically dark, and the emissive valence-band state. This results in a radiative lifetime that increases from 32 to 48ns with increasing shell thickness. Different treatments of the InP cores prior to shell deposition result in different core/shell interfaces as indicated by resonance Raman spectroscopy, as well as differences in the amplitude and timescale of the slow PL rise and the PL decay time. These are interpreted in terms of different radial distributions of the indium-based hole traps, which can be related to differences in the interfacial lattice strain.

Tuning Hot Carrier Dynamics of InP/ZnSe/ZnS Quantum Dots by Shell Morphology Control.

Isotropic InP/ZnSe/ZnS quantum dots (QDs) are prepared at a high reaction temperature, which facilitates ZnSe shell growth on random facets of the InP core. Fast crystal growth enables stacking faults elimination, which induces anisotropic growth, and as a result, improves the photoluminescence (PL) quantum yield by nearly 20%. Herein, the effect of the QD morphology on photophysical properties is investigated by observing the PL blinking and ultrafast charge carrier dynamics. It is found that hot hole trapping is considerably suppressed in isotropic InP QDs, indicating that the stacking faults in the anisotropic InP/ZnSe structures act as defects for luminescence. These results highlight the importance of understanding the correlation between QD shapes and hot carrier dynamics, and present a way to design highly luminescent QDs for further promising display applications.

Performance analysis of ITO-free PEDOT:PSS/InP nanowire hybrid solar cell

InP nanostructure based solar cells (SCs) offer high efficiency with reduced material requirement when compared to their planar/thin-film SCs. In order to further reduce the cost and improve the effectiveness of nanostructured SCs, a new approach by combining the organic and inorganic materials in the active layer has been developed. In this study, we have replaced the indium tin oxide (ITO) with PEDOT:PSS as the transparent conducting oxide since its sheet resistance is virtually comparable to that of ITO and exhibits almost similar performance as an electrode. Geometrical parameters such as diameter (D) of InP nanowire (NW) and coating thickness (T) of PEDOT:PSS are optimized by FDTD technique to achieve best absorption and optical current density (Jsc). It is observed that for PEDOT:PSS/InP NWSCs with 160 nm of InP NW diameter and coating thickness of 80 nm for PEDOT:PSS, optical current density of 34.2 mA/cm2 is achieved, which is better than the ITO/InP NWSCs. Electrical analysis of PEDOT:PSS/p-InP/n-InP NW homojunction SCs using Charge Solver Module revealed that the power conversion efficiency (PCE) of ∼ 24% can be attained with the proposed nanostructures through a carrier lifetime of 1 ns, surface recombination velocity (SRV) of 103 cm/s of InP NW (at n-InP and p-InP interface) and core doping of 5 × 1018 cm−3. We also found out that conformally coated NWs outperform fully infiltrated NW structures in terms of optical and electrical performance.

Bending of GaAs–InP Core–Shell Nanowires by Asymmetric Shell Deposition: Implications for Sensors

Bending of GaAs–InP Core–Shell Nanowires by Asymmetric Shell Deposition: Implications for Sensors

Photophysical Properties of Mn-Doped InP/ZnS Nanocrystals

InP-based nanocrystals (NCs) have received considerable attention as promising candidates to replace Cd- and Pb-based NCs because they contain no heavy metal elements. However, relevant studies on the photophysical properties of InP-based NCs are still insufficient. In this work, Mn-doped InP/ZnS NCs are synthesized, and their photophysical properties have been investigated. With the help of transient absorption spectrum measurement, it is found that Mn-doped InP/ZnS NCs exhibit efficient exciton–dopant exchange interaction. Through the temperature-dependent fluorescence lifetime measurements it is revealed that some excitons may undergo reverse crossing to the conduction band of the InP core from the long-lived 4T1 state of the Mn2+ ions with increasing temperature, resulting in the exciton fluorescence lifetime of the doped NCs to be less influenced by the temperature. In addition, compared with the Mn fluorescence, the exciton fluorescence shows an opposite intensity tendency to the temperature variation as a result of the greatly decreased energy transfer efficiency at elevated temperature. Importantly, the doped NCs exhibit a bright two-photon excited Mn fluorescence in which no size selection effect is observed. The excellent photophysical properties of Mn-doped InP/ZnS NCs imply that they are promising in the applications of highly effective optoelectronic devices and biological science.

Tuning the interfacial stoichiometry of InP core and InP/ZnSe core/shell quantum dots

We demonstrate fine-tuning of the atomic composition of InP/ZnSe quantum dots (QDs) at the core/shell interface. Specifically, we control the stoichiometry of both anions (P, As, S, and Se) and cations (In and Zn) at the InP/ZnSe core/shell interface and correlate these changes with the resultant steady-state and time-resolved optical properties of the nanocrystals. The use of reactive trimethylsilyl reagents results in surface-limited reactions that shift the nanocrystal stoichiometry to anion-rich and improve epitaxial growth of the shell layer. In general, anion deposition on the InP QD surface results in a redshift in the absorption, quenching of the excitonic photoluminescence, and a relative increase in the intensity of broad trap-based photoluminescence, consistent with delocalization of the exciton wavefunction and relaxation of exciton confinement. Time-resolved photoluminescence data for the resulting InP/ZnSe QDs show an overall small change in the decay dynamics on the ns timescale, suggesting that the relatively low photoluminescence quantum yields may be attributed to the creation of new thermally activated charge trap states and likely a dark population that is inseparable from the emissive QDs. Cluster-model density functional theory calculations show that the presence of core/shell interface anions gives rise to electronic defects contributing to the redshift in the absorption. These results highlight a general strategy to atomistically tune the interfacial stoichiometry of InP QDs using surface-limited reaction chemistry allowing for precise correlations with the electronic structure and photophysical properties.

Effects of Interfacial Oxidative Layer Removal on Charge Carrier Recombination Dynamics in InP/ZnSexS1-x Core/Shell Quantum Dots.

Red-light-emitting InP/ZnSexS1-x core/shell quantum dots (QDs) were prepared by one-pot synthesis with optimal hydrogen fluoride (HF) treatment. Most of the surficial oxidative species could be removed, and the dangling bonds would be passivated by Zn ions for the InP cores during HF treatment, which would be beneficial to the subsequent ZnSexS1-x shell coating. Three-dimensional time-resolved photoluminescence spectra of the QD samples were analyzed by singular value decomposition global fitting to determine the radiative and nonradiative lifetimes of charge carriers. A proposed model illustrated that the charge carriers in the InP/ZnSexS1-x QDs with interfacial oxidative layer removal would evidently recombine through radiative pathways, mainly from the conduction band to the valence band (lifetime, 33 ns) and partially from the trap states (lifetime, 150 ns). This work offers the important physical insight into the charge carrier dynamics of low-toxicity QDs which have the desired optical properties for optoelectronic applications.

Auger Dynamics in InP/ZnSe/ZnS Quantum Dots Having Pure and Doped Shells

In this study, we report the negative trion and biexciton Auger dynamics in very high-quality InP/ZnSe/ZnS quantum dots (QDs) having varying amounts of indium-based traps in the ZnSe shell. Negative trion times are determined by time-correlated photon-counting measurements on QDs that have been photoreduced with lithium triethylborohydride. We find that the Auger times vary from 280 to 425 ps and scale linearly with the total particle volume. Excess indium in the ZnSe shell gives rise to hole traps that are transiently populated following photoexcitation. Comparing stoichiometric and nonstoichiometric particles, we find that the negative trion lifetimes are independent of the presence of indium in the ZnSe shell. Biexciton dynamics are determined from transient absorption (TA) bleach recovery measurements. Absorption of two photons results in the formation of two types of biexcitons: those having both holes in the InP core (XX state) and those having one hole trapped in the ZnSe shell (XT state). XX state Auger times are measured in stoichiometric QDs, and for these particles, the Auger times are ∼80 ps. Nonstoichiometric QDs show an additional long TA decay component, assigned to Auger recombination of the XT state. A previous study (J. Phys. Chem. C2021, 125, 4110–4118) found that hole trapping at indium-based shell traps results in a slow photoluminescence (PL) rise component. We find that the fraction of the XT state formed by the absorption of two photon correlates with the product of the fraction of slow PL rise and the rise time constant following one-photon excitation.

Cadmium-Free and Efficient Type-II InP/ZnO/ZnS Quantum Dots and Their Application for LEDs.

It is a generally accepted perspective that type-II nanocrystal quantum dots (QDs) have low quantum yield due to the separation of the electron and hole wavefunctions. Recently, high quantum yield levels were reported for cadmium-based type-II QDs. Hence, the quest for finding non-toxic and efficient type-II QDs is continuing. Herein, we demonstrate environmentally benign type-II InP/ZnO/ZnS core/shell/shell QDs that reach a high quantum yield of ∼91%. For this, ZnO layer was grown on core InP QDs by thermal decomposition, which was followed by a ZnS layer via successive ionic layer adsorption. The small-angle X-ray scattering shows that spherical InP core and InP/ZnO core/shell QDs turn into elliptical particles with the growth of the ZnS shell. To conserve the quantum efficiency of QDs in device architectures, InP/ZnO/ZnS QDs were integrated in the liquid state on blue light-emitting diodes (LEDs) as down-converters that led to an external quantum efficiency of 9.4% and a power conversion efficiency of 6.8%, respectively, which is the most efficient QD-LED using type-II QDs. This study pointed out that cadmium-free type-II QDs can reach high efficiency levels, which can stimulate novel forms of devices and nanomaterials for bioimaging, display, and lighting.

Nanoengineering InP Quantum Dot-Based Photoactive Biointerfaces for Optical Control of Neurons.

Light-activated biointerfaces provide a non-genetic route for effective control of neural activity. InP quantum dots (QDs) have a high potential for such biomedical applications due to their uniquely tunable electronic properties, photostability, toxic-heavy-metal-free content, heterostructuring, and solution-processing ability. However, the effect of QD nanostructure and biointerface architecture on the photoelectrical cellular interfacing remained unexplored. Here, we unravel the control of the photoelectrical response of InP QD-based biointerfaces via nanoengineering from QD to device-level. At QD level, thin ZnS shell growth (∼0.65 nm) enhances the current level of biointerfaces over an order of magnitude with respect to only InP core QDs. At device-level, band alignment engineering allows for the bidirectional photoelectrochemical current generation, which enables light-induced temporally precise and rapidly reversible action potential generation and hyperpolarization on primary hippocampal neurons. Our findings show that nanoengineering QD-based biointerfaces hold great promise for next-generation neurostimulation devices.