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

High-Performance Giant InP Quantum Dots with Stress-Released Morphological ZnSe-ZnSeS-ZnS Shell

Indium phosphide (InP) quantum dots (QDs) are increasingly considered potent alternatives to traditional cadmium-based QDs. Notwithstanding, the material stability of InP, especially when juxtaposed with its cadmium-based counterparts, poses significant challenges in its application. Generally, a thick ZnS shell is applied to InP cores to thwart photo-oxidation and diminish nonradiative recombination. Yet, the pronounced lattice mismatch between the InP core and the ZnS shell can introduce lattice defects, consequently attenuating the luminescence efficiency. This makes the cultivation of a flawless thick shell a paramount challenge. In the present research, a synthetic methodology is elucidated to fabricate highly efficient InP QDs with dimensions exceeding 20 nm, achieved by alleviating the lattice mismatch strain during the shell growth. By regulating the shell composition and morphology, InP/ZnSe/ZnSeS/ZnS QDs with shield-like morphology are prepared and demonstrate a photoluminescence quantum yield (PLQY) of ≈90%, exhibiting significantly enhanced photostability and thermal stability. This discovery is expected to greatly advance the preparation of highly efficient InP-based or other QDs, expanding their potential in various applications such as environmentally friendly displays and energy-saving lighting.

Highly Luminescent Shell-Less Indium Phosphide Quantum Dots Enabled by Atomistically Tailored Surface States

Contrary to the prevailing notion that shell structures arise from the intricate chemistry and surface defects of InP quantum dots (QDs), an innovative strategy that remarkably enhances the luminescence efficiency of core-only InP QDs to over 90% is introduced. This paradigm shift is achieved through the concurrent utilization of group 2 and 3 metal-derived ligands, providing an effective remedy for surface defects and facilitating charge recombination. Specifically, a combination of Zn carboxylate and Ga chloride is employed to address the undercoordination issues associated with In and P atoms, leading to the alleviation of in-gap trap states. The intricate interplay and proportional ratio between Ga- and Zn-containing ligands play pivotal roles in attaining record-high luminescence efficiency in core-only InP QDs, as successfully demonstrated across various sizes and color emissions. Moreover, the fabrication of electroluminescent devices relying solely on InP core emission opens a new direction in optoelectronics, demonstrating the potential of the approach not only in optoelectronic applications but also in catalysis or energy conversion by charge transfer.

Transition Layer Assisted Synthesis of Defect Free Amine-Phosphine Based InP QDs

Environmentally friendly InP-based quantum dots (QDs) are promising for light-emitting diodes (LEDs) and display applications. So far, the synthesis of highly emitting InP-based QDs via safe and economically viable amine-phosphine remains a challenge. Herein, we report the synthesis of amine-phosphine based InP/ZnSe/ZnS QDs by introducing an alloyed oxidation-free In-ZnSe transition layer (TL) at the core-shell interface. The TL not only has the essential function of preventing oxidation of the core and relieving interfacial strain but also results in oriented epitaxial growth of shell. The alloyed TL significantly mitigates the nonradiative recombination at core-shell interfacial trap states, thereby boosting the photoluminescence (PL) efficiency of the QDs up to 98%. Also, the Auger recombination is suppressed, extending the biexciton lifetime from 60 to 100 ps. The electroluminescence device based on the InP-based QDs shows a high external quantum efficiency over 10%, further demonstrating high quality QDs synthesized by this process.

Nitrogen Configuration Effects on Charge Carrier Dynamics in CsPbBr3/Carbon Dots S-Scheme Heterojunction for Photocatalytic CO2 Reduction

Nitrogen-doped carbon dots (NCDs) featuring primary pyrrolic N and pyridinic N dominated configurations were prepared using hydrothermal (H-NCDs) and microwave (M-NCDs) methods, respectively. These H-NCDs and M-NCDs were subsequently applied to decorate CsPbBr3 nanocrystals (CPB NCs) individually, using a ligand-assisted reprecipitation process. Both CPB/M-NCDs and CPB/H-NCDs nanoheterostructures (NHSs) exhibited S-scheme charge transfer behavior, which enhanced their performance in photocatalytic CO2 reduction and selectivity of CO2-to-CH4 conversion, compared to pristine CPB NCs. The presence of pyrrolic N configuration at the heterojunction of CPB/H-NCDs facilitated efficient S-scheme charge transfer, leading to a remarkable 43-fold increase in photoactivity. In contrast, CPB/M-NCDs showed only a modest 3-fold enhancement in photoactivity, which was attributed to electron trapping by pyridinic N at the heterojunction. The study offers crucial insights into charge carrier dynamics within perovskite/carbon NHSs at the molecular level to advance the understanding of solar fuel generation.

Metal-Solvent Complex Formation at the Surface of InP Colloidal Quantum Dots

The surface chemistry of colloidal semiconductor nanocrystals (QDs) profoundly influences their physical and chemical attributes. The insulating organic shell ensuring colloidal stability impedes charge transfer, thus limiting optoelectronic applications. Exchanging these ligands with shorter inorganic ones enhances charge mobility and stability, which is pivotal for using these materials as active layers for LEDs, photodetectors, and transistors. Among those, InP QDs also serve as a model for surface chemistry investigations. This study focuses on group III metal salts as inorganic ligands for InP QDs. We explored the ligand exchange mechanism when metal halide, nitrate, and perchlorate salts of group III (Al, In Ga), common Lewis acids, are used as ligands for the conductive inks. Moreover, we compared the exchange mechanism for two starting model systems: InP QDs capped with myristate and oleylamine as X- and L-type native organic ligands, respectively. We found that all metal halide, nitrate, and perchlorate salts dissolved in polar solvents (such as n-methylformamide, dimethylformamide, dimethyl sulfoxide, H2O) with various polarity formed metal-solvent complex cations [M(Solvent)6]3+ (e.g., [Al(MFA)6]3+, [Ga(MFA)6]3+, [In(MFA)6]3+), which passivated the surface of InP QDs after the removal of the initial organic ligand. All metal halide capped InP/[M(Solvent)6]3+ QDs show excellent colloidal stability in polar solvents with high dielectric constant even after 6 months in concentrations up to 74 mg/mL. Our findings demonstrate the dominance of dissociation-complexation mechanisms in polar solvents, ensuring colloidal stability. This comprehensive understanding of InP QD surface chemistry paves the way for exploring more complex QD systems such as InAs and InSb QDs.

Effect of Lattice Disorder on Exciton Dynamics in Copper-Doped InP/ZnSexS1-x Core/Shell Quantum Dots

InP/ZnSexS1-x core/shell quantum dots (QDs) with varying Cu concentrations were synthesized by a one-pot hot-injection method. X-ray diffraction and high-resolution transmission electron microscopy results indicate that Cu doping did not alter the crystal structure or particle size of the QDs. The optical shifts in UV-visible absorption and photoluminescence (PL) suggest changes in the electronic structure and induction of lattice disorder due to Cu doping. Ultrafast transient absorption spectroscopy (TAS) reveled that a higher Cu-doping level leads to faster charge carrier recombination, likely due to increased nonradiative decay from defect states. Time-resolved PL (TRPL) studies show longer average lifetimes of charge carriers with increased Cu doping. These findings informed the development of a kinetic model to better understand how Cu-induced disorder affects charge carrier dynamics in the QDs, which is important for emerging applications of Cu-doped InP/ZnSexS1-x QDs in optoelectronics.

Surface Passivation toward Multiple Inherent Dangling Bonds in Indium Phosphide Quantum Dots

Indium phosphide (InP) quantum dots (QDs) have become the most recognized prospect to be less-toxic surrogates for Cd-based optoelectronic systems. Due to the particularly dangling bonds (DBs) and the undesirable oxides, the photoluminescence performance and stability of InP QDs remain to be improved. Previous investigations largely focus on eliminating P-DBs and resultant surface oxidation states; however, little attention has been paid to the adverse effects of the surface In-DBs on InP QDs. This work demonstrates a facile one-step surface peeling and passivation treatment method for both In- and P-DBs for InP QDs. Meanwhile, the surface treatment may also effectively support the encapsulation of the ZnSe shell. Finally, the generated InP/ZnSe QDs display a narrower full width at half-maximum (fwhm) of ∼48 nm, higher photoluminescence quantum yields (PLQYs) of ∼70%, and superior stability. This work enlarges the surface chemistry engineering consideration of InP QDs and considerably promotes the development of efficient and stable optoelectronic devices.

Interface defects repair of core/shell quantum dots through halide ion penetration

The interface defects of core-shell colloidal quantum dots (QDs) affect their optoelectronic properties and charge transport characteristics. However, the limited available strategies pose challenges in the comprehensive control of these interface defects. Herein, we introduce a versatile strategy that effectively addresses both surface and interface defects in QDs through simple post-synthesis treatment. Through the combination of fine chemical etching methods and spectroscopic analysis, we have revealed that halogens can diffuse within the crystal structure at elevated temperatures, acting as "repairmen" to rectify oxidation and significantly reducing interface defects within the QDs. Under the guidance of this protocol, InP core/shell QDs were synthesized by a hydrofluoric acid-free method with a full width at half-maximum of 37.0 nm and an absolute quantum yield of 86%. To further underscore the generality of this strategy, we successfully applied it to CdSe core/shell QDs as well. These findings provide fundamental insights into interface defect engineering and contribute to the advancement of innovative solutions for semiconductor nanomaterials.

Tuning the Shades of Red Emission in InP/ZnSe/ZnS Nanocrystals with Narrow Full Width for Fabrication of Light-Emitting Diodes

While Cd-based luminescent nanocrystals (NCs) are the most mature NCs for fabricating efficient red light-emitting diodes (LEDs), their toxicity related limitation is inevitable, making it necessary to find a promising alternative. From this point of view, multishell-coated, red-emissive InP-based NCs are excellent luminescent nanomaterials for use as an emissive layer in electroluminescent (EL) devices. However, due to the presence of oxidation states, they suffer from a wide emission spectrum, which limits their performance. This study uses tris(dimethylamino)phosphine (3DMA-P) as a low-cost aminophosphine precursor and a double HF treatment to suggest an upscaled, cost-effective, and one-pot hot-injection synthesis of purely red-emissive InP-based NCs. The InP core structures were coated with thick layers of ZnSe and ZnS shells to prevent charge delocalization and to create a narrow size distribution. The purified NCs showed an intense emission signal as narrow as 43 nm across the entire red wavelength range (626-670 nm) with an emission quantum efficiency of 74% at 632 nm. The purified samples also showed an emission quantum efficiency of 60% for far-red wavelengths of 670 nm with a narrow full width of 50 nm. The samples showed a relatively long average emission lifetime of 50-70 ns with a biexponential decay profile. To demonstrate the practical ability of the prepared NCs in optoelectronics, we fabricated a red-emissive InP-based LEDs. The best-performing device showed an external quantum efficiency (EQE) of 1.16%, a luminance of 1039 cd m-2, and a current efficiency of 0.88 cd A-1.

InP/ZnS quantum dot photoluminescence modulation via in situ H2S interface engineering

InP quantum dots (QDs) are attracting significant interest as a potentially less toxic alternative to Cd-based QDs in many research areas. Although InP-based core/shell QDs with excellent photoluminescence properties have been reported so far, sophisticated interface treatment to eliminate defects is often necessary. Herein, using aminophosphine as a seeding source of phosphorus, we find that H₂S can be efficiently generated from the reaction between a thiol and an alkylamine at high temperatures. Apart from general comprehension that H₂S acts as a S precursor, it is revealed that with core etching by H₂S, the interface between InP and ZnS can be reconstructed with S^2- incorporation. Such a transition layer can reduce inherent defects at the interface, resulting in significant photoluminescence (PL) enhancement. Meanwhile, the size of the InP core could be further controlled by H₂S etching, which offers a feasible process to obtain wide band gap InP-based QDs with blue emission.

Control of the Reaction Kinetics of Monodispersed InP/ZnSeS/ZnS-Based Quantum Dots Using Organophosphorus Compounds for Electroluminescent Devices

Green emissive InP-based quantum dots (QDs) remain less developed than red QDs because of the difficulty of controlling the reactivity of small InP cores. Herein, we report the synthesis of monodispersed green InP-based QDs using tris(dimethylamino)phosphine, a considerably inexpensive and safer phosphorus source compared to conventional tris(trimethylsilyl)phosphine. An organophosphorus compound, trioctylphosphine, was used to control the reaction kinetics by slowing the progression of the nucleation process, which weakened the aggregation behavior of the clusters and improved the size distribution. The synthesized green emissive InP/ZnSeS/ZnS QDs exhibited a photoluminescence (PL) peak at 515 nm with an enhancement of the full width at half-maximum from 66 to 46 nm and the PL quantum yield from 61% to 70%. An electroluminescent device was fabricated, and the electron transport layer was optimized by changing the layer thickness. The optimized device structure improved the charge balance and increased the external quantum efficiency from 2.1% to 3.5%.

Deep Blue and Highly Emissive ZnS-Passivated InP QDs: Facile Synthesis, Characterization, and Deciphering of Their Ultrafast-to-Slow Photodynamics

InP-based quantum dots (QDs) are an environment-friendly alternative to their heavy metal-ion-based counterparts. Herein we report a simple procedure for synthesizing blue emissive InP QDs using oleic acid and oleylamine as surface ligands, yielding ultrasmall QDs with average sizes of 1.74 and 1.81 nm, respectively. Consecutive thin coating with ZnS increased the size of these QDs to 4.11 and 4.15 nm, respectively, alongside a significant enhancement of their emission intensities centered at ∼410 nm and ∼430 nm, respectively. Pure phase synthesis of these deep-blue emissive QDs is confirmed by powder X-ray diffraction (PXRD), X-ray photoelectron spectroscopy (XPS), and transmission electron microscopy (TEM). Armed with femtosecond to millisecond time-resolved spectroscopic techniques, we decipher the energy pathways, reflecting the effect of successive ZnS passivation on the charge carrier (electrons and holes) dynamics in the deep-blue emissive InP, InP/ZnS, and InP/ZnS/ZnS QDs. Successive coating of the InP QDs increases the intraband relaxation times from 200 to 700 fs and the lifetime of the hot electrons from 2 to 8 ps. The lifetime of the cold holes also increase from 1 to 4 ps, and remarkably, the Auger recombination escalates from 15 to 165 ps. The coating also drastically decreases the quenching by the molecular oxygen of the trapped charge carriers at the surfaces of the QDs. Our results provide clues to push further the emission of InP QDs into more energetically spectral regions and to increase the fluorescence quantum yield, targeting the construction of efficient UV-emissive light-emitting devices (LEDs).

A Water-Free In Situ HF Treatment for Ultrabright InP Quantum Dots

Indium phosphide quantum dots are the main alternative for toxic and restricted Cd-based quantum dots for lighting and display applications, but in the absence of protecting ZnSe and/or ZnS shells, InP quantum dots suffer from low photoluminescence quantum yields. Traditionally, HF treatments have been used to improve the quantum yield of InP to ∼50%, but these treatments are dangerous and not well understood. Here, we develop a postsynthetic treatment that forms HF in situ from benzoyl fluoride, which can be used to strongly increase the quantum yield of InP core-only quantum dots. This treatment is water-free and can be performed safely. Simultaneous addition of the z-type ligand ZnCl₂ increases the photoluminescence quantum yield up to 85%. Structural analysis via XPS as well as solid state and solution NMR measurements shows that the in situ generated HF leads to a surface passivation by indium fluoride z-type ligands and removes polyphosphates, but not PO₃ and PO₄ species from the InP surface. With DFT calculations it is shown that InP QDs can be trap-free even when PO₃ and PO₄ species are present on the surface. These results show that both polyphosphate removal and z-type passivation are necessary to obtain high quantum yields in InP core-only quantum dots. They further show that core-only InP QDs can achieve photoluminescence quantum yields rivalling those of InP/ZnSe/ZnS core/shell/shell QDs and the best core-only II-VI QDs.

Quasi-Shell-Growth Strategy Achieves Stable and Efficient Green InP Quantum Dot Light-Emitting Diodes

Indium phosphide (InP) based quantum dots (QDs) have been known as an ideal alternative to heavy metals including QDs light emitters, such as cadmium selenium (CdSe) QDs, and show great promise in the next-generation solid-state lighting and displays. However, the electroluminescence performance of green InP QDs is still inferior to their red counterparts, due to the higher density of surface defects and the wider particle size distribution. Here, a quasi-shell-growth strategy for the growth of highly luminescent green InP/ZnSe/ZnS QDs is reported, in which the zinc and selenium monomers are added at the initial nucleation of InP stage to adsorb on the surface of InP cores that create a quasi-ZnSe shell rather than a bulk ZnSe shell. The quasi-ZnSe shell reduces the surface defects of InP core by passivating In-terminated vacancies, and suppresses the Ostwald ripening of InP core at high temperatures, leading to a photoluminescence quantum yield of 91% with a narrow emission linewidth of 36 nm for the synthesized InP/ZnSe/ZnS QDs. Consequently, the light-emitting diodes based on the green QDs realize a maximum luminance of 15606 cd m-2 , a peak external quantum efficiency of 10.6%, and a long half lifetime of > 5000 h.

Two-Step Process of a Crystal Facet-Modulated BiVO4 Photoanode for Efficiency Improvement in Photoelectrochemical Hydrogen Evolution

The photoactivity of nanoporous bismuth vanadate (BiVO₄, BVO) photoanodes that were fabricated by a two-step process (electrodeposition and then thermal conversion) in photoelectrochemical (PEC) hydrogen (H₂) evolution can be enhanced about 1.44-fold by improving the constitutive ratio of (111̅), (061), and (242̅) crystal facets. The PEC characterization was carried out to investigate the factors altering the performance, which revealed that the crystal facet modulation could improve the photoactivity of the BVO photoanodes. In addition, the orientation-controlled BVO thin-film electrodes are introduced as evidence that the present crystal facet modulation is the positive effect for BVO photoanodes in PEC. The investigation of energy band structures and interfacial charge carrier dynamics of the BVO photoanodes reveals that the crystal facet modulation could result in a shorter lifetime of charge carrier recombination and larger band bending at the interface between BVO and electrolytes. This outcome could improve the charge separation and charge transfer efficiencies of BVO photoanodes, promoting the efficiency of PEC H₂ evolution. Moreover, this crystal facet modulation can combine with co-catalyst decoration to further improve the solar-to-hydrogen efficiency of BVO photoanodes in PEC. This study presents a potential strategy to promote the PEC activity by crystal facet modulation and important insights into the interfacial charge transfer properties of semiconductor photoelectrodes for the application in solar fuel generation.

Advances and Challenges in Heavy-Metal-Free InP Quantum Dot Light-Emitting Diodes

Light-emitting diodes based on colloidal quantum dots (QLEDs) show a good prospect in commercial application due to their narrow spectral linewidths, wide color range, excellent luminance efficiency, and long operating lifetime. However, the toxicity of heavy-metal elements, such as Cd-based QLEDs or Pb-based perovskite QLEDs, with excellent performance, will inevitably pose a serious threat to people's health and the environment. Among heavy-metal-free materials, InP quantum dots (QDs) have been paid special attention, because of their wide emission, which can, in principle, be tuned throughout the whole visible and near-infrared range by changing their size, and InP QDs are generally regarded as one of the most promising materials for heavy-metal-free QLEDs for the next generation displays and solid-state lighting. In this review, the great progress of QLEDs, based on the fundamental structure and photophysical properties of InP QDs, is illustrated systematically. In addition, the remarkable achievements of QLEDs, based on their modification of materials, such as ligands exchange of InP QDs, and the optimization of the charge transport layer, are summarized. Finally, an outlook is shown about the challenge faced by QLED, as well as possible pathway to enhancing the device performance. This review provides an overview of the recent developments of InP QLED applications and outlines the challenges for achieving the high-performance devices.

Synergistic Effect of Halogen Ions and Shelling Temperature on Anion Exchange Induced Interfacial Restructuring for Highly Efficient Blue Emissive InP/ZnS Quantum Dots

InP quantum dots (QDs) have attracted much attention owing to their nontoxic properties and shown great potential in optoelectronic applications. Due to the surface defects and lattice mismatch, the interfacial structure of InP/ZnS QDs plays a significant role in their performance. Herein, the formation of In-S and S_x -In-P_1-x interlayers through anion exchange at the shell-growth stage is revealed. More importantly, it is proposed that the composition of interface is dependent on the synergistic effect of halogen ions and shelling temperature. High shelling temperature contributes to the optical performance improvement resulting from the formation of interlayers, besides the thicker ZnS shell. Moreover, the effect relates to the halogen ions where I^- presents more obvious enhancement than Br^- and Cl^- , owing to their different ability to coordinate with In dangling bonds, which are inclined to form In-S and S_x -In-P_1-x bonds. Further, the anion exchange under I^- -rich environment causes a blue-shift of emission wavelength with shelling temperature increasing, unobserved in a Cl^- - or Br^- -rich environment. It contributes to the preparation of highly efficient blue emissive InP/ZnS QDs with emission wavelength of 473 nm, photoluminescence quantum yield of ≈50% and full width at half maximum of 47 nm.