Unraveling the role of zinc complexes on indium phosphide nanocrystal chemistry

Single- and two-photon-induced Förster resonance energy transfer in InP-mCherry bioconjugates

Indium phosphide (InP) quantum dots (QDs) have recently garnered considerable interest in the design of bioprobes due to their non-toxic nature and excellent optical properties. Several attempts for the conjunction of InP QDs with various entities such as organic dyes and dye-labeled proteins have been reported, while that with fluorescent proteins remains largely uncharted. This study reports the development of a Förster resonance energy transfer pair comprising glutathione-capped InP/GaP/ZnS QDs [InP(G)] and the fluorescent protein mCherry. Glutathione on InP(G) undergoes effective bioconjugation with mCherry consisting of a hexahistidine tag, and the nonradiative energy transfer is investigated using steady-state and time-resolved measurements. Selective one-photon excitation of InP(G) in the presence of mCherry shows a decay of the emission of the QDs and a concomitant growth of acceptor emission. Time-resolved investigations prove the nonradiative transfer of energy between InP(G) and mCherry. Furthermore, the scope of two-photon-induced energy transfer between InP(G) and mCherry is investigated by exciting the donor in the optical transparency range. The two-photon absorption is confirmed by the quadratic relationship between the emission intensity and the excitation power. In general, near-infrared excitation provides a path for effective light penetration into the tissues and reduces the photodamage of the sample. The two-photon-induced energy transfer in such assemblies could set the stage for a wide range of biological and optoelectronic applications in the foreseeable future.

Narrow Near-Infrared Emission from InP QDs Synthesized with Indium(I) Halides and Aminophosphine

Nonpyrophoric aminophosphines reacted with indium(III) halides in the presence of zinc chloride have emerged as promising phosphorus precursors in the synthesis of colloidal indium phosphide (InP) quantum dots (QDs). Nonetheless, due to the required P/In ratio of 4:1, it remains challenging to prepare large-sized (>5 nm), near-infrared absorbing/emitting InP QDs using this synthetic scheme. Furthermore, the addition of zinc chloride leads to structural disorder and the formation of shallow trap states inducing spectral broadening. To overcome these limitations, we introduce a synthetic approach relying on the use of indium(I) halide, which acts as both the indium source and reducing agent for aminophosphine. The developed zinc-free, single-injection method gives access to tetrahedral InP QDs with an edge length > 10 nm and narrow size distribution. The first excitonic peak is tunable from 450 to 700 nm by changing the indium halide (InI, InBr, InCl). Kinetic studies using phosphorus NMR reveal the coexistence of two reaction pathways, the reduction of transaminated aminophosphine by In(I) and via redox disproportionation. Etching the surface of the obtained InP QDs at room temperature with in situ-generated hydrofluoric acid (HF) leads to strong photoluminescence (PL) emission with a quantum yield approaching 80%. Alternatively, surface passivation of the InP core QDs was achieved by low-temperature (140 °C) ZnS shelling using the monomolecular precursor zinc diethyldithiocarbamate. The obtained InP/ZnS core/shell QDs that emit in a range of 507-728 nm exhibit a small Stokes shift (110-120 meV) and a narrow PL line width (112 meV at 728 nm).

Bright InP Quantum Dots by Mid-Synthetic Modification with Zinc Halides

InP quantum dots (QDs) attract growing interest in recent years, owing to their environmental advantages upon applications in display and lighting. However, compared to Cd-based QDs and Pb-based perovskites, the synthesis of InP QDs with high optical quality is relatively more difficult. Here, we established a mid-synthetic modification approach to improve the optical properties of InP-based QDs. Tris(dimethylamino)phosphine ((DMA)₃P) and indium iodide were used to prepare InP QDs with a green emission (∼527 nm). By introducing zinc halides (ZnX₂) during the mid-synthetic process, the photoluminescence quantum yield (PLQY) of the resulting InP/ZnSeS/ZnS core/shell/shell QDs was increased to >70%, and the full-width-at-half-maximum (FWHM) could be narrowed to ∼40 nm. Transmission electron microscopy clearly showed the improvement of the QDs particle size distribution after introducing ZnX₂. It was speculated that ZnX₂ was bound to the surface of QDs as a Z-type ligand, which not only passivated surface defects and suppressed the emission of defect states but also prevented Ostwald ripening. The InP cores were also activated by ZnX₂, which made the growth of the ZnSeS shell more favorable. The photoluminescence properties started to be improved significantly only when the amount of ZnX₂ exceeded 0.5 mmol. As the amount increased, more ZnX₂ was distributed around the QDs to form a ligand layer, which prevented the shell precursor from crossing the ligand layer to the surface of the InP core, thus reducing the size of the InP/ZnSeS/ZnS QDs. This work revealed a new role of ZnX₂ and found a method for InP QDs with high brightness and low FWHM by the mid-synthetic modification, which would inspire the synthesis of even better InP QDs.

Zn-Doped P-Type InAs Nanocrystal Quantum Dots

Doped heavy metal-free III-V semiconductor nanocrystal quantum dots (QDs) are of great interest both from the fundamental aspects of doping in highly confined structures, and from the applicative side of utilizing such building blocks in the fabrication of p-n homojunction devices. InAs nanocrystals (NCs), that are of particular relevance for short-wave IR detection and emission applications, manifest heavy n-type character poising a challenge for their transition to p-type behavior. The p-type doping of InAs NCs is presented with Zn - enabling control over the charge carrier type in InAs QDs field effect transistors. The post-synthesis doping reaction mechanism is studied for Zn precursors with varying reactivity. Successful p-type doping is achieved by the more reactive precursor, diethylzinc. Substitutional doping by Zn²⁺ replacing In³⁺ is established by X-ray absorption spectroscopy analysis. Furthermore, enhanced near infrared photoluminescence is observed due to surface passivation by Zn as indicated from elemental mapping utilizing high-resolution electron microscopy corroborated by X-ray photoelectron spectroscopy study. The demonstrated ability to control the carrier type, along with the improved emission characteristics, paves the way towards fabrication of optoelectronic devices active in the short-wave infrared region utilizing heavy-metal free nanocrystal building blocks.

Investigating the role of zinc precursor during the synthesis of the core of III-V QDs

To investigate the role of Zn precursor based on hard and soft acids and bases theory, we introduced Mn and Ca precursors along with Zn precursor. The synthesis of III-V cores with these three metal precursors revealed that the roles of Zn precursor are as a reaction suppressant, a size regulator, and a dopant. Furthermore, we discovered which role was primarily played by Zn precursor at different concentrations.

Luminescent Anisotropic Wurtzite InP Nanocrystals

Indium phosphide (InP) nanocrystals are emerging as an alternative to heavy metal containing nanocrystals for optoelectronic applications but lag behind in terms of synthetic control. Herein, luminescent wurtzite InP nanocrystals with narrow size distribution were synthesized via a cation exchange reaction from hexagonal Cu₃P nanocrystals. A comprehensive surface treatment with NOBF₄ was performed, which removes excess copper while generating stoichiometric In/P nanocrystals with fluoride surface passivation. The attained InP nanocrystals manifest a highly resolved absorption spectrum with a narrow emission line of 80 meV, and photoluminescence quantum yield of up to 40%. Optical anisotropy measurements on ensemble and single particle bases show the occurrence of polarized transitions directly mirroring the anisotropic wurtzite lattice, as also manifested from modeling of the quantum confined electronic levels. This shows a green synthesis path for achieving wurtzite InP nanocrystals with desired optoelectronic properties including color purity and light polarization with potential for diverse optoelectronic applications.

Tailored growth of single-crystalline InP tetrapods

Despite the technological importance of colloidal covalent III-V nanocrystals with unique optoelectronic properties, their synthetic process still has challenges originating from the complex energy landscape of the reaction. Here, we present InP tetrapod nanocrystals as a crystalline late intermediate in the synthetic pathway that warrants controlled growth. We isolate tetrapod intermediate species with well-defined surfaces of (110) and ([Formula: see text]) via the suppression of further growth. An additional precursor supply at low temperature induces [Formula: see text]-specific growth, whereas the [110]-directional growth occurs over the activation barrier of 65.7 kJ/mol at a higher temperature, thus finalizes into the (111)-faceted tetrahedron nanocrystals. We address the use of late intermediates with well-defined facets at the sub-10 nm scale for the tailored growth of covalent III-V nanocrystals and highlight the potential for the directed approach of nanocrystal synthesis.