Composition-Defined Optical Properties and the Direct-to-Indirect Transition in Core–Shell In_1–xGa_xP/ZnS Colloidal Quantum Dots

Abstract

Semiconductors are commonly divided into materials with direct or indirect band gaps based on the relative positions of the top of the valence band and the bottom of the conduction band in crystal momentum (k) space. It has, however, been debated if k is a useful quantum number to describe the band structure in quantum-confined nanocrystalline systems, which blur the distinction between direct and indirect gap semiconductors. In bulk III−V semiconductor alloys like In 1−x Ga x P, the band structure can be tuned continuously from the direct-to indirect-gap by changing the value of x. The effect of strong quantum confinement on the direct-to-indirect transition in this system has yet to be established because high-quality colloidal nanocrystal samples have remained inaccessible. Herein, we report one of the first systematic studies of ternary III−V nanocrystals by utilizing an optimized molten-salt In-to-Ga cation exchange protocol to yield bright In 1−x Ga x P/ZnS core−shell particles with photoluminescence quantum yields exceeding 80%. We performed two-dimensional solid-state NMR studies to assess the alloy homogeneity and the extent of surface oxidation in In 1−x Ga x P cores. The radiative decay lifetime for In 1−x Ga x P/ZnS monotonically increases with higher gallium content. Transient absorption studies on In 1−x Ga x P/ZnS nanocrystals demonstrate signatures of direct-and indirect-like behavior based on the presence or absence, respectively, of excitonic bleach features. Atomistic electronic structure calculations based on the semi-empirical pseudopotential model are used to calculate absorption spectra and radiative lifetimes and evaluate band-edge degeneracy; the resulting calculated electronic properties are consistent with experimental observations. By studying photoluminescence characteristics at elevated temperatures, we demonstrate that a reduced lattice mismatch at the III−V/II−VI core−shell interface can enhance the thermal stability of emission. These insights establish cation exchange in molten inorganic salts as a viable synthetic route to nontoxic, high-quality In 1−x Ga x P/ZnS QD emitters with desirable optoelectronic properties.

Asymmetric Metal–Carboxylate Complexes for Synthesis of InGaP Alloyed Quantum Dots with Blue Emission

“…We observed a gradual decrease in the PLQY of the In 1– x Ga x P QDs as the Ga composition increased, ranging from 31.1% in InP QDs to 4.6% in In 0.62 Ga 0.38 P QDs (Figure S7). This trend supports the notion of an alloy composition for the In 1– x Ga x P QDs, rather than InP/GaP core/shell structure, as indicated by the decreasing PLQY with higher Ga composition …”

“…This trend supports the notion of an alloy composition for the In 1−x Ga x P QDs, rather than InP/GaP core/shell structure, as indicated by the decreasing PLQY with higher Ga composition. 45 This trend suggests a higher likelihood of defects on the Ga surface compared with the In surface. To improve the emission properties of In 0.62 Ga 0.38 P alloyed QDs, we employed shell materials to the QD surface for passivation (Figure 5a).…”

Synthesis of Ternary and Quaternary Group III-Arsenide Colloidal Quantum Dots via High-Temperature Cation Exchange in Molten Salts: The Importance of Molten Salt Speciation

“…In addition, for samples of InP and InAs annealed in the KGaI 4 molten salt, we observe shifts of the diffraction peaks to larger q (2θ), indicating the lattice parameter of the nanocrystals has decreased, demonstrating that gallium has diffused into the lattice. Previous work from our group has shown that the PXRD peak shifts correspond to homogeneous alloying which was further confirmed by solid-state NMR; however, for larger quantum dots in this work, a composition gradient cannot be ruled out. Further we observe nearly identical peak width for the KGaI 4 treated nanocrystals (light brown and orange lines, respectively) compared to the initial nanocrystals (black).…”

“…Importantly this methodology resulted in highly luminescent In 1– x Ga x P/ZnS and In 1– x Ga x As/CdS nanocrystals demonstrating that molten salt derived colloidal nanocrystals have high crystal quality. Importantly, molten salt solvents are thus far the only route which has resulted in highly emissive Ga-containing III–V nanocrystals over a wide composition range. , Given the potential to prepare high-quality materials, it is imperative to better understand these unconventional reaction environments to enable rational design of molten salt derived colloidal materials.…”

Aqueous Topological Synthesis of Au@semiconductor Core–Shell Nanocrystals with Morphology and Composition Engineering

“…Colloidal core–shell nanocrystals (C–S NCs), especially the noble metal@semiconductor (NMS) C–S NCs with plasmon-exciton coupling effect, are an essential class of materials due to their tunable optical and electrical properties. − NMS C–S NCs have kept the controllable localized surface plasmon resonance (LSPR) effect from the noble metal, the widely tunable absorption benefiting from various compositions, and the maximum degree of functional coupling corresponding to the most extensive contact areas. − In the past decades, many strategies have been utilized to synthesize NMS C–S NCs, such as vapor deposition, molecular beam epitaxy, and colloidal epitaxial growth. − Beyond this, it is desirable to assume a general strategy with not only flexible morphology and composition engineering but also hydrophilic surface chemistry.…”