Gupta Aritrajit, Ondry Justin C, Lin Kailai, Chen Yunhua, Hudson Margaret H, Chen Min, Schaller Richard D, Rossini Aaron J, Rabani Eran, Talapin Dmitri V
Department of Chemistry, James Franck Institute, and Pritzker School of Molecular Engineering, University of Chicago, Chicago, Illinois 60637, United States.
Department of Chemistry, University of California, Berkeley, California 94720, United States.
J Am Chem Soc. 2023 Aug 2;145(30):16429-16448. doi: 10.1021/jacs.3c02709. Epub 2023 Jul 19.
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 () space. It has, however, been debated if 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 InGaP, the band structure can be tuned continuously from the direct- to indirect-gap by changing the value of . 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 InGaP/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 InGaP cores. The radiative decay lifetime for InGaP/ZnS monotonically increases with higher gallium content. Transient absorption studies on InGaP/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 InGaP/ZnS QD emitters with desirable optoelectronic properties.
半导体通常根据价带顶和导带底在晶体动量（）空间中的相对位置分为具有直接或间接带隙的材料。然而，对于是否是描述量子限制纳米晶体系统中能带结构的有用量子数存在争议，因为这模糊了直接和间接带隙半导体之间的区别。在诸如InGaP的体相III-V族半导体合金中，通过改变的值，能带结构可以从直接带隙连续调节到间接带隙。由于高质量的胶体纳米晶体样品仍然难以获得，因此尚未确定强量子限制对该系统中直接到间接跃迁的影响。在此，我们报告了对三元III-V族纳米晶体的首批系统研究之一，通过利用优化的熔盐In到Ga阳离子交换协议，制备出光致发光量子产率超过80%的明亮InGaP/ZnS核壳颗粒。我们进行了二维固态核磁共振研究，以评估InGaP核中的合金均匀性和表面氧化程度。InGaP/ZnS的辐射衰减寿命随着镓含量的增加而单调增加。对InGaP/ZnS纳米晶体的瞬态吸收研究分别基于激子漂白特征的存在或不存在，证明了直接和类间接行为的特征。基于半经验赝势模型的原子电子结构计算用于计算吸收光谱和辐射寿命，并评估带边简并性；所得计算出的电子性质与实验观察结果一致。通过研究高温下的光致发光特性，我们证明了III-V/II-VI核壳界面处晶格失配的减小可以提高发射的热稳定性。这些见解确立了在熔融无机盐中进行阳离子交换是一种可行的合成路线，可用于制备具有理想光电性质的无毒、高质量InGaP/ZnS量子点发光体。
