量子点界的网红：III-IV族量子点（InP）
前两天和朋友聊天过程中，无意间发现今年各类杂志上关于InP的文章如雨后春笋般涌现。当之无愧可以称为量子点界的网红。InP QDs的各种优势已经在上一篇相关分享中列出（InP量子点合成相关文献总结与分析）。总结为一句话：InP是最有可能取代含Cd量子点而走向应用的材料。
首先从近期CM上的一篇综述开始。专门针对InP材料的综述很少见，最近更新的这篇综述较为全面的介绍了InP量子点的研究进展。
Chemically synthesized InP nanocrystals (NCs) are drawing a large interest as a potentially less toxic alternative to CdSe-based nanocrystals. With a bulk band gap of 1.35 eV and an exciton Bohr radius of ∼10 nm the emission wavelength of InP NCs can in principle be tuned throughout the whole visible and near-infrared range by changing their size. Furthermore, a few works reported fluorescence quantum yields exceeding 70% after overcoating the core NCs with appropriate shell materials. Therefore, InP NCs arevery promising for use in lighting and display applications. On the other hand, a number of challenges remain to be addressed in order to progress from isolated research results to robust and reproducible synthesis methods for high quality InP NCs. First of all, the size distribution of the as-synthesized NCs needs to be reduced, which directly translates into more narrow emission line widths. Next, reliable protocols are required for achieving a given emission wavelength at high reaction yield and for further improving the emission efficiency and chemical and photostability. Advances in these directions have been hampered for a long time by the specific properties of InP, such as the rather covalent nature of binding implying harsh synthesis conditions, high sensitivity toward oxidation, and limited choice of phosphorus precursors. However, in recent years a much better understanding of the precursor conversion kinetics and reaction mechanisms has been achieved, giving this field new impulse. In this review we provide a comprehensive overview from initial synthetic approaches to the most recent developments. First, we highlight the fundamental differences in the syntheses of InP-based NCs with respect to established II–VI and IV–VI semiconductor NCs comparing their nucleation and growth stages. Next, we inspect in detail the influence of the nature of the phosphorus and indium precursors used and of reaction additives, such as zinc carboxylates or alkylamines, on the properties of the NCs. Finally, core/shell systems and doped InP NCs are discussed, and perspectives in this field are given.
文中首先介绍了InP QDs合成方法的发展历程以及反应机理。紧接着介绍经典的成核与生长机理。基本发展过程和基本机理介绍完成后，作者分类总结了P 前体、In前体、配体以及添加剂在InP QDs合成中的作用。除了单纯的InPQDs的合成，文中后半部分重点介绍InP/ZnS核壳结构量子点的合成和InP的掺杂。
Wet chemical synthesis of covalent III-V colloidal quantum dots (CQDs) has been challenging because of uncontrolled surfaces and a poor understanding of surface–ligand interactions. We report a simple acid-free approach to synthesize highly crystalline indium phosphide CQDs in the unique tetrahedral shape by using tris(dimethylamino) phosphine and indium trichloride as the phosphorus and indium precursors, dissolved in oleylamine. Our chemical analyses indicate that both the oleylamine and chloride ligands participate in the stabilization of tetrahedral-shaped InP CQDs covered with cation-rich (111) facets. Based on density functional theory calculations, we propose that fractional dangling electrons of the In-rich (111) surface could be completely passivated by three halide and one primary amine ligands per the (2×2) surface unit, satisfying the 8-electron rule. This halide–amine co-passivation strategy will benefit the synthesis of stable III-V CQDs with controlled surfaces.
这篇Angewande首次报道合成出四面体InP量子点。有关量子点形貌调控的文献很多，但很少涉及InP。本文通过Cl和胺这两种配体的协同作用，成功合成出四面体InP量子点，尺寸在10nm左右。通读全文，发现该文的亮点并不在形貌的控制。首先文中只是合成出这一种形貌的InP，其次没有考察其成核与生长过程，机理解释也较为简单。个人觉得亮点在于无酸体系的引入，实现了较厚ZnS层的生长，从而提高InP/ZnS的稳定性。已有多篇文献报道在有酸存在的合成体系中，无论是单纯的InP Core量子点，还是InP/ZnS核壳量子点，都会有部分InP被氧化，从而阻碍InP的生长和包壳。本文选择以InCl3作为铟源，Cl和油胺作为配体，避免了羧酸盐和羧酸的使用，从而没有InPOx的生成。通过此方法，最终可以在InP表面生长近2nm的ZnS，从而提高材料的稳定性。
上面这篇文献通过抑制InP氧化从而实现ZnS的有效成壳。InP/ZnS核壳量子点难以合成的另外一个原因是两者的晶格参数相差大。解决晶格失配的方法有两种：一是引入过渡层；二是形成合金。下面这篇文献中，作者合成出InZnPx合金，通过控制Zn的含量，可以实现晶格参数的调控。
Colloidal quantum dots (QDs) show great promise as LED phosphors due to their tunable narrow-band emission and ability to produce high-quality white light. Currently, the most suitable QDs for lighting applications are based on cadmium, which presents a toxicity problem for consumer applications. The most promising cadmium-free candidate QDs are based on InP, but their quality lags much behind that of cadmium based QDs. This is not only because the synthesis of InP QDs is more challenging than that of Cd-based QDs, but also because the large lattice parameter of InP makes it difficult to grow an epitaxial, defect-free shell on top of such material. Here, we propose a viable approach to overcome this problem by alloying InP nanocrystals with Zn2+ ions, which enables the synthesis of InxZnyP alloy QDs having lattice constant that can be tuned from 5.93 Å (pure InP QDs) down to 5.39 Å by simply varying the concentration of the Zn precursor. This lattice engineering allows for subsequent strain-free, epitaxial growth of a ZnSezS1–z shell with lattice parameters matching that of the core. We demonstrate, for a wide range of core and shell compositions (i.e., varying x, y, and z), that the photoluminescence quantum yield is maximal (up to 60%) when lattice mismatch is minimal.
在包壳时，同样可以通过调节S的含量来控制ZnSeSx晶格参数。最终，作者将量子产率与核壳晶格匹配度关联（如下图），发现晶格匹配度越高，其QY越高。
Magic-sized nanoclusters have been implicated as mechanistically relevant intermediates in the synthesis of group III-V quantum dots. Herein we report the single-crystal X-ray diffraction structure of a carboxylate-ligated indium phosphide magic-sized nanocluster at 0.83 Å resolution. The structure of this cluster, In37P20(O2CR)51, deviates from that of known crystal phases and possesses a non-stoichiometric, charged core composed of a series of fused 6-membered rings. The cluster is completely passivated by bidentate carboxylate ligands exhibiting predominantly bridging binding modes. The absorption spectrum of the cluster shows an asymmetric line shape that is broader than what would be expected from a homogeneous sample. A combination of computational and experimental evidence suggests that the spectral line width is a result of multiple, discrete electronic transitions that couple to vibrations of the nanocrystal lattice. The product of reaction of this nanocluster with 1 equiv of water has also been structurally characterized, demonstrating site selectivity without a drastic alteration of electronic structure.
这篇JACs的亮点在于对InP Cluster的表征。文章来自于Cossairt课题组。该课题组做了大量关于InP合成方面的工作。在InP量子点合成过程中，Cluster的形成似乎不可避免，甚至可以粗浅将Cluster理解为一种中间态。本文通过单晶衍射，仔细研究了InPCluster的结构，发现其由六圆环组成，组成并非严格化学计量比。综合实验结果与理论计算，文中提出Cluster的宽半半峰宽来自晶格震动。
We demonstrate the ability of M2+ Lewis acids (M = Cd, Zn) to dramatically enhance the photoluminescence quantum yield (PL QY) of InP quantum dots. The addition of cadmium and zinc is additionally found to red- and blue-shift, respectively, the lowest energy absorption and emission of InP quantum dots while maintaining particle size. This treatment results in a facile strategy to post-synthetically tune the luminescence color in these materials. Optical and structural characterization (XRD, TEM, XAS, ICP) have led us to identify the primary mechanism of PL turn-on as surface passivation of phosphorus dangling bonds, affording PL QYs up to 49% without the growth of a type I shell or the addition of HF. This route to PL enhancement and color tuning may prove useful as a standalone treatment or as a complement to shelling strategies.
这篇JPCL同样出自Cossairt课题组，他们发现InP经过羧酸镉盐与锌盐后处理能够明显提高其效率，同时还可以调节发光波长。镉盐处理发生红移，锌盐处理发生蓝移。
成核过程难以考察，特别是成核早期。当实验难以触碰的时候，理论模拟就是一把利器。下面这篇文章利用High-Temperature Ab Initio Molecular Dynamics 方法模拟InP量子点成核与生长初期中间体的变化情况。
Colloidal quantum dots (QDs) exhibit highly desirable size- and shape-dependent properties for applications from electronic devices to imaging. Indium phosphide QDs have emerged as a primary candidate to more toxic CdSe QDs, but production of InP QDs with the desired properties lags behind other QD materials due to a poor understanding of how to tune the growth process. Using high-temperature ab initio molecular dynamics (AIMD) simulations, we report the first direct observation of early-stage intermediates and subsequent formation of an InP cluster from indium and phosphorus precursors. In our simulations, indium agglomeration precedes formation of In–P bonds. We observe a predominantly intercomplex pathway in which In–P bonds form between one set of precursor copies, and the carboxylate ligand of a second indium precursor in the agglomerated indium abstracts a ligand from the phosphorus precursor. This process produces an indium-rich cluster with structural properties comparable to those in bulk zinc-blende InP crystals. Minimum energy pathway characterization of the AIMD-sampled reaction events confirms these observations and identifies that In–carboxylate dissociation energetics solely determine the barrier along the In–P bond formation pathway, which is lower for intercomplex (13 kcal/mol) than intracomplex (21 kcal/mol) mechanisms. The phosphorus precursor chemistry, on the other hand, controls the thermodynamics of the reaction. Our observations of the different roles of precursors in controlling QD formation strongly suggest that the challenges thus far encountered in InP QD synthesis optimization may be attributed to an overlooked need for a cooperative tuning strategy that simultaneously addresses the chemistry of both indium and phosphorus precursors.
研究发现反应过程中会有In4P cluster生成。文中指出在Cluster形成的40ps内，多个In前驱体围绕一个P前驱体形成中间体。这一中间体通过协同作用形成Cluster。本文最终总结得出：成核与生长过程受In源和P源的协同控制（具体作用见原文）。如果只是调节某一前驱体的活性，难以提高InP的分散性。
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