Atomistic Description of Thiostannate-Capped CdSe Nanocrystals: Retention of Four-Coordinate SnS4 Motif and Preservation of Cd-Rich Stoichiometry

Colloidal semiconductor nanocrystals (NCs) are widely studied as building blocks for novel solid-state materials. Inorganic surface functionalization, used to displace native organic capping ligands from NC surfaces, has been a major enabler of electronic solid-state devices based on colloidal NCs. At the same time, very little is known about the atomistic details of the organic-to-inorganic ligand exchange and binding motifs at the NC surface, severely limiting further progress in designing all-inorganic NCs and NC solids. Taking thiostannates (K4SnS4, K4Sn2S6, K6Sn2S7) as typical examples of chalcogenidometallate ligands and oleate-capped CdSe NCs as a model NC system, in this study we address these questions through the combined application of solution 1H NMR spectroscopy, solution and solid-state 119Sn NMR spectroscopy, far-infrared and X-ray absorption spectroscopies, elemental analysis, and by DFT modeling. We show that through the X-type oleate-to-thiostannate ligand exchange, CdSe NCs retain their Cd-rich stoichiometry, with a stoichiometric CdSe core and surface Cd adatoms serving as binding sites for terminal S atoms of the thiostannates ligands, leading to all-inorganic (CdSe)core[Cdm(Sn2S7)yK(6y‑2m)]shell (taking Sn2S76– ligand as an example). Thiostannates SnS44– and Sn2S76– retain (distorted) tetrahedral SnS4 geometry upon binding to NC surface. At the same time, experiments and simulations point to lower stability of Sn2S64– (and SnS32–) in most solvents and its lower adaptability to the NC surface caused by rigid Sn2S2 rings.

胶体半导体纳米晶（colloidal semiconductor nanocrystals, NCs）作为新型固态材料的构筑基元，已得到广泛研究。用于取代纳米晶表面原生有机封端配体的无机表面功能化手段，是支撑基于胶体纳米晶的电子固态器件发展的关键技术之一。然而，学界对纳米晶表面有机-无机配体交换与结合基序的原子级细节仍知之甚少，这严重限制了全无机纳米晶及其固体组装体的设计研发进程。本研究以硫代锡酸盐（thiostannates，涵盖K4SnS4、K4Sn2S6、K6Sn2S7）作为硫属金属配体的典型范例，以油酸盐封端的CdSe纳米晶作为模型纳米晶体系，通过联用溶液相1H核磁共振波谱法、溶液与固态119Sn核磁共振波谱法、远红外及X射线吸收光谱、元素分析与密度泛函理论（DFT）建模，对上述问题进行了系统探究。研究结果显示，通过X型油酸盐-硫代锡酸盐配体交换，CdSe纳米晶保留了其富镉化学计量比：以化学计量比CdSe为核，表面镉吸附原子作为硫代锡酸盐配体末端硫原子的结合位点，最终形成全无机结构(CdSe)核[Cdm(Sn2S7)yK(6y‑2m)]壳层（以Sn2S76–配体为例）。硫代锡酸盐SnS44–与Sn2S76–结合至纳米晶表面后，仍保留（畸变）四面体SnS4几何构型。与此同时，实验与模拟结果均表明，Sn2S64–（及SnS32–）在多数溶剂中的稳定性较差，且受限于刚性Sn2S2环结构，其适配纳米晶表面的能力更弱。