Dynamic Nuclear Polarization Enhanced ¹¹³Cd Solid-State Nuclear Magnetic Resonance Spectroscopy Reveals CdSe Nanocrystals with Triangular Two-Dimensional Projections are Terminated by {100} Facets

Surface structure plays an important role in particle growth and determining the chemical and photophysical properties of semiconductor nanocrystals (NCs). Therefore, there is a need for structural tools that can characterize and detect different surface facets. Here, we sought to investigate the structure of {111} CdSe facets by applying dynamic nuclear polarization (DNP) enhanced 113Cd and 77Se solid-state nuclear magnetic resonance (SSNMR) spectroscopy to zinc-blende CdSe NCs that exhibit triangular two-dimensional (2D) projections within transmission electron microscope (TEM) images. It was originally hypothesized in the literature that these CdSe NCs were tetrahedral in shape and terminated by facets from the {111} family of lattice planes of the zinc-blende structure. Surprisingly, we observe 113Cd NMR spectra indicating that the primary facets are from the {100} family of lattice planes. We also obtained DNP-enhanced 113Cd and 77Se SSNMR spectra of recently reported right trigonal bipyramidal (rTriBP) CdSe NCs grown by seeded growth. The 113Cd NMR spectra rTriBP CdSe NCs are consistent with {100} surface facets. TEM images show that rTriBP NCs may also exhibit triangular 2D projections that are similar to the so-called tetrahedral NCs. Based upon these results, we conclude that the so-called tetrahedral NCs are predominantly terminated by {100} surface facets and most likely have the same shape as rTriBP NCs. These results highlight the need for multiple complementary techniques when assigning the shape and surface termination of NCs.表面结构在半导体纳米晶（semiconductor nanocrystals, NCs）的颗粒生长过程以及其化学与光物理性质的调控中发挥着关键作用。因此，亟需开发能够表征并检测各类表面晶面的结构表征手段。 本研究针对透射电子显微镜（transmission electron microscope, TEM）成像中呈现三角形二维（two-dimensional, 2D）投影的闪锌矿（zinc-blende）型硒化镉纳米晶，采用动态核极化（dynamic nuclear polarization, DNP）增强的¹¹³Cd与⁷⁷Se固态核磁共振（solid-state nuclear magnetic resonance, SSNMR）光谱技术，探究{111}晶面硒化镉纳米晶的表面结构。 此前学界曾提出假说，认为此类硒化镉纳米晶为四面体形貌，其表面终止于闪锌矿结构{111}族晶格平面。 令人意外的是，我们通过¹¹³Cd核磁共振光谱观测到，该纳米晶的主要表面晶面属于{100}族晶格平面。 我们还获取了近期报道的、通过晶种生长法制备的正三角双锥（right trigonal bipyramidal, rTriBP）型硒化镉纳米晶的DNP增强¹¹³Cd与⁷⁷Se固态核磁共振光谱。 正三角双锥型硒化镉纳米晶的¹¹³Cd核磁共振光谱与{100}表面晶面相符。 透射电子显微镜成像显示，正三角双锥型纳米晶同样可呈现与所谓“四面体”纳米晶相似的三角形二维投影。 基于上述结果，我们得出结论：所谓的“四面体”硒化镉纳米晶，其表面主要终止于{100}表面晶面，且其形貌极有可能与正三角双锥型纳米晶一致。 本研究结果凸显了在表征纳米晶形貌与表面终止结构时，采用多种互补表征手段的必要性。