Energetics at the Surface: Direct Optical Mapping of Core and Surface Electronic Structure in CdSe Quantum Dots using Broadband Electronic Sum Frequency Generation Microspectroscopy.
Energetics at the Surface: Direct Optical Mapping of Core and Surface Electronic Structure in CdSe Quantum Dots using Broadband Electronic Sum Frequency Generation Microspectroscopy.
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DOI：
10.1021/acs.nanolett.9b02201
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发表时间：
2019-08
期刊：
影响因子：
10.8
通讯作者：
Brianna R. Watson;Benjamin Doughty;T. Calhoun
中科院分区：
文献类型：
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作者：
Brianna R. Watson;Benjamin Doughty;T. Calhoun
了解和控制纳米材料的电子结构是使其在实际应用中得到广泛应用的关键。尽管有这种需要，但许多重要的电子状态对于传统的光学测量来说是不可见的，并且通常是根据它们对发光特性的推断影响间接识别的。这在纳米材料表面及其相关缺陷的研究中尤其常见和重要。表面阱态在光物理过程中起着至关重要的作用，但人们对其了解甚少。在这里，我们首次证明了宽带电子和频生成（eSFG）显微光谱可以直接绘制纳米粒子的光学亮态和暗态，包括难以捉摸的间隙下态。这种新方法被应用于模型硒化镉（CdSe）量子点（QDs），其中表面陷阱态的能量几十年来一直无法直接进行光学表征。我们的eSFG测量显示了带隙以上的电子跃迁的清晰特征，我们将其归因于先前报道的与CdSe核心相关的单光子和双光子跃迁，以及归因于表面态的带隙以下的广谱特征。除了核心状态外，该分析还揭示了间隙下状态的两种不同分布，为该系统的浅表面和深表面状态提供了第一次直接光学测量。最后，通过氧化对表面进行化学修饰，导致来自表面状态的信号相对增加。总的来说，我们的eSFG实验为直接绘制QD核心和表面电子结构的整体提供了一条途径，这有望为研究这些材料如何在原位生长以及如何控制表面状态以调整功能提供机会。
Understanding and controlling the electronic structure of nanomaterials is the key to tailoring their use in a wide range of practical applications. Despite this need, many important electronic states are invisible to conventional optical measurements and are typically identified indirectly based on their inferred impact on luminescence properties. This is especially common and important in the study of nanomaterial surfaces and their associated defects. Surface trap states play a crucial role in photophysical processes yet remain remarkably poorly understood. Here we demonstrate for the first time that broadband electronic sum frequency generation (eSFG) microspectroscopy can directly map the optically bright and dark states of nanoparticles, including the elusive below gap states. This new approach is applied to model cadmium selenide (CdSe) quantum dots (QDs), where the energies of surface trap states have eluded direct optical characterization for decades. Our eSFG measurements show clear signatures of electronic transitions both above the band gap, which we assign to previously reported one- and two-photon transitions associated with the CdSe core, as well as broad spectral signatures below the bandgap that are attributed to surface states. In addition to the core states, this analysis reveals two distinct distributions of below gap states providing the first direct optical measurement of both shallow and deep surface states on this system. Finally, chemical modification of the surfaces via oxidation results in the relative increase in the signals originating from the surface states. Overall, our eSFG experiments provide an avenue to directly map the entirety of QD core and surface electronic structure, which is expected to open up opportunities to study how these materials are grown in situ and how surface states can be controlled to tune functionality.