Watson Brianna R, Doughty Benjamin, Calhoun Tessa R
Department of Chemistry , University of Tennessee , Knoxville , Tennessee 37996 , United States.
Chemical Sciences Division , Oak Ridge National Laboratory , Oak Ridge , Tennessee 37831 , United States.
Nano Lett. 2019 Sep 11;19(9):6157-6165. doi: 10.1021/acs.nanolett.9b02201. Epub 2019 Aug 7.
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 band gap 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 the 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.
理解和控制纳米材料的电子结构是在广泛实际应用中定制其用途的关键。尽管有此需求，但许多重要的电子态对于传统光学测量是不可见的，通常是基于它们对发光特性的推断影响来间接识别。这在纳米材料表面及其相关缺陷的研究中尤为常见且重要。表面陷阱态在光物理过程中起着关键作用，但仍知之甚少。在此，我们首次证明宽带电子和频产生（eSFG）显微光谱能够直接绘制纳米颗粒的光学亮态和暗态，包括难以捉摸的带隙以下态。这种新方法应用于模拟硒化镉（CdSe）量子点（QDs），数十年来其表面陷阱态的能量一直无法通过直接光学表征确定。我们的eSFG测量显示，在带隙以上有清晰的电子跃迁特征，我们将其归因于先前报道的与CdSe核相关的单光子和双光子跃迁，以及带隙以下归因于表面态的宽光谱特征。除了核态，该分析还揭示了带隙以下态的两种不同分布，首次对该系统上的浅表面态和深表面态进行了直接光学测量。最后，通过氧化对表面进行化学修饰导致源自表面态的信号相对增加。总体而言，我们的eSFG实验提供了一条直接绘制量子点核与表面电子结构全貌的途径，有望为研究这些材料如何原位生长以及如何控制表面态以调节功能开辟机会。
