Ji Botao, Rabani Eran, Efros Alexander L, Vaxenburg Roman, Ashkenazi Or, Azulay Doron, Banin Uri, Millo Oded
Department of Chemistry and the Hebrew University Center for Nanoscience and Nanotechnology, The Hebrew University of Jerusalem, Jerusalem 91904, Israel.
Key Laboratory of 3D Micro/Nano Fabrication and Characterization of Zhejiang Province, School of Engineering, Westlake University and Institute of Advanced Technology, Westlake Institute for Advanced Study, Hangzhou 310024, China.
ACS Nano. 2020 Jul 28;14(7):8257-8265. doi: 10.1021/acsnano.0c01950. Epub 2020 Jul 1.
Quasi-two-dimensional (2D) semiconductor nanoplatelets manifest strong quantum confinement with exceptional optical characteristics of narrow photoluminescence peaks with energies tunable by thickness with monolayer precision. We employed scanning tunneling spectroscopy (STS) in conjunction with optical measurements to probe the thickness-dependent band gap and density of excited states in a series of CdSe nanoplatelets. The tunneling spectra, measured in the double-barrier tunnel junction configuration, reveal the effect of quantum confinement on the band gap taking place mainly through a blue-shift of the conduction band edge, along with a signature of 2D electronic structure intermixed with finite lateral-size and/or defects effects. The STS fundamental band gaps are larger than the optical gaps as expected from the contributions of exciton binding in the absorption, as confirmed by theoretical calculations. The calculations also point to strong valence band mixing between the light- and split-off hole levels. Strikingly, the energy difference between the heavy-hole and light-hole levels in the tunneling spectra are significantly larger than the corresponding values extracted from the absorption spectra. Possible explanations for this, including an interplay of nanoplatelet charging, dielectric confinement, and difference in exciton binding energy for light and heavy holes, are analyzed and discussed.
准二维（2D）半导体纳米片表现出强烈的量子限制效应，具有独特的光学特性，即光致发光峰窄，其能量可通过厚度以单层精度进行调节。我们结合光学测量使用扫描隧道谱（STS）来探测一系列CdSe纳米片中与厚度相关的带隙和激发态密度。在双势垒隧道结配置中测量的隧道谱揭示了量子限制对带隙的影响，主要通过导带边缘的蓝移发生，同时还有二维电子结构的特征，与有限的横向尺寸和/或缺陷效应相互交织。正如理论计算所证实的，由于吸收中激子束缚的贡献，STS基态带隙大于光学带隙。计算还表明轻空穴和分裂-off空穴能级之间存在强价带混合。令人惊讶的是，隧道谱中重空穴和轻空穴能级之间的能量差明显大于从吸收光谱中提取的相应值。对此的可能解释，包括纳米片充电、介电限制以及轻、重空穴激子结合能差异的相互作用，都进行了分析和讨论。
