Dielectric Confinement and Excitonic Effects in Two-Dimensional Nanoplatelets.,ACS Nano

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Dielectric Confinement and Excitonic Effects in Two-Dimensional Nanoplatelets.
ACS Nano ( IF 15.8 ) Pub Date : 2020-06-25 , DOI: 10.1021/acsnano.0c01950
Botao Ji _{1,

2} , Eran Rabani _{3,

4,

5} , Alexander L Efros ₆ , Roman Vaxenburg ₇ , Or Ashkenazi ₈ , Doron Azulay _{8,

9} , Uri Banin ₁ , Oded Millo ₈

Affiliation

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.
Department of Chemistry, University of California Berkeley, Berkeley, California 94720, United States.
Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States.
The Raymond and Beverly Sackler Center of Computational Molecular and Materials Science, Tel Aviv University, Tel Aviv 69978, Israel.
Center for Computational Material Science, Naval Research Laboratory, Washington, D.C. 20375, United States.
Howard Hughes Medical Institute, Janelia Research Campus, 19700 Helix Drive, Ashburn, Virginia 20147, United States.
Racah Institute of Physics and the Hebrew University Center for Nanoscience and Nanotechnology, The Hebrew University of Jerusalem, Jerusalem 91904, Israel.
Azrieli, Jerusalem College of Engineering, Jerusalem 9103501, Israel.

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.

中文翻译：

二维纳米片中的介电限制和激子效应。

准二维（2D）半导体纳米片表现出强大的量子限制，具有窄光致发光峰的出色光学特性，其能量可通过厚度以单层精度进行调节。我们将扫描隧道光谱（STS）与光学测量结合使用，以探测厚度依赖性带隙和一系列CdSe纳米片中的激发态密度。在双势垒隧道结构型中测量的隧道谱揭示了量子约束对带隙的影响，主要是通过导带边缘的蓝移，以及二维电子结构的特征和有限的横向混合-尺寸和/或缺陷影响。正如理论计算所证实的，STS基带隙大于光学间隙，这是由于激子结合对吸收的贡献所期望的。该计算还指出，在轻度孔和分离度孔位之间存在强价带混合。引人注目的是，隧道光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。分析和讨论了对此可能的解释，包括纳米片电荷的相互作用，介电限制以及轻和重空穴的激子结合能的差异。引人注目的是，隧道光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。分析和讨论了对此可能的解释，包括纳米片电荷的相互作用，介电限制以及轻和重空穴的激子结合能的差异。引人注目的是，隧道光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。分析和讨论了对此可能的解释，包括纳米片电荷的相互作用，介电限制以及轻和重空穴的激子结合能的差异。

更新日期：2020-07-28

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