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.
Bibliographical noteFunding Information:
The research in the U.B. Lab leading to these results received partial support from the ISF-NSFC joint research program (grant no. 2495/17). U.B. thanks the Alfred and Erica Larisch memorial chair. The work of E.R. was supported by the NSF DMREF Program under awards DMR-1629361 and by the University of California Lab Fee Research Program (grant LFR-17-477237). A.L.E. acknowledge support from the U.S. Office of Naval Research and the Laboratory University Collaboration Initiative of the DoD Basic Research Office. O.M. thanks support from the Israel Science Foundation (grant 661/16) and the Harry de Jur Chair in Applied Science.
Copyright © 2020 American Chemical Society.
- atomistic empirical pseudopotential
- quantum confinement
- scanning tunneling spectroscopy
- semiconductor nanoplatelets
- two-dimensional systems