Proceedings of nanoGe Fall Meeting19 (NFM19)
DOI: https://doi.org/10.29363/nanoge.nfm.2019.031
Publication date: 18th July 2019
Recent advances in colloidal synthesis and assembly1-3 allows a comparison of the strength of light absorption of semiconductor nanocrystals in three distinct electronic phases: (i) as non-interacting individual nanocrystals with strong three-dimensional quantum confinement dispersed in solution, (ii) as ligand-separated nanocrystals present in an ordered monolayer, and (iii) as nanocrystals epitaxially connected in a monolayer superlattice. We performed quantitative absorptance measurements on these three different samples, for the case of PbSe (band gap in the IR) and CdSe (band gap in the visible). The light absorption cross section of PbSe nanocrystals in a hexagonal monolayer is 5-10 fold increased versus nanocrystals in solution; this is due to far-field polar coupling, which reduces the dielectric screening of the electric field in a NC monolayer. The absorption cross section is further enhanced in superlattices as the epitaxial connection results in a two-dimensional electronic system, and thus complete quenching of dielectric screening. Nanocrystal monolayer superlattices of PbSe and CdSe on quartz show 1.6 % absorptance, a value directly related to the fine structure constant. This “quantum of light absorption” has been reported for several other two-dimensional systems, from graphene4 to III-V semiconductor quantum wells5, and explained with Fermi’s golden rule and the effective mass approximation5. We calculated the absorptance of two-dimensional II-VI and IV-VI semiconductors and superlattices with atomistic tight-binding theory, resulting in values close to the absorptance quantum.
REFERENCES
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2. Schliehe, C. et al. Ultrathin PbS Sheets by Two-Dimensional Oriented Attachment. Science 329, 550-553 (2010).
3. Peters, J. L. et al. Mono- and Multilayer Silicene-Type Honeycomb Lattices by Oriented Attachment of PbSe Nanocrystals: Synthesis, Structural Characterization, and Analysis of the Disorder. Chem. Mater. 30, 4831-4837 (2018).
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5.Fang, H. et al. Quantum of optical absorption in two-dimensional semiconductors. P.. Natl. Acad. Sci. USA 110, 11688-11691 (2013).
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