Publication date: 3rd July 2020
While early works on PbS NPLs assumed or inferred rock-salt crystal structure (as in bulk and nanocrystals), later experimental studies evidenced an orthorhombic (P nma space group) modification may be also formed.[1,2] Orthorhombic NPLs seem to present a distinct optical response, namely weak luminescence at 650 − 800nm,[1,2] which is much shorter than the emission wave-length of rock-salt NPLs (1200−1500 nm) despite havingsimilar thickness. High energy emission was also observed in earlier studies of PbS NPLs, whose crystal structure was not unambiguously defined. The nature of such a different optical response is still an open question. To gain insight into the implications of such a change over the optoelectronic properties, we calculate the band structure of orthorhombic PbS bulk using density functional theory with Perdew–Burke-Ernzerhof functional for solids and spin-orbit interaction. An indirect band gap is found, which may explain the weak luminescence reported in experiments. We infer effective masses for conduction and valence bands, and observe that quantum confinement along the a crystallographic axis reinforces the indirect band gap, but that along b and c axes favor a direct gap instead. Calculations for colloidal nanoplatelets of 1.8 nm thickness, carried out with k·p theory, show that excitonic effects are strong, with binding energies around 150 meV. Lateral confinement provides a means to manipulate the emission energies for lateral sizes under 15 nm.
[1] A. H. Khan, R. Brescia, A. Polovitsyn, I. Angeloni, B. Martı́n-Garcı́a, and I. Moreels, Near-infrared emitting colloidal pbs nanoplatelets: Lateral size control and optical spectroscopy, Chemistry of Materials 29, 2883 (2017).
[2] Q. A. Akkerman, B. Martı́n-Garcı́a, J. Buha, G. Almeida, S. Toso, S. Marras, F. Bonaccorso, U. Petralanda, I. Infante, and L. Manna, Ultrathin orthorhombic pbs nanosheets, Chemistry of Materials 31, 8145 (2019).
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