Publication date: 27th June 2014
Due to their atomic flatness and strictly quantized thickness distribution, colloidal nanoplatelets (NPL) [1] are ideal objects to understand quantum and dielectric confinement effects in well-defined ultrathin quantum wells. Previous simulations performed using empirical k.p or tight-binding models yield a first description of quantum confinement effects, with crude approximations for surface related properties [2, 3]. Density functional theory (DFT) as proposed in the present contribution, is well suited to better account for the influence of surface engineering. Various NPL surface morphologies are considered: surfaces passivated by carboxylic ligands or hydrogen atoms as well as 2x1 reconstructed surfaces. Quantum confinement effects are predicted but in addition, electronic densities of states and band diagrams exhibit a strong dependence on the surface states for ultrathin NPL.
Bare electron and hole monoelectronic states are well-known to exhibit a strong renormalization due to the “dielectric confinement” effect between the NPL and the surrounding medium, partially compensated by the increase of exciton binding. This self-energy effect was roughly taken into account in previous studies [2,3] by considering abrupt dielectric profiles. A new method based on DFT calculations is proposed to compute dielectric constant and self-energy potential profiles in these nanoscale objects, beyond the abrupt interface model (AIM). Simulations are performed using local orbital representations of the electronic wavefunctions in presence of a static electric field. It allows calculations of local polarization and nanoscale dielectric constant profiles, taking into account lattice relaxations and chemical bonding at the surface. It shows that the AIM fails for ultrathin NPL.
[1] S. Ithurria et al., J. Am. Chem. Soc. 2008, 130, 16504; S. Ithurria et al., Nature Mater. 2011, 10, 936.
[2] A. W. Achtstein et al., Nano Lett. 2012, 12, 3151; J. Zhou et al., J. Phys. Chem. C 2013, 117, 25817.
[3] R. Benchamekh et al., Phys. Rev. B 2014, 89, 035307.
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