We study the influence of surface passivating ligands on the optical and structural properties of zinc blende CdSe nanoplatelets. Ligand exchange of native oleic acid with aliphatic thiol or phosphonic acid on the surface of nanoplatelets results in a large shift of exciton transition energy for up to 240 meV. Ligand exchange also leads to structural changes (strain) in the nanoplatelet's core analysed by wide-angle X-ray diffraction. By correlating the experimental data with theoretical calculations we demonstrate that the exciton energy shift is mainly caused by the ligand-induced anisotropic transformation of the crystalline structure altering the well width of the CdSe core. Further the exciton reduced mass in these CdSe quantum wells is determined by a new method and this agrees well with the expected values substantiating that ligand-strain induced changes in the colloidal quantum well thickness are responsible for the observed spectral shifts. Our findings are important for theoretical modeling of other anisotropically strained systems and demonstrate an approach to tune the optical properties of 2D semiconductor nanocrystals over a broad region thus widening the range of possible applications of AB nanoplatelets in optics and optoelectronics.
In this paper we present a simple
method for the preparation of
highly stable colloidal solutions of individual nanoplatelets (NPls)
with increased fluorescence quantum yield and a versatile procedure
of NPls self-assembly into stacks of controlled size. Dynamic light
scattering technique has been demonstrated to be simple and accurate
method for in situ studies of the growth kinetics of NPls aggregates.
The self-assembly method introduced in this work is based on the exchange
of ligands on the surface of CdSe nanoplatelets. Hexadecylphosphonic
acid allows control of the average size (length) of NPls stacks in
a broad range by varying its concentration and reaction time. The
main mechanism governing controlled formation of NPls stacks is based
on strong van der Waals interaction between rigid brushes of alkyl
chains on the surface of neighboring NPls. The interaction strength
and, consequently, the length and colloidal stability of stacks have
been shown to be dependent on type and concentration of different
ligands.
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