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Controlling the shape anisotropy of colloidal nanocrystals to modify the effects of carrier confinement

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Colloidal nanocrystals of inorganic perovskites CsPbX3 (X = Br, Cl, I) have been the subject of intense research activity since their first synthesis in 2015. Their exceptional optical and electronic properties, which one can seek to modify in controlling the shape anisotropy, make them competitive alternatives to standard II-VI semiconductors for many applications like single photon emitters, LED technology, bio-imaging etc. It is within this framework that the members of the “Photonics and Coherence of Spin” (PHOCOS) team at INSP[1] have recently investigated the emission and absorption properties of ensembles of CsPbBr3 nanocrystals in the form of nano-platelets and nano-sticks with controlled synthesis. The nature of the confinement of carriers - in one or two dimensions - is demonstrated by a joint study combining electron microscopy and optical spectroscopy.

[1] in collaboration with the “Physico-chemistry and dynamics of surfaces” team of the INSP and the Laboratory of Materials Physics of the University from Carthage, Tunisia.

The synthesis techniques used here make it possible to obtain two classes of nanocrystals : nanosticks having a cross section of a few nm2 and lengths of tens of nms, and square nano-platelets with sides of hundreds of nm and thickness of just a few nm. We proceeded through coupled analyzes of optical microscopy images and spectroscopy data collected at low temperature. Optical studies reveal fine lines (a few tens of meV wide) which reflect the great structural homogeneity of the systems (sizes defining confinement). This property is a prerequisite which makes it possible : (i) to determine the level of anisotropy and the associated dimensions/thicknesses, (ii) to demonstrate the very strong dependence of optical responses as a function of morphologies, (iii) to characterize the observed behaviors with an intuitive physical model of carrier confinement effects in the two types of system.

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Figure 1
(a) Transmission electron microscopy images of films of nanocrystals (modified colloidal synthesis) in the form of very large square nano-platelets with identifiable thicknesses thanks to their emission responses and sometimes also absorption. (b) - (c) Absorption (in black) and emission (in red) spectra at low temperatures. In (c), the luminescence peaks are indexed by integers n corresponding to the different thicknesses Lz of the nanoplateletss (Lz = n x a where a is the interplanar distance of the quasi-cubic structure). Insert : Energies of emission peaks as a function of 1/n2. With the linear fit, a  0.58 nm is obtained in excellent agreement with the structural data.

The figure illustrates the specific case of nano-platelets with a thickness denoted Lz, and whose transition energies, here in emission, vary linearly in 1 / Lz2 as predicted within the limit of the strong confinement regime. Such an almost perfect dependence observed also in absorption is valid up to thicknesses of 4 monolayers or 2.3 nm. However, this dependence is surprising because rigorous modeling requires the consideration of « ingredients » beyond the strong confinement model. In fact, in systems where variations of the optical index occur on the scale of a few nm, two effects of electrostatic origin - called dielectric confinement effects - must be taken into account : the increase in the « self- energy « of the carriers on the one hand and the increase of the attractive Coulomb interaction between the components of the exciton on the other hand. Our experimental study appears to imply that these effects, while individually large, tend to cancel out. We have theoretically estimated the size of these corrections with the formalism of image charges, and this compensation of effects is indeed confirmed by our calculations (not yet published).

In addition to the development of nanophotonics devices taking advantage of the controllable sizes and geometries of the nano-platelets, one of the many future possibilities of these coupled optics and electron microscopy studies is for precision testing of the modeling of the effects of geometric and dielectric confinements in nano-platelets with thickness less than 4 monolayers, and in nano-sticks (pseudo-1D systems) in which taking into account the dielectric confinement in two directions remains a challenge. nfinement diélectrique dans deux directions constitue un challenge.

Reference
« Anisotropic shape of CsPbBr3 colloidal nanocrystals : from 1D to 2D confinement effects » Violette Steinmetz, Julien Ramade, Laurent Legrand, Thierry Barisien, Frédérick Bernardot, Emmanuel Lhuillier, Mathieu Bernard, Maxime Vabre, Imen Saïdi, Amal Ghribi, Kaïs Boujdaria, Christophe Testelin, Maria Chamarro Nanoscale, 12, 18978-18986 (2020).

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Contact
Laurent Legrand : legrand(at)insp.jussieu.fr