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Self-assembly of semi-conducting CdSe nanoplatelets in large needles


Adding a “bad solvent” to a colloidal suspension of CdSe nanoplatelets brings about their self-assembly in needles that can be several microns long. The physical properties of these needles are very anisotropic, as illustrated by the strong polarization of their fluorescence emission. Such anisotropy could be useful for some opto-electronic devices.

The self-assembly of nanocrystals in well-organized micronic structures is currently being studied worldwide because such structures may display new collective physical properties. Semi-conducting CdSe nanoplatelets have recently been discovered and presently raise much interest due to the sharpness and tunability of their optical absorption and fluorescence peaks (Figure 1a).


Figure 1 : A) Spectroscopic properties of the CdSe nanoplatelets ; B) TEM image of individual nanoplatelets.
 

These platelets, 10 nm wide (Figure 1b), are covered by an oleic acid brush to make them easily dispersible in apolar solvents. In collaboration with the group of B.Dubertret at ESPCI, we observed that adding a small amount of ethanol destabilizes these suspensions and brings about the precipitation of the platelets. However, the resulting aggregates are actually well-defined needles made of columns of regularly stacked nanoplatelets (Figure 2).


Figure 2 : A) Fluorescence microscopy image of the needles ; B) TEM image of the needles ; C) schematic representation of the organization of the platelets in a (vertical) needle.
 

The length of these needles can reach several microns and their diameter ranges from 100 to 300 nm, which represents about a million platelets well-organized and all aligned with their normal parallel to the needle main axis. Such self-assemblies have very anisotropic physical properties like, for example, their fluorescence emission that is highly polarized in the plane of the platelets (Figure 3).


Figure 3 : Epifluorescence microscopy images of the needles for two different polarizations (arrows) of the emitted light.
 

This effect could be used to optimize the use of these nanoplatelets in opto-electronic devices such as LEDs. More generally, adding a small amount of a “bad solvent” to a colloidal suspension of nanoparticles covered with an organic layer looks like a promising way to induce their self-assembly.

 

Contact :
Benjamin Abécassis

Reference :
Self-Assembly of CdSe Nanoplatelets into Giant Micrometer-Scale Needles Emitting Polarized Light
Benjamin Abécassis, Mickael D. Tessier, Patrick Davidson, and Benoit Dubertret.
Nanoletters, 14, 710, (2014).