Research Group Peter Schall

Institute of Physics, University of Amsterdam

Quantum-dot epitaxy

We deposit quantum dots on a substrate using critical Casimir forces. These solvent-mediated forces drive the nanocrystals to the substrate, giving rise to interactions between the nanocrystals, and between the nanocrystals and the substrate. Due to its exquisite temperature dependence, the critical Casimir force allows new temperature control of the deposition and growth process.

The nanocrystal interactions are composed of a superposition of repulsive electrostatic and attractive critical Casimir interactions. The two control parameters are thus the Debye screening length λD set by the salt concentration, and the solvent correlation length ξ set by temperature. Relevant here is the temperature difference ΔT to the solvent critical temperature. Depending on these two parameters, we can obtain crystalline and amorphous structures, as shown by the figures below. We hence achieve new in-situ control over the growth of quantum-dot layers that reminds of the epitaxial growth of atomic layers.

Images of deposited structures are shown below, together with the control parameters, indicated in a λDξ diagram.

Figure 1 Crystalline nanocrystal epitaxy at low critical Casimir forces. The scanning electron microscopy (SEM) images reveal small crystalline islands of the CdSe nanocrystals deposited on a silicon substrate. The scale bars indicate 125nm (top) and 25nm (bottom).

  

   

Figure 2 Amorphous nanocrystal epitaxy at medium and large critical Casimir forces. The scanning electron microscopy (SEM) images show amorphous islands of the CdSe nanocrystals deposited on a silicon substrate. The island size grows with increasing solvent correlation length from left to right .

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