Nano Focus: Colloidal quantum dot films show RGB lasing
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ticle that is its own antiparticle, one that could persist in the material undisturbed for long periods. Discovery of these socalled Majorana fermions would be an achievement in itself, and could also
provide a way of overcoming the main obstacle to realizing a working quantum computer, a method of indefinitely storing data as “qubits.”
Nano Focus Colloidal quantum dot films show RGB lasing
CQD-VCSEL beam
C
olloidal semiconductor quantum dots exhibit efficient luminescence and bandgap controllability due to quantum confinement effects. However, to obtain laser emission from these materials, it is necessary to achieve a high colloidal-quantum-dot (CQD) packing density, and to reduce losses arising from nonradiative, multi-excitonic (Auger) recombination. In a joint collaboration, C. Dang of Brown University, C. Breen of QD Vision, Inc., Massachusetts, and their colleagues have demonstrated how these requirements can be met to achieve red-green-blue (RGB) lasing. As published in the May issue of Nature Nanotechnology (DOI: 10.1038/ nnano.2012.61; p. 335), the researchers report lasing emission from CdSe/ ZnCdS core/shell CQD with aromatic ligands. These form densely packed films that exhibit optical gain across the visible spectrum with an average of less than one exciton per CQD. This singleexciton gain allows the films to reach the threshold of amplified spontaneous emission at very low optical pump energy densities of 90 μJ cm–2. This is more than one order of magnitude better than previously reported values. The gain of these nanocomposite films was used to produce the first colloidal quantum dot, vertical-cavity surface-emitting laser (CQD-VCSEL). In this work, the researchers prepared type I CdSe/Zn0.5Cd0.5S core/shell CQDs by high-temperature organometallic synthesis with nominal CdSe core di-
Long pass filter CQD-VCSEL
Photographic image of a red colloidal quantum dot, vertical-cavity surface-emitting laser (CQD-VCSEL) showing a spatially well-defined output beam, which is collinear with the pump beam. Reproduced with permission from Nat. Nanotechnol. 7 (2012) DOI: 10.1038/ nnano.2012.61; p. 335. © 2012 Macmillan Publishers Ltd.
ameters of 4.2 nm, 3.2 nm, and 2.5 nm. The thin (1 nm) ternary shell reduces strain and creates a moderate core/shell bandgap difference. Transmission electron microscopy images showed welldefined crystallinity and “pyramid-like” morphologies. Together, these properties modify the electronic states from those of ideal spherical CQDs, where the anisotropic shape of the CQDs is a key feature that enables lasing with one single exciton. In ideal spherical CQDs, the Auger process is typically two orders of magnitude faster than photoluminescence decay, which severely hinders the dynamic buildup of population inversion. In this work, the dynamics of optical gain in
Correction The affiliations for the authors of the article, “Survey reveals interdisciplinarity of MSE faculty,” published in MRS Bulletin 37 (June 2012) p. 541, are Parag Banerjee, Department of Mechanical Engineering a
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