Copyright 2004 by Marcel Dekker, Inc. All Rights Reserved.

nents and poor photostability of polymers. Recent work on InP NQD/polymer structures [68] indicates improvement in the performance of hybridsystems. However, a many-orders improvement in the injection efficiency isstill required for achieving the gain threshold.One possible strategy to improve injection efficiencies can be throughcombining ‘‘soft’’ colloidal methods with traditional epitaxial techniques forincorporating dots into high-quality ‘‘injection’’ layers of wide-gap semiconductors.A possible technique that is ‘‘gentle’’ enough to be compatiblewith colloidal dots is energetic neutral atom beam epitaxy. This method utilizesa beam of neutral atoms carrying significant kinetic energy of severalelectron volts. The beam energy is sufficient for the activation of nonthermalsurface chemical reactions, eliminating the need for substrate heating in orderto grow high-quality films for NQD encapsulation.Because of highly efficient nonradiative Auger decay, the realization ofinjection pumping of NQD lasing devices is significantly more difficult thanpumping simple, ‘‘subthreshold’’ light emitters. Suppression of Auger recombinationwould simplify achieving lasing in the case of both optical andelectrical pumping. One possible approach for reducing Auger rates is basedon utilizing NQD shape control [69]. Previous work indicates that in sphericalnanoparticles, Auger rates scale inversely with NQD volume [16]. If this scalingis also valid for nonspherical, elongated nanoparticles (quantum rods), theefficiency of Auger recombination should decrease with increasing nanoparticleaspect ratio. Because for elongated particles the energy gap is primarilydetermined by the length of the ‘‘short’’ axis [70], quantum rods can, inprinciple, provide independent (or weakly dependent) controls for both theposition of the emission band (by varying the dimension along the ‘‘short’’axis) and the efficiency of Auger recombination (by changing the ‘‘long’’ axislength).Another interesting problem for future research is ‘‘single-exciton’’ versus‘‘biexciton’’ mechanisms for optical gain. The ‘‘biexcitonic’’ gain origin isa consequence of the degeneracy of the lowest emitting transition. If thedegeneracy is split using, for example, interactions with magnetic impurities[71], the gain can, in principle, be realized using NQD single-exciton states.In addition to reduced threshold, ‘‘single-exciton lasing’’ would eliminate theproblem of ultrafast gain decay due to multiparticle Auger recombination.ACKNOWLEDGMENTSI would like to acknowledge contributions of A. A. Mikhailovsky, J. A.Hollingsworth, M. A. Petruska, A. V. Malko, H. Htoon, and S. Xu to thework reviewed here. I am also grateful to Al. L. Efros and M. G. Bawendi forCopyright 2004 by Marcel Dekker, Inc. All Rights Reserved.