Chemically synthesized quantum dots (QDs) can potentially enable a new class of highly flexible, spectrally tunable lasers processible from solutions [1,2]. Despite a considerable progress over the past years, colloidal-QD lasing, however, is still at the laboratory stage and an important challenge - realization of lasing with electrical injection - is still unresolved. A major complication, which hinders the progress in this field, is fast nonradiative Auger recombination of gain-active multi-carrier species [3,4]. Recently, we explored several approaches for mitigating the problem of Auger decay by taking advantage of a new generation of core/multi-shell QDs with a radially graded composition that allow for considerable (nearly complete) suppression of Auger recombination by “softening” the electron and hole confinement potentials . Using these specially engineered QDs, we have been able to realize optical gain with direct-current electrical pumping , which has been a long-standing goal in the field of colloidal nanostructures. Further, we apply these dots to practically demonstrated the viability of a “zero-threshold-optical-gain” concept using not neutral but negatively charged particles wherein the pre-existing electrons block either partially or completely ground-state absorption . Such charged QDs are optical-gain-ready without excitation and, in principle, can exhibit lasing at vanishingly small pump levels. All of these exciting recent developments demonstrate a considerable promise of colloidal nanomaterials for implementing solution-processible optically and electrically pumped laser devices operating across a wide range of wavelengths and fabricated on virtually any substrate using a variety of optical-cavity designs.
 Klimov, V. I. et al. Optical gain and stimulated emission in nanocrystal quantum dots. Science 290, 314 (2000).
 Klimov, V. I. et al. Single-exciton optical gain in semiconductor nanocrystals. Nature 447, 441 (2007).
 Klimov, V. I., Mikhailovsky, A. A., McBranch, D. W., Leatherdale, C. A. & Bawendi, M. G. Quantization of multiparticle Auger rates in semiconductor quantum dots. Science 287, 1011 (2000).
 Robel, I., Gresback, R., Kortshagen, U., Schaller, R. D. & Klimov, V. I. Universal Size-Dependent Trend in Auger Recombination in Direct-Gap and Indirect-Gap Semiconductor Nanocrystals. Phys. Rev. Lett. 102, 177404 (2009).
 Y.-S. Park, J. Lim, N. S. Makarov, V. I. Klimov, Effect of Interfacial Alloying versus “Volume Scaling” on Auger Recombination in Compositionally Graded Semiconductor Quantum Dots. Nano Lett. 17, 5607 (2017).
 Lim, J., Park, Y.-S. & Klimov, V. I. Optical Gain in Colloidal Quantum Dots Achieved by Direct-Current Charge Injection. Nat. Mater. in press (2017).
 Wu, K., Park, Y.-S., Lim, J. & Klimov, V. I. Towards zero-threshold optical gain using charged semiconductor quantum dots. Nat. Nanotechnol. DOI: 10.1038/NNANO.2017.189 (2017).