Colloidal quantum dots (CQDs) feature a low degeneracy of electronic states at the band edges compared with the corresponding bulk material, as well as a narrow emission linewidth. Unfortunately for potential laser applications, this degeneracy is incompletely lifted in the valence band, spreading the hole population among several states at room temperature. This leads to increased optical gain thresholds, demanding high photoexcitation levels to achieve population inversion (more electrons in excited states than in ground states-the condition for optical gain). This, in turn, increases Auger recombination losses, limiting the gain lifetime to sub-nanoseconds and preventing steady laser action. State degeneracy also broadens the photoluminescence linewidth at the single-particle level. Here we demonstrate a way to decrease the band-edge degeneracy and single-dot photoluminescence linewidth in CQDs by means of uniform biaxial strain. We have developed a synthetic strategy that we term facet-selective epitaxy: we first switch off, and then switch on, shell growth on the (0001) facet of wurtzite CdSe cores, producing asymmetric compressive shells that create built-in biaxial strain, while still maintaining excellent surface passivation (preventing defect formation, which otherwise would cause non-radiative recombination losses). Our synthesis spreads the excitonic fine structure uniformly and sufficiently broadly that it prevents valence-band-edge states from being thermally depopulated. We thereby reduce the optical gain threshold and demonstrate continuous-wave lasing from CQD solids, expanding the library of solution-processed materials that may be capable of continuous-wave lasing. The individual CQDs exhibit an ultra-narrow single-dot linewidth, and we successfully propagate this into the ensemble of CQDs.

Colloidal nanoplatelets, quasi-two-dimensional quantum wells, have recently been introduced as colloidal semiconductor materials with the narrowest known photoluminescence line width (∼10 nm). Unfortunately, these materials have not been shown to have continuously tunable emission but rather emit at discrete wavelengths that depend strictly on atomic-layer thickness. Herein, we report a new synthesis approach that overcomes this issue: by alloying CdSe colloidal nanoplatelets with CdS, we finely tune the emission spectrum while still leveraging atomic-scale thickness control. We proceed to demonstrate light-emitting diodes with sub-bandgap turn-on voltages (2.1 V for a device emitting at 2.4 eV) and the narrowest electroluminescence spectrum (FWHM ∼12.5 nm) reported for colloidal semiconductor LEDs.

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With rapid progress in the use of colloidal quantum dots (QDs) as light emitters, the next challenge for this field is to achieve high brightness. Unfortunately, Auger recombination militates against high emission efficiency at multiexciton excitation levels. Here, we suppress the Auger-recombinationinduced photoluminescence (PL) quantum yield (QY) loss in CdSe/CdS core− shell QDs by reducing the absorption cross section at excitation wavelengths via a thin-shell design. Studies of PL vs shell thickness reveal that thin-shell QDs better retain their QY at high excitation intensities, in stark contrast to thickershell QDs. Ultrafast transient absorption spectroscopy confirms increased Auger recombination in thicker-shell QDs under equivalent external excitation intensities. We then further grow a thin ZnS layer on thin-shell QDs to serve as a higher conduction band barrier; this allows for better passivation and exciton confinement, while providing transparency at the excitation wavelength. Finally, we develop an isolating silica matrix that acts as a spacer between dots, greatly reducing interdot energy transfer that is otherwise responsible for PL reduction in QD films. This results in the increase of film PL QY from 20% to 65% at low excitation intensity. The combination of Auger reduction and elimination of energy transfer leads to QD film PL QY in excess of 50% and absolute power conversion efficiency of 28% at excitation powers of 1 kW/cm 2 , the highest ever reported for QDs under intense illumination.