Surface-emitting red, green, and blue colloidal quantum dot distributed feedback lasers.

We demonstrate surface emitting distributed feedback (DFB) lasers across the red, green, and blue from densely packed colloidal quantum dot (CQD) films. The solid CQD films were deposited on periodic grating patterns to enable 2nd-order DFB lasing action at mere 120, 280, and 330 µJ/cm2 of optical pumping energy densities for red, green, and blue DFB lasers, respectively. The lasers operated in single mode operation with less than 1 nm of full-width-half-maximum. We measured far-field patterns showing high degree of spatial beam coherence. Specifically, by taking advantage of single exciton optical gain regime from our engineered CQDs, we can significantly suppress the Auger recombination to reduce lasing threshold and achieve quasi-steady state, optically pumped operation.

A wafer-level integrated white-light-emitting diode incorporating colloidal quantum dots as a nanocomposite luminescent material.

High-brightness, color-tunable colloidal quantum dots are incorporated in 3D nanoporous GaN to create a nanocomposite material (CQD/NP-GaN), which is demonstrated to be an effective approach for a wavelength down-conversion nanomaterial in solid-state lighting. The white-light-emitting diode (LED) made from a blue GaN-based LED and the CQD/NP-GaN shows an increase of extraction efficiency by a factor of 2, a controllable white color, and a down-conversion quantum efficiency as high as 82%.

Red, green and blue lasing enabled by single-exciton gain in colloidal quantum dot films.

Colloidal quantum dots exhibit efficient photoluminescence with widely tunable bandgaps as a result of quantum confinement effects. Such quantum dots are emerging as an appealing complement to epitaxial semiconductor laser materials, which are ubiquitous and technologically mature, but unable to cover the full visible spectrum (red, green and blue; RGB). However, the requirement for high colloidal-quantum-dot packing density, and losses due to non-radiative multiexcitonic Auger recombination, have hindered the development of lasers based on colloidal quantum dots. Here, we engineer CdSe/ZnCdS core/shell colloidal quantum dots with aromatic ligands, which form densely packed films exhibiting optical gain across the visible spectrum with less than one exciton per colloidal quantum dot on average. This single-exciton gain allows the films to reach the threshold of amplified spontaneous emission at very low optical pump energy densities of 90 µJ cm(-2), more than one order of magnitude better than previously reported values. We leverage the low-threshold gain of these nanocomposite films to produce the first colloidal-quantum-dot vertical-cavity surface-emitting lasers (CQD-VCSEL). Our results represent a significant step towards full-colour single-material lasers.

Contact printing of quantum dot light-emitting devices.

We demonstrate a solvent-free contact printing process for deposition of patterned and unpatterned colloidal quantum dot (QD) thin films as the electroluminescent layers within hybrid organic-QD light-emitting devices (QD-LEDs). Our method benefits from the simplicity, low cost, and high throughput of solution-processing methods, while eliminating exposure of device structures to solvents. Because the charge transport layers in hybrid organic/inorganic QD-LEDs consist of solvent-sensitive organic thin films, the ability to avoid solvent exposure during device growth, as presented in this study, provides a new flexibility in choosing organic materials for improved device performance. In addition, our method allows us to fabricate both monochrome and red-green-blue patterned electroluminescent structures with 25 microm critical dimension, corresponding to 1000 ppi (pixels-per-inch) print resolution.

On the mechanism of lead chalcogenide nanocrystal formation.

On the basis of evidence from 31P NMR spectroscopy, and using PbSe as a model, we propose two simultaneous mechanisms through which "monomers" are formed in preparations of lead chalcogenide nanocrystals (NCs). In one mechanism, selenium is delivered as a Se2- species, whereas in the other, Se0 reacts with metal already reduced by the organophosphine. This latter mechanism helps explain the sensitivity of NC preparations to the purity of organophosphines and allows the rational modification of batch NC reactions to increase yield.