Photoluminescent color conversion by quantum dots (QDs) makes possible the formation of spectrum-ondemand light sources by combining blue LEDs with the light generated by a specific blend of QDs. Such applications, however, require a near-unity photoluminescence quantum efficiency since self-absorption magnifies disproportionally the impact of photon losses on the overall conversion efficiency. Here, we present a synthesis protocol for forming InP-based QDs with +90% quantum efficiency across the full visible spectrum from blue/cyan to red. The central features of our approach are as follows: (1) the formation of InP core QDs through one-batch-one-size reactions based on aminophosphine as the phosphorus precursor, (2) the introduction of a core/ shell/shell InP/Zn(Se,S)/ZnS structure, and (3) the use of specific interfacial treatments, most notably the saturation of the ZnSe surface with zinc acetate prior to ZnS shell growth. Moreover, we adapted the composition of the Zn(Se,S) inner shell to attain the intended emission color while minimizing line broadening induced by the InP/ZnS lattice mismatch. The protocol is established by analysis of the QD composition and structure using multiple techniques, including solid-state nuclear magnetic resonance spectroscopy and Raman spectroscopy, and verified for reproducibility by having different researchers execute the same protocol. The realization of full-spectrum, +90% quantum efficiency will strongly facilitate research into light−matter interaction in general and luminescent color conversion in particular through InP-based QDs.

pathways for stimulated emission across the band-edge transition. [2] As demonstrated using CdSe-based QDs, progressive state filling results in a saturated gain spectrum that is the mirror image of the absorbance spectrum, [6] and that is characterized by a high material gain of several 1000 cm −1 and nearly temperature-independent gain characteristics. [7][8][9] Using such CdSe-based QDs, various types of lasers were demonstrated, including engineered devices such as vertical-cavity surface-emitting lasers, [10] distributed feedback lasers, [11,12] or integrated microdisk lasers. [13] While these QD lasers initially operated under femtosecond (fs), and later under nanosecond (ns) pulsed or quasicontinuous optical pumping, [14] recent work showed the first steps toward electrically pumped devices. [15] This progress was made possible by the introduction of innovative QD heterostructures, which were mostly aimed at reducing the Auger recombination rate of multi-excitons and thus prolong the lifetime of the inverted state. [16][17][18] Here, in particular CdSe/CdS core/shell QDs proved highly suited, for which an optimal balance between material gain and inverted-state lifetime can be found by adapting the core and shell sizes. [8,9] Following the restrictions on the use of cadmium in electronic appliances, InP-based QDs are emerging as the most viable substitutes for CdSe-based QDs as printable, spectrally narrow, and fast emitters. [19,20] Economic synthesis methods for InP and InP-based core/shell QDs have been developed, [21] and the use of these QDs for lighting and display applications, either as luminescent color convertors or electroluminescent emitters, is widely studied. [22,23] On the other hand, reports on optical gain or stimulated emission by InP-based QDs are rare and lack follow-up research, [24] a situation that may be related to the limited understanding of the opto-electronic properties of excitons and multi-excitons in these QDs. The few studies that address multi-exciton dynamics by femtosecond spectroscopy focus on Auger recombination of biexcitons or surface trapping of single excitons, [23,[25][26][27] but do not provide a quantitative analysis of state filling, the reference model for CdSe-based QDs. In fact, even the mechanism of radiative recombination in InP-based QDs remains a matter of debate. Ensemble-level and single-dot photoluminescence (PL) studies showed finestructure properties and radiative lifetimes characteristic of Colloidal InP-based quantum dots (QDs) are widely studied for luminescent color conversion or electroluminescence, yet the nature of the emitting state remains a matter of debate and reports on stimulated emission by these materials are nearly absent. Here, the properties of photo-excited InP/ZnSe QDs are investigated using femtosecond transient absorption spectroscopy. It is shown that the evolution of the band-edge bleach with increasing exciton number can be interpreted as state filling of the conduction-and valenceband edge states by delocalized electrons and...

Quantum dots (QDs) offer an interesting alternative for traditional phosphors in on-chip light-emitting diode (LED) configurations. Earlier studies showed that the spectral efficiency of white LEDs with high color rendering index (CRI) values could be considerably improved by replacing red-emitting nitride phosphors with narrowband QDs. However, the red QDs in these studies were cadmium-based, which is a restricted element in the EU and certain other countries. The use of InP-based QDs, the most promising Cd-free alternative, is often presented as an inferior solution because of the broader linewidth of these QDs. However, while narrow emission lines are the key to display applications that require a large color gamut, the spectral efficiency penalty of this broader emission is limited for lighting applications. Here, we report efficient, high-CRI white LEDs with an on-chip color converter coating based on red InP/ZnSe QDs and traditional green/yellow powder phosphors. Using InP/ZnSe QDs with a quantum yield of nearly 80% and a full width at half-maximum of 45 nm, we demonstrate high spectral efficiency for white LEDs with very high CRI values. One of the best experimental results in terms of both luminous efficacy and color rendering performance is a white LED with an efficacy of 132 lm/W, and color rendering indices of R a ≈ 90, R9 ≈ 50 for CCT ≈ 4000 K. These experimental results are critically compared with theoretical benchmark values for white LEDs with on-chip downconversion from both phosphors and red Cd-based QDs. The various loss mechanisms in the investigated white LEDs are quantified with an accurate simulation model, and the main impediments to an even higher efficacy are identified as the blue LED wall-plug efficiency and light recycling in the LED package.

We demonstrate the synthesis of copper nanocolloids by the thermal decomposition of copper formate in oleylamine under ambient conditions. By progressively increasing the loading of copper formate in the reaction mixture and imposing sufficiently high conversion rates, we demonstrate the formation of nanocrystals that are more than 97% pure copper without using an inert atmosphere. We attribute this result to the excess of copper formate relative to initially dissolved oxygen, and to the suppression of oxygen influx in the reactor. By adjusting the precursor and ligand concentrations, we obtain copper nanocrystals with sizes ranging from 10 to 200 nm. In view of applications, we show that the reaction can be upscaled to a 1 L reaction volume to produce over 1 50 grams of copper nanocrystals. Moreover, we formulate a conductive ink based on the copper nanocolloids obtained here with which we printed copper films exhibiting a resistivity of 23 µΩ • cm after thermal sintering under N 2 . We conclude that the approach presented here consititutes a next step towards the cost-effective production of metallic copper nanocrystals for printed electronics.

For their unique optical properties, quantum dots (QDs) have been extensively used as light emitters in a number of photonic and optoelectronic applications. They even met commercialization success through their implementation in high-end displays with unmatched brightness and color rendering. For such applications, however, QDs must be shielded from oxygen and water vapor, which are known to degrade their optical properties over time. Even with highly qualitative QDs, this can only be achieved through their encapsulation between barrier layers. With the emergence of mini- and microLED for higher contrast and miniaturized displays, new strategies must be found for the concomitant patterning and encapsulation of QDs, with sub-millimeter resolution. To this end, we developed a new approach for the direct patterning of QDs through maskless lithography. By combining QDs in photopolymerizable resins with digital light processing (DLP) projectors, we developed a versatile and massively parallel fabrication process for the additive manufacturing of functional structures that we refer to as QD pockets. These 3D heterostructures are designed to provide isotropic encapsulation of the QDs, and hence prevent edge ingress from the lateral sides of QD films, which remains a shortcoming of the current technologies.