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Although achieving optical gain using aqueous solutions of colloidal nanocrystals as a gain medium is exceptionally beneficial for bio-optoelectronic applications, the realization of optical gain in an aqueous medium using solution-processed nanocrystals has been extremely challenging because of the need for surface modification to make nanocrystals water dispersible while still maintaining their gain. Here, we present the achievement of optical gain in an aqueous medium using an advanced architecture of CdSe/CdS@CdxZn1−xS core/crown@gradient-alloyed shell colloidal quantum wells (CQWs) with an ultralow threshold of ∼3.4 µJ cm−2 and an ultralong gain lifetime of ∼2.6 ns. This demonstration of optical gain in an aqueous medium is a result of the carefully heterostructured CQWs having large absorption cross-section and gain cross-section in addition to inherently slow Auger recombination in these CQWs. Furthermore, we show low-threshold in-water amplified spontaneous emission (ASE) from these aqueous CQWs with a threshold of 120 µJ cm−2 In addition, we demonstrate a whispering gallery mode laser with a low threshold of ∼30 µJ cm−2 obtained by incorporating films of CQWs by exploiting layerby-layer approach on a fiber. The observation of low-threshold optical gain with ultralong gain lifetime presents a significant step toward the realization of advanced optofluidic colloidal lasers and their continuous-wave pumping. Agency for Science, Technology and Research (A*STAR) Ministry of Education (MOE) National Research Foundation (NRF) Submitted/Accepted version The authors gratefully acknowledge the financial support in part from Singapore National Research Foundation under the programs of NRF-NRFI2016-08, the Science and the Singapore Agency for Science, Technology and Research (A*STAR) SERC Pharos Program under grant number 152-73-00025 and Agency for Science, Technology and Research (A*STAR) MTC program under grant number M21J9B0085, Ministry of Education Tier 1 under grant number MOE-RG62/20 (Singapore), and in part from TUBITAK 119N343, 120N076, 121N395 and 20AG001.

Colloidal quantum wells (CQWs), also known as nanoplatelets (NPLs), are exciting material systems for numerous photonic applications, including lasers and light-emitting diodes (LEDs). Although many successful type-I NPL-LEDs with high device performance have been demonstrated, type-II NPLs are not fully exploited for LED applications, even with alloyed type-II NPLs with enhanced optical properties. Here, we present the development of CdSe/CdTe/CdSe core/crown/crown (multi-crowned) type-II NPLs and systematic investigation of their optical properties, including their comparison with the traditional core/crown counterparts. Unlike traditional type-II NPLs such as CdSe/CdTe, CdTe/CdSe, and CdSe/CdSexTe1–x core/crown heterostructures, here the proposed advanced heterostructure reaps the benefits of having two type-II transition channels, resulting in a high quantum yield (QY) of 83% and a long fluorescence lifetime of 73.3 ns. These type-II transitions were confirmed experimentally by optical measurements and theoretically using electron and hole wave function modeling. Computational study shows that the multi-crowned NPLs provide a better-distributed hole wave function along the CdTe crown, while the electron wave function is delocalized in the CdSe core and CdSe crown layers. As a proof-of-concept demonstration, NPL-LEDs based on these multi-crowned NPLs were designed and fabricated with a record high external quantum efficiency (EQE) of 7.83% among type-II NPL-LEDs. These findings are expected to induce advanced designs of NPL heterostructures to reach a fascinating level of performance, especially in LEDs and lasers. Agency for Science, Technology and Research (A*STAR) Ministry of Education (MOE) Published version This research is supported by the Singapore Agency for Science, Technology and Research (A*STAR) MTC program, Grant No. M21J9b0085, and the Ministry of Education, Singapore, under its Academic Research Fund Tier 1 (MOERG62/20), and partly from TUBITAK 119N343, 120N076, 121C266, 121N395, and 20AG001. H.V.D. also gratefully acknowledges the support from the TUBA and TUBITAK 2247-A National Leader Researchers Program (121C266). B.L. acknowledges the support from the Science and Technology Program of Guangdong Province under Grant 2021A0505110009 and the National Natural Science Foundation of China under Grant 62104265.

Micro/nanoscale semiconductor multicolor lasers offer great potential for enhanced-performance photonic circuits. Colloidal quantum wells (CQWs) are excellent candidates as active materials for these platforms owing to their superior properties including suppressed Auger recombination and large absorption cross-section. In this work, multicolor optical gain and lasing from the heterostructures of CQWs as the gain media in their own all-solution processed optical cavity are proposed and demonstrated for the first time. Here, using a simple waveguide slab consisting of the thin films of green-emitting CdSeS/Cd0.1Zn0.9S core/hot-injection-shell grown CQWs and red-emitting CdSe/CdS@CdZnS core/crown@shell CQWs, a transparent low refractive index colloidal spacing layer of silica nanoparticles (NPs) is devised that critically suppresses otherwise detrimental nonradiative energy transfer between the green and red-emitting CQWs. This multilayer configuration is key to enabling simultaneous amplified spontaneous emission behavior in two colors with low threshold levels. This layered architecture is further adapted to a whispering-gallery-mode cavity by fabricating a microdisk pattern directly out of these CQWs-NPs-CQWs colloids. The resulting device exhibits dual-color multimode lasing both at 569 and 648 nm at the same time. This unique multicolor lasing layered architecture holds great promise for on-chip photonic applications such as dual-color biological imaging. Ministry of Education (MOE) Submitted/Accepted version The authors gratefully acknowledge the financial support in part fromthe Singapore Agency for Science, Technology and Research (A*STAR)MTC program under grant number M21J9b0085, Ministry of Education,Singapore, under its Academic Research Fund Tier 1 (MOE-RG62/20),and in part from TUBITAK 119N343, 121C266, 121N395 and 20AG001. H.V.D. also acknowledges support from TUBA and TUBITAK 2247-A Na-tional Leader Researchers Program (121C266).

Cadmium selenide (CdSe) nanoplatelets (NPLs) have attracted significant attention thanks to their favorable optical properties, including narrow emission linewidths, reduced Auger recombination, and a high absorption cross section. However, the photoluminescence (PL) quantum yield (QY) in the near-infrared (NIR) region is poor as compared to that in the visible region. Doping of metal ions is proven to be a successful strategy for inducing Stokes-shifted NIR emission. Here, we report the first account of the successful doping of ytterbium (Yb) into CdSe NPLs by a modified seeded-growth method. The successful incorporation of divalent Yb ions into CdSe NPLs resulted in an additional NIR emission apart from their excitonic emission. By optimizing the dopant concentration, we observed an impressive PL QY of ∼55% for these Yb-doped NPLs. Detailed elemental and optical characterizations were conducted to understand the emerging photophysical properties of these Yb-doped NPLs. These NIR-emitting lanthanide-doped CdSe NPLs might have applications in the next-generation bioimaging, night vision, and photodetection. Agency for Science, Technology and Research (A*STAR) Ministry of Education (MOE) Published version This research was supported by the Ministry of Education, Singapore, under its Academic Research Fund Tier 1 (MOERG62/20), Singapore Agency for Science, Technology and Research (A*STAR) MTC program, grant no. M21J9b0085, and partly from TUBITAK 119N343, 120N076, 121N395, and 20AG001. H.V.D. also acknowledges the support from TUBA.

© 2022 Wiley-VCH GmbH.Colloidal quantum wells (CQWs) are excellent candidates for lighting and display applications owing to their narrow emission linewidths (60 h, respectively, which is ≈4× and ≈2× better than the drop-casted CQW films and reference (KCl) host. Color-converting LEDs of these green- and red-emitting CQWs in sucrose possess luminous efficiencies 122 and 189 lm W−1elect, respectively. With the liquid crystal display filters, this becomes 39 and 86 lm W−1elect, respectively, providing with a color gamut 25% broader than the National Television Standards Committee standard. These results prove that CQW solids enable efficient and stable color converters for display and lighting applications.

Extending the emission peak wavelength of quasi-2D colloidal quantum wells has been an important quest to fully exploit the potential of these materials, which has not been possible due to the complications arising from the partial dissolution and recrystallization during growth to date. Here, the synthetic pathway of (CdSe/CdS)@(1-4 CdS/CdZnS) (core/crown)@(colloidal atomic layer deposition shell/hot injection shell) hetero-nanoplatelets (NPLs) using multiple techniques, which together enable highly efficient emission beyond 700 nm in the deep-red region, is proposed and demonstrated. Given the challenges of using conventional hot injection procedure, a method that allows to obtain sufficiently thick and passivated NPLs as the seeds is developed. Consequently, through the final hot injection shell coating, thick NPLs with superior optical properties including a high photoluminescence quantum yield of 88% are achieved. These NPLs emitting at 701 nm exhibit a full-width-at-half-maximum of 26 nm, enabled by the successfully maintained quasi-2D shape and minimum defects of the resulting heterostructure. The deep-red light-emitting diode (LED) device fabricated with these NPLs has shown to yield a high external quantum efficiency of 6.8% at 701 nm, which is on par with other types of LEDs in this spectral range. Agency for Science, Technology and Research (A*STAR) National Research Foundation (NRF) The authors gratefully acknowledge the financial support in part from Singapore National Research Foundation under the programs of NRF-NRFI2016-08, NRF-CRP14-2014-03 and the Science and Engineering Research Council, Agency for Science, Technology and Research (A*STAR) of Singapore and in part from TUBITAK 115F297, 117E713, and 119N343. H.V.D. also acknowledges support from TUBA.

We demonstrate amplified spontaneous emission (ASE) in solution with ultralow thresholds of 30 μJ/cm2 in red and of 44 μJ/cm2 in green from engineered colloidal quantum well (CQW) heterostructures. For this purpose, CdSe/CdS core/crown CQWs, designed to hit the green region, and CdSe/CdS@CdxZn1–xS core/crown@gradient-alloyed shell CQWs, further tuned to reach the red region by shell alloying, were employed to achieve high-performance ASE in the visible range. The net modal gain of these CQWs reaches 530 cm–1 for the green and 201 cm–1 for the red, 2–3 orders of magnitude larger than those of colloidal quantum dots (QDs) in solution. To explain the root cause for ultrahigh gain coefficient in solution, we show for the first time that the gain cross sections of these CQWs is ≥3.3 × 10–14 cm2 in the green and ≥1.3 × 10–14 cm2 in the red, which are two orders of magnitude larger compared to those of CQDs. Agency for Science, Technology and Research (A*STAR) National Research Foundation (NRF) Accepted version The authors gratefully acknowledge the financial support in part from Singapore National Research Foundation under the programs of NRF-NRFI2016-08, NRF-CRP14-2014-03 and the Science and the Singapore Agency for Science, Technology and Research (A*STAR) SERC Pharos Program under Grant No. 152-73-00025 and in part from TUBITAK 115F297, 117E713, and 119N343. H.V.D. also gratefully acknowledges support from TUBA. O.E. acknowledges support from TUBITAK BIDEB. S.D. and O.E contributed equally. The authors declare no competing financial interest.

The realization of high-quality lasers in microfluidic devices is crucial for numerous applications, including biological and chemical sensors and flow cytometry, and the development of advanced lab-on-chip (LOC) devices. Herein, an ultralow-threshold microfluidic single-mode laser is proposed and demonstrated using an on-chip cavity. CdSe/CdS@Cdx Zn1-x S core/crown@gradient-alloyed shell colloidal semiconductor quantum wells (CQWs) dispersed in toluene are employed in the cavity created inside a poly(dimethylsiloxane) (PDMS) microfluidic device using SiO2 -protected Ag mirrors to achieve in-solution lasing. Lasing from such a microfluidic device having CQWs solution as a microfluidic gain medium is shown for the first time with a record-low optical gain threshold of 17.1 µJ cm- ² and lasing threshold of 68.4 µJ cm- ² among all solution-based lasing demonstrations. In addition, air-stable SiO2 protected Ag films are used and designed to form highly tunable and reflective mirrors required to attain a high-quality Fabry-Pérot cavity. These realized record-low thresholds emanate from the high-quality on-chip cavity together with the core/crown@gradient-alloyed shell CQWs having giant gain cross-section and slow Auger rates. This microfabricated CQW laser provides a compact and inexpensive coherent light source for microfluidics and integrated optics covering the visible spectral region. National Research Foundation (NRF) Accepted version

Core/shell CdSe/(Cd,Mn)S colloidal nanoplatelets containing magnetic Mn2+ ions are investigated by the optically detected magnetic resonance technique, combining 60 GHz microwave excitation and photoluminescence detection. Resonant heating of the Mn spin system is observed. We identify two mechanisms of optical detection, via variation of either the photoluminescence polarization or its intensity in an external magnetic field. The spin-lattice relaxation dynamics of the Mn spin system is measured and used for evaluation of the Mn concentration. In CdSe/(Cd,Zn,Mn)S nanoplatelets the addition of Zn in the shells significantly broadens the magnetic resonance, evidencing local strain.

Colloidal semiconductor quantum wells have emerged as a promising material platform for use in solution-processable lasers. However, applications relying on their optical gain suffer from nonradiative Auger decay due to multi-excitonic nature of light amplification in II-VI semiconductor nanocrystals. Here, we show sub-single exciton level of optical gain threshold in specially engineered CdSe/CdS@CdZnS core/crown@gradient-alloyed shell quantum wells. This sub-single exciton ensemble-averaged gain threshold of (Ng)≈ 0.84 (per particle) resulting from impeded Auger recombination, along with a large absorption cross-section of quantum wells, enables us to observe the amplified spontaneous emission starting at an ultralow pump fluence of ~ 800 nJ cm−2, at least three-folds better than previously reported values among all colloidal nanocrystals. Finally, using these gradient shelled quantum wells, we demonstrate a vertical cavity surface-emitting laser operating at a low lasing threshold of 7.5 μJ cm−2. These results represent a significant step towards the realization of solution-processable electrically-driven colloidal lasers., Colloidal quantum wells are highly promising for applications of solution-processed lasers, but their performance is limited by multi-excitonic nature of the materials. Here, the authors demonstrate optical gain in graded alloy core/shell CdSe/CdS@CdZnS quantum wells at less than one exciton per particle resulting in ultralow thresholds.

The intentional incorporation of transition metal impurities into colloidal semiconductor nanocrystals allows an extension of the host material's functionality. While dopant incorporation has been extensively investigated in zero-dimensional quantum dots, the substitutional replacement of atoms in two-dimensional (2D) nanostructures by magnetic dopants has been reported only recently. Here, we demonstrate the successful incorporation of Co

The intentional incorporation of transition metal impurities into colloidal semiconductor nanocrystals allows an extension of the host material’s functionality. While dopant incorporation has been extensively investigated in zero-dimensional quantum dots, the substitutional replacement of atoms in two-dimensional (2D) nanostructures by magnetic dopants has been reported only recently. Here, we demonstrate the successful incorporation of Co2+ ions into the shell of CdSe/CdS core/shell nanoplatelets, using these ions (i) as microscopic probes for gaining distinct structural insights and (ii) to enhance the magneto-optical functionality of the host material. Analyzing interatomic Co2+ ligand field transitions, we conclude that Co2+ is incorporated into lattice sites of the CdS shell, and effects such as diffusion of dopants into the CdSe core or diffusion of the dopants out of the heterostructure causing self-purification play a minor role. Taking advantage of the absorption-based technique of magnetic circular dichroism, we directly prove the presence of sp-d exchange interactions between the dopants and the band charge carriers in CdSe/Co2+:CdS heteronanoplatelets. Thus, our study not only demonstrates magneto-optical functionality in 2D nanocrystals by Co2+ doping but also shows that a careful choice of the dopant type paves the way for a more detailed understanding of the impurity incorporation process into these novel 2D colloidal materials.

Achieving low-threshold optical gain for solution-processed materials is crucial for their real-life applications and deployment as gain media. However, the realization of low gain threshold in the blue region has shown to be technically an extremely challenging task using colloidal nanocrystals as a result of fast nonradiative Auger rates in smaller nanocrystals. Here, ultralow-threshold blue amplified spontaneous emission (ASE) (≈2.7 µJ cm ) accompanied with a large net modal gain coefficient of 360 cm in the blue enabled by blue-emitting (≈455–465 nm) colloidal quantum rings (QRs) of inverted type-I CdS/CdSe core/crown nanoplatelets (NPLs) is proposed and demonstrated. The synthesized QRs with controlled crown size outperform the best reported ASE thresholds and net modal gain coefficients from the solution-processed materials by ≈2.5- and ≈4-fold, respectively, in the similar blue spectral window. Utilizing this QR architecture, it is also shown that the ASE peak can be spectrally tuned by controlling the lateral size of the crown and hence quantum confinement in the lateral direction. These outstanding results support the prospects of these solution-processed QRs made of 2D hetero-NPLs in the challenging blue region as colloidal gain. Agency for Science, Technology and Research (A*STAR) National Research Foundation (NRF) Accepted version

The activation processes of [CuII(bdmpza)2] in the water oxidation reaction were investigated using cyclic voltammetry and chronoamperometry. Two different paths wherein CuO is formed were distinguished. [CuII(bdmpza)2] can be oxidized at high potentials to form CuO, which was observed by a slight increase in catalytic current over time. When [CuII(bdmpza)2] is initially reduced at low potentials, a more active water oxidation catalyst is generated, yielding high catalytic currents from the moment a sufficient potential is applied. This work highlights the importance of catalyst pre-treatment and the choice of the experimental conditions in water oxidation catalysis using copper complexes.