Self-assembled semiconductor microlaser based on colloidal nanoplatelets

A semiconductor microsphere laser based on colloidal nanoplatelets is demonstrated comprising a micron-sized supraparticle obtained by self-assembly of core/shell CdSe/ CdS nanoplatelets with peak luminescence at 660nm. It shows multimode laser emission between 665 and 695nm with threshold at 200 nJ (28 ± 17 mJ.cm -2 )

Colloidal semiconductor quantum well supraparticles as low-threshold and photostable microlasers

This study introduces and compares the lasing performance of micron-sized and sphere-shaped supraparticle (SP) lasers fabricated through bottom-up assembly of II-VI semiconductor colloidal quantum wells (CQWs) with their counterparts made of quantum dots (CQDs). CQWs consist of a 4-monolayers thick CdSe core and an 8-monolayers thick CdxZn1-xS shell with a nominal size of 14 × 15 × 4.2 nm, and CQDs of CdSxSe1-x/ZnS with 6 nm diameter. SPs are optically characterized with a 0.76 ns pulse laser (spot size: 2.88 × 10−7 cm2) at 532 nm, and emit in the 620–670 nm spectral range. Results show that CQW SPs have lasing thresholds twice as low (0.1–0.3 nJ) as CQD SPs (0.3–0.6 nJ), and stress tests using a constant 0.6 nJ optical pump energy demonstrate that CQW SPs withstand lasing emission for longer than CQD SPs. Lasing emission in CQW and CQD SPs under continuous operation yields half-lives of τCQW SP ≈150 min and τCQD SP ≈22 min, respectively. The half-life of CQW SPs is further extended to τQW ≈385 min when optically pumped at 0.5 nJ. Such results compare favorably to those in the literature and highlight the performance of CdSe-based CQW SPs for laser applications

Impact of reaction variables and PEI/l-cysteine ratio on the optical properties and cytocompatibility of cationic Ag 2 S quantum dots as NIR bio-imaging probes

Near-infrared emitting semiconductor quantum dots (NIRQDs) are popular fluorescent probes due to better penetration depth and elimination of tissue autofluorescence. Here, we demonstrate one pot aqueous synthesis of cytocompatible, strongly luminescent, cationic Ag2S NIRQDs utilizing a mixed coating composed of branched polyethyleneimine (PEI)-25 kDa and L-cysteine (Cys) as in vitro luminescent tags and in vivo optical imaging agents. Ultrasmall sizes, a clear first excitonic peak in the absorption spectra, relatively narrow emission peaks with maxima between 730 and 775 nm and a Stokes shift less than 100 nm were obtained. Lifetime measurements indicate excitonic and defect-related emissions. Interestingly, not the emission maxima but the intensity was influenced by the Cys amount more dramatically. PEI/Cys 60/40 mol ratio provided the highest quantum yield reported until now for Ag2S NIRQD (157%) emitting at such a short wavelength. Low molecular weight PEI failed to produce luminescent QDs. Cytotoxicity evaluation of the most strongly luminescing NIRQDs, revealed the PEI/Cys (mol mol−1) 50/50 composition as the non-toxic composition below 2.4 μg Ag per mL concentration. Others had low-toxicity. In vitro microscopy experiments showed endosomal distribution of NIRQDs in Hela cells and strong NIR signal. In vivo imaging study demonstrated that Ag2S NIRQDs could effectively be used as strong optical imaging agents

Near-infrared emission from CdSe-based nanoplatelets induced by ytterbium doping

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 versionThis 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

Ultrahigh green and red optical gain cross sections from solutions of colloidal quantum well heterostructures

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 versionThe 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

Observation of optical gain from aqueous quantum well heterostructures in water

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 versionThe 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

Measuring the ultrafast spectral diffusion and vibronic coupling dynamics in CdSe colloidal quantum wells using two-dimensional electronic spectroscopy

We measure the ultrafast spectral diffusion, vibronic dynamics, and energy relaxation of a CdSe colloidal quantum wells (CQWs) system at room temperature using two-dimensional electronic spectroscopy (2DES). The energy relaxation of light-hole (LH) excitons and hot carriers to heavy-hole (HH) excitons is resolved with a time scale of ∼210 fs. We observe the equilibration dynamics between the spectroscopically accessible HH excitonic state and a dark state with a time scale of ∼160 fs. We use the center line slope analysis to quantify the spectral diffusion dynamics in HH excitons, which contains an apparent sub-200 fs decay together with oscillatory features resolved at 4 and 25 meV. These observations can be explained by the coupling to various lattice phonon modes. We further perform quantum calculations that can replicate and explain the observed dynamics. The 4 meV mode is observed to be in the near-critically damped regime and may be mediating the transition between the bright and dark HH excitons. These findings show that 2DES can provide a comprehensive and detailed characterization of the ultrafast spectral properties in CQWs and similar nanomaterials.Agency for Science, Technology and Research (A*STAR)Ministry of Education (MOE)Submitted/Accepted versionH.V.D. gratefully acknowledges the financial support in part from the Singapore Agency for Science, Technology and Research (A*STAR) SERC under Grant No. M21J9b0085, and the Singapore Ministry of Education Tier 1 grant (MOERG62/20). H.V.D. also gratefully acknowledges the support from TUBA. H.-S.T. gratefully acknowledges the financial support in part from the Singapore Ministry of Education Tier 1 grant (MOE-RG2/19 and MOE-RG14/20). O.V.P. acknowledges the financial support from the United States National Science Foundation under Grant No. CHE-2154367

Blue-emitting CdSe nanoplatelets enabled by sulfur-alloyed heterostructures for light-emitting diodes with low turn-on voltage

Colloidal nanoplatelets (NPLs) have emerged as the last class of semiconductor nanocrystals for their potential optoelectronic applications. The heterostructures of these nanocrystals can achieve high photoluminescence quantum yield and enhanced photostability, along with color purity. Such advantages make them a promising candidate for solution-processable light-emitting diodes (LEDs). However, to date, blue-emitting CdSe nanoplatelets (NPLs) exhibit poor photoluminescence quantum yield and also typically suffer from a rolled-up morphology. To mitigate these problems in this work, we propose and demonstrate efficient alloyed 4 ML CdSe1-xSx nanoplatelets having a CdS crown with enhanced photoluminescence quantum yields (up to 60%) in the blue region (462-487 nm). We successfully used these NPLs as an electrically driven active emitter in the blue-emitting NPL-LEDs with a low turn-on voltage of ∼4 V. The Commission Internationale de L'Eclairage (CIE) coordinates of (0.23, 0.14) were obtained for these blue-emitting NPL-LEDs. These emitters could potentially open up the opportunity for full-color displays using these NPL-based blue LEDs in conjunction with the red and green ones.Ministry of Education (MOE)National Research Foundation (NRF)Accepted versionThis research is supported by the National Research Foundation, Prime Minister’s Office, Singapore under its NRF Investigatorship Award program (NRF-NRFI2016-08) and the Ministry of Education Tier 1 grant (MOE-RG62/20). H.V.D. also gratefully acknowledges the support from TUB

Gradient type‐II CdSe/CdSeTe/CdTe core/crown/crown heteronanoplatelets with asymmetric shape and disproportional excitonic properties

Characterized by their strong 1D confinement and long-lifetime red-shifted emission spectra, colloidal nanoplatelets (NPLs) with type-II electronic structure provide an exciting ground to design complex heterostructures with remarkable properties. This work demonstrates the synthesis and optical characterization of CdSe/CdSeTe/CdTe core/crown/crown NPLs having a step-wise gradient electronic structure and disproportional wavefunction distribution, in which the excitonic properties of the electron and hole can be finely tuned through adjusting the geometry of the intermediate crown. The first crown with staggered configuration gives rise to a series of direct and indirect transition channels that activation/deactivation of each channel is possible through wavefunction engineering. Moreover, these NPLs allow for switching between active channels with temperature, where lattice contraction directly affects the electron–hole (e–h) overlap. Dominated by the indirect transition channels over direct transitions, the lifetime of the NPLs starts to increase at 9 K, indicative of low dark-bright exciton splitting energy. The charge transfer states from the two type-II interfaces promote a large number of indirect transitions, which effectively increase the absorption of low-energy photons critical for nonlinear properties. As a result, these NPLs demonstrate exceptionally high two-photon absorption cross-sections with the highest value of 12.9 × 106 GM and superlinear behavior.Agency for Science, Technology and Research (A*STAR)Ministry of Education (MOE)Submitted/Accepted versionThe authors gratefully acknowledge the financial support from Agency for Science, Technology and Research (A*STAR) MTC program, Grant No. M21J9b0085 (Singapore), Ministry of Education Tier 1 grant MOE-RG62/20 (Singapore) and TUBITAK 115F297, 117E713, 119N343, 121N395, and 20AG001. H.V.D. also acknowledges the support from TUBA

High external quantum efficiency light-emitting diodes enabled by advanced heterostructures of type-II nanoplatelets

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 versionThis 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