Production of Small, Stable PbS/CdS Quantum Dots via Room Temperature Cation Exchange Followed by a Low Temperature Annealing Processes

Here, we discuss a simple low temperature process for the synthesis of small and stable PbS/CdS QDs with emission below 1100 nm. For this, small PbS QDs with emission below 1100 nm synthesized from PbCl₂ in oleylamine with 1-dodecanethiol, as reported by our group recently, were used. A thin CdS shell was grown on PbS at room temperature (RT) via cation exchange (CE), which is a self-limiting process providing about 100 nm blue shift in the emission maxima, hence is quite practical for reaction control and production of predictable particles. RTCE process provides 6–9 times stronger emission than original PbS with better optical stability. Annealing of the PbS/CdS QDs in solid state at mild temperatures (50–100 °C) improves crystallinity of the particles. Final ligand exchange on the annealed PbS/CdS with 1-dodecanethiol (DT) enhances the long-term stability of particles further. The optimum overall process is determined as RTCE followed by annealing at 50 °C for 1 h and finished with ligand exchange with DT. Influence of these processes on QD structure and optical properties were studied as well as stability in chloroform and petroleum products (diesel and gasoline) for possible optical tagging applications of such liquids. Overall, a simple, controllable, and scalable method is developed to produce highly stable, bright, size-tunable PbS/CdS QDs with emission detectable with low cost semiconductor detectors

Ultrahigh quality microlasers from controlled self‐assembly of ultrathin colloidal semiconductor quantum wells

Colloidal quantum wells (CQWs) have emerged as a promising class of gain material in various optical feedback configurations. This is due to their unique excitonic features arising from their 1D quantum confinement. However, existing methods for integrating CQW onto microresonators will cause low laser quality due to uneven CQW coating. To overcome this, the use of liquid-interface kinetically driven self-assembly is proposed to coat ultrathin, close-packed layers of colloidal CdSe/Cd1−xZnxS core/shell CQWs between 7 and 14 nm onto the surface of silica microsphere cavities. The fabricated CQW-whispering-gallery-mode microlasers possess a commendable high quality (Q) factor of 13 000 at room temperature. Stable single-mode lasing output is demonstrated through evanescent field coupling between a CQW-coated microsphere and a thin uncoated microfiber in a 2D-3D microcavity configuration. These promising results highlight the suitability of the liquid-interface kinetically driven self-assembly method for realizing ultrathin CQW-coated microlasers and its high compatibility for integrating colloidal nanocrystals onto complex 3D microstructures for future miniaturized colloidal optoelectronic and photonic applications.Agency for Science, Technology and Research (A*STAR)Economic Development Board (EDB)Ministry of Education (MOE)National Research Foundation (NRF)Submitted/Accepted versionThis work was supportedby c and AME-IRG- A20E5c0083. The SEM imaging and EDS mapping wereperformed at the Facility for Analysis, Characterization, Testing and Sim-ulation (FACTS) at Nanyang Technological University, Singapore. H.V.D.and E.G.D. gratefully acknowledge the financial support in part from theSingapore Agency for Science, Technology and Research (A*STAR) MTCprogram under Grant No. M21J9b0085, Ministry of Education, Singapore,under its Academic Research Fund Tier 1 (MOE-RG62/20). H.V.D. alsogratefully acknowledges the support from TUBA. W.S.L. and C.X.X.L. ac-knowledge the support of EDB-IPP (REQ0165097)

Room-Temperature Lasing in Colloidal Nanoplatelets via Mie-Resonant Bound States in the Continuum

Solid-state room-temperature lasing with tunability in a wide range of wavelengths is desirable for many applications. To achieve this, besides an efficient gain material with a tunable emission wavelength, a high quality-factor optical cavity is essential. Here, we combine a film of colloidal CdSe/CdZnS core-shell nanoplatelets with square arrays of nanocylinders made of titanium dioxide to achieve optically pumped lasing at visible wavelengths and room temperature. The all-dielectric arrays support bound states in the continuum (BICs), which result from lattice-mediated Mie resonances and boast infinite quality factors in theory. In particular, we demonstrate lasing from a BIC that originates from out-of-plane magnetic dipoles oscillating in phase. By adjusting the diameter of the cylinders, we tune the lasing wavelength across the gain bandwidth of the nanoplatelets. The spectral tunability of both the cavity resonance and nanoplatelet gain, together with efficient light confinement in BICs, promises low-threshold lasing with wide selectivity in wavelengths.Agency for Science, Technology and Research (A*STAR)Submitted/Accepted versionThis work was supported by the A*STAR SERC Pharos programme (grant number 152 73 00025; Singapore). D.R.A. and J.A.S.-G. acknowledge support from the Spanish Ministerio de Ciencia e Innovación (NANOTOPO FIS2017-91413-EXP, MELODIA PGC2018-095777-B-C21, and FPU PhD Fellowship FPU15/03566, MCIU/AEI/FEDER, UE). H.V.D. gratefully acknowledges support from TUBA. The authors acknowledge Vytautas Valuckas (IMRE, A*STAR) for SEM characterization

Dual-Resonance Nanostructures for Color Downconversion of Colloidal Quantum Emitters

We present a dual-resonance nanostructure made of a titanium dioxide (TiO2) subwavelength grating to enhance the color downconversion efficiency of CdxZn1–xSeyS1–y colloidal quantum dots (QDs) emitting at ∼530 nm when excited with a blue light at ∼460 nm. A large mode volume can be created within the QD layer by the hybridization of the grating resonances and waveguide modes, resulting in large absorption and emission enhancements. Particularly, we achieved polarized light emission with a maximum photoluminescence enhancement of ∼140 times at a specific angular direction and a total enhancement of ∼34 times within a 0.55 numerical aperture (NA) of the collecting objective. The enhancement encompasses absorption, Purcell and outcoupling enhancements. We achieved a total absorption of 35% for green QDs with a remarkably thin color conversion layer of ∼400 nm. This work provides a guideline for designing large-volume cavities for absorption/fluorescence enhancement in microLED display, detector, or photovoltaic applications