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1. Low-threshold laser medium utilizing semiconductor nanoshell quantum dots

   https://doi.org/10.1039/D0NR03582C  

   Porotnikov, Dmitry; Diroll, Benjamin T.; Harankahage, Dulanjan; Obloy, Laura; Yang, Mingrui; Cassidy, James; Ellison, Cole; Miller, Emily; Rogers, Spencer; Tarnovsky, Alexander N.; et al (August 2020, Nanoscale)

   null (Ed.)

   Colloidal semiconductor nanocrystals (NCs) represent a promising class of nanomaterials for lasing applications. Currently, one of the key challenges facing the development of high-performance NC optical gain media lies in enhancing the lifetime of biexciton populations. This usually requires the employment of charge-delocalizing particle architectures, such as core/shell NCs, nanorods, and nanoplatelets. Here, we report on a two-dimensional nanoshell quantum dot (QD) morphology that enables a strong delocalization of photoinduced charges, leading to enhanced biexciton lifetimes and low lasing thresholds. A unique combination of a large exciton volume and a smoothed potential gradient across interfaces of the reported CdS bulk /CdSe/CdS shell (core/shell/shell) nanoshell QDs results in strong suppression of Auger processes, which was manifested in this work though the observation of stable amplified stimulated emission (ASE) at low pump fluences. An extensive charge delocalization in nanoshell QDs was confirmed by transient absorption measurements, showing that the presence of a bulk-size core in CdS bulk /CdSe/CdS shell QDs reduces exciton–exciton interactions. Overall, present findings demonstrate unique advantages of the nanoshell QD architecture as a promising optical gain medium in solid-state lighting and lasing applications. 

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2. Photon Upconversion at Organic-Inorganic Interfaces

   https://doi.org/10.1146/annurev-physchem-090722-011335  

   Huang, Zhiyuan; Miyashita, Tsumugi; Tang, Ming Lee (June 2024, Annual Review of Physical Chemistry)

   Photon upconversion is a process that combines low-energy photons to form useful high-energy photons. There are potential applications in photovoltaics, photocatalysis, biological imaging, etc. Semiconductor quantum dots (QDs) are promising for the absorption of these low-energy photons due to the high extinction coefficient of QDs, especially in the near infrared (NIR). This allows the intriguing use of diffuse light sources such as solar irradiation. In this review, we describe the development of this organic-QD upconversion platform based on triplet-triplet annihilation, focusing on the dark exciton in QDs with triplet character. Then we introduce the underlying energy transfer steps, starting from QD triplet photosensitization, triplet exciton transport, triplet-triplet annihilation, and ending with the upconverted emission. Design principles to improve the total upconversion efficiency are presented. We end with limitations in current reports and proposed future directions. This review provides a guide for designing efficient organic-QD upconversion platforms for future applications, including overcoming the Shockley-Queisser limit for more efficient solar energy conversion, NIR-based phototherapy, and diagnostics in vivo. 

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3. Open-shell donor–π–acceptor conjugated metal-free dyes for dye-sensitized solar cells

   https://doi.org/10.1039/D0ME00091D  

   Sabuj, Md Abdus; Rai, Neeraj (November 2020, Molecular Systems Design & Engineering)

   null (Ed.)

   Dye-sensitized solar cells (DSCs) have drawn a significant interest due to their low production cost, design flexibility, and the tunability of the sensitizer. However, the power conversion efficiency (PCE) of the metal-free organic dyes is limited due to the inability of the dye to absorb light in the near-infrared (NIR) region, leaving a large amount of energy unused. Herein, we have designed new DSC dyes with open-shell character, which significantly red-shifts the absorption spectra from their counterpart closed-shell structure. A small diradical character ( y < 0.10) is found to be beneficial in red-shifting the absorption maxima into the NIR region and broadening up to 2500 nm. Also, the open-shell dyes significantly reduce the singlet–triplet energy gaps (Δ E ST ), increase the total amount of charge-transfer to the semiconductor surface, reduce the exciton binding energy, and significantly increase the excited-state lifetimes compared to the closed-shell systems. However, the closed-shell dyes have higher injection efficiency with increased intramolecular charge transfer (ICT) character. Our study reveals the design rule for open-shell DSC dyes to be able to absorb photons in the NIR region, which can increase the efficiency of the solar cell device. 

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4. Inorganic Semiconductor Quantum Dots as a Saturated Excitation (SAX) Probe for Sub‐Diffraction Imaging

   https://doi.org/10.1002/cptc.202000195  

   Kumar_Singh, Avinash; Burkhow, Sadie_J; Wijesooriya, Chamari; Boote, Brett_W; Petrich, Jacob_W; Smith, Emily_A (November 2020, ChemPhotoChem)

   Abstract The photoluminescence (PL) saturation of CdSe/ZnS core/shell inorganic semiconductor quantum dots (QDs) and its utility as a probe for saturated excitation (SAX) microscopy are reported. Under saturating excitation power densities, the PL signal was demodulated and recorded at harmonics of the fundamental frequency. For commercially available Qdot® 655 ITK™ QDs, the power density required to achieve saturation was dependent upon the local environment of the QDs. For QDs deposited and dried on a glass substrate, the excitation power density required for PL saturation was less than 1 kW/cm². Compared to this, saturation of PL for QDs dispersed in water required an excitation power density greater than 200 kW/cm². This observation is manifested as a limitation in the imaging of hydrated samples, as demonstrated for HeLa cells labelled with biotinylated‐phalloidin followed by labelling with streptavidin‐coated QDs. As saturation affects the obtained spatial resolution in several imaging formats, including confocal imaging, the provided data will aid in obtaining the optimal spatial resolution when using QD probes to image biological samples. 

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5. Parameters of fast and high-yield InAs/GaAs quantum dot semiconductor scintillator

   https://doi.org/10.1557/s43580-021-00019-y  

   Minns, A.; Dropiewski, K.; Yakimov, M.; Tokranov, V.; Hedges, M.; Murat, P.; Oktyabrsky, S. (January 2021, MRS Advances)

   null (Ed.)

   InAs quantum dots (QDs) embedded into a waveguiding GaAs semiconductor matrix may produce scintillation detectors with exceptional speed and yield, making them valuable for nuclear security, medical imaging, and high energy physics applications. In this work, we developed thick (~25um) epitaxial heterostructres with high luminescence efficiency composed of self-assembled nano-engineered InAs QDs grown by molecular beam epitaxy. The bulk GaAs acts as a stopping material for incident particles and as a waveguide when layer-transferred onto a low-index substrate. Waveguiding and self-absorption (<1cm-1) were studied using photoluminescence with scanning laser excitation and modeled with ray optics approximation and geometrical coupling of high-index waveguide to a collection fiber. Scintillating signals from alpha-particles were analyzed with an external photodiode (PD) and an integrated PD which provided an improved optical coupling. The mean charge collected by the integrated PD corresponded to 5×1e4 photoelectrons per 1 MeV of deposited energy, or ~20% of the theoretically achievable light yield. Combined with the previously measured QD scintillation time of 0.3-0.6 ns, this makes the InAs/GaAs QD heterostructures the fastest high yield scintillation material reported. 

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