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1. Size- and temperature-dependent photoluminescence spectra of strongly confined CsPbBr ₃ quantum dots

   https://doi.org/10.1039/d0nr02711a  

   Cheng, Oscar Hsu-Cheng; Qiao, Tian; Sheldon, Matthew; Son, Dong Hee (June 2020, Nanoscale)

   null (Ed.)

   Lead-halide perovskite nanocrystals (NCs) are receiving much attention as a potential high-quality source of photons due to their superior luminescence properties in comparison to other semiconductor NCs. To date, research has focused mostly on NCs with little or no quantum confinement. Here, we measured the size- and temperature-dependent photoluminescence (PL) from strongly confined CsPbBr 3 quantum dots (QDs) with highly uniform size distributions, and examined the factors determining the evolution of the energy and linewidth of the PL with varying temperature and QD size. Compared to the extensively studied II–VI QDs, the spectral position of PL from CsPbBr 3 QDs shows an opposite dependence on temperature, with weaker dependence overall. On the other hand, the PL linewidth is much more sensitive to the temperature and size of the QDs compared to II–VI QDs, indicating much stronger coupling of excitons to the vibrational degrees of freedom both in the lattice and at the surface of the QDs. 

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2. Stacking‐Order‐Driven Optical Properties and Carrier Dynamics in ReS ₂

   https://doi.org/10.1002/adma.201908311  

   Zhou, Yongjian; Maity, Nikhilesh; Rai, Amritesh; Juneja, Rinkle; Meng, Xianghai; Roy, Anupam; Zhang, Yanyao; Xu, Xiaochuan; Lin, Jung‐Fu; Banerjee, Sanjay_K; et al (April 2020, Advanced Materials)

   Abstract Two distinct stacking orders in ReS₂are identified without ambiguity and their influence on vibrational, optical properties and carrier dynamics are investigated. With atomic resolution scanning transmission electron microscopy (STEM), two stacking orders are determined as AA stacking with negligible displacement across layers, and AB stacking with about a one‐unit cell displacement along theaaxis. First‐principles calculations confirm that these two stacking orders correspond to two local energy minima. Raman spectra inform a consistent difference of modes I & III, about 13 cm⁻¹for AA stacking, and 20 cm⁻¹for AB stacking, making a simple tool for determining the stacking orders in ReS₂. Polarized photoluminescence (PL) reveals that AB stacking possesses blueshifted PL peak positions, and broader peak widths, compared with AA stacking, indicating stronger interlayer interaction. Transient transmission measured with femtosecond pump–probe spectroscopy suggests exciton dynamics being more anisotropic in AB stacking, where excited state absorption related to Exc. III mode disappears when probe polarization aligns perpendicular tobaxis. The findings underscore the stacking‐order driven optical properties and carrier dynamics of ReS₂, mediate many seemingly contradictory results in the literature, and open up an opportunity to engineer electronic devices with new functionalities by manipulating the stacking order. 

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3. Novel Composite of Nickel Thiocyanate‐Based All‐Inorganic Lead Bromide Perovskite Nanocrystals with Enhanced Luminescent and Stability for White Light‐Emitting Diodes

   https://doi.org/10.1002/admi.202200710  

   Thapa, Saroj; Zhu, Hongyang; Grigoriev, Alexei; Zhu, Peifen (June 2022, Advanced Materials Interfaces)

   Abstract Doping of CsPbBr₃perovskite nanocrystals (PNCs) to achieve excellent material properties is accelerating due to their increasing use in optoelectronic devices. Herein, a novel composite of CsPbBr₃PNCs with nickel thiocyanate Ni(SCN)₂is reported, exhibiting stronger photoluminescence (PL) and more extended stability. The addition of Ni(SCN)₂at different molar concentrations reduces the surface trap states of the host PNCs. Therefore, the microstrain, dislocation density, PL emission linewidth, and Urbach energy decrease, resulting in an increased photoluminescence quantum yield (PLQY) from 72% to high above 90%. When stored in the ambient atmosphere for 120 days, the PLQY of doped PNCs is maintained by more than four times compared to host PNCs. A combination of 3D‐printed conversion layers containing green‐, yellow‐, and red‐emitting doped PNCs with blue light‐emitting diodes results in stable white light with superior color qualities. Hence, new composites with desired properties are developed as an alternative to conventional color phosphors. 

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4. Anomalous Stranski-Krastanov growth of (111)-oriented quantum dots with tunable wetting layer thickness

   https://doi.org/10.1038/s41598-019-54668-z  

   Schuck, Christopher F.; Roy, Simon K.; Garrett, Trent; Yuan, Qing; Wang, Ying; Cabrera, Carlos I.; Grossklaus, Kevin A.; Vandervelde, Thomas E.; Liang, Baolai; Simmonds, Paul J. (December 2019, Scientific Reports)

   Abstract Driven by tensile strain, GaAs quantum dots (QDs) self-assemble on In_0.52Al_0.48As(111)A surfaces lattice-matched to InP substrates. In this study, we show that the tensile-strained self-assembly process for these GaAs(111)A QDs unexpectedly deviates from the well-known Stranski-Krastanov (SK) growth mode. Traditionally, QDs formed via the SK growth mode form on top of a flat wetting layer (WL) whose thickness is fixed. The inability to tune WL thickness has inhibited researchers’ attempts to fully control QD-WL interactions in these hybrid 0D-2D quantum systems. In contrast, using microscopy, spectroscopy, and computational modeling, we demonstrate that for GaAs(111)A QDs, we can continually increase WL thickness with increasing GaAs deposition, even after the tensile-strained QDs (TSQDs) have begun to form. This anomalous SK behavior enables simultaneous tuning of both TSQD size and WL thickness. No such departure from the canonical SK growth regime has been reported previously. As such, we can now modify QD-WL interactions, with future benefits that include more precise control of TSQD band structure for infrared optoelectronics and quantum optics applications. 

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5. Localized interlayer excitons in MoSe2–WSe2 heterostructures without a moiré potential

   https://doi.org/10.1038/s41467-022-33082-6  

   Mahdikhanysarvejahany, Fateme; Shanks, Daniel N.; Klein, Matthew; Wang, Qian; Koehler, Michael R.; Mandrus, David G.; Taniguchi, Takashi; Watanabe, Kenji; Monti, Oliver L. A.; LeRoy, Brian J.; et al (September 2022, Nature Communications)

   Abstract Interlayer excitons (IXs) in MoSe₂–WSe₂heterobilayers have generated interest as highly tunable light emitters in transition metal dichalcogenide (TMD) heterostructures. Previous reports of spectrally narrow (<1 meV) photoluminescence (PL) emission lines at low temperature have been attributed to IXs localized by the moiré potential between the TMD layers. We show that spectrally narrow IX PL lines are present even when the moiré potential is suppressed by inserting a bilayer hexagonal boron nitride (hBN) spacer between the TMD layers. We compare the doping, electric field, magnetic field, and temperature dependence of IXs in a directly contacted MoSe₂–WSe₂region to those in a region separated by bilayer hBN. The doping, electric field, and temperature dependence of the narrow IX lines are similar for both regions, but their excitonic g-factors have opposite signs, indicating that the origin of narrow IX PL is not the moiré potential. 

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