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Encapsulating Cs4PbBr6 quantum dots in silicon nano-sheets not only stabilizes the halide perovskite, but also takes advantage of the nano-sheet for a compatible integration with the traditional silicon semiconductor. Here, we report the preparation of un-passivated Cs4PbBr6 ellipsoidal nanocrystals and pseudo-spherical quantum dots in silicon nano-sheets and their enhanced photoluminescence (PL). For a sample with low concentrations of quantum dots in silicon nano-sheets, the emission from Cs4PbBr6 pseudo-spherical quantum dots is quenched and is dominated with Pb2+ ion/silicene emission, which is very stable during the whole measurement period. For a high concentration of Cs4PbBr6 ellipsoidal nanocrystals in silicon nano-sheets, we have observed Förster resonance energy transfer with up to 87% efficiency through the oscillation of two PL peaks when UV excitation switches between on and off, using recorded video and PL lifetime measurements. In an area of a non-uniform sample containing both ellipsoidal nanocrystals and pseudo-spherical quantum dots, where Pb2+ ion/silicene emissions, broadband emissions from quantum dots, and bandgap edge emissions (515 nm) appear, the 515 nm peak intensity increases five times over 30 min of UV excitation, probably due to a photon recycling effect. This irradiated sample has been stable for one year of ambient storage. Cs4PbBr6 quantum dots encapsulated in silicon nano-sheets can lead to applications of halide perovskite light emitting diodes (PeLEDs) and integration with traditional semiconductor materials. [ABSTRACT FROM AUTHOR]

Twisted moiré photonic crystal is an optical analog of twisted graphene or twisted transition metal dichalcogenide bilayers. In this paper, we report the fabrication of twisted moiré photonic crystals and randomized moiré photonic crystals and their use in enhanced extraction of light in light-emitting diodes (LEDs). Fractional diffraction orders from randomized moiré photonic crystals are more uniform than those from moiré photonic crystals. Extraction efficiencies of 76.5%, 77.8% and 79.5% into glass substrate are predicted in simulations of LED patterned with twisted moiré photonic crystals, defect-containing photonic crystals and random moiré photonic crystals, respectively, at 584 nm. Extraction efficiencies of optically pumped LEDs with 2D perovskite (BA)2(MA) n −1PbnI3 n +1 of n = 3 and (5-(2′-pyridyl)-tetrazolato)(3-CF3−5-(2′-pyridyl)pyrazolato) platinum(II) (PtD) have been measured. [ABSTRACT FROM AUTHOR]

Herein, photoluminescence (PL) and fluorescence lifetime imaging (FLIM) in multilayer MoSe2 are studied. Strain‐activated stimulated emission via defect levels in multilayer MoSe2 under laser excitation is observed, for the first time in defects of transition metal dichalcogenides. The stimulated emission is indicated by a threshold behavior of PL emission intensity with respect to laser intensity, strong polarization effects, achieved population inversion with a difference in lifetimes of two competing excited states, and localization of the stimulated emission zone as observed in FLIM. The presented results not only demonstrate strain‐activated stimulated emission and highlight the necessity of strain engineering in tailoring 2D layered materials for optoelectronic applications, but also shed light on the design of stimulated emission in transition metal dichalcogenide's defects to tailor for potential single‐photon emission behavior. [ABSTRACT FROM AUTHOR]

The layer edge states or low energy state (LES) in 2D hybrid organic–inorganic perovskites demonstrate a prolonged carrier lifetime for better performance of optoelectronic devices. However, the fundamental understanding of LES in 2D perovskites is still inconclusive. Herein, a photoluminescence (PL) study of LES in 2D Ruddlesden–Popper perovskites is presented with n = 2 and n = 3 from their cleaved cross sections that are more stable than the natural edge. The PL measurements clearly observe reversible, and irreversible surface relaxations (case I and case II) in three laser intensity ranges, further supported by a PL excitation cycle from low to high laser intensity, and vice versa. The PL wavelength of LES is tunable with laser intensity and blueshifts with increasing laser intensity during irreversible surface relaxation process (case I). Fluorescence lifetime imaging (FLIM) shows that the LES has a longer lifetime than the band‐edge emission in the sample without a photodegradation, while the BE lifetime becomes relatively longer in the area with a photodegradation. The presented laser tunable LES and the related irreversible relaxation process provide a new insight that can help improve the photostability in 2D perovskites and understand roles of LESs in optoelectronic device performance. [ABSTRACT FROM AUTHOR]

Very recently, the synthesis of 2D MoS2 and WS2 through pulsed laser-directed thermolysis can achieve wafer-scale and large-area structures, in ambient conditions. In this paper, we report the synthesis of MoS2 and MoS2 oxides from (NH4)2MoS4 film using a visible continuous-wave (CW) laser at 532 nm, instead of the infrared pulsed laser for the laser-directed thermolysis. The (NH4)2MoS4 film is prepared by dissolving its crystal powder in DI water, sonicating the solution, and dip-coating onto a glass slide. We observed a laser intensity threshold for the laser synthesis of MoS2, however, it occurred in a narrow laser intensity range. Above that range, a mixture of MoS2 and MoO2 is formed, which can be used for a memristor device, as demonstrated by other research groups. We did not observe a mixture of MoS2 and MoO3 in the laser thermolysis of (NH4)2MoS4. The laser synthesis of MoS2 in a line pattern is also achieved through laser scanning. Due to of the ease of CW beam steering and the fine control of laser intensities, this study can lead toward the CW laser-directed thermolysis of (NH4)2MoS4 film for the fast, non-vacuum, patternable, and wafer-scale synthesis of 2D MoS2. [ABSTRACT FROM AUTHOR]

Twisted photonic crystals are photonic analogs of twisted monolayer materials such as graphene and their optical property studies are still in their infancy. This paper reports optical properties of twisted single-layer 2D+ moiré photonic crystals where there is a weak modulation in z direction, and bilayer moiré-overlapping-moiré photonic crystals. In weak-coupling bilayer moiré-overlapping-moiré photonic crystals, the light source is less localized with an increasing twist angle, similar to the results reported by the Harvard research group in References 37 and 38 on twisted bilayer photonic crystals, although there is a gradient pattern in the former case. In a strong-coupling case, however, the light source is tightly localized in AA-stacked region in bilayer PhCs with a large twist angle. For single-layer 2D+ moiré photonic crystals, the light source in Ex polarization can be localized and forms resonance modes when the single-layer 2D+ moiré photonic crystal is integrated on a glass substrate. This study leads to a potential application of 2D+ moiré photonic crystal in future on-chip optoelectronic integration. [ABSTRACT FROM AUTHOR]

Organic-inorganic perovskites hold great promise as optoelectronic semiconductors for pure color light emitting and photovoltaic devices. However, challenges persist regarding their photostability and chemical stability, which limit their extensive applications. This paper investigates the laser radiation hardening and self-healing-induced properties of aged MAPbBr3 perovskites encapsulated in NiO nanotubes (MAPbBr3@NiO) using photoluminescence (PL) and fluorescence lifetime imaging (FLIM). After deliberately subjecting the MAPbBr3@ NiO to atmospheric conditions for two years, the sample remains remarkably stable. It exhibits no changes in PL wavelength during UV laser irradiation and self-healing. Furthermore, exposure to UV light at 375 nm enhances the PL of the self-healed MAPbBr3@NiO. FLIM analysis sheds light on the mechanism behind photodegradation, self-healing, and PL enhancement. The results indicate the involvement of many carrier-trapping states with low lifetime events and an increase in peak lifetime after self-healing. The formation of trapping states at the perovskite/nanotube interface is discussed and tested. This study provides new insights into the dynamics of photo-carriers during photodegradation and self-healing in organic-inorganic perovskites. [ABSTRACT FROM AUTHOR]

Realizing stimulated emission from defects in 2D‐layered semiconductors has the potential to enhance the sensitivity of characterizing their defects. However, stimulated emission from defects in layered materials presents a different set of challenges in carrier lifetime and energy level design and is not achieved so far. Here, photoluminescence (PL) spectroscopy, Raman spectroscopy, and first‐principles theory are combined to reveal an anomalous PL intensity–temperature relation and strong polarization effects at a defect emission peak in annealed multilayer MoS2, suggesting defect‐based stimulated emission. The emergence of stimulated emission behavior is also controllable (by temperature) and reversible. The observed stimulated emission behavior is supported by a three‐level system involving two defect levels from chalcogen vacancies and a pump level from the conduction band edge. First‐principles calculations show that the special indirect gap that enables stimulated emission is not native to pristine bulk MoS2 and only emerges under thermal strain. [ABSTRACT FROM AUTHOR]

Realizing stimulated emission from defects in 2D‐layered semiconductors has the potential to enhance the sensitivity of characterizing their defects. However, stimulated emission from defects in layered materials presents a different set of challenges in carrier lifetime and energy level design and is not achieved so far. Here, photoluminescence (PL) spectroscopy, Raman spectroscopy, and first‐principles theory are combined to reveal an anomalous PL intensity–temperature relation and strong polarization effects at a defect emission peak in annealed multilayer MoS2, suggesting defect‐based stimulated emission. The emergence of stimulated emission behavior is also controllable (by temperature) and reversible. The observed stimulated emission behavior is supported by a three‐level system involving two defect levels from chalcogen vacancies and a pump level from the conduction band edge. First‐principles calculations show that the special indirect gap that enables stimulated emission is not native to pristine bulk MoS2 and only emerges under thermal strain. [ABSTRACT FROM AUTHOR]