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Metal halide perovskites have emerged as the next generation of light emitting semiconducting materials due to their excellent properties such as tunable bandgaps, high photoluminescence quantum yield, and high color purity. Nickel oxide is a hole transport material that has been used in planar light emitting diodes (LEDs). In this paper, we develop a novel method for the large scale fabrication of metal halide perovskite nanowire arrays encapsulated inside nickel oxide nanotubes. We study the structural and spectral properties of these infiltrated perovskites nanowires and, to the best of our knowledge, for the first time report on a working LED device consisting of perovskites encapsulated inside nickel oxide nanotubes. Finally, we study the photoluminescence and electroluminescence of an LED with MAPbBr 3 inside nickel oxide nanotubes and obtain an outstanding current efficiency of 5.99 Cd A −1 and external quantum efficiency of 3.9% for the LED device. 

Abstract Silicon telluride (Si 2 Te 3 ) has emerged as one of the many contenders for 2D materials ideal for the fabrication of atomically thin devices. Despite the progress which has been made in the electric and optical properties of silicon telluride, much work is still needed to better understand this material. We report here on the Raman study of Si 2 Te 3 degradation under both annealing and in situ heating with a laser. Both processes caused pristine Si 2 Te 3 to degrade into tellurium and silicon oxide in air in the absence of a protective coating. A previously unreported Raman peak at ∼140 cm −1 was observed from the degraded samples and is found to be associated with pure tellurium. This peak was previously unresolved with the peak at 144 cm −1 for pristine Si 2 Te 3 in the literature and has been erroneously assigned as a signature Raman peak of pure Si 2 Te 3 , which has caused incorrect interpretations of experimental data. Our study has led to a fundamental understanding of the Raman peaks in Si 2 Te 3 , and helps resolve the inconsistent issues in the literature. This study is not only important for fundamental understanding but also vital for material characterization and applications. 

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 withn = 2 andn = 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.

 

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 MoS₂and only emerges under thermal strain.

 

Controlling diamond structures with nanometer precision is fundamentally challenging owing to their extreme and far‐from‐equilibrium synthetic conditions. State‐of‐the‐art techniques, including detonation, chemical vapor deposition, mechanical grinding, and high‐pressure‐high‐temperature synthesis, yield nanodiamond particles with a broad distribution of sizes. Despite many efforts, the direct synthesis of nanodiamonds with precisely controlled diameters remains elusive. Here the geochemistry‐inspired synthesis of sub‐5 nm nanodiamonds with sub‐nanometer size deviation is described. High‐pressure‐high‐temperature treatment of uniform iron carbide nanoparticles embedded in iron oxide matrices yields nanodiamonds with tunable diameters down to 2.13 and 0.22 nm standard deviation. A self‐limiting, redox‐driven, and diffusion‐controlled solid‐state reaction mechanism is proposed and supported by in situ X‐ray diffraction, ex situ characterizations, and computational modeling. This work provides a unique mechanism for the precise control of nanostructured diamonds under extreme conditions and paves the road for the full realization of their potential in emerging technologies.

 

Abstract Monolayer (ML) molybdenum disulfide (MoS₂) is a novel 2-dimensional (2D) semiconductor whose properties have many applications in devices. Despite its potential, ML MoS₂ is limited in its use due to its degradation under exposure to ambient air. Therefore, studies of possible degradation prevention methods are important. It is well established that air humidity plays a major role in the degradation. In this paper, we investigate the effects of substrate hydrophobicity on the degradation of chemical vapor deposition (CVD) grown ML MoS 2 . We use optical microscopy, atomic force microscopy (AFM), and Raman mapping to investigate the degradation of ML MoS 2 grown on SiO 2 and Si 3 N 4 that are hydrophilic and hydrophobic substrates, respectively. Our results show that the degradation of ML MoS₂ on Si 3 N 4 is significantly less than the degradation on SiO 2 . These results show that using hydrophobic substrates to grow 2D transition metal dichalcogenide ML materials may diminish ambient degradation and enable improved protocols for device manufacturing. 

It is reported that chemical vapor deposition (CVD) grown bilayer (BL) MoS₂films are significantly more structurally stable in ambient air than CVD‐grown monolayer (ML) MoS₂films that have been reported to structurally degrade in ambient air. The authors present atomic force microscopy (AFM) images of preheated and as‐grown ML and multilayer MoS₂films after exposure to ambient air for periods of up to 2 years. The AFM images show that, in ambient air, preheated and as‐grown BL and thicker‐layer MoS₂films do not exhibit the growth of dendrites that is characteristic of ML degradation. Dendrites are observed to stop at the ML‐BL boundary. Raman spectra of BLs exposed for up to 2 years are similar to those reported for as‐grown BLs. The greater stability of BLs and thicker layers are attributed to their indirect band gaps that suppress Förster resonance energy transfer processes that have been proposed to be responsible for ML degradation. The results show that BL and thicker‐layer transition metal dichalcogenides with indirect band gaps may be structurally stable in air and useful for ambient‐air applications.