The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/adom.202201059.
Nanosheets of silicene, a 2D material made of silicon atoms, have great potential for use in next-generation electronic and optoelectronic devices due to their unique properties. However, issues in the large-scale production of silicene nanosheets and in their degradation still present challenges for these applications. Here, we report a method to obtain large quantities of single to fewlayer thick silicene/silicene oxide ("Si nanosheets") from calcium disilicide (CaSi 2 ). We also show that the silicene nanosheets experience oxidation in air and are highly oxidized after 12 weeks of storage. Density functional theory studies were performed and provide a detailed understanding of this oxidation. Silicene/ silicene oxide nanosheet-based photodetector devices were also fabricated for the first time. They show a broadband response in the visible spectral range with outstanding responsivity (14.3 A/W), detectivity (3 × 10 10 Jones), and external quantum efficiency (44.6%), demonstrating the promising applications of this material for optoelectronic devices.
contain dendritic structures with a fractal dimension consistent with a diffusionlimited aggregation process. [3,6] Although a variety of degradation prevention techniques have been reported such as encapsulation with hexagonal boron nitride [9] and polymers, [10] use of novel gate dielectrics, [11] and placement in an environment with a desiccant [3] or in a vacuum, [4] such techniques are not applicable for ambientair applications such as gas sensors. To our knowledge, the stability of bilayer (BL) and thicker-layer MoS 2 films in ambient air has not been extensively studied. It is important to understand the stability of such films for developing applications and having a better understanding of ML degradation. In this paper, we report on the structural stability of preheated and as-grown BL and thicker-layer MoS 2 films in ambient air. The films are grown using CVD on SiO 2 substrates and studied using atomic force microscopy (AFM), and Raman and PL spectroscopies.BL and thicker-layer MoS 2 and other TMD films, nanosheets, [12] and nanostructures, [13] although having indirect band gaps, have attracted considerable interest because of their useful properties. For example, BLs and thicker-layers have higher electrical conductivities than MLs due to their higher density of states and more effective screening of impurities in the substrate. [14] In addition, BL device yield is typically higher than ML device yield, due to the greater mechanical strength of BLs. [15,16] BL and thicker-layer TMD films offer layer control of properties such as spin-orbit coupling, [17] interlayer coupling, [18] and band gap. [19] Varying the twist angle between layers in BLs has been reported to result in twist-dependent valley and band alignment, [20] and Moiré pattern excitons. [21] The ambient-air degradation of CVD-grown ML MoS 2 and other ML TMD films was first reported by Gao et al. [3] They observed that ML MoS 2 and WS 2 grown on SiO 2 substrates developed extensive cracking, morphological changes, and quenching of PL after exposure to ambient air at room temperature (RT) for a period of about a year. The degradation was attributed to oxidation along grain boundaries and other defects. It was found that water vapor in the air was necessary for degradation to occur since films did not degrade in a dry box. In addition, Budania et al. [4] reported that mechanically exfoliated thin multilayer MoS 2 flakes on SiO 2 developed speckles in air at a high relative humidity (RH) of 60% over a period of about a year. Kotsakidis et al. [5] reported
Silicon telluride (Si2Te3) has emerged as one of the many contenders for 2D materials ideal for the fabrication of atomically thin devices. Despite the progresses which have 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 Si2Te3 degradation under both annealing and in-situ heating with laser. Both processes caused the pristine Si2Te3 to degrade to tellurium and silicon oxide in air without protection coating. A new Raman peak at ~140 cm-1 was observed from the degraded samples and is found to be associated with pure tellurium. This peak was unresolved with the peak at 144 cm-1 for pristine Si2Te3 in the literature and has been erroneously assigned as the signature Raman peak of pure Si2Te3, which caused incorrect interpretation of experimental data. Our study has led to a fundamental understanding of Raman peaks in Si2Te3, which helps resolve the inconsistent issues in the literature. This study is not only important for fundamental understanding but also vital for material characterization and application.
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...
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.
Self-assembly of block copolymers (BCPs) provides a unique platform for producing periodic and orderly structured soft materials of nanometer scale. The kinetics of the assembly process defines the accessible range of morphologies and allows for the formation of asymmetric hierarchies. Here, self-assembly of ultrahigh-molecular-weight BCPs with relatively slow molecular chain dynamics is used to fabricate a metastable asymmetric structure. More specifically, a kinetically trapped solvent vapor annealing process is applied to poly(styrene-block-2-vinylpyridine) thin films, wherein the concentration ratio of trichloroethylene to tetrahydrofuran of the mixed solvent vapor gradually increases throughout the annealing process, pushing the system away from equilibrium. Under such a dynamic process, poly(styrene-block-2-vinylpyridine) micelles rearrange into vertical protuberances that mimic moth-eye structures enhancing the light transmission. Sequential infiltration synthesis is used to convert the poly-2-vinylpyridine domain into alumina in order to verify the formation mechanism of the asymmetric protuberances. It is determined that vertically packed and merged micelles are only formed under a gradually built solvent vapor environment.
Silicon telluride (Si2Te3) and many other tellurium-containing compounds show emergent Raman peaks located at ∼120 and ∼140 cm–1 as they age. The origin of these two emergent peaks is controversial in the literature and has been attributed to myriad causes such as the intrinsic Raman modes of the telluride materials, surface oxidation, defects, double resonances, and tellurium precipitates. The controversial nature of these peaks has led to the misidentification of highly degraded materials as pristine and to the misinterpretation of changes in Raman spectra. Here, we present a comprehensive and multimodal study on Si2Te3 thin films and bulk crystals grown by a chemical vapor deposition process. We find that the two emergent Raman peaks originate from tellurium nanocrystallites formed in the degraded surface layers of Si2Te3. The formation of the tellurium nanocrystallites is shown to be a result of a hydrolysis process in which Si2Te3 reacts with atmospheric water vapor. This study unambiguously clarifies the origin of the controversial Raman peaks seen in numerous telluride compounds and provides a blueprint for the accurate characterization of these material systems in the future.
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