Synthesis of Ultrathin and Thickness-Controlled Cu_2–<i>x</i>Se Nanosheets via Cation Exchange

We demonstrate the use of cation exchange to synthesize ultrathin and thickness-controlled Cu_2–<i>x</i>Se nanosheets (NSs) beginning with CdSe NSs. In this manner, extremely thin (i.e., 1.6 nm thickness) Cu_2–<i>x</i>Se NSs, beyond which can be made directly, have been obtained. Furthermore, they represent the thinnest NSs produced via cation exchange. Notably, the exchange reaction preserves the starting morphology of the CdSe sheets and also retains their cubic crystal structure. The resulting nonstoichiometric and cubic Cu_2–<i>x</i>Se NSs are stable and do not exhibit any signs of Cu or Se oxidation after exposure to air for 2 weeks. Resulting NSs also show the existence of a localized surface plasmon resonance in the infrared due to the presence of copper vacancies. Efforts to isolate intermediates during the cation exchange reaction show that it occurs via a mechanism where entire sheets are rapidly converted into the final product once the exchange reaction commences, precluding the isolation of alloyed species

Efficient Photocatalytic Hydrogen Generation from Ni Nanoparticle Decorated CdS Nanosheets

High-quality, thickness-controlled CdS nanosheets (NSs) have been obtained through the thermal decomposition of cadmium diethyldithiocarbamate in octadecene. Ensembles with discrete thicknesses of 1.50, 1.80, and 2.16 nm have been made with corresponding lateral dimensions on the order of 90 nm × 20 nm. These latter values make the 1–3 nm NSs the largest 2D CdS specimens made to date using colloidal chemistry. Associated Ni nanoparticle decorated counterparts have been made through the photodeposition of Ni onto NSs with an average nanoparticle diameter of 6 nm. Subsequent photocatalytic hydrogen generation measurements have compared the performance of CdS NSs with that of their Ni NP decorated counterparts in water/ethanol mixtures. Apparent quantum yields as large as 25% have been seen for Ni NP decorated NSs with transient yields as large as 64% within the first 2 h of irradiation. Results from ensemble femtosecond transient differential absorption spectroscopy reveal that the origin of this high efficiency stems from efficient electron transfer from CdS to Ni. In this regard, the CdS/Ni semiconductor/metal heterojunction acts to dissociate strongly bound excitons in CdS NSs, creating free carriers needed to carry out relevant reduction chemistries

Defect-Mediated CdS Nanobelt Photoluminescence Up-Conversion

Laser cooling in semiconductors has recently been demonstrated in cadmium sulfide nanobelts (NBs) as well as in organic–inorganic lead halide perovskites. Cooling by as much as 40 K has been shown in CdS nanobelts and by as much as 58 K in hybrid perovskite films. This suggests that further progress in semiconductor-based optical refrigeration can ultimately lead to solid state cryocoolers capable of achieving sub 10 K temperatures. In CdS, highly efficient photoluminescence (PL) up-conversion has been attributed to efficient exciton–longitudinal optical (LO) phonon coupling. However, the nature of its efficient anti-Stokes emission has not been established. Consequently, developing a detailed understanding about the mechanism leading to efficient PL up-conversion in CdS NBs is essential to furthering the nascent field of semiconductor laser cooling. In this study, we describe a detailed investigation of anti-Stokes photoluminescence (ASPL) in CdS nanobelts. Temperature- and frequency-dependent band edge emission and ASPL spectroscopies conducted on individual belts as well as ensembles suggest that CdS ASPL is defect-mediated via the involvement of donor–acceptor states

Two new xanthones from the pericarp of <i>Garcinia mangostana</i>

<div><p>Two new xanthones, designated garcimangosxanthone F (<b>1</b>) and garcimangosxanthone G (<b>2</b>), were isolated from the EtOAc-soluble fraction of ethanolic extract from the pericarp of <i>Garcinia mangostana</i>. Their structures were established as 1,6,7-trihydroxy-5-(3-methylbut-2-enyl)-8-(3-hydroxy-3-methylbutyl)-6′,6′-dimethylpyrano[2′,3′:3,2]xanthone and 1,6,7-trihydroxy-5-(3-methylbut-2-enyl)-8-(3-hydroxy-3-methylbutyl)-6′,6′-dimethyl-4′,5′-dihydropyrano[2′,3′:3,2]xanthone, respectively, on the basis of their 1D, 2D NMR and MS data interpretation.</p></div

Spatially Non-uniform Trap State Densities in Solution-Processed Hybrid Perovskite Thin Films

The facile solution-processability of methylammonium lead halide (CH₃NH₃PbI₃) perovskites has catalyzed the development of inexpensive, hybrid perovskite-based optoelectronics. It is apparent, though, that solution-processed CH₃NH₃PbI₃ films possess local emission heterogeneities, stemming from electronic disorder in the material. Herein we investigate the spatially resolved emission properties of CH₃NH₃PbI₃ thin films through detailed emission intensity versus excitation intensity measurements. These studies enable us to establish the existence of nonuniform trap density variations wherein regions of CH₃NH₃PbI₃ films exhibit effective free carrier recombination while others exhibit emission dynamics strongly influenced by the presence of trap states. Such trap density variations lead to spatially varying emission quantum yields and correspondingly impact the performance of both methylammonium lead halide perovskite solar cells and other hybrid perovskite-based devices. Of additional note is that the observed spatial extent of the optical disorder extends over length scales greater than that of underlying crystalline domains, suggesting the existence of other factors, beyond grain boundary-related nonradiative recombination channels, which lead to significant intrafilm optical heterogeneities

The complete mitochondrial genome of <i>Sarcophaga angarosinica</i> (Diptera: Sarcophagidae)

Sarcophaga (Liosarcophaga) angarosinica (Rohdendorf, 1937) (Diptera: Sarcophagidae) is a species of both medical and ecological significance. In this study, the complete mitochondrial genome (mitogenome) of S. angarosinica was sequenced and characterized. The mitogenome has a total length of 15,215 bp, including 13 protein-coding genes, two ribosomal RNAs, 22 transfer RNAs, and an adenine and thymine-rich region. This mitogenome comprises 39.5% adenine, 9.4% guanine, 14.4% cytosine, and 36.8% thymine. Phylogenetic analysis revealed that S. angarosinica is closely related to Sarcophaga similis. This study enriches the genetic data on S. angarosinica and will contribute to establishing the phylogenetic relationships among flesh flies.</p

Transforming Layered to Nonlayered Two-Dimensional Materials: Cation Exchange of SnS₂ to Cu₂SnS₃

We demonstrate the chemical transformation of layered, two-dimensional (2D) SnS₂ to nonlayered Cu₂SnS₃ via cation exchange. Resulting Cu₂SnS₃ nanosheets (NSs) retain the overall starting morphology of their parent, few-layer SnS₂ templates. Specifically, they possess micrometer-sized dimensions and have controlled thicknesses dictated by the number of initial SnS₂ layers. Our demonstration shows that existing layered compounds can serve as templates for difficult-to-synthesize nonlayered 2D specimens with cation exchange providing a bridge between families of layered and nonlayered materials. New 2D systems are therefore accessible, opening the door to future explorations of low-dimensional nanostructure anisotropic optical and electrical properties

Molybdenum Carbamate Nanosheets as a New Class of Potential Phase Change Materials

We report for the first time the synthesis of large, free-standing, Mo₂O₂(μ-S)₂(Et₂dtc)₂ (MoDTC) nanosheets (NSs), which exhibit an electron-beam induced crystalline-to-amorphous phase transition. Both electron beam ionization and femtosecond (fs) optical excitation induce the phase transition, which is size-, morphology-, and composition-preserving. Resulting NSs are the largest, free-standing regularly shaped two-dimensional amorphous nanostructures made to date. More importantly, amorphization is accompanied by dramatic changes to the NS electrical and optical response wherein resulting amorphous species exhibit room-temperature conductivities 5 orders of magnitude larger than those of their crystalline counterparts. This enhancement likely stems from the amorphization-induced formation of sulfur vacancy-related defects and is supported by temperature-dependent transport measurements, which reveal efficient variable range hopping. MoDTC NSs represent one instance of a broader class of transition metal carbamates likely having applications because of their intriguing electrical properties as well as demonstrated ability to toggle metal oxidation states