ABSTRACT
Despite its simple crystal structure, layered boron nitride features a surprisingly complex variety of phonon-assisted luminescence peaks. We present a combined experimental and theoretical study on ultraviolet-light emission in hexagonal and rhombohedral bulk boron nitride crystals. Emission spectra of high-quality samples are measured via cathodoluminescence spectroscopy, displaying characteristic differences between the two polytypes. These differences are explained using a fully first-principles computational technique that takes into account radiative emission from "indirect," finite-momentum excitons via coupling to finite-momentum phonons. We show that the differences in peak positions, number of peaks, and relative intensities can be qualitatively and quantitatively explained, once a full integration over all relevant momenta of excitons and phonons is performed.
ABSTRACT
Hexagonal boron nitride nanosheets (BNNSs) are promising 2D materials due to their exceptional chemical and thermal stabilities together with their electrical insulation properties. A combined synthesis method involving the polymer-derived ceramics (PDCs) route and the spark plasma sintering (SPS) process is proposed, leading to well-crystallized and pure layered h-BN crystals, prone to be exfoliated into large BNNSs. Here we focus more specifically on the influence of two key parameters of the process to be optimized: the Li3N concentration (0-10 wt%) and the SPS temperature (1200 °C-1950 °C). The presence of Li3N, added as crystal promoter in the pre-ceramic powder, significantly improves the crystallinity level of the product, as evidenced by XRD, SEM and Raman spectrometry. SPS temperature strongly modifies the size of the resulting h-BN flakes. The influence of SPS temperature on both purity and crystallinity is studied using cathodoluminescence. h-BN flakes larger than 200 µm2 (average flake area) are obtained. Few-layered BNNSs are successfully isolated, through exfoliation process.
ABSTRACT
Spin crossover cations have been successfully synthesized in the pores of a mesoporous robust Metal-Organic Framework (MOF) MIL-100(Al) through sequential introduction of Fe(iii) cations and a sal2trien ligand. The MIL-100(Al)@Fe(sal2trien) hybrid material retains its crystallinity and partial porosity compared to the parent MOF. The spin state of the Fe(sal2trien)+ cations can be modulated at room temperature through sorption of guest molecules, paving the way to the design of a new generation of sensors based on MOF@spin crossover complex solids.
