Masoud Taleb

Phonons in hexagonal boron nitride (hBN) possess exceptionally strong oscillator strengths, giving rise to hyperbolic phonon polaritons and enabling robust phonon‐mediated nonlinear interactions. Defect states in hBN are known to interact strongly with lattice vibrations, yet the role of defects in enabling inelastic optical scattering through otherwise Raman‐inactive, infrared (IR)‐active phonon modes remains largely unexplored. Here, we demonstrate a highly efficient inelastic phonon scattering mechanism in which the general selection rules are violated, leading to a down‐converted laser emission mediated by IR‐active phonons in defect‐engineered hBN, via stimulated Raman scattering. We further show that the phonon landscape of hBN can be tailored through substrate choice, enabling multiple inelastic scattering processes and sequential down‐conversion of the excitation laser frequency. These results establish a general strategy for nonlinear defect–phonon scattering and tailoring the nonlinear‐optical selection rules in hBN.

Two-dimensional perovskites, such as Ruddlesden-Popper perovskites, exhibit outstanding optical properties and high exciton binding energies but are highly susceptible to degradation under photo- and electron-beam exposure. To overcome this limitation, we encapsulate the perovskites with mechanically exfoliated hexagonal boron nitride flakes, forming hexagonal boron nitride/perovskite heterostructures. Cathodoluminescence spectroscopy reveals that these heterostructures exhibit significantly reduced electron-beam-induced degradation, enhanced luminescence intensity, a narrower emission bandwidth, and an extended exciton decay time. Moreover, leveraging the scanning capability of our fiber-based cathodoluminescence spectroscopy technique, we demonstrate ultra-long-range exciton transport over distances of approximately 150 micrometers, attributed to exciton-defect coupling. This exciton-defect interaction not only enhances luminescence but also highlights the potential of hexagonal boron nitride/perovskite heterostructures as hybrid van-der-Waals systems with long-range exciton transport and slow radiative decay rates, paving the way for robust and efficient optoelectronic applications.

Ultrafast electron-photon spectroscopy in electron microscopes commonly requires ultrafast laser setups. Photoemission from an engineered electron source is used to generate pulsed electrons, interacting with a sample that is excited by the ultrafast laser pulse at a specified time delay. Thus, developing an ultrafast electron microscope demands the exploitation of extrinsic laser excitations and complex synchronization schemes. Here, we present an inverse approach based on cathodoluminescence spectroscopy to introduce internal radiation sources in an electron microscope. Our method is based on a sequential interaction of the electron beam with an electron-driven photon source (EDPHS) and the investigated sample. An electron-driven photon source in an electron microscope generates phase-locked photons that are mutually coherent with the near-field distribution of the swift electron. Due to their different velocities, one can readily change the delay between the photons and electrons arriving at the sample by changing the distance between the EDPHS and the sample. We demonstrate the mutual coherence between the radiations from the EDPHS and the sample by performing interferometry with a combined system of an EDPHS and a WSe2 flake. We assert the mutual frequency and momentum-dependent correlation of the EDPHS and sample radiation, and determine experimentally the degree of mutual coherence of up to 27%. This level of mutual coherence allows us to perform spectral interferometry with an electron microscope. Our method has the advantage of being simple, compact and operating with continuous electron beams. It will open the door to local electron-photon correlation spectroscopy of quantum materials, single photon systems, and coherent exciton-polaritonic samples with nanometric resolution.

Cherenkov radiation (CR) from electrons propagating in materials with a high refractive index have applications in particle-detection mechanisms and could be used for high-yield coherent electron beam-driven photon sources. However, the theory of the CR has been treated up to now using the non-recoil approximation, which neglects the effect of electron deceleration in materials. Here, we report on the effect of electron-beam deceleration on the radiated spectrum and exciton-photon interactions in nm-thick WSe₂ crystals. The calculation of the CR is performed by simulating the kinetic energy of an electron propagating in a thick sample using the Monto Carlo method combined with the Lienard–Wiechert retarded potential. Using this approach, we numerically investigate the interaction between the excitons and generated photons (CR) beyond the non-recoil approximation and are able to reproduce experimental cathodoluminescence spectra. Our findings pave the way for an accurate design of particle scintillators and detectors, based on the strong-coupling phenomenon.

Electroreduction of oxygen on the microporous-mesoporous carbon material synthesized from D-(+)-glucose applying hydrothermal carbonization and activated additionally with carbon dioxide method has been studied at the unmodified and Pt-nanoclusters modified electrodes. Electrochemical experiments have been carried out in argon or oxygen saturated 0.1 M KOH solution using the cyclic voltammetry, rotating disk electrode and electrochemical impedance spectroscopy techniques. The carbon material modified with Pt-nanoclusters shows high cathodic current densities up to −67 A m⁻² at 3000 rpm. Furthermore, the number of electrons transferred during the oxygen electroreduction is close to four, suggesting that the oxygen reduction reaction on the Pt-nanoclusters modified material could be dominated by a four-electron reduction pathway. The oxygen reduction reaction at unmodified material proceeds through a two-electron reduction pathway within the lower cathodic overpotential region, but changes to a four-electron reduction process within the higher cathodic overpotential region. Compared to the results of the same materials in 0.5 M H₂SO₄, higher catalytic activity has been determined in the case of 0.1 M KOH aqueous solution.

Microporous-mesoporous carbon materials synthesized from molybdenum and tungsten carbides by high-temperature chlorination method were used as supports for catalysts activated with platinum nanoclusters. The characteristics obtained have been compared with those for carbon material synthesized from D-glucose by hydrothermal carbonization followed by additional chemical activation with CO₂. The scanning electron microscopy, X-ray diffraction, Raman spectroscopy, high resolution transmission electron microscopy and low temperature N₂ and Ar as well as CO₂ adsorption methods were used to characterize the structure of the materials prepared. The materials exhibited high specific surface area and remarkable differences in their microporous-mesoporous structure. Platinum has been deposited onto the porous carbon materials in the form of nanoparticles or agglomerates of nanoparticles. All catalysts (studied by cyclic voltammetry and rotating disc electrode methods) showed high activity towards oxygen reduction and hydrogen adsorption from 0.1 M KOH and 0.5 M H₂SO₄ solutions.

Copper oxide-doped alumina nanofibres were fabricated by the solution combustion method. The bundled alumina nanofibres were impregnated with a copper nitrate–glycine (oxidizer–fuel) solution and heat-treated in an open-air environment at 400 °C for 30 min. The microstructure and phase composition of the final product were characterized by XRD, SEM, and EDS analyses. A uniform distribution of a fine-grained CuO film on the surface of gamma-alumina nanofibres was revealed. The obtained results showed a dramatical effect of the amount of fuel, the ratios of fuel to oxidizer and fibres to Cu(II) ions in the reaction mixture on the particle size of the combustion product, its phase composition, and microstructure morphology.