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We report the design and fabrication of a heterojunction photoanode consisting of two-dimensional (2D) functionalized hexagonal boron nitride (F-h-BN) nanoflakes, introduced at the interface between SnO2 and quantum dots (QDs) (e.g. CdS and CdS–CdSe), used as a photoanode in a photoelectrochemical cell (PEC) for hydrogen (H2) generation. We highlight the effect of 2D F-h-BN nanoflakes on the carrier recombination and performance of the PEC system. In addition, the tailoring of SnO2 film thickness and incorporation of multi-walled carbon nanotubes (MWCNTs) were investigated for their effect on carrier dynamics and overall device performance. Our results show that a PEC device based on SnO2/F-h-BN/CdS QDs exhibits a 60% improvement in the saturated photocurrent density (at 1.0 V vs. RHE) compared to the control device, due to improved electron injection and reduced carrier recombination at the metal oxide/QDs/electrolyte interface. The highest saturated photocurrent density value reached 6.35 mA cm−2 (at 1.0 V vs. RHE), after thickness optimization of the SnO2 film, incorporation (0.015 wt%) of MWCNTs, and cascade CdS–CdSe QDs. In addition, the PEC system maintains 98% of the initial value of photocurrent density after four hours of continuous one sun illumination (AM 1.5G, 100 mW cm−2). Our results offer a simple yet cost-effective and efficient strategy to enhance the performance of QD based PEC H2 generation and potentially other optoelectronic devices.

The structural details of nanoparticles at the sub-particle level are critical for our understanding of their functionalities and the basic mechanisms involved in their formation. In particular, the geometries of such features determine the particle's overall optical response. Hollow metallic nanoparticles (hollow-MNPs) that have cubic geometries, with varying morphologies on their walls and voids in their body, offer a platform to study the effects of such structural features on the properties of single nanoparticles and their ensemble. Here, we report the control over sub-particle pinholes and voids by modifying the dynamics of the galvanic reaction, and we connect these structures to the optical response of the hollow nanocubes. We observe that symmetry breakage in individual particles, caused by pinholes and voids, has a drastic effect on the plasmon-resonance peak positions in their UV-Vis-NIR spectra. Via electron microscopy imaging, statistical analyses, and electromagnetic simulations, we observe that enlargement in a pinhole's diameter and an increase in their number produce a redshift in the resonance absorption peak of the ensemble. Our results outline nanoparticle design avenues via sub-particle morphologies for several applications, including those operating in the biological window and those carrying chemical payloads in organisms.

We report on the time-domain integration of terahertz pulses obtained via the tight confinement of the radiation in a tapered two-wire waveguide. Both simulation and experimental results prove the time integration capability of this structure.

We show the time-domain integration of terahertz pulses achieved in a sub-wavelength, tapered two-wire waveguide. Both simulation and experimental results prove the time integration functionality of this waveguide topology.

We present the solid-state-biased coherent detection technique for ultra-broadband THz pulses operated via a homodyne configuration. This makes our detection method of easy implementation, suitable for cost-effective and portable THz systems.

We report the synthesis of a TiO2 hybrid structure, consisting of a combination of nanorods and nanoparticles, subsequently treated with hydrazine to enhance the performance of photoelectrochemical (PEC) hydrogen (H2) generation. The optimized TiO2 hybrid photoanode sensitized with Quantum dots (QDs), yields a saturated photocurrent density of 4.25 mA cm−2 (at 0.8 V vs RHE), which is 172% higher than that of the reference sample. The optimized hybrid photoanode treated with hydrazine exhibits an additional 28% increase in the saturated photocurrent density, reaching 5.43 mA cm−2 with CdS QDs, and 8.12 mA cm−2 with CdS/CdSe QDs (at 0.8 V vs RHE), while maintaining 80% of the initial value of photocurrent density, after 2 h of continuous one sun illumination (AM 1.5 G, 100 mW cm−2). We used Density Functional Theory with Hubbard energy correction (DFT + U) calculations to describe the mechanism that underpins this significant improvement. DFT + U results highlighted that the concentration of the hydrazine treatment plays a crucial role and affects the sites (surface or interstitial) where nitrogen may be present. This eventually affects the recombination centers within the hybrid photoanode. Thus, the results of this work define a promising strategy to optimize the morphology of the hybrid photoanodes via hydrazine surface engineering to fabricate efficient and stable PEC water splitting devices for H2 generation.

Enhanced image contrast has been seen at graphene-layered steps a few nanometers in height by means of photon-induced near-field electron microscopy (PINEM) using synchronous femtosecond pulses of light and electrons. The observed steps are formed by the edges of graphene strips lying on the surface of a graphene substrate, where the strips are hundreds of nanometers in width and many micrometers in length. PINEM measurements reflect the interaction of imaging electrons and induced (near) electric fields at the steps, and this leads to a much higher contrast than that achieved in bright-field transmission electron microscopy imaging of the same strips. Theory and numerical simulations support the experimental PINEM findings and elucidate the nature of the electric field at the steps formed by the graphene layers. These results extend the range of applications of the experimental PINEM methodology, which has previously been demonstrated for spherical, cylindrical, and triangular nanostructures, to shapes of high aspect ratio (rectangular strips), as well as into the regime of atomic layer thicknesses.

Characteristic energies of photonic modes are a sensitive function of a nanostructures’ geometrical parameters. In the case of translationally invariant planar waveguides, the eigen-energies reside in the infrared to ultraviolet parts of the optical spectrum and they sensitively depend on the thickness of the waveguide. Using swift electrons and the inherent Cherenkov radiation in dielectrics, the energies of such photonic states can be effectively probed via monochromated electron energy-loss spectroscopy (EELS). Here, by exploiting the strong photonic signals in EELS with 200 keV electrons, we correlate the energies of waveguide peaks in the 0.5–3.5 eV range with planar thicknesses of the samples. This procedure enables us to measure the thicknesses of cross-sectional transmission electron microscopy samples over a 1–500 nm range and with best-case accuracies below ±2%. The measurements are absolute with the only requirement being the optical dielectric function of the material. Furthermore, we provide empirical formulation for rapid and direct thickness estimations for a 50–500 nm range. We demonstrate the methodology for two semiconducting materials, silicon and gallium arsenide, and discuss how it can be applied to other dielectrics that produce strong optical fingerprints in EELS. The asymptotic form of the loss function for two-dimensional materials is also discussed.

Electric fields of nanoscale particles are fundamental to our understanding of nanoplasmonics and nanophotonics. Success has been made in developing methods to probe the effect of their presence, but it remains difficult to directly image optically induced electric fields at the nanoscale and especially when ensembles of particles are involved. Here, using ultrafast electron microscopy, we report the space-time visualization of photon-induced electric fields for ensembles of silver nanoparticles having different sizes, shapes, and separations. The high-field-of-view measurements enable parallel processing of many particles in the ensemble with high throughput of information. Directly in the image, the evanescent fields are observed and visualized when the particles are polarized with the optical excitation. Because the particle size is smaller than the wavelength of light, the near-fields are those of nanoplasmonics and are precursors of far-field nanophotonics. The reported results pave the way for quantitative studies of fields in ensembles of complex morphologies with the nanoparticles being embedded or interfacial.

We use a monochromated 200 keV electron beam as a nanometer-resolution probe of the photonic density of states in bulk and nanoparticle $\mathrm{Si}/{\mathrm{SiO}}_{2}$ systems, observing infrared to ultraviolet waveguided Cherenkov modes in Si slabs, but none in ${\mathrm{SiO}}_{2}$. While isolated Si nanoparticles are too small to support Cherenkov radiation, we find high densities of Si nanoparticles in ${\mathrm{SiO}}_{2}$ support a damped form of the radiation, with the modes determined by the effective medium of the $\mathrm{Si}/{\mathrm{SiO}}_{2}$ mixture. The guided nature of the radiation is confirmed by the presence of a thickness-dependent cutoff.

Silicon nanoparticles embedded in silica show promising optoelectronic properties, due to quantum confinement and/or radiative interface states that should correlate with the particles’ average size and shape. Here the authors report the combination of electron tomography with plasmon-filtered microscopy in order to reconstruct the three-dimensional morphology of silicon nanoparticles. They find that particles with complex morphologies and high surface to volume ratios are dominant, rather than the commonly assumed near-spherical structures. These results should affect quantum-confined excitons and the interface density of states. Their findings may help to explain the physical origin of the unusually broad photoluminescence bands and efficiencies.