Electron-hole recombination limits the efficiency of TiO2 photocatalysis. We have investigated the efficacy with which anatase/carbon nanotube (CNT) composite materials reduce charge recombination and enhance reactivity. We synthesized nanostructured assemblies composed of different proportions of anatase (5 or 100 nm) and either single-or multi-walled CNTs. The composites were prepared using a simple low temperature process in which CNTs and anatase nanoparticles were dispersed in water, dehydrated at 80 degrees C, and dried at 104 degrees C. The structures of the various TiO2/CNT composites were characterized by scanning electron microscopy (SEM) and their function was tested by phenol oxidation. Charge recombination was compared by measuring the photoluminescence spectra of select composites. We found that a nanostructured composite assembled from the 100 nm anatase and single-walled CNTs (SWCNTs) exhibited enhanced and selective photocatalytic oxidation of phenol in comparison to both pure anatase and Degussa P25. A mechanism for the enhanced reactivity is proposed in which electrons are shuttled from TiO2 particles to the SWCNTs as a result of an optimal TiO2/ CNT arrangement that stabilizes charge separation and reduces charge recombination. In addition, the SWCNT assembly provides better catalyst-support (dispersal and connection) than multi-walled CNTs.

Two-dimensional (2D) van der Waals (vdW) magnetic materials have recently been introduced as a new horizon in materials science and enable the potential applications for next-generation spintronic devices. Here, in this communication, the observations of stable Bloch-type magnetic skyrmions in single crystals of 2D vdW Fe3GeTe2 (FGT) are reported by using in-situ Lorentz transmission electron microscopy (TEM). We find the ground-state magnetic stripe domains in FGT transform into skyrmion bubbles when an external magnetic field is applied perpendicularly to the (001) thin plate with temperatures below the Curie-temperature TC. Most interestingly, a hexagonal lattice of skyrmion bubbles is obtained via field cooling manipulation with magnetic field applied along the [001] direction. Owing to their topological stability, the skyrmion bubble lattices are stable to large field-cooling tilted angles and further reproduced by utilizing the micromagnetic simulations. These observations directly demonstrate that the 2D vdW FGT possesses a rich variety of topological spin textures, being of a great promise candidate for future applications in the field of spintronics.KEYWORDS: magnetic skyrmions, van der Waals materials, Fe3GeTe2, Lorentz transmission electron microscopy 3 Two-dimensional (2D) van der Waals (vdW) materials are a family of quantum materials that have attracted great research attention in the past decade as they possess a diverse range of novel phenomena which are promising for technological applications. 1,2 In particular, the recent discovery of magnetic 2D vdW materials, such as Cr2Si2Te6/Cr2Ge2Te6, 3-5 CrI3/CrBr3, 6, 7 and Fe3GeTe2 (FGT), 8, 9 not only offers exciting opportunities for exploring new physical properties, but also opens up a new way for developing spintronic devices by applying magnetism as a possible altering parameter. 10 Among these materials, FGT is only ferromagnetic metal, in which a long-range ferromagnetic order has been confirmed experimentally ranging from bulk crystals down to monolayers. [11][12][13] Remarkably, bulk crystalline FGT has the highest Curie temperature TC (∼230 K) and the TC of layered FGT can be raised to room temperature via electrostatic gating 8,14 or in patterned microstructures. 13 Following this discovery, many intriguing magnetic and transport properties, such as extremely large anomalous Hall effect, 15 Planar topological Hall effect, 16 Kondo lattice physics, 17 anisotropy magnetostriction effect, 18 and spin filtered tunneling effect, 19 have been observed experimentally in exfoliated FGT nanoflakes and its heterostructures.Moreover, 2D vdW FGT exhibits a strong out-of-plane uniaxial magnetic anisotropy down to atomic-layer thicknesses, 8,9,14,20 which is very critical for spintronic applications, typically, magnetic-tunneling-junctions and magnetic randomaccess-memory devices. On the other hand, in a magnetic material, the competition between the uniaxial magnetic anisotropy and magnetic dipole-dipole interaction, can emerge and lead to a diversity of...

We utilize CdSe/CdS seeded nanorods as a tunable lumophore for luminescent concentration. Transfer-printed, ultrathin crystalline Si solar cells are embedded directly into the luminescent concentrator, allowing the study of luminescent concentrators with an area over 5000 times the area of the solar cell. By increasing the size of the CdS rod with respect to the luminescent CdSe seed, the reabsorption of propagating photons is dramatically reduced. At long luminescence propagation distances, this reduced reabsorption can overcome the diminished quantum yield inherent to the larger semiconductor structures, which is studied with lifetime spectroscopy. A Monte Carlo ray tracing model is developed to explain the performance of the luminescent concentrator and is then used as a design tool to determine the effect of luminescence trapping on the concentration of light using both CdSe/CdS nanorods and a model organic dye. We design an efficient luminescence trapping structure that should allow the luminescent concentrator based on CdSe/CdS nanorods to operate in the high-concentration regime.

Luminescent solar concentrators doped with CdSe/CdS quantum dots provide a potentially low-cost and high-performance alternative to costly highband-gap III−V semiconductor materials to serve as a top junction in multijunction photovoltaic devices for efficient utilization of blue photons. In this study, a photonic mirror was coupled with such a luminescent waveguide to form an optical cavity where emitted luminescence was trapped omnidirectionally. By mitigating escape cone and scattering losses, 82% of luminesced photons travel the length of the waveguide, creating a concentration ratio of 30.3 for blue photons in a waveguide with a geometric gain of 61. Further, we study the photon transport inside the luminescent waveguide, showing unimpeded photon collection across the entire length of the waveguide. L uminescent solar concentrators 1−4 (LSCs) have been studied extensively for the last three decades as low-cost alternatives to single-and multijunction photovoltaic (PV) devices. As silicon prices have fallen, it has become increasingly clear that future solar panels will need to have both low cost and high efficiency. One promising strategy for achieving a higher efficiency is to use different parts of the solar spectrum in photovoltaic materials with varying band gaps to minimize losses associated with carrier thermalization and incomplete photon absorption. For these multijunction (MJ) PV devices, there is a strong need for developing low-cost, high-band-gap solar cells for efficient utilization of the high-energy part of the solar spectrum. A luminescent solar concentrator could provide exactly this function, serving as the top junction in a multijunction architecture by converting blue photons into guided luminescence. Due to the concentration effect, only small amounts of high-performing but expensive III−V photovoltaic materials are needed to collect the light from an inexpensive luminescent waveguide. Such a device requires high concentration factors to reduce the cost of the III−V photovoltaic material. High concentration also allows the Stokes shift of the lumophore to be recovered in the operating voltage of the photovoltaic cell.The concentration factor and collection efficiency achieved by LSCs to date have been limited due to parasitic losses such as nonunity quantum yields of the lumophores, imperfect light trapping within the waveguide, and reabsorption and scattering of propagating photons. 5 Previous studies have sought to solve each of these parasitic losses individually, resulting in modest performance improvements. 6−15 Here we achieve a luminescent concentration ratio greater than 30 with an optical efficiency of 82% for blue photons by simultaneously addressing the materials and optical challenges of the LSC system. These concentration ratios are achieved through the combination of designer quantum dot lumophores and photonic mirrors, and microscale silicon photovoltaic cells are used to detect the concentration of light in the waveguide. To the best of our knowledge, this is the highest ...

First-principles calculations are applied to study spin Hall effect in semiconductors and simple metals. We found that intrinsic spin Hall conductivity (ISHC) in realistic materials shows rich sign changes, which may be used to distinguish the effect from the extrinsic one. The calculated ISHC in n-doped GaAs can be well compared with experiment, and it differs from the sign obtained from the extrinsic effect. On the other hand, the ISHC in W and Au, which shows opposite sign, respectively, is robust and not sensitive to the disorder.

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Resistive switching processes in HfO are studied by electron holography and in situ energy-filtered imaging. The results show that oxygen vacancies are gradually generated in the oxide layer under ramped electrical bias, and finally form several conductive channels connecting the two electrodes. It also shows that the switching process occurs at the top interface of the hafnia layer.

For decades, progress in the field of optical (including solar) energy conversion was dominated by advances in the conventional concentrating optics and materials design. In recent years, however, conceptual and technological breakthroughs in the fields of nanophotonics and plasmonics combined with better understanding of the thermodynamics of the photon energy conversion processes re-shaped the landscape of energy conversion schemes and devices. Nanostructured devices and materials that make use of size quantization effects to manipulate photon density of states offer a way to overcome the conventional light absorption limits. Novel optical spectrum splitting and photon recycling schemes reduce the entropy production in the optical energy conversion platforms and boost their efficiencies. Optical design concepts are rapidly expanding into the infrared energy band, offering new approaches to harvest waste heat, reduce the thermal emission losses, and achieve non-contact radiative cooling of solar cells as well as of optical and electronic circuitry. Light-matter interaction enabled by nanophotonics and plasmonics underlie the performance of the third-and fourth-generation energy conversion devices, including up-and downconversion of photon energy, near-field radiative energy transfer, and hot electron generation and harvesting. Finally, the increased market penetration of alternative solar energy conversion technologies amplifies the role of cost-driven and environmental considerations.This roadmap on optical energy conversion provides a snapshot of the state-of-the art in optical energy conversion, remaining challenges, and most promising approaches to address these challenges. Leading experts authored 19 focused short sections of the roadmap, where they share their vision on a specific aspect of this burgeoning research field. The roadmap opens up with a tutorial section, which introduces major concepts and terminology. It is our hope that the roadmap will serve as an important resource for the scientific community, new generations of researchers, funding agencies, industry experts and investors.