Here we demonstrate a more effective use of III–V photoconversion material to achieve an ultrahigh power-per-weight ratio from a solar cell utilizing an axial p-i-n junction GaAs/AlGaAs nanowire (NW) array grown by molecular beam epitaxy on a Si substrate. By analyzing single NW multicontact devices, we first show that an n-GaAs shell is self-formed radially outside the axial p- and i-core of the GaAs NW during n-core growth, which significantly deteriorates the rectification property of the NWs in the axial direction. When employing a selective-area ex situ etching process for the n-GaAs shell, a clear rectification of the axial NW p-i-n junction with a high on/off ratio was revealed. Such a controlled etching process of the self-formed n-GaAs shell was further introduced to fabricate axial p-i-n junction GaAs NW array solar cells. Employing this method, a GaAs NW array solar cell with only ∼1.3% areal coverage of the NWs shows a photoconversion efficiency of ∼7.7% under 1 Sun intensity (AM 1.5G), which is the highest achieved efficiency from any single junction GaAs NW solar cell grown on a Si substrate so far. This corresponds to a power-per-weight ratio of the active III–V photoconversion material as high as 560 W/g, showing great promise for high-efficiency and low-cost III–V NW solar cells and III–V NW/Si tandem solar cells.
Lithium is widely used in contemporary energy applications, but its isolation from natural reserves is plagued by time-consuming and costly processes. While polymer membranes could, in principle, circumvent these challenges by efficiently extracting lithium from aqueous solutions, they usually exhibit poor ion-specific selectivity. Toward this end, we have incorporated host–guest interactions into a tunable polynorbornene network by copolymerizing 1) 12-crown-4 ligands to impart ion selectivity, 2) poly(ethylene oxide) side chains to control water content, and 3) a crosslinker to form robust solids at room temperature. Single salt transport measurements indicate these materials exhibit unprecedented reverse permeability selectivity (∼2.3) for LiCl over NaCl—the highest documented to date for a dense, water-swollen polymer. As demonstrated by molecular dynamics simulations, this behavior originates from the ability of 12-crown-4 to bind Na+ ions more strongly than Li+ in an aqueous environment, which reduces Na+ mobility (relative to Li+) and offsets the increase in Na+ solubility due to binding with crown ethers. Under mixed salt conditions, 12-crown-4 functionalized membranes showed identical solubility selectivity relative to single salt conditions; however, the permeability and diffusivity selectivity of LiCl over NaCl decreased, presumably due to flux coupling. These results reveal insights for designing advanced membranes with solute-specific selectivity by utilizing host–guest interactions.
Catalysts are central to accelerating chemistry in biology and technology. In biochemistry, the relationship between the velocity of an enzymatic reaction and the concentration of chemical substrates is described via the Michaelis-Menten model. The modeling and benchmarking of synthetic molecular electrocatalysts are also well developed. However, such efforts have not been as rigorously extended to photoelectrosynthetic reactions, where, in addition to chemical substrates and charge carriers, light is a required reagent. In this perspective, we draw parallels between concepts involving enzyme catalytic efficiency, the benchmarking of molecular electrocatalysts, and the performance of photoelectrosynthetic assemblies, while highlighting key differences, assumptions, and limitations.
As an inexpensive monolayer archetypal member of the carbon family, graphene has triggered a new ‘gold rush’ in nanotechnology for achieving unique properties that were not available in many traditional materials. Owing to these unique features, graphene-related materials are finding new uses in nanomedicine and synthetic biology in addition to their diverse applications in electronics, optoelectronics, photonics and environmental clean-up. The increased production of graphene nanostructures and increased likelihood of exposures to these substances in environmental and occupational settings has raised concerns about adverse health outcomes. In particular, the biological effects of these materials need to be assessed to ensure risk free, sustainable development of graphene for widespread applications. In this work, for the first time, we studied the in vitro and in vivo interactions of a relatively new derivative of graphene, graphene nanopores (GNPs) in mammalian systems, to systematically elucidate the possible mechanism of their toxicity over time. This study showed that GNPs induced early apoptosis in both SKMES-1 and A549 lung cancer cells. However, late apoptosis is only induced at concentrations higher than 250 μg/ml, suggesting that, although GNPs at lower concentrations induce upregulation of phosphatidylserine on the cell surface membrane (i.e. early apoptotic event), GNPs do not significantly disintegrate the cell membrane. We also showed that rats intraperitoneally injected with GNPs suffered sub-chronic toxicity in a period of 27 days when tested at single and multiple doses of GNPs (5 and 15 mg/kg) as evidenced by blood biochemistry, organo-somatic index, liver and kidney enzymes functions analysis, oxidative stress biomarkers and histological examinations. In sum, our results show that GNPs are likely to have a low bioavailability in SKMES-1 and A549 lung cancer cells and rats. Nevertheless, this must be considered against the context of a wider lack of knowledge regarding the bioavailability, fate and behaviour of this type of new porous framework of graphene in natural systems. Therefore, a more long-term GNPs exposure regime, more relevant to real-life environmental consequences, is needed to fully determine the transport capacities of GNPs in living systems.
Recently, many studies have been conducted on the use of glassy carbon (GC) in advanced technological applications due to its excellent chemical, mechanical, electrical, and thermal properties, making possible fast advances in biomedical, pharmaceutical, electronic, and energy sectors. In this way, this review article reports the latest advances (2017–2021) in the scientific research on the use of GC in diverse technological applications, including scaffolds for tissue engineering, electrochemical sensors for molecular determination, energy storage systems, electrochemical devices for wastewater treatment, tools for precision molding, encapsulation of nuclear waste, and antistatic agent for antistatic packaging. A brief analysis of the number of published articles on the topic is presented, showing how the use of GC has evolved over the years in different technological sectors. The structure, the properties, the production of GC, and the promising areas for further research are also addressed, providing helpful information to foment scientific advances on the use of GC.
One of the recently established paradigms in condensed matter physics is examining a system’s behaviour in artificial potentials, giving insight into phenomena of quantum fluids in hard-to-reach settings. A prominent example is the matter-wave scatterer lattice, where high energy matter waves undergo transmission and reflection through narrow width barriers leading to stringent phase matching conditions with lattice band formation. In contrast to evanescently coupled lattice sites, the realisation of a scatterer lattice for macroscopic matter-wave fluids has remained elusive. Here, we implement a system of exciton-polariton condensates in a non-Hermitian Lieb lattice of scatterer potentials. By fine tuning the lattice parameters, we reveal a nonequilibrium phase transition between distinct regimes of polariton condensation: a scatterer lattice of gain guided polaritons condensing on the lattice potential maxima, and trapped polaritons condensing in the potential minima. Our results pave the way towards unexplored physics of non-Hermitian fluids in non-stationary mixtures of confined and freely expanding waves.
In contrast to light, matter-wave optics of quantum gases deals with interactions even in free space and for ensembles comprising millions of atoms. We exploit these interactions in a quantum degenerate gas as an adjustable lens for coherent atom optics. By combining an interaction-driven quadrupole-mode excitation of a Bose-Einstein condensate (BEC) with a magnetic lens, we form a time-domain matter-wave lens system. The focus is tuned by the strength of the lensing potential and the oscillatory phase of the quadrupole mode. By placing the focus at infinity, we lower the total internal kinetic energy of a BEC comprising 101(37) thousand atoms in three dimensions to 3/2 kB·38−7+6 pK. Our method paves the way for free-fall experiments lasting ten or more seconds as envisioned for tests of fundamental physics and high-precision BEC interferometry, as well as opens up a new kinetic energy regime.
The interplay between strong electron–electron interactions and band topology can produce electronic states that spontaneously break symmetries. The discovery of flat bands in magic-angle twisted bilayer graphene (MATBG)1–3 with non-trivial topology4–7 has provided a compelling platform in which to search for new symmetry-broken phases. Recent scanning tunnelling microscopy8,9 and transport experiments10–13 have revealed a sequence of topological insulating phases in MATBG near integer filling of the electronic bands produced by the moiré pattern. These correspond to a simple pattern of flavour-symmetry-breaking Chern insulators that fill bands of different flavours one after the other. Here we report the high-resolution local compressibility measurements of MATBG with a scanning single-electron transistor, which reveal an additional sequence of incompressible states with unexpected Chern numbers observed down to zero magnetic field. We find that the Chern numbers for eight of the observed incompressible states are incompatible with the simple picture in which the bands are sequentially filled. We show that the emergence of these unusual incompressible phases can be understood as a consequence of broken translation symmetry that doubles the moiré unit cell and splits each flavour band in two. Our findings expand the known phase diagram of MATBG, and shed light on the origin of the close competition between different correlated phases in the system.
Voltage control of magnetic order is desirable for spintronic device applications, but 180° magnetization switching is not straightforward because electric fields do not break time-reversal symmetry. Ferrimagnets are promising candidates for 180° switching owing to a multi-sublattice configuration with opposing magnetic moments of different magnitudes. In this study we used solid-state hydrogen gating to control the ferrimagnetic order in rare earth–transition metal thin films dynamically. Electric field-induced hydrogen loading/unloading in GdCo can shift the magnetic compensation temperature by more than 100 K, which enables control of the dominant magnetic sublattice. X-ray magnetic circular dichroism measurements and ab initio calculations indicate that the magnetization control originates from the weakening of antiferromagnetic exchange coupling that reduces the magnetization of Gd more than that of Co upon hydrogenation. We observed reversible, gate voltage-induced net magnetization switching and full 180° Néel vector reversal in the absence of external magnetic fields. Furthermore, we generated ferrimagnetic spin textures, such as chiral domain walls and skyrmions, in racetrack devices through hydrogen gating. With gating times as short as 50 μs and endurance of more than 10,000 cycles, our method provides a powerful means to tune ferrimagnetic spin textures and dynamics, with broad applicability in the rapidly emerging field of ferrimagnetic spintronics.