Two-dimensional membranes: Proton permeability

Nature 516, 227–230 (2014)

Owing to its atomic thickness, mechanical strength and chemical stability, graphene is of interest in the development of separation membranes. A pristine sheet of graphene is thought to be impermeable to all atoms and molecules, but by creating subnanometre-sized pores in the material, it could act as a filter, allowing molecules smaller than the pores to pass while impeding the progress of larger species. Marcelo Lozada-Hidalgo, Heng-An Wu and colleagues have now shown that protons can pass through pristine monolayers of graphene and, another two-dimensional material, hexagonal boron nitride, a result that suggests that the materials could be of use in the development of improved proton exchange membranes for fuel cells.


The researchers — who are based at the University of Manchester, the University of Science and Technology of China, and the University of Nijmegen — measured the transport of protons through a range of different two-dimensional crystals that were sandwiched between layers of Nafion, a polymer that can conduct protons in the presence of water. Significant proton currents were detected across both graphene and hexagonal boron nitride, but not molybdenum disulphide. Bilayers and trilayers of hexagonal boron nitride were also found to conduct protons, but bilayers of graphene did not. All of these observations could be rationalized by considering the electron density distributions of the different two-dimensional materials. Furthermore, by decorating the graphene and hexagonal boron nitride membranes with platinum nanoparticles — a catalyst often used in fuel cells — the proton conductivity could be considerably enhanced. OV

Thermal management: Staying warm with nanowires

Nano Lett. (2014)

Heating up buildings consumes nearly half of global energy and contributes significantly to the global energy crisis. To reduce indoor heating, most efforts have focused on installing better insulation in buildings, rather than managing heat loss from people. Yi Cui and colleagues at Stanford University have now shown that silver nanowire-coated cloths can reduce body heat loss and/or raise body temperatures when worn.

The researchers dipped a piece of cotton textile into a dispersion of silver nanowires, which created a porous and conducting network of nanowires on the cloth. Simulations and experiments showed that, due to the approximately 300-nm spacing between the nanowires, the coated cloth effectively reflected most of the radiation from the human body (which peaks at approximately 9 μm in wavelength), thereby allowing most of the heat to be trapped around the skin. Moreover, at this nanowire spacing, water vapour from perspiration can easily escape, making the material comfortable to wear. Further experiments showed that the silver nanowire coating offered 21% more thermal insulation than uncoated cloth. When a voltage was applied, the coated cloth could also actively warm up through Joule heating.

Cui and colleagues estimate that approximately 355 watts per person can be saved if silver-coated clothes are worn, suggesting that it could be a promising way to reduce the power demands of heating our homes. ALC

Friction: Simulating water slippage

Nano Lett. 14, 6872–6877 (2014)

Friction occurs when two surfaces slide against each other and is a phenomenon that can be observed at both small and large scales. The origin of friction is, however, microscopic and ultimately depends on the interaction between the atoms in the two surfaces. Understanding friction of water on different surfaces is important for the development of a range of applications, including water desalination and purification. Experiments are typically incapable of providing the required atomistic level information, and for this reason, simulations are often used. Angelos Michaelides and colleagues at University College London and the Université de Lyon now report using ab initio molecular dynamics simulations to study the friction of a water layer on graphene and on boron nitride, and find that the friction coefficient on boron nitride is three time larger than on graphene.

This observation is at first sight surprising because the structure of the water layers simulated on the two surfaces is very similar. A closer inspection, however, reveals that the surface energy is much more corrugated with boron nitride. In particular, the researchers calculated the difference in energy between the most stable position of a water molecule on the atomic structure of the two surfaces and its most unstable position. They found that with boron nitride the difference is 2.6 times larger than with graphene, which is comparable to the ratio between friction coefficients. FP

Silicon photonics: Berry phase rotates light

Nature Commun. 5, 5337 (2014)

The electrical control of light polarization in hybrid electronic–photonic systems is desirable because it could allow a dynamic rotation of the polarization. However, achieving this control in an on-chip, planar structure is challenging. Ronald Reano and colleagues at the Ohio State University have now shown that the rotation of light polarization can be dynamically tuned by electrical means in a silicon-on-insulator photonic chip.

The rotation is achieved by leveraging on the Berry phase, which is a phase of geometrical origin that can be acquired by light that is travelling along a closed path in momentum space. To attain a non-zero Berry phase in momentum space, the light is made to travel into a three-dimensional waveguide that has out-of-plane sections. The angle of deflection in real space of the out-of-plane portions determines the Berry phase and, in turn, the angle of rotation of light polarization. The researchers fabricate the out-of-plane waveguide in a silicon–silicon dioxide chip; the dynamic tuning of polarization rotation is enabled by electrically controlling a microheater that regulates the coupling of light to the waveguide. An almost-full rotation of polarization, from transverse electric to transverse magnetic mode, is demonstrated as a function of the electrical power through the heater. ED

Written by Ai Lin Chun, Elisa De Ranieri, Fabio Pulizzi and Owain Vaughan.