News & Comment

Recently, a new class of artificial materials known as metamaterials has emerged to manipulate light at the nanoscale. Contrary to natural materials, the physical properties of optical metamaterials are primarily dependent on the material structures rather than their chemical constituents. The structures forming the building blocks of the metamaterials are much smaller than the wavelength of light. By tailoring both effective electric permittivity and the magnetic permeability of metamaterials from a positive to a negative value, one can create negative refractive index metamaterials (NIMs).¹ Various 3D NIM structures have been previously reported,² but they are limited to small-scale samples of less than 1 mm. The unprecedented properties of metamaterials and their potential revolutionary applications, such as superlens imaging,³ remain a challenge to implement in practice due to the lack of innovative large-scale manufacturing methods. As a result, a scalable scheme that enables the large-area fabrication of 3D nanostructures must be developed.

As advancements are made in wearable technology, including the development of portable devices that can function when stretched or bent, so too must the ways by which these devices generate and store power.¹ For true portability of flexible smartphones, interactive bracelets and smart textiles for example, higher energy-density batteries need to be part of the ‘fabric’ of the device. Researchers from Fudan University in Shanghai demonstrated that a wire-shaped lithium-ion battery can be fabricated from a composite pairing of aligned yarns of active materials, sheathed in a heat-shrinkable tube.²

In crystalline solids, it is often the case that the Fermi surface consists of multiple pockets at well-separated degenerate band extrema (that is, valleys) in momentum space. The valley index constitutes a discrete degree of freedom of carrier, just like spin. Exploiting valley in addition to spin will make future electronics more versatile. Two-dimensional (2D) transition metal dichalcogenides, a new class of direct-gap semiconductors,^{1, 2} have provided an appealing laboratory to explore valley-electronics, because of the discovery of a valley optical selection rule that allows optical control and detection of valley polarization.³ Iwasa from University of Tokyo, and Riken and his team have now demonstrated in 2D WSe₂ the first electric control of valley-dependent optical emission.⁴

Materials scientists have been in an age of discovery, as two researchers in IBM Zurich Research Laboratory discovered high-temperature superconductivity in a copper oxide in 1986.¹ New materials including superconductors have been synthesized one after another, which have given renewed vigor and enthusiasm to the community. As a current trend, different functions are merged into a single material to generate novel and unique properties. A prime example is multiferroics where magnetism and ferroelectricity mutually influence.²

A breakthrough in semiconductor nanowire synthesis that allows fine control over axial heterostructuring was recently advanced by Hollingsworth and co-workers.¹ The report also reveals fascinating mechanistic aspects of catalyzed nanowire growth.

Catalyzed wire or whisker growth was discovered by Wagner and Ellis in 1964.² They found that gold droplets on a silicon substrate catalyze silicon wire growth under chemical-vapor-deposition conditions. Gaseous precursors react at the gold-droplet surfaces, depositing silicon into solution within the gold droplets. The droplets become supersaturated, inducing precipitation of crystalline silicon upon the substrate. As precipitation occurs only at the droplet–silicon interfaces, the silicon crystallites acquire pseudo-cylindrical wire morphologies as they grow upward from the substrate. The gold-catalyst droplets rise elevator-like from the substrate, riding upon the tips of the growing wires. Wagner and Ellis² named this method ‘vapor-liquid-solid’ or ‘VLS’ growth after the three participating phases: the vaporous precursors, liquid catalyst droplets and solid silicon wires.

Lui et al.¹ report new ‘perovskite-based’ solar cells having a photon-to-electron-conversion-efficiency (PCE) of a remarkable 15.4%. Traditional photovoltaic technology is dominated by c-Si, with inorganic thin film entrants CdTe, CIS and GIGS making substantial market inroads. Pipeline technologies such as dye-sensitized solar cells (DSSCs) and organic solar cells (OSCs)² promise ultra-low manufacturing costs as well as light-weight, flexible modules. As yet, the ‘pipeline’ is yet to deliver commercial product with long-term stability and module-scale PCEs being hurdles. The perovskite technology emerged from DSSCs as p-type materials, and then as combinatorial replacements for the dye and hole-transport components.³