News & Comment

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

Photodetectors are ubiquitous, found in everyday products such as TV remotes, disc players and digital cameras as well as specialized devices for fiber optic communications and astronomical observations. Similarly, graphene has seen a quick emergence in a range of prototypic devices due to its attractive electronic, optical and mechanical properties.¹ Using graphene in photodetection utilizes its high carrier mobility and zero bandgap that—among other advantages—show promise for wide-spectrum, high-speed, low-cost and flexible photosensors. Graphene-based photodetectors have typically relied on Schottky barriers formed near the metal–graphene contacts, where a built-in potential drives the separation and transport of photogenerated electron–hole pairs. However, symmetric metal–graphene–metal devices generate an equal positive and negative flow with a net zero photocurrent (Figure 1a). Using metals with asymmetric band structures breaks this equilibrium² (Figure 1b) at the cost of additional fabrication steps and is limited by the maximum difference in barrier heights. Recently, Liu and co-workers³ from Peking University presented a step forward by creating a single p–n junction in graphene itself.

The laser is arguably the most important and versatile optical device. Invented just over 50 years ago,¹ the laser has found immense number of uses from fundamental science and ultra-precision metrology to diverse applications in telecommunications, entertainment, computers, displays, biomedicine, materials processing, defense and homeland security and so on. These are based on fundamental property of the laser to generate coherent light that can be focused to microscopic areas or concentrated in pulses as short as 100 as (1 as =10⁻¹⁸ s). Still, quest for new lasers continues, in particular, to design the smallest and thinnest lasers possible. This is important in many respects, in particular, because such lasers can be directly modulated with a very high frequency. One way to achieve this goal is provided by invention of the spaser (Surface Plasmon Amplification Stimulated Emission of Radiation),² also called plasmonic laser, about 10 years ago. Replacing light quanta—photons—of the laser with electronic excitations at the surface of metals called surface plasmons, which can have atomic-scale dimensions, the spaser itself can be as small or as thin as the dimension of only hundreds of atoms.

Although materials having both excellent optical transparency and electrical conductivity at first seem counterintuitive, such substances are essential to a myriad of modern technologies. Applications for transparent conductors include electrodes for LCD, OLED and other displays, touch screens, electromagnetic interference shielding, transparent heaters (for example, automotive windshields), and photovoltaic cells.¹ For many applications, mechanical flexibility, as on plastic substrates, is also desirable for versatile form factors, impact resistance, roll-to-roll manufacture, product functionality and light weight. For many applications, the oxides of heavy post-transition metals, such as tin-doped indium oxide (ITO) or, to a lesser extent, related oxides, have traditionally served this purpose.¹ However, the cost of ITO is sensitive to fluctuating indium prices, electrical conductivity is not optimum, ITO is corroded in many environments, polycrystalline ITO coatings on plastic are brittle and less conductive, and ITO films are grown by capital-intensive sputtering processes.¹ A key issue in vapor-phase coating processes is the percentage of material actually transferred to the substrate, and for ITO this process has been heavily optimized for high yields.

In the early-to-mid 1800s, Charles Goodyear¹ discovered that natural rubbers became more robust when heated in the presence of a small amount of sulfur. We now know that this ‘vulcanization’ process stems from sulfur’s ability to form chemical bridges that strengthen the polymer chains in the rubber and is routinely used to produce tires among other common goods. An international team from the University of Arizona, Seoul National University, the University of Hamburg, and the University of Delaware developed what they refer to as ‘inverse vulcanization’.² In this process, sulfur is used as the primary ingredient and reinforced with a stryenic additive. The method involves adding 1,3-diisopropenylbenzene (DIB), which effectively intercepted radicals formed at elevated temperatures and afforded useful copolymeric materials enriched with sulfur. The real trick, however, was timing: adding the DIB after the sulfur was liquefied and in its reactive form appeared to be critical. The ratio of DIB-to-sulfur was also an important factor as the physical, electronic and optical properties displayed by the composites were dependent on their elemental compositions.

Multiple oil spill disasters over the last few years have highlighted the challenges of effective oil–water separation. The separation of oil–water micro- and nano-emulsions (emulsions with dispersed droplet sizes in the micro- or nano-meter range) can be particularly difficult.^{1, 2} Shi et al.³ from the Chinese Academy of Sciences in Suzhou and Beijing have now developed ultrathin carbon nanotube membranes that can separate a wide range of oil–water micro- and nano-emulsions with separation efficiency >99.9%. Perhaps more significantly, the separation fluxes are 2–3 orders of magnitude higher than those obtained with current commercially available separation membranes.

A storage mechanism for entangled photons with narrow wavelength distribution is another major step toward memory devices for quantum computers.

Thin, strong microcrystals containing photosensitive double bonds can flex or straighten on light exposure.

Electrochemical co-deposition of two metals followed by the selective etching of one allows for the synthesis of nanospring structures.

Competitive binding between blood proteins can reduce the toxicity of protein-coated carbon nanotubes toward cells.

Similar proteins demonstrate different charge transport characteristics offering a route to biological electronic devices.

Computer simulations predict new carbon materials that are stiffer than carbon nanotubes and tougher than diamond.

A combination of thermal and mechanical stimuli causes a single luminescent liquid-crystalline material to emit three different colors.

Magnetoresistance in silicon can be enhanced to match that of commercial devices by designing appropriate device geometries.

The combination of two types of resistive material can eliminate unwanted current paths and improve the performance of resistive memory.

Devices consisting of single metal-bound DNA strands bridged between electrodes pave the way for new biocompatible electronics.

Calculations suggest that a combination of graphene nanoribbon structures could be used to create a spin valve for spintronics applications.

Domains with different friction characteristics are observed for the first time in exfoliated graphene.