Articles in 2013

By applying temperature gradient on the temperature-responsive wettablitity surface of block copolymer, water droplet movement by the nonmechanical origin forces is realized on the surface with high hysteresis, due to the binary collaboration effect of wettability gradient force and Marangoni force. The results help to understand the motion of droplets on the surface with high hysteresis and also provide potential application in microfluidic devices.

A simple and universal DNA-based platform is developed to implement the required two logic gates of a half adder (or a half subtractor) in parallel triggered by the same set of inputs. The developed half adder and half subtractor are operated with the same DNA platform in an enzyme-free system. The investigations provide a new way for the prototypical DNA-based arithmetic operations and also the development of advanced circuits.

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.

Upconversion nanophosphors have unique ability to generate anti-Stokes luminescence. The rapid developments of nanotechnology in recent years have demonstrated the successful synthesis and optimization of such upconversion nanophosphors. This review summarizes the recent advances using upconversion nanophosphors as bioprobe for in vivo applications.

In this work, transformation optics technique was applied to the thermodynamic area by using the coordinate transformation to the time-dependent heat diffusion equation, which enables the manipulation of the heat flux by predefined diffusion paths. A transient thermal cloaking device engineered with effective thermal materials was experimentally demonstrated by hiding a centimeter-sized vacuum cavity. To facilitate the fabrication a rescaled heat equation taking into account of all the pertinent parameters of various ingredient materials was proposed to guide the practical design of complex transformed thermal devices.

Seven years ago, Wilma Eerenstein, Neil Mathur and Jim Scott published a Venn diagram (above) showing the overlap of piezoelectricity, ferroelectricity (green circle), ferromagnetism (black circle), and magnetoelectricity (blue hatched center circle); and soon thereafter Manuel Bibes put into each sector the crystals which were thought to belong. The overlap region between green and black are multiferroic ferromagnetic-ferroelectrics. Not all were correct; BiMnO₃ is NOT ferroelectric. Of these materials, only Cr₂O₃ and BiFeO₃ function at room temperature (or are magnetoelectric, and the former is neither a ferromagnet nor ferroelectric). Today’s review is an update on the new multiferroic and/or magnetoelectric materials that function at or near ambient temperatures and pressures: Cupric oxide (not actually magnetoelectric), iron magnesium hexaferrites, and perovskite oxides based upon PbTiO₃.

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.

β-MnO₂ nanorods with exposed tunnel structures have been successfully synthesized by a hydrothermal method. The as-prepared β-MnO₂ nanorods have exposed {111} crystal planes with a high density of (1 × 1) tunnels, leading to facile Na-ion insertion and extraction. When applied as cathode materials in Na-ion batteries, the β-MnO₂ nanorods exhibited a superior electrochemical performance with a high initial Na-ion storage capacity of 350 mAh g⁻¹.β-MnO₂ nanorods also demonstrated an excellent high rate capability and a good cyclability.

High-Performance Polypyrrole Functionalized PtPd Electrocatalysts.

Metal chalcogenide quantum dots (QDs) and lead halide perovskites are two types of prospective light harvesters for mesoscopic solar cells. The two most promising QD sensitizers are PbS and Sb₂S₃, for which the PCEs of their corresponding QDSCs have attained ∼7% in 2012. In 2013, the performance of a TiO₂ solar cell sensitized with lead-iodide perovskite (CH₃NH₃PbI₃) was optimized to attain an overall power conversion efficiency of 15%, which is a new milestone for solar cells of this type, with a device structure similar to that of a dye-sensitized solar cell.

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.

We demonstrate a LEGO-like assembly of free-standing film of individually operable components encapsulated in a polymer overcoat layer, leading to the integrated architecture without additional electrical connection. The free-standing components are produced by the peeling-off process. The sticky nature of polymer layer enables the construction of supercapacitor arrays and simple RLC circuits through interlocking the individual components.

Demonstration of a tunable conductivity of the LaAlO₃/SrTiO₃ interfaces by field effect drew significant attention to the development oxide-based electronics. Increase in the gate capacitance of LaAlO₃/SrTiO₃-based field-effect transistor is particularly important to the conductivity modulation and the development of logic device. Here, we demonstrate that annihilation of quantum capacitance and colossal capacitance enhancement (about 1000%) in the LaAlO₃/SrTiO₃ heterostructures by DC gate voltage at room temperature, which we attribute to the motion of oxygen vacancies through the LaAlO₃ layer thickness. The capacitor devices would provide a platform for the further development and application of metal-oxide-semiconductor transistor devices.

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.

Soft Material Helps Cancer-Cell Capture: Soft polystyrene (PS) polymer nanotube substrate has been developed to realize rapid and highly efficient breast cancer-cell capture from whole-blood samples with high cell viability, by integrating the soft nature of PS with specific capture agents and surface structures to mimic the cell microenvironment. It is anticipated that this soft material-based substrate will provide a potentially optimal candidate for high-quality breast cancer-cell capture and detection technologies, and open a new view to design cell-material interfaces for advanced cell-based diagnostic and therapeutic platforms.

A universal process for transferring planar, transparent functional oxide thin films on to elastomeric polydimethylsiloxane (PDMS) substrates is demonstrated. This overcomes the challenge of incorporating high-temperature-processed crystalline oxide materials with low-temperature organic substrates. The process is demonstrated using indium tin oxide (ITO) thin films to realise fully transparent and flexible resistors and zinc oxide thin films. The ITO thin films on PDMS are shown to withstand uniaxial strains of 15%, enabled by microstructure tectonics. This ubiquitous process will pave the way for touch sensing and energy harvesting for displays and electronics with flexible and transparent characteristics.

Plasmonic photoelectric conversion from visible to near-infrared wavelengths has been successfully demonstrated using electrodes in which gold nanorods are elaborately arrayed on a titanium dioxide (TiO₂) single crystal. Importantly, water molecules serve as the electron donor in the plasmonic photoelectric conversion system. Therefore, the stoichiometric evolution of oxygen and hydrogen peroxide as a result of the four- or two-electron oxidation of water molecules was accomplished even at near-infrared wavelengths, enabling the application of this plasmonic photoelectric system in artificial photosynthesis processes responding to a wide range of solar wavelengths.

Graphene quantum dots (GQDs) derived from double-walled carbon nanotubes with strong emission were prepared through solution chemistry. The introduction of GQDs in a bulk heterojunction polymer solar cell based on Poly (3-hexylthiophene):(6,6)-phenyl-C61 butyric acid methyl ester (P3HT:PCBM) resulted in the significant enhancement of power conversion efficiency (PCE). The efficiency can be further improved by adjusting the PCBM content in the active layer, with the highest PCE of 5.24%.

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.