Articles in 2009

Surfaces with physicochemical properties that can be modulated using external stimuli offer great promise for designing responsive or adaptive materials. Now, biocompatible dynamic scaffolds based on thin hydrogel coatings that reversibly hide and display surface chemical patterns in response to temperature changes have been fabricated.

Electrostatic control of spin polarization is a promising route for developing efficient spintronic devices, but is challenging for materials with a small spin–orbit interaction. It is now shown that an electric field can be used to vary the spin polarization in a silicon quantum well by exploiting the discrete nature of the energy levels. This route may work for other inorganic and organic materials.

Surface-enhanced Raman scattering has been widely used for chemical sensing, even though the large nonlinearity of the effect makes reproducible sensing difficult. A DNA-based assembly technique now offers a means of precise engineering of gap distances in nanoparticle dumbbells for a robust surface-enhanced Raman sensing of DNA and RNA molecules.

Demagnetization in metals occurs on very different timescales depending on the material. It is now shown that electron–phonon-mediated spin scattering describes the process of demagnetization well in every case, and the differences in timescale are mainly determined by the ratio between Curie temperature and the atomic magnetic moment.

An important challenge in medicine is the efficient delivery of drugs in the body using non-toxic nanocarriers. Porous metal–organic frameworks with imaging properties are now used as nanoscale carriers for the controlled delivery of antitumour and retroviral drugs against cancer and AIDS.

Chiral nematic liquid-crystal phases consist of rod-shaped molecules that have a preference to twist. However, applied fields force them to exist without the twist. Introducing particle-like twists, so called torons, using laser light relieves this frustration by facilitating the reappearance of the twist. The presence of torons could extend the use of liquid crystals in electro-optic and photonic devices.

The morphology and structure of polymer blends is central to charge-carrier, exciton and photon management in organic light-emitting diodes, transistors and solar cells. A broadly applicable approach, based on mixing a photocrosslinkable moiety into semiconducting polymers, enables the simple formation of heterostructured blends with control of morphology and structure for use in all types of device.

Metal nanoparticles with controlled composition and size are attractive candidates for heterogeneous catalysis. Now, a distillation-like process is shown to result in the synthesis of bimetallic PtRh nanoparticles with precise compositional control and high temperature stability.

Lithium-ion batteries have contributed to the commercial success of portable electronics, and should affect higher-volume applications such as plug-in hybrid electric vehicles. A fluorosulphate insertion positive electrode showing promising electrochemical performance is now reported.

Biocompatible, lithographically defined, ferromagnetic microdiscs that have a spin-vortex ground state oscillate when activated by an alternating magnetic field. This oscillation compromises the integrity of the cell membrane and initiates programmed cell death in ∼90% of cancer cells in vitro, even with a low-frequency field applied for only ten minutes.

The structure of magnetic nanoparticles has a strong influence on the properties of these materials at present being considered for magnetic-storage applications. It is now shown that size and shape of magnetic nanoparticles such as CoPt affect the transition from an ordered to a disordered phase, highlighting the need to take morphology into account to understand the structural properties.

Synthesizing magnetic nanostructures, which could potentially be used in spintronic applications, is quite challenging owing to the difficulty in incorporating magnetic impurities in a non-magnetic matrix. It is now shown that up to 10% Mn can be incorporated in CdSe nanoribbons by nucleation-controlled doping, giving rise to very strong magnetic effects.

Direct in situ high-resolution X-ray radiography and tomography observations now reveal instability and metastability domains in cellular solidification of colloidal suspensions and the transition to the stable phase. These results provide important insight into the study of morphological instabilities and could prove significant in the design of various types of nanostructure.

When artificial polypeptides are conjugated to a variety of hydrophobic molecules such as chemotherapeutics, the resulting molecules spontaneously self-assemble into nanoparticles. Delivering the chemotherapeutics to a murine cancer model, the nanoparticles have a fourfold higher maximum tolerated dose than the free drug, and induce nearly complete tumour regression after a single dose.

Grain boundaries are already known to have a large effect on the charge-carrier mobility of molecular semiconductors. Several experimental and computational techniques now show that the orientation of grain boundaries in a perylene diimide semiconductor modulates carrier mobility by two orders of magnitude. The results provide important guidelines for producing device-optimized molecular semiconductors.

Molecular sieves made out of cryptomelane-type manganese oxide (OMS-2) have been widely studied, but synthesizing them with a hierarchical nanostructure and precise crystal orientation is very challenging. It is now demonstrated that pulsed-laser deposition of OMS-2 on SrTiO₃ leads to the spontaneous formation of three-dimensional arrays of parallel and inclined fibres. The results open the way for lattice-engineered synthesis of multilayer materials.

Chromium nitride is very incompressible, making it ideal for industrial coatings. However, it is now shown that the material softens at high pressure and low temperature in connection with a phase transition from cubic to orthorhombic structure. The results could be fundamental in designing ways to improve the mechanical properties of superhard CrN.

Porous materials are technologically important for a wide range of applications, such as catalysis and separation. Covalently bonded organic cages can now be assembled into crystalline microporous materials, and their porosity is found to be intrinsic to their molecular cage structure.

Designing load-bearing tissues that match the mechanical performance of native ones adds extra challenges to tissue engineering. Electrospinning of biodegradable polymer fibres into oriented sheets enables the production of laminate scaffolds; when seeded with mesenchymal stem cells and cultured for 10 weeks, these scaffolds replicate the mechanical properties of native annulus fibrosus.

One of the attractions in studying oxide heterostructures is the unusual physical phenomena that they enable. It is now demonstrated that the enforced cation ordering in thin oxide superlattices leads to significantly enhanced magnetic ordering temperatures.