Research articles

According to Fourier theory, thermal transport is a diffusive process. However, this cannot be the case at length scales smaller than the mean free path of the energy carriers. The first experimental study of thermal transport at the nanoscale is now reported in the case of a point-like heat source, providing a quantitative description of the transition between the ballistic and diffusive regimes.

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.

Heat-responsive polymers grafted onto gold nanocages serve as a nanoscale delivery system for biologically important compounds. Laser irradiation of the nanocages heats the polymers by means of the photothermal effect; the polymers then change conformation and compounds are released. The polymers return to their original configuration when the laser is switched off, stopping further release.

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.

Like their optical counterparts, acoustic metamaterials are capable of manipulating sound waves in unusual ways. An acoustic hyperlens is now demonstrated that is capable of magnifying subwavelength acoustic waves, and could therefore find applications in medical imaging or underwater sonar.

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.

Conventional electroanalytical and structure-analysis techniques provide limited information about ionic fluxes in electrochemical systems. A quartz crystal microbalance is now used as a gravimetric probe of the concentration and compositional changes in microporous activated carbon.

Sodium beta-alumina (SBA) compositions are well known as ionic conductors. Nevertheless, ionic and electron conductivities perpendicular to the lattice planes in the material are very low. It is now shown that by exploiting this property, SBAs can be used as transistor gate dielectrics in solution-processed devices using oxide-based and polymer electrodes.

Bioelastomers generally show elasticity similar to that of rubber, which originates from entropic forces linked to deformation. It is now shown that in the egg capsule of a large marine shell, the elasticity is instead based on a structural transition. The results could have a significant impact on engineering protective encapsulating systems inspired by natural elastomers.

Soft embryonic stem cells respond to small localized forces by increasing cell protrusion and spreading; in contrast, cells that are differentiated from them—which are ten times stiffer—do not spread. The deformation of the cell cytoskeleton is thus shown to be an important determinant of cellular response to force.

By including small molecules with block copolymers in polymer nanocomposites, various types of nanoparticle can be positioned within the composite with unprecedented precision over several length scales. Moreover, the spatial distribution of nanoparticles within the combined material can be varied by exposure to heat or light, creating a new route to stimuli-responsive materials.

Plasmonic biosensors are either based on freely propagating surface plasmons or plasmons localized at nanostructures. Despite advantages such as quantitative detection, localized surface-plasmon sensors have shown lower overall sensitivities. A nanorod metamaterial supporting new plasmonic modes is now shown to considerably outperform earlier plasmonic biosensors by combining and expanding their respective advantages.

The efficiency of solar cells depends not only on the generated current, but also the photovoltage produced. Ground-state charge-transfer complexes are shown to have an important role in influencing the open-circuit voltage of several polymer–fullerene solar-cell blends; future chemical tuning of the polymers could maximize the complexes’ role in affecting the voltage for increased power-conversion efficiency.

The mechanical properties and corrosion behaviour of glassy metals are attractive for biodegradable implants. Magnesium-based glasses are particularly promising but they suffer from hydrogen evolution during corrosion. A distinct reduction in hydrogen evolution is now observed in zinc-rich magnesium glasses showing good tissue compatibility.

Characterizing the internal architecture of zeolites is crucial for understanding their structure–function relationships, and for acid–base heterogeneous catalysis. Using a unique combination of diffraction and microscopy techniques provides a unified picture of the morphology of intergrowth structures and confirmation of surface barriers for molecular diffusion.