Articles in 2013

Topological crystalline insulators are characterized by metallic states originating from crystal symmetries. A transistor device that takes advantage of the quantum properties of the topological crystalline insulating materials SnTe and Pb_1−xSn_xSe(Te) is now proposed.

Although it has been shown that engineering of conducting polymers such as poly(3,4-ethyldioxythiophene) can improve the Seebeck coefficient and the figure of merit ZT of these materials, the mechanisms underlying this improved thermoelectric behaviour are still not fully understood. It is now reported that the band structure of semicrystalline films of these bipolaronic polymers, resembling that of inorganic semi-metals used for thermoelectric applications, can explain these findings.

The imaging of tumours is challenging because of the wide range of different cancers. Now, the rapid detection of tumours, independent of type, is achieved using a nonlinear amplification strategy that employs ultrasensitive pH-responsive fluorescent nanoparticles that illuminate within tumour neovasculature or in response to the tumour’s acidic extracellular environment.

Photoelectrochemical water-splitting devices require integrating electrocatalysts with light-absorbing semiconductors, but understanding charge-transfer processes at interfaces has proved difficult. Ion-permeable electrocatalysts deposited onto TiO₂ photoelectrodes now result in adaptive semiconductor/electrocatalyst junctions where both the effective interface barrier height and the photovoltage output change depending on the oxidation state of the electrocatalyst.

In wound healing, skin cells collectively migrate to maintain tissue cohesion despite the existence of inhomogeneities in the extracellular environment within the wound bed. Yet how the cell collective responds to heterogeneities in the extracellular matrix is not well understood. Now, it is shown that migrating human keratinocyte cell sheets form suspended multicellular bridges over non-adhesive regions on micropatterned substrates comprising alternating strips of fibronectin and non-adherent polymer.

Failure caused by dendrite growth in rechargeable batteries with lithium metal anodes has prevented their widespread applicability. A microtomography study on lithium–polymer–lithium cells now reveals that at the early stage of dendrite formation dendritic structures lie within the electrode, underneath the polymer/electrode interface.

Layered oxides are promising for many applications ranging from energy conversion and storage to magnetic and electric devices. The oxygen storage ability of ferroelectric LuFe₂O_4+x is now demonstrated and the storage mechanism is believed to be governed by a complex oxygen intercalation/de-intercalation process with several intermediate metastable states.

The efficiency of organic blends used for photovoltaic applications depends on their ability to convert photoexcited charges into free holes and electrons. It is now demonstrated that the lowermost energetic states formed at the donor/acceptor interface can reach conversion efficiencies close to 100%, and therefore do not behave as traps for charge carriers.

The LaAlO₃/SrTiO₃ interface plays host to a diverse range of physical phenomena. By imaging the electrostatic landscape with a specially designed detector it is shown that tetragonal domains give rise to a large extrinsic piezoelectricity.

The electrochemical reduction of oxygen to hydrogen peroxide requires selective and stable electrocatalysts. It is now shown that Pt–Hg nanoparticles display an order of magnitude improvement in the mass activity for hydrogen peroxide production compared with the best performing catalyst.

The propagation of light in photonic crystals with a honeycomb structure mirrors the behaviour of charges in graphene, therefore allowing for the investigation of electronic properties that cannot otherwise be accessed in graphene itself. This approach is now used to predict unexpected edge states that localize in the bearded edges of hexagonal lattices.

Proteins from bone extracellular matrix are known to mediate the organization of apatite crystals in bone. Now, electron microscopy, X-ray scattering and nuclear magnetic resonance measurements of the structure and organization of apatite nanoparticles and intact bone samples show that water also plays a significant role in orienting the apatite crystals, and that such structuring is mediated by a disordered mineral layer that coats the crystalline core of bone apatite.

Metal fluorides/oxides are promising electrodes for lithium-ion batteries, but the mechanism by which they exhibit additional reversible capacity is still not well understood. By using high-resolution solid-state NMR techniques it is shown that extra capacity in this RuO₂ system is due to the generation of LiOH and its subsequent reversible reaction with Li to form Li₂O and LiH.

In organic semiconductors, disorder-induced traps can alter the mobility of the charges and introduce noise in transport measurements. It is now shown that simple drop-casting of perfluoropolyether on top of organic single-crystals is an effective strategy for healing charge traps. This method allows the intrinsic transport properties of these materials to be recovered as well as suppressing noise in Hall effect measurements.

Rare-earth hexagonal manganites are known for their multiferroic properties. Using parameters calculated from first principles, a theoretical description of the topological defects arising in these systems is now presented.

Somatic cells can be reprogrammed into induced pluripotent stem cells biochemically through the expression of a few transcription factors. It is now shown that aligned microgrooves or nanofibres on cell-adhesive substrates can promote the reprogramming of somatic cells more efficiently through epigenetic regulation of genes related to pluripotency and the mesenchymal-to-epithelial transition. The findings suggest that the epigenetic state can be regulated by variations in cell morphology.

Cells can sense and respond to their environment through mechanical forces. However, how the cell’s cytoskeleton transmits forces and how cytoskeletal proteins respond to forces is largely unknown. Now, a combination of mechanical perturbations and multiscale modelling offers insights into the molecular mechanisms behind the observed variations in the accumulation kinetics of the involved proteins in response to different types of deformation.

Spin-torque diodes enable the detection and rectification of radiofrequencies by means of spin-torque-induced ferromagnetic resonance between nanomagnets. Now, by using magnetic tunnel junctions with a MgO barrier and a FeB free-layer detection sensitivities in excess of those of semiconductor devices are demonstrated.

Patterning physiologically relevant proteins in three-dimensional hydrogels without affecting the activity and stability of the proteins has been difficult. Now, by using enzymatic crosslinking reactions, in situ control over the phototriggered immobilization of virtually any desired protein in a synthetic hydrogel is demonstrated. The approach can be used to manipulate cells, as demonstrated by the three-dimensional control of the invasion of mesenchymal stem cells within poly(ethylene glycol) hydrogels.

Previous studies have suggested that even in the absence of a graphene bandgap, a relaxation bottleneck at the Dirac point may allow for population inversion and lasing. Now, using time- and angle-resolved photoemission spectroscopy with femtosecond extreme-ultraviolet pulses, it is shown that interband excitations give rise to population inversion, suggesting that terahertz lasing may be possible.