Jahn–Teller chess

Appl. Phys. Lett. 89, 233120 (2006)

Credit: © AIP 2006

One of the significant promises of modern bottom-up fabrication approaches is to produce devices at a scale much smaller than those that can be obtained by conventional top-down techniques such as optical lithography. Sunmog Yeo and collaborators have now uncovered an alternative self-patterning effect that creates phase-separated columns of manganese perovskite materials that are only a few nanometres wide. The authors make use of the Jahn–Teller effect, where structural distortion of the crystal unit cell lifts the degeneracy of electronic states. This can also lead to an ionic phase-separation in alloys. In the present case, ZnGa2O4 is mixed with ZnMn2O4, which leads to a checkerboard-like separation into two phases of ZnMnGaO4 with different Ga and Mn concentrations. Each phase is only 4 nm wide but when looked at from the side, they show a long columnar, herringbone-like pattern. The small size of the checkerboard patterns and their uniformity is promising for applications, and in the case of magnetic structures could potentially lead to their use, for example, in magnetic data storage.

Fridge-on-a-chip

Lab on a Chip doi:10.1039/b617767k (2007)

In the macroworld, heaters are available in different formats. In contrast, microscale refrigerators are harder to come by. One such device that could be integrated in larger microfluidic systems has been made by group of researchers in Wisconsin, USA. They used photopolymerizable polymers to fabricate a two-layer microfluidic device in which a cooling liquid is kept circulating by a small nickel impeller driven by an external rotating magnetic stirrer. A ring made of thermoresponsive hydrogel surrounding the impeller works as a clutch: it expands and blocks the rotation of the impeller whenever the temperature drops below a given critical value, which can be tailored in advance (during hydrogel synthesis) according to the requirements of the chip in use. The device therefore provides autonomous control of the temperature by cooling rather than heating. This is a desirable function for lab-on-a-chip systems, particularly those used for biological or biochemical processes.

Anionic guests

Chem. Mater. 19, 79–87 (2007)

Multifunctional hybrid materials prepared by intercalation reactions can form lamellar inorganic/organic assemblies with structures controlled by host–guest interactions. However, these layered materials, as well as open structures such as zeolites, have anionic frameworks and can only accept cationic or basic guests, thereby limiting the range of organic molecules that can be incorporated. Tom Mallouk and colleagues now report on a type of layered material that has the ability to encapsulate anionic substances. The lamellar nanocomposites were produced by intercalation of cationic polyelectrolytes with different charge densities into synthetic fluoromica. The anion-accepting ability of these compounds was quantified by studying the encapsulation of a blue dye, which demonstrated that the composites possessed two kinds of hydrated interlayers in which polycation strands were coiled. The authors believe that the properties of these layered host materials could prove technologically significant, not only for applications in cosmetics and pigments, but also in biomedical imaging, artificial photosynthesis and photodynamic therapy.

Flip-flops in the dots

Phys. Rev. Lett. (in the press); Preprint at http://arxiv.org/cond-mat/0609371 (2006)

The spin of one electron confined in a quantum dot seems ideal for fundamental studies and applications such as quantum information. It is well localized in space, can assume only two values, and with a bit of care can be made almost inert to the environment. Except for one detail: even in the best cases, the electron spin interacts with the thousands of nuclear spins within the dot (through the so-called hyperfine interaction). Alexander Tartakovskii and colleagues have now shown that this is not always a disadvantage. They used a specific laser excitation to define the spin of an electron confined in an InAs dot. Through the hyperfine interaction, the electron spin can flip and be transferred to the nuclei, therefore changing their magnetization. On varying the laser power, the nuclear magnetization switches between two well-defined states. The researchers explain that this arises due to the effect of the nuclear magnetization on the rate at which the spin can be transferred. The results show how the apparent drawback represented by the hyperfine interaction can actually be used to define the configuration of nuclear spins to be used, for example, as quantum bits.

Our own surfactants

Langmuir doi:10.1021/la061608+ (2006)

Credit: GETTY

Our lungs are lined with a surfactant layer at the interface where gas transport between air and blood takes place. The surface tension of the layer can change for variable lengths of time, for example when we laugh or have a persistent cough. It is becoming ever more clear that this surfactant layer has a role in the oxygen-transfer process. However, with the in vitro method commonly used to measure this process it has not been possible to probe the very low surface tension range (≤25 mJ m−2) thought to be associated with the surfactant layer in the lungs. Researchers have now managed this using a technique that captures photographically the shrinkage (related to gas diffusion) of a confined bubble having a surfactant film at its interface1. They found that a sparse film of bovine lipid extract (a surfactant used therapeutically), giving a surface tension higher than 6 mJ m−2, offers minimal resistance to oxygen transport. As the surfactant molecules become more and more compressed and the surface tension decreases, the film limits oxygen transfer. This would suggest that the surfactant layer might have a specific, yet unclear role in pulmonary physiology.