System Bits: March 17

Symmetry in graphene growth
According to Rice University researchers, what lies beneath growing islands of graphene is important to its properties.

The team analyzed patterns of graphene – a single-atom-thick sheet of carbon – grown in a furnace via chemical vapor deposition and discovered that the geometric relationship between graphene and the substrate, the underlying material on which carbon assembles atom by atom, determines how the island shapes emerge.

The study shows how the crystalline arrangement of atoms in substrates commonly used in graphene growth, such as nickel or copper, controls how islands form. Graphene’s electronic properties are typically done on mechanically exfoliated graphene, which limits the flake size, as well as being expensive if a lot of material is needed. Because of this, the research community has been trying to come up with a better way to grow it from gases like methane (the source of carbon atoms) using different substrate metals. The problem is, the resulting crystals look different from substrate to substrate, even though it’s all graphene.

And because graphene’s edges are so important to its electronic properties, any step toward understanding its growth is important. Whether a graphene edge ends up as a zigzag, an armchair or something in between depends on how individual atoms fall into equilibrium as they balance energies between their neighboring carbon atoms and those of the substrate.

The atoms in metals form a specific arrangement, a crystal lattice, such as  a pure copper lattice called “face-centered cubic,” although individual grains can have different surfaces in polycrystalline material like copper foils frequently used as graphene-growth substrates.

Electron microscopy showed that all graphene islands growing on the same copper grain tend to have a similar shape, for instance, all perfect hexagons, or all elongated, and that the islands inherit the symmetry of the grains’ surfaces and grow faster in some directions, which explains the peculiar distribution of shapes.

Graphene islands formed in two distinctly different shapes on separate grains of copper (colored in blue and red) grown simultaneously because the substrates’ atomic lattices have different orientations, according to Rice University researchers. (Source: Rice University)

These results are useful, the researchers stressed, in that it gives direction on how to organize the orientation of islands, and particular grain boundaries can be designed all in an effort to control the material’s semiconducting properties.

Human hearts on a chip
UC Berkeley bioengineers have developed a network of pulsating cardiac muscle cells housed in an inch-long silicone device that effectively models human heart tissue, and have demonstrated the viability of this system as a drug-screening tool by testing it with cardiovascular medications.

This organ-on-a-chip represents a major step forward in the development of accurate, faster methods of testing for drug toxicity and could ultimately replace the use of animals to screen drugs for safety and efficacy.

The heart cells were derived from human-induced pluripotent stem cells, the adult stem cells that can be coaxed to become many different types of tissue.

The researchers designed their cardiac microphysiological system, or heart-on-a-chip, so that its 3-D structure would be comparable to the geometry and spacing of connective tissue fiber in a human heart. They added the differentiated human heart cells into the loading area, similar to passengers boarding a subway train at rush hour. The system’s confined geometry helps align the cells in multiple layers and in a single direction.

Wafers like the one shown here are used to create “organ-on-a-chip” devices to model human tissue. (Source: UC Berkeley)

Microfluidic channels on either side of the cell area serve as models for blood vessels, mimicking the exchange by diffusion of nutrients and drugs with human tissue. In the future, this setup could also allow researchers to monitor the removal of metabolic waste products from the cells.

Trapping qubits
At ETH, physicist Jonathan Home’s laboratory has a room full of equipment that traps tiny ions and places them in special quantum states – perhaps the first step towards building a quantum computer.

Home’s team of 13 researchers is aiming to achieve precision control of individual atoms so they can build them up into quantum systems. His laboratory is located at the Institute for Quantum Electronics on the Hönggerberg campus of ETH.

The launch pad for the group’s experiments is a black box, one and a half meters tall, with a laser inside. This equipment features the same technology as modern atomic clocks but they are not measuring time — they are using the atoms as qubits. They want to know what happens when they connect these them up – when they try to precisely nudge one atom with another. This takes place in what is known as an ion trap, which has been built in the room next door.

The golden rectangle in the centre of the green plate is a special ion trap designed by Jonathan Home’s research group. (Source: ETH)

Since this method is so robust, it could potentially be used to simulate complex physical systems – something classical computers can’t do. Whether precision control of individual ions’ quantum states will one day lead to the building of a quantum computer, the researchers don’t yet know.