Manufacturing Bits: August 5

Double Big Mac chips
Using molecular beam epitaxy (MBE), Cornell has devised a method of growing an emerging class of oxide films called Ruddlesden-Poppers.

These oxides are layered structures, which consist of 2D-based perovskite slabs interleaved with cations. In the future, these structures could be used in various applications, such as superconductivity, magnetoresistance and ferromagnetism.

In the lab, researchers from Cornell devised layered structures that resembled a “Double Big Mac” from fast food giant McDonald’s. The layered structure had alternating double and single layers of meat patties–strontium oxide–and bread–titanium oxide.

A thin-film stack based on Ruddlesden-Poppers phases. Using MBE, Cornell has made a perfect atom sandwich. (Source: Cornell)

Researchers used MBE to create the layers. During processing in the lab, researchers noticed that the Ruddlesden-Popper films were missing a layer of strontium oxide.

“It turned out that following a double layer of strontium and oxygen, the next layer of titanium atoms, instead of sitting on top as expected, seeps down between the two strontium oxide layers,” according to researchers. “That meant the missing first layer of strontium and oxygen ended up on the film’s surface–a subtlety overlooked for years.”

“Imagine laying down two meat patties on a bun, followed by a layer of bread, and another two meat patties, only to find that the resulting sandwich consists of just one meat patty below the layer of bread and three above it,” said Cornell researcher Yuefeng Nie, on the university’s Web site. “This is the equivalent of what we found to occur with our layers of atoms.”

To address the issues, researchers deposited an extra layer of strontium oxide. “Our dream is to control these materials with atomic precision,” said researcher Darrell Schlom. “We think that controlling interfaces between Ruddlesden-Poppers will lead to exotic and potentially useful, emergent properties.”

All optical transistors
The Max Planck Institute of Quantum Optics has taken a step towards devising the long-awaited optical transistor. The technology could pave the way towards long-haul data transmissions using an all-optical network.

Researchers from Max Planck have devised a type of optical transistor using a cloud of ultra-cold rubidium atoms. The device had a twentyfold amplification of signal variations at the one-photon level.

In an optical transistor, the input signal is a weak light pulse, called the gate pulse. This, in turn, modifies the transparency of a medium for a second pulse, dubbed the target pulse.

By exciting one atom into a Rydberg state a single photon (red wave packet) reduces the transmission of a laser pulse through a cloud of ultra-cold rubidium atoms by 20 light quanta. (Source: MPQ).

In the lab, researchers from Max Planck devised a medium, which consists of a cloud of about 150,000 rubidium atoms. The atoms are kept in an optical dipole trap. This is done by using two crossed laser beams. At 0.30 micro-Kelvins, the cloud can be held in place for several seconds.

The atomic cloud is irradiated with two light pulses of the same color. The pulses are separated in time by two microseconds. The first gate pulse is weak. It has less than one photon on average.

By comparing the intensities of the outgoing target pulses with and without a preceding gate pulse (a single photon), the reduction of the target signal was determined. “Right at the Förster-resonance, we observe a reduction of 20 photons,” said researcher Stephan Dürr, on the organization’s Web site. “This effect should make it possible–at least in principle–to cascade such transistors in order to solve complex computational tasks. In addition, the present experiment demonstrates a new and non-destructive method for the detection of Rydberg excitations. Because of the high amplification we can reveal whether a single Rydberg excitation has been created in the atomic cloud in a single shot.”

Better batteries from China
Lithium-sulfur (Li-S) batteries have three to five times the capacity and energy density of today’s commercial lithium-ion batteries. In addition, sulfur is inexpensive and environmental safe, making a viable candidate for next-generation batteries.

But Li-S battery technology also suffers from various issues. The technology has poor electrical conductivity and volume expansion of sulfur during the cycling periods, according to researchers.

To solve these problems, China’s Fujian Institute of Research on the Structure of Matter has devised porous carbon nanomaterials for encapsulating sulfur. This, in turn, could enhance the electrochemical performance of Li-S batteries. Researchers synthesized a tube-in-tube carbon nanostructure (TTCN). The technology is based on multi-walled carbon nanotubes (MWCNTs).

As it turned out, the MWCNTs and the outer porous carbon nanotube provided high electrical conductivity. It also enables a high sulfur loading and accommodates volume expansion. And the outer porous tube wall inhibits the polysulfide dissolution, according to researchers.