System Bits: Sept. 2

Thinnest semiconductor
A team of researchers from the University of Washington, the University of Hong Kong and the University of Warwick have demonstrated that two single-layer semiconductor materials can be connected in an atomically seamless fashion known as a heterojunction, which they expect could be the basis for next-generation flexible and transparent computing, better light-emitting diodes, or LEDs, and solar technologies.

As heterojunctions are fundamental elements of electronic and photonic devices, this experimental demonstration of such junctions between 2D materials should enable new kinds of transistors, LEDs, nanolasers, and solar cells to be developed for highly integrated electronic and optical circuits within a single atomic plane, the researchers explained.

As seen under an optical microscope, the heterostructures have a triangular shape. The two different monolayer semiconductors can be recognized through their different colors. (Source: University of Washington)

The researchers discovered that two flat semiconductor materials can be connected edge-to-edge with crystalline perfection. They worked with two single-layer, or monolayer, materials – molybdenum diselenide and tungsten diselenide – that have very similar structures, which was key to creating the composite 2D semiconductor.

Collaborators from the electron microscopy center at the University of Warwick in England found that all the atoms in both materials formed a single honeycomb lattice structure, without any distortions or discontinuities. This provides the strongest possible link between two single-layer materials, necessary for flexible devices. Within the same family of materials it is feasible that researchers could bond other pairs together in the same way.

The researchers stress this is a scalable technique because the materials have different properties, they evaporate and separate at different times automatically. The second material forms around the first triangle that just previously formed.

With a larger furnace, it would be possible to mass-produce sheets of these semiconductor heterostructures, the researchers said. On a small scale, it takes about five minutes to grow the crystals, with up to two hours of heating and cooling time.

This photoluminescence intensity map shows a typical piece of the lateral heterostructures. The junction region produces an enhanced light emission, indicating its application potential in optoelectronics. (Source: University of Washington)

Revealing ‘nanomechanical’ surface traits
Purdue University engineers have created a research platform that uses a laser to measure the “nanomechanical” properties of tiny structures undergoing stress and heating, which they expect to yield insights for improving designs for microelectronics and batteries.

The technique, called nanomechanical Raman spectroscopy, reveals information about how heating and the surface stress of microscale structures affect their mechanical properties, the researchers said, who have discussed the merits of surface-stress influence on mechanical properties for decades. Nanomechanical Raman spectroscopy has offered the first such measurement.

These findings are potentially important because silicon structures measured on the scale of micrometers and nanometers form essential components of semiconductor processors, sensors and the emerging class of microelectromechanical (MEMS) systems.

This work is also potentially important for the measurement of components in batteries to study stresses as they constantly expand and contract during charge-discharge cycles. Ordinary sensors are unable to withstand punishing conditions inside batteries. However, because Raman spectroscopy uses a laser to conduct measurements, it does not have to be attached to the batteries, making possible a new type of sensor removed from the harsh conditions inside batteries.

A new research platform uses a laser to measure the “nanomechanical” properties of tiny structures undergoing stress and heating, an approach likely to yield insights to improve designs for microelectronics and batteries. Clockwise from upper left, graphics of the instrument setup, and at bottom right a scanning electron microscope image of the tiny silicon cantilever used in the research. (Source: Purdue University)