Our smartphones automatically rotate their screen displays based on how we hold them, showing how different orientations can activate specific functions. In a similar way, the directional traits of electrons confer various properties to the materials they lie in. Gaining control over switching between these states is the key to powering the next generation of computing technologies.
Kuan Eng Johnson Goh, Senior Principal Scientist and Pillar Director at the A*STAR Institute of Materials Research and Engineering (A*STAR IMRE) and A*STAR Quantum Innovation Centre (A*STAR Q.InC) demonstrated a practical way to tune these directional properties in monolayer tungsten disulphide, WS2, a two-dimensional semiconductor just three atoms thick.
“Think of the material like a landscape with two distinct valleys, where the electrons can choose to sit in either valley based on their spin orientation. Herding electrons all into one valley is known as valley polarisation,” Goh explained.
Together with first author Sarthak Das—a Scientist at A*STAR IMRE and Q.InC—and colleagues, the team designed a WS2 device that contains charged biexcitons, which are special particles made of two electron-hole pairs plus an extra charge that can carry spin information related to specific energy valleys. These valleys are particularly important for computing devices because they can serve as a vessel for encoding data.
Traditionally, polarising electrons into one valley requires finely tuned lasers, magnetic field or mechanical strain, but these methods run into issues of scalability. Thanks to the charged biexcitons being robust and tuneable, however, the team could directly control the valley polarisation of the electrons by applying a simple electrical field. “Using this gate voltage is like flipping a switch to guide electrons into the right valley,” Goh said.
Each layer of the device had to be perfectly crafted such that the electrical field would target the right area and create the optimal number of extra electrons or holes for inducing a strong valley polarisation effect.
“Too few carriers, and we don’t get the charged biexcitons. Too many, and the system becomes noisy with competing processes,” said Das. Accordingly, they designed ultra-clean, atomically layered structures to allow precise control of the voltage applied.
The team also found that the device displayed unique and robust switching behaviour, leading to consistent peaks and dips in polarisation levels. “This might come from how electrons push each other around when a voltage is applied, rearranging themselves like guests shifting around at a crowded party when someone new enters,” Das added.
Strikingly, applying the electrical field only required a household battery. This kind of low-power control can bring valleytronics closer to working with standard chip-based computing systems, opening avenues for producing energy-efficient devices in future electronic and quantum technologies.
While the use of electrical fields to control valley polarisation is still at the proof-of-concept stage, the researchers believe early applications of this approach could include optoelectronic systems that modulate circularly polarised light or quantum light emitters. The team is securing intellectual property for the device, with a patent application already filed at the PCT stage.
The A*STAR-affiliated researchers contributing to this research are from the A*STAR Institute of Materials Research and Engineering (A*STAR IMRE) and A*STAR Quantum Innovation Centre (A*STAR Q.InC).
