Spinning to a 2D future

15 January 2019

Since the isolation of graphene in 2004, many two-dimensional materials have been identified, including monolayers of Transition Metal Dichalcogenides (TMDs).

Two-dimensional (2D) materials are crystalline objects made up of a single layer of atoms and feature unique electronic and optical properties arising from their quantum confinement.

The TMD monolayer, an atomically thin semiconductor with a direct optical band gap and strong spin-orbit interaction, is an emerging material attracting great interest from scientists across many fields. While its unique set of properties makes the material ideal for fundamental studies, the semiconductor has also attracted much attention due to its potential applications in nanoelectronics, optoelectronics, and quantum devices.

Immersed in research into TMDs is University of Hong Kong fundamental physicist Professor Xiaodong Cui (Croucher Senior Research Fellowship 2017). He has been experimenting with solid-state physics, with an emphasis on the electrical and optical properties of low dimensional materials and a particular focus on spin and valley physics in semiconductors.

Cui explained: “TMDs offer a platform to explore physics in a 2D world. As dimension shrinks from 3D to 2D, some novel phenomena may arise due to the enhanced Coulomb interactions (the electrostatic interactions between electric charges), and the electron’s spin and valley degree of freedom, among others.”

Unlike graphene, which conducts an electric current all the time, monolayer TMD semiconductors can be controlled externally to be either conductive or insulating of a current. This is why monolayer TMD is attracting huge interest for electrical engineering applications.

Spintronics and valleytronics

Electrons have either a negative or positive charge. Modern electron technology is based on the flow of charge and its manipulation, called the charge degree of freedom. Electrons also have additional degrees of freedom called “spin” and “valley” in TMD.

Over the years many studies have been carried out that have led to significant progress in controlling the spin angular momentum of electrons for spintronic devices. This has potential applications in processing, information storage, and quantum computing.

Physicists have been less successful in the associated area of valleytronics, a conceptual technique for information processing utilising valley degree of freedom, until the emergence of monolayer TMDs.

Cui’s team is one of the first three groups to demonstrate valley polarisation in 2D TMDs and is also a pioneer in the coupling of spin and valley degrees of freedom in the material. One area where Cui has hope regarding 2D semiconductors is the use of light polarisation to control valley degree of freedom, which in turn allows for transmission of information through 2D materials.

“Besides spin and charge degree of freedoms, electrons in 2D TMD have a new feature, valley degree of freedom. We experimentally demonstrated a manipulation of electrons’ valley degree of freedom by a light excitation,” Cui explained.

Curiosity driven

While many researchers are seeking to identify applications for 2D materials, so far none are close to reaching this goal.

Despite the hype surrounding spintonic and valleytronic devices, Cui is grounded and says much more effort is needed to realise their future in electronics.

“Two-dimensional materials have unique properties but I cannot see any immediate application or even a sign of one. I don’t think our present state of knowledge is sufficient,” he added.

However, physics is a science based on logic and mathematics. The driving force for most fundamental physicists is curiosity and desire for deep understanding of the underlying principles, without being obliged to generate applications. Cui explained: “I will be very satisfied if our team is able to take even the smallest step forward in the field of two-dimensional materials, and it doesn’t necessarily have to be a breakthrough in application, but better understanding.”

Professor Xiaodong Cui received the Croucher Senior Research Fellowship in 2017. He obtained his B.S. in Physics from the University of Science and Technology of China and PhD from Arizona State University. He undertook joint postdoctoral research at Columbia University and T.J. Watson Research Center at IBM. He joined the Physics Department at The University of Hong Kong (HKU) in 2004, first as an assistant/associate professor, and later as professor. In 2010, he was awarded Outstanding Young Researcher by HKU. HKU named him Outstanding Researcher in 2018. 

To view Professor Cui's Croucher profile, please click here