Jun 27 2017
A group of international Scientists, performing research at the Department of Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab) and UC Berkeley, synthesized an atomically thin material and recorded its durable and striking characteristics that render it to be a propitious material for an emerging division of electronics called “spintronics.”
This animated rendering shows the atomic structure of a 2D material known as 1T’-WTe2 that was created and studied at Berkeley Lab’s Advanced Light Source. (Credit: Berkeley Lab)
Named as 1T’-WTe2, the material acts as a bridge between two burgeoning fields of research: namely, 2D materials, including monolayer materials like graphene that behave in different ways than thicker equivalents and topological materials, where electrons can get arranged in calculable ways at nearly zero resistance and irrespective of defects that may normally hinder their motion.
The spin of electrons (i.e. a particle property functioning similar to a compass needle pointing toward north or south) and their momentum at the edges of this material are closely related and calculable.
This most recent experimental evidence can promote the usage of the material as an investigative material for next-generation applications, for example, a new class of electronic devices that exploit its spin characteristic to transfer and store data in an effective way than prevalent devices. These attributes are fundamental to spintronics.
The material is termed a topological insulator as its interior surface does not have the ability to conduct electricity and its electrical conductivity, or the movement of electrons, is confined to its edges.
“This material should be very useful for spintronics studies,” stated Sung-Kwan Mo, who co-led the research and is a staff scientist and physicist at Berkeley Lab’s Advanced Light Source (ALS). The study has been published in the journal Nature Physics on 26th June 2017.
The flow of electrons is completely linked with the direction of their spins, and is limited only to the edges of the material. The electrons will travel in one direction, and with one type of spin, which is a useful quality for spintronics devices.
Sung-Kwan Mo, Co-leader of the research and Staff Scientist and Physicist, Berkeley Lab’s Advanced Light Source (ALS)
These devices can conceivably transfer data fluidly, with comparatively less power requirements and heat formation than prevalent electronic devices.
“We’re excited about the fact that we have found another family of materials where we can both explore the physics of 2D topological insulators and do experiments that may lead to future applications,” stated Zhi-Xun Shen who also co-led the study and is a Professor in Physical Sciences at Stanford University and the Advisor for Science and Technology at SLAC National Accelerator Laboratory. “This general class of materials is known to be robust and to hold up well under various experimental conditions, and these qualities should allow the field to develop faster,” he further stated.
The material was synthesized and investigated at the ALS, an X-ray research facility called as a synchrotron. Shujie Tang—a co-lead author of the research as well as a visiting Postdoctoral Researcher at Berkeley Lab and Stanford University—actively contributed to fabricating the three-atom-thick crystalline samples of the material in an extremely purified, vacuum-sealed chamber at the ALS. This was achieved by employing a technique called as molecular beam epitaxy.
Then, the highly pure samples were analyzed at the ALS by means of a method called angle-resolved photoemission spectroscopy, or ARPES, which enables powerful investigation of the properties of electron in the materials.
“After we refined the growth recipe, we measured it with ARPES. We immediately recognized the characteristic electronic structure of a 2D topological insulator,” stated Tang, based on theory and predictions. “We were the first ones to perform this type of measurement on this material.”
However, as the conducting part of the material was just a few nanometers thin at its outermost edge, that is, thousands of times thinner when compared to the focus of X-ray beam— all the electronic properties of the material could not be positively identified.
Therefore, Fellow Researchers at UC Berkeley carried out additional atomic-scale measurements by means of a method called scanning tunneling microscopy, or STM. “STM measured its edge state directly, so that was a really key contribution,” stated Tang.
The study was initiated in the year 2015 and involves over 24 Scientists belonging to a range of fields. The Researchers were also assisted by computational work at Berkeley Lab’s National Energy Research Scientific Computing Center (NERSC).
The electronic properties of two-dimensional materials are distinctive, and hence are believed to be highly salient in using them for spintronics applications. Moreover, globally, there is a highly dynamic R&D work toward customizing these materials for particular applications by selectively piling up disparate types of such materials.
Researchers are trying to sandwich them on top of each other to tweak the material as they wish—like Lego blocks. Now that we have experimental proof of this material’s properties, we want to stack it up with other materials to see how these properties change.
Sung-Kwan Mo, Co-leader of the research and Staff Scientist and Physicist, Berkeley Lab’s Advanced Light Source (ALS)
A characteristic difficulty in synthesizing such designer materials by using atomically thin layers is the fact that materials normally include nanoscale flaws that can be hard to remove and may have an impact on their performance. However, due to the fact that 1T’-WTe2 is a topological insulator, naturally, its electronic properties are rigid.
At the nanoscale it may not be a perfect crystal, but the beauty of topological materials is that even when you have less than perfect crystals, the edge states survive. The imperfections don’t break the key properties.
Sung-Kwan Mo, Co-leader of the research and Staff Scientist and Physicist, Berkeley Lab’s Advanced Light Source (ALS)
In the future, the goal of the Scientists is to synthesize bigger samples of this material and to find out the way to selectively adjust and emphasize particular characteristics. Apart from its topological characteristics, its “sister materials” that have identical characteristics and were also investigated by the Researchers were found to be light-sensitive and possess functional characteristics for optoelectronics and for solar cells, which regulate light for applications in electronic devices.
The ALS and NERSC are DOE Office of Science User Facilities. Scientists from Stanford University, the Chinese Academy of Sciences, Shanghai Tech University, POSTECH in Korea and Pusan National University in Korea were also a part of this research. The Department of Energy’s Office of Science, the National Science Foundation, the National Science Foundation of China, the National Research Foundation (NRF) of Korea and the Basic Science Research Program in Korea supported the study.