Dr. Xiaolong Zou, Material Science and Mechanical Engineering at Rice University talks to AZoNano about Sub-Nanowire Formation.
KK - What is special about the Molybdenum/Sulfur lattice?
XZ - Monolayer Molybdenum Disulfide (MoS2) consists of a middle plane of triangularly packed metal atoms, sandwiched between the two layers of Sulfur atoms, also packed triangularly in their respective planes. In the most stable 2H phase, these three layers S|Mo|S` are stacked so that the frontal view displays the alternating Mo and S|S` nodes as a honeycomb lattice, with each Mo bonding with six sulfur atoms, arranged in prismatic S3MoS`3 units. The difference (or inequivalency) between Mo and S|S` nodes will give rise to interesting physical properties.
KK - Why have researchers focused on Molybdenum Disulfide?
XZ - Two-dimensional (2D) materials hold tremendous potential for revolutionizing nanoelectronic technology as they essentially eliminate device dimension in one direction and bring device miniaturization to an unprecedented level. It is well known that transistor is the building block of modern electronics. As a rule of thumb, materials for making a good transistor must have both high carrier mobility to ensure fast information transmission and high on/off ratio to have well-defined “0” and “1” states. So far MoS2 is the best known 2D material that excels in both aspects. In addition, MoS2 enables very efficient photon-electron interaction and may find applications in optoelectronics and energy harvesting.
Two-dimensional molybdenum/sulfur is a hexagonal lattice when seen from above, but when viewed edge on, as it is here, its three-layer form is apparent. When two sheets of the material meet, three-dimensional dislocations appear at the grain boundaries. When the sheets meet at a 60-degree angle, those boundaries are metallic, and conductive. Image credit: Yakobson Group/Rice University
KK - How did you discover sub-nanowire formation in two-dimensional materials?
XZ - We first recognize that it is very difficult, if not impossible, to grow perfect, defect-free MoS2 in the laboratory. Borrowing knowledge from crystal growth and theory of dislocation, we expect that a common and important defect formed during MoS2 growth would be grain boundary, which is the interface between two MoS2 layers with different crystallographic orientations that nucleate, grow and later meet on the same substrate. The presence of grain boundaries in MoS2 could have considerable impact on its performance, e.g. resulting in low carrier mobility. We thus decided to carry out theoretical investigation to gain more understanding on the structures and properties of such defects.
During this quest we made a surprising discovery that unique type of grain boundaries, which we refer to as the sub-nanowire, can be formed when the orientations of the two abutting MoS2 layers differ by 60o. Compared to other types of grain boundaries, the sub-nanowire possesses very unusual electronic properties and improves rather than hurts the material conductivity.
KK - How can you predict conductivity of the two-dimensional material?
XZ - We use state-of-the-art first-principles calculations to compute the electronic band structure of this material, from which we can accurately determine its conductivity without any experimental input.
KK - How does this new two-dimensional material compare to graphene?
XZ - Graphene and MoS2 have very different electronic properties. Graphene is a very good electron conductor, and MoS2 is a semiconductor. They can be used to build different elements in 2D electronic devices in the future. While graphene can function like metal connectors in conventional electronic circuits, MoS2 will be mainly used to make transistors in the circuits.
KK - What are the main applications for this two-dimensional material?
XZ - Commercially, MoS2 has already been successfully used as a solid lubricant and catalyst for hydrodesulfurization. Because of its many other unique properties, researchers have envisioned its potential application in nanoelectronics, optoelectronics, catalyst for hydrogen evolution reactions, piezoelectronics and valleytronics, just to name a few.
KK - Are there any aspects of the new two-dimensional material that require further design and development attention before commercial application can be considered?
XZ - There are two important issues we need to consider here. First, we need to learn how to control the synthesis conditions to accurately tune the defect (such as grain boundaries) population, distribution and properties in these 2D materials to make sure they deliver desired performance. Secondly, nanoelectronic devices built upon these 2D materials will contain a large amount of interfaces between them and other materials. We need to develop a better understanding of the critical effects of these interfaces on the device performance.
KK - This research has motivated the search for other two-dimensional materials. Can you explain these?
XZ - An important lesson we learned from this research is that topological defects in 2D materials do not necessarily result in defective or inferior material properties. Instead, carefully tailored defect structure may even improve material performance in certain aspects such as electronic mobility. Such a discovery calls for more intensive study on the formation, stability and properties of various types of defects in 2D materials, which may create a new dimension in the search of 2D materials through the “defect engineering” approach.
KK - What factors can affect the functional capacity of this form of sub-nanowire formation?
XZ - The sub-nanowire may have its own defects. For example, a kink may form on the sub-nanowire and makes it no longer a straight line. Such “defect in a defect” is expected to have some influence on the electronic properties of the sub-nanowire, which remains unknown at present. We plan to study this in the next stage of our research project.
KK - Why is it difficult to achieve an ideal lattice structure for such sub-nanowire formation?
XZ - A perfect sub-nanowire is formed when two growing MoS2 layers whose orientations differ by 60o meet each other along a straight line. In reality, however, it is difficult to control 1) the misorientation of the two MoS2 to be exactly 60o, and 2) the two layers to form a perfectly flat interface. They are the two main challenges that need to be addressed in fabrication.
KK - What will metal disulfides promise for the future of electronic devices?
XZ - Compared to other 2D materials like graphene, metal disulfides are a class of compounds that show semiconductor-like behaviour and are particularly suitable for transistor applications.
They will have an important place in the future of 2D electronic devices with small size, flexibility and lower power consumption.
About Dr. Xiaolong Zou
Dr. Xiaolong Zou received his PhD (2011) in Physics from Tsinghua University, China, under the guidance of Professor Wenhui Duan. He is currently a Research Associate in Professor Boris Yakobson's laboratory at Rice University, Houston, USA.
Xialong Zou's research focuses on 2D materials for new electronic and optical applications. He is also interested in energy storage and conversion, such as Li storage, thermoelectricity, and photo-related applications.