Recently, in cooperation with the researchers in McGill University and Shanxi University, Prof. Xiaohong Zheng¡¯s group has made new progress in the study of edge state control in two-dimensional (2D) materials and device design. Parts of the results have been published in Appl. Phys. Lett. 108, 233106(2016) and 2D Materials 4, 025013 (2017).
Edge physics and edge state induced phenomena are always attractive research topics in 2D materials. One of the most interesting observations is half-metallicity (HM), which means 100% spin polarized transport at the Fermi level and is extremely important for spintronics. HM in 2D materials was first predicted in graphene by a transverse electrical field (Y. W. Son, et al., Nature 444, 347-349(2006)) and later by some other ways such as edge decoration, BN/graphene heterostructure (S. Dutta, et al., Phys. Rev. Lett. 102, 096601(2009)) and B-N co-doping (X. H. Zheng, et al., J. Phys. Chem. C 114, 4190 (2010)) in the same material.
Recent research indicates that monolayer zigzag SiC nanoribbons also present peculiar edge states and more noteworthy is that the anti-ferromagnetic (AFM) ground state of the pristine zigzag SiC nanoribons is half-metallic, with no need of other controls. However, there is also a ferromagnetic (FM) and non-half-metallic state which is very close in energy to the AFM state. Thus, the HM in the SiC nanoribbons is almost undetectable and it is hard to achieve 100% spin polarized transport since finite temperature can easily switch the ribbon between these two states.
In order to observe stable and 100% spin polarized transport which is independent of the edge state magnetic configurations of the SiC nanoribbons, the researchers in Prof. Zheng¡¯s group proposed an idea of ¡°single edge transport¡±, which means blocking the transport channel on one edge and keeping that on the other edge conducting. This has been implemented by two schemes.
In the first scheme, one edge C atom is replaced by a N atom in the scattering region. The strong scattering potential introduced by the N dopant completely blocks the edge state transport channel on the C edge and meanwhile, the transport channel on the Si edge is not affected. Since the edge states in zigzag SiC nanoribbons with different spins are localized on opposite edges, 100% spin polarized transport is achieved.
In the second scheme, two zigzag SiC nanoribbons are connected by a C¨CSi¨CC¨CSi tetramer to form a transport junction where the right ribbon is turned by 180¡ã around the transport direction.
It is found that 100% spin polarization with nearly perfect transmission is obtained due to perfect momentum k-matching across the transport junction. Such a transport is independent of the magnetic configurations of the two ribbons.
A concomitant property is that 100% valley polarized charge transport is also achieved. In principle, the structures of the proposed transport junctions above turn SiC nanoribbons into promising, robust, and essentially perfect spin filters. These results reflect unique opportunities provided by the 2D material SiC and may generate considerable interest and impact in research communities of nano-science, device physics, materials chemistry and electronic engineering.
The research was funded by the Natural Science Foundation of China and Chinese Scholarship Council.
Figure 1. The transmission functions and scattering states with different connections of the two ribbons.
Figure 2. The band alignment and k-matching of the left and right leads with different connections.