Recently, a research team led by Prof. ZHENG Xiaohong working with Institute of Solid State Physics, Hefei Institutes of Physical Science conducted a joint study with the researchers in McGill University and Shanxi University to computationally demonstrate a new scheme for generating perfect spin-polarized quantum transport in zigzag-edged graphene nanoribbons(ZGNRs) by construction of van der Waals heterostructures.

The joint study was published in Nanoscale entitled "h-BN/Graphene van der Waals vertical heterostructure: A fully spin-polarized photocurrent generator". 

It is a central issue to achieve large spin polarization in spintronics and single spin transport is particularly interesting.

Thus far, quite a few schemes have been suggested to achieve half-metallicity (a property with only one spin conducting) in zigzag edged graphene nanoribbon (ZGNR), such as electrical field, edge decoration and B-N co-doping.

These schemes can be divided into two categories, one by applying electrical field and the other by chemical modification.

However, it has been shown that the strength of electrical field should be extremely high to achieve half-metallicity in ZGNR, which is almost unavailable in laboratory.

Meanwhile, with chemical modification it is hard to precisely control the doping or adsorption sites and structural stability.

More seriously, chemical modification can lead to dramatic decrease or even the disappearance of the energy difference between the antiferromagnetic (AFM) and ferromagnetic (FM) edge configurations, whereas the half-metallicity can only be obtained from the AFM ground state.

Thus, a finite temperature can easily lead to paramagnetism in ZGNRs, making the half-metallicity practically unobservable.

Consequently, it is still very important to design new schemes to avoid the utilization of either electrical field or chemical modification for achieving fully spin polarized transport with ZGNRs.

In this work, researchers proposed a better approach using graphene based van der Waals (vdW) vertical heterostructures.

Firstly, the team constructed a vdW heterostructure h-BN/Graphene(Gr)/h-BN by sandwiching a ZGNR between two hexagonal BN sheets with AA stacking. The two BN sheets produce a stagger potential at their middle plane which acts differently on the two sublattices of the ZGNR.

Since the edge states of the two spins are also located at different sublattices, the energies of the edge states of the two spins shift oppositely, which leads to the lift of the spin degeneracy of the edge states and different energy gaps Eg↑ and Eg↓ (with Eg↑ < Eg↓) for the two spins.

Secondly, they proposed to shed a linearly polarized light on the central region of a device. The photons with properly selected energy excite electrons of only one spin from the valence band to the conduction band.

Thirdly, they applied a small positive bias across the light irradiated region to drive the excited electrons in the conduction band to flow to the right lead, which results in fully spin polarized photocurrent.

This scheme is rather robust in that fully spin polarized transport is always achievable no matter whether they decrease or increase the interlayer distance by applying compressive or tensile strain vertically to the sheets or shift the BN sheets in-plane relative to the graphene nanoribbon.

In a word, by combining with photon irradiation, their study demonstrates the great importance and potential of 2D vdW heterostructures in spintronics, which should be taken into consideration in the design of spintronic devices based on 2D materials.

This work was supported by the National Natural Science Foundation of China and the calculations were performed at the ScGrid of Supercomputing Center and Computer Network Information Center of the Chinese Academy of Sciences.

Fig. 1: (a) Schematic plot of the vdW vertical heterostructure; (b)The band structure of the h-BN/Gr/h-BN vdW vertical heterostructure (Image by ZHENG Xiaohong) 

Fig. 2: (a) The effective potential at the graphene plane produced by two BN nanoribbons; (b) Two spin components of the photocurrent versus the photon energy (Image by ZHENG Xiaohong)