Nature Electronics of Shanghai Institute of Microsystems reports major progress in research on basic physical properties of new carbon-based two-dimensional semiconductor materials

Carbon-based two-dimensional materials represented by graphene have received extensive attention since their discovery. However, the zero-bandgap semiconducting properties of graphene severely limit its application in the field of microelectronic devices. In response to this situation, Dr. Yang Siwei, researcher Ding Guqiao and others from Shanghai Institute of Microsystems and Information Technology, Chinese Academy of Sciences have carried out research on the preparation of new carbon-based two-dimensional semiconductor materials since 2013. In January 2014, they successfully prepared carbon and nitrogen atomically formed graphene-like honeycomb-like non-porous ordered structure semiconductor C3N monolayer material (Fig. 1), and found the ferromagnetic long order of the material after electron injection. The related results were published in Adv. Mater. as a cover paper in February 2017 (C3N—A 2D Crystalline, Hole‐Free, Tunable‐Narrow‐Bandgap semiconductor with Ferromagnetic Properties, Advanced Materials). The successful synthesis of C3N makes up for the major defect that graphene has no bandgap, provides a new option for the application of carbon-based nanomaterials in microelectronic devices, and has attracted widespread attention. However, compared with graphene, which has been relatively mature in research, the research on C3N started late, and there are still a lot of gaps in the basic physical properties of this material to be filled.

Figure 1. C3N lattice structure and ferromagnetic long program after hydrogenation

At the same time as the above work was published, Dr. Yang Siwei initially realized the preparation of AA’ and AB’ stacking bilayer CN3N in 2016 (Fig. 2). On this basis, researcher Ding Guqiao’s team from Shanghai Institute of Microsystems and Information Technology, Chinese Academy of Sciences and Yuan Qinghong’s team from East China Normal University have worked hard for nearly 5 years, with the help of experimental technology and theoretical research, in the band gap properties of double-layer C3N, the output Significant breakthroughs have been made in research fields such as its operational properties, and the results further demonstrate the important application potential of bilayer C3N in nanoelectronics and other fields.

Figure 2. HAADF-STEM images of AA’ (ac) and AB’ (df) stacked bilayer CNs

This work demonstrates the feasibility of transitioning bilayer C3N from semiconducting to metallicity through controlled stacking. Compared with single-layer C3N with an intrinsic bandgap of 1.23 eV, the band gaps of double-layer C3N can be roughly divided into three types: 1) AA and AA’ stacking close to metallicity; 2) the band gap is reduced by nearly 30% AB and AB’ stacking; 3) bilayer molar stacking close to the band gap of the monolayer. The above band gap change can be attributed to the splitting of the energy band near the Fermi level under the pz orbital coupling between the top and bottom C3N. Under the premise that the interaction potentials between the two layers are close, the number of overlapping wave functions of the top of the valence band and the bottom of the conduction band determines the degree of band splitting, which in turn affects the band gap. Among them, in the double-layer C3N such as AA, AA’, AB, AB’, the number of overlapping wave functions of the two layers has a double relationship, and the band gap splitting value is approximately doubled. For the bilayer Moiré-rotated stripe structure, the upper and lower atoms are basically staggered, and the overlapping of pz orbitals is limited, so the band gap is close to that of monolayer C3N.

More importantly, the study also found that the modulation of the band gap of AB’ stacking bilayer C3N can be achieved by applying an external electric field. Experimental results show that under an applied electric field of 1.4 V nm-1, the band gap of the AB’-stacked bilayer C3N drops by about 0.6 eV, realizing the transition from semiconducting to metallic (Fig. 3).

Figure 3. Bandgap modulation of bilayer C3N under electric field

The above work is another important breakthrough in the experimental and theoretical research of C3N materials, and provides an important support for the further construction of new all-carbon microelectronic devices. The related research results were published online in Nature Electronics under the title of “Stacking-Induced Bandgap Engineering of 2D-Bilayer C3N”.

The first authors of the paper are Wei Wenya, a doctoral student at East China Normal University, Dr. Yang Siwei, an assistant researcher at the Shanghai Institute of Microsystems, Chinese Academy of Sciences, and Dr. Wang Gang, an associate professor at Ningbo University. The corresponding authors are Professor Yuan Qinghong from East China Normal University, Researcher Ding Guqiao from Shanghai Institute of Microsystems, Chinese Academy of Sciences, Professor Kang Zhenhui from Soochow University, and Academician Debra J. Searles from the University of Queensland. During the research period, the team of Ding Guqiao and Yang Siwei of the Institute of Microsystems was responsible for the preparation, stacking modulation, structural characterization, and device preparation of double-layer C3N; Yuan Qinghong’s team of East China Normal University was responsible for the elaboration of the physical mechanism of double-layer C3N; Associate Professor Wang Gang of Ningbo University The team is responsible for the demonstration of double-layer C3N photodetection applications.