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Surface functionalization, strain tune electronic and magnetic properties of 2D polyaniline

By Eva Karatairi December 6, 2019
C3N polyaniline
(a) Geometric atomic structure of C3N, with carbon atoms represented as brown and nitrogen atoms as grey spheres. The red parallelogram surrounds a structural unit cell. The high charge density (blue) around the N atoms, projecting toward the C–N bonds, suggests a charge transfer from C to N atoms. (b) Simulated scanning tunnelling microscopy image of the C3N structure. (c) Structure with various H coverage of the structural unit cell. (From left to right): 1H/C3N (3.1% coverage), 2H/C3N (6.2%), and 3H/C3N (9.3%). In each case the H atom is located on top of a C atom (CH), with a single covalent bond CH–H forming between them. The length of the bond (1.12 Å) shows that H is chemically adsorbed on C3N. The C–C and C–N bonds are elongated to 1.5 Å, in comparison with the ones in pure C3N (1.404 and 1.403 Å, respectively). The H–CH–C angles display the characteristic angle of sp3 hybridization (105–110°). The CH atom moves out of the C3N plane in the upward direction. Credit: Advanced Electronic Materials

Graphene is well-known as a two-dimensional (2D), crystalline, one-atom-thick layered material, with remarkable conductivity: at room temperature it can conduct current 100 times better than silicon. This is because it is a zero bandgap semiconductor. Its electrons can move freely and uncontrollably between the valence and the conduction bands. But the very same reason that makes the electrons of graphene so fast also hinders its use for the production of semiconductor switching devices, like transistors, which need a bandgap as an on-and-off switch. To unleash its full potential and to make electronic devices thinner, faster, and more efficient than their silicon analogues, researchers look for ways to tame graphene’s electrons. In the process, they have created a new category of graphene-like 2D materials, as in the case of polyaniline.   

Two-dimensional polyaniline possesses a huge advantage over graphene due to the presence of evenly distributed nitrogen atoms in its structure. It has a C3N structural unit and a framework that resembles the planar honeycomb lattice of graphene. The repetition of the C3N unit creates a pattern of six nitrogen atoms that surround a phenyl ring (Fig. a). After 2D polyaniline was synthesized in 2016, scientists kept looking for ways to control its properties in order to engineer new practical applications. A research team from the University of Guilan (Iran), the University of Sydney (Australia), and the University of Antwerp (Belgium) have carried out theoretical simulations, which predict that the basic electronic and magnetic properties of 2D polyaniline can vary widely by surface functionalization, that is, by way of interventions such as by adhering specific atoms to its surface. The results—published recently in Advanced Electronic Materials—reveal a high potential for applications in various fields, including catalysis, energy storage, and nanoelectronics. 

“The idea was that functionalization by adatom adsorption, by topological defects, and by strain engineering would be efficient ways to improve and tune the properties of polyaniline,” says Asad Bafekry of the University of Guilan and first author of the article. “We knew, for example, atom impurities and strain have an effect on the electronic properties of a material. This can be very useful to design novel materials, with an optimal performance, for advanced strain and chemical nanosensors, which can be used in nanoelectronics, energy storage, and photo-catalysts,” he says.

To investigate changes in the atomic, electronic, and magnetic structure of C3N, the team used systematic first-principles calculations. They also simulated scanning tunnelling microscopy images (Fig. b), to produce a visible guide for future experimental observations. 

The results for the adsorption of hydrogen (H) and oxygen (O) adatoms, for different coverages of the structural unit cell, on C3N, showed that the structural and electronic properties of C3N strongly depend on adatom coverage (Fig. c). When H adheres to the surface of polyaniline, it imparts a metallic character to the material, which can be continuously tuned by varying the H coverage from 3.1% to 12.5%. A coverage of 12.5% results in a transition from a semiconductor to a topological insulator (TI) state. 

The researchers also tested O adsorption on the polyaniline. They checked what happens when two atoms were adsorbed on the top (t) and bottom (b) of the polyaniline plane, with three two-side configurations including HtHb, OtOb, and HtOb. In the first case, they saw that the width of the energy bandgap becomes more narrow and decreases with an increasing O/C ratio, while the magnetic properties of C3N are also affected. In the HH case, polyaniline acquires metallic characteristics. For the OO configuration the results showed an indirect semiconductor and for H–O the calculated structure was found to be a nonmagnetic metal. 

The team investigated the surfaces of C3N as well as nanoribbons of C3N being functionalized and semi- or fully-covered with H, O, and F atoms. C3N nanoribbons are thin strips of C3N, with a finite width and a very high length-to-width ratio, which for these experiments was in the range of of 2-4 nm. The researchers found that the adatoms are adsorbed approximately perpendicular to the C3N plane, forming sp3 hybridized bonds with C, which leads to a distortion of the C3N plane and to the opening of a bandgap.  In this case, adsorption of H transforms the material from semiconducting to metallic. Finally, investigations showed that the bandgap can be significantly altered and controlled by the application of strain. 

Javeed Mahmood, a professor at Ulsan National Institute of Science & Technology (UNIST) in South Korea and Jong-Beom Baek, director of the School of Energy and Chemical Engineering at UNIST, are two of the researchers who led the efforts that resulted in the synthesis of polyaniline and did not participate in this work. “This fundamental study shows the true potential of the C3N structure and how simple functionalization or strain can completely modify its electronic and physical properties,” Mahmood says. Baek strongly believes that “if a facile synthesis method for the formation of large area single-layered C3N [can be] developed, it possesses a strong potential from wet-chemistry to device applications, which graphene cannot do. The theoretical study by Bafekry et al. on the C3N structure highlights the real potential of this material and new avenues for future research.” 

Read the abstract in Advanced Electronic Materials.