Structures and functions of two-dimensional materials: From theoretical prediction to experimental realization

Abstract

<p indent="0mm">Two-dimensional (2D) materials have shown promising applications in optoelectronics, energy storage and conversion, electronic informatics, aerospace, and biomedicine due to their unique physical, chemical, and mechanical properties. The past decade has witnessed the increasing power and the more widely applications of theoretical approaches, especially those based on the density functional theory (DFT), in the research field of 2D materials, and the close interplay between theory and experiment has become a new paradigm. Herein we briefly reviewed the computational design of the structures and functions of 2D materials, and their experimental realizations, paying special attention to how theoretical predictions guided experimental explorations. The structural design strategy for 2D materials mainly includes bottom-up (constructing 2D materials from 0D or 1D materials) and top-down (exfoliating bulk to monolayers) approaches. In a bottom-up approach, 2D frameworks were constructed by linking macrocyclic conjugated molecules, such as metalloporphyrin and metallophthalocyanine, whose typical MN_{<italic>x</italic>} metal centers can serve as active sites of catalysts. In addition, nanoribbons, which can be viewed as quasi-2D materials or derivatives of 2D materials, were achieved by unzipping the 1D nanotubes. For example, the interaction between the B and its adjacent N atoms of a BN nanotube can be weakened by attaching an external lone electron pair to the B site, thus aqueous ammonia solution was used as an effective reagent to unzip boron nitride nanotubes into BN nanoribbons upon sonication treatment. In the experiment, the top-down approach is a straightforward manner to obtain 2D materials by exfoliating its bulk counterparts. Among others, guided by theoretical predictions, arsenene and antimonene have been exfoliated from the experimentally available bulk material, and various applications have been explored. In addition, the combination of DFT computations with the global minimum search, exemplified by the particle swarm optimization (PSO), in the 2D space, is also an effective method for searching new 2D materials. By this method, the novel pentagonal PdS₂ and PdSe₂ monolayers were theoretically predicted, shortly followed by the experimental fabrication of 2D penta-PdSe₂. We introduced several important methods to manipulate the band structure, magnetism, catalytic activity, etc. of 2D materials, including surface hydrogenation and halogenation, doping with heteroatoms, modulating single-atom catalytic centers, forming van der Waals heterojunctions. These functionalizations enable 2D materials for various applications, such as nanoelectronic devices, catalysts, gas separation, and electrode materials for rechargeable batteries. Among others, we discussed the hydrogenation of h-BN and how their electronic structures and magnetism performance were modulated. For example, fully hydrogenated armchair BN nanoribbons are nonmagnetic semiconductors, while the zigzag counterparts are magnetic and metallic. Moreover, single-doping, co-doping, and doping at different sites were theoretically explored to improve the catalytic performance of the 2D materials, and the corresponding experimental verifications were highlighted. Almost all the theoretically designed materials under review have been experimentally realized, demonstrating the power of the theoretical studies in the research field of 2D materials. With the further development of computer software and hardware and increasing usage of machine learning and big-data techniques, theoretical studies will play an even more important role and accelerate the research and development of 2D materials in a faster pace.