2D materials: Rising star for future applications

Abstract

Two-dimensional (2D) materials have attracted enormous research interest due to their predominant quantum effects and fascinating materials properties, which potentially lead to various important applications. Meanwhile, the sheer openness of 2D materials makes their interactions with external stimuli particularly efficient. It is much more convenient to modulate the materials properties of 2D systems by mechanical, electronic, optical, and magnetic modulations than in 3D bulks, advantageous for device control. Compared with 3D counterparts, 2D materials that generally include monolayer and few-layer materials exhibit diverse unprecedented characteristics. First, their reduced length in the vertical direction (∼1 nm) is particularly useful for pushing the miniaturization limit of electronic and optoelectronic devices. Successful applications have been demonstrated by prototypical field-effect transistors (FET), tunnel FET (Figures 1A and 1B ), photodetectors, and logic circuits, as well as novel brain-inspired devices. Second, the Coulomb screening in 2D systems is significantly reduced, which greatly enhances many-body interactions and leads to emergent properties. For example, the Coulomb interaction between electrons and holes in 2D is usually several hundreds of meV or more, one order of magnitude stronger than that in 3D. Benefitting from the strong Coulomb binding and the improved lifetime of interlayer excitons with electrons and holes separated in different layers of heterostructures, it is possible to not only fabricate room-temperature excitonic devices, but also observe high-temperature excitonic quantum behaviors including excitonic Bose-Einstein condensation, superfluidity, and electron-hole liquid, which have been long sought since the early studies on 3D semiconductor-based quantum wells. Third, different degrees of freedom of quasiparticles in 2D systems, including valley, orbital, spin, topology, and layer, could couple with each other and also with external fields to give rise to intriguing new states and applications1Xu X. Yao W. Xiao D. et al.Spin and pseudospins in layered transition metal dichalcogenides.Nat. Phys. 2014; 10: 343-350Crossref Scopus (1673) Google Scholar (Figures 1C and 1D). Representatives are spin-valley-coupled transport and valley-selective circular dichroism in transition metal dichalcogenides, which can be adopted for the realization of valley-Hall transistors. Fourth, as ideal model systems, 2D magnetic materials present great opportunities to explore the delicate magnetic interactions and their controls, as well as new devices in the 2D limit,2Burch K.S. Mandrus D. Park J.-G. Magnetism in two-dimensional van der Waals materials.Nature. 2018; 563: 47-52Crossref PubMed Scopus (445) Google Scholar including spin valve, spin filter, spin tunnel FET (Figures 1E and 1F), and magnetic nano electromechanical devices. Fifth, taking advantage of the unique layered structures and distinct properties of 2D materials, it is feasible to construct various heterostructures with rich quantum states and interactions. Besides their applications in excitonic devices shown above, p-n junctions for electronic, photovoltaic (Figure 1G), and light emitting applications have been showcased. Furthermore, the introduction of twist degree of freedom (Figure 1H) brings in a new controllable length and interaction scale endowed by long-wavelength moiré superlattices, providing a new knob to modulate electronic interactions. Through doping, displacement field, and strain control on top of twisting, many novel collective states, including correlated insulator, unconventional superconductor, nontrivial band topology, and orbital magnetism, have been realized in twisted graphene or transition metal dichalcogenides.3Kennes D.M. Claassen M. Xian L. et al.Moiré heterostructures as a condensed-matter quantum simulator.Nat. Phys. 2021; 17: 155-163Crossref Scopus (39) Google Scholar Regarding optical phenomena, twisting in 2D semiconducting systems leads to the observation of moiré excitons, offering new platforms to design artificial excitonic crystals, to study topological and correlated excitons, and to explore potential quantum information applications. Given the vast choices of layered materials, many more emergent phenomena with different dimensionalities and their transitions could be expected in the near future. Moreover, the interplay of topology and symmetry creates exciting opportunities for exploring novel 2D quantum states (Figure 1I). Quantization, symmetry, and phase factor, as three thematic melodies of twentieth-century theoretical physics, have fundamentally revolutionized modern understanding of condensed matter. This is well manifested by the research of topological quantum matter, where the topology is characterized by quantized geometry phase and symmetry or symmetry breaking plays an essential role in defining topological properties.4Hasan M.Z. Kane C.L. Colloquium: topological insulators.Rev. Mod. Phys. 2010; 82: 3045-3067Crossref Scopus (12119) Google Scholar,5Qi X.-L. Zhang S.-C. Topological insulators and superconductors.Rev. Mod. Phys. 2011; 83: 1057-1110Crossref Scopus (8627) Google Scholar Graphene is an ideal example, which displays a plethora of exotic quantum effects (e.g., the anomalous integer quantum Hall effect, Klein tunneling, and weak anti-localization) owing to the existence of massless Dirac fermions with quantized Berry phase π protected by PT (space inversion and time reversal) symmetry. The inclusion of spin-orbit coupling (SOC) introduces momentum-dependent effective magnetic field into the system, which breaks the time reversal symmetry (TRS) of each spin subspace but preserves the TRS as a whole. Thus a spin-dependent Dirac gap opens and the quantum spin Hall (QSH) effect emerges, as found in graphene or graphene-like materials. Alternatively, the coupling with magnetism can open the Dirac gap as well. Distinctly, the anomalous Hall conductance becomes nonzero for broken TRS and quantized to integer multiples of quantum conductance (Ce2/h, C is the first Chern number), as required by topological theory. Another new state of topological matter—quantum anomalous Hall (QAH) insulator (also called Chern insulator) arises, which distinguishes from normal insulators by the nonzero topological Chern number. The QAH effect has been experimentally observed in magnetically doped topological insulator (TI) films, MnBi2Te4 films, and twisted bilayer graphene at low temperatures, and intensive efforts have been devoted to realizing the exotic QAH physics at high temperatures. The 2D Chern insulator is intimately related to SOC, topology, and magnetism and can serve as the parent state in the topological world. Many novel 3D topological states can be built from Chern insulator, including Weyl semimetal, 3D QAH insulator, antiferromagnetic TI, high-order magnetic TI, etc. The introduction of strong correlation effects is able to create fractional Chern insulator that hosts the fractional quantum Hall effect under zero magnetic field. Other quantum states with nontrivial topological order could also be generated by many-body interactions, such as quantum spin liquid, useful for realizing fractionalization and long-range entanglement. Moreover, the topological concept can be generalized from momentum space to other parameter spaces, giving rise to new types of topological entities (e.g., magnetic skyrmions, floquet [fractional] Chern insulator of light). Furthermore, the topological physics is greatly enriched by interacting topology with distinct kinds of spontaneous symmetry-breaking states (e.g., ferroelectricity, excitonic insulator, and superconductivity), which offers a promising platform to explore emergent quantum phenomena (e.g., topological excitonic insulator, topological superconductivity, and Majorana fermion). In this context, the emergent topological physics opens a new era for scientific research of 2D materials, which sheds light on developing unprecedentedly new quantum applications, such as low-power electronics and topological quantum computing.4Hasan M.Z. Kane C.L. Colloquium: topological insulators.Rev. Mod. Phys. 2010; 82: 3045-3067Crossref Scopus (12119) Google Scholar,5Qi X.-L. Zhang S.-C. Topological insulators and superconductors.Rev. Mod. Phys. 2011; 83: 1057-1110Crossref Scopus (8627) Google Scholar Despite great advances in related fields, there remain great challenges for the practical applications of 2D materials. First, the poor long-term stability of many 2D materials with fascinating properties greatly prohibits their practical applications. Second, the low carrier mobility and the low absorption rate of single-layer 2D materials largely limit their applications in electronics and optoelectronics/optics. Third, the weak magnetic interaction and low conductivity of 2D magnetic materials, partly inherited from the localized d orbitals, remain as serious obstacles to tackle with. Fourth, the low-cost growth of various 2D materials with controlled quality and doping, and the scalable integration of channel, contact, and dielectric materials with low contamination and industry compatibility still require considerable research efforts. These challenges also bring up new opportunities for researchers, including searching for new materials, developing new growth routes to synthesize high-quality large-scale samples, and exploring effective approaches to improve the stability in either intrinsic or extrinsic ways. In particular, thanks to the recent achievement in databases for 2D materials, high-throughput calculations combined with advanced machine learning techniques could help facilitate the design of new materials and optimization of their properties for applications.

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