Graphene’s success has shown that it is possible to create stable, single and few-atom-thick layers of van der Waals materials, and also that these materials can exhibit fascinating and technologically useful properties. Here we review the state-of-the-art of 2D materials beyond graphene. Initially, we will outline the different chemical classes of 2D materials and discuss the various strategies to prepare single-layer, few-layer, and multilayer assembly materials in solution, on substrates, and on the wafer scale. Additionally, we present an experimental guide for identifying and characterizing single-layer-thick materials, as well as outlining emerging techniques that yield both local and global information. We describe the differences that occur in the electronic structure between the bulk and the single layer and discuss various methods of tuning their electronic properties by manipulating the surface. Finally, we highlight the properties and advantages of single-, few-, and many-layer 2D materials in...

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Progress, Challenges, and Opportunities in Two-Dimensional Materials Beyond Graphene

Graphene-Like Two-Dimensional Materials

Two-dimensional layered materials (2DLMs) have been a central focus of materials research since the discovery of graphene just over a decade ago. Each layer in 2DLMs consists of a covalently bonded, dangling-bond-free lattice and is weakly bound to neighbouring layers by van der Waals interactions. This makes it feasible to isolate, mix and match highly disparate atomic layers to create a wide range of van der Waals heterostructures (vdWHs) without the constraints of lattice matching and processing compatibility. Exploiting the novel properties in these vdWHs with diverse layering of metals, semiconductors or insulators, new designs of electronic devices emerge, including tunnelling transistors, barristors and flexible electronics, as well as optoelectronic devices, including photodetectors, photovoltaics and light-emitting devices with unprecedented characteristics or unique functionalities. We review the recent progress and challenges, and offer our perspective on the exploration of 2DLM-based vdWHs for future application in electronics and optoelectronics. With a dangling-bond-free surface, two dimensional layered materials (2DLMs) can enable the creation of diverse van der Waals heterostructures (vdWHs) without the conventional constraint of lattice matching or process compatibility. This Review discusses the recent advances in exploring 2DLM vdWHs for future electronics and optoelectronics.

Van der Waals heterostructures and devices

Graphene and monolayer transition metal dichalcogenides (TMDs) are promising materials for next-generation ultrathin optoelectronic devices. Although visually transparent, graphene is an excellent sunlight absorber, achieving 2.3% visible light absorbance in just 3.3 A thickness. TMD monolayers also hold potential as sunlight absorbers, and may enable ultrathin photovoltaic (PV) devices due to their semiconducting character. In this work, we show that the three TMD monolayers MoS2, MoSe2, and WS2 can absorb up to 5–10% incident sunlight in a thickness of less than 1 nm, thus achieving 1 order of magnitude higher sunlight absorption than GaAs and Si. We further study PV devices based on just two stacked monolayers: (1) a Schottky barrier solar cell between MoS2 and graphene and (2) an excitonic solar cell based on a MoS2/WS2 bilayer. We demonstrate that such 1 nm thick active layers can attain power conversion efficiencies of up to ∼1%, corresponding to approximately 1–3 orders of magnitude higher power de...

Extraordinary Sunlight Absorption and One Nanometer Thick Photovoltaics Using Two-Dimensional Monolayer Materials

Individual monolayers of metal dichalcogenides are atomically thin two-dimensional crystals with attractive physical properties different from those of their bulk counterparts. Here we describe the direct synthesis of WS2 monolayers with triangular morphologies and strong room-temperature photoluminescence (PL). The Raman response as well as the luminescence as a function of the number of S–W–S layers is also reported. The PL weakens with increasing number of layers due to a transition from direct band gap in a monolayer to indirect gap in multilayers. The edges of WS2 monolayers exhibit PL signals with extraordinary intensity, around 25 times stronger than that at the platelet’s center. The structure and chemical composition of the platelet edges appear to be critical for PL enhancement.

Extraordinary room-temperature photoluminescence in triangular WS2 monolayers.

First-principles calculations are performed to study the electronic and magnetic properties of VX2 monolayers (X = S, Se). Our results unveil that VX2 monolayers exhibit exciting ferromagnetic behavior, offering evidence of the existence of magnetic behavior in pristine 2D monolayers. Furthermore, interestingly, both the magnetic moments and strength of magnetic coupling increase rapidly with increasing isotropic strain from −5% to 5% for VX2 monolayers. It is proposed that the strain-dependent magnetic moment is related to the strong ionic–covalent bonds, while both the ferromagnetism and the variation in strength of magnetic coupling with strain arise from the combined effects of both through-bond and through-space interactions. These findings suggest a new route to facilitate the design of nanoelectronic devices for complementing graphene.

Evidence of the existence of magnetism in pristine VX₂ monolayers (X = S, Se) and their strain-induced tunable magnetic properties.

Very recently, two-dimensional nanosheets of MoSe(2), MoTe(2) and WS(2) were successfully synthesized experimentally [Science, 2011, 331, 568]. In the present work, the electronic and magnetic properties of perfect, vacancy-doped, and nonmetal element (H, B, C, N, O, and F) adsorbed MoSe(2), MoTe(2) and WS(2) monolayers are systematically investigated by means of first-principles calculations to give a detailed understanding of these materials. It is found that: (1) MoSe(2), MoTe(2) and WS(2) exhibit surprising confinement-induced indirect-direct-gap crossover; (2) among all the neutral native vacancies of MoSe(2), MoTe(2) and WS(2) monolayers, only the Mo vacancy in MoSe(2) can induce spin-polarization and long-range antiferromagnetic coupling; (3) adsorption of nonmetal elements on the surface of MoSe(2), MoTe(2) and WS(2) nanosheets can induce a local magnetic moment; H-absorbed WS(2), MoSe(2), and MoTe(2) monolayers and F-adsorbed WS(2) and MoSe(2) monolayers show long-range antiferromagnetic coupling between local moments even when their distance is as long as ∼12 A. These findings are a useful addition to the experimental studies of these new synthesized two-dimensional nanosheets, and suggest a new route to facilitate the design of spintronic devices for complementing graphene. Further experimental studies are expected to confirm the attractive predictions.

Electronic and magnetic properties of perfect, vacancy-doped, and nonmetal adsorbed MoSe2, MoTe2 and WS2 monolayers

The geometric and electronic structures of graphene adsorption on MoS2 monolayer have been studied by using density functional theory. It is found that graphene is bound to MoS2 with an interlayer spacing of 3.32 A and with a binding energy of −23 meV per C atom irrespective of adsorption arrangement, indicating a weak interaction between graphene and MoS2. A detailed analysis of the electronic structure indicates that the nearly linear band dispersion relation of graphene can be preserved in MoS2/graphene hybrid accompanied by a small band-gap (2 meV) opening due to the variation of on-site energy induced by MoS2. These findings are useful complement to experimental studies of this new synthesize system and suggest a new route to facilitate the design of devices where both finite band-gap and high carrier mobility are needed.

Graphene adhesion on MoS₂ monolayer: an ab initio study.

Very recently, the graphene@MoSe2 heterobilayers [G@MS HBLs] were successfully synthesized experimentally. In this work, the adhesion and electronic properties of the G@MS HBLs have been studied by using density functional theory. It is found that the graphene is weakly bound to the MoSe2 monolayer without any site selectivity. The bands of G@MS HBLs have characteristic graphene-like features with a small band gap (2 meV) opening at K. However, the gap value is significantly lower than kBT at room temperature and can even vanish at room temperature, which suggests that the unusual electronic properties of graphene in G@MS HBLs are comparable to those in the free-standing graphene. Thus, the MoSe2 monolayer might be considered as a promising substrate for potential graphene-based devices. The physical origin of the semiconducting property is related to the inhomogeneity of the onsite energy of C atoms induced by MoSe2. Our results provide a detailed understanding of the interfacial properties of G@MS HBLs ...

First-Principles Study of the Graphene@MoSe2 Heterobilayers

Recently, extensive experimental and theoretical studies on single layers of BN, GaN and graphene have stimulated enormous interest in exploring the properties of these sheets by decorating their surfaces. In the present work we discuss half-fluorinated single layers of BN, GaN and graphene, in the context of intercoupling between strain and magnetic property. First-principles calculations reveal that the energy difference between ferromagnetic and antiferromagnetic couplings increases significantly with strain increasing for half-fluorinated BN, GaN and graphene sheets. More surprisingly, the half-fluorinated BN and GaN sheets exhibit intriguing magnetic transitions between ferromagnetism and antiferromagnetism by applying strain, even giving rise to half-metal when the sheets are under compression of 6%. It is found that the magnetic coupling as well as the strain-dependent magnetic transition behavior arise from the combined effects of both through-bond and p–p direct interactions. Our work offers a new avenue to facilitate the design of controllable and tunable spin devices.

Chengwang Niu

Papers

Evidence of the existence of magnetism in pristine VX₂ monolayers (X = S, Se) and their strain-induced tunable magnetic properties.

Electronic and magnetic properties of perfect, vacancy-doped, and nonmetal adsorbed MoSe2, MoTe2 and WS2 monolayers

Graphene adhesion on MoS₂ monolayer: an ab initio study.

First-Principles Study of the Graphene@MoSe2 Heterobilayers

Strain-induced magnetic transitions in half-fluorinated single layers of BN, GaN and graphene