Electronics and optoelectronics of two-dimensional transition metal dichalcogenides

Abstract

699M any two-dimensional (2D) materials exist in bulk form as stacks of strongly bonded layers with weak interlayer attraction, allowing exfoliation into individual, atomically thin layers 1 . The form receiving the most attention today is graphene, the monolayer counterpart of graphite. The electronic band structure of graphene has a linear dispersion near the K point, and charge carriers can be described as massless Dirac fermions, providing scientists with an abundance of new physics 2,3 . Graphene is a unique example of an extremely thin electrical and thermal conductor 4 , with high carrier mobility 5 , and surprising molecular barrier properties 6,7 .Many other 2D materials are known, such as the TMDCs 8,9 , transition metal oxides including titania-and perovskite-based oxides 10,11 , and graphene analogues such as boron nitride (BN) 12,13 . In particular, TMDCs show a wide range of electronic, optical, mechanical, chemical and thermal properties that have been studied by researchers for decades 9,14,15 . There is at present a resurgence of scientific and engineering interest in TMDCs in their atomically thin 2D forms because of recent advances in sample preparation, optical detection, transfer and manipulation of 2D materials, and physical understanding of 2D materials learned from graphene.The 2D exfoliated versions of TMDCs offer properties that are complementary to yet distinct from those in graphene. Graphene displays an exceptionally high carrier mobility exceeding 10 6 cm 2 V -1 s -1 at 2 K (ref. 16) and exceeding 10 5 cm 2 V -1 s -1 at room temperature for devices encapsulated in BN dielectric layers 5 ; because pristine graphene lacks a bandgap, however, fieldeffect transistors (FETs) made from graphene cannot be effectively switched off and have low on/off switching ratios. Bandgaps can be engineered in graphene using nanostructuring [17][18][19] , chemical functionalization 20 and applying a high electric field to bilayer graphene 21 , but these methods add complexity and diminish mobility. In contrast, several 2D TMDCs possess sizable bandgaps around 1-2 eV (refs 9,14), promising interesting new FET and optoelectronic devices.TMDCs are a class of materials with the formula MX 2 , where M is a transition metal element from group IV (Ti, Zr, Hf and so on), group V (for instance V, Nb or Ta) or group VI (Mo, W and so on), and X is a chalcogen (S, Se or Te). These materials form layered structures of the form X-M-X, with the chalcogen atoms in two hexagonal planes separated by a plane of metal atoms, as shown in Fig. 1a. Adjacent layers are weakly held together to form the bulk crystal in a variety of polytypes, which vary in stacking orders and metal atom coordination, as shown in Fig. 1e. The overall symmetry of TMDCs is hexagonal or rhombohedral, and the metal atoms have octahedral or trigonal prismatic coordination. The electronic properties of TMDCs range from metallic to semiconducting, as summarized in Table 1. There are also TMDCs that exhibit exotic behaviours such as charge density waves ...

Electronic and Optoelectronic Applications Based on 2D Novel Anisotropic Transition Metal Dichalcogenides

“…Third, the indirect–direct bandgap transition between the bulk and monolayer in some TMDs affords various optoelectronic applications, from photodetectors to light emitters 5, 11, 12, 13. The common chemical formula of TMDs is MX 2 , where M is a transition metal (group IVB–VIIB; M = Mo, W, Re, and so on) and X is a chalcogen (group VIA; X = S, Se, Te) 14, 15. The sandwich structure of TMDs leads to excellent electronic and optoelectronic properties 14, 15, 16, 17, 18.…”

Ultrahigh, Ultrafast, and Self‐Powered Visible‐Near‐Infrared Optical Position‐Sensitive Detector Based on a CVD‐Prepared Vertically Standing Few‐Layer MoS2/Si Heterojunction

“…Since the discovery of graphene in 2004, 2D materials have attracted significant interest due to their unique electronic and optical properties and numerous potential applications in optoelectronic devices 1, 2, 3, 4. Among various 2D materials, MoS 2 has shown excellent properties in optoelectronic applications due to its suitable band gap value,5, 6 relatively high carrier mobility,2, 7 high light absorbance,3, 8 and, more importantly, good stability,9, 10 and brilliant optoelectronic properties 1, 7, 11, 12.…”

“…Among various 2D materials, MoS 2 has shown excellent properties in optoelectronic applications due to its suitable band gap value,5, 6 relatively high carrier mobility,2, 7 high light absorbance,3, 8 and, more importantly, good stability,9, 10 and brilliant optoelectronic properties 1, 7, 11, 12. However, pure MoS 2 ‐based optoelectronic devices are usually limited to infrared light detection and lower photoelectric conversion efficiency (PCE) because of the direct band gap of 1.8 eV for single‐layered MoS 2 sheet5, 6 and the picosecond ultrashort carrier lifetime 13, 14.…”

Few‐Layer Antimonene by Liquid‐Phase Exfoliation

“…Apart from various well‐stablished 2D materials, such as graphene, h‐BN, and MoS 2 , black phosphorus (BP) has received considerable attention over the last two years 2. This is due to the fact that whereas graphene is a non‐band‐gap material and transition‐metal dichalcogenides have a relatively large band gap for certain optoelectronic applications (1.5–2.5 eV),3 the direct band gap of few‐ and single‐layer BP is approximately 1.5 eV,4 and this material therefore has appealing properties for electronic and ultrafast optoelectronic applications. However, isolated layers of BP are extremely sensitive to the surroundings, and strongly degrade upon air exposure, which limits their application 2b.…”