The rise of two-dimensional (2D) materials research took place following the isolation of graphene in 2004. These new 2D materials include transition metal dichalcogenides, mono-elemental 2D sheets, and several carbide-and nitride-based materials. The number of publications related to these emerging materials has been drastically increasing over the last five years. Thus, through this comprehensive review, we aim to discuss the most recent groundbreaking discoveries as well as emerging opportunities and remaining challenges. This review starts out by delving into the improved methods of producing these new 2D materials via controlled exfoliation, metal organic chemical vapor deposition, and wet chemical means. We look into recent studies of doping as well as the optical properties of 2D materials and their heterostructures. Recent advances towards applications of these materials in 2D electronics are also reviewed, and include the tunnel MOSFET and ways to reduce the contact resistance for fabricating highquality devices. Finally, several unique and innovative applications recently explored are discussed as well as perspectives of this exciting and fast moving field.

Atomically thin two-dimensional (2D) metals may be key ingredients in next-generation quantum and optoelectronic devices. However, 2D metals must be stabilized against environmental degradation and integrated into heterostructure devices at the wafer scale. The high-energy interface between silicon carbide and epitaxial graphene provides an intriguing framework for stabilizing a diverse range of 2D metals. Here we demonstrate large-area, environmentally stable, epitaxial graphene/single-crystal 2D gallium, indium, and tin heterostructures. The 2D metals are covalently bonded to SiC below but present a non-bonded interface to graphene overlayer, i.e. they are "half van der Waals" metals with strong internal gradients in bonding character. These non-centrosymmetric 2D metals open compelling opportunities for superconducting devices, topological phenomena, and advanced optoelectronic properties. For example, the reported 2D-Ga is a superconductor that combines six strongly coupled Ga-derived electron pockets with a large nearly-freeelectron Fermi surface that closely approaches the Dirac points of the graphene overlayer.Major advances in fundamental science have followed from the exfoliation, stacking, and encapsulation of atomically thin 2D layers 1 . The next step towards technological impact of 2D layers and heterostructures is to transition sophisticated "pick and place" devices to a wafer-scale platform. However, the sensitivity of 2D systems to interfacial reactions and environmental influences -especially for two-dimensional metals or small-gap semiconductors -poses challenges for large-scale integration. Very few metals resist degradation of their top few atomic layers upon environmental exposure, and for a 2D metal, these layers constitute the entire system. A general platform for producing environmentally stable and wafer-scale 2D metals that are not prone to interfacial interactions would represent a significant advance. Inspired by the success of wide-bandgap 2D gallium nitride 2 , we turn focus onto the metal alone and demonstrate a platform dubbed confinement heteroepitaxy (CHet), where the interface between epitaxial graphene (EG) and silicon carbide (SiC) stabilizes crystalline 2D forms of Group-III (Ga, In) and group-IV (Sn) elements. Defect engineering of the graphene overlayer enables uniform, large-area intercalation at the high-energy SiC/EG interface; this interface then templates intercalant crystallization at a thermodynamically defined number of atomic layers. The unreactive nature of as-grown EG on SiC (graphene plus buffer layer) performs multiple services: (1) it only partially passivates the SiC surface underneath, thereby sustaining the high-energy interface that drives intercalation; (2) it lowers the energy of the (otherwise exposed) upper surface of the metal, thus facilitating 2D morphologies; (3) it protects the newly formed 2D metal from environmental degradation after intercalation through in situ healing of the graphene defects. Stability of these 2D metals in air over months gr...

Since their modern debut in 2004, 2-dimensional (2D) materials continue to exhibit scientific and industrial promise, providing a broad materials platform for scientific investigation, and development of nano-and atomic-scale devices. A significant focus of the last decade's research in this field has been 2D semiconductors, whose electronic properties can be tuned through manipulation of dimensionality, substrate engineering, strain, and doping. 1-8 2D semiconductors such as molybdenum disulfide (MoS2) and tungsten diselenide (WSe2) have dominated recent interest for potential integration in electronic technologies, due to their intrinsic and tunable properties, atomic-scale thicknesses, and relative ease of stacking to create new and custom structures. However, to go "beyond the bench", advances in large-scale, 2D layer synthesis and engineering must lead to "exfoliation-quality" 2D layers at the wafer scale. This roadmap aims to address this grand challenge by identifying key technology drivers where 2D layers can have an impact, and to discuss synthesis and layer engineering for the realization of electronic-grade, 2D materials. We focus on three fundamental areas of research that must be heavily pursued in both experiment and computation to achieve high-quality materials for electronic and optoelectronic applications. The document is organized as follows:

Doping is a fundamental requirement for tuning and improving the properties of conventional semiconductors. Recent doping studies including niobium (Nb) doping of molybdenum disulfide (MoS 2 ) and tungsten (W) doping of molybdenum diselenide (MoSe 2 ) have suggested that substitutional doping may provide an efficient route to tune the doping type and suppress deep trap levels of two dimensional (2D) materials. To date, the impact of the doping on the structural, electronic and photonic properties of in-situ doped monolayers remains unanswered due to challenges This article is protected by copyright. All rights reserved.2 including strong film-substrate charge transfer, and difficulty achieving doping concentrations greater than 0.3 at%. Here, we demonstrate in-situ rhenium (Re) doping of synthetic monolayer MoS 2 with ~1 at% Re. To limit substrate-film charge transfer r-plane sapphire is used. Electronic measurements demonstrate that 1 at% Re doping achieves nearly degenerate n-type doping, which agrees with density functional theory calculations. Moreover, low-temperature photoluminescence (PL) indicates a significant quench of the defect-bound emission when Re is introduced, which is attributed to the Mo-O bond and sulfur vacancies passivation and reduction in gap states due to the presence of Re.The work presented here demonstrates that Re doping of MoS 2 is a promising route towards electronic and photonic engineering of 2D materials.