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.

The relationship between structure, PEO mobility, and ionic conductivity is investigated for the solid polymer electrolyte, PEO/LiClO 4 . Amorphous and semicrystalline samples with ether-oxygen-to-lithium ratios ranging from 4:1 to 100:1 are measured. Previous X-ray diffraction results show that three crystalline phases can form in this system depending on the LiClO 4 concentration: (PEO) 3 :LiClO 4 , pure PEO, and (PEO) 6 : LiClO 4 . We use SANS to determine that the (PEO) 3 :LiClO 4 phase forms cylinders with a radius of 125 Å and a length of 700 Å. We also measure the amount and size of pure PEO lamellae by exploiting the neutron scattering length density contrast that arises because of crystallization. The samples are thermally treated such that the (PEO) 6 :LiClO 4 phase does not form. QENS is used to measure PEO mobility directly in amorphous and semicrystalline samples, and it reveals two processes. The first process at short times is attributed to the segmental mobility of PEO, and the second process at longer times is attributed to the restricted rotation of protons around the Li + ions. The type of motion and the radius of rotation are consistent with a cylindrical structure observed by diffraction: two PEO chains wrapping around Li + ions in an ether-oxygen-to-lithium ratio of 6:1. By directly comparing structure, mobility, and conductivity of the same samples, we determine that at 50 °C, a semicrystalline sample (concentration of 14:1) has the highest conductivity despite being less mobile, partially crystalline, and having less charge carriers than amorphous samples at the same temperature. The results suggest a decoupling of ionic conductivity and polymer mobility.

Atomically thin transition metal dichalcogenides (TMDs) are of interest for next-generation electronics and optoelectronics. Here, we demonstrate device-ready synthetic tungsten diselenide (WSe) via metal-organic chemical vapor deposition and provide key insights into the phenomena that control the properties of large-area, epitaxial TMDs. When epitaxy is achieved, the sapphire surface reconstructs, leading to strong 2D/3D (i.e., TMD/substrate) interactions that impact carrier transport. Furthermore, we demonstrate that substrate step edges are a major source of carrier doping and scattering. Even with 2D/3D coupling, transistors utilizing transfer-free epitaxial WSe/sapphire exhibit ambipolar behavior with excellent on/off ratios (∼10), high current density (1-10 μA·μm), and good field-effect transistor mobility (∼30 cm·V·s) at room temperature. This work establishes that realization of electronic-grade epitaxial TMDs must consider the impact of the TMD precursors, substrate, and the 2D/3D interface as leading factors in electronic performance.

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.

Transition metal dichalcogenides are relevant for electronic devices owing to their sizable band gaps and absence of dangling bonds on their surfaces. For device development, a controllable method for doping these materials is essential. In this paper, we demonstrate an electrostatic gating method using a solid polymer electrolyte, poly(ethylene oxide) and CsClO4, on exfoliated, multilayer 2H-MoTe2. The electrolyte enables the device to be efficiently reconfigured between n- and p-channel operation with ON/OFF ratios of approximately 5 decades. Sheet carrier densities as high as 1.6 × 10(13) cm(-2) can be achieved because of a large electric double layer capacitance (measured as 4 μF/cm(2)). Further, we show that an in-plane electric field can be used to establish a cation/anion transition region between source and drain, forming a p-n junction in the 2H-MoTe2 channel. This junction is locked in place by decreasing the temperature of the device below the glass transition temperature of the electrolyte. The ideality factor of the p-n junction is 2.3, suggesting that the junction is recombination dominated.