Controlled substitutional doping of two-dimensional transition-metal dichalcogenides (TMDs) is of fundamental importance for their applications in electronics and optoelectronics. However, achieving p-type conductivity in MoS2 and WS2 is challenging because of their natural tendency to form n-type vacancy defects. Here, we report versatile growth of p-type monolayer WS2 by liquid-phase mixing of a host tungsten source and niobium dopant. We show that crystallites of WS2 with different concentrations of substitutionally doped Nb up to 1014 cm–2 can be grown by reacting solution-deposited precursor film with sulfur vapor at 850 °C, reflecting the good miscibility of the precursors in the liquid phase. Atomic-resolution characterization with aberration-corrected scanning transmission electron microscopy reveals that the Nb concentration along the outer edge region of the flakes increases consistently with the molar concentration of Nb in the precursor solution. We further demonstrate that ambipolar field-effect transistors can be fabricated based on Nb-doped monolayer WS2.

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Two-dimensional (2D) van der Waals transition metal dichalcogenides (TMDs) are a new class of electronic materials offering tremendous opportunities for advanced technologies and fundamental studies. Similar to conventional semiconductors, substitutional doping is key to tailoring their electronic properties and enabling their device applications. Here, we review recent progress in doping methods and understanding of doping effects in group 6 TMDs (MX 2 , M = Mo, W; X = S, Se, Te), which are the most widely studied model 2D semiconductor system. Experimental and theoretical studies have shown that a number of different elements can substitute either M or X atoms in these materials and act as n-or p-type dopants. This review will survey the impact of substitutional doping on the electrical and optical properties of these materials, discuss open questions, and provide an outlook for further studies.

Impurity doping is a viable route toward achieving desired subgap optical response in semiconductors. In strongly excitonic two-dimensional (2D) semiconductors such as transition metal dichalcogenides (TMDs), impurities are expected to result in bound-exciton emission. However, doped TMDs often exhibit a broad Stokes-shifted emission without characteristic features, hampering strategic materials engineering. Here we report observation of a well-defined impurity-induced emission in monolayer WS 2 substitutionally doped with rhenium (Re), which is an electron donor. The emission exhibits characteristics of localized states and dominates the spectrum up to 200 K. Gate dependence reveals that neutral impurity centers are responsible for the observed emission. Using GW-Bethe−Salpeter equation (GW-BSE) calculations, we attribute the emission to transitions between spin-split upper Re band and valence band edge.

Unlike traditional water splitting in an aqueous medium, direct decomposition of atmospheric water is a promising way to simultaneously dehumidify the living space and generate power. Here, a tailored superhygroscopic hydrogel, a catalyst, and a solar cell are integrated into a humidity digester that can break down ambient moisture into hydrogen and oxygen, creating an efficient electrochemical cell. The function of the hydrogel is to harvest moisture from ambient humidity and transfer the collected water to the catalyst. Barium titanate and vertical 2D MoS2 nanosheets are integrated as the catalyst: the negatively polarized cathode can enhance the electron transport and attract H+ to the MoS2 surface for water reduction, while water oxidation takes place at the positively polarized anode. By employing this mechanism, it is possible to maintain the relative humidity in a medium‐sized room at <60% without any additional energy input, and a stable current of 12.5 mA cm−2 is generated by the humidity digester when exposed to ambient light.

atomically thin 2D layered materials, such as monolayer transition metal dichalcogenides (TMDs), [5][6][7][8] hexagonal boron nitride (hBN). [9,10] and gallium selenide, [11] are attractive alternative hosts to overcome such fundamental limitations of bulk counterparts.Following the initial reports on single photon emitters observed in naturally occurring defects in as-grown and as-exfoliated 2D TMDs, [5][6][7][8] various strain and crystal defect engineering approaches have been developed to deterministically generate these quantum emitters. While local strain introduced by nano-pillars/ holes or nano-indents commonly results in single photon emitters in a variety of 2D materials including hBN, WS 2 , WSe 2 , and MoSe 2 , [12][13][14][15][16][17] creation of point defects by ion [10,18] and electron [9,19] beam irradiation has proved to be a viable route to inducing similar quantum emitters. Further, direct writing of quantum emitter arrays on monolayer MoS 2 with precisely controlled positions has been demonstrated using a focused helium ion beam. [20][21][22] Chalcogen vacancies (V X , X = S/Se), which are commonly present in TMDs, are known to introduce in-gap states. [23][24][25][26] Studies on helium-ion treated MoS 2 (Refs. [18,(20)(21)(22)27,28]) found that transitions involving such in-gap states can be optically bright and yield anti-bunched photons at sufficiently low-density defects. However, emission peaks commonly attributed to chalcogen vacancies in other TMDs are often broad (FWHM > 100 meV), lacking typical features of quantum emitters. [26,29,30] Defect engineering of atomically thin semiconducting crystals is an attractive route to developing single-photon sources and valleytronic devices. For these applications, defects with well-defined optical characteristics need to be generated in a precisely controlled manner. However, defect-induced optical features are often complicated by the presence of multiple defect species, hindering the identification of their structural origin. Here, we report systematic generation of optically active atomic defects in monolayer MoS 2 , WS 2 , MoSe 2 , and WSe 2 via proton-beam irradiation. Defect-induced emissions are found to occur ≈100 to 200 meV below the neutral exciton peak, showing typical characteristics of localized excitons such as saturation at high-excitation rates and long lifetime. Using scanning transmission electron microscopy, it is shown that freshly created chalcogen vacancies are responsible for the localized exciton emission. Density functional theory and ab initio GW plus Bethe-Salpeter-equation calculations reveal that the observed emission can be attributed to transitions involving defect levels of chalcogen vacancy and the valence band edge state.

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Hexagonal boron nitride (hBN) has been a centre of interest due to its ability to host several bright quantum emitters at room temperature. However, the identification of the observed emitters remains challenging due to spectral variability as well as the lack of atomic defect structure information. In this work, we report two new blue quantum emitters with zero phonon line (ZPL) centred around 460 nm and 490 nm in hBN powders. We further demonstrate that the blue emissions can be created by high temperature annealing or high energy ion irradiation in exfoliated hBN flakes. Scanning transmission electron microscopy (STEM) reveals that the dominant defect structures present in ion irradiated sample are vacancy-type (V­x­) and adatom(intercalant)-type (A­x­). Together with first principle GW-BSE (Bethe-Salpeter equation) calculation, the observed blue emissions at 490 nm may be due to boron intercalants (Bint). Our results not only discover a new group of blue quantum emissions in hBN, but also provide an insight on the physical origin of the emissions by correlating the emission wavelength with local atomic structures in hBN.