We report on a strong photoluminescence (PL) enhancement of monolayer MoS2 through defect engineering and oxygen bonding. Micro-PL and Raman images clearly reveal that the PL enhancement occurs at cracks/defects formed during high-temperature annealing. The PL enhancement at crack/defect sites could be as high as thousands of times after considering the laser spot size. The main reasons of such huge PL enhancement include the following: (1) the oxygen chemical adsorption induced heavy p doping and the conversion from trion to exciton; (2) the suppression of nonradiative recombination of excitons at defect sites, which was verified by low-temperature PL measurements. First-principle calculations reveal a strong binding energy of ∼2.395 eV for an oxygen molecule adsorbed on a S vacancy of MoS2. The chemically adsorbed oxygen also provides a much more effective charge transfer (0.997 electrons per O2) compared to physically adsorbed oxygen on an ideal MoS2 surface. We also demonstrate that the defect engineering and oxygen bonding could be easily realized by mild oxygen plasma irradiation. X-ray photoelectron spectroscopy further confirms the formation of Mo-O bonding. Our results provide a new route for modulating the optical properties of two-dimensional semiconductors. The strong and stable PL from defects sites of MoS2 may have promising applications in optoelectronic devices.

There have been continuous efforts to seek for novel functional two-dimensional semiconductors with high performance for future applications in nanoelectronics and optoelectronics. In this work, we introduce a successful experimental approach to fabricate monolayer phosphorene by mechanical cleavage and the following Ar + plasma thinning process. The thickness of phosphorene is unambiguously determined by optical contrast combined with atomic force microscope (AFM). Raman spectroscopy is used to characterize the pristine and plasma-treated samples. The Raman frequency of A 2 g mode stiffens, and the intensity ratio of A 2 g to A 1 g modes shows monotonic discrete increase with the decrease of phosphorene thickness down to monolayer. All those phenomena can be used to identify the thickness of this novel two-dimensional semiconductor efficiently. This work for monolayer phosphorene fabrication and thickness determination will facilitates the research of phosphorene.

The electronic structures of two-dimensional materials are strongly dependent on their thicknesses; for example, there is an indirect to direct band gap transition from multilayer to single-layer MoS2. A simple, efficient, and nondestructive way to control the thickness of MoS2 is highly desirable for the study of thickness-dependent properties as well as for applications. Here, we present layer-by-layer thinning of MoS2 nanosheets down to monolayer by using Ar(+) plasma. Atomic force microscopy, high-resolution transmission electron microscopy, optical contrast, Raman, and photoluminescence spectra suggest that the top layer MoS2 is totally removed by plasma while the bottom layer remains almost unaffected. The evolution of Raman and photoluminescence spectra of MoS2 with thickness change is also investigated. Finally, we demonstrate that this method can be used to prepare two-dimensional heterostructures with periodical single-layer and bilayer MoS2. The plasma thinning of MoS2 is very reliable (with almost 100% success rate), can be easily scaled up, and is compatible with standard semiconductor process to generate heterostructures/patterns at nanometer scale, which may bring out interesting properties and new physics.

Epitaxially grown ultrathin organic semiconductors on graphene show great promise as highly efficient phototransistors. The devices exhibit a strong photoresponse down to the limit of a monolayer organic crystal, with a photoresponsivity higher than 10(4) A W(-1) and a photoconductive gain over 10(8) . The excellent performance is attributed to the high quality of the organic crystal and interface, a unique feature of van der Waals epitaxy.

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