Monolayer molybdenum disulfide (MoS2) is a two-dimensional direct band gap semiconductor with unique mechanical, electronic, optical, and chemical properties that can be utilized for novel nanoelectronics and optoelectronics devices. The performance of these devices strongly depends on the quality and defect morphology of the MoS2 layers. Here we provide a systematic study of intrinsic structural defects in chemical vapor phase grown monolayer MoS2, including point defects, dislocations, grain boundaries, and edges, via direct atomic resolution imaging, and explore their energy landscape and electronic properties using first-principles calculations. A rich variety of point defects and dislocation cores, distinct from those present in graphene, were observed in MoS2. We discover that one-dimensional metallic wires can be created via two different types of 60° grain boundaries consisting of distinct 4-fold ring chains. A new type of edge reconstruction, representing a transition state during growth, was also identified, providing insights into the material growth mechanism. The atomic scale study of structural defects presented here brings new opportunities to tailor the properties of MoS2 via controlled synthesis and defect engineering.

Single layered molybdenum disulfide with a direct bandgap is a promising twodimensional material that goes beyond graphene for next generation nanoelectronics. Here, we report the controlled vapor phase synthesis of molybdenum disulfide atomic layers and elucidate a fundamental mechanism for the nucleation, growth, and grain boundary formation in its crystalline monolayers. Atomic layered graphene has shown many fascinating properties as a supplement to silicon-based semiconductor technologies [1][2][3][4] . Consequently, great effort has been devoted to the development and understanding of its synthetic processes [5][6][7][8] . However, graphene with its high leaking current, due to its zero bandgap energy, is not suitable for many applications in electronics and optics 9, 10 . Recent developments in two different classes of materials -transition metal oxides and sulfides -have shown many promises to fill the existing gaps [10][11][12] . For example, the successful demonstration of molybdenum disulfide (MoS 2 )-based field-effect transistors (FET) 11 , has prompted an intense exploration of the physical properties of few-layered MoS 2 films [13][14][15][16][17] .MoS 2 is a layered semiconductor with a bandgap in the range of 1.2-1.8 eV, whose physical properties are significantly thickness-dependent 13,14 . For instance, a considerable enhancement in the photoluminescence of MoS 2 has been observed as the thickness of the material decreases 14 . The lack of inversion symmetry in single-layer Initially, small triangular domains were nucleated at random locations on the bare substrate (Fig. 1a). Then, the nucleation sites continued to grow and formed boundaries when two or more domains met (Figs. 1b and 1c), resulting in a partially continuous film.This process can eventually extend into large-area single-layered MoS 2 continuous films if sufficient precursor supply and denser nucleation sites are provided (Fig. 1d) In the quest for feasible strategies to control the nucleation process, we take advantage of some of our common experimental observations. Our experiments show that the MoS 2 triangular domains and films are commonly nucleated and formed in the vicinity of the substrates' edges, scratches, dust particles, or rough surfaces (supplementary Fig. S4).We utilized this phenomenon to control the nucleation by strategically creating step edges on substrates using conventional lithography processes (Fig. 1e). The patterned substrates with uniform distribution of rectangular SiO 2 pillars (40×40 μm 2 in size, 40 μm apart, and ~40 nm thick) were directly used in the CVD process for MoS 2 growth ( The inherent dependence of this approach on the edge-based nucleation resembles some of the observations and theoretical predictions in the growth other layered materials [29][30] .Theoretical studies have revealed a significant reduction in the energy barrier of graphene nucleation close to the step edges, as compared to flat surfaces of transition metal substrates 30 . We propose that similar edge-based catalytic pr...

Monolayer Molybdenum disulfide (MoS 2 ), a two-dimensional crystal with a direct bandgap, is a promising candidate for 2D nanoelectronic devices complementing graphene. There have been recent attempts to produce MoS 2 layers via chemical and mechanical exfoliation of bulk material. Here we demonstrate the large area growth of

Phosphorene, an elemental 2D material, which is the monolayer of black phosphorus, has been mechanically exfoliated recently. In its bulk form, black phosphorus shows high carrier mobility (~10000 cm 2 /V· s) and a ~0.3 eV direct bandgap. Well-behaved p-type field-effect transistors with mobilities of up to 1000 cm 2 /V· s, as well as phototransistors, have been demonstrated on few-layer black phosphorus, showing its promise for electronics and optoelectronics applications due to its high hole mobility and thickness-dependence direct bandgap. However, p-n junctions, the basic building blocks of modern electronic and optoelectronic devices, have not yet been realized based on black phosphorus. In this paper, we demonstrate a gate tunable p-n diode based on a p-type black phosphorus/n-type monolayer MoS2 van der Waals p-n heterojunction. Upon illumination, these ultra-thin p-n diodes show a maximum photodetection responsivity of 418 mA/W at the wavelength of 633 nm, and photovoltaic energy conversion with an external quantum efficiency of 0.3%. These p-n diodes show promise for broadband photodetection and solar energy harvesting.Key words: black phosphorus, phosphorene, MoS2, p-n diode, van der Waals heterojunction, photodetection, solar cell 3 The successful isolation of graphene from graphite has led to its extensive study in physics, materials, and nano-engineering due to its extraordinary electrical and mechanical properties. [1][2][3][4] However, a lack of a bandgap limits its potential for electronic device applications, and has inspired the exploration of other 2D layered materials. [5][6][7] Among them, transition metal dichalcogenides (TMDCs), such as MoS2, are the most studied materials. [8][9][10][11] Recently, phosphorene, the monolayer form of black phosphorus, has been successfully isolated. 12 Analogous to graphite and graphene, black phosphorus is a stack of phosphorene monolayers, bound together by van der Waals interactions. 12,13 Bulk black phosphorus shows a ~0.3 eV direct bandgap and a mobility of up to ~10000 cm 2 /V· s. 14-17 Its bandgap increases as its thickness decreases, and is predicted to have a >1 eV direct bandgap in its monolayer form. 12,13 Well-behaved p-type field-effect transistors with mobilities of up to 1000 cm 2 /V· s, as well as inverters, have been demonstrated on few-layer black phosphorus. 12,13,[18][19][20] Based on its direct bandgap, few-layer black phosphorus phototransistors have been demonstrated with a responsivity of 4.8 mA/W. 19 These results indicate that black phosphorus is a promising candidate for both high performance electronics and optoelectronics applications due to its ultra-thin 2D nature, high hole mobility, and narrower direct bandgap compared to most of TMDCs. P-N junctions are the basic building blocks of modern semiconductor devices, including diodes, bipolar transistors, photodiodes, light-emitting diodes, and solar cells. In the conventional p-n homo-junction, the p-and n-type regions are formed by 4 chemically doping a bulk semiconduct...

We show that the lack of inversion symmetry in monolayer MoS2 allows strong optical second harmonic generation. Second harmonic of an 810-nm pulse is generated in a mechanically exfoliated monolayer, with a nonlinear susceptibility on the order of 10 −7 m/V. The susceptibility reduces by a factor of seven in trilayers, and by about two orders of magnitude in even layers. A proofof-principle second harmonic microscopy measurement is performed on samples grown by chemical vapor deposition, which illustrates potential applications of this effect in fast and non-invasive detection of crystalline orientation, thickness uniformity, layer stacking, and single-crystal domain size of atomically thin films of MoS2 and similar materials.

Monolayer molybdenum disulfide (MoS 2 ) has attracted tremendous attention due to its promising applications in high-performance field-effect transistors, phototransistors, spintronic devices and nonlinear optics. The enhanced photoluminescence effect in monolayer MoS 2 was discovered and, as a strong tool, was employed for strain and defect analysis in MoS 2 . Recently, large-size monolayer MoS 2 has been produced by chemical vapour deposition, but has not yet been fully explored. Here we systematically characterize chemical vapour deposition-grown MoS 2 by photoluminescence spectroscopy and mapping and demonstrate non-uniform strain in single-crystalline monolayer MoS 2 and strain-induced bandgap engineering. We also evaluate the effective strain transferred from polymer substrates to MoS 2 by three-dimensional finite element analysis. Furthermore, our work demonstrates that photoluminescence mapping can be used as a non-contact approach for quick identification of grain boundaries in MoS 2 .

Atomic layers of two-dimensional (2D) materials have recently been the focus of extensive research. This follows from the footsteps of graphene, which has shown great potential for ultrathin optoelectronic devices. In this paper, we present a comprehensive study on the synthesis, characterization, and thin film photodetector application of atomic layers of InSe. Correlation between resonance Raman spectroscopy and photoconductivity measurements allows us to systematically track the evolution of the electronic band structure of 2D InSe as its thickness approaches few atomic layers. Analysis of photoconductivity spectra suggests that few-layered InSe has an indirect band gap of 1.4 eV, which is 200 meV higher than bulk InSe due to the suppressed interlayer electron orbital coupling. Temperature-dependent photocurrent measurements reveal that the suppressed interlayer interaction also results in more localized pz-like orbitals, and these orbitals couple strongly with the in-plane E' and E″ phonons. Finally, we measured a strong photoresponse of 34.7 mA/W and fast response time of 488 μs for a few layered InSe, suggesting that it is a good material for thin film optoelectronic applications.

Ternary two-dimensional dichalcogenide alloys exhibit compositionally modulated electronic structure, and hence, control of dopant concentration within each individual layer of these compounds provides a powerful tool to efficiently modify their physical and chemical properties. The main challenge arises when quantifying and locating the dopant atoms within each layer in order to better understand and fine-tune the desired properties. Here we report the synthesis of molybdenum disulfide substitutionally doped with a broad range of selenium concentrations, resulting in over 10% optical band gap modulations in atomic layers. Chemical analysis using Z-contrast imaging provides direct maps of the dopant atom distribution in individual MoS2 layers and hence a measure of the local optical band gaps. Furthermore, in a bilayer structure, the dopant distribution is imaged layer-by-layer. This work demonstrates that each layer in the bilayer system contains similar local Se concentrations, randomly distributed, providing new insights into the growth mechanism and alloying behavior in two-dimensional dichalcogenide atomic layers. The results show that growth of uniform, ternary, two-dimensional dichalcogenide alloy films with tunable electronic properties is feasible.