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.

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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...

Graphene quantum dots (GQDs), which are edge-bound nanometer-size graphene pieces, have fascinating optical and electronic properties. These have been synthesized either by nanolithography or from starting materials such as graphene oxide (GO) by the chemical breakdown of their extended planar structure, both of which are multistep tedious processes. Here, we report that during the acid treatment and chemical exfoliation of traditional pitch-based carbon fibers, that are both cheap and commercially available, the stacked graphitic submicrometer domains of the fibers are easily broken down, leading to the creation of GQDs with different size distribution in scalable amounts. The as-produced GQDs, in the size range of 1-4 nm, show two-dimensional morphology, most of which present zigzag edge structure, and are 1-3 atomic layers thick. The photoluminescence of the GQDs can be tailored through varying the size of the GQDs by changing process parameters. Due to the luminescence stability, nanosecond lifetime, biocompatibility, low toxicity, and high water solubility, these GQDs are demonstrated to be excellent probes for high contrast bioimaging and biosensing applications.

Nanostructured carbon materials are potentially of great technological interest for the development of electronic, catalytic and hydrogen-storage systems. Here we describe a general strategy for the synthesis of highly ordered, rigid arrays of nanoporous carbon having uniform but tunable diameters (typically 6 nanometres inside and 9 nanometres outside). These structures are formed by using ordered mesoporous silicas as templates, the removal of which leaves a partially ordered graphitic framework. The resulting material supports a high dispersion of platinum nanoparticles, exceeding that of other common microporous carbon materials (such as carbon black, charcoal and activated carbon fibres). The platinum cluster diameter can be controlled to below 3 nanometres, and the high dispersion of these metal clusters gives rise to promising electrocatalytic activity for oxygen reduction, which could prove to be practically relevant for fuel-cell technologies. These nanomaterials can also be prepared in the form of free-standing films by using ordered silica films as the templates.