Three-dimensional organic-inorganic perovskites have emerged as one of the most promising thin-film solar cell materials owing to their remarkable photophysical properties, which have led to power conversion efficiencies exceeding 20 per cent, with the prospect of further improvements towards the Shockley-Queisser limit for a single‐junction solar cell (33.5 per cent). Besides efficiency, another critical factor for photovoltaics and other optoelectronic applications is environmental stability and photostability under operating conditions. In contrast to their three-dimensional counterparts, Ruddlesden-Popper phases--layered two-dimensional perovskite films--have shown promising stability, but poor efficiency at only 4.73 per cent. This relatively poor efficiency is attributed to the inhibition of out-of-plane charge transport by the organic cations, which act like insulating spacing layers between the conducting inorganic slabs. Here we overcome this issue in layered perovskites by producing thin films of near-single-crystalline quality, in which the crystallographic planes of the inorganic perovskite component have a strongly preferential out-of-plane alignment with respect to the contacts in planar solar cells to facilitate efficient charge transport. We report a photovoltaic efficiency of 12.52 per cent with no hysteresis, and the devices exhibit greatly improved stability in comparison to their three-dimensional counterparts when subjected to light, humidity and heat stress tests. Unencapsulated two-dimensional perovskite devices retain over 60 per cent of their efficiency for over 2,250 hours under constant, standard (AM1.5G) illumination, and exhibit greater tolerance to 65 per cent relative humidity than do three-dimensional equivalents. When the devices are encapsulated, the layered devices do not show any degradation under constant AM1.5G illumination or humidity. We anticipate that these results will lead to the growth of single-crystalline, solution-processed, layered, hybrid, perovskite thin films, which are essential for high-performance opto-electronic devices with technologically relevant long-term stability.

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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Hexagonal boron nitride (h-BN), a layered material similar to graphite, is a promising dielectric. Monolayer h-BN, so-called "white graphene", has been isolated from bulk BN and could be useful as a complementary two-dimensional dielectric substrate for graphene electronics. Here we report the large area synthesis of h-BN films consisting of two to five atomic layers, using chemical vapor deposition. These atomic films show a large optical energy band gap of 5.5 eV and are highly transparent over a broad wavelength range. The mechanical properties of the h-BN films, measured by nanoindentation, show 2D elastic modulus in the range of 200-500 N/m, which is corroborated by corresponding theoretical calculations.

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...