Using first-principles atomistic simulations, we study the response of atomically-thin layers of transition metal dichalcogenides (TMDs) -a new class of two-dimensional inorganic materials with unique electronic properties -to electron irradiation. We calculate displacement threshold energies for atoms in 21 different compounds and estimate the corresponding electron energies required to produce defects. For a representative structure of MoS2, we carry out high-resolution transmission electron microscopy experiments and validate our theoretical predictions via observations of vacancy formation under exposure to a 80 keV electron beam. We further show that TMDs can be doped by filling the vacancies created by the electron beam with impurity atoms. Thereby, our results not only shed light on the radiation response of a system with reduced dimensionality, but also suggest new ways for engineering the electronic structure of TMDs. [6,7]. Recently, TMDs with a common structural formula MeX 2 , where Me stands for transition metals (Mo, W, Ti, etc.) and X for chalcogens (S, Se, Te), have received considerable attention. These 2D materials are expected to have electronic properties varying from metals to wide-gap semiconductors, similar to their bulk counterparts [8,9], and excellent mechanical characteristics [10]. The monolayer TMD materials have already shown a good potential in nanoelectronic [3,11,12] and photonic [4,13,14] applications.Characterization of the h-BN [15-17] and TMD [5,6,18] samples has extensively been carried out using high-resolution transmission electron microscopy (HR-TEM). During imaging, however, energetic electrons in the TEM can give rise to production of defects due to ballistic displacements of atoms from the sample and beam-stimulated chemical etching [19], as studies on h-BN membranes also indicate [15][16][17]20].Contrary to h-BN, very little is known about the effects of electron irradiation on TMDs. So far, atomic defects have been observed via HR-TEM in WS 2 nanoribbons encapsulated inside carbon nanotubes at electron acceleration voltage of 60 kV [21] as well as at the edges of MoS 2 clusters under 80 kV irradiation [22], while no significant damage or amorphization was reported for MoS 2 sheets at 200 kV [18] -a surprising result taking into account the relatively low atomic mass of the S atom. Clearly, precise microscopic knowledge of defect production in TMDs under electron irradiation is highly desirable for assessing the effects of the beam on the samples. This knowledge would allow designing experimental conditions required to minimize damage, as well as developing beam-mediated post-synthesis doping techniques. Moreover, information on the displacement thresholds is important in the context of fundamental aspects of the interaction of beams of energetic particles with solids, as the reduced dimensionality may give rise to an irradiation response different from that in the bulk counterpart of the 2D material [23].Here, by employing first-principles simulations, we study the ...

Adsorbed layers of water are ubiquitously present at surfaces and fill in microscopic pores, playing a central role in many phenomena in such diverse fields as materials science, geology, biology, tribology, nanotechnology, etc. Despite such importance, the crystal structure of nanoconfined water remains largely unknown. Here we report high-resolution electron microscopy of mono-and few-layers of water confined between two graphene sheets, an archetypal example of hydrophobic confinement. Confined water is found to form square ice at room temperature -a phase with symmetry principally different from the conventional tetrahedral geometry of hydrogen bonding. The square ice has a high packing density with a lattice constant of 2.83 Å and during TEM observation assembles in bi-and trilayer crystallites exhibiting AA stacking. Our findings are important for understanding of interfacial phenomena and, in particular, shed light on ultrafast transport of water through hydrophobic nanocapillaries. Our MD simulations suggest that square ice is likely to be common inside hydrophobic nanochannels, independent of their exact atomic makeup.Three-dimensional (3D) water exists in many forms, as liquid, vapor and as many as 15 crystalline and some amorphous phases of ice, with the commonly found hexagonal ice alone being responsible for the fascinating variety of snowflakes [1,2]. Less noticeable but equally ubiquitous is water present at interfaces and in microscopic pores where nanometer-scale confinement makes the structure of water and its dynamics radically different from bulk water [3,4]. Confined and interfacial water has attracted dedicated interest in fields ranging from life to earth to materials sciences, playing a crucial role in such diverse phenomena as protein assembly, nanofriction, filtration, dissolving, frost heaving, detergent cleaning, heterogeneous catalysis and so on [5][6][7][8]. It is now well established that near a solid surface, whether hydrophilic or hydrophobic, water forms a layered structure made up of distinct monolayers [3][4][5][9][10][11][12][13][14][15][16][17][18][19][20][21][22]. However, the structure within these layers remains largely unknown. Molecular dynamics (MD) simulations [9-17] predicted a great variety of phases, although the results are sensitive to modelled conditions and some seem conflicting. For example, a buckled monolayer ice was found inside hydrophilic nanochannels [11] and a flat hexagonal ice inside hydrophobic ones below room temperature [12,15,22]. On the other hand, no in-plane order was observed inside mica (hydrophilic) and graphite (hydrophobic) nanochannels at and above room temperature [11,14]. A close analogue of planar square ice was reported in MD simulations of water inside carbon nanotubes [9,10,17]. In this case, water molecules

Using magnetization measurements, we show that point defects in graphene - fluorine adatoms and irradiation defects (vacancies) - carry magnetic moments with spin 1/2. Both types of defects lead to notable paramagnetism but no magnetic ordering could be detected down to liquid helium temperatures. The induced paramagnetism dominates graphene's low-temperature magnetic properties despite the fact that maximum response we could achieve was limited to one moment per approximately 1000 carbon atoms. This limitation is explained by clustering of adatoms and, for the case of vacancies, by losing graphene's structural stability.Comment: 14 pages, 14 figure