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 ...

Using high resolution transmission electron microscopy, we identify the specific atomic scale features in chemically derived graphene monolayers that originate from the oxidation-reduction treatment of graphene. The layers are found to comprise defectfree graphene areas with sizes of a few nanometers interspersed with defect areas dominated by clustered pentagons and heptagons. Interestingly, all carbon atoms in these defective areas are bonded to three neighbors maintaining a planar sp 2 -configuration, which makes them undetectable by spectroscopic techniques. Furthermore, we observe that they introduce significant in-plane distortions and strain in the surrounding lattice.

We present an accurate measurement and a quantitative analysis of electron-beam induced displacements of carbon atoms in single-layer graphene. We directly measure the atomic displacement ("knock-on") cross section by counting the lost atoms as a function of the electron beam energy and applied dose. Further, we separate knock-on damage (originating from the collision of the beam electrons with the nucleus of the target atom) from other radiation damage mechanisms (e.g. ionization damage or chemical etching) by the comparison of ordinary ( 12 C) and heavy ( 13 C) graphene. Our analysis shows that a static lattice approximation is not sufficient to describe knock-on damage in this material, while a very good agreement between calculated and experimental cross sections is obtained if lattice vibrations are taken into account.

By combining high-resolution transmission electron microscopy experiments and first-principles calculations, we study production, diffusion, and agglomeration of sulfur vacancies in monolayer MoS 2 under electron irradiation. Single vacancies are found to be mobile under the electron beam and tend to agglomerate into lines. Different kinds of such extended defects are identified in the experiments, and their atomic structures and electronic properties are determined with the help of calculations. The orientation of line defects is found to be sensitive to mechanical strain. Our calculations also indicate that the electronic properties of the extended defects can be tuned by filling vacancy lines with other atomic species, thereby suggesting a way for strain and electron-beam-assisted engineering of MoS 2 -based nanostructures.

Large-area graphene substrates provide a promising lab bench for synthesizing, manipulating, and characterizing low-dimensional materials, opening the door to high-resolution analyses of novel structures, such as two-dimensional (2D) glasses, that cannot be exfoliated and may not occur naturally. Here, we report the accidental discovery of a 2D silica glass supported on graphene. The 2D nature of this material enables the first atomic resolution transmission electron microscopy of a glass, producing images that strikingly resemble Zachariasen's original 1932 cartoon models of 2D continuous random network glasses. Atomic-resolution electron spectroscopy identifies the glass as SiO(2) formed from a bilayer of (SiO(4))(2-) tetrahedra and without detectable covalent bonding to the graphene. From these images, we directly obtain ring statistics and pair distribution functions that span short-, medium-, and long-range order. Ab initio calculations indicate that van der Waals interactions with graphene energetically stabilizes the 2D structure with respect to bulk SiO(2). These results demonstrate a new class of 2D glasses that can be applied in layered graphene devices and studied at the atomic scale.