Crystallographic defects play a key role in determining the properties of crystalline materials. The new class of two-dimensional materials, foremost graphene, have enabled atomically resolved studies of defects, such as vacancies, 1-4 grain boundaries, 5-7 dislocations, 8,9 and foreign atom substitutions. 10-14 However, atomic resolution imaging of implanted selfinterstitials has so far not been reported in any three-but also not in any two-dimensional material. Here, we deposit extra carbon into single-layer graphene at soft landing energies of ∼1 eV using a standard carbon coater. We identify all the self-interstitial dimer structures theoretically predicted earlier, 15-17 employing 80 kV aberration-corrected high-resolution transmission electron microscopy. We demonstrate accumulation of the interstitials into larger aggregates and dislocation dipoles, which we predict to have strong local curvature by atomistic modeling, and to be energetically favourable configurations as compared to isolated interstitial dimers. Our results contribute to the basic knowledge on crystallographic defects, and lay out a pathway into engineering the properties of graphene by pushing the crystal into a state of metastable supersaturation.

Formation and characterization of low-dimensional nanostructures is crucial for controlling the properties of two-dimensional (2D) materials such as graphene. Here, we study the structure of low-dimensional adsorbates of cesium iodide (CsI) on free-standing graphene using aberration-corrected transmission electron microscopy at atomic resolution. CsI is deposited onto graphene as charged clusters by electrospray ion-beam deposition. The interaction with the electron beam forms two-dimensional CsI crystals only on bilayer graphene, while CsI clusters consisting of 4, 6, 7, and 8 ions are exclusively observed on single-layer graphene. Chemical characterization by electron energy-loss spectroscopy imaging and precise structural measurements evidence the possible influence of charge transfer on the structure formation of the CsI clusters and layers, leading to different distances of the Cs and I to the graphene.

Aberration-corrected high-resolution transmission electron microscopy (AC-HRTEM) has enabled atomically resolved imaging of molecules adsorbed on low-dimensional materials like carbon nanotubes, graphene oxide and few-layer-graphene. However, conventional methods for depositing molecules onto such supports lack selectivity and specificity. Here, we describe the chemically selective preparation and deposition of molecules-like polyoxometalate (POM) anions [PWO] using electrospray ion-beam deposition (ES-IBD) along with high-resolution TEM imaging. This approach provides access to sub-monolayer coatings of intact molecules on freestanding graphene, which enables their atomically resolved ex situ characterization by low-voltage AC-HRTEM. The capability to tune the deposition parameters in either soft or reactive landing mode, combined with the well-defined high-vacuum deposition conditions, renders the ES-IBD based method advantageous over alternative methods such as drop-casting. Furthermore, it might be expanded towards depositing and imaging large and nonvolatile molecules with complex structures.

SAC has played a vital role in electronbeam-assisted defect-formation reactions in 2D-materials decorated with atomic species. [2] The reported defect structures, such as, vacancies, holes, and nanoribbons, have promising applications such as molecular sieving, [3,4] confinement, [5] magnetism, [6] electrochemistry, [7] and for DNA translocation sensors. [8] SAC was shown to be triggered by transition-metal atoms and silicon, [9][10][11] which form covalent bonds with the underlying graphene and thus lead to some charge redistribution. [11] This weakens the CC atom bonding in the graphene lattice and facilitates vacancy formation in the graphene lattice under high-energy electron irradiation. The dangling bonds at the vacancy site have the tendency to attract other SAC atoms due to their high reactivity compared to pristine graphene, [12] hence, resulting in additional vacancy formation ultimately leading to the formation of holes. [13][14][15][16] Experimental studies highlight that group-I (alkali), group-III, and group-VIII (noble gas) atoms do not show SAC; instead, group-I atomic species form pure ionic bonds, and group-III atoms form partial ionic and covalent bonds with graphene. [17,18] However, the CC bond-weakening reaction in single-layer graphene (SLG) has not been studied in case of molecular species. Of particular interest are Atomic design of a 2D-material such as graphene can be substantially influenced by etching, deliberately induced in a transmission electron microscope. It is achieved primarily by overcoming the threshold energy for defect formation by controlling the kinetic energy and current density of the fast electrons.Recent studies have demonstrated that the presence of certain species of atoms can catalyze atomic bond dissociation processes under the electron beam by reducing their threshold energy. Most of the reported catalytic atom species are single atoms, which have strong interaction with single-layer graphene (SLG). Yet, no such behavior has been reported for molecular species. This work shows by experimentally comparing the interaction of alkali and halide species separately and conjointly with SLG, that in the presence of electron irradiation, etching of SLG is drastically enhanced by the simultaneous presence of alkali and iodine atoms. Density functional theory and first principles molecular dynamics calculations reveal that due to chargetransfer phenomena the CC bonds weaken close to the alkali-iodide species, which increases the carbon displacement cross-section. This study ascribes pronounced etching activity observed in SLG to the catalytic behavior of the alkali-iodide species in the presence of electron irradiation.