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

Graphitic carbon nitride has been predicted to be structurally analogous to carbon-only graphite, yet with an inherent bandgap. We have grown, for the first time, macroscopically large crystalline thin films of triazine-based, graphitic carbon nitride (TGCN) using an ionothermal, interfacial reaction starting with the abundant monomer dicyandiamide. The films consist of stacked, two-dimensional (2D) crystals between a few and several hundreds of atomic layers in thickness. Scanning force and transmission electron microscopy show long-range, in-plane order, while optical spectroscopy, X-ray photoelectron spectroscopy, and density functional theory calculations corroborate a direct bandgap between 1.6 and 2.0 eV. Thus TGCN is of interest for electronic devices, such as field-effect transistors and light-emitting diodes.

We report on the preparation, atomic resolution imaging, and element selective damage mechanism in atomically thin boron nitride membranes. Flakes of less than 10 layers are prepared by mechanical cleavage and are thinned down to single layers in a high-energy electron beam. At our beam energies, we observe a highly selective sputtering of only one of the elements and predominantly at the exit surface of the specimen, and then subsequent removal of atoms next to a defect. Triangle-shaped holes appear in accordance with the crystallographic orientation of each layer. Defects are compared to those observed in graphene membranes. The observation of clean single-layer membranes shows that hexagonal boron nitride is a further material (in addition to graphene) that can exist in a quasi-two-dimensional allotrope without the need for a substrate.

We present an atomic-resolution observation and analysis of graphene constrictions and ribbons with sub-nanometer width. Graphene membranes are studied by imaging side spherical aberration-corrected transmission electron microscopy at 80 kV. Holes are formed in the honeycomb-like structure due to radiation damage. As the holes grow and two holes approach each other, the hexagonal structure that lies between them narrows down. Transitions and deviations from the hexagonal structure in this graphene ribbon occur as its width shrinks below one nanometer. Some reconstructions, involving more pentagons and heptagons than hexagons, turn out to be surprisingly stable. Finally, single carbon atom chain bridges between graphene contacts are observed. The dynamics are observed in real time at atomic resolution with enough sensitivity to detect every carbon atom that remains stable for a sufficient amount of time. The carbon chains appear reproducibly and in various configurations from graphene bridges, between adsorbates, or at open edges and seem to represent one of the most stable configurations that a few-atomic carbon system accomodates in the presence of continuous energy input from the electron beam. * These authors contributed equally to this work

The electronic charge density distribution or the electrostatic atomic potential of a solid or molecule contains information not only on the atomic structure, but also on the electronic properties, such as the nature of the chemical bonds or the degree of ionization of atoms. However, the redistribution of charge due to chemical bonding is small compared with the total charge density, and therefore difficult to measure. Here, we demonstrate an experimental analysis of charge redistribution due to chemical bonding by means of high-resolution transmission electron microscopy (HRTEM). We analyse charge transfer on the single-atom level for nitrogen-substitution point defects in graphene, and confirm the ionicity of single-layer hexagonal boron nitride. Our combination of HRTEM experiments and first-principles electronic structure calculations opens a new way to investigate electronic configurations of point defects, other non-periodic arrangements or nanoscale objects that cannot be studied by an electron or X-ray diffraction analysis.