Plasmon spectroscopy of the thinnest possible membrane, a single layer of carbon atoms: graphene, has been carried out in conjunction with ab initio calculations of the low loss function. We observe and +-surface plasmon modes in free-standing single sheets at 4.7 and 14.6 eV, which are substantially redshifted from their values in graphite. These modes are in very good agreement with the theoretical spectra, which find theand + in-plane modes of graphene at 4.8 and 14.5 eV. We also find that there is little loss caused by out-of-plane modes for energies less than about 10 eV.

Research interest in graphene, a two-dimensional crystal consisting of a single atomic plane of carbon atoms, has been driven by its extraordinary properties, including charge carriers that mimic ultra-relativistic elementary particles. Moreover, graphene exhibits ballistic electron transport on the submicrometre scale, even at room temperature, which has allowed the demonstration of graphene-based field-effect transistors and the observation of a room-temperature quantum Hall effect. Here we confirm the presence of free-standing, single-layer graphene with directly interpretable atomic-resolution imaging combined with the spatially resolved study of both the p ! p* transition and the p 1 s plasmon. We also present atomic-scale observations of the morphology of free-standing graphene and explore the role of microstructural peculiarities that affect the stability of the sheets. We also follow the evolution and interaction of point defects and suggest a mechanism by which they form ring defects.Recent measurements of the remarkable electronic properties of graphene have resulted in intense research activity on twodimensional (2D) crystals [1][2][3][4][5] . Unlike most materials in condensed matter physics, where the Schrödinger equation can be used to describe their electronic properties, for graphene the charge carriers mimic relativistic particles and can thus be described using the Dirac equation 3 . The ability of extended 2D structures to exist is the subject of a long-standing theoretical debate, and it has previously been suggested that 2D films embedded in three-dimensional (3D) space can be stabilized by out-of-plane undulations 6,7 . Elucidating the atomic structure of graphene may seem blindingly obvious at first consideration, but, given that it is necessarily an 'imperfect' 2D crystal, it offers insight in three important ways. First, direct imaging of atoms combined with energy-loss spectroscopy provides further corroboration of the existence of areas of free-standing monolayers of carbon atoms. Second, revealing the atomic structure of the edges of graphene and the fundamental topological defects within adds insight to the stability issues, as does the characterization of the surface contamination believed to consist mainly of hydrocarbons ubiquitously found on graphene. This last point may also provide clues as to certain limitations in the electronic behaviour of graphene films.Evidence of the existence of free-standing graphene has been obtained from electron diffraction experiments 3 , which, in this case, was averaged over approximately a square micrometre of material. Recently, others have presented defect configurations in suspended graphene using bright-field phase contrast 8 . The appearance of atomic structure in phase contrast in the case of 3D crystals is not immediately interpretable, and even in 2D crystals, is sensitive to focusing conditions. However, the atomic lattice seen in high-angle annular dark-field (HAADF) images acquired in a scanning transmission electron microscope (STEM)...

The effect of strain on the phonon modes of monolayer and few-layer MoS 2 has been investigated by observing the strain-induced shifts of the Raman-active modes. Uniaxial strain was applied to a sample of thin-layer MoS 2 sandwiched between two layers of optically transparent polymer. The resulting band shifts of the E 2 1 g (∼385.3 cm −1) and A 1g (∼402.4 cm −1) Raman modes were found to be small but observable. First-principles plane-wave calculations based on density functional perturbation theory were used to determine the Grüneisen parameters for the E 1g , E 2 1 g , A 1g , and A 2u modes and predict the experimentally observed band shifts for the monolayer material. The polymer-MoS 2 interface is found to remain intact through several strain cycles. As an emerging 2D material with potential in future nanoelectronics, these results have important consequences for the incorporation of thin-layer MoS 2 into devices.

The properties of suspended graphene are currently attracting enormous interest, but the small size of available samples and the difficulties in making them severely restrict the number of experimental techniques that can be used to study the optical, mechanical, electronic, thermal and other characteristics of this one-atom-thick material. Here we describe a new and highly-reliable approach for making graphene membranes of a macroscopic size (currently up to 100 µm in diameter) and their characterization by transmission electron microscopy. In particular, we have found that long graphene beams supported by one side only do not scroll or fold, in striking contrast to the current perception of graphene as a supple thin fabric, but demonstrate sufficient stiffness to support extremely large loads, millions of times exceeding their own weight, in agreement with the presented theory. Our work opens many avenues for studying suspended graphene and using it in various micromechanical systems and electron microscopy.