Transmission electron microscopy (TEM) is a microscopy method in which an electron beam images a specimen by transmission. The specimen is usually an ultrathin section less than 100 nm thick or a suspension on a grid. An image is formed from the interaction of the electrons with the sample as the beam is transmitted.
Because of the small de Broglie wavelength of electrons, TEM offers resolution thousands of times higher than the resolution of light microscopes. Thus since its invention in 1931, TEM has become a prime analytical method in the physical, chemical and biological sciences, having found applications in cancer research, virology, and materials science as well as pollution, nanotechnology and semiconductor research.
Because TEM grids have holes with diameters on the order of micrometers, sometimes they are not suitable for imaging very small objects that can fall through the holes. In the past years, graphene has become something of a standard support material for samples on TEM grids. Graphene can be suspended over the holes in the grid, covering the entire grid, and thus be used as support for objects that rest on the graphene above the holes. Because of the small atomic number Z of the carbon atoms that constitute graphene, it interacts very little with the electron beam, yielding sharper images.
An experimental demonstration of high-resolution imaging of single low-atomic number atoms such as hydrogen and carbon on graphene-covered TEM grids was first published in 2008. On a clean single-layer graphene membrane, adsorbates such as atomic hydrogen and carbon could be seen as if they were suspended in free space. Such precision is not possible using other material membranes on TEM grids, for example SiN, because those membranes are much thicker than graphene.
Graphene-covered TEM grids are especially proving to be useful for biotechnology, because biological samples tend to stick to the walls of the holes in uncovered TEM grids, distorting shape and adversely affecting resolution. Graphene solves that problem and is even compatible with cryogenic TEM (cryoTEM) because it can sustain large temperature variations.
Image: Sketch of graphene on TEM grid membrane used for XPS (Velasco-Vélez et al, Rev. Sci. Instrum. 87, 053121 (2016)).
Graphenea markets single layer and bilayer graphene suspended on TEM grids. Such TEM grids have enabled atmospheric-pressure X-ray photoemission spectroscopy (XPS), alleviating the need for high vacuum conditions for samples studied with XPS. Atmospheric operation allows studies of biological samples as well as solid/gas interfaces in a reaction environment, which is extremely important for catalysis, a vital process in energy generation. The graphene forms a barrier between the atmospheric side where the sample is housed and the vacuum side that houses the X-Ray components. Atmospheric XPS with graphene TEM barriers has for example been used to study extremely small particles such as iridium nanoparticles. The oxidation reaction of these nanoparticles is an important process in hydrogen harvesting from electrochemical oxidation of water, a prospective candidate for future energy generation devices. Graphene on TEM grids also enables studies of hydrogenation of Pd-black catalysts (see Velasco-Vélez et al, Topics in Catalysis 61, 2052 (2018)).
Image: High resolution TEM images of iridium nanoparticles on bilayer graphene TEM grid (Velasco-Vélez et al, Surface Science 681 (2019) 1–8).
The unprecedented resolution enabled by graphene-on-TEM allowed imaging of single catalyst atoms, such as silicon adatoms chiselling atomic structures in graphene (Li Wang et al, Nano Lett. 2014, 14, 450−455), as well as resolving atomic-scale damage induced by electron beams on bilayer graphene (Zubeltzu et al, Phys. Rev. B 88, 245407 (2013).
Image: Molecular-scale graphene pores, with catalyst Si adatoms on pore edges (Li Wang et al, Nano Lett. 2014, 14, 450−455).
Image: Atomic-scale defects on graphene induced by electron beam (Zubeltzu et al, Phys. Rev. B 88, 245407 (2013)).