Z-contrast atom counting using the few-layer graphene sheets

Upon the development of the quantitative scanning transmission electron microscopy (Q-STEM), individually counting the gold atom has been achieved. The correlation of the signals to the atom quantity (N), i.e. calibration, is its crucial work One has to seek the ultra-fine mass standards, where the size-selected gold clusters rank the best recently [1]. However, practically this is difficult for light elements, since STEM intensity is proportional to Z^α, where Z is atomic number of the elements and a is in the range of 1.5-1.9 depending on the detector collection angle, sample thickness, and the Debye-Waller factor of the atomic species. As a result, the light elements would have a much weaker image contrast than that of heavy elements. This could be even more difficult for the carbon particles supported by the Formvar films. Graphene layers are featured by large scale uniformity and discrete layer steps, which shed light on precision calibration of the carbon atomic scales.