ABSTRACT
Low-energy ion scattering (LEIS) is known for its extreme surface sensitivity, as it yields a quantitative analysis of the outermost surface as well as highly resolved in-depth information for ultrathin surface layers. Hence, it could have been generally considered to be a suitable technique for the analysis of graphene samples. However, due to the low scattering cross section for light elements such as carbon, LEIS has not become a common technique for the characterization of graphene. In the present study we use a high-sensitivity LEIS instrument with parallel energy analysis for the characterization of CVD graphene transferred to thermal silica/silicon substrates. Thanks to its high sensitivity and the exceptional depth resolution typical of LEIS, the graphene layer closure was verified, and different kinds of contaminants were detected, quantified, and localized within the graphene structure. Utilizing the extraordinarily strong neutralization of helium by carbon atoms in graphene, LEIS experiments performed at several primary ion energies permit us to distinguish carbon in graphene from that in nongraphitic forms (e.g., the remains of a resist). Furthermore, metal impurities such as Fe, Sn, and Na located at the graphene-silica interface (intercalated) are detected, and the coverages of Fe and Sn are determined. Hence, high-resolution LEIS is capable of both checking the purity of graphene surfaces and detecting impurities incorporated into graphene layers or their interfaces. Thus, it is a suitable method for monitoring the quality of the whole fabrication process of graphene, including its transfer on various substrates.
ABSTRACT
The distribution of specific atoms and molecules within living cells is of high interest in bio-medical research. Laser secondary neutral mass spectrometry (laser-SNMS) and time-of-flight secondary ion mass spectrometry (TOF-SIMS) detect atoms with high sensitivity and spatial resolution. The application of these methods to cultured cells requires special preparation techniques preserving morphological and chemical integrity of the living cells. The cells should, therefore, be grown on a conducting material preventing charging of the sample during ion bombardment. Silicon is currently used as the preferred support material for non-biological samples in mass spectrometry. This study investigates (1) the influence of silicon surfaces on cell growth and (2) the suitability of a sandwiched, rapid freezing method to analyse transmembrane ion gradients. Human melanoma cells were grown on silicon with polished or etched surfaces. Growth kinetics were studied using the Sulforhodamine-B assay. Number, shape, and morphology of the cells were assessed by epifluorescence microscopy of calcein AM- and DAPI-stained cells. Cells were subjected to rapid freezing, freeze-fracturing, and freeze-drying prior to analysis by TOF-SIMS and laser-SNMS. While cell numbers and morphology on the rough silicon wafers were impaired, morphology and growth kinetics of cells on polished silicon were identical to control cells on cell culture tested polystyrene. TOF-SIMS and laser-SNMS resulted in high-resolution elemental images and mass spectra. Measurement of the intracellular Na+ and K+ concentrations revealed a ratio as observed in living cells. In conclusion, culturing cells on polished silicon wafers followed by sandwiched, rapid freezing is an adequate preparation method to study intracellular ion distribution with mass spectrometry.
