Hypervelocity cluster ion impacts on free standing graphene: Experiment, theory, and applications.

We present results from experiments and molecular dynamics (MD) simulations obtained with C and Au impacting on free-standing graphene, graphene oxide (GO), and graphene-supported molecular layers. The experiments were run on custom-built ToF reflectron mass spectrometers with C and Au-LMIS sources with acceleration potentials generating 50 keV C and 440-540 keV Au . Bombardment-detection was in the same mode as MD simulation, i.e., a sequence of individual projectile impacts with separate collection/identification of the ejecta from each impact in either the forward (transmission) or backward (reflection) direction. For C impacts on single layer graphene, the secondary ion (SI) yields for C and C emitted in transmission are ∼0.1 (10%). Similar yields were observed for analyte-specific ions from submonolayer deposits of phenylalanine. MD simulations show that graphene acts as a trampoline, i.e., they can be ejected without destruction. Another topic investigated dealt with the chemical composition of free-standing GO. The elemental composition was found to be approximately COH. We have also studied the impact of Au clusters on graphene. Again SI yields were high (e.g., 1.25 C/impact). 90-100 Au atoms evaporate off the exiting projectile which experiences an energy loss of ∼72 keV. The latter is a summation of energy spent on rupturing the graphene, ejecting carbon atoms and clusters and a dipole projectile/hole interaction. The charge distribution of the exiting projectiles is ∼50% neutrals and ∼25% either negatively or positively charged. We infer that free-standing graphene enables detection of attomole to zeptomole deposits of analyte via cluster-SI mass spectrometry.