Investigation of Hypervelocity Massive Projectile- Graphene Interaction and Characterization of Individual Free-standing Nanoparticles in the Transmission Direction by Massive Cluster Secondary Ion Mass Spectrometry

Abstract

It has been shown that secondary ion emission from ultra-thin foils is notably enhanced in the transmission direction. This feature should be of interest for examining nano-objects. A pre-requisite is to deposit them on as thin a support as possible. For this study graphene was chosen. Free-standing graphene was bombarded alone and with deposition of dispersed nanoparticles in a setup enabling bombardment at 0⁰ and secondary ion (SI) detection in transmission in-line with the incident projectiles. C^1,2+ 60 and Au^4+ 400 at impact energies of ∼0.4, 0.8 and 1.2 keV/atom respectively were used as primary ions. The experiments were run as a sequence of single projectile impacts with each time separate recording of the SIs identified via ToF-MS.

In order to improve the understanding of the graphene as a potential quasi-immaterial substrate for the deposition of sub-monolayer nanoparticles, the 1-layer and 4-layer graphene were impacted by the individual 25 and 50 keV C^1,2+ 60 projectiles and negative SIs and secondary electrons (SEs) were collected in the transmission direction. The yields of C− n (n ≤ 4) are above 10% and decrease exponentially with n. The results are explained with the aid of molecular dynamics (MD) simulation. The ionization probability was estimated by comparing the SI yields of C− n to the yields of C 0 n from MD simulation. The ions come from the thermally excited rim of the impact hole damped by cluster fragmentation and electron detachment. The SE probability distributions are Poisson-like, and on average 3 thermal electrons are emitted per impact. The interaction of a 2D projectile on a 2D target is fundamentally different from that on a 3D material.

1-layer graphene was also impacted by the 440-540 keV Au4+ 400 projectiles in both positive and negative ion modes in the transmission direction. The projectiles penetrated the graphene and the Au± 1−3 fragment ions were observed as well as C± n. During the impact, ∼15% of the initial kinetic energy is lost. The Au projectiles are neutralized when approaching the graphene, and then partially ionized again (positively and negatively) via electron tunneling from the hot rims of the impact hole on graphene. The projectiles obtain an internal energy of ∼500 eV (∼4900 K) after the impact. They undergo a ∼90 step fragmentation with the ejection of Au1 atoms in the experimental time range of ∼0.1 µs.

Individual free-standing 5 nm gold nanoparticles coated with dodecanethiol were deposited on graphene film and bombarded with Au^4+ 400 and C^1,2+ 60 . The graphene substrate contributed few SIs beyond m/z 120, facilitating the detection of moieties attached to the nanoparticles. Compared to reflection SIMS, transmission SIMS shows a ∼4 times higher effective yield of molecular ions from the dodecanethiol coating. The SI yields from Au^4+ 400 impact are ∼3 times higher than those from C^2+ 60 impact. The yield of the dodecanethiol molecular ion is 1.0 × 10^−4 from the Au^4+ 400 bombardment and 3.0 × 10^−5 from the C^2+ 60 bombardment. In this case, assuming the Au nanoparticles are perfectly coated by the dodecanethiol molecules, the limit of detection is ∼5×10^4 dodecanethiol molecules with Au^4+ 400 bombardment.