Effects of Metal Nanoparticles on the Secondary Ion Yields of a Model Alkane Molecule upon Atomic and Polyatomic Projectiles in Secondary Ion Mass Spectrometry

Abstract

A model alkane molecule, triacontane, is used to assess the effects of condensed gold and silver nanoparticles on the molecular ion yields upon atomic (Ga(+) and In(+)) and polyatomic (C60(+) and Bi3(+)) ion bombardment in metal-assisted secondary ion mass spectrometry (MetA-SIMS). Molecular films spin-coated on silicon were metallized using a sputter-coater system, in order to deposit controlled quantities of gold and silver on the surface (from 0 to 15 nm equivalent thickness). The effects of gold and silver islets condensed on triacontane are also compared to the situation of thin triacontane overlayers on metallic substrates (gold and silver). The results focus primarily on the measured yields of quasi-molecular ions, such as (M - H)(+) and (2M - 2H)(+), and metal-cationized molecules, such as (M + Au)(+) and (M + Ag)(+), as a function of the quantity of metal on the surface. They confirm the absence of a simple rule to explain the secondary ion yield improvement in MetA-SIMS. The behavior is strongly dependent on the specific projectile/metal couple used for the experiment. Under atomic bombardment (Ga(+), In(+)), the characteristic ion yields an increase with the gold dose up to approximately 6 nm equivalent thickness. The yield enhancement factor between gold-metallized and pristine samples can be as large as approximately 70 (for (M - H)(+) under Ga(+) bombardment; 10 nm of Au). In contrast, with cluster projectiles such as Bi3(+) and C60(+), the presence of gold and silver leads to a dramatic molecular ion yield decrease. Cluster projectiles prove to be beneficial for triacontane overlayers spin-coated on silicon or metal substrates (Au, Ag) but not in the situation of MetA-SIMS. The fundamental difference of behavior between atomic and cluster primary ions is tentatively explained by arguments involving the different energy deposition mechanisms of these projectiles. Our results also show that Au and Ag nanoparticles do not induce the same behavior in MetA-SIMS of triacontane. The microstructures of the metallized layers are also different. While metallic substrates provide higher yields than metal islet overlayers in the case of silver, whatever the projectile used, the situation is reversed with gold.

Influence of the organic layer thickness in (Metal-Assisted) secondary ion mass spectrometry using Ga+ and C60+ projectiles

“…For thick layers, the yield values are more than two orders of magnitude higher that those measured upon Ga ϩ bombardment. Another striking difference with the Ga ϩ results, already observed in a previous study of triacontane samples [7], occurs after Au evaporation. With C 60 ϩ projectiles, metallization leads to a yield decrease even after evaporation of 0.5 nm Au.…”

“…This behavior has been already observed for molecular alkane layers metallized with various gold thicknesses [7]. In that study, the Au n ϩ cluster yields decreased continuously as the gold coverage increased for polyatomic projectiles such as C 60 ϩ .…”

“…On the other hand, the yields increase or decrease continuously with the amount of evaporated gold depending whether Ga ϩ or C 60 ϩ , respectively, are used as projectiles (Figures 2 and 3). An interpretation has already been proposed for this differential effect of the gold metallization in previous papers [7,16,23] [16]. They found that the use of polyatomic ion such as Bi 3 ϩ , SF 5 ϩ , and C 60 ϩ leads to a decrease of the secondary ion yields with increasing gold deposition.…”

“…However, existing reports indicate that the yield enhancement seems to be much more limited than for bulk organics [21,22]. In a similar manner, the efficiency of MetA-SIMS could only be proved in the case of atomic projectiles [7,23].…”

“…Specific substrates such as noble metals strongly enhance the characteristic secondary ions yields upon atomic projectile bombardment [4,7,8]. Moreover, the signal intensities from a given compound are affected by the surface coverage but they are not always proportional to the amount of the compound available at the surface [6, 9 -13].…”