Interaction of multiply charged ions with large free silver nanoparticles: Multielectron capture, fragmentation, and sputtering phenomena

We report on the interaction of multiply charged ions at keV energies with free Ag nanoparticles with diameters of $\sim 6$ nm (containing about 6600 Ag atoms). As the ionization energy increases only very slowly with the degree of ionization, multielectron capture processes are very likely to occur in peripheral collisions with large cluster sizes. However, due to the large particle size, the produced highly charged Ag nanoparticles are mostly stable. For projectile charge states below $q=8$ the geometrical cross section overcomes the cross section for peripheral electron transfer, and penetrating collisions become dominant. Therefore, these collisions can be described as ion collision with a nanosurface, where the distribution of small-size fragments with $n<20$ is due to sputtering events from the nanoparticle surface. This is in contrast to nanoparticles with a smaller diameter ($<2$ nm), where small fragments are produced by fission of multiply charged clusters.

Figure 1

(a) AFM image of Ag nanoparticles deposited on a ${\mathrm{SiO}}_2$ substrate. (b) Size histogram with a center at a diameter of about 6 nm. (c) Structure of a 5-nm Ag NP obtained by TEM microscopy.

Figure 2

(a) Scheme of the collision system, showing the impact parameter $b$ and the nanoparticle radius $R_{\mathrm{NP}}$. (b) Geometrical cross section (horizontal lines) and cross sections for the total electron capture in peripheral collisions as a function of the projectile charge, shown for two Ag nanoparticle with diameters of 2 and 6 nm.

Figure 3

Time-of-flight spectrum of silver nanoparticles produced in ${\mathrm{Ar}}^{9+}-{\mathrm{Ag}}_n$ collisions at a collision energy of 135 keV.

Figure 4

(a) Stability diagram for multiply charged Ag nanoparticles. The appearance sizes for $q=2$ and $q=3$ are taken from Schulze et al. [33]. The size distribution of the initial nanoparticles is shown as a dashed line. (b) COB cross sections for multielectron capture by ${\mathrm{Ar}}^{9+}$ projectiles from ${\mathrm{Ag}}_n$ ($D_{\mathrm{NP}}=6$ nm) and Ar atoms.

Figure 5

Distribution of small singly charged fragments produced in collisions of ${\mathrm{Xe}}^{17+}$ projectiles with neutral Ag nanoparticles at 255 keV.

Figure 6

Integrated peak intensities as a function of the cluster size $n$ normalized to the monomer intensity. Squares: spectrum taken from Fig. 5 for ${\mathrm{Xe}}^{17+}$ ion collisions with Ag nanoparticles; dots: spectrum taken obtained in ${\mathrm{Xe}}^{+}$-surface collisions [40].

Figure 7

Comparison of sputter yields. The values are normalized to that for collisions with ${\mathrm{Xe}}^{25+}$ projectiles. Squares: integrated intensities for monomers, dimers, and trimers measured in the present experiment for different projectiles; dots: yield values calculated with the empirical formula for ion/Ag-surface collisions [43].