Nanoscale Volcanoes: Accretion of Matter at Ion-Sculpted Nanopores

We demonstrate the formation of nanoscale volcanolike structures induced by ion-beam irradiation of nanoscale pores in freestanding silicon nitride membranes. Accreted matter is delivered to the volcanoes from micrometer distances along the surface. Volcano formation accompanies nanopore shrinking and depends on geometrical factors and the presence of a conducting layer on the membrane’s back surface. We argue that surface electric fields play an important role in accounting for the experimental observations.

Figure 1

(a) Schematic of ion-beam sculpting experiment. (b) Nanopore closing curve. (c) AFM surface scans after 0, 4, 8, and 15 s beam exposures.

Figure 2

(a) A pore making a 20° angle to surface. (b) A volcano forms on the acute angle side of the pore.

Figure 3

(a)–(d) Array of pores after successive equal exposures to an ${\mathrm{Ar}}^{+}$ ion beam. The original, presputtered level is at $\sim 5\mathrm{nm}$, as calculated from the known sputter yield of ${\mathrm{Ar}}^{+}$ on silicon nitride. (e) Volcanoes on a gold backed membrane.

Figure 4

Individual nanopore TEM images overlaid on the AFM image of the Fig. 3b nanopore array.

Figure 5

Plot of in-plane electric field $E_T$ near a nanopore in units of $\sigma /ϵ$. Pore radius is at $r/r_0=1$.