Force and energy dissipation variations in noncontact atomic force spectroscopy of composite carbon nanotube systems

UHV dynamic force and energy dissipation spectroscopy in noncontact atomic force microscopy were used to probe specific interactions with composite systems formed by encapsulating inorganic compounds inside single-walled carbon nanotubes. It is found that forces due to nanoscale van der Waals interaction can be made to decrease by combining an Ag core and a carbon nanotube shell in the silver-filled single-walled carbon nanotube (Ag@SWNT) system. This specific behavior was attributed to a significantly different effective dielectric function compared to the individual constituents, evaluated using a core-shell model. Energy dissipation measurements showed that by filling dissipation increases. Aside from minimized Joule dissipation, such an effect was attributed to two short-range mechanisms due to metal filling: softening of C-C bonds resulting in a more deformable nanotube cage, and an increased mechanical damping of the nanotube’s bending oscillation mode. Thus, filled and unfilled nanotubes can be discriminated based on force and dissipation measurements. These findings have two different implications for potential applications: tuning the effective dielectric properties and tuning the interaction force for molecular absorption by appropriately choosing the filling with respect to the nanotube.

Figure 1

(Color online) Typical spectroscopic mapping and curves over an individual, unfilled single-walled nanotube. (a), (b) Frequency and dissipation maps ($0.6\times 1.6{\mathrm{nm}}^2$ resolution), respectively, corresponding to a tip’s position $-0.5\mathrm{nm}$ below the topographic profile. (c) Representative force-distance curves for various lateral positions of the tip, on the flat surface and across the nanotube. The curves are symmetric relative to the center of the tube’s topographic profile. The strength of the interaction decreases from the sides toward the top of the nanotube, where it reaches its minimum.

Figure 2

(Color online) Spectroscopic maps ($1\times 1{\mathrm{nm}}^2$ resolution) and topography of a bundle from the Ag-filled material. (a), (b) Topography and topographic profile, respectively, taken with a set point of $-20\mathrm{Hz}$ in the original frequency curves. (c), (d) Frequency and dissipation maps corresponding to the tip being positioned $-0.45\mathrm{nm}$ below the topographic profile from (a). (e), (f) Frequency and dissipation maps corresponding to the tip being positioned above the topographic profile from (a). Region “f,” with lowest force and higher dissipation, is attributed to an Ag-filled part of a nanotube.

Figure 3

(Color online) Selected force-distance (a) and dissipation-distance (b) curves corresponding to the bundle from Fig. 2, taken at various lateral positions of the tip: on the flat graphite surface (curve 1), and across the nanotube bundle (2 at the very edges of the bundle, 3 at the very top of the bundle, and “f” over the region “f”). Curves with stronger interaction force correspond to curves with stronger dissipation, except for region “f,” where dissipation is not the lowest, despite the force being the lowest. The force values are compatible with nanoscale van der Waals interaction between $1\mathrm{nm}$ radius sphere and tube/plane. Inset of (b) shows similar slopes for curves 1, 2, and “f,” and a lower slope for curve 3 in the large separation regime.

Figure 4

(Color online) Frequency maps (a) and (c), and dissipation maps (b) and (d) below and above the topographic profile, respectively. $1\times 1{\mathrm{nm}}^2$ resolution. The marked regions “${}^{*}$” from (c) are attributed to Ag-filled nanocomposites. (e) The superposition of (a) and (c). (f) Force -distance curves corresponding to a Ag-filled region (“${}^{*}$”), the edge of the bundle (1), and on a normal region on the bundle (2).

Figure 5

(Color online) (a) Simulated van der Waals interaction between a mesoscopic tip, of $11\mathrm{nm}$ radius, and the single-walled carbon nanotube from Fig. 1. The short-range part of the interaction is being kept fixed, as described in the text. Curves correspond to different points across the topographic profile shown in the inset. The inset shows the contact profile (continuous line) and the topographic profile (dashed line) obtained for an arbitrary low-attractive-force set point. (b) A higher attractive force for the feedback.

Figure 6

(a) Complex dielectric functions for Ag, and corresponding to the transverse direction for a (23,0) SWCNT (Ref. 43). (b) Effective dielectric function for the Ag filling-SWCNT shell system. A spherical system was considered instead of the real cylindrical geometry. (c) $\epsilon \left(i\nu \right)$ functions for Ag, unfilled SWCNT, and the combined Ag/SWCNT system.