Frictional duality of metallic nanoparticles: Influence of particle morphology, orientation, and air exposure

The contact area dependence of the interfacial friction experienced during the translation of antimony nanoparticles deposited on a graphite substrate is studied under different conditions using the tip of an atomic force microscope as a manipulation tool. In vacuum a dual behavior of the friction-area curves is found, characterized by the observation that some particles exhibit friction below the detection limit while other similarly sized particles showed constant shear stress values. Detailed investigations prove the reproducibility of this effect, revealing that neither the particle’s morphology nor their alignment relative to the substrate lattice influence the findings. In contrast, we observe that a temporary exposure to ambient air can lead to a drastic increase in the particle’s friction.

Figure 1

(Color online) Evolution of particle morphology with size. The contact areas of the particles shown in (a)-(d) are (i) $60000{\text{nm}}^2$, (ii) $66000{\text{nm}}^2$, (iii) $135000{\text{nm}}^2$, (iv) $360000{\text{nm}}^2$, (v) $60000{\text{nm}}^2$, (vi) $1180000{\text{nm}}^2$, and (vii) $240000{\text{nm}}^2$. While smaller particles are usually round and compact (e.g., particles (i), (ii), larger particles perform a transition to more complex branched structures (e.g., particle (vi). The color scale ranges from 0 nm (dark blue) to $\approx 60\text{nm}$ (bright yellow).

Figure 2

(Color online) Conventional friction force microscopy performed on top of a large Sb nanoparticle with $A_{\text{contact}}=120000{\text{nm}}^2$ (a; image size: $5\times 5{\text{nm}}^2$) and the adjacent HOPG substrate (b); $2.5\times 2.5{\text{nm}}^2$). Additionally, friction loops measured along the fast scan axis are shown for both the antimony nanoparticle (c) and the HOPG substrate (d). In both cases, the lateral force signals show regular stick-slip behavior.

Figure 3

Structural analysis of antimony nanoparticles. (a) TEM measurements performed on small and round nanoparticles. (b) Selected area electron diffraction of the nanoparticles shown in (a). (c) and (d) TEM (c) and SAED (d) measurements of larger, branched nanoparticles.

Figure 4

(Color online) Schematic of the manipulation process. Left: The AFM tip, which profiles the substrate in contact mode, approaches the nanoparticle. Right: The probe tip reaches the nanoparticle and pushes it along its scan path. An increased cantilever torsion is observed due to the additional lateral force acting on the tip during the nanoparticle translation.

Figure 5

(Color online) Example of a particle manipulation event. (a) A small nanoparticle with $A_{\text{contact}}=48000{\text{nm}}^2$ is imaged using a low cantilever load. (b) Exerting a higher load causes manipulation of the nanoparticle. After the nanoparticle has been profiled by the AFM tip for a few lines in this bottom-to-top scan, it is pushed along the line indicated by the white dashed arrow and ultimately removed from the field of view. (c) Lateral force signal acquired simultaneously with the topographical data shown in (b). The increased lateral force due to the nanoparticle manipulation is clearly visible. (d) A quantitative interfacial friction value can be extracted by analyzing the scanline at which manipulation occurred. The interfacial friction is calculated from the difference between the average lateral force signal before and during manipulation. The same nanoparticle was manipulated two more times (e) and (f) with very similar results.

Figure 6

(Color online) Topography signals (top row) and lateral force signals (bottom row): (a) and (d) measured before particle dislocation, (b), (c), (e), (f) measured during the dislocation of two nanoparticles. (b), (e): dislocation of upper particle with $A_{\text{contact}}=\left(60000\pm 5000\right){\text{nm}}^2$; (c), (f): dislocation of lower particle with $A_{\text{contact}}=\left(84000\pm 5000\right){\text{nm}}^2$. (g) Direct comparison of the respective lateral force signals during particle dislocation. While the red curve, corresponding to the event depicted in (c) and (f), shows considerable finite friction (labeled in the figure as “regular translation event”), no apparent friction other than a small initial sticking is visible for the blue curve representing the translation of the particle pushed in (b) and (e).

Figure 7

(Color online) Interfacial friction force vs. contact area measured for Sb nanoparticles manipulated on an HOPG substrate. All measurements have been performed under UHV conditions. The full markers (squares) represent unoxidized nanoparticles kept permanently under UHV conditions. These particle show either a constant, finite shear stress (black), or vanishing friction (red). The open symbols (triangles and circles) represent Sb nanoparticles that had been exposed to ambient conditions prior to the friction measurements. Note that the triangles (gray) are located on the branch of finite shear stress, while the circles (blue) form a second branch of high shear stress.

Figure 8

(Color online) Perimeter (a) and form factor ${\gamma }_{\text{form}}$ (b) plotted vs nanoparticle contact area with round red markers for particles featuring low friction and square black markers for high friction particles. The solid blue curve in (a) indicates the theoretical lower limit of perimeter for perfectly round particles. The two AFM images used as inset in (b) illustrate the particle shapes for different form factors (left particle: $A_{\text{contact}}=25000{\text{nm}}^2$, ${\gamma }_{\text{form}}=0.88$; right particle: $A_{\text{contact}}=178000{\text{nm}}^2$, ${\gamma }_{\text{form}}=0.59$).

Figure 9

(Color online) Examples of particle rotations occurring during the manipulation of antimony nanoparticles. In (a) and (b), the same nanoparticle as featured earlier in Fig. 5 is displayed before the first (a) and before the third (b) manipulation with an effective rotation of $\cong 15°$, whereas the nanoparticle imaged in (c) and (d) is shown at the start (c) and after completion (d) of a series of multiple manipulations $\left(n>10\right)$ that resulted in an accumulated rotation of $\approx 135°$.