Thermally Activated Sliding of C60 on Gold

Abstract

Gold nanoclusters are known to slide easily on a graphite surface. In this study, we confirm the slipperiness of the gold-carbon interface by studying the sliding behavior of fullerene adsorbates on gold by using a quartz crystal microbalance (QCM). More precisely, we transfer high-quality gold electrodes deposited on an atomically flat mica substrate to the QCM. By means of an effusion cell, we deposit C₆₀ molecules on the QCM gold electrode kept in ultrahigh vacuum. We observe the pinning of the fullerene adsorbates at room temperature. As the temperature increases above 320 K, the fullerene adsorbates begin to slide. This thermally activated sliding is explained in terms of a simple diffusive model.

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(a) AFM image of the surface of gold directly transferred on the gold electrode (1 × 1 μm²); (b) Derivative of the height signal of the panel (a). The scale bars are 250 nm.

2

(a) STM image of the gold surface transferred to the QCM electrode. The terrace steps are monatomic and the total height range is about 2 nm. (b) The derivative signal, although characterized by a low contrast, highlights the “herringbone” reconstruction of the Au(111) surface. The scale bars 50 nm. (c) Height profile along the blue line in panel (a), showing the atomic step. (d) Frequency spectrum of the electrical impedance Z of the quartz before (red) and after (blue) the transfer procedure. The corresponding Q values are 100780 and 105500 respectively.

3

(a) Sketch of the QCM oscillating in the fundamental shear mode. (b) Normalized resonance curve of a quartz crystal. (c) Schematic of the evaporation process: high-purity C₆₀ powder is loaded into the crucible and sublimed at T = 670 K. Opening the shutter S starts the dosing of C₆₀ on the QCM. (d) STM image showing clustering of C₆₀ in proximity of crystal edges. The scale bar is 10 nm.

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Dosing scans representing the resonance frequency (gray squares, left axes) and the amplitude (red circles, right axes) as C₆₀ is deposited on Au(111). (a) The two jumps in resonance frequency observed at T = 355 K occur when the effusion cell shutter is open and closed, respectively. The overall frequency shift corresponds to a C₆₀ coverage of about 0.75 monolayers. (b) The staircase curves at T = 324 K are due to repeated openings and closings of the shutter up to approximately 1.5 C₆₀ monolayers. The corresponding values are taken by waiting for equilibrium to be reached after closing the effusion cell shutter at each step.

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Slip time of C₆₀ on Au(111) as a function of film coverage. The scans are taken at various temperatures, with all data taken at room temperature and below showing no slippage. Each scan refers to a gold layer deposited on a different quartz crystal.

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Temperature dependence of the slip time corresponding to a C₆₀ film coverage θ = 0.25 ML. The continuous blue line is the best fit to the experimental data calculated according to the thermally activated diffusive model described in the text. In particular, the energy barrier against lateral motion is found to be U ₀ = 333 meV.