Long-term stability of Cu surface nanotips

Abstract

Sharp nanoscale tips on metal surfaces of electrodes enhance locally applied electric fields. Strongly enhanced electric fields trigger electron field emission and atom evaporation from the apexes of the nanotips. Combined together, these processes may explain electric discharges in form of small local arcs observed near metal surfaces in the presence of electric fields even in ultra high vacuum conditions. In the present work we investigate the stability of nanoscale tips by means of computer simulations of surface diffusion processes on copper (Cu), the main material of high voltage electronics. We study the stability and life-time of thin Cu surface nanotips at different temperatures in terms of diffusion processes. For this purpose, we have developed a surface Kinetic Monte Carlo (KMC) model where the jump processes are described by tabulated precalculated energy barriers. We show that tall surface features with high aspect ratios can be fairly stable at room temperature. However, the stability was found to depend strongly on the temperature: 13 nm nanotips with the major axes in the <110> crystallographic directions were found to flatten down to half of the original height in less than 100 ns at temperatures close to the melting point, whereas no significant change in the height of these nanotips was observed after 10 $\mu$s at room temperature. Moreover, the nanotips built up along the <110> crystallographic directions were found significantly more stable than those oriented in the <100> or <111> crystallographic directions. The proposed KMC model has been found well suited for simulating atomic surfaces processes and was validated against Molecular Dynamics (MD) simulation results via the comparison of the flattening times obtained by both methods. We also note that the KMC simulations were two orders of magnitude computationally faster than the corresponding MD calculations.