Decay of nanoripples on Au(111) studied by He atom scattering

The morphology of the nanostructured Au(111) surface has been investigated in the temperature range between 170 and 400 K by means of thermal energy He atom scattering. The gold surface has been irradiated with 3 keV Ar+ ions along the azimuthal direction < 110 > at grazing incidence (about 75 degrees). After sputtering at temperatures greater than 180 K, a well-defined diffraction pattern has been measured, which is consistent with the development of a ripple structure with ridges oriented along the incoming ion-beam direction. The evolution with time of the diffraction pattern has been followed at different crystal temperatures T-C in the range 280-400 K. The results show that the rippled structure is stable up to 350 K. Instead, between 370 and 390 K, the structure changes with time with a rate that depends on temperature, and the ripple wavelength is constant. The decay is well described by an exponential function, and the estimated activation energy is about 0.85 eV. At 400 K, the data show an increase of the ripple wavelength and this suggests that shorter wavelength modes, which contribute to the ripple profile, decay faster than longer ones with a shift toward greater values of the average ripple wavelength.