Nitric oxide molecule adsorbes dissociatively on Si(111)7x7 surface at room temperature (RT) [1]. Recently, the migration picture of N and O atoms after the dissociation was suggested [2]: the kinetic energies of the N and O atoms dissipate into the substrate and finally trap to Si adatoms but not Si rest atoms. Ref. 2 reported preferential site-selectivity for the adsorption on faulted half (FH) unit-cell than unfaulted half (UH), and almost equal selectivity for corner (Co) and center (Ce) adatoms at RT, from STM empty-state images obtained at positive sample-bias voltages Vs. If the hopping N and O atoms interact in each other, the interaction should result in some correlation between the adsorption sites. To study the correlation we require sequential images in the same area as a function of NO exposure.
In experiments using a pulse jet valve for NO dosage, we have obtained the same area images in STM at RT as functions of the pulse shot and Vs. We confirmed the trend of increasing of darken adatoms with the shot number in negative and positive Vs (filled and empty states, respectively) implying N or O adsorption, however, found that darken sites in both Vs do not coincide; there are darken and darken (DD), darken and normal (DN), and normal and darken (ND) type-sites for negative and positive Vs, respectively. The number of DD type-sites was close to the sum of DN and ND type-sites, and the number of ND type-sites was quit smaller than that of DN type-sites. We suggest that the atop type adsorption (possibly due to N atom) leads to DD type-site and the backbond-bridge type adsorption (possibly due to O atom) leads to DN or ND type-sites. The site-selectivity in positive Vs (for DD and ND type-sites) was consistent with Ref. 2.
From the analysis of spatial distribution of these DD, DN and ND type-sites in 3,000 observed adatoms at 2.5-9.0% in coverage, we found strong correlation 1) between DD-type Ce adatom in FH and DN-type Ce adatom in UH, and 2) between DN-type Ce adatoms in the same UH.
References:
[1] M. Nishijima et al., Surf. Sci. 137, 473 (1984).
[2] H.-J. Liu et al., Phys. Rev. B 73, 165421 (2006). |