Ultra-high vacuum scanning tunnelling microscopy investigation of free radical adsorption to theSi(111)-7 × 7 surface

Abstract

Room-temperature ultra-high vacuum (UHV) scanning tunnelling microscopy(STM) has been employed to investigate free radical chemistry on theSi(111)-7 × 7 surface with atomic-scale spatial resolution. In particular, due to itssingle-site binding mechanism and extensive previous study on theSi(100)-2 × 1 surface, the nitroxyl free radical 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO)was explored. UHV STM imaging of isolated molecules revealed that TEMPOcovalently reacts with adatom dangling bonds with high affinity. By monitoringTEMPO adsorption as a function of surface coverage, TEMPO was also found topreferentially bind to centre adatom sites at the initial stages of adsorption. On theother hand, as the surface coverage increased, TEMPO molecules adsorbed tocentre adatoms and corner adatoms approached a ratio of 1:1. At all surfacecoverage levels, TEMPO showed minimal preference for binding to either thefaulted or unfaulted half of the unit cell. Consequently, upon saturation, theTEMPO adlayer exhibited long-range ordering and preserved the underlying7 × 7 surface reconstruction. This study provides fundamental insight into free radical surface chemistryand suggests a direct pathway for forming nearly perfectly ordered organic adlayers on theSi(111)-7 × 7 surface.