It was noted that dissolved O, in 40% aqueous NH4F solution, initiated the formation of etch pits on the terraces of otherwise ideal H-Si(111) surfaces. The etch pits were observed by means of  ex situ  scanning tunnelling microscopy in an Ar atmosphere, following removal from the fluoride solution. The purging of O from the solution, by using Ar, substantially reduced the initiation of etch pits. It was proposed that the etch-pits were initiated when O molecules were reduced, to superoxide anion radicals, at the negative open-circuit potential of the Si surface. A small fraction (less than 0.4%) of these superoxide anions abstracted H atoms from the H-Si(111) terraces so as to form Si radicals (dangling bonds) which were then susceptible to etching in neutral or basic aqueous solutions.

C.P.Wade, C.E.D.Chidsey: Applied Physics Letters, 1997, 71[12], 1679-81