Cooperative molecular dynamics in surface reactions

Abstract

The controlled imprinting of surfaces with specified patterns is important in the development of nanoscale devices. Previously, such patterns were created using self-assembled physisorbed adsorbate molecules that can be stabilized on the surface by subsequent chemical bonding. Here we show a first step towards use of the bonding within a surface to propagate reactions for patterning, namely the cooperative reaction of adjacent silicon atoms. We exploit the double-bonded silicon dimer pairs present on the surface of Si(100)-2×1 and show that the halogenation of one silicon atom (induced by electrons or heat) results in cooperative halogenation of the neighbouring silicon atom with unit efficiency. The reactants used were two 1-halopentane molecules physisorbed over a pair of silicon atoms. This cooperative pair of halogenation reactions was shown by ab initio calculation to be sequential on a timescale of femtoseconds.

Imprinting self-assembled patterns of lines at a semiconductor surface, using heat, light, or electrons

“…Surface buckling similar to the features marked "b" in Fig 1 was previously observed for 1,5-dichloropentane and also for pairs of 1-fluoropentane molecules (13,14). Exhaustive calculations showed that the buckling arose because of the asymmetry of these molecular pairs, which caused a buildup of charge and hence brightness at a silicon dimer located to one side of the adsorption site (13,14).…”

“…Linear self-assembly was found to be due to dipole-directed assembly (DDA) in which the displacement of surface charge, caused by an initial physisorption event, rendered an adjacent surface site attractive to subsequent physisorption (13). A further finding pertinent to the present study was the identification of "cooperative" surface reaction in which the first molecule of a physisorbed pair of halides (fluoropentane molecules) invariably triggered the reaction of the second molecule of the pair (14). Cooperative reaction was shown to occur for reactions initiated by heat or electrons.…”

“…In the remaining 40% of cases, we always observed one of the two pentyl radicals, R, attaching to the surface; never both. In related work on fluoropentane pairs on Sið100Þ-2×1 (14) we found that the attachment of the pentyl radical to the surface happens late during the reaction and does not affect the barrier to dihalogenation.…”

“…The adsorption product that we image by STM consists, however, of stable CP pairs. In previous work (13,14) such stabilization of pairs was shown by theory to be due to charge-transfer in the surface. The rate of CP pair formation and subsequent line formation depends quadratically on the concentration of mobile CP (ad), whereas the competing rate of desorption back to the gas phase depends linearly on this quantity.…”

“…Exhaustive calculations showed that the buckling arose because of the asymmetry of these molecular pairs, which caused a buildup of charge and hence brightness at a silicon dimer located to one side of the adsorption site (13,14). The surface buckling was shown to attract adsorbates and constitute the growing point for the line formation (13).…”