Facile Charge-Displacement at Silicon Gives Spaced-out Reaction

Abstract

Adsorbates on metals, but not previously on semiconductors, have been observed to display long-range repulsive interactions. On metals, due to efficient dissipation, the repulsions are weak, typically on the order of 5 meV at 10 Å. On the 7×7 reconstruction of the Si(111) surface, charge transport through the surface has been demonstrated by others using charge injection by STM tips. Here we show that for both physisorbed brominated molecules, and for chemisorbed Br-atoms, induced charge-transfer in the Si(111)-7×7 surface can lead to a strong repulsive interaction between adsorbates, calculated as 200 meV at 13.4 Å. This large repulsive interaction must be channeled through the surface since it causes widely spaced "one-per-corner-hole" patterns of physisorption (three cases--directly observed here) and subsequent chemisorption (four cases observed). The patterns were observed by ultrahigh vacuum scanning tunneling microscopy for four different brominated hydrocarbon adsorbates; 1,2-dibromoethane, 1-bromopropane, 1-bromopentane, and bromobenzene, deposited individually on the surface. In every case, adsorbates were overwhelmingly more likely to be found singly than multiply adjacent to a corner-hole, constituting a distinctive pattern having a probability p = 7 × 10(-5) compared to a random distribution.

Charge-Transfer in Silicon Governs the Pattern of Dissociative Attachment of Hydrogen Halides: HCl, HBr, and HI

“…Charge-transfer through the surface has been studied extensively for various reactions on metal and on semiconductor surfaces. ,,,− For single H-atoms at Si(100), Radny et al found, as here, that the charge in the adjacent dangling bond was transferred along the dimer row rather than across . Charge flow along dimer-rows has also previously been noted in theoretical studies of adsorption of O- and N-containing molecules on the surface studied here; Si(100). − …”

“…We calculated the substantial difference of 0.5 electrons between the OD site and the ID site. Since Br radicals are known to prefer an electron-rich environment, ,, the increased charge adjacent to a reacted H favored ID reaction by the Br-atom.…”

Stereo-isomerism controls surface reactivity: 1-chloropentane-pairs on Si(100)-2×1

“…The 70 meV difference in E a corresponds to a fifteen-fold difference in reactivity at room temperature. The structures shown here were calculated at a lower level of theory (see ESIz), known from other work, 12 to give good physisorption geometries and energetics for halogenated alkanes on silicon surfaces, but which does not recover the surface buckling at dimers adjacent to the adsorbates.…”

Adsorbate Alignment in Surface Halogenation: Standing Up is Better than Lying Down

“…The products of thermal reaction are chemisorbed bromine atoms, identified by the characteristic lighting‐up at a sample bias between +1.0 V and +2.5 V,5a and confirmed by the thermal stability of the features for days at 300 K, in contrast to the spontaneous dissociation6b of physisorbed PeBr at the same temperature. Product bromine atoms exclusively chemisorbed at the prior physisorption sites, as shown in Figure 2 a–d.…”

“…Studies in this laboratory of the adsorption and reaction of haloaromatics5 and haloalkanes6 at silicon surfaces, using scanning tunneling microscopy (STM), provided evidence of vertical, v , and horizontal, h , states of single adsorbates at Si(111)‐7×7; examples being 1‐bromopropane,6a,b 1‐bromopentane,6c 1‐chlorododecane,6d–f and 1‐bromododecane 6e,f. Experimentally, the assignment of adsorbate alignment ( v vs h ) is based on contrasting surface mobilities6a–f ( v more mobile than h ) and sometimes differing STM imaging6a–c ( v higher than h ).…”