The frustrated motion of benzene on the surface of Si(111)

Abstract

Benzene adsorption on Si͑111͒-7ϫ7 is studied with scanning tunneling microscopy. Benzene diffusion is found to be inhibited. Ordinarily surface diffusion is controlled by a substantially lower energy of activation than is desorption. In this case diffusion is frustrated by a barrier to diffusion that is comparable to that for desorption. Both desorption and diffusion are monitored. On average, for every two adsorbate disappearances, one readsorption is observed and one molecule desorbs. Site-specific activation barriers of 0.94Ϯ0.01 eV and 0.95Ϯ0.01 eV for center faulted and corner faulted adatom sites, respectively, are extracted. Residence times increase as coverage decreases, implying adsorbate crowding causes the strength of the surface-adsorbate bond to weaken. Diffusion is generally found to involve jumps to sites beyond nearest neighbors. It emerges that the adsorbate largely breaks its existing bond to a surface site before forming a substantial bonding interaction with a new site. We surmise that this ''break before make'' scheme leaves the adsorbate in an intermediate, essentially physisorbed state, where it is sufficiently mobile to make longer than nearest neighbor jumps, or from which it desorbs.

Mapping the site-specific potential energy landscape for chemisorbed and physisorbed aromatic molecules on the Si(1 1 1)-7 × 7 surface by time-lapse STM

“…These aromatic molecules chemisorb on the Si(1 1 1)-7×7 surface at room temperature and are imaged as 'missing' Si adatom features (i.e. black spots) in STM [14,15]. Each molecule forms a pair of covalent bonds to two silicon atoms, one to a silicon adatom and one to a silicon restatom, so called di-σ bonding [20].…”

“…filled states image displaying the STM signature of the two distinct halves of the unit cell-the faulted half of the unit cell images more brightly than the unfaulted. The adsorption of chlorobenzene, benzene and toluene on the Si(1 1 1)-7×7 surface has been actively studied by many experimental and theoretical techniques [14][15][16][17][18][19][20][21]. These aromatic molecules chemisorb on the Si(1 1 1)-7×7 surface at room temperature and are imaged as 'missing' Si adatom features (i.e.…”

“…While in the precursor state a molecule may laterally explore the surface [14,19,22]. Even at low temperatures, e.g.…”

“…We acknowledge that any molecules that leave the scan field via diffusion between images would be mistakenly counted as undergoing desorption than diffusion (increasing N G over N x C ). Given a scan size of 60 nm and diffusive step size of 2.3 nm [14], a simple calculation that gives molecules at the perimeter of the scan a 25% chance of leaving the field (via the closest edge), 3.6% of active molecules would leave the field. We can build a more sophisticated computational model of molecules on the Si(1 1 1)-7×7 surface: at a coverage of 2 molecules per unit cell, molecules diffuse to a random available site within 2.3 nm.…”

Dissociative adsorption of pyrrole on Si(111)-(7×7)

“…[7][8][9][10][11][12][13][14][15][16][17] Compared to metal surfaces, Si(111)-(7ϫ7) exhibits a diversity of reactive sites and is expected to have a rich surface chemistry. In one unit cell, there are various Si atoms with dangling bonds ͑dbs͒, including adatoms in the first layer, rest atoms in the second layer, and corner holes in the fourth layer.…”