Surface structures of normal paraffins and cyclohexane monolayers and thin crystals grown on the (111) crystal face of platinum. A low-energy electron diffraction study

Abstract

The surfaces of the normal paraffins (C 3-C 8) and cyclohexane have been studied using low-energy electron diffraction (LEED). The sampl~s were prepared by vapor deposition on the (III) face of a platinum single crystal in ultrahigh vacuum. and were studied both as thick films and as adsorbed monolayers. These molecules form ordered monolayers on the clean metal surface in the temperature range 100-220 K and at a vapor flux corresponding to 10-7 Torr. In the adsorbed monolayers of the normal paraffins (C.-C s). the molecules lie with their chain axes parallel to the Pt surface and Pt[l To]. The paraffin monolayer structures undergo order-disorder transitions as a function of temperature. Multilayers condensed upon the ordered monolayers maintained the same orientation and packing as found in the monolayers. The surface structures of the growing organic crystals do not corresond to planes in their reported bulk crystal structures and are evidence for epitaxial growth of pseudomorphic crystal forms. Multilayers of n-octane and n-heptane condensed upon disordered monolayers have also grown with the (001) plane of the triclinic bulk crystal structures parallel to the surface. n-Butane has three monolayer structures on Pt(lll) and one of the three is maintained during growth of the crystal. Cyclohexane forms an ordered monolayer. upon which a multilayer of cyclohexane grows exhibiting the (001) surface orientation of the monoclinic bulk crystal structure. Surface structures of saturated hydrocarbons are found to be very susceptible to electron beam induced damage. Surface charging interferes with LEED only at sample thicknesses greater than 200 A.

Video-STM, LEED and X-ray diffraction investigations of PTCDA on graphite

“…In addition, structural aspects of adsorbatesubstrate interactions have been reported in a number of low energy electron diffraction (LEED) and scanning tunneling microscopy (STM) studies of other aromatic molecules, such as benzene [3,4] and naphthalene [5][6][7], perylene and coronene [8], and of several phthalocyanines [9,10] on various metal surfaces with different reactivities and symmetries. Examples showing the dependence of molecular order on the size of the molecules are LEED-investigations of n-paraffines of different length on a Pt(lll)-surface [11,12] and of coronene and hexa-peri-benzocoronene on graphite(0001) and MoS2(0001 ) [13]. These experiments also proved that in LEED experiments beam damage, though present, does not in principle prohibit investigations of organic monolayers, if a suitable registration technique is used.…”

Two-Dimensional Crystals of Alkanes Formed on Au(111) Surface in Neat Liquid: Structural Investigation by Scanning Tunneling Microscopy

“…The structure of the alkane adlayer was investigated by lowenergy electron diffraction on Pt(111), 4 Ag(111), 5 and Cu(111) 13 as summarized in Table 2. The odd-even effect on the alkane adlayer was observed on all substrates although the structure was dependent on the substrate.…”

“…Firment and Somorjai showed that the alkanes formed ordered monolayers on Pt(111) 4 and Ag(111) 5 under UHV conditions by low-energy electron diffraction (LEED). In addition, the physisorption of alkanes has been studied on Au(111), 6 Pt(111), 7,8 Pt(110), 9 Ir(110), 10 Cu(100), 11 Ru(001), 12 and Cu(111), 13 under UHV conditions.…”

Structures of self-assembled monolayer of alkanes adsorbed on Au(111) surfaces

“…For gold substrate, the interface between C 16 H 34 and the surface studied by molecular dynamic simulation consists of molecules parallel to the surface. 12,13 Alkanes deposited on metal surfaces in an ultrahigh vacuum (UHV) have been studied by low-energy electron diffraction (LEED) for short chains on Pt(111) at low temperature 14 and by STM for three-dimensional islands on Au(111). 15 Very recently, we have investigated the structure of monolayers of long-alkane molecules adsorbed on reconstructed Au(111) surfaces (details available from the author upon request).…”