Incommensurate Crystalline phase of n-Alkane Monolayers on Graphite (0001)

Abstract

An incommensurate crystalline phase of the (sub)monolayer of n-C 36 H 74 lying on graphite (0001) is observed on cooling the smectic phase using normal incidence near edge X-ray absorption fine structure spectroscopy at the carbon K-edge (NI C K-NEXAFS) and scanning tunneling microscopy (STM). The orientation of the CCC plane of the all-trans alkyl chain with respect to the substrate is confirmed by the 1s f σ CH */R resonance detected by the NI C K-NEXAFS spectra at the absorption edge. Almost all molecules take the parallel (flat-on) orientation in the smectic phase, and at least half of them change to the perpendicular (edge-on) orientation in the incommensurate crystalline phase. A lamellar structure with a width corresponding to the chain length and no internal structure is observed by STM in the smectic phase, whereas that in the incommensurate crystalline phase exhibits a ladder-like structure with a periodicity of ca. 2 nm transverse to the chain direction. The periodicity is incommensurate with the substrate lattice. The molecular orientation in the ladder-like structure is related to the molecular width varying from 0.43 to 0.63 nm in the magnified STM image.

Compression-Induced Conformation and Orientation Changes in an n-Alkane Monolayer on a Au(111) Surface

“…The herringbone ridges are a result of uniaxial compression along one atomic row, with this row being orthogonal to the herringbone ridge. Therefore, the lamellar axis, which makes an angle of 60° with the herringbone ridges, indicates that the molecular chains are aligned along another atomic row of the Au(111) surface, in agreement with previous results. ,,− The lack of the molecular image is reasonable because the molecules are mobile in the lower density phase as in the smectic phase on the graphite basal plane …”

“…As a result, the interaction between the detached methylene group and the substrate becomes ineffective and the final state orbital is not modified much. The calculated σ* CH /R ∥ and σ* CH /R ⊥ resonances for the methylene in the edge-on molecules on a graphite basal plane appear at 287.2 and 288.5 eV, respectively, with higher intensity than that of the modified σ* CH /R resonance for the flat-on adsorption . Therefore, the enhanced σ* CH /R resonances are attributed to the inclined carbon plane.…”

Epitaxially Self‐Assembled Alkane Layers for Graphene Electronics

“…To characterize the HTC structure at the HTC‐graphene interface, the scanning tunneling microscope (STM) sample was prepared on the Si substrate coated with a 280 nm thick SiO 2 layer using a typical mechanical exfoliation and e‐beam lithography method. The monolayer of HTC on graphene could be obtained by removing multilayers by heating at 400 K in ultra‐high vacuum (UHV) where the desorption temperature of multilayers is ≈350 K and the desorption temperature of monolayer is above 450 K . The STM image of Figure b shows that the monolayer of HTC is self‐assembled and closely packed on a monolayer graphene supported on a SiO 2 ‐coated silicon wafer.…”

Chiral Adsorption Conformations of Long-Chain n-Alkanes Induced by Lattice Mismatch

“…Whereas most STM studies indeed claim flat-on adsorption, edge-on adsorption 28 (i.e., carbon backbone plane orthogonal to the surface) as well as mixed adsorption orientations have also been reported. 29,31,32 Theoretical calculations indicate that the flat-on adsorption geometry is energetically preferred over edge-on adsorption, although the energy difference is small. 33,34 Owing to a lower degree of orbital hybridization, the carbon zigzag backbone of graphite exhibits a slightly shorter periodicity (2.46 Å) than those of n-alkanes in the all-trans conformation (2.58 Å, value taken from ref 33).…”

“…Whereas most STM studies indeed claim flat-on adsorption, edge-on adsorption ( i.e. , carbon backbone plane orthogonal to the surface) as well as mixed adsorption orientations have also been reported. ,, Theoretical calculations indicate that the flat-on adsorption geometry is energetically preferred over edge-on adsorption, although the energy difference is small. , …”