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

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 -> sigma(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.