Phase-dependent premelting of self-assembled phosphonic acid multilayers

Melting and premelting phenomena in self-organized organic systems have been extensively explored in the literature, exploring distinct behaviors of different molecule lengths and morphologies. Nevertheless, the influence of the supramolecular assembly configuration on the occurrence of premelting remains poorly explored. Here we use phosphonic acids as model systems for self-organized molecular assemblies. These molecules exhibit long-range order on different types of substrates. The balance between chain-to-chain and head-to-head interactions leads to distinct types of stackings. Although their structural configurations are well understood, very little is known about their behavior near the melting transition. We show here that premelting occurs in lamellar structures and that its behavior depends directly on the ordered configuration assumed in the studied multilayers. Two molecules with different chain lengths were investigated: octadecyl phosphonic and octyl phosphonic acids. Although almost no dependence on the molecule length was observed, the occurrence of premelting is strongly influenced by their lamellar packing configuration. For tilted packings premelting is unfavored while in straight configurations, where alkyl chain interactions are weakened with respect to head-to-head interactions, strong premelting is observed. We find that the onset of premelting occurs at the domain boundaries with straight lamellar configurations and the domain sizes exhibit power law temperature dependences.

Metastable phase formation and structural evolution of epitaxial graphene grown on SiC(100) under a temperature gradient

Multilayer epitaxial graphene was obtained from a 6H-SiC(001) substrate subjected to a temperature gradient from 1250 to 1450 °C. Scanning tunneling microscopy and x-ray diffraction were used to identify the structure and morphology of the surface, from which the formation of a metastable phase was inferred. By a comparison between microscopy and diffraction data, we report the appearance of misoriented Si-doped graphene in cold regions (1250 °C) of the substrate. This metastable phase occurs in domains where silicon sublimation is incomplete and it coexists with small domains of epitaxial graphene. At 1350 °C this phase disappears and one observes complete graphene-like layers (although misoriented), where rotational registry between the underlying epitaxial graphene and additional layers is absent. At 1450 °C the stacking among layers is established and the formation of highly oriented single crystalline graphite is complete. The stability of this Si-rich metastable phase at 1250 °C was confirmed by first-principles calculations based on the density functional theory.

Nanowires and nanoribbons formed by methylphosphonic acid

The production and physical properties of nanowires and nanoribbons formed by methylphosphonic acid (MPA)--CH3PO(OH)2--were investigated. These structures are formed on an aluminum coated substrate when immersed in an ethanolic solution of MPA for several days. A careful investigation of the growth conditions resulted in a narrow window of solution concentrations and temperatures for the successful development of nanowires and nanoribbons. Several different techniques were employed to characterize these nanostructures: (1) Photoluminescence experiments showed a strong emission at 2.3 eV (green), which is visible to the naked eye; (2) X-ray diffraction experiments indicated a significant cristalinity, in agreement with atomic force microscopy (AFM) and transmission electron microscopy (TEM) morphology images, which show organized nano-scale wires and ribbons, (furthermore, AFM-Phase and TEM images also suggest that nanoribbons are formed by well-aligned nanowires); (3) Conductive-AFM experiments revealed an intermediary conductivity for these structures (10(-1)/Ohm x m), which is similar to some intrinsic semiconductors and; (4) finally, Infrared, Raman, and X-Ray Photoelectron Spectroscopies produced information about the contents, structure, and composition of both wires and ribbons.

3D composition of epitaxial nanocrystals by anomalous X-ray diffraction: observation of a Si-rich core in Ge domes on Si(100)

Three-dimensional composition maps of nominally pure Ge domes grown on Si(001) at 600 degrees C were obtained from grazing incidence anomalous x-ray scattering data at the Ge K edge. The data were analyzed in terms of a stack of layers with laterally varying concentration. The results demonstrated that the domes contained a Si-rich core covered by a Ge-rich shell and were independently supported by selective etch experiments. The composition profile resulted from substrate Si alloying into the Ge during growth to partially relax the stress in and under the domes.