Surface modification of ZnO using triethoxysilane-based molecules.

Zinc oxide (ZnO) is an important material for hybrid inorganic-organic devices in which the characteristics of the interface can dominate both the structural and electronic properties of the system. These characteristics can be modified through chemical functionalization of the ZnO surface. One of the possible strategies involves covalent bonding of the modifier using silane chemistry. Whereas a significant body of work has been published regarding silane attachments to glass and SiO2, there is less information about the efficacy of this method for controlling the surface of metal oxides. Here we report our investigation of molecular layers attached to polycrystalline ZnO through silane bonding, controlled by an amine catalyst. The catalyst enables us to use triethoxysilane precursors and thereby avoid undesirable multilayer formation. The polycrystalline surface is a practical material, grown by sol-gel processing, that is under active exploration for device applications. Our study included terminations with alkyl and phenyl groups. We used water contact angles, infrared spectroscopy, and X-ray photoemission spectroscopy to evaluate the modified surfaces. Alkyltriethoxysilane functionalization of ZnO produced molecular layers with submonolayer coverage and evidence of disorder. Nevertheless, a very stable hydrophobic surface with contact angles approaching 106 degrees resulted. Phenyltriethoxysilane was found to deposit in a similar manner. The resulting surface, however, exhibited significantly different wetting as a result of the nature of the end group. Molecular layers of this type, with a variety of surface terminations that use the same molecular attachment scheme, should enable interface engineering that optimizes the chemical selectivity of ZnO biosensors or the charge-transfer properties of ZnO-polymer interfaces found in oxide-organic electronics.

Giant discrete steps in metal-insulator transition in perovskite manganite wires.

Optical lithography is used to fabricate LPCMO wires starting from a single (La(5/8-0.3)Pr(0.3))Ca3/8MnO3 (LPCMO) film epitaxially grown on a LaAlO3(100) substrate. As the width of the wires is decreased, the resistivity of the LPCMO wires exhibits giant and ultrasharp steps upon varying temperature and magnetic field in the vicinity of the metal-insulator transition. The origin of the ultrasharp transitions is attributed to the effect of spatial confinement on the percolative transport in manganites.

Visualization of localized holes in manganite thin films with atomic resolution.

The magnetic and transport behaviors of manganites are critically related to the spatial distribution and correlation of doped holes. Using in situ scanning tunneling microscopy, we have imaged both occupied and unoccupied states simultaneously in a hole-doped (La(5/8-0.3)Pr0.3)Ca(3/8)MnO3 epitaxial thin film grown by laser molecular beam epitaxy. Doped holes localized on Mn4+ ion sites were directly observed with atomic resolution in the paramagnetic state at room temperature. In contrast to a random distribution, these doped holes show strong short-range correlation and clear preference of forming nanoscale CE-type charge-order-like clusters. The results provide direct visualization of the nature of intriguing electronic inhomogeneity in transition metal oxides.