Structural and morphological investigation of (R)-α-phenylethylammonium-oxalate in bulk vs. nanowires on a modified substrate surface

Abstract

A chiral organic insulator, (R)-α-phenylethylammonium-oxalate (RAPEAO), was prepared in the forms of single-crystal, powder and spin-coated layers on silicon substrate surfaces modified by plasma treatment or a (3-aminopropyl)triethoxysilane (APTES) polymer layer. For spin-coated samples, different deposition conditions have been investigated – various thicknesses controlled by speed and the number of repeated cycles, deposited continuously or by a layer-by-layer technique. The chemistry of this compound did not allow the deposition of the continuous thin film, yet, it caused the formation of a few nuclei on the substrate surface. Modification of the substrate with low temperature plasma caused the increased number of nuclei as well as enabled the growth of the nanowires, which was confirmed by atomic force microscopy (AFM) images. The same effect has been observed from the X-ray diffraction (XRD) measurements, where preferential growth of the studied compound in one direction was confirmed by grazing incidence, as well as wide reciprocal space mapping (WRSM). XRD studies confirmed the structural similarity of the compound, disregarding the compound form ranging from nanowires on the substrate to the bulk. Finally, the substrate covered by APTES thin film has had increased coverage of the substrate surface by the studied compound. Impedance spectroscopy revealed that the electrical conductivity of the sample in bulk at 20 °C is 6.3 × 10⁻¹⁵ (Ω cm)⁻¹, indicating the insulating properties of the material.