Semiconductor nanorod layers aligned through mechanical rubbing

Abstract

Large-scale lateral alignment of nanorods (NRs) is of interest for manifestation of their anistropic properties including polarized emission and directional electrical transport. This study investigates the utility of mechanical rubbing for macroscopic scale alignment of colloidal semiconductor NRs. CdSe/CdS seeded-rods, exhibiting linearly polarized emission, are aligned by mechanical rubbing of a spin-coated glass substrate. The dragging force exerted by the rubbing fibers results in deflection and reorientation of the NRs along the rubbing direction. The rubbed samples were characterized by various methods including absorption, polarized emission, optical fluorescence microscopy, atomic force microscopy, and ultra-high resolution scanning electron microscopy. The emission polarization contrast ratio (CR), defined as the ratio between emission intensities parallel and perpendicular to the rubbing direction, was used to characterize the rods alignment. The effects of substrate treatments on the CR were studied, showing that partially hydrophobic surface provides optimal conditions for alignment. Excess organic ligands added to the deposited NR solution strongly affect the extent of alignment. This was studied for a series of NR samples of different dimensions and an optimal additive ratio of ∼3 ligand molecules per 1 nm² NR surface area was found to yield the highest CR. Average CR values of 3.5 were detected over the entire 6 cm² substrate area, with local values exceeding 4.5. While samples of rubbed spherical quantum dots and spin-coated films of NRs show no emission polarization, the emission intensity from rubbed NR samples is polarized obeying Malus' law (wherein, the intensity is proportional to cos²(θ)). Mechanical rubbing, well known for its use in LC devices, may be considered as a method for large-scale alignment of NRs on substrates.