Polarized Raman scattering and SEM combined full characterization of self-assembled nematic thin films

Abstract

Elongated particles are predestined for a fast transfer of optical and electronical signals in a preferred direction, which is mandatory for a quick response in optoelectronic devices. The performance of the material is based on the quality of defect less alignment of the particles. On this account we present an easy non-invasive methodology for characterization of both surface and bulk order. The characterization of bulk order is performed by orientation dependent variation of the polarized Raman scattering signal on large areas by mapping. Scanning electron microscopy and image analysis on the surface complete the characterization. New insights in dip coated nematic structures clearly show the interplay of evaporation induced and shear-induced self-assembly and reveal a comprehensive mechanistic picture for nanorod assembly: the shear force dominated regime orients the particle in direction of withdrawal. At low withdrawal velocity, however, shear forces and evaporation counteract to produce a three-layered film where the top and bottom layers are oriented perpendicular to each other. The middle layer gives a clear evidence for a reorientation by convective flow.

From evaporation-induced self-assembly to shear-induced alignment

“…26 For all experiments the relative humidity was set to 10%. Pre-cleaned silicon wafer slides coated with 20 nm Ti, 100 nm TiN and 500 nm Al were used as substrates.…”

“…The spectra were fitted by Lorentzian functions and the integrated band intensity of A 1 (TO) was compared to the sum of all other relevant modes. 26 100 individual spectra were analysed in order to obtain a mean value and a standard deviation for the BO. Further details can be found elsewhere.…”

“…In the EISA regime the c-axis of the ZnO nanorods align parallel to the contact line in order to maximize the interfacial coverage per particle, which in turn minimizes the Helmholtz free energy of the system. 26 Low volume fractions were selected to ensure…”

“…At low withdrawal velocities EISA and SIA are competing and counter-acting mechanisms. 26 In dip coating a substrate is withdrawn from a colloidal reservoir at a constant velocity. This results in a parabolic dependency of the film thickness on the withdrawal velocity.…”

“…33 With respect to the dip coating of nanorods we have recently reported that the capillary regime is EISA dominated and SIA prevails in the viscous drag regime. 26 For EISA a critical nanorod volume fraction in the suspension is required. From a theoretical point of view this critical volume fraction can be described for monodisperse spherocylinders on the basis of the Onsager theory.…”