1. Electrical and optical properties of organic materials are closely related to its molecular orientation. SE is employed in the understanding of molecular orientation of an anisotropic organic material with and without pre-deposition aligning treatment. By taking SE measurements at varying sample rotation angles (rotation along the sample normal), we observed that the organic thin film deposited on rubbed monolayer displays not only out-of-plane anisotropy but also in-plane anisotropy. However, thin film grown on un-rubbed monolayer does not show any orientation (sample rotation) dependence. Nanostructures and Molecular Orientation Studied by Spectroscopic Ellipsometry Shih-Hsin Hsu 1, Yia-Chung Chang 1, Yuh-Jen Cheng 1, Ching-Hua Chiu 2, Hao-Chung Kuo 2, Tien-Chang Lu 2, Shing-Chung Wang 2, Chih-Wei Huang 3, and Yu-Tai Tao 3 1 Research Center for Applied Sciences, Academia Sinica, Taipei, Taiwan 2 Department of Photonics and Institute of Electro-Optical Engineering, National Chiao Tung University, Hsinchu, Taiwan 3 Institute of Chemistry, Academia Sinica, Taipei, Taiwan Spectroscopic ellipsometry (SE) is a nondestructive optical technique and conventionally used for characterizing thin films and bulk materials. Here, we extend its applications to the characterization of nanorods and the study of molecular orientation of organic thin films. Figure 6. Layer structure of the organic thin film deposited on top of a monolayer on a SiO 2 /Si substrate. GaN Nanorods Molecular Orientation Figure 7. SE measurement of organic thin films with (left) and without rubbing pre- treatment (right). The refractive index profiles of GaN nanorods extracted from the SE analysis suggest the broad spectral and angular antireflection is mainly attributed to the gradually varying porous structure. The orientation-resolved SE measurement of organic thin films clearly reveals the anisotropic property and the pre-deposition treatment is crucial to its orientation. This study demonstrates that SE could be a useful and non-contact tool for the characterization of structures in nanometer scale and potentially in molecular level. Summary Figure 4. Measurement data and model fitting by describing the GaN nanorods as a 3- node graded EMA layer. GaN nanorods demonstrating broad angular and spectral antireflection were characterized by SE. The GaN samples were first epitaxially grown by MOCVD on c-plane sapphire substrates. Thin Ni films with various thicknesses ranging from 5 to 20 nm were subsequently evaporated, and followed by a rapid thermal annealing process under N 2 gas to form Ni nano-dots of different sizes, which served as the etching masks. After being etched by an RIE process, the samples were dipped into a heated HNO 3 to remove the residual Ni. Optical reflection measurements show the reflectance for both p- and s-polarizations is held well below 10% from UV to IR wavelengths and at incident angles up to 60º. Effective medium approximation (EMA) theory was employed in the analysis and the nanorods were modeled as a graded-index layer, in which each sub-layer is modeled as a mixture of uniaxial GaN and voids with varying porosity fraction. The model fitting based on a 3-node, graded EMA layer model works well in the IR region, and can be extended to visible region for samples with smaller rod sizes. Figure 3. Optical reflection measurements of GaN nanorods. Left to right panels correspond to the samples shown in Fig. 2 (a), (b), (c), respectively. Figure 1. Fabrication process of GaN nanorods. RTARIE Figure 2. SEM images of the GaN nanorods samples: top view (a, b, c) with a scale bar corresponding to 200 nm; cross-sectional view (d) with a scale bar corresponding to 1  m. abc d Figure 5. The depth profiles for the refractive index of GaN nanorods at 632.8 nm (left) and 1000 nm (right). rubbed or un-rubbed GaN nanorods GaN film