2020同步年報

Soft Matter 033 Molecular Orientation of a π-Conjugated Polymer Monolayer The face-on orientation dominates the surface-segregated monolayer, as revealed by NEXAFS and GIWAXS spectra. A synthetic-polymer thin film plays a crucial role in the modern economy. Although many artificial-polymer- based technologies have been successfully integrated into industrial processes, electronic applications based on a conjugated polymer are still in a dawn. Many believe that π-conjugated polymers have the potential to revolutionize electronic technologies, as their peculiar properties can be engineered with a highly developed synthesis toolbox from polymer science. The charge-transport property of a π-conjugated polymer is governed by not only the chemi- cal composition of the polymeric macromolecules but also their conformation inside the membrane. Intensive effort has consequently been devoted to manipulate the crystal- linity and molecular orientation of the membrane. Among various approaches, the packing structure and molecular orientation of a surface-segregated monolayer are sus- ceptible to molecular design, allowing its properties to be tailored from the bottom up. In this report, Yaw-Wen Yang (NSRRC), Chain-Shu Hsu (National Chiao Tung University) and Keisuke Tajima (RIKEN Center, Japan) and their coworkers take advantage of the surface-segregated monolayers (SSM) forming mechanism to set up a monolayer of π-conjugated polymer. By replac- ing the alkyl side chain of N2200 with a semifluoroalkyl chain, the surface energy of the product, named FNDIT2, is decreased (see Fig. 1(a) for the monomer structures). The difference in surface energy inside the spin-coated blended FNDIT2/N2200 film drives a phase separation, resulting in an enhanced surface concentration of FNDIT2 over that of N2200. Depth profiling of fluorine with X-ray photoelectron spectroscopy (XPS) etching experiment concluded that the thickness of the surface layer is about 1.8 nm. This condi- tion indicates a single-layer thickness for the FNDIT2 over- layer, as illustrated in Fig. 1(b) . The literature shows that the SSM mechanism can induce an ordered single-layer polymer with its π-conjugated plane oriented to the substrate surface in either face-on or edge-on or end-on manner, as illustrated in Fig. 2(a) . For electronic applications, this orientation is of the ut- most importance, as it strongly influences the anisotropic charge-transport properties. In this work, the π-plane ori- entation about the surface region was characterized with near-edge X-ray absorption fine structure (NEXAFS) spectra. Quantum mechanics indicates that the X-ray absorbance depends on how well the electric-field vector overlaps with the participating molecular orbitals. As the initial state is a symmetrical C 1 s orbital in this case, only the final state, the carbon π* orbital, accounts for the degree of such overlap. As the carbon π* orbital is perpendicular to the molecular π plane, a dependency between the electric-field vector and the molecular π-plane is established, formulated in this equation: , in which I is absorbance, P is degree of polarization, which is 0.9 for the bending magnet used in the present experiment, θ is Fig. 1 : (a) Molecular structure of the monomer unit of N2200 and FNDIT2. (b) A FNDIT2/N2200 surface-segregated monolayer. [Reproduced from Ref. 1] I = A [ Pcos 2 θ (1-— sin 2 γ )+— sin 2 γ ] 3 2 1 2 Fig. 2 : (a) Schematic diagram of the NEXAFS experiment geometry. (b) Schematic diagram of molecular π-plane orientations. [Repro- duced from Ref. 1]