Sukekatsu Ushioda

Surface anisotropy of polyimide film irradiated with linearly polarized ultraviolet light

Using polarized infrared (IR) absorption, we have investigated the surface anisotropy of a poly [4, 4′-oxydiphenylene-pyromellitimide] (PMDA-ODA) film that arises from anisotropic decomposition of the polyimide chain during irradiation with linearly polarized ultraviolet (LPUV) light. To monitor the surface anisotropy, we designed the sample structure so that the polyimide films decomposed uniformly over the entire film thickness. The surface anisotropy has a maximum at an irradiation energy of 105 J/cm2. For PMDA-ODA, the maximum surface anisotropy is significantly smaller than the surface anisotropy generated by rubbing. By analyzing the irradiation energy dependence of an IR absorption band, we found that the decomposition rate of the polyimide chain oriented parallel to the polarization direction of the LPUV light is greater only by ∼23% than that oriented perpendicular to it. This is the reason for the small surface anisotropy induced by the LPUV light irradiation.

Determination of the molecular orientation of very thin films on solid substrates: surface liquid crystal layers and rubbed polyimide films

Abstract The molecular orientation of very thin films on solid substrates can be determined quantitatively by measuring the polarized infrared (IR) absorption spectra of samples as a function of angle of incidence. The quantitative molecular orientation is derived by fitting the incident angle dependence and the dichroic ratio with theoretical calculations. We applied this method to a technologically important system: liquid crystal (LC)/rubbed polyimide film. To understand the alignment mechanism of LC molecules in contact with rubbed polyimide films, we have quantitatively determined the molecular orientation of rubbed polyimide films and a surface LC layer in contact with a rubbed polyimide film. In this paper two relations are discussed: (1) correlation between the inclination angle of polyimide backbone structures in rubbed films and the pretilt angle of bulk LC in contact with them, and (2) relation among the molecular orientation of a rubbed polyimide film and those of surface and bulk LC layers in contact with it.

Relation between the molecular orientations of a very thin liquid crystal layer and an underlying rubbed polyimide film

By polarized infrared absorption spectroscopy, we have determined the relation between the molecular orientation of a rubbed polyimide film and that of a very thin liquid crystal (LC) layer (approximately a monolayer of 8CB molecules) in contact with the polyimide film. The molecular orientation of the rubbed polyimide film was determined by fitting theoretically the incident angle dependence of the infrared absorption for molecular vibrations polarized parallel to the length of the polymer. Approximately a monolayer of 8CB molecules was deposited on a rubbed polyimide film with previously measured molecular orientation. The molecular orientation of this LC layer was also determined by polarized infrared absorption. We found that the LC molecules and the polyimide chains are oriented, on average, along the rubbing direction and tilted up from the surface, and that the molecular order of the LC layer is higher than that of the underlying rubbed polyimide film.

Relation between the molecular orientation of a liquid crystal monolayer and the underlying polyimide film exposed to linearly polarized ultraviolet light

We have determined the relation between the in-plane anisotropy of the molecular orientation of a liquid crystal (LC) monolayer and the underlying polyimide film exposed to linearly polarized ultraviolet light (LPUVL). To evaluate the anisotropy of the LC monolayer and the polyimide film, the sample orientation dependence of the polarized infrared absorption spectrum was measured. The in-plane anisotropy of the LC monolayer was found to be proportional to that of the polyimide film, the proportionality factor being about 75%. This result strongly suggests that the alignment of the LC molecules in contact with the LPUVL-exposed polyimide film is induced by an interaction between the polyimide and LC molecules.

Multiple-Fiber Collection System for Scanning Tunneling Microscope Light Emission Spectroscopy

We have constructed an optical fiber system for collecting light emission (LE) from the gap between a sample and the tip of a scanning tunneling microscope (STM). The collection system consists of four optical fibers with a core diameter of 600 µm and a numerical aperture (NA) of 0.2. The four optical fibers point radially at the tip-sample gap from four directions. To demonstrate the performance of the collection system, we evaluated its effective solid angle of collection by measuring the STM-LE from a Au evaporated film. The effective solid angle was estimated to be 0.33±0.03 sr, which corresponds to about 2.6 times that of a single optical fiber determined by the NA (0.13 sr). Although the solid angle of collection did not increase by a factor of four, the increase of the collection solid angle will improve the detection limit by reducing the accumulation time for spectral measurements.

Molecular Orientation of Liquid Crystal Monolayers on Polyimide Films Exposed to Linearly Polarized UV Light

By using optical second harmonic generation (SHG), we have investigated the molecular orientation of liquid crystal (LC) monolayers in contact with polyimide films exposed to linearly polarized ultraviolet light (LPUVL) at the wavelength of 266 nm. For different exposures at 3, 8, 20, and 80 J/cm2, the second harmonic (SH) signal from the LC monolayer was measured as a function of the rotation angle of the sample around the surface normal. The SH signal has no rotation angle dependence, independent of the energy density of exposure. On the other hand, uniform parallel (homogeneous) alignment of bulk LC was observed for a LC cell made with two polyimide-coated substrates exposed to LPUVL at 8 J/cm2. From these results we found that the in-plane anisotropy of the LC monolayer in contact with the polyimide film is very small, if any, even though the polyimide film can induce the homogeneous alignment of bulk LC. The average tilt angle of LC molecules in the monolayers in contact with the polyimide films was also determined. We found that the average tilt angle of LC molecules measured from the surface normal decreases with the increase of UV exposure.

Roughness-Induced Magnetic Domain in Fe Thin Films on Land-and-Groove Structures Studied by Spin-Polarized Secondary Electron Microscopy

The magnetic domain structures of Fe thin films on two-dimensionally arranged land-and-groove structures have been studied by spin-polarized secondary electron microscopy (SP-SEM) under an applied dc field. The coercive force on the land area was found to be higher than that on the groove area under magnetization reversal. The surface roughness measured by atomic force microscopy (AFM) was greater on the land area than on the groove area. The roughness-induced high-coercivity on the land prevented the reversed magnetic domain on the groove from spreading over the land in the initial magnetization reversal. This result indicates that surface roughness is an important factor in domain size control of thin magnetic films.

Optical Second Harmonic Generation in Si1-ninxGex Film Epitaxially Grown on Si(100)

We have observed the optical second harmonic generation (SHG) from Si1-x Gex epitaxial films for the first time. The SH intensity was measured as a function of Ge fraction x and the azimuthal angle of rotation of the film about the surface normal. The SH intensity has a maximum around x=0.8~0.9. The SH intensity curve as a function of the azimuthal angle shows a four-fold pattern for the Ge fraction x=0.8~0.9. The enhancement in the SH intensity and the signal anisotropy are believed to be due to the resonance of the incident and second harmonic photon energies with the E0 and E1 band gaps of Si1-x Gex , respectively.

Very high resolution photoelectron spectra of NEA-GaAs

The energy distribution of photoelectrons emitted from NEA-GaAs has been measured with a very high energy resolution (ΔE ≈ 5 meV). The sample consists of epitaxially grown p-type GaAs (hole concentration 1 × 1019 cm−3) with a (100) surface. After cleaning the surface by heating in UHV (7 × 10−11 Torr), Cs and O2 were coadsorbed to obtain a NEA surface. A pulsed GaAlAs laser (λ = 820 nm, pulsewidth = 60 ps) was used for the light source, and the photoelectron energy distribution was measured by a high resolution time-of-flight electron spectrometer. A series of peaks with an approximately regular energy interval of 50 meV was observed in the spectra. Theoretical considerations reveal that the origin of the regular structure are the two-dimensional subbands of electrons at the X-point in the band-bending region near the surface.

Raman scattering study of surface band-bending at the NEA Cs/p-GaAs(110) surface

We have investigated the band-bending behaviour of a caesium-covered p-GaAs (110) surface by electric-field-induced Raman scattering. A high potential barrier height (approximately 1.5±0.2 eV) was found at high caesium coverage (about 2 monolayers), and we conclude that the metallic property of caesium plays a significant role in this system when the overlayer is thick.