S. Ushioda

Determination of molecular orientation of very thin rubbed and unrubbed polyimide films

To determine the molecular orientation of rubbed and unrubbed poly[4,4′‐oxydiphenylene‐ pyromellitimide] (PMDA‐ODA) films, we have measured the infrared (IR) absorption spectra of PMDA‐ODA films on Si substrates as a function of the incident angle. The molecular orientation was determined by fitting the incident angle dependence and the IR dichroic ratio with the theoretical curves. The IR absorption was calculated by a transfer matrix method to take account of the multiple reflection and refraction at the interfaces. The vibrational response of the polyimide film in the IR region was represented by a Lorentz oscillator model. We found that the molecules in a spin‐coated polyimide film are oriented parallel to the substrate surface with a standard deviation of 6.5°. For a rubbed film, the polymer chains are oriented along the rubbing direction and are tilted up on average by 8.5° from the surface plane. We observed a blueshift of the C=O asymmetric stretching band at oblique incidence. The blueshift is ex...

CORRELATION BETWEEN THE PRETILT ANGLE OF LIQUID CRYSTAL AND THE INCLINATION ANGLE OF THE POLYIMIDE BACKBONE STRUCTURE

We have determined the inclination angle of the backbone structure of polyimide with alkyl side-chains in rubbed films. Four different polyimides with the same backbone structure but different lengths of alkyl side-chains were used in this study. The inclination angle of the backbone structure increases with the number of carbon atoms in the alkyl side-chains. We found a linear relation between the inclination angle of the backbone structure and the pretilt angle of liquid crystal (LC) that is in contact with the polyimide films. We conclude that the inclination of the polyimide backbone structure determines the pretilt angle of LC.

Visible light emission spectra of individual microstructures of porous Si

We have measured the spectra of visible light emitted from the individual structures of porous Si (PS) below the probe tip of the scanning tunneling microscope (STM), and found that the peak energy of the emission spectrum shifts with the size of nanometer‐scale structures on the PS surface. Samples with a PS layer ∼50 nm thick were formed by anodic etching of p+Si(100) substrates (∼0.005 Ωu2009cm). The STM images show that protrusions whose dimensions are 3–10 nm in diameter are distributed on the PS surface. The peak energy of the STM light emission spectrum shifts from ∼1.7 to ∼2.1 eV as the diameter of the structure below the STM tip decreases from ∼9 to ∼3 nm. The measured peak shift with the size of the structure is consistent with the shift of the energy gap predicted on the basis of a quantum confinement model.

STM light emission spectroscopy of surface micro-structures on granular Au films

Abstract Light emission spectra from the gap region of a scanning tunneling microscope (STM) have been measured simultaneously with the cross-section of the surface topography. The spectra consist of two components centered about 1.7 and 2.0 eV. The intensity ratio between these components varies as the tip moves along different surface structures on the sample. We have found a correlation between the light emission spectra and the surface topography. We suggest a mechanism that can qualitatively explain the observed position-dependent spectra.

Molecular Orientation of Rubbed and Unrubbed Polyimide Films Determined by Polarized Infrared Absorption

We have determined the orientations of poly[ 4,4-oxydiphenylene-pyromellitimide] (PMDA-ODA) molecules in rubbed and unrubbed polyimide films ( ~120 A) on Si substrates by polarized IR absorption spectroscopy. We found that for rubbed films the polymer chains are oriented along the rubbing direction and are tilted up on average by 8.5° from the surface plane. We conclude that the polymer chain inclination induced by rubbing determines the tilt of the director of the bulk liquid crystal.

Light emission spectra of AlGaAs/GaAs multiquantum wells induced by scanning tunneling microscope

We have investigated the scanning-tunneling-microscope light emission (STM-LE) spectra of p-Al0.4Ga0.6As/p-GaAs multiquantum wells. The injection current level was kept as low as 0.1–0.5 nA to ensure that the sample is not damaged by the tunneling current. This is the current level ordinarily used for taking STM images. The peak energy of the emission shifts to the high energy side with decreasing well widths. A corresponding peak shift behavior was also observed in the photoluminescence (PL) spectra for the same samples. From comparisons of the STM-LE and the PL spectra, we find that although there is a difference in the excitation process, the final recombination process is identical in both cases.

Single molecule spectrum of rhodamine 6G on highly oriented pyrolytic graphite

We have measured the scanning tunneling microscope (STM) light emission spectrum of a single molecule of rhodamine 6G (R6G) adsorbed on highly oriented pyrolytic graphite (HOPG). Since the HOPG substrate radiates no STM light, we have succeeded in observing the spectrum radiated by R6G alone. The spectrum agrees well with the photoluminescence spectrum of R6G on HOPG with the exception of two structures that may arise from a triplet state whose transition is forbidden in photoluminescence. Based on this agreement, we have determined the STM light emission mechanism of adsorbed R6G.

Light emission spectra of the monolayer-island of C60 molecules on Au(111) induced by scanning tunneling microscope

Abstract We have measured the light emission (LE) spectra of the ordered C 60 monolayer-island on the Au(1xa01xa01) surface and the bare Au surface, induced by the scanning tunneling microscope (STM). The optical measurements were performed at 78 K using electrochemically etched W-tips at a constant tunneling current of 2.0 nA for a sample bias voltage with respect to the tip of 2.3 V. For the bare Au surface, we observed an asymmetric peak at ∼2.01 eV, which is the typical STM-LE spectrum for the Au sample–W tip system. For the C 60 monolayer-island, we observed a broad peak centered at ∼1.7 eV, which consists of the red-shifted emission from the underlying Au substrate (∼1.93 eV) and a new emission peak around 1.65 eV. The energy range of the new emission is consistent with that of the photoluminescence of the isolated molecules and a 100 nm-thick film of C 60 . Thus we conclude that the new emission peak is the fluorescence from the C 60 molecules in the monolayer-island.

Servomechanism for locking scanning tunneling microscope tip over surface nanostructures

Measurement of the light emission spectrum from a scanning tunneling microscope (STM) requires a long exposure time due to its extremely low intensity, and thermal drift of the tip during the exposure time limits the spatial resolution. To improve the resolution, a computer controlled servomechanism that locks the STM tip over a target position has been developed. We have measured the light emission spectra from individual nanometer scale structures on an evaporated Au film with and without this mechanism, and demonstrated the effectiveness of the servomechanism.

Light emission spectra of individual GaAs quantum wells induced by scanning tunneling microscope

We have investigated the light emission from individual single GaAs quantum wells of cleaved (110) AlGaAs/GaAs heterostructures, using the scanning tunneling microscope tip as a local injection source of minority carriers. Single emission peaks were observed to shift to the high-energy side with decreasing well width. The emission peaks are assigned to the transition between n=1 single-quantum-well electron and heavy-hole states of the respective wells.