Showing papers by "Y. R. Shen published in 1998"

Icelike Water Monolayer Adsorbed on Mica at Room Temperature

Abstract: The structure of a water film formed on mica at room temperature, in equilibrium with water vapor at various relative humidities (RH), was studied using sum-frequency-generation (SFG) vibrational spectroscopy and scanning polarization force microscopy (SPFM). Analysis of the O-D stretch modes in the SFG spectra of D{sub 2}O on mica indicates that as RH increases, the submonolayer water structure evolves into a more ordered hydrogen-bonding network. At full monolayer coverage ({approximately} 90{percent} RH) , the SFG spectrum suggests an icelike film with no dangling O-D groups, in agreement with a recent molecular dynamics simulation. {copyright} {ital 1998} {ital The American Physical Society}

Chain−Chain Interaction between Surfactant Monolayers and Alkanes or Alcohols at Solid/Liquid Interfaces

Abstract: Sum-frequency surface vibrational spectroscopy was used to study chain conformation of self-assembled surfactant monolayers at various quartz/alkane and quartz/alcohol interfaces as a function of surfactant surface density and chain lengths of both surfactant and alkane or alcohol molecules. We found that, in comparison with fully packed monolayers such as octadecyltrichlorosilane (OTS) on glass or alkanethiols on gold, the chain conformation of loosely packed monolayers is considerably more sensitive to their environment. With sufficiently high surfactant surface density and sufficiently long chain lengths of both surfactant and alkane, the chain−chain interaction between surfactant and alkane molecules can effectively eliminate all the gauche defects initially present in the surfactant chains. With alkanes replaced by alcohols the hydrophobic effect appears to be dominant, as the alcohol molecules like to form a hydrogen-bonding network at an interface with the hydroxyl groups facing the surfactant mono...

Surface Studies of Polymer Blends by Sum Frequency Vibrational Spectroscopy, Atomic Force Microscopy, and Contact Angle Goniometry

Abstract: The surface composition, surface structure, and wetting properties of a polymer blend were studied by a combination of surface-sensitive techniques: IR + visible sum-frequency generation vibrational spectroscopy (SFG), atomic force microscopy (AFM), and contact angle goniometry. The polymer blend studied is composed of Biospan-S (BS) and a phenoxy base polymer (BP). BS is a polyurethane, capped with poly(dimethylsiloxane) (PDMS) end groups, and has a water contact angle of 94°. BP is more hydrophilic with a water contact angle of 74°. The SFG study shows that the more hydrophobic BS component likes to segregate to the blend surface in air and approaches a full surface coverage when the BS bulk concentration is only 1.7 wt %. The surface density of BS decreases with the decrease of its bulk concentration from 1.7 wt % and becomes undetectable at the surface, at a bulk concentration of 0.17 wt %. The surface free energy of the blend varies accordingly, as seen from the contact angle measurements. The AFM s...

Surface studies of polyimide thin films via surface-enhanced Raman scattering and second harmonic generation

Abstract: Two polyimides having the same backbone chemical structure and different pendant side groups at the 2- and 2′-positions of the diamine, the six methylene units capped with 4-cyanobiphenyl end groups and trifluoromethyl, were synthesized (6FDA-6CBO and 6FDA-PFMB). Surface-enhanced Raman scattering and surface optical second harmonic generation measurements show that after rubbing the major change in 6FDA-PFMB surface appears in the orientation of the dianhydride, which was originally planar, but becomes tilted with respect to the surface plane. In the case of 6FDA-6CBO, rubbing also causes the originally planar 4-cyanobiphenyls to tilt away from the surface and assume an azimuthally anisotropic distribution.

Two-photon-excited photoluminescence from porous silicon

Abstract: Two-photon-excited photoluminescence can be readily observed from porous silicon (PSi) with pulsed lasers. While its spectrum and lifetime are identical to those under one-photon excitation, it has a degree of polarization significantly higher than the latter and depending on the orientation of the input polarization with respect to the crystalline axes of the sample. The degree of polarization is a maximum when the input polarization is along [110] in the surface plane of PSi prepared from a Si (100) wafer and a minimum along [010]. The results can be understood from selective excitation of ellipsoidal nanoparticles by linearly polarized light and intrinsic anisotropy in two-photon excitation of crystalline Si. {copyright} {ital 1998} {ital The American Physical Society}

Surface-induced ferroelectric ice

Abstract: Summary form only given. Can ice be ferroelectric! This question has long attracted much attention. In crystalline ice, water molecules are held together by tetrahedralhydrogen bonding. The molecular orientations at the lattice points should obey the ice rules, which require that each molecule donate two protons to two of the attached water molecules and accept two protons from the other two. We used infrared-visible sum-frequency generation (SFG) spectroscopy as a probe in our experiment. As a second-order nonlinear optical process, SFG is forbidden in a medium with inversion symmetry. In its application to an ice film, the spectrum could be weak if the water molecules in the film are non-polar oriented.

Guest-host-interaction-induced enhancement of optical nonlinearity of liquid crystals and isotropic liquids

Abstract: Optical nonlinearity of liquid crystals (LC) in the isotropic phase can be enhanced by one order of magnitude by dissolving 0. 1% of anthraquinone dye in the LC. The enhancement decreases when the LC transforms into the nematic phase. The sameguest-host effect also exists in non-LC liquids. It can be explained by a model based on selective excitation of dye moleculesand the change of guest-host interaction induced by optical excitations of the dye.Keywords: liquid crystals, dye, optical Kerr effect, Freedericktz transition, optical nonlinearity, guest-host interaction INTRODUCTION Guest-host interaction is a subject ofhigh interest in many disciplines. Recently, it has received a great deal of attention in thefield of liquid crystals (LC) because of its potential applications to displays and other optoelectronic devices. In one case, itwas found that the unusually strong optical nonlinearity of nematics LCs can be further enhanced dramatically by a smallamount of dye dissolved in LC '.

Dye-Induced Enhancement of Optical Nonlinearity in Liquid Crystals and Ordinary Liquids

Abstract: Optical nonlinearity of liquid crystals (LC) in the isotropic phase can be enhanced by one order of magnitude by dissolving 0.1% of anthraquinone dye in the LC. The enhancement decreases when the L...

Correlation of Structure and Mechanical Properties of Polyolefin Surfaces by Ir + Visible Sum Frequency Generation Vibrational Spectroscopy and Atomic Force Microscopy

Abstract: The surface vibrational spectrum of atactic polypropylene was measured using IR+Visible Sum Frequency Generation (SFG) vibrational spectroscopy, in the temperature range 23 to -50 °C. A sharp rise in the intensity of the C-H symmetric stretch of CH2 groups is observed in the temperature range 0 to -20 °C. The elastic modulus and friction of the same polymer were measured with Atomic Force Microscopy (AFM) in the same temperature range i.e. 23 to -50 °C. There is a sharp rise in the modulus and a sharp decrease in the friction coefficient, at temperatures between 23 and 0 °C. These changes are attributed to the glass transition in atactic polypropylene which is expected to occur in the temperature range 10 to -10 °C. The temperature at which the glass transition is observed, from changes detected in AFM experiments, is higher than that observed by SFG experiments and that expected from bulk measurements. This elevation in the glass transition temperature has been attributed to the high pressure applied to the polymer under the AFM tip.

Guest-host-interaction-induced enhancement of optical nonlinearity of liquids

Abstract: Guest-host interaction has received a great deal of attention in the field of liquid crystals because of its potential. applications to displays and other optoelectronic devices. In one case, it was found that the unusually strong optical nonlinearity of nematic liquid crystals (LCs) can be further enhanced dramatically by a small amount of dye dissolved in the host. It was suggested that the enhancement is caused by optical excitation of dye molecules and dye-LC interaction. An important question unanswered is whether this effect is specific to liquid crystalline host material or it may have a more generic perspective. In this work, we show that the same guest-host effect also exists in the isotropic phase of liquid crystals, and more generally, in ordinary liquids without a mesomorphic phase. We demonstrate that by using 1-amino-anthroquinone dye as guest material and alkylcyanobiphenyls CN(C/sub 6/H/sub 4/)/sub 2/C/sub n/H/sub 2n+1/ as host materials, and performing optical Kerr effect measurement on such guest-host systems. To understand this guest-host effect on a microscopic level, we use a model based on the selective excitation of dye molecules and the change of guest-host interaction upon excitation.

Nonlinear optical spectroscopy for surface studies of rubbed polyimide

Abstract: Summary form only given. In fabrication of liquid-crystal (LC) display devices, rubbed polyimide films on glass substrates are widely used to align bulk liquid crystal molecules in a preferred direction. It is believed that rubbing induces anisotropic chain orientation on the polymer surfaces, and the subsequent LC-polymer interaction leads to the bulk LC alignment, but the detailed surface structure of the rubbed polyimide is not known. Polarized Fourier transform infrared (FTIR) measurements have shown that rubbing indeed induces anisotropy in the IR absorption of the polyimide film.