Seunghwan Lee

Structural characterization and frictional properties of C60-terminated self-assembled monolayers on Au(111)

Abstract C60-terminated self-assembled monolayers (SAMs) were generated on the surface of Au(111) by the adsorption of unsymmetrical disulfides having saturated hydrocarbon backbone structures comprised of six (C-6) and eleven (C-11) carbon atoms. The formation of oriented monolayer films from these adsorbates was confirmed by ellipsometric thickness and contact angle measurements. The morphologies and frictional properties of the C60-terminated SAMs were examined using atomic force microscopy (AFM). The SAMs derived from the C-11 disulfides exhibited smooth morphologies and homogeneous frictional force distributions, which proved ideal for the study of intrinsic frictional properties. The SAMs derived from the C-6 disulfides, however, exhibited rough and inhomogeneous topographic and frictional force distributions. In addition, the shorter SAMs exhibited erratic frictional behavior and partial film delamination. The relative frictional forces observed on these immobilized C60 films were higher than those observed on normal methyl-terminated SAMs and on the surface of graphite.

Chemisorption site of methanethiol on Pt{111}

The chemisorption site of the simplest prototypical model alkanethiol compound, methanethiol [CH3SH], on a Pt{111} surface in the temperature range 298–1073 K has been investigated by means of time-of-flight scattering and recoiling spectrometry (TOF-SARS) and low-energy electron diffraction (LEED). TOF-SARS spectra of the scattered and recoiled ions plus fast neutrals were collected as a function of crystal azimuthal rotation angle δ and beam incident angle α using 4 keV Ar+ primary ions. At room temperature, the adsorption of methanethiol produces a partially disordered overlayer that gives rise to a diffuse (3×3)R30° LEED pattern and three-fold symmetry in the scattering profiles. Heating this surface layer results in the sequential dehydrogenation of the methanethiol and the formation of S–C species at elevated temperatures. By ∼373 K, hydrogen is absent from the TOF-SARS spectra and a sharp (3×3)R30° LEED pattern is observed. The model developed from the scattering data is consistent with the preserv...

The relationships between interfacial friction and the conformational order of organic thin films

This report describes studies of the relationships between the structures of organic monolayers and their molecular-scale frictional properties. Several distinct self-assembled monolayers (SAMs) were formed by the adsorption of a series of spiroalkanedithiols and a single structurally related normal alkanethiol. Measurements of hexadecane wettability, infrared vibrational spectroscopy, and X-ray photoelectron spectroscopy revealed that the films possessed a wide range of interfacial structures and conformational orders. Atomic force microscopy was used to measure the frictional properties of the well-characterized SAMs on the molecular scale. Comparison of the frictional data with structural information derived from complementary analytical techniques revealed a high correlation between the conformational order of the films and the observed frictional response.

Light-Induced Fluorescence Modulation of Quantum Dot-Crystal Violet Conjugates: Stochastic Off–On–Off Cycles for Multicolor Patterning and Super-Resolution

Photoswitching or modulation of quantum dots (QDs) can be promising for many fields that include display, memory, and super-resolution imaging. However, such modulations have mostly relied on photomodulations of conjugated molecules in QD vicinity, which typically require high power of high energy photons at UV. We report a visible light-induced facile modulation route for QD-dye conjugates. QD crystal violets conjugates (QD-CVs) were prepared and the crystal violet (CV) molecules on QD quenched the fluorescence efficiently. The fluorescence of QD-CVs showed a single cycle of emission burst as they go through three stages of (i) initially quenched off to (ii) photoactivated on as the result of chemical change of CVs induced by photoelectrons from QD and (iii) back to photodarkened off by radical-associated reactions. Multicolor on-demand photopatterning was demonstrated using QD-CV solid films. QD-CVs were introduced into cells, and excitation with visible light yielded photomodulation from off to on and off by nearly ten fold. Individual photoluminescence dynamics of QD-CVs was investigated using fluorescence correlation spectroscopy and single QD emission analysis, which revealed temporally stochastic photoactivations and photodarkenings. Exploiting the stochastic fluorescence burst of QD-CVs, simultaneous multicolor super-resolution localizations were demonstrated.