Hyunjin Lim

Structures of Ionic Liquids with Different Anions Studied by Infrared Vibration Spectroscopy

We investigated the structures of ionic liquids (1-butyl-3-methylimidazolium iodide [BMIM][I] and 1-butyl-3-methylimidazolium tetrafluoroborate [BMIM][BF4]) and their aqueous mixtures using attenuated total reflection (ATR) infrared absorption and Raman spectroscopy. The ATR spectrum in the CHx (x = 1, 2, 3) vibration region from 2800 to 3200 cm-1 was very different between [BMIM][BF4] and [BMIM][I] even though all the spectral features in this region were from the butyl chain and the imidazolium ring of the same cation. The spectrum did not change appreciably irrespective of the water concentration for [BMIM][BF4], whereas the spectrum from [BMIM][I] showed significant changes as the water concentration was increased, especially in CH-vibration modes from the imidazolium ring. For very diluted solutions both aqueous mixtures of [BMIM][I] and [BMIM][BF4] showed very similar spectra. Mixing of [BMIM][I] with heavy water (D2O) facilitated the isotopic exchange of the proton attached to the most acidic carbon of the imidazolium ring into deuterium from D2O, whereas even prolonged exposure to D2O did not induce any isotopic exchange for [BMIM][BF4]. Raman spectra around 600 cm(-1) indicative of the butyl chain conformation also changed differently as the water concentration was increased between [BMIM][I] and [BMIM][BF4]. These differences are considered to come from the variation in the position of the anion, where I- is expected to be closer to the C(2) hydrogen of the imidazolium cation and interacting more specifically as compared to BF(4-).

Aligned networks of cadmium sulfide nanowires for highly flexible photodetectors with improved photoconductive responses

We developed a simple but efficient method to mass-produce highly flexible and high-performance photodetectors based on aligned cadmium sulfide (CdS) nanowire (NW) networks. In this method, the CdS NWs were selectively aligned along the molecular patterns on flexible substrates via a direct assembly method, and the aligned CdS NW patterns were utilized as the channels of flexible photodetectors. The photodetectors based on the aligned CdS NWs exhibited ∼10 times higher photosensitivity and ∼100 times faster photoresponse than those based on randomly oriented NW networks. In addition, the flexible photodetectors exhibited stable photoconductive characteristics even when these were bent down to the radius of curvature of 0.2 mm. This research may pave the way for the large-scale fabrication of low-cost and high performance flexible photodetectors based on the aligned NW networks.

New Insights into ETS-10 and Titanate Quantum Wire: A Comprehensive Characterization

The titanate quantum wires in ETS-10 crystals remain intact during ion exchange of the pristine cations (Na(+)(0.47) + K(+)(0.53)) with M(n+) ions (M(n+) = Na(+), K(+), Mg(2+), Ca(2+), Sr(2+), Ba(2+), Pb(2+), Cd(2+), Zn(2+)) and during reverse exchange of the newly exchanged cations with Na(+). The binding energies of O(1s) and Ti(2p) decrease as the electronegativity of the cation decreases, and they are inversely proportional to the negative partial charge of the framework oxygen [-delta(O(f))]. At least five different oxygen species were identified, and their binding energies (526.1-531.9 eV) indicate that the titanate-forming oxides are much more basic than those of aluminosilicate zeolites (530.2-533.3 eV), which explains the vulnerability of the quantum wire to acids and oxidants. The chemical shifts of the five NMR-spectroscopically nonequivalent Si sites, delta(I(A)), delta(I(B)), delta(II(A)), delta(II(B)), and delta(III), shift downfield as -delta(O(f)) increases, with slopes of 2.5, 18.6, 133.5, 216.3, and 93.8 ppm/[-delta(O(f))], respectively. The nonuniform responses of the chemical shifts to -delta(O(f)) arise from the phenomenon that the cations in the 12-membered-ring channels shift to the interiors of the cages surrounded by four seven-membered-ring windows. On the basis of the above, we assign delta(I(A)), delta(I(B)), delta(II(A)), and delta(II(B)) to the chemical shifts arising from Si(12,12), Si(12,7), Si(7,12), and Si(7,7) atoms, respectively. The frequency of the longitudinal stretching vibration of the titanate quantum wire increases linearly and the bandwidth decreases nonlinearly with increasing -delta(O(f)), indicating that the titanate quantum wire resembles a metallic carbon nanotube. As the degree of hydration increases, the vibrational frequency shifts linearly to higher frequencies while the bandwidth decreases. We identified another normal mode of vibration of the quantum wire, which vibrates in the region of 274-280 cm(-1). In the dehydrated state, the band-gap energy and the first absorption maximum shift to lower energies as -delta(O(f)) increases, indicating the oxide-to-titanium(IV) charge-transfer nature of the transitions.

Complete suppression of large InAs island formation on GaAs by metal organic chemical vapor deposition with periodic AsH3 interruption

Self-assembled InAs quantum dots (QDs) on GaAs substrates were grown by metal organic chemical vapor deposition with periodic AsH3 interruption. In contrast to the conventional InAs QD growth method, AsH3 was interrupted periodically while TMIn was introduced into the reactor continuously. By interrupting AsH3 periodically, the growth surface is modulated between As-stabilized surface and In-stabilized one, resulting in complete suppression of relaxed large island formation and significant improvement in photoluminescence intensity. With further optimization of growth parameters, the authors obtained the emission at 1.32μm and narrow linewidth of 32meV at room temperature.

Distribution Pattern of Length, Length Uniformity, and Density of TiO32− Quantum Wires in an ETS-10 Crystal Revealed by Laser-Scanning Confocal Polarized Micro-Raman Spectroscopy†

ETS-10 is a highly intriguing microporous titanosilicate that has shown an excellent propensity for the selective removal of harmful heavy-metal ions, the potential to work as an effective catalyst for various reactions, and that can be used as a material for solar cells. Such important features arise from the TiO3 2 quantum wires with the diameter (d) of approximately 0.67 nm running along the [110] and [110] directions in the crystal (Figure 1). The TiO3 2 quantum wire is a one-dimensional (1D) extreme of three-dimensional (3D) bulk titanates, which are widely used in industry as, for example, capacitors. It also exhibits an interesting 1D quantum confinement effect. The TiO3 2 quantum wires are not expected to be connected all the way from one face to the opposite face of a crystal owing to the large number of randomly distributed defects. Now the questions are what is the average length of the wires, to what degree do the lengths vary (how does the length homogeneity vary), how does the local density of the quantum wire vary from one region to another within a crystal, do they vary randomly or in accordance with a certain pattern? Answers to the above questions will be highly useful for understanding the mechanism of ETS-10 formation and growth, the refinement of its structure, improvements of its catalytic activities, and its future applications. However, there have been no methods to gain such information. The TiO3 2 quantum wire in ETS-10 gives a strong Raman shift band between 724 and 840 cm , arising from a longitudinal vibrational mode of the -Ti-O-Ti-Ochain. Its frequency at the band maximum (nmax), its bandwidth (full width at half maximum, fwhm), and intensity (I) reflect the relative average length, length homogeneity, and density of the quantum wire, respectively. The Raman band frequency decreases as the length increases, owing to the increase in the reduced mass of the quantum wire. The smallest frequency ever observed is 724 cm . Bandwidths between 23 and 120 cm 1 have been observed, and the bandwidth decreases as the length uniformity increases. The intensity increases as the number of the TiO3 2 quantum wire increases. Accordingly, the frequency, bandwidth, and intensity have served as the three important criteria for comparison of the relative average lengths, relative average length uniformities, and relative average densities of the TiO3 2 quantum wires in the ETS-10 crystals. This information indicates that we can also apply the same principle to obtain their distribution pattern within an ETS-10 crystal if we can obtain a matrix of Raman spectra measured from a large number of artificially divided very small sections of a crystal. Furthermore, the obtained data would be more informative if we can obtain a map of these three data sets for the TiO3 2 quantum wires running along the [110] and [110] directions, respectively. We now report that laser scanning confocal polarized micro-Raman (LSC-PMR) spectroscopy is a highly useful tool for the above purpose and the novel fact that the TiO3 2 quantum wires are not evenly distributed within ETS-10 crystals but distributed in a symmetrical manner according to an interesting pattern. Figure 1. a) Illustrations of a typical morphology (truncated bipyramid) of an ETS-10 crystal and three-dimensional networks of SiO2 channels (cyan) and TiO3 2 quantum wires (red) in the case of polymorph B and b) a single TiO3 2 quantum wire.

Growth of ZnO-Nanorod Grating on the Seed Grating Produced by Femtosecond Laser Pulses

In this research, we successfully fabricated ZnO-nanorod grating by carrying out femtosecond-laser modification of the seed layer. First, a Ag-doped ZnO seed layer was deposited on a glass substrate by dc/rf magnetron co-sputtering, in which rf and dc power sources were utilized for the ZnO and the Ag targets, respectively. Next, a seed grating was produced on the seed layer by using the two-beam interference of femtosecond-laser pulses. Finally, a ZnO-nanorod grating was grown on the seed grating by chemical bath deposition in an aqueous solution of Zn(NO3)2 and hexamethyltetramine. The scanning-electron-microscopy images indicate that the ZnO-nanorod grating can be regarded as a spatially periodic structure consisting of alternating bands of ZnO nanorods with relatively large and small diameters. The selected-area electron-diffraction patterns of the seed grating reveal that the formation of the ZnO-nanorod grating is attributable to the spatially selective modification of the seed layer produced by femtosecond-laser pulses.