Body weight is a major risk determinant of frailty, but the effect of obesity on frailty is controversial. The present study aimed to confirm the hypothesis that the risk of frailty is positively associated with obesity (BMI ≥ 30 kg/m2), but the association is mediated by the waist:height ratio (WHtR) in older women and men. A total of 2862 community-dwelling older individuals aged 70–84 years were assessed for frailty using the Korean version of Fatigue, Resistance, Ambulation, Illnesses, and Loss of weight index. Obesity (BMI ≥ 30 kg/m2) was associated with a higher risk of frailty compared with BMI 18·5–<23 kg/m2 in all the older individuals (OR 1·88; 95 % CI 1·11, 3·17; P = 0·018) and in older women (OR 1·86; 95 % CI 1·01, 3·42; P = 0·047) before adjusting for WHtR but was not associated with BMI after adjusting for WHtR. Additionally, obesity was not significantly associated with the risk of frailty before and after adjusting for WHtR in older men. Mediation analysis revealed that the association between BMI and frailty score was mediated by WHtR. Moreover, the mediating effect of WHtR on frailty score was positive in both women and men, but the frailty score was associated with BMI positively in women and negatively in men. The present study suggests that the risk of frailty is higher in obese women, which is mediated by WHtR, but not in obese men.

Room temperature photoluminescence (PL) spectra from zinc oxide (ZnO) nanostructures were studied. ZnO samples were produced via thermal chemical vapor deposition (thermal-CVD) and a variety of ZnO nanostructures were synthesized by adjusting the oxygen content during the growth process. All samples exhibit a sharp and strong ultra-violet near-band-edge (NBE) emission at about 3.18 eV. The visible emission from the samples deposited under an oxygen-deficient condition were dominated by blue-green band emission at 2.34 eV. The intensity of the blue-green band was greatly reduced (so-called green band free) for the ZnO deposited at the center of the wafer while strong violet-blue emission bands and broad bands at yellow-orange-red range were collected from the ZnO grown along the edge of the wafer. We believe that the spatial inhomogeniety was caused by turbulent gas flow in the reaction chamber, which resulted in different local oxygen concentration. Origin of visible luminescence from ZnO nanostructures will be discussed and a model to explain the observed visible luminescence process will be presented.

Light emitting diodes (LEDs) are normally operated at high current levels resulting in substantial junction heating. However, the junction temperature cannot be measured directly. In the study reported here, it was attempted to estimate junction temperature in LEDs with peak emission 400 nm. Temperature was estimated from current-voltage (I-V) measurements as a function temperature and peak-shift of optical emission spectra with increasing temperature. For a diode operated at a current-level of 100 mA, a temperature of 100 °C was estimate from current voltage measurement and 160 °C from the peak-shift in the optical emission spectra. The difference in temperature estimated using the two different technique is not understood at this point.

A micro-reflectance spectroscopy was performed on a single island of photonic crystal array. An array of islands of photonic crystal with colloidal polystyrene beads was assembled on Si using ink-jet printing. The polystyrene colloids with three different sizes (190 nm, 210 nm, and 270 nm in diameter) were used and the polystyrene colloidal particles were self-assembled to form fcc lattices with three different lattice spacing. It was observed from the reflectance spectra that the position of the optical stop band shifts as the size of the colloidal particle changes. Effective medium approximations were used to model the dielectric properties of the colloid/air composite. The theoretically expected reflectance peak position agrees well with those of the experimentally observed peaks. The effect of finite size of the photonic crystal island on its reflectance spectroscopy was investigated by comparing the reflectance spectra collected from four different photonic crystal islands assembled from the same polystyrene colloidal particles, but with different lateral size for each island. It was found that the primary reflection peak was broadened and its intensity was reduced when the lateral size of the island was decreased.

Blue light emitting diodes (LEDs) based on an AlGaN/GaN/AlGaN double heterojunction structure were electrically characterized as a function of temperature. Current-voltage (I-V), capacitance-voltage (C-V) and reverse recovery storage time measurements were conducted at temperatures in the range between -8° and 75° C. Capacitance-voltage measurements as a function frequency (20 Hz – 1 MHz) and electroluminescence study at room temperature were also performed. It was observed that the diode turn-on voltage decreased with increasing temperature, however, reverse leakage currents monitored at -1, -5 and -10 V showed only a slight increase with increasing temperature. The concentration of deep states and their position in the bandgap, as extracted from logarithmic plots of the forward characteristics, were not influenced by the measurement temperature. Recombination lifetimes, as obtained from experimentally determined reverse recovery storage times, remained constant over the range of temperature considered. A higher value of diode capacitance was observed at low measurement frequencies (20 Hz – 1 kHz), gradually dropping to a lower value over a frequency range between (1 kHz – 100 kHz) and remained constant from 100 kHz to 1 MHz. A loss peak centered about 10 kHz was observed in the corresponding plot of gm/ω as a function of frequency, f. The position of the peak in the gm/ω - frequency (f) plot and dC/dω(for f in the range 1 kHz – 100 kHz ), yielded a concentration of deep-states of approximately 2.2 × 1015/cm3, located at 0.39 eV above the valence bandedge.

In this investigation, Schottky diodes with different device sizes (150μm, 420μm and 700μm) were fabricated on the Ga-face of a free-standing n--GaN wafer produced by Kyma Technologies, Inc. Full area back side ohmic contact was prepared on the N-face of the bulk GaN using Ti/Al. Without any edge-termination scheme, a relatively high reverse breakdown voltage of 240V was achieved. The reverse breakdown voltage decreases as the device size increases. The forward turn-on voltage was as low as 2.4V at room temperature for the 150μm diameter Schottky diodes. The best on-state resistance was 7.56 mΩcm2 for diodes with VB=240V, producing a figure-of-merit (VB2/RON) of 7.6 MWcm-2. The Schottky diode also showed an extremely short reverse recovery time (< 20 ns) switching from forward bias to reverse bias.

This study presents results of wavelength-dependent Raman scattering from amorphous silicon carbon (a-Si:C:H). The a-Si:C:H films were produced by radio-frequency plasma-enhanced chemical vapor deposition. Prior results with amorphous carbon indicate that laser excitation selectively probes clusters with differing sizes. Our measurements with a-Si:C:H indicate that when using red (632.8 nm), green (514.5 nm), and blue (488.0 nm) excitation, the Raman D and G peaks shift to higher wave numbers as the excitation energy increases. The higher frequency is associated with smaller clusters that are preferentially excited with higher photon energy. It appears that photoluminescence occurs due to radiative recombination from intracluster transitions in Si-alloyed sp2-bonded carbon clusters

The thick films of GaN were investigated using X-ray diffraction, micro-Raman spectroscopy and photoluminescence spectroscopy. The thick films of GaN were prepared on (0001) sapphire using high rate magnetron sputter epitaxy with growth rates as high as 10–60 m/min. The width of the X-ray rocking curve ((0002) reflection) for the sample produced by this method is ∼300 arc-sec. Only the allowed modes were observed in the polarized Raman spectra. The background electron concentration is lower than 3×1016 cm−3, which was determined from the Raman spectra. The phonon lifetime determined from Raman E2(2) mode was 1.6 ps, which is comparable to that of bulk single crystal GaN grown by sublimation (1.4 ps). The full-width-at-half-maximum of the near band-edge photoluminescence peak obtained at 77K is ∼100 meV.