Tanakorn Osotchan

Electron mobilities in gallium, indium, and aluminum nitrides

Electron mobilities in GaN and InN are calculated, by variational principle, as a function of temperature for carrier concentrations of 1016, 1017, and 1018 cm−3 with compensation ratio as a parameter. Both GaN and InN have maximum mobilities between 100 and 200 K, depending on the electron density and compensation ratio, with lower electron density peaking at lower temperature. This is due to the interplay of piezoelectric acoustic phonon scattering at low carrier concentrations and ionized impurity scattering at higher carrier concentrations. Above 200 K, polar mode optical phonon scattering is the mobility limiting process. The 300 and 77 K electron and Hall mobilities as functions of carrier concentration in the range of 1016–1020 cm−3 and compensation ratio are also calculated. The theoretical maximum mobilities in GaN and InN at 300 K are about 1000 and 4400 cm2 V−1 s−1, respectively, while at 77 K the limits are beyond 6000 and 30 000 cm2 V−1 s−1, respectively. We compare the results with experimen...

Correlation between the orthorhombicity and the transition temperatures of the “123” rare earth series superconductors

Abstract The influence of the tetragonal-to-orthorhombic phase transition on the normal-to-superconducting phase transition in REBa 2 Cu 3 O 7− y (RE = rare earth element) is studied within the framework of a Ginzburg-Landau (GL) approach. Through a minimization of the GL free energy, an expression for the superconducting critical temperature is obtained which predicts that T c should vary as the square of the orthorhombicity. It is found that the correlation between the observed T c s and orthorhombicities of the “123” RE series superconductors require the two phase transitions to be competitive. This is unlike the case for the “124” RE series superconductors, where the two phase transitions enhance each other.

Synthesis and Reactivity of Nitrogen Nucleophiles-Induced Cage-Rearrangement Silsesquioxanes

Novel phthalimide and o-sulfobenzimide-functionalized silsesquioxanes were successfully synthesized via nucleophilic substitution reactions from octakis(3-chloropropyl)octasilsesquioxane. Surprisingly, the formation of deca- and dodecasilsesquioxanes cages was discovered during substitution with phthalimide, but only octasilsesquioxane maintained a cage in the o-sulfobenzimide substitution reaction. Moreover, we report the electronic effect of nitrogen nucleophiles to promote cage-rearrangement of inorganic silsesquioxane core for the first time. Structures of products were confirmed by (1)H, (13)C, and (29)Si NMR spectroscopy, ESI-MS analysis, and single-crystal X-ray diffraction.

Tunable Porosity of Cross-Linked-Polyhedral Oligomeric Silsesquioxane Supports for Palladium-Catalyzed Aerobic Alcohol Oxidation in Water

Polyhedral oligomeric silsesquioxane (POSS)-based materials, poly-POSS-Tn [n = 8 (1), 10 (2), 12 (3), and mix (4)], were prepared in high yields via free radical polymerization of corresponding pure forms of methacrylate-functionalized POSS monomers, MMA-POSS-Tn (n = 8, 10, 12), and the mixture form, MMA-POSS-Tmix. Powder X-ray diffraction (XRD) spectra and BET analysis indicate that 1-4 are amorphous materials with high surface areas (683-839 m2 g-1). The surface areas and total pore volumes follow the trend: poly-POSS-T12 > poly-POSS-T10 > poly-POSS-Tmix > poly-POSS-T8. In addition, on the basis of Barrett-Joyner-Halenda (BJH) analysis, poly-POSS-T12 contains the highest amount of mesopores. The Pd nanoparticles immobilized on poly-POSS-Tn [n = 8 (5), 10 (6), 12 (7), and mix (8)] are well dispersed with 4-6 wt % Pd content and similar average particle sizes of 6.2-6.5 nm, according to transmission electron microscopy-energy dispersive X-ray analysis (TEM-EDX) and microwave plasma-atomic emission spectroscopy (MP-AES). At 90 °C, the stabilized Pd nanoparticles in 5-8 catalyzed aerobic oxidation of benzyl alcohol to benzaldehyde in 72-100% yields at 6 h using a mixture of a H2O/Pluronic (P123) solution. The PdNp@poly-POSS-T8 catalyst (5) exhibited the lowest catalytic activity, as a result of its lowest surface areas, total pore volumes, and amounts of mesopores. With the catalyst 8, various benzyl alcohol derivatives were converted to the corresponding aldehydes in good to excellent yields. However, with alcoholic substrates featuring electron-withdrawing substituents, high conversions were achieved with 1 equiv of K2CO3 additive and longer reaction times.

Hall and drift mobilities in molecular beam epitaxial grown GaAs

A series of nominally undoped and Si-doped GaAs samples have been grown by molecular beam epitaxy (MBE) with Hall concentrations ranging from 1015 to 1019 cm−3 and mobilities measured at 77 and 300K by Hall-van der Pauw methods. Drift mobilities were calculated using the variational principle method and Hall scattering factors obtained from a relaxation-time approximation to permit cross-correlation of experimental data with drift or Hall mobilities and actual or Hall electron concentrations. At 77K, both high purity and heavily doped samples are well represented by either drift or Hall values since piezoelectric acoustic phonon scattering and strongly screened ionized impurity scattering hold the Hall factor close to unity in the respective regimes. Between n≊1015 and 1017 cm−3, where lightly screened ionized impority scattering predominates, Hall mobility overestimates drift mobility by up to 50 percent and Hall concentration similarly underestimates n. At 300K, polar optical phonons limit mobility and a Hall factor up to 1.4 is found in the lowest doped material, falling close to unity above about 1016 cm−3. Our calculation also agrees remarkably well with the Hall mobility of the highest purity MBE grown sample reported to date.

Blue Flexible Transparent Organic Light-Emitting Devices

Blue transparent organic light-emitting devices (TOLEDs) and blue flexible OLEDs (FOLEDs) have been fabricated on glass and plastic films, respectively. We have also fabricated blue flexible transparent OLEDs (FTOLEDs) having both flexibility and transparency by using indium-tin-oxide:cesium (ITO:Cs) as a cathode. A Cs-incorporated ITO electrode is useful for the fabrication of not only a TOLED on a glass substrate but also a flexible TOLED on a plastic film.

ELECTRONIC STRUCTURE OF THE FINITE-SIZED SINGLE-WALLED CARBON NANOTUBES

The effects of tubule length and terminal capping on the geometrical and electronic properties of finite-sized zig-zag (9, 0) single-walled carbon nanotubes (SWNT), which length varying from 2 up to 12 unit cells (~50 A), were investigated using molecular mechanics, semi-empirical methods (AM1 and EHMO) and density functional theory (B3LYP). AM1 method indicates how the nanotube ends are capped affects strongly the tubule geometric parameters. Although these effects seem to decrease exponentially as the tube gets longer, the converging values for C–C bond length in the open- and closed-end structures are slightly different. It was learned that combination of low-level methods like AM1 and EHMO (which tend to overestimate and underestimate the HOMO–LUMO energy gap, respectively) together with high-level method such as DFT is efficient to estimate band gap for finite-sized nanostructures. The HOMO–LUMO energy gaps obtained from semi-empirical and DFT methods decrease as the tubule length increases. Terminal capping also affects strongly the electronic structure of finite-sized nanotube. Thus, closing the terminal ends by fullerene hemisphere broadens the energy gap of the hydrogen-saturated open-end nanotube. Although the open-end SWNT has much lower AM1 HOMO–LUMO energy gap than the closed-end SWNT, these orbitals unfortunately are localized near the capping hydrogen, thereby do not provide conducting channels for electrons. By comparing only the delocalized frontier orbitals, both structures yield closer energy gap. Analysis of the energy gap based on EHMO, AM1 and DFT results suggests that both open- and closed-end finite-sized SWNT are semiconductor, in agreement with recent scanning tunneling experiment. It was found that the slight accumulated negative charges are likely to locate at the nanotubes fullerene tips.

FLUCTUATIONS IN THE LOCAL FIELDS DUE TO AL3+ IONS SUBSTITUTION IN THE M-TYPE BARIUM HEXAFERRITES, BAFE12-XALXO12

The effects of aluminum substitution into the M-type Ba-hexaferrites on the saturation magnetization and Mossbauer spectrum of these ferrites are studied. The saturation magnetization Ms of the BaFe12-xAlxO12 is seen to decrease linearly with increasing Al substitution. The Mossbauer spectrums for the x = 0.1, 0.2 and 0.6 specimens were all resolved as superpositions of four sub-spectrums (12k, 4fiv+2a, 4fvi and 4e). The half width of the Mossbauer lines of the 4fiv+2a sextets is seen to broadening as Al ions are substituted into the ferrite. No broadening is seen in the lines of the other sextets. These behaviors (the line broadening and the linear decrease in Ms) are consistent with the Al3+ ions substituting preferentially into the 2a sites.

On the calculation of electron mobility in In0.53Ga0.47As

Abstract The electron mobility in In 0.53 Ga 0.47 As is calculated using both relaxation-time-approximation and variational-principle approaches and compared with that calculated by the Matthiessen rule method and with experiment. By using either the compensation ratio or alloy scattering potential, or both, as fitting parameters, all three techniques can be brought to good agreement with experimental data for carrier concentration in the 5 × 10 15 –1 × 10 17 cm −3 range. The fitted parameters are also shown to be comparable across the different techniques used and with those determined experimentally. Using all three methods, the effect of compensation on electron mobility at 77 and 300 K is also calculated as a function of net electron density. These curves are then compared with the multitude of experimentally determined mobilities in In 0.53 Ga 0.47 As grown by various techniques. In general, the use of the variational principle to calculate electron mobility give only slightly better correlation with experiments than the more approximate methods.

Study of low power deposition of ITO for top emission OLED with facing target and RF sputtering systems

Deposition of ITO as top transparent electrode was studied using two deposition systems with and without direct contact to working plasma; namely with conventional RF-magnetron planar (RSS) and pulsed-DC facing target sputtering systems (FTS). Test devices were made on glass substrates and consisted of (from bottom up) ITO/4 Organic Layers/ITO. Depositions were performed at low deposition powers; 30 and 60 watts, to reduce damages by energetic sputtered particles to underlying organic layers. Test devices from both sputtering systems were found to function well. Leakage current density at -5 V reverse bias were relatively constant from 0.3 and 0.4 mA/cm2 at 30 W and 60 W in FTS, while the values were found to increase from 0.001 to 0.2 mA/cm2 at 30 W and 60 W in RSS.