David S. Y. Hsu

Structure and luminescence of annealed nanoparticles of ZnS:Mn

Structural and light-emitting properties of nanoparticles of ZnS:Mn annealed in vacuum at temperatures up to 525 °C are presented. Annealing the 3.5 nm particles at temperatures up to 350 °C caused growth of some particles without substantial change in the luminescence or ZnS lattice. After annealing at 400–525 °C, the high-temperature wurtzite phase of ZnS appeared, accompanied by an increase of the average particle diameter to approximately 100 nm and a rearrangement of the Mn ions. Dramatic increase in cathodoluminescence emission was also observed and is compared to the structural information obtained from electron microscopy, x-ray diffraction, x-ray absorption fine structure, and electron paramagnetic resonance measurements.

Integrally gated carbon nanotube-on-post field emitter arrays

Multiwalled carbon nanotubes were grown using chemical vapor deposition on the tops of blunt vertical silicon posts in cells having a horizontal gate aperture of conventional field emitter design. We obtained over 1 mA total emission current from a single array, or 0.3 μA per cell at 40 V. In addition to the low voltage operation, the most distinctive differences from conventional field emitter arrays include their stability and the lack of catastrophic arcing without any special sample preparation.

Microgating carbon nanotube field emitters by in situ growth inside open aperture arrays

Multiwalled carbon nanotubes were grown using chemical vapor deposition inside small apertures having a horizontal gate and a sidewall insulator spacer. Emission currents up to 140 nA per cell at 63 V have been obtained. These arrays have exhibited a gate current as low as 2.5% of the anode current throughout the entire gate voltage range, representing the lowest gate to anode current ratio of gated nanotube emitters reported to date. We attribute this feature to the emitter geometry and method of fabrication. The overall fabrication method required only a few and simple processing steps.

Dynamics of OH desorption from single crystal Pt(111) and polycrystalline Pt foil surfaces

The laser-induced fluorescence technique in conjunction with an UHV apparatus was used to probe the desorption dynamics of OH radicals formed in the association reaction of chemisorbed O and H atoms on single crystal Pt(111) and polycrystalline Pt foil surfaces. The apparent activation energies of desorption and the rotational energy distributions of OH radicals were measured in these experiments. The apparent OH desorption energies for both surfaces were observed to increase by about 10 kcal/mol as the O/H reactant gas mixture ratio is decreased by a factor of 100, with the values for the Pt(111) single crystal at 5–10 kcal/mol below those for the Pt foil. Computer kinetic modeling of the temperature dependence of OH desorption rate as a function of O/H ratio showed similar trends as those observed in the experiments. Boltzmann-like OH rotational energy distributions with relatively high rotational to surface temperature ratios, T r / T s =0.85, were measured over the temperature range 1227–1479 K for the Pt(111) single crystal and over 1283–1475 K for the polycrystalline Pt foil. For both surfaces the OH spin-orbit temperatures were essentially the same as the rotational temperatures and no preference for any lambda-doublet state was observed. The T r / T s ratio does not seem to depend on the O/H reactant gas mixture ratio for each surface, suggesting that the rotational energy accommodation is insensitive to the apparent Pt-OH bond strength.

Electronic-to-vibrational energy transfer reactions: Na(3 2P) + CO (X1Σ+, υ=0)

Abstract The vibrational distribution of CO produced from the electronic-to-vibrational energy transfer reaction: Na(32P) + CO(X1Σ+, υ=0)→Na(32S) + CO(X1Σ+, υ⩽8) has been determined by means of infrared resonance absorption measurements employing a cw CO laser. A flash-lamp-pumped dye laser is used to excite the ground state Na to the 32P 1 2 and 32P 3 2 states. The CO molecules formed in the reaction were found to be vibrationally excited up to the limits of available electronic energies carried by the excited Na atoms, and the vibrational population exhibits a maximum at υ=2. The efficiency of E→V energy transfer was determined to be 35%. Our present results were found to be consistent with the impulsive (half-collision) and curve-crossing models.

Dynamics of reactions of O(3P) atoms with CS, CS2 and OCS

Abstract The reactions of CS(X 1Σ+), CS2(X 1Σ+g) and OCS(X 1Σ+) with O(3P) were studied at 298 K by means of a CO laser resonance absorption technique. The CO(ν) population distribution produced from the reaction O(3P) + CS(X 1Σ+) studied in a quartz flash photolysis tube (λ>/ 200 nm) is similar to distributions observed previously for ν> 7. For ν / 300 nm) in which the O(3P) + CS2(X 1Σ+g) reaction is the only one which can occur confirmed that the colder population observed is attributable to this process. The branching ratio for the reaction channel O(3P) + CS2(X 1Σ+g) → CO(X 1Σ+) + S2(3Σ−g) has been measured. We find that 1.4 ± 0.2% of the O + CS2 reaction proceeds through this channel, and that the rate constant for this reaction channel is, k = 3.5 (±0.5) × 1010 cm3/mole s. Isotope labeled experiments using 18O atoms show that the O(3P) + OCS(X 1Σ+) reaction takes place by a direct stripping mechanism, wherein CO(ν) is produced exclusively from the parent OCS molecule. The CO(ν) formed in this reaction carries about 9% of the total available energy.

A comparison of rotational energy accommodation in catalytically produced OD and OH radicals desorbing from a smooth Pt(111) single crystal

Rotational energy distributions of OD and OH radical reaction products desorbing from a smooth Pt(111) single crystal surface have been measured under closely similar experimental conditions by using the laser‐induced fluorescence technique in conjunction with an UHV apparatus. Over the temperature ranges studied, nearly full rotational energy accommodation was observed for OD with a rotational to surface temperature ratio, Tr/Ts =0.96±0.04, and a somewhat lower ratio of 0.86±0.05 was obtained for OH. For both OD and OH the spin–orbit temperatures were the same as the rotational temperatures and no preference for any lambda‐doublet state was observed.

Thermal decomposition of formic acid at high temperatures in shock waves

Thermal decomposition of HCOOH has been studied from 1280 to 2030 K over a broad range of initial concentrations (0.07 to 1.6% in Ar) in a shock tube using a stabilized cw CO laser to monitor CO formation. The observed CO concentration profiles over the whole range of temperature and pressureemployed could be satisfactorily accounted for by the following two-channel molecular decomposition mechanism: HCOOH + M → 1 CO + H 2 O + M HCOOH + M → 2 CO 2 + H 2 O + M 89-1 with the following apparent second order rate constants in the pressure range 0.75–2.8 atm. k 1 = 2.3 x 10 15 exp ( − 50 , 000 ± 1700 / RT ) c c mole − 1 sec − 1 k 2 = 1.5 x 10 16 exp ( − 57 , 000 ± 2800 / RT ) c c mole − 1 sec − 1 The values of these two rate constants were evaluated by the non-linear least squares fitting of the observed CO concentration profiles. The possibility of an alternative decomposition route involving the initial production of H atoms, instead of H2, has also been considered. However, the rate constant evaluated from the kintic modeling of the CO formed at high temperatures cannot satisfactorily account for that observed at lower temperatures.

Chemical processes involved in the etching of silicon by xenon difluoride

The spontaneous etching of silicon by xenon difluoride has been investigated under single collision conditions using multiphoton ionization mass spectrometry for the detection of radical products. SiF2 is the only gas phase radical observed with an apparent activation energy of 6.7±0.5 kcal/mol. Electron ionization techniques were used to determine an apparent activation energy for gas phase production of SiF4 of 5.6±0.8 kcal/mol. The identification of SiF2 as the sole reactive desorbing species during the steady‐state etching process is useful for understanding many aspects of primary surface processes and secondary gas phase reactions in etching.

MPI/MS detection of SiF and SiF2 radicals produced from the reaction of F2 and NF3 with silicon

Abstract SiF and SiF 2 radicals, produced in the thermal reaction of F 2 and NF 3 with solid silicon, have been detected by multiphoton ionization mass spectrometry in the 438 and 321 nm wavelength regions, respectively. The reaction pathway to the production of SiF radicals is of minor significance compared with SiF 2 production over a wide range of temperatures and pressures. In both cases it was found that little or no fragmentation of the parent ion occurs upon ionization. MPI/MS is thus shown to be a useful diagnostic tool for studies of fluorine-silicon etching reactions.