Cheow-Keong Choo

CaWO4 thin films synthesized by pulsed laser deposition

Thin films of CaWO4 were synthesized by pulsed laser deposition (PLD) using targets composed of CaO and WO3 oxide mixtures. The crystallinity of CaWO4 thin films strongly depended on PLD conditions, that is, substrate temperature, ambient oxygen pressure, and laser fluence. Congruent thin films with stoichiometry were obtained at 873 K. They showed photoluminescence (PL) at room temperature. PL spectra were characteristic with excitation wavelength, peaked at 440 and 520 nm, which were excited with 340–360 and 300–330 nm, respectively. Cathodeluminescence (CL) spectra could be recorded at room temperature on the PLD thin films and were composed of the same species as observed in PL spectra. Intensity of CL spectra increased drastically after postannealing the PLD thin films at 873 K. PLD experiments were also carried out using CaWO4 targets at the same conditions. Crystalline CaWO4 thin films were obtained much easier than PLD using targets composed of CaO and WO3 oxide mixtures.

Characterization of nitrogen terminated silicon nanoparticles on AFI zeolite with X-ray and ultraviolet photoelectron spectroscopies

Abstract Nitrogen terminated silicon nanoparticles were synthesized on AFI AlPO 4 -5 zeolite by pulsed laser ablation of silicon target in the presence of 1.33×10 −5 Pa of ammonia, and were characterized in situ by X-ray photoemission spectroscopy (XPS) and ultraviolet photoelectron spectroscopy (UPS). Nonstoichiometric silicon nitride and unterminated silicon nanoparticles with size of estimated to be ∼5.4 A in diameter were existed. They were found to migrate and diffuse into the AlPO 4 -5 channels at 373 K and migrate back to the external surface at 503 K. The effect of sample annealing was studied at 373 K. The change of silicon surface state (unterminated silicon nanoparticles) to hydride species was observed concurrently with breaking of the NH x species. This result implies that the NH x species remained on the silicon particles form Si 3 N 4 like bonds and the dissociated H-species are bound to the unterminated silicon surface.

Time-resolved photoluminescence on ion doped CdWO4 thin films prepared with pulsed laser deposition

Samarium-, nickel-, cesium-ion-doped and -undoped CdWO4 thin films are prepared by pulsed laser deposition (PLD) method. Photoluminescence (PL), cathode luminescence (CL), and x-ray excited luminescence (XL) are studied on these thin films. Luminescence spectra are deconvoluted to three luminescence species typically at 2.77, 2.48, and 2.18 eV. Luminescence intensities and the relative contribution of three deconvoluted components are compared with those on the undoped CdWO4 (010) single crystal surface. PL intensity on the undoped thin film can be compared to that on the (010) single crystal surface. Not only CL and XL intensities but also PL intensity drops drastically on ion-doped CdWO4 thin films. However, the emission component at about 2.2 eV is dominant and the relative contribution of 2.77 eV is diminished on the ion-doped PLD thin films. The x-ray diffraction data indicates that the (002) plane is most intense on these films and the peak shifts to lower 2θ value on the ion-doped CdWO4 thin films....

Sm-doped CdWO4 thin films synthesized by pulsed laser deposition

Samarium doped CdWO4 (Sm-CdWO4) thin films were synthesized by pulsed laser deposition (PLD) using pressed powder targets composed of Sm2O3, CdO, and WO3 mixtures. Congruent, stoichiometric, and crystalline thin films were obtained on glass substrates at 873 K using targets with 0.01 at. % Sm. The x-ray activated photoluminescence (x-ray PL) spectra were redshifted by about 40 nm at 298 K. Changes of the PL component at 2.85 eV into those at 2.20 and 2.52 eV were typical in their convolutions. These PL components showed very similar dependencies on excitation wavelengths, indicating that CdWO4 related PL emissions in the Sm-doped PLD films are relaxed from the same midband excited state. The PL peaks characteristic to Sm3+ had two sidebands shifted to both lower and higher energy sides as much as 0.03 eV. Plume emission spectra were analyzed to elucidate effects of Sm on our PLD processes. It was interpreted that energy transfer should occur between excited neutral Cd to the excited state of Sm3+ in laser...

Negatively charged subnanometer-sized silicon clusters and their reversible migration into AFI zeolite pores studied with X-ray photoelectron spectroscopy and ultraviolet photoelectron spectroscopy

Abstract Subnanometer sized silicon clusters were deposited on AFI zeolite (AlPO4-5: one-dimensional channel diameter

Size selective filtration of silicon nano-structures using AFI AlPO4-5 zeolite pores

Abstract The ablation of Si has been carried out in the presence of ammonia ambient. Non-stoichiometric (silicon rich) silicon nitride composed of Si–Si, NH x ( x =1,2) and SiH y ( y =1–3) are formed. Photoluminescence at visible region based on these nano-structures was observed at room temperature. AlPO 4 -5 zeolite was used as substrate. It is shown that zeolite can selectively control the size of the Si nano-structure and the emission wavelength as well, and that the deposited SiN nano-structures can be migrated into the AlPO 4 -5 pores by annealing treatment. The photoluminescence red-shifted after annealing suggests that, the Si nano-crystals luminescence is ruled in the luminescence center. A theoretical model is proposed to explain the PL peak shift of the silicon nano-structures.

Effects of nitrogen substitution in amorphous carbon films on electronic structure and surface reactivity studied with x-ray and ultra-violet photoelectron spectroscopies

We investigated the effects of incorporating a low percentage of nitrogen on the local and the electronic structures of amorphous carbon (a-C) using X-ray photoelectron spectroscopy and ultra-violet photoelectron spectroscopy (UPS). Nitrogen-doped amorphous carbon films (a-CNx) with varying nitrogen contents were prepared by a thermal decomposition method using a mixture of CH4 + NH3 under atmosphere. A slight shift of the C 1s core-level spectrum toward the higher binding energy side was detected in a-CNx as a function of nitrogen content, whereas a shift of the Fermi level (EF) cannot be confirmed from the UPS results. This was interpreted as a chemical shift between carbon and nitrogen atoms rather than as a shift of the EF. The C 1s peak shifts can be explained by the presence of two kinds of C–N local structures and the charge transferred bulk C–C components by nitrogen atoms. The two kinds of deconvoluted C 1s components could be well correlated with the two N 1s components. Two localized states wer...

Time-resolved photoluminescence on samarium, nickel, and cesium ion doped scintillator CdWO4 (010) single crystal surfaces

Photoluminescence (PL), cathode luminescence (CL), and x-ray excited luminescence (XL) are studied on Sm-, Ni-, and Cs-doped CdWO4 (010) single crystal surfaces. Luminescence spectra are deconvoluted to three constituent species at 2.77, 2.48, and 2.18 eV. Their intensities of the ion-doped CdWO4 crystals are compared with the undoped crystals. CL and XL intensities on Sm- and Ni-doped single crystals are suppressed to be about 20–40% of the undoped crystal. However, the luminescence component at 2.18 eV is dominant on the Sm-ion doped CdWO4 crystal. The x-ray diffraction peak at (020) face shifts to lower 2θ value on the Sm-ion doped CdWO4 single crystal, indicating that the crystal structure of CdWO4 is disturbed by Sm ions and the distance between (020) faces is enlarged. The distortion of the crystal structure will be induced by the exchange of W6+ ions into Sm3+ ions with larger ion diameter. Time-resolved PL decay analyses are studied on the doped and undoped crystals under excitation below the band...

Pulsed laser ultrahigh vacuum deposited silicon in the presence of excess cesium and oxygen studied with x-ray photoelectron spectroscopy and atomic force microscopy

Si/Cs/O clusters were prepared in an ultrahigh vacuum system with pulsed laser ablation of Si in the presence of Cs vapor and molecular oxygen. The effects of an O2 ambient, the pressure, and the deposition sequence of Cs and O2 on the oxidation state of cesium oxides and silicon oxides, as well as on the formation of Si/Cs/O clusters, were studied with x-ray photoelectron spectroscopy and atomic force microscopy (AFM). Oxygen 1s spectra were deconvoluted to cesium oxide(s) at around 530.5 eV and silicon oxide(s) at 532.3 eV. From the low binding energy side to the high binding energy side, Si 2p spectra were assigned to negatively charged Si clusters (94 and 96 eV), silicon suboxide (102 eV), SiO2 (104 eV), and Si/Cs/O clusters (106 eV), respectively. The high binding energy species was explained by the formation of Si(CsO1+x)n clusters. Surface morphologies were influenced by the oxidation state of cesium oxides and their amount. The work function decrease as much as 0.5±0.1 eV was measured with ex situ...

Oxygen containing silicon clusters on Teflon and their work functions studied with X-ray photoelectron spectroscopy and ultraviolet photoelectron spectroscopy

Silicon species were deposited on tetrafluoroethylene (Teflon) with pulsed laser silicon ablation at 532 nm under UHV. Surface species were studied with X-ray photoelectron spectroscopy (XPS) and ultraviolet photoelectron spectroscopy (UPS). Tetrafluorocarbon (CF4) was easily desorbed by the decomposition reaction of Teflon with flying silicon species. Surface species were characterized as a function of pulsed laser shots. Silicon species were terminated with oxygen in gas phase to form two kinds of SiOx clusters (x<0.3) in addition to oxygen terminated silicon networks. Their work functions could be estimated by the shift of their Fermi energies.