P.P. Ong

Photoluminescence of coumarin 540 dye confined in mesoporous silica

Coumarin 540 dye was introduced into ordered mesoporous silica (MCM-41) by thermal diffusion in vacuum. Photoluminescence responses around 535 nm and 483 nm were detected from the mesoporous silica loaded with coumarin 540, arising from the confined dye and mesoporous silica, respectively. The photoluminescence lifetimes were found to be affected by the bonding between coumarin 540 molecules and the internal surface of the MCM-41.

TOF study of pulsed-laser ablation of aluminum nitride for thin film growth

Abstract Laser ionization time-of-flight mass spectrometry (TOF-MS) was used to study the process of pulsed-laser deposition (PLD) of Aluminum Nitride (AlN) thin films. The components of the plume from the AlN target obviously changed under different deposition conditions. For the same impurity level, positive AlN ions were freer from impurities than negative ions. However, negative ions were more likely to form larger clusters. In particular, a novel cluster, comprising 10 AlN, was observed in negative TOF, suggesting that it is likely to yield highly oriented crystalline thin films when deposited on a substrate.

High-resolution FTIR spectroscopy of the v11 and v2 + v7 bands of ethylene-d4

The Fourier transform infrared spectrum of the v11 band of ethylene-d4 (C2D4) has been recorded with an unapodized resolution of 0.006 cm−1 in the frequency range 2150 to 2250cm−1. The v11 band, with a band centre of about 2201 cm−1, was found to be perturbed by the nearby v2 + v7 band centred at about 2235 cm−1 by a b-type Coriolis interaction. By fitting a total of 772 infrared transitions of v11 using a Watsons A-reduced Hamiltonian in the Ir representation with the inclusion of b-type Coriolis interaction term, two sets of constants, up to quartic distortion constants for the v11 = 1 state, and principal rotational constants for the v2 + v7 = 1 dark state, were derived. The inertia defect of the v11 state was found to be 0.0693 ± 0.0004u Å2.

The effects of Cu doping on the magnetoresistive behavior of perovskites La0.7Ca0.3MnO3

Abstract The electronic and magnetic properties of Cu-doped perovskite La 0.7 Ca 0.3 Mn 1− x Cu x O 3 obtained by doping Cu on its Mn sites have been studied. The perovskite structure was found to remain intact up to the highest doping level of x =0.20. At low Cu concentration ( x =0.05) the temperature-dependence of resistivity of the material exhibited up to two peaks corresponding to the magnetic transitions from the PM to the FM phase, and from the FM to the AFM phase. In general, the doping level was found to suppress the ferromagnetic ordering of the material, increase its resistivity, and produce large values of magnetoresistance near the resistivity peak. These results were explained as due to the formation of the antiferromagnetic phase.

High-resolution Fourier transform infrared spectroscopy and analysis of the ν12 fundamental band of ethylene-d4

Abstract The Fourier transform infrared (IR) spectrum of the ν 12 fundamental band of ethylene- d 4 (C 2 D 4 ) has been measured with an unapodized resolution of 0.004 cm −1 in the frequency range of 1030–1130 cm −1 . A total of 1340 assigned transitions have been analyzed and fitted using a Watsons A-reduced Hamiltonian in the I r representation to derive rovibrational constants for the upper state ( v 12 =1) up to five quartic terms with a standard deviation of 0.00042 cm −1 . They represent the most accurate constants for the band thus far. The ground state rovibrational constants were also further improved by a fit of combination–differences from the IR measurements. The relatively unperturbed band was found to be basically A-type with a band centre at 1076.98492±0.00003 cm −1 .

Enhancement of room-temperature magnetoresistance in Sr2FeMoO6 by reducing its grain size and adjusting its tunnel-barrier thickness

The intergrain magnetoresistance (IMR) of polycrystalline Sr2FeMoO6 is known to depend on its grain size and the tunnel barrier thickness formed during fabrication. Further enhancement of the IMR of Sr2FeMoO6 is found possible by further decreasing its grain size using high-energy ball milling, as well as by judiciously adjusting the tunnel barrier thickness. The tunnel barrier thickness depends on the amount of insulating nonmagnetic SrMoO4 impurity formed during fabrication, which in turn can be controlled by varying the mixture ratio of the ambient gaseous H2–Ar stream during annealing. With smaller grain size, the magnetic domain size decreases correspondingly and so can be more easily rotated to produce a lower coercive field. The increase in the SrMoO4 impurity phase increases the number of grain boundary barriers in the Sr2FeMoO6 matrix, leading to a larger low-field magnetoresistance effect (LFMR). A significant enhancement of the LFMR was found when the SrMoO4 impurity was close to the conduction...

Coexisting photoluminescence of Si and Ge nanocrystals in Ge/Si thin film

Thin films of composite germanium/silicon (Ge/Si) were prepared by pulsed laser ablation alternately on Ge and Si materials on a rotary target, followed by vacuum deposition of the ablated materials on an ultraclean glass substrate. X-ray diffraction and atomic force microscopy phase analysis confirmed that the film structure consisted of a mixture of Si and Ge nanoparticles which could exist in two possible phases. Most of the particles are of less than 30 nm in diameter even after the sample was annealed at 500 °C for 6 h. With different excitation light of wavelengths 280 and 380 nm the composite film yielded independent photoluminescence emissions corresponding, respectively, to the Si and Ge nanoparticles which did not interfere with each other. These results demonstrate that there was very little interaction between the Si and Ge emissions arising from their coexisting mixture in the thin film, even after high temperature annealing of the film in the atmosphere. It opens up the possibility for appli...

Room temperature visible photoluminescence from undoped ZnS nanoparticles embedded in SiO2 matrices

Thin films of ZnS nanoparticles embedded in SiO2 matrices were prepared by the pulsed laser deposition method using a rotating target. The as-prepared samples contained dispersed ZnS nanocrystallites mostly of less than 8 nm in diameter embedded in the SiO2 host. The nanoparticle size of the film increased with increasing sample annealing temperature and/or duration in the vacuum. Room temperature visible photoluminescence (PL) was found from the films without the need for any dopant. Contrary to the usual redshift following annealing, there was, instead, a blueshift of the PL which increased with increasing sample annealing. The mechanism of the PL and its blueshift is attributed to the S vacancies and the formation of ZnS(O2−) in the surface layers of the nanoparticles adjacent to the SiO2. The S vacancies arose partly from the laser ablation during the film preparation stage, and partly from the S-bond disruption at the surface of the ZnS particles during annealing. The ZnS(O2−) could be formed by the ...

Analysis of the coriolis interaction of the ν12 band with 2ν10 of cis-d2-ethylene by high-resolution Fourier transform infrared spectroscopy

Abstract The Fourier transform infrared spectrum of the ν 12 band of cis-d 2 -ethylene ( cis -C 2 H 2 D 2 ) has been recorded with an unapodized resolution of 0.0024 cm −1 in the frequency range of 1280–1400 cm −1 . This band was found to be mutually coupled by Coriolis interaction with the unobserved 2 ν 10 band situated approximately 10 cm −1 below ν 12 . By fitting a total of 771 infrared transitions of ν 12 with a standard deviation of 0.00075 cm −1 using the Watsons Hamiltonian with the inclusion of a c -type Coriolis resonance term, a set of accurate rovibrational constants for V 12 =1 state was derived. The ν 12 band is A type with a band centre at 1341.1512±0.0001 cm −1 . Accurate rovibrational constants for the V 10 =2 state were also derived.

Grain boundary effects on the magneto-transport properties of Sr2FeMoO6 induced by variation of the ambient H2–Ar mixture ratio during annealing

Abstract Using the sol–gel method, polycrystalline double perovskites Sr2FeMoO6 with nanometer-scale grain size have been synthesized in the ambience of a controlled stream of different gaseous H2–Ar mixtures during annealing. X-ray diffraction (XRD) results indicated that with an increasing proportion of H2, the component of the SrMoO4 impurity present in the crystallites first decreased, but then increased again with further increase of the H2 proportion. The XRD results were further supported by independent sample analysis using Raman spectroscopy. Systematic investigation of the transport properties of the samples indicated that the SrMoO4 impurity at the grain boundary played a key role in determining the measured resistance and magnetoresistance of Sr2FeMoO6. This impurity effect was found to be correlated to the variation of the mixture ratio of the ambient H2–Ar gas stream surrounding the sample during annealing.