Matrix-assisted pulsed laser evaporation of polyfluorene thin films

Poly(9,9-dioctylfluorene) (PF8) thin films have been deposited by matrix-assisted pulsed laser evaporation (MAPLE) using a KrF excimer laser. The influence of the laser fluence (50-500 mJ/cm²) and the nature of the solvent (chloroform, toluene, tetrahydrofuran) on the films properties have been studied. The chemical composition of the deposited films was investigated by Fourier transform infrared (FTIR) spectroscopy and compared with the one of spin coated films. To investigate the effect of the deposition parameters on the optical properties of the films, photoluminescence (PL) measurements were performed. Poor structural and optical properties were observed for films deposited starting from chloroform solutions. When using toluene as solvent, the spectra characteristics improved with increasing laser fluence, while wide PL spectra were observed. The characteristic emission bands of the PF8 polymer were nicely detected for films deposited starting from a tetrahydrofuran (THF) solution. Moreover, in this last case, the PF8 structure is preserved at high laser fluences, too.     

Zinc oxide nanostructured layers for gas sensing applications

Various kinds of zinc oxide (ZnO) nanostructures, such as columns, pencils, hexagonal pyramids, hexagonal hierarchical structures, as well as smooth and rough films, were grown by pulsed laser deposition using KrF and ArF excimer lasers, without use of any catalyst. ZnO films were deposited at substrate temperatures from 500 to 700°C and oxygen background pressures of 1, 5, 50, and 100 Pa. Quite different morphologies of the deposited films were observed using scanning electron microscopy when different laser wavelengths (248 or 193 nm) were used to ablate the bulk ZnO target. Photoluminescence studies were performed at different temperatures (down to 7 K). The gas sensing properties of the different nanostructures were tested against low concentrations of NO₂. The variation in the photoluminescence emission of the films when exposed to NO₂ was used as transduction mechanism to reveal the presence of the gas. The nanostructured films with higher surface-to-volume ratio and higher total surface available for gas adsorption presented higher responses, detecting NO₂ concentrations down to 3 ppm at room temperature.     

Precise mass measurement of a double-stranded 500 base-pair (309 kDa) polymerase chain reaction product by negative ion electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry

Electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry (ESI-FTICRMS) has been used to determine the mass of a double-stranded 500 base-pair (bp) polymerase chain reaction (PCR) product with an average theoretical mass of the blunt-ended (i.e. unadenylated) species of 308 859.35 Da. The PCR product was generated from the linearized bacteriophage Lambda genome which is a double-stranded template. Utilization of ethanol precipitation in tandem with a rapid microdialysis step to purify and desalt the PCR product was crucial to obtain a precise mass measurement. The PCR product (0.8 pmol/μL) was electrosprayed from a solution containing 75% acetonitrile, 25 mM piperidine, and 25 mM imidazole and was infused at a rate of 200 nL/min. The average molecular mass and the corresponding precision were determined using the charge-states ranging from 172 to 235 net negative charges. The experimental mass and corresponding precision (reported as the 95% confidence interval of the mean) was 309 406 +/- 27 Da (87 ppm). The mass accuracy was compromised due to the fact that the PCR generates multiple products when using Taq polymerase due to the non-template directed 3'-adenylation. This results in a mixture of three PCR products with nearly identical mass (i.e. blunt-ended, mono-adenylated and di-adenylated) with unknown relative abundances that were not resolved in the spectrum. Thus, the experimental mass will be a weighted average of the three species which, under our experimental conditions, reflects a nearly equal concentration of the mono- and di-adenylated species. This report demonstrates that precise mass measurements of PCR products up to 309 kDa (500 bp) can be routinely obtained by ESI-FTICR requiring low femtomole amounts. Copyright 1999 John Wiley & Sons, Ltd.     

Improved formulation,branch-and-cut and tabu search heuristic for single loop material flow system design

The single loop material flow system design is a combinatorial optimization problem, arising in material handling system design, which amounts to designing an unidirectional loop flow pattern as well as to locate pickup and delivery stations. The objective is to minimize the time required to carry out all material flow movements between cells. In this paper, we develop valid inequalities for a previously proposed formulation. The valid inequalities are then embedded into a branch-and-cut framework which is shown to solve much larger instances to optimality than those reported in the literature. A tailored tabu search heuristic is also illustrated and computationally assessed.     

Dependence of the surface roughness of MAPLE-deposited films on the solvent parameters

Matrix-assisted pulsed laser evaporation (MAPLE) was used to deposit layers of poly(9,9-dioctylfluorene) (PFO) to study the relation between the solvent properties (laser light absorption, boiling temperature and solubility parameters) and the morphology of the deposited films. To this end, the polymer was diluted (0.5 wt%) in tetrahydrofuran—THF, toluene and toluene/hexane mixtures. The thickness of the films was equal to 70±20 nm. The morphology and uniformity of the films was investigated by Atomic Force Microscopy and by the photoluminescence emission properties of the polymer films, respectively. It is shown that, although the solubility parameters of the solvents are important in controlling the film roughness and morphology, the optical absorption properties and boiling temperature play a very important role, too. In fact, for matrices characterized by the same total solubility parameter, lower roughness values are obtained for films prepared using solvents with lower penetration depth of the laser radiation and higher boiling temperatures.     

Semiempirical (ZINDO-PCM) approach to predict the radiative and nonradiative decay rates of a molecule close to metal particles

In this work, we present an extension of a model previously proposed (Andreussi et al. J. Chem. Phys. 2004, 121, 10190) to treat the effect of a metal particle on the optical properties of a molecule in solution (close to such a particle) in the framework of the polarizable continuum model (PCM). This extension concerns the combination of such a model with the semiempirical method Zerner's intermediate neglect of differential overlap (ZINDO), which allows us to treat large size molecular systems, as the ones normally used in the experiments. A refinement of the model is also introduced to take into account the effect of the metal specimen on the absorption process of the molecular system, which affects the probability that a molecule reaches the excited state. Numerical tests are presented to validate the reliability of the ZINDO results with respect to quantum-mechanical DFT methods. Comparisons with experimental results on two different large molecular systems are reported, and the effect of the metal on the absorption process is discussed.     

Uniform thin films of TiO2 nanoparticles deposited by matrix-assisted pulsed laser evaporation

We report morphological and optical properties of a colloidal TiO₂ nanoparticle film, deposited on a quartz substrate by using the Matrix-Assisted Pulsed Laser Evaporation (MAPLE) technique. Atomic Force Microscopy demonstrated that a good uniformity of the deposition can be obtained. The presence of agglomerates with dimensions of about 1 μm in size was noticed. Form UV-vis transmission spectra, recorded in the 200-800 nm range, the optical constants and the energy gap were determined besides the film thickness. The optical constants resulted in agreement with the values reported in literature for TiO₂ nanoparticle thin films.     

Pulsed laser deposition of magnetic films by ablation of Co- and Fe-based amorphous alloys

Magnetic films were prepared by pulsed laser ablation of amorphous magnetic ribbons (composition Co₆₇Cr₇Fe₄Si₈B₁₄, Fe_73.5Nb₃Cu₁Si_13.5B₉, and Fe₄₀Ni₄₀B₂₀) and permalloy foils (composition Fe_xNi_1-x with x=22,50). Depositions were performed in a vacuum of (2–4)×10^-5 Pa by KrF excimer laser pulses at fluences of between 2 and 7 Jcm². Films were deposited on oxidized silicon wafers, placed 60–80 mm apart from the target. Films were analyzed by SEM, XRD, RBS, and EDS. Ferromagnetic resonance (FMR) spectra were studied at GHz frequencies. From RBS and EDS measurements it follows that the stoichiometry of the targets is preserved in the films to a large extent. The films deposited from amorphous targets remain amorphous. From FMR studies it follows that Fe and Fe-Ni rich films exhibit properties close to those of bulk alloys, having very low magnetization motion damping parameter () of 7.0–7.8×10⁷ rad/s, which are appropriate for fast magnetic sensors. PACS 79.20.Ds; 75.50.Kj; 75.70.-i