Papers by Author: Luiz Pereira

Abstract: Recently, it was demonstrated that copper thin films show good adsorption characteristics for organic polar and non-polar compounds. Also, these films when used in small cavities can favor preconcentration of these organic compounds. It is also known that copper oxide can provide catalysis of organic compounds. Therefore, the aim of this work is the study of copper thin film catalysis when used in small cavities. Copper thin films, 25 nm thick, were deposited on silicon and/or rough silicon. These films do not show oxide on the surface when analyzed by Rutherford backscattering. Also, Raman analysis of these films showed only silicon bands, due to the substrate, however infrared spectroscopy shows oxide bands for films exposed to organic compound aqueous solutions. Cavities with copper films deposited inside were tested with a continuous flow of n-hexane, acetone or 2-propanol admitted in the system. The effluent was analyzed by Quartz Crystal Microbalance. It was shown that n-hexane or acetone can be trapped. The system also shows good reproducibility. Tests of catalysis were carried out using Raman spectroscopy and heating the films up to 300°C during 3 minutes after exposure to n-hexane, 2- propanol and acetophenone – pure or saturated aqueous solution. After the exposure, Raman spectra present intense bands only for 2-propanol, indicating that adsorption easily occurs. However, after heating with all solutions it was not found only silicon bands. Raman microscopy after heating also showed copper oxide cluster formation and, eventually, graphite formation. Although the heating provides oxide copper formation, this reaction does not produce a high amount of residues, which means that catalysis is possible in this condition. Thus, a simple device using copper thin films can be useful as sample pretreatment on microTAS development.

Abstract: Films produced by plasma polymerization of ethyl ether and methyl or ethyl acetate show good adsorption characteristic for polar and non-polar organic compounds. These films when used in microchannels machined in a 3D-structure present some preconcentration of organic compounds. Therefore, the aim of this work is to investigate the physical-chemical preconcentration mechanisms on this structure. The test molecules used were n-hexane and 2-propanol. Quartz crystal microbalance and mass spectrometry were used to measure preconcentration. Two different procedures for reactant injection on the structure were used: a continuous flow during several minutes or a small amount injected on a single pulse and in a few seconds. The microchannels were also modified by the introduction of small ceramic particles for enhancement of the flow dispersion. It was possible to notice for all films a similar kinetic of retention. The main removal mechanism is adsorption. Although all films can provide the removal of the adsorbents molecules, the most important characteristic for the adsorption and/or retention is the surface condition. Thus, the retention of polar compound can be troubled if a non-polar compound was used previously. The most promising films for retention are ethyl ether and ethyl acetate when n-hexane and 2-propanol are used as test molecules. The results using n-hexane or 2-propanol point out the use of low-cost microchannels for preconcentration development.

Abstract: In the present work, the nature of the electrical mechanism for carrier transport in Alq3 is studied by current-voltage measurements and broadband dielectric spectroscopy. The d.c. currentvoltage characteristics at low applied electrical field exhibits a classical “N” shape due to interfacial states located at metal-organic interface, but tend to disappear when successive higher forward bias is applied. Using dielectric spectroscopy it is possible to observe that the main relaxation peak shifts to a higher frequency with the increase of the applied d.c. voltage (from approximately 100 Hz with 0 V d.c. bias to approximately 400 Hz with 6 V d.c. bias) indicating a semiconductor structure change. The logarithmic Z’’ vs. Z’ plot has a slope about 0.7 that decreases to 0.5 with the increasing applied d.c. voltage, reaching a classic Debye relaxation. An attempt to correlate with some structural changes is made.

Abstract: In this work fluorinated Diamond Like Carbon (DLC) films have been grown with different CF4 concentrations and have been studied by electrical DC measurements in a temperature range from 30 to 300 K. It was found that the samples grown with lower CF4 concentration show a small rectification, with a potential barrier lower than 0.3 V. The bulk conduction shows a trapcontrolled Space Charge Limited Current (SCLC), with characteristic trap energy between 0.08 and 0.13 eV, confirmed by the differential conductivity analysis. The activation energy (ranging from 50 to 140 meV) is also dependent on the sample fluorine concentration, decreasing with the fluorine concentration increase.

Abstract: In this work we present an equipotential surface map with a resolution higher than 100 nm, resulting from conductive atomic force microscopy (AFM). The images, obtained in polycrystalline MPCVD (Microwave Plasma Assisted Chemical Vapor Deposition) diamond devices, are clearly structure dependent. Diamond and non-diamond phases were identified by micro-Raman spectroscopy and correlated with the electrical conduction map. The application of the non-homogeneous contact theory lead to the extraction of two Schottky Barrier Heights (SBH), one close to 0.6 V with dispersion ~ 0.1 V, attributed to the diamond / metal interface, and the other close to 0.4 V with dispersion ~ 0.04 V, due to the non-diamond phases / metal interface. The diamond / non-diamond phase ratio is about 85:15, taken from the micro-Raman spectra. Combining these results with the electrical conduction map, a general model to predict the macroscopic electrical behavior of polycrystalline diamond based devices is proposed.