Jaime Ortiz-Lopez

Optical phonons in ZnxCd1−xSe thin films

Abstract A calculation is presented on the evolution of longitudinal and transverse optical phonons as a function of the concentration parameter x in the system ZnxCd1−xSe. The calculation is based in an isodisplacement model of the atoms which include interactions to first and second neighbors. The model is fitted to Raman measurements carried out on thin-film samples grown by Close Spaced Vapor Transport (CSVT). Comparison to the experimental data allows to conclude that the x-dependence of these optical modes corresponds to an intermediate-mode behavior.

Synthesis of Carbon Nanostructures by Microwave Irradiation

Carbon nanotubes (CNT) have been synthesized with various techniques of which the most common ones are laser ablation, electric arc discharge, and chemical vapor deposition. These methods produce CNTs with different characteristics, sometimes involving complex experimental setups that add to their cost of production. It is of current general interest the development of new techniques for the efficient and selective synthesis of CNTs and other carbon nanostructures at the cheapest possible cost. One such possibility is the use of microwave radiation, which over the past few years has played an important role as a thermal tool in organic synthesis due to considerable advantages over conventional methods (Lidstrom, et al., 2001). The use of microwave radiation in the synthesis and functionalization of carbon nanotubes or other nanostructures is advantageous because it provides a fast and uniform heating rate that can be selectively directed towards a targeted area. The first report of the production of carbon nanostructures with microwaves was made by Ikeda et al (Ikeda et al., 1995), who synthesized fullerenes from microwave-induced naphthalene-nitrogen plasma at atmospheric pressure inside a cylindrical coaxial cavity. O. Kharissova has reported the synthesis of vertically aligned carbon nanotubes using a domestic microwave oven (Kharissova, 2004). Graphite is a good microwave radiation absorber. It has been used in military applications as radar-absorbing material and in anti-electromagnetic interference coatings for civil purposes. Milled flake graphite and carbon nanotubes have microwave absorption maxima in the 10-15 GHz frequency range (Fan et al., 2009). Microwave radiation can heat or cause arcing in many objects and powdered samples can absorb such radiation and be heated efficiently. Short-time direct exposure to microwave irradiation has been used to produce exfoliated graphite as well as to reduce graphite oxide (Zhu et al., 2010). In graphite powder, absorbed microwave radiation is converted into heat via dielectric loss and conductive loss mechanisms. Graphite powder is oxidized by long exposure to ambient air and may become partly electrically insulating. Microwaves are absorbed with energy dissipation through the coupling of the radiation electric field with local electric dipoles associated with structural defects in graphite powder particles such as particle edges, dangling bonds, C-O bonds, impurities and others. The electric field of microwaves also drives electric currents with efficient generation of heat due to the highly diffusive transport

X-Ray Microanalysis of Human Cementum

An energy dispersive x-ray microanalysis study was performed throughout the total length of cementum on five impacted human teeth. Mineral content of calcium, phosphorous, and magnesium were determined with an electron probe from the cemento-enamel junction to the root apex on the external surface of the cementum. The concentration profiles for calcium, phosphorous, and magnesium were compared by using Ca/P and Mg/Ca atomic percent ratio. Our findings demonstrated that the Ca/P ratio at the cemento-enamel junction showed the highest values (1.8-2.2). However, the area corresponding to the acellular extrinsic fiber cementum (AEFC) usually located on the coronal one-third of the root surface showed a Ca/P media value of 1.65. Nevertheless, on the area representing the fulcrum of the root there is an abrupt change in the Ca/P ratio, which decreases to 1.3. Our results revealed that Mg(2+) distribution throughout the length of human cementum reached its maximum Mg/Ca ratio value of 1.3-1.4 at.% around the fulcrum of the root and an average value of 0.03%. A remarkable finding was that the Mg/Ca ratio pattern distribution showed that in the region where the Ca/P ratio showed a decreasing tendency, the Mg/Ca ratio reached its maximum value, showing a negative correlation. In conclusion, this study has established that clear compositional differences exist between AEFC and cellular mixed stratified cementum varieties and adds new knowledge about Mg(2+) distribution and suggests its provocative role regulating human cementum metabolism.

Characterization of Functionalized Multiwalled Carbon Nanotubes for Use in an Enzymatic Sensor

Carbon nanotubes (CNT) have proven to be materials with great potential for the construction of biosensors. Development of fast, simple, and low cost biosensors to follow reactions in bioprocesses, or to detect food contaminants such as toxins, chemical compounds, and microorganisms, is presently an important research topic. This report includes microscopy and spectroscopy to characterize raw and chemically modified multiwall carbon nanotubes (MWCNTs) synthesized by chemical vapor deposition with the intention of using them as the active transducer in bioprocessing sensors. MWCNT were simultaneously purified and functionalized by an acid mixture involving HNO3-H2SO4 and amyloglucosidase attached onto the chemically modified MWCNT surface. A 49.0% decrease in its enzymatic activity was observed. Raw, purified, and enzyme-modified MWCNTs were analyzed by scanning and transmission electron microscopy and Raman and X-ray photoelectron spectroscopy. These studies confirmed purification and functionalization of the CNTs. Finally, cyclic voltammetry electrochemistry was used for electrical characterization of CNTs, which showed promising results that can be useful for construction of electrochemical biosensors applied to biological areas.

Hydrogen Storage on Beryllium-Coated Toroidal Carbon Nanostructure C120 Modeled with Density Functional Theory

Ab initio density functional calculations were performed on a toroidal carbon C120 nanostructure with a single beryllium atom bonded to its outer surface. These calculations are based on DFT with the generalized gradient approximation PW91 (Perdew and Wang) as implemented in the Materials Studio v.4.3 code. The Dmol3 module was used to calculate, among others, total energy, charge density, HOMO-LUMO and Mulliken population analysis. On the basis of these results, the beryllium-coated toroidal carbon C120 nanostructure appears to be a good candidate for H2 storage with moderate adsorption energy.

Dielectric Studies of CN– Dipolar Reorientation and Order–Disorder Behavior in RbCN1–x:KCNx and KCN1–x:NaCNx

The evolution of phase transitions and CN - dipolar reorientation are studied in two mixed alkalicyanides RbCN 1-x :KCN x and KCN 1-x :NaCN x for 0 < x < 1 with dielectric constant and loss measurements in the frequency range 10 1 to 10 5 Hz from room temperature to 4 K. For RbCN 1-x :KCN x , transition into an CN- elastically ordered state occurs under cooling for all concentrations x, while CN- antiferroelectric ordering takes place only in the 0.50 < x < 1 range. For KCN 1-x :NaCN x , CN- elastic ordering takes place only close to the pure compounds (0 < x < 0.10 and 0.86 < x < 1), while in the remaining concentration range (0.10 < x < 0.86), CN- orientations freeze into a glass-like disordered state at low temperatures. Antiferroelectric ordering is detected in this case only in the 0 < x < 0.06 and 0.86 < x < 1 ranges. Multiple dielectric relaxation phenomena are observed in the elastically ordered concentration regimes of KCN 1-x :NaCN x due to CN- reorientation occurring in different coexisting structural phases. Dielectric relaxation data for both mixed systems are modeled either with a single or a superposition of two or three Cole-Cole type distributions of relaxation times. The obtained phase diagram for KCN 1-x :NaCN x disagrees with the one constructed from neutron powder diffraction because in that technique residual stresses introduced by sample grinding can modify the structure of phases.

Anisotropies in carbon nanotube synthesis by the hydrogen arc plasma jet method

Abstract Single‐wall carbon nanotubes were prepared under hydrogen gas atmosphere with a DC arc discharge maintained between the tip of a sharpened graphite cathode and an anode formed by a catalytic mixture of graphite, FeS, Ni, Fe and Co compressed powders. The cathode is placed with an inclination with respect to the anode, so that the plasma jet of the discharge is deviated towards the empty space of the growth chamber. Samples were classified and analyzed according to their type and growth positions relative to the direction of the plasma jet flow. Characterization of the samples was performed with mass spectrometry, Raman spectroscopy and transmission electron microscopy. A subtle variation is found in the properties of the nanotubes obtained in different locations due to the directionality of the plasma jet flow. Observed differences in sample properties may be qualitatively understood in terms of accepted mechanisms of SWNT growth.

AC Conductivity and Dielectric Response of Polycrystalline and Amorphous C70

Abstract The AC conductivity and dielectric constant of polycrystalline and amorphous C70 samples were measured in the 75–300 K temperature range and in the 100 Hz to 1 MHz frequency range. For polycrystalline samples, we observe effects caused by O2 intercalation due to prolonged exposure to ambient air. The conductivity σ of these samples around 300 K depends on the measuring frequency ν as a σ ∼ ν n with n ≈ 0.88, implying a strong reduction of DC conductivity to less than 10−12 S/cm. The dielectric constant of polycrystalline samples shows an anomaly at 285 K which is interpreted as due to the transition from its intermediate rhombohedral phase into its monoclinic low‐temperature phase. In contrast with the polycrystalline samples, the amorphous C70 samples prepared by sublimation do not contain interstitial 02, their conductivity at 300 K is of about 10−6 S/cm, is independent of frequency, and is well described by the hopping mechanism (Davis–Mott T 1/4 law) in the 200–300 K range. All evidence of phase transitions disappears in the amorphous samples.