Samuel A. Hevia

Gold nanoparticles grown inside carbon nanotubes: synthesis and electrical transport measurements

The hybrid structures composed of gold nanoparticles and carbon nanotubes were prepared using porous alumina membranes as templates. Carbon nanotubes were synthesized inside the pores of these templates by the non-catalytic decomposition of acetylene. The inner cavity of the supported tubes was used as nanoreactors to grow gold particles by impregnation with a gold salt, followed by a calcination-reduction process. The samples were characterized by transmission electron microscopy and X-ray energy dispersion spectroscopy techniques. The resulting hybrid products are mainly encapsulated gold nanoparticles with different shapes and dimensions depending on the concentration of the gold precursor and the impregnation procedure. In order to understand the electronic transport mechanisms in these nanostructures, their conductance was measured as a function of temperature. The samples exhibit a ‘non-metallic’ temperature dependence where the dominant electron transport mechanism is 1D hopping. Depending on the impregnation procedure, the inclusion of gold nanoparticles inside the CNTs can introduce significant changes in the structure of the tubes and the mechanisms for electronic transport. The electrical resistance of these hybrid structures was monitored under different gas atmospheres at ambient pressure. Using this hybrid nanostructures, small amounts of acetylene and hydrogen were detected with an increased sensibility compared with pristine carbon nanotubes. Although the sensitivity of these hybrid nanostructures is rather low compared to alternative sensing elements, their response is remarkably fast under changing gas atmospheres.

Characterization of a laser plasma produced from a graphite target

In order to improve the understanding of pulsed laser deposition (PLD) of diamondlike carbon (DLC) films, we have initiated a detailed study of the plasma dynamics of laser produced carbon plasmas. The carbon plasma is produced by focusing a Nd:YAG laser pulse, 380 mJ, 4 ns at 1.06 μm, onto a graphite target, at a background pressure of 0.3 mTorr. Time resolved optical emission spectroscopic (OES) observations of the carbon plasma plume are obtained, with time and space resolution, using a SpectraPro 275 spectrograph, with a 15 ns MCP gated OMA. Line emission from CII to CIV carbon ions is identified at different stages of the plasma evolution. Line intensity ratios of successive ionization stages, CIII/CIV, was used to estimate the electron temperature throughout the Saha-Boltzmann equation, under the assumption of local thermodynamic equilibrium (LTE), and Stark broadening of CII lines was used to obtain measurements of the electron density. Characteristic plasma parameters, short after plasma formation, are 3.0 eV and 2-1017 cm−3which after 60 ns of plasma expansion decay to 2.7 eV and 5·10 cm−3, respectively.

Pulsed laser deposition of carbon films in low pressure neutral gas background

We have investigated Pulsed Laser Deposition (PLD) of thin carbon films using a graphite target, in a low pressure Argon gas background. Raman spectroscopy based structural analysis of the films shows a correlation between films properties and pressure of the buffer gas background. The morphology of the resulting thin films was characterized with Raman spectroscopy. Time resolved Optical Emission Spectroscopy (OES) observations indicate that, at the substrate position, and for background pressures above 80 mTorr, the dominant species in the expanding carbon plasma are C2 molecules. This is also consistent with Faraday cup observations, which show a strong decrease in the carbon ions content, as the pressure is increased. Our main result is the observation of a sharp transition in the film morphology, around 300 mTorr, from Diamond like Carbon (DLC) at lower pressures, to amorphous Carbon at higher pressures.

Pulsed laser deposition of carbon nanodots

We present preliminary results on carbon nanodots growth using pulsed laser deposition (PLD), with a graphite target, in a low pressure argon background. In the process thin carbon films are deposited on Porous Alumina Membranes (PAM) positioned over a Silicon substrate. Reproducible carbon nanodots arrays standing on Silicon are obtained by later removal of the PAM. Atomic force microscopy (AFM) was used to characterize the nanodots morphology. The laser carbon plasma was characterized by optical emission spectroscopy (OES), and a fast response, negative biased, Faraday cup was used to measure the characteristic energy and flux of the plasma ions depositing onto the PAM. Reliable Carbon nanodots growth was only observed with Argon background pressures below 18 mTorr.