Raman spectra of multilayer graphene under high temperatures.

For graphitic materials, Raman technique is a common method for temperature measurements through analysis of phonon frequencies. Temperature (T) induced downshift of the bond-stretching G mode (ΔG) is well known, but experimentally obtained thermal coefficients ΔG/ΔT vary considerably between diverse works. Further, ΔG/ΔT coefficients usually were evaluated for relatively low temperatures and found to differ strongly for mono, a few and multilayer graphene. We studied G band behavior in freely suspended multilayer graphene flakes (or graphite nanoflakes) under localized heating by a laser beam. Analysis of Stokes and anti-Stokes signals showed that G band has a complex structure and can be deconvoluted into several peaks that demonstrate distinctly different behavior under heating. A plausible assumption is that these peaks correspond to several groups of graphitic layers (surface, near-surface and bulk) and then different thermal coefficients were determined for these groups. This behavior can be explained by decreasing interaction between surface layers and underlying material at high temperatures that affects especially vibrational properties of a few outermost layers. Estimates of temperatures using anti-Stokes/Stokes intensity ratio (I aS/I S) were also done to give results comparable with those obtained from G band downshift, T ΔG ≈ T aS/S, supporting the proposed model. The range of temperatures obtained by laser heating, as evaluated by both methods, was from 450 to 1200 K.

A wearable, highly stable, strain and bending sensor based on high aspect ratio graphite nanobelts.

A simple and scalable method was developed for the fabrication of wearable strain and bending sensors, based on high aspect ratio (length/thickness ∼10(3)) graphite nanobelt thin films deposited by a modified Langmuir-Blodgett technique onto flexible polymer substrates. The sensing mechanism is based on the changes in contact resistance between individual nanobelts upon substrate deformation. Very high sensor response stability for more than 5000 strain-release cycles and a device power consumption as low as 1 nW were achieved. The device maximum stretchability is limited by the metal electrodes and the polymer substrate; the maximum strain that could be applied to the polymer used in this work was 40%. Bending tests carried out for various radii of curvature demonstrated distinct sensor responses for positive and negative curvatures. The graphite nanobelt thin flexible films were successfully tested for acoustic vibration and heartbeat sensing.

Micro-reactors for characterization of nanostructure-based sensors.

Fabrication and testing of micro-reactors for the characterization of nanosensors is presented in this work. The reactors have a small volume (100 µl) and are equipped with gas input/output channels. They were machined from a single piece of kovar in order to avoid leaks in the system due to additional welding. The contact pins were electrically insulated from the body of the reactor using a borosilicate sealing glass and the reactor was hermetically sealed using a lid and an elastomeric o-ring. One of the advantages of the reactor lies in its simple assembly and ease of use with any vacuum/gas system, allowing the connection of more than one device. Moreover, the lid can be modified in order to fit a window for in situ optical characterization. In order to prove its versatility, carbon nanotube-based sensors were tested using this micro-reactor. The devices were fabricated by depositing carbon nanotubes over 1 µm thick gold electrodes patterned onto Si/SiO(2) substrates. The sensors were tested using oxygen and nitrogen atmospheres, in the pressure range between 10(-5) and 10(-1) mbar. The small chamber volume allowed the measurement of fast sensor characteristic times, with the sensors showing good sensitivity towards gas and pressure as well as high reproducibility.

Electrical characterization of multi-walled carbon nanotubes.

Electrical characteristics of multi-walled carbon nanotubes (MWNTs) grown by chemical vapor deposition have been investigated as a function of the bias voltage, nanotubes length and temperature, in 2 and 4 terminal configurations. Nanotubes were deposited over metal electrodes using ac dielectrophoresis method. For better contacts between the nanotubes and electrodes, Ni and Pd films were deposited by an electroless deposition technique. Differential conductance was found to rise considerably with bias, and this effect was more pronounced for Ni. Using 2 and 4 terminal configurations, electrical resistance measurements for individual MWNTs were performed, and the results were interpreted using the model of nanotube as a resistive transmission line, where current at low bias flows mainly through the two outermost shells.

Formation of catalyst nanoparticles and nucleation of carbon nanotubes in chemical vapor deposition.

Multi-walled carbon nanotubes and other carbon nanostructures have been grown using catalytic thermal chemical vapor deposition method in a horizontal tubular quartz furnace at atmospheric pressure. The mechanisms of nanotubes/nanofibers nucleation and growth are analyzed. A new model explaining the nanotube nucleation as a specific instability occurring on the catalyst particle surface supersaturated with carbon is presented. It is also shown that an axially symmetric instability, giving rise to the nanotube nucleation, is developed when certain critical conditions such as temperature, supersaturation and catalyst volume are achieved. For smaller temperatures, another mechanism of carbon segregation from supersaturated catalyst particles has been observed. In this case, flat rather than tubular graphitic layers are formed. These findings are important for better understanding and control of the synthesis of different carbon nanoforms using chemical vapor deposition.