M. C. Llaguno

Carbon nanotube composites for thermal management

Single-wall carbon nanotubes (SWNTs) were used to augment the thermal transport properties of industrial epoxy. Samples loaded with 1 wt % unpurified SWNT material showed a 70% increase in thermal conductivity at 40 K, rising to 125% at room temperature; the enhancement due to 1 wt % loading of vapor grown carbon fibers was three times smaller. Electrical conductivity data showed a percolation threshold between 0.1 and 0.2 wt % SWNT loading. The Vickers hardness rose monotonically with SWNT loading up to a factor of 3.5 at 2 wt %. These results suggest that the thermal and mechanical properties of SWNT-epoxy composites are improved, without the need to chemically functionalize the nanotubes.

Electrical and thermal transport properties of magnetically aligned single wall carbon nanotube films

Dense, thick films of aligned single wall carbon nanotubes and nanotube ropes have been produced by filtration/deposition from suspension in strong magnetic fields. Electrical resistivity exhibits moderate anisotropy with respect to the alignment axis, while the thermopower is the same when measured parallel or perpendicular to this axis. Both parameters have identical temperature dependencies in the two orientations. Thermal conductivity in the parallel direction exceeds 200 W/mK, within a decade of graphite.

Magnetically aligned single wall carbon nanotube films: preferred orientation and anisotropic transport properties

Thick films of single wall carbon nanotubes (SWNT) exhibiting in-plane preferred orientation have been produced by filter deposition from suspension in strong magnetic fields. We characterize the field-induced alignment with x-ray fiber diagrams and polarized Raman scattering, using a model which includes a completely unaligned fraction. We correlate the texture parameters with resistivity and thermal conductivity measured parallel and perpendicular to the alignment direction. Results obtained with 7 and 26 T fields are compared. We find no significant field dependence of the distribution width, while the aligned fraction is slightly greater at the higher field. Anisotropy in both transport properties is modest, with ratios in the range 5–9, consistent with the measured texture parameters assuming a simple model of rigid rod conductors. We suggest that further enhancements in anisotropic properties will require optimizing the filter deposition process rather than larger magnetic fields. We show that both ...

Electrical and thermal properties of C60-filled single-wall carbon nanotubes

We report measurements of electrical resistivity, thermopower, and thermal conductivity of highly C60-filled single-wall carbon nanotubes and unfilled controls, from 1.5 to 300 K. The data suggest that the C60 chains provide additional conductive paths for charge carriers, increase the rate of phonon scattering, and block interior sites from sorbing other gas molecules.

Single-wall carbon nanotube circuits assembled with an atomic force microscope

We have developed a method to assemble single-wall carbon nanotube (SWNT) circuits using a tapping mode atomic force microscope. Nanotubes can be controllably translated, rotated, cut, and placed on top of one another by varying the tip–sample force, and the tip speed. These operations let us construct complex nanotube circuits, which are contacted using electron beam lithography. We present data from a circuit of two crossed SWNT bundles. The lower bundle behaves as two quantum dots in series, separated by a tunnel barrier created at the junction. Gate voltages can tune the number of charges on each dot and the tunnel barrier transmission.

Single-wall carbon nanotube based devices

Abstract We have developed a variety of fabrication techniques for single-wall nanotube (SWNT) circuits. Our methods range from variants of electron beam lithography to AFM nanomanipulations. In this talk, we present our most recent data on three different types of SWNT based devices: the SWNT with a local impurity, the tube–tube junction and the SWNT contacted with electrodes whose separation is less than 30 nm. Each has a specific behavior ranging from a rectifying diode to a double quantum dot in series to an ultra short quantum wire. The functionality of each device can be ascribed to specific molecular adsorbates or controlled mechanical deformation.

Thermal Conductivity of Single Wall Carbon Nanotubes: Diameter and Annealing Dependence

The thermal conductivity, k(T), of bulk single-wall carbon nanotubes (SWNTs) displays a linear temperature dependence at low T that has been attributed to 1D quantization of phonons. To explore this issue further, we have measured the k(T) of samples with varying average tube diameters. We observe linear k(T) up to higher temperatures in samples with smaller diameters, in agreement with a quantization picture. In addition, we have examined the effect of annealing on k(T). We observe an enhancement in k(T) for annealed samples which we attribute to healing of defects and removal of impurities. These measurements demonstrate how the thermal properties of an SWNT material can be controlled by manipulating its intrinsic nanoscale properties.

Electrical and Thermal Properties of C60‐filled Single Wall Carbon Nanotubes

We measured temperature‐dependent resistivity ρ, thermal conductivity κ and thermopower S of C60@SWNT (“peapods”). C60 filling reduces ρ much less than alkali metal doping at all T, indicating weak charge transfer. κ is enhanced by filling, the enhancement showing interesting structure vs. T for which we give a tentative explanation. S is reduced at all T by C60 filling, which we explain by a) C60@SWNT blocking some of the oxygen doping sites, and b) weak disorder on the 1‐D chain which reduces the mean free path of tube phonons and thus the phonon drag contribution.

Single carbon nanotube electronic devices

We review recent progress towards the fabrication of engineered single nanotube circuits. Single wall carbon nanotubes are manipulated into circuits on a silicon dioxide surface using an AFM. Nanotubes can also be incorporated into an electron beam lithography resist system and used as “shadow masks” to create electrode pairs with sub-20 nm separation. Measurements of very short channel nanotube FETs indicate that the molecules may be highly doped due to exposure to the atmosphere, a fact not taken into account in earlier models of nanotube FETs.