M. Radosavljevic

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.

Nonvolatile Molecular Memory Elements Based on Ambipolar Nanotube Field Effect Transistors

We have fabricated air-stable n-type, ambipolar carbon nanotube field effect transistors (CNFETs) and used them in nanoscale memory cells. n-Type transistors are achieved by annealing nanotubes in hydrogen gas and contacting them by cobalt electrodes. Scanning gate microscopy reveals that the bulk response of these devices is similar to gold-contacted p-CNFETs, confirming that Schottky barrier formation at the contact interface determines accessibility of electron and hole transport regimes. The transfer characteristics and Coulomb blockade (CB) spectroscopy in ambipolar devices show strongly enhanced gate coupling, most likely due to reduction of defect density at the silicon/silicon-dioxide interface during hydrogen anneal. The CB data in the “on”-state indicates that these CNFETs are nearly ballistic conductors at high electrostatic doping. Due to their nanoscale capacitance, CNFETs are extremely sensitive to the presence of individual charges around the channel. We demonstrate that this property can b...

Controlled creation of a carbon nanotube diode by a scanned gate

We use scanning gate microscopy to precisely locate the gating response in field-effect transistors (FETs) made from semiconducting single-wall carbon nanotubes. A dramatic increase in transport current occurs when the device is electrostatically doped with holes near the positively biased electrode. We ascribe this behavior to the turn-on of a reverse biased Schottky barrier at the interface between the p-doped nanotube and the electrode. By positioning the gate near one of the contacts, we convert the nanotube FET into a rectifying nanotube diode. These experiments both clarify a longstanding debate over the gating mechanism for nanotube FETs and indicate a strategy for diode fabrication based on controlled placement of acceptor impurities near a contact.

Fabrication of nanometer size gaps in a metallic wire

We present a simple shadow mask method to fabricate electrodes with nanometer scale separation. Metal wires with gaps are made by incorporating multiwall carbon nanotubes or single-wall carbon nanotube (SWNT) bundles into a trilayer electron beam lithography process. The simple, highly controllable, and scaleable method has been used to make gaps with widths between 20 and 100 nm and may be extended to gap sizes of 1 nm. We report electron transport measurements of individual SWNTs bridging nanogaps with electrode spacings of approximately 20 nm. Metallic SWNTs exhibit quantum dot behavior with an 80 meV charging energy and a 20 meV energy level splitting. We observe a strong field effect behavior in short semiconducting SWNT segments, evidence for diffusive electron transport in these samples.

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.

Local electronic properties of single-wall nanotube circuits measured by conducting-tip AFM

We use conducting-tip atomic force microscopy (AFM) to measure local electronic properties of single-wall carbon nanotube (SWNT) circuits on insulating substrates. When a voltage is applied to the tip and AFM feedback is used to position the tip, images formed from the tip-sample tunnel current have single tube resolution (near 1 nm diameter), more than an order of magnitude better than simultaneously acquired topographic AFM images. By finding points where the tip-sample current is zero, it is possible to measure the electrochemical potential within the circuit, again with nanometer resolution. Such measurements provide compelling evidence that nanotubes within a bundle have only weak electronic coupling. Finally, the AFM tip is used as a local electrostatic gate, and the gating action can be correlated with the structure of the SWNT bundle sample. This technique should be useful for a broad range of circuits containing SWNTs and other molecules.

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.

High bias transport in single-wall carbon nanotubes

We study transport and scanning probe properties on single-wall nanotube (SWNT) devices at high voltage bias. Measurements of contact resistance indicate that current saturation at high bias is an intrinsic property of SWNTs. We use a combination of scanning probe and transport techniques to measure a voltage drop along a nanotube at high bias. Together, these two observations provide experimental confirmation of the phonon-emission model of current saturation. Surprisingly, some SWNT devices show roll-off to finite resistance at high bias, instead of current saturation. We analyze these devices by including electron-phonon coupling in the phonon-emission model and use this modification to extract the value for the associated scattering length, Iph=45 nm.

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.

Temperature dependent resistivity of large ropes of single walled carbon nanotubes

The electrical resistance of ropes of single walled carbon nanotubes is presented as a function of temperature, magnetic field and electrostatic doping. The entire set of data obtained on many samples can be understood on the basis of data already published for individual nanotubes. The main conclusion is that a rope is well described on the basis of two independent parallel channels, one of semiconducting and one of metallic nanotubes.