Carbon nanodevices for sensors, actuators, and electronics

The electrical properties of carbon nanotubes¹ (CNTs) and graphene² make them exceptional candidates for developing novel devices with functionality and efficiency that is orders-of-magnitude better than state-of-the-art technologies. These next-generation electronics will significantly increase the capabilities of high-throughput information systems while simultaneously decreasing their size, weight, cost, and assembly complexity.

Despite the promise of vastly superior performance of CNT and graphene-based devices, several fabrication issues need to be resolved to realize their full potential. Our research focuses on nanodevices based on CNT and graphene nanostructures for use in nanoelectronic devices, nanoactuator systems, and nanosensors.

For example, we are developing nano-segmented, in-plane CNT structures and investigating their quantized electron energy properties. In-plane CNTs are formed using chemical vapor deposition between catalyst patterns. Then they are segmented using a voltage-applied atomic force microscope (AFM) process³ to form CNT-based quantum dots (QDs). During segmentation, an AFM tip applies a negative voltage (approximately 5V) to create defects at specified locations along the multiwall nanotube (MWNT).

Other researchers are also pursuing the development of CNT-based QDs,⁴ but our fabrication technique is compatible with forming CNT-QD arrays, which will enable single-electron memory states with high electron charging energies that are stable up to room temperature. This type of device is critical for high-speed, ultra-low-power electronics as well as highly sensitive nanosensor applications. An example is shown in Figure 1, where four sets of CNT-QDs could combine to yield a memory cell.

Figure 1. Conceptual schematic of a projected nanoscale memory cell consisting of four sets of CNT-QDs.

The quantized electronic energy levels of CNT-QDs depend strongly on the dimensions, chirality, and electron charging energy E_c of the QD (i.e. single electron confinement). Detailed knowledge of such properties is necessary to understand the single electron transport and storage properties of CNT-QDs, particularly as a function of the CNT growth and segmentation parameters.

We have found that MWNTs display a room temperature photocurrent that is slightly dependent on excitation wavelength. Conductance measurements of a MWNT sample (as a function of source-drain and back-gate voltage) carried out at 80K showed a nonlinear response, suggesting a MWNT 1D density of states.

We are in the process of characterizing the conductance of individual CNTs at temperatures from 4 to 300K, as a function of bias and gate voltage. Photoconductivity will also be measured at low temperatures to characterize the transport behavior of MWNT samples before and after segmentation.

In other work, we are investigating the field emission properties of graphene nanostructures for vacuum electronics applications. Graphene is a two-dimensional carbon honeycomb-structured single crystal showing ballistic transport, zero band gap, and electric spin transport characteristics. To measure field emission from graphene, we prepared sheets using mechanical exfoliation and placed them on an insulation layer. The resulting field emission behavior was analyzed using a Zyvex Nanomanipulator operating inside a scanning electron microscope (SEM). We are currently studying the field emission properties of different numbers of graphene layers and directions of the atomic crystal, including a triode with a gate electrode.

Nanoscale actuators can be used for many applications, such as determining how external forces applied to a cell membrane affect mechanisms like intercell signaling, cell growth, and cell adhesion. For this type of study, it is important to produce a large number of actuators with repeatable and controllable properties.

We fabricated thermal bimorph nanoactuators based on MWNTs coated with a thin metal film on one side using pulsed laser deposition (PLD), as shown in Figure 2.⁵ Thermal actuation and characterization was conducted using a temperature stage operating within an SEM. As the temperature increased from 300 to 550K, the nanoactuator's displacement was 500±100nm. The forces generated from the tip were found to be on the order of micronewtons (as measured using lateral force microscopy). We are currently developing vertical nanoactuator arrays with various actuation modes to continue this work.

Figure 2. Nanoactuator fabrication using a MWNT with a uniform PLD-based aluminum (Al) film. (a) Transmission electron microscope views of an MWNT-Al bimorph nanoactuator fabricated using a Nd:YAG (neodymium-doped yttrium aluminum garnet) laser. (b) SEM image taken at room temperature. (c) SEM image taken at 550K. (d) Overlap of images (b) and (c).

Our future work will pursue the fabrication, assembly, and manipulation of CNT and graphene nanostructures for nanosensors, actuators, and nanoelectronic device applications. Overcoming the technical challenges of working with these materials will enable wider exploitation of their outstanding electrical properties. This will lead to next-generation devices with unrivaled functionality in sensor, detector, system-on-a-chip, and system-in-a-package applications, as well as programmable logic controls and energy storage systems.

Part of this work was supported by the Air Force Office for Scientific Research.