The team from Technische Universität Darmstadt and Battenberg ROBOTIC is studying the electrical and mechanical properties of vertical CNT arrays for use in 3D mechanical sensors. Applications for these devices include pressure, tactile and vibration measurement.

High bending elasticity and structural flexibility of the microstructured 3D CNT array are the key requirements for mounting such components on any curved surface and achieving human-like tactile sensitivity. Alumina template-based CVD growth was used to fabricate parallel carbon nanotubes with homogeneous length, diameter and density. It is essential that the individual CNTs are connected by integrated carbon layers on the top and bottom of the tube. These properties allow simple micro to nano integration of the CNTs.

Wiring up the array

Shown in the figure is a schematic view of the tactile sensor array with vertically aligned CNTs arranged in a 3D array structure. The top contacts define the spatial resolution of the sensor matrix. To analyse the spatial sensitivity, it was necessary to completely decouple the individual sensor cells to eliminate any cross sensitivities. The bottom of the array was fully covered by gold metallization and serves as the actual tactile surface.

The resulting flexible matrix element is a sensitive, stable and 3D sensor suitable for mounting on arbitrary surfaces. The force applied for characterization was set by the so-called Frey filament, which mimics typical forces applied during human sensing (typically in the mN range). The complete device was then mounted on a calibrated robot arm, which allows the application of forces in a highly reproducible manner.

Bundle conductance

The local and spatial sensitivity were measured close to the sensitive area of the nano-pressure sensor array. The mechanical pressure on the surface causes an increase of CNT bundle conductance. The bent CNT bundle touches its neighbouring free-standing tubes and presents some lateral cross-tube coupling, which generates additional current paths parallel to the CNTs and increases the conductance of the sensor element.

Individually addressed devices were built using the developed process. The device dimensions are adjustable and defined by photolithography for high spatial resolution. The studies performed will allow a “skin-like” application by further optimization of the wiring with a grid of resistances.

More information can be found in the journal Nanotechnology.