Ulf Werner

NanoNi@C: Hochempfindliche Funktionsschicht für Druck- und Kraftsensoren

Zusammenfassung Die Arbeit beschreibt die Entwicklung eines neuen Materials, das auf eine mechanische Belastung zehnmal empfindlicher reagiert als die heute genutzten metallischen DMS-Materialen. In diesem Kompositmaterial liegen mit Kohlenstoff umhüllte Nickelcluster mit einem Durchmesser zwischen 10 nm und 20 nm vor. Die Umhüllungen bestehen aus einigen gebogen Graphenlagen, die die Nickelcluster wie Zwiebelschalen umgeben. Elektronenmikroskopische Aufnahmen zeigten, dass die Metallcluster isoliert voneinander vorliegen, sich also nicht berühren, so dass der Stromtransport durch die Hüllen aus Kohlenstoff beeinträchtigt ist. Wird nun eine Dehnung oder Verzerrung dieses komplexen Materials zB durch einen äußeren Druck hervorgerufen, so beeinflusst dies den Stromtransport durch das Material, also seinen elektrischen Widerstand, sehr stark. Dies ermöglicht eine sehr empfindliche elektrische Messung des Drucks oder der Kraft. Neben einer um den Faktor elf erhöhten Dehnungsempfindlichkeit (k-Faktor bis zu 22) im Gegensatz zu herkömmlichen DMS-Materialen weisen diese Dünnschichten einen auf null einstellbaren Temperaturkoeffizienten des elektrischen Widerstands auf (TKR kleiner als ±25 ppm/K). Erste Prototypen in Form von Stahlmembran-Drucksensoren zeigen eindrucksvoll das Potenzial dieser Dünnschichten für Sensoranwendungen auf. Die hohe Dehnungsempfindlichkeit konnte bestätigt werden, auch viele weitere Sensorparameter wie zB das Kriechen, die Linearität der Kennlinie und das Signalrauschen konnten untersucht werden und liegen in für diese Sensoren typischen Bereichen. Abstract A new material with a strain sensitivity enhanced by a factor of eleven compared to common strain gauges is presented. Nickel clusters with a diameter of 10 nm to 20 nm are encapsulated by shells of carbon in a hexagonal structure like graphite or graphene. Transmission electron microscopy studies reveal separated metal clusters, indicating that the electron transport is impeded. By applying strain onto this complex material the carrier transport mechanism may be influenced strongly. Hence the electrical measurement of pressure or force is possible with high sensitivity. These functional layers show high strain sensitivity (gauge factor up to 22) in combination with a low temperature coefficient of resistance (TCR, approximately ±25 ppm/K). The TCR can be adjusted to zero by controlling the nickel composition of the thin film. First pressure sensors with these nanoNi@C functional layers are produced to demonstrate the high potential of this material. The high strain sensitivity could be confirmed, linearity and hysteresis measurements, current noise and creep errors exhibit typical values for these sensor devices.

New perspectives for pressure and force sensors thin films combining high gauge factor and low TCR

Metal containing carbon thin films can be prepared to exhibit piezoresistive properties with a high sensitivity to mechanical strain and with a temperature independent resistance. This unique combination of properties predisposes these films to be used in sensors for pressure, force, weight and torque. For the case of nickel containing carbon films (often termed as Ni containing hydrogenated amorphous carbon, shortly Ni:a-C:H) we are able to demonstrate a strain sensitivity (gauge factor) of approx. 20 together with a temperature coefficient of resistivity (TCR) below ±50 ppm/K in the wide temperature range of 100 K to 400 K. The sensitivity of our films is thus enhanced by a factor of 10 compared with standard metallic thin films in todays sensors. The films consist of crystalline nanoclusters of nickel with a diameter of 10 - 20 nm which are encapsulated by only few atomic layers of graphene (or turbostratic graphite) as revealed by transmission electron microscopy (TEM). These carbon encapsulated clusters are embedded in a matrix of carbon. The new type of films are named nanoNi@C i.e. nano-Nickel clusters encapsulated by carbon.

Low Temperature Gas Phase Synthesis of Germanium Nanowires

Single crystal Ge nanowires (NWs) were obtained in high yield by gas phase decomposition of germanium di-cyclopentadienylide ([Ge(C 5 H 5 ) 2 ]), at 325 °C on iron substrates. Highresolution electron microscopy (SEM/TEM) showed Ge NWs to be uniform in terms of diameter (20 nm) and length (> 25 μm). The wire growth is selective and appears to be governed by a Ge-Fe alloy epilayer formed by the reaction between Ge clusters and iron substrate, during the initial stages of the CVD process. The supersaturation of Ge-Fe solid-solution with respect to Ge content induces the spontaneous formation of single crystal germanium nuclei that act as templates for the nanowire growth. X-ray and electron diffraction revealed the NWs to be single crystals of cubic germanium with a preferred growth direction[11–2]. The proposed base-growth model on Fe substrate is supported by TEM, EDX and XPS studies.