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Insulator/metal Core Shell Nanocomposites: A New Approach for Sensing Materials

Evagelos K. Athanassiou and Wendelin J. Stark. Chemistry and Applied Biosciences, ETH Zurich, Wolfgang-Pauli-Str. 10, ETH Hönggerberg, Zurich, 8093, Switzerland

Inexpensive, reliable temperature and pressure sensors are a crucial part in most manufacturing processes. Moreover, the current demand for increasingly more integrated consumer products has created a strong need for low cost materials with exceptional sensitivity and mechanical or thermal stability.

At present, temperature and pressure sensing relies on advanced spinel-type metal oxides or on conductive polymer composites. Here we present a new approach, the production of core-shell type materials for the development of highly sensitive sensors. Such a composite consists of a conductive metal core and a protective shell exhibiting an insulating behavior. The different electronic properties of the shell induces the presence of an energy band gap, resulting to materials with a highly sensitive pressure or temperature depending electrical resistivity. Such sensors can be produced by reducing flame synthesis, a one-step process at very large scales (10-30 g h-1). The versatility of the process allows the production of nanocomposites with different types of shell (thickness: 0.6 to 1.2 nm) covering the metal core (15 to 30 nm). The highly interesting piezoresistance and theroresistive effects (NTC behavior) of carbon coated copper nanoparticles[1,2] (Figure 1a) and ceramic coated nickel-molybdenum nanoparticles[3] (Figure 1b) prove that the material-independent principle of a core-shell type metamaterial could offer an interesting alternative for the one-step development of sensing materials (Figure 1c). Theoretical analysis suggests a tunneling based conduction mechanism for these insulator / metal core-shell materials.

Fig. 1: (a) Transmission electron micrograph of as prepared insulating carbon coated copper and ceria-zirconia coated nickel-molybdenum nanoparticles (b). The core-shell type nanomaterial allows the manufacture of a metal based sensor. Upon connecting the material with an electrical circuit and measuring its electrical resistivity it is possible to predict the surrounding temperature and the applied pressure (c).

[1] E.K. Athanassiou, R.N. Grass and W.J. Stark, Nanotechnology, 2006, 17, 1668.

[2] E.K. Athanassiou, C. Mensing and W.J. Stark, Sens. Actuators A, 2007, 138, 120.

[3] E.K. Athanassiou, F. Krumeich, R.N. Grass and W.J. Stark, Phys. Rev. Lett., 2008, submitted.