H. Schlicke, S. Kunze, H. Hartmann, T. Vossmeyer
Fraunhofer Center for Applied Nanotechnology CAN,
Keywords: nanoparticles, composite materials, gold, chemical sensors, pressure sensors
Summary:Composite materials consisting of organically capped or cross-linked nanoparticles offer several features rendering them interesting for the use in future micro- and nanoelectromechanical (MEMS/NEMS) devices. Their hybrid organic-inorganic nature enables a unique and tunable set of mechanical, optical and charge transport properties as well as chemical sorption behavior. The interplay of these characteristics can be exploited for the fabrication of novel sensors and actuators. In this contribution we present membranes consisting of dithiol cross-linked gold nanoparticles (GNPs), their electromechanical characterization and their use in first prototypical pressure/force and chemical sensors. Nanometer-thin GNP films can be easily deposited via solution-based processes, such as spin-coating or ink-jet printing. Subsequently, they can be detached from their initial substrates and transferred onto 3d-microstructures, yielding freestanding membranes. Here, the membranes' mechanical properties can be directly addressed, and their coupling to other characteristics can be exploited. First, we demonstrate the mechanical characterization of such GNP membranes using atomic force microscopy based micro-bulge testing. We show that the materials exhibit a tunable soft elastic behavior, which can be rather compared to polymers than to their bulk metal counterparts. Due to the organic matrix surrounding the GNPs, charge transport through GNP composite materials is based on tunneling between neighboring particles. As this mechanism is strongly distance-dependent, strain acting on the materials results in pronounced conductivity changes. Along with the soft elastic behavior and the nanometer thickness this renders the membranes promising for highly sensitive force and pressure sensing applications. Here we report differential pressure sensors employing GNP membranes, showing exceptional resistive sensitivities exceeding 1x10^-3 mbar^-1 as well as their in-operando electromechanical charaterization using AFM. Second, the conductivity of GNP nanomembranes enables electrostatic actuation using counter electrodes. We show the excitation of quasistatic deflections and resonant vibrations using DC and AC signals, respectively, and their characterization using laser interferometry. GNP membrane based actuators and resonators have a great potential for chemical sensing applications, as the molecular organic matrix surrounding the nanoparticles is able to sorb analyte molecules. This process is accompanied by changes in the nanomembrane's mechanical properties and reflected in variations of the deflection amplitudes and resonance frequencies of GNP membrane based actuators and resonators. We demonstrate first prototypical electromechanical chemical sensors, capable of detecting volatile organic compounds (VOCs) at concentrations in the low ppm regime.  H. Schlicke, S. Kunze, M. Finsel, E. W. Leib, C. J. Schröter, M. Blankenburg, H. Noei, T. Vossmeyer, J. Phys. Chem. C 2019, 123, 19165–19174.  H. Schlicke, M. Rebber, S. Kunze, T. Vossmeyer, Nanoscale 2016, 8, 183–186.  H. Schlicke, S. Kunze, et al., unpublished work.  H. Schlicke, C. J. Schröter, T. Vossmeyer, Nanoscale 2016, 8, 15880–15887.  H. Schlicke, M. Behrens, C. J. Schröter, G. T. Dahl, H. Hartmann, T. Vossmeyer, ACS Sens., 2017, 2, 540–546.