Abstract
Hydrothermal growth of ZnO nanorods has been widely used for the development of tactile sensors, with the aid of ZnO seed layers, favoring the growth of dense and vertically aligned nanorods. However, seed layers represent an additional fabrication step in the sensor design. In this study, a seedless hydrothermal growth of ZnO nanorods was carried out on Au-coated Si and polyimide substrates. The effects of both the Au morphology and the growth temperature on the characteristics of the nanorods were investigated, finding that smaller Au grains produced tilted rods, while larger grains provided vertical rods. Highly dense and high-aspect-ratio nanorods with hexagonal prismatic shape were obtained at 75 °C and 85 °C, while pyramid-like rods were grown when the temperature was set to 95 °C. Finite-element simulations demonstrated that prismatic rods produce higher voltage responses than the pyramid-shaped ones. A tactile sensor, with an active area of 1 cm2, was fabricated on flexible polyimide substrate and embedding the nanorods forest in a polydimethylsiloxane matrix as a separation layer between the bottom and the top Au electrodes. The prototype showed clear responses upon applied loads of 2–4 N and vibrations over frequencies in the range of 20–800 Hz.
Highlights
In recent years, with the development of sensors able to measure different parameters, mimicking the functioning of human skin receptors, tactile sensing technologies have been widely explored to reproduce the human sense of touch [1,2,3,4,5,6,7,8,9,10,11]
Finite-element simulations were carried out to provide useful guidelines for the design of the ZnO NRs-based tactile sensors with enhanced electromechanical performance
The small grainy structures obtained from the as-deposited Au (Figure 3a), evolved into larger grains (Figure 3b), and the surface roughness of the Au layer increased from a RMS value of 0.5 nm to 1.1 nm, before and after annealing
Summary
With the development of sensors able to measure different parameters (e.g., temperature, pressure, and humidity), mimicking the functioning of human skin receptors, tactile sensing technologies have been widely explored to reproduce the human sense of touch [1,2,3,4,5,6,7,8,9,10,11] Such devices are mainly classified according to the transduction principle that they exploit, distinguishing between piezoresistive [12], capacitive [13], optical [9,14,15], and piezoelectric sensors [7,16,17,18]. Several researchers investigated the electromechanical performance of ZnO NRs, finding that the electromechanical behavior of these nanostructures strongly depends on their morphological features, such as shape and aspect ratio [30,31,32]
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