Ultrafast pressure sensing with transient tunnelling currents

Abstract

We report and systematically study large amplitude piezoresistance spikes in thin composite films under stress. These spikes are characterized by a unique double exponential decay which we demonstrate to be the signature of transient tunnelling currents. We establish an expression that predicts the dynamic conductivity of the composite with only three material parameters and use it to infer the magnitude of applied stress from resistance spikes, thus achieving quasi-instantaneous readout unhindered by viscoelastic relaxation. We demonstrate the proof of principle of ultrafast mechanoreceptors based on this effect by making a sensor array which images pressure at close to cinematic speeds with a sensitivity of 50 Pa.

Piezoresistance of flexible tunneling-percolation networks

“…Firstly the dependence of the d.c. conductivity on HOPG filling fraction suggests that tunneling is the interparticle conduction mechanism. The composite conductivity was found to depart from the universal power law, G ∼ (ν − ν c ) α , as the critical conductivity exponent increases from 2 to 4.7 when the HOPG filling fraction increases from 24% to 32% [16]. Grimaldi and Balberg [5] have theoretically shown that this behavior may be ascribed to percolation through an extended network of nanoparticles electrically connected to one another through quantum tunneling.…”

“…Our choice of model is drawn from observations of the resistance of composites subjected to stress cycles [16] and sequences of stress steps [17,18]. The stress protocols applied to composites of graphitic nanoparticles (HOPG) in polydimethylsiloxane (PDMS) are detailed by Chauhan et al [16]. In this section, we recall the remarkable features of these experiments which our theory will describe.…”

“…Recent theoretical work [5][6][7] identified this departure from universality as the signature of tunneling-percolation transport. The exceptional sensitivity of electrical properties to changes in pressure is currently motivating the synthesis of novel composites [2,4,[8][9][10][11][12] for making high performance sensors [11][12][13][14][15][16]]. An quantitative model of the tunnelingpercolation conductivity under stress is now needed to synthesize composite materials with appropriate piezoresistive properties.…”

“…An quantitative model of the tunnelingpercolation conductivity under stress is now needed to synthesize composite materials with appropriate piezoresistive properties. A piezoresistance model would further allow making constructive use of viscoelastic properties which currently limit the response time of conductive polymers [16][17][18][19][20].…”

“…We also report on the symmetry of the piezoresistance with respect to strain reversal. In this way, we determine that the hopping rate must be independent of the orientation of tunneling bonds in spite of the presence of the electric field applied during resistance measurements [16].…”