Negative Gauge Factor Piezoresistive Composites Based on Polymers Filled with MoS₂ Nanosheets

Abstract

Nanocomposite strain sensors, particularly those consisting of polymer–graphene composites, are increasingly common and are of great interest in the area of wearable sensors. In such sensors, application of strain yields an increase in resistance due to the effect of deformation on interparticle junctions. Typically, widening of interparticle separation is thought to increase the junction resistance by reducing the probability of tunnelling between conducting particles. However, an alternative approach would be to use piezoresistive fillers, where an applied strain modifies the intrinsic filler resistance and so the overall composite resistance. Such an approach would broaden sensing capabilities, as using negative piezoresistive fillers could yield strain-induced resistance reductions rather than the usual resistance increases. Here, we introduce nanocomposites based on polyethylene oxide (PEO) filled with MoS2 nanosheets. Doping of the MoS2 by the PEO yields nanocomposites which are conductive enough to act as sensors, while efficient stress transfer leads to nanosheet deformation in response to an external strain. The intrinsic negative piezoresistance of the MoS2 leads to a reduction of the composite resistance on the application of small tensile strains. However, at higher strain the resistance grows due to increases in junction resistance. MoS2–PEO composite gauge factors are approximately −25 but fall to −12 for WS2–PEO composites and roughly −2 for PEO filled with MoSe2 or WSe2. We develop a simple model, which describes all these observations. Finally, we show that these composites can be used as dynamic strain sensors.

Smart molecular/MoS2 Heterostructures Featuring Light and Thermally-Induced Strain Driven by Spin Switching

“…In this mechanism, the volume change of the nanoparticles induced by the spin transition is expected to strain the flakes, resulting in an intrinsic modulation of the band structure of the layers, and thus modifying their conductivity (for further details and discussion see Supplementary pages 11-16). 29 Furthermore, because of the direct gap semiconductor nature of the MoS2 monolayers (2Hphase), it is possible to gain direct information on their band gap energy through PL measurements. 51 In fact, it is well known that when a tensive strain is applied to a MoS2 layer, its PL redshifts and weakens its intensity as consequence of a narrowing in the band gap and a transition in the semiconductor from direct to indirect band gap behavior occurs.…”

“…27,28 In addition, an intrinsic negative piezoresistivity has been observed 24 and exploited to develop a piezoresistive composite in which the resistance decreases when the strain increases (negative Gauge factor). 29 In this scenario, we have envisioned the possibility of preparing MoS2-based heterostructures where the second component is an active molecular system that can directly and reversibly tune the strain applied on the 2D material and, therefore, its electronic structure and electric conductivity. We have chosen switchable molecular-based spin-crossover (SCO) materials for this purpose.…”

Negative Piezoresistive Effect in a Stretchable Device Based on a Soft Tunneling Barrier

“…We compared the electromechanical performance of our device with those of state-of-the previously reported values have only been reported in a small number of papers. 16,17 Notably, the negative piezoresistive range of our sample extends to 130% strain, and no pGF features are observed at any applied strain.…”

“…13,14,15 The fabrication of composite stretchable negative-piezoresistive materials by, for example, utilizing a negative-piezoresistive filler embedded in an elastic matrix has been suggested. 16,17,18 However, the increased interspacing of the filler upon application of strain within a certain range counteracts the resistance reduction of the composite and the nGF strain range is limited to extremely small strain variations (ɛ = ~%). 16,17 The low off-mode resistance is not favorable for low power consumption in the standby mode.…”

“…16,17,18 However, the increased interspacing of the filler upon application of strain within a certain range counteracts the resistance reduction of the composite and the nGF strain range is limited to extremely small strain variations (ɛ = ~%). 16,17 The low off-mode resistance is not favorable for low power consumption in the standby mode. In addition, the resistance modulation in the on-state (with strain) is limited by the elastic matrix of the surrounding insulator; therefore the performance of stretchable composite materials with negative piezoresistivity is hampered by a poor on-to-off ratio (10 2 -10 3 ) and a low operating nGF strain range.…”