We report the exfoliation of graphite in aqueous solutions under high shear rate [∼ 108 s–1] turbulent flow conditions, with a 100% exfoliation yield. The material is stabilized without centrifugation at concentrations up to 100 g/L using carboxymethylcellulose sodium salt to formulate conductive printable inks. The sheet resistance of blade coated films is below ∼2Ω/□. This is a simple and scalable production route for conductive inks for large-area printing in flexible electronics.

A systematic approach for understanding the electron transport mechanisms in resistive strain gauges based on assemblies of gold colloidal nanoparticles (NPs) protected by organic ligands is described. The strain gauges were fabricated from parallel micrometer wide wires made of 14 nm gold (Au) colloidal NPs on polyethylene terephthalate substrates, elaborated by convective self-assembly. Electron transport in such devices occurs by inter-particle electron tunneling through the tunnel barrier imposed by the organic ligands protecting the NPs. This tunnel barrier was varied by changing the nature of organic ligands coating the nanoparticles: citrate (CIT), phosphines (BSPP, TDSP) and thiols (MPA, MUDA). Electro-mechanical tests indicate that only the gold NPs protected by phosphine and thiol ligands yield high gauge sensitivity. Temperature-dependent resistance measurements are explained using the 'regular island array model' that extracts transport parameters, i.e., the tunneling decay constant β and the Coulomb charging energy E(C). This reveals that the Au@CIT nanoparticle assemblies exhibit a behavior characteristic of a strong-coupling regime, whereas those of Au@BSPP, Au@TDSP, Au@MPA and Au@MUDA nanoparticles manifest a weak-coupling regime. A comparison of the parameters extracted from the two methods indicates that the most sensitive gauges in the weak-coupling regime feature the highest β. Moreover, the E(C) values of these 14 nm NPs cannot be neglected in determining the β values.

The effect of nanoparticle (NP) size on the performance of resistive strain gauges fabricated from gold colloidal NPs is evaluated. The active area of the strain gauges consists of colloidal NPs assembled into multilayered wire arrays on a flexible substrate using Stop&Go convective self-assembly (SG-CSA). The strain sensing in such gauges relies on the exponential variation of tunnel resistance with interparticle gap. The sensitivity of the fabricated strain gauges is found to increase linearly with the size of the gold NPs. A 15 times increase in sensitivity is observed on going from 5 to 97 nm NPs. However, the hysteresis under cycling tests is higher for the most sensitive gauges constructed from larger NPs because of increased disorders in the assemblies of larger NPs, which leads to their irreversible displacement under applied strain. This investigation reveals that the strain gauges derived from 15 nm gold NPs have the best overall performance, with a gauge sensitivity ten times higher than the conventional metal foil gauges at 0.5% strain without undergoing damage under cycling tests.