Large and Pressure-Dependent c-Axis Piezoresistivity of Highly Oriented Pyrolytic Graphite near Zero Pressure.

The c-axis piezoresistivity is a fundamental and important parameter of graphite, but its value near zero pressure has not been well determined. Herein, a new method for studying the c-axis piezoresistivity of van der Waals materials near zero pressure is developed on the basis of in situ scanning electron microscopy and finite element simulation. The c-axis piezoresistivity of microscale highly oriented pyrolytic graphite (HOPG) is found to show a large value of 5.68 × 10-5 kPa-1 near zero pressure and decreases by 2 orders of magnitude to the established value of ∼10-7 kPa-1 when the pressure increases to 200 MPa. By modulating the serial tunneling barrier model on the basis of the stacking faults, we describe the c-axis electrical transport of HOPG under compression. The large c-axis piezoresistivity near zero pressure and its large decrease in magnitude with pressure are attributed to the rapid stiffening of the electromechanical properties under compression.

An Overstretch Strategy to Double the Designed Elastic Stretchability of Stretchable Electronics.

Achieving or enhancing the elastic stretchability of inorganic stretchable electronics is critically significant. However, only two types of fundamental strategies-using the prestrained elastic substrate and designing the geometric layouts-are exploited thus far. This study proposes a third strategy, an overstretch strategy, applied beyond the designed elastic range of stretchable structures after transfer printing and bonding to a soft substrate. The theoretical, numerical, and experimental results collectively prove that the overstretch strategy can double the designed elastic stretchability of fabricated stretchable electronics and is valid for various geometrical interconnects with both thick and thin cross-sections. The underlying mechanism is that the elastic range of the critical part of the stretchable structure is doubled, owing to the evolution of the elastoplastic constitutive relation during overstretching. The overstretch strategy can be easily executed and combined with the other two strategies to enhance elastic stretchability, which has profound implications for the design, fabrication, and applications of inorganic stretchable electronics.