In situ mechanical and electrical testing of piezoresistive nanomaterials

Abstract

Piezoresistive materials have been widely used in applications related to electromechanical systems, owing to their variable electrical properties under mechanical strain. The physical nature behind the piezoresistive effect ensures the success of strain engineering in improving the performance of electronic and photonic devices, as it allows for enhanced carrier mobilities and direct change of band structure of semiconductors. The ability to withstand high elastic strain far beyond the limit of their bulk counterparts and the minuscule size possibly below the threshold for external and bulk defects make nanomaterials excellent platforms for the exploration of fundamental piezoresistive properties. However, quantitative characterization of mechanical properties and electromechanical coupling is challenging at the nanoscale, which requires specially designed testing systems. In this thesis, an in situ tensile testing platform was realized in a focused ion beam-scanning electron microscope (FIB-SEM) for probing the mechanical, electrical, and coupled electromechanical properties of individual nanostructures. The constructed testing system and methods were verified by testing nanomaterials with well-studied properties, i.e., germanium (Ge) and silicon (Si) nanowires (NWs), and are proved to be reliable, flexible, and able to provide high resolution measurements. During material investigations, electromechanical coupling in highly tensile strained <111> Ge NWs was studied. Under tensile strain, the conductivity of Ge NWs is enhanced exponentially, reaching an enhancement factor of ~130 at ~3.5% of strain. Under strains larger than ~2.5%, the electrical property of Ge also exhibits a dependence on the electric field. The conductivity can be further enhanced by ~2.2× with a high bias condition at ~3.5% of strain. Testing on multiple NWs and cyclic loading tests confirm that the observed electromechanical responses are repeatable, reversible, and related to the changing electronic band structure. Mechanical properties and piezoresistivity of [0001] tellurium (Te) NWs were investigated. An average elastic modulus is estimated to be 38.6 ± 4.7 GPa. Both elastic and elastic-plastic behaviours are observed in tested NWs, with a large fracture strain of up to 18% achieved in the latter case. Regardless of the deformation types, electromechanical tests of Te NWs show a trend of decreasing resistance with increasing strain at low-to-moderate tensile strains (0 to 4%), evidencing the piezoresistive effect. Furthermore, properties of nanoscale nickel (Ni)/ (cobalt (Co)) metal-organic frameworks (MOFs) were explored. Linear stress-strain relations are observed up to 17% of tension in Ni(TCPP)-pyridine MOF (TCPP = 5,10,15,20-tetrakis(4-carboxyl-phenyl)-porphyrin) nanobelts. The measured elastic modulus increases from 10.6 to 31.8 GPa with increasing length-to-width ratio and shows a stiffening effect upon unloading from tensile states. Electromechanical tests of these nanobelts show linear and reversible resistance increase and the magnitude of increase is higher than the change caused purely by the geometry effect. Anisotropic electrical conductivity is found for Ni/Co(NDC) MOF (Ni:Co = 1:1, NDC = 2,6- naphthalenedicarboxylate) nanosheets. The conductivity enhanced direction is consistent with the crystal orientation with constrained lattice spacing. These experimental results reveal excellent prospects for utilizing the investigated piezoresistive nanomaterials in strain-related nanodevices and provide important information to uncover the underlying mechanisms for the novel mechanical behaviours and electromechanical interactions of these nanostructures.