Morphology-dependent terahertz electrical conductivity of liquid-exfoliated graphite

Abstract

Liquid-phase exfoliation (LPE) is a promising and scalable technique to produce low-cost dispersible nanosheets of graphene and nano-graphite for electronic, optoelectronics, and photonics applications. Fundamental information about how LPE affects the electrical properties is lacking. Here, a relationship is provided between the morphology of nano-graphite flakes resulting from LPE and cascade centrifugation to the charge-carrier transport properties. A range of process parameters, such as centrifuge force and exfoliation solvent, are employed, leading to a range of flake sizes. Morphology is characterized by scanning electron microscopy, atomic force microscopy and optical profilometry. Raman spectroscopy is used to confirm morphology, crystallite size, and chemical properties. Terahertz time-domain spectroscopy with a Drude-Smith conduction model provides the charge-carrier concentration and scattering times from AC conductivity. Carrier concentration increases with a reduction in flake area, potentially resulting from the introduction of electronic defect states at the edge of the nano-crystallites. Meanwhile, the carrier scattering time decreases with decreased flake size, similarly due to this self-doping that increases the carrier-carrier scattering. The approach and results serve as a foundation for understanding the processing-dependent electrical characteristics of LPE flakes and nanosheets.