Two-dimensional materials for micro/nanoscale active matter
Artificial micro/nanoscale active matter capable of harnessing and converting energy from the environment into mechanical motions, mimicking the functions of live cells and microorganisms in nature, presents as desirable models for the study of the fundamental mechanisms of locomotion. The integration of functional materials, for instance, the emerging two-dimensional (2D) materials, into such an area so that a miniaturized motor/machine could exhibit unique functionalities to carry out a targeted service, is among the most intriguing and remarkable topics in nanoscience and nanotechnology. In this dissertation, two types of micro/nanoscale active matters fabricated of 2D materials are explored. For one type, free-standing longitudinal MoS₂ nanoribbons, as well as their oxide hybrids, are designed and fabricated. The synthesized MoS₂ nanoribbons can be well dispersed into a solution and activated by electric fields to move along a designed trajectory. With the assistance of “click-chemistry” at the “press of a light button”, the nanoribbon can be assembled onto pre-patterned electrodes, demonstrating its potential as photodetectors with a rapid photoresponse ~16 ms. Moreover, the MoS₂ nanoribbons rotate both clockwise and counterclockwise in an aqueous solution depending on the frequency of the applied rotating electric field. With the understanding of theoretical modeling based on both Maxwell-Wagner polarization and induced electrical double layer effect, the measured electro-rotation spectrum can be readily analyzed to extract the electrical conductivity of the MoS₂, agreeing with that of four-probe measurements on the order of magnitude, serving as a rapid method to characterize the electronic properties of nanomaterials in a non-contact and non-destructive fashion. Janus MoS₂/TiO₂ micromotors is another type of 2D material integrated micromachines, which propel themselves by harnessing energy from light and chemical fuels from the environment, are delicately designed. Especially, the Janus motors exhibit enhanced speed in response to a higher ionic strength. The unprecedented phenomenon can be attributed to the competing self-diffusiophoresis (DP) and self-electrophoresis (EP) induced by the unique bandgap alignment between TiO₂ and MoS₂. The design, fabrication, propulsion, and application of 2D-materials-based active matter and the underlying mechanistic understanding demonstrate the high potentials that 2D materials can bring to the field of micro/nanorobotics in terms of materials integration, multi-functionality, and unique properties. With the continuous discovery of new physics and chemistry in this new class of materials, we expect 2D materials to enable new breakthroughs in micro/nanorobotics.