Microfluidic Synthesis of Graphene Quantum Dots and the Subsequent Ultrafast Photophysics: Building Blocks Towards Autonomous Nanomaterial Synthesis Platforms

Abstract

Autonomous synthesis of nanomaterials is a harmonious pursuit which assembles various building blocks from a toolbox consisting of microfluidics, robotics, artificial-intelligence, and spectroscopy, amongst a plethora of others. Archetypally, an autonomous synthesis platform is composed of three components: the reactor, a detector and a control scheme. In this work, the viability of some of these building blocks were investigated in the context of graphene quantum dot (GQD) synthesis. Three building blocks were investigated: the design and performance of the microfluidic reactor, the neural-network driven detection of microfluidic droplets, and the photophysics of the graphene quantum dots.

A microfluidic reactor was designed with on-chip microwave electronics to synthesize GQDs. The various degrees of freedom available in the reactor allowed for not only synthesis, but tuning of the optoelectronic properties via modulation of particle size and fundamental electronic structure modification in the form of nitrogen-doping into N-GQDs.

Within this reactor, the micro-scale droplets serve as discrete reaction vessels which benefit from the superior rates of mass and energy transfer characteristic of microfluidic systems. The next step is then developing the detector, to interrogate the optical properties of the GQDs thus allowing the control scheme to act on the aforementioned degrees of freedom in the reactor. Optical interrogation of individual droplets requires precise detection and positional tracking of the droplets. Thus, a neural-network was trained to detect and feature extract critical positional and size information from the droplets.

The final building block involves mapping the controversial photophysics of the GQDs. The origin of photoluminescence, ranging from the ultraviolet to visible regions, is a direct result of modulation of the synthesis parameters. Mapping the spectral tunability requires fingerprinting the photophysics which occur on ultrafast timescales. Several spectroscopic techniques were used to investigate the absorption, emission and interfacial charge transfer dynamics which contribute to these controversial optoelectronic properties. Fingerprinting and understanding these techniques is an essential step to developing a truly autonomous synthesis platform.