Graphene and its derivatives are very important materials for electronic and photonic applications. While the physics and applications of these materials is being explored world-wide at a rapid pace, the reproducibility of materials properties is very difficult because of the poorly controlled chemical procedures used to isolate graphene and graphene oxide (GO) sheets. GO has been recently shown to have different photoluminescent properties depending on the extent of surface oxidation. This EAGER project, funded by both the Solid State and Materials Chemistry and Electronic and Photonic Materials Programs, aims to exploit this surface functionality and separate fractions of GO according to oxidation level and correlate that oxidation level in a more controlled manner with the electronic and photonic properties. Standard oxidation methodologies will be applied but in a systematic and controlled way. Surface chemistries will be matched with appropriate surfactants for optimizing the sheet separations. Samples will be analyzed using an AFM technique that allows visualization of the arrangements of surfactants on the surface of GO. This will provide feedback about how to design better surfactants for the separation. This work is important because it will not only provide important information about the chemical oxidation graphite , but it will also provide a procedure for isolating better chemical defined GO materials for physical property (especially photonic) applications.
NON-TECHNICAL SUMMARY: The goal of this work is the development of a new optoelectronic material with an adjustable electronic band gap. Optoelectronic materials are used to convert light into electric currents or vice versa. Typical applications are solar cells, photo detectors, digital cameras, light emitting diodes or solid state lasers. The size of the band gap largely determines which wavelengths of light are involved in these processes. An existing problem is that there are only a limited number of optoelectronic materials available, and their band gaps are fixed. For instance, this determines the colors that are available for light emitting diodes, or the fraction of sunlight which can be efficiently converted to electricity using solar cells. In this project graphene oxide (GO) is investigated as a new material with adjustable band gap; broader impacts of this work will include developing materials that will enable the design of solar cells with higher conversion efficiency, or energy-efficient light sources of many different colors. While GO can be made from graphite economically, the obtained material is not very pure and thus has a great variety of many different band gaps. In this project new separation approaches will be explored to purify this material to the level needed for electronic applications.
The central outcome of this project was that ionic surfactants (detergents) show selective adsorption on graphene oxide nanoparticles. The ultimate goal is to employ this phenomenon to produce highly purified graphene oxide that can be applied for optoelectronic applications such as solar cells, photo detectors, light emitting devices, and sensors. These materials can potentially be produced very economically and in a scalable way. These graphene oxide nanoparticles are single-atomic layers of oxidized graphene, which are produced by first oxidizing graphite and then exfoliating it in water using in an ultrasonic bath. The problem with as-produced graphene oxide is that the material is very heterogeneous and impure, leading to inferior optoelectronic properties. We were the first to reveal a phenomenon can potentially be used to produced graphene oxide of much higher quality, which can successfully be used for many applications. Using liquid-cell atomic force microscopy, we were able to show directly that certain ionic surfactant molecules only adsorb on graphene oxide sheets with particular properties, so that these sheets can potentially be separated using additional steps, such as centrifugation. We produced high-resolution images showing molecularly small aggregates of such surfactant molecules adsorbing to particular locations on these graphene oxide sheets (see attached Figure). These aggregates have diameters on the order of only 5 nanometers.
