Fabricating Vertically-Aligned Monolayer Graphene Helices for Chiral Metamaterial Development

Graphene has been heavily investigated as one of the chiral metamaterials because it demonstrates a strong response in the THz range, which is useful for detecting and analyzing low concentrations of chemical and biological materials. However, graphene's structure is achiral, making it unsuitable as a chiral material for analyzing chiral molecules. Moreover, its extremely thin thickness results in weak light interaction. To achieve strong interaction between incident light and chiral molecules, the graphene surface should be aligned parallel to the incident light. This means that the graphene should be vertically-aligned on the substrate when the incident light is vertically illuminated from the substrate. To satisfy the requirements, in this work, a dual-component self-assembly method that utilizes stress gradient induced by ultraviolet (UV) light is developed. Graphene, together with a thin SU-8 photoresist, are both self-twisted and curved vertically, forming vertically-aligned graphene helices. This assembly process enables the creation of graphene helices with helix heights up to 4 mm and an aspect ratio (height/width) of 2700.This height is equivalent to 4 million times the height of monolayer graphene. Raman measurement and SEM secondary electron test were performed to prove the pristine graphene property on the entire surface of the helix. Furthermore, to explore the chiral properties of the 3D graphene helix, simulations were performed, with the results distinctly emphasizing its unique chiral characteristics. It is believed that this work is the first time to realize graphene self-twist to a vertically-aligned monolayer graphene helix as a chiral sensor. The developed new fabrication technique can be further applied to other 2D materials like MXene, hBN and MoS₂.