Graphene strains to increase its photoabsorption

Researchers from the University of Illinois at Urbana-Champaign have demonstrated a new approach to modifying the light absorption and stretchability of atomically-thin two-dimensional (2D) materials like graphene by engineering their surfaces using mechanical strain. When combined with flexible light-emitting diodes, this new approach could lead to the development of novel wearable technology and integrated biomedical optical sensing technology.

"Increasing graphene's low light absorption in visible range is an important prerequisite for its broad potential applications in photonics and sensing," explained SungWoo Nam, an assistant professor of mechanical science and engineering at the University of Illinois. "This is the very first stretchable photodetector based exclusively on graphene with strain-tunable photoresponsivity and wavelength selectivity."

Graphene is an atomically-thin layer of hexagonally-bonded carbon atoms and has been extensively investigated for use in advanced photodetectors due to its broadband absorption, high carrier mobility and mechanical flexibility. But graphene has a low optical absorptivity, and so graphene photodetector research has so far focused on hybrid systems to increase photoabsorption. However, such hybrid systems require a complicated integration process, while the interfaces between the different materials reduce the mobility of the charge carriers.

Another option, however, is to increase graphene’s optical absorption and stretchability. According to Nam, the key to doing this is to engineering the 2D material into three-dimensional (3D) ‘crumpled structures’, thereby increasing the graphene's mass per unit area, also known as area density. With a higher area density, the continuously undulating 3D surface generates higher optical absorption per unit area, thereby improving graphene’s photoresponsivity.

The density, height and pitch of the crumpled structures are modulated by applied strain and the crumpling is fully reversible during cyclical stretching and release. This crumpling approach thus offers a new way to enhance graphene’s photoabsorption and allowed the creation of a highly-responsive photodetector based on a single graphene layer.

"We achieved more than an order-of-magnitude enhancement of the optical extinction via the buckled 3D structure, which led to an approximately 400% enhancement in photoresponsivity," stated Pilgyu Kang, a member of Nam’s research group and first author of a paper on this work in Advanced Materials. "The new strain-tunable photoresponsivity resulted in a 100% modulation in photoresponsivity with a 200% applied strain. By integrating colloidal photonic crystal – a strain-tunable optomechanical filter – with the stretchable graphene photodetector, we also demonstrated a unique strain-tunable wavelength selectivity."

"This work demonstrates a robust approach for stretchable and flexible graphene photodetector devices," Nam added. "We are the first to report a stretchable photodetector with stretching capability to 200% of its original length and no limit on detection wavelength. Furthermore, our approach to enhancing photoabsorption by crumpled structures can be applied not only to graphene, but also to other emerging 2D materials."

This story is adapted from material from the University of Illinois at Urbana-Champaign, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.