A team of international scientists has demonstrated that a new type of “glassy” graphene may be an appealing option for flexible, transparent integrated circuits. As reported in a recent issue of Science Advances, the material in thin film form is an intermediate state between graphene and glassy carbon, which is pure carbon with a disordered structure displaying glass, ceramic, and graphite properties. Glassy graphene brings together the attractive properties of both graphene and glassy carbon. Like graphene, glassy graphene has high conductivity, transparency, and flexibility. Like glassy carbon, it features high degrees of mechanical, thermal, and chemical stability.

The glassy graphene film was created through a process the researchers call polymer-assisted deposition (PAD). They begin by spin coating a precursor polymer onto a quartz substrate. After annealing it to 1000°C, they evaporate a thin layer of nickel onto the film to promote crystallization. The film is annealed to 850°C and the nickel is removed through etching. The result is a glassy graphene film that is crystalline, but with twisted and bent lattices.

When the nickel-coated film is instead annealed to 1000°C, spectral analysis and transmission electron microscopy show the high crystallinity and well-ordered lattice of graphene. When the nickel is left out completely and the precursor film is simply annealed to 1000°C, the result is a partially crystallized and disordered glassy carbon.

Optical and scanning electron microscopy studies show that PAD-deposited glassy graphene is dense and smooth, with no pores or cracks on the millimeter to nanometer scale. Atomic force microscopy calculations show a surface roughness of less than 0.7 nm. Together, these features demonstrate that glassy graphene is well-suited for use in electronic devices.

“The unique chemistry and processing design of PAD deliver stable and homogeneous aqueous solutions at a molecular level that allows the controllable growth of glassy graphene,” according to Guifu Zou from Soochow University in China, who led the research team. “This method is not only scalable, environment-friendly, and cost-effective, but also as-deposited glassy graphene is ultra-smooth and easily patterned through laser direct writing.”

In a demonstration of its potential, the researchers created patterns out of glassy graphene, including a complete graphene field-effect transistor. To do this, they used an infrared laser to cure a pattern onto the precursor polymer layer before annealing. The uncured part of the film was dissolved in water, and after processing the remaining precursor film, the circuit was transferred from the quartz substrate onto a flexible substrate. Bending and twisting tests suggest that the resistance of glassy graphene film is as reliable and flexible as graphene.

The circuits can be transferred to any flexible substrate. Their reliable performance indicates great promise for electronic applications, Zou says. His team is currently working on incorporating glassy graphene circuits into electronic skin and wearable electronics.  

Key challenges remain in the process of growing materials with suitable transparency, conductivity, and flexibility for wearable electronics and other applications, particularly with respect to crystal quality engineering says Zhongfan Liu of Peking University, an expert in low-dimensional carbon materials and two-dimensional atomic crystals with applications in nanoelectronics and energy conversion devices.

“Graphene is one of the most flexible and transparent conductive materials for wearable and flexible applications,” according to Liu. “Combining the complementary intrinsic properties of glassy carbon and graphene, the authors present a new candidate for flexible transparent conducting materials, bearing superior stability, uniformity and inch-scale flatness.”

Read the article in Science Advances.