High-speed spectroscopic transient absorption imaging of nanoscale defects in graphene


Graphene materials have found a wide range of applications in various fields such as electronics as a result of their favorable electrical, chemical and mechanical properties. However, their functionalities and properties can be compromised by nanodefects problems such as the grain boundaries. Due to their 2-dimensional layer, they can be formed through various processes one of them being chemical vapor deposition. During such processes, some of the defects observed in the graphene are formed. Grain boundaries, for example, are formed as a result of the intersection of the islands structures emanating from the multiple sites during the nucleation process of the chemical vapor deposition.

To understand the formation and effects of the grain boundaries on the properties and functionality of the graphene materials, several methods have therefore been developed to help in the imaging of the grain boundaries. Such methods apply different principles in imagining the defects. Transient absorption technique, a graphene measuring method depends on the pump to excite the electrons from the valence band to the conduction band. With the rapid growth and demand for graphene materials, large manufacturing processes are inevitable. Thus, transient absorption techniques can be modified to enable imaging of the grain boundaries and other defects in the materials at high and convenient speeds.

Boston University researchers: PhD student Kai-Chih Huang, Professor Chen Yang, Professor Ji-Xin Cheng and Postdoctoral fellow Chien-Sheng Liano in collaboration with PhD student Jeremy McCall and Technician Gregory Eakins at Purdue University together with Professor Pu Wang from Beihang University in China developed a new method based on the use of transient absorption microscopy for imaging the grain boundaries and other nanodefects in graphene materials. They also investigated the imaging speeds that could be used for the characterization of the nanodefects during the manufacturing processes of graphene such as the large area chemical vapor deposition. The research work is currently published in the journal, Nano Letters.

To significantly improve the functionality of the experimental setup regarding the spatial speeds, the authors used line-illuminated scanning method rather than the conventional raster scanning technique. They also used a tuned amplifier and photodiode array detection system invented by Ji-Xin Cheng and coworkers.

The authors observed that line-illumination scanning method increased the imaging speed of the transient absorption for instance by 1000 frames for every second. This is far much higher than the other speeds obtained in the previous studies through other methods. The system was also capable of characterizing the graphene nanodefects depending on the transient decay rate.

The research team successfully developed a new method that enables the detailed study and characterization of the grain boundaries of the graphene materials depending on the different available manufacturing techniques. For instance, the excellent speeds achieved in this work can resolve some of the defects associated with the grain boundaries like the surface wrinkles, surface coverage, and the grain numbers. Besides, their system is versatile and flexible and hence not limited to graphene and be used to study for example biological systems.

- Advances in Engineering

About the author

Prof. Chen Yang received her doctoral degree in Chemistry from Harvard in 2006. She worked as an associate in McKinsey & Co., a world leading business consulting company, from 2006 to 2007. Prof. Yang was an assistant professor and then an associate professor in Department of Chemistry and Department of Physics and Astronomy at Purdue University from 2007 and 2017 and she is an associate professor in Department of Electrical and Computer Engineering and Department of Chemistry Boston University since 2017.

Prof. Yang’s research interest is focusing on functional nanomaterials for their potential applications in biomedical, photonics and solar energy applications. Her research has been published in many high profile journals, including Science, Nature, Nature Photonics, Physics Review Letter and Nano Letters,  and been featured by public press releases, including Chemical and Engineering News, Harvard Gazette. She was a NSF Career Award recipient in 2009. She served as a guest editor for Journal of Electronic Materials and currently serve on Editorial Board of Scientific Reports.

About the author

Ji-Xin Cheng, PhD

Moustakas Chair Professor in Photonics and Optoelectronics
Professor of Biomedical Engineering
Professor of Electrical & Computer Engineering
Professor of Chemistry
Professor of Physics
Boston University Photonics Center, 8 Saint Mary’s Street, Boston, MA 02459

Ji-Xin Cheng was born in Jixi, Anhui Province, P. R. China in 1971. He attended University of Science and Technology of China (USTC) from 1989 to 1994. From 1994 to 1998, he carried out his PhD study on bond-selective chemistry under the supervision of Qingshi Zhu at USTC. As a graduate student, he worked as a research assistant at Universite Paris-sud (France) on vibrational spectroscopy and the Hong Kong University of Science and Technology (HKUST) on quantum dynamics theory. After postdoctoral training on ultrafast spectroscopy in Yijing Yan’s group at HKUST, he joined Sunney Xie’s group at Harvard University as a postdoc, where he and others developed CARS microscopy that allows high-speed vibrational imaging of cells and tissues. Cheng joined Purdue University in 2003 as Assistant Professor in Weldon School of Biomedical Engineering and Department of Chemistry, promoted to Associate Professor in 2009 and Full Professor in 2013.

He joined Boston University as the Moustakas Chair Professor in Photonics and Optoelectronics in summer 2017. Professor Cheng and his research team has been constantly at the most forefront of the rising field of label-free chemical imaging in technology, science, and clinical translation


Huang, K., McCall, J., Wang, P., Liao, C., Eakins, G., Cheng, J., & Yang, C. (2018). High-Speed Spectroscopic Transient Absorption Imaging of Defects in Graphene. Nano Letters, 18(2), 1489-1497.


Go To Nano Letters

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