Materials Horizons Emerging Investigator Series: Dr Danqing Liu, Shenzhen University, China

Danqing Liu is an Assistant Professor in the College of Material Science and Engineering of Shenzhen University. She received her BSc degree in 2010 from the University of Science and Technology of China, and obtained her PhD in Chemistry at the Chinese University of Hong Kong in 2014 under the direction of Prof. Qian Miao. After one year at CUHK as a research assistant, she joined Shenzhen University in 2015. Currently, her research group focuses on organic semiconductors and electronic devices, including organic field effect transistors and organic/hybrid thermoelectrics, by using tools from molecular engineering, processing engineering and interface engineering.

Read Danqing Liu's Emerging Investigator Series article ‘Novel butterfly-shaped organic semiconductor and single-walled carbon nanotube composites for high performance thermoelectric generators’ and read more about her in the interview below:

MH: Your recent Materials Horizons Communication reports a series of novel butterfly-shaped small-molecule organic semiconductors (OSCs) complexed with single-walled carbon nanotubes. This new composite design sets a new record for p-type thermoelectric composites based on small-molecule OSCs. How has your research evolved from your first article to this most recent article and where do you see your research going in future?

DL: I began my research on organic semiconductors during my PhD studies. We developed a series of self-assembled monolayers as novel interface engineering strategies for organic thin film transistors (OTFTs), which were generally applicable for solution-processed and vacuum-deposited n-type and p-type OSCs. When I started my independent research at Shenzhen University, I had been thinking about how to use my well-trained knowledge and the technology of organic semiconductors and devices to carry out new original work. At that time, there had been growing interest in employing organic semiconductors, especially conductive polymers, as thermoelectric (TE) materials. However, challenges still remained in understanding the molecular structure–property relationships. Our group then started researching the thermoelectric properties of small-molecule OSCs, which possess the merits of exact structure and various derivations, which aids the exploration of their structure–property relationship. We published our first work in 2019 on organic TE materials based on benchmark small-molecule organic semiconductors by controlling the doping process. In that work, we used single-walled carbon nanotubes (SWCNTs) complexed with small-molecule OSCs to obtain large-scale high electrical conductivity. With the in-depth studies of the molecular interactions between the OSCs and SWCNTs, we found that novel molecular design is highly desired for studies on structure–property relationships.

Our recent work in Materials Horizons designs a series of novel butterfly-shaped small-molecule OSCs, which are complexed with SWCNTs showing high performances for thermoelectric materials and generators. Unlike the widely employed planar-core-based OSCs, the butterfly-shaped OSCs have contorted molecular structures. The introduction of carbonyl and crowded thienyl groups generally bends the molecular backbones, leading to the enlarged activation energy that contributes to the high Seebeck coefficients. Meanwhile, the curved conjugated backbones are also beneficial for enhancing the interactions between the butterfly-shaped molecules and the curved surfaces of the SWCNTs, resulting in improved doping levels. This work inspires new molecular design criteria for organic and hybrid TE materials. Currently, we are working to develop novel non-planar small-molecule OSCs for TE applications, and investigating the relationship between their molecular structures and their TE properties.

MH: What aspect of your work are you most excited about at the moment?

DL: I am particularly excited about the curved molecular structures of the butterfly-shaped OSCs, which are obtained by the unexpected rearrangement of diols under strong acid. The polar carbonyl group and non-planar backbones in the butterfly-shaped molecules are commonly undesired for p-type organic semiconductors. Luckily, we have obtained high TE performances before the butterfly-shaped structures are confirmed by single crystal crystallography. This work inspires me to believe that the polar groups and non-planar backbones in OSCs would promote the TE performances of the organic TE composites, which suggests different molecular design criteria other than traditional planar OSCs for organic and hybrid TE materials.

MH: In your opinion, what are the most important questions to be asked/answered in this field of research?

DL: We have witnessed remarkable progress in organic thermoelectrics in recent years, however, challenges still remain. A fundamental understanding of the relationship between OSC structure and TE performance is highly desired for the development of potential organic thermoelectric materials. A set of general guidelines for the molecular design of organic semiconductors, especially for organic/hybrid thermoelectrics, is still a task to be completed. Meanwhile, the development of n-type organic TE materials is still lagging behind due to their intrinsic poor air and operation stabilities. New design strategies for high performance and stable n-type organic semiconductors are valuable. Moreover, more attention should be paid to the mechanisms of doping and charge transport.

MH: What do you find most challenging about your research?

DL: I think that the study on the intrinsic TE properties of small-molecule OSCs is the most challenging issue in my research work. Although we are now studying the TE properties of small-molecule OSCs by complexing with SWCNTs, the study of their intrinsic TE properties can help to better understand the structure–property relationship. However, organic small molecules always suffer from poor film-forming properties, low mechanical properties and electrical conductivity due to their small molecular weight and low degrees of conjugation. The charge transport properties of pristine small-molecule OSCs can only be detected at the micro-scale, where the reliable and high-accuracy measurement of temperature gradients is challenging.

MH: In which upcoming conferences or events may our readers meet you?

DL: I will probably attend the 32nd Chinese Chemical Society Congress from April 19–22, 2021 in Zhuhai, China. I look forward to meeting colleagues working on organic semiconductors and thermoelectrics.

MH: How do you spend your spare time?

DL: I spend most of my spare time with my family. My husband and I enjoy spending time and having adventures with our 4-year-old son. We organize activities every weekend, such as camping, hiking and handicrafts. My child's laughter and outdoor scenery can refresh me from the stress of work.

MH: Can you share one piece of career-related advice or wisdom with other early career scientists?

DL: I would like to suggest that early career scientists establish their own distinctive research, and always follow the guidance of questions from others.

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