Materials Horizons Emerging Investigator Series: Professor Derek Ho, City University of Hong Kong, China

Derek Ho is currently an associate professor at the Department of Materials Science and Engineering at City University of Hong Kong, where he leads the Atoms-to-Systems Laboratory (ATS Lab, https://www.atomstosystems.com). He is also cross appointed to the Department of Materials Science and Engineering at Yonsei University, Seoul, South Korea. Derek is a founding member of the Hong Kong Centre for Cerebro-Cardiovascular Health Engineering, where he is a co-principal investigator.

He received his BASc with first-class honours and MASc from the University of British Columbia, and his PhD in 2013 from the University of Toronto, all in electrical and computer engineering. His research interest is in materials and devices for wearable electronics, with emphasis on sensing and energy storage applications. Over the years, Derek's group has made notable advances in multi-length scale architectures for high-precision and wide-range physical, chemical and bio sensors. In the energy storage area, his group has also developed a method of combining both pseudo-capacitance and redox capabilities into a single electrode, which led to a new device, the micro-redoxcapacitor.

Aside from scientific research, Derek is also passionate about pedagogical innovation. He was the recipient of the prestigious Teaching and Learning Grant from the Hong Kong University Grants Committee for “transforming science and engineering talents into Technopreneurs”, where he led a network of educators across universities internationally in improving teaching and learning outcomes in engineering entrepreneurship education.

Read Derek Ho's Emerging Investigator Series article ‘Order–disorder engineering of RuO ₂ nanosheets towards pH-universal oxygen evolution’ ( https://doi.org/10.1039/D3MH00339F ) and read more about him in the interview below:

MH: Your recent Materials Horizons Communication presents an order–disorder optimisation strategy as a way to enhance electrocatalyst performance. How has the direction of your research evolved and where do you see your own research going into the future?

DH: I have always been fascinated by electronics. My main research interests are in electronic materials and devices, emphasizing two application areas: sensing as well as energy storage and conversion. My group and I build advanced electronic devices through optimizing a materials structure–property relationship and device architecture, for example, synthesizing new crystallographic phases,¹ studying crystal growth and nucleation,² engineering the interlayer space in 2D materials,³ molecular design,⁴ building hierarchical structures across multi length-scales,⁵ and creating new device architectures.⁶

In our effort, it dawned on us that there is a recurring theme, especially true for sensors and electrocatalysts, which is the idea of balancing order against disorder. The concept of order refers to the degree and spatial distribution of crystallinity. In many electronic materials, disorder is purposefully introduced through engineered defects, grain boundaries, and interfaces. Disorder sounds like something bad, so you may ask “why disorder”? Because the defect sites are where the reactions or functionalities occur. But having too many defects reduces signal strength and integrity. The critical role of this balance is especially evident in the catalytic material that we recently published here in Materials Horizons. Therefore, my student and I decided to call it as it is, hence the current title of the paper.

From here on, our group is moving towards ionic materials and devices. While mankind has a relatively established means to investigate and control electron transport in materials, which has given rise to modern electronics, our control of ionic flow as a signal or energy carrier in a solid, is much less understood and has been mainly confined to the fields of electrolytes and membranes. Since many processes in biology are fundamentally ionic and we are increasingly dependent on medical technologies, I believe the future will have plenty of applications where ionic devices (also known as iontronics) can serve us well, working alongside or combined with electronics in hybrid systems.

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

DH: We are most excited with piezoionics at the moment, which is an example of the ionic system mentioned above. A piezoionic device provides transduction from mechanical to electrical signal or energy, and vice versa, which has wide applications in biosensing, soft robotics, and energy harvesting. It works like this: upon applying an external stress, a pressure gradient within a (hydrogel) material is generated, which produces a corresponding fluidic flow that carries the ionic species within the material. Simultaneously, the polymer matrix exerts a hydrodynamic drag force on the ionic species. The difference in mobility of anionic and cationic species leads to their separation over time. The separation produces a measurable ionic voltage or current.

Early piezoionic devices have been implemented using hydrogels, where the difference in mobility of anions and cations is a main design focus. The greater the difference, the greater the resultant output signal. But, there are many open questions, for example, how ionic transport is affected by various parameters such as ion size, ion charge, ion concentration, and the interaction between the ions and the polymeric matrix.

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

DH: Today's stretchable electronics, i.e., functional devices that are made mainly from soft materials, to put it bluntly, they don’t really work well. It's incredibly hard to make complex electronic devices largely out of polymeric materials due to two main reasons:

The first major challenge is a lack of a controllable fabrication process. Synthesizing (ionic) polymeric materials using wet chemistry is still currently the mainstream method. However, compared to top-down fabrication techniques used in conventional semiconductor processing, the variability of wet chemistry as a method of fabrication is still very wide. This leads to a large variation in device performance. A significant portion of the fabricated devices may not fall within performance targets, leading to a low yield.

Then, there is the problem with lack of compatible components. While sensors (and actuators) are relatively easy to make out of soft materials, making equally soft components to perform signal processing and (wireless) data communication is not easy due to their complexity. But for a new technology to really take off, it must be supported by an ecosystem of compatible components to implement the rest of the signal processing chain. Now, is a system made of both soft and rigid components viable? I’d say the interface between the domains often suffers from mechanical and signal integrity issues, therefore the hybrid approach isn’t really a long-term solution.

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

DH: The leadership aspect of running a lab, coaching lab members to be the next generation of researchers, can be as challenging as the scientific aspects. To give just one of many examples, while a PhD thesis should primarily be the student's endeavour, as an advisor, I am responsible for guiding the definition of the thesis topic and coaching the research effort. Designing a great thesis topic is not easy: it has to be an effective vehicle to advance science fundamentally, so that the resultant impact is broad enough; the work needs to be executed well in our lab or with the help of collaborators; and most importantly, a reasonably significant milestone should be achieved within the PhD study period so the student can proceed well to the next stage of their career. Fortunately, and credit goes to my dedicated students, a good number of them, including the main authors of the papers I cite above, have themselves become independent academics.

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

DH: I hope to regain my routine of physically attending either the Materials Research Society (MRS) Spring or Fall Meeting annually. But if there are any interesting ideas we could explore together, why wait? Drop me an email to bounce some ideas around or come pay us a visit, this has worked out numerous times resulting in great opportunities to kick-start international collaborations.

MH: How do you spend your spare time?

DH: Playing with my 5-year-old daughter, Kaitlyn, and my 10-year-old son, Aiden. Being a dad makes me a better coach at work and being a coach makes me a better dad. The difference in technical and maturity levels provide a great range of interactions. But the two roles are quite similar in that it's all about bringing the best out of each individual.

I also enjoy sports. Since I couldn’t bring the Canadian snow with me as I relocated to Hong Kong, I found tennis. Or maybe tennis found me! It is strangely liberating to hit a few hundred ground strokes at my weekly training with my coach.

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

DH: In many ways, the journey to discovery is analogous to how an artist goes about art: creative and ambiguous, exercising their full faculties in dialogue with a community. Therefore, I’d encourage young scientists to adopt an “open door” policy, which has two aspects:

A research career is taxing on surprisingly many aspects of intelligence, not only on technical abilities. Therefore, it is important to be open to develop holistically by exploring ways to be effective in working with others, communicating thought, and benefiting from feedback which takes both humility and self-confidence. If you persevere in the above practice, over time, you gain the perspective and maturity necessary to filter signal from noise.

Secondly, knowledge is surprisingly interconnected and evolutionary. Therefore, throughout our career, we should go after what we are passionate about. Don’t be afraid to expand or even pivot into new areas. While it seems that a good number of researchers evolve their careers from the fundamental disciplines (e.g., physics or chemistry) to more application-orientated areas, my journey has been the opposite. When I was in grad school, I worked on microsystems for chemical and biological sensing. Back then, I was mainly concerned with putting together systems that integrated nanomaterials, optics, and microelectronics to achieve the highest performance possible. I realized that the sensory system's performance can be no better than that of the front-end signal transduction component, which in turn fundamentally depends on the material properties. This got me thoroughly convinced that materials innovation is the key, therefore my lab is now centred on materials research.

References

1. B. Tian, L. Sun and D. Ho, Hexagonal Co₉S₈: Experimental and mechanistic study of enhanced electrocatalytic hydrogen evolution of a new crystallographic phase, Adv. Funct. Mater., 2023, 33, 2210298.
2. J. Xu, D. J. Srolovitz and D. Ho, The adatom concentration profile: a paradigm for understanding two-dimensional MoS₂ morphological evolution in chemical vapor deposition growth, ACS Nano, 2021, 15(4), 6839–6848.
3. W. Cheng, J. Fu, H. Hu and D. Ho, Interlayer structure engineering of MXene-based capacitor-type electrode for hybrid micro-supercapacitor towards battery-level energy density, Adv. Sci., 2021, 8, 2100775.
4. H. Zhang, P. Xiao, Y. T. Wong, W. Shen, M. Chhabra, R. Peltier, Y. Jiang, Y. He, J. He, Y. Tan, Y. Xie, D. Ho, Y. W. Lam, J. Sun and H. Sun, Construction of an alkaline phosphatase-specific two-photon probe and its imaging application in living cells and tissues, Biomaterials, 2017, 140, 220–229.
5. Q. Tian, W. Yan, Y. Li and D. Ho, Bean pod-inspired ultra-sensitive and self-healing pressure sensor based on laser induced graphene and polystyrene microspheres sandwiched structure, ACS Appl. Mater. Interfaces, 2020, 12(8), 9710–9717.
6. Z. Cao, H. Hu and D. Ho, Micro-redoxcapacitor: A hybrid architecture out of the notorious energy-power density dilemma, Adv. Funct. Mater., 2022, 32(19), 2111805.

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