(7dz) Wearable/Implantable Ultrathin Electronic/Optoelectronic Devices with Engineered Semiconductor Nanocrystals

The pervasiveness of electronic devices in our everyday life is increasing, heading towards a ubiquitous environment involving cutting-edge technology in fields of artificial intelligence, the “Internet of Things,” autonomous vehicles, etc. In particular, wireless communication between the human body and objects is an emerging multi-disciplinary field that finds numerous applications from many diverse research areas. Merging electronic devices with our human body can be realized through the development of wearable/implantable ultrathin electronic devices which conform to the soft human tissues. It is expected that the wearable/implantable technology will lead to electronic skin displays, real-time health monitoring systems, brain-computer interfaces, and many others. Colloidal semiconductor nanocrystals have drawn considerable interest as attractive building blocks for flexible and printable electronic/optoelectronic devices. A critical barrier to commercialization, however, is the toxicity associated with usage of heavy metals (e.g. Pb or Cd), which makes RoHS compliant nanocrystals (e.g. CISeS, CZTS, Si, Ge etc.) particularly attractive for potentially biocompatible electronic devices. Hence, understanding the charge carrier transport characteristics of non-toxic semiconductor nanocrystals thin films and their practical application are crucial to their implementation in wearable/implantable electronic/optoelectronic technologies.

I have previously demonstrated the first switchable-logic devices (CMOS inverter, and NAND or NOR gate devices) using engineered non-toxic colloidal semiconductor nanocrystals. To expand on this, starting from the functionalized nanocrystals, I seek to develop ultrathin printable logic-gate and optoelectronic devices on flexible substrate for achieving wearable/implantable electronic devices. This research will involve: 1) synthesis of solution-processed non-toxic semiconductor nanocrystals; 2) exploration of suitable surface ligands for biocompatible electronic devices; and 3) design and fabrication of ultrathin logic-gate and optoelectronic device architectures. In particular, engineering the surface ligands of tailored nanocrystals strongly influences the carrier transport polarity (unipolar p- or n-type, or ambipolar) of electronic devices, which are basic characteristic for fabricating efficient logic-gate devices and other devices such as light-emitting field-effect transistors (FETs). These devices are highly applicable to wearable/implantable ultrathin electronic gadgets. Therefore, understanding the ligand chemistry for non-toxic semiconductor nanocrystals will be crucial for fabricating high-performance electronic/optoelectronic devices. Recently, inorganic ligands such as halide, chalcogenide and metal-chalcogenide complex ions have been extensively used to produce strong electronic coupling between nanocrystals, and tune the transport polarity. My research group will employ various classes of atomic inorganic ligands to non-toxic semiconductor nanocrystals for achieving high performance logic-gate and opto-electronic devices on the ultrathin substrates. The versatile tunability of transport polarity of nanocrystal film with applying various inorganic ligands is expected to enable inexpensive and high-throughput solution-based processing of ultrathin electronic/optoelectronic devices in ambient environment. In particular, this significant simplification of integration processes implies the feasibility of high profile fabrication of various device geometries.

In order to pursue such ambitious lines of research, significant financial resources will be needed. All funding opportunities available to me will be pursued, starting with National Science Foundation (NSF) fellowship applications for all grad students with a reasonable chance of success. In the beginning phases, I will also apply for early career grants such as the ACS PRF Doctoral New Investigator Grant and the NSF Early Career Award. Furthermore, national research foundation of South Korea (my home country) opens the chance to make international collaboration with exceptional research groups. I will tackle the main funding agencies with my previous collaborator in Seoul National University, KAIST, Samsung, and so on.

Research Experiences:

My research focus has been on the synthesis of semiconductor nanomaterials and characterization and control of carrier interfacial transfer and mesoscale transport behavior. My PhD studies at Seoul National University involved the development of semiconductor nanomaterials to enhance both the photo-generation of electrons and the charge transfer kinetics for solar energy conversion. I developed various impurity-doped TiO₂ photocatalysts and designed a biomimetic Z-scheme photocatalyst by combining two visible light responsive photocatalysts. At University of Pennsylvania, I focused on designing and demonstrating new variants on CdSe nanocrystal quantum-dot-sensitized liquid junction solar cells. I first applied atomic scale inorganic ligand exchange techniques to fabricate a high performance photoanode to enhance the rate of charge transfer. I am currently conducting research on the characterization of carrier transport in quantum dot thin films in Los Alamos National Laboratory. Specifically, I fabricate field-effect transistors (FETs) and photoresponsive switches for investigating the carrier dynamics of various semiconductor nanocrystals. Very recently, I successfully fabricated the first environmentally-friendly CMOS inverter, NAND and NOR gate logic devices by combining p- and n-channel non-toxic quantum dot FETs, whose polarities are well-tuned by controlling their intrinsic defect states.

Teaching Interests:

As a teaching assistant, I helped students at undergraduate courses of Chemical Engineering Experiments and Transport Phenomena in both Ajou University and Seoul National University. In addition, I mentored undergraduate and graduate students in research when I was senior PhD graduate students and post-doctoral associate. I was also invited to give research talks several universities and high school. Based on my research career, I can give lectures related semiconductor science and technology, inorganic chemistry, renewable energy, surface science and engineering, and so on.

I have three important key words as mentoring philosophy. 1) Socratic elenchus. As mentor, I lead a mentee to find their own answer by themselves. For this great endeavor, I asked and got answers to stimulate critical thinking. 2) Open mind. In research society, somebody was really unwilling to show their results because of a fear of being scooped. But I emphasized we are studying to make better world, not to get a fund or good papers. Open mind always makes scientific revolution in various research fields. 3) Beyond the ivory tower. As an engineer, I lead mentee to link our knowledge from text book and our real life. This kind of linkage made the learner feel fun of studying and motivate their creativity.

Postdoctoral Project:

“Transport Behavior of Non-Toxic CuInSe_xS_2-x Quantum Dot Films”

Under the supervision of Victor I. Klimov, Division of Chemistry, Physical Chemistry and Applied Spectroscopy

“Effective Conversion of Solar Light Using A Variety of Semiconductor and Metal Nanocrystals”

Under the supervision of Christopher B. Murray, Department of Chemistry, University of Pennsylvania

PhD Dissertation: “Preparation and Characterization of TiO₂-Based Photocatalytic Materials for the Solar Light Utilization”

Under the supervision of Jongheop Yi, School of Chemical and Biological Engineering, Seoul National University

Successful Proposals: Postdoctoral fellowship from the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education

Selected Publications:

• H. J. Yun, J. Lim, S. Keene, A. Fuhr, M. Law, J. M. Pietryga, and V. I. Klimov, “Defect Control in CuInSe_xS_2-x Quantum Dot Films for Tuning Their Carrier Transport Behavior”, In Preparation
• Q. Lin^†, H. J. Yun^†, W. Liu, H.-J. Song, N. Makarov, O. Isaienko, T. Nakotte, G. Chen, H. Luo, V. I. Klimov, and J. Pietryga, “Phase-Transfer Ligand Exchange of Lead Chalcogenide Quantum Dots for Direct Deposition of Thick, Highly Conductive Films”, J. Am. Chem. Soc. 2017, 139, 6644. (^†Qianglu Lin and Hyeong Jin Yun contributed equally to this work.)
• H. J. Yun, T. Paik, M. Edley, J. Baxter, and C. B. Murray, “Size-Dependent Efficiency of Quantum Dot Sensitized Solar Cell in Strongly-Coupled CdSe Nanocrystals/TiO₂ System”, ACS Appl. Mater. Interfaces 2016, 8, 14692.
• H. J. Yun, T. Paik, M. Edley, J. Baxter, and C. B. Murray, “Enhanced Charge Transfer Kinetics of CdSe Quantum Dot Sensitized Solar Cell by Inorganic Ligand Exchange Treatments”, ACS Appl. Mater. Interfaces 2014, 6, 3721.
• H. J. Yun, H. Lee, N. D. Kim, D. M. Lee, S. Yu, and J. Yi, “A Combination of Two Visible-Light Responsive Photocatalysts for Achieving the Z-Scheme in the Solid State”, ACS Nano 2011, 5, 4084.
• H. J. Yun, H. Lee, J. B. Joo, N. D. Kim, M. Y. Kang, and J. Yi, “Facile Preparation of High Performance Visible Light Sensitive Photo-Catalysts”, Appl. Catal. B 2010, 94(3-4), 241.