Field 1. Next generation entropy-stabilized materials
Our lab has developed various high entropy oxides (HEOx) through combinatorial methodology and solution-based processes to pursue various applications in the fields of electrical, dielectric, and piezoelectric devices. Extra values are also added to this study in parallel by collaborating with two world-renowned labs at National Institute of Standards and Technology (NIST; Dr. Green) in USA and National Institute for Materials Science (NIMS; Dr. Chikyow) in Japan through the development in combinatorial methodology, which is an essential experimental component involved in Materials Genome Initiative (MGI) launched by USA in 2011.
(a) High-entropy high-dielectric-constant (high-k) oxide films
Our lab has fabricated high-k HEO films on Si substrates using combinatorial RF sputtering and solution-based processes to study their dielectric properties for application in advanced metal–oxide–semiconductor (MOS) gate stacks. The dielectric properties are characterized through capacitance–voltage (C–V) and current–voltage (I–V) characteristics. Their thermal stability is also studied through a rapid thermal anneal at 900°C.
[Representative ref.]: Z.-W. Huang and K.-S. Chang*, “Spin-Coating Synthesis of High-entropy High-k (Al,Ti,V,Zr,Hf)Ox Films on Si for Advanced Gate Stacks,” Ceramics International 47, 22558 (2021).
(b) High-entropy piezoelectric films
We have also developed a facile spin-coating and hydrothermal approach to fabricate high-entropy piezoelectric films and investigated the influence of their morphology, film orientation, and crystallinity on the resulting piezoelectricity to gain insight into the optimization of piezoelectric devices.
[Representative ref.]: Y.-W. Chen, J.-J. Ruan, J.-M. Ting, Y.-H. Su, and K.-S. Chang*, “Solution-based fabrication of high-entropy Ba(Ti,Hf,Zr,Fe,Sn)O3 films on fluorine-doped tin oxide substrates and their piezoelectric responses,” Ceramics International 47, 11451 (2021).
Field 2. Photocatalytic disinfection
We have collaborated with Doctor of Dental Surgery (DDS) Chen at NCKU Hospital and Director Prof. Lin in Department of Environmental and Occupational Health at NCKU to develop novel photocatalysts for disinfection of microorganisms. Our goal is to explore novel visible-light photocatalysts through junction and surface modification approaches. Resulting nanocomposites are expected to distribute uniformly and actively in designated area to destroy pathogens. Both air and surface samples are collected to verify the disinfection capability of innovative photocatalytic nanoparticles/films.
Field 3. Piezophotocatalysis
Piezophotocatalysis features the simultaneous coupling of four distinctive characteristics: semiconduction, piezoelectricity, photoexcitation, and photocatalysis. The concept differs from that of piezocatalysis and sonocatalysis, in which no illumination and no piezotronic effect are respectively considered. High-piezoelectric-coefficient materials exhibiting excellent chemical stability are ideal for application in piezophotocatalysis.
[Representative ref.]: Thi Nghi Nhan Nguyen and K.-S. Chang*, “Piezoelectric and Piezophotocatalytic Study of Hydrothermally Grown BiFeO3 Films with Various Morphologies on Fluorine-Doped Tin Oxide Substrates,” Journal of Environmental Chemical Engineering (accepted).
Field 4. High-k material enhanced photocatalysis
A novel application of high-k materials as a photocatalyst was first proposed by our group. Nanocomposites of (high-k)–TiO2 nanorod arrays were fabricated using reactive sputtering. The idea is to use defects in high-k materials to trap charge carriers from TiO2 to minimize recombination of electron-hole pairs for achieving synergistic photocatalysis.
[Representative ref.]: N. V. B. Rosell III, Y.-T. Chen, and K.-S. Chang*, “Defective Y2O3-x–driven Anomalous Photocatalytic Enhancement Using Y2O3-x–TiO2-x Nanorod Composite Composition Spreads,” Journal of The American Ceramic Society 100, 5548 (2017).
Field 5. Fabrication of nano-structured thin films
(a) Solution based process
Facile polymerized complex reactions together with a hydrothermal reaction were implemented to make single crystalline TiO2 nanorods for the first time. Chromium (Cr) and nitrogen (N2) co-doping was performed to tailor the physical properties. An investigation of the photocatalytic properties exhibited high efficiency of photodegradation of methylene blue (MB) in 15 minutes under pH = 10, using a nanocomposite of (7 % Cr, 0.0021 % N) codoped and 3 % Cr doped TiO2 nanorods.
[Representative ref.]: W.-C. Lu, H.-D. Nguyen, C.-Y. Wu, K.-S. Chang*, and M. Yoshimura “Modulation of Physical And Photocatalytic Properties of (Cr,N) Codoped TiO2 Nanorods Using Soft Solution Processing,” J. of Applied Physics 115, 144305 (2014).
(b) Sputtering process
A reactive sputtering was directly applied to grow TiO2 nanorods, without any exceptional treatment, applied in photocatalysis for the first time. Three critical measures were considered to enhance photocatalysis of TiO2 nanorods, including morphology, band gap tuning, and chemical environment.
[Representative ref.]: Z.-A. Lin, W.-C. Lu, C.-Y. Wu and K.-S. Chang*, “Facile fabrication and tuning of a TiO2 nanoarchitectured morphology using magnetron sputtering and its application to photocatalysis,” Ceramics International 40, 15523 (2014).
Another example was ZnSnN2 (ZTN) fabricated through natural Sn3N4 and Zn thickness gradients deposited oppositely on a fluorine-doped tin oxide substrate to form Zn–Sn3N4 composition spreads to enhance the relative variation of the cation ratios and to promote the formation of orthorhombic ZTN.
[Representative ref.]: A.-J. Hsu and K.-S. Chang*, “Physical, photochemical, and extended piezoelectric studies of orthorhombic ZnSnN2 nanocolumn arrays,” Applied Surface Science 470, 19 (2019).
Field 6. Development of Combinatorial Methodology at CTRL
Our lab has developed physical and chemical combinatorial methodology at NCKU, which is a high throughput fabrication system for manufacturing new functional materials in thin film forms. One of the invaluable characteristics of this technology is that a variety of different thin films (“libraries”) can be made on a single chip at one time. Conventional approaches through the one-by-one trial-and-error method are too slow and out of date for modern advanced technologies, where stringent demands for performance are made for new materials in a variety of technological fields. Thus, the high throughput fabrication system provides the right platform to address this problem.
[Representative ref.]: H.-Y. Lee, S.-Y. Lee, and K.-S. Chang*, “Enhancement of Piezo-Related Properties of AlN Through Combinatorial AlN-TiN Nanocolumn Composite Composition Spread,” Ceramics International 45, 22744 (2019).
[Representative ref.]: W.-C. Lu, L.-C. Tseng, and K.-S. Chang*, “Fabrication of TiO2–Reduced Graphene Oxide Nanorod Composition Spreads Using Combinatorial Hydrothermal Synthesis and Their Photocatalytic and Photoelectrochemical Applications,” ACS Combinatorial Science 19, 585 (2017). (Journal cover story).
Field 7. Advanced Gate Stacks
High-throughput sputtering was used to fabricate high-quality, amorphous, thin HfO2-TiO2 and N2-doped HfO2-TiO2 (HfON-TiON) gate dielectric libraries. A k value of approximately 54, a leakage current density < 10-6 A/cm2, and an equivalent oxide thickness of approximately 1 nm were identified in an HfON-TiON library within a composition range of 68–80 at% Ti. This library exhibits promise for application in highly advanced metal-oxide-semiconductor (higher-k) gate stacks.
[Representative ref.]: K.-S. Chang*, W.-C. Lu, C.-Y. Wu, and H.-C. Feng, “High-throughput identification of higher-k dielectrics from an amorphous N2-doped HfO2-TiO2 library,” J. Alloys and Compounds 615, 386 (2014).