Welcome to the Functional Materials Laboratory at the Department of Materials Science and Engineering, National Cheng Kung University (NCKU). We are developing a range of functional materials for various applications, including magnetoelectric multiferroics, white-LED phosphors, solar photovoltaic cells, dielectric energy storage capacitors, and high-Tc superconductors. Headed by Prof. Xiaoding Qi, the research team includes two Ph.D. and nine MSc’s students.
Highlights of Recent Results
High recoverable energy density (Wrec) and efficiency (η) in (1-x)(0.3BiFeO3-0.7SrTiO3)-xK0.5Na0.5NbO3 (BFO-STO-xKNN, x=1.0%) ceramics: https://doi.org/10.1016/j.ceramint.2022.05.240. The samples exhibit good thermal stability of Wrec and η, which vary only 4.0% and 0.8%, respectively, over the temperature range between 25-100°C, ideal for practical application.
The BFO-STO-xKNN ceramics also show good charge/discharge performance, which is fast, in the order of 100s nanoseconds.
Magnetoelectric voltage coefficient (αE) vs. frequency of BFO-NZFO composite films grown on the LNO nanocolumnar buffers, showing very large αE (~3.2 V/Oe•cm) can be achieved in thin-film owing to lack of substrate clamping.
(a) low-resolution cross-sectional TEM, (b) high-resolution TEM at BFO/LNO interface, (c) high-resolution TEM at the region enclosing all three interfaces of NZFO/LNO, BFO/ LNO and NZFO/BFO, (d) magnified image of LNO layer to highlight the columnar nanostructure, inset: SAED pattern.
The architecture of an all-oxide spin valve with the ferroelectric anti-ferromagnet BFO as the pinning layer. The multi-layered heterostructure was grown epitaxially on a (001)STO substrate and magneto-resistance was achieved at room temperature, which was switchable magnetically in a similar way to conventional metallic spin valves.
XPS compositional depth profiling for the ZNFO/BFO interface grown at (a) 600°C, (b) 550°C, and (c) 550°C with the BFO film being annealed in 13000 Pa oxygen for 10 min prior to the deposition of ZNFO, showing a gradual reduction in Bi diffusion from BFO into ZNFO. (d–f) High-resolution TEM images of the ZNFO/BFO interface grown under the same conditions as (a), (b), and (c), respectively. (g–i) M–H hysteresis loops recorded after field annealing for the same three samples as in (d)–(f), respectively.
(a) AFM image, image area 2 × 2 µm2, RMS roughness 2.1 nm, film thickness 300 nm.
(b) PFM image of a thinner sample (100 nm), image area 2.8 × 2.8µm2. Edge areas: virgin surface. Middle white square: scanned with −10 V dc bias. Dark stripes: scanned subsequently with +10 V dc bias.
(c) P–E hysteresis loops of epitaxial BFO films on LNO/STO
Hydrothermal synthesis of pure BiFeO3 with the typical hysteresis loops of ideal ferroelectric.
(a) Surface SEM images of as-grown BiFeO3 thick films on Nb:STO substrates.
(b) Ferroelectric hysteresis loops measured at a frequency of 1.67 kHz and temperature of 300K.
(c) Cross-sectional SEM of as-grown BiFeO3 thick films on Nb:STO substrates.
(d) Cross-sectional SEM BiFeO3 thick film after mechanical polishing and ion milling.
Mo-doped SnO2 thin films for gas sensor applications. SEM of the films sputtered at room temperature from metallic targets of Sn and Mo and then annealed in air at 500 °C.
The ethanol response and recovery curves of SnO2:7%Mo films under different operating temperatures.
The sensitivity of SnO2:7%Mo films in various gases.
(a) Thick film (2 um) of pure beta phase FeSe grown on LaAlO3 by liquid-phase processing at about 900 °C.
(b) EBSD patterns matched with the tetragonal P4/nmm. Inset: EBSD mapping, showing a twin orientation of (101) and (201).
(c) R-T plot of pure beta-FeSe.
Excitation spectra of the two emissions, (a) 490 nm (b) 609 nm. Both can be excited by the same wavelength in the range of 440-480 nm.
PL spectra of La1-xPrxTiNbO6 phosphors, showing two dominant emissions (490nm, 609nm) that can be blended to give out white light for blue LEDs (~447 nm) conversion.
Relative intensities of the two can be tuned by Pr3+ concentration to give out the desired chromaticity. 2% Pr3+ gives out a near pure white light (0.35, 0.32).