Browsing by Author "Pi, Tun-Wen"

Now showing 1 - 6 of 6

Canted Antiferromagnetism on Rectangular Layers of Fe2+ in Polymorphic CaFeSeO
(2017-04-17)
Lai, Kwing To
;
Komarek, Alexander Christoph
;
Fernandez-Diaz, Maria Teresa
;
Chang, Pi-Shan
;
Huh, Sungjoon
From stoichiometric amounts of CaO, Fe, and Se, pure powders and single crystals of quaternary Ca-infinity(2)[FeSe2/2O2/2] can be obtained by solid-state reaction and self-flux growth, respectively. The as-synthesized compound exhibits a polymorphic crystal structure, where the two modifications have different stacking sequences of (2)(infinity)[FeSe2/2O2/2](2-) layers. The two polymorphs have similar unit cells but different crystal symmetries (Cmc2(1), and Pnma), of which the former is non-centrosymmetric. Fe is divalent (d(6)) and high-spin, as proven by X-ray spectroscopy, Mossbauer spectroscopy, and powder neutron diffraction data. The latter two, in combination with magnetic susceptibility and specific heat data, reveal a long-range antiferromagnetic spin order (T-N = 160 K) with a minor spin canting. CaFeSeO is an electronic insulator, as confirmed by resistivity measurements and density functional theory calculations. The latter also suggest a relatively small energy difference between the two polymorphs, explaining their intimate intergrowth.
Cesium doping at C-60/rubrene heterointerfaces for improving the performance of organic light- and current-generating devices
(2015-01-07)
Cheng, Chiu-Ping
;
Lu, Meng-Han
;
Chu, Yu-Ya
;
Wei, Ching-Hsuan
;
Pi, Tun-Wen
This study examined the effect of adding cesium (Cs) at C-60/rubrene heterointerfaces by using synchrotron-radiation photoelectron spectroscopy. A C-60/rubrene heterostructure is the basis of a novel organic dual device found to facilitate efficient integration of both electroluminescent and photovoltaic functions. With Cs doping, the interfacial dipole potential was considerably enlarged, as was the separation between the lowest unoccupied molecular orbital of C-60 and the highest occupied molecular orbital of rubrene. However, the energy-level diagram indicated that a high Cs concentration prevents the formation of photoexcitons. By contrast, adding a small amount of Cs can effectively improve the efficiency of light-and current-generating devices. In particular, the deficiency of the dopants at the heterointerface may benefit the survival of photoexcitons. (C) 2015 AIP Publishing LLC.
Electrically enhanced magnetization in highly strained BiFeO3 films
(2016-05)
Yang, Jan-Chi
;
Kuo, Chang-Yang
;
Liu, Heng-Jui
;
Ding, Hang-Chen
;
Duan, Chun-Gang
The control of magnetism via an electric field has attracted substantial attention because of potential applications in magnetoelectronics, spintronics and high-frequency devices. In this study, we demonstrate a new approach to enhance and control the magnetization of multiferroic thin film by an electric stimulus. First, to reduce the strength of the antiferromagnetic superexchange interaction in BiFeO3, we applied strain engineering to stabilize a highly strained phase. Second, the direction of the ferroelectric polarization was controlled by an electric field to enhance the Dzyaloshinskii-Moriya interaction in the highly strained BiFeO3 phase. Because of the magnetoelectric coupling in BiFeO3, a strong correlation between the modulated ferroelectricity and enhanced magnetization was observed. The tunability of this strong correlation by an electric field provides an intriguing route to control ferromagnetism in a single-phase multiferroic.
Interfacial electronic structure of Na deposited on rubrene thin film studied by synchrotron radiation photoemission
(2015-12-01)
Wei, Ching-Hsuan
;
Cheng, Chiu-Ping
;
Lin, Hong-Cheu
;
Pi, Tun-Wen
;
材料科學與工程學系
The electronic structure of rubrene doped with various concentrations of Na was studied by synchrotronradiation photoemission. Three stages of development were found with increasing Na concentration; Na penetrating deep into the organic film, followed by development of gap states, and ended with a metallic Na film. The charge transfer from Na to rubrene resulted in a vacuum-level shift. By doping Na into rubrene, we could control the IP of the organic molecule, which is favorable for application in organic semiconductor devices. (C) 2015 Elsevier B.V. All rights reserved.
Synthesis and Characterization of Ba[CoSO]: Magnetic Complexity in the Presence of Chalcogen Ordering
(2015-07-20)
Valldor, Martin
;
Roessler, Ulrich K.
;
Prots, Yurii
;
Kuo, Chang-Yang
;
Chiang, Jen-Che
Barium thio-oxocobaltate(II), Ba[CoS2/2O2/2], was synthesized by the reaction of equimolar amounts of BaO, Co, and S in closed silica ampoules. The title compound (Cmcm, a=3.98808(3), b=12.75518(9), c=6.10697(4)angstrom) is isostructural to Ba[ZnSO]. The use of soft X-ray absorption spectroscopy confirmed that cobalt is in the oxidation state +2 and tetrahedrally coordinated. Its coordination consists of two sulfur and two oxygen atoms in an ordered fashion. High-temperature magnetic susceptibility data indicate strong low-dimensional spin-spin interactions, which are suggested to be closely related to the layer-type crystal structure and perhaps the ordered distribution of sulfur and oxygen. Antiferromagnetic ordering below T-N=222K is observed as an anomaly in the specific heat, coinciding with a significant lowering of the magnetic susceptibility. Density functional theory calculations within a generalized-gradient approximation (GGA)+U approach identify an antiferromagnetic ground state within the square-like two-dimensional layers of Co, and antiferromagnetic correlations for nearest and next nearest neighbors along bonds mediated by oxygen or sulfur. However, this magnetic state is subject to frustration by relatively strong interlayer couplings.
The effect of magnesium added at C-60/Rubrene heterointerfaces
(2013-12-28)
Cheng, Chiu-Ping
;
Lee, Cheng-Wei
;
Chu, Yu-Ya
;
Wei, Ching-Hsuan
;
Pi, Tun-Wen
This study examines the effect of adding magnesium (Mg) at Codrubrene heterointerfaces by using synchrotron-radiation photoetnission spectroscopy. The heterointerface was obtained by depositing C-60 on a 4-angstrom Mg/rubrene surface. The photoemission spectra showed that the added Mg preferentially interacts with and transfers negative charges to C-60. The interfacial dipole potential was significantly enlarged, as was the separation between the lowest unoccupied molecular orbital of C-60 and the highest occupied molecular orbital of rubrene. The results demonstrate that the addition of Mg should be an effective method for improving the efficiency of light- and current-generating devices. (C) 2013 AlP Publishing LLC.