Browsing by Author "Yang, Chao-Yao"

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A facile green antisolvent approach to Cu2+-doped ZnO nanocrystals with visible-light-responsive photoactivities
(2014-08-07)
Lu, Yi-Hsuan
;
Lin, Wei-Hao
;
Yang, Chao-Yao
;
Chiu, Yi-Hsuan
;
Pu, Ying-Chih
An environmentally benign antisolvent method has been developed to prepare Cu2+-doped ZnO nanocrystals with controllable dopant concentrations. A room temperature ionic liquid, known as a deep eutectic solvent (DES), was used as the solvent to dissolve ZnO powders. Upon the introduction of the ZnO-containing DES into a bad solvent which shows no solvation to ZnO, ZnO was precipitated and grown due to the dramatic decrease of solubility. By adding Cu2+ ions to the bad solvent, the growth of ZnO from the antisolvent process was accompanied by Cu2+ introduction, resulting in the formation of Cu2+-doped ZnO nanocrystals. The as-prepared Cu2+-doped ZnO showed an additional absorption band in the visible range (400-800 nm), which conduced to an improvement in the overall photon harvesting efficiency. Time-resolved photoluminescence spectra, together with the photovoltage information, suggested that the doped Cu2+ may otherwise trap photoexcited electrons during the charge transfer process, inevitably depressing the photoconversion efficiency. The photoactivity of Cu2+-doped ZnO nanocrystals for photoelectrochemical water oxidation was effectively enhanced in the visible region, which achieved the highest at 2.0 at% of Cu2+. A further increase in the Cu2+ concentration however led to a decrease in the photocatalytic performance, which was ascribed to the significant carrier trapping caused by the increased states given by excessive Cu2+. The photocurrent action spectra illustrated that the enhanced photoactivity of the Cu2+-doped ZnO nanocrystals was mainly due to the improved visible photon harvesting achieved by Cu2+ doping. These results may facilitate the use of transition metal ion-doped ZnO in other photoconversion applications, such as ZnO based dye-sensitized solar cells and magnetism-assisted photocatalytic systems.
A facile green antisolvent approach to Cu2+-doped ZnO nanocrystals with visible-light-responsive photoactivities
(2014-08-07)
Lu, Yi-Hsuan
;
Lin, Wei-Hao
;
Yang, Chao-Yao
;
Chiu, Yi-Hsuan
;
Pu, Ying-Chih
An environmentally benign antisolvent method has been developed to prepare Cu2+-doped ZnO nanocrystals with controllable dopant concentrations. A room temperature ionic liquid, known as a deep eutectic solvent (DES), was used as the solvent to dissolve ZnO powders. Upon the introduction of the ZnO-containing DES into a bad solvent which shows no solvation to ZnO, ZnO was precipitated and grown due to the dramatic decrease of solubility. By adding Cu2+ ions to the bad solvent, the growth of ZnO from the antisolvent process was accompanied by Cu2+ introduction, resulting in the formation of Cu2+-doped ZnO nanocrystals. The as-prepared Cu2+-doped ZnO showed an additional absorption band in the visible range (400-800 nm), which conduced to an improvement in the overall photon harvesting efficiency. Time-resolved photoluminescence spectra, together with the photovoltage information, suggested that the doped Cu2+ may otherwise trap photoexcited electrons during the charge transfer process, inevitably depressing the photoconversion efficiency. The photoactivity of Cu2+-doped ZnO nanocrystals for photoelectrochemical water oxidation was effectively enhanced in the visible region, which achieved the highest at 2.0 at% of Cu2+. A further increase in the Cu2+ concentration however led to a decrease in the photocatalytic performance, which was ascribed to the significant carrier trapping caused by the increased states given by excessive Cu2+. The photocurrent action spectra illustrated that the enhanced photoactivity of the Cu2+-doped ZnO nanocrystals was mainly due to the improved visible photon harvesting achieved by Cu2+ doping. These results may facilitate the use of transition metal ion-doped ZnO in other photoconversion applications, such as ZnO based dye-sensitized solar cells and magnetism-assisted photocatalytic systems.
Competing Anisotropy-Tunneling Correlation of the CoFeB/MgO Perpendicular Magnetic Tunnel Junction: An Electronic Approach
(2015-11-24)
Yang, Chao-Yao
;
Chang, Shu-Jui
;
Lee, Min-Han
;
Shen, Kuei-Hung
;
Yang, Shan-Yi
We intensively investigate the physical principles regulating the tunneling magneto-resistance (TMR) and perpendicular magnetic anisotropy (PMA) of the CoFeB/MgO magnetic tunnel junction (MTJ) by means of angle-resolved x-ray magnetic spectroscopy. The angle-resolved capability was easily achieved, and it provided greater sensitivity to symmetry-related d-band occupation compared to traditional x-ray spectroscopy. This added degree of freedom successfully solved the unclear mechanism of this MTJ system renowned for controllable PMA and excellent TMR. As a surprising discovery, these two physical characteristics interact in a competing manner because of opposite band-filling preference in space-correlated symmetry of the 3d-orbital. An overlooked but harmful superparamagnetic phase resulting from magnetic inhomogeneity was also observed. This important finding reveals that simultaneously achieving fast switching and a high tunneling efficiency at an ultimate level is improbable for this MTJ system owing to its fundamental limit in physics. We suggest that the development of independent TMR and PMA mechanisms is critical towards a complementary relationship between the two physical characteristics, as well as the realization of superior performance, of this perpendicular MTJ. Furthermore, this study provides an easy approach to evaluate the futurity of any emerging spintronic candidates by electronically examining the relationship between their magnetic anisotropy and transport.
Complex magnetic interactions and charge transfer effects in highly ordered NixFe1-x nano-wires
(2013-04-01)
Chang, Shu-Jui
;
Yang, Chao-Yao
;
Ma, Hao-Chung
;
Tseng, Yuan-Chieh
;
材料科學與工程學系
This work investigates the subtle magnetic interactions within the NixFe1-x (x=0.3, 0.5, and 1.0) nano-wires by probing spin-dependent behaviors of the two constituted elements. The wires were fabricated by electro-deposition and an anode aluminum oxide template to produce free-standing nature, and the Ni-Fe interactions were probed by x-ray magnetic spectroscopy across a BCC -> FCC structural transition. The wires' magneto-structural properties were predominated by Ni, as reflected by a decrease but an increase in total magnetization and FCC x-ray intensity with increasing x, even if the Fe moment increased simultaneously. Upon annealing, a prominent charge transfer, together with the changes of spin-dependent states, took place in the Ni and Fe 3d orbitals, and a structural disordering was also obtained, for the wires at x=0.3. The charge transfer led to a local magnetic-compensation for the two elements, explaining the minor change in total magnetization for x=0.3 probed by a vibrational sample magnetometer. When x was increased to 0.5, however, the charge transfer became inactive due to persistent structural stability supported by Ni, albeit resulting in nearly invariant magnetization similar to that of x=0.3. The complexity of the Ni-Fe interactions varied with the composition and involved the modifications of the coupled magnetic, electronic and structural degrees of freedom. The study identifies the roles of Ni and Fe as unequally-influential in NixFe1-x, which provides opportunities to re-investigate the compound's properties concerning its technological applications. (C) 2012 Elsevier B.V. All rights reserved.
Coupled microstructural and magnetic transition in Co-doped Ni nano-arrays
(2011-10-01)
Yang, Chao-Yao
;
Huang, Chun-Chao
;
Tseng, Yuan-Chieh
;
Liu, Chien-Min
;
Chen, Chih
A superparamagnetic (SM) to ferromagnetic (FM) phase transition was investigated in Co-doped (similar to 6%) electroless plated Ni arrays. The introduction of Co altered the microstructure of the Ni arrays from nanocrystalline to polycrystalline, resulting in a SM -> FM transition. This Co-induced magnetic phase transition is similar to that observed after heat treatment of undoped samples [C. M. Liu, Y. C. Tseng, C. Chen, M. C. Hsu, T. Y. Chao, and Y. T. Cheng, Nanotechnology 20, 415703 (2009); C. C. Huang, C. C. Lo, Y. C. Tseng, C. M. Liu, and C. Chen, J. Appl. Phys. 109, 113905 (2011)]. The role of Co dopant was identified electronically using x-ray magnetic spectroscopy, revealing that the transition modified the Ni host's electronic structure and enhanced its moment by effectively spin-polarizing the Ni 3d conduction band. This was distinctly different than in the heat treatment case, which underwent an electronically independent phase transition. The element-specific magnetic hysteresis of Co and Ni was also probed, which showed that the two elements were magnetically coupled. (C) 2011 American Institute of Physics. [doi: 10.1063/1.3647753]
Effect of Si doping and applied pressure upon magnetostructural properties of Tb(5)(Si(x)Ge(1-x))(4) magnetocaloric compounds
(2011-03-28)
Tseng, Yuan-Chieh
;
Ma, Hao-Jhong
;
Yang, Chao-Yao
;
Mudryk, Yaroslav
;
Pecharsky, Vitalij K.
The composition-and pressure-dependent magnetostructural properties of Tb(5)(Si(x)Ge(1-x))(4) (x = 0.4, 0.485, 0.625, and 0.7) were investigated using x-ray powder diffraction and x-ray magnetic circular dichroism in a diamond anvil cell, respectively. Substituting the smaller-size Si for Ge stabilizes a single-phase, ferromagnetic (FM) orthorhombic O(I) structure for x >= 0.7. Similarly, application of external pressure causes a canted antiferromagnetic orthorhombic O(II) sample (x=0.4) to transform into an FMO(I) phase at 4 GPa. The element- and orbital-specific x-ray absorption data indicate that the Tb 4f orbital occupation changes with external pressure, likely through 4f-5d electronic mixing, yet no changes in Tb 4f electronic structure are observed with Si doping. The results point to different mechanisms behind the enhancement of FM exchange interactions in Tb(5)(Si(x)Ge(1-x))(4) with chemical and applied pressure, respectively.
Effect of Si doping and applied pressure upon magnetostructural properties of Tb-5(SixGe1-x)(4) magnetocaloric compounds
(2011-03-28)
Tseng, Yuan-Chieh
;
Ma, Hao-Jhong
;
Yang, Chao-Yao
;
Mudryk, Yaroslav
;
Pecharsky, Vitalij K.
The composition-and pressure-dependent magnetostructural properties of Tb-5(SixGe1-x)(4) (x = 0.4, 0.485, 0.625, and 0.7) were investigated using x-ray powder diffraction and x-ray magnetic circular dichroism in a diamond anvil cell, respectively. Substituting the smaller-size Si for Ge stabilizes a single-phase, ferromagnetic (FM) orthorhombic O(I) structure for x >= 0.7. Similarly, application of external pressure causes a canted antiferromagnetic orthorhombic O(II) sample (x=0.4) to transform into an FMO(I) phase at 4 GPa. The element- and orbital-specific x-ray absorption data indicate that the Tb 4f orbital occupation changes with external pressure, likely through 4f-5d electronic mixing, yet no changes in Tb 4f electronic structure are observed with Si doping. The results point to different mechanisms behind the enhancement of FM exchange interactions in Tb-5(SixGe1-x)(4) with chemical and applied pressure, respectively.
Element-specific study of the coupled magneto-structural and magneto-electronic properties of CoNi nanoarrays
(2013-04-01)
Yang, Chao-Yao
;
Tseng, Yuan-Chieh
;
Lin, Hong-Ji
;
材料科學與工程學系
;
Department of Materials Science and Engineering
The magneto-structural (MS) and magneto-electronic (ME) effects, as well as their coupling relationship, were investigated in electroless-plated (EL) Co0.5Ni0.5 arrays treated by post N-2 annealing and in situ field plating. Separately and combined, these two treatments have been widely employed to improve the properties of magnetic nanostructures. This work aimed to discriminate between treatments with respect to electronic and structural properties, and magnetic degrees of freedom of Co0.5Ni0.5 nanostructures. The field-plated sample exhibited a strong MS-ME coupling due to magneto-crystalline anisotropy (MCA), arising from a FCC (111) preferred orientation with lattice planes stacking orthogonally to the long axial direction of the arrays. A large coercivity was observed in this structure, arising from high magnetic stability. X-ray magnetic circular dichroism revealed that magnetization was enhanced primarily by Co magnetism, while the field-plated sample underwent a MS/ME transition with corresponding increase of the plating field. Conversely, the heat-treated sample comprised isotropically oriented nanocrystals approximately 20 +/- 3 nm in diameter, coated with an oxidation layer (approximately 5 +/- 2 nm thick). The absence of MCA in these samples ensured a weak MS-ME coupling. Although the Ni magnetization of heat-treated samples remained close to that of the field-plated sample, the Co constituent exhibited CoO and Co3O4 phases in addition to the metallic state. By contrast, the Co constituent of the field-plated sample was mainly metallic. The lack of MCA, combined with a complex Co magnetic state, appears responsible for the divergent macroscopic magnetic behaviors of the heat-treated and the field-plated samples. By isolating changes in local magnetic moments of Ni and Co, we gained a fundamental understanding of the effects of post-N-2 annealing and field plating on CoNi. Such knowledge may assist researches in improving the magnetic properties of bimetallic nanostructures.
Instrument for x-ray absorption spectroscopy with in situ electrical control characterizations
(2013-12-01)
Huang, Chun-Chao
;
Chang, Shu-Jui
;
Yang, Chao-Yao
;
Chou, Hsiung
;
Tseng, Yuan-Chieh
We report a synchrotron-based setup capable of performing x-ray absorption spectroscopy and x-ray magnetic circular dichroism with simultaneous electrical control characterizations. The setup can enable research concerning electrical transport, element-and orbital-selective magnetization with an in situ fashion. It is a unique approach to the real-time change of spin-polarized electronic state of a material/device exhibiting magneto-electric responses. The performance of the setup was tested by probing the spin-polarized states of cobalt and oxygen of Zn1-xCoxO dilute magnetic semiconductor under applied voltages, both at low (similar to 20 K) and room temperatures, and signal variations upon the change of applied voltage were clearly detected. (C) 2013 AIP Publishing LLC.
Phase-driven magneto-electrical characteristics of single-layer MoS2
(2016)
Yang, Chao-Yao
;
Chiu, Kuan-Chang
;
Chang, Shu-Jui
;
Zhang, Xin-Quan
;
Liang, Jaw-Yeu
Magnetism of the MoS2 semiconducting atomic layer was highlighted for its great potential in the applications of spintronics and valleytronics. In this study, we demonstrate an evolution of magneto-electrical properties of single layer MoS2 with the modulation of defect configurations and formation of a partial 1T phase. With Ar treatment, sulfur was depleted within the MoS2 flake leading to a 2H (low-spin) -> partial 1T (high-spin) phase transition. The phase transition was accompanied by the development of a ferro-magnetic phase. Alternatively, the phase transition could be driven by the desorption of S atoms at the edge of MoS2 via O-2 treatment while with a different ordering magnitude in magnetism. The edge-sensitive magnetism of the single-layer MoS2 was monitored by magnetic force microscopy and validated by a first-principle calculation with graded-Vs (sulfur vacancy) terminals set at the edge, where band-splitting appeared more prominent with increasing Vs. Treatment with Ar and O-2 enabled a dual electrical characteristic of the field effect transistor (FET) that featured linear and saturated responses of different magnitudes in the I-ds-V-ds curves, whereas the pristine MoS2 FET displayed only a linear electrical dependency. The correlation and tuning of the Vs-1T phase transition would provide a playground for tailoring the phase-driven properties of MoS2 semiconducting atomic layers in spintronic applications.
Sharp variation in coercivity and magnetic interactions in patterned CoxNi1-x nanoarrays
(2013-08-14)
Yang, Chao-Yao
;
Wang, Liang-Wei
;
Chen, Po-An
;
Lin, Hong-Ji
;
Lai, Chih-Huang
We present a study concerning the unexpectedly large coercivity increase and associated magneto-structural properties of CoxNi1-x patterned arrays. An increase in x led to an face-center-cubic (FCC)-> hexagonal-close-packed (HCP) transition in CoxNi1-x arrays, accompanied by a 6-fold increase in coercivity and strong 3d exchange interactions probed by x-ray magnetic circular dichroism. Sum-rule analysis revealed that orbital moment involved very little in the variable coercivity and magnetic anisotropy; this is distinct from other nanostructures displaying variable coercivity. The sharp rise in coercivity can be attributed to the geometrical confinement of the arrays, causing the microstructure of the nano-clusters to switch magnetization reversal mechanism from fanning to coherent with increasing x, based on the chain-of-spheres model. First-order-reversal curves revealed that the FCC and HCP arrays comprised both soft and hard ferromagnetic components; however, the soft component of the FCC was much more pronounced, leading to differences in reversibility. This type of nanostructure provides a sharp control of magnetic hardness that could be tailored in related technologies. (C) 2013 AIP Publishing LLC.
Soft and hard natures of Nd2Fe14B permanent magnet explored by first-order-reversal-curves
(2014-12-01)
Chen, Po-An
;
Yang, Chao-Yao
;
Chang, Shu-Jui
;
Lee, Min-Han
;
Tang, Nai-Kuang
Two commercial Nd2Fe14B samples, MQP-B and sintered-NdFeB were investigated using synchrotron based x-ray diffraction and first-order-reversal-curves (FORCs). Despite differing in magnetic and structural properties, the two samples were found to comprise two major ferromagnetic components in FORCs. For the sintered-NdFeB case, the soft component may originate from the intrinsically soft Nd-f site which was coupled with its local Fe atomic environment that differs in magnetic anisotropy from the Nd-g site (intrinsically hard). It may directly originate from the Nd-rich phase or microstructural imperfection, while the former possibility (Nd-f sire) appears greater than the latter While for the MQP-B, the minor second phase resulting from high structural disorder was likely in charge of the presence of the soft component. Sophisticated FORCs analyses revealed the natures of the soft and hard components, soft hard coupling and switching reversibility of the two cases, irrespective of the origins of their two components. This provides insights to the origin of magnetic stability and reversal dynamics of Nd2Fe14B that have not been fully understood by conventional magnetic analyses. The coexistence of the two components led to an incoherent reversal undermining the magnetic stability of Nd2Fe14B. This is a fundamental problem as to why the performance extremity can only be improved finitely through extrinsic tuning. From FORCs simulation we understand that the soft-hard coupling was moderate in a real Nd2Fe14B compound. A stronger soft-hard coupling is necessary to conquer the anisotropic competition to enable a coherent reversal that will promote the magnetic hardness. (C) 2014 Elsevier B.V. All rights reserved,
Structural imperfections and attendant localized/itinerant ferromagnetism in ZnO nanoparticles
(2014-08-30)
Yang, Chao-Yao
;
Lu, Yi-Hsuan
;
Lin, Wei-Hao
;
Lee, Min-Han
;
Hsu, Yung-Jung
Using synchrotron-based x-ray magnetic spectroscopy, we report a study focusing on the local symmetry of Cu-dopant and resultant structural imperfections in mediating Cu-doped ZnO nanoparticles\' ferromagnetism (FM). Prepared by an antisolvent method, Cu appeared to preferably populate on the basal plane of ZnO with a local symmetry of [CuO4]. This unique symmetry was antiferromagnetic in nature, while electronically and structurally coupled to surrounded oxygen vacancies (V-o) that yielded a localized FM, because of a strong dependency on the number/location of the [CuO4] symmetry. Surprisingly, the FM of undoped but oxygen-deficient ZnO appeared to be more itinerant and long-range, where V-o percolated the FM effectively and isotropically through oxygen\'s delocalized orbital. By adopting the approach of structural imperfection, this study clearly identifies V-o\'s (defect\'s) true characters in mediating the FM of magnetic semiconductors which has been thought of as a long-standing debate, and thus provides a different thinking about the traditional extrinsic ferromagnetic-tuning in the semiconductors. It even illuminates recent research concerning the intrinsic FM of low-dimensional systems that contain defects but non-magnetic elements.
Superparamagnetic ground state of CoFeB/MgO magnetic tunnel junction with dual-barrier
(2018-11-01)
Thanh-Nga Tran
;
Tu-Ngoc Lam
;
Yang, Chao-Yao
;
Lin, Wen-Chin
;
Chen, Po-Wen
CoFeB/MgO-based magnetic tunnel junctions (MTJs) have considerable potential in magnetic random access memory (MRAM), thanks to their tunable perpendicular magnetic anisotropy (PMA). We found significant reduction of dead-layer by inserting additional MgO into the MTJ structure. Interface, electronic and transport characterizations were utilized to approach the modified magnetic properties driven by the dual-MgO structure in this work. The dual-MgO structure appeared to hinder boron (B) diffusion into the metallic layer and prevent capping-layer (Ta) penetration across the interface. This suppressed the dead-layer effect and promoted overall magnetization despite PMA degradation. A robust BOx phase that formed within the dual-MgO structure presented a superparamagnetic ground state. In the single-MgO structure, any reduction in the thickness of the CoFeB promoted PMA, albeit at the cost of spin-polarization. The dual-MgO structure could restore spin-polarization by preferentially populating spin electrons into Fe/Co minority states. X-ray magnetic spectroscopy and anomalous Hall effect suggest that, the dual-MgO differs from the single-MgO with a favorable longitudinal polarized spin-channel. This makes the dual-MgO structure applicable to applications requiring in-plane rather than out-of-plane sensing.
Tailor magnetic order and spin-polarized gap states of opto-spintronic compounds by carrier mediation
(2018-08-15)
Yang, Chao-Yao
;
Chiu, Yi-Hsuan
;
Tsai, Kai-An
;
Hsu, Hua-Shu
;
Lin, Jun-Xiao
We investigate the role of carriers in the presence of oxygen vacancies (Vo) in mediating the ferromagnetic order and spin-polarized gap states of Cu-doped ZnO nanoparticles. We demonstrate the tuning of magnetic order via post-annealing under Ar/O-2. Carrier (Vo)-rich condition (Ar-treatment) is found to favor higher magnetic order of the nanoparticles with enhanced thermal stability and wide spin-polarized gap states from ultra-violet (UV) to visible energy range. Conversely, carrier (Vo)-poor condition (O-2-treatment) reduces resistance to thermal fluctuations and quenched spin-polarization due to lack of carrier mediation. The proposed method is effective to tailor the magnetic order as well as the wide polarized band-gap states, of oxide opto-spintronic compounds by finding the physical equivalency between charge carriers and spin electrons. (C) 2018 Elsevier B.V. All rights reserved.