Chen, I-Wei
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Publication A promising p-type transparent conducting material: Layered oxysulfide(2007-12-01) Liu, Min-Ling; Wu, Li-Bin; Chen, Qiang; Chen, Li-Dong; Chen, I-WeiSr3Cu2Sc2O5S2, a layered oxysulfide, composed of anti-PbO-like [Cu2S2] slabs alternating with perovskitelike [Sr3Sc2O5] slabs, was systematically studied as a p-type transparent conducting material. The material has a wide energy gap of 3.1 eV and a p-type electrical conductivity of 2.8 S cm−1 at room temperature. The hole mobility of +150 cm2 V−1 S−1 at room temperature, which is much higher than the typical value of ~10−1–10 cm2 V−1 S−1 found in other copper compounds. The performances of bulk undoped Sr3Cu2Sc2O5S2 show the promise of copper oxysulfides as a class of p-type transparent conductive materials that is essential for optoelectronic applications.Publication Magnetic impurities in conducting oxides. II. (Sr1-xLax)(Ru1-xCox)O3 system(2004-09-20) Mamchik, Alexander; Dmowski, Wotjek; Egami, Takeshi; Chen, I-WeiThe perovskite solid solution between ferromagnetic SrRuO3 and antiferromagnetic LaCoO3 is studied and its structural, electronic,and magnetic properties are compared with (Sr1-xLax)(Ru1-xFex)O3. The lower 3d energy levels of Co3+ cause a local charge transfer from 4d Ru4+, a reaction that has the novel feature of being sensitive to the local atomic structure such as cation order. Despite such a complication, Co, like Fe, spin-polarizes the itinerant electrons in SrRuO3 to form a large local magnetic moment that is switchable at high fields. In the spin glass regime when Anderson localization dominates, a large negative magnetoresistance emerges as a result of spin polarization of mobile electronic carriers that occupy states beyond the mobility edge. A phenomenological model predicting an inverse relation between magnetoresistance and saturation magnetization is proposed to explain the composition dependence of magnetoresistance for both (Sr1-xLax)(Ru1-xCOx)O3 and (Sr1-xLax)(Ru1-xFex)O3 systems.Publication Electron localization and magnetism in SrRuO3 with non-magnetic cation substitution(2011-01-02) Tong, W.; Huang, F Q; Chen, I-WeiThe destruction of the ferromagnetism of alloyed SrRuO3 can be caused by electron localization at the substitution sites. Among all the non-magnetic cations that enter the B site, Zr4+ is the least disruptive to conductivity and ferromagnetism. This is because Zr4+ does not cause any charge disorder, and its empty d electron states which are poorly matched in energy with the Ru t2g4 states cause the least resonance scattering of Ru’s d electrons. Conducting Sr(Ru, Zr)O3 may be used as an electrode for perovskite-based thin film devices, while its insulating counterpart provides unprecedented magnetoresistance, seldom seen in other non-manganite and non-cobaltite perovskites. (Some figures in this article are in colour only in the electronic version)Publication Paraffin-Based Process for Producing Layered Composites with Cellular Microstructures(2002-04-01) Dakskobler, Aleš; Kosmac, Tomaz; Chen, I-WeiA paraffin-based process that results in high-strength bimaterial ceramic layered composites is reported. The process facilitates rolling, folding, and shape retention at room temperature and allows the transition from a laminar to a cellular microstructure during deformation. The strength of sintered alumina/zirconia/alumina composites reached 700 MPa, higher than that of conventional zirconia-toughened alumina composites containing dispersed particles.Publication Development of Tough Alpha-SiAlON(2003-01-01) Chen, I-Wei; Shuba, Roman; Zenotchkine, Misha YThe development of tough α-SiAlON with elongated grains in the last five years is summarized. This progress has been guided by the improved understanding of phase relations and nucleation/growth kinetics in SiAlON ceramics. Although most α-SiAlON compositions can be processed to contain some elongated grains, their microstructure, fracture toughness and R-curve behavior vary greatly. Such variability is due to the different phase stability of α-SiAlONs and the varying physical chemistry of the competing phases, including the transient/residual liquid. For this reason, microstructure control of α-SiAlON must pay close attention to the composition, starting powder and heating schedule. Seeding with single crystals of an appropriate α-SiAlON composition provides an attractive alternative that simplifies the task of microstructure control, since such seeds are thermodynamically stable and they completely dominate the nucleation statistics. Tough and hard α-SiAlON ceramics containing Ca, Y, Nd, and Yb stabilizers have been obtained using this method, some with toughness exceeding 10 MPam1/2. The ability of maintaining a uniform microstructure of highly elongated grains is the key to high toughness material.Publication Control of strain relaxation in tensile and compressive oxide thin films(2008-10-01) Wang, Yudi; Kim, Soo Gil; Chen, I-WeiTensile and compressive solid-solution thin films based on LaAlO3 and CaZrO3 compositions were grown on perovskite oxide substrates using pulsed laser deposition to study growth mode transitions and strain relaxation. A buried layer of SrRuO3 between the thin film and the SrTiO3 substrate was also introduced to provide an auxiliary embedded strain gauge, which helps identify the critical conditions for the onset of catastrophic strain relaxation events – cracking and dislocation cascades. The results are compared with theoretical predictions to provide guidelines on some general deposition conditions that may be used to obtain smooth, crystalline and defect-free thin films of interest to perovskite-based heterostructures.Publication Optical evidence for transient photoinduced magnetization in La0.7Ca0.3MnO3(2005-02-01) McGill, S. A.; Miller, R. I.; Torrens, O. N.; Mamchik, Alexander; Chen, I-Wei; Kikkawa, J. M.Pump-probe Kerr spectroscopy reveals evidence for transient photoinduced magnetization (PIM) near Tc in the colossally magnetoresistive manganite La0.7Ca0.3MnO3. For temperatures above Tc, the PIM signature reaches full strength at the field-driven magnetic phase transition, while below Tc it exhibits a magnetic activation threshold of ~0.5 T. We compare the temperature dependence of the effect with diffuse spin scattering previously measured with neutrons and discuss these findings in the context of phase separation near Tc.Publication Elimination of Grain Boundary Glass in α-Sialon by Adding Aluminium(2005-10-19) Shuba, Roman; Chen, I-WeiThe elimination of residual glass in hot-pressed Y-α-SiAlON was demonstrated by varying the nitride composition. This was readily achieved by adding excess aluminium nitride (AlN), while the addition of excess Si3N4 had little effect. The amount of excess AlN required for this purpose increased with increasing Al and O content in α-SiAlON, and was higher if α-SiAlON contained larger interstitial cations. These results were explained by the thermodynamics of α-SiAlON precipitation. The elimination of liquid also led to suppressed grain growth, refined microstructure, and improved oxidation resistance. Grain coarsening by post-sintering annealing was required to obtain high fracture toughness.Publication Nucleation and growth mechanism of ferroelectric domain-wall motion(2007-10-07) Shin, Young-Han; Grinberg, Ilya; Chen, I-Wei; Rappe, Andrew MThe motion of domain walls is critical to many applications involving ferroelectric materials, such as fast high-density non-volatile random access memory. In memories of this sort, storing a data bit means increasing the size of one polar region at the expense of another, and hence the movement of a domain wall separating these regions. Experimental measurements of domain growth rates in the well-established ferroelectrics PbTiO3 and BaTiO3 have been performed, but the development of new materials has been hampered by a lack of microscopic understanding of how domain walls move. Despite some success in interpreting domain-wall motion in terms of classical nucleation and growth models, these models were formulated without insight from first-principles-based calculations, and they portray a picture of a large, triangular nucleus that leads to unrealistically large depolarization and nucleation energies. Here we use atomistic molecular dynamics and coarse-grained Monte Carlo simulations to analyse these processes, and demonstrate that the prevailing models are incorrect. Our multi-scale simulations reproduce experimental domain growth rates in PbTiO3 and reveal small, square critical nuclei with a diffuse interface. A simple analytic model is also proposed, relating bulk polarization and gradient energies to wall nucleation and growth, and thus rationalizing all experimental rate measurements in PbTiO3 and BaTiO3.Publication Biomedical nanoparticle carriers with combined thermal and magnetic responses(2008-10-01) Liu, Ting-Yu; Hu, Shang-Hsiu; Liu, Dean-Mo; Chen, San-Yuan; Chen, I-WeiSeveral biocompatible polymers are capable of large responses to small temperature changes around 37ºC. In water, their responses include shrinkage and swelling as well as transitions in wettability. These properties have been harnessed for biomedical applications such as tissue engineering scaffolds and drug delivery carriers. A soft material/hard material hybrid in which a magnetic metal or oxide is embedded in a temperature-responsive polymer matrix can combine the thermal sensitivity with magnetic signatures. Importantly, nanosizing such construct brings about new desirable features of extremely fast thermal response time, small magnetic hysteresis and enhanced magnetic susceptibility. Remote magnetic maneuvering and heating of the hybrid nanocolloids makes possible such applications as high-throughput enzyme separation and cell screening. Robust drug release on demand may also be obtained using these colloids and nanoparticle-derived thin film devices of combined thermal magnetic sensitivity.