Summer School of Advanced Functional Materials 2009 International Centre for Materials Physics, Chinese Academy of Sciences ABSTRACTS Multiferroic BiFeO3 Sang-Wook Cheong Department of Physics and Astronomy and Rutgers Center for Emergent Materials, Rutgers University, Piscataway, New Jersey 08854, USA BiFeO3 (BFO) is a unique multiferroic in the sense that the magnitude of ferroelectric polarization is large (about 90 C/cm) - similar with that of standard ferroelectrics such as BaTiO3 and PbTiO3. In addition, both magnetic and ferroelectric temperatures are much high than room temperature. BFO has been extensively studied, but mostly in the form of films. In order to explore the intrinsic properties of BFO and also properties that cannot be measured in film forms, we have investigated comprehensive physical properties of bulk BFO single crystals using a number of techniques such as neutron scattering, piezoelectric force microcopy and transport property measurement. The results show unprecedented coupling phenomena among charge carriers, magnetism and ferroelectricity. Multiferroic materials and tunnel junctions Qi Li Department of Physics, Pennsylvania State University, University Park, PA 16803 Multiferroic materials can provide a possibility of manipulation of magnetic states electrically or polarization states magnetically, which may be very significant for many applications, such as spintronic devices. However, there are very limited numbers of such materials exist. Alternatively, artificially synthesized multilayers or composites with both components have shown more promising results. In this talk, I will discuss in detail our recent work on multiferroic tunnel junctions. Multiferroic tunnel junction refers to a ferromagnetic tunnel junction (MTJ) with a ferroelectric barrier. Spin polarized tunneling in magnetic tunnel junctions (MTJ) has attracted much attentions in the last decade due to both fundamental interest in spin polarized transport and its vast applications in spintronics, such as computer recording and memories. It was theoretically predicted four years ago that tunnel junctions using ferroelectric barrier may display resistance switching behavior. Combining the ferromagnetic and ferroelectric effects, the multiferroic junctions, may display new properties, such as quaternary or octal logic state instead of binary state as in a dielectric MTJ, which may be used in quantum information processing and new device concepts. We have recently achieved multiferroic tunnel junctions using ferromagnetic La0.7Ca0.3MnO3 as the electrodes and ferroelectric (Ba, Sr)TiO3 as the barrier in a trilayer junction structure. Besides displaying standard magnetoresistance of a MTJ, the junction shows ~ 50 % electro-resistance in response to the polarization reversal of the barrier for both magnetic parallel and antiparallel states. We show that this type of junction has four stable resistance states, corresponding to combination of two charge polarizing states and two (parallel and antiparallel) magnetic states. The four states can be manipulated by the magnetic and electric fields. In addition, both the parallel and antiparallel magnetic state may be switched to the other state using an electrical field in certain range of magnetic fields. This is likely due to electro-magnetic coupling in the multiferroic system. The tunneling magnetoresistance, tunneling electroresistance, and the possible electro-magnetic coupling in the junctions will be discussed. Mixed-Metal Oxide Hybrid Semiconductor Nano-Composites: Surface Chemistry, Photoreactivity, and Practical Applications Michael R. Hoffmann Engineering and Applied Science, W. M. Keck Laboratories California Institute of Technology, Pasadena, CA 91125 USA [email protected] The chemical and physical properties of mixed metal oxide semiconductors will be discussed along with details of their surface chemistry and their photochemical and photophysical properties. Examples of practical systems that employ mixed-metal oxides as light absorbers will be presented. In addition, the same materials can be used as surface-bound catalysts for applications in electrochemistry such as the electrolytic splitting of water to produce hydrogen and oxygen. The topics to be address specifically include: band-gap energies, energetic limits for practical applications, energies and long-term stability, useful metal oxides, wide vs. narrow band-gap materials, effects of doping of metal oxides, photoexcitation, lifetimes of excited states, trapping of electrons and holes, range of lifetimes, identification of trapping states, and surface chemistry and reactivity. Selective growth of well aligned semiconducting single-walled carbon nanotubes Lei Ding, Alexander Tselev, Jinyong Wang, Dongning Yuan, Thomas P. McNicholas, Yan Li and Jie Liu Department of Chemistry, Duke University, Durham, North Carolina 27708, USA Even though the devices made from individual nanotubes have shown outstanding performances such as high mobility, high current, high thermal conductivity, good chemical and mechanical stability, the high hope for the next generation of carbon nanotube based electronics is hampered by several major problems. Among them are the lack of reliable methods to control the alignment and position of nanotubes as well as and perhaps most problematically, the simultaneous growth of nanotubes with different chiralities, yielding random mixtures of metallic and semiconducting nanotubes. Even though the post-growth separation of metallic from semiconducting SWNTs have made good progress, the alignment and assembly of the separated nanotubes into devices are still challenging and not suitable for large scale fabrication. Consequently, a method that can directly produce well aligned arrays of pure semiconducting nanotubes is thought to be the ideal choice for large scale fabrication of nanotubes FETs. In this talk, we show that such a method is not a dream. We developed a chemical vapor deposition (CVD) approach, which allows selective growth of high-density arrays of well-aligned SWNTs with almost exclusively semiconducting SWNTs. Analysis of the samples shows that at least over 95% of nanotubes are semiconducting. This method demonstrates great promise to solve two of the most difficult problems which limit application of carbon nanotubes in nanoelectronics – the coexistence of metallic and semiconducting nanotubes in samples produced by most, if not all, growth methods and the simultaneous control of the alignment of the nanotubes. Condensation Processes on Ag Core/ SiO2 Shell Microstructures and Some Related Issues Z.X. Cao Beijing National Laboratory for Condensed Matters, Institute of Physics, Chinese Academy of Science, Beijing 100190, China The Ag core/SiO2 shell structure is a very interesting system that can serve as a catalyst for many organic reactions, while the Ag particle can itself catalyze the growth of Si and SiO 2 nanostructures. By manipulating the cooling process of Ag core/SiO2 shell microstructures, various stressed-driven patterns, including the triangular tessellation and Fibonacci spirals, can be obtained. Furthermore, various fruit morphologies can be reproduced by simulation along this line, which provide a strong justification to the mechanical mechanism of phyllotaxis. The study of this core/shell system can also offer analogous solutions to the well-known Thomson problem, to the bubble coalesce problem, and it even allows obtaining a snapshot of the initial stage of Ag-catalyzed growth of the SiO2 nanowires. Also the defect management strategy for assembled architectures on curved surfaces can be tested on this system. Some relevant results have been published on Science (2005), APL (2004, 2007, 2008), PRL(2008), and PNAS (2008)。 Deformation and Toughening Mechanisms of Natural Biological Nanocomposites Lessons from Nature Xiaodong Li Department of Mechanical Engineering, University of South Carolina, Columbia, SC 29208, USA Seashells are natural biological nanocomposites with superior mechanical strength and toughness. What mechanisms and processes does Mother Nature use to fabricate seashells? What roles do the nanoscale structures play in the inelasticity and toughening of seashells? Can we learn from this to produce seashell-like nanocomposites? Here we limit our focus to nacre (mother-of-pearl). This presentation summarizes the recent discovery of nanoparticles in nacre and elucidates the roles that these nanoparticles play in nacre’s toughness. We found that aragonite nanoparticles can rotate, which contributes to energy dissipation in nacre’s deformation and fracture. The mechanical properties of nacre’s biopolymer layer were measured by in situ AFM mechanical testing. This presentation also presents future challenges in the study of nacre’s nanoscale structure and mechanical properties. Application and formation mechanism of self-assembly composite materials X.J. Liu Department of Materials Science and Engineering, Collage of Materials, Xiamen University, Xiamen 361005, P.R.China. [email protected], Tel: 0592-2187888 More recently, we had successfully produced self-organized core/shell composite powers by conventional gas atomization method in liquid immiscible alloy systems (Science, Vol.297, (2002), pp.990-993). The kinetic mechanism of the formation of the egg-type microstructure, however, is a relatively unexplored problem. Due to the extensive technological interests and potential applications of such kind of composite powders, better understanding of the mechanism governing the formation of the core/shell microstructure, which can also increase the engineer’s level of control over the process, is urgently needed. In the present work, mechanisms responsible for the formation of the core/shell microstructure from liquid immiscible alloys are investigated by computer simulation of phase separation in liquid parent droplets using the phase field method. The model accounts simultaneously for spinodal decomposition and subsequent coarsening, fluid flow, Marangoni motion of second-phase droplets caused by temperature dependence of the interfacial energy between the two liquid phases, collision and coalescence among the droplets, and the surface effect. The simulation results show, for the first time, the entire phase separation process leading to the core/shell microstructures. The role played by each of the concurrent phenomena at different stages of the phase separation and coupling between different mechanisms are analyzed. The wetting effect and the phase inversion phenomenon due to surface energy difference between two liquid phases are also investigated. This work provides an important theory basis for the design and development of core/shell composite materials. Interface Engineered Nanostructural Metamaterials with Anomalous Physical Phenomena Chonglin Chen Department of Physics and Astronomy, University of Texas at San Antonio, Texas and The Texas Center for Superconductivity and Department of Physics, University of Houston, Texas Interface engineered materials have attracted more and more attention in the multifunctional materials research and active device fabrication. It plays a key role to control the physical properties of advanced nanomaterials and results in the discovery of various new physical phenomena with excellent opportunity for developing new metamaterials for active devices and engineered nanosystems. We have focused on the systematical studies on the formations and the characterizations of various highly epitaxial oxide thin films and multilayered layered structures to understand the nature of interface induced anomalous physical phenomena. Recently, we have achieved an excellent dielectric tunability of 80% from highly epitaxial ferroelectric Mn:(Ba,Sr)TiO3 thin films from the interface controlled nano domain structures and observed strong anisotropic phenomena in highly epitaxial (Pb,Sr)TiO3 thin films; setup a new record of giant magnetoresistance ratio of 1010 (four order higher than the previous record) from the artificial interface domain structured (La,Ca)MnO3 epitaxial thin films; and observed an anomalous clamped domain ferroelectric phenomena from the multilayered BaTiO3/SrTiO3 superlattices. A series of models were developed to understand these interface phenomena. Photochemistry and Photophysics of Semiconductor Quantum Dots: Effects of Quantum Confinement, Defect States, Doping, Shape and Morphology Michael R. Hoffmann Engineering and Applied Science, W. M. Keck Laboratories California Institute of Technology, Pasadena, CA 91125 USA [email protected] Nanoparticulate materials have been synthesized for more 100 years. During early stages of research they were explored primarily as a scientific curiosity in terms of their physicochemical properties, aggregation tendencies in colloidal suspensions, photochemical and catalytic properties, non-linear optical effects, and interfacial chemistry. At the present time, nanometer-sized semi-conducting particle arrays (i.e., quantum dots) are used for the protective coating of a wide-variety of surfaces, for their self-cleaning properties when coated on glass, and for other diverse applications in environmental remediation, for the photo-splitting of water to produce hydrogen and oxygen, as semiconductor electrodes, as photo-initiator for polymerization reactions, and non-linear optical switches. In addition, a review of the quantum-sizing effects on particle reactivity with decreasing size will be presented. Interface Engineered Metamaterials with Optimized Ionic Transport Properties for Solid State Fuel Cells Chonglin Chen Department of Physics and Astronomy, University of Texas at San Antonio One UTSA Circle, San Antonio, TX 78249-1644 and The Texas Center for Superconductivity, University of Houston, Houston, TX 77204 The last few decades have seen an explosion in the development of new materials chemistry for sustainable energy devices, driven in parallel by the demands of technology and the inquisitiveness of basic sciences and engineering. In particular, Solid State Fuel Cells (SOFCs) have promised a high-energy efficiency and can provide society with sustainable energy producing technology. In the past ten years, we have systematically studied various highly mixed conductive and ionic conductive materials and focused on the acquisition of fundamental understanding the physical properties and chemical stability of advanced nanostructured materials when used in multilayered structures for the development of an intermediate temperature solid oxide fuel cell (IT-SOFC). Our efforts have demonstrated that the highly ionic conductive PrBaCo2O5 is a good cathode candidate for IT- SOFC. The interface engineered YSZ/GCO multilayered structures can significantly enhance the oxygen exchange in the electrolyte. Also, various interesting transport properties have been observed in the mixed conductive LaBaCo2O5.5. Details will be discussed in the talk. Multiferroics with Symmetric or Antisymmetric couplings Sang-Wook Cheong Department of Physics and Astronomy and Rutgers Center for Emergent Materials, Rutgers University, Piscataway, New Jersey 08854, USA Discoveries of extraordinary cross-coupling effects between magnetic and lattice degrees of freedom in new multiferroics such as 90 or 180 flipping of polarization or drastic change of dielectric constant with external magnetic fields have recently excited our community. These significant cross-coupling effects have been observed in the so-called magnetism-driven ferroelectrics. Lattice relaxation in magnetically-ordered states with broken inversion magnetic symmetry can induce non-centrosymmetric lattice distortions through exchange-striction, leading to the presence of electric polarization. In these magnetically-driven ferroelectrics, external magnetic fields can influence the configuration of magnetic order, and can consequently change the properties related with ferroelectricity. Both symmetric and antisymmetric exchange coupling can be involved in the exchange-striction. Magnetically-driven ferroelectrics with the symmetric exchange striction are associated with “acentric” spin order, and the antisymmetric exchange striction, relevant to the Dzyaloshinskii-Moriya-type interaction, becomes active when ferroelectricity is induced by spiral magnetic orders. A few examples of magnetically-driven ferroelectrics, exhibiting high tunability of dielectric properties in magnetic fields, will be discussed. Instability of Carbon Nanotubes: From Atomic Simulations to Continuum Mechanics Q. Wang Department of Mechanical and Manufacturing Engineering University of Manitoba, Canada Carbon Nanotubes (CNTs) are graphene sheet(s) rolled into a seamless tube with a diameter in the order of nanometres discovered in 1991. With the acknowledgement of CNTs’ uniquely strong mechanical properties came immense interest in further exploration of their nano-mechanics. Traditional CNT stability analysis with continuum models is limited in capturing CNT atomic characteristics and thus lacks accuracy in stability analysis of CNTs. This seminar will cover the latest development of a hybrid model that incorporates both continuum mechanics and molecular mechanics in predicting the critical strain, stress, and buckling load of the inelastic buckling of CNTs. The hybrid model can analyze both beam- and shell-like buckling behaviour of CNTs. The buckling solutions from the hybrid model can be verified by molecular dynamics simulations and available research findings. The existence of the optimum diameter, at which the buckling load reaches its maximum, can now be uncovered with this model. The simplicity and effectiveness of the proposed model are not only able to reveal the chiral and size-dependent buckling solutions for CNTs, but also enable a thorough understanding of the stability behaviour of CNTs for potential applications. The presentation will also briefly cover a few recent findings in other research fields such as atomic transportation and partition with CNTs, structural health monitoring and repair of structures with smart materials. Phase diagram and isotope effect in pnictide superconductors X.H. Chen Hefei National Laboratory for Physical Science at Microscale and Department of Physics, University of Science and Technology of China, Hefei, Anhui 230026, China We will talk about the discovery of superconductivity with Tc higher than 40 K in Fe-based superconductors. In this talk, we show you the transport properties (resistivity, Hall coefficient, TEP ) in normal state. These results suggest a quantum critical point around x=0.14 in SmFeAsO1-xFx system. Such behavior is confirmed by structure, netron scattering, transport properties under high magnetic field and muSR study. A electronic phase diagram is proposed in SmFeAsO1-xFx and Ba-122 system. Isotope effect is discussed in the SmFeAsO1-xFx system and Ba1-xKxFe2As2 with iron isotope exchange. Materials and Physics in Pnictide Superconductors Hai-Hu Wen National Lab for Superconductivity, Institute of Physics, Chinese Academy of Science, Beijing 100190, China Superconductivity in the pnictides has shown itself to be very interesting and attractive. Some experimental results have revealed that the superconducting mechanism could be unconventional, and the transition temperature higher. In this talk I will survey our recent progress of both material synthesizing and physical properties of this rich family. First I will give an introduction to the history and basic structures in the FeAs families. By using solid state reaction method, we have made several major contributions to the synthesizing of new materials of pnictide superconductors. (1) Fabrication of the hole doped RE1-xSrxFeAsO samples (RE=La and Pr). We found that the upper critical field is much higher in the hole doped samples than that in the electron doped ones. (2) Fabrication of a series of new parent compounds DvFeAsF (Dv=divalent metals: Sr, Ca, Eu etc.) and many new superconductors with Tc beyond 50 K by doping electrons into the system: 32 K in Sr0.6La0.4FeAsF, 52.8 K in Ca0.4Pr0.6FeAsF and 57.4 K in Ca0.4Nd0.6FeAsF. (3) Invention of the new material (Sr3Sc2O5)Fe2As2 with rather large spacing distance between the FeAs planes. We fabricated the new superconductor Sr4V2O6Fe2As2 with Tc=38 K. This new superconductor has a large anisotropy and vortex liquid region. In collaboration with Gang Mu, Zhaosheng Wang, Huiqian Luo, Huan Yang, Xiyu Zhu, Fei Han, Ying Jia, Bing Zeng, Bing Shen, Cong Ren, Lei Shan References:  H. H. Wen et al., EPL82, 17009 (2008).  G. Mu et al., Phys. Rev. B 79, 104501 (2009).  F. Han et al., Phys. Rev. B 78, 180503 (R) (2008). Editor selection article.  P. Cheng et al., Europhys. Lett. 85, 67003 (2009).  X. Y. Zhu et al., Europhys. Lett. 85, 17011 (2009)..  X. Y. Zhu et al., Phys. Rev. B 79, 024516 (2009).  X. Y. Zhu et al., PRB Rapid Communication 2009, in press, Editor’s selection article. Synthesis, characterization and application explorations of graphene Zhongshuai Wu, Libo Gao, Dawei Wang, Wencai Ren and Hui-Ming Cheng* Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences, 72 Wenhua Road, Shenyang 110016, P. R. China *Corresponding author: [email protected] Graphene, as a one-atom-thick flat allotrope of carbon, has been attracting increasing interests because of its unique structure, excellent properties and a variety of promising applications. However, the properties of graphene are very sensitive to their detailed structure. Therefore, controllable synthesis and structural characterization of graphene are essential for their further fundamental studies and extensive applications. In this presentation, our recent progresses on the synthesis of graphene and graphene nanoribbons (GNRs) will be reported. We proposed a simple and effective strategy to tune the number of graphene layers  and to improve electrical conductivity by chemical exfoliation . We realized the facile synthesis of narrow GNRs with smooth edges , and controlled cutting of graphene with preferential crystallographic directions. For rapid and accurate identification of graphene layers, we developed a total color difference method based on a combination of reflection spectrum and International Commission on Illumination color space , and a surface and interference co-enhanced Raman scattering technique to dramatically enhance the Raman signals of graphene by using a specifically designed substrate . Finally, our explorations on the applications of graphene will be reported. We proposed an electrophoretic deposition technique to fabricate homogeneous graphene films , and found that the graphene film shows excellent field emission properties, comparable to carbon nanotubes . The good capacitance performance of graphene as a flexible energy storage material is also demonstrated . References: . Z. S. Wu, W. C. Ren, L. B. Gao, B. L. Liu, C. B. Jiang, H. M. Cheng, Carbon 47 (2), 493-499 (2009). . Z. S. Wu, W. C. Ren, L. B. Gao, J. P. Zhao, Z. P. Chen, B. L. Liu, D. M. Tang, B. Yu, C. B. Jiang, H. M. Cheng, ACS Nano 3 (2), 411-417 (2009). . Z. S. Wu, W. C. Ren, L. B. Gao, B. L. Liu, C. B. Jiang, H. M. Cheng, submitted (2009). . L. B. Gao, W. C. Ren, F. Li, H. M. Cheng, ACS Nano 2 (8), 1625-1633 (2008). . L. B. Gao, W. C. Ren, B. L. Liu, R. Saito, Z. S. Wu, S. S. Li, C. B. Jiang, F. Li, H. M. Cheng, ACS Nano 3 (4), 933-939 (2009). . Z. S. Wu, S. F. Pei, W. C. Ren, D. M. Tang, L. B. Gao, B. L. Liu, F. Li, C. Liu, H. M. Cheng, Advanced Materials 21 (17), 1756-1760 (2009). . D. W. Wang, F. Li, J. P. Zhao, W. C. Ren, Z. G. Chen, J. Tan, Z. S. Wu, I. Gentle, G. Q. Lu, H. M. Cheng, ACS Nano, in press (2009). Dominance of broken bond and nonbonding electrons in low-dimensional functional materials Chang Q Sun School of Electrical and Electronic Engineering, Nanyang Technological University, Singapore 639798 and School of Physical Science, Xiangtan University, Hunan 411105, China Although they exist ubiquitously in our daily activities, the significance of nonbonding states, such as broken bonds, nonbonding lone pair and unpaired electrons, and antibonding dipoles, are often overlooked in practice. Recent research revealed that the presence of these nonbonding states and the associated energetics dominate the fascinating behavior of low-dimensional functional materials including biologic and organic species. It is my hope that this presentation will help increase the awareness of the importance of these electrons and revive interest in them. At the same time, an emphasis is also placed on the essentiality of interpreting the macroscopic properties of a material from the perspective of bond and nonbond formation, dissociation, relaxation and vibration, and the associated energetics and dynamics of charge repopulation, polarization, densification and localization. Multiferroicity ----Coupling between spin and ferroelectric orders J. –M. Liu Nanjing National Laboratory of Microstructures, Nanjing University, Nanjing 210093, China International Center for Materials Physics, Chinese Academy of Sciences, Shenyang, China http://pld.nju.edu.cn/ E-mail: [email protected] Recently, the multiferroic materials, in which the ferroelectric (FE) and magnetic orders coexist and are intimately coupled, have attracted much attention due to their technological relevance and fundamental science challenges. Among the single phase multiferroic materials, those multiferroics with noncollinear (spiral, helical, conical etc) and collinear spin orders, which exhibit novel and spin-order induced ferroelectricity, have been receiving special attentions. In this talk, I am going to have a brief overview of the major progresses in this stimulating field by addressing several substantial issues and our own preliminary work: (1) Thermodynamics and microscopic mechanisms of noncollinear spin order generated ferroelectricity: one-, two- and three-dimensional cases. (2) Origin of spiral spin order in multiferroic manganites and quantitative Monte Carlo simulation. (3) Mean-field theory of collinear spin order generated ferroelectricity: Ca3CoMnO6 as an example. (4) Routes to enhance multiferroicity. Hopefully, this overview allows a comprehensive understanding of the multifold interactions which eventually have impact on the magnitude of ferroelectric polarization in those multiferroics. References:  K. F. Wang, J. –M. Liu, and Z. F. Ren, Adv. Phys. 59 (4), 321-448 (2009).