News & Events

Speaker: Wei Su-Huai
Abstract Details: The kesterite-structured semiconductors Cu2ZnSnS4, Cu2ZnSnSe4 and their alloys are drawing considerable attention recently as the active layers in earth-abundant, low-cost thin-film solar cells. The additional number of elements in these quaternary compounds, relative to binary and ternary semiconductors, results in increased flexibility in the material properties. Conversely, a large variety of intrinsic lattice defects can also be formed, which have important influence on their optical and electrical properties, and hence their photovoltaic performance. I will review our recent theoretical studies on the structural, electronic and defect properties of the kesterite materials based on systematical density functional theory calculations and compare them with the better studied chalcopyrite materials CuGaSe2 and CuInSe2. Some highlights of the results observed are: (i) the strong phase-competition between the kesterites and the coexisting secondary compounds; (ii) the intrinsic p-type conductivity determined by the high population of acceptor CuZnantisites and Cu vacancies, and their dependence on the Cu/(Zn+Sn) and Zn/Sn elemental ratio; (iii) the role of charge-compensated defect clusters such as [2CuZn+SnZn], [VCu+ZnCu] and [ZnSn+2ZnCu] and their contribution to non-stoichiometry; (iv) the electron-trapping effect of the abundant [2CuZn+SnZn] clusters, especially in Cu2ZnSnS4; (v) the absolute surface energies of the kesterites and the origin of the p-type to n-type inversion at the surfaces. The calculated properties explain the experimental observation that Cu poor and Zn rich conditions result in the highest solar cell efficiency, as well as why there is an efficiency limitation in Cu2ZnSn(S,Se)4 cells when the S composition is high.
About the Speaker: Su-Huai Wei received his B.S. in Physics from Fudan University in 1981 and Ph.D. from the College of William and Mary in 1985. He joined the National Renewable Energy Laboratory in 1985 and is currently a Principal Scientist and Group Manager for the Theoretical Materials Science Group. His research is focused on developing electronic structure theory of materials, especially for semiconductors and energy related materials and applications. He has published more than 340 papers in leading scientific journals, including more than 58 in Physical Review Letters with an H index of 67. He is a Fellow of the American Physical Society.
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About the Speaker:

The Abdus Salam International Centre for Theoretical Physics (ICTP), in collaboration with the Nanyang Technological University (NTU) and Graphene Research Centre (GC) at the National University of Singapore (NUS), is jointly organizing a School on Modern Topics in Condensed Matter Physics to take place in Singapore from 28th January to 8th February 2013.

This school aims to introduce modern concepts and methods in condensed matter physics, with a focus towards the physics in low dimensions, covering areas such as

• Strong correlations and frustration in low dimensions
• Topological phases and topological insulators
• Interfaces, heterostructures and their new phases
• Two dimensional crystals: Graphene and others
• Competing and exotic orders in complex materials
• Techniques and advances in material science

from both experimental and theoretical points of view. The school is intended predominantly for young scientists (student and researchers). Limited funds are available for participants from Asia.

The application form can be accessed at the activity website, http://agenda.ictp.it/smr.php?2504

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Speaker: Anjan Soumyanarayanan
Affiliation: Harvard University & MIT, USA
Abstract Details: The last four years have witnessed the emergence of a new class of electronic materials whose properties are determined by the mathematical notion of topology. These `topological materials’ host surface states that mimic spin-polarized Dirac particles and are expected to be insensitive to material disorder. To harness the properties of these surface states and develop topological materials towards spin-based applications, it is crucial to understand and quantify their behavior on the nanoscale. Using scanning tunneling microscopy (STM), we visualize surface state Dirac fermions in antimony, and report the first simultaneous observation of Landau quantization and quasiparticle interference in any material. We establish the spectroscopic equivalence of these two momentum-resolved phenomena, and combine them to quantitatively reconstruct the surface state band structure. We use our observations to quantify the robustness of surface states to disorder, thereby informing the development of new topological materials. We further examine the spectroscopic effects of single atom impurities on the surface states.
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Speaker: Warren Pickett
Affiliation: UC Davies and GRC
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Speaker: Dietrich Belitz
Affiliation: University of Oregon, USA
Abstract Details: Many observables in a Fermi liquid, such as the density of states, the specific heat, and the spin susceptibility, are nonanalytic functions of the temperature, the frequency, or the wave number. I will discuss a general theoretical description of these anomalies, their origins, and their consequences. One overarching concept is the description of the Fermi-liquid state as an ordered state with a spontaneously broken continuous symmetry and the density of states as the order parameter. The corresponding Goldstone modes are soft two-particle excitations that cause the nonanalyticities. These get stronger with decreasing dimensionality, and suggest a possible correlation-induced instability of the Fermi-liquid state in d=2. They also have profound consequences for the ferromagnetic quantum phase transition in metals, which is found to be generically of first order. Various examples of these effects, as well as predictions of the theory, will be discussed, with emphasis on low-dimensional systems.
About the Speaker: http://physics.uoregon.edu/~belitz/db_virtual/db.html
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Speaker: J. M. D. Coey
Affiliation: School of Physics and CRANN, Trinity College, Ireland
Abstract Details: There are many reports in the literature of ferromagnetism in thin films and nanoparticles of nonmagnetic oxides doped with a few percent of transition metal cations. In some cases, the d0 materials, samples are ferromagnetic even when undoped. The reports are controversial, not least because they fly in the face of received wisdom regarding the magnetism of oxides. Based on consideration of superexchange, these materials would normally be expected to be paramagnetic when doped below the percolation threshold. The ferromagnetism is unusual, in that it a high-temperature phenomenon, which is largely anhysteretic. The magnetization process is controlled by dipolar interactions, and it can be shown that no only small fraction of the volume of the films or nanoparticles is actually magnetic. Based on this experimental analysis, two quite different models are proposed. One is a Stoner model of wandering axis ferromagnetism, where the magnetism resides in an impurity band associated with defects such as grain boundaries, and the associate density of states is poulated by charge transfer from a proximate charge reservoir. The other is quite different; it depends on giant orbital moments associated with pseudospin excitations in a graphene-like lattice with topological defects.
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Speaker: Slaven Garaj
Affiliation: NUS Department of Physics and Bioengineering and GRC
Abstract Details: Nanopores are versatile platform for studying structure and behavior of individual biomolecules, such as DNA, RNA and proteins. Here we present a new class of nanopores – graphene nanopores – which show unique molecular sensing properties. Graphene nanopore sensor, fabricated in freestanding atomically thin graphene membrane, is used to deduce the nano-fludic properties of graphene, and to detected conformation of many individual DNA molecules. We show that graphene nanopores have extremely high sensitivity (0.5nA/Angstrom) to small changes in the DNA diameter, when the diameter of the graphene nanopore closely matches that of a DNA molecule. Experimental results and theoretical modeling indicate that the graphene nanopores have potential to resolve features along the length of a DNA molecule whose separation is comparable to the distance between nucleobases. Those results underscore graphene nanopore platform as an important research route towards the development of physical (electrical) DNA sequencing techniques. At the end, I will give a broad overview of nanopore physics, and discuss the other applications of nanopores in understanding nano-fluidic flows and local surface-liquid interaction, all crucial for energy science and technology.
About the Speaker: Slaven Garaj received PhD from Swiss Federal institute of Technology Lausanne (Switzerland) in the field of solid-state physics. He continued his research career at Harvard University, working at the intersection of nano-electronics and biophysics, particularly by developing novel methods for electrical (4th generation) DNA sequencing based on nanopores. Throughout his career, his different research projects attracted general public attention and were featured in international media and professional magazines (such as BBC News, New Scientist, Technology Review, MRS Bulletin, etc). Dr. Garaj is currently a NRF Fellow, affiliated with Department of Physics and Department of Bioengineering of the National University of Singapore.
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Speaker: Litao SUN SEU-FEI
Affiliation: Nano-Pico Center, Key Laboratory of MEMS, China
Abstract Details: With the continuous improvement of in situ techniques inside transmission electron microscope (TEM), the capabilities of TEM extend beyond structurual characterization to high-precision nanofabrication and property measurement, which not only enriches the experimental methods of nanoresearch, but also provides new opportunities for the development in nanoscience and nanotechnology. Based on the idea of 'setting up a nanolab inside a TEM', we present our recent progress in carbon related nanomaterials including in situ growth, nanofabrication with atomic resolution, in situ property characterization, nanodevice construction and other possible applications (e.g. a 5nm-diameter hole on graphene for third-generation gene sequencing, the spongy graphene as an ultra-efficient sorbent for oils and organic solvents, etc.). References Science 312, 1199 (2006); Nature Nanotechnology 2, 307 (2007); Phys. Rev. Lett. 101, 156101 (2008); Phys. Rev. Lett. 105, 196102 (2010); Adv. Mater. 24, 1844 (2012); Adv. Mater. 24, 5124 (2012); Adv. Funct. Mater. 22, 4421 (2012).Â
About the Speaker: Prof. Litao Sun currently serves as vice dean of School of Electronic Science and Engineering, Southeast University (SEU), the director of SEU-FEI Nano-Pico Center, and founding chairman of IEEE Nanotechnology Council Nanjing Chapter. He received his PhD from the Shanghai Institute of Applied Physics, Chinese Academy of Sciences in 2005. He worked as a research fellow at University of Mainz, Germany from 2005 to 2008, and a visiting professor at University of Strasbourg, France from 2009 to 2010. Since 2008, he joined SEU and honored as a Distinguished Professor. His research currently focus on controlled synthesis and characterization of carbon-related nanomaterials, in-situ experimentation in the electron microscope, and fundamental understanding and applications of nanostructures in micro/nano systems. He is the author and co-author of over 70 papers and review articles on international journals including Science, Nature Nanotechnology, Phys. Rev. Lett., etc.
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Speaker: Dr. KOH Yee Kan
Abstract Details: Many emerging devices including graphene devices contain a high density of interfaces, which are usually located in close vicinity of active functioning areas of the devices. Thus, knowledge on how heat is transported across the interfaces is crucial for the design and performance of these novel devices. Unfortunately, understanding of heat transfer at nanometer length scales is still incomplete, partly due to the challenges in measuring heat transport at such a small length scale. As a result, thermal management of the emerging devices is often designed in an ad hoc manner. In this talk, I will present a state-of-the-art characterization technique, called time-domain thermoreflectance (TDTR), to measure the heat transport across interfaces. Using the novel technique, we studied the physics of heat transport across different interfaces. I will first discuss heat transport across atomically discrete semiconductor/semiconductor interfaces and how phonons are scattered by these interfaces. Then, I will present our TDTR measurements on metal/graphene interfaces, which advance our understanding on heat dissipation through graphene interfaces. Finally, I will present some preliminary measurements on heat transport across different metal contacts for graphene devices. Our results facilitate choices of metal contacts for better thermal management of future graphene devices.
About the Speaker: KOH Yee Kan received a B.S. and a M.Eng. in Mechanical Engineering from the University of Technology Malaysia. In 2004, he enrolled in the University of Illinois at Urbana-Champaign, and obtained a M.S. in Physics in 2007 and a Ph.D. in Materials Science and Engineering in 2010 from the university. After his graduation in 2010, Koh joined the Department of Mechanical Engineering in National University of Singapore (NUS) as an assistant professor. Koh's expertise is in heat transport in nanostructures and across interfaces, with emphasis on applications in thermoelectric energy conversion and thermal management of emerging electronic devices (e.g., graphene devices). Koh is an expert in time-domain thermoreflectance (TDTR), a modulated pump-probe technique to measure heat conduction on nanometer length scales. He has published 13 papers on nanoscale heat transport and has received numerous awards, including the Ross J. Martin award (2010), SMF-NUS Research Horizons Award (2010), NUS Young Investigator Award (2011), and the prestigious Fulbright fellowship (2004).
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About the Speaker: The Industry Liasion Office, a division of NUS Enterprise, will present “Building Value on Your Basic Research on Graphene”.
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