News & Events

Speaker: Wong Toon Suan
Abstract Details: Mr. Wong has more than 35 years of experience in water-energy nexus and infrastructure, spanning across both private and public sectors. He most recently served as Senior Vice President of the Singapore Power Group and was formerly Director of MEWR/PUB.  Mr. Wong is an alumnus, registered professional engineer and senior member of the Institute of Engineers of Singapore. He also attended the Advanced Management Program at Harvard Business School. He is the current President of the Gas Association of Singapore and a member of SPRING's National Energy Standard Committee. He was the Assistant Executive Director of the Environment and Water Industry Council (EWI) assisting in managing the water R&D ecosystem and promoting Singapore as global hydrohub (funded by NRF (2011:$330M)). While he was with Singapore Power Group, he held various senior positions, including posting to a listed company in Australia.
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Speaker: Giovanni Vignale
Affiliation: University of Missouri-Columbia
Abstract Details: Layered structures of materials with different magnetic and/or spin orbital properties are rapidly emerging as most promising candidates for spintronic applications. A slew of new effects is under study, which have no analogue in bulk materials or differ subtly from their three-dimensional counterparts. Spin polarization and spin transfer torque can be generated by passing an electric current near an interface. Novel types of anisotropic magnetoresistance (e.g., the recently observed unidirectional spin Hall magnetore- sistance are possible. Spin polarized currents may produce a nonlocal form of the anomalous Hall effect. In this talk I review our recent theoretical work on these effects vis-a-vis experiment. In particular, I present a new theory of the nonlocal anomalous Hall effect, which occurs in a non-magnetic heavy metal (eg. Pt) that is interfaced with a ferromagnet.About the speaker: Giovanni Vignale is Curators' Professor of Physics at the University of Missouri-Columbia where he leads the Theoretical Condensed Matter Physics group. After graduating from the Scuola Normale Superiore in Pisa (Italy) in 1979 and gaining his Ph.D. at Northwestern University (USA) in 1984, he carried out research at the Max-Planck-Institute for Solid State Research in Stuttgart (Germany), and Oak Ridge National Laboratory (USA). He became a Fellow of the American Physical Society in 1997. Professor Vignale's main areas of research are many-body theory and density-functional theory, where he is recognized by the community for his seminal contributions on spintronics, on the density functional theory of electronic systems in the presence of magnetic fields, and on the time-dependent current density functional theory. He is the author of two books, the well known textbook Quantum Theory of the Electron Liquid and The Beautiful Invisible, and has published more than 200 research papers."
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Speaker: Catalin Spataru
Affiliation: Sandia National Laboratories, USA
Abstract Details: Metal contacts play an important role for thin-film thermoelectric (TE) devices, especially in high heat-flux applications (e.g. chip cooling) where reduced contact resistivity (?C ) is critical to device performance. In the first part of the talk I will present our effort towards understanding the limits of low-?C in realistic metal-contacts to TE materials. Using a combination of approaches ranging from ab initio calculations to higher-level continuum theories we investigate the structural, electronic and transport properties of electrical contacts to TE materials. The study benefits from experimental input that provides atomic scale structure validation of the interfaces considered computationally. Various properties of supported graphene films depend strongly on the exact positions of carbon atoms with respect to the underlying substrate. While density functional theory (DFT) can predict atom position in many systems, it cannot be applied straightforwardly to systems that are incommensurate or have large unit cells, such as graphene on a BN surface. In the second part of the talk I will present our effort in addressing these limitations by developing a simple moiré model with parameters derived from DFT calculations for systems strained into commensurate structures. Our moiré model, which takes into account the flexural rigidity of graphene and includes the influence of the substrate, is able to reproduce the DFT-relaxed carbon positions with an accuracy of <0.01 Å.
About the Speaker: Catalin Spataru is an expert in ab initio modeling of material properties. He has over 15 years of experience with studies of structural, electronic and optical properties of solids and novel materials of various kinds, ranging from bulk to low-dimensional systems, and from metals to insulators, in both equilibrium and out-of-equilibrium conditions. He has extensive experience on the use of ab initio Density Functional Theory and quantum many-particle methods such as the GW approximation for the electron self-energy and the Bethe-Salpeter method for excitonic effects. Catalin obtained his PhD in Physics at the University of California at Berkley (USA) in 2004, following which he held postdoctoral positions at the Lawrence Berkeley National Laboratory and Columbia University. In 2008 he became a member of the technical staff at the Sandia National Laboratories.
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Speaker: Norbert Klein
Affiliation: Imperial College, London
Abstract Details: Passive microwave-to-terahertz resonators and transmission line structures offer a wide potential for contact-free material characterization and sensor applications. As an example, our semi-open dielectric loaded microwave cavities based in functional ceramics have been successfully commercialized for liquid explosive detection in passenger checkpoints, and are still in use for our research, for example as biosensor for accurate haemoglobin measurements on sub-microlitre blood samples and as method for contact-free characterization and quality assessment of large area graphene layers and FET structures. At terahertz frequencies, our photonic bandgap and spoof plasmon structures have been successfully used for nanolitre bioliquid detection, aiming towards label-free cancer cell detection within microfluidic lab-on-chip systems. Graphene has a huge potential to be used as material for THz modulators and detectors, potentially enabling low-cost THz communication and imaging systems. First results with transparent microwave modulators based on large-area self-gated transparent G-FET structures will be presented. In combination with graphene as biointerface, the microwave-to-terahertz frequency range offers challenging opportunities for sensor applications within health and security.
About the Speaker: Prof. Norbert Klein is Professor for Electromagnetic Materials and Sensors in the Department of Materials at Imperial College London. He is founder and Director of Imperial’s Centre for Terahertz Science and Engineering and he spun out a company which successfully commercializes microwave sensor systems for airport security (www.emisens.com). Prof. Klein is author or more than 160 publications in science- and engineering journals and he holds a large number of patents in microwave/ THz sensor and communication technology. His current research comprises label-free electromagnetic biochemical sensors including microfluidics, electromagnetic material characterization and graphene device fabrication and device physics.
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Speaker: Du Wei
Affiliation: The Nijhuis Group
Abstract Details: Molecular electronic control over plasmons offers a promising route for on-chip integrated molecular-plasmonic devices for information processing and computing. In this seminar, I will discuss on-chip molecular electronic plasmon sources consisting of tunnel junctions based on self-assembled monolayers (SAMs) sandwiched between two metallic electrodes that excite localized plasmons and surface plasmon polaritons by tunnelling electrons. The plasmons originate from single, diffraction-limited spots within the junctions, follow power-law distributed photon statistics, and have well-defined polarisation orientations. The structure of the SAM and the applied bias influence the observed polarization. We also show molecular electronic control of the plasmon intensity by changing the chemical structure of the molecules and by bias-selective excitation of plasmons using molecular diodes.
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Speaker: Luo Xin
Affiliation: Centre for Advanced 2D Materials, NUS
Abstract Details: Phonons are important collective excitations that can affect carrier scattering and thermal conductivity. Here, we explore from first principles calculations the properties of phonons in 2D layered materials, and compare our results with experimental Raman spectra. Our calculations explain the physical origin of frequency shifts observed due to changing film thickness,[1-3] or due to adsorbates.[4] We find that the formation of a surface in the 2D material leads to larger interatomic force constants at the surface, which results in experimentally observed anomalous blue-shifts of the E12g mode in MoS2,[5] and the E12g and B12g modes in WSe2.[6] The effect of a substrate on the phonon frequencies will also be discussed. Finally, we also explore the ultra-low frequency interlayer phonon modes that are peculiar to 2D materials, and show that these frequencies can be described well using a nearest neighbor force constant model. We find that the frequencies of the interlayer shear modes blue-shift for AB stacked materials, and red-shift for ABC stacked materials, as the number of layers increases.[7] This prediction is observed in experiments[8] and can be understood from an intuitive bond polarizability model. Our studies shed light on a fundamental understanding of phonons in 2D layered materials and guide experimentalists to use Raman spectroscopy as a sensitive probe of the film thickness, stacking order, as well as environmental effects in 2D materials. [1] Nano Lett. 13, 1007 (2013) [2] Nano Lett. 15, 3931 (2015) [3] Phys. Rev. B 90, 245428 (2014) [4] Adv. Electron. Mater. 1400037 (2015) [5] Phys. Rev. B 88, 075320 (2013) [6] Phys. Rev. B 88, 195313 (2013) [7] Scientific reports 5, 14565 (2015) [8] Adv. Mater. 27, 4502 (2015)
About the Speaker: Dr. Luo Xin graduated from Sun Yat-sen University where he obtained his bachelor degree in 2006 and doctoral degree in 2011, both in physics. During his PhD, Luo Xin used first principles calculations and non-equilibrium Green function’s to investigate the strain tunable electroresistance of ferroelectric tunneling junctions and the 2D electron gas between perovskite oxide interfaces. After his graduation, he worked as a research scientist in the Institute of High Performance Computing, A*STAR, Singapore. Since 2013, he moved to the Centre for Advanced 2D materials and Graphene research center in the National University of Singapore as a research fellow. In his research in Singapore Luo Xin collaborates closely with experimentalists using first principles approaches and simple models to make predictions and explain the vibrational and transport properties of 2D layer materials such as Bi2Te3, MoS2 and few layer black phosphorus.
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Speaker: Mansoor B. A. Jalil
Affiliation: CA2DM & Faculty of Engineering, NUS
Abstract Details: The concepts of gauge fields and gauge symmetry are pervasive in modern physics [1]. Recently, the concepts of gauge potentials have been introduced to the field of spintronics as a means to describe spin transport and dynamics, especially in spin-orbit coupling systems [2,3]. In this talk, I will present the gauge field description of spin-orbit torques (SOT). At present, SOT is being heavily-researched as it can potentially lead to a highly efficient method of switching magnetic memories. The phenomenon of SOT was first predicted by us in a Rashba system in 2007, via the gauge field description [4,5]. The SOT formula was later re-derived via the equivalent Boltzmann model by Manchon in 2008 [6], and since then has been experimentally investigated in a variety of systems, beginning with a heavy metal interfacial system in 2010 [7]. I will also present the gauge description of SOT in other two-dimensional systems like graphene [8] and topological insulators [9]. Secondly, I would also describe the application of gauge theory to the spin Hall effect (SHE) [10,11], and introduce the associated concept of the Yang-Mill’s spin motive force [12]. Finally, the equivalence to the Kubo treatment will be discussed [13]. [1] C. N. Yang, Physics Today 67, 45 (2014). [2] T. Fujita, M. B. A. Jalil, S. G. Tan, and S. Murakami, Journal of Applied Physics 110, 121301 (2011). [3] S. G. Tan and M. B. A. Jalil, Introduction to the Physics of Nanoelectronics (Elsevier, ISBN 9780857095114, 2012). [4] S. G. Tan, M. B. A. Jalil, and X.-J. Liu, arXiv preprint arXiv:0705.3502 (2007). [5] S. G. Tan, M. B. A. Jalil, T. Fujita, and X.-J. Liu, Annals of Physics 326, 207 (2011). [6] A. Manchon and S. Zhang, Physical Review B 78, 212405 (2008). [7] I. M. Miron et al., Nature 476, 189 (2011). [8] J. Chen, M. B. A. Jalil, and S. G. Tan, AIP Advances 3, 062127 (2013). [9] J. Chen, M. B. A. Jalil, and S. G. Tan, Journal of the Physical Society of Japan 83, 064710 (2014). [10] S. G. Tan and M. B. A. Jalil, Journal of the Physical Society of Japan 82, 094714 (2013). [11] S. G. Tan, M. B. Jalil, C. S. Ho, Z. Siu, and S. Murakami, Scientific Reports 5, 18409 (2015). [12] C. S. Ho, M. B. A. Jalil, and S. G. Tan, EPL (Europhysics Letters) 107, 37005 (2014). [13] D. Xiao, M.-C. Chang, and Q. Niu, Reviews of Modern Physics 82, 1959 (2010).
About the Speaker: Mansoor B. A. Jalil is an Associate Professor of the NUS Department of Electronic and Computer Engineering. His expertise in the theory of nanoscale electronic devices, particularly electronic charge and spin transport, is reflected in more than 200 journal publications and several book contributions in this field, and is recognized by various research-based awards over the years. Mansoor’s research interests comprise topics such as (i) semiconductor spintronics, including spin-orbit effects, spin Hall effect, and spin injection, (ii) metal-based spintronics, including spin transfer switching, magnetic random access memory, and spin-orbit coupling effects, (iii) transport in graphene devices including Klein tunneling, helical and magnetoresistive transport, (iv) topological insulators, including spin torque and magnetoresistance.
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Speaker: Mirco Milletari
Affiliation: Department of Physics & CA2DM, NUS
Abstract Details: Spintronics, the science that aims at utilising the spin degrees of freedom in addition to the charge of electrons for low-power operation and novel device functionalities, has seen rapid developments in the past decade. In particular, the spin Hall (SH) effect, i.e. the emergence of a transverse spin current in response to an applied longitudinal electric field, has attracted much interest for the possibility of building all-electric spin manipulation devices [1]. The efficiency of the Spin current generation is measured by the SH angle. While in semiconductors the SH angle is quite small (0.0001-0.001) [2], it was shown that a giant SH conductivity can be achieved in graphene decorated with small doses of resonant, spin-orbit active adatoms [3]. It was argued that in this case, the effect is mostly due to the semiclassical skew-scattering mechanism, where electrons of different spins are scattered asymmetrically. In this talk, I will present present a rigorous microscopic theory of the extrinsic spin Hall effect in disordered graphene based on a nonperturbative quantum diagrammatic treatment incorporating skew scattering and anomalous – impurity concentration-independent – quantum corrections on equal footing [4, 5]. Our self-consistent approach – where all topologically equivalent noncrossing diagrams are resummed – unveils that the skewness generated by spin-orbit-active impurities deeply influences the anomalous component of the SH conductivity, even in the weak scattering regime. This seemingly counterintuitive result is due to the symmetry structure induced by spin-orbit coupling, for which the commonly Gaussian white noise approximation is generally invalid. Our treatment shows that it is possible to experimentally access regions in parameter space where anomalous quantum contributions to the SH conductivity are dominant. Finally, we assess the role of quantum interference corrections by evaluating an important subclass of crossing diagrams, considered only recently in the context of the anomalous Hall effect [6]. We show that diagrams encoding quantum coherent skew scattering events, display a strong Fermi energy dependence, dominating the anomalous spin Hall component away from the Dirac point. Our findings open up the intriguing prospect of measuring quantum interference fingerprints in nonlocal spin signals. [1] J. Sinova, S. O. Valenzuela, J. Wunderlich, C. H. Back, T. Jungwirth, Rev. Mod. Phys. 87 (2015). [2] Y. K. Kato, R.C. Myers, A.C. Gossard and D. D. Awschalom, Science 306, 1910 (2004). [3] A. Ferreira, T. G. Rappoport, M. A. Cazalilla, and A. H. Castro Neto, Phys. Rev. Lett. 112, 066601 (2014). [4] M. Milletari and A. Ferreira, arXiv:1601.08076 (2016). [5] M. Milletari and A. Ferreira, arXiv:1604.03111 (2016). [6] A. Ado, I.A. Dmitriev, P. M. Ostrovsky and M. Titov, EPL 111, 37004 (2015).
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Speaker: Dr. Henrique Rosa, Photonics Group
Abstract Details: Nonlinear optics research is of fundamental importance for nanomaterials science, to understand how materials electronic and optical properties behave and can be controlled under high intensity field interaction. In this seminar, I will present some of the latest results from the Photonics Group in 2D Materials nonlinear optics characterization and applications. I will discuss how nonlinear optical parametric process can be used to probe nanomaterials properties, showing our results and publications about graphene saturable absorbers for ultrafast fiber lasers application, ultrafast pump-probe spectroscopy in stacked CVD graphene, and Second- and Third-Harmonic Generation (SHG/THG) in 2D crystals.
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Speaker: Endre Horva?th
Affiliation: EPFL, Switzerland
Abstract Details: The discovery of photocatalytic water splitting and photocatalysis dates back to the 1970s. Since then, several semiconductor nanoparticles have been found to have remarkable photocatalytic activity to eliminate health-threatening bacteria, viruses, worms and persistent, bioaccumulative organic water pollutants as pharmaceuticals, pesticides and endocrine disruptors. Surprisingly, despite significant research efforts, studies are mainly remained in the stage of laboratory experiments and only limited number of products using this technology can be found on the market. Materials and devices with sufficient efficiency, stability and low cost are yet to be demonstrated. Among the known semiconductor photocatalysts, titanium dioxide (TiO2) is the most popular owing to its excellent chemical stability, low toxicity and low cost. This talk will introduce this functional material in nanowire shape. First, characterization of individual, single crystalline titanium oxide nanofilaments, as well as 3D porous composites built from elongated titanium oxide particles will be shown. I will discuss the main technical barriers that impede the commercialization of this technology and show several strategies as potential countermeasures to improve the photocatalytic efficiency of titania based solar environmental purification systems. As an example, the first prototype of a low cost, durable and easy to operate solar- thermal water purification device will also be presented, which allows the production of bio-hazard-free drinking water from contaminated water resources.
About the Speaker: Endre Horvath is research scientist at the Institute of Physics, École Polytechnique Fédérale de Lausanne (Switzeland), and a founder of a Swoxid, a start-up company focused on photocatalytic water purification technologies."
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