News & Events

Speaker: Jens Martin
Affiliation: NUS Physics
Abstract Details: Measurements of thermodynamic quantities provide complentary information to transport experiments. In particular, measurements of the local electronic compressibility gives unique insight into the microscopic physics of electronic interactions of two dimensional electron systems (2DES). In this talk I will explain the measurement approach and review results on high mobility 2DES in semiconducting hetero-structures and graphene.
Click HERE for directions

—
To view all the upcoming seminars, you can visit: https://graphene.nus.edu.sg/news-events/events/
You may also Like & Subscribe our following channels below to receive instant notifications for new announcements.

Speaker: Jiwoong Park
Affiliation: Cornell University, USA
Abstract Details: In the past few years, the science and application of graphene, a hexagonal crystal entirely made of carbon atoms, has experienced a revolutionary change, mainly due to the development of large scale growth methods based on chemical vapor deposition (CVD). This enabled production of single-atom-thick conducting films in meter scale. However, it still remains difficult to characterize and process graphene films, while maintaining many of their excellent properties. Namely, the materials science and engineering of graphene is still in its infancy. First, these CVD graphene films are polycrystalline as grown. We recently reported the first atomic-resolution imaging of individual grain boundaries in graphene membranes as well as a dark-field transmission electron microscopy (TEM) technique that rapidly characterizes graphene grain structures. Using these new synthesis, fabrication and TEM characterization methods, our group can now investigate the structure of grain boundaries and probe their effects on the mechanical, electrical, and chemical properties of CVD graphene. In addition, CVD graphene produces bilayer regions with different stacking structures. We showed that the angle dependent interlayer interaction leads to fascinating electrical and mechanical properties of bilayer graphene in different configurations (vertical junctions). More recently, we used some of these newly developed capabilities in order to demonstrate atomically thin circuitry entirely made of graphene (conductor) and hexagonal boron nitride (insulator), which presents exiting new directions for these materials. Our results represent an important step towards developing atomically thin integrated circuitry and enable the fabrication of electrically isolated active and passive elements embedded in continuous, one-atom-thick sheets, which could be manipulated and stacked to form complex devices at the ultimate thickness limit. References [1] M. P. Levendorf, C.-J. Kim, L. Brown, P. Y. Huang, R. W. Havener, D. A. Muller, and J. Park, “Graphene and Boron Nitride Lateral Heterostructures for Atomically Thin Circuitry”, Nature, 488, 627-632 (2012). [2] R. W. Havener, H. Zhuang, L. Brown, R. Hennig, and J. Park, “Angle-Resolved Raman Imaging of Interlayer Rotations and Interactions in Twisted Bilayer Graphene”, Nano Letters, 12, 3162-3167 (2012). [3] A. W. Tsen, L. Brown, M. P. Levendorf, F. Ghahari, P. Y. Huang, C. S. Ruiz-Vargas, R. W. Havener, D. A. Muller, P. Kim, and J. Park, 'Tailoring Electrical Transport across Grain Boundaries in Polycrystalline Graphene', Science, 336, 1143-1146 (2012). [4] P. Y. Huang, C. S. Ruiz-Vargas, A. M. van der Zande, W. S. Whitney,  M. P. Levendorf, J. W. Kevek, S. Garg, J. S. Alden, C. J. Hustedt, Y. Zhu, J. Park, P. L. McEuen, D. A. Muller, “Grains and Grain Boundaries in Single-Layer Graphene Atomic Patchwork Quilts,” Nature 469, 389-392 (2011).
About the Speaker: Dr. Jiwoong Park is an associate professor in the Department of Chemistry and Chemical Biology and an executive member of Kavli Institute for Nanoscale Science at Cornell University, with a PhD in physics from the University of California, Berkeley. Before coming to Cornell, he was a Junior Fellow at Rowland Institute at Harvard University. His research interests focus on the electronic and optical properties of nanoscale materials including semiconductor nanowires, carbon nanotubes and graphene. He made contributions on understanding photoelectic and thermoelectric properties of 1D and 2D nanostructures synthesized by his group. Dr. Park is a recipient of NSF CAREER award (2008), Presidential Early Career Awards for Scientists and Engineers (2009) and Alfred P. Sloan Research Fellowship (2010).
Click HERE for directions

—
To view all the upcoming seminars, you can visit: https://graphene.nus.edu.sg/news-events/events/
You may also Like & Subscribe our following channels below to receive instant notifications for new announcements.

Speaker: Sergei Manzhos
Affiliation: Dept. Mech. Eng. NUS
Abstract Details: Ability to model reactions at interfaces is critical for the development of new catalysts and energy conversion and storage technologies. Specifically, graphene, doped graphene, and other carbon allotropes have shown potential as effective noble metal-free catalysts for oxygen reduction1. Calculations of processes at graphene-like materials have the potential to guide functional material design. Most theoretical studies focus on static calculations of structures and energetics to describe or predict essentially dynamic outcomes (the volcano curve is an example). This is because the modeling of dynamics requires a potential energy surface (PES), and for the vast majority of molecule-surface systems, PES's do not exist. This makes theoretical predictions shaky. I will focus on methods to predict dynamics at the surfaces, including reaction dynamics and vibrational dynamics. I will describe a method to build potential energy surfaces, including multi-body effects and without imposing a pre-determined functional form, so that all chemical interactions can be accounted for 2. A catalyst can be characterized by vibrational signatures of reactants, intermediates, and products. If anharmonicity and coupling of modes are significant, the harmonic frequencies from DFT codes can be in error of dozens or hundreds cm-1 (on top of the error due of the electronic structure code), thwarting spectral assignment and thereby material design. Today there do not exist computational tools to compute routinely anharmonic spectra. I will present a method we have been developing to compute anharmonic vibrational spectra for molecules at interfaces3. I will also discuss strain and its effect on the phonon spectrum of graphene. Stress-strain relations including only Young modulus and third-order elastic moduli are still used4. Even a quick calculation shows higher-order expansions are needed. For planar graphene, a comprehensive theoretical analysis of stress-strain exists5. It still needs be computed for other graphene-like materials. We are also interested in out-of-plane distortions: a recent high-profile study reported changes in the phonon spectrum of graphene when under stress in non-planar configurations6. Our preliminary calculations show that the frequency shift is a non-linear function of strain and is also mode-dependent. This suggests an approach to characterize graphene-like materials. 1. S. Kattel, P. Atanassov, B. Kiefer, J Phys Chem C 116, 17378 (2012); P. Wu, P. DU, H. Zhang, C. Cai, J Phys Chem C 116, 20472 (2012); L. Qu, Y. Liu, J.-B. Baek, L. Dai, ACS Nano 4, 1321 (2010); Z. Yang, Z. Yao, G. Li, G. Fang, H. Nie, Z. Liu, X. Zhou, X. Chen, S. Huang, ACS Nano 6, 205 (2012) 2. S. Manzhos, K. Yamashita, Surf Sci 604, 555 (2010); S. Manzhos, K. Yamashita, T. Carrington, Comput Phys Commun 180, 2002 (2009) 3. S. Manzhos, T. Carrington, K.  Yamashita, Surf Sci 605, 616 (2011); M. Chan, K. Yamashita, T. Carrington, S. Manzhos, MRS Proceedings, in print. 4. C. Lee, X. Wei, J. W. Kysar, J. Hone, Science 321, 385 (2008) 5. X. Wei, B. Fragneaud, C. A. Marianetti, J. W. Kysar, Phys Rev B 80, 205407 (2009) 6. J.-U. Lee, D. Yoon, H. Cheong, Nano Lett 12, 4444 (2012)
Click HERE for directions

—
To view all the upcoming seminars, you can visit: https://graphene.nus.edu.sg/news-events/events/
You may also Like & Subscribe our following channels below to receive instant notifications for new announcements.

Speaker: David B. Mitzi
Affiliation: IBM T.J. Watson Research Center
Abstract Details: While chalcopyrite and kesterite materials offer a promising pathway toward commercialized photovoltaic technology, the complicated multi-element nature of these materials generally leads to the requirement of more complex and costly deposition processes. This talk will focus on the development of a relatively simple liquid-based deposition process that has enabled the fabrication of high-performance CuIn1-xGaxSe2-ySy (CIGS) and Cu2ZnSnSe4-ySy (CZTS) absorber layers, with resulting device power conversion efficiencies of as high as 15% and 11%, respectively. The devices are compared using a variety of physical characterization tools, including temperature-dependent J-V, external quantum efficiency and capacitance spectroscopy, leading to a better understanding of factors limiting device performance. For CZTS, the combination of new record efficiency, earth abundant metal starting materials, and solution-based processing opens opportunities for development of a potentially pervasive PV technology.
Click HERE for directions

—
To view all the upcoming seminars, you can visit: https://graphene.nus.edu.sg/news-events/events/
You may also Like & Subscribe our following channels below to receive instant notifications for new announcements.

Speaker: Ong Zhun Yong
Affiliation: University of Texas at Dallas
Abstract Details: In the modeling of graphene, the substrate is usually regarded as nothing more than a static object providing physical support and a different effective permittivity. However, the properties of graphene are modified through its interaction with the substrate. In the first half of this talk, I discuss carrier scattering by interfacial plasmon-phonons and charged impurities in supported and top-gated graphene. The effect of dielectric material and thickness are analyzed. In the latter half, the lattice and electronic contribution to the Kapitza conductance are presented. In particular, I show how dynamic screening affects the electronic Kapitza conductance.
About the Speaker: The speaker graduated from the National University of Singapore in 2004 with Bachelor of Science (First Class Honours) in physics and computational science. Following a one-year stint in the Institute of Materials Research and Engineering, he entered the doctoral programme of the physics department of the University of Illinois at Urbana-Champaign in 2005 and graduated with a PhD in 2011. He is currently a research associate in the department of materials science and engineering of the University of Texas at Dallas. His research interests are in the theory and simulation of electro-thermal transport in low-dimensional heterostructure materials. Among other things, he has worked on the theory of remote impurity and phonon scattering in graphene heterostructures, the simulation of Kapitza conductance in low-dimensional nanomaterials, the electro-thermal transport through grain boundaries, and experimental and simulation studies of high-field transport in graphene.
Click HERE for directions

—
To view all the upcoming seminars, you can visit: https://graphene.nus.edu.sg/news-events/events/
You may also Like & Subscribe our following channels below to receive instant notifications for new announcements.

Speaker: William Yim Wai Leung
Affiliation: IHPC, A*STAR
Abstract Details: Hydrogen under high pressure received much attention recently because of its role in superconductivity and energy storage. Novel phenomena were discovered by various high pressure experimental methods, e.g. H2 vibron softening induced by SiH4...H2 interaction. To help understand the interaction mechanisms in the compressed solid, quantum mechanical calculations were used to investigate the electronic, optical and thermodynamic properties. For this purpose, density functional theory (DFT) calculations were widely used. However, DFT should be used with great care especially when strong electron correlation and non-local interaction are important. In this talk, I will report our recent advance on understanding the H2 vibron softening mechanism in SiH4...H2 at low pressure regime, electronic band structures of compressed AlH3 and the thermodynamics of dense hydrogen. The challenges of theoretical prediction in compressed materials will also be discussed.
Click HERE for directions

—
To view all the upcoming seminars, you can visit: https://graphene.nus.edu.sg/news-events/events/
You may also Like & Subscribe our following channels below to receive instant notifications for new announcements.

Speaker: Prof. Pinaki Sengupta
Affiliation: Nanyang Technological University
Abstract Details: Geometrically frustrated spin systems are known to exhibit novel quantum phenomena. One example is the unique non-monotonic field dependence of the magnetization and the associated emergence of magnetization plateuas in a class of frustrated spin compounds commonly known as the Shastry-Sutherland compounds after their underlying magnetic lattice. In this talk, I shall discuss the mechanism for the formation of these plateaus. In the first half of the talk, I shall compare the quantitative predictions from our recently developed unconstrained Chern-Simons theory with experimental observations in SrCu2(BO3)2. Interestingly, our theoretical calculations predict that at the plateaus, the elementary magnetic excitations, triplons, form well-defined stripes. Evidence of such stripes have recently been reported in TmB4, another compound belonging to this family. In the second half of the talk, I shall discuss the formation of magnetizaion plateaus in a new family of Shastry-Sutherland compounds, viz., the rare-earth tetraborides. The bare Shastry-Sutherland model needs to be supplemented by additional longer range interactions to describe the magnetic properties of these compounds. I shall discuss he underlying microscopic models and our recent studies of these models.
Click HERE for directions

—
To view all the upcoming seminars, you can visit: https://graphene.nus.edu.sg/news-events/events/
You may also Like & Subscribe our following channels below to receive instant notifications for new announcements.

Speaker: Goki Eda
Abstract Details: Layered compounds in which individual layers are held together by weak van der Waals forces represent a rich source of two-dimensional (2D) molecular crystals. The ability to isolate atomically thin 2D crystals from layered parent structures enables observation of intriguing physical phenomena associated with its dimensionality while also opening up new perspectives on potential applications. Solution-based exfoliation of layered materials is a scalable route to producing 2D crystals and manipulating them in bulk quantities. In this talk, I will discuss chemical exfoliation of graphite and molybdenum disulfide (MoS2) and their unique properties. Chemical exfoliation often leads to chemical and physical modification of the initial material. The electronic properties of the derived 2D crystals deviate from those of the pristine materials, thus leading to emergence of properties absent in the pristine materials. In the case of graphene, strong covalent modification by oxygen functionalization in the exfoliation process leads to localization of pi electrons and formation of local energy gaps. This manifests as unique photoluminescence behavior in graphene oxide. Similar but non-chemical modification takes place in some layered transition metal dichalcogenides (LTMD) leading to remarkable changes in their electronic properties. Chemical exfoliation of MoS2 and WS2 compounds via lithium intercalation leads to formation of 2D sheets with unique mixed phase structures. Potential applications of these materials will be discussed.
Click HERE for directions

—
To view all the upcoming seminars, you can visit: https://graphene.nus.edu.sg/news-events/events/
You may also Like & Subscribe our following channels below to receive instant notifications for new announcements.

Speaker: Ivana Petkovi?
Affiliation: SPEC-CEA Saclay, France
Abstract Details: Edges in two dimensional electron systems (2DES) are central to the quantum Hall effect which depends for its existence on chiral edge currents with vanishing backscattering. But edges also host collective excitations in the form of chiral, globally neutral edge magnetoplasmons (EMP). Much investigated in conventional 2DES, little is known about them in the newly discovered graphene system where electrons obey massless relativistic dynamics. By timing the propagation of narrow wave-packets along the edge of an exfoliated graphene sample on picosecond time scales, we find that the propagation is chiral with low attenuation and the velocity is quantized on the Hall plateaus. We extract the electron drift velocity, which we show to be slightly less than the Fermi velocity, as expected for the abrupt edge of graphene. It is substantially higher than in conventional quantum 2DES, which makes it experimentally accessible. Spatial spread of charge imbalance is shown to be narrower than for conventional soft edged systems.
Click HERE for directions

—
To view all the upcoming seminars, you can visit: https://graphene.nus.edu.sg/news-events/events/
You may also Like & Subscribe our following channels below to receive instant notifications for new announcements.

Speaker: Prof Shun-Qing Shen
Affiliation: Hong Kong University
Abstract Details: Topological insulator is an insulator that always has a metallic boundary. These metallic boundaries originate from the topology of the band structure of solids, which is insensitive to the geometry of system and cannot change as long as the material remains insulating. A topological insulator is characteristic of topological invariant, not the order parameters such as in ferromagnet and superconductors. The first topological state of matter is the quantum Hall state, the Hall conductance of which is insensitive to continuous changes in the parameters and depends only on the number of edge states, which are unidirectional because of the breaking of the time reversal symmetry due to the magnetic field. This effect was generalized to the system with time reversal symmetry, such as graphene with spin orbit coupling and an 'inverted' semiconductor HgTe/CdTe quantum well, exhibiting the phenomenon of the quantum spin Hall effect. The generalization of this new state from two dimensions to three dimension is non-trivial. In three dimensional topological insulators, the electron momentum in the surface states is locked in with the electron spin and electrons form a Dirac cone due to strong spin orbit coupling. A series of materials such as Bi2Se3 and Bi2Te3 have been verified to be topological insulators. Now we have realized that the topological phase also exists in one-dimensional conducting polymers, and superfluid of Helium 3. In this talk we first present an introduction to topological insulator and then present a simple but unified description for a large family of topological insulators based on a modified Dirac equation. A series of solutions are presented to demonstrate the existence of edge and surface states in topological insulators and superconductors.
About the Speaker: Professor Shun-Qing Shen, an expert in the field of condensed matter physics, is distinguished for his research works on topological insulator, spintronics of semiconductors, quantum magnetism and orbital physics in transition metal oxides, and novel quantum states of condensed matters. He proposed theory of topological Anderson insulator, spin transverse force, resonant spin Hall effect and theory of phase separation in colossal magnetoresistive (CMR) materials. He proved existence of antiferromagnetic long-range order and off-diagonal long-range order in itinerant electron systems. Professor Shen has been a professor of physics at The University of Hong Kong since July 2007. He received his BS (1986), MS (1989), and PhD (1992) from Fudan University in Shanghai. He was a postdoctorial fellow (1992 – 1995) in China Center of Advanced Science and Technology (CCAST), Beijing, Alexander von Humboldt fellow (1995 – 1997) in Max Planck Institute for Physics of Complex Systems, Dresden, Germany, and JSPS research fellow (1997) in Tokyo Institute of Technology, Japan. He was awarded Croucher Senior Research Fellowship (Croucher Prize) in 2010.
Click HERE for directions

—
To view all the upcoming seminars, you can visit: https://graphene.nus.edu.sg/news-events/events/
You may also Like & Subscribe our following channels below to receive instant notifications for new announcements.