News & Events

Speaker: Xiang Du
Abstract Details: Attributing to the large surface-to-volume ratio of nanomaterials, surface plays a crucial role in determining their electronic and optical properties. Surface modification shows great potential to modulate the performance of various 1D and 2D nanomaterials based electronic and optoelectronic devices. In this presentation, I will introduce two effective surface engineering approaches which are H2 annealing and surface transfer doping, to significantly enhance the performance of 1D (MoO3 nanobelts) and 2D (black phosphorus) nanomaterials based functional devices, respectively. Through H2 annealing, a high-performance photodetector fabricated by gap states assisted single MoO3 nanobelt with wide visible spectrum response was successfully achieved. On the other hand, the ambipolar characteristics of BP based FETs were effectively tuned via surface functionalization of Cs2CO3 and MoO3. This doping can also modulate the Schottky junctions formed between metal contacts and BP flakes, and hence to enhance the photo responsivity of BP based photodetectors.
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Speaker: Kedar Hippalgaonkar
Affiliation: (IMRE, A*STAR
Abstract Details: Thermoelectric devices require large Seebeck and simultaneously large electrical conductivity, while maintaining a low thermal conductivity. Significant progress in the thermoelectric performance of materials has been made by exploring ultralow thermal conductivity at high temperature, reducing thermal conductivity by nanostructuring, resonant doping and energy-dependent scattering. For a given thermal conductivity and temperature, thermoelectric powerfactor is determined by the electronic structure of the material.  Low dimensionality (1D and 2D) opens new routes to high powerfactor due to the unique density of states (DOS) of confined electrons and holes. Emerging 2D transition metal dichalcogenide (TMDC) semiconductors represent a new class of thermoelectric materials not only because of their discretized density of states, but also due to their large effective masses and high carrier mobilities, different from gapless semi-metallic graphene. We have observed 2D crystals of MoS2 with a powerfactor as large as 8.5 mWm−1K−2 at room temperature, which is the highest among all thermoelectric materials. Moreover, measurement of thermoelectric properties of monolayer MoS2 in the metallic regime allows us to determine the confined 2D DOS near the conduction band edge for the first time, which cannot be measured by electrical conductivity alone. Further, we measure the interlayer thermal resistance in MoS2/hBN heterostructures, which can be used to tune the in-plane thermal conductivity allowing for an additional tunable knob for future thermoelectrics. The demonstrated high electronically modulated powerfactor in 2D TMDCs with tunable thermal conductivity holds promise for efficient thermoelectric energy conversion.
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Speaker: Justin Song
Affiliation: Caltech, USA
Abstract Details: Charge carriers in materials are often described as quasiparticles similar to free electrons and can be characterized by effective quantities such as an effective mass. However, electrons in topological materials acquire an additional quantum mechanical property - Berry curvature - that is the key ingredient in a range of new phenomena. A striking example is carrier dynamics in gapped Dirac systems, such as graphene on hexagonal-boron-nitride (G/h-BN). I will discuss how Berry curvature gives rise to transverse valley currents even in the absence of a magnetic field in these systems. Crucially, these valley currents do not depend on the presence of edge states, and persist even in the gapped system bulk. These anomalous carrier dynamics manifest naturally in G/h-BN, displaying large non-local resistances mediated by valley currents in G/h-BN devices, yielding a new platform/scheme to access topological characteristics in layered 2D stacks of materials."
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Speaker: Siegfried Eigler
Affiliation: Friedrich Alexander University, Germany
Abstract Details: The bulk chemistry of graphene is based on the oxidation of graphite, with a tradition of more than 150 years.[1] It turned out that all developed oxidation methods resulted in functionalized layers of graphene with plenty of defects within the carbon framework. Thereby the procedures are accompanied by over-oxidation leading to more amorphous structures with manifold functional groups. This over-oxidation can even lead to disintegration of flakes and the generation of oxidative debrides. Thus, chemical functionalization of graphene oxide is dominated by reactivity of edge-groups of defects. Approximately one missing carbon atom was determined in graphene oxide on about 25 lattice atoms using a conventional synthetic protocol. Thus, chemistry was predominantly driven by the functionalization of defects. We optimized the oxidative procedure to avoid the formation of defects and now a residual average density of defects of about 0.05% was reached.[2] Statistical Raman microscopy is now available to monitor the impact of reaction on the integrity of the carbon lattice.[3] With such materials, the chemistry of graphene can be explored while defects play a minor role. We used oxo-functionalized graphene with organosulfate groups to prepare a composite that can be used as charge storing layer in floating gate memory devices operating at 3V.[4] This goal was only reached by controlled chemistry. In another recent study we used graphite sulfate, which is a long known intercalation compound that bears a C24 subunit with one positive charge. This charge can be used for chemical functionalization and an oxo-functionalized graphene derivative with an idealized subunit of C24(OH)(H2O)2 is yielded.[5] Chemical reduction and statistical Raman analysis reveals that the best average quality of graphene can be prepared from this novel derivative of graphene. Overcoming performance limits, as determined for graphene oxide, is in reach using controlled chemistry of oxo-functionalized graphene. [1] S. Eigler, A. Hirsch, Angew. Chem. Int. Ed. 2014, 53, 7720. [2] S. Eigler, M. Enzelberger-Heim, S. Grimm, P. Hofmann, W. Kroener, A. Geworski, C. Dotzer, M. Rockert, J. Xiao, C. Papp, O. Lytken, H. P. Steinrück, P. Müller, A. Hirsch, Adv. Mater. 2013, 25, 3583. [3] S. Eigler, F. Hof, M. Enzelberger-Heim, S. Grimm, P. Müller, A. Hirsch, J. Phys. Chem. C 2014, 118, 7698. [4] Z. Wang, S. Eigler, Y. Ishii, Y. Hu, C. Papp, O. Lytken, H.-P. Steinrück, M. Halik, J. Mater. Chem. C 2015, 3, 8595. [5] S. Eigler, Chem. Commun. 2015, 51, 3162."
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Speaker: Hilmi Volkan Demir
Affiliation: CA2DM, NUS & Luminous, NTU
Abstract Details: Solution-processed semiconductor nanocrystals have attracted great interest in photonics including color conversion and enrichment in quality lighting and display backlighting [1]. In this talk, we will present architectures of colloidal nanocrystals obtained by tailoring and controlling the dimensionality, size, and composition of these nanostructures in an effort to realize high performance in light generation and lasing [2]. These cover types of colloidal quantum dots, quantum rods and quantum wells. Based on the rational design and control of excitonic processes in these nanocrystals, we successfully demonstrated highly efficient light- emitting diodes [3] and lasers [4,5]. To this end, we systematically studied and showed that electronic-type tuning in colloidal quantum heterostructures allows for fine tunability [6]. Here ultra-low threshold stimulated emission was achieved using engineered core/shell architectures enabling substantially suppressed Auger recombination, enabling the first liquid laser of nanocrystals [4]. Also, we developed an all-colloidal solid laser using these nanocrystals as the optical gain media for the first time in a fully colloidal resonator [5]. As an extreme case of solution-processed highly-confined quasi-2D colloids, we showed that the atomically flat heteronanoplatelets uniquely combine ultra-low threshold stimulated emission and record high optical gain coefficients and the controlled stacking of these nanoplatelets further tune their excitonic properties [7]. The recent progress in the colloidal optoelectronics suggest that solution-processed quantum materials hold great promise to challenge epitaxial counterparts in the near future. References: [1] H. V. Demir et al., Nano Today 6, 632 (2011); T. Erdem and H. V. Demir, Nature Photonics 5, 126 (2011). [2] B. Guzelturk et al., Laser & Photonics Reviews 8, 73 (2014); and J. Phys. Chem. Lett. 5, 2214 (2014). [3] X. Yang et al., Advanced Materials 24, 4180 (2012); Advanced Functional Materials 24, 5977 (2014); ACS Nano 8, 8224 (2014); and Small 10, 246 (2014). [4] Y. Wang et al., Advanced Materials 27, 169 (2015). [5] B. Guzelturk et al., Advanced Materials in press (2015). DOI: 10.1002/adma.201500418 [6] A. F. Cihan et al. ACS Nano 7, 4799 (2013); and J. Phys. Chem. Lett. 4, 4146 (2013). [7] B. Guzelturk et al. ACS Nano 8, 6599 (2014); and ACS Nano 8, 12524 (2014).
About the Speaker: Dr. Hilmi Volkan Demir is a professor of electrical engineering and physics. He is an NRF Fellow of Singapore, and serves as the Founding Director of LUMINOUS! Center of Excellence for Semiconductor Lighting and Displays at NTU Singapore. Demir earned his PhD (2004) and MSc (2000) degrees in electrical engineering from Stanford University, CA, and his BSc (1998) degree in electrical and electronics engineering from Bilkent University (one of the top ranking science and engineering schools in Turkey). His current research interests include the science and technology of semiconductor lighting; nanocrystal optoelectronics; excitonics and plasmonics for high- efficiency light generation and harvesting; and wireless in vivo sensing and smart implants for future healthcare. Demir published over 200 peer-reviewed research articles in major scientific journals and delivered over 150 invited seminars, lectures and colloquia on the topics of LED lighting, nanophotonics, in vivo sensing, and nanoparticles research in industry and academia. Demir has contributed to commercialization and licensing of several new enabling technologies as well as establishing two successful companies and led to >30 patent applications (granted and pending), several of which have currently been used, owned or licensed by the industry. These scientific and entrepreneurship activities resulted in several important international and national awards including Nanyang Award for Research Excellence, ESF European Young Investigator Award, TUBITAK Scientific and Technological Research Council of Turkey Young Investigator TESVIK Award, and TUBA-GEBIP Distinguished Young Scientist Award. He has been selected The Outstanding Young Person in the World (TOYP Award) of Junior Chamber International (JCI) Federation of Young Leaders and Entrepreneurs Worldwide in the category of academic achievement and leadership. He is the PI of Singapore NRF Competitive Research Program on future lighting using excitonics. He is the SPRINGER-VERLAG Series Editor of Nanoscience and Nanotechnology and an OSA editor of Optics Express. He will serve as the Technical Chair (Washington DC 2015), Member-at-Large (Hawaii 2016), and General Chair (2017) of the IEEE Photonics Society’s flagship program IEEE Photonics Conference (IPC). He is a selected partner of European Union FP7 Nanophotonics for Energy Network of Excellence (N4E NoE) and an elected Associate Member of the Turkish National Academy of Sciences.
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Speaker: Daniel Ucko
Abstract Details: Physical Review Letters (PRL) is the Letters journal of the Physical Review series, and is published by the American Physical Society. It is one of the most prestigious journals in physics. As a Letters journal publishing short reports of high importance, impact, and interest across all of physics, it is a unique publication, and no other journal has PRL’s scope and coverage of physics. We enjoy a healthy amount of submissions and recently celebrated our 50th anniversary. However, the inner workings of PRL are however sometimes a bit mysterious to the authors and referees that make the journal what it is. The role of editor is really concerned with triad of author, referee, and editor, and managing the relationships between each of these. I will be explaining what is expected of each of these, and how they can best work together. I will also be talking about the state of the journal, and show on some new developments at PRL in view of the changing publication landscape. A question and answer session will follow the main presentation.
About the Speaker: Daniel received his Ph.D. in physics at University College London, UK, in magnetism in 2001. Before joining PRL in 2004, he had a postdoctoral research position at the University of Birmingham, UK, and at the low-energy muon beam at the Paul Scherrer Institut in Switzerland. Daniel handles papers on condensed matter.
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Speaker: Alexandra Carvalho, Research Fellow
Affiliation: CA2DM, NUS
Abstract Details: We used first principles calculations to investigate systematically the structural, electronic and optical properties of `phosphorene analogues', the family of group-IV monochalcogenides SnS, SnSe, GeS, and GeSe. We show that all are semiconducting, with bandgap energies covering part of the infra-red and visible range, and in most cases higher than phosphorene. Further, we analysed the changes associated with the reduction of dimensionality, from bulk to monolayer or bilayer form. The prospects of using these materials for spintronics and valleytronics are discussed.
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Speaker: Clas Persson
Affiliation: University of Oslo
Abstract Details: The talk will briefly present the different projects and research activities within theoretical modeling of solar energy materials at the Univesity of Oslo in Norway. One of the main research project is on Cu-based solar cell materials, like Cu(In,Ga)Se2 and Cu2ZnSnS4, and the talk will therefore discuss a little more deepdly why this type of materials has fascinated researchers for the last three decades. [1] [review article] S. Siebentritt, M. Igalson, C. Persson, and S. Lany Prog. Photovoltaics Res. Appl. 18, 390 (2010)."
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Speaker: D.N. Basov
Affiliation: University of California San Diego
Abstract Details: Layered van der Waals (vdW) crystals, which consist of individual atomic planes weakly coupled by vdW interaction similar to graphene monolayers in solid graphite, can harbor remarkable properties: viz. superconductivity and ferromagnetism with high transition temperatures, light emission, and topologically protected surface states. Such artificial crystals provide building blocks for stacked heterostructures where each such block delivers layer-specific attributes. In examples assembled from atomically thin layers of graphene and hexagonal boron nitride (hBN), a rich variety of optical effects arise from the confluence of unusual elementary excitations: viz. surface plasmons in graphene and hyperbolic phonon polaritons in hBN. We have launched, detected and imaged plasmonic, phonon polaritonic and hybrid plasmon-phonon polariton waves in a setting of an antenna based nano-infrared apparatus. The nano-scale exploration of polaritonic modes has offered a new perspective on fundamental physics behind electronic phenomena in graphene. For example, by interferometric infrared imaging of plasmonic standing waves we were able to quantify the electronic losses in graphene. This latter result highlights the important role of many body effects  that were not anticipated theoretically. Recent publications: Fei et al. Nature 487, 82 (2012), Basov et al. Reviews of Modern Physics 86, 959 (2014) Dai et al Science 343, 1125 (2014).
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Speaker: Taniguchi Takashi
Abstract Details: Hexagonal BN (hBN) and cubic BN (cBN) are known as the representative crystal structures of BN. The former is chemically and thermally stable, and has been widely used as an electrical insulator and heat-resistant materials. The latter, which is a high-density phase, is an ultra-hard material second only to diamond. Some progresses in the synthesis of high purity BN crystals were achieved by using Ba-BN as a solvent material at high pressure crystal growth [1].  Band-edge natures (cBN Eg=6.2eV and hBN Eg=6.4eV) were characterized by their optical properties. The key issue to obtain high purity crystals is to reduce oxygen and carbon contamination in the growth circumstances. It should be emphasized that hBN exhibits attractive potential for deep ultraviolet (DUV) light emitter [2,3] and also superior properties as substrate of graphene devices. In this presentation, recent trials for high quality hBN, cBN crystals growth by using flux process under high pressure will be introduced. After somehow achieved to obtain high purity crystals, artificial doping to realize new function should be a next important step. The study for rare earth element doping in cBN and their characterization by ultimate analysis and ab-initio study will be also introduced. [1] T.Taniguchi, K.Watanabe, J.Cryst.Growth 303,525 (2007).  T.Taniguchi, in Comprehensive Hard Mater. ed.by V.K.Sarin et.al, (Elsevier, 2014) pp.587. [2] K.Watanabe, T.Taniguchi and H.Kanda, Nature Materials, 3,404 (2004). [3] K.Watanabe,T.Taniguchi,A.Niiyama,K.Miya, M.Taniguchi, Nature Photonics 3,591(2009). [4] C.R.Dean, A.F.Young, K.Watanabe, T.Taniguchi, P.Kim, et.al.,Nat.Nanotech, 5, 722(2010).
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