Speaker: Jaroslav Fabian
Affiliation: University of Regensburg, Germany
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Speaker: Jaroslav Fabian
Affiliation: University of Regensburg, Germany
Abstract Details: Two dimensional materials, such as graphene, transition metal dichalcogenides, or black phosphorous, offer immense opportunities for electronics and spintronics [1]. Being ultimately thin these materials could make the thinnest diodes and transistors, or the thinnest magnetic sensors and read heads. Being essentially a surface, they are also susceptible to adatoms and admolecules which can induce local magnetic moments and giant spin-orbit coupling [2]. This is in fact a great opportunity, allowing us to decorate (functionalize) graphene and like materials with specific defects to make desired properties. I will review the essential spin physics of novel two dimensional materials, including spin-orbit coupling and magnetic moments, and discuss the ramifications of the (intended and non-intended) functionalization for spin transport experiments. Most of the results are obtained by performing first principles calculations on large atomic supercells, necessary to study the physics in the dilute defect limit. These calculations show a nice agreement with experiments regarding spin relaxation in single [3] and bilayer [4] graphene, but also make authoritative predictions for future realistic charge and spin based device---an example is given by optospintronics in graphene/TMC structures.
[1] W. Han, R. Kawakami, M. Gmitra, and J. Fabian, Nature Nanotechnology 9, 794 (2014).
[2] M. Gmitra, D. Kochan, and J. Fabian, Phys. Rev. Lett. 110, 246602 (2013).
[3] D. Kochan, M. Gmitra, and J. Fabian, Phys. Rev. Lett. 112, 116602 (2014).
[4] D. Kochan, S. Irmer, M. Gmitra, and J. Fabian, arXiv:1504.03898
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Speaker: Wong Pei Yu Calvin
Abstract Details: Diffusion barriers prevent materials from intermixing (e.g., undesired doping) in electronic devices. Most diffusion barrier materials are often very specific for a certain combination of materials and/or change the energetics of the interface because they are insulating or add to the contact resistances. In my talk, I present graphene (Gr) as an electronically transparent diffusion barrier in metal/semiconductor devices, where Gr prevents Au and Cu from diffusion into Si, and unintentionally dope Si. We studied the electronic properties of the n-Si(111)/Gr/M Schottky barriers (M = Au or Cu) by I(V) measurements and at the nanoscale by ballistic electron emission microscopy (BEEM). The layer of Gr does not change the Schottky barrier of these junctions.
The Gr barrier was thermally and mechanically stable enough to withstand the harsh fabrication methods typically encountered in clean room processes (e.g., deposition of metals in high vacuum conditions at high temperatures), it is electronically transparent (it does not change the energetics of the Si/Au or Si/Cu Schottky barriers), and effectively prevented diffusion of the Cu or Au into the Si at elevated temperatures. Applications of the Gr diffusion barrier in c-Si based photovoltaics and molecular electronics will be discussed.
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Speaker: Xu Qing-Hua
Affiliation: Department of Chemistry, NUS
Abstract Details: Noble metal nanoparticles, such as gold and silver, display unique properties known as localized surface Plasmon resonance, which could be utilized to enhance linear and nonlinear optical properties of nearby chromophores and metal nanoparticles themselves. Our group has done extensive work on plasmon enhanced one- and two-photon excitation fluorescence and their applications. In particular, we found an interesting phenomenon that non-fluorescence metal nanoparticles started to emit strong two-photon photoluminescence (TPPL) upon plasmon coupling in the aggregated state. We have demonstrated that this kind of plasmon coupling enhanced TPPL is a general phenomenon for Au and Ag nanoparticles of different morphologies. TPPL of these metal nanoparticles was found to be enhanced by up to hundreds of times in the colloid solution and five orders of magnitude on single particle level upon plasmon coupling. As many biologically important species can induce aggregation of metal nanoparticles, this phenomenon has been further utilized to develop various two-photon sensing and imaging applications to take their unique advantages of deep penetration into biological tissues and 3-dimensional confined excitation. We have also employed ultrafast spectroscopy techniques to understand the underlying enhancement mechanisms.
About the Speaker: XU Qing-Hua received his B.S. from Zhejiang University (1993), M.S. from Peking University (1996) and University of Chicago (1997), Ph.D. from UC Berkeley (2001), and conducted the postdoctoral research at Stanford University and UC Santa Barbara. He joined NUS Chemistry in 2005 and became an Associate Professor since 2011. His primary research interest is development of various light based applications such as sensing, imaging, photosensitization and optoelectronics using various nanomaterials and organic/polymer materials, as well as investigation of the underlying fundamental mechanisms using various novel optical spectroscopy and imaging techniques.
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Speaker: Thomas G. Pedersen
Affiliation: Aalborg University, Denmark
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Speaker: John Robertson
Affiliation: Cambridge University, UK
Abstract Details: The Schottky barrier heights of the layered transition metal dichalcogenides (TMDs) have been calculated using density functional theory. The calculated Schottky Barrier heights are found to depend quite weakly on the metal work function, with a pinning factor of S~0.3. Â This indicates that TMD Schottky barriers follow the metal induced gap state (MIGS) model, like three-dimensional semiconductors, despite the two-dimensional bonding of TMD layers. This is because the bonding between the contact metal atoms and the TMD chalcogen atoms is covalent/ionic, and not of a van der Waals type. The metal Fermi level is pinned in the upper gap in MoS2, but pinned near midgap in most other TMDs. Thus, ambipolar contacts can be achieved by avoiding MoS2, or by using a high work function electrodes like MoO3 on MoS2. Contacts for black phosphorus are also covered."
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Speaker: Silvija Gradecak
Affiliation: MIT, USA
Abstract Details: Functionality of novel nanomaterials – including two dimensional (2D) ultrathin films, one dimensional (1D) nanowires/nanotubes, and zero dimensional (0D) nanocrystals – and their impact on society will be ultimately dictated by our understanding and ability to precisely control their structural properties, size uniformity, and dopant distribution at the atomic level. Over the past several years, we have revealed several important insights into the fundamental growth mechanisms in several nanowire compound materials systems, and more recently, we have used this knowledge to control composition and morphology of nanowires in-situ during the growth (“nanowires on demandâ€). These findings were enabled by the development of a unique growth system, as well as by direct structure-property correlation using state of the art electron microscopy techniques, some of which we have developed. In this talk, I will also discuss development of flexible and transparent nanostructured photovoltaic devices with power conversion efficiencies exceeding 9%. This new class of solar cells takes advantage of transparent graphene electrodes, quantum dot absorbers, and nanowire transport layers."
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Speaker: Brian Spector
Abstract Details: NAG Technical Consultant Brian Spector will be visiting Singapore during the week of the 17th March to visit NUS and participate in Supercomputing Frontiers 2015.
He has offered to give this lecture about NAG at CA2DM, in particular about the NAG Toolbox for MATLAB.
Besides this lecture at CA2DM, Brian will deliver a workshop on Friday 20th March at the Supercomputing Frontiers 2015 conference where he will talk about how users can tune their code for the Intel Xeon and Intel Xeon Phi. Â
Finally, NAG would like to remind everyone that NUS is fully licensed for NAG's software products which include the NAG Library and the Fortran Compiler. This software may be installed on staff and student personal machines as well as University owned hardware such as clusters and supercomputers. Faculty and students can make full use of NAG Technical Support, contact support@nag.co.uk"
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Speaker: Thiti Taychatanapat
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Speaker: Alexander Petrovi?
Affiliation: Nanyang Technological University
Abstract Details: Fractals are scale-invariant spatial distributions which are ubiquitous in nature, governing the shapes of objects as diverse as snowflakes, trees and coastlines. A less well-known instance of fractal ordering occurs in strongly disordered materials, where electronic wavefunctions develop multifractal spatial distributions in the vicinity of the Anderson transition between extended and localised states. Such multifractal ordering has been predicted to enhance electron-electron interactions. In materials with instabilities to correlated electron phase formation (such as ferromagnets or superconductors), one may therefore envisage the possibility of tuning the quantum ground state via disorder.
In disordered superconductors, a large multifractal enhancement is expected in the pairing interaction (and hence the critical temperature Tc), provided that the Coulomb repulsion is weak. However, no such rise in Tc has ever been observed experimentally, due to the suppression of superconductivity by emergent granularity and a dynamically-augmented Coulomb repulsion in highly disordered materials. Using a range of experimental and numerical techniques (including electrical transport, magnetization, X-ray diffraction/scattering and density functional theory), we demonstrate that multifractal pairing enhancement does in fact occur in the quasi-one-dimensional superconductor Na2-δMo6Se6, due to the combination of random Na vacancy disorder with an intrinsically screened Coulomb repulsion. The pairing temperature Tons rises monotonically as the Anderson-Mott mobility edge is approached from the metallic side, in quantitative agreement with a multifractal enhancement model. Strikingly, Tons continues to rise in the localised phase after crossing the mobility edge, in accordance with theoretical predictions. The upper critical field Hc2 exceeds the weak-coupling Pauli limit by a factor of at least 4 in the localised regime, indicating a large increase in the superconducting gap energy.
Our results provide the first experimental perspective onto the unknown physics of correlated electron materials in the absence of Coulomb repulsion. We also show that the unique interplay between superconductivity and localisation in nanofilamentary materials renders them ideal building blocks for functional superconductors. Electron delocalisation drives an intrinsic stabilisation of phase fluctuations upon raising the temperature, magnetic field or electric current, in direct contrast to the behaviour of conventional homogeneous superconductors.
About the Speaker: Alex graduated from Clare College, Cambridge, with a BA and MPhys in Experimental and Theoretical Physics, spending his final year working on muon spin relaxation and thermal hysteresis in underdoped cuprates. After moving to Geneva, Switzerland for his PhD, he spent the next 6 years building a helium-3 high-field scanning tunnelling microscope for use on unconventional superconductors. These labours eventually bore fruit, yielding the first real- space images of a vortex glass in a type-II superconductor, a multi-band scenario to explain the extremely high upper critical fields in Chevrel phases and a completely new type of vortex core. Since joining NTU in 2009, he has worked on a variety of projects including structural and magnetic phase transitions in magnetoelectric EuTiO3, ARPES in quasi-one-dimensional metals, phase emergence at the LaAlO3/SrTiO3 interface, manganite superlattices and the development of a 10mK 17T UHV local probe microscope inside a 350-ton floating laboratory."
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