Seminars 2008

Title: Electrostatics of water at the nanoscale
Speaker: Professor Gerard Wong
Date:12 December 2008
Time:1.30pm – 2.30pm
Venue:Hilbert Space (PAP-02-02)
Host:Dr Sum Tze Chien
Abstract: Current understanding of electrostatics in water is based on mean-field theories like the Poisson-Boltzmann formalism and its approximations, which are routinely employed in colloid science and computational biology. This approach, however, breaks down for highly charged systems, which exhibit counterintuitive phenomena such as overcharging and like-charge attraction. We examine the collective spatial and temporal correlations of multivalent ions that mediate like-charge attraction between strongly-charged polyelectrolytes using inelastic x-ray scattering. We find a new acoustic phonon mode associated with correlated counterions. The excitation spectra at high wave-vector Q reveal unexpected liquid-like dynamics due to ions interacting with their ‘cages’ of nearest neighbors. The measured speed of sound and collective relaxation rates in this liquid agree surprisingly well with simple model calculations until the nanoscopic regime, where the granularity of water is important. It is well-known that the role of water in ‘wet’ electrostatics is difficult to access theoretically and experimentally at molecular time-scales and length-scales. Using 3rd generation synchrotron sources, it is possible to combine spectroscopy and diffraction in an inelastic x-ray scattering experiment. We show that it is possible to do experiments in the frequency and momentum domains and directly measure the Greens function for water. Using this, we will show that it is possible to reconstruct water dynamics at sub-Angstrom spatial and 50 femtosecond temporal resolution.
The dynamic hydration structure of a negatively charged ion moving at 250m/s and 1000m/s. Red and blue shifts represents increases and decreases in average induced oxygen density respectively. The hydration structure evolves from an isotropic shell at rest to a hydration cap at 250m/s, and finally to a hydration ring at 1000m/s, which is near typical thermal speeds.


Title: Theoretical Studies Ranging from Quantum Dots to On‐water Catalysis
Speaker: Professor Rudolph A. Marcus
Date:17 November 2008
Time:10.00am - 11.00am 
Venue:MAS Executive Classroom 2 , SPMS‐MAS‐03‐07
Host:Professor Alfred Huan
Abstract: With the advent of new experimental techniques, many experimental puzzles have arisen where theory plays a useful interactive and predictive role. Examples will be drawn from a broad range of recent studies by our group, such as the catalysis of certain organic ‘on‐water’ reactions (green chemistry),1 the intermittent fluorescence of nanoparticles (quantum dots),2 fluctuations in single molecule properties of proteins,3 temperature independence of the H/D kinetic isotope effect for some enzymes operating under their natural conditions4 and an abnormal Arrhenius pre‐exponential factor for a thermophilic enzyme operating below its break‐point temperature,4 and a mass‐independent isotope effect in ozone formation in the stratosphere.5

1Y. Jung & RAM, J. Am. Chem. Soc. 129, 549 (2007); 2J. Tang & RAM, J.Chem.Phys. 123, 05470 (2005) and P. Frantsuzov & RAM, Phys. Rev. B 72, 155321 (2005); 3M. K. Prakash & RAM, Proc. Nat. Acad. Sci. USA 104, 15982 (2007) and J. Phys.Chem. B 112, 399 (2008); 4RAM, in Quantum Tunneling in Enzyme Catalyzed Reactions, R. Allemann and N. S. Scrutton, eds. (to be published, 2009); 5Y. Q. Gao & RAM, J. Chem. Phys. 116, 137 (2002) and 127, 244318 (2007).


Title: High power THz Generation and THz Nonlinear Spectroscopy
Speaker: Dr Matthias C. Hoffmann
Date:14 November 2008
Time:11.00am - 12.00pm 
Venue:Hilbert Space (PAP-02-02)
Host:Professor Alfred Huan
Abstract: The use of ultrashort terahertz pulses has facilitated terahertz spectroscopy of a wide range of physical, chemical and biological samples, and has enabled timeresolved measurements in which the terahertz pulse is used to probe dynamical responses to an optical excitation pulse. In almost all cases reported to date, the THz pulses have had rather low energy, field amplitude, and average power. This has slowed the development of nonlinear THz optics and spectroscopy as well as THz signal processing, spectroscopic imaging and screening, and other applications.

Using an extremely effective terahertz radiation generation scheme pioneered by Hebling et al., we obtained the highest energy tabletop terahertz source to date. By using strong THz radiation we have demonstrated nonlinear effects in semiconductors such as saturated absorption, intervalley scattering, self-phase modulation and impact ionization.

These effects can be monitored time and frequency resolved using a novel THzpump/THz probe scheme in the absence of band-to-band excitation. Our data show the effect of highly accelerated carriers and the strong coupling of the electronic system to the lattice on the picosecond timescale.


Title: The Spin Supersolid Phase
Speaker: Dr Pinaki Sengupta
Date:13 November 2008
Time:11.00am - 12.00pm 
Venue:Hilbert Space (PAP‐02‐02)
Host:Professor Alfred Huan
Abstract: There has been a revival in interest in the phenomenon of supersolidity following the experiments of Kim and Chan on solid He. While it is still unclear whether a supersolid (SS) state can be stabilized in the continuum, recent theoretical studies have shown that such a phase can be stabilized in the presence of an underlying lattice (or a periodic potential). I shall present a simple and intutive mechanism leading to the formation of a lattice supersolid, focussing primarily on spin systems. I'll talk about the possible experimental signatures of such a phase and discuss several conditions under which a spin‐supersolid can be realized in real spin compounds.


Title: Electronic Properties of Graphene
Speaker: Professor Zhang Yuanbo
Date:7 November 2008
Time:1.30pm – 2.30pm
Venue:Hilbert Space (PAP-02-02)
Host:Professor Shen Zexiang
Abstract: Graphene, a single atomic layer of carbon, has been arguably one of the hottest topics in condensed matter physics recently. This worldwide enthusiasm has been fueled by its novel electronic, optical and mechanical properties discovered in the past few years. Most strikingly, the electrons in graphene behave like two-dimensional Dirac fermions, which lead to unusual quantum Hall effect in high magnetic fields. Such quantum effect persists even at room temperature owing to graphene’s exceptionally high quality.

These collective phenomena originate from graphene’s unique honey-comb lattice and its intricate interplay with impurities and defects. Scanning tunneling microscopy and spectroscopy (STS) offer ideal tools to study the local electronic properties of graphene with unparalleled spatial resolution. Graphene keeps giving us surprises at such atomic length scale. We observe an unexpected energy gap feature in the graphene tunneling spectrum that is pinned to the Fermi level. This feature coexists with an additional depression in the graphene density-of-states (DOS) that shifts energetically with gate voltage. Our analysis reveals that the pinned gap-feature is due to phonon-assisted inelastic tunneling, while the DOS depression directly marks the location of the graphene Dirac point. Our ability to locally probe the energetic location of the Dirac point via STM spectroscopy allows us to spatially map out electron density inhomogeneities in graphene with a resolution that is two orders of magnitude higher than previous experiments. We observe charge inhomogeneities that coexist with energy-dependent electronic interference patterns, and which can be directly correlated with nanometer-scale topographic features, thus revealing new impurity scattering behavior for Dirac fermions in graphene. These results are significant for understanding the sources of electron COLLEGE OF SCIENCE NANYANG TECHNOLOGICAL UNIVERSITY SPMS-04-01, 21 NANYANG LINK, SINGAPORE 637371 FAX: +65 6515 8229 TEL: +65 6513 8459 density inhomogeneity and scattering in graphene, as well as the microscopic mechanisms that determine graphene electron mobility.

1. Y. Zhang, V. W. Brar, F. Wang, C. Girit, Y. Yayon, M. Panlasigui, A. Zettl, M. F. Crommie, “Giant phonon-induced conductance in scanning tunneling spectroscopy of gate-tunable graphene” Nature Physics 4, 627 (2008)
2. F. Wang, Y. Zhang, C. Tian, C. Girit, A. Zettl, M. F. Crommie, Y. R. Shen, “GateVariable Optical Transitions in Graphene” Science, 320, 206 (2008)
3. V. W. Brar, Y. Zhang et. al., “Scanning tunneling spectroscopy of inhomogeneous electronic structure in monolayer and bilayer graphene on SiC” Appl. Phys. Lett. 91, 122102 (2007)
4. K. S. Novoselov, Z. Jiang, Y. Zhang, S. V. Morozov, H. L. Stormer, U. Zeitler, J. C. Maan, G. S. Boebinger, P. Kim, A. K. Geim, “Room-Temperature Quantum Hall Effect in Graphene” Science, 315, 1379 (2007). Brevia.
5. Y.-W. Tan, Y. Zhang, K. Bolotin, Y. Zhao, S. Adam, E. H. Hwang, S. Das Sarma, H. L. Stormer, and P. Kim, “Measurement of Scattering Rate and Minimum Conductivity in Graphene” Phys.. Rev. Lett. 99, 246803 (2007)
6. Z. Jiang, Y. Zhang, Y.-W. Tan, J. A. Jaszczak, H. L. Stormer, and P. Kim, “Graphene in extremely high magnetic fields” Int. J. Mod. Phys. B, 21, 1123 (2007)
7. Z. Jiang, Y. Zhang, H. L. Stormer and, P. Kim, “Quantum Hall States near the Charge Neutral Dirac Point in Graphene” Phys. Rev. Lett. 99, 106802 (2007)


Title: Opportunities in Nanomagnetism
Speaker: Professor S. D. Bader
Date:23 October 2008
Time:11.00am - 12.00pm 
Venue:Hilbert Space (PAP‐02‐02)
Host:Professor Alfred Huan
Abstract: This talk addresses the challenges and scientific problems in the emerging area of nanomagnetism. [1] Included are fabrication strategies, and experiments that explore new spin‐related behavior in metallic systems, as well as efforts to understand the observed phenomena. As a subfield of nanoscience, nanomagnetism shares many of the same basic organizing principles, such as geometric confinement, physical proximity, and chemical self‐organization. These principles are illustrated by means of examples drawn from the quests for ultra‐strong permanent magnets, ultra‐high‐density magnetic recording media, and nanobiomagnetic sensing strategies. As a final example showing the synergetic relationship to other fields of science, the manipulation of viruses to fabricate magnetic nanoparticles is presented.

* Argonne, a U.S. Department of Energy Office of Science laboratory, is operated under Contract No. DE‐AC02‐06CH11357.

[1] S. D. Bader, Rev. Mod. Phys. 78, 1 (2006).


Title: Short coherence length superconductivity and its Short coherence length superconductivity and its consequences in the extreme type-II compound SnMo6S8
Speaker: Dr Alexander Petrović
Date:21 October 2008
Time:11.00am - 12.00pm 
Venue:Hilbert Space (PAP‐02‐02)
Host:Nanyang Associate Professor Christos Panagopoulos
Abstract: Members of the Chevrel Phase family of superconductors may have unusually high upper critical fields (Hc2) reaching 70T, which is comparable to some high-Tc cuprates and pnictides. These materials unfortunately fell out of favour in the mid-1980s despite their superconducting mechanism and large Hc2 remaining unexplained. However, a return to these relatively uncomplicated compounds armed with 21st century experimental techniques may yield valuable insight into other “more fashionable” poorly-understood superconductors. With this in mind, we have performed a detailed study of the Meissner and mixed states of single-crystal SnMo6S8 (Hc2 ~ 40T) using scanning tunnelling spectroscopy (STS), specific heat measurements in fields up to 28T and conventional magnetic techniques. This material presents compelling spectroscopic evidence for two-band superconductivity and also exhibits a novel form of gapped vortex core spectra which may be related to the complex order parameter. Our specific heat data yield further evidence for double-gap superconductivity, thus providing an explanation for the large observed Hc2. Mixed state STS measurements at 400mK reveal the first real-space image of a vortex glass phase in a low-Tc superconductor and enable us to quantitatively compare its topology with the low-field Bragg phase. The glass formation appears to be mediated by a crossover from single to collective vortex pinning and is not associated with any pinning enhancement or magnetization peak effect. Finally, we evaluate the entire magnetic phase diagram for SnMo6S8, hence clarifying the extent of magnetic disorder and the nature of the peak effect in quasi-3D superconductors with nonnegligible thermal fluctuations.


Title: Will Carbon based devices bring in the Next Micro-electronics Revolution?
Speaker: Professor T. C. Choy
Date:2 September 2008
Time:11.00am - 12.00pm 
Venue:PAP Conference Room (Hilbert Space #02-02)
Host:Assistant Professor Cheong Siew Ann
Abstract: Microelectronics has almost become synonymous with silicon technology. But that needn’t be so. Carbon in its various forms - but not in any single form - can beat silicon for almost any performance measure [1]. Diamond has high carrier mobility, and marvellous thermal and optical properties. Semiconducting polymers are changing light-emitting devices with promising developments in several other areas such as low speed FETs [2]. Carbon nanotubes seem the new electronic material of the future especially for interconnects. Graphene structures are introducing new ways of fabricating two dimensional electronic systems and will present a challenge to traditional GaAs hetereostructure devices. Printable carbon field emitters (PFE) have brought in a new technology for electronic displays due to their unusually high cold field emission currents under modest electric fields, but they are teeming with theoretical questions [3]. Nanodiamond could combine features of special interest, with its small (nanoscale) diamond particles separated by ultra-thin intergrannular carbon that is probably graphitic (or even graphene-like). It is a mix of highly insulating grains and intervening conducting carbon that none the less has variations in structure on the nanoscale. For any electronic application one needs to know how the electronic carriers behave, and how they can be manipulated by applied fields. Nanodiamond’s observed electrical properties for example can be puzzling, especially behaviour in a magnetic field [4]. Boron-doped nanodiamonds are now found to be superconducting and there is an urgent need to understand the mechanism. In this talk I shall first present a perspective on the areas in carbon electronics that interest us and why, and then I shall discuss the open theoretical and experimental challenges ahead, with an outlook on future research and applications.

[1] A M Stoneham, Nature Materials 3, 3-5 (2004)
[2] T C Choy and A M Stoneham, Materials Today, 72, 64,April (2004)
[3] T C Choy,A M Stoneham and A H Harker, J. Phys.: Condens. Matter 17, 1505-1528 (2005).
[4] T C Choy,A M Stoneham, M Ortuño, and A M Somoza, Appl Phys Lett 92, 02120 (2008)


Title: Functional Nanomaterials: Shapes, Forms and Functions
Speaker: Professor Satishchandra B. Ogale
Date:15 August 2008
Time:11.00am - 12.00pm 
Venue:Hilbert Space (SPMS‐PAP‐02‐02)
Host:Assistant Professor Tom Wu
Abstract: The emergence and rapid progress of nanoscience during the past decade has led to enormous possibilities of materials manipulation and control to suit specific application requirements. This has also led to an unprecedented upsurge of activity in the field of Advanced Materials which embodies the notion of objective oriented materials science. In my presentation I will discuss several examples of our recent work in the field of nanoscience which captures this spirit of designer materials. The broad areas of current interest to us feature energy (solar cells), environment (photo‐catalysis, sensors) and health science (biomedicine/drug delivery/ hyperthermia). Funding Support: Council of Scientific and Industrial Research (CSIR), Department of Science and Technology (DST), Department of Atomic Energy (DAE) and Department of Information Technology (DIT), Government of India.


Speaker: Dr Serge Bouaziz
Date:13 August 2008
Time:10.00am - 11.00am 
Venue:PAP Conference Room (Hilbert Space) #02‐02
Host:Assistant Professor Phan Anh Tuan
Abstract: The human immunodeficiency virus type 1 (HIV‐1) genome encodes a highly conserved regulatory gene product, Vpr (96 residues, 14 kDa), which is incorporated into virions. In the infected cells, Vpr, expressed late in the virus cycle, is believed to function in the early phases of HIV‐1 replication, such as nuclear migration of pre‐integration complex, transcription of the proviral genome, viral multiplication by blocking cells in G2 phase and regulation of apoptosis phenomenon. Vpr has a critical role in long term AIDS disease by inducing infection in non‐dividing cells such as monocytes and macrophages.

To gain insight into the structure–function relationships of Vpr, the (1–96)Vpr protein was synthesized by automated solid phase synthesis, purified by reversed‐phase HPLC and its 3D structure was analyzed in the presence of CD3CN and in pure water at low pH and refined by restrained simulated annealing. The structure of the protein is characterized by three well‐defined alpha‐helices folded around a hydrophobic core and surrounded by flexible N‐ and C‐terminal domains.

Vpr is a good target for new drug discovery and its structure is of first interest for the study of its interaction with different partners as nucleic acids, ANT and p6.


Title: RaPID (Random Peptide Integrated Discovery) system: A new Emerging Technology for the Development of Non‐standard Peptide Drugs
Speaker: Professor Hiroaki SUGA
Date:26 June 2008
Time:10.30am - 11.30am 
Venue:PAP Conference Room
Host:Assistant Professor Phan Anh Tuan
Abstract: We have invented a highly flexible tool for tRNA acylation based on an artificial ribozyme, referred to as flexizyme. This novel tool enables us to charge virtually any desired amino and hydroxy acids onto any arbitrarily chosen tRNAs, and thus the genetic code can be completely reprogrammed. Using this tool, we are able to synthesize non‐standard peptides containing various non‐proteinogenic (non‐natural) amino acids using the combination of the flexizyme system a reconstituted cell‐free translation system, enabling us to reprogram the genetic code. Genetic code reprogramming involves the reassignment of codons from proteinogenic amino acids to non‐proteinogenic ones via multiple sense suppressions. Thus, this methodology enables us to simultaneously incorporate multiple non‐proteinogenic amino acids into peptides, which represents a major advantage over the previous non‐sense suppression methodology. I shall present its technology development and application toward the synthesis of non‐standard peptides for drug discovery.

• H. Xiao, H. Murakami, H. Suga, A. R. Ferré‐D’Amaré “Structural basis of specific tRNA aminoacylation by a small in vitro selected ribozyme” Nature in press (2008).
• Y. Goto, H. Murakami, H. Suga “Initiating translation with D‐amino acids” RNA in press (2008).
• Y. Sako, J. Morimoto, H. Murakami, H. Suga “Ribosomal synthesis of bicyclic peptides with two orthogonal inter‐sidechain reactions” Journal of American Chemical Society in press (2008).
• T.‐J. Kang, H. Suga “Ribosomal synthesis of nonstandard peptides” Biochemistry and Cell Biology 86, 92‐99 (2008).
• A. Ohta, Y. Yamagishi, H. Suga “Synthesis of biopolymers using genetic code reprogramming” Current Opinion in Chemical Biology 12, 159‐167 (2008).
• Y. Sako, Y. Goto, H. Murakami, H. Suga “Ribosomal synthesis of peptidase‐resistant peptides closed by a non‐reducible inter‐sidechain bond” ACS Chemical Biology 3, 241‐249 (2008).
• Y. Goto, A. Ohta, Y. Sako, Y. Yamagishi, H. Murakami, H. Suga “Reprogramming the initiation event in translation for the synthesis of physiologically stable cyclic peptides” ACS Chemical Biology 3, 120‐129 (2008).
• T. Kawakami, H. Murakami, H. Suga “Messenger RNA‐directed incorporation of multiple N‐methyl‐amino acids into linear and cyclic peptides” Chemistry & Biology 15, 32‐42 (2008).
• A. Ohta, H. Murakami, E. Higashimura, H. Suga “Synthesis of polyester by means of genetic code reprogramming” Chemistry & Biology 14, 1315‐1322 (2007).
• M. Ohuchi, H. Murakami, H. Suga “The flexizyme system: A highly flexible tRNA aminoacyation tool for the translation apparatus” Current Opinion in Chemical Biology 11, 135‐144 (2007).
• Murakami H., Ohta A., Ashigai H., and H. Suga “A highly flexible tRNA acylation method nonnatural polypeptide synthesis” Nature Methods 3, 357‐359 (2006).


Title: Macroscopic Super-radiance and Squeezed Photons from Exciton Condensate in Electron-Hole Bilayer Systems
Speaker: Dr Jinwu YE
Date:12 June 2008
Time:3.00pm - 4.00pm 
Venue:PAP Conference Room (SPMS-PAP-02-02)
Host:Professor Alfred Huan
Abstract: We study quantum nature of photons emitted from the excitonic superfluid phase in semiconductor electron-hole bilayer systems(EHBL). We show that the anomalous Green function of the exciton lead to a two mode squeezed state of the emitted photon along all tilted directions. When the exciton decay rate is sufficiently large, the angle resolved power spectrum shows a macroscopic super-radiance form which is proportional to the square of the number of excitons. We demonstrate explicitly that the photoluminescence from the exciton in EHBL systems at zero temperature is a very natural, feasible and unambiguous internal probe of the nature of quantum phases of excitons in EHBL such as the ground state and the Bogoliubov quasi-particle excitations above the ground state.


Title: Dynamics of Electrorheological (ER) Fluid and Electron Transport in Magnetic Multilayers
Speaker: Dr Jianwei Zhang
Date:23 April 2008
Time:10.30am - 11.30am 
Venue:Math Computation Lab (SBS b2n‐02)
Host:Professor Alfred Huan
Abstract: This talk will focus on the following two parts. The first part is dynamics of Electrorheological (ER) fluid. ER fluid consists of solid particles dispersed in an insulating liquid. Their rhelogical characters can be varied by applied electrical field. Starting from Onsager principle of minimum energy dissipation, we derive a two‐phase electrical‐ hydrodynamic model for ER fluid dynamics. We consider the energetic dipole‐dipole interaction between solid particles in terms of continuum field variable and couple hydrodynamic equation with continuity equation. Our numerical solution of the relevant equations yields prediction that displays very realistic behaviors observed experimentally.

The second part of this presentation is electron transport in noncollinear magnetic multilayers. When a spin current goes through a ferromagnetic metallic layer, the spin current transfers angular momentum to the magnetic layer. The Boltzmann equation is used in the layer‐by‐layer approach. By including the out‐of‐equilibrium spin accumulations, we build a self‐consistent model for electron transport in noncollinear magnetic multilayers. The current induced spin flip by spin accumulation at the interface allows more spin torque to be generated and less current dispassion. We present a continuous fashion for polarization of the spin current across the interface between magnetic layers.


Title: Molecular Self‐Assembly on Surfaces and Molecule‐Substrate Interface
Speaker: Dr Wei Chen
Date:15 April 2008
Time:10.30am - 11.30am 
Venue:PAP Meeting Room (SBS b3n‐19)
Host:Assistant Professor Kuo Jer Lai
Abstract: This talk will focus on two topics: (i) Molecular self‐assembly on surfaces and (ii) Molecule‐substrate interface.

(i) Molecular self‐assembly on surfaces
Creation of well‐ordered functional molecular arrays at the nanometer scale is one of the key issues in the development for future molecular‐ or nano‐electronic devices, solid‐state quantum computation, single‐ electron devices, and biosensors. One promising approach to create well‐ordered molecular nanostructure arrays is the selective coupling of functional molecules at preferential adsorption sites of pre‐patterned surface nanotemplates. Here, we demonstrate the formation of various wellordered C60 superstructures with tunable periodicity and symmetry using different molecular surface nanotemplates.[1] It is found that that the formation of the tunable C60 molecular arrays arises from the delicate balance between the homointermolecular (van‐der‐Waals forces), hetero‐intermolecular (charge transfer) and molecule‐substrate interfacial interactions under different experimental conditions, which can be simply adjusted by choosing appropriate C60 and 6T, 6P or pentacene coverage and post annealing temperature. Our results suggest that self‐assembling of molecular superstructures on surface nanotemplates represents a simple and effective method for the construction of highly ordered functional molecular nanostructure arrays, and offers a versatile route towards the fabrication of novel molecular interconnects and devices.

(ii) Molecule‐substrate interface
We also use synchrotron‐based photoemission spectroscopy (PES) and near‐edge x‐ray absorption spectroscopy (NEXAFS) to investigate the organic‐electronics related interface problems, including interface dipole formation, interface energy level alignment, molecular orientation and surface transfer doping of graphene.[2] For example, we use functionalized self‐assembled aromatic thiols to reduce the hole injection barrier between CuPc thin film and Au(111) from 0. 9 eV to 0.15 eV; CuPc always adopts the standing up configuration on SAMs terminated Au(111) in spite of interface charge transfer.

(a) Small 3, 2015 (2007); (b) Appl. Phys. Lett. 92, 023105 (2008); (c) Advanced Materials. 20, 484 (2008); (d) J. Am. Chem. Soc. 130, 2720 (2008).
(a) J. Am. Chem. Soc. 128, 935 (2006); (b) Appl. Phys. Lett. 88, 184102 (2006); (c) J. Phys. Chem. B. 110, 26075 (2006); (d) Advanced Functional Materials, 17, 1339 (2007)


Title: Nanoscale Torque Measurements of F1 ATP‐Synthase
Speaker: Dr Akilan Palanisami
Date:1 April 2008
Time:10.30am - 11.30am 
Venue:PAP Meeting Room (SBS b3n‐19)
Host:Dr Elbert Chia
Abstract: What is the efficiency of a biological molecular motor? The enzyme F1 ATP‐ synthase is a rotary motor/generator found in a huge variety of organisms (plants, animals, and bacteria). Being a major player in cellular respiration, F1 has evolved to optimize efficiency. However, a thermodynamic measurement of F1's efficiency has been difficult to obtain due to the 10 nm nanometer size scale of F1. To address this, we have constructed a new type of magnetic torque manipulator, involving the binding of specially designed nanoscale magnetic rods to glass surface anchored F1 . By controlling the external magnetic field and observing the rod's rotation via optical microscopy, kBT scale torque can be both applied and measured. In this way, we show the F1 torque vs. angle profile not to be featureless, as is commonly thought, but to have an unexpectedly strong angle dependence. Despite this, the thermodynamic efficiency appears to approach 100%.


Title: Gallium Nitride Micro-optoelectronics
Speaker: Dr Anthony Choi
Date:28 March 2008
Time:3.00pm - 4.00pm 
Venue:PAP Meeting Room (SBS b3n-19)
Host:Dr Sun Handong
Abstract: The recent findings of the Gallium Nitride micro-optoelectronics group at the University of Hong Kong will be presented in this talk. The group focuses on the study novel light-emitting diode (LED) structures and geometries at the micrometer and nanometer scale. Various low-dimensional features have been incorporated to enhance the performance and functionality of GaNbased visible and ultraviolet LEDs. For instance, LEDs consisting of an array of hexagonal micro-pixels have been designed to boost light extraction significantly. Fluorescent microspheres have been adopted as colorconversion agents for white light generation; white light LEDs with luminous efficacies exceeding 60 lumens per watt have been demonstrated. Using the technique of microsphere lithography, arrays of nano-pillars, nano-tips and more recently, photonic crystal structures have been integrated in LED structures. We have also created free-standing GaN microdisks structures pivoted on Si substrates, which exhibited photo-pumped lasing characteristics.


Title: Ultrafast Spectroscopy of Semiconducting Nanoparticles and Organic Polymers
Speaker: Dr Sum Tze Chien
Date:28 March 2008
Time:9.30am - 10.30am 
Venue:PAP Meeting Room (SBS b3n‐19)
Host:Professor Alfred Huan
Abstract: Our research involves investigating the energy transfer processes in semiconducting nanoparticles, electroluminescent polymers and organic‐based photovoltaic compounds. The samples are probed by femtosecond laser pulses as well as short electrical pulses, which generate non‐equilibrium states that can be monitored by time‐resolved spectroscopy. Work on doped semiconducting nanoparticles and organic polymers will be presented in this talk:

1. Semiconducting Nanoparticles ‐ Nanoparticles display a variety of unique optical and electronic properties that result from their quantum confinement effects and high surface/volume ratios. In this work, we investigate the decay lifetimes of the dopant and host semiconductor in the Mn2+‐doped ZnS nanoparticle system. In this system, the Mn2+ ion acts as a luminescent center, emitting near 590 nm (orange) as a result of the 4T1 – 6A1 transition. We seek to better understand the energy transfer mechanisms between the host and the dopants in nanoparticle systems so as to enhance their luminescence efficiencies.

2. Organic Polymers – Rare‐earth (RE) doped organic molecules display unique narrow band emissions that originates from the 4f transitions of the RE ions. RE complexes also possesses emissive bands which span a wide wavelength range (from visible to NIR), and can be useful for application in OLED. In this work we seek to better understand the energy transfer processes between the host and the RE dopants through ultrafast spectroscopy. The knowledge gained will lead to more efficient and durable OLEDs. Results from the newly established streak camera based time‐resolved electroluminescence technique will also be presented.


Title: Cleaning in Nano-Electronics by Physical Forces
Speaker: Dr P.W. Mertens
Date:19 March 2008
Time:10.30am - 11.30am 
Venue:Classroom 6, SBS‐01n‐26
Host:Dr Claus‐Dieter‐Ohl
Abstract: State-of-art fabrication of electronic integrated circuits, commonly referred to as micro-chips, has become a very large nano-electronics industry. The fabrication process involves multiple cleaning steps in which very small contaminating residues such as particles or flakes with dimensions down to the order of ten or a few tens of nm need to be removed with very high efficiency. Historically this was obtained by a pure liquid chemical treatment in which particles were undercut by a weak etching (order of a few nm) of the substrate. Thereby the Van der Waals attractive forces are reduced and electrostatic repulsive forces make the particle detach and transport away from the substrate.

In state of the art and future nano-electronic devices the dimension of the structures are so small that the amount of substrate etching involved in a cleaning step should be kept below typically 0.05nm. Therefore other cleaning mechanism involving a mechanical force have to be considered such as acoustic agitation of the cleaning liquid or high-velocity aerosol bombardment of the circuit surface. As these mechanical forces are engaged it was discovered that they also can damage fine structures already manufactured on the substrate, particularly if they are “up-features” sticking outward of the surface plain. This implies that a careful trade-off needs to be made between avoiding damage to fine manufactured structured and still obtaining an acceptable cleaning performance. This presentation also covers some typical hardware configurations and possible mechanisms for particle removal. It will be shown that a lot of cleaning tanks used today suffer from significant undesirable non-uniformity in the megasonic performance . While some alternative configurations feature improved uniformity. Finally also the use of agitation in non-aqueous liquid solutions will be shown for dedicated applications such as removal of photo-resist.


Speaker: Dr Chun-Hua Yan
Date:26 February 2008
Time:10.30am - 11.30am 
Venue:PAP Meeting Room (SBSB3n-19)
Host:Dr Yu Ting
Abstract: In this talk, we will discuss the controlled synthesis and properties of novel rare earth nanomaterials. With using some solution-based synthetic methods including solvothermal treatment and the thermolysis of metal complex precursors, a series of novel nanostructured rare-earth compounds, such as ultra-small colloidal ceria nanoparticles, highly homogenous and stable ceria-zirconia solid solutions, and high-quality rare-earth oxide and fluoride nanocrystals, etc., have been prepared by elaborately controlling the synthetic parameters and reaction kinetics. In order to reveal the mechanisms of synthesis and properties, the phase, microstructure, texture, and surface state have been characterized systematically. It is demonstrated that these rare earth nanocrystals could serve as good candidates candidates for wide applications applications such as UV-absorbents absorbents, three way catalysts, high efficiency up-conversion phosphors and biolabel materials.


Title: Magnetic Coupling and Quantum Well States
Speaker: Professor Z. Q. Qiu
Date:1 February 2008
Time:11.00am - 12.00pm 
Venue:PAP Meeting Room (SBS B3n-19)
Host:Professor Alfred Huan
Abstract: Electrons in ultrathin films are confined to form quantum well states (QWS). Photoemission provides the most direct observation of QWS in k-space. The unique capabilities now available at the Advanced Light Source (ALS) at Berkeley make it possible to measure the QW states with atomic layer resolution. In this talk, the photoemission result from the ALS on the Cu/Co(100) QW system is presented. Firstly, using a Ni monolayer to probe the Cu QWS at different positions, it is shown that the QWS in metallic thin films can be described by the envelope function of the Bloch wave. Secondly, by measuring the magnetic coupling using the magnetic dichroism, the relationship is demonstrated between the oscillatory magnetic coupling and the oscillations of the density of states at the Fermi level.


Title: BN nanotubes, interaction with molecules and their polymeric composites
Speaker: Dr Zhi Chunyi 
Date:19 January 2008 


Title: Probing Charge Transfer from a Metal Electrode into a Molecule/Polymer/Insulator with Nanometer Resolution
Speaker: Dr N Chandrasekhar
Date:18 January 2008
Time:11.00am - 12.00pm 
Venue:PAP Meeting Room (SBS B3n-19)
Host:Professor Alfred Huan
Abstract: Charge transport from a metal to a molecule/polymer is necessary to realize devices made from organic materials. It is important to study this charge transfer phenomenon with nanometer scale lateral resolution due to trend in miniaturization of devices. BEEM is a technique capable to providing such lateral resolution, coupled with milli-electron volt energy resolution. BEEM has been used extensively to study metal-inorganic semiconductor interfaces. Results from two oligomer systems and two self-assembled monolayer molecular system will be presented and discussed. We show that BEEM, in combination with V-z spectroscopy is capable of determining the entire band alignment picture for SAM’s. V-z probes the molecule-substrate interface, whereas BEEM probes the metalmolecule interface. Notable features of charge transfer across metal-organic interfaces -influence of traps, inhomogeneous charge transfer, multiple resonances in quantum mechanical transmission will be discussed.

I will also illustrate the application of BEEM to study industrially relevant materials such as Hafnium dioxide, a gate dielectric. Band offsets with a metal electrode and effective mass of carriers in the dielectric can be determined. In addition, BEEM can be used to locally stress the oxide and study breakdown phenomena.


Speaker: Dr Nobuo Maeda
Date:11 January 2008
Time:11.00am - 12.00pm 
Venue:PAP Meeting Room (SBS B3n-19)
Host:Professor Alfred Huan
Abstract: Capillary condensation of a wetting liquid from undersaturated vapor is perhaps the best known example of a surfaceinduced phase transition, and plays an important role in determining the behavior of systems ranging from hard disk drives to sand castles. Here we present the results of Surface Forces Apparatus experiments on the formation and disappearance of capillary-held liquids, with an emphasis on dynamics. The observed condensation rates are consistent with a model which takes into account both vapor diffusion and flow in thin liquid films adsorbed on the pore surfaces.

Furthermore, our measurements reveal the presence of two distinct modes for the disappearance of liquids held in a slit-like pore of adjustable slit-width, dependent on the slit-opening rate. In contrast to the first mode (‘bridge rupture’), which is well-documented in terms of the Young-Laplace equation, unexpected behavior was observed in the second mode (‘evaporation’), where the liquid bridge was held in the vicinity of the thermodynamic phase boundary (equilibrium Kelvin length). We will discuss potential implications of these results in the behavior of a wide range of systems.


Title: Spin Dynamics in Semiconductor Nanostructures
Speaker: Dr M. W. Wu
Date:6 January 2008
Time:11.00am - 12.00pm 
Venue:PAP Meeting Room (SBS B3n-19)
Host:Assistant Professor Tom Wu
Abstract: In this talk we are going to present our theoretical investigations on spin dynamics of semiconductor nanostructures under various conditions. It is shown from a fully microscopic kinetic-spin-Bloch-equation (KSBE) approach that the single-particle approach is inadequate in accounting for the spin relaxation/dephasing (R/D) both in the time domain and the spacial domain. The momentum dependence of the effective magnetic field (the Dresselhaus and the Rashba terms) and the momentum dependence of the spin diffusion rate along the spacial gradient all serve as inhomogeneous broadenings.

It is pointed out that in the presence of inhomogeneous broadening, any scattering, including the carrier-carrier Coulomb scattering, can cause irreversible spin R/D. Moreover, besides the spin R/D channel the scattering provides, it also gives rise to the counter effect to the inhomogeneous broadening. The scattering tends to drive carriers to a more homogeneous states and therefore suppresses the inhomogeneous broadening. Finally, this approach is valid in both strong and weak scattering regime and can be used to study systems far away from the equilibrium such as electrons of high spin polarization and/or electrons with strong electric field (hot electrons). Many novel effects are predicted from our theory and some have been realized experimentally very recently.


Title: Controllable Synthesis, Characterization and Optical Properties of Semiconductor Nanowires
Speaker: Dr Guanzhong Wang
Date:6  January 2008
Time:2.30pm - 3.30pm 
Venue:PAP Meeting Room (SBS B3n-19)
Host:Assistant Professor Tom Wu
Abstract: We present the synthesis of high-density well-aligned ZnO nanorod arrays on ZnO film coated Si substrates, their Raman spectra, temperature dependence of and time-resolved photoluminescence spectra, and laser emission; synthesis and optical properties of the doped ZnO nanobelts with (0001) polar plane as dominated surface and ZnO:In superlattice nanowires. We also category superlattice nanowires and nanobelts into three types and demonstrate the first characterization of planar superlattice nanobelts by X-ray diffraction and cross section transmission electron microscopy techniques.