Archive: Seminars 2008
April 22, 2008 |
"How Macromolecules Pass Through a Nanopore or Ultrafiltration of Macromolecules through Nanopores," Chi
(Qi) Wu, The Chinese University of Hong Kong, hosted by Yugang Sun and Gary Wiederrecht
Abstract: Using a special double-layer membrane to avoid interaction among flow fields generated by different
pores, we have, for the first time, observed the predicted discontinuous first-order transition in ultrafiltration of flexible
linear polymer chains. That is, the chain could pass through a pore much smaller than its unperturbed radius only when the flow
rate is higher than a certain value. When only one chain and one pore are considered, in theory, such a threshold is surprisingly
independent of both the chain length and the pore size. Our results reveal that for a membrane with many pores and at a microscopic
flow rate (q) lower than the threshold, the inevitable blocking of some pores by longer nonstretched coiled chains increases q
in those nonblocked pores because the macroscopic flow rate (Q) is a constant.
Our results reveal that the force needed to stretch a polymer coil in an athermal solvent is only ~10 fN.
Further, using this method, we are able to measure how "soft" a polymer chain is and how strong the interchain interaction
is when they are collapsed and entangled with each other. |
April 17, 2008 |
"MEMS-Based Spatial Light Modulators: Recent Developments and Future Directions," Daniel
Lopez, Alcatel-Lucent, hosted by Eric Isaacs
Abstract: Micro electro mechanical dystems (MEMS) technology enables mass production of microscopic mechanical
systems through batch fabrication techniques similar to the ones used in the electronics integrated circuit (IC) industry. Similar
to ICs containing millions of individual transistors, mechanical systems with millions of independent moving parts (degrees of
freedom) can now be fabricated on a single small silicon chip. While a variety of applications will benefit from this technology,
nowhere is the potential as clear and compelling as in optical systems, at the intersection of electronic, mechanical and optical
domains.
The unique advantages of MEMS are high-speed and excellent optical quality, wavelength and polarization independence,
and low optical loss. Dense integration of millions of individually controlled micro-mirrors onto a single silicon chip leads
to creation of a reconfigurable fast digital diffractive optical element that allows an unprecedented degree of control and manipulation
of the optical field. This new kind of spatial light modulator (SLM) reveals potential to bring revolutionary new capabilities
for projection, information processing, and telecommunications.
In this talk I will describe the fundamentals of this technology and our recent work in the development and
fabrication of a MEMS-based phase-only SLM originally envisioned as a substitute for expensive optical photomasks. I will also
describe the new and distinctive capabilities that these SLMs offer for miniature projection systems, optical nano-manipulation,
holographic data storage, and free space communications. |
April 14, 2008 |
"Device Implications of Spin Transfer Torques," Jordan Katine, Hitachi San Jose Research
Center, hosted by Axel Hoffman
Abstract: This presentation looks at spin transfer torques from the perspective of three technological applications:
hard disk drives, MRAM, and current-tunable high-frequency oscillators. In hard disk drives, spin transfer torques are a source
of noise, and I will discuss the implications spin transfer noise will have on future sensor designs. For MRAM, I evaluate the
feasibility of spin transfer driven switching. Finally, I consider the possibility of GHz communication applications enabled by
nanoscale spin transfer oscillators, including Hitachi's recent results in MgO-based devices. |
March 19, 2008 |
"Shape Changes induced by Chemical Transformation of Nanocrystals," Can K. Erdonmez, Massachusetts
Institute of Technology, hosted by Yugang Sun and Gary Wiederrecht
Abstract: Colloidal nanoparticles (NPs) of a great number of materials can now be produced with well-controlled
size, shape and surface properties. However, it is still often difficult to extend an existing synthetic recipe. For example,
a particular size range and shape might be easy to produce for one composition, but not another, similar one. Analogous to cases
in organic chemistry, inorganic NPs can participate in chemical reactions as preformed precursors and form product particles with
modified size, composition, and properties. Recent research shows that this approach not only allows the realization of new compositions,
but can also yield morphologies derived from, but also distinct from, the shapes of the starting materials.
I will present several examples of morphological evolution of NPs as a consequence of chemical transformation.
The major focus is on the transformation of solid particles into hollow nanoshells due to directional diffusive mass transport
(the Kirkendall effect) accompanying chemical transformation. This process is predicted to be easily achieved for a large number
of materials and experimental results support the general nature of the effect. In one case, the transformation of cobalt NPs
into Co3S4 nanoshells, existing bulk studies are complete enough and the nanoshell synthesis mature enough for making inferences
about the shell formation process.
I will also highlight two more examples of simultaneous composition and shape modification and discuss their
likely mechanisms: transformation of solid silver particles into octahedral cages upon galvanic exchange and formation of periodically
ordered Ag2S inclusions in CdS nanorods upon partial exchange of Cd2+ with dissolved Ag+. The former process illustrates one possible
end result of etching and redeposition occurring simultaneously in a nanoscale system; the latter involves a complex and still
not fully understood interplay between elastic stress, nucleation and diffusion-limited segregation kinetics in a one-dimensional
system. |
February 21, 2008 |
"Quantum dots based energy transfer to photodynamic therapy agents," Smita Dayal, Case
Western Reserve University, hosted by Matthew Pelton |
February 12, 2008 |
"Nanoscale Imaging with the Coherent Diffraction Microscope," Changyong Song, University
of California, Los Angeles, hosted by Jorg Maser and Ian McNulty
Abstract: The coherent diffraction microscope (CDM) is a versatile imaging probe with applications spanning
a wide range of nano- and biosystems at nanometer resolution. The simple experimental schemes of the CDM have led to immediate
adaptations over broad spectra of coherent light sources including hard X-ray, soft X-ray, and tabletop EUV lasers. Extensive
research over the past several years has advanced the technique significantly – three-dimensional tomography, resonant imaging
and tabletop diffraction microscopy – to address certain scientific questions as a practical microscope. The ultimate interest
for the technique is to resolve structures within single macromolecule complexes at near atomic resolution, which will be feasible
with the emergent X-ray free electron lasers (XFELs). The coherent diffraction microscope combined with the XFELs will shed a
new light on nano- and biotechnology. |
January 29, 2008 |
"Spectroscopic Studies of Novel Nanomagnetic Materials," Saritha Nellutla, National High
Magnetic Field Laboratory, hosted by Tijana Rajh
Abstract: Nanomagnets are fascinating materials not only from a basic research point of view but also
because of their relevance in memory storage, quantum computation, spintronics, and sensors. They can be thought of as "bridges" between
the isolated single-ion spin systems and bulk magnetic materials.
The talk will focus on magnetic and electron paramagnetic resonance (EPR) studies of polynuclear transition
metal ion clusters, Cu3, Cu20, and Mn6, while addressing such potential applications as catalysts and nanomagnets.
The two extreme diluted spin systems compared with correlated magnetic lattices will also be highlighted.
Potassium niobate doped with Cr5+ ions, a dilute spin S = 1/2 system, has been characterized by using pulsed EPR and electron
nuclear double resonance techniques and will be introduced as a new transition metal-ion-based electron spin qubit. As illustrative
examples of correlated magnetic lattices, thermomagnetic data of spin S = 1/2 and S = 1 antiferromagnetic peroxychromates will
be presented and nature and origin of the three-dimensional magnetic ordering in these compounds will be discussed. |
January 15, 2008 |
"Nanocrystal based 'artificial solids': a modular approach to materials design," Dmitri
Talapin, The University of Chicago, hosted by Tijana Rajh
Abstract: The development of applications ranging from displays and photovoltaic cells to thermoelectric,
light-emitting devices and sensors could be accelerated by introducing lower cost alternatives to conventional silicon technology.
Chemically synthesized semiconductor nanocrystals are considered promising candidates that allow inexpensive
solution-based device fabrication with precise engineering of electronic structure due to quantum size and shape effects. Self-assembly
of chemically synthesized nanocrystals can yield complex long-range ordered and quasicrystalline structures that can be used as
model systems for studying transport in low-dimensional materials. At the same time, employing nanocrystals in these and other
electronic and optoelectronic applications require deep understanding of charge transport and collective phenomena in nanocrystal
solids.
We propose the techniques for engineering nanocrystal surfaces to improve exchange coupling in self-assembled
nanocrystal solids. The conductivity of nanocrystal solids can be switched between n- and p-type transports by surface transfer
doping. Thus, hydrazine-capped PbSe nanocrystal solids show n-type conductivity ~1.4 S cm -1 with electron mobility of 2.5 cm2V-1s-1,
successfully competing with organic electronic materials. Doping of PbSe and PbTe nanocrystal solids can also occur through the
exchange coupling with other semiconductor (Ag 2Te) or metal (Au) nanocrystals intentionally introduced in the nanocrystal solids.
By using various approaches to nanoparticle surface engineering, we demonstrated n- and p-channel field-effect transistors based
on PbS, PbSe, PbTe, CdSe, and SnTe nanocrystals.
We develop a general approach to solution-processed semiconductor nanocomposite materials based on incorporation
of nanocrystals into a matrix of another crystalline inorganic semiconductor. This approach opens up broad avenues for designing
novel functional materials. We demonstrate memory devices using CdSe/ZnS core-shell nanocrystals as floating gates, solar cells
employing CdS nanorods integrated into CuIn(1-x)Ga(x)Se 2 matrix and Sb2Te3-Bi2Te3 nanocomposites promising for thermoelectric
applications. |
January 9, 2008 |
"Development of novel photocatalytically active materials on the base of porous silica and titania," Galyna
Krylova, Université de Rennes, hosted by Tijana Rajh and Elena Shevchenko
Abstract: Photocatalysis is a promising, energy-saving approach for purification of the environment
from toxic pollutants. The most current studies are based on TiO2 photocatalyst as a rather inexpensive and chemically stable
material. Surface modification of TiO2 photocatalyst allows efficient use of sunlight, leads to an increase in the active surface,
and improves charge separation. We discovered that the addition of 3-aminopropyltrimethoxysilane (APTES) to the precursors of
the mixed TiO2/ZnO sol-gel nanostructure films drastically affects the size of crystallites and their crystal lattices, leading
to much smaller (6-10 times) domains of Zn2TiO4 spinel and TiO2 anatase, instead of relatively large ZnTiO3 and TiO2 rutile crystallites.
As compared with widely used titania films prepared from P25 Degussa sol, our APTES-modified mixed-oxide nanostructures demonstrate
two times higher photocatalytical efficiency in such model reactions as decomposition of methylene blue (model pollutant of waste
water) and stearic acid (model reagent to study antifouling properties). The excellent hydrophobic properties of TiO2/ZnO sol-gel
films allows their use as self-cleaning, antifogging glass and mirror coatings.
Highly adsorptive porous silica is another excellent platform for designing hybrid photocatalysts. Modification
of porous silica with photoactive organic molecules promotes the reduction of metals cations noble and allows the formation of
nanoparticles of noble metals. Surface modification of mesoporous powders of films of silica with benzophenone shifts the barrier
of photosensitized reduction of Ag+, AuCl4–, Pd2+, Сu2+, Cr2O72–, and Hg2+ ions from far (253.7 nm) to near
(365 nm) ultraviolet region. The mechanism of photosensitized reduction in these systems was proposed. Variation in the reaction
conditions allows both synthesis of stable colloidal solutions of noble metals with controllable sizes and shapes and silica-noble
metal nanostructures composites. As compared witih the chemical synthesis of such structures, our photo-induced approach has a
number of advantages associated with low energy consumption and the possibility of removing side reaction products by absorption
at silica surface. |
January 4, 2008 |
"Enhancing the open-circuit voltage of dye-sensitized solar cells by coadsorbents and alternative redox
couples," Zhipan Zhang, Swiss Federal Institute of Technology, hosted by Gary Wiederrecht
Abstract: The interface between a nanocrystalline semiconducting metal oxide film and a redox electrolyte was
optimized in order to enhance the photovoltage and performance of dye-sensitized solar cells. It features the application of ω-guanidino
acids as the coadsorbent with ruthenium amphiphilic sensitizers.
Meanwhile, fast one-electron-transfer couples were employed as alternative redox mediators to the normal iodide/iodine
system to reduce the potential mismatch between the Nernst potential of the dye cation and that of the redox mediator. Although
a much faster recombination was observed as compared with I- / I3- redox, the device with 2,2,6,6 -Tetramethylpiperidine-1-oxyl
(TEMPO/TEMPO+) showed an overall solar to electric power conversion efficiency of 5.4 % under AM 1.5 illumination at 100 mW/cm2. |
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