Argonne National Laboratory Center for Nanoscale Materials U.S. Department of Energy

Archive: Seminars 2005

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Dec. 8, 2005

“Ultrafast spectroscopy of single-walled carbon nanotubes," Libai Huang-Stevenson, University of Rochester, hosted by Gary Wiederrecht

Abstract: The optical and electronic properties of single-walled carbon nanotubes (SWNTs) were studied by ultrafast spectroscopy. The photoexcited carriers in isolated SWNTs relaxed through many channels with decay time ranging from subpicosecond to over 100 ps. The magnitude of the longest-lived component in the ultrafast signal exhibited strong resonance dependence, thus suggesting that this lifetime corresponds to the band-edge relaxation time.

Excitons are expected to play an important role in reduced dimension materials. Exciton effects in SWNTs were investigated through Auger recombination. Quantized Auger recombination was observed, and Auger lifetimes for two and three e-h pair states were determined. Auger recombination in SWNTs is extremely fast, with lifetimes of only a few picoseconds for SWNTs excited with two and three e-h pairs. Experimental results conclusively show that Auger recombination in SWNTs is due to interaction between one-dimensional excitons and occurs as a two-particle process; a one-electron picture cannot explain our data. Thus, this provides experimental evidence of discrete one-dimensional exciton-excited states in SWNTs.

Nov. 14, 2005

"High-Efficiency Carrier Multiplication in Semiconductor Nanocrystals," Richard D. Schaller, Los Alamos National Laboratory, hosted by Gary Wiederrecht

Abstract: The efficiency with which photons are converted into charge carriers determines the ultimate efficiencies of various photo-induced physical and chemical processes including photo-generation of electricity (photovoltaics) and solar fuels, optically pumped lasing, generation of nonlinear-optical responses, etc. Normally it is assumed that the absorption of a single light quantum (a photon) by a semiconductor produces a single electron-hole pair (an exciton), meaning that the quantum efficiency (QE) in generating charge carries is 100%.  However, we have found that quantum-confined semiconductor nanocrystals of PbSe can produce two or even three excitons (QE > 200%) in response to a single absorbed photon via the process referred to as carrier multiplication (CM) [Phys. Rev. Lett. 92, 186601 (2004)]. To address the issues of generality of CM to other material compositions and the mechanism for this phenomenon, comparative studies of CM in nanocrystals of PbSe and CdSe that are characterized by a significant difference in both electronic structures and carrier relaxation behaviors were performed. Despite these differences, both compositions exhibit CM with comparable efficiencies (defined in terms of the slope of the QE dependence on photon energy above the CM threshold), which is indicative of broad generality of this phenomenon to quantum-confined, semiconductor nanoparticles. We demonstrate that CdSe nanocrystals show a lower activation threshold for CM than PbSe nanocrystals (~2.5 vs. ~2.9 energy gaps), which can be explained using simple carrier effective-mass arguments. Furthermore, we observe a monotonic increase in QE with increasing excess energy above the CM threshold up to ~700% in PbSe nanocrystals and ~160% in CdSe nanocrystals; and we expect that these values can be increased further by using still higher photon energies and/or by decreasing the energy gap. High exciton multiplicity produced by a single light quantum has numerous potential applications in physics and chemistry ranging from high-efficiency photovoltaics and single-photon oxidation of water molecules (water splitting) to low-threshold lasing and generation of entangled photon pairs.

Oct. 27, 2005

"Progress and Prospects in Coherent X-Ray Diffraction," Malcolm Howells, Advanced Light Source, Lawrence Berkeley National Laboratory

Abstract (14kb pdf)

Oct. 10, 2005

"Nanofabrication and Directed Self-Assembly," J. Alexander Liddle, Lawrence Berkeley National Laboratory, hosted by Leonida Ocala

Abstract: Lithographic resolution for dense features is typically limited by the blur from adjacent features overlapping, causing a critical loss in contrast. In the first part of this talk I will discuss how this limit can be overcome by using sequential, precisely aligned exposures. I will describe the alignment process and present results from a 30-nm pitch zone plate. Although this technique is effective, its usefulness is constrained by its inherently low throughput. It also becomes increasingly difficult as the dimensions are reduced. Many applications in nanotechnology require very large areas of nanoscale features. In the second part of the talk, I will discuss two approaches to generating ordered nanostructures using a minimal amount of conventional lithography. The first makes use of chemically patterned surfaces to control the structure of diblock copolymer materials, while the second relies on fluidic forces to assemble solution-synthesized nanostructures in prefabricated features.

June 27, 2005

Nano-Optics: From Surface Plasmons to Photoinduced Molecular Mass Transport,” Alexandre Bouhelier, CNM/Chemistry Division, hosted by Eric Isaacs

Abstract: We investigated two-photon-induced photoluminescence in single-resonant nanoparticles to assess the electromagnetic field enhancement associated with surface plasmon resonances. We demonstrate an anisotropic polarization sensitivity of the photoluminescence in gold nanorods as well as shape-induced spectra of the photluminescence.

Surface plasmons excited and traveling in planar metal surface offer the potential for developing new types of photonic device. Toward this end, we introduce a far-field optical method for exciting and observing surface plasmon intensity distribution. This approach provided a direct measurement of the propagation length of plasmons and the damping mechanisms involved in accordance with theoretical modeling. Surface plasmons also find applications in the context of near-field optical microscopy. The electromagnetic field enhancement and the high degree of confinement occurring at nanometric gold structures such as tips are particularly attractive to amplify weak optical cross-sections. In this context, we use photochemical mass transport to produces topographical changes in a polymer to study (a) the excitation conditions leading to an enhancement effect and (b) the resulting small-scale photomodification due to the high degree of field localization.

June 24, 2005

“Na, na, na, nano,” Andries Meijerink, Condensed Matter and Interfaces, Debye Institute, The Netherlands, hosted by Lynn Soderholm

Abstract: The popularity of everything that involves "nano" has contributed to the rapid increase in research on the luminescence of nanocrystalline semiconductor particles. This nanohype is a valid excuse to increase the chances for funding and the money is used to do exciting new physics and chemistry. The quantum size effects that occur in nanocrystalline semiconductors (also called quantum dots) are responsible for a change in the electronic structure in the nanocrystals as a function of particle size. This results in fascinating size-dependent optical and electrical properties. These properties are not only of great fundamental interest but also offer possibilities for applications of quantum dots in the fields of chemistry, physics, and biology.

Quantum dots of CdSe and CdTe can be made with a size that can be controlled with great accuracy between 2 and 6 nm and size dispersion below 15%. The quantum dots show a bright photoluminescence (with quantum efficiencies up to 80%) of a color that depends on the size of the crystallites. The possibility to measure emission spectra of a single quantum dots has contributed to a better understanding. The luminescence of a single quantum dot shows interesting phenomena: the emission shows an on/off behavior (blinking), a gradual shift to shorter wavelengths (blueing), and finally the emission disappears (bleaching). This behavior is important for one of the promising applications of quantum dots as luminescent labels of biomolecules.

The luminescence efficiency of quantum dots depends critically on the surface passivation. The subtle influence of the surface capping layer is illustrated by the quenching of the luminescence as the quantum dots are cooled. This is a unique phenomenon as in any other system the luminescence quenches upon heating (temperature quenching of luminescence). The temperature anti-quenching of luminescence that is observed for CdSe and CdTe quantum dots is related to subtle changes in surface layer of the quantum dot. Finally it will be shown how quantum dots can be used to probe the influence of the refractive index on the radiative decay rate for electric dipole transitions and give an opportunity for experimental verification of decades of work in the field of theoretical physics.

June 8, 2005

"Infrared Metamaterials and Nanophotonics," Shuang Zhang, Center for High Technology Materials, University of New Mexico, sponsored by Leonidas Ocola

Abstract: Fabrication and characterization of several novel metal-based structures that show unconventional optical properties and have great potential applications are discussed. While metals provide a negative permittivity at frequencies below the plasma frequency, naturally occurring materials with negative permeability at optical frequencies are not available. Composite electromagnetic materials with resonant structures with sizes much less than the wavelength can act as an effective homogeneous media with a negative permeability. The fabrication, characterization, and modeling of arrays of a new nanostructure design with resonances in the mid-infrared (IR) region and properties that demonstrate strong magnetic activity indicative of negative permeability are described. This is among the first experimental works on negative permeability reported in the IR.

By combining structures with magnetic response and electrical response, a negative refractive index metamaterial is designed and fabricated. The negative index of the fabricated structure is obtained uniquely from the experimental results. This is the first demonstration of negative index metamaterials in the near-IR, about four orders of magnitude shorter than previously reported work.

Finally, the fabrication and measurement of an array of metallic coaxial apertures will be presented. Over 80% transmission with very small fractional openings is achieved. Interaction between the propagating mode supported by the coaxial aperture and the surface plasmon mode is observed. The potential application of this structure will be discussed.

April 25, 2005

"Current-Driven Phenomena in Magnetic Multilayers," Sergei Urazhdin, John Hopkins University, hosted by John Freeland

Abstract: The effects of magnetism on electronic transport in ferromagnets, such as anisotropic, giant, and tunneling magnetoresistances, are now well understood and find applications in various sensors and prototype memory devices. Recent advances in nanofabrication technology enabled us to study the reverse effect of current on the magnetic order, called spin-transfer, which becomes significant at extremely high current densities of about 10E7-10E8 A/cm squared. I will give a brief introduction to spin-transfer in magnetic nanostructures and describe our recent findings: identification and separation of different current-driven behaviors, the effects of temperature and magnetic field. I will introduce an effective temperature model of the current-driven phenomena, and describe a series of experiments demonstrating the central role played in these phenomena by spin currents. I will also outline the possible future applications of spin-transfer.

April 28, 2005

"Automated High-Content Screening of Cell and Chip Based Assays – The Pathfinder System," Khuong Truong, IMSTAR SA, Paris, France, hosted by Millie Firestone and Lahsen Assoufid

Abstract: High-content screening is being used several areas of research, drug discovery and in vitro diagnostics. Automated screening allows robust and timesaving cell detention and signal quantitation. We will present an automated imaging system, the PATHFINDER system designed for high-content screening of cellular assays within tissue sections and cell spreads on slides, multiwell microplates and microarrays. The analysis can be performed for multiple categories of cell markers matching criteria of accuracy, objectivity and laboratory productivity.

April 5, 2005

"Advances in Computational Approaches to Homogeneous Nucleation: The Importance of Nanoscale Particles," Dr. Bruce C. Garrett, Pacific Northwest National Laboratory

April 5, 2005

"Recent Developments in AFM Technology at Veeco Instruments, " Bob Tench, Director of Product Management

"Atomic Force Microscopy of Soft Materials: High Resolution Imaging and Mapping of Local Properties," Sergei Magonov, Senior Staff Scientist

March 11, 2005

"Measurements of Brownian Motion of Nanoparticles in Suspension Using Micro/Nano PTV Techniques," Kenneth D. Hihm, Magnavox, University of Tennessee, hosted by Derrick Mancini

Abstract: A recent experiment conducted at the MIcro/Nano-Scale Fluidics and Energy Transport (MINSFET) Laboratory at the University of Tennessee shows that the thermal conductivity of nanofluids dramatically increases with decreasing nanoparticle sizes suspended in the base coolant. Since the dynamic and thermal phoreses of nanoparticles of different sizes are believed to be crucial to explain the basic physics of the aforementioned findings, micro/nano PTV techniques have been developed and applied to non-intrusively measure the nanoparticle sizes and their dynamic locations, and the suspension temperature.

Feb. 9, 2005

“Towards in vivo Dynamical Observations of Single-Protein Molecules Using X-Rays," Yuji C. Sasaki, Spring-8/JASRI Biomedical Group, JST/CREST, hosted by Eric Isaacs

Abstract: In vitro time-resolved X-ray observations of picometer-scale slow Brownian motion of individual protein molecules in aqueous solutions have been obtained with a new single molecular detection system, diffracted X-ray tracking (DXT).

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