CASP Distinguished Colloquium Series

This Distinguished Colloquium series includes external scientists presenting important recent advances relevant to Center for Advanced Solar Photophysics (CASP) research. Each speaker has been selected from the broad scientific community nationwide based on recommendations from Center members.

Title and Abstract TBA

Prof. Markus Raschke
Department of Physics
University of Colorado at Boulder

Chemistry Division Auditorium, TA-46, Bld. 535, Rm. 103
Monday, January 30, 2012, at 10:00 am

Infrared Metamaterials and Their Applications to Energy Harvesting and Bio-Sensing

Prof. Gennady Shvets
Department of Physics
The University of Texas at Austin

Chemistry Division Auditorium, TA-46, Bld. 535, Rm. 103
Wednesday, December 7, 2011 at 10:00 am

Abstract: Multi-resonant plasmonic structures are novel building blocks of optical metamaterials. In this talk I will discuss the potential of such metamaterials as platforms for optical devices designed to efficiently harvest and compress the solar spectrum for solar thermophotovoltaic (STPV) applications and to enable highly-sensitive infrared bio-sensing of molecular monolayers. The theoretical framework for describing the so-called Fano resonances will be discussed and experimental results presented. Both metal- and dielectric-based metamaterials will be reviewed and compared. Long-range coherence effects in periodic and disordered arrays of Fano-resonant structures will be discussed. Both spectroscopic and near-field characterization results of various plasmonic metamaterials will be presented. Examples of metamaterials used for highly-efficient STPV applications and protein fingerprinting will be discussed.

Colloidal Core/Shell Heterostructures with Alloy Composition: Synthesis, Magneto-Optical Characterization and Theoretical Electronic Band Structure

Professor Efrat Lifshitz
Schulich Faculty of Chemistry
Russell Berrie Nanotechnology Institute, Solid State Institute
Technion, Haifa, 32000, Israel

Chemistry Division Auditorium, TA-46, Bld. 535, Rm. 103
Wednesday, June 22, 2011, 2 pm

Abstract: Two decades of research were devoted to explore the electronic properties of colloidal quantum dots (CQDs) by varying their size, shape and surface coverage. The option to engineer the properties is of  significant importance for the CQDs implementation as absorbers or emitters in photovoltaic cells, light emitting diodes, optical switches, gain devices, photodetectors, thermoelectric, spintronics devices and biological platforms. However, some of these applications impose constrains on the size and shape, say, incorporating them into biological membrane or a mesoporous substrate, while retaining the demand for a suitable emission color. These restrictions can be overcome by new strategies gaining property control using: (a) alloyed ternary or quaternary compounds, when a ternary material is comprised of two different cations/anions with a common anion/cation, while quaternary includes four elements. In all, the elements can be either distributed homogeneously or exhibit a graded composition along a selective direction; (b) core/shell heterostructures, comprised of a semiconductor core, covered by a shell (sphere or rod shape), of another semiconductor, when the band edge offset at the core/shell interface, can be tuned from a type-I (when shell ban-edge is rapping that of the core), through quasi-type-II, to a type-II (when, band-edge of the constituents are staggered) alignment. Moreover, one of the constituents (core or shell) may have alloyed composition. Recent years showed a progressive effort in the synthesis of alloyed colloidal quantum dots (CQDs) or nanowires, by employing an effective hightemperature synthetic strategy with balancing of precursors' reactivity. Further on, unique alloyed core/shell heterostructures, such as PbSe/PbSexS1-x,1 CdTe/CdTexSe1-x, and CdSe/CdSexS1-x/CdS,2 were developed lately, offering better crystallographic and dielectric match at the core/shell interface, regulating carriers' delocalization and/or charge separation by tunability of the band off-set, showing an exceptionally high emission quantum yield, chemical stability, and an option to stabilize an emission intensity (blinking free behavior),2 as well as sustain the biexciton lifetime over a nanosecond. The last can be of a valuable benefit in the use of CQDs in gain devices and photovoltaic cells. Significant knowledge on carriers' localization is gained by the use of the following magnetooptical methodologies: (a) optically detected magnetic resonance (ODMR), supplying a magnetic resonance identification of a carrier and its surrounding, phenomenological g-factor, electron-hole exchange interaction and crystal anisotropy. (b) optically detected orbit resonance (ODOR) enabling a direct measure of an effective mass, viz., reflecting knowledge on composition (e.g. alloying); (c) Theoretical description of the electronic band structure of alloyed core and core/shell CQDs, using a k*p model, covered a wide physical aspects, including an effective mass anisotropy, dielectric variation between the constituents, a sharp or a smooth off-set at the core/shell interface and electron-hole Coulomb interactions, laid a ground for tailoring heterostructures with the desired composition and optical properties.

Ultrafast Single and Multiple Electron Transfer from Quantum Dots

Professor Tianquan Lian
Department of Chemistry
Emory University, Atlanta, GA 30322

Chemistry Division Auditorium, TA-46, Bld. 535, Rm. 103
Wednesday, March 9, 2011, 11 am

Abstract: Charge transfer to and from quantum dots (QDs) is of intense interest because of its important roles in QD-based devices, such as solar cells and light emitting diodes. Recent reports of multiple exciton generation (MEG) by one absorbed photon in some QDs offer an exciting new approach to improve the efficiency of QD-based solar cells and to design novel multi-electron/hole photocatalysts. However, two main challenges remain. First, the efficiency of MEG process needs to be significantly improved for practical applications. Second, the utilization of the MEG process requires ultrafast exciton dissociation prior to the exciton-exciton annihilation process, which occurs on the 10s to 100s ps time scale. In this presentation we report a series of studies of exciton dissociation dynamics in quantum dots by electron transfer to adsorbed electron acceptors. We show that excitons in QDs (CdSe, PbS and others) can be dissociated on the picosecond timescale to various adsorbates, competes effectively with exciton-exciton annihilation. As a proof of principle, we demonstrate that multiple excitons per QD (generated by multiple photons) can be transferred to adsorbed acceptors. We will discuss the dependence of the electron transfer rates on the size and the nature of the quantum dots and possible approaches to optimize the multiple exciton dissociation efficiency.

Theoretical Study of Carrier Multiplication in Semiconductor Nanocrystals

Dr. Andrei Piryatinski

Chemistry Division Auditorium, TA-46, Bld. 535, Rm. 103
Wednesday, October 27, 2010, 10 am

Abstract: Recently, carrier multiplication (CM) in semiconductor nanocrystals (NC) became a focus of extensive studies motivated by potential applications in new efficient photovoltaic devices. Understanding a variety of CM spectroscopic measurements requires unified theoretical approach. In this lecture, we will briefly discuss different theoretical models developed to clarify the CM mechanisms. Then the exciton scattering model1 will be introduced. The model represents a unified approach that recovers previously developed models as limiting cases. Based on this model, we performed systematic numerical study of CM dynamics in PbSe NCs and bulk whose results will be discussed in details. Finally, new two-dimensional double-quantum coherence spectroscopy which according to our calculations has potential of resolving interband Coulomb interactions giving rise to CM will be discussed.

Spectroscopic and Electrical Imaging of Disordered Polymer Solar Cells: Bridging the Gap in Understanding Structure-Function Relationships over the Nano- to Microscales

Professor John K. Grey
Department of Chemistry
University of New Mexico
Albuquerque, NM 87131

Chemistry Division Auditorium, TA-46, Bld. 535, Rm. 103
Wednesday, September 15, 2010, 2 pm

Abstract: Polymer based materials are rapidly gaining prominence as viable candidates for excitonic solar cells owing to their improved performance and low cost of processing. However, there are still many unresolved issues pertaining to the role of polymer molecule conformation and packing properties on energy and charge transfer processes. We use resonance Raman-photocurrent imaging to study polymer packing characteristics in model solar cells based on a prototypical polythiophene system. These measurements report sub-micron scale fluctuations in local morphology that can be directly correlated to device performance. To surpass diffraction limited spatial resolution, we are currently exploring new nanoscale functional materials forms that permit a highly detailed view of structure-dependent excited state processes. These studies will help bridge the gap in understanding of material structure-function relationships over the crucial nano- to micrometer size scales.

Synthesis and Properties of Si and Ge Nanowires

S. Tom Picraux
Center for Integrated Nanotechnologies
Los Alamos National Laboratory
Los Alamos, NM 87545

Chemistry Division Auditorium, TA-46, Bld. 535, Rm. 103
Wednesday, June 16, 10:00 am

Abstract: Semiconducting nanowires promise a wide variety of potential applications, including novel electronic, photovoltaic, and energy storage devices. In this seminar I will review current understanding of the favored approach, the vapor-liquid-solid (VLS) method, for the growth of semiconducting nanowires and the associated materials science issues. Examples will be given of our research on Si, Ge, and Si/Ge heterostructured nanowires and their properties.1 Recent work on size-dependent growth rates at small nanowire diameters will be highlighted, including the thermodynamic limited minimum diameter for growth.2 One unique aspect of the VLS growth is the formation of axial and radial (core/shell) heterogeneous structures which can't be obtained by conventional 2D strained layer growth. Since nanowires are not laterally confined, this allows large strains to be incorporated into the structures, providing a new approach to band structure engineering. In the concluding part I will briefly discuss current interest in nanowires for energy applications.
1 Silicon and Germanium Nanowires: Growth, Properties and Integration (Invited Overview), S. T. Picraux, S. Dayeh, P. Manandhar D. E. Perea and S. G. Choi, JOM, 62, (4) 35 (2010).
2 S.A. Dayeh, S.T. Picraux, (under review).

Plasmons in Strongly Coupled Nanostructures: Hybridization, Fano Resonances, and Plexcitons

Peter Nordlander
Laboratory for Nanophotonics
Department of Physics and Astronomy and Electrical and Computer Engineering
Rice University, Houston TX 77005, USA

Chemistry Division Auditorium, TA-46, Bld. 535, Rm. 103
Monday, June 7, 2010 at 10:00 am

Abstract: Plasmonic nanostructures such as narrow plasmonic cavities, strongly interacting nanoparticle aggregates, and hybrid plasmonic/excitonic systems offer highly tunable platforms for the study of radiative interference and coherence effects such as subradiance, superradiance, and electromagnetically induced transparency (EIT). In structures with reduced symmetry, narrow Fano resonances can appear in their extinction spectra resulting from the interference between superradiant and subradiant modes. Apart from their fundamental importance, such phenomena are also of practical interest in metamaterial and chemical and sensing applications because of the extraordinarily narrow linewidths and strong sensitivities to the dielectric properties of the environment. In this talk, I will present a general framework for the description of radiative interference effects in plasmonic systems and illustrate the concepts with examples from recent applications to symmetry broken nanoshells[1], small nanoparticle clusters of D6h symmetry (Heptamers) [2], planar ring-disk systems (Fanocavities) [3], plasmonic heterodimers,[4] and quantum plasmonics and plexcitonics.[5]
[1] H. Wang et al., Proc. Nat. Acad. Sci. USA 103(2006)10856.
[2] N. A. Mirin et al., J. Phys. Chem. A 113(2009) 4028 and TBP
[3] F. Hao et al., Nano Lett. 8(2008)3983; ibid. 9(2009)1663; ACS Nano 3(2009)643 and Y. Sonnefraud et al., ibid. ACS Nano 4(2010)1664.
[4] L. Brown et al., ACS Nano 4(2010)819
[5] J. Zuloaga et al., Nano Lett. 9(2009)887, N.T. Fofang et al., Nano Lett. 8(2008)3481, ibid. 9(2009)887

Nanocrystal Solids: A Modular Approach to Materials Design

Prof. Dmitri V. Talapin
Department of Chemistry, The University of Chicago

Chemistry Division Auditorium, TA-46, Bld. 535, Rm. 103
Tuesday, March 23, 11am

Abstract: Nanoparticles of different metals, semiconductors and magnetic materials can self-assemble from colloidal solutions into long range ordered periodic structures (superlattices). Combining two types of nanoparticles yields binary nanoparticle superlattices (BNSLs) exhibiting very rich phase diagrams with a multitude of close-packed and non-close-packed phases, including quasicrystalline arrangements. Through a series of systematic studies of self-assembly phenomena in single- and multicomponent nanoparticle assemblies we demonstrate that observed structural diversity is a result of the intricate interplay of entropy-driven crystallization with isotropic and anisotropic interparticle interactions, such as van der Waals, Coulombic and dipolar forces. Colloidal nanocrystals are considered promising building blocks for electronic and optoelectronic devices. Potentially, they can combine the advantages of crystalline inorganic semiconductors with size-tunable electronic structure and inexpensive solution-based device fabrication. However, the insulating nature of the surface ligands used for nanocrystal synthesis typically results in the poor electronic coupling between individual nanocrystals. In an attempt to address this fundamental problem, we demonstrated that molecular metal chalcogenide complexes can serve as versatile ligands for a broad range of inorganic nanomaterials. This new class of nanocrystal colloids provides a set of advantages such as all-inorganic design and diverse compositional tunability for both nanocrystal and ligand constituents. We observed electron mobility of ∼10 cm2/Vs in arrays of CdSe nanocrystals and ∼200 S/cm conductivity in arrays of 5 nm gold nanocrystals capped with metal chalcogenide Zintl ions. We demonstrate the power of this approach on several examples of prospective thermoelectric and photovoltaic materials.

Superconducting Devices for Detection of Single Photons

Dr. Sae Woo Nam
National Institute of Standards and Technology (NIST), Boulder, CO

Chemistry Division Auditorium, TA-46, Bld. 535, Rm. 103
Wednesday, February 10, 2010, 10am

Abstract: There is increasing interest in using superconducting optical photon detectors in a variety of applications. These applications require detectors that have extremely low dark count rates, high count rates, and high quantum efficiency. I will describe our work on two types of superconducting detectors, the Superconducting Nanowire Single Photon Detector (SNSPD) and superconducting Transition-Edge Sensor (TES). An SSPD is an ultra-thin, ultra-narrow (nm scale) superconducting meander that is current biased just below its critical current density. When one or more photon is absorbed, a hot spot is formed that causes the superconductor to develop a resistance and consequently a voltage pulse. By exploiting the sharp superconducting-to-normal resistive transtion of tungsten at 100mK, TES detectors give an output signal that is proportional to the cumulative energy in an absorption event. This proportional pulse-height enables the determination of the energy absorbed by the TES and the direct conversion of sensor pulse-height into photon number. I will discuss our progress towards developing detectors with quantum efficiencies approaching 100% as well as describe several experiments and applications that are enhanced by using these detectors.