CASP Lecture Series

This lecture series includes presentations by experts in a variety of fields with focus on nanomaterials and their applications in solar energy conversion. The lectures will take place at Los Alamos National Laboratory, TA-46, Bldg. 535, Room 103.

Lecture 12: Quantum Chemistry of Quantum Dots

Andrei Piryatinski
Theoretical Division, Los Alamos National Laboratory, Los Alamos, New Mexico 87545, USA
Monday, May 17, 2011, 10am
Chemistry Division Auditorium, TA-46, Bld. 535, Rm. 103

Abstract: The field of nanoplasmonics has recently emerged as a result of the advances in understanding the nanoscale phenomena. The motivation is to understand how the light-matter interactions can be affected on the nanoscale by the presence of metal nanoparticles containing confined but still mobile electron gas. The variation of these electron densities by the optical fields results in the creation of highly enhanced local electric fields. The local fields further change optical properties of the nanomaterials (e.g., photon absorption and emission rates). The possibility to control such optical properties using precisely tailored metal nanostructures opens a variety of potential applications in new photonic and optical information processing devices, sensors, and biolabels. In this lectures, we will cover the basics of surface plasmon physics in nanostructured materials using semiclassical electrodynamic models as well as overview a number of important applications.

Lecture 11: Quantum Chemistry of Quantum Dots

Jennifer Hollingsworth
Center for Integrated Nanotechnologies, Los Alamos National Laboratory, Los Alamos, New Mexico 87545, USA
Monday, May 10, 2011, 10am
Chemistry Division Auditorium, TA-46, Bld. 535, Rm. 103

Abstract: Nanoscale semiconductor materials exhibit unique properties useful for energy-harvesting applications that contrast them with their respective bulk-phase counterparts, such as size-tunable band-gap energies for tuning the energy-onset of absorption, a concentration of the density of states into quantized energy levels resulting in high oscillator strengths for high absorptivities, and ultra-high surface-to-volume ratios. While much of the early work on the special properties of confined dimensionality has involved three-dimensionally (3D) confined nanoparticles (NPs), or quantum dots (QDs), 2D confined semiconductor nanowires (SC-NWs) can exhibit similar enabling characteristics. Furthermore, by virtue of their "wire-like" geometry, SC-NWs offer the additional advantage of providing a built-in conduit for charge transport-a key characteristic for applications in energy harvesting. SC-NWs can be fabricated using either vapor- or solution-phase methods, where solution-phase approaches offer advantages in lower-cost simplified processing and scale-up, as well as almost unlimited choice of materials systems from Group III-V to Groups IV, II-VI, IV-VI, and I-III-VI2 semi-conductors. Here, I will review aspects of the field of SC-NW synthesis, including a look at some future directions.

Lecture 10: Quantum Chemistry of Quantum Dots

Sergei Tretiak
Theoretical Division, CNLS, Los Alamos National Laboratory, Los Alamos, New Mexico 87545, USA
Wednesday, April 21, 2011, 10am
Chemistry Division Auditorium, TA-46, Bld. 535, Rm. 103

Abstract: Over the years, electronic structure calculations, such as density functional theory (DFT), transformed theoretical chemistry and materials science by creating a new capacity to describe the electronic structure and complex dynamics in molecules with hundreds of atoms. This talk will overview applications of modern quantum chemical methodologies to electronic structure of semiconductor nanocrystals. I will start with a brief introduction to the wavefunction and DFT methodologies and their extensions to the excited state calculations. Then I will overview several recent literature reports on quantum chemical studies of quantum dots (QDs) and will discuss possible advantages and sources of errors in these applications. In the second part of my talk I will analyze our benchmark of several computational methods versus high level ab initio techniques for the minimal CdSe clusters to assess an accuracy of different theories. Finally I will present our studies on the surface ligands effects on the QDs electronic structure, where we observe strong surface-ligand interactions leading to formation of hybridized states. This potentially opens new relaxation channels for high energy photoexcitations.

Lecture 9: Nanocrystals in Photovoltaics and Photocatalysis

Milan Sykora
Softmatter Nanotechnology and Advanced Spectroscopy, Chemistry Division
Los Alamos National Laboratory, Los Alamos, New Mexico 87545, USA
Monday, March 29, 2011, 10am
Chemistry Division Auditorium, TA-46, Bld. 535, Rm. 103

Abstract: Over the past several years, the search for more efficient solutions to solar energy conversion has intensified, in large part driven by concerns over the impact of fossil energy sources on global climate. In my presentation I will provide a brief overview of the state-of-the-art in photovoltaics, and discuss key drivers in the development of new types of solar cells. I will also describe methods used in the characterization of solar cell performance. I will then give a brief overview of several categories of emerging solar cell technologies based on quantum confined semiconductor nanocrystals, and discuss some of the challenges associated with the optimization of their performance. In the second part of my talk I will explain the concept of photocatalysis, provide a brief historical overview of the field, and discuss several examples of photocatalytic systems based on quantum confined semiconductor nanocrystals.

Lecture 8: Carrier Multiplication: Experimental Aspects and Practical Implications

Victor Klimov
Softmatter Nanotechnology and Advanced Spectroscopy,
Chemistry Division
Los Alamos National Laboratory, Los Alamos, New Mexico 87545, USA
Monday, March 29, 2011, 10am
Chemistry Division Auditorium, TA-46, Bld. 535, Rm. 103

Abstract: The efficient conversion of photon energy into electrical charges is a central goal of much research in physics, chemistry, and biology, especially in areas such as photovoltaics, photocatalysis, and photosynthesis. The usual assumption is that absorption of a single photon by a semiconductor or a molecule produces a single electron-hole pair (exciton), while the photon energy in excess of the energy gap is dissipated as heat by exciting molecular or lattice vibrations (phonons). Under this assumption, the maximum power-conversion efficiency of solar cells is limited to ∼31% (the Shockley-Queisser limit). In principle, one can surpass this limit using carrier multiplication, a process in which absorption of a single photon produces not one, but multiple excitons. In this lecture, I will review the current status of carrier multiplication research and describe some of the challenges concerning experimental measurements of multiexciton yields and understanding the mechanisms for multiexciton generation and competing energy relaxation processes. I will also briefly discuss the practical implications of this effect for solar-energy conversion technologies.

Lecture 7: Auger Recombination and Nanocrystal Lasing

Victor Klimov
Softmatter Nanotechnology and Advanced Spectroscopy,
Chemistry Division
Los Alamos National Laboratory, Los Alamos, New Mexico 87545, USA
Monday, March 29, 2011, 10am
Chemistry Division Auditorium, TA-46, Bld. 535, Rm. 103

Abstract: Using semiconductor nanocrystals (NCs), one can produce extremely strong spatial confinement of electronic wave functions not accessible with other types of nanostructures. One consequence of this effect is a significant enhancement in carrier-carrier interactions that leads to a number of novel physical phenomena, including ultrafast multiexciton decay due to Auger recombination. Auger recombination is a process in which the electron-hole recombination energy is not emitted as a photon but is transferred to a third carrier. In bulk semiconductors, Auger recombination is inhibited by kinematic restrictions imposed by energy and translational-momentum conservation that lead to the development of a thermal activation threshold for this process. However, because of relaxation of momentum conservation, the activation threshold is removed in NCs, which leads to a significant enhancement in the efficiency of Auger recombination. One consequence of fast Auger decay is very short lifetimes of optical gain in NCs that greatly complicate the applications of these nanostructures in practical lasing technologies. In this lecture, I will start with fundamentals of both Auger recombination and optical gain in strongly confined systems, and then talk on the most recent approaches to suppression of Auger decay using various types of engineered heteronanostructures. I will also describe a novel concept for NC lasing using single-exciton optical gain enabled by giant exciton-exciton repulsion in type-II hetero-NCs that produce efficient spatial separation of electrons and holes.

Lecture 6: Optical Spectroscopy of Individual Nanocrystal Quantum Dots

Han Htoon
Softmatter Nanotechnology and Advanced Spectroscopy,
Los Alamos National Laboratory, Los Alamos, New Mexico 87545, USA
Monday, March 22, 2011, 10am
Chemistry Division Auditorium, TA-46, Bld. 535, Rm. 103

Abstract: Optical spectroscopy has been an indispensable tool in probing fundamental photophysics of nanoscale materials. Conventional optical spectroscopy approaches that usually require sampling hundreds to thousands of individual nano-objects, are effective only in measuring averaged characteristics of an ensemble. As a result, these approaches are incapable of probing variations in optical characteristics that result from strong quantum confinement effects in different individual nanostructures. Furthermore, they are also powerless in probing electronic fine structures with energy splittings smaller than the inhomogeneously broadened ensemble spectral features. Here I will review single nanostructure optical spectroscopy approaches that are designed to overcome these problems of ensemble averaging. Specifically I will review how single NQD photoluminescence (PL) imaging, low temperature PL and magnetoPL, PL excitation and time resolved PL techniques help attain a profound understanding on the fine structure of band-edge exciton states as well as the competition between radiative and non-radiative processes.

Lecture 5: Semiconductor Nanocrystals and High Magnetic Fields

Scott Crooker
National High Magnetic Field Laboratory,
Los Alamos National Laboratory, Los Alamos, New Mexico 87545, USA
Monday, February 22, 2011, 10am
Chemistry Division Auditorium, TA-46, Bld. 535, Rm. 103

Abstract: The strong zero-dimensional quantum confinement realized in colloidal semiconductor nanocrystals leads to very large spin-exchange interactions between electrons and holes (ie, excitons). Whereas typical exchange energies in bulk semiconductors are on the order of tens of micro-eV, those in nanocrystals can be 100-1000 times larger, leading to sizable milli-eV scale "fine structure" splittings of the lowest 1S exciton that manifest at modest temperatures (1 meV == 12 Kelvin). For example, exchange interactions in CdSe nanocrystals lead to a well-separated lowest exciton state that has angular momentum projection J=2 and is therefore optically forbidden, or "dark", resulting in very long photoluminescence lifetimes at cryogenic temperatures.

High magnetic fields couple to the spin degrees of freedom of electrons and holes, and are therefore a useful tool for addressing the large spin-exchange interactions that exist in nanocrystals (1 meV == ∼10 Tesla, or 10∧5 Gauss). This talk will discuss recent PL, fluorescence-line-narrowing, and magnetic-circular-dichroism studies of nanocrystals in both dc (to 33 Tesla) and pulsed magnetic fields (to 60 Tesla). High fields have been used to reveal, for example, i) dark excitons in both CdSe and PbSe nanocrystals, ii) anisotropic exchange in CdSe nanocrystals, iii) tunable sp-d exchange in magnetically-doped nanocrystals, and iv) Zeeman splittings of single excitons in individual nanocrystals.

Some fraction of this talk will survey the various methods utilized at the National High Magnetic Field Laboratory for generating the world's highest magnetic fields using both nondestructive (to 100 Tesla) as well as destructive (>1000 Tesla) techniques… amusing videos will be featured.

Lecture 4: Nanocrystals Under Hydrostatic Pressure

Richard Schaller
Softmatter Nanotechnology and Advanced Spectroscopy,
Chemistry Division
Los Alamos National Laboratory, Los Alamos, New Mexico 87545, USA
Monday, February 22, 2011, 10am
Chemistry Division Auditorium, TA-46, Bld. 535, Rm. 103

Abstract: Hydrostatic pressure provides a convenient means of controllably manipulating many material properties such unit cell size, crystal structure, and energy gap. In this presentation, I will review the experimental aspects of generating and characterizing high pressures and present literature-based studies of relevance to nanocrystal materials research.

Lecture 3: The Chemistry of Colloidal Nanocrystal Quantum Dots

Jeffrey Pietryga
Softmatter Nanotechnology and Advanced Spectroscopy,
Chemistry Division
Los Alamos National Laboratory, Los Alamos, New Mexico 87545, USA
Monday, February 22, 2011, 10am
Chemistry Division Auditorium, TA-46, Bld. 535, Rm. 103

Abstract: Colloidal nanocrystal quantum dots (NQDs) are a unique class of materials that are under widespread investigation for applications ranging from bio-labeling to solid-state lighting and photovoltaics. Their versatility stems from an elegant marriage of physics and chemistry in which the principles of chemical synthesis, centuries in development, are brought to bear directly on the problem of creating nanoscale structures with single atom-layer precision for the purposes of controlling the sub-nanosecond dynamics of electrons and holes. This talk will examine the unique hybrid inorganic/organic structure of NQDs and the basics of the colloidal growth process. We will then discuss the major developments in the field, with particular attention to their relevance to the problem of creating NQD-based solar cells. Topics will include: the syntheses of NQDs of II-VI, IV-VI, III-V and other classes of semiconductor materials; heterostructuring; shape control; ligand exchange and assembly into thin films.

Lecture 2: Nanocrystal Quantum Dots: Electronic Structures and Relaxation Pathways

Victor Klimov
Softmatter Nanotechnology and Advanced Spectroscopy,
Chemistry Division
Los Alamos National Laboratory, Los Alamos, New Mexico 87545, USA
Monday, February 8, 2011, 10am
Chemistry Division Auditorium, TA-46, Bld. 535, Rm. 103

Abstract: Semiconductor nanocrystals are nanometer-size crystalline particles that contain approximately 100 to 10,000 atoms. Using chemical syntheses they can be fabricated with almost atomic precision as nearly spherical nanoparticles (quantum dots), elongated nano-sized crystals (quantum rods), or nanostructures of other more complex shapes such as tetrapods. The ability to precisely control the composition, size and shape of the nanocrystals provides great flexibility in engineering their electronic and optical properties by directly manipulating electronic wavefunctions. This lecture will provide an introduction into electronic and optical properties of nanocrystal quantum dots, including the structure of electronic states, optical absorption and emission spectra, and carrier relaxation and recombination pathways. It will also discuss the use of ultrafast spectroscopic methods for studies of electronic spectra and carrier dynamics in nanocrystals.

Lecture 1: Center for Advanced Solar Photophysics: Overview of Research Thrusts

Victor Klimov
Softmatter Nanotechnology and Advanced Spectroscopy,
Chemistry Division
Los Alamos National Laboratory, Los Alamos, New Mexico 87545, USA
Monday, February 8, 2011, 10am
Chemistry Division Auditorium, TA-46, Bld. 535, Rm. 103

Abstract: Center for Advanced Solar Photophysics (CASP) is part of the recent DOE initiative in Energy Frontier Research Centers (EFRCs) launched in August of 2009. The goal of CASP is to explore and exploit the unique physics of nanostructured materials to boost the efficiency of solar energy conversion through novel light-matter interactions, controlled excited state dynamics, and engineered carrier-carrier coupling. The research of our Center concentrates in three main thrust areas: i) novel nanoscale physical phenomena for efficient capture and conversion of light into electrical charges via quantum confinement, plasmonic and photonic effects, ii) new means for charge manipulation in nano-assemblies for rapid charge extraction and low-loss transport, and iii) proof-of-principle solar-energy conversion schemes that exploit the emergent physics of the nanoscale size regime. In this lecture, I will overview the current research directions in the Center as well as our plans for future research.