Alexander O. Govorov Ohio University
Zhiming M. Wang University of Arkansas
Andrey L. Rogach Ludwig-Maximilians-Universtitaet Muenchen
Harry Ruda University of Toronto
Mark Brongersma Stanford University
GG1: Self-assembled Quantum Dots and Quantum Phenomena I
Monday PM, November 26, 2007
Room 310 (Hynes)
9:30 AM - **GG1.1
Optical Properties of Quantum Dots and Quantum Posts.
Pierre Petroff 1 4 , H. Krenner 1 , J. He 1 , C. Pryor 2 , C. Morris 3 , M. Sherwin 3 Show Abstract
1 Department of Materials, University of California-Santa Barbara, Santa Barbara, California, United States, 4 Department of Electrical & Computer Engineering, University of California-Santa Barbara, Santa Barbara, California, United States, 2 Department of Physics and Astronomy, University of Iowa, Iowa City, Iowa, United States, 3 Department of Physics, University of California-Santa Barbara, Santa Barbara, California, United States
We discuss the growth and optical properties of InGaAs/GaAs self assembled quantum posts (QPs). The MBE grown QP is formed of a seed quantum dot (QD) connected to a short quantum wire and is capped by another QD. The QP length along the growth direction can be adjusted between 10 and 60nm. We briefly discuss the QP structural and chemical composition. Their optical properties measured by micro-photoluminescence (micro-PL) are compared to an 8-bands strain-dependent k.p model incorporating the detailed structure and alloy composition. The calculations for QPs shorter than 40nm, show full electron delocalization in the quantum wire part of the quantum post and the hole localization in the strain-induced regions at the ends of the QP. By embedding the QPs inside an n-i-p structure, measurements of the bias dependent micro-PL spectra show strongly tunable excitons transitions due to the Quantum Confined Stark effect. In addition, we find anti-crossings, which are consistent with delocalized electron and localized holes states. Thus, QP offers the possibility of controlling the strength of the electric dipole moment in the structure. We have measured dipole moments 40 times larger than those of isolated QDs. This opens up new possibilities for the studies of light matter interactions in the strong coupling regime.
10:00 AM - **GG1.2
Directed Self-Assembly of Coupled Nanostructures and their Behavior.
Gregory Salamo 1 Show Abstract
1 Physics, University Arkansas, Fayetteville, Arkansas, United States
Novel and clever techniques for fabricating nanosize materials, one-atomic-layer-at-a-time, have simultaneously opened a door to a fantastic adventure. Nanosize materials simply do not behave as the bulk. Indeed, the rules that govern the growth and behavior of these tiny structures are only now being explored by many researchers with fascinating results throughout the world. In this talk we focus on semiconductor quantum dots that were produced by molecular beam epitaxy via the Stranski-Krastanov or droplet epitaxy growth techniques. In addition, we focus on developing an understanding of the underlying physics that gives rise to the quantum dot electronic structure and dynamical carrier processes both of which are important for device applications. For example, application of semiconductor quantum dots in high-performance lasers, quantum computations, or single electron transistors, implies carrier transfer from a continuum of states into the discrete atomic-like states of quantum dots. Efficiency of such transfer is determined to a great extent by the strength of the coupling between the quantum dot array and the carrier reservoir. The importance of this coupling to a carrier reservoir extends well beyond quantum dots to more complex structures, such as, quantum dot molecules, chains, three dimensional arrays, and even quantum rings. Excitingly, the ability to design and engineer such complex arrays of self-assembled of nanostructures is creating new opportunities to explore the underlying physics of carrier transfer and can potentially lead to novel devices. However, as is most often the case, it would be wise to investigate coupling between a carrier reservoir and quantum dots of a simple system before investigating more complex structures like quantum dot three dimensional arrays. Certainly a quantum well or wetting layer that collects carriers can act as a carrier reservoir from which lateral diffusion results in carrier capture by quantum dots. One might think that a simple example is a coupled quantum well-quantum dot array structure. However, even in this simple system the energy level structure suffers from a clear picture of carrier behavior. For example, quantum dots used in application are normally dense and generally not well separated spatially by long distances or infinite potential barriers. As a result, the electronic wavefunction of adjacent quantum dots can overlap, thus allowing carriers to travel from one quantum dot to another. This carrier transfer, even if slow, can affect carrier dynamics after optical excitation. In this talk we examine the underlying physics of coupling, both laterally and vertically, in semiconductor bilayer structures.
10:30 AM - **GG1.3
Electric Field Tuning of Exciton-biexciton Cascade in a Single Quantum Dot for Entangled Photon Pair Generation.
Marek Korkusinski 1 , Michal Zielinski 1 , Michael Reimer 1 , Robin Williams 1 , Pawel Hawrylak 1 Show Abstract
1 Institute for Microstructural Sciences, National Research Council, Ottawa, Ontario, Canada
We present theory and experiment describing the effect of the lateral electric field on excitonic and bi-excitonic resonances in a single self-assembled quantum dot. In model calculations the electron and hole single particle states of a single quantum dot in an electric field are described exactly by displaced harmonic oscillators. The multi-exciton states are expanded in terms of exact electron-hole configurations for any applied electric field strength. The Coulomb matrix elements for same particles are calculated exactly, and electron-hole Coulomb matrix elements are calculated numerically. The multi-exciton complexes are calculated using configuration interaction method with up to three shells for electrons and for holes. Electron-hole exchange is included and parametrized in terms of zero electric field anisotropic exchange splitting (AES). Results of calculations show that the lateral electric field modifies the attractive Coulomb interactions in exciton and biexciton while keeping the repulsive contribution unchanged. At a critical value of electric field the bi-exciton binding energy vanishes, leading to two indistinguishable recombination pathways for the biexciton. This produces entangled photon pair even in the presence of exciton states split by anisotropic exchange. The theoretical results are successfully compared with microscopic tight-binding calculations and with experiment on a single, site-selected InAs/InP quantum dots emitting in the wavelength range between 1300nm and 1550nm.  Michael E. Reimer , Marek Korkusinski, Jacques Lefebvre, Jean Lapointe, Philip Poole, Geof Aers, Dan Dalacu, W. Ross McKinnon, Simon Frédérick, Pawel Hawrylak, Robin L. Williams, submitted to Phys.Rev.Letters.
11:30 AM - GG1.4
Controlling the Optical Properties of Self-Assembled Quantum Dots Using External Strain.
Garnett Bryant 1 , Michal Zielinski 2 3 , W. Jaskolski 3 , Javier Aizpurua 4 Show Abstract
1 Atomic Physics Division, National Institute of Standards and Technology, Gaithersburg, Maryland, United States, 2 Instytut Fizyki, Uniwersytet Mikolaja Kopernika, Torun Poland, 3 Institute of Microstructural Sciences, National Research Council of Canada, Ottawa, Ontario, Canada, 4 , Donostia International Physics Center, San Sebastian Spain
Passive control of the optical properties of self-assembled semiconductor quantum dots is achieved by controlling dot size, shape and composition via growth. Active, dynamical control is needed. In self-assembled quantum dots, the local strain due to lattice mismatch between the quantum dot and barrier materials critically influences the dot electronic properties and optical response. Active control could be achieved via imposed external strain to induce or split level degeneracies, polarize optical transitions, or modify coupling in closely spaced dots, all critical capabilities for the use of dots in quantum information processing. Conversely, it was proposed recently that quantum dots could be used in optical cooling schemes to bring nanomechanical oscillators and cantilevers into the quantum limit for mechanical systems. To understand the coupling between externally imposed strains and the electronic states of the self-assembled quantum dots, we have developed an atomistic tight-binding theory of the confined states in quantum dots that incorporates the local strain due to atomistic lattice mismatch and the externally imposed strain due to applied stressors or to the bend in a vibrating nanomechanical oscillator. Both local strain and the externally imposed strain are important, so we include them on an equal footing via an atomistic valence force field approach, with the externally imposed strain modeled as a distortion of the lattice on the boundary of structure. A full tight-binding model including an sp3s*d5 orbital model and spin-orbit effects is used. To understand how applied stress can be used to actively control dot optical properties, we consider dots buried at different points in a nanobridge oscillator or cantilever. We study the dependence of the quantum dot electronic states and optical transitions on the coupling to bending modes of the oscillator. Both isolated and coupled dots are considered. Ten meV energy shifts of both the electron and hole state states are possible. Redshifts or blueshifts are possible, depending on how the stress is applied to the dot. Level crossings for hole states are found. State symmetries can be significantly distorted by the applied stress. Transitions can be strongly polarized parallel or perpendicular to the external strain or suppressed, depending on how the dots are stressed. The dependence on applied stress can be understood as a competition between the internal and external stress that can either enhance or suppress the local strain at the dot, depending on how the external stress is applied. These results are discussed to show how much active control is feasible.
11:45 AM - **GG1.5
Resonantly Controlled Light Emission Of Quantum Dots In Cavities.
Ken Shih 1 Show Abstract
1 Department of Physics, University of Texas at Austin, Austin, Texas, United States
12:15 PM - **GG1.6
Time-resolved Measurements of Single Electron Spin Coherence in a Quantum Dot.
Maiken Mikkelsen 1 , Jesse Berezovsky 1 , Oliver Gywat 1 , Nick Stoltz 1 , Larry Coldren 1 , David Awschalom 1 Show Abstract
1 Center for Spintronics and Quantum Computation, University of California, Santa Barbara, California, United States
The non-destructive detection of a single electron spin in a quantum dot (QD) was recently demonstrated using a time-averaged magneto-optical Kerr rotation measurement . This technique provides a means to directly probe the spin off-resonance, thus minimally disturbing the system. Furthermore, the ability to sequentially initialize, manipulate, and read out the state of a qubit, such as an electron spin in a quantum dot, is necessary for virtually any scheme for quantum information processing. Here, in contrast to previous time-averaged measurements, we have extended the single dot KR technique into the time domain with pulsed pump and probe lasers, allowing the observation of the coherent evolution of an electron spin state . The dot is formed by interface fluctuations of a GaAs quantum well and embedded in a diode structure to allow controllable charging of the QD. To enhance the small single spin signal, the QD is positioned within a vertical optical cavity. Observations of coherent single spin precession in an applied magnetic field allow a direct measurement of the electron g-factor and a transverse spin lifetime of ~10 ns. These measurements reveal information about the relevant spin decoherence mechanisms, while also providing a sensitive probe of the local nuclear spin environment. The results represent progress toward the manipulation and coupling of single spins and photons for quantum information processing as well as quantum non-demolition measurements of a single spin.1. J. Berezovsky, M. H. Mikkelsen, O. Gywat, N. G. Stoltz, L. A. Coldren, and D. D. Awschalom, Science 314, 1916 (2006).2. M. H. Mikkelsen, J. Berezovsky, N. G. Stoltz, L. A. Coldren, and D. D. Awschalom, submitted for publication (2007).
12:45 PM - GG1.7
Optical Properties of Ordered Self-assembled Wires, Mono- and Bi- chains of InAs Quantum Dots.
Emanuele Uccelli 1 , Max Bichler 1 , Gerhard Abstreiter 1 , Anna Fontcuberta i Morral 1 Show Abstract
1 Walter Schottky Institut, Technische Universität München, Garching Germany
Semiconductor quantum dots (QDs) have attracted in the past years significant interest worldwide as promising active media for advanced device applications and as systems enabling the investigation of new quantum physics phenomena. For all these studies and applications, controlling the assembly of QDs in a deterministic way is highly desirable. As previously shown, we were able to fabricate long range ordered chains of InAs QDs by combining selective growth with self-assembly. InAs nucleation was realized on a (110) facet consisting of AlAs nanostripes embedded in GaAs, that was obtained by in situ cleaving of a previously MBE grown AlAs/GaAs (001) heterostructure .Recently, we were able to “depict” a detailed phase diagram that shows under which growth conditions the formation of InAs QDs chain can be optimized. Moreover, we also obtained new and different nanostructures configurations on the cleaved surface, such as InAs wire array as well as mono- and bi- chain of InAs quantum dots. Indeed, combining high As4-vapor overpressure and very low InAs amount, the geometry of the AlAs stripes pattern acts as the selective factor for the nucleation of InAs wire or QDs chain structures [2,3].Here, we present a detailed investigation of the optical properties of the InAs nanostructures configurations at the cleaved facet. Intense sharp peaks around 1.3 eV at low temperature (10 K) have been registered for all the different growth scenarios (wire, mono- and bi- QDs chains), on samples capped by a thin GaAs layer. As expected, the trend in the excitonic emission for QDs located on AlAs stripe of different thickness confirms the geometrical evolution in the QDs dimensions. Differences in the QDs signals from similar and long studied InAs/GaAs (001) QDs system have been observed. To explain them, detailed Raman investigations have been started and pointed out an high In-Al intermixing in the dots and wire system, that also confirms our previous theoretical assumptions regarding the starting factor for the InAs nucleation only on the AlAs stripes.References: Bauer J. et al., Long-range ordered self-assembled InAs quantum dots epitaxially grown on (110) GaAs, Applied Physics Letters 85, 4750 (2004) Uccelli E. et al., Guided self assembly of InAs quantum dots on a cleaved facet, Proceeding of Materials Research Society 959 M 16.02 (2007) Uccelli E. et al., Growth mechanisms of self assembled InAs quantum dots on (110) AlAs/GaAs cleaved facet, submitted to Superlattices and Microstructures (2007)
GG2: Self-assembled Quantum Dots and Quantum Phenomena II
Monday PM, November 26, 2007
Room 310 (Hynes)
2:30 PM - **GG2.1
Mode Locking of Electron Spin Coherence in Singly Charged Quantum Dots.
Alexander Efros 1 Show Abstract
1 Center for Computational Material Science, NRL, Washington, District of Columbia, United States
The fast dephasing of electron spins in an ensemble of quantum dots is detrimental for applications in quantum-information processing. We show that dephasing can be overcome by using a periodic train of light pulses to synchronize the phases of the precessing spins, and demonstrate this effect in an ensemble of singly charged (In,Ga)As/GaAs quantum dots . We first discuss the physical mechanism of this synchronization for pulses of different intensity [2, 3]. A periodic train of circularly polarized light pulses from a mode-locked laser synchronizes the precession of the spins to the laser repetition rate, transferring the mode-locking into the spin system. The mode-locking technique allows us to measure the single-spin coherence time to be 3 microseconds , which is four orders of magnitude longer than the ensemble dephasing time of 400 picoseconds. The interference also gives the possibility for all-optical coherent manipulation of spin ensembles, in which the electron spins can be clocked by two trains of pump pulses with a fixed temporal delay. After this pulse sequence, the quantum dot ensemble shows multiple echo-like Faraday rotation signals with a period equal to the pump pulse separation. A. Greilich, D. R. Yakovlev, A. Shabaev, Al. L. Efros, I. A. Yugova, R. Oulton, V. Stavarache, D. Reuter, A. Wieck, and M. Bayer, Science 313, 341 (2006). A. Greilich, R. Oulton, E. A. Zhukov, . A. Yugova, D. R. Yakovlev, M. Bayer, A. Shabaev, Al. L. Efros, I. A. Merkulov, V. Stavarache, D. Reuter, and A. Wieck, Phys.Rev. Lett., 96, 227401 (2006). A. Shabaev, Al. L. Efros, D. Gammon, and I. A. Merkulov, Phys. Rev. B 68, 201305(R) (2003).
3:00 PM - GG2.2
Multi-color InAs and InGaAs Quantum Dot Photodetectors for Mid and Long-wavelength Infrared Detection.
Brandon Passmore 1 , J. Wu 1 , O. Manasreh 1 , Vas. Kunets 2 , G. Salamo 2 Show Abstract
1 Department of Electrical Engineering and Microelectronics and Photonics, University of Arkansas, Fayetteville, Arkansas, United States, 2 Department of Physics, University of Arkansas, Fayetteville, Arkansas, United States
The intersubband transitions in self-assembled InAs and In0.3Ga0.7As quantum dots grown by molecular beam epitaxy have been investigated for their use in mid- and long-wavelength infrared detection applications. The materials were characterized using x-ray diffractometry, photoluminescence, electrochemical capacitance-voltage and optical absorption. Devices were fabricated from the multiple quantum dot structures in order to measure the temperature dependence (4 – 300 K) of the photoresponse for normal incident operation. In addition, the dark current and photocurrent was measured in the temperature range of 4 – 300 K for the devices. For the two band infrared photodetector consisting of two stacks of n-type InAs and In0.3Ga0.7As multiple quantum dots, the photoresponse peaks were measured to be 5 – 7 µm and 10 – 14 µm, respectively. However, a broad-band photoresponse from InAs quantum dots embedded in an InGaAs graded well was measured in the spectral range of 3 – 12 µm. The transfer matrix method is used to estimate the peak position energies of the intersubband transitions in the two multi-color quantum dot infrared photodetector structures.
3:15 PM - GG2.3
Coupling Ga Nanoparticles with Semiconductors: The Impact of Charge-transfer Phenomena on the Ga Surface Plasmon Resonance.
Pae Wu 1 , Maria Losurdo 2 , Tong-Ho Kim 1 , Maria Giangregorio 2 , Giovanni Bruno 2 , April Brown 1 Show Abstract
1 Electrical and Computer Engineering, Duke University, Durham, North Carolina, United States, 2 , Institute of Inorganic Methodologies and of Plasmas -- CNR, Bari Italy
Surface plasmon resonant metal nanoparticles (NPs) are widely exploited for a variety of applications including biosensors, waveguides, photon emission enhancement, and surface-enhanced Raman scattering. Traditionally Ag and Au, both noble metals, have been employed for plasmonic applications and their formation through chemistries in solution, with a high degree of shape and size control, is well-established. Gallium (Ga) plasmonic nanoparticles have recently gained traction in nonlinear optics and plasmonic applications such as optical switches and plasmonic waveguides. Ga NPs are also advantageous for device applications as they can be deposited in situ in the same apparatus where III-V semiconductor heterostructures are grown. This enables the pristine coupling of Ga NPs, which exhibit a wide plasmon resonance range unlike Au and Ag, with GaAs- and GaN-based photonic devices. We demonstrate the tailoring and exploitation of self-assembled plasmonic Ga NPs supported on both polar (GaN, N-polar, Ga-polar; SiC, Si-polar, C-polar; ZnO, Zn-polar, O-polar) and non-polar (e.g. Si) semiconductors. Novel factors presented and discussed in this contribution are:- splitting and tailoring of two plasmon modes, the longitudinal plasmon and the transverse plasmon that can be tuned from the far UV to the IR range; a correlation between the Ga NPs size and the wavelength of the two modes is established.- Thermal stability of Ga nanoparticles under very extreme temperature ranges (from -90 to 600°C), which enables Ga plasmon applications for harsh environments.- Demonstration of plasmonic ellipsometry employing a phase modulated spectroscopic ellipsometer (UVISEL – Jobin Yvon) that provides information both on the phase and amplitude of the reflected light. This powerful technique allows us to highlight and investigate, in real-time, plasmon tailoring during NP formation and plasmon perturbation phenomena due to interaction with surrounding surfaces and media.- Modification of the Ga plasmon resonance due to charge-transfer mechanisms. We examine the electronic phenomena due to charge transfer between the Ga NPs and the semiconductor surface affecting both the position and amplitude of the plasmon modes and demonstrate the sensitivity of the plasmonic behavior in response to semiconductor polarity and the accompanying semiconductor surface charge. We also investigate the modification of metallic nanoparticle optical behavior induced by charge-perturbation during interaction with electron donor and electron acceptor molecules. Finally, the plasmon-based detection of NO by hemin-functionalized Ga nanoparticles mediated by a charge-transfer mechanism is demonstrated.
3:30 PM - **GG2.4
Optical Spectroscopy of Quantum Dot Molecules.
Dan Gammon 1 Show Abstract
1 , Naval Research Laboratory, Washington, District of Columbia, United States
In this talk I will present our recent experiments on coherently coupled ‘diatomic’ quantum dot molecules. Individual pairs of vertically stacked InAs/GaAs QDs with tunnel barrier thicknesses ranging from 2 – 18 nm are being studied using optical spectroscopy through shadow masks. We find that the molecular spectra, though rich in structure, can be largely understood with a few key conceptual elements, involving coherent tunneling of either an electron or hole, Coulomb interactions, and spin. We have measured exciton and biexciton spectra of neutral and charged dots (both positively charged and negatively charged). Many of the properties that have been intensely studied over the last decade in single dots can now be systematically measured in molecules, including Stark shifts, Zeeman splittings, charged excitons, biexcitons, spin fine structure, and excited states. Each of these properties is greatly enriched by the coherent coupling between the two dots. I will present an overview of several of these results, emphasizing the similarities and differences between electron and hole tunneling.
GG3: Colloidal Nanocrystals: Plasmons, Excitons, and Thermal Effects I
Monday PM, November 26, 2007
Room 310 (Hynes)
4:30 PM - **GG3.1
Optical Manipulation using Gold Nanoparticle Aggregates.
Jochen Feldmann 1 Show Abstract
1 Photonics and Optoelectronics Group, Physics Department and CeNS, Ludwig-Maximilians-Universität (LMU), Munich Germany
Aggregates of gold nanoparticles provide unique electromagnetic properties. In so-called "hot spots" intense Raman signals can be generated allowing for a detection of Raman spectra on a single molecule level.In this contribution it is shown that optical excitations of a protein-linked gold nanoparticle dimer not only leads to intense Raman signals but also provokes a change of the inter-nanoparticle distance on the order of 0.5 nm. As a second example gold nanoparticle aggregates linked by double stranded DNA are optically excited by nanosecond laser pulses. Herewith a controlled melting of the double stranded DNA can be achieved. This fast optothermal method allows for single basepair mismatch detection even in the presence of perfectly matching DNA.
5:15 PM - GG3.3
Size-Dependent Energy Transfer Processes in Mn(II)-Doped CdSe.
Remi Beaulac 1 , Paul Archer 1 , G. Salley 1 , Daniel Gamelin 1 Show Abstract
1 Chemistry, University of Washington, Seattle, Washington, United States
Colloidal Mn(II)-doped II-VI quantum dots are interesting materials for the study of magnetic and luminescent phenomena in quantum confined semiconductor nanostructures. In recent years, several reports have described luminescence, absorption and magnetism of Mn(II)-doped ZnS, CdS and ZnSe quantum dots. In general, the emission properties of these nano-scale materials behave much like their bulk counterparts, showing a size insensitive Mn(II) ligand-field emission with a lifetime of a few microseconds. In contrast, Mn(II)-doped CdSe nanoparticles are expected to behave differently from bulk because of the possibility of size-tuning the band-gap energy from below to above the Mn(II) emitting levels. For this reason, Mn(II)-doped CdSe offers an interesting opportunity for fundamental strudies of quantum confinement effects in doped semiconductors. Curiously, although photoluminescence spectra of self-assembled Mn(II) quantum dots prepared by vacuum deposition have been reported, the Mn(II) is either absent of only tentatively reported in the case of very high Mn(II) concentrations. Moreover, CdSe excitonic emission is observed despite the fact that the energy gap is greater than the Mn(II) excitation energy, in contrast with Mn(II)-doped CdS, ZnS and ZnSe. We recently presented a new method for preparing colloidal doped CdSe quantum dots.[1,2] Importantly, these particules show a giant Zeeman splitting of their excitonic transitions, as is expected for diluted magnetic semiconductor (DMS). The use of electronic absorption and magnetic circular dichroism (MCD) spectroscopies as probes of 3d transition metal dopant speciation in DMS quantum dots will be briefly described. Temperature-dependent photoluminescence of these particules has been measured and gives an insight on the kinetics of the energy transfer processes between the excited states of Mn(II)-CdSe. We propose a kinetic model that explains the paradoxical absence of Mn(II) emission reported before in Mn(II)-doped CdSe.  Archer, P. I.; Santangelo, S. A.; Gamelin, D. R., Nano. Lett., 7, 1037-1043 (2007). Archer, P. I.; Santangelo, S. A.; Gamelin, D. R., J. Amer. Chem. Soc., accepted.
5:30 PM - GG3.4
CdSe:Te Nanocrystals: Band-Edge versus Te-Related Emission.
Andrey Rogach 1 , Thomas Franzl 1 , Josef Mueller 1 , Thomas Klar 1 , Jochen Feldmann 1 , Dmitri Talapin 2 3 , Horst Weller 2 Show Abstract
1 Department of Physics and Center for Nanoscience (CeNS), Ludwig-Maximilians-Universitaet Muenchen, Munich Germany, 2 Institute of Physical Chemistry, University of Hamburg, Hamburg Germany, 3 The Molecular Foundry, Lawrence Berkeley National Laboratory, Berkeley, California, United States
Strongly luminescent monodisperse CdSe nanocrystals in which a few Se atoms are substituted with Te atoms (CdSe:Te) provide a model system for studies of both band-edge and trap-related luminescence. Ensemble photoluminescence spectra of CdSe:Te nanocrystals are asymmetrically broadened and red-shifted in comparison to bare CdSe nanoparticles. Single particle luminescence measurements show that the bare CdSe and the CdSe:Te nanocrystals emit at distinctly different wavelengths and differ in line shape and linewidth. Individual CdSe:Te nanocrystals show two kinds of emission spectra, which have been ascribed by us to particles with one Te and with a few Te atoms per nanocrystal. Single particle measurements furthermore show that a single CdSe:Te nanocrystal can emit either from the band-edge states or from trap state(s) created by the Te atom(s), but not from both.
5:45 PM - GG3.5
Exploiting Bimodal Distribution to Enhance Photoluminescence Efficiency of Quantum Dot Arrays.
Krisztian Kohary 1 , Victor Burlakov 1 , David Pettifor 1 Show Abstract
1 Materials, Oxford University, Oxford, Oxfordshire, United Kingdom
Photoluminescence of semiconductor quantum dot arrays is significantly deteriorated by the presence of bad quantum dots quenching optical excitons via non-radiative decay channels. This process is highly facilitated by easy migration of excitons between aggregated quantum dots due to resonant Forster transfer processes. We propose to decrease the role of bad quantum dots by slowing down the exciton migration using a binary mixture of quantum dots, where one type of quantum dots serves as spacer quantum dots (SQDs) between the primary quantum dots (PQDs). We identified the maximum and minimum values of the photoluminescence efficiency for different spacer/primary quantum dot compositions. We found that for a given composition the photoluminescence efficiency is the highest for the random distribution of spacer and primary quantum dots. However, the photoluminescence efficiency dramatically decreases when clusters of SQDs and PQDs are formed. We studied and analyzed this cluster formation of quantum dots and the corresponding photoluminescence of the bimodal quantum dot array using the kinetic Monte Carlo simulation technique. By comparing our simulation results with those obtained experimentally we were able to determine the magnitude of the attractive pair-wise interaction between the quantum dots and explore possible strategies to achieve the highest photoluminescence efficiency of the PQDs in hybrid organic-inorganic materials for photo-electronics. (This research is funded by Hewlett-Packard Laboratories, Palo Alto, USA.)
Alexander O. Govorov Ohio University
Zhiming M. Wang University of Arkansas
Andrey L. Rogach Ludwig-Maximilians-Universtitaet Muenchen
Harry Ruda University of Toronto
Mark Brongersma Stanford University
GG4: Colloidal Nanocrystals: Plasmons, Excitons, and Thermal Effects II
Tuesday AM, November 27, 2007
Room 310 (Hynes)
10:00 AM - **GG4.1
Characterization of Heat Generation from Single Gold Nanoparticle Complexes.
Hugh Richardson 1 Show Abstract
1 Chemistry and Biochemistry, Ohio University, Athens, Ohio, United States
Metal nanoparticles efficiently generate heat in the presence of electromagnetic radiation. This process becomes strongly enhanced under plasmon resonance and also depends on the shape and organization of the nanoparticles. In particular, the amount of heat generated and temperature increase depends on the number of nanoparticles in a complex. Metal nanoparticles can be used to induce a phase transformations when they are in thermal contact with a solid matrix, such as ice, and are optically driven. This effect can be used to quantitatively determine the amount of heat generated from single metal nanoparticle complexes using confocal Raman and photoluminescence microscopy. We use the phase transformation of ice to determine the temperature increase around a gold NP complex. With this, we can measure, not only the optical response of the NPs, but also thermal response. Because the heat generation is dependent upon the mesoscopic character of different clusters of NPs, the gold NPs are first immobilized on a glass substrate and then characterized with single particle spectroscopy. After characterization, the temperature profile around the NP complex during optical excitation is determined by measuring the relative amount of water and ice within the excitation volume. The thermal properties of the optically-excited gold clusters are then established by combining theoretical calculations with experimental results. This approach yields a quantitative measure of the amount of heat generation. Our results show for gold NP complexes in ice that at relatively low laser power (less than 10^5 W/cm2) liquid water encases the photo-excited gold particle and the temperature profile agrees with recent theoretical calculations. But at larger laser powers, a vapor cocoon surrounds the excited particle that shuts down thermal transport from the particle to the surrounding ice matrix, causing superheating of the particle. This phenomenon is also observed for gold NP complexes in liquid water where an insulating vapor cocoon is formed for laser powers in excess of 10^5 W/cm2.
10:30 AM - GG4.2
Plasmon-assisted Nanoscale Thermal Engineering: Principle and Applications.
Linyou Cao 1 2 , Mark Brongersma 1 2 Show Abstract
1 Department of Materials Science and Engineering, Stanford University, Stanford, California, United States, 2 Geballe Laboratory of Advanced Materials , Stanford University, Stanford, California, United States
The realization of controlled, nanoscale thermal environments has great fundamental and practical importance. Research in this area is largely driven by a desire to better control and monitor physicochemical or biochemical reactions and to develop thermally-controlled nanoscale devices. Here we present a novel approach by exploiting light-induced surface plasmon excitations in metallic nanostructures to generate localized, nanoscale controllable thermal environments. Theoretical calculations and experimental measurements collectively show highly spatial and temporal control over temperature field can be thereby achieved. This plasmon-assited nanoscale thermal engineering (PANTE) can be easily combined with other patterning techniques to generate pre-specified thermal environment. The potential capability of the PANTE in gaining superior control over chemical reaction or physical phase change are illustrated by its application in patterned local growth of individual semiconductor nanowires and carbon nanotubes and as pump in nanofluidic devices.
10:45 AM - GG4.3
Gold Nanoparticle Protein Conjugates: Study of Pulsed Laser Heating.
Joshua Alper 1 , Andy Wijaya 2 , Lauren DeFlores 3 , Andrei Tokmakoff 3 , Kimberly Hamad-Schifferli 1 4 Show Abstract
1 Department of Mechanical Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States, 2 Department of Chemical Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States, 3 Department of Chemistry, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States, 4 Department of Biological Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States
Irradiation of Au nanorods with femtosecond laser pulses at the plasmon resonance can be used to heat the particles. We present here a study of how it can be used to selectively control activity of proteins that are conjugated to the nanorods. Au nanorod synthesis and linking to biomolecules is described, along with a biophysical characterization of the nanorod-biomolecule conjugates by electrophoresis, circular dichroism, and optical spectroscopy. Proteins include Ribonuclease S, Ribonuclease A and cytochrome c. The conjugates are irradiated with approximately 1 - 1000 femtosecond laser pulses close to the longitudinal plasmon frequency of the nanorods (λ = 790 nm). We study the effect of irradiation on the biomolecular structure and activity, as well as the resulting change in nanorod morphology. Effects of laser fluence, number of pulses and protein conjugation are explored.
11:30 AM - GG4.4
Tuning Exchange Interaction in Colloidal Nanocrystals.
Stefan Rohrmoser 1 , Andrei Susha 2 , Andrey Rogach 2 , Dmitri Talapin 3 , Horst Weller 4 , Richard Harley 1 , Pavlos Lagoudakis 1 Show Abstract
1 School of Physics and Astronomy, University of Southampton, Southampton United Kingdom, 2 Photonics and Optoelectronics Group, Ludwig-Maximilians-Universität München, Munich Germany, 3 Molecular Foundry, Lawrence Berkeley National Laboratory, Berkeley, California, United States, 4 Institute of Physical Chemistry, Universität Hamburg, Hamburg Germany
Compared to self-assembled quantum dots made by molecular beam epitaxy, colloidal nanocrystals can be produced with controlled size and shape, such as in quantum dots, rods or even tetrapods, while retaining a high optical and electronic stability. Shape control in the synthesis of colloidal nanocrystals offers unprecedented abilities to tune the interaction of solid state quantum structures with the environment, opening up the possibility of performing truly nanoscale manipulations of the optical and electronic properties. Entanglement schemes that utilise the broken degeneracy of the exciton fine structure in quantum dots have been proposed for applications in quantum information processing, where the degeneracy of exciton states is lifted by the structural shape asymmetry. The naturally occurring shape asymmetry of colloidal wurtzite nanocrystals allows for the growth of nanorods with precise control over the aspect ratio of the nanostructure, while advances in synthetic chemistry have made it possible for the growth of a novel class of core/shell nanocrystals that consist of two different materials grown with a strongly asymmetric shape. These nanorods facilitate electron penetration into the elongated CdS shell whereas the hole is confined inside the spherical CdSe core, which is preferentially situated at one end of the wurtzite structure. The ability to tune the aspect ratio of these core/shell nanorods allows us to control the strength of the exchange interaction, a powerful tool for investigating the electronic structure that dominate their optoelectronic properties. Application of strong external magnetic fields reveals a rich exciton fine structure and its dependence on the aspect ratio of the nanorods. Furthermore, we tune the exchange interaction between carriers in a single nanocrystal by modulating the electron-hole wavefunction overlap under an external electric field parallel to the nanorods and at the same time we probe the effect under an external magnetic field. The simultaneous application of external electric and magnetic fields on nanorods of fixed aspect ratio allows us to actively manipulate the exciton fine structure. This opens applications in spin-polarized magneto-electronics, spintronics and quantum computation.
11:45 AM - GG4.5
Self-Assembly and Conductivity of Nanocrystal Solids.
Dmitri Talapin 1 2 , Elena Shevchenko 2 , Maksym Kovalenko 2 3 , Jong-Soo Lee 2 Show Abstract
1 Department of Chemistry, University of Chicago, Chicago, Illinois, United States, 2 The Molecular Foundry, LBNL, Berkeley, California, United States, 3 Institute of Semiconductor and Solid State Physics, University Linz, Linz Austria
12:00 PM - GG4.6
Dipolar Interactions Between PbSe Nanocrystals and Their Impact on Synthesis and Self-assembly.
Arjan Houtepen 1 3 , Rolf Koole 1 , Mark Klokkenburg 2 , Ben Erne 2 , Daniel Vanmaekelbergh 1 Show Abstract
1 Condensed Matter and Interfaces, Utrecht University, Utrecht Netherlands, 3 Opto-Electronic Materials, Delft University of Technology, Delft Netherlands, 2 Van 't Hoff Laboratory for Physical and Colloid Chemistry, Utrecht University, Utrecht Netherlands
PbSe is one of the most studied nanocrystal materials because of its unique optical properties in the near infrared (NIR). In addition, PbSe quantum dots (QDs) hold great promise for the field of photovoltaics because of the recently observed multiple exciton generation (MEG) in these IV-VI nanocrystals. A controlled and reproducible synthesis of PbSe QDs is crucial to tailor the electronic or optical properties. We show that a small contamination of acetic acid in the synthesis mixture leads to star-shaped QDs, and that a perfect drying of the precursors is crucial for the synthesis of spherical nanocrystals with high monodispersity (< 5%). By tuning the amount of acetic acid one can tune the size of the star-shaped QDs from 10-150 nm. It is shown that monodisperse star-shaped nanocrystals form hexagonal close packed arrays with full alignment of their crystal planes. The formation of star-shaped nanocrystals can be explained by the presence of a permanent dipole moment in PbSe QDs, which causes an oriented attachment of clusters in the <100> direction. Using Wide Angle Electron Diffraction we have observed a high degree of atomic alignment between PbSe NCs in 2D and 3D self-assembled structures. This alignment can also be explained by a significant dipole moment on the nanocrystals. However, the origin and magnitude of the dipole moment in PbSe QDs is still under debate. To obtain quantitative information on the dipole moment, we have performed Cryo-TEM experiments on colloidal dispersions of PbSe NCs of various shapes. In Situ Cryo-TEM images reveal the spontaneous formation of linear chains of QDs in dispersion. We have performed a quantitative real-space analysis on a single-particle level and show that these chains correspond to one-dimensional dipolar equilibrium structures. The dipolar pair-attraction is around 8 kBT at room temperature and is significantly larger than has been previously assumed.The results are relevant for all experiments on relatively concentrated NC dispersions, since the distribution of chain lengths can influence the response of the system to optical and electrical signals. For example, efficient energy-transfer may occur within clusters. Understanding the chain formation and the distribution of chain lengths is also important for the synthesis of anisotropic nanocrystals such as wires, stars and rings. Finally, the presence of significant amount