Bulletin of the American Physical Society
APS March Meeting 2011
Volume 56, Number 1
Monday–Friday, March 21–25, 2011; Dallas, Texas
Session T34: Optical and Electronic Properties of Nanocrystals and Wires |
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Sponsoring Units: DCMP Chair: Steve Erwin, Naval Research Laboratory Room: C141 |
Wednesday, March 23, 2011 2:30PM - 2:42PM |
T34.00001: Colloidal Quantum Dot Red-Shifting on Textured Metal Surfaces Christopher Ferri, Anthony Grimes, Sayantani Ghosh We have studied the influence of textured metal surfaces on the emission of an ensemble of colloidal CdSe/ZnS core-shell quantum dots (QDs). The texture was generated by sputter coating a thin film of Gold Paladium (AuPd) on a thermoplastic Polydimethylsiloxane (PDMS) sheet. We used two separate protocols to generate two types of surfaces. We constrained some substrates such that they shrunk along only one planar dimension (uniaxial) while some were allowed to shrink along both planar directions (biaxial). The uniaxial substrates forced the metal to buckle along one dimension and the biaxial substrates buckled into a pseudorandom texture. We found that the QDs deposited on the biaxial substrates had a general red shift in the emission wavelength compared to their emission in solution, which also corresponded to a change in the temporal dynamics of the emission. The QDs on the uniaxial substrates showed a change in their temporal dynamics corresponding to plasmonic coupling, but no spectral shift. We hypothesize that the effects observed on the biaxial substrates are caused by the Franz-Keldysh effect. [Preview Abstract] |
Wednesday, March 23, 2011 2:42PM - 2:54PM |
T34.00002: Single-molecule spectroscopy study of interfacial charge separation and energy transfer between quantum dots and conjugated polymers Zhihua Xu, Mircea Cotlet Blends of semiconducting quantum dots (Qdots) and conjugated polymers (CPs) are promising materials for light-emitting diodes or photovoltaic devices. Effective design of optoelectronic devices relies on further understanding of the photophysics in these hybrid materials, including charge separation (CS) and energy transfer (ET). We have studied the photophysics of the blends of two water-soluble conjugated polymers and CdSe/ZnS quantum dots with varying shell thickness, which enable us to control the competitive CS and ET processes by tuning energy-band alignment and interfacial distance. Combining single --molecule spectroscopy with ensemble-based measurements provides deeper understanding of the dynamic interactions at inhomogeneous interfaces. [Preview Abstract] |
Wednesday, March 23, 2011 2:54PM - 3:06PM |
T34.00003: Broadband ultrafast transient absorption of multiple exciton dynamics in lead sulfide nanocrystals Felice Gesuele, Chee Wei Wong, Matthew Sfeir, James Misewich, Weonkyu Koh, Christopher Murray Multiple exciton generation (MEG) is under intense investigation as potential third-generation solar photovoltaics with efficiencies beyond the Shockley-Queisser limit. We examine PbS nanocrystals, dispersed and vigorously stirred in TCE solution, by means of supercontinuum femtosecond transient absorption (TA). TA spectra show the presence of first and second order bleaches for the 1Sh-Se and 1Ph-Pe excitonic transition while photoinduced absorption for the 1Sh,e-Ph,e transitions. We found evidence of carrier multiplication (MEG for single absorbed photon) from the analysis of the first and second order bleaches, in the limit of low number of absorbed photons (Nabs$\sim $0.01), for energy three times and four times the Energy gap. The MEG efficiency, derived from the ratio between early-time to long-time TA signal, presents a strongly dispersive behavior with maximum red shifted respect the first absorption peak. Analysis of population dynamics shows that in presence of biexciton, the 1Sh-Se bleach peak is red-shifted indicating a positive binding energy. MEG efficiency estimation will be discussed with regards to spectral integration, correlated higher-order and first excitonic transitions, as well as the nanocrystal morphologies. [Preview Abstract] |
Wednesday, March 23, 2011 3:06PM - 3:18PM |
T34.00004: The effects of controlled charging on photoluminescence of individual ``giant'' core/shell nanocrystals Christophe Galland, Yagna Ghosh, Bhola Pal, Jennifer Hollingsworth, Milan Sykora, Victor Klimov, Han Htoon We report the first single-nanocrystal photoluminescence (PL) study under controlled charge injection. We investigate so-called ``giant'' nanocrystal quantum dots (g-NQDs) that comprise a CdSe core and a thick CdS shell. We use solid-state gated devices as well as electrochemical cells for charged injection. When the gate bias or electrochemical potential is tuned, we observe dramatic changes in the PL dynamics that are accompanied by spectral shifts and intensity modulations. Our initial results suggest that negatively charged excitons are at least as bright as neutral excitons in these g-NQDs for which Auger recombination has been shown to be strongly suppressed. Surprisingly, hole injection leads primarily to quenching of the NQD emission, which might explain the appearance of ``gray'' or ``dark'' periods in the single-dot PL trajectories. [Preview Abstract] |
Wednesday, March 23, 2011 3:18PM - 3:30PM |
T34.00005: Near-Unity Biexciton Quantum Yields in Individual Giant Nanocrystal Quantum Dots Young-Shin Park, Anton Malko, Javier Vela, Yongfen Chen, Yagnaseni Ghosh, Florencio Garcia-Santamaria, Jennifer Hollingsworth, Victor Klimov, Han Htoon We report quantitative studies of photoluminescence (PL) quantum yields of biexcitons ($Q_{BX})$ in individual CdSe/CdS core/shell nanocrystal quantum dots (NQDs) as a function of shell thickness. $Q_{BX}$s measured by two independent techniques show a gradual increase with increasing shell thickness, reaching a near-unity value of $\sim $0.9 for the NQDs with a 19 monolayer-thick shell. These results imply a strong suppression of Auger decay.~ However, $Q_{BX}$s show a wide variation among nominally identical NQDs implying a strong dependence of $Q_{BX}$ on subtle structural differences of the core/shell interfaces. Surprisingly, despite a wide variation in $Q_{BX}$, all thick-shell NQDs exhibit a complete suppression of PL blinking, implying that this non-blinking behavior does not result from the suppression of Auger decay and instead may simply arise from a reduced likelihood of photocharging. [Preview Abstract] |
Wednesday, March 23, 2011 3:30PM - 3:42PM |
T34.00006: Axially localized optical properties of individual nanowires Junping Zhuang, Yao Liang, S.K. Hark Nanowires are not always uniform in their properties. Studying the localized optical properties of the nanowires is thus important for their characterization and potential applications. Micro-photoluminescence spectra and fluorescence lifetime along individual ZnSe nanowires were measured and studied using laser scanning confocal microscopy. The nanowires were selected from an array that was synthesized via self catalyzed VLS mode on GaAs substrates. Through fluorescent imaging, the distribution of the deep-level emissions along the nanowires, in which a bright-tip and a dim-tail were observed, is found to be very different from the relatively uniform distribution of the near-band-edge emissions. Using fluorescence lifetime imaging, we found that the fluorescence decay behaviors are very different between the two emission bands and have position dependence. We believe that the unintended Ga diffusion during the growth of the nanowires should be responsible for the observed distributions. [Preview Abstract] |
Wednesday, March 23, 2011 3:42PM - 3:54PM |
T34.00007: Polarization dependence of ultrafast dynamics in single Si nanowires M.A. Seo, S.A. Dayeh, P.C. Upadhya, S.T. Picraux, J. Martinez, B.S. Swartzentruber, A.J. Taylor, R.P. Prasankumar Understanding how light interacts with individual nanowires (NWs), particularly depending on its polarization with respect to the NW alignment, is essential for a wide range of applications. We present the first ultrafast time-resolved, polarization-dependent experiments on both single- and ensemble-silicon nanowires using non-degenerate pump-probe spectroscopy to excite and probe carriers above the indirect band gap. Polarization sensitive pump-probe excitation and detection reveal a clear anisotropy in the ultrafast dynamics measured parallel and perpendicular to the long axis of a single nanowire. In addition, the magnitude of the photoinduced change in ensembles of NWs varies for four different sets of pump and probe polarization, without an anisotropy in relaxation time. The comparison of ultrafast dynamics between single and ensemble nanowires provides great insight into the influence of incident light polarization on different absorption and interaction mechanisms. The observed anisotropy in single NWs could enable advanced applications, such as optical switching and polarization sensitive photo detection, on the nanoscale, where directional control and high spatial resolution are much desired [Preview Abstract] |
Wednesday, March 23, 2011 3:54PM - 4:06PM |
T34.00008: Space-and-Time-Resolved Spectroscopy of Single GaN Nanowires Prashanth Upadhya, Julio Martinez, Qiming Li, George Wang, Brian Swartzentruber, Antoinette Taylor, Rohit Prasankumar Understanding the carrier relaxation pathways in individual semiconductor nanowires (NWs) is crucial, since the geometry of these nanostructures can significantly influence carrier recombination and trapping. In particular, GaN NWs are promising wide bandgap semiconductors for applications in nanophotonics, but the efficiency and lifetime of GaN-based devices are largely affected by the presence of structural and point defects. In this study we employ wavelength-tunable femtosecond optical pump-probe spectroscopy to study carrier relaxation through the defect states responsible for yellow luminescence in both a single GaN NW and NW ensembles. These are the first ultrafast optical experiments on single group III-V NWs, revealing spatially resolved carrier dynamics along the length of an individual wire. [Preview Abstract] |
Wednesday, March 23, 2011 4:06PM - 4:18PM |
T34.00009: Electronic Structure of PbSe Nanorods Adam Bartnik, Alexander L. Efros, Weon-kyu Koh, Jun Yang, Christopher Murray, Frank Wise In spherical lead-salt (PbS and PbSe) nanocrystals, their large dielectric constant and mirror-like band structure significantly weaken the Coulomb interaction, while their large exciton Bohr radii place them at the limit of strong confinement. But in a 1-dimensional structure, the Coulomb interaction can act primarily through the medium, greatly reducing screening. Thus, by controlling their length, the lead-salts can uniquely switch from strong confinement to strong Coulomb binding. To investigate this, we develop a 4-band k-p model of the electronic structure of lead-salt nanorods (NRs), which includes the Coulomb interaction through an effective 1D potential along the NR axis. Perpendicular to the cylindrical axis, confinement dominates and is the major determinant of the location of peaks in the optical spectra. Along the rod axis, the effective Coulomb potential dominates, highly correlating the electron and hole in this direction and enhancing multiparticle interactions, with the observable effect of producing isolated peaks in optical spectra. Predictions of the locations of these enhanced transitions are shown to have good agreement with measured optical spectra of recently synthesized colloidal PbSe NRs. [Preview Abstract] |
Wednesday, March 23, 2011 4:18PM - 4:30PM |
T34.00010: Atomistic calculations of the exciton-biexciton mixing and biexciton lifetime in CdSe nanocrystals Marek Korkusinski, Oleksandr Voznyy, Pawel Hawrylak We present an atomistic tight-binding theory of multi-exciton complexes in spherical CdSe nanocrystals. As shown previously [1] the properties of exciton (X) and biexciton (XX) are determined by the shell of four quasi-degenerate states at the top of the valence band, resulting in a band of correlated XX states. This XX fine structure affects the Coulomb mixing of the low-lying XX with excited X states. Here we compare different approaches to computation of the XX ground state lifetime. The Fermi's golden rule accounting only for directly coupled XX and X configurations is compared to configuration-interaction approach where XX and X with energy close to 2Eg are taken into account. We show that the expansion of the basis of single particle configurations used to describe XX leads to a significant increase of the amount of X configurations to which XX can couple. The effect of inclusion of the X configurations coupled to XX indirectly via the intermediate X states is also discussed. \\[4pt] [1] M. Korkusinski, O. Voznyy, and P. Hawrylak, arXiv:1010.0021 (Phys. Rev. B, 82, 2010, in press). [Preview Abstract] |
Wednesday, March 23, 2011 4:30PM - 4:42PM |
T34.00011: Can nanocrystals be charge-doped using surface adsorbates? Alex Kutana, Steve Erwin We study theoretically the possibility that lead selenide nanocrystals can be doped with electrons or holes by charge transfer from molecules (hydrazine, N2H4) adsorbed on their surface. Despite experiments showing that hydrazine adsorption greatly increases the conductivity of PbSe nanocrystal films, our density-functional theory results show no evidence that carriers are directly created by adsorption. Instead, we find that PbSe always remains intrinsic for a variety of hydrazine coverages and PbSe surface orientations. Moreover, analysis of the charge density shows negligible electron transfer from hydrazine to the surface. We tentatively attribute the discrepancy between experiment and theory to extrinsic factors such as surface defects, doping by surface ligands, or dopant activation by hydrazine. For example, we predict that adsorption of acetic acid will create mobile holes in PbSe, and that surface adsorption of lead atoms will result in mobile electrons. [Preview Abstract] |
Wednesday, March 23, 2011 4:42PM - 4:54PM |
T34.00012: Surface states in CdSe nanocrystals with carboxylic acid ligands: an ab initio study Oleksandr Voznyy The electronic properties of the realistic Cd-rich CdSe quantum dots with covalently bound (X-type) carboxylic ligands are investigated using density functional theory for the nanocrystal (NC) sizes sufficient to distinguish core and surface states. We find that Cd and Se atoms with only one dangling bond do not create surface traps. The amount of ligands and the crystal shape is dictated by the overall electronic balance of the NC rather than by the surface free energies of particular ligated facets. To achieve this balance more ligands are required than there are ``ideal'' adsorption sites for them, creating ligand-induced trap states near the valence band maximum. These extra ligands are mobile on surface, resulting in spectral diffusion of the trap levels, providing first atomistic example of diffusion and activated recombination centers models for blinking. [Preview Abstract] |
Wednesday, March 23, 2011 4:54PM - 5:06PM |
T34.00013: First-Principles Calculations of Lattice-Strained Core-Shell Nanocrystals K.H. Khoo, J.T. Arantes, James R. Chelikowsky, G.M. Dalpian We have studied the properties of CdS-ZnS and ZnS-CdS core-shell nanocrystals over a range of shell thicknesses using real-space pseudopotential density functional theory. The effect of structural relaxation was shown to be important as it leads to significant changes in the HOMO-LUMO gap and frontier orbital localizations. Also, strains due to lattice mismatch are predicted to be highly localized around the core-shell interface, giving rise to a thin shell regime where both confinement and strain effects are important and a thick shell regime where confinement effects dominate. This has interesting implications for the evolution of the HOMO-LUMO gap with shell thickness. [Preview Abstract] |
Wednesday, March 23, 2011 5:06PM - 5:18PM |
T34.00014: First Principles Study of Core-Shell Semiconductor Nanocrystals Igor Vasiliev Core-shell nanocrystals composed of two different semiconductors have recently attracted considerable attention. These structures provide enhanced functionality and possess more degrees of freedom than single-component semiconductor nanocrystals and quantum dots. I present the results of {\it ab initio} density functional calculations for the structures, electronic densities of states, and optical absorption gaps of core-shell nanocrystals composed of group II-VI semiconductors, such as CdSe, CdTe, ZnSe, and ZnTe. The outer surfaces of the nanocrystals are passivated using partially charged hydrogen atoms. The calculations are performed for ``traditional'' core-shell nanocrystals, in which a core a narrow gap semiconductor is covered with a shell of a wide gap material, and ``inverted'' core-shell nanocrystals, in which a wide-gap core is enclosed in a narrow-gap shell. [Preview Abstract] |
Wednesday, March 23, 2011 5:18PM - 5:30PM |
T34.00015: Jahn-Teller distortion in semiconductor nanocrystals Gustavo M. Dalpian, Aline L. Schoenhalz, Jeverson T. Arantes Semiconductor nanocrystals or quantum dots can present properties that are very different from their bulk counterparts. Quantum confinement and surface effects play important roles in determining these unusual properties. Here we report a Jahn-Teller distortion in pristine nanocrystals upon the addition of charges. Results will be presented for Si, InAs and CdSe nanocrystals. In order to observe this effect, we have performed calculations using the Density Functional Theory and hybrid functionals. We will discuss the implications of this distortion on the electronic and optical properties of these materials. [Preview Abstract] |
Wednesday, March 23, 2011 5:30PM - 5:42PM |
T34.00016: Memory, Photoconductivity, and Traps in Semiconducting Nanocrystal Arrays Jessamyn Fairfield, Lauren Willis, Tali Dadosh, Marija Drndic Nanoscale devices are extensively studied for their tunable electronic and optical properties, but the influence of impurities and defects is amplified at these length scales and can lead to poorly understood variations in material characteristics. By performing a large ensemble of photoconductivity measurements in nanogaps bridged by core-shell CdSe/ZnS semiconductor nanocrystals, we discover optoelectronic methods for affecting solid-state charge trap populations. We show that the magnitude and temperature dependence of the photocurrent depends on the illumination and electric field history on a few-hour timescale. Subband gap illumination of nanocrystals prior to measurements modifies the photocurrent more than band gap illumination. We introduce a model that unifies previous work and transforms the problem of irreproducibility in nanocrystal electronic properties into a robust photocurrent response due to trap state manipulation. Because traps dominate many physical processes, these findings may lead to improved performance and device tunability for nanoscale applications through the control and optimization of impurities and defects. [Preview Abstract] |
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