Bulletin of the American Physical Society
2005 APS March Meeting
Monday–Friday, March 21–25, 2005; Los Angeles, CA
Session W15: Focus Session: Optical properties of QDs |
Hide Abstracts |
Sponsoring Units: FIAP Chair: P. Guyot Sionnest, University of Chicago Room: LACC 405 |
Thursday, March 24, 2005 2:30PM - 2:42PM |
W15.00001: On the nature of ultraviolet photoluminescence in ZnO quantum dots Vladimir A. Fonoberov, Alexander A. Balandin ZnO quantum dots (QDs) have recently attracted significant attention for applications in ultraviolet (UV) light-emitting diodes, lasers, varistors, as well as QD-based nano-electronic components. However, the nature of the UV photoluminescence (PL) from ZnO QDs has not been fully understood [1]. We address this issue by examining theoretically the optical properties of technologically important 2-6 nm in diameter ZnO QDs [2] with and without ionized impurities at the surface. It is found that depending on the surface passivation technique, the UV PL in ZnO QDs can be attributed to either confined excitons or surface-bound ionized acceptor-exciton complexes. In the latter case the Stokes shift of the order of 100-200 meV is observed in PL spectrum. We also find that the exciton radiative lifetime can be used as a probe of the exciton localization. The authors acknowledge the support of MARCO and its Functional Engineered Nano Architectonics (FENA) Focus Center. [1] V.A. Fonoberov and A.A. Balandin, Appl. Phys. Lett. 85, in press (Dec. 20, 2004). [2] V.A. Fonoberov and A.A. Balandin, Phys. Rev. B 70, 195410 (2004). [Preview Abstract] |
Thursday, March 24, 2005 2:42PM - 2:54PM |
W15.00002: The Structure of Excited-State Transitions in Individual Nanocrystals Probed by Photoluminescence Excitation Spectroscopy Han Htoon, P.J. Cox, V.I. Klimov We study the structure of excited-state transitions in CdSe nanocrystals (NCs) using our newly developed, single-NC, photoluminescence excitation (PLE) technique that mitigates the problem of blinking and spectral wandering of NC emission. The low- temperature (4K) PLE spectra of individual NCs show a few sharp band-edge peaks with 3 to 4 meV linewidths. The higher energy transitions, however, are not manifested as narrow peaks but they rather merge into a dense quasi-continuum. These spectroscopic observations can be rationalized based on results of intraband hole relaxation studies. Specifically, these studies indicate that the energy relaxation rate is very large (1 -- 2 eV/ps) at high spectral energies, leading to significant lifetime broadening of optical transitions that merge into a structureless quasi-continuum. The relaxation rate sharply drops near the band-edge, which is consistent with our observations of the reduced density of states at low spectral energies. Overall, our studies indicate that, while the idealized description of NC excited-state transitions in terms of sharp atomic-like resonances may be applicable at near-band-edge spectral energies, this description is invalid at high spectral energies where optical transitions are significantly broadened because of ultrafast intraband relaxation. [Preview Abstract] |
Thursday, March 24, 2005 2:54PM - 3:06PM |
W15.00003: Fluorescence from Individual PbS Quantum Dots Jeffrey Peterson, Todd Krauss Due to their extremely large electron, hole, and exciton Bohr radii, PbS quantum dots (QDs) can achieve levels of quantum confinement not accessible to III-V and II-VI QDs. Thus, the strong confinement regime is attained for relatively large particles, which may mitigate deleterious surface effects and impart novel properties. PbS QDs are also optically active in the near-infrared region, making these materials potentially useful for telecommunications and biotechnological applications. We will present investigations of single PbS QD fluorescence using far-field microscopy. PbS QDs were synthesized with a size-tunable exciton absorbance ranging between 765 nm and 1800 nm. Of particular note is the ability to synthesize highly luminescent, small radii QDs, allowing for fluorescence detection with high sensitivity silicon CCDs. Upon spincoating QDs onto glass substrates at densities near the single dot level, we observe fluorescence intermittency, or “blinking” and a narrowing of the fluorescence spectra relative to the ensemble, both hallmarks of single fluorophores. The fluorescence energy irreversibly blue shifts with longer integration times and higher excitation intensities, indicative of a photo-induced degradation. Photobleaching of the majority of PbS QDs occurred in ~30 sec. An analysis of the blinking statistics will be discussed. [Preview Abstract] |
Thursday, March 24, 2005 3:06PM - 3:18PM |
W15.00004: Photoluminescence studies of colloidal PbSe nanocrystals Jeffrey Harbold, Frank Wise, Chris Murray Nanocrystals of the IV-VI semiconductors PbS, PbSe, and PbTe provide unique opportunities to investigate the effects of strong confinements on both electrons and holes. The sparse electronic structure of IV-VI quantum dots (QDs) was expected to dramatically decrease the intraband relaxation rate of carriers; however, picosecond time-scale relaxation is observed experimentally. One possible explanation could be the presence of states between the P and S energy levels. Therefore, we undertook photoluminescence studies of PbSe QDs to elucidate their band-edge electronic structure. Our measurements reveal a splitting of the photoluminescence spectrum that increases from 20 to 120 meV when decreasing the QD diameter from 4.6 to 3.2 nm. We also observe relatively broad emission lines (approximately 40 meV) even when selectively exciting only the largest QDs within the size distribution. We present two possible explanations based on different calculations of the electronic structure of PbSe QDs and also comment on the relevance of these findings for applications of these materials. [Preview Abstract] |
Thursday, March 24, 2005 3:18PM - 3:30PM |
W15.00005: Fluorescence Resonance Energy Transfer in PbS Nanocrystal Films Stephen Clark, Jeffrey Harbold, Frank Wise We report a study of time-resolved photoluminescence of colloidal PbS nanocrystals both in solvent and dried into films. Lifetimes of the PbS nanocrystals in solvent are measured to be around 2.5 microseconds, which is two orders of magnitude larger than expected based on the transition dipole moment. The luminescence spectrum of PbS nanocrystals in the film is red-shifted with respect to the spectrum from the solvent. Spectrally resolving the photoluminescence dynamics of the films clearly shows a decay from the high-energy side of the spectrum, and a rise in the low energy side, on a time scale of around 200 nanoseconds. These measurements provide direct evidence of fluorescence resonance energy transfer in the presence of dielectric screening. [Preview Abstract] |
Thursday, March 24, 2005 3:30PM - 3:42PM |
W15.00006: Amplified spontaneous emission in an opal infiltrated with CdSe/titania sol-gel Garry Maskaly, Melissa Petruska, Jagjit Nanda, Ilya Bezel, Richard Schaller, Victor Klimov The emission spectra of semiconductor, colloidal nanocrystals (NCs) and the stop band positions in photonic crystals (PCs) are individually tunable through the respective sizes of the NCs and the particles comprising the PC lattice. This tuning allows the two components to be matched over a wide variety of wavelengths. An increased photonic density of states and a reduced group velocity at the photonic band edge (even a pseudogap edge) increases the interaction time between the light and the active material, potentially lowering the threshold for amplified spontaneous emission (ASE) and lasing. In this work, we present a composite consisting of a polystyrene opal infiltrated with CdSe NC/titania sol-gel. This structure exhibits ASE despite a NC volume loading of less than 3%. A red shift of the center of the PL spectrum and a blue shift in the ASE of the sol-gel is observed relative to a CdSe/sol-gel control. The two wavelengths shift to an identical frequency at the pseudogap edge where the group velocity is expected to be a minimum. This work is a step toward NC/PC lasers tunable over a wide spectral range. [Preview Abstract] |
Thursday, March 24, 2005 3:42PM - 3:54PM |
W15.00007: Electric field induced narrowing of exciton line width Ilya Ponomarev, Lev Deych, Alexander Lisyansky Considering effects of electric field on the low temperature absorption line of quantum well excitons, we show [1] that, for moderate strength of the electric field, the main contribution to the field dependence of the line-width results from field induced modifications of inhomogeneous broadening of excitons. In the presence of disorder, the center-of-mass motion of the exciton in a QW structure is determined by the Schr\"{o}dinger equation with random effective potential. The variance, $\sigma ^2=\left\langle {V_{eff} \left( {\rm {\bf R}} \right)^2} \right\rangle $ of the effective potential is an important characteristic determining the total exciton inhomogeneous line width. Application of the electric field in $z-$direction considerably modifies the excitonic wave functions, which in turn change the properties of the effective random potential. We show that in the Stark regime of moderate fields for the case of compositional disorder the potential variance changes with the field as$\sigma ^{(\mbox{com})2}(E)=\sigma ^{(\mbox{com})2}(0)-\alpha E^2$, while the contribution of the interface disorder to the variance is $\sigma ^{(\mbox{int})2}(E)=\sigma ^{(\mbox{int})2}(0)+\beta E+\gamma E^2$. Magnitudes and signs of $\alpha ,\beta ,\gamma $ depend on the thickness and interface qualities of the well. We show that under certain conditions one can observe narrowing of exciton spectral lines in electric field. This means that the electric field can become an important tool for identification of the disorder mechanism and monitoring the interface quality in the process of growth of the QW structures. [1], I. V. Ponomarev, L. I. Deych, A. A. Lisyansky, cond-mat/0408296. [Preview Abstract] |
Thursday, March 24, 2005 3:54PM - 4:06PM |
W15.00008: Dielectric Confinement and Optical Properties of ZnO Quantum Rods Igor L. Kuskovsky, M. Yin, Y. Gu, Y. Gong, T. Andelman, S. O’Brien, A. Shabaev ZnO has been of great interest for UV RT applications due to its wide band gap and high exciton binding energy. The device performance is often improved by the use of low-dimensional structures. There have been many reports on the fabrication and the optical properties of ZnO nanorods; however, no quantum confinement was reported until recently [1, 2]. At the same time, it has been shown [3] that the dielectric confinement must be considered for the correct analysis of absorption and emission from 1D quantum systems. The role of dielectric confinement in the optical properties of ZnO quantum rods is studied here. Specifically, we compared optical absorption of ZnO quantum rods dissolved in hexane and chloroform. We have found that the absorption energy is higher when hexane is used as a solvent. This is explained, qualitatively, by the increase in the single particle energies due to the dielectric confinement [3], since the ZnO dielectric constant is larger than that of hexane, but almost equal to that of chloroform. Finally, we shall discuss the role of confinement on the green emission observed in these quantum rods. 1. M. Yin, \textit{et al.}, J. Am. Chem. Soc. \textbf{126}, 6206 (2004). 2. Y. Gu, \textit{et al.}, Appl. Phys. Lett$.$ \textbf{85}, 3383 (2004). 3. A. Shabaev and Al. L. Efros, Nano Lett., \textbf{4}, 1821 (2004). [Preview Abstract] |
Thursday, March 24, 2005 4:06PM - 4:18PM |
W15.00009: Toward Single-Exciton Lasing through Engineered Exciton-Exciton Interactions Jagjit Nanda, Sergei Ivanov, Han Htoon, Ilya Bezel, Andrei Piryantinski, Sergei Tretiak, Victor Klimov We synthesize and study ``inverted,'' core-shell nanocrystals (NCs), in which a core of a wide-gap semiconductor (ZnSe) is overcoated with a shell of a narrower gap material (CdSe). Compared to monocomponent or traditional, ``noninverted'' hetero-NCs, these novel, ``inverted'' structures provide new capabilities for controlling their functionalities via a direct control of the spatial distribution of electron and hole wavefunctions. Specifically, by increasing the thickness of the NC shell (for a fixed NC core size), one can continuously tune the carrier localization regime from type-I for thin shells to type-II for intermediate shells and finally back to type-I for thick shells. We show that these heterostructures can be used to significantly increase absorption cross sections and simultaneously decrease the efficiency of Auger recombination compared to monocomponent NCs emitting at the same wavelength. Furthermore, appropriately designed hetero-NCs can exhibit repulsive exciton-exciton interactions that lead to reduced excited-state absorption associated with singly-excited NCs. This effect leads to reduced optical-gain thresholds and can potentially allow lasing in the single-exciton regime, for which Auger recombination is inactive. We use these hetero-NCs to demonstrate efficient light amplification that is tunable across a ``difficult'' range of green and blue colors. [Preview Abstract] |
Thursday, March 24, 2005 4:18PM - 4:30PM |
W15.00010: Non-linear Optical Properties of Semiconductor Nanoparticle-Doped Glass Microspheres Anuranjita Tewary, April Montoya Vaverka, Risbud Subhash, Mark Brongersma Microspheres were fabricated by heating with a CO$_{2}$ laser ($\lambda $ = 10.6 $\mu $m) the sharp tip of a fiber pulled from CdTe-doped or Silicon-doped borosilicate glass. Nanoparticles were formed in the microspheres during the few seconds that it took for the microsphere to form under CO$_{2}$-laser irradiation. We have fabricated microspheres ranging in diameter from 10 $\mu $m to 300 $\mu $m. These microspheres contain nanoparticles ranging in diameter from 5 nm to 10 nm, as indicated by TEM. We have resonantly coupled light into the whispering gallery modes (WGM) of the microspheres using a tapered-fiber coupler. We have measured high optical quality factors (Q) of 2X10$^{4}$ for a 30 $\mu $m diameter CdTe nanoparticle-doped microsphere and 5X10$^{4}$ for a 300 $\mu $m diameter silicon nanoparticle-doped microsphere. We will present our results on a number of non-linear optical studies conducted on these microspheres and discuss the implications of these results towards making active optical devices. [Preview Abstract] |
Thursday, March 24, 2005 4:30PM - 4:42PM |
W15.00011: Optical behavior of nanocrystal quantum dots adsorbed on crystalline semiconductor substrates: Evidence for energy transfer Siyuan Lu, Atul Konkar, Anupam Madhukar Integration of epitaxical and colloidal semiconductor nanostructures into hybrid structures can potentially open unprecedented functionalities that combine the strengths of the epitaxical nanostructures in optoelectronics with the versatility of the nanocrystal quantum dots (NCQDs) and their application in solution environment. Here we report on a photoluminescence study of InAs/ZnSe NCQDs adsorbed on GaAs(001) surface with or without buried near surface quantum nanostructures. Quenching of NCQDs luminescence is observed when the excitation energy goes above the GaAs bandgap, providing the first evidence of excitation energy transfer from high excited states of NCQDs into the substrate. work is supported by DARPA/AFOSR under the DURINT program. [Preview Abstract] |
Thursday, March 24, 2005 4:42PM - 4:54PM |
W15.00012: Mapping Exciton Spin States in CdSe Nanocrystals with Spin-Polarized, Resonant Photoluminescence Spectroscopy Madalina Furis, Patrick Robbins, Todd Barrick, Scott Crooker, Serguei Goupalov, Melissa Petruska, Victor Klimov, Alexander Efros We report on high-resolution, spin-polarized resonant photoluminescence (PL) studies of exciton spin states in CdSe nanocrystals in high magnetic fields up to 33 T. Optically allowed spin-up or spin-down ``bright'' (spin-1) excitons are resonantly pumped using a circularly-polarized, narrowband, tunable dye laser. Quasi-resonant PL from the optically forbidden ``dark'' (spin-2) excitons is analyzed as a function of polarization and magnetic field, allowing us to map the evolution of both ``dark'' and ``bright'' excitons with magnetic field. Strikingly, the PL develops a sharp, circularly- polarized ``spin flip-like'' peak for B $>$ 10 T, which we associate with the splitting of the "bright" exciton state. This energy splitting scales inversely with nanocrystal size and extrapolates to a finite value at zero magnetic fields. This large, zero-field splitting of the ``bright'' exciton (1-2 meV) likely originates from the anisotropy of the nanocrystal confining potential. [Preview Abstract] |
Follow Us |
Engage
Become an APS Member |
My APS
Renew Membership |
Information for |
About APSThe American Physical Society (APS) is a non-profit membership organization working to advance the knowledge of physics. |
© 2024 American Physical Society
| All rights reserved | Terms of Use
| Contact Us
Headquarters
1 Physics Ellipse, College Park, MD 20740-3844
(301) 209-3200
Editorial Office
100 Motor Pkwy, Suite 110, Hauppauge, NY 11788
(631) 591-4000
Office of Public Affairs
529 14th St NW, Suite 1050, Washington, D.C. 20045-2001
(202) 662-8700