Bulletin of the American Physical Society
APS March Meeting 2015
Volume 60, Number 1
Monday–Friday, March 2–6, 2015; San Antonio, Texas
Session D13: Focus Session: Optical Properties of Semiconductor Nanostructures |
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Sponsoring Units: DCMP DMP Chair: Thomas Reinecke, Naval Research Laboratory Room: 007D |
Monday, March 2, 2015 2:30PM - 2:42PM |
D13.00001: Comparative study of the electronic and optical properties of core-shell nanocrystals Vancho Kocevski, Jan Rusz, Olle Eriksson, D.D. Sarma The photoluminescence (PL) properties of semiconducting nanocrystals (NCs) can be notably improved by capping the NCs with a shell of another semiconductor, making core-shell structures. Furthermore, their PL properties can be manipulated by changing the core type or the interface between the core and the shell. Here we present a comparative first-principles study of the electronic and optical properties of two different types of core-shell NCs, CdSe/CdS and CdS/ZnS, with four different structural models: pure core, graded core, alloyed interface and graded interface. For the purpose of the study we made NCs with two different diameters, 2.4 nm and 3.0 nm. We show that the electronic and optical properties of the CdS/ZnS NCs are influenced more by the different structural models, compared to the properties of CdSe/CdS NCs. We further looked into the spatial confinement of the HOMO and LUMO wavefunctions (WFs) within the core of the NCs, and the e-h Coulomb interaction energies. We argue that although both types of core-shell NCs have similar confinement of the WFs, the lowering of e-h Coulomb interaction energies, hence increasing the AR lifetimes, in the CdSe/CdS NCs, compared to the CdS/ZnS NCs, is one of the main reasons for the higher quantum yield of the CdSe/CdS NCs. [Preview Abstract] |
Monday, March 2, 2015 2:42PM - 2:54PM |
D13.00002: Effect of Different Ligands on Carrier Dynamics of CdSe Quatum Dots for Solar Cells Applications Baichhabi R. Yakami, Urice Togha, Meg Mahat, Shashank R. Nandyala, Milan Balaz, Jon M. Pikal We have carried out steady state absorption and photoluminescence (PL), as well as time resolved PL and ultrafast transient absorption (TA) studies of CdSe quantum dots (QD) with five different capping ligands: trioctylphosphine oxide (TOPO), oleic acid (OA), dodecanethiol (DDT), mercaptopropionic acid (MPA), and L-cysteine (Cys). These ligands have different chemical structures and which effects the optical properties of the QDs. Measurements were conducted on QD sizes ranging from {\O} $=$ 2.5nm to 4.6nm with smaller QDs showing an excitonic PL and a broad surface trap state PL. The ligand exchange of OA CdSe QDs with MPA, DDT and Cys leads to quenching of excitonic PL intensity accompanied by a larger surface trap state to excitonic PL intensity ratio. This is consistent with the TRPL measurements, which show faster exciton PL decays for CdSe QDs with MPA, DDT and Cys ligands compared to OA and TOPO. The PL decay shows multi-exponential behavior with the average lifetime decreasing with increasing QD size. Data from TA experiments using a white light probe is also used to study the picosecond carrier dynamics. These measurements shed light on the role of capping ligands on the carrier dynamics of the QD used as sensitizers in solar cells. [Preview Abstract] |
Monday, March 2, 2015 2:54PM - 3:06PM |
D13.00003: Ultrafast Electron Trapping in Ligand-Exchanged Quantum Dot Assemblies J. M. Kikkawa, M. E. Turk, P. M. Vora, A. T. Fafarman, B. T. Diroll, C. B. Murray, C. R. Kagan We use time-integrated and time-resolved photoluminescence and absorption to characterize the low-temperature (10 K) optical properties of CdSe quantum dot (QD) solids with different ligand and annealing preparation. Close-packed CdSe quantum dot solids are prepared with native aliphatic ligands and with thiocyanate with and without thermal annealing. Using sub-picosecond, broadband time-resolved photoluminescence and absorption, we find that ligand exchange increases the rate of carrier surface trapping. We further determine that holes within the QD core, rather than electrons, can bleach the band-edge transition in these samples at low temperature, a finding that comes as a surprise given what is known about the surface treatment in these QDs. We find that our ligand treatments lead to faster electron trapping to the quantum dot surface, a greater proportion of surface photoluminescence, and an increased rate of nonradiative decay due to enhanced interparticle coupling upon exchange and annealing. [Preview Abstract] |
Monday, March 2, 2015 3:06PM - 3:18PM |
D13.00004: Van der Waals materials for the passivation of monolayer closed-packed films of CdSe quantum dots Dennis Zi-Ren Wang, Datong Zhang, Richard Creswell, Chenguang Lu, Jiayang Hu, Irving P. Herman Van der Waals (vdW) materials are shown to protect CdSe quantum dots (QDs) from oxidization. Few-layer vdW materials, e.g. graphene and MoS2, were transferred onto a monolayer closed-packed CdSe quantum dots and were examined by photoluminescence (PL) after different time periods. By comparing the PL of CdSe QDs in uncovered areas and those covered by different numbers of layers of graphene and MoS2, we saw that vdW encapsulation slows down the aging of CdSe QDs dramatically. PL mapping results clearly showed better protection of the CdSe QDs under the central part of the vdW material compared to that at the edge; this can be explained by the diffusion of oxygen and water vapor from the edge of the vdW materials. [Preview Abstract] |
Monday, March 2, 2015 3:18PM - 3:30PM |
D13.00005: Very Low Threshold ASE and Lasing Using Auger-Suppressed Nanocrystal Quantum Dots Young-Shin Park, Wan Ki Bae, Andrew Fidler, Tomas Baker, Jaehoon Lim, Jeffrey Pietryga, Victor Klimov We report amplified spontaneous emission (ASE) and lasing with very low thresholds obtained using thin films made of engineered thick-shell CdSe/CdS QDs that have a CdSeS alloyed layer between the CdSe core and the CdS shell. These ``alloyed'' QDs exhibit considerable reduction of Auger decay rates, which results in high biexciton emission quantum yields (Q$_{\mathrm{BX}}$ of $\sim$ 12{\%}) and extended biexciton lifetimes ($\tau _{\mathrm{BX}}$ of $\sim$ 4ns). By using a fs laser (400 nm at 1 kHz repetition rate) as a pump source, we measured the threshold intensity of biexciton ASE as low as 5 $\mu $J/cm$^{2}$, which is about 5 times lower than the lowest ASE thresholds reported for thick-shell QDs without interfacial alloying. Interestingly, we also observed biexciton random lasing from the same QD film. Lasing spectrum comprises several sharp peaks (linewidth $\sim$0.2 nm), and the heights and the spectral positions of these peaks show strong dependence on the exact position of the excitation spot on the QD film. Our study suggests that further suppression of nonradiative Auger decay rates via even finer grading of the core/shell interface could lead to a further reduction in the lasing threshold and potentially realization of lasing under continuous-wave excitation. [Preview Abstract] |
Monday, March 2, 2015 3:30PM - 3:42PM |
D13.00006: Energy Up-conversion in Photo-luminescent Processes of CdSe and CdSe/ZnS Quantum Dots with High Energy Shift Muchuan Hua, Ricardo Decca, Rajesh Sardar, Meghan Teunis, Daniel Minner Photo-luminescent(PL) spectra of CdSe and CdSe/ZnS quantum dots(QDs), with radius in the range of $2.5$ nm to $4$ nm, have been obtained, where energy up-conversion is noticeable. When sweeping the excitation energy around the center of the PL of the QDs samples, the up-conversion of PL is constant and close to the energy of a single longitudinal optical phonon of bulk CdSe. Meanwhile, the PL intensity depends linearly on the excitation intensity, precluding multi-photon absorption processes. These observations indicate phonon-assisted PL might be responsible for the up-conversion processes. On the other hand, when the samples were excited around the tail of the PL spectra, the energy shift of PL up-conversion increased and showed a strong sample dependence. Potential mechanisms to understand these processes will be discussed. [Preview Abstract] |
Monday, March 2, 2015 3:42PM - 3:54PM |
D13.00007: Molecular Polaron Formation of Acoustic Phonons in Quantum Dot Molecules Joshua Casara, Andrew Jacobs, Cyprian Czarnocki, Alessandro Monteros, Thomas Peev, Joshua Tin Yau Tse, Youstina Gad, Michael Scheibner In a recent experimental study, coherent and non-dissipative behavior of optical phonons was achieved via the generation of molecular polarons in a coupled quantum dot pair. An optical transparency caused by a Fano-type resonant quantum interference between discrete interdot excitons and continuum single dot-like polaron states [1] revealed the molecular polaron. It has been shown that the phonon-induced transparency is highly controllable by electric field, excitation energy and power. Here we review the molecular polaron formation via optical phonons and we investigate an analogous transparency induced by acoustic phonons. Photoluminescence excitation spectroscopy is used to probe the characteristics of the transparency. The study tests the molecular polaron formation as a function of the longitudinal acoustic phonon density of states in the range from 10 meV to 20 meV above the bare single dot-like neutral exciton ground state transition. [1] M. L. Kerfoot et al., Nat. Commun. 5, 3299 (2014). [Preview Abstract] |
Monday, March 2, 2015 3:54PM - 4:06PM |
D13.00008: Dephasing and Population Dynamics of Electron-Hole-Pairs in AlGaAs/GaAs Core-Shell Nanowires H.P. Wagner, M. Kaveh, Q. Gao, C. Jagadish, W. Langbein, G. Duscher We investigate the dephasing and population dynamics of electron-hole-pairs (EHPs) in VLS grown zincblende AlGaAs/GaAs core-shell nanowires (NWs) using heterodyne four-wave mixing (HFWM) in three-beam configuration at liquid nitrogen temperature. In the experiments the photon energy of the 100 fs excitation pulses was set resonant (1.51 eV) and below (1.49 eV) the band-gap energy. At 1.51 eV photon energy the HFWM amplitude shows a rapid initial decay on a sub-ps time-scale indicating a fast relaxation of photo-excited EHPs to their band edges via carrier-carrier scattering. For longer delays the HFWM amplitude reveals a decay time of 500 ps which is attributed to the EHP lifetime. At 1.49 eV pulse energy the signal due to EHPs is significantly reduced and the HFWM amplitude shows an increase with maximum at 10 ps and subsequent decay with a time constant of more than 20 ns. The observed signal is attributed to the excitation of background carriers with subsequent redistribution and capture at impurities with long residence time. Photon echo experiments of resonantly excited EHPs reveal a dephasing time in the order of 100 fs. We attribute this fast dephasing to carrier scattering with a high carrier background provided by the impurities in these NWs. [Preview Abstract] |
Monday, March 2, 2015 4:06PM - 4:18PM |
D13.00009: Quantum-confined Stark shifts of quantum-dot like states in GaAs/AlGaAs core multi-shell nanowires Teng Shi, Bekele Badada, Howard Jackson, Leigh Smith, Changlin Zheng, Joanne Etheridge, Nian Jiang, Qiang Gao, Hoe Tan, Chennupati Jagadish A 4nm GaAs quantum well tube sandwiched by AlGaAs barriers is formed surrounding a central 50nm GaAs core. The GaAs/AlGaAs core multi-shell nanowires were grown by MOCVD. Single nanowire devices were fabricated through photolithography followed by deposition of Ti/Al metal contacts. We observed photoluminescence (PL) emission with multiple sharp peaks on a single nanowire device at 10K. We attribute these quantum-dot (QD) like states to well width and alloy fluctuations. We apply a bias across the device to investigate the quenching of PL due to external field ionization of excitons and the Stark shifts in these QD like states. Integrated PL emission show quenching on the high energy sideat a lower bias voltage compared to the low energy side.. Quantum confined Stark shifts on individual QDs are observed in the range of couple hundreds of micro-eV, suggesting QD sizes varing from 7 to 15nm. We acknowledge the NSF through DMR-1105362, 1105121 and ECCS-1100489, and the Australian Research Council. [Preview Abstract] |
Monday, March 2, 2015 4:18PM - 4:30PM |
D13.00010: Probing the band-structures and carrier dynamics of single GaAsSb nanowire heterostructures Yuda Wang, Bekele Badada, Howard Jackson, Leigh Smith, Xiaoming Yuan, Philippe Caroff, Lan Fu, Hoe Tan, Chennupati Jagadish We present the band structure and carrier relaxation of MOVCD grown single GaAs1-xSbx using photocurrent (PC) spectroscopy and transient Rayleigh Scattering (TRS) spectroscopy techniques. The PC spectroscopy was performed on nanowire devices fabricated using e-beam lithography and deposition of Ti/Au as contacts. The devices show nearly Ohmic behavior and are photosensitive. PC spectra shows an onset of absorption at room temperature in agreement with reported values of bulk GaAs0.6Sb0.4. We also used low temperature (10K) transient Rayleigh scattering (TRS) spectroscopy to measure the band structure as well as carrier relaxation dynamics of individual GaAsSb (x=30\% and 40\%) nanowires with and without InP passivation layers. The band gaps extracted from the TRS experiments are consistent with both photoluminescence (PL) measurements and theoretical predictions. The InP passivated GaAsSb shows smaller Eg due to the tensile strain from InP on GaAsSb as well as longer lifetimes due to the surface passivation. The carrier density and temperature are extracted by a phenomenological fitting model based on band to band transition theory. We acknowledge the NSF through DMR-1105362, 1105121 and ECCS-1100489, and the Australian Research Council. [Preview Abstract] |
Monday, March 2, 2015 4:30PM - 4:42PM |
D13.00011: Superfluorescence from semiconductor quantum wells: magnetic field, temperature, and density dependence Kankan Cong, Ji-Hee Kim, G. Timothy Noe II, Stephen A. McGill, Yongrui Wang, Alexey A. Belyanin, Junichiro Kono In the phenomenon of superfluorescence (SF), a macroscopic polarization spontaneously builds up from an initially incoherent ensemble of excited dipoles and then cooperatively decays, producing giant pulses of coherent radiation. SF arising from electron-hole recombination has recently been observed in semiconductor quantum wells, but its observability conditions have not been fully understood. Here, by fully mapping out the magnetic field ($B)$, temperature ($T)$, and pump power ($P)$ dependence of SF intensity and linewidth, we have constructed a ``phase' diagram, showing the $B$-$T$-$P$ region in which SF is observable. In general, SF can be observed only at low enough temperatures, high enough magnetic fields, and high enough laser powers with characteristic threshold behaviors. For example, for the (11) inter-Landau-level transition, when $B=17.5T$ and $P=4mW$, SF can be observed only when $T < 105K$; at $B=17.5T$ and $T=4K$, SF can only be induced by excitation power $P > 0.05mW$. These results lay the foundation of our understanding of electron-hole SF and provide guidelines for our search for a Bardeen-Cooper-Schrieffer state of excitons. [Preview Abstract] |
Monday, March 2, 2015 4:42PM - 4:54PM |
D13.00012: Artificial graphene in nanopatterned GaAs Quantum Wells Sheng Wang, Diego Scarabelli, Antonio Levy, Loren Pfeiffer, Ken West, Vittorio Pellegrini, Michael J. Manfra, Shalom Wind, Aron Pinczuk Electrons in graphene have linear energy-momentum dispersion, making them massless Dirac fermions. An alternative way to achieve massless Dirac-fermions in a controlled and tunable manner is to construct a honeycomb lattice potential for a 2D electron gas in a semiconductor quantum well. We report realization of very short period (as small as 40 nm) honeycomb lattice pattern using e-beam lithography and drying etching on a GaAs quantum well and spectroscopy data of electron states under this potential modulation. The study is carried out using photoluminescence and light scattering at low temperature (about 4K). Inter mini-band transitions are observed by resonant inelastic light scattering and interpreted with calculated mini-band structure. Control over parameters such as Fermi level should permit manipulation of massless fermions. This will provide a platform for novel behavior such as topological states in a semiconductor quantum simulator. [Preview Abstract] |
Monday, March 2, 2015 4:54PM - 5:06PM |
D13.00013: Nanoscale light emission: direct bandgap versus indirect bandgap Jun-Wei Luo, Shu-Shen Li The electron-hole Coulomb interaction bounds an electron and a hole together forming an exciton and the e-h exchange interaction lifts the spin-allowed bright exciton state up with respect to the spin-forbidden dark state by an energy of ~1 meV to tens meV depending on the NCs size. The recombination dynamics of the NC exciton remains open to debate, especially at high temperatures (T > 20 K). One perception is a weak exchange interaction of dark excitons with the ensemble of dangling bonds on the NC surface, resulting in spin flip assisted recombination directly from the dark state. Another perception is thermal redistribution of excitons between the dark and bright states. By performing atomistic pseudopotential calculations of indirect bandgap Si NCs and direct bandgap InAs NCs in a large range of NC sizes, we found that the predicted recombination rates of NC excitons are in excellent agreement with experimental data provided by various groups. This agreement confirms the explanation of thermal activation of bright state of excition recombination dynamics in NCs. We also found that the exciton recombination rates, as function of confinement energy, of indirect bandgap NCs is distinct from direct bandgap NCs. More detail theoretical analyses will be presented. [Preview Abstract] |
Monday, March 2, 2015 5:06PM - 5:18PM |
D13.00014: White and Red Light Photoluminescence of ZnS:Eu$^{3+}$ - CMC Nanophosphors Ahemen Ikorkya, Dilip De, Osita Meludu, V. Bruno White and red photoluminescence based on europium-doped zinc sulfide nanocrystals capped with sodium carboxymethyl cellulose (ZnS: Eu$^{3+}$ - CMC) was synthesized using precipitation technique with Eu$^{3+}$ ions doping concentrations of 1 mol{\%} and 5 mol{\%}. Some portions of the doped samples were annealed at 300 $^{\circ}$C in a sulfur-rich atmosphere. All samples show cubic (zinc blende) structure with crystal sizes; 2.56 nm and 2.91 nm, for the as-synthesized samples, 4.35 nm and 3.65 nm for thermally treated samples, respectively. The as-synthesized samples have equal energy band gap of 4.2 eV, but decreased to 3.76 eV and 3.81 eV after heat treatment. Photoluminescence studies indicate defect emission bands and Eu$^{3+}$ ion lines for the as-synthesized samples. The as-synthesized samples gave pure orange-red emission when excited at wavelength of 394 nm and 465 nm. After thermal annealing of the samples, a broad emission band in the blue-green region assigned to defect related states emerged or were enhanced. Also enhanced were the emission lines of Eu$^{3+}$ ions in the orange-red region. A combination of these two transitions gave white light of different shades depending on Eu concentration or excitation wavelength. Different shades of white light from cool white through Day-light to warm white light were recorded on the CIE 1931 chromaticity diagram. The source excitation wavelengths range from UV-330 nm through near UV -- 396 nm to blue - 465 nm wavelengths which are in the range of InGaN --based LEDs emissions. [Preview Abstract] |
Monday, March 2, 2015 5:18PM - 5:30PM |
D13.00015: The Influence of Impurity Doping On the Phase Transition and Morphology Tuning of Sr/CaFCl: Yb3$+$,Er3$+$ Nanoparticles Yue Cui, Suling Zhao, Wenxiao Huang, Zheng Xu, Yuan Li, David Carroll Upconversion nanoparticles (UCNPs) are well-known for their unique luminescent properties that enable the conversion of low-energy photons into high-energy photons by multiphoton processes. In this work, Sr/CaFCl: Yb3$+$,Er3$+$ NCs with a wide range of ions dopant concentrations were synthesized, and strong green and red upconversion fluorescence were observed under laser excitation at a wavelength of 980 nm. The influence and mechanism of ions dopant are demonstrated and discussed. The ions doped concentration has a significant influence on the phase-transfer of the host material and on the corresponding upconversion emissions, and the mechanism of which was studied. In addition, the optimized concentration represents a good balance between the occurrence of the phase transition and concentration quenching. These high-efficiency nanoparticles have potential applications in the fields of optical nanodevices and biomedicine. [Preview Abstract] |
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