Bulletin of the American Physical Society
APS March Meeting 2018
Volume 63, Number 1
Monday–Friday, March 5–9, 2018; Los Angeles, California
Session S18: Quantum Dots and Nanocrystals: Structural and Optical Properties |
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Sponsoring Units: DCMP DMP Chair: Patrick Vora, George Mason University Room: LACC 306B |
Thursday, March 8, 2018 11:15AM - 11:27AM |
S18.00001: Tensile-Strained Germanium Quantum Dots on (111) Surfaces Kathryn Sautter, Christopher Schuck, Paul Simmonds Si and Ge are ubiquitous in electronics, but their indirect bandgaps make them unsuitable for optoelectronic devices. Theory shows that placing Ge under tensile strain reduces its semiconductor bandgap by reducing the Γ-valley in Ge’s conduction band faster than the L-valley. Once at ~2% tensile strain, Ge should acquire a direct bandgap. Researchers have therefore tried various ingenious methods to create tensile strain in Ge, but these attempts typically generate strain-induced defects and do not result in viable optoelectronic materials. Our approach to this problem is to synthesize Ge quantum dots (QDs) that self-assemble as a result of biaxial tensile strains on (111) surfaces. We have previously developed a method to grow defect-free GaAs(111) QDs at ~4% tensile strain with molecular beam epitaxy (MBE). Since GaAs and Ge have similar lattice constants, we simply replace GaAs with Ge in these structures. Initial data suggest spontaneous formation of Ge QDs under 3.7% tensile strain, leading to optically active Ge with a reduced bandgap. We will present results demonstrating control of the structural and optoelectronic properties of tensile-strained Ge QDs with MBE parameters. |
Thursday, March 8, 2018 11:27AM - 11:39AM |
S18.00002: Manipulation of the excitonic properties of GaAs quantum dots via strain fields Fritz Weyhausen-Brinkmann, Jun-Wei Luo, Alex Zunger, Armando Rastelli, Xueyong Yuan, Gabriel Bester Epitaxial semiconductor quantum dots like InAs/GaAs or GaAs/AlGaAs are excellent quantum emitters and because of their field-tunable properties they can provide key components of quantum information devices, such as quantum repeaters. A solid understanding of the effect of applied external fields on the exciton states is crucial for the optimized operation of devices. Based on atomistic million-atom empirical pseudopotential calculations we show the effect of biaxial and uniaxial external strain fields on the exciton levels and relate them to the change of the hole states. It turns out, that uniaxial strain can change the natural hole quantization axis, while biaxial strain leads to an interesting heavy-hole light-hole transition. This transition comes along with a non-trivial spin-mixing and optical polarization anisotropies. Both effects may be useful for new technical devices in quantum information networks. |
Thursday, March 8, 2018 11:39AM - 11:51AM |
S18.00003: Strain-controlled Quantum Dots in Atomically-Thin Semiconductors Leo Yu, Sven Borghardt, Jingyuan Linda Zhang, Jelena Vuckovic, Tony Heinz The creation of laterally confined 0D structures in 2D van-der-Waals semiconductor layers opens many new possibilities for control of electronic properties and the localization of excitations. One approach to this challenge is to grow small islands of 2D materials. But this scheme is difficult to achieve site controllability and also creates edge states that must be passivated. Alternatively achieving lateral confinement, we present a novel method to produce a highly localized strain field in the layer, and describe the resulting change in the local optical band gap of the material: We first suspend a monolayer over a nanoscale cavity in the substrate, then deform the layer using high pressure gas. The strain profile is frozen in and maintained when the monolayer adheres to the surface of the cavity. Applying our method to a monolayer WSe2, we demonstrate controlled biaxial strain up to 2.8%, localized within a 250-nm width and produced at various defined positions on chip. The resulting reduction in the local bandgap is distributed symmetrically in space, yielding maximal red shifts in photoluminescence energy up to 200 meV, observable both under ambient conditions and at low temperature. |
Thursday, March 8, 2018 11:51AM - 12:03PM |
S18.00004: Linewidth Broadening of Coupled Quantum Dot Pairs Parveen Kumar, Cameron Jennings, Cyprian Czarnocki, Joshua Casara, Andrew Jacobs, Allan Bracker, Brennan Pursley, Daniel Gammon, Sophia Economou, Samuel Carter, Michael Scheibner Solid state atoms are promising systems for emerging quantum technologies. The harnessing of coherences in these systems offers a route to quantum-enhanced devices for sensing, information processing and communications. Crucial for the realization of such technology is the understanding of the dephasing, relaxation and fluctuation processes affecting quantum states in the solid state environment in order to mitigate, eliminate and utilize these processes. In the optical spectra these phenomena are prominent via the linewidth broadening and line profile of optical transitions. We perform a detailed analysis of linewidth broadening of spatially direct, indirect excitons and examine the linewidth as function of electric field including the tunneling resonances. The study yields new insights on the broadening mechanisms for different charge and spin configurations in coupled quantum dots systems as a function of temperature and tunnel coupling strength. |
Thursday, March 8, 2018 12:03PM - 12:15PM |
S18.00005: Optophononic Polarization Rotation in Coupled Quantum Dots Andrew Jacobs, Joshua Casara, Cameron Jennings, Parveen Kumar, Michael Scheibner We theoretically study the use of coherent behavior of phonons in coupled quantum dots. A coherent Fano-type quantum interference between a single-dot polaron state and a spatially indirect exciton gives rise to a spectrally asymmetric optical response of the coupled dot system and a phonon-induced optical transparency [1]. Optical selection rules dictate which exciton spin state participates in this Fano-type interference. Consequently, the optical response of the optically driven system to a weak probe is polarization dependent, with a Fano- or a Lorentzian-shape spectral profile for orthogonal polarizations. These two spectral configurations have different refractive indices, and result in a situation similar to the magneto-optic Kerr or Faraday effects. We theoretically demonstrate an optophononic polarization due to the Fano-type interference induced by the coherent phonon behavior. We extend the scope of the treatment to a wide range of Fano asymmetries. We calculate the asymmetric spectrum using a fully quantum mechanical, master equation formalism that accounts for both weak- and strong-coupling regimes of the exciton with the lattice vibrations of the dots. |
Thursday, March 8, 2018 12:15PM - 12:27PM |
S18.00006: Phonon Decoherence of Quantum Dots in Photonic Structures: Fundamental Limits to Photon Indistinguishability Petru Tighineanu, Chris Dreeßen, Christian Flindt, Peter Lodahl, Anders Sørensen There has been outstanding progress towards devising a deterministic interface between stationary and flying qubits with semiconductor quantum dots (QDs). Exploiting this interface at the ultimate quantum level requires a profound understanding and precise control of the sources of noise that decohere the QDs. Here we present the first unifying description of the interaction between QDs and phonons in photonic structures with a particular emphasis on the attainable indistinguishability of the emitted photons [1]. We show that the coherence depends critically on the geometric dimensionality of the structure for QDs embedded in a photonic sphere (0D), waveguide (1D), membrane (2D), or bulk medium (3D). In bulk, the dephasing rate scales drastically with temperature as T11 and is negligible at low temperatures. In contrast, the stepwise phonon density of states of 1D structures leads to a linear temperature scaling and low photon indistinguishability even at sub-Kelvin temperatures. |
Thursday, March 8, 2018 12:27PM - 12:39PM |
S18.00007: Laterally Gated Quantum Ring as a Tunable Double Quantum Well Thomas Collier, Vasil Saroka, Mikhail Portnoi We theoretically investigate the optical functionality of a semiconducting quantum ring manipulated by two electrostatic lateral gates used to induce a double quantum well along the ring. The well parameters and corresponding inter-level spacings, which lie in the THz range, are highly sensitive to the gate voltages. Our analysis shows that selection rules for inter-level dipole transitions, caused by linearly polarized excitations, depend on the polarization vector angle with respect to the gates. In striking difference from the conventional symmetric double well potential, the ring geometry permits polarization-dependent transitions between the ground and second excited states, allowing the use of this structure in a three-level lasing scheme. |
Thursday, March 8, 2018 12:39PM - 12:51PM |
S18.00008: Characterizing buried nano-interfaces in nanocrystal solids at the atomistic level: a coupled theoretical-experimental approach Emilio Scalise, Giulia Galli, Dmitri Talapin, Stefan Wippermann The properties of nanomaterials, e.g. nanocrystal (NC)-solids, are dominated by their large surface to volume ratio. Detailed knowledge of the nanoscale interfacial chemistry is required to enable rational device design. By combining electronic structure calculations of surfaces with experiments and first principles molecular dynamics, we investigate fully inorganic NC-solids obtained by chemical solution processing of InAs NCs capped with Sn2S6 ligands. Contrary to organic ligands, which are adsorbed as intact structural units on the NC surface, we find inorganic ligands to dissociate on the NC surface, forming first a surface passivation layer and then a matrix around the NCs with complex structural details depending on the synthesis conditions. Thus the ligands and the matrix form an integral part of the NC-solid, with the NCs not having the same identity as when they are isolated or capped by organic ligands. We also find important atomistic details, such as a sulfur subsurface layer formed below the NC surface and a sulfur chain network inside the matrix, which determine the composite electronic properties and are expected to at least partially explain current experimental observations in InAs nanocrystal solids formed from sulfide-based inorganic ligands. |
Thursday, March 8, 2018 12:51PM - 1:03PM |
S18.00009: Surface-bound states and zero-point motion effect in nanodiamonds and diamondoids Gabriel Bester, Peng Han, J. Wrachtrup, Denis Antonov The positive electron affinity of bulk diamond becomes negative for hydrogen passivated nanodiamonds and leads to a type-II offset with a vacuum level at nearly midgap. We use ab initio density functional theory and a screened configuration interaction approach to show [1] that in this case and with three further conditions: (i) a surface dipole |
Thursday, March 8, 2018 1:03PM - 1:15PM |
S18.00010: Atomistic theory of nanostructures: beyond 10-million atoms in simulation Michal Zielinski Optical and electronic properties of novel nanostructures arise from atomic scale contributions related to alloy randomness, substrate orientation or thin monolayer-thick interfaces. All these effects must be accounted for by an accurate, atomistic approach. However, computations for realistic size nanostructures involve number of atoms going above 1-million atoms for nanowire quantum dots, exceeding 10-million atoms for crystal phase quantum dots and may reach 100-millon for future silicon nanodevices doped with chains of individual phosphorus atoms. In this presentation I present a step-by-step solution to this problem: empirical tight-binding and exact diagonalization scheme that unites linearly scaling computational time with the essentials of the atomistic modelling. I illustrate our method on the example of self-assembled quantum dot, with the emphasis on the dark exciton spectra. Next, I show that alloy randomness alone is sufficient to trigger substantial excitonic fine structure even in cylindrical nanowire quantum dots. |
Thursday, March 8, 2018 1:15PM - 1:27PM |
S18.00011: The Electronic Properties of 30nm Meta-lattice Made by High Pressure CVD ZhaoHui Huang, Vincent Crespi A nanoscale 3D superlattice, called meta-lattice, can be synthesized by infiltrating a template of close-packed nanometer-scale silica spheres with Si or Ge by high pressure chemical vapor deposition. Their structures can be controlled by using different size spheres; accordingly, their electronic properties are geometry-dependent, hence they offer a platform to create systems with both quantum confinement and extended electronic states in three dimensions. We employ a tight binding method to calculate the electronic structure of a silicon meta-lattice containing more than 68,000 atoms in the unit cell formed from a 30-nm diameter sphere template. Our structural model is designed to minimize the surface free energy while respecting the template geometry. The Implicitly Restarting Arnoldi Method is implemented with a MPI code to solve the large Hamiltonian. The resulting electronic structure is compared to experimental spatially resolved EELS measurements showing location-dependent quantum confinement. |
Thursday, March 8, 2018 1:27PM - 1:39PM |
S18.00012: The origins of Stokes shift in PbS nanocrystals Yun Liu, Donghum Kim, Owen Morris, David Zhitomirsky, Jeffrey Grossman The Stokes shifts in lead chalcogenides family of QDs are anomalously large and its origin has not been fully resolved. In this work we show, using ab initio calculations that the presence of intrinsic defects can cause substantial electron density localization of the bandedge states, causing excessive Franck-Condon (FC) shifts. In addition, the energetics and size disorders of a polydisperse QD film were found to contribute a 20 to 50 meV increase in Stokes shift compared to isolated QDs. FC shifts increase as the electronegativities of the of passivating ligands increase. These effects are small, and mainly due to the stronger bindings between surface Pb atoms and the ligands, resulting in smaller effective QD sizes. Our findings are significant to improving the understanding of the optical properties of PbS QDs and are important to designing future optoelectronics and photovoltaics devices. |
Thursday, March 8, 2018 1:39PM - 1:51PM |
S18.00013: Signature of Lasing and Polarized Emission from CsPbBr3 Quantum Dots Shailaja Mahamuni, Richa Gahlaut, Aparna Shinde A comparative study of quantum confined cesium lead halide pervoskite (CsPbBr3) nanocrystals is carried out. Quantum dots having size 5.5 nm reveal stimulated emission at low temperature for excitation fluence as low as ~4.16 µW (Xe lamp excitation). Moreover, about 48% linear polarized emission is also observed in the colloidal solution besides 95 % photoluminescence (PL) quantum yield at room temperature. Even though, CsPbBr3 quantum dots reveal the red shift in band gap at low temperature, similar to the single crystal, the exciton-phonon interaction is profoundly affected by the quantum size effects. Temperature dependent optical studies reveal an anomalous decrease in exciton-LO phonon coupling in small sized quantum dots aside from expected higher exciton binding energy. Observed stimulated emission in low sized CsPbBr3 quantum dots has implications in realizing quantum dot based laser. |
Thursday, March 8, 2018 1:51PM - 2:03PM |
S18.00014: Optical Orientation and Alignment of Excitons in Ensembles of Inorganic Perovskite Nanocrystals Mikhail Nestoklon, Serguei Goupalov, Roslan Dzhioev, Olga Ken, Vladimir Korenev, Yuri Kusrayev, Victor Sapega, Chris de Weerd, Leyre Gomez, Tom Gregorkiewicz, Junhao Lin, Kazu Suenaga, Yasufumi Fujiwara, Lev Matyushkin, Irina Yassievich We demonstrate the optical orientation and alignment of excitons in a two-dimensional layer of CsPbI3 perovskite nanocrystals prepared by colloidal synthesis, and measure the anisotropic exchange splitting of exciton levels in the nanocrystals. From the experimental data at low temperature (2 K), we obtain the average value of anisotropic splitting of bright exciton states of the order of 80 microeV. Our calculations demonstrate that there is a significant contribution to the splitting due to the nanocrystal shape anisotropy for all inorganic perovskite nanocrystrals. |
Thursday, March 8, 2018 2:03PM - 2:15PM |
S18.00015: Tunable Emission from Organolead Bromide Perovskite Quantum Dots for White Light-Emitting Diodes Gopi Adhikari, Preston Vargas, Hongyang Zhu, Peifen Zhu Perovskites are very promising materials for use in optoelectronic devices due to their ability to tune through the visible colors with high quantum efficiency. Chloride based perovskites are most often used to get blue emission. However, working with Chlorine is unnecessarily difficult. To date, there is no significant literature discussing an effective method of obtaining blue emission from Organolead bromide perovskite (MAPbBr3, MA-CH3NH3) quantum dots via increasing the total amount of ligands used. This would make the use of Chlorine unnecessary. Ligand variation has the effect of controlling the crystal growth to a greater degree than previous works have, allowing for very specifically changing the band gap from the standard green to the mid-violet range. Here we demonstrate the Bromide-based quantum dot’s tunability throughout the violet-green color range with high color purity and narrow line-width emission. This narrow emission is accomplished by a facile solution process for MAPbBr3 quantum dots via changing the amount of ligands used in a room temperature synthesis. |
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