Bulletin of the American Physical Society
APS March Meeting 2016
Volume 61, Number 2
Monday–Friday, March 14–18, 2016; Baltimore, Maryland
Session F24: Electronic and Optical Properties of Nanoparticle AssembliesFocus
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Sponsoring Units: DMP Room: 323 |
Tuesday, March 15, 2016 11:15AM - 11:51AM |
F24.00001: Theory of light scattering at nanoparticles and optical forces between small particles Invited Speaker: Juan Jose Saenz Appropriate combinations of laser beams can be used to trap and manipulate small particles with “optical tweezers” as well as to induce significant “optical binding” forces between particles. Here we review some basic concepts related to the optical forces on small (subwavelength) particles, focusing on the interplay between scattering asymmetry and momentum transfer. These forces are, in general, non-conservative (curl forces) which lead to a number of intriguing predictions regarding the dynamics of nanoparticles. Optical forces between small particles are usually strongly anisotropic depending on the interference landscape of the external fields. This is in contrast with the familiar isotropic van der Waals and, in general, Casimir-Lifshitz interactions between neutral bodies arising from random electromagnetic waves generated by equilibrium quantum and thermal fluctuations. As we will see, artificially created random fluctuating light fields can be used to induce and control dispersion forces between small colloidal particles. Interestingly, for relatively high refractive index semiconductor nanoparticles, the interactions can be tuned from attractive to strongly repulsive when the frequency of the external fluctuating field is tuned near the first magnetic Mie-resonance. Interactions induced by randomly fluctuating light fields open a path towards the control of translational invariant interactions with tuneable strength and range in colloidal systems. [Preview Abstract] |
Tuesday, March 15, 2016 11:51AM - 12:03PM |
F24.00002: Redox-driven conductance modulation of a single quantum dot in an electrolytic environment Giacomo Lovat, Boyeon Choi, Xavier Roy, Latha Venkataraman Electrons confined in zero-dimensional systems exhibit shape and size-dependent electronic and optical properties of interest for many technological applications. A realization of molecular-scale quantum dots having precise shape and size is provided by the synthesis of atomically defined isostructural metal chalcogenide clusters functionalized with organic connectors, which opens the possibility of wiring up these dots without altering significantly their electronic structure. Here, we characterize the charge transport in single molecule junctions fabricated with Co$_{6}$Se$_{8}$ clusters via the scanning tunneling microscope break junction technique. The cluster structure consists of an octahedron of Co atoms concentric with a cube of Se atoms; the electrical connection to the Au leads is provided by aurophilic thiol-terminated ligands attached at the Co sites. We demonstrate that conductance modulation in a cluster junction can be achieved by controlling the charge state of the cluster. The conductance of the oxidized species differs from that of the neutral ones, consistent with the value obtained in a control experiment with chemically oxidized clusters. [Preview Abstract] |
Tuesday, March 15, 2016 12:03PM - 12:15PM |
F24.00003: First Principles Simulations of Nanoparticle Solids Arin Greenwood, M\'arton V\"or\"os, Giulia Galli Nanoparticle solids are gaining popularity as materials for optoelectronic devices such as solar cells [1, 2]. However, there is still much debate regarding the transport regime governing the charge carriers. To date, no comprehensive description of transport mechanisms in nanoparticle solids has been established, and there is a lack of computational studies predicting electron mobilities and transport rates at the \textit{ab initio} level. In order to understand electron transport properties, it is an essential prerequisite to build realistic structural models of nanoparticle solids to use for prediction of electronic structure and eventually transport properties. Here we present Ab Initio Molecular Dynamics simulations of lead chalcogenide nanoparticles and surrounding ligands to extract relevant electronic structure properties for charge transport calculations. We tested the validity of recently observed "band-like" transport [3] by assessing the formation of bands and their dependence on nanoparticle surface structure and ligands. \newline [1] Crisp, Ryan et al. Scientific Reports, 2014, 5: 9945. [2] Ning, Zhijun et al. Nature Materials, 2014, 13, pp 822-828. [3] Choi, Ji-Hyuk et al. Nano Letters, 2012, 12(5), pp 2631-2638. [Preview Abstract] |
Tuesday, March 15, 2016 12:15PM - 12:27PM |
F24.00004: Molecularly-linked gold nanoparticle films across the insulator-to-metal transition: from hopping to strong electron correlations Monique Tie, Al-Amin Dhirani Materials which have strong electron-electron interactions are known to display a wide variety of exotic behaviours. We have found that molecularly-linked gold nanoparticle films represent a new class of materials that exhibit correlated electronic behaviours. Most notably, (a) they undergo a percolation insulator-to-metal transition as a function of film thickness, (b) as the system transitions from an insulator to a metal, a previously unobserved zero-bias conductance peak emerges, attributed to electron correlations, and (c) Coulomb effects play an important role in the conductance on both the insulating and metallic sides near the transition. On the insulating side near the transition, we observe hopping transport with significant Coulomb charging barriers (Efros-Shklovskii variable range hopping). On the metallic side, we have found that conductance behaves as a Fermi liquid with disorder mediated electron-electron interactions. Remarkably, in this barely-metallic phase, we have found elastic scattering lengths smaller than inter-atomic Au-Au separation, violating the Ioffe-Regel limit and signalling strong electron-electron interactions. These results show that gold nanoparticle films can serve as a new test bed for studying correlated electrons and illustrate the promis [Preview Abstract] |
Tuesday, March 15, 2016 12:27PM - 12:39PM |
F24.00005: Towards the Ultimate Limit of Connectivity in Quantum Dots with High Mobility and Clean Gaps Huashan Li, David Zhitomirsky, Shreya Dave, Jeffrey Grossman Colloidal quantum dots (CQDs) are especially promising for commercial electronic and optoelectronic applications, yet there is a considerable lack of fundamental understanding of their electronic structure as they couple within thin films. In this work, we applied a combination of computational and experimental techniques to gain insight into the impact of connectivity in CQD assemblies. High Resolution Transmission Electron Microscopy demonstrates that a range of connectivity between dots in the film is attainable by tuning the CQD size and ligand treatment. These results were complemented by ab-initio simulations within the phonon-assisted charge hopping scenario. We find that both the orbital hybridization and interfacial dipole moment can change the electronic structure substantially; thus, control over the interface structure beyond stoichiometry is necessary to eliminate trap states. In addition, carrier mobility has a strong dependence on the type of connectivity (i.e., bridge vs. necking), the connectivity orientation, carrier energy, and defect states. Based on our calculations, we propose a scheme for improved carrier mobility, by necking the dots for the advantage of large electron coupling, followed by excess I ligand passivation to recover the wavefunction delocalization. [Preview Abstract] |
Tuesday, March 15, 2016 12:39PM - 12:51PM |
F24.00006: Metal-Insulator Transition in nanoparticle solids: a kinetic Monte Carlo study Gergely Zimanyi, Luman Qu, Marton Voros Nanoparticle (NP) solids recently emerged as a promising platform for high performance electronic/optoelectronic devices, including third generation solar cells, light emitting diodes and field effect transistors. A challenge of NP films is that their charge transport is in the unfavorable hopping/insulating regime. Recent experiments showed that it is possible to tune the NP solids through a Metal-Insulator Transition (MIT) via ligand engineering and ALD matrix infilling. However, the microscopic understanding of this transition is not yet clear. To address this challenge, we developed a Kinetic Monte Carlo transport modeling framework that builds on determining NP parameters from ab initio-based calculations of the energy level structures, charging energies and overlaps, and then uses these to compute the hopping mobility across a disordered NP array by the Marcus and Miller-Abrahams mechanisms. We reproduced and explained the observed non-monotonous dependence of the mobility on the NP diameter. Centrally, we extended our platform to be able to capture the MIT. We determined the MIT phase boundary on the (NP-NP overlap - Electron density) plane. We demonstrated that all mobilities fall on a universal scaling curve, allowing us to determine the critical behavior across the MIT. [Preview Abstract] |
Tuesday, March 15, 2016 12:51PM - 1:03PM |
F24.00007: The effect of oxidation on charge carrier motion in PbS quantum dot thin films studied with Kelvin Probe Microscopy Lan Phuong Nguyen Hoang, Pheona Williams, Jason Moscatello, Kathy Aidala We developed a technique that uses scanning probe microscopy (SPM) to study the real-time injection and extraction of charge carriers in thin film devices. We investigate the effects of oxidation on thin films of Lead Sulfide (PbS) quantum dots with tetrabutyl-ammonium-iodide (TBAI) ligands in an inverted field effect transistor geometry with gold electrodes. By positioning the SPM tip at an individual location and using Kelvin Probe Force Microscopy (KPFM) to measure the potential over time, we can record how the charge carriers respond to changing the backgate voltage with grounded source and drain electrodes. We see relatively fast screening for negative backgate voltages because holes are quickly injected into the PbS film. The screening is slower for positive gate voltages, because some of these holes are trapped and therefore less mobile. We probe these trapped holes by applying different gate voltages and recording the change in potential at the surface. There are mixed reports about the effect of air exposure on thin films of PbS quantum dots, with initial exposure appearing to be beneficial to device characteristics. We study the change in current, mobility, and charge injection and extraction as measured by KPFM over hours and days of exposure to air. [Preview Abstract] |
Tuesday, March 15, 2016 1:03PM - 1:15PM |
F24.00008: Effects of electron-phonon interactions on the electron tunneling spectrum of PbS quantum dots A. Zimmers, H. Wang, E. Lhuillier, Q. Yu, A. Mottaghizadeh, C. Ulysse, A. Descamps-Mandine, B. Dubertret, H. Aubin We present a tunnel spectroscopy study of single PbS and HgSe quantum dots (QDs) as a function of temperature and gate voltage. The samples are fabricated through high-vacuum projection of the QDs on the chip circuit. For PbS, three distinct signatures of strong electron-phonon coupling are observed in the electron tunneling spectrum (ETS) of these QDs. In the shell-filling regime, the 8x degeneracy of the electronic levels is lifted by the Coulomb interactions and allows the observation of phonon subbands that result from the emission of optical phonons. At low bias, a gap is observed in the ETS that cannot be closed with the gate voltage, which is a distinguishing feature of the Franck-Condon blockade. From the data, a Huang-Rhys factor in the range S similar to 1.7-2.5 is obtained. Finally, in the shell-tunneling regime, the optical phonons appear in the inelastic ETS d(2)I/dV(2). For HgSe, the data show that the inter-band and intra-band gap can be clearly identified in the ETS. [Preview Abstract] |
Tuesday, March 15, 2016 1:15PM - 1:27PM |
F24.00009: Enhancement of pumped current in quantum dots. Juan Pablo Ramos, Luis Foa, Victor Marcelo Apel, Pedro Orellana A direct current usually requires the application of a non-zero potential difference between source and drain, but on nanoscale systems (NSS) it is possible to obtain a non-zero current while the potential difference is zero. The effect is known as quantum charge pumping (QCP) and it is due to the interference provided by the existence of a time-dependent potential (TDP). QCP can be generated by a TDP in non-adiabatic limit. An example of this is a system composed by a ring with a dot embedded on it, under the application of an oscillating TDP. By the action of a magnetic field across the system, a pumped current is generated, since time reversal symmetry is broken. Decoherence is crucial, both from a scientific and technological point of view. In NSS it is expected that decoherence, among others things, decreases the QCP amplitude. In this context, we study what is the effect of a bath on the pumped current in our system. We find that for certain values of magnetic flux, the bath-system produce amplification of the pumped current. [1] J. P. Ramos \textit{et al.} J. of appl. Phys. \textbf{115}, 124507, (2014). [2] M. Moskalets \textit{et al.} Phys. Rev. B \textbf{64}, 201305, (2001). [Preview Abstract] |
Tuesday, March 15, 2016 1:27PM - 1:39PM |
F24.00010: Strong coupling effects in coherent electron transport in periodic quantum nanostructures T. V. Shahbazyan, L. S. Petrosyan We study coherent transport in a system of periodic linear chain of quantum dots placed between two parallel quantum wires. We show that resonant-tunneling conductance between the wires exhibits Rabi splitting of the resonance peak as a function of Fermi energy in the wires. This effect is an electron transport analogue of the Rabi splitting in optical spectra of two interacting systems. The conductance peak splitting originates from anticrossing of Bloch bands in a periodic system caused by strong coupling between electron states in the quantum dot chain and quantum wires. A perpendicular magnetic field, by breaking the system left-right symmetry, gives rise to multiple Bloch band anticrossings leading to the appearance of a fine structure in the conductance lineshape. [Preview Abstract] |
Tuesday, March 15, 2016 1:39PM - 1:51PM |
F24.00011: How many electrons make a semiconductor nanocrystal film metallic Konstantin Reich, Ting Chen, Nicolaas Kramer, Han Fu, Uwe Kortshagen, Boris Shklovskii For films of semiconductor nanocrystals to achieve their potential as novel, low-cost electronic materials, a better understanding of their doping to tune their conductivity is required. So far, it not known how many dopants will turn a nanocrystal film from semiconducting to metallic. In bulk semiconductors, the critical concentration $n_M$ of electrons at the metal-insulator transition is described by the famous Mott criterion: $n_M a_B^3 \simeq 0.02$, where $a_B$ is the effective Bohr radius. We show theoretically that in a dense NC film, where NCs touch each other by small facets, the concentration of electrons $n_c \gg n_M$ at the metal-insulator transition satisfies the condition: $n_c \rho^3\simeq 0.3$, where $\rho$ is a radius of contact facets. In the accompanying experiments, we investigate the conduction mechanism in films of phosphorus-doped, ligand-free silicon nanocrystals. At the largest electron concentration achieved in our samples, which is half the predicted $n_c$, we find that the localization length of hopping electrons is close to three times the nanocrystals diameter, indicating that the film approaches the metal-insulator transition. [Preview Abstract] |
Tuesday, March 15, 2016 1:51PM - 2:03PM |
F24.00012: \textbf{Anomalous hopping conduction in nanocrystalline/amorphous composites and amorphous semiconductor thin films} James Kakalios, Kent Bodurtha Composite nanostructured materials consisting of nanocrystals (nc) embedded within a thin film amorphous matrix can exhibit novel opto-electronic properties. Composite films are synthesized in a dual-chamber co-deposition PECVD system capable of producing nanocrystals of material A and embedding then within a thin film matrix of material B. Electronic conduction in composite thin films of hydrogenated amorphous silicon (a-Si:H) containing nc-germanium or nc-silicon inclusions, as well as in undoped a-Si:H, does not follow an Arrhenius temperature dependence, but rather is better described by an anomalous hopping expression (exp[-(To/T)$^{\mathrm{3/4}})$, as determined from the ``reduced activation energy'' proposed by Zabrodskii and Shlimak. This temperature dependence has been observed in other thin film resistive materials, such as ultra-thin disordered films of Ag, Bi, Pb and Pd; carbon-black polymer composites; and weakly coupled Au and ZnO quantum dot arrays. There is presently no accepted theoretical understanding of this expression. The concept of a mobility edge, accepted for over four decades, appears to not be necessary to account for charge transport in amorphous semiconductors. [Preview Abstract] |
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