Charge and Energy Transport in Nanocrystal Assemblies (CETNA) 2017

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    Symmetry breaking induced activation of the nanocrystal photoluminescence
    (2017-05) Efros, Alexander
    We have shown that the descent of the nanocrystal symmetry from spherical to point group Cs, which is characterized by just one mirror plane symmetry element, leads step by step to activation of all five F =2, Fz= 2,1, 0 excitons. Even the ground exciton becomes optically active, which should be observable in low-temperature photoluminescence measurements. For several intermediate symmetries the band edge exciton fine structure consists of sets of three linearly polarized mutually orthogonal dipoles plus a dark exciton, one of which is always the ground state. We quantify the effect of symmetry descent on the exciton fine structure by introducing a charged Coulomb impurity in the nanocrystals. The calculations show that the nanocrystal symmetry breaking by a Coulomb impurity, particularly a positively charged center, shortens the radiative decay of nanocrystals even at room temperatures in qualitative agreement with the increase in PL efficiency observed in nanocrystals doped with positive Ag charge centers.
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    Role of Surface Chemistry on Charge Carrier Transport in Quantum Dot Solids
    (2017-05) Kagan, Cherie
    Colloidal semiconductor quantum dots (QDs) are promising materials for electronic and optoelectronic devices due to their size tunable electronic and optical properties and the solution-based processes that enable the integration of these materials into devices. However, the long, insulating ligands commonly employed in the synthesis of colloidal QDs inhibit strong interparticle coupling and charge transport once QDs are assembled into the solid state as QD arrays. A general approach to increase carrier mobility is to reduce the interparticle spacing by ligand exchange. During solution-based deposition and ligand exchange of QD thin films, the QD surfaces are often “attacked” by solvents or ligands, creating surface defect sites. These surface defects generate in-gap states that may scatter mobile carriers and reduce the lifetime of photogenerated carriers by trapping. In this talk, I will describe methods to synthetically control and spectroscopically probe the density and occupancy of defect states at the QD surface and at QD-device interfaces and their importance to creating high mobility and long lifetime QD materials for electronic and optoelectronic devices.
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    Charge and energy transport in films of touching nanocrystals
    (2017-05) Reich, Kostya
    This talk deals with films of nanocrystals (NC), which touch each other by small facets with radius ρ. First I calculate the matrix element for electron tunneling from one NC to another and show that it is proportional to ρ3. I use this matrix element to calculate two transport properties of NC films: conductivity and exciton diffusion length. In the first case I focus on the critical concentration of electrons for the insulator-metal transition (IMT) in the film. The famous Mott's criterion answers this question only in bulk materials. The same critical concentration as in a bulk material is not sufficient for the IMT in NC films because of the weak coupling between NCs. For IMT in NC films, one needs much larger concentration at which the typical electron wave packet becomes smaller than the facet radius ρ and can pass through the facet. The predicted critical concentration is proportional to 1/ρ3 and is in good agreement with experimental data obtained by Kortshagen's group for silicon NC films. In the second part of the talk, I consider the exciton diffusion length in NC films. In photovoltaic devices based on NC films, absorption of a light quantum creates of an exciton (electron-hole pair) in a NC. The diffusion length of an exciton is important parameter determining the volume from which photons are harvested. It is known that an exciton can hop between nearest-neighbor NC via the Forster mechanism. For touching NC, I propose another mechanism where the electron and hole tunnel through the small contact facet in tandem. The tandem tunneling occurs through the intermediate state in which the electron and hole are in different NCs. I show that for majority of materials the tandem tunneling exciton transfer rate is comparable with the Forster rate, while for silicon the tandem tunneling dominates.
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    Photogeneration, Diffusion and Decay The life and fate of optical excitations in quantum-dot solids
    (2017-05) Houtepen, Arjan
    In recent years we have studied the photoconductivity in quantum-dot solids with a combination of terahertz spectroscopy, transient absorption spectroscopy and time-resolved microwave conductivity. Appropriate surface treatments induce controlled necking between Quantum Dots resulting in significant electronic coupling and charge carrier mobilities as high as ~10 cm2/Vs. We demonstrate that at room temperature photogenerated excitons readily dissociate into mobile charge carriers.1-3 Carrier Multiplication (CM) is observed in the photoconductivity of these films.4 The number of surviving free charges that result from CM increases with increasing charge carrier mobility as a result of the competition between Auger recombination and multi-exciton dissociation.5, 6 Finally I will discuss charge trapping at surface sites of IV-VI QD films. Using a combination of electrochemically controlling the Fermi level in QD films with ultrafast transient absorption and photoluminescence spectroscopy we demonstrate that electron trapping can be controlled and even avoided altogether.7 This also allows us to determine the density of trap states in the band gap of the quantum dots and, by comparison with density function theory calculations, to identify the chemical nature of these traps as well as the physical mechanism of charge trapping.8, 9 (1) Gao, Y.; Aerts, M.; Sandeep, C. S. S.; Talgorn, E.; Savenije, T. J.; Kinge, S.; Siebbeles, L. D. A.; Houtepen, A. J. ACS Nano 2012, 6, 9606-9614. (2) Talgorn, E.; Gao, Y.; Aerts, M.; Kunneman, L. T.; Schins, J. M.; Savenije, T. J.; van Huis, M. A.; van der Zant, H. S. J.; Houtepen, A. J.; Siebbeles, L. D. A. Nat. Nanotechnol. 2011, 6, 733-739. (3) Sandeep, C. S. S.; Azpiroz, J. M.; Evers, W. H.; Boehme, S.-C.; Moreels, I.; Kinge, S.; Siebbeles, L. D. A.; Infante, I.; Houtepen, A. J. ACS Nano 2014, 8, 11499-11511. (4) Aerts, M.; Suchand Sandeep, C. S.; Gao, Y.; Savenije, T. J.; Schins, J. M.; Houtepen, A. J.; Kinge, S.; Siebbeles, L. D. A. Nano Lett. 2011, 11, 4485-4489. (5) Suchand Sandeep, C. S.; Cate, S. t.; Schins, J. M.; Savenije, T. J.; Liu, Y.; Law, M.; Kinge, S.; Houtepen, A. J.; Siebbeles, L. D. A. Nat. Commun. 2013, 4, 2360. (6) Gao, Y.; Sandeep, C. S. S.; Schins, J. M.; Houtepen, A. J.; Siebbeles, L. D. A. Nat. Commun. 2013, 4. (7) Boehme, S. C.; Walvis, T. A.; Infante, I.; Grozema, F. C.; Vanmaekelbergh, D.; Siebbeles, L. D. A.; Houtepen, A. J. ACS Nano 2014, 8, 7067-7077. (8) Boehme, S. C.; Mikel Azpiroz, J.; Aulin, Y. V.; Grozema, F. C.; Vanmaekelbergh, D.; Siebbeles, L. D. A.; Infante, I.; Houtepen, A. J. Nano Lett. 2015, 15, 3056-3066. (9) Houtepen, A. J.; Hens,
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    Tracking the Energy Flow on Nanoscale via Sample-Transmitted Excitation Photoluminescence (STEP) Spectroscopy
    (2017-05) Zamkov, Mikhail
    Monitoring the energy flow in nanoscale materials is an important yet challenging goal. Experimental methods for probing the intermolecular energy transfer (ET) are often burdened by the spectral crosstalk between donor and acceptor species, which complicates unraveling their individual contributions. This issue is particularly prominent in inorganic nanoparticles and biological macromolecules featuring broad absorbing profiles. Here, we demonstrate a general spectroscopic strategy for measuring the energy transfer efficiency between nanostructured or molecular dyes exhibiting a significant donor-acceptor spectral overlap. The reported approach is enabled through spectral shaping of the broadband excitation light using solutions of donor molecules, which helps suppressing the excitation of respective donor species in the sample. The resulting changes in the acceptor emission induced by the spectral modulation of the excitation beam allow determining the quantum efficiency and the rate of ET processes between arbitrary fluorophores (molecules, nanoparticles, polymers) with high accuracy. The feasibility of the reported method is demonstrated using two control donor-acceptor systems: a low-overlap protein-bridged Cy3-Cy5 dye pair, and high-overlap CdSe560-CdSe600 nanocrystal film.
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    Visualizing current flow at the mesoscale in disordered assemblies of touching semiconductor nanocrystals
    (2017-05) Thimsen, Elijah
    The transport of electrons through assemblies of nanocrystals is important for the performance of these materials in optoelectronic applications. The transport of electrons has primarily been studied by focusing on single nanocrystals or transitions between pairs of nanocrystals. There is a gap in knowledge of how large numbers of nanocrystals in an assembly behave collectively, and how this collective behavior manifests at the mesoscale. In this work, the transport of electrons in assemblies of touching, heavily doped ZnO nanocrystals was visualized as a function of temperature at the mesoscale theoretically using the model of Skinner, Chen and Shklovskii (SCS); and experimentally by conductive atomic force microscopy on ultrathin films only a few particle layers thick. Agreement was obtained between the model and experiments, with a few notable exceptions. The SCS model predicts that a single network within the nanocrystal assembly, comprised of sites connected by small resistances, dominates conduction - namely the well-known optimum band from variable range hopping theory. However, our experiments revealed that in addition to the optimum band, there are subnetworks that appear as additional peaks in the resistance histogram; which were not observed in the model calculations. Furthermore, the connections of these subnetworks to the optimum band changes in time. As time proceeds, some isolated subnetworks become connected to the optimum band; while some of the connected subnetworks become disconnected and then isolated from the optimum band. The subset of nanocrystals comprising the optimum band is dynamic.
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    Bright and dark exciton transfer in quantum dot arrays
    (2017-05) Rodina, Anna
    We theoretically study nonradiative and radiative resonance energy transfer between two localized quantum emitters, a donor (initially excited quantum dot) and an acceptor (quantum dot receiving the excitation). We find that the donor lifetime can be significantly modified only due to the nonradiative dipole-dipole Förster Resonance energy transfer process to the short living energy level in acceptor dot at donor - acceptor separations of approximately 5-15 nm (depending on the acceptor radiative lifetime) and for the energy detuning not larger than ~1-2 meV. The dark (spin forbidden) exciton states can participate in the Forster energy transfer due to the weak admixture of the bright exciton states to the dark states, which also allows the radiative recombination of the dark excitons. We demonstrate the dominant role of the dark exciton in the low temperature energy transfer in ensemble of closely packed CdTe nanocrystals by time-dependent photoluminescence spectroscopy in applied external magnetic field which considerably enhances this admixture. The Förster Resonance energy transfer in inhomogeneous dense arrays of epitaxial CdSe/ZnSe quantum dots is also demonstrated by time-and space-resolved photoluminescence spectroscopy. The work was supported by Russian Science Foundation (Project number 14-22-00107).
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    Early Time Photoconductance in Quantum Dot Solids Probed by Ultrafast Photocurrent Spectroscopy
    (2017-05) Klimov, Victor
    Understanding and controlling carrier transport and recombination dynamics in colloidal quantum dot (QD) films is key to their application in electronic and optoelectronic devices. Towards this end, we have conducted transient photocurrent measurements (10-50 ps time resolution) to monitor charge-transport dynamics in lead selenide QD films as a function of pump fluence, temperature, electrical bias, and surface treatment [1, 2]. Room temperature dynamics reveal two distinct timescales: one sub-nanosecond and the other, tens-to-hundreds of nanoseconds. The first of these processes is assigned to relaxation of one type of carriers (presumably, electrons) into low-mobility intra-gap states, which pins the corresponding quasi-Fermi level at below band-edge energies [3]. This effect is likely responsible for a considerable photovoltage deficit typical of QD PVs. A longer-time transient photocurrent exhibits memory-less decay due to nongeminate recombination of the remaining mobile charges with the trapped carriers of the opposite sign (pre-existing and photogenerated). In addition to potentially modifying the chemical nature and/or abundance of trapping sites, application of different QD surface treatments also alters the initial (“dark”) occupancy of intra-gap states, which has a profound effect on mobile-carrier lifetimes. The peak photoconductance observed immediately after short-pulse excitation is temperature-independent suggesting a tunnelling mechanism of early time phototransport. Further, low temperature measurements reveal an important role of the excitonic fine structure and, specifically, the electron-hole exchange interaction (exchange blockade) in early time photocurrent dynamics. This effect is likely universal as it necessarily arises following photoexcitation when an electron and a hole are generated in the same QD and hence are strongly coupled by the exchange interaction, which creates a barrier to their separation between adjacent QDs. Finally, side-by-side comparison of photocurrent transients using excitation with low- and high-photon energies (1.5 vs. 3.0 eV) reveals clear signatures of carrier multiplication (CM), that is, generation of multiple excitons by single photons [2]. Based on photocurrent measurements of QD solids and optical measurements of solution based samples, we conclude that the CM efficiency is unaffected by inter-dot coupling, and therefore, the results of previous numerous spectroscopic CM studies conducted on dilute QD suspensions should, in principle, be reproducible in electronically coupled QD films used in devices. 1. Fidler, A.F., J. Gao, and V.I. Klimov, Electron-hole exchange blockade and memory-less recombination in photoexcited films of colloidal quantum dots. Nat. Phys, 2017. advance online publication. 2. Gao, J.B., A.F. Fidler, and V.I. Klimov, Carrier multiplication detected through transient photocurrent in device-grade films of lead selenide quantum dots. Nat. Comm. 2015. 6. 3. Nagpal, P. and V.I. Klimov, Role of mid-gap states in charge transport and photoconductivity in semiconductor nanocrystal films. Nat. Comm. 2011. 2: p. 486.
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    Electronic Transport Phenomena in Composite Nanocrystalline/Amorphous and Free-Standing Nanocrystalline Thin Films
    (2017-05) Kakalios, James
    Composite materials consisting of nanocrystalline semiconductors embedded within a bulk amorphous semiconductor or an insulator have attracted interest for applications ranging from photovoltaics, thermoelectrics, thin film transistors, particle detectors and electroluminescent devices. These materials combine the best of both worlds – the thin film large area advantages of disordered semiconductors with the superior opto-electronic properties of crystals, and often display electronic properties not observed in either material separately. Using a unique dual-chamber Plasma Enhanced Chemical Vapor Deposition system, we have synthesized nanocrystals of silicon or germanium in a surrounding hydrogenated amorphous silicon (a-Si:H) matrix (a/nc-Si:H). The dark conductivity of n-type doped a/nc-Si:H films displays three distinct conduction mechanisms: thermally activated conduction, multi-phonon hopping and Mott variable range hopping, as the crystal fraction and temperature of these films is varied. Studies of the thermopower of composite films of a-Si:H containing germanium nanocrystals find that transport changes from n-type to p-type as the nc-Ge concentration is increased, with a transition sharper than expected from a standard two-channel model for charge transport. Using the Zabrodskii analysis technique, the conductivity in the nc-Ge/a-Si:H films is described by an anomalous hopping expression, ~ exp[(To/T)k] where k = ¾, suggesting an entirely new conduction mechanism. Similar studies of free-standing nc Si films, deposited without a surrounding matrix, find that the conduction mechanism varies with the film’s exposure to atmosphere. This research done in collaboration with Uwe Kortshagen, C. Blackwell, Y. Adjallah, L. Wienkes, K. Bodurtha, C. Anderson and J. Trask. This work was partially supported by NSF grants NER-DMI-0403887, DMR-0705675, the NINN Characterization Facility, the Xcel Energy grant under RDF contract #RD3-25, NREL XEA-9-99012-01 and the University of Minnesota.