Bulletin of the American Physical Society
APS March Meeting 2016
Volume 61, Number 2
Monday–Friday, March 14–18, 2016; Baltimore, Maryland
Session C32: Device Characterization of Nanostructured Devices and HeterostructuresFocus
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Sponsoring Units: DCP Chair: Matt Law, UC-Irvine Room: 332 |
Monday, March 14, 2016 2:30PM - 3:06PM |
C32.00001: Designer Nanocrystal Materials for Photovoltaics Invited Speaker: Cherie Kagan Advances in synthetic methods allow a wide range of semiconductor nanocrystals (NCs) to be tailored in size and shape and to be used as building blocks in the design of NC solids. However, the long, insulating ligands commonly employed in the synthesis of colloidal NCs inhibit strong interparticle coupling and charge transport once NCs are assembled into the solids state as NC arrays. We will describe the range of short, compact ligand chemistries we employ to exchange the long, insulating ligands used in synthesis and to increase interparticle coupling. These ligand exchange processes can have a dramatic influence on NC surface chemistry as well as NC organization in the solids, showing examples of short-range order. Synergistically, we use 1) thermal evaporation and diffusion and 2) wet-chemical methods to introduce extrinsic impurities and non-stoichiometry to passivate surface traps and dope NC solids. NC coupling and doping provide control over the density of states and the carrier type, concentration, mobility, and lifetime, which we characterize by a range of electronic and spectroscopic techniques. We will describe the importance of engineering device interfaces to design NC materials for solar photovoltaics. [Preview Abstract] |
Monday, March 14, 2016 3:06PM - 3:18PM |
C32.00002: Towards a Dithiocarbamate Ligand for CdS Nanoparticle-based Photocatalysis Andrew O'Hara, Andrew D. LaCroix, Sokrates T. Pantelides, Janet E. Macdonald Photocatalysis of water into H$_{2}$ and O$_{2}$ presents a clean, renewable route for energy storage and production. Traditionally, most semiconducting nanoparticle research on photocatalysis has focused on the ability to reduce chemical systems using the photoexcited electron. Here we employ a combination of theory and experiments to develop a possible route towards the oxidation of chemical systems via the hole from photoexcitation using an asymmetric bipyridine ligand with conjugated dithiocarbamate ligand bound to the surface of cadmium sulfide nanorods. In particular, we use density functional theory to calculate the electronic levels and optical absorption of the designer ligand, free from the cadmium sulfide surface as well as attached to the surface, with and without the copper center. These calculations are compared with experimental UV/VIS absorption and fluorescence spectroscopy measurements to understand the role of copper chelation. Furthermore, theoretical comparisons are made with a related ligand known to oxidize water under an applied potential bias. Finally, we discuss whether we expect photocatalysis from the ligand and possible improvements to its design. [Preview Abstract] |
Monday, March 14, 2016 3:18PM - 3:30PM |
C32.00003: Influence of Defect States on Charge Transport in CuInSe$_{\mathrm{2-x}}$S$_{\mathrm{x}}$ Quantum Dot Films Hyeong Jin Yun, Andrew Fidler, Jaehoon Lim, Addis Fuhr, Jeffrey Pietryga, Sam Keene, Matt Law, Victor Klimov CuInSe$_{\mathrm{2-x}}$S$_{\mathrm{x}}$ quantum dots (QDs) are environmental-friendly alternatives to Cd- or Pb-based QDs for solar energy applications. The key to using QD thin films in opto-electronic devices like solar cells is understanding their charge-transport properties, which are known to be influenced by defects that can serve as carrier traps. Here, we combine field effect transistor (FET) and ultrafast transient photocurrent (u-TPC) measurements to obtain a more complete picture of the nature and role of trap sates in CuInSe$_{\mathrm{2-x}}$S$_{\mathrm{x}}$ QD thin films. FET devices employing indium contacts exhibit $n$-type transport with electron mobility of 5.34 \texttimes 10$^{\mathrm{-4}}$ cm$^{\mathrm{2}}$/Vs, but they also indicate high concentrations of electrons in the films. Early-time dynamical signatures revealed in u-TPC suggest that this high carrier density arises from the presence of trap states in CuInSe$_{\mathrm{2-x}}$S$_{\mathrm{x}}$ QDs. In order to reduce the density of trap states, atomic layer deposition was used to infill the CuInSe$_{\mathrm{2-x}}$S$_{\mathrm{x}}$-based devices with amorphous alumina, which results in both higher FET mobilities, and a reduction in trap-related decay signatures in u-TPC measurements. [Preview Abstract] |
Monday, March 14, 2016 3:30PM - 3:42PM |
C32.00004: Probing the interface between semiconducting nanocrystals and molecular metal chalcogenide surface ligands: insights from first principles Emilio Scalise, Stefan Wippermann, Giulia Galli, Dmitri Talapin Colloidal nanocrystals (NCs) are emerging as cost-effective materials offering exciting prospects for solar energy conversion, light emission and electronic applications. Recent experimental advances demonstrate the synthesis of fully inorganic nanocrystal solids from chemical solution processing. The properties of the NC-solids are heavily determined by the NCs surface and their interactions with the host matrix. However, information on the atomistic structure of such composites is hard to obtain, due to the complexity of the synthesis conditions and the unavailability of robust experimental techniques to probe nanointerfaces at the microscopic level. Here we present a systematic theoretical study of the interaction between InAs and InP NCs with Sn$_2$S$_6^{4-}$ ligands. Employing a grand canonical ab initio thermodynamic approach we investigate the relative stability of a multitude of configurations possibly realized at the NC-ligand interface. Our study highlights the importance of different structural details and their strong impact on the resulting composite's properties. We show that to obtain a detailed understanding of experimental data it is necessary to take into account complex interfacial structures beyond simplified NC-ligand model interfaces. [Preview Abstract] |
Monday, March 14, 2016 3:42PM - 3:54PM |
C32.00005: Lifetime, Mobility, and Diffusion of Photoexcited Carriers in Ligand-Exchanged PbSe Nanocrystal Films Measured by Time-Resolved Terahertz Spectroscopy Siming Li, Glenn Guglietta, Yaoting Wu, Natalie Gogotsi, Christopher Murray, Jason Baxter Colloidal semiconductor nanocrystals have been used as building blocks for electronic and optoelectronic devices ranging from field effect transistors to solar cells. Properties of the nanocrystal films depend sensitively on the choice of capping ligand to replace the insulating synthesis ligands. Thus far, ligands leading to the best performance in transistors result in poor solar cell performance, and vice versa. To understand this dichotomy, we used time-resolved terahertz spectroscopy to study the mobility and lifetime of PbSe nanocrystal films with five common ligand-exchange reagents. The films treated with different displacing ligands show more than an order of magnitude difference in the peak conductivities and a bifurcation of time-dynamics. Inorganic chalcogenide ligand-exchanges with Na$_{2}$S or NH$_{4}$SCN show high mobilities but nearly complete decay of transient photocurrent in 1.4 ns. In contrast, ligand exchanges with EDA, EDT, and TBAI show lower mobilities but longer lifetimes, resulting in longer diffusion lengths. This bifurcated behavior may explain the divergent performance of field-effect transistors and photovoltaics constructed from nanocrystal building blocks with different ligand exchanges. Ref: Guglietta et al., ACS Nano, 2015. [Preview Abstract] |
Monday, March 14, 2016 3:54PM - 4:06PM |
C32.00006: Biexciton Dissociation Efficiency at Quantum Dot-Oxide Interfaces Mischa Bonn, Hai Wang, Enrique Canovas Harvesting multiexcitons populating semiconductor quantum dots (generated by carrier multiplication, CM) has been proposed as a path towards higher efficiencies in photovoltaic devices. Although CM efficiency has been widely interrogated in colloidal QD solutions, less focus has been placed on the physics regarding biexciton collection at electrodes. We investigate interfacial biexciton transfer dynamics from PbS quantum dots directly nucleated onto mesoporous SnO$_{\mathrm{2}}$ films as a function of impinging photon flux and photon energy. A priori, this system seems very well-suited for achieving efficient biexciton dissociation, as the ultrafast QD-to-oxide transfer rate for 800nm excitation is substantially faster than Auger relaxation. Remarkably, the biexciton dissociation efficiency is below the detection efficiency, i.e. essentially zero. This seemingly counterintuitive result can be understood by noting that efficient hot electron transfer at the QD-oxide interface can compete with CM within the QDs. Hot electron transfer is observed to occur on sub-100 fs timescales, nulling the CM efficiency. Implications of these results for solar energy conversion are discussed. [Preview Abstract] |
Monday, March 14, 2016 4:06PM - 4:42PM |
C32.00007: Matrix engineering, state filling, and charge transport in PbSe quantum dot solids Invited Speaker: Matt Law Colloidal semiconductor quantum dots (QDs) are attractive building blocks for solar photovoltaics (PV). In this talk, I will highlight our recent progress in designing PbX (X $=$ S, Se, Te) QD thin film absorbers for next-generation PV. Basic requirements for QD absorber layers include efficient light absorption, charge separation, charge transport, and long-term stability. I begin by discussing QD film fabrication, charge transport physics, insights from theory, and evidence that the carrier diffusion length is short and limited by electronic states in the QD band gap. Studies of carrier mobility as a function of basic film parameters such as inter-QD spacing, QD size, and QD size distribution have led to a better understanding of charge transport within highly disordered QD films. Efforts to improve carrier mobility by enhancing inter-dot electronic coupling, passivating surface states, and implementing surface doping will be highlighted. Engineering the inter-QD matrix to produce QD/inorganic or QD/organic nanocomposites is presented as a powerful way to optimize coupling, remove surface states, eliminate hysteretic charge trapping and ion motion, and achieve long-term environmental stability for high-performance, robust QD films that feature good carrier multiplication efficiency. New results on the use of atomic layer deposition infilling of QD films to yield all-inorganic QD transistors free of the bias-stress effect will be presented, and the likely role of ion transport in QD optoelectronics discussed. The use of infrared transmission spectroscopy to understand state filling and study charge transport in QD thin film transistors will be presented. [Preview Abstract] |
Monday, March 14, 2016 4:42PM - 4:54PM |
C32.00008: "Flash" synthesis of "giant" Mn-doped CdS/ZnSe/ZnS nanocrystals with ZnSe layer as hole quantum-well Ruilin Xu, Jiayu Zhang Usually, exciton-Mn energy transfer in Mn-doped CdS/ZnS nanocrystals (NCs) can readily outcompete the exciton trapping by an order of magnitude. However, with the accumulation of non-radiative defects in the giant shell during the rapid growth of the thick shell (up to \textasciitilde 20 monolayers in no more than 10 minutes), the photoluminescence (PL) quantum yield of this kind of “giant” NCs is significantly reduced by the accumulation of non-radiative defects during the rapid growth of thick shell. That is because the exciton-Mn energy transfer in Mn-doped CdS/ZnS NCs is significantly inhibited by the hole trapping as the major competing process, resulting from the insufficient hole-confinement in CdS/ZnS NCs. Accordingly “flash” synthesis of giant Mn-doped CdS/ZnSe/ZnS NCs with ZnSe layer as hole quantum-well is developed to suppress the inhibition. Meanwhile Mn$^{\mathrm{2+}}$ PL peak changes profoundly from \textasciitilde 620 nm to \textasciitilde 540 nm after addition of ZnSe layer. Studies are under the way to explore the relevant mechanisms. [Preview Abstract] |
Monday, March 14, 2016 4:54PM - 5:06PM |
C32.00009: Development of an improved molecular dynamics force field for surface-adsorption simulations of molybdenum disulfide Gary Leuty, Rajiv Berry, Christopher Muratore, Vikas Varshney, Heath Turner Transition metal dichalcogenides (TMDs) such as molybdenum disulfide (MoS$_2$) have garnered significant interest in recent years. With a layered structure similar to graphene, TMDs also have an intrinsic band gap. This band gap makes them an attractive alternative to graphene in many applications. MoS$_2$ in particular has received attention due to the placement and tenability of its band gap, via functionalization, mechanical manipulation or physisorption. The latter of these is of interest in biosensor devices. Such applications are dependent on understanding physisorption on the MoS$_2$ surface at the molecular level. This can be difficult experimentally but is possible via computer simulation techniques such as molecular dynamics (MD) simulations. MD simulations, however, require a force field accurate to the process modeled. Such a force field must correctly describe non-bonded interactions between substrate layers and between the surface and adsorbates. The force fields we are aware of have focused on intra-layer covalent bonding for structural and vibrational analysis. This work seeks to develop, through DFT and MD simulations with experimental characterization of surface adsorption, a more accurate parameterization for non-bonded interactions for MoS$_2$. [Preview Abstract] |
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