Bulletin of the American Physical Society
APS March Meeting 2014
Volume 59, Number 1
Monday–Friday, March 3–7, 2014; Denver, Colorado
Session Y53: Focus Session: Electron, Ion, and Exciton Transport in Nanostructures IV |
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Sponsoring Units: DMP Chair: William Vandenberghe, University of Texas at Dallas Room: Mile High Ballroom 2C |
Friday, March 7, 2014 8:00AM - 8:12AM |
Y53.00001: In situ studies of transient photoconductivity in PbSe quantum dot solar cells Jianbo Gao, Weon-Kyu Koh, Nikolay Makarov, Jeffrey Pietryga, Victor Klimov PbSe quantum dot (QD) solar cells have attracted significant interest due to their band gap tunability, easy-processing and flexibility. Efficiencies have risen from 1{\%} just a few years ago to nearly 9{\%} today. Furthermore, the novel concept of multiple exciton generation (MEG) resulting from quantum confinement makes these materials scientifically interesting counterparts to bulk semiconductors. Recent observations of more than 100{\%} external quantum efficiency in PbSe QD solar cells confirm direct relevance of MEG to practical photovoltaics. However, in order to take full advantage of this effect, one needs a better understanding of photogeneration dynamics and carrier transport in QD solar cells. In this talk, we discuss a new technique for in situ measurements of transient photoconductivity with fast response time (\textless 50 ps) applied to study carrier transport and photogeneration dynamics in PbSe QD solar cells. These measurements complement traditional photoconductivity techniques such as time-resolved microwave conductivity and time-of-flight. Based on the analysis of temperature, excitation wavelength and electrical field dependence measurements, we derive parameters such as MEG efficiency, carrier lifetime, trap-free mobility and carrier emission rate from trap states. [Preview Abstract] |
Friday, March 7, 2014 8:12AM - 8:24AM |
Y53.00002: Highly Nonlinear Photocurrent and Efficient Charge Separation in Lead Sulfide Nanowire Field Effect Transistors Yiming Yang, Xingyue Peng, Dong Yu We present our scanning photocurrent microscopy (SPCM) study of lead sulfide (PbS) NW field effect transistors (FETs). PbS NWs were synthesized via chemical vapor deposition (CVD) method, with controlled ambipolar doping from 10$^{19}$ cm$^{-3}$ (n-type) to 10$^{18}$ cm$^{-3}$ (p-type) [1]. We have observed highly nonlinear photocurrent in single PbS NW FETs under high-intensity optical excitation. The spatially resolved photocurrent images obtained from SPCM showed complex patterns, indicating reversal of the photocurrent direction at high excitation intensity, attributable to the non-equilibrium band structure modulated by the photo-injected carriers. Our numerical simulations agree well with the experimental results, when considering the electric field created by the bulk metal contact. In addition, we have also achieved high charge separation efficiency at the Schottky contact to NWs. The wavelength dependent photocurrent can be understood from the absorption cross-section of the NW obtained by the Finite-difference time-domain (FDTD) simulation. [1] Yang, Y. M.; Li, J.; Wu, H. K.; Oh, E.; Yu, D. Nano Lett. 2012, 12, 5890-5896. [Preview Abstract] |
Friday, March 7, 2014 8:24AM - 8:36AM |
Y53.00003: Measuring charge transport in nanopatterned PbS colloidal quantum dots using charge sensing Nirat Ray, Neal E. Staley, Darcy D. Wanger, Moungi G. Bawendi, Marc A. Kastner Colloidal quantum dots (CQDs) can self-assemble from solution into close-packed arrays, where the motion of electrons is expected to be correlated due to long-range coulomb interactions. In order to study electron transport in these arrays, measurement of conductance around zero bias is required. Devices fabricated using CQDs, however, tend to be highly resistive (owing to large tunnel barriers from the organic ligands), and techniques to increase the conductance, such as annealing, often lead to large scale cracking. We nanopattern PbS CQDs, using electron beam lithography and a liftoff process, adjacent to a charge sensor. The patterning process helps to eliminate cracking, and improve packing of the dots. By performing a time resolved measurement of charge through the dots, using the sensor, we are able to measure conductance values as low as 10$^{\mathrm{-19\thinspace }}\Omega^{\mathrm{-1\thinspace }}$with a voltage bias of just 100mV. Our technique also allows us to map out the current voltage characteristics, even at low temperatures where the current becomes immeasurably small. We present the first transport measurements, near zero bias, on nanopatterned PbS quantum dots. [Preview Abstract] |
Friday, March 7, 2014 8:36AM - 8:48AM |
Y53.00004: Properties and Modeling of Graphene/CdSe Nanoparticle Film/Graphene Tunneling Device Structures Datong Zhang, Chenguang Lu, Philip Kim, Irving P. Herman We fabricated graphene/monolayer CdSe nanoparticle film/graphene sandwich device structures through a multi-step procedure. The monolayer CdSe nanoparticle film is formed on a liquid-air surface before transfer onto the bottom graphene layer that had been micro-exfoliated onto a 285 nm SiO$_{\mathrm{2}}$/Si substrate. The top graphene layer is transferred to the targeted area on the CdSe nanoparticle film via a dry transfer technique. Current-voltage measurements across the device suggest tunneling-type transport; the I-V curves are fit by tunneling models with an effective thin insulator with barrier height of about 1.6 eV and a tunneling distance of about 2.8 nm, which matches the nanoparticle dimension. In photoconductivity measurement, the source-drain current is greatly enhanced when the laser is on the junction area. The magnitude of the photocurrent is in agreement with that estimated using the nanoparticle absorption coefficient and laser intensity. [Preview Abstract] |
Friday, March 7, 2014 8:48AM - 9:00AM |
Y53.00005: Energy Level Alignment for Efficient Carrier Transport in PbS Nanoparticles Capped with Cross-linking Ligand Danylo Zherebetskyy, Marcus Scheele, David Hanifi, Yi Liu, Paul Alivisatos, Lin-Wang Wang Arrays of inorganic nanoparticle (NP) can be used for different applications from solar cell to LED. The connection between the NP by organic linker molecule and the resulting carrier transport is a major issue in such applications. We theoretically investigate the electronic coupling between the NP and tetrathiafulvalene-tetraacid (TTFTA) as a function of energy level alignment using multiscale theoretical approach. First, standard DFT calculations are applied to get the geometry of TTFTA on NP surface. Second, the correct band structure is obtained for the molecule and surface from GW formalism including relativistic effects. Third, a long range polarization effect due to the NP dielectric media is included. Fourth, the quantum confinement effect is added to the PbS 1S$_{\mathrm{h}}$ and 1S$_{\mathrm{e}}$ levels. Finally, charge transport rate between NP through the TTFTA cross-linking molecules is calculated under Marcus theory. The resonant alignment between molecular HOMO and 1Sh state of NP is observed for 9.8 nm NP. The alignment is confirmed experimentally using cyclic voltammetry and ambient pressure X-ray photoelectron spectroscopy. [Preview Abstract] |
Friday, March 7, 2014 9:00AM - 9:12AM |
Y53.00006: Charge transfer between a CdSe/CdS quantum rod and a tethered ferrocene molecule linwang Wang, Kartick Tarafder, Yogesh Surendranath, Jacob Olshansky, Paul Alivisatos Hole transfer between a CdSe/CdS core/shell semiconductor nanorod and a surface-ligated alkyl ferrocene is investigated by a combination of ab initio quantum chemistry calculations and experimental measurements. The calculated driving force for hole transfer corresponds well with electrochemical measurements of nanorods partially ligated by 6-ferrocenylhexanethiolate. The calculations and the experiment suggest that the hole transfer from the valence band maximum to ferrocene is through a direct coherent hopping, not through any intermediate steps, and this hopping is in the Marcus inverted region. The calculated rate of hole transfer is in line with the photoinduced hole transfer rate determined experimentally, and the calculated state energy alignment agrees excellently with the experiments. Together, the calculations suggest that holes may be extracted more efficiently from well-passivated nanocrystals by reducing the energetic driving force for hole transfer, thus minimizing energetic losses. [Preview Abstract] |
Friday, March 7, 2014 9:12AM - 9:48AM |
Y53.00007: Chemical Synthesis, Computational Modeling, and Surface Reactions of Silicon Nanotube Anodes and Silicate Cathodes for Lithium Ion Batteries Invited Speaker: Christopher Hinkle Nanostructured materials show significant promise in enhancing the performance and safety of Li-ion batteries at greatly reduced cost. We highlight certain classes of materials for next generation anodes, cathodes, and solid electrolytes in addition to interface reactions and show how advanced chemical spectroscopy and first principles modeling can be utilized to improve battery performance and stability. In this work, we utilize advanced materials characterization techniques (in-situ XPS and FTIR, Raman, AFM, XRD) to elucidate the chemical bonding, nanostructure, and electrochemical properties that lead to improved storage capabilities in these materials. We describe the recent progress in chemical synthesis methods of fabricating hydrogenated amorphous-Si nanotube anodes and tetrahedral transition metal silicate cathodes (Li$_{2}$MSiO$_{4})$, which may be well-suited for future technologies. Additionally, insight into the redox potentials and ionic and electronic conductivities has been investigated using first-principles modeling. Our findings suggest that high-voltage, multi-component Li$_{2}$MSiO$_{4}$ cathodes (M $=$ Fe, Mn, Ni) with high Mn content are strong candidates for future Li-ion batteries. Inorganic solid electrolytes are also discussed highlighting their potential for improved safety, increased ionic conductivities, and stability against adverse reactions with the electrodes. Finally, we illustrate the complexity of interfacial chemistry in these new materials and the need for advanced spectroscopic characterization to make progress on all aspects of electrode and electrolyte development. [Preview Abstract] |
Friday, March 7, 2014 9:48AM - 10:00AM |
Y53.00008: Dielectric relaxation studies of ion diffusion into low-k dielectrics Archana Raja, Thomas Shaw, Eric Liniger, Fen Chen, Alfred Grill, Juan Borja, Griselda Bonilla, Joel Plawsky, Tony Heinz, Robert Laibowitz High speed interconnects in advanced integrated circuits require ultra-low-k dielectrics to reduce the RC time constant. Reduction of the dielectric constant in these films is typically achieved via incorporation of nanopores in materials containing silicon, carbon, oxygen and hydrogen (SiCOH). Trap states build-up as dielectric breakdown is approached and increased leakage is observed. To understand the mechanism of breakdown we study nanoporous SiCOH films of k=2.4 to 2.7 primarily using dielectric relaxation. Dielectric films, in the thickness range of 40 nm, are incorporated into interwoven capacitor structures. To quantify dielectric relaxation in the pre-breakdown regime, capacitance and dielectric losses are determined as a function of frequency and temperature. Through these dielectric measurements, we have obtained activation energies in the range of 0.1-0.2 eV for humidified and annealed capacitors; and 0.9-1.2 eV for copper ion incursion into the dielectric. We also deduce a charge center density of 10$^{15}$/cm$^3$. Our measurements provide an estimate of the impurity content and changes in activation energy with annealing and other fabrication parameters. [Preview Abstract] |
Friday, March 7, 2014 10:00AM - 10:12AM |
Y53.00009: Focused helium and neon ion beam induced etching for advance EUV lithography and mask repair Rajendra Timilsina, Carlos Gongalez, Philip Rack The gas field ion microscope was used to investigate helium and neon ion beam induced etching (IBIE) of nickel as a candidate technique for extreme ultraviolet (EUV) lithography mask editing. No discernable nickel etching was observed for room temperature helium exposures at 16 and 30 keV in the range of 1x10$^{\mathrm{15}}$-1x10$^{\mathrm{18}}$ He$^{\mathrm{+}}$/cm$^{\mathrm{2}}$, however transmission electron microscopy (TEM) revealed subsurface damage to the underlying Mo-Si multilayer EUV mirror. Subsequently, neon beam induced etching at 30 keV was investigated over a similar dose range and successfully removed the entire 50 nm nickel top absorber film at a dose of approximately 3x10$^{\mathrm{17}}$ Ne$^{\mathrm{+}}$/cm$^{\mathrm{2}}$. TEM also revealed subsurface damage in the underlying Mo-Si multilayer. To further understand the helium and neon damage, we simulated the ion-solid interactions with our EnvizION Monte Carlo sputtering program which reasonably correlated the observed damage and bubble formation to the nuclear energy loss and the implanted inert gas concentration, respectively. A critical nuclear energy density loss of approximately 80 eV/nm$^{\mathrm{3}}$ and critical implant concentration of approximately 10$^{\mathrm{20}}$ atoms/cm$^{\mathrm{3}}$ have been calculated for damage generation in the multilayer structure. [Preview Abstract] |
Friday, March 7, 2014 10:12AM - 10:24AM |
Y53.00010: The role of interfacial effects on enhanced catalytic performance of TiO2-graphene nanocomposites Dinko Chakarov, Raja Sellappan Graphene-containining TiO2 nanocomposites have significantly higher photocatalytic activity compared to bare TiO2 films. The enhancement is result of improved transport and higher efficiency of the charge carries separation at carbon-TiO2 interface. These effects were assessed by comparison of six anatase-graphene structures, fabricated by different synthesizing techniques and referenced to the performance of TiO2--graphitic-carbon and TiO2--Au thin films. [Preview Abstract] |
Friday, March 7, 2014 10:24AM - 10:36AM |
Y53.00011: High Surface Area Dendrite Nanoelectrodes for Electrochemistry Nathan Nesbitt, Jennifer Glover, Saurabh Goyal, Svetoslav Simidjiysky, Michael Naughton Solution-based electrodeposition of metal using a low ion concentration, surface passivation agents, and/or electrochemical crystal conditioning has allowed for the formation of high surface area metal electrodes, useful for Raman spectroscopy and electrochemical sensors. Additionally, high frequency electrical oscillations have been used to electrically connect co-planar electrodes, a process called directed electrochemical nanowire assembly (DENA). These approaches aim to control the crystal face that metal atoms in solution will nucleate onto, thus causing anisotropic growth of metal crystals. However, DENA has not been used to create high surface area electrodes, and no study has been conducted on the effect of micron-scale surface topography on the initial nucleation of metal crystals on the electrode surface. When DENA is used to create a high surface area electrode, such a texture has a strong impact on the subsequent topography of the three dimensional dendritic structures by limiting the areal density of crystals on the electrode surface. Such structures both demonstrate unique physics concerning the nucleation of metal dendrites, and offer a unique and highly facile fabrication method of high surface area electrodes, useful for chemical and biological sensing. [Preview Abstract] |
Friday, March 7, 2014 10:36AM - 10:48AM |
Y53.00012: First-principles studies of conformation and solution effects on DNA transport Bikan Tan, Miroslav Hodak, Wenchang Lu, Jerry Bernholc The electrical conductivity of DNA molecules is of fundamental interest in the life sciences. We use first-principles techniques combined with molecular dynamical (MD) simulations to calculate transport properties of B-DNA connected to carbon nanotubes via alkane linkers. The quantum transport properties are calculated for over a hundred of snapshots recorded in MD trajectories. We discover that the DNA conformation and especially the overlaps between sequential guanine bases play a critical role in electron transport. DNA charge transport is indeed governed by charge delocalization with wavefunctions extent controlled by geometrical overlaps. Solvent atoms also affect the conductivity, with counterions decreasing the conductance by a factor of 2-3. In addition, we find that water molecules around the double helix screen the negatively-charged phosphate groups suppressing the conductance of DNA. Comparing transport properties of 4-base-pair (BP) with 10-BP DNA, we find weak distance dependence of the conductivity. Finally, we discuss the effect of sequence on DNA conductivity. [Preview Abstract] |
Friday, March 7, 2014 10:48AM - 11:00AM |
Y53.00013: Functionalization of hybrid organic-inorganic materials for highly efficient photovoltaic materials Levi Lentz, Alexie Kolpak Low mobility and high recombination rates limit the incident photon conversion efficiency (IPCE) of organic-based photovoltaics. In this work we employ first-principles density functional theory calculations to investigate hybrid organic-inorganic materials designed to directly ameliorate these issues. By constructing superlattices composed of 2D transition metal phosphate sheets separated by ordered regions of well-known organic dyes, we show that one can significantly decrease recombination and increase charge carrier mobilities relative to typical organic photovoltaic materials. ~We discuss how functionalization of the molecules in the organic region can be used to simultaneously enhance exciton separation and tune the organic-inorganic band alignment to encourage charge transfer into the inorganic regions, which can act as high-mobility charge carrier channels. ~Our results suggest that nanostructured hybrid materials could significantly improve IPCE over traditional organic photovoltaics. [Preview Abstract] |
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