Bulletin of the American Physical Society
APS March Meeting 2014
Volume 59, Number 1
Monday–Friday, March 3–7, 2014; Denver, Colorado
Session M2: Focus Session: Charge & Energy Transfer I |
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Sponsoring Units: DCP Chair: Nathan Gabor, Massachusetts Institute of Technology Room: 102 |
Wednesday, March 5, 2014 11:15AM - 11:51AM |
M2.00001: The optoelectronic properties of in-plane grain boundaries and out-of-plane interfaces in two-dimensional transition metal dichalcogenides Invited Speaker: Arend van der Zande Two-dimensional monolayer transition metal dichacogenides (TMDs) are a promising new class of nanomaterial for energy harvesting systems. Monolayer group 6 TMDs (MX2, M $=$ Mo, W, X $=$ S, Se) are direct bandgap semiconductors with strongly bound excitons giving potential for diverse physical phenomena such as multiple exciton generation, trion formation, spin valley coupling and hot electron extraction. In this talk, we will study the behavior of electrons and excitons along in- and out-of-plane interfaces of monolayer materials. Using chemical vapor deposition, we produce high-quality, large-area monolayer molybdenum disulfide. Using electron microscopy, optical spectroscopy and electrical transport, we show that these monolayers contain in-plane grain boundaries composed of 8-4-4 ring defects in the hexagonal lattice and that that these grain boundaries impact the local optical and electronic properties of the material. We examine the role of interlayer coupling by building heterostructures of similar and dissimilar monolayer materials using ultra-clean transfer techniques. First, we build bilayers of molybdenum disulfide with a well-defined interlayer twist. Using optical spectroscopy, we observe that the layers electronically hybridize to form an indirect optical transition and that we can continuously tune electronic and optical properties of the bilayer with the twist angle. Next, we study the properties of an atomically thin p-n junction formed by a MoS2-WSe2 heterostucture. From photoluminescence and scanned photocurrent measurements, we demonstrate a photovoltaic response in the junction, mediated by charge transfer across the van der Waals interface. [Preview Abstract] |
Wednesday, March 5, 2014 11:51AM - 12:03PM |
M2.00002: Photosynthesis Revisited: Optimization of Charge and Energy Transfer in Quantum Materials Nathaniel Gabor The integration of new nano- and molecular-scale quantum materials into ultra-efficient energy harvesting devices presents significant scientific challenges. Of the many challenges, the most difficult is achieving high photon-to-electron conversion efficiency while maintaining broadband absorption. Due to exciton effects, devices composed of quantum materials may allow near-unity optical absorption efficiency yet require the choice of precisely one fundamental energy (HOMO-LUMO gap). To maximize absorption, the simplest device would absorb at the peak of the solar spectrum, which spans the visible wavelengths. If the peak of the solar spectrum spans the visible wavelengths, then why are terrestrial plants green? Here, I discuss a physical model of photosynthetic absorption and photoprotection in which the cell utilizes active feedback to optimize charge and energy transfer, thus maximizing stored energy rather than absorption. This model, which addresses the question of terrestrial greenness, is supported by several recent results that have begun to unravel the details of photoprotection in higher plants. More importantly, this model indicates a novel route for the design of next-generation energy harvesting systems based on nano- and molecular-scale quantum materials. [Preview Abstract] |
Wednesday, March 5, 2014 12:03PM - 12:15PM |
M2.00003: Charge and spin transport in porphyrin-based molecular junctions Zhenfei Liu, Sujun Wei, Olgun Adak, Hongsik Yoon, Luis M. Campos, Woo-Dong Jang, Latha Venkataraman, Jeffrey B. Neaton We study weak-bias charge and spin transport behavior of four metal-porphyrin molecules in molecular junctions, using a combination of break-junction experiments and a self-energy corrected first-principles approach based on density functional theory. Some of the molecules are open-shell, and they are of potential interest to spin filtering and solar energy conversion. Optimally-tuned range-separated hybrid functionals are used in combination with a correction for static polarization effects to yield accurate level alignment between Fermi level and dominating conducting orbital energies in the junction. We find that the conductance can change by up to a factor of two when different metal cations are used. Our calculations of low-bias conductance generate similar trends and are in quantitative agreement with experimental measurements. Implications for spin transport are discussed. [Preview Abstract] |
Wednesday, March 5, 2014 12:15PM - 12:27PM |
M2.00004: Two Dimensional White Light Spectroscopy Reveals Energy Transfer Pathways in Semiconducting Carbon Nanotube Thin Films Randy Mehlenbacher, Thomas McDonough, Maksim Grechko, Nicholas Kearns, Meng-Yin Wu, Michael Arnold, Martin Zanni Carbon nanotubes are promising materials for the active layer in photovoltaic devices because of their tunable bandgaps and large exciton diffusion lengths. We are studying thin films of coupled semiconducting nanotubes to understand the dynamics of exciton transfer. Previous work used transient absorption to follow photoexcitation transfer from large to small bandgap nanotubes, but a comprehensive mechanism could not be obtained, largely due to the wide range of wavelengths over which these films absorb. We have developed two-dimensional white light spectroscopy (2D WL) as a novel probe of these films and many other systems in the solar energy sciences. By studying the evolution (as a function of waiting time) of crosspeaks between the E$_{ii}$ states for different bandgap nanotubes, we are able to map out the energy transfer pathway. The advantage to using 2D WL over traditional 2D electronic spectroscopies is that the spectral bandwidth produced from supercontinuum generation is significantly larger than that accessible from an optical parametric amplifier. Thus, we are able to cover the entire film absorption simultaneously, thereby obtaining a map of the instantaneous exciton distribution. Several surprising results will be reported. [Preview Abstract] |
Wednesday, March 5, 2014 12:27PM - 1:03PM |
M2.00005: Optoelectronics of Two-Dimensional Transition Metal Dichalcogenides Invited Speaker: Xiaodong Xu Monolayer transition metal dichalcogenides (TMDs) are a new class of 2D semiconductors with the band edge at the corners of the hexagonal Brillouin zone. There has been rapid progress in demonstrating the interesting 2D excitonic properties of TMDs, such as tunable exciton charging effects, large exciton and trion binding energies, and valley exciton polarization and coherence. In this talk, I will discuss the role of excitons in solid--state light emitting devices made from monolayer TMDs, as well as intralayer and interlayer excitonic properties in both TMD bilayers and heterostructure devices. The results are relevant for energy-efficient optoelectronics based on 2D layered materials. [Preview Abstract] |
Wednesday, March 5, 2014 1:03PM - 1:15PM |
M2.00006: ABSTRACT WITHDRAWN |
Wednesday, March 5, 2014 1:15PM - 1:27PM |
M2.00007: Probing Transport through Single Molecule Junctions by Electrolytic Gating Brian Capozzi, Q. Chen, P. Darancet, M. Buzzeo, J.B. Neaton, C. Nuckolls, L. Venkataraman Organic field effect transistors made using ionic liquids or electrolyte solutions as gate dielectrics have received significant attention due to their ability to generate huge interfacial capacitances at the nano-scale. We apply this technique to single-molecule junctions created using the scanning tunneling microscope-based break-junction technique. We demonstrate that we can tune the transport characteristics of single-molecule junctions, modulating the conductance of junctions with molecules that are electrochemically inactive, within the gate bias range probed. For molecules that conduct through the highest occupied molecular orbital (HOMO), we see a decreasing conductance while applying a positive electrochemical gate potential while those that conduct though the lowest unoccupied molecular orbital (LUMO) show the opposite trend. Furthermore, we are able to fit the experimental gating data with a Lorentzian transmission function, and find the fitting parameters to be in quantitative agreement with self-energy corrected density functional theory calculations. This work shows that electrolyte gating can directly modulate the alignment of the conducting orbital relative to the metal Fermi energy, thereby changing the junction transmission characteristics. [Preview Abstract] |
Wednesday, March 5, 2014 1:27PM - 1:39PM |
M2.00008: Projection operators - non-equilibrium Green functions approach to quantum transport Maicol Ochoa, Michael Galperin, Mark Ratner We consider projection operator approach to non-equilbrium Green function equation-of-motion (PO-NEGF EOM) method. The technique resolves problem of arbitrariness in truncation of an infinite chain of EOMs, and prevents violation of symmetry relations resulting from the truncation. The approach, originally developed by Tserkovnikov [Theor. Math. Phys. \textbf{118}, 85 (1999)] for equilibrium systems, is reformulated to be applicable to time-dependent non-equilibrium situations. We derived canonical form of EOMs, thus explicitly demonstrating a proper introduction of the non-equilibrium atomic limit in junction problems. A simple practical scheme applicable to quantum transport simulations is formulated. We perform numerical simulations within simple models, and compare results of the approach to other techniques, and where available also to exact results. [Preview Abstract] |
Wednesday, March 5, 2014 1:39PM - 2:15PM |
M2.00009: Optical Properties of Two-Dimensional Crystals and Heterostructures Invited Speaker: Feng Wang Atomically thin two-dimensional materials, including graphene, boron nitride, metal dichalcogenides and their heterostructures, can exhibit novel optical phenomena that are distinctly different from bulk materials. In this talk, I will present our recent results on tunable optical properties in graphene/boron nitride heterostructures, where the coupling between graphene and boron nitride gives rise to new functionality. I will also discuss new optical behavior observed in metal dichalcogenide materials. [Preview Abstract] |
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