Bulletin of the American Physical Society
APS March Meeting 2017
Volume 62, Number 4
Monday–Friday, March 13–17, 2017; New Orleans, Louisiana
Session X25: Focus Session Chemical Physics Frontiers at Interfaces IVFocus
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Sponsoring Units: DCP Chair: William Tisdale, Massachusetts Institute of Technology Room: 288 |
Friday, March 17, 2017 8:00AM - 8:36AM |
X25.00001: Energy transfer in solar cells made from semiconducting carbon nanotubes studied using 2D White-Light Spectroscopy Invited Speaker: Martin Zanni It has recently become possible to purify carbon nanotubes into semiconducting thin films, making their use possible in energy harvesting and optoelectronic devices. Individual carbon nanotubes are well known to have exceptional transport and optical properties, but the properties in bulk materials are unknown. They are essentially a new mesoscale material. In this talk, I will report a method that we have developed, called two-dimensional white light spectroscopy (2D WL), which uses a broadband continuum as both the pump and probe source, enabling us to simultaneously examine a spectral range spanning from 500-1400 nm. The 2D WL spectra resolve energy transfer between all possible combinations of excitonic states in the chirality-selected nanotubes, thereby providing an instantaneous and comprehensive snapshot of the dynamical pathways. We observe exciton hopping, exciton dissociation, and anti-correlated energy levels; all of which have important implications in the development of carbon nanotube electronics and optoelectronics. [Preview Abstract] |
Friday, March 17, 2017 8:36AM - 8:48AM |
X25.00002: Trion Photoluminescence in Individual Single-Walled Carbon Nanotubes Amanda Amori, Todd Krauss Recently, charge carrier doping has garnered interest as a means of modifying the optical and electronic properties of single-walled carbon nanotubes (SWNTs). Specifically, chemically doping SWNTs with extra holes has been shown to quench the bright singlet exciton feature while a red-shifted, weaker feature appears. While this feature has been attributed to formation of a positively charged exciton, or trion, it has been observed that ensemble trion PL does not follow the standard line narrowing or T$^{\mathrm{-1/2}}$ dependence of delocalized excitons. In this presentation, we will discuss an alternative hypothesis; specifically, that this feature may arise as a consequence of recombination of an exciton at a charged defect site along the surface of the nanotube. Using scanning confocal microscopy of individual doped SWNTs at both room temperature (RT) and 10 K, we will show PL spectra and correlated intensity statistics for trions versus excitons. Spatially resolving PL from the trion for compassion to PL from the exciton at both RT and 10 K allows for a direct assessment of the localization of the trion. Localized trion PL suggests that the trion is more likely arising from exciton recombination at a charged defect and not from a three-body delocalized quasi-particle. [Preview Abstract] |
Friday, March 17, 2017 8:48AM - 9:00AM |
X25.00003: Extreme Tellurium nanowires encapsulated within narrow-diameter single-walled carbon nanotubes: Theory and experiments. Paulo V C Medeiros, Samuel Marks, Jamie Wynn, Andrij Vasylenko, Quantin Ramasse3, David Quigley, Jeremy Sloan, Andrew Morris Extreme nanowires are the ultimate class of crystalline materials: They are the smallest possible periodic materials. With atom-wide motifs repeated along one single ~dimension, they offer a unique perspective into the Physics and Chemistry of low-dimensional systems. The interior of narrow single-walled carbon nanotubes (NSWCNTs), on the other hand, ~provides an ideal environment for the creation of such materials. We report the observation of extreme Te nanowires grown inside NSWCNTs. We start by discussing how implicit single-walled carbon nanotubes SWCNTs can be introduced to speed up the structural searches on SWCNT-encapsulated structures. Then, using ~high- precision, high-throughput \textit{ab initio} calculations, along with state-of-the-art imaging techniques, we unambiguously determine the phase evolution of encapsulated Te as a function of the diameters of the encapsulating NSWCNTs. From 1-atom-wide linear chains -- the ultimate extreme nanowires, elemental Te evolves into zigzag chains and, still within very narrow SWCNTs, forms helical structures that are the one-dimensional analogues of bulk Tellurium. [Preview Abstract] |
Friday, March 17, 2017 9:00AM - 9:12AM |
X25.00004: Exciton dissociation by fullerene versus non-fullerene acceptors at organic photovoltaic interfaces Steven Robey Extensive development of polymer and small molecule donors has produced a steady increase in the efficiency of organic photovoltaic (OPV) devices. However, OPV technology would benefit from the introduction of non-fullerene acceptors. Recent progress has been promising, but efforts to replace fullerenes often lead to reduced efficiencies, possibly due to unfavorable non-fullerene morphologies and/or to more favorable excitation/carrier delocalization in fullerenes. Significantly increased exciton dissociation with fullerenes, associated with the fullerene molecular excited electronic structure, has also been predicted as the source of increased efficiency. This would provide a more critical barrier to achieving comparable performance with non-fullerenes. This hypothesis was tested using time-resolved two-photon photoemission (TR-2PPE) to compare exciton dissociation at interfaces between zinc phthalocyanine (ZnPc) and the non-fullerene acceptor, perylene tetracarboxylic dianhydride (PTCDA) versus dissociation at the analogous interface with C$_{\mathrm{60}}$. Exciton dissociation rates were found to be comparable for phthalocyanine interfaces with both classes of acceptors, suggesting that other effects dominate higher efficiencies with fullerene acceptors. [Preview Abstract] |
(Author Not Attending)
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X25.00005: The Dual Role of Disorder on the Dissociation of Interfacial Charge Transfer Excitons Liang Shi, Chee-Kong Lee, Adam Willard In organic-based photovoltaics (OPV), dissociation of neutral photo-excitations (i.e., Frenkel excitons) into free charge carriers requires the excitons to overcome binding energy that can significantly exceed thermal energies. The inability of bound charges to overcome this large binding energy has been implicated as a primary source of efficiency loss in OPVs. Despite the potential impact on the performance of organic solar cells much remains to be understood about the microscopic mechanism of exciton dissociation in OPV materials. Here we explore the role of static molecular disorder in mediating this charge dissociation process. Using a simple lattice model of exciton dynamics we demonstrate that random spatial variations in the energetic landscape can mitigate the effects of the exciton binding energy by lowering the free energy barrier. By considering the competition between this thermodynamic effect and the disorder-induced slowing of dissociation kinetics we demonstrate that exciton dissociation yields are expected to depend non-monotonically on the degree of static disorder. We conclude that a certain amount of molecular-scale disorder is desirable in order to optimize the performance of organic photovoltaic materials. [Preview Abstract] |
Friday, March 17, 2017 9:24AM - 9:36AM |
X25.00006: How Number of Layers and Relative Position Modulate the Interlayer Electron Transfer in $\pi$-Staked 2D Materials Marco Caricato, Alessandro Biancardi Understanding the photoinduced electron transfer (ET) between 2D layers, e.g. from $\pi$-stacked molecular films to few-layers graphene, is central to both technological and biological applications. While electron transfer has been extensively studied in the case of isolated molecules, its description in the case of extended solids is still unsatisfying. Here, using our recent extension of the Fock/Kohn-Sham matrix reconstruction within periodic boundary conditions, we describe the ET as a function of both number of layers and relative positions. Specifically, we consider the photoinduced ET from a zinc phthalocyanine film deposited over few-layer graphene with number of layers ranging from one to four. We find the ET critically dependent on both number of layers, staking of layers and relative position between donor and acceptor layers. In agreement with experiment, we show that the ET to a single-layer graphene is faster than to a double-layer graphene due to interference effects between layers in the latter arrangement. These results shed light on the subtle interplay between 2D structure and interlayer transfer, which may lead to more effective strategies for the “bottom up” design of these materials. [Preview Abstract] |
Friday, March 17, 2017 9:36AM - 10:12AM |
X25.00007: Frontiers of controlling energy levels at interfaces Invited Speaker: Norbert Koch The alignment of electron energy levels at interfaces between semiconductors, dielectrics, and electrodes determines the function and efficiency of all electronic and optoelectronic devices. Reliable guidelines for predicting the level alignment for a given material combination and methods to adjust the intrinsic energy landscape are needed to enable efficient engineering approaches. These are sufficiently understood for established electronic materials, e.g., Si, but for the increasing number of emerging materials, e.g., organic and 2D semiconductors, perovskites, this is work in progress. The intrinsic level alignment and the underlying mechanisms at interfaces between organic and inorganic semiconductors are discussed first. Next, methods to alter the level alignment are introduced, which all base on proper charge density rearrangement at a heterojunction. As interface modification agents we use molecular electron acceptors and donors, as well as molecular photochromic switches that add a dynamic aspect and allow device multifunctionality. For 2D semiconductors surface transfer doping with molecular acceptors/donors transpires as viable method to locally tune the Fermi-level position in the energy gap. The fundamental electronic properties of a prototypical 1D interface between intrinsic and p-doped 2D semiconductor regions are derived from local (scanning probe) and area-averaged (photoemission) spectroscopy experiments. Future research opportunities for attaining unsurpassed interface control through charge density management are discussed. [Preview Abstract] |
Friday, March 17, 2017 10:12AM - 10:24AM |
X25.00008: Stacking the Deck: Leveraging Surface Interactions to Tune Interfacial Electronic Structure Bret Maughan, Calley Eads, Percy Zahl, Peter Sutter, Oliver Monti We present results from a series of experiments aimed at understanding and controlling molecular interactions in phthalocyanine (Pc) thin-films on Cu(110) to tailor the interfacial electronic structure. Using low-temperature scanning tunneling microscopy (LT-STM), we identify interactions that drive surface-molecule coupling, molecular self-assembly and thin-film order. We provide evidence that interactions with native Cu adatoms play a pivotal role in self-assembly of Pc systems, along with anisotropic nanoribbon growth dynamics, supported by an agent-based kinetic Monte Carlo (AB-KMC) simulation. We show further that self-assembled nanoribbon length can be controlled using surface diffusion barriers and that ordered 2D thin-film growth is promoted by diminishing surface-molecule interactions that otherwise dominate native Cu(110) interfaces. Altogether, this detailed structural understanding allows us to interpret interfacial electronic structure and dynamics, uncovered through ultraviolet (UPS) and two-photon photoemission (2PPE) spectroscopy experiments, in molecular configuration-specific detail. In all, our understanding of interfacial processes guides strategic modifications to both surface and molecule to harness interfacial interactions and thereby modify the collective electronic structure of the interface. [Preview Abstract] |
Friday, March 17, 2017 10:24AM - 10:36AM |
X25.00009: Electronic structural properties of phenol adsorption of Fe$_{\mathrm{2}}$O$_{\mathrm{3}}$ nanoclusters and (0001) surface Nadra Sakr, Matthew Patterson, Orhan Kizilkaya, Richard Kurtz, Phillp Sprunger Temperature dependent electronic structure of phenol adsorbed on single-crystal Fe$_{\mathrm{2}}$O$_{\mathrm{3}}$(0001) and Fe$_{\mathrm{2}}$O$_{\mathrm{3}}$ nanoparticles is investigated. in an effort to further understand how environmentally persistent free radicals (EPFRs) are formed. EPFR formation on metal oxide powders is typically accompanied by a reduction of metal cations as electrons are transferred from the aromatic precursor. The current study takes a surface science approach to study the atomic-scale formation of EPFRs on single-crystal Fe$_{\mathrm{2}}$O$_{\mathrm{3}}$(0001) and 18 nm Fe$_{\mathrm{2}}$O$_{\mathrm{3}}$ nanoparticles in order to guide a more fundamental understanding of the mechanism of radical formation. Here we use synchrotron-based photoemission (UPS), XPS, FTIR, and EELS to probe the surface electronic and vibrational structure of phenol adsorbed on an environmentally abundant metal oxide in order to develop an atomic-scale understanding of the electronic structure of the composite organic/metal oxide system and better elucidate the physical interactions that produce known trends in the lifetime, reactivity, and biological activity of EPFRs. [Preview Abstract] |
Friday, March 17, 2017 10:36AM - 10:48AM |
X25.00010: Raman Enhancement Effect on Thin GaSe Flake and Its Thickness Dependence Lin Quan, Yuqing Song, Guanghui Zhang, Yukun Wu, Ke Jin, Huaiyi Ding, Nan Pan, Yi Luo, Xiaoping Wang Chemical enhancement is one of the important mechanisms in surface-enhanced Raman spectroscopy, however, its origin is still under debate. Two dimensional (2D) layered material is thought to be a strong candidate to investigate the chemical mechanism of Raman enhancement because it has flat surface, well defined structure and without the interference of electromagnetic enhancement. Herein we report the systematic studies of Raman enhancement effect on the gallium selenide (GaSe) flake by using copper phthalocyanine (CuPc) molecule as a probe. It is found that the Raman signal of CuPc on the monolayer GaSe can be significantly increased by one order of magnitude than that on the SiO2/Si substrate. Meanwhile, the enhancement effect is found to decrease with increasing the thickness of GaSe flake. The origin of the Raman enhancement is attributed to the chemical mechanism resulted from the charge transfer between the GaSe flake and the detected molecules. The supposition is further verified by the investigation of Raman enhancement effect of CuPc with different thicknesses on the GaSe flake. Our work will shed more light on the understanding of the chemical mechanism for Raman enhancement and expand more practical applications of GaSe. [Preview Abstract] |
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