Bulletin of the American Physical Society
APS March Meeting 2017
Volume 62, Number 4
Monday–Friday, March 13–17, 2017; New Orleans, Louisiana
Session V25: Focus Session Chemical Physics Frontiers at Interfaces IIIFocus
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Sponsoring Units: DCP Chair: Xiaoyang Zhu, Columbia University Room: 288 |
Thursday, March 16, 2017 2:30PM - 3:06PM |
V25.00001: Interfacial Exciton Dynamics in Atomically Thin Semiconductors Invited Speaker: William Tisdale Dielectric confinement in atomically thin transition metal dichalcogenides (TMDs) leads to excitonic behavior that differs markedly from bulk semiconductors. Through time-resolved optical microscopy studies of exciton transport and interfacial energy transfer in monolayer MoS$_{\mathrm{2}}$ and WS$_{\mathrm{2}}$, I will demonstrate the dominant role of dielectric screening in exciton-exciton interactions in this system, and how this affects our understanding of interfacial dynamics in atomically thin semiconductors. [Preview Abstract] |
Thursday, March 16, 2017 3:06PM - 3:18PM |
V25.00002: Using oxide nanostructures to control carrier transport properties Anders Hellman Selecting suitable material(s) for water splitting is an intricate dilemma, as materials with high solar-to-hydrogen (STH) conversion are typically not stable in aqueous environment and/or are scarce, whereas stable and abundant materials often exhibit unacceptable performance for commercialization [1]. For example, Fe$_{\mathrm{2}}$O$_{\mathrm{3}}$ is an abundant n-type semiconductor that has excellent stability in neutral and alkaline electrolytes, but so far the reported STH conversion efficiency has not exceeded 3{\%} [2]. A major factor hampering the performance Fe$_{\mathrm{2}}$O$_{\mathrm{3}}$, is the high charge recombination rate inside the semiconductor. Thus, there is a need to develop methods and designs to reduce charge recombination. Recently we showed that by joining different oxides we were able to control the charge recombination rate [3]. The control mechanism relies on the formation of dipole-like electric fields at the interface which, depending on the field direction, attract or repel minority carriers from the interface. Here we investigate the built-in electric field generated at the interfaces of Fe$_{\mathrm{2}}$O$_{\mathrm{3}}$/TiO$_{\mathrm{2}}$ and Fe$_{\mathrm{2}}$O$_{\mathrm{3}}$/Cu$_{\mathrm{2}}$O using first-principles methods. The results show how electronic band alignment and defects doping at the interface determine the direction and strength of the built-in field. Our understanding of the oxide nanostructures can be employed for designing and improving the performance of water-splitting photoelectrodes. [1]: A. Fujishima, et al., \textit{Surf. Sci. Rep.}, \textbf{2008}, 6, 515-582. [2]: D. K. Bora, et al., \textit{Energy Environ. Sci.}, \textbf{2013}, 6, 407-425. [3]: B. Iandolo, et al., \textit{Nano Lett.},~\textbf{2016},~\textit{16}, 2381--2386. [Preview Abstract] |
Thursday, March 16, 2017 3:18PM - 3:30PM |
V25.00003: Probing Ultrafast Electron Dynamics at Surfaces Using Soft X-Ray Transient Reflectivity Spectroscopy L. Robert Baker, Jakub Husek, Somnath Biswas, Anthony Cirri The ability to probe electron dynamics with surface sensitivity on the ultrafast time scale is critical for understanding processes such as charge separation, injection, and surface trapping that mediate efficiency in catalytic and energy conversion materials. Toward this goal, we have developed a high harmonic generation (HHG) light source for femtosecond soft x-ray reflectivity. Using this light source we investigated the ultrafast carrier dynamics at the surface of single crystalline $\alpha $-Fe$_{2}$O$_{3}$, polycrystalline $\alpha $-Fe$_{2}$O$_{3}$, and the mixed metal oxide, CuFeO$_{2}$. We have recently demonstrated that CuFeO$_{2}$ in particular is a selective catalyst for photo-electrochemical CO$_{2}$ reduction to acetate; however, the role of electronic structure and charge carrier dynamics in mediating catalytic selectivity has not been well understood. Soft x-ray reflectivity measurements probe the M$_{2}$,$_{3}$ edges of the \textit{3d} transition metals, which provide oxidation and spin state resolution with element specificity. In addition to chemical state specificity, these measurements are also surface sensitive, and by independently simulating the contributions of the real and imaginary components of the complex refractive index, we can differentiate between surface and sub-surface contributions to the excited state spectrum. Accordingly, this work demonstrates the ability to probe ultrafast carrier dynamics in catalytic materials with element and chemical state specificity and with surface sensitivity. [Preview Abstract] |
Thursday, March 16, 2017 3:30PM - 3:42PM |
V25.00004: Nonlinear Optical Detections of Structural Distortions at Degraded Fe-doped SrTiO$_{3}$ Interfaces Onur Kurt, David Ascienzo, Zehra Cevher, Steve Greenbaum, Thorsten Bayer, Clive Randall, Nicholas Madamopoulos, Yuhang Ren We report on the detection of structural distortions at anode and cathode interfaces in degraded reduced and oxidized Fe-doped SrTiO$_{3}$ (Fe:STO) crystals using optical second harmonic generation (SHG) and Raman spectroscopy in the reflection geometry. SHG spectra were collected from various regions across the interfaces. SHG intensity changes are associated to the formation of centrosymmetric Fe$^{4+}$:Ti$^{4+}$-O$_{6}$ octahedra and non-centrosymmetric Jahn-Teller distortions in the inner region of the reduced anode and the central region of the oxidized anode, respectively. These results are supported by Raman measurements taken from the anode interfaces of both crystals, which present Fe$^{4+}$ concentration and structural changes across the probed regions. Moreover, the accumulation of oxygen vacancies is identified across the cathode interfaces of both crystals. These structural distortions and Fe$^{4+}$ concentration changes are well correlated with the migration of oxygen ions and vacancies. [Preview Abstract] |
Thursday, March 16, 2017 3:42PM - 3:54PM |
V25.00005: Investigation of Electric Field--Induced Second Harmonic Generation from Fe-Doped SrTiO$_{\mathrm{3}}$ Interfaces David Ascienzo, Haochen Yuan, Steve Greenbaum, Thorsten Bayer, Russell Maier, Jian-Jun Wang, Clive Randall, Elizabeth Dickey, Haibin Zhao, Yuhang Ren Oxygen vacancy electromigration is a leading contributor to breakdown mechanisms such as resistance degradation in perovskite oxide dielectrics. Greater understanding of oxygen vacancy migration and the correlated formation of structural defects/strain at dielectric interfaces is crucial for improving lifetime and reliability in these materials. We report on structural changes at reduced and oxidized Fe-doped SrTiO$_{\mathrm{3}}$ interfaces detected by electric field-induced second harmonic generation (EFISHG). Under a dc-field, oxygen ions and holes migrate to the anode while oxygen vacancies and electrons migrate to the cathode. Vacancy migration to the cathode leads to electrostrictive distortions, described as Fe:Ti-O bond stretching and bending, in FeTiO$_{\mathrm{6}}$ octahedra. Differences in EFISHG responses are explained by intrinsic electric fields present at the interfaces whose local strength and polarity are influenced by the oxygen vacancy, Fe$^{\mathrm{3+}}$, and Fe$^{\mathrm{4+}}$ concentrations of the crystals. Results show optical SHG is a powerful tool for detecting structural changes at perovskite oxide interfaces due to oxygen vacancy migration. [Preview Abstract] |
Thursday, March 16, 2017 3:54PM - 4:30PM |
V25.00006: Efficient Hot Electron Transfer by Plasmon Induced Interfacial Charge Transfer Transition Invited Speaker: Tianquan Lian Surface plasmon resonance in metal nanostructures has been widely used to enhance the efficiency of semiconductors and/or molecular chromophore based solar energy conversion devices by increasing the absorption or energy transfer rate through the enhanced local field strength. In more recent years, it has been shown that excitation of plasmons in metal nanostructures can lead to the injection of hot electrons into semiconductors and enhanced photochemistry. This novel mechanism suggests that plasmonic nanostructures can potentially function as a new class of widely tunable and robust light harvesting materials for solar energy conversion. However, plasmon-induced hot electron injections from metal to semiconductor or molecules are still inefficient because of the competing ultrafast hot electron relaxation processes within the metallic domain. In this paper we discuss a recent study on the plasmon-exciton interaction mechanisms in colloidal quantum-confined semiconductor-gold nanorod heterostructures. In CdSe NRs with Au tips, the distinct plasmon band of the Au nanoparticles was completely damped due to strong interaction with the CdSe domain. Using transient absorption spectroscopy, we show that optical excitation of plasmons in the Au tip leads to efficient hot electron injection into the semiconductor nanorod. In the presence of sacrificial electron donors, this plasmon induced hot electron transfer process can be utilized to drive photoreduction reactions under continuous illumination. We propose that the strong metal/semiconductor coupling in CdSe/Au hetersostructures leads to a new pathway for this surprising efficient hot electron transfer. In this plasmon induced interfacial charge transfer transition (PICTT) the a plasmon decay by direct excitation of an electron from the metal to semiconductor, bypassing the competition with hot electron transfer in metal. Ongoing studies are examining the generality of this mechanism and exploring possible approaches for improving its efficiency through controlling the size and shape of the plasmonic and excitonic domains. [Preview Abstract] |
Thursday, March 16, 2017 4:30PM - 4:42PM |
V25.00007: Photo-induced surface reactivity and metallic nanoparticle deposition on ferroelectric Strontium Barium Niobate. Eftihia Barnes, Lawrence Christopher Mimun, Jonathon Brame, Aimee Poda, Christopher Warner In recent years, ferroelectric photocatalysts have attracted considerable attention due to their internal dipolar fields which separate photo-induced charge carriers, potentially leading to increased photocatalytic activity. In addition, photochemically generated reaction products have been shown to be spatially localized on ferroelectric surfaces, indicating that electrons and holes are spatially separated in the bulk. In this work, we studied the photo-induced surface reactivity of ferroelectric Sr$_{\mathrm{0.4}}$Ba$_{\mathrm{0.6}}$Nb$_{\mathrm{2}}$O$_{\mathrm{6}}$ (SBN:40) and Sr$_{\mathrm{0.61}}$Ba$_{\mathrm{0.39}}$Nb$_{\mathrm{2}}$O$_{\mathrm{6}}$ (SBN:61) single crystals by employing photochemical marker reactions. In more detail, we investigated the effect of the excitation wavelength, light intensity, exposure time, concentration of the reaction solution, and domain configuration on the morphology and spatial distribution of photo-deposited metallic nanostructures. By tuning various parameters, such as the solution concentration and light intensity, we can achieve spatially uniform nanoparticle deposition or favor the formation of nanoparticle chains. [Preview Abstract] |
Thursday, March 16, 2017 4:42PM - 4:54PM |
V25.00008: The electrocatalytic properties of doped $\mathbf{TiO_{2}}$ Rasmus Karlsson, Heine Hansen, Thomas Bligaard, Ann Cornell, Lars Pettersson "Dimensionally stable anodes" (DSA), rutile oxide electrode coatings of 30\% $\mathrm{RuO_{2}}$ and 70\% $\mathrm{TiO_{2}}$, deposited on Ti substrates, have been used industrially for electrosynthesis of chlorine gas since the 1970s. Nevertheless, a fundamental understanding of the origin of their high activity and chlorine selectivity has been lacking. In this presentation, I will indicate how calculations using density functional theory and the computational hydrogen electrode have been used to model the electrocatalysis of both the oxygen evolution reaction (OER) and the chlorine evolution reaction (ClER) on surfaces with the typical DSA composition. Our calculations suggest a fundamental reason for the high ClER selectivity of Ru-doped $\mathrm{TiO_{2}}$: Ru dopants activate nearby Ti surface sites, and create active sites with the optimal descriptor value for high ClER activity and selectivity. I will also present results from a broad screening study, where we examine the effects from doping $\mathrm{TiO_{2}}$ with each of the fourth, fifth and sixth-row transition metals. We suggest several dopants that can activate $\mathrm{TiO_{2}}$ for either the OER or the ClER, including dopants not previously tested experimentally. [Preview Abstract] |
Thursday, March 16, 2017 4:54PM - 5:06PM |
V25.00009: Single Molecule Electrochemistry: Impact of Surface Site Heterogeneity. Bo Fu, Colin Van Dyck, Stephanie Zaleski, Richard Van Duyne, Mark Ratner The efforts on coupling the electrochemistry with single molecule spectroscopy increased significantly during the past decade in observing the redox processes at the nanoscale level. Zaleski et al recently studied the reduction reaction of the rhodamine 6G (R6G) molecule by electrochemical single molecule surface-enhanced Raman spectroscopy (EC-SMSERS) and showed a broader reduction potential distribution than that observed in the ensemble electrochemistry experiment. In order to understand and explain this experiment theoretically, we related the adsorption energy of R6G cation on AgNP with its reduction potential and computed this binding energy at various local surface sites using periodic density functional theory. Eight local surface sites and five molecular configurations at each site were explored. A semi-quantitative agreement was obtained between the reduction potential distribution of the Faradaic events and the adsorption energy distribution of the strongest binding states at each explored surface site. We further inferred that the underpotential reduction events corresponds to the less stable binding states compared to those matched with Faradaic events. [Preview Abstract] |
Thursday, March 16, 2017 5:06PM - 5:18PM |
V25.00010: Adsorption Behavior, Thermodynamics, and Kinetics of the Methanol Decomposition Reaction on defective graphene-supported Pt13 Raymond Gasper, Ashwin Ramasubramaniam Defective graphene has been shown experimentally to be an excellent support for transition-metal electrocatalysts in direct methanol fuel cells. Prior computational modeling has shown that the improved catalytic activity of graphene-supported metal clusters is in part due to increased resistance to catalyst sintering and CO poisoning, but the increased reaction rate for the methanol decomposition reaction (MDR) is not yet fully explained. Using DFT, we investigate the adsorption of MDR intermediates and reaction thermodynamics on defective graphene-supported Pt13 nanoclusters with realistic, low-symmetry morphologies. We find that the support-induced shifts in Pt13 electronic structure correlate well with a rigid shift in adsorption of MDR intermediates, and that adsorption energy scaling relationships perform well on the low-symmetry surface. We investigate the reaction kinetics and thermodynamics, including testing the effectiveness of scaling relationships for predicting reaction barriers on the nanoclusters. Using these fundamental data, we perform microkinetic modeling to quantify the effect of the support on the MDR, and to understand how the support influences surface coverages, CO poisoning, and the relationships between reaction pathways. [Preview Abstract] |
Thursday, March 16, 2017 5:18PM - 5:30PM |
V25.00011: Engineering Surface Properties of Reactive Materials Richa Padhye The effect of processing liquids on particle surface hydration and subsequent reactivity of mixtures containing aluminum (Al) with different oxidizing agents was investigated. Processing mixtures of Al and PTFE using a non-polar liquid limited surface hydration and produced significantly lower reactivity than the same mixture processed in a polar liquid. This understanding has been extended toward assessing the influence of processing liquid on reactivity of Al with other oxidizing agents, specifically CuO and MoO3. Al$+$CuO showed no difference in reactivity as a function of processing liquid. But MoO3 was shown to be more hydration sensitive than CuO and the added water concentration in the overall mixture aided combustion and increased the flame speed when compared to Al$+$MoO3 processed in a non-polar liquid. To better comprehend this behavior, density functional theory (DFT) calculations were performed to understand molecular variations on an alumina surface. The analysis has strong implications for the reactivity of aluminum (Al) particles passivated by an alumina shell. [Preview Abstract] |
Thursday, March 16, 2017 5:30PM - 6:06PM |
V25.00012: Broadband Deep UV Spectra of Interfacial Aqueous Iodide Invited Speaker: Richard Saykally The behavior of ions at aqueous interfaces influences vital processes in many fields, but has long remained a subject of controversy. Over the past decade, counterintuitive surface concentration enhancement of several ions in aqueous solution has been demonstrated via nonlinear laser spectroscopy and mass spectrometry. While the evidence for significant ion enhancement at the air-water interface is convincing, the mechanism remains incompletely understood. Toward this end, we present the full charge-transfer-to-solvent (CTTS) spectrum of interfacial aqueous iodide measured in a single laser shot with a newly developed broadband deep UV-SFG technique, clearly revealing a \textasciitilde 7 nm redshift and a significant linewidth narrowing relative to bulk solution spectra. KI and NaI solutions yield indistinguishable results. Additionally, we observe a dramatic change in the relative intensities of the J $=$ 3/2 and J $=$ 1/2 CTTS transitions. $\backslash $Rizzuto, A.M., Irgen-Gioro, Shawn, Eftekhari-Bafrooei, A., Saykally, R. J. "Broadband Deep UV Spectra of Interfacial Aqueous Iodide" \textit{J. Phys. Chem. Lett.}, \textbf{7}, 3882-3885 (2016).$\backslash $D.E. Otten, P. Shaffer, P. Geissler, R.J. Saykally, \textit{Elucidating the Mechanism of Selective Ion Adsorption to the Liquid Water Surface}, PNAS \textbf{109}, 701(2012). [Preview Abstract] |
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