Bulletin of the American Physical Society
APS March Meeting 2016
Volume 61, Number 2
Monday–Friday, March 14–18, 2016; Baltimore, Maryland
Session L32: Dynamic Interactions Between NanostructuresFocus
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Sponsoring Units: DCP Chair: Todd Krauss, University of Rochester Room: 332 |
Wednesday, March 16, 2016 11:15AM - 11:51AM |
L32.00001: \textbf{Efficient Light-driven Long Distance Charge Separation and H}$_{\mathbf{2}}$\textbf{ Generation in Semiconductor Quantum Rods and Nanoplatelets} Invited Speaker: Tianquan Lian Quantum confined semiconductor nanocrystals (0D quantum dots, 1D quantum rods and 2D quantum platlets) have been intensively investigated as light harvesting and charge separation materials for photovoltaic and photocatalytic applications. The efficiency of these semiconductor nanocrystal-based devices depends on many fundamental processes, including light harvesting, carrier relaxation, exciton localization and transport, charge separation and charge recombination. The competition between these processes determines the overall solar energy conversion (solar to electricity or fuel) efficiency. Semiconductor nano-heterostructures, combining two or more material components, offer unique opportunities to control their charge separation properties by tailoring their compositions, dimensions and spatial arrangement. Further integration of catalysts (heterogeneous or homogeneous) to these materials form multifunctional nano-heterostructures. Using 0D, 1D and 2D CdSe/CdS/Pt heterostructures as model systems, we directly probe the above-mentioned fundamental exciton and carrier processes by transient absorption and time-resolved fluorescence spectroscopy. We are examining how to control these fundamental processes through the design of heterostructures to achieve long-lived charge separation and efficient H$_{2}$ generation. In this talk, we will discuss a new model for exciton dissociation by charge transfer in quantum dots (i.e. Auger assisted electron transfer), mechanism of 1D and 2D exciton transport and dissociation in nanorods, and key factors limiting H$_{2}$ generation efficiency in CdSe/CdS/Pt nanorod heterostructures. [Preview Abstract] |
Wednesday, March 16, 2016 11:51AM - 12:03PM |
L32.00002: How to harvest solar energy with the photosynthetic reaction center. Alexander Balaeff, Justin Reyes Photosynthetic reaction center (PRC) is a protein complex that performs a key step in photosynthesis: the electron-hole separation driven by photon absorbtion. The PRC has a great promise for applications in solar energy harvesting and photosensing. Such applications, however, are hampered by the difficulty in extracting the photogenerated electric charge from the PRC. To that end, it was proposed to attach the PRC to a molecular wire through which the charge could be collected. In order to find the attachment point for the wire that would maximize the rate of charge outflow from the PRC, we performed a computational study of the PRC from the R. virdis bacterium. An ensemble of PRC structures generated by a molecular dynamics simulation was used to calculate the rate of charge transport from the site of initial charge separation to several trial sites on the protein surface. The Pathways model was used to calculate the charge transfer rate in each step of the network of heme co-factors through which the charge transport was presumed to proceed. A simple kinetic model was then used to determine the overall rate of the multistep charge transport. The calculations revealed several candidate sites for the molecular wire attachment, recommended for experimental verification. [Preview Abstract] |
Wednesday, March 16, 2016 12:03PM - 12:15PM |
L32.00003: Managing photons and carriers for photocatalysis Isabell Thomann, Hossein Robatjazi, Shah Bahauddin, Chloe Doiron, Xuejun Liu, Thejaswi Tumkur, Wei-Ren Wang, Parker Wray While small plasmonic nanoparticles efficiently generate energetic hot carriers, light absorption in a monolayer of such particles is inefficient, and practical utilization of the hot carriers in addition requires efficient charge-separation. Here we describe our approach to address both challenges. By designing an optical cavity structure for the plasmonic photoelectrode [1], light absorption in these particles can be significantly enhanced, resulting in efficient hot electron generation. Rather than utilizing a Schottky barrier to preserve the energy of the carriers, our structure allows for their direct injection into the adjacent electrolyte. On the substrate side, the plasmonic particles are in contact with a wide band gap oxide film that serves as an electron blocking layer but accepts holes and transfers them to the counter electrode. The observed photocurrent spectra follow the plasmon spectrum, and demonstrate that the extracted electrons are energetic enough to drive the hydrogen evolution reaction. A similar structure can be designed to achieve broadband absorption enhancement in monolayer MoS2 [2]. Time permitting, I will discuss charge carrier dynamics in hybrid nanoparticles composed of plasmonic / two-dimensional materials, and applications of photo-induced force microscopy to study photocatalytic processes. [1] Nano Letters, 2015, 15 (9), p 6155 [2] Photon management strategies for monolayer MoS$_{\mathrm{2}}$, submitted [Preview Abstract] |
Wednesday, March 16, 2016 12:15PM - 12:51PM |
L32.00004: Charge Transfer Dynamics in Semiconductor Quantum Dots Relevant to Solar Hydrogen Production. Invited Speaker: Todd Krauss Artificial conversion of sunlight to chemical fuels has attracted attention for several decades as a potential source of clean, renewable energy. For example, in light-driven proton reduction to molecular hydrogen, a light-absorbing molecule (the photosensitizer) rapidly transfers a photoexcited electron to a catalyst for reducing protons. We recently found that CdSe quantum dots (QDs) and simple aqueous Ni$^{\mathrm{2+}}$ salts in the presence of a sacrificial electron donor form a highly efficient, active, and robust system for photochemical reduction of protons to molecular hydrogen. To understand why this system has such extraordinary catalytic behavior, ultrafast transient absorption (TA) spectroscopy studies of electron transfer (ET) processes from the QDs to the Ni catalysts were performed. CdSe QDs transfer photoexcited electrons to a Ni-dihydrolipoic acid (Ni-DHLA) catalyst complex extremely fast and with high efficiency. Even under high fluence, the relative fraction of TA signal due to ET remains well over 80{\%}, and depopulation from exciton-exciton annihilation is minimal (6{\%}). We also found that increasing QD size and/or shelling the core CdSe QDs with a shell of CdS slowed the ET rate, in agreement with the relative efficiency of photochemical H$_{\mathrm{2}}$ generation. The extremely fast ET provides a fundamental explanation for the exceptional photocatalytic H$_{\mathrm{2}}$ activity of the CdSe QD/Ni-DHLA system and guides new directions for further improvements. [Preview Abstract] |
Wednesday, March 16, 2016 12:51PM - 1:03PM |
L32.00005: Multi-photon Photoemission Dynamics in TiO$_{2}$ Adam Argondizzo, Xuefeng Cui, Cong Wang, Huijuan Sun, Honghui Shang, Jin Zhao, Hrvoje Petek TiO$_{2}$ is a material of interest in photocatalytic and photovoltaic applications. Until recently, however, the ability to probe the electron dynamics of this system has been limited to optical experiments. By probing the rutile TiO$_{2}$(110) surface using two-photon photoemission (2PP) with a tunable ultrashort ($\sim$20 fs) laser pulse we investigated the dynamics of electrons excited to its conduction band. Previous 2PP experiments on protic solvent covered TiO$_{2}$ surfaces using 400 nm (3.1 eV) light revealed the presence of an unoccupied surface adsorbate-induced “wet electron” state. By expanding such measurements at higher photon energy we have found a pair of new nearly degenerate unoccupied states located at 2.7 and 2.8 eV above the Fermi level. Based on the calculated electronic structure and optical transition moments, as well as related spectroscopic evidence, we assign these resonances to transitions between Ti-3d bands of nominally t$_{2g}$ and e$_{g}$ symmetry, which are split by crystal field. A detailed understanding of the t$_{2g}$-e$_{g}$ transitions is essential for the characterization of electron dynamics and adsorbate induced resonances in photocatalytic processes on TiO$_{2}$. [Preview Abstract] |
Wednesday, March 16, 2016 1:03PM - 1:15PM |
L32.00006: ABSTRACT WITHDRAWN |
Wednesday, March 16, 2016 1:15PM - 1:51PM |
L32.00007: Hole transfer dynamics from QDs to tethered ferrocene derivatives Invited Speaker: A. Paul Alivisatos Quantum dots (QDs) have shown particular promise in recent years as light absorbers in solar energy conversion schemes. However, in solution junction solar devices such as QD-sensitized solar cells and photocatalytic water splitting systems, efficiencies are often limited by hole transfer from the photoexcited QD. This process is sluggish and can lead to oxidative photocorrosion of the QD material. In order to design highly efficient nanocrystal systems with hole transfer rates that outcompete these undesirable processes, a fundamental understanding of the parameters that control these rates is imperative.\\ \\We have developed a model system to study charge transfer from QDs to surface bound acceptors, to fundamentally understand the charge transfer processes for QD systems, namely electronic coupling between the donor and acceptor and the thermodynamic driving force for the hole transfer process. Specifically, we examine hole transfer from the nearly spherical CdSe-core CdS-shell QDs with photoluminescence (PL) quantum yields over 80{\%} to ferrocene derivatives bound to the QD surface via an alkane thiol linker. In this system, we mitigate the ill-defined nonradiative charge dynamic pathways that are intrinsic to native CdSe cores, and then controllably engineer on the surface charge acceptors with well-defined oxidation potentials, spatial distribution, and quantity. By Measuring the PL lifetime decay and calibrating the number of hole acceptor ligands per QD via quantitative `H NMR, we extracted the hole transfer rate per acceptor. This rate per acceptor could be varied over four orders of magnitude by changing the coupling between donor and acceptor through modulations in the CdS shell thickness and alkane chain length of the molecule. Furthermore, owning to the large number of acceptors on the surface, we achieve systems in which \textasciitilde 99{\%} of the photoexcited holes are transferred to these well-defined mediators.\\ \\We further mapped the relationship between the thermodynamic driving force and hole transfer rate. We systematically tune the driving force over nearly 1 eV by varying the redox potentials of the ferrocene ligands through functionalization of the cyclopentadiene rings. Our results show a monotonic increase in rate as a function of the increasing driving force with no observed inverted region. This behavior is understood by considering the residual electron in the QD conduction band, which could exhibit intraband excitations coupled to the hole transfer, thus creating a many-state system that would eliminate the inverted region. The resulting relationship between rate and energetic driving force for hole transfer can be used to design QD-molecular systems that maximize interfacial charge transfer rates while minimizing energetic losses associated with the driving force. [Preview Abstract] |
Wednesday, March 16, 2016 1:51PM - 2:03PM |
L32.00008: Cooperative Electron-Hole Dynamics at the Organic Donor-Acceptor Interface Chee Kong Lee, Shi Liang, Adam Willard Charge transfer (CT) excitons are Coulombically bound electron and hole pairs located in spatially separate regions. They play an important role in both light emission of organic light emitting devices and the generation of photocurrent in organic photovoltaic. For some donor/acceptor blends the lowest electronic excitations are triplet CT states, and in these materials the photoluminescence and photocurrent generation exhibit a non-trivial magnetic field dependence due to its effect on singlet-triplet intersystem crossing and reverse intersystem crossing rates. Recent experiments have demonstrated that in these materials bound electron-hole pairs can move geminately over distance of 5-10nm confirming the transport of CT excitons despite strong Coulombic attraction. These experiments can be understood with a numerical model combining kinetic Monte Carlo and the quantum master equation. The model contains a minimal set of physical elements, and yet is able to quantitatively reproduce the experimental results. More importantly, the model provides insights into properties that otherwise cannot be obtained from experiments. Here I present the details of this model along with the physical insights it has provided into this particular class of materials. [Preview Abstract] |
Wednesday, March 16, 2016 2:03PM - 2:15PM |
L32.00009: Probing of Charge Transfer States at Buried Organic Interfaces with Even-Order Spectroscopy Ravindra Pandey, Aaron Moon, Sean Roberts Organic thin film photovoltaics (OPV) are an emerging economically competitive technology that combines manufacturing adaptability, low-cost processing and a lightweight, flexible device end-product. At junctions formed between organic electron-donating and electron-accepting materials, the abrupt change in the dielectric properties can strongly perturb the density of states of the OPV. This can substantially alter the driving force for charge transfer between these materials. Electronic Sum Frequency Generation (ESFG), owing to its inherent interfacial sensitivity, is ideally suited to probe buried interfaces. Here, we report the ESFG spectra of Copper Phthalocyanine (CuPc) films, deposited on SiO$_{\mathrm{2}}$ measured for both reflection and transmission geometries. Three peaks are observed that roughly correlate with resonances that comprise CuPc's Q-band absorption but display slight shifts and amplitude changes with respect to CuPc's bulk absorption spectrum. Experimental results are compared with calculations based on a thin film interference model that accounts for ESFG emitted from both the CuPc:Air and CuPc:SiO$_{\mathrm{2}}$ interface as well as contributions to the signal from higher order source terms from the bulk. The model reveals a difference in the density of states between the two interfaces and suggests that by combining experimental transmission and reflection data it is possible to separate bulk and interfacial contributions to ESFG spectra. [Preview Abstract] |
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