Bulletin of the American Physical Society
APS March Meeting 2016
Volume 61, Number 2
Monday–Friday, March 14–18, 2016; Baltimore, Maryland
Session Y33: Organic Electronics and Photonics - Spin Transport and PhotophysicsFocus
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Sponsoring Units: FIAP Chair: Bryan Boudouris, Purdue University Room: 336 |
Friday, March 18, 2016 11:15AM - 11:27AM |
Y33.00001: Engineering of spin injection and spin transport in organic spin valves using $\pi $-conjugated polymer brushes. Rugang Geng, Anandi Roy, Ram Subedi, Jason Locklin, Tho Nguyen, Wenbo Zhao, Xiaoguang Li Charge transport in amorphous organic semiconductors is governed by carriers hopping between localized states with small spin diffusion length. Furthermore, the spin interfacial resistance of organic spin valves (OSVs) is poorly controlled resulting in controversial reports of the magnetoresistance response. Here, we used surface initiated Kumada transfer polycondensation to covalently graft $\pi $-conjugated poly(3-methylthiophene) brushes from the La$_{0.67}$Sr$_{0.33}$MnO$_{3}$ (LSMO) bottom electrode. The covalent attachment along with the brush morphology allows for more control over the LSMO/brush interfacial resistance and large spacer mobility. Remarkably, with 15 nm brush spacer layer, we observed an optimum magnetoresistance (MR) effect of 70{\%} at cryogenic temperatures and a MR of 2.7{\%} at 280K. The temperature dependence of the MR is nearly an order of magnitude weaker than that found in control OSVs made from spin-coated poly(3-hexylthiophene). Using a variety of different brush layer thicknesses, the thickness dependent MR at 20K was investigated. A spin diffusion length of 20 nm at 5 mV junction voltage rapidly increases to 55 nm at -280 mV. [Preview Abstract] |
Friday, March 18, 2016 11:27AM - 11:39AM |
Y33.00002: Intrinsic spin and momentum relaxation in organic single-crystalline semiconductors probed by ESR and Hall measurements Junto Tsurumi, Roger Häusermann, Shun Watanabe, Chikahiko Mitsui, Toshihiro Okamoto, Hiroyuki Matsui, Jun Takeya Spin and charge momentum relaxation mechanism has been argued among organic semiconductors with various methods, devices, and materials. However, little is known in organic single-crystalline semiconductors because it has been hard to obtain an ideal organic crystal with an excellent crystallinity and controllability required for accurate measurements. By using more than 1-inch sized single crystals which are fabricated via contentious edge-casting method developed by our group, we have successfully demonstrated a simultaneous determination of spin and momentum relaxation time for gate-induced charges of 3,11-didecyldinaphtho[2,3-$d$:2',3'-$d$']benzo[1,2-$b$:4,5-$b$']dithiophene, by combining electron spin resonance (ESR) and Hall effect measurements. The obtained temperature dependences of spin and momentum relaxation times are in good agreement in terms of power law with a factor of approximately -2. It is concluded that Elliott-Yafet spin relaxation mechanism can be dominant at room temperature regime (200 -- 300 K). Probing characteristic time scales such as spin-lattice, spin-spin, and momentum relaxation times, demonstrated in the present work, would be a powerful tool to elucidate fundamental spin and charge transport mechanisms. [Preview Abstract] |
Friday, March 18, 2016 11:39AM - 11:51AM |
Y33.00003: Vast Hole- and Electron-Polaron Spatial Extent in Oligomeric $\pi $-Conjugated Porphyrin Arrays Paul Angiolillo, Jeff Rawson, Michael Therien \textit{meso}-Ethyne bridged $\pi $-conjugated zinc porphyrin oligomers (PZn$_{\mathrm{n}}$ compounds) have been demonstrated to evince lowest excited singlet states that are globally delocalized. It has also previously been shown that hole-polaron states of these oligomers exhibit delocalization lengths that mirror the molecular spatial dimension, 7.5 nm in the case of the heptamer. Here we demonstrate that the electron-polaron states in PZn$_{\mathrm{n}}$ compounds also feature vast areal delocalization. This finding is evidenced by concurrent optical and electron spin resonance measurements, coupled with electronic structure calculations that suggest atypically small reorganization energies for one-electron reduction of these materials. These results are buttressed by electron spin relaxation measurements of PZn$_{\mathrm{n}}$ electron polarons that show that both T$_{\mathrm{1}}$ and T$_{\mathrm{2}}$ relaxation times are unusually large, on the order of 10$^{\mathrm{3}}$ ns and 10$^{\mathrm{2}}$ ns, respectively. Since rapid charge delocalization defines an important mechanism that mitigates Coulombic stabilization of photogenerated electron-hole pairs to create separated free charge carriers, and spin polarization lifetimes feature prominently in spin currents, these findings identify conjugated materials with exceptional optical, electronic, and spintronic properties. [Preview Abstract] |
Friday, March 18, 2016 11:51AM - 12:03PM |
Y33.00004: Magnetization Dynamics of Organic-based Magnetic Heterostructures Michael Chilcote, Yu Lu, Hailong Wang, Fengyuan Yang, Ezekiel Johnston-Halperin We present temperature dependent ferromagnetic resonance measurements of both isolated magnetic films and bilayers, including all organic and organic/inorganic hybrid magnetic heterostructures. These results establish organic magnetic heterostructures as an exciting new materials platform for the exploration of the fundamental mechanisms driving magnetic ordering in organic-based materials and promise the extension of organic spintronics into the regime of dynamically-driven spin currents, such as those found in spin pumping. The low cost, low-temperature conformal deposition of organic-based thin film magnets makes them an attractive class of materials for device applications. For example, they offer the potential for novel applications in high frequency magnetoelectronics on flexible substrates. Our materials are of the form M[Acceptor]$_{x}$ (M $=$ transition metal, x $\approx $ 2), exhibit room temperature magnetic ordering, and provide the opportunity to tailor magnetic properties through the selection of the transition metal ions and organic ligands. In particular, we focus on ferrimagnetic films and heterostructures where M $=$ vanadium and the organic ligands are tetracyanoethylene (TCNE), ethyl tricyanoethylene carboxylate (ETCEC), and methyl tricyanoethylene carboxylate (MeTCEC). [Preview Abstract] |
Friday, March 18, 2016 12:03PM - 12:15PM |
Y33.00005: Large Magnetoresistance at High Bias Voltage in Double-layer Organic Spin Valves R. C. Subedi, S. H. Liang, R. Geng, Q.T. Zhang, L. Lou, J. Wang, X. F. Han, T. D. Nguyen We report studies of magnetoresistance (MR) in double-layer organic spin valves (DOSV) using tris (8-hydroxyquinolinato) aluminum (Alq$_{\mathrm{3}})$ spacers. The device exhibits three distinct resistance levels depending on the relative magnetizations of the ferromagnetic electrodes. We observed a much weaker bias voltage dependence of MR in the device compared to that in the conventional organic spin valve (OSV). The MR magnitude reduces by the factor of two at 0.7~V bias voltage in the DOSV compared to 0.02~V in the conventional OSV. Remarkably, the MR magnitude reaches 0.3{\%} at 6~V bias in the DOSVs, the largest MR response ever reported in OSVs at this bias. Our finding may have a significant impact on achieving high efficient bipolar OSVs strictly performed at high voltages. [Preview Abstract] |
Friday, March 18, 2016 12:15PM - 12:27PM |
Y33.00006: Multi-scale modeling of spin transport in organic semiconductors Shayan Hemmatiyan, Amaury Souza, Pascal Kordt, Erik McNellis, Denis Andrienko, Jairo Sinova In this work, we present our theoretical framework to simulate simultaneously spin and charge transport in amorphous organic semiconductors. By combining several techniques e.g. molecular dynamics, density functional theory and kinetic Monte Carlo, we are be able to study spin transport in the presence of anisotropy, thermal effects, magnetic and electric field effects in a realistic morphologies of amorphous organic systems. We apply our multi-scale approach to investigate the spin transport in amorphous Alq3 (Tris(8-hydroxyquinolinato)aluminum) and address the underlying spin relaxation mechanism in this system as a function of temperature, bias voltage, magnetic field and sample thickness. [Preview Abstract] |
Friday, March 18, 2016 12:27PM - 12:39PM |
Y33.00007: Chromophoric disorder in conjugated polymers: the cursious case of P3HT Lena Simine, Peter Rossky The origin of the broad absorption spectrum of conjugated polymers is discussed. Motivated by the open questions posed in the recent experimental literature, we investigate theoretically the chromophoric disorder in single molecule poly(3hexyl)thiophene (P3HT) at the atomic level using quantumclassical simulations. We reproduce the absorption spectrum and confirm qualitatively the prediction of simplified models the localization length of the first excited state decreases with increasing temperature. Counter to expectation, the same trend is observed for the the average energy of the chromophore: in spite of a shorter localization length, the spectrum of the hot chromophore is redshifted with respect to its cold counterpart. We trace this peculiarity to the anharmonicity of the underlying torsional potential which allows preferential access to more planar interring conformations at high temperature. The contributions of bending to the transitional energies, the origi n of inhomogeneous broadening and the possibility of classification of the chromophore as planar/twisted/bent atlow and high temperatures are discussed. [Preview Abstract] |
Friday, March 18, 2016 12:39PM - 12:51PM |
Y33.00008: \textbf{Varying the apparent conduction mechanism in polymer semiconductors} Emily G. Bittle, Hyun Wook Ro, James I. Basham, Dean Delongchamp, David Gundlach, Oana Jurchescu The weak van der Waals inter-molecular interactions in organic semiconductors (OSCs) result in large variations in transport behavior ranging from hopping to band-like. Accurately measuring and modelling charge transport is a prerequisite to establishing robust transport-microstructure correlations and developing predictive structure-function relationships for optimized materials design and processing. Field-effect transistors have become a favored test structure for parameterizing and benchmarking the electronic properties of OSCs due to their ease of fabrication, measurement, and possible use in commercial applications. However, correctly analyzing transistor current-voltage measurements to extract material properties has proven difficult, as parasitic effects influence the device electrical properties and mask intrinsic material properties. Here, we use impedance spectroscopy to evaluate the effects of contacts on device operation and extract the properties of the channel which we compare with conventional DC measurements. We apply this approach to model systems of the widely studied polymer regioregular poly(3-hexylthiophene-2,5-diyl) which we engineer through different solidification kinetics to achieve distinct, well characterized degrees of molecular order. When increasing the order we find that the transport changes from field enhanced to field independent. This study addresses the origins of transport behavior seen in OSCs while discerning non-linear contact effects from field dependent transport. [Preview Abstract] |
Friday, March 18, 2016 12:51PM - 1:03PM |
Y33.00009: Suppression of molecular vibrations in strained organic semiconductors Takayoshi Kubo, Roger Häusermann, Junto Tsurumi, Junshi Soeda, Yugo Okada, Yu Yamashita, Chikahiko Mitsui, Toshihiro Okamoto, Hiroyuki Matsui, Jun Takeya, Susumu Yanagisawa, Norihisa Akamatsu, Atsushi Shishido The inherent softness of organic semiconductors, which makes them totally suited for flexible applications, also offers us an idea on the modulation of the charge transport in largely strained crystal structure. Here, solution-processed single crystal of 3,11-didecyldinaphto[2,3-{\it d}:2',3'-{\it d}']benzo[1,2-{\it b}:4,5-{\it b}']dithiophene (C$_{10}$-DNBDT-NW)\footnote{C. Mitsui, \textbf{Adv. Mater.} 6, 4546} is uniaxially and compressively strained for 3\% simply by bending the flexible substrate. With this strain, the field-effect mobility increases dramatically from 9.7 cm$^{2}$/Vs to 16.5 cm$^{2}$/Vs by up to 70\%, which is reversible and repeatable for successive bending. Combined with X-ray diffraction and low temperature measurement, a series of calculations based on density functional theory reveal the origin of the enhanced charge transport to be the suppression of the thermal fluctuation of the molecules rather than the slight change of the band structure. Our work shows that compression of the crystal structure directly hinders molecular vibrations and thus leads to the suppression of dynamic disorder, which is a unique mechanism in soft organic semiconductors. [Preview Abstract] |
Friday, March 18, 2016 1:03PM - 1:15PM |
Y33.00010: Illumination of Conjugated Polymer in Solution Alters its Conformation and Thermodynamics: The Role of Incident Light Intensity, Wavelength and Exposure Time Brian Morgan, Mark Dadmun The importance of chain structure in conjugated polymer-based material active layers and its relation to device efficiencies in OPVs, organic field transistors, and OLEDs, has been well established. However the influence of light absorption on the conjugated polymer structure is not well understood. We have employed small angle neutron scattering to investigate structural changes occurring in solutions of poly(3-hexylthiophene-2,5-diyl) with exposure to white light. Our previous results indicate significant change in the structure of the polymer upon illumination, an effect we attribute to an alteration in the thermodynamic interactions of the polymer with the surrounding solvent. In order to further our understanding of this phenomenon, we have studied the modulation of these light/dark structural changes as a function of solvent choice, incident light intensity, illumination wavelength, and light exposure duration. Analysis of this data allows refinement and increased control of these light-initiated effects, moving our efforts closer to the development of a powerful, non-destructive, and tunable method for controlling polymer conformation in solution and novel light-responsive materials. [Preview Abstract] |
Friday, March 18, 2016 1:15PM - 1:27PM |
Y33.00011: Real-time charge carrier motion in P3HT studied with Kelvin Probe Microscopy Chloe Castaneda, Alyina Zaidi, Jason Moscatello, Katherine Aidala We have developed a technique that uses scanning probe microscopy (SPM) to study the real-time injection and extraction of charge carriers in organic semiconductor devices. We investigate P3HT (full name) in an inverted field effect transistor geometry with gold electrodes. By positioning the SPM tip at an individual location and using Kelvin probe microscopy to record the potential over time, we can record how the charge carriers respond to changing the backgate voltage while the source and drain electrodes are grounded. We see relatively fast screening for negative backgate voltages because holes are quickly injected into the P3HT film.~The screening is slower for positive gate voltages, because some of these holes are trapped and therefore less mobile. We compare P3HT transistors with different fabrication procedures that are expected to change the trap distribution: no silanization of the oxide and no annealing, silanization and no annealing, and both silanization and annealing. By incrementally stepping the gate voltage, we probe different trap depths. The recorded change in potential over time is best fit by a double exponential, suggesting two physical mechanisms involved in screening. [Preview Abstract] |
Friday, March 18, 2016 1:27PM - 1:39PM |
Y33.00012: Spatially resolved charge transport study in discotic liquid crystalline organic semiconductors Sanjoy Paul, Alexander Semyonov, Nathan J. Dawson, Kenneth D. Singer, Robert J. Twieg, Brett Ellman Spatially resolved time-of-flight photogeneration and mobility have been measured on a discotic liquid crystalline organic semiconductor using scanning time-of-flight microscopy (STOFm). STOFm simultaneously obtains time-of-flight transients and polarized transmittance across the sample. Various shapes in time-of-flight transients were observed and extracted charge transport parameters such as photogeneration efficiency, mobility, and trapping show significant spatial variation. In some cases these can be linked to electrode surface inhomogeneities. Detailed measurement methodology, experimental results and challenges in their analysis will be discussed. [Preview Abstract] |
Friday, March 18, 2016 1:39PM - 1:51PM |
Y33.00013: The Effects of Stoichiometry on the Optical Properties of PTZ-TCNQ Charge Transfer Crystals Iris Stone, Jaydeep Joshi, Robert Smith, Scott Melis, Edward Van Keuren, Patrick Vora Charge transfer (CT) crystals are two-component organic materials formed by stacked pairs of donor and acceptor molecules. Depending on the choice of donor and acceptor molecules it is possible to achieve semiconducting, insulating, or metallic characteristics, making the CT crystal platform potentially transformative for applications in low-cost flexible electronics. The use of phenothiazine (PTZ) donors and tetracyanoquinodimethane (TCNQ) acceptors is predicted to result in a semiconducting state with high electron and hole mobilities, properties that are ideal for ambipolar transistors. Here, we seek to understand the effect of stoichiometry on the optical and electronic properties of PTZ:TCNQ CT crystals by comparing nanowires with 1:1 stoichiometry to a novel 3:1 stoichiometry using temperature-dependent optical spectroscopy. Ensemble photoluminescence and absorption measurements indicate that a CT state forms in the 1:1 sample, whereas the 3:1 sample exhibits weaker coupling between TCNQ and PTZ. These results support a strong correlation between stoichiometry and optical properties. Our observations give important insight into how the intermolecular coupling varies with stoichiometry and are crucial to future efforts to realize an organic ambipolar transistor. [Preview Abstract] |
Friday, March 18, 2016 1:51PM - 2:03PM |
Y33.00014: Quantitative Probes of Electron-Phonon Coupling in an Organic Charge-Transfer Material Aaron Rury, Shayne Sorenson, Eric Driscoll, Jahan Dawlaty While organic charge transfer (CT) materials may provide alternatives to inorganic materials in electronics and photonics applications, properties central to applications remain understudied in these organic materials. Specifically, electron-phonon coupling plays a pivotal role in electronic applications yet this coupling in CT materials remains difficult to directly characterize. To better understand the suitability of organic CT materials for electronic applications, we have devised an experimental technique that can directly assess electron-phonon coupling in a model organic CT material. Upon non-resonant interaction with an ultrafast laser pulse, we show that coherent excitation of Raman-active lattice vibrations of quinhydrone, a 1:1 co-crystal of the hydroquinone and p-benzoquinone, modulates the energies of electronic transitions probed by a white light pulse. Using a well-established theoretical framework of vibrational quantum beat spectra across the probe bandwidth, we quantitatively extract the parameters describing these electronic transitions to characterize electron-phonon coupling in this material. In conjunction with temperature-dependent resonance Raman measurements, we assess the hypothesis that several sharp transitions in the near-IR correspond to previously unknown excitonic states of this material. These results and their interpretation set the foundation for further elucidation of the one of the most important parameters in the application of organic charge-transfer materials to electronics and photonics. [Preview Abstract] |
Friday, March 18, 2016 2:03PM - 2:15PM |
Y33.00015: Influence of structural fluctuations on lifetimes of adsorbate states at hybrid organic-semiconductor interfaces M. M\"uller, D. S\'{a}nchez-Portal, H. Lin, G. Fratesi, G.P. Brivio, A. Selloni On the road towards a more realistic description of charge transfer processes at hybrid organic-semiconductor interfaces for photovoltaic applications we extend our first-principles scheme for the extraction of elastic linewidths to include the effects of structural fluctuations. Based on snapshots obtained from Car-Parinello molecular dynamics simulations at room temperature, we set up geometries in which dye molecules at interfaces are attached to a semi-infinite TiO$_{2}$ substrate. The elastic linewidths are computed using a Green's function method. This effectively introduces the coupling to a continuum of states in the substrate. In particular we investigate catechol and isonicotinic acid on rutile(110) and anatase(101) at the level of semi-local density functional theory. We perform multiple calculations of linewidths and peak-positions associated with the adsorbate's frontier orbitals for different geometric configurations to obtain a time-averaged analysis of such physical properties. We compare the results from the considered systems to understand the effects of dynamics onto interfacial charge transfer and systematically assess the dependence of the extracted elastic lifetimes on the relative alignment between adsorbate and substrate states. [Preview Abstract] |
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