Bulletin of the American Physical Society
APS March Meeting 2010
Volume 55, Number 2
Monday–Friday, March 15–19, 2010; Portland, Oregon
Session L33: Focus Session: Spin Dependent Physics in Organic-Based Materials II |
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Sponsoring Units: GMAG Chair: Anthony Caruso, University of Missouri - Kansas City Room: E143 |
Tuesday, March 16, 2010 2:30PM - 2:42PM |
L33.00001: Extremely weak length-dependent spin transport along alkane chain in Fe3O4/monolayer molecules nanoparticles Di Wu, Shen Wang, Fenjuan Yue, An Hu, Youwei Du Spin transport along alkane chain is investigated using self-assembled monolayers (SAMs) saturated molecules on magnetic nanoparticles. Transmission electron microscopy shows the interparticle distance is about the length of the molecules. Temperature dependent resistivity exhibits variable range hopping behavior, which is attributed to the hopping in the nanoparticle. The resistivity exponentially increases with increasing the molecular length, indicating tunneling is dominant conduction mechanism in the saturated molecules. However, the magnetoresistance is almost constant even though resistivity changes two orders of magnitude at room temperature with varying the molecular length. At low temperature the magnetoresistance gradually increases due to the increase of the surface spin polarization of the nanoparticles. [Preview Abstract] |
Tuesday, March 16, 2010 2:42PM - 2:54PM |
L33.00002: Giant tunneling magnetoresistance in organic spin valves Dali Sun, Lifeng Yin, Chengjun Sun, Hangwen Guo, Zheng Gai, X.-G Zhang, Thomas Ward, Zhaohua Cheng, Jian Shen Using buffer layer assist growth, we have successfully fabricated vertical organic spin valves with much sharper interface between top electrode and organic material. Organic spin valves prepared by this method maintain a simple and clean trilayer structure, allowing us to study the mechanism of the MR. Analysis of the current-voltage characteristics at different spacer layer thicknesses indicates that the spin-dependent carrier injection correlates strongly with the observed MR. [Preview Abstract] |
Tuesday, March 16, 2010 2:54PM - 3:06PM |
L33.00003: Understanding spin dependent transport through Alq3 molecules C. Barraud, P. Seneor, R. Mattana, S. Fusil, K. Bouzehouane, R. Guillemet, C. Deranlot, P. Graziosi, L. Hueso, I. Bergenti, V. Dediu, F. Petroff, A. Fert Molecular and organic spintronics offers the opportunity to play with chemical versatility and to bring the spin degree of freedom to electronics devices. We will show how, as a contender to commonly used inorganic materials, organic/molecular based spintronics devices can exhibit very large magnetoresistance and lead to tailored spin polarizations. First we will report on giant tunnel magnetoresistance of up to 300{\%} in a (La,Sr)MnO$_{3}$/Alq$_{3}$/Co nanometer size magnetic tunnel junctions. Samples are nanofabricated using a conductive tip AFM nanolithography process in order to circumvent problems such as inhomogeneity and strong metal diffusion. In a second part we will present a spin dependent transport model giving a new understanding of spin injection into organic materials. We will show how one could tune spin injection by molecular engineering. [Preview Abstract] |
Tuesday, March 16, 2010 3:06PM - 3:18PM |
L33.00004: Spin-polarized charge carrier injection by tunneling from ferromagnetic metals into organic semiconductors Mohammad Yunus, P. Paul Ruden, Darryl L. Smith Efficient spin-polarized charge carrier injection from a ferromagnetic metal (FM) into a semiconductor is a challenging task. Because of the large differences between the conductivities of metals and semiconductors a spin-dependent injection mechanism, such as tunneling, is a critical requirement. We discuss a new model for such a mechanism for the specific case of organic semiconductors (OS), such as conjugated hydrocarbons. Spin injection is modeled as tunneling through an interfacial layer into localized molecular states and subsequent thermally activated hopping of the charge carriers out of these localized states into the bulk of the semiconductor, where the transport can be described by macroscopic device equations. We explore the sensitivity of spin-injection to the parameters describing the FM and the OS. We also discuss the magneto-resistance that can result from spin-polarized injection and spin-transport if the extraction process is analogous to the injection mechanism. [Preview Abstract] |
Tuesday, March 16, 2010 3:18PM - 3:30PM |
L33.00005: Spin-orbit couplings and spin relaxation in organic electronic materials Zhi-Gang Yu Understanding spin relaxation in organic materials requires a reliable estimate of the spin-orbit coupling (SOC) strengths in these system. It is often argued that the SOC in $\pi$-conjugated organic materials is weak because of the small atomic SOC strength of C atom. However, most organic materials used in spin transport structures, such as T$_6$, MEH-PPV, Alq$_3$, and P3HT, contain N, O, and S atoms that have a much stronger atomic SOC than C atom. Here we carry out first-principles calculation to determine the SOC strengths of several representative organic molecules. We find that in MEH-PPV and Alq3 the SOC originates mainly from the O atoms, whose lone pairs of electrons strongly couple with the C $p_z$ orbitals. In T$_6$ and P3HT, the $\pi$-orbitals have a significant contribution from S atoms and therefore a sizable SOC. Using the obtained SOC strengths, we calculate the spin relaxation times and spin diffusion lengths caused by carrier hopping in these organic materials. Our results are consistent with recently measured spin relaxation times in MEH-PPV and temperature-dependent spin diffusion lengths in Alq$_3$. [Preview Abstract] |
Tuesday, March 16, 2010 3:30PM - 4:06PM |
L33.00006: Isotope Effect in Organic Magneto-Transport; the Role of Hyperfine Interaction Invited Speaker: Organic semiconductors have been used as active layer in devices such as organic light-emitting diodes, photovoltaic cells, and field-effect transistors. Recently there has been a growing interest in spin and magnetic field effects (MFE) in these materials. This include optically detected magnetic resonance (ODMR), where long spin coherence time was demonstrated; organic light emitting diodes (OLED), where substantive magneto-electroluminescence and magneto-conductance were obtained; and organic spin valves (OSV), where spin injection from ferromagnetic electrodes was verified. The interest in organic semiconductors has been motivated by the weak spin-orbit interaction that is caused by the light building block elements such as carbon and hydrogen. However, the role of the hyperfine interaction (HFI) between the injected spin-$\raise.5ex\hbox{$\scriptstyle 1$}\kern-.1em/ \kern-.15em\lower.25ex\hbox{$\scriptstyle 2$} $ carriers and various nuclear spins in organic magneto-transport has not been experimentally tested. Using the chemical versatility advantage of the organics, we studied and compare the magnetic-field effects in films, OLED and OSV devices based on polymers made of protonated, H-, and deuterated, D-hydrogen having a weaker HFI strength. We demonstrate that the HFI indeed plays a \textit{crucial role} in all three magnetic-field effects. OLEDs [films] based on the D-polymers show substantial narrower MFE [ODMR] response; whereas due to the longer measured spin diffusion, OSV devices based on D-polymers show substantially larger magnetoresistance response. [Preview Abstract] |
Tuesday, March 16, 2010 4:06PM - 4:18PM |
L33.00007: STM studies of hybrid inorganic-organic molecular magnets on an ultrathin insulating film. Taeyoung Choi, Jay Gupta The interplay of electronic structure and magnetic properties is of interest in various organic materials. For example, transition metal -- tetracyanoethylene (TCNE) complexes form a family of organic magnets with Curie temperatures exceeding room temperature. TCNE has a strong electron affinity that facilitates chemical bond formation and charge transfer with metals. However, the chemical bonding and its influence on electronic and magnetic properties is not well understood at the atomic scale. We use scanning tunneling microscopy to build Co-TCNE and Fe-TCNE complexes with atomic manipulation on an ultrathin insulating layer (Cu2N on Cu(100)). Cu2N decouples the complexes from the conducting substrate, which impacts their electronic and magnetic properties. Tunneling spectroscopy shows molecular orbitals and inelastic steps due to various vibrational modes and spin excitations. The ability to connect such complexes with additional metal atom chains provides an opportunity to study spin and charge transport through single molecules with atomically precise contacts. http://www.physics.ohio-state.edu/$\sim $jgupta [Preview Abstract] |
Tuesday, March 16, 2010 4:18PM - 4:30PM |
L33.00008: {\em Ab initio} design of spin-filters using single organic molecules Nicolae Atodiresei, Predrag Lazic, Vasile Caciuc, Stefan Bl\"ugel The possibility of using the electron`s spin in addition to its charge as information carrying physical quantity in future electronic devices has stimulated extensive experimental and theoretical studies over the last decade. The design of nanoscale spintronic elements in multifunctional devices relies on a clear understanding of the physics at electrode-molecule interfaces and in particular, the functionality of specific molecules in a given organic-metal surface environment. Using density functional theory simulations we have performed systematic studies on several organic molecules (e.g. benzene, cyclopentadiene radical and cyclooctatetraene) adsorbed on a ferromagnetic surface (e.g. 2ML Fe on W(110)). We show how the magnetic information can be transmitted through an interface formed even by a non-magnetic molecule adsorbed on the ferromagnetic metal surfaces. Furthermore, our calculations demonstrate that as for other aromatic molecules on metal surfaces [1], taking into account the van der Waals interaction is essential to precisely follow the charge transfer at the interface and the formation of spin- split molecule-metal hybrid states. Our results demonstrate that even in the case of non-magnetic molecules, a molecule-electrode system can act as an efficient molecular spin-filter if the electrode is magnetic. [1] N. Atodiresei et al. Phys. Rev. Lett. 102, (136809) (2009). [Preview Abstract] |
Tuesday, March 16, 2010 4:30PM - 4:42PM |
L33.00009: Role of photogenerated meta-stable polarons in organic magnetoresistance: evidence for polaron pair mechanism Bhoj Gautam, Tho Nguyen, Z. Valy Vardeny We studied the magneto-conductance (MC) in \textit{homopolar} organic diodes based on semiconductor polymers MEH-PPV and DOO-PPV. In dark we measured negative MC in both MEH-PPV and DOO-PPV homopolar devices, which was previously interpreted as due to magnetic field effect on singlet yield of polaron pairs having the same charge, known in the literature as the ``bipolaron'' mechanism. We investigated the role of photogenerated meta-stable polarons on the MC, when illuminating the device with a cw laser beam at various intensities and illumination times. Such illumination is known to produce metastable polarons that are deep-trapped in MEH-PPV polymer, but less so in DOO-PPV polymer. Upon illumination we obtained a gradual change in the MC magnitude and magnetic field response, where the MC first decreases then changes sign from negative to positive with the illumination time. Similar effects were not obtained in DOO-PPV devices. We therefore conclude that the metastable polarons in the illuminated polymer initiate the formation of polaron pairs with opposite charge in the homopolar device upon current injection; and these are therefore responsible for positive MC. This photoinduced MC is in agreement with a similar effect found in MC of \textit{bipolar} organic diodes upon increasing the bias voltage beyond the threshold for bipolar injection. [Preview Abstract] |
Tuesday, March 16, 2010 4:42PM - 4:54PM |
L33.00010: Optical and Magnetic Resonance Studies of Regio-Regular and Regio-Random Poly (3-hexylthiophene)/PCBM Blends Golda Hukic-Markosian, Ye Zhang, Sanjeev Singh, Valy Vardeny Regio-regular (RR) P3HT has been successfully used as donor polymer in organic bulk heterojunction photovoltaic cells based on blends with fullerene acceptors; with power conversion efficiencies of over 6{\%}. However, when regio-random (RR-a) P3HT is used as donor polymer in the blend, the power conversion efficiency drops to less than 0.5{\%}. We have used various optical and magnetic resonance techniques to elucidate the charge photogeneration in the two polymer/fullerene blends. Using tunneling electron microscopy we conclude that phase separation takes place in blends based on RR P3HT but not in blends based on RR-a P3HT. Photoluminescence spectrum shows a prominent band in RR-a P3HT blend at 1.32 eV, indicating the dominance of charge transfer exciton recombination. Photoinduced absorption shows higher localization of polarons in RRa-P3HT blend, with a distinct PA band due to negative polaron on PCBM molecules. Photoinduced absorption detected magnetic resonance resolves the contributions of RR-a P3HT and PCBM as two resonances indicating positive polarons on the polymer and negative polaron on the fullerene. A model based on our experimental results will be discussed. [Preview Abstract] |
Tuesday, March 16, 2010 4:54PM - 5:06PM |
L33.00011: Coherent quantum control of photoluminescence in organic semiconductors Dane McCamey, S.-Y. Lee, S.-Y. Paik, J.M. Lupton, C. Boehme Understanding the fundamental spin process which impact the optoelectronic properties of organic semiconductors will inform the future development of devices which utilize the spin degree of freedom in such materials. Recently, we have demonstrated that the conductivity of organic light emitting diodes can be controlled by coherent manipulation of spins within polaron pairs [1], the precursor states to light emission. In this talk, we test the model proposed there by investigating coherent spin control of the photoluminescent properties of the organic semiconductor MEH-PPV. We coherently evolve the spin symmetry of polaron pairs between the singlet and triplet configurations, and demonstrate that doing so directly modulates the photoluminescence. We show, using spin-beating experiments, that the signal is due to spin pairs, as proposed in [1]. We will also discuss the time-dependent form of the photoluminescence change observed, and in doing so scrutinize the existing models of spin-dependent excitonic recombination found in the literature.\\[4pt] [1] McCamey, D. R. et al, \it{Nature Mater.} $\bf{7},$ 723 (2008) [Preview Abstract] |
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