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2009, 113, 400–404 Published on Web 12/11/2008
Optically-Controlled Spin Valves in Conjugated Polymers Sheng Li,*,†,‡ Thomas F. George,*,‡ Xiao-Ling He,§ Bin-Ping Xie,| and Xin Sun# Department of Physics, Zhejiang Normal UniVersity, Zhejiang 310004, China, Office of the Chancellor and Center for Nanoscience, Departments of Chemistry and Biochemistry and Physics and Astronomy, UniVersity of Missouri-St. Louis, St. Louis, Missouri 63121, School of Science, Zhejiang UniVersity of Science and Technology, Hangzhou 310023, China, School of Physics and Astronomy, UniVersity of St. Andrews North Haugh, St. Andrews, Fife KY16 9SS, U.K., and Research Center for Quantum Manipulation and Key Laboratory for Surface Physics, Fudan UniVersity, Shanghai 200433, China ReceiVed: October 17, 2008; ReVised Manuscript ReceiVed: NoVember 11, 2008
In this article, two optically-controlled spin transfer effects are proposed for π-conjugated polymers. When such a polymeric molecule undergoes two-photon excitation, the charge of a spin carrier can be reversed, and simultaneously an applied external electric field drives the charge-reversed spin carrier to move in the opposite direction. As for a spinless carrier, the photoexcitation dissociates it into two spin carriers, forming entanglement. The coupling between the newly produced spin carriers and a ferromagnet will change the magnetoresistance. Both the fissions of spinless and spin carriers are ultrafast dynamical processes. By combining an electric field, magnetic field, and photoexcitation, two generic optically-controlled ultrafast response organic spin valves are designed. I. Introduction The carrier in conductors or semiconductors generally has two freedoms: charge and spin. The so-called “electronics” is to apply external field to manipulate the charge transfer of carrier. If the spin transfer also can be by controlled by an external field, the controlled spin property will open a gate to “spintronics”. The discovery of giant magnetoresistance1 and tunneling magnetoresistance in metallic spin valves2 has helped launch a new research field known as inorganic “spintronics”.3 By contrast to inorganic materials, organic materials possess strong electron-lattice coupling. Because of this, the energy spectrum and bond distortion of a polymeric molecule depend on electron excitation, leading to the self-trapping effect. After halogen doping, the resultant carriers in the conjugated polymer are composite particles called charged spin polarons or charged spinless bipolarons. They can be characterized by the lattice configuration4,5 and form their own electronic structure with localized states near the center of the energy gap. At a Manganite/polymer junction, when the Fermi level of Manganite lies below the bipolaron level of the polymer, the completely polarized spin can be injected into the polymer to form spin carriers-polarons.6 Furthermore, due to the extremely weak spin-orbit interaction and weak hyperfine interaction in π-conjugated organic semiconductors (OSEs), the spin diffusion length is especially long,7 which makes it possible to realize spin-coherent transport in OSEs. Because of the appealing properties of OSEs, a nanosize planar spin injection junction * To whom correspondence should be addressed. E-mail: (S.L.)
[email protected]; (T.F.G.)
[email protected]. † Zhejiang Normal University. ‡ University of Missouri-St. Louis. § Zhejiang University of Science and Technology. | University of St. Andrews North Haugh. # Fudan University.
10.1021/jp809219j CCC: $40.75
LSMO/T6/LSMO (LSMO stands for La0.7Sr0.3MnO3 and T6 for sexa-thiophene) was constructed.8 Following this, Yu developed a comprehensive theory to explain the magnetoresistance and I-V characteristics in this device.9 To date, several new organic spin-dependent devices have been fabricated, such as spin valves10 and organic tunable magnetoresistance transistors.11,12 Recently, based on poly(9,9-dioctylfluorenyl-2,7-diyl)(PFO), Francis et al. realized a magnetoresistance effect even at room temperature.13 Although the applied magnetic field is weak compared with other organic materials, the magnetoresistance change is strong, by as much as up to 10%. This has opened an avenue for promising plastic spintronics. Since 2004, scientists have conducted extensive research on organic magnetoresistance.14-16 There are two approaches to realize this phenomenon: one is to use the spin effect of an exciton,14,15 and the other is to capitalize on the spin properties of the charged carriers as bipolarons or polarons.17 In OSEs, the introduction of spin-orbit coupling largely suppresses the production of triplet excitons, yielding more singlet excitons,18 which is called the electrophosphorescent effect. Following this principle, the mixture of the strong spin-orbit coupling molecule fac-tris(2-phenylpyridinato) iridium [Ir(ppy)3] and polymer poly(N-vinyl carbazole)(PVK) changes the ratio between singlet and triplet excitons. Because of the different spin polarizations of the singlet and triplet excitons, the magnetoresistance effect can be tuned by a modification of the singlet-to-triplet exciton ratio.19 Besides the effect caused by excitons, recent experiments have confirmed that the mobility of holes and electrons, that is, positive and negative polarons, can lead to positive and negative magnetoresistance OSEs.20 This work shows that organic spintronics can be controlled by changing the transport of carriers in organic materials. The main shortage of the above method is the slow response of the charge injection into the conjugated polymers. In order to resolve this, we consider on 2009 American Chemical Society
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J. Phys. Chem. B, Vol. 113, No. 2, 2009 401
Figure 1. (a) Lattice configuration of a positive polaron in a polymer light-emitting diode. The vertical axis is the lattice configuration in the unit Å, and N refers to the sites. (b) Electronic spectrum of the positive polaron.
effect and strong electron-phonon coupling of conjugated polymerscannowbequantitativelydescribed.ABrazovskii-Kirova symmetry-breaking term23 can be added to this model to describe the confinement effect of a nondegenerate polymer
H)-
+ cl,s + ∑ {t0 + R(ul+1 - ul) + (-1)lte} × [cl+1,s l,s
K 2
Hc] +
H ) U
∑ (ul+1
∑ nl,vnl,V + V ∑ nl,snl+1,s
(2)
∑ Ee(l - N +2 1 )anl,s
(3)
l,s,s
l,s
HE )
- ul)2 + H′ + HE (1)
l
l,s
Figure 2. (a) Lattice configuration of a positive bipolaron in a polymer light-emitting diode. The vertical axis is the lattice configuration in the unit Å, and N refers to the sites. (b) Charge distribution of a positive bipolaron in a polymer light-emitting diode. The vertical axis is the charge distribution in the unit +|e|, and N refers to the sites. (b) Electronic spectrum of the positive bipolaron.
another typical characteristic of the conjugated polymer: the selftrapping effect mentioned earlier. As is well known, once a conducting polymeric molecule is doped, a charged carrier, such as a positive polaron or bipolaron, can be formed along the polymer chain. Once these carriers undergo photoexcitation, the self-trapping effect can induce a change in their properties.21 It is appropriate to apply photoexcitation to control the properties of plastic spintronics. Furthermore, the ultrafast process of photoexcitation will highly improve the response speed of the plastic spintronics. Here, we suggest two methods of controlling the spin transport, based on photoinduced carrier fission,22 to lead to an opticallycontrolled spin valve effect.
The parameters used in the above Hamiltonian are conventional values as determined by previous research on conducting polymers: t0 is a hopping constant; R is an electron-lattice + (cl,s) denotes the electron creation (ancoupling constant; cl,s nihilation) operator at site l with spin s; a is a lattice constant; ul is the displacement of atom l with mass M; K is an elastic constant; and te is the Brazovskii-Kirova term reflecting the confinement effect in a polymer with a nondegenerate ground state, which ensures that composite particles in the polymer, such as bipolarons or polarons, are stable.21 H′ is the electron-electron interaction, which can be treated by the Hartree-Fock approximation since the polymer is not a strongly correlated system.22,23 HE is the interaction of the electrons with the electric field b E directed along the polymer chain. The electron’s energy spectrum εµ and wave function Φµ are functionals of the lattice displacement ul and are determined by the eigenequation
HΦµ ) εµΦµ
Since atoms are much heavier than electrons, based on the Feynman-Hellmann theorem, an atom’s movement can be described by classical dynamics as
II. Model Following the development of the extended Su-SchreifferHeeger-Hubbard Hamiltonian,4 the prominent self-trapping
(4)
M
d2ul dt2
occ
) -
∂ε
∑ ∂uµl + K(2ul - ul+1 - ul-1) µ
(5)
402 J. Phys. Chem. B, Vol. 113, No. 2, 2009
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Figure 3. Time-dependent charge distribution of a conjugated polymer after the original positive polaron undergoes two-photon excitation.
Figure 4. Motion of a spin polaron under an external field E ) 5.0 × 10-4 V/cm. When the polymeric molecule absorbs two photons at 300 fs, the charge sign of the spin carrier is reversed to negative, driving the spin to move in the opposite direction. The vertical axis measures time in the unit femtoseconds, and N refers to the sites. The orientation of the electric field is along the polymer chain. s Assuming an electronic wave function Φµ ) {Z n,µ }, the R s 2 charge distribution can be represented as Fn ) ∑µ|Zn,µ| -n0, 2 and the spin distribution is Sn ) (p/2)∑µ|Z sn,µ|2 - (p/2) ∑µ|Z-s n,µ| , where n0 is the density of the positively charged background. Using the Hartree-Fock approximation, the electron interaction term H′ can be transformed to
∑ l,s {U( ∑ µocc |Zl,µ-s|2 - 21 ) + occ -s 2 occ -s 2 † | + ∑ µ |Zl+1,µ | - 2)]}al,s al,s × V[ ∑ s ( ∑ µ |Zl-1,µ s )(al,s? al,s + Hc) (6) ∑ l,s (V ∑ µocc Zl,µs Zl+1,µ
H )
where occ stands for the occupation or population of electrons. Because ul is a time-dependent variable describing lattice displacement, Hamiltonian also becomes a time-dependent variable after substituting ul of eq 5 into the Hamiltonian eq 1 and 6. These coupled equations can quantitatively describe the dynamics of a conjugated polymer chain.
Figure 5. Organic optically-controlled spin valve. Before photoexcitation, the positive spin polaron remains in the polymeric material B as shown in (a), and after photoexcitation, the spin is injected into material A in (b).
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Figure 6. (a) Time-dependent spin distribution of a conjugated polymer after the original positive bipolaron undergoes photoexcitation. The unit of spin distribution is p/2, and N refers to the sites. (b) Time-dependent charge distribution of a conjugated polymer after the original positive bipolaron undergoes photoexcitation. The unit of charge distribution is +|e|, and N refers to the sites.
Figure 7. Organic optically-controlled spin valve. Before photoexcitation, there is no spin in the spacer B as shown in (a), and after photoexcitation, the spin is generated in the spacer as shown in (b).
III. Results and Discussion When a conjugated polymeric molecule is in its ground state, the homogeneous dimerization of the lattice configuration leads to valence and conduction bands, reflecting its semiconducting property. After one electron at the top of valence band (the highest-occupied molecular orbital known as HOMO) is removed, the original homogeneous dimerization of the lattice configuration is no longer stable, yielding a localized configuration of the bond structure as shown in Figure 1a. The strong electron-lattice interaction then produces two localized states at the center of the gap between the valence and conduction bands, Φu and Φd, where Φd is occupied by only one electron and the other is empty, as shown in Figure 1b, indicating a spin carrier, namely, a positively charged polaron. Afterward, if one more electron is removed from Φd of the positive polaron, the distortion of the localized lattice is more serious, producing a new carrier-bipolaron which is a spinless carrier with two positive charges, and both Φu and Φd are empty, all of which are illustrated in Figure 2. Here, without the external electric field, when the positive polaron undergoes two-photon excitation, two electrons in the HOMO are excited to Φu through STIRAP (stimulated Raman adiabatic passage) technology or an external laser pump. The induced strong lattice oscillation completely changes the original charge distribution and lattice configuration, leading to carrier fission. The time-dependent charge distribution during its relaxation process following the two-photon excitation of a positive polaron is depicted in Figure 3. From this figure, the original carrier is separated into two parts, where the one at the left is a positively charged carrier, and the one at the right is
negatively charged. In order to conserve total charge, it is found that for the resultant carriers, the positive carrier is a bipolaron with two charges, and the other is a negative polaron lying on the right of the polymer chain. This effect is the result of symmetry breaking.15 As mentioned above, when the Fermi level of Manganite lies below the bipolaron level at the interface of the Manganite/ conducting polymer, the completely polarized spin can be injected into the polymer to form spin carriers, that is, polarons.8 If the original polaron is positively charged, the spin carrier is a positive polaron with p/2 spin. After two-photon excitation, dynamical relaxation splits the original carrier into two parts: one is a spinless carrier with +2|e| charge (positive bipolaron), and the other is a spin carrier with -|e| charge (negative polaron), which also conserves the total spin p/2. We now look at the spin property of the conjugated polymers. Through photoexcitation and dynamic relaxation, the charge of the spin carrier in a conducting polymer can be reversed from +|e| to -|e|. When an electric field is applied along with the polymer chain with E ) 5.0 × 10-4 V/cm, the spin of the positively charged polaron is driven to move in the same direction as the field. However, at 300 fs when the spin carrier undergoes two-photon excitation, the relaxation splits this spin carrier into two parts and reverses the charge sign of the spinpositive polaron to a negative one, but still keeps the same spin sign. The electric field drives the spin in the opposite direction along the polymer chain. The whole process is shown in Figure 4. Combining lattice relaxation and the spin transport process, the spin carrier-positive polaron can be separated into two parts within 100 fs, shown in Figure 3. At the same time, the spintransfer direction is also reversed, which apparently is an ultrafast process. On the basis of this optically-controlled backward spintransfer effect in a conjugated polymer, a generic polymer spin valve can be designed as illustrated in Figure 5. The valve consists of two different conjugated polymeric materials, A and B. Here, we apply an external magnetic field to determine the spin polarizations. Meanwhile, the bias voltage between A and B always makes the voltage in A higher than in B. Without photoexcitation, the positive spin polaron remains in B due to the applied bias voltage, as shown in Figure 5a. Once the positive polaron undergoes photoexcitation, the charge of the spin polaron is reversed, namely, the excitation transforms the positive spin into a negative-spin polaron. Then, the bias voltage in the junction drives the spin to transfer backward, easily injecting the spin of a negative polaron from B into A, as shown in Figure 5b. Let us now turn our focus to the other spinless charged carrierbipolaron. Once the positive bipolaron undergoes photoexcita-
404 J. Phys. Chem. B, Vol. 113, No. 2, 2009 tion, namely one electron from the HOMO is excited to the state Φd, the original lattice configuration is no longer stable. It is found that the original spinless carrier is split into two spin carriers, where one is a polaron with positive spin sign, and the other corresponds to a negative spin sign, as shown in Figure 6a. In this case, although spin can be generated through external excitation, the total spin is still conserved. Most importantly, the resultant carriers caused by the external excitation are still carriers in the ground state, which maintains the generated spin for a long time, thus prolonging the spin coherent time. Moreover, the charge distribution of the positive bipolaron is completely separated into two parts after 100 fs relaxation, as shown in Figure 6a. Combining the properties of spin and charge, the resultant carriers correspond to two positively charged polarons. Actually, the result of the photoexcitation is an entanglement state. If the polaron with spin up is written as P(v) and P(V) stands for the one with spin down, this state becomes P(v)P(V) + P(V)P(v). Once a given polaron’s spin sign is fixed by an external magnetic field, the other polaron’s spin sign is the opposite. In the meantime, due to the same charge of the resultant polarons, these carriers with opposite spins will keep far away each other, as shown in Figure 6b. Because recent experiments have shown that the magnetoresistance in organic semiconductors is dominated by charged polarons while the spinless bipolaron does not contribute to this effect,20 the photoexcitation not only induces bipolarons to produce polarons, but also provides an appropriate method to control the magnetoresistance, which is an ultrafast response spin valve effect. The above photoinduced spin generation provides a possible approach to realize an ultraresponse organic spin valve mentioned in the Introduction. A conventional organic magnetoresistance device is a sandwich A/B/C structure as illustrated in Figure 7a, where A is a pinned layer, B is a spacer, and C is a free layer. Generally, a pinned layer is made of ferromagnetic materials, whose spin direction is fixed. A spacer consists of overdoped conjugated polymeric materials. Because of the overdoping, the carriers in spacer are spinless bipolarons. The free layer is also composed of ferromagnetic materials whose spin direction can be changed by the external magnetic field B. Before photoexcitation, the device is shown as Figure 7a. After the polymeric materials spacer undergoes the photoexcitation, the spinless bipolaron in the spacer will be split into two polarons with opposite spins. Following this, the ferromagnetic materials in the pinned and free layers couple with the newly produced spin polarons. The coupling induces the change of magnetoresistance and finally changes the current in this circuit, as shown in Figure 7b. The whole process finishes within 100 fs, which should be an ultrafast response valve effect. In conclusion, a photoexcitation-induced backward spintransfer effect in a π-conjugated polymer is predicted: When a spin-polarized positive polaron absorbs two photons, the charge of the spin carrier can be reversed, and simultaneously, the applied external electric field drives the charge-reversed spin carrier to move in the opposite direction. Meanwhile, when a conducting conjugated polymeric molecule undergoes photoexcitation, the positively charged spinless bipolaron dissociates into two positive polarons with opposite spin signs, forming an entanglement. The coupling between the newly produced spin
Letters polarons and ferromagnet will change the magnetoresistance. By combining an electric field, magnetic field and photoexcitation, two generic optically-controlled ultrafast response organic spin valves are designed. Acknowledgment. This work was supported by the U.S. Army Research Office under Contact W911NF-04-1-0383, the National Science Foundation of China under Grants 20804039, 20674010 and 10747157, and the Science Foundation of Zhejiang Province of China under Grant Y4080300. Note Added after ASAP Publication. This paper was published ASAP on December 11, 2008. The content of Figures 6 and 7 needed to be switched. The updated paper was reposted on December 16, 2008. References and Notes (1) Baibich, M. N.; Broto, J. M.; Fert, A.; Nguyen Van Dau, F.; Petroff, F. Phys. ReV. Lett. 1988, 61, 2472. (2) Moodera, J.; Kinder, L.; Wong, T.; Meservey, R. Phys. ReV. Lett. 1995, 74, 3273. (3) (a) Wolf, S. A.; Awschalom, D. D.; Buhrman, R. A.; Daughton, J. M.; von Molner, S.; Roukes, M. L.; Chtchelkanova, A. Y.; Treger, D. M. Science 2001, 294, 1488. (b) Kikkawa, J. M.; Awschalom, D. D. Nature 1999, 397, 139. (c) Ohno, Y.; Young, D. K.; Beschoten, B.; Matsukura, F.; Ohno, H.; Awschalom, D. D. Nature 1999, 402, 790. (d) Zˇutic´, I.; Fabian, J,; Sarma, S. D. ReV. Mod. Phys. 2004, 76, 323. (4) Heeger, A. J.; Kivelson, S.; Schrieffer, J. R.; Su, W. P. ReV. Mod. Phys. 1988, 60, 781. (5) (a) Scott, J. C.; Pfluger, P.; Krounbi, M. T.; Street, G. B. Phys. ReV. B 1983, 28, 2140. (b) Scott, J. C.; Bredas, J. L.; Yakushi, K.; Pfluger, P.; Street, G. B. Synth. Met. 1984, 9, 165. (6) Xie, S. J.; Ahn, K. H.; Smith, D. L.; Bishop, A. R.; Saxenal, A. Phys. ReV. B 2003, 67, 125202. (7) Krinichnyi, V. I. Synth. Met. 2000, 108, 173. (8) Dediu, V.; Murgia, M.; Matacotta, F. C.; Taliani, C.; Barbanera, S. Solid State Commun. 2002, 122, 181. (9) Yu, Z. J.; Berding, M. A.; Krishnamurthy, S. Phys. ReV. B 2005, 71, 060408. (10) Xiong, Z. H.; Wu, D.; Vardeny, Z. V.; Shi, J. Nature 2004, 427, 821. (11) Luo, F.; Song, W.; Wang, Z. M.; Yan, C. H. Appl. Phys. Lett. 2004, 84, 1719. (12) Santos, T. S.; Lee, J. S.; Migdal, P.; Lekshmi, I. C.; Satpati, B.; Moodera, J. S. Phys. ReV. Lett. 2007, 98, 016601. (13) Francis, T. L.; Mermer, Veerarghavan, G.; Wohlgenannt, M. New J. Phys. 2004, 6, 185. (14) Prigodin, V. N.; Bergeson, J. D.; Lincoln, D. M.; Epstein, A. J. Synth. Met. 2006, 156, 757. (15) Desai, P.; Shakya, P.; Kreouzis, T.; Gillin, W. P.; Morley, N. A.; Gibbs, M. R. J. Phys. ReV. B 2007, 75, 094423. (16) Wu, Y.; Xu, Z.; Hu, B.; Howe, J. Phys. ReV. B 2007, 75, 035214. (17) Bobbert, P. A.; Nguyen, T. D.; Oost, F. W.; Koopmans, van B.; Wohlgennant, M. Phys. ReV. Lett. 2007, 99, 216801. (18) (a) Kido, J.; Haromichi, H.; Hongawa, K.; Nagai, K.; Okuyama, K. Appl. Phys. Lett. 1994, 65, 2124. (b) Hoshino, S.; Suzuki, H. Appl. Phys. Lett. 1996, 69, 224. (c) Zhang, X.; Sun, R. Q. Zheng; Kobayashi, T.; Li, W. Appl. Phys. Lett. 1997, 71, 2596. (19) Sheng, Y.; Nguyen, T. D.; Veeraraghavan, G.; Mermer, O.; Wohlgenannt, M. Phys. ReV. B 2007, 75, 035202. (20) (a) Bloom, F. L.; Wagemans, W.; Kemerink, M.; Koopmans, B. Phys. ReV. Lett. 2007, 99, 257201. (b) Bloom, F. L.; Wagemans, W.; Koopmans, B. J. Appl. Phys. 2008, 103, 07F320. (21) Sun, X.; Fu, R. L.; Yonemitsu, K.; Nasu, K. Phys. ReV. Lett. 2000, 84, 2830. (22) (a) Li, S.; George, T. F.; Sun, X.; Chen, L. S. J. Phys. Chem. B 2007, 111, 6097. (b) Li, S.; Chen, L. S.; George, T. F.; Sun, X. Phys. ReV. B 2004, 70, 075201. (23) Brazovskii, S. A.; Kirova, N. N. JETP Lett. 1981, 33, 6.
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