Organic Semiconductors for Spintronic Applications. V. Alek DEDIU

Transcription

1 Italian School of Magnetism, Pavia 2012 Organic Semiconductors for Spintronic Applications V. Alek DEDIU Institute for Nanostructered Materials CNR, BOLOGNA, ITALY

2 Outline - Motivation - General notions on Organic SC - Brief introduction to injection and transport in OSC - Main achievements in Organic Spintronics > Spin Injection > New (multifunctional) devices - Conclusions: Problems and Possibilities

3 OS in Spintronics - MOTIVATION Most organic semiconductors are characterized by very weak spin scattering strength: - low Spin-Orbit Coupling due to low Z values ( SOC Z 4 ) - (McClure 1952 J Chem Phys) long spin relaxation times sec -> transport of spin polarized signals to nm) even for very low mobility materials Technological advantages: Easy to grow, low sensitivity to impurities Nature Mater. 8, 707 (2009)

4 OSC in Spintronics - MOTIVATION What is perhaps the most attractive aspect: Stable and easily controllable interfaces with many inorganic materials tuning of the spin injection ability via interface engineering - backed by an enormous variety of molecules - Tailoring interface spin selectivity

5 Intriguing complementarity of Organic and Inorganic materials Added: Alq3 our data Chart representing the spin diffusion length l S as a function of the corresponding spin diffusion time, for various spintronics materials. The organic semiconductors cluster in the top-left corner. They have a long spin lifetime but, due to the typically low mobilities, short spin diffusion lengths. [S. Sanvito et al. Nat Mater 8, 963 (2009)].

6 Organic field-effect transistors Sony and Samsung 55-inch OLED tv Flexible PV cell (Konarka) Sony Develops a "Rollable OTFT-driven OLED Display

7 Charges in OSC: intramolecular Carbon - C - configuration: sp 2 -hybridised orbitals form a triangle within a plane and the p z - orbitals are in the plane perpendicular to it. Example: in a benzene rings the π -bonds become delocalized

8 Small molecules vs polymers

9 Organic Semiconductors C 60 α -4T α-sexithiophene α -6T S S S S S S S S S S a-quartertiophene Pentacene Rubrene π-conjugated Molecules (oligomers) Tris(8-hydroxyquinoline)aluminium (Alq3)

10 Charges in OSC: intermolecular This leads to: - strong carrier localization - very narrow bands < 0.1 ev - soft mechanical properties Intermolecular interactions: van der Waals (vdw) WEAK ( mev) vdw: either (permanent dipole) x (induced dipole) or instantaneous (induced dipole) x (induced dipole)

11 Injection-Transport Properties _ cathode LUMO HOMO V van der Waals intermolecular anode + Charge injection: vanishingly low density of intrinsic carriers about cm -3. The electrodes provide carriers: molecules can easily accommodate an extra charge

12 Injection-Transport Properties _ cathode LUMO HOMO V van der Waals intermolecular anode + Carrier injection into OSC is best described in terms of thermally and field assisted charge tunnelling across the inorganic/ organic interface followed by carrier diffusion into the bulk of OSC Arkhipov et al. JAP84, 848 (1998)

13 Injection-Transport Properties _ cathode LUMO HOMO V van der Waals intermolecular anode + The current J can either be injection limited or space charge limited (SCLC). The injection limited current cannot be expressed by an unique formula and has to be analyzed case by case- i.e. for any given interfacing VERY IMPORTANT PROPERTY: THICKNESS INDEPENDENT interface resistance dominates

14 Injection-Transport Properties _ cathode LUMO HOMO V van der Waals intermolecular anode + The current J can either be injection limited or space charge limited (SCLC). The SCLC current is characterized by a strong thickness (d) dependence. It requires at least one Ohmic contact (unlimited injection efficiency): J SCLC = µ(e) x V 2 /d 3

15 Injection-Transport Properties _ cathode LUMO HOMO V van der Waals intermolecular anode + The Diffusion plays a significant role in disordered low mobility OSC materials J X = qnµe X + qd(dn/dx) Drift Diffusion Considering carriers time of flight and hence spin relaxation one has to analyze carefully the diffusion time

16 Charge Transport Properties: intermolecular

17 Tunneling vs Injection (devices) Speaking about Electrical Injection of the Spin Polarization we can evidence two conceptually different approaches: - Tunneling of SP carriers THROUGH OSC barriers (TMR) - Injection of SP carriers INTO the OSC electronic states (GMR) Tunneling devices provide apparently higher MR signals: sensors, magnetic switching elements, Injection devices provide the possibility to implement spintronic effects into OLEDs, OFETs, allow spin manipulation, Tunneling-like injection into organics generated some confusion and sometimes these two processes are confounded. Sometimes tunneling was claimed in materials and thickness well known for light emission.

18 Spin Injection in OSC where we are? Brief overview

19 V. Dediu, C. Taliani et al. Sol. St. Comm.122 (2002),181 Patent US First Organic SP Injection device w= nm LSMO T 6 Large negative magnetoresistance measured Advantage: NO short circuits! Problem: not possible (at least not at all easy) to reach AP configuration. Spin relaxation length L S 100 nm Spin relaxation time τ 10-6 s

20 Demonstration of the Spin Valve effect Univ. of Utah, Valy Vardeny group: La 0.7 Sr 0.3 MnO 3 /Alq3(130nm)/Co Inverse spin valve effect Z. H. Xiong, V. Vardeny et al. Nature 427, 821 (2004) The Spin Valve effects were registered in the ±1 V interval, up to 240 K

21 ISMN-CNR- Bologna La 0.7 Sr 0.3 MnO 3 manganite d = 12 nm Z scale: 3 nm R q = 0,23 nm R pv = 1,27 nm Supercon. Sci. Technol. 8, 160 (1995) Phys. Stat. Sol. 215, 1443 (1999) Φ K (mdeg) nm In col. with R. Sessoli group H (mt)

22 Further developments La 0.7 Sr 0.3 MnO 3 /Alq3(130nm)/Co, both electrodes and Alq3, was a lucky choice It becomes very fast the most used device in organic spintronics in an attempt to understand the physics rather than tsting new materials (or discovering new properties of old ones) Although the second approach becomes also vital and important. It generates new devices (see below)

23 Vertical Spin Valves: interface engineering Long channels injection! LiF, AlOx 2 nm Co 15 nm Alq nm La 0.7 Sr 0.3 MnO nm NdGaO 3 / SrTiO 3

24

25 Evolution of LSMO-Alq3-Al 2 O 3 -Co spin valves at 100 K 2007 data Rough Alq3 rms 5-10 nm 2008 data Smooth Alq3 rms 1 nm arxiv:cond-mat/ PRB78, (2008) 380k 360k optimized interfaces Room T GMR in 100nm Alq about 1-2 % Resistance(Ω) paper in preparation, k 320k 300k 280k 22 % 260k Field (Oe)

26 Various OSC in spintronics

27 Spin Injection Is the SPIN INJECTION in OSC recently fully demonstrated? MR alone is not a sufficient prove. No LEDs (actually OLEDs), not yet Hanle not yet non-local detection BUT: Two photon photoemission spectroscopy (checking right injection) Muon Spin Rotation in a vertical injection device (injection/transport)

28 Further developments Cinchetti et al. Nature Mater.8,115(2009) Drew et al. Nature Mater.8,109(2009)

29 Cinchetti et al. Nature Mater.8,115(2009)

30 What about OLEDs No circularly polarized light is expected, but an efficiency enhancement Unless one works on triplet emission: E Shikoh, E Nakagawa and A Fujiwara, JoP 200 (2010)

31 Spin Polarized injection in Organic LEDs NON polarised electrodes e( )p( )+e( )p( )+e( )p( )+e( )p( ) = 1/2(S+T)+T+1/2(S+T)+T=S+3T = 25% ONE polarised electrode e( )+p( ),p( ) ->e( )p( ) +e( )p( ) = 1/2(S+T)+T=1/2S+3/2T = 25% TWO polarised electrodes e( )+p( ) -> e( )p( ) = 1/2(S+T) = 50% e( )+p( ) -> e( )p ( ) = T = 100%

32 Manganite based OLEDs Al LiF (2 nm) Alq3 (70 nm) TPD (70 nm) OLED off I La 0.7 Sr 0.3 MnO 3 SrTiO 3 (0.5 mm) OLED on SPECTROMETER J. Appl. Phys. 93, 7682 (2003) J. Lumin. 110, 384 (2004) Org. Electron. 5, 309 (2004) (Bologna group) PRB 70, (2004) (IBM, Zurich) In spite of many attempts, many OLEDs and much LIGHT: :::::::::::::::::::::::::::::::::::::::::::::::: no convincing SP effects detected

33 ISMN-Bologna Fundamental (?) limitation Signal Intensity Light emission Spin injection Light emission Need to 1 V? 2 V? Voltage increase maximal voltage for high density spin injection Possible solution (?): polarizing carriers injected by efficient non magnetic injectors Will SP injection in organics be ever applicable to OPTOELECTRONICS?

34 New Devices from Organic Spintronics

35 Magnetically Enhanced Memristor (MEM)

36 What is Memristor?

37 Memristors are resistors with memory Memristor was formalized as the fourth basic electrical element by Chua in 1971 V Normal Resistor v = Ri i q Memristor Chua, IEEE Trans. Circuit Theory, 507,18 (1971) ϕ v i = = M ( q) i G( ϕ)v M= Memristance G= Memductance

38 Resistive Memories are Memristors Pinched I-V Hysteresis Loop At least two different non-volatile Resistive states Chua L., Appl. Phys. A, 102, 765 (2011)

39 First Experimental Memristor Formally found in 2008 (bistable sytems are known from 60s) TiO 2 resistors. Memristance due To oxygen migration Strukov, Nature, 453, 80 (2008) 9/28/11

40 Stateful logics from memristors In 2010 Stateful Logics scheme was published: With a Memristor only crossbar array it is possible to compute and store data at the same device Borghetti et Al., Nature, 464, 873 (2010) Memristors are good candidates for future computing and memory applications

41 Electric bistability spintronic device Long channels injection! LiF, AlOx 2 nm Co 15 nm Alq nm La 0.7 Sr 0.3 MnO nm NdGaO 3 / SrTiO 3

42 Current (A) Current[mA] Our devices: Bistability in I-V curves Vth- 4 Vth Voltage [V] K 150K 175K 200K 225K 250K 275K Bias (V)

43 MR can be switched OFF.. Resistance (Ω) Resistance (Ω) 380k 360k 340k 320k 300k 280k 680k 660k 640k 620k 600k 580k 100K -100mV SV 22% -3k -2k -1k 0 1k 2k 3k Field (Oe) 100K -100mV SV 11.2% 22% 11% -3k -2k -1k 0 1k 2k 3k Field (Oe) Applying -1.5V BEFORE measure Applying 2.5V BEFORE measure All measurement taken at -100mV Resistance (Ω) 4.6M 4.4M 4.2M 4.0M 3.8M..and ON 100K -100mV 0% -3k -2k -1k 0 1k 2k 3k Field (Oe) M. Prezioso et al. Adv Mat. (2011), 23, 1371

44 Resistance (Ω) Resistance (Ω) It is possible to recover original MR Resistance (Ω) 4.6M 4.4M 4.2M 4.0M 3.8M 0% -3k -2k -1k 0 1k 2k 3k Field (Oe) 2.5V Resistance (Ω) 680k 660k 640k 620k 600k 580k 11.2% -3k -2k -1k 0 1k 2k 3k Field (Oe) 3V 580k 560k 540k 520k 500k 480k 460k 440k -3.5k -3.0k -2.5k -2.0k -1.5k -1.0k k 1.5k 2.0k 2.5k 3.0k 3.5k Field (Oe) 21% 3.5V 540k 520k 500k 480k 460k 440k 420k 18.8% -3k -2k 0 2k 3k Field (Oe)

45 Resistance (Ω) Resistance (Ω) Voltage Dependent 540k 520k 3V 500k 480k 580k 460k 440k 420k 18.8% -3k -2k 0 2k 3k Field (Oe) 560k 540k 520k 500k 480k 18.6% 460k 440k -3k -2k -1k 0 1k 2k 3k Field (Oe)

46 behavior Scheme

47 Bistability mechanisms in Organics Filamentary // tends to have multiple discrete levels Redox reactions // difficult to imagine a reversible redox reaction at such interfaces that can give such effect Cobalt inclusions // ruled out by interfacial analyses Conformational modifications // requires really high electric fields Charge trapping // Trapped charge can lower the mobility by a sort of coulomb blockade

48 Simple model for the description of both GMR-RM M. J. Rozenberg et al. PRL (2004), 92,

49 MEM as Logic Gate A (E) B(H) AND Patent Pending A B MR

50 The experimental AND truth table

51 The experimental AND truth table

52 Current (A) Switching can be done repeatedly 1.0µ µ -2.0µ -3.0µ turn on pulse +1.5V turn off pulse -2V time (s) 1E-5 (different sample different switching biases) WRER Cycles +-4V Writing/reading/Erasing Cycles. It can be done many times. The OFF state seems more reproducible NON VOLATILE - Current [A] 1E-6 1E-7 1E-8 1E-9 OFF ON Cycle Electrical bistability Tested up to Cycles Roff/Ron up to 10 4 Retention time at least 24h@100K

53 New Physics (o Chemistry?)

54 Inverse spin valve effect spin-up spin-up LSMO(10nm)/ Alq3(200nm)/AlOx (2nm)/Co(30nm) 580k Resistance (Ohms) 560k 540k 520k 500k 480k 460k 440k 22 % 100K, V = -100mV Field (Oe)

55 Direct spin valve effect spin-up spin-up LSMO(10nm)/ Alq3 (2 nm)/co(30nm) C. Barraud, P. Seneor, AD, A. Fert, Nature Physics 6, 615 (2010)

56 Injection into organic levels: Interface Spin selection ON Resistance (Ohms) 580k 560k 540k 520k 500k 480k 460k 440k LSMO/Alq3 200nm/AlOx/Co R(H) at 100 K Negative (G)MR Field (Oe) Direct tunneling allowed: Interface Spin selection OFF Positive (T)MR C. Barraud, P. Seneor, AD, A. Fert, Nature Physics 6, 615 (2010)

57 Interface bonding: various hybridization effects S. Sanvito, News&Views, Nature Physics 6 (2010)

58 Atodiresei, N. et al. Phys. Rev. Lett. 105, (2010).

59 First direct demonstration: Proximity induced SP states Fe Alq3 Y. Zhan, M. Fahlman, et al. Adv. Materials 21, 1, 2009

60 Recently proposed spin scattering mechanism The hyperfine scattering model(bobbert et al. PRL 102, (2009)) features two important characteristics: - weak temperature dependence - - spin transport weakly dependent on mobility - The latter results from the fact that hyperfine spin scattering is an intramolecular process, while momentum scattering (mobility) is exclusively intermolecular hopping effect

61 Recently proposed spin scattering mechanisms

62 CONCLUSIONS Organic Spintronics it is still a young science much to be understood Spin Injection straightforward demonstration still missing, but many indirect evidences support that Fascinating interface physics/chemistry many possibilities for spintuning at hybrid interfaces Nonetheless many questions still open see above - new device paradigms are already coming out

63 Thanks to Colleagues: I. Bergenti, A. Riminucci, M. Prezioso, P. Graziosi, F. Borgatti, A. Gambardella, R. Cecchini, E. Lunedei EC projects: OFSPIN, HINTS, IFOX MIUR: few FIRB and PRIN projects Many collaborators from differnt EU countries, USA, Japan and Russia