Spin-dependent tunneling through organic molecules: spin scattering by organic radicals

Transcription

1 Spin-dependent tunneling through organic molecules: spin scattering by organic radicals P. LeClair Physics & Astronomy, MINT, University of Alabama G.J. Szulczewski, A. Gupta, S. Street Weihao Xu Chemistry, MINT, University of Alabama Dina Genkina, Abel Demisse REU interns The University of Alabama A NSF Materials Research Science and Engineering Center

2 Organic Spintronics Tunneling processes more crucial as length scales decrease Control with multiple degrees of freedom Spin transport & more general applications enable organic-based spintronics Organic materials are unique: - properties tailored through synthesis Fundamental understanding of transport - one molecular building block at a time - multiple degrees of freedom - interplay with optoelectronics? Create toolkit for device design The University of Alabama A NSF Materials Research Science and Engineering Center

3 Organic Spintronics Tunneling processes more crucial as length scales decrease Control with multiple degrees of freedom Spin transport & more general applications enable organic-based spintronics Organic materials are unique: - properties tailored through synthesis Fundamental understanding of transport - one molecular building block at a time - multiple degrees of freedom - interplay with optoelectronics? Create toolkit for device design The University of Alabama A NSF Materials Research Science and Engineering Center

4 Organic semiconductors an NSF Materials Science and Engineering Center

5 Organic semiconductors Alq 3 an NSF Materials Science and Engineering Center

6 Organic semiconductors Alq 3 H 2 TPP an NSF Materials Science and Engineering Center

7 Organic semiconductors Alq 3 H 2 TPP Pc an NSF Materials Science and Engineering Center

8 Organic semiconductors Alq 3 H 2 TPP Pc PTCDA an NSF Materials Science and Engineering Center

9 Sample Preparation La 0.67 Sr 0.33 MnO 3 film ( nm) SrTiO 3 an NSF Materials Science and Engineering Center

10 Sample Preparation La 0.67 Sr 0.33 MnO 3 film ( nm) SrTiO 3 Pattern an NSF Materials Science and Engineering Center

11 Sample Preparation La 0.67 Sr 0.33 MnO 3 film ( nm) SrTiO 3 Pattern Clean with ~0.1% Br 2 in ethanol for 30 sec an NSF Materials Science and Engineering Center

12 Sample Preparation La 0.67 Sr 0.33 MnO 3 film ( nm) SrTiO 3 Pattern Vapor deposit organic Clean with ~0.1% Br 2 in ethanol for 30 sec an NSF Materials Science and Engineering Center

13 Sample Preparation La 0.67 Sr 0.33 MnO 3 film ( nm) SrTiO 3 Pattern Vapor deposit organic Clean with ~0.1% Br 2 in ethanol for 30 sec Vapor deposit Co and Al through shadow mask an NSF Materials Science and Engineering Center

14 Sample Preparation La 0.67 Sr 0.33 MnO 3 film ( nm) SrTiO 3 Pattern Vapor deposit organic Clean with ~0.1% Br 2 in ethanol for 30 sec Cross-bar Area = 0.10 mm 2 Vapor deposit Co and Al through shadow mask an NSF Materials Science and Engineering Center

15 an NSF Materials Science and Engineering Center

16 Current-voltage and conductance curves 0.5 K Co 15nm TPP 11 K LSMO Xu et al. Appl. Phys. Lett. 90, (2007) superconducting electrodes confirms tunneling an NSF Materials Science and Engineering Center

17 Normalized MR versus LSMO surface polarization Normalized MR LSMO surface polarization (Park) Normalized MR for LSMO/TPP/Co no apparent spin loss through TPP T dependence just LSMO Temperature (K) Park et al. PRL 81, 1953 (1998) an NSF Materials Science and Engineering Center

18 Meservey-Tedrow tunneling thin film SC in H (~2-3T) Zeeman split quasiparticle DOS measure di/dv ~ N sc N fm nonmagnetic : symmetric -- equal # of spins magnetic: asymmetry = spin polarization barrier FM SC robust, microscopic theory directly gives spin imbalance of tunneling current THE UNIVERSITY OF ALABAMA CENTER FOR MATERIALS FOR INFORMATION TECHNOLOGY An NSF Science and Engineering Center

19 T = 500mK P ~ 37% T= 0.36 K H= 3 T H= 0 T H = 0 H = 3T Al / AlOx / TPP / Co polarization is not decreased by TPP The University of Alabama A NSF Materials Research Science and Engineering Center

20 di/dv V magnon scattering increased conductance at low bias, low T... evidence of magnetic excitations! an NSF Materials Science and Engineering Center

21 Magnetoresistance of Co/20 nm TPP/LSMO sizable MR observed up to 40% at 10K Xu et al. Appl. Phys. Lett. 90, (2007) an NSF Materials Science and Engineering Center

22 so what s the problem? still to complicated need a simpler starting point for structureproperty correlation so we go back to the future (again) an NSF Materials Science and Engineering Center

23 Molecular Tunnel Junctions One layer of a simple, flexible molecule - benzoic acid derivatives Model system for more complex molecules - scaffolding Correlate transport and molecular structure Structure Photoemission (UPS,XPS) Raman/IR Inelastic electron tunneling free FM organic Al2O3 pinned FM Transport Conductance (V,H,T) Spin-polarized tunneling The University of Alabama A NSF Materials Research Science and Engineering Center

24 Molecular Tunnel Junctions One layer of a simple, flexible molecule - benzoic acid derivatives Model system for more complex molecules - scaffolding Correlate transport and molecular structure Structure Photoemission (UPS,XPS) Raman/IR Inelastic electron tunneling free FM organic Al2O3 pinned FM Transport Conductance (V,H,T) Spin-polarized tunneling The University of Alabama A NSF Materials Research Science and Engineering Center

25 Molecular Tunnel Junctions One layer of a simple, flexible molecule - benzoic acid derivatives Model system for more complex molecules - scaffolding Correlate transport and molecular structure Structure Photoemission (UPS,XPS) Raman/IR Inelastic electron tunneling free FM organic Al2O3 pinned FM Transport Conductance (V,H,T) Spin-polarized tunneling The University of Alabama Self-assembly yields wellordered monolayers Al2O3 Al A NSF Materials Research Science and Engineering Center

26 Sample Preparation Al (~10 nm) shadow mask an NSF Materials Science and Engineering Center

27 Sample Preparation Al (~10 nm) shadow mask deposit benzoic acid from solution an NSF Materials Science and Engineering Center

28 Sample Preparation Al (~10 nm) shadow mask deposit benzoic acid from solution Vapor deposit Co or Pb through shadow mask an NSF Materials Science and Engineering Center

29 Sample Preparation Al (~10 nm) shadow mask deposit benzoic acid from solution Cross-bar Area = 0.10 mm 2 Vapor deposit Co or Pb through shadow mask an NSF Materials Science and Engineering Center

30 IETS: is there transport through the molecule? tunneling electrons excite vibrational modes see Raman & IR modes no strong selection rules when ev hv stepped increase in di/dv new set of final states available unambiguous - transport through molecule example: studying model catalysts CO on Rh/Al 2 O 3 -- CO vibrations tunneling energy loss spectroscopy detail: tiny signals challenging THE UNIVERSITY OF ALABAMA CENTER FOR MATERIALS FOR INFORMATION TECHNOLOGY An NSF Science and Engineering Center

31 IETS: is there transport through the molecule? tunneling electrons excite vibrational modes see Raman & IR modes no strong selection rules when ev hv stepped increase in di/dv new set of final states available unambiguous - transport through molecule example: studying model catalysts CO on Rh/Al 2 O 3 -- CO vibrations tunneling energy loss spectroscopy detail: tiny signals challenging THE UNIVERSITY OF ALABAMA CENTER FOR MATERIALS FOR INFORMATION TECHNOLOGY An NSF Science and Engineering Center

32 IETS: is there transport through the molecule? tunneling electrons excite vibrational modes see Raman & IR modes no strong selection rules when ev hv stepped increase in di/dv new set of final states available unambiguous - transport through molecule example: studying model catalysts CO on Rh/Al 2 O 3 -- CO vibrations di/dv hv d 2 I/dV 2 V tunneling energy loss spectroscopy detail: tiny signals challenging hv V THE UNIVERSITY OF ALABAMA CENTER FOR MATERIALS FOR INFORMATION TECHNOLOGY An NSF Science and Engineering Center

33 IETS: is there transport through the molecule? tunneling electrons excite vibrational modes see Raman & IR modes no strong selection rules when ev hv stepped increase in di/dv new set of final states available unambiguous - transport through molecule example: studying model catalysts CO on Rh/Al 2 O 3 -- CO vibrations tunneling energy loss spectroscopy detail: tiny signals challenging THE UNIVERSITY OF ALABAMA CENTER FOR MATERIALS FOR INFORMATION TECHNOLOGY An NSF Science and Engineering Center

34 IETS: is there transport through the molecule? tunneling electrons excite vibrational modes see Raman & IR modes no strong selection rules when ev hv stepped increase in di/dv new set of final states available unambiguous - transport through molecule example: studying model catalysts CO on Rh/Al 2 O 3 -- CO vibrations tunneling energy loss spectroscopy detail: tiny signals challenging THE UNIVERSITY OF ALABAMA CENTER FOR MATERIALS FOR INFORMATION TECHNOLOGY An NSF Science and Engineering Center

35 How to measure a derivative? I(V dc + δv ac cosωt) = I(V dc ) + di ( dv δv cosωt ac ) + d 2 I dv 2 2 δi ac 4 ( ) cos 2ωt +K drive system with V dc + δv ac sin ωt measure response at ω frequency / phase sensitive technique lock-in amplifiers (talk to Dr. Enders) THE UNIVERSITY OF ALABAMA CENTER FOR MATERIALS FOR INFORMATION TECHNOLOGY An NSF Science and Engineering Center

36 (1) original nice idea... THE UNIVERSITY OF ALABAMA CENTER FOR MATERIALS FOR INFORMATION TECHNOLOGY An NSF Science and Engineering Center

37 (1) original nice idea... THE UNIVERSITY OF ALABAMA CENTER FOR MATERIALS FOR INFORMATION TECHNOLOGY An NSF Science and Engineering Center

38 (1) original nice idea... (2) the Danny filter... pin DC switch 20k 40k 400k 1M THE UNIVERSITY OF ALABAMA AC switch 1k 10k 100k 1M 10M board connector 1 ac input from SW 2 dc input from SW 3 BNC ground 4 -V 5 +V 6-10 NC 11 com/bnc ground 12 to pin 4 LH0063K 13 to pin 3 LH0063K feedback U1-U3-U4-U5 = OP07 U2 = LF351 U6 = LH0063K CENTER FOR MATERIALS FOR INFORMATION TECHNOLOGY An NSF Science and Engineering Center

39 (1) original nice idea... (3) implementation... (2) the Danny filter... pin DC switch 20k 40k 400k 1M THE UNIVERSITY OF ALABAMA AC switch 1k 10k 100k 1M 10M board connector 1 ac input from SW 2 dc input from SW 3 BNC ground 4 -V 5 +V 6-10 NC 11 com/bnc ground 12 to pin 4 LH0063K 13 to pin 3 LH0063K feedback U1-U3-U4-U5 = OP07 U2 = LF351 U6 = LH0063K CENTER FOR MATERIALS FOR INFORMATION TECHNOLOGY An NSF Science and Engineering Center

40 V dc IETS setup + V & dv V ac LIA DC R thing di/dv: lock in to f d 2 I/dV 2 (IETS): lock in to 2f constant resolution mode software feedback to maintain modulation R precise LIA I & di DC

41 Quantum Design PPMS R(T,H) T: 350mK K 3He fridge H: T scriptable + Home-built electronics di/dv(v,t) d 2 I/dV 2 (V,T) Transport facilities THE UNIVERSITY OF ALABAMA CENTER FOR MATERIALS FOR INFORMATION TECHNOLOGY An NSF Science and Engineering Center

42 T = 2K IETS: The University we really of have Alabama transport through the molecule A NSF Materials Research Science and Engineering Center

43 T = 2K IETS: The University we really of have Alabama transport through the molecule A NSF Materials Research Science and Engineering Center

44 ATR-FTIR what is the state of the molecule? is it still there? Evanescent wave 1 um IR beam Sample To detector ATR crystal The University of Alabama JASCO FT/IR-4100_ATR Scan #: 128 Resolution: 2 cm -1 A NSF Materials Research Science and Engineering Center

45 COO- symmetric modes - monodentate T = 2K The University of Alabama ν: stretch; β: in-plane bend; γ: out-of-plane bend. A NSF Materials Research Science and Engineering Center

46 Co Co I 3d I I I I Al2O3 Al Al2O3 Al N 1s Al/Al2O3/BA/(Co or Pb) Co reacts with heavier halogens Pb does not halogen substitution leads to organic radicals The University of Alabama A NSF Materials Research Science and Engineering Center

47 Conductance Through Benzoic Acids Vary para- substituent Controlled introduction of radicals Co Co I I I I Al2O3 Al Al2O3 Al The University of Alabama A NSF Materials Research Science and Engineering Center

48 size of zero-bias anomaly scales with degree of reaction only very tiny effects for fluorobenzoic acid (or cyano) slightly larger for chlorobenzoic acid huge for iodobenzoic acid The University of Alabama A NSF Materials Research Science and Engineering Center

49 when we have a reaction there is an unpaired electron which means an unpaired spin which gives a Kondo resonance (TK~20K) (wacky quantum interference) (many body effect) The University of Alabama A NSF Materials Research Science and Engineering Center

50 Al/Al2O3/ 4-iodobenzoic acid / Co T = 2K H=0 H=7T The University of Alabama in a field, Kondo peak suppressed small well carved out of it... A NSF Materials Research Science and Engineering Center

51 Spin-dependent Scattering by Organic Radicals Organic radicals = unpaired electron Should have normal spin flip scattering Revealed through application of H fit gives g~2 consistent The University of Alabama A NSF Materials Research Science and Engineering Center

52 More details: fit to high field data gives g ~ 2 expected signature logarithmic (V,T) dependence of Kondo peak in H=0 Kondo, H-dependent part are small terms V-independent parts much larger spin scattering dominates conductance The University of Alabama A NSF Materials Research Science and Engineering Center

53 Building a Molecular Spin Transport Toolkit Porphyrin derivatives (E. Galoppini, Rutgers) - control orientation - Dr. Choe... Phthalocyanine derivatives -CoPc, F16CuPc -tune spin-orbit scattering via TM ion Lanthanide complexes - RE 3+ controls spin dynamics - strong exchange The University of Alabama A NSF Materials Research Science and Engineering Center

54 Building a Molecular Spin Transport Toolkit new families of (known) molecules spin transport is unknown build knowledge block by block systematic variation new tools and synergy transport structure designer toolkit for spin transport Spin-dependent scattering dominates with radicals - General mechanism! The University of Alabama thanks to: NSF MRSEC grant No. DMR A NSF Materials Research Science and Engineering Center

55 The University of Alabama A NSF Materials Research Science and Engineering Center

56 The University of Alabama A NSF Materials Research Science and Engineering Center

57 Organic Spintronics Understand spin injection & transport Enable organic-based spintronics Organic materials are unique: - properties tailored through synthesis Fundamental understanding of transport - one molecular building block at a time - multiple degrees of freedom - interplay with optoelectronics? Create toolkit for device design The University of Alabama A NSF Materials Research Science and Engineering Center

58 Organic Spintronics Understand spin injection & transport Enable organic-based spintronics Organic materials are unique: - properties tailored through synthesis Fundamental understanding of transport - one molecular building block at a time - multiple degrees of freedom - interplay with optoelectronics? Create toolkit for device design free FM organic pinned FM Two terminal: OLED, MTJ, spin valve The University of Alabama A NSF Materials Research Science and Engineering Center

59 Organic Spintronics Understand spin injection & transport Enable organic-based spintronics Organic materials are unique: - properties tailored through synthesis Fundamental understanding of transport - one molecular building block at a time - multiple degrees of freedom - interplay with optoelectronics? Create toolkit for device design Al2O3 FM organic gate FM free FM organic pinned FM Three terminal: FET, logic, memory The University of Alabama Two terminal: OLED, MTJ, spin valve A NSF Materials Research Science and Engineering Center