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Why Bottom-Up ?

Tuning Molecule-mediated Spin Coupling in Bottom-up Fabricated Vanadium-TCNE Nanostructures Daniel Wegner Institute of Physics and Center for Nanotechnology (CeNTech) University of M ü nster Germany. Why Bottom-Up ?. Challenges Low temperatures Leads Molecule orientation Deposition.

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Why Bottom-Up ?

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  1. Tuning Molecule-mediated Spin Couplingin Bottom-up FabricatedVanadium-TCNE NanostructuresDaniel WegnerInstitute of Physics and Center for Nanotechnology (CeNTech)University of MünsterGermany

  2. Why Bottom-Up ? Challenges Low temperatures Leads Molecule orientation Deposition Controlled model systems − + (drosophila of SMMs) Single molecule Magnet: Mn12-acetate Bottom-up STM

  3. Our Goal • Bottom-up design by STM manipulation • Spin-coupling via molecules 3d 3d 3d molecule molecule

  4. Flexibility Engineer spin structures (1D, 2D) Tune spin coupling triangular spin lattice spin chain Kagome spin lattice spin ladder square spin lattice

  5. Outline Motivation: molecular spin networks V + TCNE – promising candidates Building Vx(TCNE)y molecules Magnetic properties of Vx(TCNE)y Outlook

  6. Vanadium + TCNE 300 K magnetization molecule = ? magnetic field = V 3d Bulk Vx[TCNE]y • Room-temperature magnet (TC≈ 400 K) • Crystal structure unclear magnetic coupling unclear! tetracyanoethylene Manriquez et al., Science (1991)

  7. Sample Preparation V e-beam evaporator in-situ LT-STM (T = 7 K) Molecules via leak valve at 300 K V deposition atlow-temperature leak valve UHV TCNE gas Ag(100) crystalline TCNE

  8. Outline Motivation: molecular spin networks V + TCNE– promising candidates Building Vx(TCNE)y molecules Magnetic properties of Vx(TCNE)y Outlook

  9. Building Vx(TCNE)y Complexes TCNE TCNE LUMO TCNE V-TCNE -0.3 Å 0 Å 0.5 Å 1.0 Å 1.5 Å V2TCNE V V V-TCNE V-TCNE 20 Å d2I/dV2 -80 -40 0 40 80 sample bias (mV) Wegner et al., Nano Lett. 8, 131 (2008) topo dI/dV -0.6V Synthesis of V-TCNE Wegner et al., PRL 103, 087205 (2009) Reaction of V-TCNE with V  trans-V2TCNE V

  10. Vx(TCNE)y Structure Analysis -0.3 Å 0 Å 0.5 Å 1.0 Å 1.5 Å Model “rules” • TCNE on bridge site • V on hollow site • V-N bond 1.8-2.4 Å V-V distances@27° > @11°11.9 Å > 10.4 Å long V2TCNE short V2TCNE Wegner et al., PRL 103, 087205 (2009)

  11. Outline Motivation: molecular spin networks V + TCNE– promising candidates Building Vx(TCNE)y molecules Magnetic properties of Vx(TCNE)y Outlook

  12. STS Probes the Spin V atom metal U EF Majority @V dI/dV V-LDOS dI/dV @TCNE Minority -80 -40 0 40 80 Kondo resonance sample bias (mV) -1 -0.5 0 0.5 1 -1 -0.5 0 0.5 1 sample bias (V) E−EF (eV)      dI/dV dI/dV 5 Å 5 Å conduction electronsscreen impurity spin -1 -0.5 0 0.5 1 -80 -40 0 40 80 sample bias (V) sample bias (mV) V-TCNE V(TCNE)2 DFT -0.2 V state two local probes of V-spin Wegner et al., PRL 103, 087205 (2009)

  13. Spin in V2TCNE long V2TCNE short V2TCNE       5 Å 5 Å @TCNE @V dI/dV dI/dV -1 -0.5 0 0.5 1 -1 -0.5 0 0.5 1 sample bias (V) sample bias (V) ! dI/dV dI/dV -50 0 50 -50 0 50 sample bias (mV) sample bias (mV) Long V2TCNE • -0.2 V state • Kondo resonance Short V2TCNE • -0.2 V state • STS virtually identical • No Kondo resonance! Magnetic coupling! Wegner et al., PRL 103, 087205 (2009)

  14. Magnetic Coupling and Kondo Effect Two-impurity Kondo problem Jayaprakash et al., PRL (1981) Single spin impurity Two spin impurities weakly coupled Two spin impurities strongly coupled Texp weak coupling strong FMcoupling → TKFM«TK weakcoupling → TK strong AFMcoupling → no long V2TCNE strong coupling short V2TCNE

  15. FM vs. AFM Coupling short V2TCNE Symmetry arguments • Even number of electrons on TCNE(TCNE0, TCNE2−) • Odd number of electrons on TCNE(TCNE−) V-spins couple AFM 5 Å V-spins couple FM Determine the charge state

  16. DFT vs. Experiment V2TCNE V2TCNE exp. short dI/dV long -1 -0.5 0 0.5 1 sample bias (V) • (V2TCNE)− DOS fits best • ferromagneticcoupling Wegner et al., PRL 103, 087205 (2009)

  17. Double Check: V-TCNE @V dI/dV @TCNE -1 -0.5 0 0.5 1 sample bias (V) • (V-TCNE)− DOS fits best • isolated molecule: TCNE− V-TCNE exp. Wegner et al., PRL 103, 087205 (2009)

  18. Outline Motivation: molecular spin networks V + TCNE– promising candidates Building Vx(TCNE)y molecules Magnetic properties of Vx(TCNE)y Outlook

  19. Outlook V TCNE TCNE Insulating monolayer Substrate Weaken substrate coupling • reduce screening • increase spin coupling Build larger structures • various symmetries, geometries • engineered spin structures

  20. Summary @TCNE @V dI/dV V majority d-state -1 -0.5 0 0.5 1 sample bias (V) Kondo resonance -50 0 50 sample bias (mV) Create V+TCNE complexes: - atomic-scale precision- strong chemical bond STS: two spin probes short V2TCNE longV2TCNE V2TCNE molecules: Increase orbital overlap  Ferromagnetic coupling Wegner et al., PRL 103, 087205 (2009)

  21. Acknowledgements Michael F. CrommieRyan YamachikaXiaowei Zhang(UC Berkeley Physics) Jeffrey R. LongBart M. Bartlett(UC Berkeley Chemistry) Mark PedersonTunna Baruah(NRL Washington DC / UT El Paso) Alexander von Humboldt Foundation National Science Foundation Department of Energy

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