why bottom-up ?

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Tuning Molecule-mediated Spin Couplingin Bottom-up FabricatedVanadium-TCNE Nanostructures

Daniel Wegner

Institute of Physics and Center for Nanotechnology (CeNTech)University of MünsterGermany

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(drosophila of SMMs)

Why Bottom-Up ?

Challenges

• Low temperatures

• Leads

• Molecule orientation

• Deposition

Controlled model systems

Single molecule Magnet: Mn12-acetate+−

Bottom-up

STM

3

molecule

Our Goal

• Bottom-up design by STM manipulation

• Spin-coupling via molecules

molecule3d3d 3d

4

Flexibility

Engineer spin structures (1D, 2D)

Tune spin coupling

spin chain

spin ladder

triangular spin lattice

squarespinlattice

Kagome 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

Bulk Vx[TCNE]y

• Room-temperature magnet (TC ≈ 400 K)

• Crystal structure unclear magnetic coupling unclear!

Manriquez et al., Science (1991)

tetracyanoethylene

?V3d =

N N

N N

C C

C C

C Cmolecule =

300 K

mag

netiz

atio

nmagnetic field

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Sample Preparation

Molecules via leak valve at 300 K

V deposition atlow-temperature

crystalline TCNE

TCNE gas

Ag(100)

UHV leak valve

LT-STM(T = 7 K)

in-situ

V e-beamevaporator

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Outline

Motivation: molecular spin networks

V + TCNE– promising candidates

Building Vx(TCNE)y molecules

Magnetic properties of Vx(TCNE)y

Outlook

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V

N N

N N

C C

C C

C C

-80 -40 0 40 80

sample bias (mV)

d2 I/d

V2

TCNE

V

20 Å

Building Vx(TCNE)y Complexes

Synthesis of V-TCNE

V-TCNE

V-TCNE

V

TCNE TCNE

V-TCNE

V2TCNE

-0.3 Å 0 Å 1.5 Å0.5 Å 1.0 Å

Reaction of V-TCNE with V trans-V2TCNE

LUMO

Wegner et al.,Nano Lett. 8, 131 (2008)

Wegner et al.,PRL 103, 087205 (2009)

topo dI/dV -0.6V

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Vx(TCNE)y Structure Analysis

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

-0.3 Å 0 Å 1.5 Å0.5 Å 1.0 Å

long V2TCNE short V2TCNE

Wegner et al., PRL 103,087205 (2009)

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Outline

Motivation: molecular spin networks

V + TCNE– promising candidates

Building Vx(TCNE)y molecules

Magnetic properties of Vx(TCNE)y

Outlook

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Kondo resonance

conduction electronsscreen impurity spin

STS Probes the Spin

5 Å

V-TCNE

-1 -0.5 0 0.5 1

dI/d

V

sample bias (V)

@TCNE

@V -0.2 V state

E−EF (eV)

Majority

Minority

V-L

DO

S

-1 -0.5 0 0.5 1

dI/d

V

-80 -40 0 40 80sample bias (mV)

EF

metal

U

V atom

two local probes of V-spin

DFT

5 Å

-1 -0.5 0 0.5 1

dI/d

V

sample bias (V)

V(TCNE)2

dI/d

V

-80 -40 0 40 80sample bias (mV)

Wegner et al., PRL 103,087205 (2009)

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Spin in V2TCNE

Long V2TCNE

• -0.2 V state

• Kondo resonance

Short V2TCNE

• -0.2 V state

• STS virtually identical

• No Kondo resonance!-0.5 0 0.5sample bias (V)

1-1

dI/d

V5 Å

long V2TCNE

5 Å

short V2TCNE

-0.5 0 0.5sample bias (V)

1-1

dI/d

V

@TCNE

@V

Magnetic coupling!

-50 0 50sample bias (mV)

dI/d

V

-50 0 50sample bias (mV)

dI/d

V !

Wegner et al., PRL 103,087205 (2009)

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Magnetic Coupling and Kondo Effect

Single spin impurity

Two spin impurities weakly coupled

Two spin impurities strongly coupled

weak

coupling

strong

coupling

long V2TCNE

short V2TCNE

Two-impurity Kondo problemJayaprakash et al., PRL (1981)

strong AFMcoupling

→ no

weakcoupling

→ TK

strong FMcoupling

→ TKFM « TK

Texp

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Symmetry arguments

• Even number of electrons on TCNE(TCNE0, TCNE2−)

• Odd number of electrons on TCNE(TCNE−)

FM vs. AFM Coupling

5 Å

short V2TCNE

V-spins couple AFM

V-spins couple FM

Determine the charge state

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DFT vs. Experiment V2TCNE

-1 0 1

Majority

(V2TCNE)2–

PD

OS

(ar

b. u

nits

)

E - EF (eV)

total vanadium

Minority

-1 0 1

Majority

(V2TCNE)0

PD

OS

(ar

b. u

nits

)

E - EF (eV)

total vanadium

Minority

-1 -0.5 0 0.5 1

dI/d

V

sample bias (V)

V2TCNEexp.

short

long

-1 0 1

Majority

(V2TCNE)–

PD

OS

(arb

. units

)E - E

F (eV)

total vanadium

Minority

• (V2TCNE)− DOS fits best

• ferromagneticcoupling

Wegner et al., PRL 103,087205 (2009)

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Double Check: V-TCNE

-1 0 1

Majority

(V-TCNE)2–

PD

OS

(arb

. units

)

E - EF (eV)

total vanadium

Minority

-1 0 1

Majority

(V-TCNE)–

PD

OS

(ar

b. u

nits

)E - E

F (eV)

total vanadium

Minority

-1 -0.5 0 0.5 1

dI/d

V

sample bias (V)

@TCNE

@V V-TCNEexp.

-1 0 1

Majority

(V-TCNE)0

PD

OS

(ar

b. u

nits

)

E - EF (eV)

total vanadium

Minority

• (V-TCNE)− DOS fits best

• isolated molecule: TCNE−

Wegner et al., PRL 103,087205 (2009)

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Outline

Motivation: molecular spin networks

V + TCNE– promising candidates

Building Vx(TCNE)y molecules

Magnetic properties of Vx(TCNE)y

Outlook

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Outlook

Weaken substrate coupling

• reduce screening

• increase spin coupling

Build larger structures

• various symmetries, geometries

• engineered spin structures

Substrate

TCNEVTCNEInsulating monolayer

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Summary

Create V+TCNE complexes:

- atomic-scale precision- strong chemical bond

short V2TCNE

V2TCNE molecules:

Increase orbital overlap

Ferromagnetic coupling

STS: two spin probes

longV2TCNE

-0.5 0 0.5sample bias (V)

1-1

dI/

dV

@TCNE

@V V majority d-state

Kondo resonance

-50 0 50sample bias (mV) Wegner et al., PRL 103, 087205 (2009)

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Acknowledgements

Michael F. CrommieRyan YamachikaXiaowei Zhang

(UC Berkeley Physics)

Jeffrey R. Long Bart M. Bartlett

(UC Berkeley Chemistry)

Mark PedersonTunna Baruah

(NRL Washington DC / UT El Paso)

Alexander von Humboldt Foundation

National Science Foundation

Department of Energy