1

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

Daniel Wegner

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

2

(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

7

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

8

Outline

Motivation: molecular spin networks

V + TCNE– promising candidates

Building Vx(TCNE)y molecules

Magnetic properties of Vx(TCNE)y

Outlook

9

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

10

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)

11

Outline

Motivation: molecular spin networks

V + TCNE– promising candidates

Building Vx(TCNE)y molecules

Magnetic properties of Vx(TCNE)y

Outlook

12

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)

13

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)

14

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

15

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

16

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)

17

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)

18

Outline

Motivation: molecular spin networks

V + TCNE– promising candidates

Building Vx(TCNE)y molecules

Magnetic properties of Vx(TCNE)y

Outlook

19

Outlook

Weaken substrate coupling

• reduce screening

• increase spin coupling

Build larger structures

• various symmetries, geometries

• engineered spin structures

Substrate

TCNEVTCNEInsulating monolayer

20

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)

21

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