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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
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
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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
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
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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)
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)
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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)
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)
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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
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