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Magnetism in single metalloorganic complexes formed by

atom manipulation

Journal: Nano Letters

Manuscript ID: nl-2013-04054v

Manuscript Type: Communication

Date Submitted by the Author: 31-Oct-2013

Complete List of Authors: Choi, Taeyoung; Ohio state university, Physics Badal, Marion; Ohio state university, Physics; University of California, Merced, School of Natural Sciences

Loth, Sebastian; IBM Almaden Research Center, Yoo, Jung-Woo; Ohio state university, ; Ulsan National Institute of Science and Technology, School of Mechanical and Advanced Material Engineering-Low dimensional Carbon Materials Center Lutz, Chris; IBM Almaden Research Center, Heinrich, Andreas; IBM Almaden Research Center, Epstein, Arthur; Ohio state university, Physics Stroud, David; Ohio state university, Physics Gupta, Jay; Ohio state university, Physics

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Magnetism in single metalloorganic complexes formed by atom manipulation

T. Choi†,┴, M. Badal†,‡, S. Loth§,¶, J.-W. Yoo†,||, C.P. Lutz§, A.J. Heinrich§, A.J. Epstein†, D.G. Stroud†,

and J.A. Gupta*,†

† Department of Physics, The Ohio State University, Columbus, OH 43210 ‡School of Natural Sciences, University of California, Merced, CA 95343 § IBM Research Division, Almaden Research Center, San Jose, CA 95120

||School of Mechanical and Advanced Material Engineering-Low dimensional Carbon Materials Center,

Ulsan National Institute of Science and Technology, Ulsan, Republic of Korea, 688-798

Abstract

The magnetic properties of molecular structures can be tailored by chemical synthesis or bottom-

up assembly at the atomic scale. We used scanning tunneling microscopy to study charge and

spin transfer in individual complexes of transition metals with the charge acceptor,

tetracyanoethylene (TCNE). The complexes were formed on a thin insulator, Cu2N on Cu(100), by

manipulation of individual atoms and molecules. The Cu2N layer decouples the complexes from

Cu electron density, enabling direct imaging of the TCNE molecular orbitals as well as spin-flip

inelastic electron tunneling spectroscopy. Results were obtained at low temperature (>1 K) and in

magnetic fields up to 7 T in order to resolve splitting of spin states in the complexes. We also

performed spin-polarized density functional theory calculations to compare with the experimental

data. Our results indicate that charge transfer to TCNE leads to a change in spin magnitude,

Kondo resonance, and magnetic anisotropy for the metal atoms.

An atomic-level understanding of spin and charge transfer across metal/organic interfaces is

fundamental for a variety of pursuits, including information technologies (e.g. molecular electronics 1,2,

molecular spintronics 3, quantum computing 4), as well as surface chemistry (e.g. heterogeneous

catalysis 5). For example, in the family of organic-based semiconductors M[TCNE]x~2, where M is a

transition metal and TCNE is the strong charge acceptor tetracyanoethylene (C6N4), a variety of

phenomena coupling electronic, optical and magnetic degrees of freedom are observed as one varies

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transition metal (V, Cr, Mn, Fe, Co, Ni) across the periodic table 6-8. Disorder is a central issue in such

materials, and arises from variability in the charge state of both metal and organic components, chemical

stoichiometry, and purely structural contributions from materials growth in solvent, or solvent-free

conditions 9. It is perhaps counterintuitive, that highly disordered V[TCNE]~2 exhibits the highest

ferromagnetic TC in the family (~400 K), while crystalline Mn[TCNE]~2, or Fe[TCNE]~2 compounds

with higher-spin transition metal atoms have much lower TC (107 K and 121 K respectively).

Such uncertainty motivates scanning tunneling microscopy (STM) studies of the smallest

possible sub-units of these materials, namely individual metallo-organic complexes 10-15. Atomic-

resolution imaging and atom/molecule manipulation provide detailed information on the bonding

between metal atoms and molecules, while tunneling spectroscopy can be used to probe how the

electronic and magnetic properties evolve upon bond formation 13,16. For example, magnetism in single

TCNE (tetracyanoethylene) molecules adsorbed on Cu(111) 14 and in V[TCNE] complexes on Ag(100)

formed by atom manipulation 13, produces a Kondo resonance in tunneling spectroscopy due to

interaction with the substrate electrons. On Cu(111), multistable binding configurations with distinct

charge and spin transfer were observed 14, echoing the variable oxidation states coexisting in the bulk

materials. Here, we report STM studies of individual M[TCNE] complexes, where M is a magnetic (Co,

Fe) or nonmagnetic (Cu) transition metal atom. Atomic manipulation is used to synthesize these

complexes on an ultrathin insulating layer of Cu2N islands grown on Cu(100). Such layers have proven

to be useful for decoupling adsorbates from the substrate electron density, thus enabling direct imaging

of molecular orbitals similar to the gas-phase molecule 16,17 and spin-flip spectroscopy of single atoms

and clusters 18,19. STM imaging allows us to visualize bonding in the complexes, and high resolution

tunneling spectroscopy, combined with spin-polarized density functional theory calculations, allows us

to study how charge transfer to TCNE influences the spin state and magnetic anisotropy of the magnetic

atoms.

Figure 1a shows an STM image of a Cu2N island, surrounded by bare Cu(100) surface. Under

our deposition conditions (c.f. Methods), we find metal atoms on both Cu2N and Cu regions, but TCNE

only on Cu regions, or rarely, adsorbed atop defects in the Cu2N islands (Figure 2a and Supporting

Information Figure S2). This adsorption behavior reflects the fact that thin insulating films such as Cu2N

typically have low sticking coefficients for adsorbates. We therefore used atomic manipulation to form

individual M[TCNE] complexes on the Cu2N island, as illustrated in Figure 1. First, the STM tip is

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parked over a TCNE molecule adsorbed on the Cu substrate. After disabling the STM feedback loop, the

tip is brought closer (~0.3 nm) to the molecule while ramping the bias voltage (Figure 1f). This process

results in the transfer of the TCNE molecule from the surface to the apex of the STM tip (nearly 100%

success rate). Figure 1b shows an STM image taken with the TCNE terminated tip; the enhanced spatial

resolution results from chemical interactions between the functionalized tip and the surface 20. Next, the

TCNE molecule can be transferred back to the sample surface to form a complex with a metal atom on a

Cu2N island. To achieve this, the TCNE-terminated tip is parked over the metal atom, and is brought

closer by lowering the bias voltage (~10 mV) and increasing the set current (~7 nA) with the STM

feedback enabled. As the tip is moved closer to the metal atom, a sharp reduction in tunneling current

marks the drop-off of the TCNE molecule (~80% success rate). Figure 1c shows an STM image of the

area after forming a Co[TCNE] complex. We also formed a Cu[TCNE] complex on the same Cu2N

island by first depositing a Cu atom from the STM tip (c.f. Figure 1d and Supporting Information

Figures S1 and S8), and then repeating the TCNE transfer procedure (Figure 1e). Complexes were

formed with one of four possible orientations with respect to the Cu2N lattice, but always with the cyano

group (C≡N) closest to the metal atom (Supporting Information Figure S3).

Figure 2a shows a Co[TCNE] complex and TCNE molecule on Cu2N. Both closely resemble

the lowest unoccupied molecular orbital (LUMO) of gas-phase TCNE (c.f. Supporting Information

Figure S2), except that one cyano group (C≡N) appears larger in Co[TCNE] due to interaction with the

metal atom. Perturbation by the STM tip during imaging or spectroscopy can rotate TCNE within the

complexes without detachment, suggesting some degree of bonding (c.f. Supporting Information Figure

S3). Tunneling spectra from individual Co, TCNE, and Co[TCNE] indicate differences in electronic

structure also suggestive of bond formation (Figure 2b). For example, a prominent occupied orbital

resonance appears at -0.15V for Co in Co[TCNE] that is absent for the isolated atom. Spectra taken with

the tip over the central C=C bond in isolated TCNE shows steps at ~32 meV(~258 cm-1) due to inelastic

scattering of tunneling electrons from e.g. vibrational modes 20. These steps are assigned to the =C-

(CN)2 in-plane rocking mode 13,21, and are suppressed after complex formation 13. Similarly strong

differences are observed with the tip positioned over the cyano groups. Unfortunately, we did not

observe steps due to the C=C bond (expected in the 170-200 mV range) or C≡N bond (~280 mV), as

prior Raman studies attributed changes in these mode energies to charge transfer in alkali[TCNE] salts 21.

Selection rules for IETS are still not well understood, and it is often the case that vibrational modes

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(1) )ˆˆ(ˆˆˆ 222yxzB SSESDSBgH −++⋅−=

rµ

visible with other techniques do not appear in STM-IETS. Tunneling spectra were found to be

independent of orientation within the complex (Supporting Information Figure S3), suggesting that

interaction with Cu2N is secondary to the M-TCNE bonding itself.

Figures 3a-c show STM images of Co, Fe, and Cu[TCNE] complexes, formed by

atom/molecule manipulation as in Figure 1. We characterized the magnetic properties of the isolated

metal atoms (Co, Fe, and Cu) and the metal atoms in complexes (Co[TCNE], Fe[TCNE], and

Cu[TCNE]) by spin-flip tunneling spectroscopy as seen in Figures 3d-f. We observe step-like features in

conductance for the magnetic atoms due to the opening of an inelastic tunneling channel associated with

spin-flip excitations. These spectra can be understood by an empirical spin-Hamiltonian 19,22,23:

Here g is the Lande g-value, µB is the Bohr magneton, B is the applied magnetic field, S is the atomic

spin, D is an axial anisotropy, and E is a transverse anisotropy. The anisotropy terms result from the

Cu2N surface, and break the degeneracy of ms levels, where ms is the magnetic quantum number along

the z axis. For example, the step at ±5.5 mV for Co corresponds to the transition between multiplets ms=

±1/2 and ms= ±3/2. Previous studies with applied magnetic fields give the values (g=2.2, S=3/2, D=2.75

meV, and E=0 meV) for Co on Cu2N 23. The positive value of D for the Co atom represents hard-axis

and easy-plane anisotropy. The peak at 0 mV for Co is a Kondo resonance, which is not included in the

spin-Hamiltonian model. Compared to Co on Cu(100) with a Kondo temperature of 88K 24, the lower

Kondo temperature (2.6 K) for Co on Cu2N indicates the decoupling effect of the intervening Cu2N 23.

We now discuss tunneling spectroscopy of the M[TCNE] complexes after synthesis by atomic

manipulation. Data taken on the Co atom of Co[TCNE] shown in Figure 3d show steps at a reduced

voltage (~1.9 meV), and no Kondo resonance. Similarly, spectra on the Fe atom of Fe[TCNE] show

steps at a reduced voltage (Figure 3e). Coordination of these atoms to TCNE may introduce charge

transfer and change the local electrostatic environment, both factors which can significantly change the

magnetic anisotropy. Control measurements of nonmagnetic Cu atoms and Cu[TCNE] complexes reveal

no steps in low voltage range (Figure 3f), suggesting that the steps are not associated with low energy

vibrational modes 20.

To confirm the spin-related origin of the low-energy steps, and determine the spin magnitude and

magnetic anisotropy, we performed measurements at lower temperature (1 K) and in magnetic fields up

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to 7 T. Figures 4a-b show an STM image and schematic of the Co[TCNE] complex, as well as the

magnetic field orientation with respect to the Cu2N lattice. As the magnetic field is increased, the steps

in dI/dV systematically shift toward larger voltage (Figure 4c).

These spectra can be analyzed within the spin-Hamiltonian model (Equation 1) to understand

how magnetic properties have changed as a result of complex formation. The coordinate system was

chosen following the convention for isolated Co on Cu2N 23. As a first approximation, we assume that

the spin-spin interaction between Co and TCNE is weak compared to the magnetic anisotropic energy of

the Co atom. This parameter is thought to be ~0.5 meV in bulk Co[TCNE]2 25, but is not known for

Co[TCNE] on surfaces as we have here. Our assumption is consistent with the independence of the spin-

flip spectra on molecular orientation (c.f. Supporting Information Figure S3), as well as the lack of any

spin-flip steps or Kondo features on TCNE within the complexes (c.f. Figure 2). A priori, the magnitude

of the Co atomic spin is not known. Therefore, we simulated the field-dependent spectra with varying g,

D, and E values for the two most likely Co spin values (S=3/2 and S=1) (c.f. Supporting Information

Figure S7) 23. The deviation between simulated and experimental curves is then calculated to evaluate

the quality of the fit. Parameter-space plots of the deviation constrain g, D, and E to two local minima

for each value of S. Comparably good fits are obtained for either S value (Figure 4c). Though we cannot

more uniquely determine these parameters without further measurements with different field directions,

our constraints clearly indicate significant differences in magnetic anisotropy between the isolated Co

atom on Cu2N and the Co atom in Co[TCNE] complex (Figure 3 and Supporting Information Figure S6).

To further support this conclusion and gain insight into the underlying mechanism, we

performed spin-polarized DFT calculations (c.f. Methods). The spin value of the Co atom in the

Co[TCNE] complex can differ from the bare Co atom due to charge transfer from the metal atom to

TCNE, which is a strong charge acceptor. We find that the fully relaxed Co[TCNE] complex lies in a

plane parallel to the Cu2N surface, with Co (0.17 nm) and TCNE (0.32 nm) above as seen in Figure 5a.

The Co atom of the Co[TCNE] complex attracts its neighboring two N atoms, and the Cu atom

underneath the Co atom moves toward the bulk (Figure 5b) 22,26. The calculated atomic positions of the

Co atom and the neighboring atoms in the Cu2N island are very similar to those in the Co/Cu2N system 26, suggesting that introduction of TCNE does not cause further surface relaxation. Thus, we attribute the

changes in magnetic anisotropy to bond formation rather than structural deformation due to TCNE. To

confirm this, we calculated charge densities of the Co[TCNE]/Cu2N system and find that there is a

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significant charge transfer from the Co atom (+0.72e) to the TCNE molecule (-0.58e) and Cu2N surface

(-0.14e) as seen in Figure 5b. This suggests that the Co atom forms chemical bonds with the TCNE

molecule and Cu2N surface. The simulated STM topographic image agrees well with the experimental

image (Figure 5d); both show a LUMO-like orbital associated with TCNE, and a bulge in one of the -

C≡N lobes where the Co atom is bonded. We note that although the molecular orientation for the

theoretical modeling differs from that in Fig. 4, the comparison remains valid because the tunneling

spectra were independent of TCNE orientation (c.f. Supporting Information Figure S3).

We estimate the Co spin by calculating the magnetic moment distribution and the orbital-

projected LDOS. Figure 5b shows a magnetic moment of 2µB on the Co atom of the Co[TCNE]-

complex, which corresponds to a spin S=1 on the Co atom. Additional moment density occurs on the

TCNE molecule as well as nearest neighbor Cu and N atoms in the Cu2N surface. Using the same

coordinate system as for the spin-Hamiltonian, the orbital-projected LDOS on the Co atom of the

complex indicates that the 3dxz, 3dxy and 3dy2-z2 orbitals lie primarily below EF (0 eV) for both spin up

and down, suggesting that these orbitals are fully occupied, while the 3dyz and 3dx2 orbitals have

significant LDOS above EF for spin down, and are only partially occupied (Figure 5c). We compare the

calculated 3d LDOS with our tunneling spectra taken on isolated Co and Co in the Co[TCNE] complex

(Figure 5c, bottom panel). The prominence of occupied states for Co[TCNE] compared to isolated Co

suggests significant charge transfer. These occupied states are in good agreement with the calculated

LDOS, allowing us to estimate a valence electron configuration of 3d84s0 for the Co atom in Co[TCNE],

compared to a configuration of 3d74s2 for a free Co atom 26. This qualitative analysis is consistent with

the charge distribution (transfer), the magnetic moment, and the orbital-projected LDOS of the Co atom.

Therefore, our calculations suggest that the Co spin is reduced from S=3/2 for the isolated Co on Cu2N,

to S=1 in the Co[TCNE] complex. Given S=1, our data (c.f. Figure 4c and Supporting Information

Figure S7) indicate axial and transverse anisotropy parameters that are significantly different than for the

isolated atom (D=2.75, E=0), suggesting the presence of TCNE can break the symmetry of the local

electrostatic environment and thus change the magnetic anisotropy as well.

The negatively charged TCNE molecule in the complex could have an unpaired spin in

molecular orbitals, leading one to expect a Kondo resonance 14 or spin-IETS signal. However, no such

features are observed in tunneling spectroscopy (c.f. Figure 2b). The absence of a Kondo resonance

could be attributed to factors including an unfavorable spin ground state 23 or simply a weak interaction

with Cu substrate electron density. This latter point is consistent with the relatively large calculated

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adsorption height for TCNE in the complex (0.32 nm) as seen in Figure 5a. The absence of spin-IETS

steps in tunneling spectroscopy could reflect that the local magnetic anisotropy is very different for the

TCNE molecular spin than the Co or Fe atomic spins, also to be expected for a larger adsorption height.

In summary, our studies illustrate a method for studying charge transfer in metallo-organic

complexes at the atomic scale. A combination of atomic-resolution imaging, tunneling spectroscopy and

density functional theory is used to develop a model for charge and spin redistribution in the complexes.

We demonstrate atomic manipulation methods for constructing these complexes on a thin insulating

surface, which facilitates detailed studies of the magnetic properties (spin magnitude, Kondo resonance,

and magnetic anisotropy). This approach provides valuable information toward a microscopic

understanding of metal/organic interfaces, relevant for information technologies and heterogeneous

catalysis.

Methods.

Data were taken in ultra-high vacuum (<1×10-10 mbar) with a Createc STM (at 5.3 K) and a

custom-built STM operating at 1 K and in magnetic fields up to 7 T. In both instruments, a cut Ir tip was

prepared with field emission and controlled contact with the sample. The latter step generally produces a

tip terminated with Cu from the substrate. For tunneling spectroscopy at constant tip height, the STM

feedback loop was disabled, and a modulation voltage (0.5 mVrms at 5.3 K, 0.1 mVrms at 1 K, ~800 Hz)

was added to the sample voltage V. The differential conductance dI/dV, was then measured using a lock-

in amplifier. Experimental broadening limits the energy resolution in tunneling spectroscopy, and

reflects contributions from the temperature, modulation voltage (chosen to be small) and spurious

electrical/vibrational noise. In our measurements at 1 K, the effective resolution was limited by this last

contribution, as estimated by measuring spin-flip IETS spectra on isolated Co atoms on Cu2N 23. This

empirical broadening was added when using the spin Hamiltonian to fit experimental data. Voltage and

set current conditions for imaging and tunneling spectroscopy were limited by the stability of the

complexes, which were readily perturbed by the STM tip.

A clean Cu(100) surface was prepared by repeated Ar+ sputtering and annealing cycles. Cu2N

islands were grown by additional sputtering in N2 (<1 × 10−5 mbar) for two minutes, followed by

annealing at 350 °C for one minute 27. Auger electron spectroscopy was used to monitor sample

cleanliness and N adsorption. Co or Fe was deposited by electron beam evaporation onto the surface at a

temperature <10 K. TCNE molecules were introduced through a UHV leak valve, also in situ at <10 K.

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Single Cu atoms were transferred onto Cu2N islands by ramping voltage to ~ -0.5 V while moving the

Cu-terminated tip closer by sub-nanometer (c.f. Supporting Information Figure S8b).

In the density functional theory calculation, we used the projector augmented wave (PAW)

method 28,29. A Perdew-Burke-Ernzerhof (PBE) exchange correlation functional was employed with the

generalized gradient approximation (GGA) 30, as implemented in the Vienna ab-initio Simulation

Package (VASP) 31. All the results presented and structure relaxation were obtained using spin-polarized

DFT. Charge and spin densities were partitioned using the Bader scheme 32, as implemented in the code

Bader 33, and visualized using VESTA 34. The unit cell consisted of a symmetric slab of five Cu layers,

with N adsorbed to both faces to reduce unphysical multipoles. The Co[TCNE] complex was adsorbed

to one face of the slab and a vacuum region of thickness equal to the Cu slab plus complex was used.

The Co[TCNE] complex periodic image was repeated every four Cu lattice constants, with minimum

distance between images equal to two lattice constants. Convergence of the total energy to <10−5 eV and

ionic forces <0.0095 eV/Å was achieved using a 500 eV plane-wave cutoff energy, and a supercell

Brillouin zone k-points mesh of at least 3×3×1 for relaxation and 5×5×1 for the local density of states

(LDOS) determination.

■ ASSOCIATED CONTENT

Supporting Information

Methods, Characterization, Materials, and Supplementary contents.

■ AUTHOR INFORMATION

Corresponding Author

*Email: gupta.208@osu.edu.

Present Address

┴ T. Choi is now a post-doctoral researcher at the University of Maryland, College Park and Joint

Quantum Institute, MD, USA. ¶ S. Loth is now a group leader of Dynamics of Nanoelectronic System at the Center for Free-Electron

Laser Science, Hamburg and the Max-Planck Institute for Solid State Research, Stuttgart in Germany.

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Notes

The authors declare no competing financial interest.

■ ACKNOWLEDGEMENTS

We acknowledge C.Y. Chen for providing 99% purified TCNE sample and thank J.W. Nicklas, A.

Wadehra, H. Park, and J.W. Wilkins for helpful discussions. We are grateful for support from NSF

CAREER Award No. DMR-0645451, the Center for Emergent Materials NSF-funded MRSEC (DMR-

0820414), and the Ohio Supercomputer Center. T. Choi thanks the Ohio State University Presidential

Fellowship for funding. A.J. Heinrich and C.P. Lutz thank the Office of Naval Research.

■REFERENCES

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Co[TCNE]

c

TCNE

TCNE

Co

Cu2N

Cu(100)

1nm

a

TCNE

Cob

Cu[TCNE]

e Co[TCNE]

TCNE

Cu

dFig. 1

f

Figure 1. Building metal-TCNE complexes on Cu2N/Cu(100). (a) STM image of the surface as prepared,

showing a Co atom on Cu2N, and two TCNE molecules on the Cu(100) substrate. (b) One TCNE is picked up

(dashed circle) onto the tip by varying voltage V and tip height Δz as shown in (f). Enhanced contrast is attributed to

the TCNE-terminated tip. (c) A Co[TCNE] complex is formed after dropping the TCNE molecule off onto the Co

atom. (d) A Cu atom is dropped off from the tip. (e) A nonmagnetic control, Cu[TCNE] is formed by picking up

another TCNE molecule and putting it down onto the Cu atom. All images are taken at (0.15V, 20pA) and are

Laplace filtered to emphasize local contrast. Scale bars=1 nm. All images are taken at 5.3K. (f) Plot of typical bias

voltage, tip height and measured tunneling current for picking up TCNE molecules from Cu substrate. The initial tip

height is set at (0.1V, 50pA).

0.0 0.5 1.0 1.5 2.0

-1.0

-0.5

0.0

Va

lue

time (s)

Voltage (V)

z (nm)

current (A)

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0.5nm

Co[TCNE]

TCNE

a

Figure 2. Comparison of spectra before/after bond formation taken with various tip locations. (a) STM

image of Co[TCNE] on Cu2N (0.2V, 40pA) and TCNE adsorbed on a N vacancy in Cu2N island (0.15V, 20pA).

All scale bars = 0.5nm. (b) dI/dV spectroscopy on Co, TCNE, and Co[TCNE]. The tip height was set at 0.2V,

50pA. For clarity, red curves are offset by 0.1pA/mV. Inset shows numerical derivative of the dI/dV signal

(middle of (b)) enhancing the difference of vibrational IETS signals before and after bond formation (blue

arrows). All data were taken at 5.3K.

Fig. 2

0.0

0.3

0.6

0.9

Individual Co atom

Co atom of Co[TCNE]

0.0

0.3 C=C of individual TCNE

C=C of Co[TCNE]

dI/

dV

(p

A/m

V)

-200 -150 -100 -50 0 50 100 150 2000.0

0.3 CN of individual TCNE

CN of Co[TCNE]

Voltage (mV)

b

-200 -100 0 100 200-8

-4

0

4

8 C=C of individual TCNE

C=C of Co[TCNE]

d2I/

dV

2 (

fA/m

V2)

Voltage (mV)

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Fe[TCNE]b

Fig. 3

Cu[TCNE]

c

-10 -5 0 5 10

1.0

1.5

2.0

2.5

dI/

dV

(p

A/m

V)

Voltage (mV)

Fe atom on Cu2N

Fe atom of Fe[TCNE]

e

-10 -5 0 5 10

0.2

0.3

0.4

0.5

dI/

dV

(p

A/m

V)

Voltage (mV)

Co atom on Cu2N

Co atom of Co[TCNE]

d

-10 -5 0 5 10

0.2

0.3

0.4

0.5

dI/

dV

(p

A/m

V)

Voltage (mV)

Cu atom on Cu2N

Cu atom of Cu[TCNE]

f

Figure 3. Characterization of M[TCNE] complexes (M=Co, Fe, and Cu). (a-c) STM images of

Co[TCNE], Fe[TCNE] and Cu[TCNE] complexes formed by atom manipulation (0.2V, 20pA, 0.25V, 40pA,

and 0.2V, 20pA respectively). Scale bars = 1nm. Inset of (a) shows the chemical structure of TCNE and

calculated molecular orbital (LUMO) of gas-phase TCNE 14. (d-f) dI/dV spectroscopy on M[TCNE]

complexes and isolated M atoms on Cu2N (M=Co, Fe, Cu respectively). The tip height was set at (60mV,

30pA), (40mV, 50pA), and (40mV, 10pA) in (d-f) respectively. For clarity, colored curves are offset by

0.15pA/mV in (d,f) and 0.2pA/mV in (e). All data were taken at 5.3K.

a

1nm

Co[TCNE]TCNE

LUMO

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Fig. 4

c

Bext

Cu2N

bb yz

x

0.5nm

a

Figure 4. Magnetic field dependence of spin-IETS steps (a) STM image of a Co atom and a Co[TCNE]

complex (0.2V, 20pA). (b) Corresponding surface model showing Cu (red), N (blue) and Co (yellow) atoms, as

well as a scale model of TCNE. The black arrow indicates the magnetic field orientation (Bext). Also shown is the

coordinate system used in the spin-Hamiltonian model. (c) Spin-IETS steps as a function of magnetic field, with

the tip position indicated by the black dot in (a). The tip height was set at (30mV, 1nA). All spectra were taken at

1 K and offset by 0.15 along the y-axis for clarity. Solid and dotted lines indicate spin Hamiltonian calculations

with (S=3/2, D=-0.45, E=0, g=1.95) and (S=1, D=0.3, E=0.8, g=2.3) respectively.

-5 0 50.8

1.0

1.2

1.4

No

rma

lize

d d

I/d

V

Voltage (mV)

7 T

4 T

0 T

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Fig. 5

b

Co

Cu

N

TCNE

(+1.99)+0.72

(+0.53)-0.58

(+0.2)-1.1

(+0.04)+0.48

(+0.05)+0.05

Cu

N

(+0.2)-1.1 Cu

(+0.04)+0.48

(Magnetic moment)Charge

d

Experiment

DFT calculationa

Co

TCNE

CuN

CoCu N

y

z

x

0

3

3d spin down

3d spin down total 3d spin up total

LD

OS

(a

rb.

un

it.)

-3 -2 -1 0 1 2 3

0

3

3dyz 3dy

2-z

2

3dxz

3dx2

3dxy

Energy (eV)

EFc

-1.0 -0.5 0.0 0.5 1.0

0

1

dI/

dV

(n

A/V

)

Co of Co[TCNE] (STS)

dI/

dV

(a

.u.)

Voltage (V)

Co on Cu2N (STS)Experiment

0

1 Co 3d spin total

of Co[TCNE] (DFT)

DFT

Figure 5. Spin-polarized DFT calculations of Co[TCNE]. (a) Side and top view of fully relaxed Co[TCNE]

complex on Cu2N/Cu(100). (b) Bader analysis of charge and magnetic moment (parentheses) distribution. Here, we

only display charge and magnetic moment distribution of nearest neighboring atoms of the Co atom. Total charge of

Cu2N slab is -0.14 e. (c) Top panel is the calculated 3d orbital local density of states (LDOS) for the Co atom in

Co[TCNE]. The five 3d orbitals for spin up lie below EF while 3dyz and 3dx2 for spin down exhibit significant LDOS

above EF (lower panel). Bottom panel shows comparison between the calculated 3d LDOS and dI/dV spectra. (d) A

comparison of the DFT-simulated and experimental STM images (V=0.2V).

0.17nm

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