Supplementary Information
Improving MgO/Fe Insulator-Metal Interface Structure throughOxygen-Precoating of Fe(001)Nana K. M. Nazriq1, Peter Krüger1,2, and Toyo Kazu Yamada1,2*
1. Department of Materials Science, Chiba University, 1-33 Yayoi-Cho, Inage-ku, Chiba 263-8522, Japan.
2. Molecular Chirality Research Centre, Chiba University, 1-33 Yayoi-cho, Inage-ku, Chiba 263-8522, Japan.

Author Information,
Corresponding Author: Toyo Kazu Yamada *E-mail: toyoyamada@faculty.chiba-u.jp
Version 2023.1.20

Cleaning process of Fe(001) whisker single crystal surface.
A Fe(001) whisker single crystal was cleaned via Ar+ sputtering process (see Fig. S1a) with theevolution of temperature between room temperature and 350, 500, 600 degrees Celsius. The cleaned
surface showed the squared bcc(001) symmetry spots in the low-energy electron diffraction (LEED)
images (see Fig. S1b).

a Sputter & Anneal cycleb Fe(001) LEED spots

Figure S1. (a) UHV cleaning cycles of the Fe(001) whisker surface using Ar + sputtering at different
substrate temperatures. (b) LEED images on the cleaned Fe(001) surface at 300 K in UHV.

Magnesium evaporation control
A magnesium rod (length 50 mm, diameter 1.6 mm, purity 99.95%) was set in the electronbombardment type evaporator (see Fig. S2a). The top of the Mg rod was heated by the accelerated hotelectrons (HV 0.5 kV, EC 3-4 mA) from the tungsten filament. The Mg deposition rate was controlled
by monitoring the quartz crystal microbalance and the evaporated Mg flux (see Fig. S2b). Figure S2ce shows STM results of 1.5 ML Mg films deposited on the O/Fe(001). The absorbed Mg atoms
thermally diffused on the surface and the 1st Mg layer grew in a step flow mode. However, the second
Mg layer started to grow before the first Mg layer completely covered the substrate O/Fe(001) surface.
This means that the Mg films grew in a layer-plus-island mode. Also, both first and second Mg layers
have a height of ~0.15 nm.
acd10 nmFe-O

Mg 2ndMg 1st40 nm

20nm

e40nm
0.4

30z [nm]bThickness [Å]Flux: 14.1 nA
20Before

Mg 1st layer

0.20.1

Flux: 0 nA

AfterMg 2nd layer

0.30010Flux: 8.0 nA

2030x [nm]4050f10Flux: 3.1 nA

Adjust rod position(+ 3-4 mm forward)

Mg 2nd layer

Mg 1st layer
Fe - O00200400600800

Time [s]Figure S2. (a) A sketch of the magnesium (Mg) evaporator. A constant flux was provided by adjusting therod position. (b) The Mg deposition rate obtained by the quartz crystal microbalance was controlled by
the Mg flux of the evaporator. (c, d) STM topographic images obtained on the surface of the Fe(001)whisker substrate covered by 1.4 ML Mg: (c) 100×100 nm2, Vs = -1.0 V, I = 80 pA (d) 50×50 nm2, Vs = 1.0 V, I = 100 pA. Blue -yellow contrast illustrates the low-high height scale. (e) Height profile along thearrow in (d). (f) The cross section view model corresponds to the height profile in (e).

Electronic structures of Mg films on Fe(001)-p(1×1)Oabc
dI/dV map @ +1.6 V

20 nm

20nm
d 8.0
7.0

dI/dV [nA/V]

6.0

Overlaid

20nm

20nm

Topo.

eFe-OMg

20nm
dI/dV signalHigh5.0Low
4.0x

3.0
2.0

Mg 2nd layer

1.00-1.5

Mg 1st layer-1.0-0.5

00.5

1.0

1.5

Fe - O

Bias [V]Figure S3. STM/STS results obtained on the 1.4 ML Mg films deposited on the O/Fe(001) substrate. (a, b)
Simultaneously obtained (a) STM topographic image and (a) dI/dV map at +1.6 V: 100×100 nm2, Vs = -1.0
V, I = 520 pA. In (a), blue -yellow contrast illustrates the low-high height scale. In (b), blue -yellow contrast
illustrates the low-high differential tunneling conductance (dI/dV) scale. (c) Overlaid image of (a) and (b).
(d) dI/dV curves obtained at the substrate O/Fe(001) area (black lines) and the Mg films (blue lines). (e)Comparison between dI/dV signal and the cross section view model of the Mg films on O/Fe(001).

Line profiles in dI/dV maps across the atomic defect in the MgO nanoisland

MgO island

2000

4
3.711
3.614
3.486
3.197
3.004
2.811
2.49
2.008
1.494
1.012
0.498
0.016
-0.016-0.498-1.012-1.494-2.008-2.49-3.004-3.486-4

Defect 1

1800
1600
1400

dI/dV [nA/V]

1200
1000
800
600
400
2000-200-400-6000102030Position [nm]405060

Defect 1dI/dV map at -4.0 V

9 nm

9.0nm

Figure S4. Line profiles along the black arrow in the dI/dV maps at various bias voltages between +4 and 4 V (Vs = +4.0 V, I = 1 nA), which across the regularly shaped MgO nanoisland. Defect 1 and 2 positions
are marked by red and blue arrows, respectively.

DFT calculations of CuPc DOS

Free-standing CuPc
DOS(spin-up)
DOS(spin-down)
30

LUMO

DOS

20HOMO
100−5−4−3−2−1012345E - EF [eV]CuPc on MgODOS(spin-up)
DOS(spin-down)
80

DOS60

40

LUMO
20HOMO
0−5−4−3−2−1012345E - EF [eV]
Figure S5. DFT calculation results of CuPc DOS. Upper and lower panels denote free-standing CuPc andCuPc adsorbed on MgO surface, respectively.