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Improving MgO/Fe Insulator-Metal Interface Structure through Oxygen-Precoating of Fe(001)Nana K. M. Nazriq, Peter Krüger, Toyo Kazu Yamada
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S0169-4332(23)00304-5
https://doi.org/10.1016/j.apsusc.2023.156628
APSUSC 156628

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Applied Surface Science

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21 December 2022
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29 January 2023

Please cite this article as: N. K. M. Nazriq, P. Krüger, T. Kazu Yamada, Improving MgO/Fe Insulator-Metal
Interface Structure through Oxygen-Precoating of Fe(001), Applied Surface Science (2023), doi: https://doi.org/
10.1016/j.apsusc.2023.156628

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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 2023.1.28

Highlights* Controlled p(1×1) oxygen pre-coating of Fe(001) drastically improves MgO film growth.
* Upon oxygen pre-coating, the MgO island size is increased by a factor of 10.
* Atomically-flat high-quality insulating MgO islands contain only few atomic defects.
* MgO islands also work as an electronic decoupling layer from the metal substrate.

Abstract
We grew atomically flat and electronically uniform MgO island terraces on a Fe(001) whisker
substrate precoated by p(1×1) oxygen monolayer film. This interface engineering process increased
about 10 times the size of MgO islands compared to the size of MgO islands directly grown on Fe(001),
showing that the interface oxygen monolayer plays an essential role in MgO growth. Importantly, eachsquared-shape atomically flat MgO island includes only a few atomic defects, which provides highquality insulating layers. Further, the MgO islands have an electronic decoupling functionality fromthe metal substrate. Surface morphology and local electronic structures were studied using a homebuilt ultrahigh vacuum scanning tunneling microscopy (STM) at a temperature of 5 K.

Keywords: magnesium oxide, iron, oxygen coating, scanning tunneling microscopy, phthalocyanine,organic molecule

MgO nanoclusters

MgO

Fe(001)Oxygenmonolayer>10 times larger

MgO
Fe(001)20 nm

1. Introduction
A remarkable aspect of using the insulator films is their extremely high spin filter effect. Especially,
the Fe/MgO(001) interface provided the theoretically predicted TMR values of 1000% 1,2 and theexperimental TMR values of 200-600% at room temperature3–6. Thus Fe/MgO has been widely used
as a tunnel magnetoresistance (TMR) head in a hard disk drive (HDD) and finds applications inmagnetoresistive random access memory (MRAM). These large TMR values are generated by therealization of an atomically flat interface between Fe and MgO films. The cross-section view of
atomically-resolved transmission electron microscopy images through the Fe/MgO(001) films showedatomically precise stacking of the MgO atomic layers on the Fe(001) film 3,4.
However, there is a problem with the reversed MgO/Fe interface, because, on the Fe(001) substrate,
MgO grows as islands in the Volmer-Weber (VW) mode 7–9. Therefore, MgO nanoclusters with adiameter of a few nanometers are obtained instead of a flat film. Small MgO nanocluster growth was
also observed on the (001) surface of bcc molybdenum 10.
In this study, we introduced an ordered p(1×1) oxygen monolayer film at the interface between the
MgO islands and Fe(001) whisker substrate 11–19. This interface engineering drastically modified the
MgO growth. It successfully increased the MgO island size to 10 times larger than MgO islands grown
directly on Fe(001) and produced regularly shaped atomically flat and electronically uniform MgOnanoislands. This oxygen precoating weakens the bonding between adsorbed MgO atoms and thesubstrate Fe(001), enhancing the thermal diffusion of the Mg and O atoms. The diffusing atoms weretrapped by the descending steps, forming nucleation. Other deposited Mg and O atoms were trappedby the nuclei, and the MgO island size increased until the MgO island reached another ascending step.
The growth and electronic structures of the MgO islands were studied by using a home-built ultrahighvacuum (UHV) scanning tunneling microscopy (STM) and scanning tunneling spectroscopy (STS)
setup at a cryogenic temperature of 5 K 20–23.
Another important role of using such an atomically flat insulating surface with a thickness of severalmonolayers is an electronic decoupling functionality from its substrate, which allows direct
visualization of free-standing electronic structures of adsorbents on a solid-state surface 24–35. Whenadsorbed on noble metal substrates, NaCl and MgO films with a thickness of 2-4 monolayers (MLs)follow the Frank-van der Merwe (FM) layer-by-layer growth mode and atomically flat insulating
terraces with a 20-100 nm size can be achieved. These insulating surfaces have been widely used toinvestigate quantum spin states of isolated 3d/4f transition metal atoms or nodal planes of molecularorbitals of single organic molecules36–38. Herein we also demonstrate the electronic decoupling abilityof the MgO islands grown on Fe(001)- p(1×1)O using single organic CuPc molecules.

2. Methods
2.1 Home-built low-temperature UHV STM setup.
The experiment was performed with a home-built low-temperature STM at 5 K under UHV. The setup
consists of the introduction, preparation, and analytical chambers with the base pressure below 10-8
Pa20–23. A UHV cryostat (CryoVac) including an outer liquid nitrogen tank (8 liters) and inner heliumtank (4 liters) is docked on the analytical chamber. The STM was placed at the center of the analyticalchamber thermally contacted with the cryostat. Heat radiation from the analytical chamber to the STM
was cut with two cylindrical cooling shields, which maintained the STM temperature of 5 K for 40 husing 4 liters of liquid helium.

2.2 STM / STS measurements.
STM and scanning tunneling spectroscopy (STS) measurements were performed with a combined
Nanonis SPM controller and software. The topographic images were obtained in a constant current
mode. The obtained STM data were analyzed using WSxM 5.0 Develop 10.0 software39 and Gwyddion
2.56. STS was done by measuring tunneling current as a function sample bias voltage (I(V)) at eachpixel position in an STM topographic image (feedback-off grid mode). Differential conductance(dI/dV) curves, which are proportional to sample local density of states (LDOS), were obtained using
WSxM 5.0 Develop 10.0 software39 via numerical differentiation of I(V) by the sample bias voltage(V).

2.3 Fabrication of W tip.
STM tips were fabricated from polycrystalline W and Mo wires with a diameter of 0.3 mm (purity
99.95 %) via electrochemical etching using KOH aq.40 as well as the flame-etching process41,42, andsubsequently transferred into the UHV preparation chamber. Then, the tip apex was flashed at 2000 K
to exclude oxide layers40. The cleaned tip was again transferred to the UHV analytical chamber without
breaking UHV and set into the STM using a wobble stick.

2.4 Fe(001) preparation (see Supplementary Information, Fig.S1 and Fig. S3).
A Fe(001) single crystal whisker, which was grown via chemical vapor deposition12,14, was cleaned byrepeated cycles of Ar+ sputtering (+0.8 kV, + 0.80 µA) at 820 K. The cleaned surface was checked by
STM /STS, which showed atomically-flat terraces with a wide width of 50-100 nm and the Fe(001) 3d
surface state peak at +0.2 eV in the dI/dV curves12,15,18,43,44. Low energy electron diffraction (LEED)
spots showed the bcc(001) four-fold symmetry.

2.5 Fe(001)-p(1×1)O preparation (see Supplementary Information, Fig.S3).
The Fe(001)-p(1×1)O surface was prepared in the preparation chamber by exposing the clean Fe(001)surface to the oxygen gas of about 2 Langmuir (60 sec, 4.0×10-6 Pa ) while maintaining the substratetemperature at 300 K. Then the sample was annealed up to 850 K for 15 min. STM topographic image
showed a clear p(1×1) structure (see Supplementary Information, Fig.S3), where dark spots correspondto the oxygen atom position 8,13,19. This oxygen monolayer coating prevents further oxidization andinterface roughening.

2.6 Ultrathin MgO / Fe(001)-p(1 × 1)O preparation (see Supplementary Information, Fig.S2 andFig.S3).
The Fe(001)-p(1×1)O surface was exposed again to the oxygen gas (60 s, 1.0×10-6 Pa) in the
preparation chamber, and Mg atoms were deposited from the evaporator (AEV-3, AVC Co., Ltd.) at
the sample temperature of 750 K (see Supplementary Information, Figs. S2 and S3). STM images
showed growth of atomically-flat ultrathin squared-shaped nanoislands with a terrace width of about
40 nm and a uniform height of about 1 nm. Fe(001)-p(1×1)O terraces were still observed between theislands.

2.7 CuPc singles molecule deposition.

Although the CuPc molecule evaporation took place in the UHV chamber at room temperature
condition, the sample was precooled at 5 K before the evaporation. CuPc was set in a crucible with aheat of 553 K. During the evaporation, the chamber pressure went up to 3.1×10-7 Pa. We spent threehours degassing and opened the shutter shortly only 3-5 seconds for deposition.

2.8 DFT calculation.
Density functional theory (DFT) calculations of CuPc molecules, either free or on MgO, were
performed using the projector-augmented wave method as implemented in the VASP package 45,46.
The structures were optimized using the PBE exchange-correlation functional together with Grimme's
DFT-D3 van der Waals corrections. The cut-off energy was set to 400 eV. We have modelled the
CuPc/MgO system with a MgO substrate of two atomic layers. The coordinates of the lower layer are
fixed to the bulk structure (with theoretical lattice constant a = 0.4208 nm) and the upper layer was
fully relaxed. For the CuPc monolayer on MgO, we use a 5 × 5 supercell with 50 atoms per MgO layer
and a 2 × 2 × 1 Monkhorst-Pack k-point mesh.
After structural optimization with the PBE functional, the electronic structure of the system wasrecalculated using the hybrid HSE06 functional47,48. The density of states plots in Fig. 4 correspond to
the HSE functional which was found to provide a good description of the electronic state of CuPc 49.
Hybrid functionals also give much better band gap values than pure DFT functionals such as PBE. As
a check, we also optimized the structure of the free CuPc molecule using HSE and found that theelectronic structure changes between the PBE-optimized and the HSE-optimized molecule are
negligible.

3. Results and Discussion
3.1 Growth of large atomically-flat MgO islands.
First, a Fe(001)-whisker single crystal, which was grown via chemical vapor deposition12,14, wascleaned by repeated cycles of Ar+ sputtering (+0.8 kV, + 0.80 µA) at 820 K. Then, at the final cycle,
we maintained the sample temperature 820 K for 5-10 min after the sputtering, which increased thesize of the Fe(001) terraces and cooled down to ~300 K for 15 min. Subsequently, the sample wastransferred without breaking UHV to the STM cooling shield (~100 K) and waited about 3 hours.
Finally, it was set into STM operated at 5 K (Methods and Supplementary Information, Fig.
S1)12,13,16,50–52. Figure 1a shows an STM topographic image obtained on the Fe(001) surface, where
several atomically-flat terraces are observed. The STS dI/dV curve measured on this surface indicatesa 3d surface state peak at +0.2 eV12,15,16, identifying the bcc-Fe(001).
It is known that Mg has a very low surface energy of 790 mJ/m2 compared to the surface energy of Fe:
2550 mJ/m2. 53 Therefore, when Mg films are grown directly on Fe(001) they follow the layer-by-layer
FM mode as shown in Supplementary Information, Fig. S2. The surface energy of MgO (~1500
mJ/m2)54 is also much lower than that of Fe and so a layer-by-layer growth mode was expected for
MgO, too. However, the direct growth of MgO on Fe(001) produced a rough surface. Figure 1b shows
an STM topographic image obtained on the Fe(001) surface after Mg deposition in the oxygen
atmosphere (~2×10-6 Pa). The whole surface was coated by 1-3 nm size nanoclusters, similar toprevious studies 9.
This experimental evidence means that the MgO growth on Fe(001) is determined not only bythermodynamic stability, i.e. differences in surface energy, but also by kinetic aspects. The roughsurface of MgO/Fe(001) in Fig. 1b was obtained by depositing Mg on Fe(001) in the oxygen
atmosphere. In contrast, here the substrate Fe(001) was first exposed to oxygen at 300 K and then Mg
was deposited in oxygen atmosphere. In previous experiments, we found that all Fe(001) terraces werecovered by disordered oxide patches after the oxidization at 300 K, producing a rough surface13. If asimilar oxidization process occurred during the MgO growth on Fe(001), because of the interface
roughening, plenty of atomic defects were generated. These could trap the thermally diffused MgO,

resulting in and many nucleation sites, which leads to the growth of many independent MgOnanoislands of small size.
Therefore, if one first pre-coats the Fe(001) surface by an ordered oxygen monolayer with preserving
an atomically-flat interface (less atomic defects), adsorbed MgO can thermally diffuse on the surface,which leads to a larger island. A ball-and-stick model is shown in Fig. 1c, where the adsorbed Mg andO atoms have thermally diffused on the surface. Once they are trapped, nucleation can occur, andsubsequently, island growth starts. Previous studies reported an atomic layer distance of 0.143 nm inbulk bcc-Fe(001), while the distance at the interface changes to 0.17 nm and 0.21 nm between Fe and
FeO, and FeO and MgO, respectively 55.
In order to use the oxygen passivation effect as depicted in the model in Fig. 1c, the Fe(001) surface
was covered by one monolayer oxygen film by introducing oxygen gas of about 2 Langmuir (60 sec,
4.0×10-6 Pa) while maintaining the substrate temperature at 300 K. Then the sample was annealed up
to 850 K for 15 min. In our previous study of temperature control of Fe-oxide growth on Fe(001)13,
only the Fe(001) oxidization at a substrate temperature of 300 K with a subsequent post-annealing to850 K for 15 min produced the p(1×1) ordered oxygen coating film on Fe(001). Other conditions
generated Fe3O4 (magnetite) nanoislands. Figure 1c shows an STM topographic image obtained after
the oxidization of the Fe(001) surface. Several atomically-flat terraces were observed, while here all
surface was covered by oxygen atoms by following the bcc(001) p(1×1) structure (dark spots in theinset in Fig. 1c). This oxygen coating changed the electronic structure of the surface as shown in the
dI/dV curve in the inset in Fig. 1c, where the Fe(001) 3d surface state peak at +0.2 eV is quenched anda broad shoulder peak around +1 eV could be observed, in agreement with the previously reported
Fe(001)- p(1×1)O structure. 13,19
Then, we deposited MgO. The Fe(001)-p(1×1)O surface was exposed again to the oxygen gas (60 s,
1.0×10-6 Pa) in the preparation chamber. The magnesium (Mg) rod was evaporated in UHV (seeSupplementary Information, Fig. S2), and the Mg atoms were oxidized in the UHV chamber byintroducing the oxygen gas (2×10-6 Pa) and MgO atoms were adsorbed on the O/Fe(001) surface at the
sample temperature of 750 K.

Figure 1d-h shows STM/STS results obtained on the Fe(001)-p(1×1)O surface after the Mg depositionat 750 K in oxygen gas dozing condition (2×10-6 Pa). The island growth was controlled by the Mg
deposition time (t). Figure 1d-f shows STM topographic images obtained after t = 5 s, 60 s, and 120 s,
respectively, at a sample bias of >3 V, well above the band gap. Height profiles along the arrows in
Fig. 1d-f are shown in Fig. 1g, which clearly indicates that not only the size but also the height of the
MgO islands increased.
At t = 5 s (Fig. 1d), all MgO islands are located along descending steps. This indicates that the diffused
Mg and O atoms were first trapped at the descending steps and the island nucleation occurred. Theseislands had a non-uniform shape as depicted in lower panels in Fig. 1d, while the bcc(001) atomic
lattices were locally observed. Another aspect is that the density of the growing islands was quite low:
18 / (100×100 nm2) = 0.0018 nm-2, indicating a low trapping rate of the diffused MgO atoms on
O/Fe(001) compared to Fe(001). In Fig. 1g, the black line denotes the height profile along the arrow
in Fig. 1d, which shows a corrugation on the island of ±50 pm. Such a small corrugation, much lessthan the atomic step height of 143 pm, could be caused by a nonuniform atomic arrangement inside
the island.
The islands started to have a uniform regular shape from t = 60 s, namely a square shape following thebcc(001) square lattice as seen in Fig. 1e. The height profile along the arrow shows an atomically-flat
surface on the MgO island (red line in Fig. 1g). A wide range STM topographic image shown in thelower panel in Fig. 1e indicates that the MgO island intensively grew at the descending steps. Therefore,
the island density at the step area (σstep = 10 / (200×100 nm2) = 0.0005 nm-2) is about 10 times higher
than the density at the terrace area (σterrace = 1 / (200×100 nm2) = 0.00005 nm-2). The size and the heightof the islands are about 20×20 nm2 and about 0.4 nm, respectively.
Figure 1f shows the oxygen pre-coated Fe(001) surface after the Mg deposition for t = 120 s. Comparedto the surface at t = 60 s in Fig. 1e, the size of the island becomes about four times larger, about 40×40
nm2. A line profile across one of the islands shows a height of about 0.8±0.1 nm (blue line in Fig. 1h).
Although the size becomes larger, the island density remains the same: σstep = 20 / (200×200 nm2) =0.0005 nm-2, indicating an increase in island size.

The STM images in Fig. 1d-f also show the robustness and energetical stability of the Fe(001)-p(1×1)O
surface since it is clear that there is no damage on the Fe(001)-p(1×1)O surface/interface after the MgOgrowth.
STM results of the MgO growth on Fe(001) (Fig. 1b) and Fe(001)-p(1×1)O (Fig. 1f) showed thatoxygen passivation effect maintains the atomically-flat interface during the MgO growth at 750 K.
Such a high temperature is required to suppress the Fe3O4 nanocluster generation in an oxygen
atmosphere at lower temperatures13. There is a good contrast to the case of the insulating NaCl films
directly grown on Fe(001) and O/Fe(001)56. The former and later follow the Stranski-Krastanov (SK)and FM mode, respectively, but the VW mode was never observed.

3.2 Electronic structures of insulating MgO islands.
The electronic structures of each pixel position on the surface were immediately measured by the STS
method (see Methods). Namely, the STM tip was fixed on each pixel and measured the sample local
density of states (LDOS) in the vicinity of the Fermi energy. The dI/dV curve is proportional to the
LDOS: zero, positive, and negative bias sample voltages correspond to the Fermi energy, unoccupied,and occupied LDOS, respectively. Figure 1h shows dI/dV curves obtained on the oxygen-precoated
Fe(001) surface (blue line) and the MgO island (red line). While the O/Fe(001) has LDOS peaks near
the Fermi energy, the MgO has negligible LDOS (dI/dV ~ 0.0 nA/V) near the Fermi energy, indicating
an insulating bandgap. A careful analysis of the electronic structures of these MgO islands is shown in
Fig. 2.
We performed dI/dV curve measurements at each pixel position in the topographic image of Fig. 2a,
where six MgO islands are observed. A simultaneously obtained dI/dV map at +2.45 V in Fig. 2bclearly shows a darker contrast for the MgO islands compared to the O/Fe(001) substrate. Since thebrighter-darker contrast in dI/dV maps denotes higher and lower LDOS, respectively, it indicates thatMgO could have a wide bandgap. Bulk MgO is an insulator with a large bandgap of 7.8 eV and adielectric constant of 9.87–9,57,58. All MgO nanoislands in the dI/dV map clearly show identical dark
contrast, indicating that they also have a large gap. dI/dV curves corresponding to the four MgOnanoislands in Fig. 2a are shown in Fig. 2d, such that cyan, blue, red, and green colors denote eachisland. All nanoislands have a bandgap of about 2.9 eV between -2.0 eV and +0.9 eV. This gap value
is comparable to the previously reported gap width of 2.5 eV at two monolayers and 3.6 eV at six
monolayers of MgO films on Fe(001) 7. The dI/dV curve obtained on the substrate terrace is shown as
a dashed line in Fig. 2d; it shows no bandgap.
Because of the large difference in the differential conductivity between the MgO islands and thesubstrate O/Fe(001), the apparent height (dz) of the MgO islands from the substrate changes at different
bias voltages. Fig. 2e shows that the MgO islands have a height of 0.18±0.02 nm within the gap. The
height of the MgO nanoisland started to increase from 0.9 V, which is just above the bandgap, as dz =0.30±0.02 nm at +1 V, 0.62±0.02 nm at +2 V, and 1.00±0.04 nm at +4 V.
Although the apparent height of the MgO islands varies as a function of the sample bias voltage, theheight obtained at +4 V, which is dz ~ 1 nm, could represent the real height of the MgO islands as
dI/dV values of the MgO islands and the substrate are comparable above +3 V (Fig. 2d). Also, we
could estimate the thickness of the MgO island using previous reported layer distances. The distance
between the first MgO layer and the Fe(001)-p(1×1)O substrate is 0.253 nm and the distance between
the subsequent MgO layers is 0.239 nm 55,59. dMgO can be estimated from the four MgO monolayer(ML) stacking: dMgO = 0.253 + 0.239*3 = 0.97 nm ~ 1 nm. Additionally, we deposited about 1.4 MLsof Mg on the substrate to grow the MgO nanoislands (Supplementary Information, Figs. S2 and S3).
However, the MgO nanoislands cover only about 30% of the surface. This means that the nanoisland
could have a height of 1.4/0.3 ~ 4.6 ML.
It should be noted that five MgO islands in Fig. 2a were grown along the descending steps, but only
one island (left-lower side) was grown across the step, which generated an irregularly shaped island.
This irregularly shaped island appeared much brighter in the dI/dV map at +3.36 V in Fig. 2c, indicating
different LDOS from other regularly shaped islands. Figures 2f shows a comparison between dI/dV
curves of regularly (black dots) and irregularly (red dots) shaped islands. Since the irregularly shaped
islands have a LDOS peak at +3.2 eV above the Fermi energy, this could be a cause of the brightappearance in the dI/dV map in Fig. 2c. A height profile along the arrow in Fig. 2a is shown in theinset in Fig. 2f. Because of the higher LDOS, the apparent height of the irregularly shaped island isobserved as an about 0.1 nm higher island (the inset in Fig. 2f). Furthermore, magnified spectra showed
an additional shoulder and about 0.7 eV shift of the valence band are observed near the Fermi energy

(right panel in Fig. 2f), while both islands have no LDOS near the Fermi energy. They are observed as
darker regions in the dI/dV map at 16 mV (the inset in the right panel).

3.3 Atomic defects inside MgO islands.
The dI/dV map at +3.36 eV in Fig. 2c reveals another aspect of the regular-shaped islands. Inside the
MgO islands, some dark spots exist, which indicates a different LDOS from the surroundings. The
electronic structure inside the MgO island was carefully investigated by measuring simultaneously
topographic image and dI/dV curves at each pixel position (STS grid mode), which provides 200 dI/dV
maps between ±4 V. In Fig. 3a, dI/dV maps obtained at -4.0, -1.6, +3.5, and +4.0 V are shown. With
careful checks of these dI/dV maps, we found two different types of spots, which are marked by red(defect No.1) and blue (defect No.2) arrows in Fig. 3a. In the dI/dV maps at negative bias voltages (4.0 V and -1.6 eV), brighter spots (red arrows, defect No.1) appeared in the dark MgO island. However,in the positive bias voltages, other dark spots (blue arrows, defect No.2) appeared. dI/dV curves
obtained at defects No.1 and No.2 are shown in Fig. 3b. It is clear that LDOS at defect No.2 and MgO
are very similar, but the LDOS at No.1 defect is clearly different, which has almost no bandgap but
the LDOS decay exponentially to the Fermi energy. Series of dI/dV line profiles across the island
clearly showed that the bright spot was visible at ±2 V (Supplementary, Fig. S4).
These indicate that defect No.1 could be an atomic defect, such as an atomic vacancy or misalignmentof the local crystalline structure inside the island that significantly changes the electronic structure,causing in-gap states. However, defect No.2 seems to be the physisorbed impurity on the surface
because of the tiny difference in the electronic structures. Therefore, such physisorbed impurities(defect No.2) could diffuse or desorb at room temperature. At the measurement temperature of 5 K,
however, such processes are negligible and the impurities can clearly be observed in the STM image
as dark spots.
Thus, atomic defects that significantly modify the electronic structure of MgO islands are rare, only
about three in one MgO island, meaning that the defect concentration is much lower than 1%, thus
providing uniform LDOS.

3.4 Electronic decoupling of adsorbed molecules from the metal substrate by MgO island

Here we show that when organic molecules are adsorbed on the MgO islands, the adsorbed molecules
are well decoupled from the metal substrate such that their electronic structure is close to that of thegas phase free molecules. Thus, the large MgO islands obtained here by O-precoating of Fe(001)provide an efficient electronic decoupling layer which allows visualizing the molecular orbitals using
STS.
Figure 4 shows Cu-phthalocyanine (CuPc) molecules deposited on the MgO nanoislands and theO/Fe(001) substrate terraces. Figure 4a shows a wide-range STM topographic image, where three MgOislands surrounded by the dotted curves were observed. It should be noted that nodal planes of theindividual CuPc single molecules were clearly observed only on the MgO islands, which is actuallyclear evidence that the MgO islands grown on O/Fe(001) has an ability to decouple the electronicinteraction from the metallic substrate. In contrast, the CuPc molecules adsorbed on the O/Fe(001)terrace were hardly identified as individual molecular shapes due to the direct electronic interaction
with the metallic substrate.
Figure 4b shows a dI/dV spectrum obtained on the single CuPc molecule on MgO. The molecular nodal
planes are clearly visible in the dI/dV maps around the HOMO (-1 eV) and LUMO (+2 eV) position.
Between HOMO and LUMO peaks, dI/dV spectrum shows no differential conductivity (dI/dV ~ 0
pA/V), indicating a gap and a decoupling from the metallic Fe(001) substrate. These STM observations
are in good agreement with STM results obtained on phthalocyanines adsorbed on insulating NaClfilms27,28,33,34.
In the present CuPc/MgO/O/Fe(001) system, the HOMO peak is located around -1 eV, while the
LUMO peak is hidden in the conduction band of the substrate MgO/O/Fe. Note that in the case of
CuPc/NaCl/Ag(111), HOMO and LUMO peaks were found at -2.4 eV and +0.9 V60, and for
CuPc/NaCl/Cu(111) at -2.2 eV and +1 V 61, respectively. The about 1 eV higher HOMO energy of
CuPc/MgO/O/Fe(001) indicates that the charge transfer mechanism is different from the
CuPc/NaCl/noble metal systems.
We have performed DFT calculations for the free CuPc molecule and for a CuPc monolayer on MgO.
The PBE exchange-correlation functional was used for structural optimization and the HSE hybrid
functional for the electronic state calculation, see the Methods section for further computational details.

We tested two adsorption sites, one with the Cu atom over Mg and one with Cu over O, which is shownin Fig. 4. This structure is more stable by 0.145 eV per molecule. The CuPc molecule is 0.30 ± 0.01
nm above the MgO substrate. The molecular is a little bent and the Cu atom relaxes slightly out of themolecular plane towards the substrate by 0.01 nm (with respect to the N atoms). Such a relaxation may
be expected from the attractive Coulomb interaction between the Cu2+ ion and nearest neighbour O2ion on the substrate.
The calculated density of states (DOS) of free CuPc and CuPc/MgO are shown in Figs 4c and 4d,
respectively. As the Fe substrate was not taken into account in the calculation, the exact position of
the Fermi-level between HOMO and LUMO could not be determined theoretically. Therefore, we haveshifted the DOS rigidly such as to align the HOMO peak with the experimental one. The HSEfunctional gives a HOMO-LUMO gap of 1.8 eV in reasonable agreement with experiment.
Importantly, it is seen that the HOMO-LUMO gap does not change upon adsorption on MgO. This
result fully confirms the experimental observation that the electronic structure of the adsorbed CuPc
is close to that of the free molecule, i.e. the molecule can be electronically decoupled from the metal
substrate by insertion of a MgO film.

4. Conclusions
In summary, we have demonstrated that regularly-shaped and electronically uniform insulating MgOislands can be grown on a Fe(001) whisker substrate by inserting the interface p(1×1) oxygenmonolayer film, which drastically increased the MgO island size by a factor of 10, compared to MgOislands grown directly on Fe(001). This interface engineering restricts the trapping probability of theadsorbed Mg and O atoms by Fe(001) and thus enhances adatom diffusion. The MgO island nucleatednear the descending steps of the substrate. After nucleation, more Mg and O atoms accumulated at thenucleation site and the size of the islands increased until they reached other ascending steps. The
regularly-shaped islands have identical LDOS with a gap of 2.9 eV and contain only a few atomic
defects. However, when the MgO island grew after crossing the substrate step, irregularly-shaped
islands were formed with different LDOS. Finally, it was demonstrated that the atomically-flat MgOisland surface breaks the electronic coupling between adsorbed CuPc molecules and the metal substrate,
such that the CuPc molecular orbitals could be clearly visualized.

CRediT authorship contribution statement
Nana Nazriq: Conceptualization, Methodology, Validation, Analysis, Investigation, Writing –original draft. Peter Krüger: Validation, Analysis, Calculation, Validation. Toyo Kazu Yamada:Conceptualization, Methodology, Validation, Analysis, Investigation, Writing – review & editing,
Supervision, Project administration, Funding acquisition.

Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationshipsthat could have appeared to influence the work reported in this paper.

Acknowledgments

This work was supported by JSPS KAKENHI Grant Numbers 17K19023 and the Murata Science
Foundation. We thank Mr. Yuji Nonaka, Mr. Sakaguchi, and Ms. Kumi Kobayashi for their assistanceat the early stage of the experiments. We acknowledge Dr. Lukas Gerhard (Institute for Quantum
Materials and Technologies, Karlsruhe Institute of Technology, Germany) and Dr. Eiichi Inami(School of Systems Engineering, Kochi University of Technology, Japan) for the fruitful discussion.

Appendix A. Supplementary material
The Supporting Information is available.
Figures S1-S5 give additional experimental information.
Figure S1: Cleaning process of the Fe(001) whisker single crystal surface.
Figure S2: Magnesium evaporation control using quartz crystal microbalance and flux monitor, andSTM study of 1.4 ML Mg / O / Fe(001).
Figure S3: STM/STS study of 1.4 ML Mg / O / Fe(001).
Figure S4: Line profiles of the STS dI/dV maps at various bias voltages across a defect in the regularlyshaped MgO island on O/Fe(001).
Figure S5. DFT calculated DOS results of free-standing CuPc and CuPc adsorbed on MgO surface.

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Figures / Figure captions
Figure 1ab

Fe(001)c
MgO/Fe(001)

Fe(001)-p(1×1)O
STM tip

0.3 nmMg
atom

dI/dV [nA/V]

40 nm

dI/dV [pA/V]

Oxygen10-6 Pa
0.2 eV SS

10 nm

40 nm

Sample voltage [V]MgO
islandMgO
diffusion

Oxygenmonolayer

Sample voltage [V]Fe(001)-whisker

MgO / Fe(001)-p(1×1)O
t=5s

e

f

t = 60 sg

t = 120 s
O/Fe(001)

O/Fe(001)
MgO
20 nm

Height [nm]

d

MgO
10 nm
20 nm

17 nm
O/Fe(001)

h

MgO

10 nm

Terrace

MgO
MgO

0.5 nm40nm
Steps

σterrace = 0.00005 [nm-2]
σstep = 0.0005 [nm-2]

5s
60 s
120 s

20

0

0

20

40 nm

σ = 0.0005 [nm-2]

60
40
x 40
[nm] 60

80

80

0.5
0.4
0.3

MgO
island0.2
0.1

O/Fe(001)
40 nm

dI/dV [nA/V]

σ ~ 0.002 [nm-2]

1.2
1.2
1.0
0.8
0.8
0.6
0.4
0.4
0.2
0.0
0.0

O/Fe(001)

0.0
-3

-2

-1 0
1
2
Sample voltage [V]3

Figure 1. UHV-STM imaging at 5 K of MgO islands grown on Fe(001)-p(1×1)O substrate. (a-c) STM
topographic images obtained on (a) the cleaned Fe(001) surface (200×200 nm2, Vs = -1 V, I = 200 pA), (b)MgO nanoislands on Fe(001) (50×50 nm2, Vs = 3.5 V, I = 50 pA), and (c) the oxygen-precoated Fe(001)p(1×1)O surface (200×200 nm2, Vs = -0.5 V, I = 200 pA). Inset in (a) shows the dI/dV spectrum obtained on
the Fe(001) surface with a peak at 0.2 eV arising from the surface state present on the clean Fe(001) surface.
Insets in (c) show an atomically-resolved image and the dI/dV curve of the Fe(001)-p(1×1)O lattice. Right
panle in (c) denotes a side view of the spherical model. White, red, and grey spheres denote W, Mg, and Fe
atoms, respectively. Black dots denote oxygen atoms. (d-h) STM/STS results obtained on the O/Fe(001)surface after Mg deposition for (d) 5 s, (e) 60 s, and (f) 120 s in oxygen atmosphere (10-6 Pa). (d) STM
topographic images from top to down: 100×100 nm2, 50×25 nm2, 2.9×1.5 nm2 (Vs = +3.0 V, I = 810 pA).
(e) STM topographic images: higher panel 50×30 nm2 and lower panel 200×200 nm2 (Vs = +3.5 V, I = 950
pA). (f) STM topographic images: higher panel 80×50 nm2 and lower panel 200×200 nm2 (Vs = +3 V, I =50 pA). (g) Height profiles along the black, red, and blue arrows in (d), (e), and (f), respectively. (h) dI/dV
curves obtained on the Fe(001)-p(1×1)O substrate (blue line) and the MgO island (black line).

Figure 2006-007-008.3ds

d
MgO

MgO
MgO

e

10
0.1

-1

1.2

0.01

MgO

dI/dV [nA/V]

MgO

0.001

MgO

10-3

islands

10-40.0001MgO

20 nm

b dI/dV map at +2.45 V

MgO

-3

0.4

0.30 nm02-1
1-1 0 1 2
Sample voltage [V]

-2

3

-0.2
0

3

5

0.4

1

15

20

0

0.7 eV shift

0.003 308040x [nm]

dI/dV at +16 mV

dI/dV [pA/V]

Height [nm]dI/dV [nA/V]

0.5

0.002 2

0.3

Irregularly shaped island

0.001 1

Regularly shaped island

0.1
0.0

10
x [nm]

0.004 40.2

Irregularly shaped
island

0.6

f

MgO

0.5 V
0.4 V
0.3 V
0.2 V
0.1 V

0.62 nm0.0

0.6

c dI/dV map at +3.36 V

0.8

0.18 nm~2.9 eV

-3 -2

MgO

MgO

1.00 nm-510-60.000001

4V
2V
1V

0.2

10
0.00001
MgO

MgO

island1.0

terrace

10-2

Height [nm]a STM topographic image

-3

-2

-3 -2

-1-1 00 11 22
Sample voltage [V]330.000 0

-3

-1
-3 -2
-2 -1

00

11

22

Sample voltage [V]33

Figure 2. Electronic structure of MgO islands grown on Fe(001)-p(1×1)O substrate. (a) STM topographic
image: 100×100 nm2, Vs = +4.0 V, I = 1 nA, and (b, c) simultaneously obtained dI/dV maps at (b) +2.45 V
and (c) +3.36 V. (d) dI/dV curves obtained on the four MgO islands marked by cyan, blue, red and green
circles in (a), indicating a bandgap of about 2.9 eV (from -2 V to +0.9 V). The dI/dV curve obtained on thesubstrate is shown as a dashed line. (e) Apparent height (dz) of the observed MgO islands at different sample
voltages. (f) dI/dV curves obtained on the regularly (black) and the irregularly (red) shaped MgOnanoislands. The right panel denotes the magnified spectra. The inset shows dI/dV map within the gap at+16 mV.

Figure 3

Atomic defects in MgOadI/dV map at -4.0 V

Topographic image

dI/dV map at +3.5 V

9.0nm
9 nm

9.0nm
9 nm

9 nm

b

dI/dV map at -1.6 V

1.4

9.0nm
9 nm

dI/dV map at +4.0 V

9.0nm
9 nm

0.05
1

1.2

0.04
0.1

dI/dV [nA/V]

1

defect 1 defect 2
0.8

0.03

0.01

0.6

0.02

MgO

defect 1

0.001

0.4

2

0.2

3

4

0.01

defect 1

0

0

MgO
defect 2

defect 2
-0.2

-0.01
-4

-3

-2

-1

0

1

Sample voltage [V]

2

3

4

-4

-3

-2

-1

0

1

2

3

4

Sample voltage [V]Figure 3. Atomic defects in the regularly shaped MgO island grown on Fe(001)-p(1×1)O substrate. (a) STM
topographic image: 45×56 nm2, Vs = +4.0 V, I = 1 nA, and simultaneously obtained dI/dV maps at -4.0 V, 1.6 V, +3.5 V, and +4.0 eV. Red and blue arrows denote two kinds of defects: defect 1 and 2, respectively.
(b) dI/dV curves obtained at the MgO surface (black dots), defect 1 (red dots), and defect 2 (blue dots)
positions. The right panel denotes magnified spectra.

Figure 4a
MgO islandb7

STS on CuPc /MgO/O/Fe(001)dI/dV map at +2.1 V

dI/dV map at -1.1 V

6

dI/dV [pA/V]

5
4
3

6 nm

2

6 nm

LUMO

HOMO

1
0
−1

O/Fe(001)

−1.5

−1.0

−0.5

0.0

0.5

1.0

1.5

E - EF [eV]c

DFT free-standing CuPc DOS
DOS(spin-up)
DOS(spin-down)
30

LUMO

DOS

20

HOMO
10

MgO island

0
−1.5

−1.0

−0.5

0.0

0.5

1.0

1.5

E - EF [eV]
Side
view

d

DOS(spin-up)
DOS(spin-down)
80

Top
view

MgO

DFT CuPc / MgO DOS

CuPc
DOS

60

40

LUMO
20

HOMO

0
−1.5

−1.0

−0.5

0.0

0.5

1.0

1.5

E - EF [eV]

Figure 4. UHV-STM imaging at 5 K of Cu-phthalocyanine (CuPc) molecules adsorption on MgO islands
grown on Fe(001)-p(1×1)O substrate. (a) STM topographic image: 100×100 nm2, Vs = -1.0 V, I = 100 pA.
The inset shows the magnified image of CuPc at -2 V. (b) Upper panel: dI/dV curve obtained on single CuPc
molecules. dI/dV values of about 0 pA/V means a gap, indicating an electronic decoupling from the metallic
Fe(001) substrate. Insets denote dI/dV maps around -1 V and +2 V. Lower panel: DFT calculated density ofstates (DOS) of free-standing CuPc.

A. Namiki, N. Nakashima, K. Yoshihara, Fluorescence quenching due to the electron transfer. Indolechloromethanes in rigid ethanol glass, J. Chem. Phys. 71(2) (1979) 925–930. https://doi.org/10.1063/1.438383.