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ArticleDirect Imaging of Precursor Adcomplex States during CryogenicTemperature On-Surface Metalation: Scanning TunnelingMicroscopy Study on Porphyrin Array with Fe Adsorption at 78.5 K
Eiichi Inami,* Masataka Yamaguchi, Ryohei Nemoto, Hideki Yorimitsu, Peter Krüger,
and Toyo Kazu Yamada*

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sı Supporting Information*

ABSTRACT: On-surface metalation of metal-free π-conjugatedplanar molecules, such as metal-free tetraphenylporphyrin(2HTPP), using 3d transition metals prepared in ultrahigh vacuum(UHV), attracted signiﬁcant attention as a newly developedbottom-up method to create a building block for 1-nm-size nanodevices, one- or two-dimensional nano networks, as a precursor offurther on-surface synthesis or a magnetic atom array aligned on awell-ordered molecular template with an extreme low impurity
concentration. Experimental and theoretical studies for this type ofon-surface metalation via dehydrogenation (e.g., 2HTPP + Fe →H2 + FeTPP) have been successfully demonstrated by depositing 3d
metal atoms on metal-free π-conjugated planar molecular array at
300 K with subsequent annealing of 350−700 K in UHV.
Signiﬁcantly, photoemission spectroscopy study for the on-surface metalation process at cryogenic temperatures suggested a
precursor adcomplex state diﬀerent from the normal metalation state. In this study, we demonstrate that on-surface metalation at
cryogenic temperature reduces thermal diﬀusion of both metal atoms and molecules, which allows gentle adsorption of the 3d metal
atoms inside the molecule, providing precursor adcomplex states while preserving the original well-ordering molecular arraystructure. Using scanning tunneling microscopy (STM) we investigated Fe deposition on a well-ordered metal-freetetraphenylporphyrin (2HTPP) monolayer array islands with an Au(111) substrate temperature maintained at 78.5 K. Throughdirect STM topographic and spectroscopy imaging supported by density functional theory calculations, we identify three types of
TPP appearance: (1) original 2HTPP, (2) 2HTPP with an Fe atom on top (precursor adcomplex “α” state), and (3) 2HTPP withan Fe atom inside (precursor adcomplex “γ” state, where a clear LUMO peak change was observed).
vast range of functionalities such as chemical sensor,6−11
catalysts,12−15 solar cells,16−18 and information storage
devices.19−22
Conventionally, metal−molecule complexes synthesized via
ex-situ methods including chemical reactions in solvents23−25
and physical vapor deposition26−29 are drop-casted on various
substrates and form a well-ordered self-assembly molecularmonolayer ﬁlm in air at room temperature. Alternatively, onsurface metalation is a newly developed approach where guest
metal atoms are deposited on the preadsorbed molecules on
atomically ﬂat and cleaned noble metal substrates, which
realize an extremely low impurity concentration because all insitu growth and metalation processes are performed in

1. INTRODUCTION
On-surface metalation of metal-free π-conjugated planarmolecules, such as porphyrins (TPP) or phthalocyanines(Pc), with 3d transition metal adsorption has attracted
research attention as an excellent method to create wellordered magnetic molecular nanostructures where planarmolecules serve as hosts for metal atoms (see ref 1 andreferences therein).
Obtained metal−molecule complexes via on-surface metalation play an important role in extreme miniaturization offunctional units and their atomically precise bottom-up
fabrication process toward future high-functional devices. For
this task, increasing attention focuses on porphyrin [see Figure1] and similar planar π-conjugated molecules as promisingbuilding blocks to self-assemble well-ordered one- or twodimensional templates,2,3 which allow us to design supramolecular architectures.4,5 The synthesized metal−molecule
complexes exhibit tremendous diversity in physical/chemical
properties based on the guest metal species and thus provide a© 2020 American Chemical Society

Received: October 18, 2019
Revised: January 21, 2020
Published: January 23, 2020

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ArticleFigure 1. (A) Schematic illustration of typical reaction routes for metalation of metal-free tetraphenylporphyrin (2HTPP) molecules with anadsorption of guest metal atoms or substrate metal atoms. (B) STM topography images obtained on a clean Au(111) surface at 78.5 K. Upper
panel: 164 × 164 nm2, Vs = −300 mV, and It = 50 pA. Right-lower panel: Au(111) surface reconstruction pattern, 40 × 40 nm2, Vs = −1400 mV,
and It = 500 pA. Left-lower panel: Au atoms, 1.5 × 1.5 nm2, Vs = −70 mV, and It = 100 pA.

they form a 1.4 nm × 1.4 nm square lattice, meaning only 51
molecules are enough to cover the 10 × 10 nm2 area. If one
2HTPP captures one 3d metal atom during the metalation,
only 0.037 (=51/1387) monolayers (ML) of 3d metal
deposition is enough to change all 2HTPP to 3d-TPP.
However, when more than 0.037 ML of 3d metal was
deposited at 300 K, the excess metal atoms play extra roles34
since they thermally diﬀuse along the surface. For example, Fe
or Co atoms on noble metal surfaces are known to diﬀuse toform triangular islands at room temperature.20,48,49 Also,
molecular diﬀusion should be considered in the case ofmolecular nanoisland. For example, a previous STM study
indicated that the metal-free tetraphenylporphyrin (2HTPP)
monolayer islands on Au(111) surface vary over time at theedge parts owing to thermal diﬀusion of individual molecules
at room temperature.3 These types of high-mobility metal
atoms and molecules should react with each other via in-plane
lateral interaction [see route no. 3 in Figure 1A],42 and thus
they can form a variety of metal−molecules complexes.
Importantly, this type of metalation can destroy well-orderedmolecular structures due to multiple routes of metalation
process. For example, an STM study of the metalation of mesotetramesitylporphyrin island on Cu(100) with 0.1−0.3
monolayers (ML) of Fe adsorption at room temperature,
where hundreds of Fe atoms were deposited on about 50
molecules, demonstrated how the generation of Fe islands andFe−porphyrin complex clusters causes disorder of the ﬁlm
structure.34 Also, a 0.1 ML Fe deposition on a well-ordered
metal-free phthalocyanine (H2Pc) monolayer ﬁlm at 300 K in
UHV destroyed the original molecular lattice and produced
Fe−molecule nanoclusters (see Supporting Information,
Figure S5).
On-surface metalation processes (route nos. 1−3) at 300 K
with suﬃcient annealing (350−700 K) produce a change the
2HTPP to 3d-TPP. However, another type of on-surface

ultrahigh vacuum (UHV). Furthermore, even metal organic
molecules that are highly reactive at ambient pressure, e.g.,
FePc,30−33 can be used in UHV as building blocks to create aclean molecular monolayer ﬁlm.
On-surface metalation also stimulated fundamental researchof chemical reactions. Speciﬁcally, scanning tunneling microscopy (STM) is used to reduce the problem of complicatedinteractions between huge amounts of molecules and metal
atoms to single-molecule level. Hence, in situ metalations on
several metal surfaces are examined via extensive STM studies,such as direct imaging,34−40 spectroscopy,2,41 and manipulation,33,42,43 which unveiled reaction routes between single
molecules and metal atoms and characterized a variety ofmetal−molecule complexes.
In the aforementioned studies (see, e.g., ref 1 and references
therein), the metalations proceeded on a surface maintained atroom temperature or higher and typically required substratetemperatures of 300−700 K. With respect to the well-orderedmolecular monolayer ﬁlm, two possible metalation processes
are reported (see Figure 1A). The one is direct contact of 3d
metal atoms on the preadsorbed target molecule [route no. 1
in Figure 1A]34,38,39,44−46 where the metal atoms were
evaporated from the evaporator located in UHV chamber.
The other is the interaction of molecule with the substrate 3d
metal atom (see route no. 2 in Figure 1A).39,47 For both routeno. 1 and route no. 2, X-ray photoelectron spectroscopy (XPS)
clearly showed that additional annealing at 350−700 K
stimulates a chemical reaction; e.g., 2HTPP + Fe → FeTPP+ H2 (gas).1
Previously, it was demonstrated that metalation can beachieved with precise control of the amount of deposited 3d
metal by using well-ordered arrays of planar molecules.1,44−46
For example, fcc-Au(111) is composed of 1387 Au atoms in a
10 × 10 nm2 area. When metal-free tetraphenylporphyrin(2HTPP) molecules are adsorbed on the Au(111) surface,3622

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ArticleFigure 2. 2HTPP monolayer islands and single molecules grown on Au(111) at 300 K. STM images are obtained at 78.5 K in UHV. Deposition
amounts of 2HTPP are (A) > 1 ML and (B and C) 0.3 ML. (A) 39 × 39 nm2, Vs = +1000 mV, and It = 10 pA, (B) 130 × 130 nm2, Vs = −1400
mV, and It = 10 pA. (C) 46 × 46 nm2, Vs = −1400 mV, and It = 10 pA. In part C, three types of islands are shown, which are rotational symmetriesof each other. (D) Height proﬁle along the white arrow in part C. (E) Left panels: STM image of single 2HTPP molecule (top) and thecorresponding molecular model (bottom). Right panel: height proﬁle along the white arrow in the left panel.

Fe atom inside (precursor adcomplex “γ” state; a clear LUMO
peak change was observed).
Further, we have succeeded in maintaining the original wellordered molecular structures even when depositing more Fe
atoms than the numbers of preadsorbed 2HTPP owing tocryogenic temperature where in-plane mobilities of both
adsorbed molecules and metal atoms are considerably
decreased.

metalation was reported from XPS and calculation studies of
3d metals of Fe and Co atoms deposited on the 2HTPP ﬁlms
at cryogenic temperature (100 K).44,50 It was found that themetal atom inserted inside the 2HTPP is coordinated to thetwo iminic nitrogen atoms, while the two hydrogen atoms arestill bonded to the two aminic nitrogen atoms, which is called aprecursor adcomplex state [route no. 4 in Figure 1A].
In this study, we show ﬁrst real space imaging on the surfaceof the precursor adcomplex state of the on-surface metalation
process by means of a home-built low-temperature (78.5 K)
UHV STM setup. About 100 Fe atoms were deposited on
about 50 2HTPP per 30 × 15 nm2 area (6243 Au atoms
located) at the substrate temperature maintained at 78.5 K.
Supported by density functional theory, our STM topographic
and spectroscopy images reveal three diﬀerent TPP appearances: (1) original 2HTPP, (2) 2HTPP with an Fe atom on
top (precursor adcomplex “α” state), and (3) 2HTPP with an

2. METHODS
2.1. UHV-LT-STM Setup. On-surface metalation of metalfree tetraphenylporphyrin (2HTPP) monolayer islands on
Au(111) was demonstrated with 3d-metal adsorption at room
temperature and liquid nitrogen temperature of 78.5 K in UHV
via a home-built low-temperature UHV-STM setup (basepressure <8 × 10−9 Pa). 51 Our system included an
introduction, a preparation, and an STM analytical chamber.
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Article5B and Supporting Information, Figures S3 and S4], using
density functional theory (DFT) with the PBE exchangecorrelation potential and the VASP software.56 For Fe-2HTPP
and FeTPP, DFT+U was used with U = 4 eV and J = 1 eV for
the Fe d states. Previously, these values were found to give thebest results for various Fe−porphyrin molecules.57 The energy
cutoﬀ was set to 400 eV, and the simulation box was 20 × 20 ×
14 Å. The atomic positions were fully optimized until theforces on all atoms were smaller than 0.01 eV/Å. Fe-2HTPP
and FeTPP are in an intermediate spin (S = 1) state.

They were interconnected for performing sample preparations
and their STM characterization without breaking UHV. The
STM stage in the analytical chamber was surrounded by acryostat, where two Cu shields (Au-coated) cut heart radiation.
The sample was placed on the outer cooling shield for 15 min,and then set into the STM head by opening the outer andinner shield doors shortly. The head was kept at 78.5 K byﬁlling the liquid nitrogen in the cryostat bath inner (8 L) andouter (17 L) tanks (CryoVac Co. Ltd.). Typically, we waitedfor 2−3 h to cool both tip and sample to 78.5 K. Subsequently,
the W tip was approached and scanned the sample surface.
2.2. Sample Preparations. As a metal substrate, Au(111)
was subjected to several cycles of Ar+ ion sputtering (1 kV)
and annealing at 873 K in the preparation chamber by which aclean and atomically ﬂat surface, including herringbone
reconstruction,52,53 was obtained (see Figure 1B).
The growth of the 2HTPP submonolayer ﬁlms was
performed using homemade molecular evaporator set in the
preparation chamber. 2HTPP was purchased from TCI and
was chromatographically puriﬁed on silica gel (Wako gel C200) with a mixture of dichloromethane and hexane (v/v =1:1) as an eluent. The eluent containing 2HTPP was
evaporated under a reduced pressure on a rotary evaporator.
The 2HTPP thus obtained was further puriﬁed by sublimationin vacuum (∼10−4 Pa). Recrystallized 2HTPP powders (5−10mg) were set into the crucible. Powdered 2HTPP in an Al2O3
crucible was heated to 423 K over 1 h for degassing and thendeposited on the Au(111) substrate maintained at room
temperature for 198 s (∼0.3 ML 2HTPP) in the preparationchamber. After the deposition, the substrate was moderately
annealed at 373 K for 5 min to facilitate the ﬁlm formation
with well-ordered square islands via self-assembly [same as
Figure 2].
The cobalt deposition at 300 K was performed as follows. In
the preparation chamber, ∼0.28 ML cobalt (Co) was
deposited at 300 K from the evaporator (Co rod: purity
99.95%). With respect to iron deposition at 78.5 K, iron (Ferod: purity 99.995%) was evaporated from a multisource
evaporator set in the STM analytical chamber. The outlet ofthe evaporator points toward the sample located on the STM
stage. We opened the cooling shield door only 5 s to deposit
only small amounts of Fe (approximately 0.02 ML,
corresponding to about 100 atoms per 30 × 15 nm2 area).
2.3. STM Characterization. The STM measurements
were performed at 78.5 K. Electrochemically etched W-tip (ϕ= 0.3 mm, purity 99.99%) was carefully cleaned via annealing
in the introduction chamber prior to utilization54 and thensubsequently set into the STM stage. The STM topographic
images were obtained in a constant current mode. Sample local
density of states (LDOS) was investigated via current imaging
tunneling spectroscopy (CITS) measurement. In the CITS,
topography and current−voltage characteristics (I−V) wererecorded at each pixel. Each I−V spectrum was numericallydiﬀerentiated to obtain diﬀerential conductance (dI/dV)
curves and dI/dV maps, where dI/dV is considered as
proportional to LDOS.55 Therefore, the obtained data enableda direct comparison of the topography and LDOS in the same
area. Speciﬁcally, dI/dV values at positive and negative voltage
sides denote unoccupied and occupied LDOS, respectively,
near the Fermi energy (EF).
2.4. Density Functional Theory Calculations. We havecomputed the structure and electronic states of 2HTPP, Fe2HTPP adcomplex, and FeTPP single molecules [see Figure

3. RESULTS AND DISCUSSION
3.1. 2HTPP Monolayer Islands on Au(111). Figure 2
shows STM topographic images obtained on the Au(111)atomic terraces after deposition of >1 ML [Figure 2A] and 0.3
ML [Figure 2, parts B and C] of 2HTPP molecules. Most of
the adsorbed molecules formed well-ordered rectangularshaped islands on the surface via self-assembly when the
2HTPP single molecules touch down on Au(111) at 300 K
with subsequent annealing at 373 K. Currently, we focused on
the submonolayer regime. Figure 2C shows an enlarged image(46 nm × 46 nm) where approximately 10 square monolayer
islands were placed on the (111) terrace while some molecules
remained isolated or aligned (trapped) along the step edges.
Figure 2D shows the height proﬁle across one molecular island[white arrow in Figure 2C], thereby revealing the apparent
height from the Au(111) substrate corresponding toapproximately 0.22 nm, which is identical to the single
2HTPP height shown in Figure 2E. The heights werecomparable with the monolayer height.3 Additionally, it waspossible to identify that the 2HTPP ﬁlms were rotated by 120°
relative to each other as labeled by type-I, type II, and type III
in Figure 2C. Statistical analysis of the images surveyed over
250 nm × 250 nm area conﬁrmed that given 740 2HTPP
islands in total, the ratios of type-I, type II, and type III islands
corresponded to 29%, 38%, and 33%, respectively. The result
suggested that the three types were energetically equivalent,
and their rotational diﬀerences were simply due to 3-foldsymmetry of the Au(111) surface.3
3.2. Cobalt Deposition at 300 K on 2HTPP Array. The
previously reported photoemission spectroscopy and STM
studies demonstrated successful on-surface metalation bydepositing 3d metal atoms on a π-conjugated molecularmonolayer array at 300 K with subsequent annealing at 350−
700 K.1 However, in this process, the number of deposited 3d
atoms should not exceed the number of preadsorbed
molecules. If it does, the original well-ordered molecular
structures can be disordered at 300 K. Indeed, the reported
STM study directly indicated that the adsorption of 0.1−0.3
monolayers of Fe at 300 K deconstructed well-ordered mesotetramesitylporphyrin islands on Cu(100) to generate Fe
islands and isolated Fe−porphyrin complex clusters.34 In thatcase, hundreds of Fe atoms were deposited on 51 molecules ina 10 × 10 nm2 area. We also demonstrated that well-ordered
metal-free phthalocyanine (H2Pc) monolayer ﬁlm wasdestroyed by a 0.1 ML Fe deposition at 300 K in UHV and
Fe−molecule nanoclusters were grown (see Supporting
Information, Figure S5).
To ensure the occurrence of the aforementioned type of adisordering process due to 3d-atom metalation for othersystems, we examined 0.28 ML cobalt deposition on a 0.3 ML
2HTPP array wherein the substrate was maintained at 300 K.
Figure 3A shows the resulting STM image (40 × 40 nm2), in
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ArticleFigure 3. Co deposition at 300 K on 2HTPP array in UHV. STM images are obtained at 78.5 K. (A) STM topography image: 40 × 40 nm2, Vs =−1400 mV, and It = 30 pA. Three types of single molecules induced by Co deposition were marked by white boxes in (A). Height proﬁles of thesethree types of isolated complexes are shown in the right panels. (B) Schematic illustration of two possible reaction routes of Co-2HTPP complexes.
Ordered complexes are produced via route no. 1, while the isolated clusters were grown via route no. 3. (C) Upper panel: STM image of triangularshaped intercalated Co nanoisland (20 × 13 nm2, Vs = −1400 mV, and It = 10 pA). Lower panel: height proﬁle along the arrow in the upper panel.
(D) Schematic illustration of the Co intercalation, which is the metalation process route no. 5.

of the molecule appeared as characteristic four lobes alignment.
The height proﬁles across the aforementioned three types ofmolecules along the arrows in Figure 3A exhibit a comparable
height from the substrate: I, 0.32 nm; II, 0.36 nm; and III, 0.32
nm (Figure 3A, right panels), which are ∼0.1 nm higher than
2HTPP single molecule (∼0.22 nm). The experimental results
suggest that type I, II, and III are not pure 2HTPP and insteadcorrespond to incorporated Co atom inside via the route no. 1
or route no. 3 in Figure 1 with diﬀerent appearances, which canbe attributed to diﬀerently coordinated Co-2HTPP complexes.
However, gentle adsorption via route no. 1 cannot explain thedecomposition of the well-ordered molecular array. Route no.
3 could be the cause of the decomposition. At 300 K, the TPP
molecules located at the edge of the island diﬀuse,3 which
provides an opportunity to meet with a diﬀusing 3d atom andwhich about 6200 Co atoms were deposited on about 100 TPP
molecules. Compared to Figure 2, the observations in Figure
3A were drastically changed. Only one well-ordered squareisland remained (marked by a dotted rectangle), while 90% of
the molecules are disordered or decomposed into isolated
molecules. Further, as also shown in Figure 3A, Co deposition
alters the appearance of large fraction of 2HTPP molecules,
which are classiﬁed into three types, labeled I, II, and III in
Figure 3A. The type-I molecule was characterized by fourlobed shapes corresponding to phenyl groups with a slightly
brighter spot at the center of the molecule. With respect to thetype-II molecules, four phenyl groups appear slightly darkrelative to type-I, and a brighter spot resides in the oﬀ-center
position. With respect to the type-III molecules, four phenylgroups exhibited similar intensity with type-II while the center
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ArticleFigure 4. Fe deposition at 78.5 K on 2HTPP array in UHV. STM images are obtained at 78.5 K. (A, B) STM topography images obtained at thesame area at diﬀerent times: t = 0 min 0 s and t = 9 min 21 s (30 × 30 nm2, Vs = −1400 mV, and It = 200 pA). Between two images, one of two
bright spots denoted by the box is manipulated by the STM tip. (C) Height proﬁles between two green arrows in parts A and B before (solid line)
and after (dashed line) the manipulation. (D) Scanning tunneling spectroscopy results (30 × 15 nm2, Vs = −1400 mV, and It = 500 pA). At eachpixel position, topography and dI/dV curves are simultaneously measured. Obtained STM topography and dI/dV maps at HOMO (−1.0 eV) and
LUMO (+0.9 eV) are shown. Three diﬀerent observations (α, β, and γ states) are found. (E) Line proﬁles of height (blue) and dI/dV intensity(red) between two green arrows marked in the STM topographic image and the dI/dV LUMO map in part D. (F) dI/dV curves obtained on the α[blue line], β [red line], γ state [black line], and the Au(111) substrate [dotted line].

at room temperature. One interesting point is that we
frequently observed an atomically ﬂat area at approximately
the center of the island, and TPP molecules were located on
the step edge of the island. The height proﬁle across the island
consisted of two layers (see Figure 3C). The atomically ﬂatarea exhibited a height of approximately 280 pm (marked by
the arrow), which is half of the height of bilayer Co
nanoislands on Au(111): ∼550 pm, thereby indicating theﬁrst layer of the island consisted of Co atoms. The single
molecule located on the triangular island exhibited a height ofapproximately 200 pm, which was comparable to the height of
2HTPP on Au(111): ∼220 pm.
If one 2HTPP can hold only one atom inside, then excess
Co atoms diﬀuse at 300 K on the surface until Co meetsanother Co. Once they form Co dimers, they continuously
grow and form Co monolayer triangular-shape island following
the substrate fcc-Au(111) 3-fold symmetry. The growth of thetriangular-shaped island during the metalation indicated theexistence of the route no. 5 (see Figure 3D). The DFTform a Co-2HTPP complex. Once the Co-2HTPP was
generated, the molecules stop diﬀusion; thus, the moleculararray gradually decomposed.
It should be noted that about 6200 Co atoms were
deposited on about 100 TPP molecules in Figure 3A, but we
did not see any isolated Co atoms on the Au(111) terraces,
which means that all of deposited Co atoms must be placed
inside the TPP molecules, producing a variety of Co-2HTPP
complexes with diﬀerent numbers of metal atoms orintercalating below the molecules.
The indication of the intercalation is visible in Figure 3A,
which is marked by a dotted triangle. Figure 3C shows one of
the triangular-shaped islands (dotted lines are guides for eyes
showing a 60° angle), while the triangular-shaped islands are
not observed in Figure 2. Namely, the triangular island wascaused by Co adsorption. It is well-known that the 3d metal
deposition on fcc(111) substrate at 300 K forms triangularshaped bilayer islands,20,49,58,59 which indicates high diﬀusivityof the adsorbed 3d atoms on a closed packed fcc(111) surface
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Articlearrows in Figure 4B is shown as the dotted line in Figure 4C
where the height of the no. 7 molecule evidently decreases
from 280 to 180 pm. The STM demonstrations in Figure 4A−
C indicate a weak bonding between the Fe atom and 2HTPP
located below, thereby suggesting physisorption. The brighter
spots are denoted as “α” spots.
Additionally, we observe a scratch-like noise [denoted bydotted circles in Figure 4, parts A and B] surroundings of the
2HTPP island, which was not observed on the surface inFigure 3. The scratches indicate that excess isolated Fe atoms
remain on the ﬂat bare Au terraces. At the cryogenic
temperature, Fe atoms stay on the terrace, while they can be
manipulated by the STM tip, which are known to be observed
as the scratches in the STM topography.
The local density of states (LDOS) of Fe-adsorbed TPP
islands at 78.5 K was measured using CITS method (seeMethods). Figure 4D shows simultaneously obtained a
topographic image and dI/dV maps at the highest occupied
molecular orbital (HOMO) states: −1.0 eV and the lowest
unoccupied molecular orbital (LUMO) states: + 0.9 eV, wherethe 0 eV denotes the Fermi energy, obtained from the samesquare TPP island. The full CITS and the dI/dV data are given
in Figures S1 and S2, respectively. The diﬀerential conductance: dI/dV is proportional to sample LDOS, and thus thecontrast in the dI/dV map denotes diﬀerences in LDOS ateach atomic position of the sample surface.
The dI/dV map at −1.0 eV in Figure 4D shows clearlybrighter areas, which correspond to the TPP molecule island
location, thereby indicating a presence of the HOMO states. In
the map, the “α” spot position appears no clear diﬀerence incontrast (see the dotted circles), thereby indicating no
signiﬁcant diﬀerence in LDOS at the “α” spot. However,signiﬁcant new spots [named “β”] suddenly appeared when the
dI/dV maps were checked at the LUMO state. The dI/dV map
at +0.9 eV in Figure 4D shows clear 12 brighter spots in theisland (pointed by red arrows “β”).
Figure 4E shows the height proﬁle [blue line] and dI/dV
intensity proﬁle [red line] obtained from the identical line asdenoted by two green arrows in the topographic image and the
dI/dV maps at +0.9 eV in Figure 4D. We identiﬁed themolecule positions corresponding to no. 1−10 from the heightline proﬁle. The “α” spot was conﬁrmed only at the no. 6
molecule position with the apparent height corresponding toapproximately 0.2 nm. However, along the line, three “β” spots
were observed at molecules no. 2, no. 5, and no. 9, whereapproximately 1 nA/V higher dI/dV intensity was observed,
albeit an almost negligible dI/dV value was conﬁrmed at theno. 6 molecular position (“α”). Speciﬁcally, it is not possible toidentify “β” in the topography. The experimental results clearlysuggest “α” and “β” spots can be caused by diﬀerent origins.
Namely, these STM topography images and dI/dV maps tellus that Fe deposition on 2HTPP array at 78.5 K produces
three types of TPP complexes. (1) “α” state: an Fe atom
weakly bonded on a 2HTPP (physisorption). (2) “β” state: nosigniﬁcant change in the topography, but a clear diﬀerence in
LDOS. (3) “γ” state: TPP molecules which do not belong toneither “α” nor “β”. In Figure 4D, the larger island (total 42
TPP) consists of 6 “α”, 16 “β”, and 20 “γ” and the smaller
island (total 11 TPP) consists of 2 “α”, 5 “β”, and 4 “γ”, whichindicate that the majority are “β” and “γ” states.
dI/dV curves obtained at “α” [blue line], “β” [red line], and“γ” [black line] states are shown in Figure 4F, where EF
denotes the Fermi energy. The Au(111) curve [dotted line]

calculations indicated that on the noble metal substrate, the [πconjugated molecule on 3d-atom] system was energeticallysigniﬁcantly stable (∼4 eV) when compared to the [3d-atomon π-conjugated molecule] system.60 Therefore, it is energetically suitable for the adsorbed Co atoms to place below themolecule. The Co atoms inside three Co-2HTPP complexes in
Figure 3A can be located below the 2HTPP molecules.
Additionally, the formation of the Co-induced triangular-shapeisland suggested that adsorbed Co atoms not only preferred to
be located below the molecule but also nucleated and formed
the Co monolayer island.
An observation is that the TPP molecules were located on
the island rim. Following the nucleation of 3d atoms andfurther island formation, the diﬀusing TPP molecules contact
to the Co island edge. Subsequently, TPP molecules are placedon the Co and TPP diﬀusion stops. Therefore, the TPP
molecules are located on the island rim. This also indicates alower surface energy of the molecules when compared to thatof metals: Co (2550 mJ/m2) or Au (1550 mJ/m2).61
As a result, the 300 K metalation can be categorized into two
diﬀerent regimes. First, when the number of deposited 3d
metal atoms is less than the number of preadsorbed molecules,
3d atoms are inserted into the molecules one by one andsubsequent annealing (at 350−700 K) successfully produces
3dTPP complexes while keeping the well-ordered moleculararray, as reported before.1 Second, however, when there aremuch more 3d metal atoms than preadsorbed molecules, avariety of metalation processes occur: (1) 3dTPP complex
array with the original well-ordered square array structure
remaining (only 10%), (2) isolated 3dTPP complexes (weconﬁrmed at least three types) (80%), and (3) 3dTPP
complexes placed on an intercalated 3d metal monolayer island(10%).
3.3. Iron Deposition at 78.5 K on 2HTPP Array. As
shown in Figure 3, the metalation processes at 300 K provide avariety of 3dTPP complexes and strong 3d metal interactions
change the morphology and form three types of new molecular
structures, namely, (1) isolated clusters, (2) 3d metalintercalation, and (3) well-ordered 3dTPP array. Since these
could be due to the thermal diﬀusion of the deposited 3d metal
atoms at 300 K, we demonstrated an STM study on wellordered square-shape 2HTPP molecular arrays (about 50
molecules) with an adsorption of about 100 Fe atoms at thesubstrate temperature maintained at 78.5 K. Interestingly, atthe cryogenic temperature, previous X-ray photoemission
spectroscopy results44,50 suggested the precursor adcomplex
state of Fe-2HTPP instead of FeTPP formation.
Figure 4A shows the resulting STM image. In contrast to
Figure 3: the case of 3d-metal deposition at 300 K, 2HTPP
islands were observed to maintain their square shapes (notriangular shaped islands were observed). Currently, we
focused on an island located at the center of the image in
Figure 4A. On the island, brighter spots marked by the box canbe observed. Height proﬁle between two green arrows in
Figure 4A is denoted as a black line in Figure 4C. Elevenmolecules (nos. 1−11) were placed. no. 6 and no. 7 molecules
are 100−200 pm higher than other molecules. The results
indicated that the brighter spots can correspond to adsorbed
Fe atoms on 2HTPP. Figure 4B is obtained after 9 min 21 s at
the same area as Figure 4A wherein eight brighter spots
decreased to seven. Speciﬁcally, we manipulated one of the twobrighter spots (no. 7) as denoted by the dotted box by closing
the STM tip apex. The height proﬁle between two green
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ArticleFigure 5. Fe-2HTPP adcomplex states at 78.5 K. (A) dI/dV curves obtained on the β [red line] and γ state [black line]. (B) Calculated density ofstates (DOS) of 2HTPP [red line] and Fe-2HTPP [black line]. Dotted line denotes FeTPP DOS. (C) Fe atom insertion into 2HTPP molecules
during the STM measurement. Upper and lower panels denote STM topographic image and LUMO dI/dV maps at +0.9 eV before (t = 0 h) andafter (t = 6 h) the insertion, respectively. (D) dI/dV curves obtained at the positions a [red lines] and b [black lines] in part C before (t = 0 h, solidline) and after (t = 6 h, dashed line) the insertion. (E) Schematic illustration of “α”, “β”, and “γ” states.

shows the Shockley surface states at approximately −0.5
eV.53,62,63 On 2HTPP island, clear HOMO states atapproximately −1.0 eV and LUMO states approximately+1.0 eV can be identiﬁed. These energy positions are good
agreement with the previously reported HOMO/LUMO peaks
positions for 2HTPP.64,65 The origin of the −1.0 eV HOMO
peaks are conﬁrmed also by the dI/dV map obtained at −1.0
eV as shown in Figure 4D. Clear diﬀerences between “α”, “β”,
and “γ” states are observed at the positive bias side in Figure
4F. The “β” state has a larger dI/dV intensity peak comparedto “α” and “γ”, and the “α” and “γ” states show similar curves.
Figure 5A shows dI/dV curves of the majority states of “β”[red line] and “γ” [black line] states, which can be comparedwith the calculated LDOS of 2HTPP and Fe-2HTPP
adcomplex as shown in Figure 5B. One should be noted thatthe dI/dV curve of the “γ” state shows a clear shoulder peak(shaded area) around +0.3 eV in Figure 5A.
Figure 5B shows the theoretical free molecule DOS of
2HTPP [red line], Fe-2HTPP adcomplex [black solid line]
and FeTPP [black dotted line]. Details of the calculated DOS
of 2HTPP, Fe-2HTPP, and FeTPP are shown in the
Supporting Information, Figure S3.The Fermi-level is set tothe center of the HOMO−LUMO gap of 2HTPP (3.2 eV
below the calculated vacuum level). The calculated LDOS
reproduces well the HOMO and LUMO peaks of 2HTPP

around −1 and +1 eV, respectively. For FeTPP and Fe2HTPP, the same energy scale was used, such that the main
molecular levels are well aligned between all three molecules.
For Fe-2HTPP, this deﬁnition of the Fermi level (EF) implies
that the level at EF + 0.3 eV is actually occupied in the freemolecule calculation. In the real Fe-2HTPP/substrate system,
it is quite possible that Fe is in a Fe2+ state, and the weakly
bound Fe-4s electrons are transferred to the metal substrate. In
the Fe-2HTPP DOS, the LUMO peak (at +0.9 eV in 2HTPP)
is split into two peaks at 0.3 and 1 eV. The lower peak ﬁts well
the ﬁrst peak at 0.3 eV in the experimental curves (the shadedpeak shoulder in Figure 5A) and the main LUMO peak at 1 eV
has much lower intensity than the LUMO peak of 2HTPP.
The large intensity diﬀerence around 1 eV agrees well with theexperimental data and suggests that “β” and “γ” molecules
correspond to 2HTPP and Fe-2HTPP, respectively.
Further, we calculated the DOS of two types of Fe-2HTPP
adcomplex (see Supporting Information, Figure S4); Type A[black line in Figure S4]: two hydrogen atoms at the core
position point upward. Type B [gray line in Figure S4]: one of
the two hydrogen atoms at the core position points upward,
while other points downward. Since both types of Fe-2HTPP
adcomplex showed similar DOS, we used the DOS of the type
A in Figure 5B.
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Article4. CONCLUSIONS
On-surface metalation in UHV produces metal atom arrayfollowing a well-ordered molecular platform with extreme
lower concentration of impurity, which can be useful as abuilding block of designing 1 nm-size nanodevice and 1D- or
2D-lattice networks, and a precursor for further on-surface
synthesis. In the study, direct imaging during the cryogenic
temperature on-surface metalation process was successfully
performed using the home-built low-temperature UHV STM
setup. Fe deposition on 2HTPP array and STM/STS
measurements were done at the sample temperature
maintained at 78.5 K (about 100 Fe atoms on about 50
molecules). STM topographic images and HOMO/LUMO
maps combined with DFT calculations unveiled the cryogenic
temperature on-surface metalation process and identiﬁed three
types of TPP appearance: (1) original 2HTPP (40%), (2)2HTPP with an Fe atom on top (precursor adcomplex “α”
state, 15%), and (3) 2HTPP with an Fe atom inside (precursor
adcomplex “γ” state, 45%).

The DFT calculations show that the Fe-2HTPP adcomplex
state is unstable against dehydrogenation; i.e., Fe-2HTPP is 1.6
eV higher in energy than FeTPP + H2. Our STM experimental
data exhibits that this type of metastable state can be produced
at cryogenic temperatures.
Parts A and B of Figure 5 indicate that the experimentallyobserved the “β” state could be the 2HTPP without Fe
insertion, while the “γ” state could be the Fe-2HTPP
adcomplex state. Since the “α” state has similar LDOS as the“γ” state, these “α” and “γ” states could be Fe-2HTPP
adcomplex states, while the diﬀerence was observed only in the
topology: “α” is about 100−200 higher in height. Note that we
succeeded to manipulate the “α” state between Figure 4A and
4B. Remarkably after the manipulation, the “α” state changed
to “γ” state. This indicates that using the STM tip thephysisorbed Fe atom could be inserted into the molecule. The
STM observations of “α” and “γ” suggest there are at least two
types of Fe-2HTPP adcomplex states during the cryogenic
temperature metalation process.
Another signiﬁcant point is that state switching from “β” to“γ” was observed during the STM measurement. All the STM
topographic images shown in this manuscript were measured
at −1.4 eV (HOMO states), no signiﬁcant diﬀerence wasobserved in the topographic images between “β” and “γ” states.
Figure 5C shows a rectangle-shape island in consist of ten TPP
molecules. From the STM topographic image and the LUMO
map, we identify the island includes six “β” (=2HTPP) and
four “γ” states (=Fe-2HTPP). The upper panels in Figure 5C
were obtained at t = 0 h, but after 6 h, the observations
changed as depicted in the lower panels in Figure 5C. Now thesame island consists of four “β” (=2HTPP) and six “γ” states(=Fe-2HTPP). Namely, during the STM observation, Fe
atoms located on the terrace was inserted into the 2HTPP via
the route no. 4 and formed the precursor adcomplex. This stateswitching was clearly visible in the dI/dV curves as shown in
Figure 5D, where the dI/dV curves obtained at positions a andb are shown. Solid and dotted lines denote before (t = 0 h) andafter (t = 6 h) the state switching. The dI/dV curves obtainedat the position b [black line] are identical to those for the “γ”
state for all time, but the dI/dV curves obtained at position a[red line] clearly shows the LDOS change from 2HTPP to Fe2HTPP adcomplex state.
In summary, the present study of cryogenic temperature(78.5 K) deposition of the Fe atom (∼100 atoms) on a
2HTPP molecular array (∼50 molecules) has provided threeimportant insights. (1) No signiﬁcant change was observed in
the molecular morphology after the Fe deposition at 78.5 K on
2HTPP array, which preserved the original well-orderedmolecular structures in contrast to the 300-K metalation
process. (2) In spite of the fact that we deposited about twice
as many Fe atoms as 2HTPP molecules, about 40% (=21/53 in
Figure 4D) of the TPP molecules remain as 2HTPP without
Fe atom insertion, which indicates that the on-surface
metalation process no. 1 dominates at cryogenic temperatures.
(3) Two types of precursor adcomplex Fe-2HTPP states wereobserved. One could be the physisorbed Fe atom located on
the 2HTPP (15% in Figure 4D), and the other could be the Fe
atom inserted into the 2HTPP (45% in Figure 4D), while the
Fe insertion into the 2HTPP is still processing during the STM
measurements. The model in Figure 5E shows the summary of
the possible produced adcomplexes.

■

ASSOCIATED CONTENT

sı Supporting Information*

The Supporting Information is available free of charge athttps://pubs.acs.org/doi/10.1021/acs.jpcc.9b09795.

■

Figure S1, tunneling current maps of Fe atom adsorption
on 2HTPP at 78.5 K in UHV; Figure S2, diﬀerential
conductance maps (dI/dV maps) of Fe atom adsorption
on 2HTPP at 78.5 K in UHV; Figure S3, DFT
calculation results of 2HTPP, FeTPP, and Fe-2HTPP
adcomplex; Figure S4, DFT calculation results of two
types of Fe-2HTPP adcomplex; and Figure S5, STM
topographic images of one ML H2Pc ﬁlm grown on
Cu(111) before and after the 0.1 ML Fe deposition at
300 K in UHV (PDF)

AUTHOR INFORMATION

Corresponding Authors

Eiichi Inami − Department of Materials Science, Chiba
University, Chiba 263-8522, Japan; School of Systems
Engineering, Kochi University of Technology, Kochi 782-8502,
Japan; orcid.org/0000-0002-2167-8510; Phone: +81887-57-2307; Email: inami.eiichi@kochi-tech.ac.jp
Toyo Kazu Yamada − Department of Materials Science andMolecular Chirality Research Center, Chiba University, Chiba
263-8522, Japan; orcid.org/0000-0001-5185-6472;
Phone: +81-43-290-3915; Email: toyoyamada@faculty.chiba-u.jp
Authors

Masataka Yamaguchi − Department of Materials Science, Chiba
University, Chiba 263-8522, Japan
Ryohei Nemoto − Department of Materials Science, Chiba
University, Chiba 263-8522, Japan
Hideki Yorimitsu − Department of Chemistry, Graduate Schoolof Science, Kyoto University, Kyoto 606-8502, Japan;
orcid.org/0000-0002-0153-1888
Peter Krüger − Department of Materials Science and Molecular
Chirality Research Center, Chiba University, Chiba 263-8522,
Japan; orcid.org/0000-0002-1247-9886
Complete contact information is available at:https://pubs.acs.org/10.1021/acs.jpcc.9b09795
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The authors declare no competing ﬁnancial interest.

■

ACKNOWLEDGMENTS
This work was supported by JSPS KAKENHI Grant Numbers
19H05789, 17K19023 15K13357, 15H03531, 15H06092,
25110011, and 25600012, and the Murata Science Foundation.
We thank Professor Dr. Hisao Ishii (Chiba University) for
sublimation puriﬁcation of 2HTPP molecules, Mr. Mikio
Shimasaki for sublimation control of 2HTPP molecules, andProf. Dr. Kohji Nakamura (Mie University) for supporting
calculations.

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