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Cite this: DOI: 10.1039/d3tc03339b

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On-surface growth of transition-metal cobalt
nanoclusters using a 2D crown-ether array†
Toyo Kazu Yamada, *ab Ryohei Nemoto,a Fumi Nishino,a Takuya Hosokai,
Chi-Hsien Wang, d Masaki Horie, d Yuri Hasegawa,e Satoshi Kerae andPeter Krüger abcAdvanced methods of producing nanometer-size transition metal clusters have enhanced the functionality ofnanoclusters (NCs), leading to single-atom catalysis, miniaturization of spintronics media, and biomedicine.
Herein, on-surface growth of transition-metal cobalt (Co) NCs in a vacuum was demonstrated using a
structurally flexible two-dimensional (2D) 4,40 ,5,50 -tetrabromodibenzo[18]crown-6 ether (BrCR) ring moleculararray, prepared on an atomically flat noble metal Cu(111) surface. Then, we successfully produced two diﬀerent
sizes (B1.5 nm and B3.6 nm) of Co NCs using the trapping ability of BrCR. The growth and electronic
structures of Co NCs were investigated by low-temperature scanning tunneling microscopy/spectroscopy (STM/
Received 14th September 2023,
Accepted 10th December 2023
DOI: 10.1039/d3tc03339b

STS) and angle-resolved photoelectron spectroscopy (ARPES) in an ultrahigh vacuum. DFT calculations unveiled
the energetically stable adsorption sites. The growth process of Co NCs is governed by the electronic p–d hybridization between BrCR and Co. Once the Co atom was trapped, it became a nucleation center, around which
other Co atoms gathered to form a nanocluster. In contrast to the standard behavior of crown ether molecules
in solution, the stable position of the trapped metal atom is not at the center of the crown ring but at the BrCR

rsc.li/materials-cedge, where the Co atom is located below the side substituents.

1. Introduction
Transition-metal nanoclusters (NCs) using 3d metals, e.g., Co,
Ni, CoPt, FePt, Fe3O4, and Fe2O3, with a size of 10–100 nm
coated by chemical compounds have drastically advanced in awide variety of promising applications such as biomedicine,1,2drug delivery,3,4 catalysts,5–7 and water purification.8 Downsizing
the cluster to a few nanometers or atomic size has encouraged
further functionality such as single-atom catalysis9,10 or miniaturization of spintronics media.11,12 For that, the coordination oforganic molecules with transition-metal atoms could be an essential key factor. Since p-conjugated molecules, e.g., phthalocyanines(Pc) or tetraphenyl porphyrins (TPP), have a size of about 1 nm,
the metalation of self-assembled monolayer (SAM) moleculara

Department of Materials Science, Chiba University, 1-33 Yayoi-Cho, Inage-ku,
Chiba 263-8522, Japan. E-mail: toyoyamada@faculty.chiba-u.jpbMolecular Chirality Research Centre, Chiba University, 1-33 Yayoi-cho, Inage-ku,
Chiba 263-8522, Japanc
National Metrology Institute of Japan, National Institute of Advanced Industrial
Science and Technology, Tsukuba Central 5, 1-1-1, Higashi, Tsukuba,
Ibaraki 305-8565, Japand
Department of Chemical Engineering, National Tsing Hua University, 101, Sec. 2,
Kuang-Fu Road, Hsinchu, 30013, TaiwaneInstitute for Molecular Science, Myodaiji, Okazaki 444-8585, Japan† Electronic supplementary information (ESI) available. See DOI: https://doi.org/
10.1039/d3tc03339b

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films on 2D surfaces could be a novel method to promote thedownsizing of the nanocluster.13–15 However, the SAM films can
trap only one atom per molecule. Suppose the number ofadsorbed transition metal atoms exceeds the number of molecules. In that case, the SAM orderliness is broken, and otheratoms grow independently from the molecules, leading to a
messy surface.16
In this study, we demonstrated the adsorption of the ‘‘guest’’ 3dtransition-metal cobalt (Co) on the ‘‘host’’ well-ordered ring molecular array at room temperature in ultra-high vacuum (UHV). Crown
ether molecules in 4,40 ,5,50 -tetrabromodibenzo[18]crown-6 ether(BrCR) SAM films were used as a ‘‘host,’’ which have been studied
since the 1960s as cyclic molecules with an intramolecular ring inbulk crystal and liquid solution.17–23 This kind of ring is well-known
as a host, i.e., a molecule that can trap guest metal atoms, molecules,
or ions. Because of the flexibility of the O–C–O bond angles, thecrown ring is easily deformed, and BrCR has different bent conformations in the gas phase and the bulk crystal. However, it wasrecently shown that the crown ring can be unfolded when BrCR
forms a self-assembled monolayer (SAM) ring array on a metal
Cu(111) surface.24 Herein, Co was selected as the ‘‘guest’’ since thegrowth, electronic, and magnetic structures of bilayer triangularshape Co nanoislands on Cu(111) substrate are well known.25–28
Although there are reports on the adsorption of Co ions on crownether molecules in bulk,29 no studies have been conducted on crownether and metal clathrate compounds on solid surfaces in a vacuum.

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Here, we demonstrate that two sizes of equivalent-sized Co
NCs can be produced on 2D surfaces using the trapping abilityof BrCR, and the growth process was elucidated using complementary techniques of both experimental and theoretical simulations, such as a home-built low-temperature (78 K) UHV
scanning tunneling microscopy/spectroscopy (STM/STS) setup,
a UHV angle-resolved photoelectron spectroscopy (ARPES)
setup with low energy electron diﬀraction (LEED), and density
functional theory (DFT) calculation.

Journal of Materials Chemistry C
2.3

Characterization data of the used BrCR molecules are summarized as follows: 1H NMR (500 MHz, CDCl3, r.t.): dH 3.94 (t, 8H,
OCH2, J(HH) = 5 Hz), 4.09 (t, 8H, OCH2, J(HH) = 5 Hz), 7.01
(s, 4H, C6H2(catechol)) (Fig. S1, ESI†). HRFDMS calcd. For
C20H20Br4O6 [M]+ (m/z): 671.79989, found: 671.80053 (error
0.96 ppm) (Fig. S2, ESI†). CCDC 1901496 contains supplementary crystallographic data (Fig. S3, ESI†).
2.4

2. Experimental section
A home-built STM setup combined with the Nanonis SPM
controller BP4 (ESI†) was used to obtain topographic imagesof sample surfaces in a constant current mode. All STM results
were obtained at 78 K in UHV. We used sharp tungsten tips as
STM probe tips.30–32 ARPES and LEED measurements wereconducted in the home-built UHV apparatus (Supplementary
Notes, ESI†).

2.1

STM/STS measurements

Since the tunneling current (I) detected by the tip is exponentially proportional to the tip-sample separation (z) via exp((8m(F  eV/2))1/2z/h), where m: electron mass, h: Planck constant,
F: apparent barrier height between tip and sample, and V: setpoint sample bias voltage,33–35 STM can measure the surface
corrugations in the order of 1 pm. STS measures the sample
surface local density of states (LDOS). Tunneling current, I, as afunction of sample bias voltages (typically, from 2 V to +2 V),
was measured under a feedback off condition (one single curve
was measured within 50–200 ms) by fixing the tip-sample
separation (z = zc), which was determined by the setpoint
voltage (Vs) and the current (Is). The obtained I(V) curves werenumerically differentiated, and differential conductance (dI/dV) curves were obtained, which were proportional to LDOS. In
the dI/dV curve, positive and negative bias voltages mean
unoccupied and occupied LDOS, respectively. Zero eV corresponds to the Fermi energy. The STM/STS data were analyzed
using WSxM 5.0 Develop 10.2 software,36 and Gwyddion 2.53.

2.2

Cu(111) cleaning process

We used two Cu(111) single crystals, one for ARPES/LEED andanother for STM. Both Cu(111) single crystals (diameter 6 mm,
MatecK, 99.999%) were carefully sputtered and annealed toobtain clean and atomically flat surfaces in the preparationchambers (o5.0  108 Pa). Cleaning parameters were Ar+
sputtering (+1.0 keV, 0.4 mA) and annealing (B820 K) for the
STM setup, and Ar+ sputtering (+0.6 keV, 3.0 mA) and annealing(B770 K) for the ARPES setup. The Cu(111) was cleaned using a
conventional way of repeating Ar+ sputtering and annealing in
the preparation chamber, and Co was deposited on Cu(111) inthe same chamber.

J. Mater. Chem. C

Characterization of the synthesized BrCR

BrCR molecule evaporation in UHVBrCR molecules were deposited in the deposition chamber(o3.0  107 Pa) with sublimation rate control using a quartz
crystal microbalance (QCM).37 We used two diﬀerent UHV
setups: one for ARUPS and LEED and another one for lowtemperature STM. Therefore, it was necessary to control precisely the amount of deposited molecules on the Cu(111)substrate at 300 K in both setups. We used the quartz SiO2
crucible and homebuilt molecular evaporator (thermal radiation heating) to obtain the same crucible temperature andpressure (1  106 Pa). For the QCM calibration for checking
the sublimation rate, parameters were set to z-ratio = 1,
density = 1. The quarts crucible was radiatively heated byflowing current through a tungsten wire filament (diameter
0.3 mm). The crucible temperature was monitored using thealumel-chromel thermocouple contacting the bottom of the
crucible. The Cu(111) substrate was put B110 mm above thecrucible. Before the deposition, we always checked the BrCR
evaporation rate by setting a QCM at the same distance position. We carefully increased the crucible temperature in time,and precisely above the crucible temperature of 357 K, and
QCM detected the molecule deposition. The evaporation speedsof 0.03 nm min1 and 0.08 nm min1 were reproducibly
obtained at the crucible temperature of B361 K and B369 K,
respectively. Since the QCM parameters of z-ratio and molecule
density, the estimated deposition amounts, and the adsorption
probability on the substrate were unknown, the molecule
thickness estimated by QCM was only used as an index. STM
checked the precise deposited molecular monolayer (ML).
2.5

Cobalt deposition in UHV

Co atoms were deposited from an electron bombardment typeevaporator (AEV-3, AVC Co., Ltd) at a sample temperature of
300 K in UHV. Hot electrons from a W wire ring located above a
Co rod (diameter 2.0 mm, purity 99.99%) were accelerated by
B800 eV to the apex, which melted and evaporated only the
apex of the Co rod, leading to a Co evaporation flux of about
5 nA. The deposition amounts of Co were controlled from STM
topographic images of Co island growth on Cu(111).
2.6

DFT calculation

Density functional theory calculations were performed with the
projector augmented wave method as implemented in the VASP
code.38,39 The numerical settings were the same as in ourprevious study on BrCR/Cu(111).24 We used the PBE
exchange–correlation potential and Grimme’s DFT-D2 van derThis journal is © The Royal Society of Chemistry 2023

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Journal of Materials Chemistry C
Waals correction, where the C6 parameter of Cu was rescaledby 50%.
This scaling parameter was determined through test calculations on benzene/Cu(111), in order to correct overbinding
between benzene-type pi-orbital and metal surfaces within
DFT-D2.24 More recent dispersion correction schemes, such
as DFT-D3, have been found to be superior to D2 for metals40,41
and should give reliable results without the need for parameter
rescaling. Nonetheless, here we chose to keep the same computational settings as in our previous work24 for consistency
and easy comparison.
A plane-wave energy cut-oﬀ of 400 eV and a 2  4  1 k-point
grid for the Cu(111) 7  4 supercell were used. The Cu(111)substrate was modeled with four atomic layers. Starting fromthe stable structure of BrCR/Cu(111), one Co atom was put atvarious initial positions in the proximity of the molecule. Then,
all atomic positions, except those in the two lowest Cu layers,
were optimized until the forces were less than 0.2 eV per atom.

Paper
Fig. 1(a) shows STM topographic images acquired on thepristine Cu(111) terraces, where triangular-shaped Co
bilayer islands grew after the Co deposition. Here, theadsorbed Co atoms are thermally diﬀused on the Cu(111)terraces and form ordered structures following the fcc(111)symmetry.
Next, we demonstrated the interaction balance between Co
and BrCR and between Cu and BrCR. If the Cu–BrCR interaction is stronger than the Co–BrCR interaction, BrCR and Co
islands could grow separately and not interact. Here, wedeposited BrCR molecules on the Co bilayer islands as depictedin the model in Fig. 1(b). Fig. 1(c) shows the result. Interestingly, we observed no BrCR islands. In contrast, new protrusions appear on the top and edge of the Co islands while theislands remain triangular. Namely, most adsorbed BrCR
islands preferentially contact the Co islands. This means that
the Co–BrCR interaction is stronger than the Cu–BrCR interaction. This experimental evidence in Fig. 1(c) further provides
an idea of producing nanoclusters (NCs) by depositing Co
atoms on the BrCR array.

3. Results and discussion
3.1

STM imaging of Co/Cu(111) and BrCR adsorption

The nanocluster (NC) formation using the transition metal
cobalt (Co) and BrCR ring molecules is governed by twointeractions between Co and BrCR and Co and the Cu substrate. To clarify the substrate interaction eﬀect, we firstchecked the growth of Co on Cu(111). Then, we tested BrCR
molecules adsorption on the Co nanoislands.

3.2

Co-adsorption on BrCR 2D array

To adsorb Co atoms on the 2D BrCR array to create transition
metal NCs, we first prepared the BrCR 2D array.17 Fig. 1(d)shows BrCR array self-assembled monolayer (SAM) islands on
Cu(111). The magnified image in Fig. 1(e) shows periodic
ordered patterns on the BrCR islands, corresponding to the(7  4) superstructure.24

Fig. 1 (a) STM topographic image of Co bilayer nanoislands on Cu(111) (66  66 nm2, 1 V, 87 pA). (b) BrCR molecule adsorption model on these Co
islands. (c) STM topographic image of Co nanoislands on Cu(111) after the BrCR deposition (100  100 nm2, 1 V, 10 pA). (d) and (e) STM topographic
images obtained on the surfaces of BrCR islands on Cu(111): (d) 300  300 nm2, 2 V, 10 pA and (e) the ordered BrCR array structure (5  5 nm2, 1.4 V,
10 pA). (f) STM topographic image obtained on the surface of Co islands on Cu(111) after the BrCR deposition (200  200 nm2, 2 V, 10 pA).

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Journal of Materials Chemistry C

Fig. 2 (a) Schematic model of Co atom deposition on the BrCR ordered array. (b) and (c) STM topographic images of Co nanoclusters (NCs) on the BrCR
array: (b) 10  5 nm2 and (c) 10  10 nm2, 2 V, 10 pA. The inset in (b) denotes height profiles crossing five diﬀerent NCs. (d) STS results obtained on the
Co NCs. The left panels denote simultaneous obtained topographic image (20  7 nm2, 2.5 V, 200 pA) and dI/dV maps at 1.5 V and +1.5 V. The rightpanel denotes dI/dV curves obtained on the NCs (blue, red, and grey dots) and the BrCR array (green dots).

About 0.01 cobalt monolayers (MLs) were deposited on the
BrCR array at 300 K, and subsequently, STM/STS measurements
were performed at 78 K without breaking UHV. Here, the ratioof the guest Co atoms to the host BrCR ring molecules wastuned to about two to nine (Fig. 2(a)). Fig. 1(f) represents an
STM topographic image of the BrCR array islands after the Co
deposition, where the observation clearly changed. Many brightspots appeared, indicating the Co NC formation instead of
forming the triangular Co islands on Cu(111) depicted inFig. 1(a). This suggests that the BrCR molecule trapped theadsorbed Co atoms, restringing their thermal diﬀusion andcould form the metal–organic NCs. Also, bright spots werelocated at random positions, and the BrCR array lattice wasmaintained. This means trapped Co atoms could not be moved
or intercalated even at 300 K. Namely, the Co–BrCR NC formation could be energetically stable owing to the strong Co–
BrCR bonding.
Fig. 2(b) shows a magnified STM topographic image inside
one BrCR domain after Co deposition. The white lines inFig. 2(b) denote the BrCR lattice. Here, NCs observed as brightspots are kept within the BrCR 2D array. Height profiles crossing randomly selected five diﬀerent NCs as depicted in the inset
in Fig. 2(b) indicating that the NCs have rather uniform size,

J. Mater. Chem. C

namely B200 pm height and B1.5 nm diameter. Still, they are
not the same shape, indicating diﬀerent orientations and/or
coordination of Co–BrCR NC structures because of the flexibility of the crown ether ring. Another indication of BrCR deformation can be explored by observing the darker area next to thespot, which could be peeled off the BrCR from the Cu(111)
surface during the NC formation. Because the NC size iscomparable to the one-unit cell size of the BrCR lattice, oneby-one trapping of the Co atom by the BrCR could occur.
Fig. 2(c) shows an STM topographic image of another BrCR
domain, in which white dotted lines indicate the remaining
unit cell of the BrCR lattice, and the intersection indicates thecenter position of the crown ring. BrCR molecule models arealso shown in Fig. 2(c) as a guide to the eye. A one-unit cell
contains one BrCR molecule. By comparing the BrCR lattice
and the NC position, it is clear that the NCs do not grow at thecenter of the crown ether ring but are close to the intermolecular position.
We could estimate the number of Co atoms consisting of
one NC. In the area shown in Fig. 2(c), the 0.01 MLs Co/Cu(111)
corresponds to about 18 Co atoms deposited on about
136 BrCR molecules, while we could observe only eight NCs.
This means that a single NC contains, on average, 18/8 Z 2 CoThis journal is © The Royal Society of Chemistry 2023

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Journal of Materials Chemistry C
atoms. So, the NC includes not only one but more than two atoms.
This is the clear diﬀerence compared to the previously studied pconjugated Pc/TPP molecules, where only one atom can betrapped at the core, but extra Co atoms diﬀused and disordered
the SAM film.16 In this sense, it is remarkable that the molecules
surrounding the NC kept the original (7  4) superstructure, as
shown by the white box in Fig. 2(c), including 5 by 3 BrCR
molecules without disordering or deformation. Since a Co atom
adsorbed and started to diﬀuse on the BrCR array surface, if thebonding between BrCR and Co is too strong, the Co atom should
not move. However, the experimental results showed that one NC
has more than two Co atoms. This growth can occur due to two Co
atoms diﬀusing and meeting each other on the surface. Namely,
the results in Fig. 2(c) indicate a low trapping ability of the Co
atom by the crown ether ring in BrCR but suggest that once a Co
atom was trapped, other diﬀusing Co atoms gathered, forming ananocluster.
3.3Electronic structures of Co–BrCR nanoclusters

The metal–organic NC formation process was also directly
imaged in the electronic structures. As suggested in Fig. 1,

Paper
the formation of Co–BrCR NCs is energetically more stable thanthe co-existence of isolated Co atoms and BrCR molecules.
Namely, the 3d Co atom prefers to couple with BrCR moleculesand could form a bonding state via p–d hybridization.
We used scanning tunneling spectroscopy (STS) to measure
the sample local density of states (LDOS) at each pixel
position.33–35 Fig. 2(d) shows, from top to bottom, an STM
topographic image and dI/dV maps at 1.5 eV and +1.5 eV
obtained on the same BrCR surface after the Co deposition,
where two NCs exist.
Since the BrCR array has HOMO and LUMO peaks around1.5 eV and +1.5 eV, which was checked from the dI/dV curve
obtained on the pristine BrCR film on Cu(111) (see the ESI,†
Fig. S8), the dI/dV maps obtained at 1.5 V and +1.5 V (leftpanels in Fig. 2(d)), whose brightness corresponds to the LDOS
intensity, clearly showed the BrCR ordered pattern. This alsomeans that the orderliness of the BrCR array in the electronic
structure was preserved even after the Co nanocluster formation. Namely, the Co NC formation did not drasticallychange the overall surface electronic structure, but it changed
only the local electronic structure around the Co NC. Because of

Fig. 3 (a) and (b) Simultaneously obtained STM topographic image and current map at the edge of the BrCR array island, where two Co nanoclusters(NCs) are observed (10  5 nm2, 2 V, 10 pA). (c) Height profiles along the red and blue arrows in (a). (d) DFT calculation results of Co atom adsorption
into the BrCR/Cu(111) system. Ball-and-stick models of five configurations are shown with Co, Br, O, C, H, and Cu atoms in green, magenta, red, grey,
blue, and golden, respectively. Side and top views are shown. The Co atom is placed between Br atoms and the benzene ring (no. 1), below the benzene
ring (no. 2), below or outside the Br atoms (no. 3), below the crown ring (no. 4), and above the benzene ring (no. 5). DE denotes the energy value
diﬀerence from the most stable configuration. (e) DFT calculation suggested a model of how the adsorbed Co atom moved to the stable position.

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the diﬀerent contrast between the BrCR and Co NCs, NCs could
have diﬀerent LDOS from the BrCR.
We checked dI/dV curves obtained from the Co NCs, whichwere found to be classified into three types named a (67%, redline), b (22%, blue line), and g (11%, grey line) as depicted in
the right panel in Fig. 2(d). The green line denotes the dI/dV
curve obtained on the BrCR array.
Interestingly, all dI/dV curves reveal peaks/shoulders at
0.73 and +0.25 eV despite the non-uniform topology of theNCs. The diﬀerence in the dI/dV might occur because of thenon-flat surface of the NCs, which changes the local
barrier height (F) of each nanocluster and varies the exponential background in the dI/dV curves,34 namely, dI/dV B
rs((8m(F  eV/2))1/2z/h), where rs denotes the sample LDOS,
m is the electron mass, and z is the tip-sample separation. Thismeans that the type a NCs (red line in Fig. 2(d)) have a lower
local barrier height compared to the type b and g NCs, increasing the background. The obtained LDOS features at 0.73 and+0.25 eV could indicate Co–BrCR bonding and antibonding
states, respectively.
Another point to be noted is that the LDOS of the remaining
BrCR-ordered molecular array is still preserved after the Co
deposition. Only the LDOS of limited BrCR areas near the NCs
were disturbed. These areas are encircled by a black line in the
dI/dV maps in Fig. 2(d). The STM topographic image in Fig. 2(d)shows that the BrCR array of these areas is disordered. This
could be a trace from when the NC formation pulled the BrCR
from the 2D array.
3.4

Initial growth processes in nanocluster formation

Fig. 3(a)–(c) represents that the NCs located at the edge of the
BrCR array could grow twice as fast. Fig. 3(a) and (b) show an
STM topographic image and a current map obtained in thesame area. Here, the latter visualized atom and molecule
patterns. Height profiles along the blue and red arrows in
Fig. 3(a) represent B200 pm and B400 pm in height andB1.5 nm and B2.0 nm in width, respectively (see Fig. 3(c)).
The current map in Fig. 3(b) denotes the BrCR unit cell lattice,indicating the edge NC has a size of about two BrCR unit cells.
Since Br atoms are located at the BrCR island’s edges instead of
the crown rings, these observations in Fig. 3(a)–(c) indicate ahigher trapping ability by the edge instead of the crown ring.
To clarify the mechanism of each growing mode, density
functional theory (DFT) calculations were performed with the
projector-augmented wave method implemented in the VASP
code.38,39 The calculated DFT results shown in Fig. 3(d) shedfurther light on the growth processes of Co–BrCR NCs. First,
the energetically stable configuration of the (7  4) BrCR
superlattice on Cu(111) was calculated. Then, one Co atom
was added to the BrCR lattice unit cell, and the adsorptionenergy (E) was calculated. Five configurations shown in Fig. 3(d)were examined to determine the energetically most stableconfiguration. Ball-and-stick models of five configurations areshown with Co, Br, O, C, H, and Cu atoms in green, magenta,
red, grey, blue, and golden, respectively. Hexagonal lines
denote the substrate fcc-Cu(111) lattice. The most energetically

J. Mater. Chem. C

Journal of Materials Chemistry C
stable configuration among them was the Co atom placed
between the benzene ring and bromine atoms, where the
C–Br bond was broken, and a new C–Co–Br bond was formed.
We indicate the structural energy (DE) relative to this structure(which has DE = 0.0 eV). The other configurations shown in
Fig. 3(d) have higher energies of DE = 2.5 eV, 3.5 eV, 3.5 eV, and
5.1 eV, respectively. The second stable configuration was the Co
atom located below the benzene ring. Although we expectedintercalation of the Co atom below the BrCR molecules, the Co
atom below the Br atoms (+3.5 eV) and the crown ring (+3.5 eV)
revealed much higher energy. The most unstable position was
the Co atom located on the BrCR molecule (+5.1 eV); thisdefinitely rules out the possibility of Co atom adsorption on
top of the BrCR molecules. These might indicate a similar
behavior to the bulk crystal of Co(H2O6) in CR.29
Fig. 3(e) shows a schematic image of possible Co atom
adsorption processes when a Co atom adsorbs on the centercrown ring. First, the Co atom cannot stay on the molecule(+5.1 eV). Thus, the Co atom may intercalate below the crownring. However, the position below the crown ring is still
unstable (+3.5 eV). Hence, the Co atom starts to diﬀuse andmoves to the side of the molecule, around the benzene
ring (+2.5 eV), and could be trapped by the formation of the
C–Co–Br bond (DE = 0.0 eV). These DFT results suggest theonset of the nanocluster growth.
3.5

Mass production of transition-metal nanoclusters

Finally, we evaluated how the on-surface method using a 2D
BrCR array could produce plenty of equivalent-sized NCs. Fig. 4
shows experimental results. Fig. 4(a) and (b) demonstrate two
diﬀerent cases of Co adsorption on the BrCR array. The BrCR
covered 75% of the former surface, but the latter surface wascovered by only 15%. Then, it is found that Co NCs located on
the Cu(111) terrace, apart from the BrCR array, have a larger
size. Fig. 4(c) summarizes the diameter of NCs located inside
the BrCR array. We checked the diameter of hundreds of the Co
NCs. Then, we classified the sizes into 1.0, 1.5, and 2.0 nm forthe smaller NCs, and 3, 4, and 5 nm for the larger NCs, which
are shown in Fig. 4(c). Gaussian fittings to these distributions
led to the average value of 1.5  0.5 nm for the smaller-size NCs
and 3.6  1.2 nm for the larger-size NCs.
In Fig. 4, it is essential to know that many NCs were isolatedon the Cu terrace. This indicates a necessary message that theisolated NCs could be energetically stable. First, the absorbed
Co atom was trapped by the BrCR array’s edges, similar to
Fig. 3(a). Second, other Co atoms gathered, and the NC’s size
increased. The NC could be released when the NC thermal
fluctuation overcame the bonding with the BrCR array. Repeating the drawing out of BrCR from the array and isolating NC
formation made the array shrink and disarrange thearray edges.
Fig. 4(e) shows dI/dV curves obtained on the larger-size(B3.6 nm) isolated NCs, whose energy positions are identicalto the initial growth of the smaller-size (B1.5 nm) NCs in
Fig. 2(d). Increasing the amount of the Co atoms inside BrCR
could lead to enhanced charge transfer. However, both smallThis journal is © The Royal Society of Chemistry 2023

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Fig. 4 Co nanocluster formation on BrCR/Cu(111). (a) and (b) STM topographic images of Co atom adsorption on (a) 0.75 monolayers (MLs) and(b) 0.15 MLs of BrCR on Cu(111) (100  100 nm2, 2 V, 10 pA). (c) Nanocluster diameter histogram, indicating two diﬀerent sizes of nanoclusters (NCs). (d)STM topographic image of Co NCs (50  50 nm2, 2 V, 10 pA). (e) dI/dV curves obtained on the larger-size Co NCs (blue and red dots) and BrCR array(green dots). (f) PES spectra obtained from the surfaces of the bare Cu(111) surface (black line), 0.57 MLs Co/Cu(111) (grey line), 1.0 ML BrCR film on
Cu(111) (blue line), and 0.57 MLs Co film on the 1.0 ML Br-CR film/Cu(111) (red line). These PES spectra were measured in the G–K direction in the Brillouinzone of the Cu(111) surface and summed up from 0.0 to 1.1 Å1. Experimentally obtained work function (WF) values are also shown. (g) Schematic modelof two types of NCs.

(B1.5 nm in Fig. 2(d)) and large (B3.6 nm in Fig. 4(e)) Co NCs
have similar dI/dV curves with LDOS peaks around 0.73 and+0.25 eV. No significant change was observed, suggesting thatthe charge transfer eﬀect is rather small.
While STS measures the sample surface LDOS with highspatial resolution in the vicinity of the Fermi energy for both
occupied and unoccupied states, angle-resolved photoemission
spectroscopy (ARPES) measures the occupied DOS of a large
surface area, detecting macroscopic surface and interface electronic structures. Here, averaging information of the massproduced NCs was further investigated.
Fig. 4(f) shows the PES spectra obtained on the bare Cu(111)(black line), 0.57 MLs Co/Cu(111) (grey line), and 1 ML BrCR
film on Cu(111) before (blue line) and after (red line) the
0.57 MLs Co deposition. The PES intensity curves were obtainedby summing up ARPES data from 0.0 to 1.1 Å1 along the G–K
direction in the Brillouin zone (see the ESI,† Fig. S4 and S5).

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The binding energy at zero eV denotes the Fermi energy. The
Cu(111) has a surface state at 0.4 eV (a small peak in the
spectrum) and 3d band peaks between 2.5 and 4.5 eV with awork function (WF) of 4.90 eV. When the Cu(111) surface wascovered with the 1 ML BrCR film, the work function was
lowered to 4.43 eV due to the interface dipole induced by amolecular permanent dipole and charge transfer at the interface. The Cu surface state peak disappeared, and a HOMO peakat 1.48 eV appeared, which corresponds to the energy positionof the HOMO peak measured by STS.24 Also, the spectrum peak
shape changes to two-dip and one-peak features after the 3dband region around 3.8 eV, indicating the superposition ofmolecular orbital features.
We deposited 0.57 MLs Co on the BrCR film. The red
spectrum shows that the BrCR HOMO peak at 1.48 eV isstrongly quenched. This could indicate the disordering of the
BrCR ordered array, and the BrCR nanocluster formation would

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give a broadened feature of HOMO-derived states. On the other
hand, the intensity between 1 eV and 0 eV increased compared to the blue spectrum obtained on the bare BrCR (see thered spectrum in Fig. 4(f)), which could have the exact origin as
the LDOS peak at 0.73 eV in the STS curve. The PES spectra
diﬀerence between before and after Co deposition is also
shown in the ESI,† Fig. S4. A similar intensity increase wasobserved on the Co/Cu(111) surface (grey line), indicating ashoulder near the Fermi energy (around 0.2 eV). In contrast,
the band peaks between 2.0 eV and 3.5 eV show no diﬀerence before and after the Co deposition, mainly dominated by
the Cu 3d state.42 Unfortunately, it is not easy to give any
information on the spectral diﬀerence from types of Co NCs
with the present experimental conditions. The dips at 3.8 eVand 5 eV became flat, which could be due to the shift andbroadening behavior of deeper-lying orbital states as found for
the HOMO, interacting weakly among the 3d Co states and thesubstrate Cu 3d states.
LEED spot patterns of the 2D BrCR array wholly disappeared
after the Co deposition (ESI,† Fig. S6), in agreement with the
mass-productive NCs on Cu(111) (see also the complete NCformation on the surface in the ESI,† Fig. S7).
Finally, a growth model of the two types of NCs is shown in
Fig. 4(g). The smaller 1.5 nm size NCs, whose size is comparable to one BrCR molecule, could consist of one BrCR including
more than two Co atoms below the BrCR, increasing the heightof +200 pm. The larger 3.6 nm size NCs seem to include morethan two BrCR molecules, and the Co bilayer likely grew below
molecules since the height increase is comparable to the Co
bilayer height (B400 pm) in Fig. 1(a). This suggests the larger
NC includes more than 10 Co atoms.

4. Conclusions
We demonstrated the growth of transition-metal cobalt (Co)nanoclusters (NCs) using the trapping ability of the 2D crownether (BrCR) (7  4) array on Cu(111). The sample was preparedat 300 K in UHV, while all STM/STS and ARPES experiments
were performed at cryogenic temperatures in UHV. DFT calculations confirmed the energetically stable adsorption site of the
Co atom into the BrCR molecule on Cu(111).
In contrast to the host–guest coupling of the CR ringmolecule in bulk or solution, the NC formation or supramolecular chemistry becomes substantially modified in the presenceof a solid surface. The adsorbed Co atoms were trapped not at
the center ring but at the side benzene region via p–dbonding. The NCs inside the BrCR array have a smaller size
(1.5  0.5 nm), but the isolated NCs apart from the array have alarger size (3.6  1.2 nm). The latter could be formed byrepeating the drawing out of BrCR from the array with simultaneous Co bonding.
Thus, we found an on-surface method to lead the mass
product of isolated transition-metal Co NCs by depositing Co
on the 2D BrCR array on Cu(111) in UHV at 300 K.

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Journal of Materials Chemistry C

Conflicts of interest
There are no conflicts to declare.

Acknowledgements
This work was supported by JSPS KAKENHI grant numbers
17K19023 and 23H02033, the Murata Science Foundation, theShorai Foundation for Science and Technology, TEPCO Memorial Foundation, Casio Science Promotion Foundation, andToshiaki Ogasawara Memorial Foundation. We thank Dr Eiichi
Inami for carefully reading our manuscript and for the fruitful
discussion.

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