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ArticleGrowth of Transition-Metal Cobalt Nanoclusters on 2D Covalent
Organic Frameworks
Toyo Kazu Yamada,* Shingo Kanazawa, Keisuke Fukutani, and Satoshi Kera
Cite This: https://doi.org/10.1021/acs.jpcc.3c07435

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sı Supporting Information*

ABSTRACT: Two-dimensional (2D) covalent organic frameworks (COFs) fabricated through on-surface synthesis were
investigated as a honeycomb nanopore template for the growthof 3d-transition-metal nanoclusters (NCs) with a size of 2 nm on ametallic substrate. The evolution of these NCs and their electronic
characteristics were studied employing scanning tunnelingmicroscopy/spectroscopy (STM/STS), angle-resolved ultraviolet
photoelectron spectroscopy (ARUPS), and X-ray photoelectron
spectroscopy (XPS) under an ultrahigh-vacuum (UHV) condition at room temperature. The 2D COFs were synthesized on
Cu(111) substrate utilizing 1,3,5-tris(4-bromophenyl)benzene (TBB) precursors, which engendered a honeycomb nanopore arrayof approximately 2 nm in size. In contrast to the behavior observed in the Co/Cu(111) system producing triangular-shaped bilayerstacking nanoclusters measuring 10−20 nm, STM imaging of Co/COFs/Cu(111) revealed the growth of Co NCs of approximately
1.5 nm within a single COF nanopore. This growth occurred without forming a monolayer film beneath the COFs, providing direct
evidence that the 2D COFs on Cu(111) can effectively entrap Co atoms within the nanopore, giving rise to Co NCs. Spectroscopy
measurements, including STS/UPS/XPS, confirmed the different local densities of states for Co NCs and COFs, corroborating thecoexistence of Co NCs and COFs on the surface.
spectrum of applications, encompassing quantum dots,25,26
quantum bits,27,28 spintronics,29,30 biomedicine,31,32 drugdelivery,33,34 catalysts,35−37 and water purification.38
The utilization of organic compounds to immobilize
magnetic atoms on a surface was initially demonstrated on
the surface of π-conjugated molecular self-assembled monolayer (SAM) films, such as phthalocyanines or porphyrins.39−41
However, a single molecule can only accommodate one atom
at its core in this metalation process. Suppose the deposition oftransition-metal atoms surpasses the number of organic
molecules within the SAM film. In that case, excess 3d
atoms push out the molecule and prefer to contact the noblemetal substrate. This results in intercalation, the formation ofmultiple adcomplexes, and the disruption of the ordered SAM
array, ultimately yielding an uncontrolled and disorderedsurface.39,40,42 In essence, SAM films lack the capability tocapture dozens to hundreds of magnetic atoms.
In this study, our focus is directed toward the experimental
investigations of the initial growth of submonolayer cobalt(Co) on a 2D COFs film prepared on an atomically flat and

1. INTRODUCTION
Among the robust intermolecular interactions, covalentbonding between carbon atoms has attracted notable attention.
Recently, this bonding has been harnessed to create covalentorganic frameworks (COFs), a remarkable nanostructure thathas emerged as a pioneering innovation in the past few
decades. COFs serve as nanometer-scale templates, adept at
accommodating guest nanomaterials with an exceptionally highpacking density. This property holds great promise for various
advanced applications, including gas storage and separation,1−3catalysis,4−6 drug delivery,7−9 photoelectronic devices,10−12
and batteries.13,14
A thorough comprehension of the mechanisms by which
guest materials become entrapped within COFs, whether
through electronic bonding or otherwise, is pivotal for
determining the functionality of COF-based applications.
Notably, both three-dimensional (3D) COFs, synthesized insolution, and two-dimensional (2D) COFs, prepared on noblemetal surfaces through on-surface synthesis, have beendeveloped.15−24 In the case of 2D COFs, precursor molecules
are utilized on atomically flat metal substrates, triggering
thermal diffusion and chemical reactions that culminate informing a 2D nanopore network.
Our primary focus lies in discerning whether these 2D COFs
possess the capability to capture adsorbed transition-metal
atoms within their nanopores, thereby engendering artificial
magnetic nanoclusters (NCs) measuring a mere few nanometers in size. Such NCs hold tremendous potential for a broad© XXXX American Chemical Society

Received: November 9, 2023
Revised: December 19, 2023
Accepted: December 20, 2023

A

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ArticleFigure 1. (a) UHV-STM topographic images obtained on the surface of TBB film grown on Cu(111) at 300 K (left panel: 100 × 100 nm2, Vs = +2
V, I = 10 pA; right panel: 30 × 30 nm2, Vs = +2 V, I = 10 pA). The lower panel denotes a honeycomb lattice formation model via the Ullman
reaction. The inset in the right panel denotes the FFT map, showing no symmetric spots. (b) STM topographic image of the ordered 2D-COFs
islands on Cu(111) after the thermal annealing in UHV (50 × 50 nm2, Vs = −2 V, I = 443 pA). The inset denotes the FFT map, representing broad
hexagonal spots corresponding to the honeycomb symmetry. The lower panel denotes the height profile along the arrow in the topographic image,indicating the size of the nanopore. (c) STM topographic images of the same COFs obtained with different tip conditions (No. 1 and No. 2) (15 ×
7.5 nm2, Vs = −2 V, I = 10 pA). The former and the latter revealed the COFs as bright and dark lines, respectively. (d) Bias dependence in STM
topographic images (50 × 50 nm2, I = 10 pA). Left and right panels were taken at positive (Vs = +2 V) and negative (Vs = −2 V) bias voltages.

2. METHODS
2.1. Home-Built UHV STM Setup. A custom-built UHVSTM apparatus featuring analytical, preparation, and introduction chambers was employed for this study.50,51 Eachchamber maintained a base pressure of approximately 10−8 Pa.
The STM instrument was situated within the analyticalchamber. We meticulously cleaned the STM tips, sample
surfaces, and film formations in the preparation chamber.
Transferring the cleaned STM tip and prepared samples was
performed without breaking the UHV, so the cleaned tip andpristine Cu(111) substrate in ultrahigh vacuum (UHV) atroom temperature. As our template, we employed a conventional 2D COF synthesized via on-surface synthesis throughthe Ullmann reaction,43 using 1,3,5-tris(4-bromophenyl)benzene (TBB) precursors.20,44−49 This particular COF
configuration offers nanopores of approximately 2 nm in size,with the capacity to accommodate approximately 60 Co atoms
within its structure.
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sample were set into the STM. STM measurements wereconducted at 300 K in UHV using a combined Nanonis SPM
controller and software. The topographic images were acquired
in a constant current mode, and the data obtained from the
STM measurements were subsequently analyzed using WSxM
5.0 Develop 10.0 software.52
2.2. UHV ARUPS/XPS/LEED Setup. ARUPS, XPS, and
LEED measurements were performed in the custom-built
UHV apparatus. All ARUPS and XPS measurements were
executed at room temperature, using an ultralow-background,high-sensitivity ARUPS apparatus with a hemisphericalelectron energy analyzer (MBS A-1) and two monochromators(MBS M-1) for two wavelength regions as setup.53,54 He-Iα(hν = 21.218 eV) and He-IIα (hν = 40.814 eV) radiation
sources were performed by a high-density plasma lamp (MBS
L-1) with an Al filter. XPS was measured by irradiating X-ray(1486.6 eV) on the sample surface in the same UHV chamber.
Doniach−Sunjic function was used for fitting the XPS data to
determine peak positions. LEED with a multichannel plate(MCP) equipped in the preparation chamber was used todetect weak electron signals from the monolayer molecularfilm. A beam energy ranging from 10 to 50 eV was employed
to minimize potential damage to the deposited molecular film.
The sample current during LEED measurements remained
below 100 nA.
2.3. Fabrication of STM Tips. STM tips were fabricatedfrom polycrystalline W (purity 99.95%), Mo (purity 99.95%),
and Pt80Ir20 wires with a diameter of 0.3 mm. This fabrication
process involved both electrochemical etching55 and flameetching techniques.50,56 Subsequently, the prepared tips were
carefully transferred into the UHV preparation chamber.
The tip apex underwent a thermal treatment, subjected to atemperature of 2000 K in UHV to eliminate any oxide layers
on the apex of tungsten and molybdenum tips.55 Following thiscleaning procedure, the prepared and pristine tip was installed
into the STM stage within the UHV analytical chamber.
2.4. Cu(111) Substrate Preparations. Our experiments
employed two Cu(111) single crystals, each serving distinct
purposes�one for ARUPS/XPS/LEED measurements andthe other for STM.53,54,57,58 Both Cu(111) single crystals,
having a diameter of 6 mm (MatecK, purity 99.9999%),
underwent a meticulous process of sputtering and annealing toachieve clean and atomically flat surfaces in the preparationchambers (<5.0 × 10−8 Pa) (as detailed in the Supporting
Information, Figure S1).
The cleaning treatment for the Cu(111) single crystals
involved Ar+ sputtering (+1.0 keV, 400 nA) with subsequentannealing (∼820 K) utilizing electron bombardment55 for the
STM setup, and Ar+ sputtering (+0.6 keV, 3000 nA), followedby annealing (∼800 K) using the infrared heater for the
ARUPS setup.
2.5. On-Surface Synthesis of 2D COFs. The precursor
employed in this study was 1,3,5-tris(4-bromophenyl)benzene(TBB, Tokyo Chemical Industry Co., Ltd., CAS RN: 7511-491, purity >98%) (Figure S4). TBB molecules, in the form ofwhite powder, were placed within the alumina crucible of ourcustom-built molecular evaporator.59 The crucible’s temperature was raised through irradiation by the surroundingtungsten (W) filament loops. A type K thermocouple, incontact with the bottom of the crucible, closely monitored the
crucible’s temperature. We maintained the crucible temperature at approximately 390 K (filament power: 1.32 W = 1.10
A × 1.20 V). This setup resulted in a sublimation rate of 0.155

Articlenm/min, which we accurately monitored using a quartz crystal
microbalance (QCM) with parameters z impedance = 1 anddensity = 1.59
In this study, approximately 0.8 nm of TBB precursors,
ascertained via QCM analysis, was deposited on both Au(001)and Cu(111) substrates. On the Au(001) substrate, thedeposition of this 0.8 nm layer yielded a self-assembled
monolayer (SAM) film at 300 K (Figure S2). Consequently, inthe scope of this study, it can be posited that a 0.8 nm
deposition is adequate to envelop the entire surface with amonolayer of TBB precursors.
Nevertheless, when we undertook the deposition of 0.8 nm
of TBB molecules on Cu(111), only approximately 30% of thesurface exhibited the formation of ordered COFs. This was
attributed to the Ullmann reaction occurring at 300 K, which
led to the formation of disordered COFs, attributed to theexistence of an intermediate state (see Figure 1a). It isnoteworthy, however, that the intermediate state evolved into
the final state upon thermal annealing at approximately 430 K
for a duration of 20 min in UHV60 (as detailed in Figures 1
and S5). XPS measurements disclosed the presence of Br peaks
at 182 eV (3p3/2) and 190 eV (3p1/2)61 (refer to Figure 3b),indicating the persistence of residual Br atoms on the surface
even following the annealing process.
2.6. Cobalt Deposition in UHV. Cobalt (Co) deposition
was performed using two distinct evaporators: the AEV-3
evaporator from AVC Co., Ltd., integrated with the STM
setup, and the EMF3/4 evaporator manufactured by Omicron.
These depositions were conducted at a sample temperature of
300 K,54,62 as depicted in Figure S3. The same deposition rate
was employed for both setups, amounting to 2.8 monolayers
per minute (with a flux of 5.0 nA). To ensure precise
quantification of the deposited Co, the deposition quantities
were additionally calibrated through STM topographic images
depicting the growth of Co islands on Cu(111), as displayed in
Figure S3.

3. RESULTS AND DISCUSSION
The proliferation of two-dimensional covalent organic frameworks (2D COFs) on noble metallic substrates has undergone
extensive scrutiny in recent decades. This exploration has
unveiled the potential of inert surfaces, such as gold (Au) orsilver (Ag), to catalyze the formation of 2D COFs through theadsorption of precursor molecules at ambient conditions,followed by annealing in an ultrahigh-vacuum (UHV)
environment, employing the Ullmann reaction as the pivotal
synthetic route.44,48
The Ullmann reaction mechanism governing this hexagonallattice structure may be elucidated by referencing the model
presented in the lower panel in Figure 1a. The precursor,
comprising a single TBB molecule, is characterized by a central
benzene ring connected to three peripheral benzene rings,
exhibiting C3 symmetry, with bromine (Br) atoms capping its
termini. After thermal annealing atop the noble metallic
substrate, the desorption of Br atoms transpires. Concurrently,
thermal diffusion augments the probability of precursor
encounters, thereby facilitating the formation of novel covalent
linkages between precursor molecules. The iterative executionof the Ullmann reaction culminates in the generation of 2D
COFs.
Our initial investigation entailed the examination of TBB
precursors’ adsorption onto a meticulously prepared, atomically flat Au(001) surface, conducted in UHV at 300 K. ThisC

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ArticleFigure 2. (a) UHV-STM topographic image obtained on the surface of triangular-shape bilayer Co islands grown on Cu(111) at 300 K (100 × 70
nm2, Vs = −1 V, I = 50 pA). The lower panel denotes the height profile along the arrow. (b) UHV-STM topographic image of the COFs/Cu(111)
surface after the Co deposition at 300 K (100 × 70 nm2, Vs = +2 V, I = 52 pA). The lower panel denotes the height profiles along the blue and red
arrows in (c). (c) Magnified STM topographic images. Upper panel: COFs appear as bright lines using the tip condition No. 1 (50 × 32 nm2, Vs =−2 V, I = 41 pA). Lower panel: COFs appear as dark lines using the tip condition No. 2, in which the Co nanoclusters (Co NCs) inside the COFs
are sharply visible and distinguished from COFs (20 × 13 nm2, Vs = +2 V, I = 100 pA). (d) Histogram of different size Co NCs. Blue, red, andgreen bars denote the number of Co NCs grown inside COFs, outside COFs, and bare Cu(111), respectively. The inset denotes the NC formation
model inside COFs. (e) Scanning tunneling spectroscopy (STS) results obtained on the Co NCs. The upper panels show an STM topographic
image (20 × 12 nm2, Vs = −2 V, I = 1 nA) and simultaneously obtained dI/dV map at +0.15 V. The lower panel represents dI/dV curves
proportional to the sample LDOS. Blue and black lines were measured on the Co NCs and COFs/Cu(111), respectively. Dashed lines and arrows
denote the Co surface state peak at −0.35 eV, TBB HOMO, and Cu(111) surface state (SS) peak around −0.45 eV.

procedure resulted in the establishment of a self-assembled
monolayer (SAM) film, as depicted in Figure S2a,b.
Subsequent thermal annealing in UHV induced the desorptionof Br atoms from the precursors, thereby fostering covalentinteractions among the precursor molecules. Consequently,
this process facilitated the formation of 2D COFs on thesurface, as elucidated in Figure S2c.
The primary objective of our investigation in this study is toleverage 2D COFs as a nanoporous honeycomb template forthe growth of transition-metal nanoclusters (NCs). The
realization of magnetic NCs necessitates the fulfillment of

two critical prerequisites: (1) the prevention of intercalation ofadsorbed transition-metal atoms beneath the COFs, which
could yield a monolayer film beneath the COFs, as opposed tothe intended NCs, and (2) the avoidance of chemical bonding
between the adsorbed transition-metal atoms and the COFs, assuch interactions could jeopardize the structural integrity andregularity of the COFs.
To assess the capacity of 2D COFs in capturing transitionmetal atoms, we conducted a comparative analysis of two
distinct systems: one devoid of COFs and the other featuring
the presence of COFs.
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For the former system, we selected Co NCs on a Cu(111)substrate, a system well-documented for forming triangularshaped bilayer NC islands with sizes ranging between 10 and
20 nm upon depositing approximately 0.2 monolayers (MLs)of Co at 300 K in UHV,54,62,63 as demonstrated in Figure S3.
Another rationale for selecting Cu(111) as our substrate lies
in its capacity to prevent intermixing or interdiffusion between
the adsorbed transition metals and noble-metal substrates.64
This stems from the fact that Cu boasts a higher surface energyof 1850 mJ/m2, compared to the surface energies of Au (1550mJ/m2) and Ag (1250 mJ/m2).65 It is worth noting that inert
Au or Ag surfaces may exhibit weaker interactions with the 2D
COFs, potentially leading to the unintended formation of amonolayer film beneath the COFs through intercalation.
Our initial step involved the meticulous preparation of aCu(111) surface characterized by atomic flatness andcleanliness (>100 nm terrace width) in UHV (Figure S1).
The verification of the substrate’s morphology was executed
through the utilization of scanning tunneling microscopy(STM), low-energy electron diffraction (LEED) analysis,
which revealed 6-fold fcc(111) symmetry, and angle-resolved
ultraviolet photoelectron spectroscopy (UPS), affirming thepresence of the parabolic Shockley surface state, in agreementwith previous studies.66,67
Figure 1a shows STM topographic images acquired after
introducing TBB precursors at 300 K in UHV onto theCu(111) surface. On the Cu(111) atomic terrace, a discernible
contrast between brighter and darker regions becomes evident,
denoting the coexistence of two distinct materials on thesurface, even at the relatively modest temperature of 300 K.
This stands in marked contrast to the formation of the SAM on
the Au surface (cf. Figure S2).
The magnified view presented in the right panel of Figure 1a
reveals the formation of a network structure, suggesting that
the Ullmann reaction has already transpired at 300 K on theCu surface. However, the ordered nature of this network
remains elusive, as the FFT map exhibits an absence ofsymmetric spots. This irregularity may be attributed to theintermediate state formation resulting from the Ullmann
reaction,44,48 particularly the creation of C−Cu−C bonds
between TBB precursors, which includes a copper atom, as
illustrated in the lower panel of Figure 1a. Notably, numerous
scratch lines are observable on the TBB/Cu surface within
Figure 1a, potentially indicating the diffusion of desorbed Br
atoms from the TBB precursors across the surface. Thisphenomenon could impede the further development of COFs,
as the presence of Br atoms might constrain additional TBB
adsorption on the surface. This observation is substantiated by
XPS, which detected the residual Br atoms on this surface, as
depicted in Figure 3 (we discuss later).
The energetically precarious intermediate state depicted in
Figure 1a wholly transformed into the energetically favorable
final state following annealing to 430 K (20 min) in UHV. Thisannealing process facilitated the formation of a honeycomb
nanopore array through the establishment of covalent bonds
among the precursor molecules. The STM topographic image
in Figure 1b vividly illustrates the emergence of the 2D COFs.
The FFT map reveals the presence of 6-fold spots, indicative of
the ordered structure. A height profile along the arrow crossing
three COFs nanopores, as depicted in the lower panel of
Figure 1b, yields an approximate dimension of about 2 nm.
An additional intriguing facet worthy of mention pertains to
the inherent instability observed during STM imaging of these

ArticleCOFs on Cu(111) at 300 K in UHV when employing
conventional PtIr and W metal tips. This instability may be
attributed to the presence of residual Br atoms on the surface,which could adhere to the tip apex, thereby inducing
alterations in the contrast within the STM images. Figure 1c
illustrates this phenomenon. In the upper panel, the image
portrays the COFs as bright lines, while, upon repeatedscanning of the same region with the same tip, the appearanceof the same COFs undergoes a transformation, as evidenced in
the lower panel of Figure 1c, where the COFs are depicted as
dark lines.
Furthermore, it is noteworthy that a contrast inversion was
also detected upon altering the bias voltage, as exemplified in
Figure 1d. Under identical tip conditions, the COFs wereobserved as bright lines at a bias voltage (Vs) = +2 V (left panelin Figure 1d), while they appeared as dark lines when the bias
voltage was set to Vs = −2 V (right panel in Figure 1d).
While the visualization of the COFs does indeed hinge on
the condition of the tip and the applied bias voltage, it isessential to highlight that these variables can be judiciously
utilized to discern the positions of Co adsorption within the
COFs region.
In assessing the efficacy of 2D COFs as trapping agents forgenerating transition-metal NCs, we undertook a comparative
examination of submonolayer growth processes involving Co
on Cu(111) and Co/COFs/Cu(111). As an initial step,
approximately 0.2 monolayers (MLs) of Co was deposited on
Cu(111) at 300 K in UHV (Figure 2a). During this process,the adsorbed Co atoms diffused thermally across the surface,culminating in the formation of triangular-shaped bilayer
nanoislands, characterized by dimensions of 10−20 nm. Thisobservation is corroborated by the height profile along the pathtraversing one such nanoisland, which reveals a height ofapproximately 400 pm and a width of roughly 12 nm.
Next, we deposited an equivalent quantity of Co onto the
COFs/Cu(111) substrate at 300 K in UHV. Figures 2a and 2b
have been presented at identical dimensions to clarify the
nanocluster size. As illustrated in Figure 2b, the introduction of
COFs engendered a marked transformation in the system,
resulting in a reduction in the size of the NCs and a heightened
proliferation of clusters. The height profiles, as displayed in thelower panel in Figure 2b and traversing two different NCs (asindicated by the blue and red arrows in Figure 2c), disclose adimension of approximately 1.5−2.0 nm, aligning with thedimensions of the nanopores within the COFs, as previously
observed in Figure 1b. These profiles were obtained using tip
conditions No. 1 and No. 2. The tip conditions No. 1 and No.
2 appeared in the COFs as bright and dark lines (Figure 1d),which could resolve the Co and COFs positions. Moreover,the height measurement approximates around half (∼200 pm)of the bilayer Co NC on Cu(111), as illustrated in Figure 2b,
suggesting a monolayer configuration of cobalt within the Co
NCs encapsulated within the COFs. The empirical evidencecompellingly establishes the COFs on Cu(111) as proficient inentrapping Co atoms within their nanopores. The magnified
images in Figure 2c provide a glimpse into cluster growth’s
progression.
It is discernible that a significant portion of the Co NCs issequestered by the COFs islands, which effectively conceal theunderlying COFs, thereby impeding a clear observation of the
COFs’ morphology. Nonetheless, within the area demarcatedby the dotted circle in Figure 2c, we are able to discern theE

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ArticleFigure 3. UHV-LEED, UPS, and XPS results. (a) LEED images obtained from the clean Cu(111) substrate at the electron beam energy of 178.9eV (top panel) and COFs/Cu(111) at the beam energy of 27.1 eV (bottom panel). (b) XPS spectra for the Br 3p peaks obtained at the nominal
molecular thicknesses of 1.6 nm (left panel) and 0.8 nm (right panel). For each thickness, the spectra for “as grown” (black) and “annealed” (gray)samples are shown. The vertical lines are shown to indicate the approximate positions of each of the peaks. The intermediate state before theannealing includes C−Cu−C and Br−C bonding, but the final state consists of C−C covalent and Br−Cu bonding. After the annealing in UHV, Br
atoms remained on the surface. (c) UPS spectra for the Cu(111) (dotted line), COFs/Cu(111) (gray line), and Co/COFs/Cu(111) (black line).
The dashed line marks the COFs flat HOMO position (2.20 eV). Arrows denote the Cu(111) surface states, in-gap state of COFs, and Co 3d
surface states. (d) UPS spectra in the vicinity of the Fermi energy obtained on the Co/Cu(111) surface (gray line) and the Co/COFs/Cu(111)
surface (black line). (e) XPS results around Co 2p orbitals obtained on the Co/Cu(111) surface (gray line) and the Co/COFs/Cu(111) surface(black line).

unaltered COFs, an observation that carries a pivotal
implication.
As initially posited, it remains imperative to ensure that the
COFs do not engage in chemical bonding with the adsorbed
Co atoms, which might otherwise lead to the production of Co
NCs and potentially culminate in the decomposition of the
COFs. In this context, the outcome in the upper panel of
Figure 2c serves as compelling evidence affirming therobustness of the COFs, specifically in terms of covalentbonding, in withstanding the demands of adsorption by active
transition metals. This stands in stark contrast to thedeposition of Co on SAM films, as reported previously.40
Furthermore, within the delineated area encircled by thedotted circle, diminutive Co NCs were detected along theperiphery of the COFs. This observation suggests the potential

initiation of NC growth commencing from the periphery, as
opposed to the core of the nanopore.
Additional insights into the formation of the Co NC were
gleaned through the utilization of a tip with contrast-reversible
capabilities, as evidenced in Figures 1c and 1d. This specialized
tip facilitated a clear differentiation between the COFs and theCo NC regions, with the COFs manifesting as dark lines in thelower panel of Figure 2c. The presence of these dark lines
encircling each Co NC (indicated by bright dots) provides
further evidence that the COFs’ nanopores effectively
entrapped the Co atoms, fostering the in situ growth of NCs
within these nanopores.
Crucially, it is noteworthy that this area in the lower panel of
Figure 2c produced three distinct Co NCs while other COFs
nanopores remained unoccupied. This observation suggests
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that adsorbed Co atoms may be able to migrate or diffuse
across the surface of the COFs yet come to a halt when theyencounter a suitable Co nucleation site. It is imperative toemphasize that they might not intercalate beneath the COFs,
an outcome indicative of stronger coupling between the πorbitals of the COFs and the underlying Cu surface, thus
serving as a constraint against Co intercalation beneath the
COFs, as depicted in the inset models within Figure 2d.
In our quest for a comprehensive understanding of the size
distribution of the formed Co NCs, we conducted a statisticalanalysis. Figure 2d encapsulates a complete summary,illustrating the quantity of Co NCs plotted against theirrespective diameters. A substantial majority of the Co clusters
exhibit a size of approximately 1.5 nm, closely corresponding
to the dimensions of the COFs’ nanopores. This empiricalfinding serves as additional evidence substantiating Co atoms’
confinement within the COFs’ nanopores.
The inset model in Figure 2d proffers plausible mechanisms
for forming Co NCs. As Co atoms adhere to the surface at 300
K, they can undergo thermal diffusion and migration across thenanopores. However, when these diffusing Co atoms become
ensnared by the edges of the pores, instigating nucleation
events, they serve as nuclei around which other thermallydiffused Co atoms aggregate, ultimately culminating in thegrowth of a nanocluster.
Building upon the experimental findings illustrated in Figure
2c, it is reasonable to deduce that the electronic interaction
between Co and the COFs should exhibit notable weakness.
This hypothesis is directly affirmed through scanning tunnelingspectroscopy (STS) measurements performed on this surface,as depicted in Figure 2e.
The simultaneous presentation of the topographic image andthe differential conductance (dI/dV) map within Figure 2eeffectively conveys the differentiation in the local density ofstates (LDOS) between Co NCs and the adjacent COFs on
Cu(111). Notably, the former manifests an LDOS peak
positioned at −0.35 eV (blue line in Figure 2e), while the latter
exhibits some peaks within proximity of the Fermi energy,
along with an additional peak around −2 eV.
Initially, the presence of the dI/dV curve obtained from theCo NCs within the COFs exhibited a peak at −0.35 eV. This
substantiates the cobalt composition within the nanoclusters,
as Co/Cu(111) is recognized for manifesting a spin-polarized
peak at −0.35 eV.54,62,63 Subsequently, the dI/dV curve
obtained on the COFs manifested several peaks near the Fermi
energy. Because the well-known Cu(111) surface state (SS)peak is located at −0.45 eV, the observed peaks near −0.45 eV
could potentially stem from the Cu(111) surface states
penetrating the COFs’ gap. Conversely, discernible LDOS
peaks between +0.2 and +0.4 eV might represent in-gap states
due to the formation of COFs. UPS findings in Figure 3c,marked by the arrow, also identified an escalated intensity
around the Fermi energy on the COFs/Cu(111) compared tothe pristine Cu(111), which is clearly visible in the ARUPS
maps around 0 eV (= Fermi energy) in Figure S7.
In addition to the microscopic STM/STS measurements in
Figures 1 and 2, LEED, UPS, and XPS measurements were
further performed to understand macroscopic information,encompassing both the surface and interface regions. Figure 3aillustrates the LEED patterns obtained from the unalteredCu(111) surface and the COFs/Cu(111) surface. The
presence of distinct and well-defined six spots, aligning withthe fcc-Cu(111) symmetry, characterizes the pristine Cu(111)

Articlesurface. However, following the thermal annealing of the TBB/
Cu(111) surface, these sharp six spots vanish, signifying thecomprehensive coverage of the surfaces by both the COFs andresidual Br atoms. The appearance of broad, hexagonallysymmetric LEED spots aligns with the ordered structure of the
COFs on Cu(111) and is consistent with the 6-fold spots
discernible in the fast Fourier transform (FFT) map in Figure
2b.
The XPS results pertaining to the Br 3p orbitals, as acquired
during the deposition of TBB at two distinct thicknesses (0.8
and 1.6 nm) on Cu(111), both prior to and following thermal
annealing, reveal the residual Br atoms, as depicted in Figure
3b. As a single nanopore within the COFs comprises 18
benzene rings (= 108 carbon atoms), achieved by excising 12
Br atoms from the TBB precursors using the Ullmann reaction(as depicted in the lower panel of Figure 1a), it can be inferredthat Br atoms could cover approximately ∼11% (= 12/108) of
the surface, significantly below the coverage of a monolayer.
Additionally, the intensity of the Br peak diminishedsubsequent to annealing (Figure 3b), suggesting a reductionin the quantity of Br atoms present on the surface during theannealing process. Although Br atoms are not visually apparent
in the STM topographic images, as indicated in Figure 1c, wehave previously noted frequent changes in the tip’s behavior,
which might suggest the incidental pickup of diffusing Br
atoms on the Cu surface at a temperature of 300 K.
An intriguing feature is the peak shift of approximately 1.3
eV, which becomes evident when comparing the XPS spectra
before and after annealing. This shift may be attributed totransitioning from the intermediate to the final state. In otherwords, it implies that a significant portion of the chemicalbonding has transitioned from Br−C bonding involving the
TBB precursors to Br−Cu bonding with the underlying copper
surface.
The UPS spectra have disclosed the presence of a discernible
highest occupied molecular orbital (HOMO) peak, positioned
at 2.20 eV below the Fermi energy on COFs/Cu(111).
Furthermore, an ARUPS map, presented in a second-derivativeplot, showcases the flat dispersion of the HOMO in k-space, as
depicted in Figures S6 and S7. Notably, the position of this
HOMO peak remains unaltered both prior to and following
the deposition of Co, as illustrated in Figure 3c. However,there is a noticeable increase in intensity at approximately 0.35
eV, as evidenced in Figures 3c and 3d, aligning with the −0.35
eV peak in the dI/dV curve from Figure 2e. This alignment
suggests a correlation with the spin-polarized Co 3d surface
state of Co/Cu(111).54,62 Furthermore, a meticulous examination of the Co 2p orbitals and the impact of Co adsorption
was conducted. Nevertheless, the XPS outcomes revealed noclear distinctions between Co/Cu(111) (gray line) and Co/
COFs (black line), as presented in Figure 3e. This implies that
Co might not engage in electronic hybridization with the
HOMO of the COFs. This finding aligns with prior XPS
results obtained from Co NCs situated within metal−organic
frameworks, which likewise failed to indicate any chemicalbonding between cobalt and the carbon, nitrogen, and sulfur
atoms within those systems.68 We further checked that there
was no significant shift of the carbon 1s peak of the COFs
before and after the Co deposition (Figure S8), indicating
weak electronic interactions between COFs and Co.
Consequently, we can assert that no clear electronic
hybridization exists between the transition metal and the 2D
COFs on Cu(111). This outcome can be attributed to theG

https://doi.org/10.1021/acs.jpcc.3c07435
J. Phys. Chem. C XXXX, XXX, XXX−XXXThe Journal of Physical Chemistry C

pubs.acs.org/JPCC

Keisuke Fukutani − Institute for Molecular Science, Okazaki444-8585, Japan
Satoshi Kera − Institute for Molecular Science, Okazaki 4448585, Japan; orcid.org/0000-0003-0353-5863

pronounced covalent bonding within the COFs, which prevails
over the interaction between the molecule and the transition
metal’s 3d orbitals. This covalent bonding enables the thermal
diffusion of Co atoms to traverse through the COFs without
undergoing significant electronic interaction with the COFs.

Complete contact information is available at:https://pubs.acs.org/10.1021/acs.jpcc.3c07435

4. CONCLUSIONS
Our investigation has demonstrated that the 2D COFs
prepared on Cu(111) exhibit a remarkable capability forentrapping Co NCs of equivalent size (∼2 nm) within theirnanopores. All STM/STS, UPS/XPS, and LEED measurements were conducted at a temperature of 300 K within a
UHV environment.
This Co NC formation can be attributed to several key
factors: (1) the absence of monolayer film growth below the
COFs; (2) the preservation of COFs even after Co adsorption;(3) Co atoms can migrate or diffuse across the COFs until
they are captured by nucleation sites located at the edges of the
COFs; (4) the aggregation of other diffusing Co atoms at these
nucleation sites results in larger NCs. However, the size of theNCs remains constrained by the dimensions of the COFs’
nanopores.
These compelling experimental findings chart a novel pathfor producing Co NCs composed of dozens of atoms utilizing
2D COFs. This breakthrough holds immense promise across a
myriad of application domains, including quantum dots,
quantum bits, spintronics, biomedicine, drug delivery, catalysts,
and water purification.

■

Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS
This work was supported by JSPS KAKENHI Grants
17K19023, 23H02033, and 23H05461, the Murata Science
Foundation, the Shorai Foundation for Science and Technology, TEPCO Memorial Foundation, Casio Science Promotion
Foundation, and the Toshiaki Ogasawara Memorial Foundation. Shingo Kanazawa was supported by the Special InterUniversity Researcher program organized by the Institute for
Molecular Science. We thank Dr. Eiichi Inami for carefully
reading our manuscript and fruitful discussions.

■

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ASSOCIATED CONTENT

sı Supporting Information*

The Supporting Information is available free of charge athttps://pubs.acs.org/doi/10.1021/acs.jpcc.3c07435.
UHV-STM, LEED, and ARUPS results of the cleaned
Cu(111) substrate; UHV-STM results of TBB SAM
films on Au(001) and 2D COFs on Au(001); UHVSTM, UPS, and XPS results of cobalt deposition on
Cu(111); precursor molecule deposition process on
Cu(111) in UHV at 300 K; UHV-STM and LEED
results obtained before and after the post-annealing of
the TBB/Cu(111); angle-resolved UPS maps obtainedon Cu(111) and COFs/Cu(111) before and after the
post-annealing; angle-resolved UPS maps in a differential
mode obtained on Cu(111) and COFs/Cu(111) before
and after the post-annealing; XPS spectra for the C 1s
and Co 2p peaks before and after the thermal annealingof TBB films and before and after the Co deposition on
TBB/Cu(111) (PDF)■

ArticleAUTHOR INFORMATION

Corresponding Author

Toyo Kazu Yamada − Department of Materials Science, Chiba
University, Chiba 263-8522, Japan; Molecular Chirality
Research Centre, Chiba University, Chiba 263-8522, Japan;
orcid.org/0000-0001-5185-6472; Email: toyoyamada@faculty.chiba-u.jp
Authors

Shingo Kanazawa − Department of Materials Science, Chiba
University, Chiba 263-8522, Japan; Institute for Molecular
Science, Okazaki 444-8585, Japan
H

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