Nanoscale
Horizons

Volume 9
Number 5
May 2024
Pages 659–874

The home for rapid reports of exceptional significance in nanoscience and nanotechnology
rsc.li/nanoscale-horizons

ISSN 2055-6756

COMMUNICATION
Toyo Kazu Yamada et al.
Designing 2D stripe winding network through crown-ether
intermediate Ullmann coupling on Cu(111) surface

Nanoscale
Horizons
COMMUNICATION

Cite this: Nanoscale Horiz., 2024,
9, 718
Received 23rd December 2023,
Accepted 21st March 2024

Designing 2D stripe winding network throughcrown-ether intermediate Ullmann couplingon Cu(111) surface†
Toyo Kazu Yamada, *ab Ryohei Nemoto,a Haruki Ishii,a Fumi Nishino,a
Yu-Hsin Chang,c Chi-Hsien Wang, c Peter Krüger ab and Masaki Horiec

DOI: 10.1039/d3nh00586k
rsc.li/nanoscale-horizons

Chemical synthesis typically yields the most thermodynamicallystable ordered arrangement, a principle also governing surface

New concepts

synthesis on an atomically level two-dimensional (2D) surface,

The STM experimental investigation illustrated the creation of a 2D
monolayer film comprising randomly connected stripe networks formedby linking individual tetrabrominated crown ether molecules throughthree distinct connections. While intermediate states have conventionallybeen deemed energetically unstable, our experiments substantiated thestability of the resultant 2D network at room temperature (300 K) and its
formation feasibility at a relatively lower temperature of 453 K. This sheds
new light on a novel approach for 2D network synthesis. Leveraging theflexibility and resilience of these stripes, a densely packed 2D network
film was achieved on a flat Cu(111) surface. Given the inherent flexibilityof the crown ether molecule’s core ring, this breakthrough holds
significant promise as a foundational building block in fabricating ring
host supramolecules, thereby enhancing its capacity to encapsulate guest
atoms, molecules, or ions.

fostering the creation of structured 2D formations. The linear
connection arising from energetically stable chemical bonding
precludes the generation of a 2D random network comprised of
one-dimensional (1D) convoluted stripes through on-surface synthesis. Nonetheless, we underscored that on-surface synthesis
possesses the capability not solely to fashion a 2D ordered linear
network but also to fabricate a winding 2D network employing
a precursor with a soft ring and intermediate state bonding within
the Ullmann reaction. Here, on-surface synthesis was exhibitedon Cu(111) employing a 2D self-assembled monolayer array of
4,4 0 ,5,5 0 -tetrabromodibenzo[18]crown-6 ether (BrCR) precursors.
These precursors were purposefully structured, with a crown ether
ring at the core and Br atoms positioned at the head and tail ends,
facilitating preferential connections along the elongated axis tofoster a 1D stripe configuration. We illustrate how adjustments inthe quantities of the intermediate state, serving as a primary
linkage, can yield a labyrinthine, convoluted winding 2D networkof stripes. The progression of growth, underlying mechanisms, andelectronic structures were scrutinized using an ultrahigh vacuum
low-temperature scanning tunneling microscopy and spectroscopy(STM/STS) setup combined with density functional theory (DFT)
calculations. This experimental evidence opens a novel functionality in leveraging on-surface synthesis for the formation of a 2D
random network. This discovery holds promise as a pioneering
constituent in the construction of a ring host supramolecule,augmenting its capability to ensnare guest atoms, molecules,
or ions.

1. Introduction
Exploration of on-surface synthesis stands as an advanced
bottom-up approach for fabricating expansive two-dimensional(2D) molecular assemblies,1–12 notably graphene (Gr) or graphene
nanoribbons (GNR)13–22 and porous organic frameworks(POFs)23,24 encompassing covalent organic molecular frameworks (COFs).25,26
This method harnesses highly functional, adaptable monolayer organic films, significantly broadening their potential
applications across diverse fields, including gas storage andseparation, catalysis, optoelectronics, sensing, and adsorptionof small molecules.27–39
The Ullmann reaction40–42 conducted on noble metal surfaces extensively employs inflexible and planar benzene ringaDepartment of Materials Science, Chiba University, 1-33 Yayoi-Cho, Inage-ku, Chiba 263-8522, Japan. E-mail: toyoyamada@faculty.chiba-u.jp
Molecular Chirality Research Centre, Chiba University, 1-33 Yayoi-cho, Inage-ku, Chiba 263-8522, Japanc
Department of Chemical Engineering, National Tsing Hua University, 101, Sec 2, Kuang-Fu Road, Hsinchu, 30013, Taiwan† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d3nh00586kb718 | Nanoscale Horiz., 2024, 9, 718–730

This journal is © The Royal Society of Chemistry 2024

Communication
compounds as precursors for COFs creation. Principally, thereaction mechanism on Cu surfaces has undergone rigorous
investigation, leading to the elucidation of its procedural
intricacies.43–45 For example, in the case of the Ullmann reaction from bromotriphenylene to bistriphenylene on a Cu(111)
surface, it was found that halogen, first, dissociates andadsorbs on Cu and leaves a phenyl radical. Second, a phenyl–
Cu–phenyl intermediate state is formed from a Cu adatom.43,46
Third, when heating above 660 K, the C–C (phenyl–phenyl)
bonding is made, and the Cu atom disappears. However,organic molecules can be easily damaged or sublimatedat such a high temperature. Therefore, understanding andcontrolling the on-surface synthesis process at appropriate
temperatures in the range of 300–500 K could be crucial.
In this study, we demonstrate the generation of a nonsuperimposing 2D stripe random network via on-surface synthesis
employing pliable precursors and their intermediate-state bonding. Such random networks elude formation through the useof rigid, durable, and flat precursors like phthalocyanines orporphyrins, which, however, yield ordered networks.47
Furthermore, this growth process prohibits the emergenceof a second-layer stripe formation, thereby precluding threedimensional (3D) phase formation. This stands in contrast to
the construction of 3D overlapping stripes achieved throughthe conventional drop-casting of graphene nanoribbons orcarbon nanotube solutions.48–50
The internal ring within the crown ether molecule stands asa potential candidate among flexible precursors for the generation of a random network. A 4,4 0 ,5,5 0 -tetrabromodibenzo[18]crown-6 ether (BrCR) molecule, consisting of a center crownether ring with two phenyl rings at both sides, was used.51
Owing to the pliability in the crown-ether ring, both bulk crystal
and gas phase configurations exhibit bent ring structures.52
Moreover, the design of 2D stripes utilizing BrCR precursors
poses a challenge due to conventional bulk methods that yieldaggregation when employing BrCR and Cu powder synthesis inbulk, rendering the synthesis of the stripe unattainable.
The precise alignment control of BrCR precursors on
Cu(111) assumes a pivotal role, as it is imperative for stripe
formation that the Ullmann reaction selectively occurs between
two BrCR molecules orientated in the same direction. Thisorientation ensures that the ‘head’ and ‘tail’ ends of themolecules, housing the Br atoms, face each other—a condition
satisfied by the BrCR monolayer on Cu(111).52–55 Repetition ofsuch a linear connection in a consistent direction yields stripes.
By selecting the annealing temperature conducive to intermediate state connections, successful growth of a 2D random
stripe network becomes feasible, thereby exploring thepotential of on-surface synthesis in generating amorphouslike, asymmetrical nanopatterns.
This CR ring polymer exhibits significantly greater durability
compared to individual molecules, demonstrating stability
even at 300 K. Moreover, the crown ring component within
the CR polymer possesses the capacity to encapsulate guest
atoms, molecules, and ions, presenting vast potential for adiverse array of applications. For instance, incorporating aThis journal is © The Royal Society of Chemistry 2024

Nanoscale Horizons
guest transition-metal atom could yield polymers with enhanced
functionality,56 spanning quantum dot networks,57–60 spintronics,61,62 biomedicine,63,64 drug delivery,65,66 catalysts,67–69 andwater purification.70 Furthermore, integrating the host ‘‘wheel’’
BrCR ring molecule with the guest ‘‘axel’’ molecule could result in
the formation of nanomolecular machine polymers.71–73

2. Experimental section
2.1

Characterization of synthesized BrCR precursors

First, BrCR was synthesized and characterized based on themethod described in the literature.29 Field-desorption mass
spectra (FDMS) of Ullmann coupling products were observed by
JEOL JMS-T200GC AccuTOF GCx. CCDC 1901496 contains the
supplementary crystallographic data (see Fig. S4–S6, ESI†).
2.2

Sample preparations

A Cu(111) single crystal (diameter 6 mm, MatecK, purity
99.999%) was carefully sputtered and annealed to obtain clean
and atomically flat surfaces in the preparation chambers(o5.0  108 Pa). Cleaning parameters were Ar+ sputtering(+1.0 keV, 0.4 mA) with subsequent annealing (B820 K). Then,BrCR molecules were deposited on the Cu(111) substrate at
300 K in the deposition chamber (o3.0  107 Pa). We used aquartz SiO2 crucible and a homebuilt molecular evaporator.
The quartz crystal microbalance (QCM) parameters (z-ratio =1, density = 1) were taken to operate QCM.74 The quarts crucible
was radiatively heated by flowing current through a tungsten
wire filament (diameter 0.3 mm). The crucible temperature wasmonitored by using the alumel–chromel thermocouple contacting the bottom of the crucible. The Cu(111) substrate was put
B110 mm above the crucible. Before the deposition, we always
checked the BrCR evaporation rate by setting a QCM at
the same distance position. We used the evaporation speed of
0.03 nm min1 at B361 K. Since the QCM parameters of z-ratio
and molecule density, the estimated deposition amounts, aswell as the adsorption probability on the substrate, were
unknown, the molecule thickness estimated by QCM was onlyused as an index. STM checked the precise deposited molecularmonolayer (ML).
2.3

STM/STS measurements

STM measurements combined with the Nanonis SPM controller BP4 were used to obtain topographic images of sample
surfaces in a constant current mode. The STM data wereanalyzed using WSxM 5.0 Develop 10.2 software and Gwyddion
2.56.75 The home-built UHV-STM equipment consisted of STM,
preparation, and deposition chambers. The base pressures ofeach chamber were below 5.0  108 Pa, 2.0  108 Pa, and1.0  107 Pa, respectively.76 Samples and STM tips weretransferred between chambers using transfer rods without
breaking the UHV. Gate valves separated each chamber.
A UHV cryostat (CryoVAC) in the STM chamber was used tocool down the STM setup. All STM results were obtained at 78 K
in UHV. We used sharp tungsten tips as STM probe tips.77–80

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2.4

DFT calculation

DFT calculations of possible intermediate and final conformations of the BrCR Ullmann coupling have been performed using
the projector-augmented wave method as implemented in the
VASP package.81,82 The PBE exchange–correlation function was
employed with a plane wave cut-off energy of 400 eV. Sufficiently large supercells were used such that the distance
between periodic images was at least 1 nm. All structures were
fully optimized until the forces on all atoms were less than
0.1 eV Å1. The energies of all these systems were computed
using the same method (VASP, PBE-functional, same planewave energy cut-off). The adsorption energy of Br/Cu(111) was
computed using a five monolayer Cu(111) slab with one Br atom in
a 4  4 surface supercell and a 6  6  1 Gamma-centered k-point

Communication
mesh. Similar computational settings were recently used for F
adsorption on Cu(001).83 When adsorbed at the stable hollow
fcc site, the adsorption energy per Br atom is 1.70 eV for theBr2 molecule in the gas phase or 2.96 eV for Br single atoms.
This agrees well with the value of 3.05 eV reported by Migani
and Illas.84

3. Results and discussion
3.1

On-surface design of 2D stripe random network

Fig. 1(a)–(c) shows how the precursor symmetry determines theon-surface synthesized 2D network coordination. Here, C3, C2,
and C4 symmetry precursors could ideally produce honeycomb,
straight stripes, and squared lattice structures on a 2D surface.

Fig. 1 (a)–(c) Bonding models of precursors. (a) Honeycomb network with C3, 1D stripes with C2, and 2D squares with C4 precursors. (d) Single-crystal
X-ray crystallographic structure of BrCR. Stick and space-fill models are overlaid. (e) Sphere models of 2D BrCR self-assembled monolayer array on
Cu(111). (f)–(h) STM topographic images obtained at 77.8 K on BrCR grown on Cu(111) (f) before and after the thermal annealing of (g) 433 K and (h) 453 K:(f) left panel: 20  10 nm2, 0.8 V, 10 pA, right panel: 100  50 nm2, 2.5 V, 10 pA. (g) 100  100 nm2, 2.5 V, 10 pA, (h) 100  100 nm2, 1.6 V, 100 pA.

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During the on-surface synthesis, the connection reaction between
precursors was governed by the Arrhenius equation. This means
the reaction process is strongly dependent on the temperature.
For example, 1,3,5-tris(4-bromophenyl)benzene (TBB) precursors,12,85,86 known as a precursor to creating the honeycomb
porous lattice (Fig. 1(a)), generate not only hexagons but
also pentagons and heptagons, producing an incomplete 2D
honeycomb network because bromine (Br) desorption,12,85
thermal diﬀusion, and covalent bonding formation simultaneously occur.
Still, rigid and flat benzene ring compounds produced a flat
2D linear structure because of the strong covalent bonding
between benzene rings. The p-orbital from the benzene ring ispreferentially coupled with s-orbitals of the noble metal substrate, leading to a flat surface, i.e., the benzene ring parallel tothe substrate surface. This means the 2D random networkof tortuous stripes cannot be produced using the normal rigid
p-conjugated precursors.
To design the 2D stripe random network, (1) the precursor
requires a flexible structure with the functionality of bending or
twisting, and (2) the formation of a one-dimensional (1D) stripe
requires that the Ullman reaction happens only at the head andtail position of the precursor. Thus, we designed the precursor
with a crown ether at the center, Br atoms were set at head
and tail positions, and two benzene rings were used to bondwith the substrate, keeping the flatness. The BrCR molecule(see Fig. 1(d)) consists of a center crown ether ring with two
phenyl rings on both sides.51
In a bulk state, Ullmann coupling of BrCR in the presenceof Cu powder produced 3D aggregations, including various
compounds of mono-coupling, biphenylene, and triphenylene
products observed by field desorption mass spectra (FDMS,
Fig. S4–S6, ESI†), suggesting uncontrollability of the chemical
reactions (Fig. S7, ESI†).
In this study, we used the ordered self-assembled BrCR
monolayer film formed on Cu(111).52–55 Fig. 1(e) shows aspherical model of an ordered (7  4) 2D BrCR array. Although
the BrCR in the gas phase has a C2 symmetry, the BrCR single
molecules are slightly distorted, losing the symmetry.
Notably, the long axis of the BrCR molecule on the Cu(111)
surface aligns parallel to each other, producing Br atom rows.
Namely, when the thermal annealing could preferentially occur
the Ullmann reaction at these rows, the BrCR precursors could
connect along the long axis since one single BrCR has two Br
atoms on each side. Therefore, the free-bonding hands could
have a higher chance of bonding with neighbors after the
Ullmann reaction.
Fig. 1(f) presents the STM topographic depiction acquiredon the BrCR array structure prepared on Cu(111) at 300 K under
UHV conditions, distinctly demonstrating the absence ofon-surface synthesis at this temperature. This stands in stark
contrast to observations with 4,400 -dibromo-p-terphenyl87 or
TBB,12,85,86 wherein Br deposition at 300 K on Cu(111) initiatedon-surface synthesis, yielding a planar polymer chain. However,the BrCR arrangement on Cu(111) at 300 K did not manifest
such synthesis, as evidenced in Fig. 1(f), implying that theThis journal is © The Royal Society of Chemistry 2024

Nanoscale Horizonscrown ether ring attenuates the interaction with the Cu substrate, thus inhibiting Br desorption. Nonetheless, the BrCR
array on Cu(111) does exhibit on-surface synthesis commencing at an annealing temperature of 433 K, as depicted inFig. 1(g) and (h).
Before our annealing experiments, the following results werereported: (1) ortho-dibromobenzene molecules conducted onsurface Ullmann coupling produce biphenylene and triphenylene groups.88 (2) TBB precursors generate intermediate bonds
on Cu(111) at the annealing temperature of 370 K, and thisintermediate state transforms to the final C–C bonding states
above 410 K.89 Therefore, we expected a 2D network consistingof the biphenylene and triphenylene groups, suggesting major
straight and trident connections. Then, we set the annealing
temperature at 433–453 K.
After the thermal annealing treatment at 433 K in UHV for
the BrCR array on Cu(111) provided the 1D stripe formation, as
depicted in Fig. 1(g). Markedly, we observed a drastic change on
the surface after the further thermal annealing at 453 K in UHV.
In Fig. 1(h), the ordered array disappeared, but a 2D stripe
random network was produced.
3.2

Theoretically predicted stable connections

To better understand the chemical reactions of BrCR precursors on Cu(111), DFT calculations were performed to investigate the energetically stable connections (Fig. 2). Except for theadsorption of Br on Cu(111), all calculations were done in thegas phase.
We consider a product molecule X, formed by the Ullmann
coupling reaction between n BrCR precursor molecules (n = 2 or
3). In the process, m Br atoms are dissociated, which we assume
will adsorb on the Cu surface. Thus, the reaction can be written
as: nBrCR + Cu(111) - X + (mBr)/Cu(111) or, when X is a Cucontaining intermediate: nBrCR + Cu (S) + Cu(111) - X +(m Br)/Cu(111).
In Fig. 2, the DFT optimized structures are shown together
with their reaction energies DE, which are given per one BrCR
reactant molecule and with respect to the following reference
states: BrCR: free molecule, Br: adsorbed on Cu(111), Cu: bulk.
All energies are negative, which means exothermic reactions.
The reaction energy of the Cu-containing intermediate state(no. 2) is minimal, making this configuration rather unlikely or
short-lived.
It should be noted that the Cu-containing intermediates(no. 2 and no. 4) are gas phase models. In reality, the Cu atoms
in the intermediate states of the Ullmann coupling are likely to
be attached to the Cu surface, either as a Cu adatom or apartially lifted surface Cu atom.43 Such adsorbed structures
should be more stable than the gas phase models consideredhere. All reactions in Fig. 2 (except for no. 2) are exothermic andthus thermodynamically possible. Note that the Br adsorption
on Cu (calculated adsorption energy 1.70 eV per half Br2
molecule) contributes a large part to the reaction energy.
If the reaction product is Br2 gas instead of adsorbed Br, allreaction energies will increase by m/n  1.70 eV. The corresponding reaction energies become: (2) bent 1: 1.97 eV, (3) kink:

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Fig. 2 DFT calculation results of possible optimized bonding configurations of two BrCR monomers via Ullmann reaction. From left to right,(1) monomer, (2) 1201 bent structure no. 1 with a Cu atom, (3) kink structure, (4) 1201 bent structure no. 2 with a Cu atom, (5) twist structure,(6) 1801 straight structure, and (7) trident structure. Although energetically stable configurations are (6) and (7), intermediate states of (4) and (5) introduce
the 1201 bent structure. Especially, the intermediate ‘‘twist’’ configuration provides corrugation of approximately 0.5 nm. Blue energy values denote
reaction energies DE, given per reactant BrCR molecule concerning the reference states Br/Cu(111) and bulk Cu for structures (2) and (4). D E o 0 means
an exothermic reaction.

0.28 eV, (4) bent 2: 1.59 eV, (5) twist: 0.02 eV, (6) straight:
1.01 eV, and (7) trident: 0.27 eV. Then, all reactions except
trident formation would become endothermic, and no linear
connections could be formed. This implies that apart from theordered arrangement of the BrCR monolayer on Cu, a second
important factor for the formation of a 2D stripe network is thatfor a monolayer of BrCR molecules, the dissociated Br atoms
can eﬃciently adsorb on the Cu surface, which makes thestraight connection energetically stable. Indeed, the present
calculations suggest that when Br adsorption on Cu is impossible (or saturated), a straight connection cannot be formed.
Thus, the DFT results in Fig. 2 suggested that the mostenergetically stable formation using Ullmann coupling could be(6) straight and (7) trident in agreement with the previously
reported results of ortho-dibromobenzene molecules conducted
on-surface Ullmann coupling producing biphenylene and triphenylene groups.88 However, 2D tortuous stripe networks presented
in Fig. 1(h) cannot be made using only straight and trident
connections. This leads to a requirement of intermediate 1201
bent connections suggested by the DFT results in Fig. 2.
Another crucial aspect highlighted by the theoretical projections in Fig. 2 is that solely the intermediate states of the (4)bent 2 and (5) twisted configurations exhibit corrugation, whilealternative configurations such as the (2) bent 1, (3) kink,(6) straight, and (7) trident maintain flatness subsequent tobonding. The (5) twisted arrangement demonstrates a maximum
width of approximately 0.5 nm between H–H atoms. Subsequently,
this corrugation will be discernible in the STM images.
3.3 Necessity of intermediate states for 2D stripe random
network formation
Fig. 3 shows STM experimental results obtained on the 2D
stripe network grown on Cu(111). Fig. 3(a) shows a magnified

722 | Nanoscale Horiz., 2024, 9, 718–730

STM topographic image. Single chains have an identical widthof approximately 1 nm, as shown by the height profile along thearrow in smaller-right panel, which is comparable to the BrCR
width estimated from a single-crystal X-ray crystallographic
spacefill model and the BrCR unit cell vector of a = 0.93 nm
as depicted in Fig. 1(d).
We additionally assessed the height profile along a single
long chain, marked by the green line in Fig. 3(a). This heightprofile, presented in the right panel, conspicuously reveals asurface that is not atomically flat on the polymer chain. This
stands in stark contrast to polymer chains synthesized using
precursors composed solely of benzene rings.12,85–87 Suchcorrugations, reaching a maximum height of approximately
200 pm, may signify the existence of the intermediate state as
anticipated in Fig. 2 (twist structure).
We measured the angle between the connections and summarized in the histogram (ESI,† Fig. S8). It is clear that theconnection angle is widely distributed between 60–180 degrees
and therefore this could a proof of the randomness of the
network. Such a random network cannot be produced by using
the rigid precursors, such as phthalocyanines or porphyrins.47
The Fourier transformed (FT) image displayed no apparent
symmetry (no spots). (Other images are also shown in ESI,†
Fig. S1–S3.)
From the image in Fig. 3(a), we found the following facts.
(1) One stripe structure continuously connects and grows with alength longer than 50 nm. (2) All stripes are separated and do
not bind together, indicating a repulsive interaction. (3) Interestingly, the stripes seem connected randomly, as depicted by
the magnified image in Fig. 3(c) (12  12 nm2); however, thisindicates that the local chains consist of mainly three connections. Namely, 1201 bent, 1801 straight, and trident connections. One BrCR precursor size (B1.5 nm) is marked as a box inThis journal is © The Royal Society of Chemistry 2024

Communication

Nanoscale HorizonsFig. 3 (a) STM topographic image obtained at 77.8 K on the BrCR/Cu(111) surface after the thermal annealing at 453 K for 10 min in UHV (50  50 nm2,
1.6 V, 100 pA). The right panels denote the height profiles along the arrow and the green line. (b) Histogram of the connections: stable straight (26%) andtrident (26%) connections, but the intermediate state of the 1201 bent structures are found to be majority (48%). (c) Magnified STM topographic image:18  18 nm2. The chain polymers consist of three connections: straight, bent, and trident connections. A box denotes the single BrCR precursor size.
(d) 2D network models formed with diﬀerent precursors. From left to right, the network consisting of 16 bent and 16 trident precursors, the network
composed of 32 bent and 16 straight precursors, the network consisting of 32 bent, eight trident, and 32 straight precursors, and the network consistingof 32 bent, 16 trident, and 16 straight precursors. The last one corresponds to the experimentally obtained ratio in (b).

Fig. 3(c). We classified 746 connections in the STM topographic
images, and Fig. 3(b) shows the results. Surprisingly, thetheoretically predicted energetically stable straight and trident
connections were limited to 26%. On the contrary, the intermediate 1201 bent connection was found to be the majorconnection, 48%.
Fig. 3(d) shows network models using 1201 bent, 1801
straight, and trident connections. These simple models help
us understand the network formation process. The two networks on the upper side consist of only two connections. These
networks do not fit the experimental observation; the first has
closed stripe loops, and the second has only 1D stripes. Thus,
these two models exclude the possibility of creating a 2D
network using only two connections. The third and fourthmodels used three connections, but the ratio was modified:bent : trident : straight = 32 : 8 : 32 and 32 : 16 : 16; the former led

This journal is © The Royal Society of Chemistry 2024

to a more considerable distance between stripes, but the latterproduced a dense stripe network, in agreement with theexperimental results, and this ratio (2 : 1 : 1) corresponds tothe experimentally obtained ratio of 48% : 26% : 26%. Therefore, the network models in Fig. 3(d) strongly indicate that theformation of the 2D stripe random network is governed by thepresence of the intermediate 1201 bent connection.
Moreover, the ratio of 48% : 26% : 26% following the 453 K
annealing closely mirrors that of 54% : 20% : 26% following the433 K annealing. No substantial alteration was discerned,
suggesting the stability of intermediate states within thistemperature range.
3.4

Electronic structures in 2D random network

Another aspect of the stripes lies in the corrugation observed on
the polymer surface, as depicted in Fig. 3(a), stemming from

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the formation of intermediate states, which could potentiallyalter the molecular orbitals. To investigate this phenomenon,
scanning tunneling spectroscopy (STS) measurements wereconducted on the polymer surface depicted in Fig. 4.
Fig. 4(a) shows an STM topographic image showing 2D
stripes on Cu(111). At each pixel position in this area, wemeasured dI/dV curves, proportional to the sample LDOS, asa function of the bias voltage. Fig. 4(b) shows dI/dV curves,in which EF denotes the Fermi energy and positive andnegative bias sides represent unoccupied and occupied LDOS,
respectively. We observed a peak around 0.3 eV just below
the Fermi energy, which might be an in-gap state inside
the molecular gap or the Cu(111) surface state (S.S.) locatedat 0.35 eV below the Fermi energy,90 through the molecular gap.
The dI/dV curves delineated in Fig. 4(b), extracted from twopolymers demarcated by blue (I) and red (II) boxes in Fig. 4(a),exhibit identical dI/dV variation with respect to the energy. This
indicates that the on-surface synthesized 1D stripe polymers
using the BrCR could possess a uniform electronic structure
despite the corrugated surface topology. Since the HOMO andLUMO peaks are often obscured by the exponential backgroundin the dI/dV curve, namely, the tunneling probability function(T), a well-established normalization method is employed toCommunication
recover the highest occupied molecular orbital (HOMO) and thelowest unoccupied molecular orbital (LUMO) peaks.91–94
The normalized (dI/dV)/T curves depicted in the lower panelof Fig. 4(b) elucidates the recovered HOMO and LUMO peaks
positioned at approximately 1.1 eV and +0.7 eV, respectively.
These energy levels are slightly shifted from the HOMO/LUMO
positions of the original 2D BrCR array: 1.5 eV and+1.4 eV.52,54 This implies that the original HOMO–LUMO gap
of approximately 2.9 eV has been narrowed to approximately
1.8 eV, in agreement with theoretical predictions. Density ofstates (DOS) plots of the BrCR monomer and dimers in Fig. 2(gas phase) shown in ESI,† Fig. S9 suggest that all dimer
connections exhibit a narrower band gap compared to themonomer.
Fig. 4(c) and (d) depict dI/dV maps acquired at 1.1 V and+0.7 eV, corresponding to the energy positions of the HOMO
and LUMO peaks, respectively. Three strips are delineated by I,
II, and III, wherein the majority of regions exhibit uniform
HOMO levels, while local disparities are discernible in the
LUMO energy map, suggesting the presence of at least two
types of chemical bonding within the stripe.
The black dashed line in Fig. 4(b) represents the dI/dV curve
derived from the region marked by the arrow in the stripe III,aligning with the location of heightened corrugation (brightFig. 4 STS results obtained on the 2D stripe network. (a) Topographic image (20  10 nm2). (b) dI/dV spectra, corresponding to the surface LDOS,
obtained on the regular stripe (blue and red lines) and the bright spot position in the stripe (black dotted line). (c) and (d) dI/dV maps obtained on the same
area as (a) at (c) 1.1 V and (d) +0.7 V. (e) dI/dV 3D plot along the stripe no. III in (a) as a function of the energy: x-axis: distance (nm), y-axis: E  EF (eV),and z-axis: dI/dV value.

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Communicationspot), which manifests an additional peak around 1.5 eV andthe suppression of the LUMO peak. As the DFT calculated DOS
in ESI,† Fig. S9, shows, the straight and the bent 2 connections
have slightly diﬀerent HOMO and LUMO peak positions. If thebright spot with a high corrugation (B200 pm) in the strips
every B5 nm, corresponding to the length of about threeprecursors, it suggests that about 30% of the connections (ESI,†
Fig. S3) could be originated by the intermediate bent configuration, thus aligning with both DFT and experimentallyobtained results. A similar heightened appearance due to theintermediate state was observed for other on-surface
syntheses,89 which could also suggests a metal atom from thesubstrate in the covalent bonding, possibly CuBr2 (bent structures in Fig. 2).
We further investigate the variation of LDOS concerning theposition within the stripe. Fig. 4(e) illustrates a 3D plot of dI/dV
acquired along the stripe III the x-axis represents distance, they-axis denotes energy relative to the Fermi energy, and the z-axis
illustrates dI/dV values (with higher and lower values depictedin blue and red, respectively). This 3D plot suggests that the brightspot within the stripe may exhibit a locally distinct LDOS.
3.5

Initial growth of stripe formation

Although the complete 2D stripe random network was accomplished using the thermal annealing temperature of 453 K for
10 min (Fig. 1(h)), tuning the temperature by thermal heating
could control the intermediate states since the chemical reaction follows the Arrhenius equation. To investigate the initial

Nanoscale Horizons
growth of the stripes, we reduced the heating temperature from453 K to 433 K.
Fig. 5(a) and (b) show the STM results after heating at 433 K
for 480 s. In contrast to Fig. 1(h), Cu terraces in Fig. 5(a) are notcoated by stripes, and we could observe the original BrCR
monolayer array. Interestingly, the STM image magnified
inside the array showed an identical orderliness withthe original BrCR array, the same as Fig. 1(f). It is entirely
unexpected that the stripe formation did not start from thearray. Still, the stripe began to grow from Cu atomic steps, only
at descending steps (no ascending steps). This indicates thatthe onset of the stripe formation requires a Cu atom since theCu atom at the steps has a higher chance of contact with thediﬀusing BrCR molecules, leading to the intermediate stateconnection. These stripes along the steps observed in Fig. 5(a)consist of only straight and 1201 bent connections (no trident).
A height profile along the arrow in the upper image, as depictedin the lower panel of Fig. 5(a), represents the stripe formationthat occurred only at the descending steps.
We found a tiny change inside the array in one out of
62 BrCR molecules in Fig. 5(b) (marked by a dotted circle).
Here, the diﬀerence includes 30 pm higher spots with 20 pm
deeper holes, indicating one Ullmann reaction occurred
between two BrCR molecules. Interestingly, the dimer formation already produced a buckled structure instead of maintaining a flat surface. Such a buckling should not happen by
the stable straight or trident structures suggested by DFT in
Fig. 2. Namely, the intermediate state generated a non-flat

Fig. 5 (a) STM topographic images obtained at 77.8 K on BrCR/Cu(111) after the thermal annealing at 433 K for 480 s (200  100 nm2, 2.5 V, 10 pA). The
right panel denotes the height profile along the arrow. (b) Magnified STM image inside the CR array (5  3 nm2, 1.5 V, 11 pA). (c) and (d) STM topographic
images obtained on the same sample surface after the thermal annealing at 433 K for 1080 s; (c) 100  100 nm2 and (d) 30  30 nm2, 2.5 V, 10 pA. The
model shows the stripe formation started from the BrCR island edge.

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Nanoscale Horizons
molecular surface and dominated the connection from the
beginning.
Next, we maintained the sample temperature at 433 K but
kept it longer. Fig. 5(c) and (d) show STM topographic images
obtained after 1080 s annealing time. Here, many stripes
started to grow and covered the terraces while the BrCR arrayremained, and the orderliness was also maintained. So how
were the BrCR precursors supplied to grow the stripes? The hint
was shown in Fig. 5(c) where stripes grew not equivalently but
densely at local areas between the BrCR array and Cu steps.
Also, many stripes were produced from the Cu steps, andtheir length became longer. Then, one could imagine that thegrowing stripes contact with the BrCR array, more and more
BrCR connected to the stripe, making the stripe longer andscraped oﬀ the BrCR array one by one, as shown in the model inFig. 5(d). These results could indicate that the stripe formation
requires step defects on the substrate, which becomes a trigger
to form the stripe. Also, no Ullmann reaction occurred at thistemperature for the BrCR array; the step defects could have an
essential role in lowering the activation energy of the Ullmann
reaction. When a growing stripe touches the BrCR array, thestripe absorbs the BrCR precursors to change the stripe,
producing dense stripes on the surface. This could repeat until
all the BrCRs were completely switched to CR stripe polymers.
It should be noted that there are no second monolayers of the
stripe; this stripe formation is only limited to one monolayer
film. Thus, this on-surface synthesis occurred only on the Cu
surface, leading to an ultrathin one-monolayer-thickness 2D
stripe random network.

4. Conclusions
The growth of the 2D stripe random network using BrCR
precursors was eﬀectively showcased on the atomically flatand pristine Cu(111) surface. This process underwent scrutiny
employing experimental STM/STS configurations and DFT calculations. Success ensued by employing a precursor embedding
a pliable crown ether ring and adjusting the thermal annealing
temperature to enhance the prevalence of the intermediate 120degree bent structure as a primary connection.
The initiation of stripe formation was ascertained to beprompted by Cu step defects, with the intermediate state
assuming dominance from the inception of dimer formation.
As the stripe extends, it assimilates the BrCR array, elongating
itself and engendering densely patterned stripes on the surface.
This process can iterate until all BrCRs are entirely transformed
into CR stripe polymers. The pliability of the crown ether ringallows for facile bending of the stripe, contributing to therandomness of the pattern. Notably, this stripe formation is
confined to a sole monolayer film; there are no second monolayer of the stripe observed. Our empirical evidence substantiates that on-surface synthesis is not confined solely to thecreation of linearly ordered systems via rigid and planar precursors like phthalocyanines or porphyrins, but also extends tothe production of a 2D winding network using precursors

726 | Nanoscale Horiz., 2024, 9, 718–730

Communication
featuring a soft ring and intermediate state connections in
the Ullmann reaction.
The creation of a 1D polymer composed of crown ether
hoops formed on the 2D surface oﬀers promising prospects asfoundational units for molecular complexes. Moreover, it facilitates the modulation of surface-based electric and morphological properties. Given the chain’s incorporation of numerous
rings, these structures possess the capacity to encapsulate
supplementary guest materials within, engendering diverse
functionalities, such as the formation of single-atom catalysts
through the introduction of metal atoms.

Author contributions
T. K. Y. conceived and designed the project. T. K. Y., R. N., andF. N. performed all STM/STS experiments. C. H. W., Y. H. C.,
and M. H. synthesized and evaluated the BrCR molecules. P. K.
performed DFT calculations. T. K. Y, H. I., and M. H. analyzed
the data. T. K. Y. wrote the first draft, and M. H. and P. K.
corrected and added synthesis and theoretical discussion in the
manuscript. All authors discussed the results and commented
on the manuscript.

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,
the Shorai Foundation for Science and Technology, TEPCO
Memorial Foundation, Casio Science Promotion Foundation,
and Toshiaki Ogasawara Memorial Foundation. We thank Dr
Eiichi Inami for carefully reading our manuscript and fruitful
discussion. We thank Ms Ayano Yoshida for STM experimental
support.

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