Atomic-scale visualization of stepwise growth mechanism of metal-alkynyl networks on surfaces

Article abstract of DOI:10.1021/jacs.0c04311

One of the most appealing topics in the study of metal?organic networks is the growth mechanism. However, its study is still considered a significant challenge. Herein, using scanning tunneling microscopy, the growth mechanisms of metal? alkynyl networks

Full text of DOI:10.1021/jacs.0c04311

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Article
Atomic-Scale Visualization of Stepwise Growth
Mechanism of Metal-Alkynyl Networks on Surfaces
Chen-Hui Shu, Yan He, Ruo-Xi Zhang, Jian-Le Chen, An Wang, and Pei-Nian Liu
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.0c04311 • Publication Date (Web): 08 Sep 2020
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Atomic-Scale Visualization of Stepwise Growth Mechanism of
Metal-Alkynyl Networks on Surfaces
Chen-Hui Shu,† Yan He,† Ruo-Xi Zhang, Jian-Le Chen, An Wang and Pei-Nian Liu*
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Key Laboratory for Advanced Materials and Feringa Nobel Prize Scientist Joint Research Center, State Key Laboratory of
Chemical Engineering, School of Chemistry and Molecular Engineering, East China University of Science & Technology,
Meilong Road 130, Shanghai, 200237, China
KEYWORDS: Organometallic networks, Surfaces, Growth mechanism, Scanning tunneling microscopy, Density functional
theory calculations
ABSTRACT: One of the most appealing topics in the study of metal-organic networks is the growth mechanism. However, its study
is still considered a significant challenge. Herein, using scanning tunneling microscopy, the growth mechanisms of metal-alkynyl
networks on Ag(111) and Au(111) surfaces were investigated at the atomic scale. During the reaction of 1,3,5-
tris(chloroethynyl)benzene on Ag(111), honeycomb Ag-alkynyl networks formed at 393 K, and only short chain intermediates were
observed. By contrast, the same precursor formed honeycomb Au-alkynyl networks on Au(111) at 503 K. Progression annealing led
to a stepwise evolution process, in which the sequential activation of three Cl-alkynyl bonds led to the formation of dimers, zigzag
chains, and novel chiral networks as the intermediates. Moreover, density functional theory calculations indicate that chlorine atoms
are crucial in assisting the breakage of metal-alkynyl bonds to form Cl-metal-alkynyl, which guarantees the reversibility of the
break/formation equilibration as the key to forming regular large-scale organometallic networks.
INTRODUCTION
previously reported from a precursor of threefold symmetry
containing Br-alkynyl through annealing on Au(111) at 320
K.11 Similarly, the formation of honeycomb Ag-alkynyl
networks has been achieved through the activation of Br-
alkynyl bonds on Ag(111) at room temperature.12 Interestingly,
honeycomb Ag-alkynyl networks can also be prepared through
an oxygen-mediated surface reaction of H-alkynyl bonds in
1,3,5-tris(4-ethynylphenyl)benzene.13 Recently, Wu and
coworkers reported a dissymmetric reaction of a bifunctional
precursor with H-alkynyl and Br-phenyl substituents for the
construction of binodal organometallic networks, which contain
both alkynyl-Ag-alkynyl and alkynyl-Ag-phenyl nodes.14
Metal-organic networks (MONs) have attracted significant
attention recently as part of the emerging field of two-
dimensional (2D) materials.1 Furthermore, remarkable progress
has been made in the design and preparation of MONs, with
superb structural, chemical, and functional tunability.2 One of
the vital aspects in achieving novel structures and prominent
properties for MONs is the in-depth understanding of their
growth mechanism, which has thus far remained a significant
challenge.3 Taking advantage of the atomic resolution of
scanning tunneling microscopy (STM),4 MONs constructed on
well-defined surfaces can be directly visualized,5 which
provides the opportunity to investigate their growth mechanism
at the atomic scale. For example, the hierarchical assembly of
1,3,5-tricarboxylic benzoic acid and Fe atoms was reported on
Cu(100), and successive aufbau structures with increasing
complexities were observed after progression annealing.6 Later,
distinct MONs were reported using varying ratios of europium
and linkers, including four-, five-, and six-fold vertices.5a We
Because the organometallic bond strength of Ag(Au)-alkynyl
is stronger than those of the coordination bonds such as M-N or
M-O,12 we assume that the intermediates formed during the
organometallic network formation process may be more stable
and easier to identify. Moreover, the formation of regular
organometallic
networks
comprising
Ag(Au)-alkynyl
constituents requires higher temperatures, which may offer an
opportunity to monitor the growth mechanism using a
temperature progression approach.
also reported
a
multi-step nonreversible structural
transformation of MONs by increasing the surface molecular
density.7 However, because the MON formation process is
highly complex and dynamic,8 acquiring an in-depth
understanding of the stepwise growth mechanism of MONs is a
significant challenge.
Because of the higher bond strength and resulting lower
reactivity of the C-Cl bond in comparison with those of the C-
Br bond,15 we observed a sequential and stepwise activation of
the C-Cl bond during the coupling process of the CCl3 group.16
Owing to the pπ-dπ overlap between metals and alkynyl
groups, metal-alkynyl moiety demonstrates a strengthened
metal-carbon bond and an extended π-electron delocalization.9
Thus, organometallic structures containing metal-alkynyl
moieties exhibit intriguing conductive, magnetic, and optical
properties.10 The on-surface preparation of organometallic
honeycomb networks comprising Au-alkynyl units has been
Thus,
in
this
study,
we
designed
1,3,5-
tris(chloroethynyl)benzene (tCEB) (Scheme 1) as the precursor
and investigated the assembly pathways during the formation of
honeycomb Ag(Au)-alkynyl networks on Ag(111) and Au(111)
surfaces. Remarkably, these pathways were different for
Ag(111) and Au(111). Distinct transient products and regular
network phases were observed on Au(111), including chiral
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motifs and network types. The experimental observations and
density functional theory (DFT) calculations suggested that
chlorine atoms play important roles in surface chemical
conversions and are a decisive factor for forming regular metal-
alkynyl networks.
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Scheme 1. Stepwise growth mechanism of metal-alkynyl networks on Ag(111) and Au(111)
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By increasing the annealing temperature to 393 K, the
RESULTS AND DISCUSSION
irregular organometallic network transformed to a single
domain of large-scale honeycomb Ag-alkynyl networks (Fig. 1e
and Fig. S1 in the Supporting Information). The magnified
image in Fig. 1f shows the detailed structures of the honeycomb
Ag-alkynyl networks (unit cell of a = b = 21.3 ± 0.8 Å, θ = 120°),
which match well with the chemical structure fitted on it.
Detached chlorine atoms (marked with the green dashed circle)
were observed to assemble along the honeycomb networks. The
distance between the benzene rim and the center of the chlorine
atom was measured to be 2.7 ± 0.2 Å, which agrees with the
DFT-calculated distance (see Fig. S2 and S3). Moreover, the
atoms-in-molecules (AIM) analysis suggested that the bond
energy of the Cl···H interaction was -71.1 meV, which
confirmed the stabilization effect of the chlorine atoms on the
honeycomb networks (see Fig. S3). The efficient
transformation from irregular structures to honeycomb Ag-
alkynyl networks at a mild annealing temperature of 393 K
demonstrated a good reversibility of the break/formation
reactions of the Ag-alkynyl bond on the Ag(111) surface. This
facilitated the self-healing process for repairing irregular rings
that were observed in the irregular organometallic networks to
afford thermodynamically stable honeycomb networks.
Growth mechanism of honeycomb Ag-alkynyl networks on
Ag(111). The deposition of tCEB onto the surface of Ag(111)
at 150 K resulted in the formation of disordered short chain
structures (Fig. 1a). Fig. 1b shows a magnified STM image, in
which a chemical structure is fitted and the protrusions in the
chains are attributed to the Ag atoms.12,17 The measured length
(12 ± 0.5 Å) between the two benzene rings agrees with the
DFT-calculated length (12.1 Å). Therefore, in the chain
structure, two Cl-alkynyl bonds are activated, and one Cl-
alkynyl bond remains intact in the tCEB unit.
When the sample was annealed at 300 K, irregular
organometallic networks formed, as shown in Fig. 1c. A
magnified image (Fig. 1d) shows a network in which all three
Cl-alkynyl bonds of tCEB are activated, and the structural
fitting corroborates the STM results. A large-scale statistical
analysis indicated that 74% of the networks were hexagonal,
whereas the other 26% were irregular (four-, five-, and seven-
membered) rings, thereby suggesting the occurrence of kinetic
trapping during the formation of Ag-alkynyl networks at room
temperature.18 The results indicate that the formation of
organometallic networks on Ag(111) is a fast process, in which
the stepwise growth mechanism is difficult to monitor when
using the temperature progression approach.
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Figure 1. Growth process of tCEB forming honeycomb Ag-alkynyl networks on Ag(111). (a, b) Large-scale and magnified STM
images of tCEB deposited on Ag(111) at 150 K. (c, d) Large-scale and magnified STM images of irregular Ag-alkynyl networks on
Ag(111) annealed at 300 K. (e, f) Large-scale and magnified STM images of honeycomb Ag-alkynyl networks on Ag(111) annealed
at 393 K. The detached chlorine atoms are marked by the green dashed circles. Scanning parameters (a-f): U = -1.0 V, I = 0.1 nA.
Growth mechanism of honeycomb Au-alkynyl networks on
Au(111). The formation of honeycomb Au-alkynyl networks on
the surface of Au(111) may require a higher annealing
temperature owing to the lower reactivity of gold compared to
that of silver.18b Thus, the potential intermediates may be more
stable and easier to monitor with the temperature progression
approach. Next, we discuss the on-surface synthesis of large-
scale honeycomb Au-alkynyl networks, which is achieved by
the thermal treatment of tCEB on the less reactive surface of
Au(111).
dimers of tCEB. The magnified image in Fig. 2d shows that the
alkynyl groups of two tCEB units in a dimer are linked by a
gold atom. The length between the two benzene rings obtained
by STM measurement was 12 ± 0.5 Å; this value was consistent
with that obtained through the DFT calculations (12.0 Å).
When the sample was annealed at 300 K, the resulting
structures were composed of 46% dimers and 54% short zigzag
chains (Fig. 2e). The magnified image in Fig. 2f shows short
zigzag chains fitted with chemical structures. Subsequently, the
activation of the second Cl-alkynyl bond occurred, which
linked the tCEB units into zigzag chains. As indicated by the
white dashed circle, two Cl-Au-alkynyl substitutes assemble in
the vicinity position, thereby suggesting that a further reaction
might occur between the two Cl-Au-alkynyl substitutes to form
a longer zigzag chain. It is worth noting that the deposition of
tCEB on the Au(111) surface at 150 K had no obvious impact
on the Au(111) herringbone reconstruction. However, after
annealing the sample at 300 K, the Au(111) herringbone
reconstruction was altered, as shown in Fig. S4. This
phenomenon may be attributed to the activation of Cl-alkynyl,
which may lift the Au atoms from the Au(111) herringbone
reconstruction.20 When the sample was annealed to 393 K, the
tCEB units were linked to form significantly longer zigzag
chains, which self-assembled in a staggered arrangement to
form a compact structure, as shown in Fig. 2g. The structural
fitting in the magnified image (Fig. 2h) illustrates that the
zigzag chains are Au-alkynyl complexes, in which two Cl-
alkynyl bonds are activated and only one Cl-alkynyl bond
remains intact.
We deposited a sub-monolayer of tCEB on Au(111) at 150
K, which resulted in the formation of two phases. A close
packing of tCEB monomers and dimers is shown in Fig. 2a, in
which unreacted tCEB molecules can be found (as indicated by
the white dashed circle). As shown in image (1) of Fig. 2b, the
length from the benzene center to the Cl end was measured as
5.6 ± 0.2 Å, which was consistent with the distance obtained by
the DFT calculations (5.7 Å). It is worth noting that the
unreacted tCEB had a low concentration of less than 5%,
suggesting that the Cl-alkynyl bond can also be efficiently
activated on the Au(111) surface at 150 K. Image (2) of Fig. 2b
shows a tCEB in which one Cl-alkynyl is activated to form Cl-
Au-alkynyl. The Cl-Au-alkynyl moiety is evidently different
from Cl-alkynyl; this is because the Au atom appears as a bright
round protrusion, whereas the adjacent C≡C group becomes
dim.11,19 The distance between the benzene center and the Cl end
in the reacted tCEB was measured as 8.4 ± 0.2 Å, which was
consistent with the distance obtained through the DFT
calculations (8.3 Å). Fig. 2c shows another phase observed in
the same sample at 150 K, which consists of self-assembled
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Figure 2. Growth process of tCEB on Au(111) surface. (a, b) Large-scale and magnified STM images containing tCEB monomers
on Au(111) at 150 K. (c, d) Large-scale and magnified STM images of dimers on Au(111) at 150 K. (e, f) Large-scale and magnified
STM images of short zigzag chains on Au(111) annealed at 300 K. (g, h) Large-scale and magnified STM images of long zigzag
chains on Au(111) annealed at 393 K. The detached chlorine atoms were marked with the green dashed circles. Scanning parameters
(a), (c–h): U = -1.0 V, I = 0.1 nA; b, U = -0.5 V, I = 0.1 nA.
When the sample was annealed at 423 K, a part of the ordered
self-assembled zigzag chains broke down to form a mixture of
hexagons and chain structures (Fig. 3a). As shown in the
magnified image in Fig. 3b, a hexagonal structure is composed
of six tCEB units and six Au atoms as linkers. It is worth noting
that Cl-alkynyl (white dashed circle) and Cl-Au-alkynyl (red
dashed circle) substituents coexisted at this annealing
temperature.
It is worth noting that the honeycomb Au-alkynyl networks
can also be observed at the annealing temperature of 453 K, and
the ratio of the chiral Au-alkynyl networks to the honeycomb
Au-alkynyl networks is 7:1. When the sample was further
annealed to 473 K, many organometallic fragments could be
found in the gap between the blue and white dashed lines (Fig.
3e), thereby suggesting that the chiral networks might be
fragmented and then transform into the honeycomb networks.
The magnified image in Fig. 3f shows detailed structures of the
fragments and the newly formed honeycomb Au-alkynyl
networks. A statistical analysis of the STM data showed that the
ratio of the chiral Au-alkynyl networks to the honeycomb Au-
alkynyl networks was increased to 9:7 at 473 K. To clarify the
driving force responsible for the transformation of the chiral
networks to the honeycomb networks, we conducted DFT
calculations to compare the energies of the nonlinear alkynyl-
Au-alkynyl moiety with a distortion angle of 28° and the linear
alkynyl-Au-alkynyl moiety (Fig. S5 in the Supporting
Information). The results demonstrated that the linear moiety
was more stable than the nonlinear moiety, and that the
stabilization energy was 0.18 eV.
Upon annealing the sample at 453 K, the activation of three
Cl-alkynyl bonds was completed and the hexagons were
unexpectedly connected to form novel chiral Au-alkynyl
networks with a unit cell of a = b = 42.0 ± 0.8 Å, θ = 120° (Fig.
3c and 3d). As shown in Fig. 3d, this interesting arrangement
can be regarded as a regular hexagon core with six
circumambient-distorted hexagons. Note that two enantiomeric
networks, which have been indicated as clockwise (CW) and
counterclockwise (CCW), coexist in the sample (Fig. 3c). A
careful inspection of the chiral Au-alkynyl networks reveals
that every set of three regular hexagons is linked by a tCEB unit
that has been rotated at an angle of 28° (Fig. 3d). In the sample
annealed at 423 K, single hexagons are prevalent, whereas a
single molecule with three Cl-Au-alkynyl substituents (see the
inset in image 2 of Fig. 3i) can also be found. Therefore, as
depicted in Fig. 3i, we propose that the formation of the chiral
Au-alkynyl networks may proceed through the following
processes: the assembly of three hexagons and a tCEB linker
(image 2), the subsequent rotation of the tCEB linker (image 3)
and its reaction with three hexagons (image 4). The DFT
calculations of the structures shown in images 3 and 4 indicate
that the Au and benzene rings are located in the stable bridge
and hollow sites of the Au(111) surface, respectively.
Afterwards, the resulting structure (image 5) may propagate to
afford chiral Au-alkynyl networks (image 6).
When the sample was further annealed to 503 K, the chiral
Au-alkynyl networks disappeared and the more stable
honeycomb Au-alkynyl networks were dominant on the surface
as the thermodynamic product (Fig. 3g). Note that the
honeycomb Au-alkynyl networks formed on the Au(111)
surface represent multiple domains (20–55 nm2, Fig. S6), which
is in contrast to the single domain of honeycomb Ag-alkynyl
networks formed on the Ag(111) surface. This can be attributed
to the higher stability of the Au-alkynyl bond formed on the
Au(111) surface, which leads to a relatively poor reversibility
of the break/formation equilibration. The magnified STM
image presented in Fig. 3h shows the detailed structure of the
honeycomb Au-alkynyl networks with a unit cell of a = b = 21.3
± 0.8 Å, θ = 120°, which is in agreement with that obtained in a
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previous study.11 Similar to the case of honeycomb Ag-alkynyl
networks, the detached chlorine atoms were observed to
assemble along the intermediate structures and honeycomb
networks on Au (111). The stabilization effect of chlorine atoms
on the structures was revealed through AIM analysis. The
Cl···H interactions were detected, and their bond energies
varied from -57.0 to -90.1 meV (Fig. S7).
We also tried a higher annealing temperature to convert
honeycomb Au-alkynyl networks into covalent networks. After
annealing at 563 K, the desorption of chlorine atoms occurred,
and the organometallic networks converted into complicated
and irregular structures in which only small patches of covalent
networks could be observed (see Fig. S8). Thus, high-
temperature annealing is not a good way to convert current
organometallic networks into 2D covalent networks.
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Figure 3. Growth process forming honeycomb Au-alkynyl networks on Au(111). (a, b) Large-scale and magnified STM images on
Au(111) annealed at 423 K. (c, d) Large-scale and magnified STM images of chiral Au-alkynyl networks on Au(111) annealed at
453 K. (e, f) Large-scale and magnified STM images on Au(111) annealed at 473 K. (g, h) Large-scale and magnified STM images
of honeycomb Au-alkynyl networks on Au(111) annealed at 503 K. (i) Proposed formation mechanism of chiral Au-alkynyl networks.
The detached chlorine atoms were marked with the green dashed circles. Scanning parameters (a–i): U = -1.0 V, I = 0.1 nA.
DFT calculations for the breakage of metal-alkynyl bonds.
The reversibility of the break/formation of metal-alkynyl bonds
plays a key role in the formation of thermodynamic products on
metal surfaces. Therefore, DFT calculations were conducted on
the Au(111) and Ag(111) surfaces to evaluate the breaking of
the metal-alkynyl bond using a simplified molecular model,
(chloroethynyl)benzene (CEB). The reaction barrier and
reaction energy required to break the Ag-alkynyl bond on the
Ag(111) surface were calculated as 1.02 and 0.68 eV,
respectively (Fig. 4a, blue line). By contrast, breaking the Au-
alkynyl bond on the Au(111) surface requires a high barrier
energy of 1.43 eV, and the reaction is highly endothermic with
a large energy consumption of 1.24 eV (Fig. 4a, brown line).
The high reaction barrier suggests that breaking the Au-alkynyl
bond on Au(111) would be rather difficult. However, this is
contradictory to the experimental fact that the reversibility of
the breaking/forming of the Au-alkynyl bond is rather good for
forming large-scale chiral and honeycomb Au-alkynyl
networks at 453 and 503 K, respectively.
Numerous detached chlorine atoms were observed to
assemble alongside the Au-alkynyl bond during the
experiments, and the complexes incorporating Cl-Au-alkynyl
were frequently detected during the growth process on the
Au(111) surface. Therefore, we suspected that the chlorine
atoms may assist the breaking of the Au-alkynyl bond by
forming Cl-Au-alkynyl complexes. We conducted DFT
calculations according to this assumption. The barrier and
reaction energy decreased to 0.90 and 0.25 eV, respectively, for
the breaking of the Au-alkynyl bond (Fig. 4b, brown line).
These results are in accordance with our experimental findings.
Furthermore, the reaction barrier and reaction energy required
to break the Ag-alkynyl bond with the assistance of chlorine
atoms were also reduced to 0.77 and 0.25 eV (Fig. 4b, blue line),
respectively. This result is also in good agreement with the
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Figure 4. Breaking of metal-alkynyl bonds on Ag(111) and Au(111) surfaces. a) Breaking of metal-alkynyl bond without the
assistance of a chlorine atom. b) Breaking of metal-alkynyl bond with the assistance of a chlorine atom. The brown and blue lines
represent the reactions occurring on Au(111) and Ag(111) surfaces, respectively. Top and side views of the initial state (IS), transition
state (TS), and final state (FS) are for the reaction on Au(111).
CONCLUSION
ACKNOWLEDGMENT
In summary, the stepwise growth mechanism of organometallic
This work was supported by the National Natural Science
metal-alkynyl networks, utilizing the tCEB precursor with a
Foundation of China (Nos. 21421004, 21925201, 91845110 and
threefold symmetry, was visualized at the atomic scale by
21672059), Shanghai Municipal Science and Technology Major
progression annealing. On Ag(111), the honeycomb Ag-alkynyl
networks formed at 393 K and only short-chain intermediates
were observed during the formation process. On Au(111),
honeycomb Au-alkynyl networks were formed at 503 K, and
the sequential activation of three Cl-alkynyl bonds led to the
generation of intermediates such as dimers, zigzag chains, and
novel chiral Au-alkynyl networks. By combining the
experimental observations and DFT calculations, the formation
mechanism of the novel chiral Au-alkynyl networks is
elucidated along with the driving force that forms honeycomb
Au-alkynyl networks. In addition, the DFT calculations indicate
that chlorine atoms assist the breakage of metal-alkynyl bonds,
reduce the reaction barrier, and stabilize the networks on
Au(111) and Ag(111). These results are coherent with the large-
scale growth of regular metal-alkynyl networks and the
stepwise formation process on Au(111). Furthermore, our
findings provide important new insights into the growth
mechanism and dynamics of organometallic networks, which
may lead to novel approaches to realize high-quality 2D
networks with superb structural, chemical, and functional
tunability.
Project (Grant No.2018SHZDZX03), the Programme of
Introducing Talents of Discipline to Universities (B16017),
Program of the Shanghai Committee of Sci. & Tech. (Project No.
18520760700) and the Program for Eastern Scholar Distinguished
Professor. We thank Dr. Shao-Ze Zhang in Kunming University of
Science and Technology for his help in AIM analysis. We thank
Research Center of Analysis and Test of East China University of
Science and Technology for the help on the characterization.
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ASSOCIATED CONTENT
Supporting Information. Detailed descriptions of experimental
and theoretical procedures, and additional STM and DFT
calculation results. This material is available free of charge via the
Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*liupn@ecust.edu.cn
Author Contributions
†These two authors contributed equally to this work.
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