Steering Surface Reaction at Specific Sites with Self-Assembly

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Steering Surface Reaction at Specific Sites with Self-Assembly Strategy Xiong Zhou, Fabian Bebensee, Mingmei Yang, Regine Bebensee, Fang Cheng, Yang He, Qian Shen, Jian Shang, Zhirong Liu, Flemming Besenbacher, Trolle R. Linderoth, and Kai Wu ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.7b04900 • Publication Date (Web): 15 Aug 2017 Downloaded from http://pubs.acs.org on August 15, 2017

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Steering Surface Reaction at Specific Sites with Self-Assembly Strategy Xiong Zhou, † Fabian Bebensee, ‡ Mingmei Yang, † Regine Bebensee, ‡ Fang Cheng, † Yang He, † Qian Shen, † Jian Shang, † Zhirong Liu, † Flemming Besenbacher, ‡ Trolle R. Linderoth, ‡,* Kai Wu†,* †

BNLMS, College of Chemistry and Molecular Engineering, Peking University, Beijing 100871,

China, ‡Sino-Danish Center for Molecular Nanostructures on Surfaces and Interdisciplinary Nanoscience Center, Aarhus University, DK-8000 Aarhus C, Denmark. E-mails: [email protected] (Kai Wu), [email protected] (Trolle R. Linderoth).

KEYWORDS: steering surface reaction · self-assembly strategy · active site · ullmann coupling·scanning tunneling microscopy

ABSTRACT To discern the catalytic activity of different active sites, self-assembly strategy is applied to confine the involved species which are “attached” to specific surface sites. The employed probe reaction system is the Ullmann coupling of 4-bromobiphenyl, C6H5C6H4Br, on atomically flat Ag(111) surface, which is explored by combined scanning tunneling microscopy, synchrotron X-ray photoelectron spectroscopy and density functional theory calculations. The catalytic cycle involves the detachment of the Br atom from the initial reactant to form an

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organometallic intermediate, C6H5C6H4AgC6H4C6H5, which subsequently self-assembles with its central Ag atom residing either on two-fold bridge or three-fold hollow sites at full coverage. The hollow site turns out to be catalytically more active than the bridge one, allowing us to achieve site-steered reaction control from the intermediate to the final coupling product, pquaterphenyl, at 390 and 410 K, respectively.


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Precisely discerning catalytic sites and associating them to specific chemical reactions are two immensely difficult tasks which are, however, prerequisites to fundamental understanding of heterogeneous catalysis and the ultimate control of catalytic reactions. Many previous studies have shown that low-coordinated surface defect sites such as steps,1-3 kinks,4 corners and edges,57

elbows of reconstructed Au(111)8 or even vacancies9 and adatoms10 can function as active sites

for a wide variety of catalytic reactions. On ordered surfaces, some special sites, i.e. the “C7” centers on Fe(111) and Fe(211) for ammonia synthesis11,12 may exhibit special catalytic activity as suggested by various indirect experimental measurements13-15 and theoretical calculations.16,17 Hollow, bridge and top sites on atomically flat surfaces also show different adsorption or activation ability.18-21 Direct observation of the catalytic difference between those sites on atomically flat surfaces remains a great challenge. This challenge is further compounded by the fact that most previously studied catalytic reactions involve small molecules such as NH3, CO and O2 which are extremely mobile and therefore cannot be directly detected by scanning tunneling microscopy (STM) under reaction conditions. In this work, we instead choose a target molecule that can be “fixed” at reactive sites through an assembly confinement effect so that sub-molecular resolution images can be obtained with STM before and after the reaction. Substrate-induced confinement effect has been already proved effective to tune surface reactions.22-25 Besides, the confinement effect may also be created by the mobile reacting molecules themselves with the assembly strategy.26,27 Recently, the self-assembly strategy were successfully applied to steer the regio-selectivity of the dehydrocyclization reaction and suppress defluorinated coupling of 4,4’-Bis(2,6-difluoropyridin-4-yl)-1,1´:4’,1”-terphenyl (BDFPTP) molecules.26 It’s anticipated that the molecular assembly may also invoke two-dimensional assembly-confinement effect to “pin down” the target molecules around active sites. Though the


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reaction molecules may still be able to slightly diffuse off from the active site, their reaction kinteics can be profoundly different, which provides us a possible method to steer the surface reaction sites and directly compare their activities. Ullmann coupling reaction is of great significance in industry and nanotechnology and has therefore attracted numerous studies28-33 in the past century. It is proposed that an organometallic intermediate be involved in the catalytic reaction.34-41 In this study, the activities of hollow and bridge sites on the Ag(111) surface for the Ullmann coupling of aryl halides, i.e., 4bromobiphenyl (C6H5C6H4Br, denoted as BBP, Fig. 1a) have been extensively explored. Combined STM and synchrotron X-ray photoelectron spectroscopy (XPS) experiements, together with density functional theory (DFT) calculations, show that the aforementioned catalytic sites on atomically flat Ag(111) can be unequivocally identified. The difference in the activities of these two sites can be further exploited to steer the thermally triggered surface reactions in a stepwise manner.

RESULTS AND DISCUSSION When deposited on Ag(111) at low temperatures (STM: 120 K; XPS: 110 K, see Experimental Methods for more details for the molecular deposition), the BBP molecules form various molecular clusters, as shown by the STM image in Fig. 1b (all STM images hereafter adopt the same color scale bar in Fig. 1). 42,43 The clusters involve two distinct arrangements of the BBP molecules: head-to-head clovers and head-to-tail triangles in which the molecules are held together by halogen and weak hydrogen bonds.44-46 Corresponding XPS measurements (Fig. 2a) show peaks originating from C 1s and Br 3d. The XPS peak in the C 1s region can be


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fitted with a majority contribution at a binding energy (BE) of 284.5 eV, attributed to benzyl ring carbon not directly bonded to Br, and a minority contribution at a higher BE of 285.5 eV, ascribed to C-Br. These two contributions have an 11:1 intensity ratio, in good agreement with the stoichiometry of BBP. The Br 3d BE features (3d5/2 = 71.7 eV and 3d3/2 = 70.6 eV) indicate no abstraction of Br from the molecules. The BBP molecules are thus molecularly adsorbed at the surface.

Figure 1. Sequential evolution of the Ullmann coupling of BBP molecules on Ag(111) via thermal activation. V = 0.2 V, I = 0.1 nA. (a) The catalytic reaction process. (b) Image of the intact BBP molecules adsorbed on Ag(111) at 120 K. (c) Assembly structure of Ag-COIs formed


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by RT annealing. The variation of the brightness of the central parts for the intermediates shows the co-existence of two distinct surface species, H type (hollow sites, marked by black arrows) and B type (bridge sites, marked by red arrows) intermediates. (d) Some H type Ag-COIs (black elliptical circles) converted into the final QP product at 390 K. (e) All H type Ag-COIs converted into the QP product at 400 K and all left B type Ag-COIs remained unreacted. (f) Most B type Ag-COIs, except for those highlighted by red elliptical circles, converted into QP at 410 K. (g) All Ag-COIs converted into the product QP at 420 K. Black arrows indicate the cracks generated after the completion of the coupling reaction on Ag(111). Inset: STM image of an enlarged area. Black dash lines indicate the molecular axis orientations of the corresponding species.

When annealed at mild temperatures (STM: 300 K for extended time; XPS: 323 K for 10 minutes), a new assembly structure evolves (Fig. 1c) where each entitiy is in the shape of threekernel-peanut with a length of ~ 2.1 nm, about twice as long as the intact BBP molecule (~ 1.0 nm), and a distinct bright protrusion appears at the center of the entity. Corresponding Br 3d XPS measurement shows a new dominating doublet peak at lower BE (3d5/2 = 69.0 eV and 3d3/2 = 68.0 eV), characteristic of AgBr,47 indicating that Br is detached from the BBP molecule and bound to the Ag substrate (Fig. 2b).


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Figure 2. XPS data of the corresponding Br 3d (left panel) and C 1s (right panel) features of the BBP reactant molecularly adsorbed on Ag(111) single crystal at 110 K at full coverage (a). The Ag-COI was formed at 323 K (b) and the final QP product was formed at 483 K (c).

Under proper scanning conditions, the detached Br atoms could be clearly imaged between the peanut-shaped entities (see Figs. S1b and S1c). In the C1s XPS, the main peak significantly shifts towards lower BE, as compared with the situation prior to annealing, and displays a new shoulder at its low BE side. Qualitatively, these changes in BE indicate that biphenyl moieties, C6H5C6H4-, generated by the Br detachment instead couple to Ag atoms which are considerably less electronegative than Br, and hence can donate electrons to cause a downshift in BE. The increase of the work function for the sample may also cause the downshift of the BE for C1s in this reaction step,48 however, the BE variation for C1s is likely determined by the electronegativitiy of its connecting species like Br or Ag in our experiments because the BE


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shifts upward during the formation of the QP. The peanut-shaped entities are therefore attributed to Ag-coordinated organometallic intermediates (denoted as Ag-COI), C6H5C6H4AgC6H4C6H5. This assignment is supported by the C1s XPS peak which can be fitted with components at a BE of 284.0 eV (ascribed to C in the phenyl rings not attached to hetero-atoms) and a small feature at 283.0 eV (ascribed to C-Ag). Similar organometallic intermediate has been previously proposed

for

the

Ullmann

coupling

reaction

of

4,

4’’-dibromo-p-terphenyl,

BrC6H5C6H4C6H5Br.35 To prove that the bright central protrusion is indeed an Ag atom, we targeted one intermediate by the STM tip and applied a sudden voltage pulse. As shown in Fig. 3, this manipulation resulted in formation of three fragments on the surface (three items in the right part), one Ag atom and two identical biphenyls in close proximity. The knockoff Ag atom (top-right Ag atom, marked K-Ag, in Fig. 3) appeared identical to the one dropped by tender contact of an Ag tip with the Ag substrate (the central Ag atom, marked T-Ag). This provides direct experimental evidence to confirm the existence of the organometallic intermediate.

Figure 3. STM images (0.5 V, 0.1 nA) of two intact Ag-COIs (left) and one Ag-COI broken into three pieces (right) after a sudden voltage pulse (3 V, 1 s) on the central Ag atom. The detached Br atom near the top-left Ag-COI is clearly visible. The knockoff Ag atom (noted as K-Ag) is the same as the one picked from the Ag(111) substrate and dropped onto the surface again by the STM tip (noted as T-Ag). The two broken C6H5C6H4- groups stay near the K-Ag atom.


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The structural unit cell for the Ag-COIs (Figs. S1b and S1c) contains two Ag-COI complexes and four Br atoms, in agreement with the stoichiometry of the deposited BBP molecules. The unit cell parameters are: a = 1.85 ± 0.05 nm, b = 1.76 ± 0.05 nm and θ = 85 ± 4o (Fig. 6a), corresponding to a = |7c1+6c2|=1.89 nm, b = |3c1+4c2|=1.76 nm and θ = 87.1o (Fig. 6b), where c1 and c2 are unit vectors of the unit cell for the Ag(111) surface lattice.

Figure 4. Determination of the adsorption sites of the intermediates on Ag(111). (a) STM image of an area showing the co-existence of a densely packed Ag-COI island and sparse Ag-COIs nearby (0.2 V, 0.1 nA). (b) Close-up STM image of the white square area in (a) showing two types of differently oriented Ag-COIs (0.1 V, 0.4 nA). (c) STM image of the same area as in (b) clearly showing the Ag(111) lattice near the Ag-COI island (-0.2V, 3 nA). (d) Superposition of the bottom Ag-COI island in (b) and the top Ag lattice in (c) separated by the light blue dash line.


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The white grid superposed indicates the substrate lattice deduced from the Ag atom image in the upper part. Two distinct adsorption sites can now be clearly distinguished, i.e. B type Ag-COI located at the bridge sites (with bright central protrusion) and H type Ag-COI located at the hollow sites (with dim central protrusion). Black solid and dash lines indicate the underlying Ag lattice and molecule axis orientations, respectively. The hollow and bridge sites are highlighted by blue dots representing the Ag atoms.

A close look at the STM images in Fig. 1c reveals that the Ag-COIs systematically exhibit two different brightness levels of the central protrusion. Interestingly, these two species organize into arrays according to the brightness of the central protrusion such that we can distinguish B arrays (red arrows) and H arrays (black arrows) in Fig. 1c where the B species has a larger apparent height. To precisely determine the adsorption sites with an atomic resolution for the Ag-COIs, the coverage of the reactant BBP is controlled to be about 0.8 monolayer so that both the surface intermediate island and its nearby Ag substrate could be simultaneously imaged and directly compared on the same terrace. We first spot an area where a densely packed Ag-COI island and nearby sparse Ag-COIs co-exist (Fig. 4a). By controlling the scanning conditions, we are able to clearly observe either the differently oriented Ag-COIs in the molecular island (Figs. 4b and 1c) and at the periphery of the island or the periodically arranged Ag atoms in the Ag(111) substrate (Fig. 4c) in the same area. A simple superposition of both images (Figs. 4b and 4c) gives Fig. 4d. By extending the surface Ag lattice across the Ag-COI island (white grid in Fig. 4d), its immediately noticed that the central Ag atoms in the H species are positioned at three-fold hollow sites (formed by the three underlying Ag atoms in triangular arrangement, as indicated by blue dots) and the B species, at two-fold bridge sites (formed by two nearest-neighbor Ag


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atoms), respectively. Lineprofiles over these two types of species (Fig. 5a) quantify the apparent height difference to be 0.05±0.02 Å. A proposed model is given later on. This explanation for the apparent height difference finds further convincing evidence from STM manipulation experiments of the Ag-COI, as depicted in Fig. 5b. The protrusion of the intermediate (the bottom one) in the image lights up immediately after its central Ag atom is re-positioned from hollow to bridge site and the manipulated molecule orients against the un-manipulated one (the top one) at the angle, 46o, exactly identical to that shown in Fig. 1c.

Figure 5. (a) Line profiles of the Ag-COIs marked along the red and black lines in Fig. 4b for the bridge and hollow sites, respectively. (b) STM tip manipulation of a Ag-COI from the hollow site to the bridge site, demonstrating the site-dependence of the central Ag protrusion (0.5 V, 0.1 nA). The superimposed white grid represents the underlying Ag(111) surface lattice according to real experimental measurements. The hollow and bridge sites are highlighted by


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blue dots representing the Ag atoms. (c) Adsorption energy of the Ag-COIs at different sites on Ag(111) surface as a function of the angle between the long molecular axis of the intermediate and the underlying Ag lattice along either of the three equivalent surface directions. (d) Optimized perspective and top-view models of the Ag-COIs adsorbed at the hollow and bridge sites on Ag(111).

To examine the adsorption energy of the Ag-COI on the Ag(111) surface, DFT calculations are performed with the central Ag atom located on top, bridge and hollow sites (Fig. 5c). The top site possesses the highest adsorption energy (0.26 eV higher with respect to the hollow site). These results support the assignment of the two adsorption states to the hollow and bridge sites.31 The calculated heights of the central Ag atoms are 2.496 Å (bridge) and 2.443 Å (hollow), respectively. This is in agreement with the detected difference in the apparent height, 0.05 Å. Moreover, at the hollow site the adsorption angle between the Ag-COI and underlying Ag lattice is 4o while at bridge site it is 10o, in agreement with the DFT calculations.


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Figure 6. Evolution of the surface species on Ag(111) and their correspondingly proposed models during the reaction process. (a) STM image of two types of differently oriented Ag-COIs (0.5 V, 0.1 nA). Unit cell (yellow parallelogram) parameters: a = 1.85 ± 0.05 nm, b = 1.76 ± 0.05 nm and θ = 85 ± 4o. (b) Corresponding unit cell model for the structure in (a) with parameters: a = |7c1 + 6c2| = 1.89 nm, b = |3c1 + 4c2|= 1.76 nm and θ = 87.1o. Ag atoms forming the hollow and bridge sties to anchor the central Ag atoms in the Ag-COIs are marked in yellow to enhance visibility. (c) All H type Ag-COIs converted into QP while all B type Ag-COIs remain intact on Ag(111). Unit cell parameters: a = 1.83 ± 0.05 nm, b = 1.74 ± 0.05 nm and θ = 83 ± 4o. (d) Proposed model for the unit cell in (c). (e) All Ag-COIs converted into the final product, QP. Since the QP molecule is shorter than Ag-COI, the unit cell parameters of the QP structure changed into a = 2.00 ± 0.05 nm, b =1.55 ± 0.05 nm and θ = 90 ± 2o. (f) Proposed model for (e) with unit cell parameters: a = |7c1| = 2.02 nm, b = |3c1 + 6c2| = 1.50 nm and θ = 90o,


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in good agreement with the observed ones shown in (e). Black solid and dash lines indicate the underlying Ag lattice and molecule axis orientations, respectively.

Ther treatment of the sample at 390 K for 5 minutes caused some of the H type intermediates to be transformed into oval-shaped molecules (~ 1.8 nm in length) without a central protrusion (highlighted by black ellipses in Fig. 1d, ~ 40% H type intermediates being transformed). We ascribe this change to elimination of the central Ag atom in the Ag-COI on the hollow sites to form the final p-quaterphenyl product, C6H5C6H4C6H4C6H5 (abbreviated as QP). The on-surface generated QP species form an angle of 50o, an increase of about 4o, against the intact B type intermediate in neighboring rows. Further annealing to 400 K for 5 minutes leads to that virtually all H type Ag-COIs on the hollow sites transform into the final QP products (Fig. 1e), resulting in alternating rows, as indicated by black and red arrows (~2% B type intermediates transformed). The co-existence of the Ag-COIs on bridge sites and the generated QP products on the hollow sites at the atomically flat Ag(111) surface has never been experimentally observed before. A temperature elevation up to 410 K for 5 minutes causes the majority of the B type Ag-COIs on the bridge sites to transform into the QP molecules, as shown in Fig. 1f where some unreacted Ag-COIs are marked by the red ellipses (~ 70% B type intermediates transformed). The produced QP molecules form an angle of 60o in neighboring rows. Final heating to 420 K for 5 minutes results in complete conversion of all Ag-COIs into the QP products. In the corresponding XPS measurements (Fig. 2c), the C 1s peak remarkably narrows down and loses its shoulder at low BE, indicating a uniform chemical state for the C atoms. The peak position


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(BE 284.1 eV) is in between the features for BBP and Ag-COI in which the phenyl groups are connected to electronegative (Br) and electropositive (Ag) species, respectively. As a final control experiment, commercially available QP molecules are deposited onto Ag(111) for comparison (Fig. S2d), resulting in assembly structures with the same STM appearance as for the on-surface generated QP molecules, except for the cracks in the monolayer of the latter (which is discussed later on). We therefore conclude that the Ullmann coupling reaction has completed and attribute the oval features observed in Figs. 1d-1g to the final QP molecules. Annealing up to 500 K leads to that the QP molecules begin to desorb from the surface. The experimental observations described above strongly suggest that the conversion from AgCOI to QP be site-steered with a reaction rate that depends on the adsorption site for the central Ag atom in the Ag-COI. If the molecules became rather mobile at the reaction temperature of 390 K such that the mixed structure in Fig. 1e is formed by re-assembly upon cooling down, one would expect the Ag-COIs to occupy the most preferred adsorption sites, the hollow ones, according to the calculations. In our experiment, however, the Ag-COIs mainly sit at the bridge sites (red arrows). The main reason lies in that the molecule mobility is largely suppressed by the surface spatial “confinement” of the molecules in the intermediate assembly, and the in-plane rotation of the species is restricted along a specific direction.27 To verify that the molecules are indeed “attached” around the assumed reaction sites, STM images of the Ag-COIs are recorded at room temperature, i.e., the reaction temperature for the formation of the Ag-COIs (Fig. S3). Due to thermally enhanced drift and molecular vibrations at surface, molecules in the image becomes fuzzy, compared with those in Fig. 1c imaged at low


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temperatures. However, the intermediate packing patterns remain the same at both room and low temperatures. In order to achieve sub-molecular resolution, our STM imaging is acquired at low temperatures after the sample is swiftly cooled down from high temperatures where the Ullmann coupling reaction takes place. If the Ag-COIs were not turned into the coupling products, QP, on specific sites, then a randomly mixed structure rather than the alternately packed one (Fig. 1e) of the Ag-COI and QP species should have been experimentally observed by STM because a sudden temperature quench of the sample should freeze most surface species in position. This also indicates that the QP products are in situ generated by the Ag-COIs sitting on the hollow sites. In addtion, large-scale STM images of the QP herringbone-like structure due to on-surface reaction depict irregular cracks, as marked by the black arrows in Fig. 1 g (large-scale images of Figs. 1c and 1e being given in Fig. S2). These cracks are not present in the control experiment using the commercial QP molecules (Fig. S2). The detailed structural information provided in Figs. 6a, 6c and 6e show that the superstructure unit cells for the Ag-COI and QP structures are commensurate (see the corresponding models in Figs. 6b, 6d and 6f). The unit cell for the final QP structure is slightly smaller than that for the Ag-COI. The changes of the angle between adjacent molecules support these models. The cracks in the final QP monolayer are thus formed by release of the yield stress which is caused by the shrinkage of the QP molecules after completion of the catalytic reaction on Ag(111). If the product molecules were free to diffuse on the surface, these structural defects would have been annealed out. The observed cracks therefore provide further evidence that the QP product is indeed generated on site.


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Figure 7. Large-area STM image of randomly appeared QP (,arked by light blue elliptical circles) converted from the C6H5C6H4CuC6H4C6H5 intermediates (peanut-shaped features) on Cu(111) at 450 K (V= 0.2 V, I= 0.1 nA). This is completely different from cases shown in Figs. 1d and 1e on Ag(111) where the QP products are initially generated from the Ag-COIs sitting on the bridge sites.

Finally, due to the different lattice parameters of the Cu(111) substrate, though the intermediate packing mode on Cu(111) (Fig. 7) looks similar to that on Ag(111) (Fig. 1c), random occurrence of the QP formation at surface is routinely observed. This means that the steered surface reactions at active sites on Ag(111) are controlled in synergy by the sizes of the BBP, Ag-COIs and QP species involved, the underlyging substrate lattice, and the self-assembly strategy as well.


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CONCLUSIONS In summary, the Ullmann coupling reaction of the BBP molecules on Ag(111) undergoes a stepwise catalytic procedure as the reaction temperature elevates. In specific, the molecularly adsorbed BBP reactant detaches the Br atom to form the Ag-COI whose central Ag atom resides either on the bridge or hollow site at full coverage, as steered by the self-assembly strategy. The hollow site is catalytically more active than the bridge one, enabling us to achieve site-steered reaction from the Ag-COI to the final coupling product QP at 390 and 410 K, respectively. This study demonstrates that the self-assembly strategy can be well employed to confine reaction species around the active sites even on an atomically flat surface, allowing to steer and observe surface reactons on specific sites at the sub-molecular level and advancing our understanding of surface chemistry and catalysis.

EXPERIMENTAL METHODS STM experiments. The STM images and STS data were acquired on a Unisoku ultra-high vacuum (UHV) low-temperature STM. Single crystal- substrates were cleaned by cycles of Ar+ sputtering and then annealed at about 500 oC. The flatness and cleanness of the substrates were verified by STM imaging. Most STM experiments were performed by using a W tip which was prepared by electrochemical corrosion in 2 mol/L NaOH solution, except for the experiment in Fig. 2b where a Ag tip was employed and fabricated by electrochemical corrosion in mixed solution with a volume ratio, 25%~28% aqueous ammonia: pure ethyl alcohol: 3% hydrogen peroxide solution=1: 1: 1.


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BBP was bought from Alfa Aesar (≥98% purity) and dosed onto the substrate from a selfmade Ta boat at room temperature without further purification. In order to avoid undesired dosing, the Ta boat was kept in an individually pumped compartment separated by a gate valve from the main chamber. The coverage was controlled by the dosing time. XPS measurements. The XPS experiments were performed at the bending magnet beamline SX-700 at the ASTRID storage ring. The UHV chamber with a base pressure of about 2×1010

mbar, equipped with a ion-sputtering gun and sample heating facilities to clean the Ag(111)

substrate. Clean samples were obtained after repeated cycles of sputtering with Ar+ ions at a kinetic energy of 1.5 keV for 20 minutes (sample current ≈ 8 µA) and annealing to 823 K for 10 minutes. The cleaning cycles were repeated until no carbon and bromine contaminations could be detected in XPS. BBP was bought from ACROS (≥99% purity) and dosed onto the Ag(111) substrate from a quartz crucible at room temperature which was quite similar to the procedure used in the STM chamber. All spectra were collected at normal emission with the X-rays impinging onto the sample at an angle of 45° with respect to the plane of the sample. Each Br 3d spectrum was obtained by adding up 10 sweeps of the electron energy range with a step size of 0.1 eV and an acquisition time of 1 s per step. In order to obtain a high signal-to-noise ratio to capture the slight differences in the C 1s spectra, 120 sweeps were added up per spectrum and the step size was reduced to 0.05 eV/step. The XPS data for molecular adsorption was collected at 110 K while other XPS data were collected at 300 K which is either equal to or lower than the annealing temperature so that the molecular chemical states would retain. Curve fitting was carried out by using the software package OriginPro 8. All curves were fitted with Voigt profiles, since the peak shapes are


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typically determined by a Lorentzian contribution due to the limited lifetime of the core hole state and a Gaussian broadening, mostly due to line shape of the exciting X rays from an X-ray monochromator, electron scattering and detection in the spectrometer.49 Gaussian contributions may also be related to thermal excitation processes. Chemical, structural and electronic (by dopants) inhomogeneities in the surroundings of the emitting atoms often contribute to the Gaussian broadening. DFT Calculations. DFT calculations were carried out using the Vienna ab initio simulation package (VASP)50 with projector augmented waves (PAW) potentials.51 A kinetic energy cutoff of 400 eV was found to cover the total energy of our systems. The exchange-correlation functional was treated with the generalized gradient approximation in the Perdew-BurkeErnzerhof (PBE) form.52 Van der Waals (vdW) forces between the biphenyl groups and the surface were not included due to limited computation resources. In computation, a three-layered slab model with a 3 × 9 supercell was employed to describe the Ag(111) surface. During the relaxation, the two bottom layers of Ag(111) were fixed at their bulk values. Adjacent slabs were separated by about 10 Å to avoid the interaction between them. A Monkhorst-Pack k-point mesh of 1×5×1 was used for k-point sampling of the Brillouin zone. STM images were simulated within the Tersoff-Hamann approximation.53

Conflict of Interest: The authors declare no competing financial interest. Acknowledgement. This work was jointly supported by NSFC (91527303, 21333001) and MOST (2017YFA0204702), China. We acknowledge support from the Danish Council for Independent Research Natural Sciences and the Danish National Research Foundation, and National Research Foundation of Singapore for the SPURc program (R-143-001-205-592). F. Bebensee thanks the


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Alexander von Humboldt-Foundation for a Feodor Lynen-Fellowship. We thank Zheshen Li for technical support in the synchrotron XPS measurements at the SX-700 beamline at ASTRID. Supporting Information Available: STM imaging of surface species, large-area STM imaging of surface species, and room temperature imaging of Ag-COIs. This material is available free of charge via the Internet at http://pubs.acs.org. CORRESPONDING AUTHOR * Address correspondence to [email protected] (Kai Wu), [email protected] (Trolle R. Linderoth). REFERENCES AND NOTES 1.

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Steering Surface Reaction at Specific Sites with Self-Assembly

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