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C: Surfaces, Interfaces, Porous Materials, and Catalysis

Thermal-Driven Formation of 2D Nano-Porous Networks on Metal Surfaces
Lu Lyu, Maniraj Mahalingam, Sina Mousavion, Sebastian Becker,
Han Huang, Martin Aeschlimann, and Benjamin Stadtmüller
J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.9b06327 • Publication Date (Web): 03 Oct 2019
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Page 1 of 27 The Journal of Physical Chemistry

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4 Thermal-driven Formation of 2D Nano-porous Networks on
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7 Metal Surfaces
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11 Lu Lyu,1 Maniraj Mahalingam,1 Sina Mousavion,1 Sebastian Becker,1, 2 Han Huang,*,3
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13 Martin Aeschlimann,1 and Benjamin Stadtmüller§,1,4
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1Department of Physics and Research Center OPTIMAS, University of Kaiserslautern,
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19 Erwin-Schrödinger-Straße 46, 67663 Kaiserslautern, Germany
20 2Department
21 of Chemistry, University of Kaiserslautern, Erwin-Schrödinger-Straße 52,
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23 67663 Kaiserslautern, Germany
24 3Institute
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of Super-microstructure and Ultrafast Process in Advanced Materials, School
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of Physics and Electronics, the Central South University, Changsha, Hunan 410083, P.
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R. China
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30 4Graduate School of Excellence Materials Science in Mainz, Erwin-Schrödinger-
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32 Straβe 46, 67663 Kaiserslautern, Germany
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36 *Email: physhh@csu.edu.cn
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38 §Email: bstadtmueller@physik.uni-kl.de
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43 ABSTRACT:
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46 Controlling the quantum confinement of (spin-dependent) electronic states by material
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48 design opens a unique avenue to accelerate the implementation of quantum technology
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51 in next generation photonic and spintronic applications. In the nano-world, two-
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dimensional porous molecular networks have emerged as highly tunable material
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56 platform for the realization of exotic quantum phases for which the nano-pores can be
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59 tuned by chemical functionalization of the size and shape of the molecules. Here, we
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4 demonstrate a new approach to control the periodicity, size, and barrier width in 2D
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7 porous molecular networks on surface by tuning the balance between intermolecular-
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9 and molecule-surface interactions using temperature. At 106 K, the prototypical TPT
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12 molecules form nano-porous networks with different periodicity and barrier width
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depending on the surface reactivity. The network structures continuously transform into
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17 a close-packed molecular structure at room temperature. This reversible structural
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20 phase transition can be attributed to an entropy driven loss of long-range order at higher
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22 temperature coinciding with a modification of the molecular adsorption site on the
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25 surface. Our findings hence open a new way to design the quantum confinement of
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electrons in porous structures on surfaces by external stimuli such as temperature, or
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30 laser assisted thermal activation of the molecule-metal hybrid system.
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35 Introduction:
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38 In the last decade, organic molecular-based porous materials, such as covalent organic
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frameworks (COFs),1 metal-organic frameworks (MOFs),2 and two-dimensional (2D)
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43 molecular porous networks (MPNs)3 have emerged as a highly flexible platform with
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46 exceptional properties in molecular catalysis, adsorption, separation, and energy
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48 storage.4 Among these materials, low dimensional molecular porous networks on
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51 surfaces are of particular interest for future applications since they can be easily
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fabricated with long-range order using molecular self-assembly processes.5 The
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56 structural tunability of the size and the shape of the pores in the molecular networks
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59 opens a diverse possibility to template surfaces, for instance, for hosting guest
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4 molecules and atoms in the pores,6, 7 to create nano-reactors for surface catalysis,8 and
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7 to confine surface electrons in tunable 2D quantum well structures.9, 10 The latter can
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9 lead to formation of new tunable quantum states at surfaces, such as topological or
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12 quantum anomalous Hall insulators, which is highly desirable for future applications.11,
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14 12
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17 So far, a tremendous effort was devoted to the fabrication of 2D porous networks
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20 on surfaces and to determine the fundamental interactions stabilizing the structural
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22 order of these molecular networks.3, 13 The most stabile network structures can be
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25 realized by either a direct covalent bonding between neighboring molecules or by
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coordinated interactions between molecular units and metallic centers in metal-organic
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30 networks. In these cases, the network structure is mainly determined by the
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33 directionality of the intermolecular bonding, while the molecule-surface interaction
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35 does not play any decisive role for the network structure. The latter interaction mainly
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38 comes into play for 2D porous networks with significantly weaker intermolecular
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interactions mediated either by hydrogen bonding or by van der Waals interactions
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43 between the network molecules. In these cases, the shape and periodicity of the 2D
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46 porous network is determined by a delicate balance between the inter-molecular and
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48 molecule-surface interactions.14, 15 This offers the unique opportunity to controllably
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51 alter the structure of the molecular films on surfaces by tuning the strength of the
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molecule-surface interaction, for instance, by changing the surface reactivity, or by
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56 employing external stimuli such as light and/or temperature. In turn, this high flexibility
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59 allows one to tailor the lateral dimensions of the nano-pores in these networks (e.g.
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4 porous size and sharp) and hence to alter the potential well for the quantum confinement
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7 of electrons in these low dimensional structures on surfaces. However, clear strategies
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9 to alter the balance between the different interfacial interaction mechanisms of 2D
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12 molecular networks on surfaces is still elusive so far.
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Therefore, in this work, we will address the influence of the surface reactivity and
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17 temperature on the supramolecular self-assembly process that potentially plays the key
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20 role for designing porous nano-platforms.16 As a prototypical molecular adsorbate, we
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22 use the C3 symmetric molecule 2,4,6-triphenyl-1,3,5-triazine (TPT), which consists of
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25 a central triazine ring and three peripheral phenyl groups as shown in Figure 1. The
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absence of specific functional end groups or oxygen atoms prevents the formation of
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30 covalent or hydrogen-bonds between neighboring molecules. As a consequence, the
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33 ultimate structure formation is expected to be determined by the interplay between
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35 intermolecular and molecule-surface interactions which is also supported by a previous
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38 study.17 Here, we study the temperature-dependent phase transition of a monolayer film
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of TPT molecules on Cu(111) and Ag(111) surfaces using variable-temperature
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43 scanning tunnelling microscopy (VT-STM) and low energy electron diffraction
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46 (LEED).
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3 Figure 1. (a) The ball-and-stick model of TPT molecule and (b) the schematic sample preparation of
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5 TPT on Cu(111) and Ag(111) surfaces.
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8 EXPERIMENTAL METHODS
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11 All experiments were carried out in a custom-built multichamber ultra-high-vacuum
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13 (UHV) system housing on Omicron MULTIPROBE.18 Prior to the molecule deposition,
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16 the single crystal surfaces Cu(111) and Ag(111) were processed by several cycles of
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18 Ar ion sputtering and subsequent annealing (~ 820 K, resistive heating). The
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21 cleanliness and the surface morphology were inspected by recording large-scale STM
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24 images and conducting low-energy electron diffraction (LEED) experiments. The TPT
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26 powder (Sigma-Aldrich, purity 98%) was further purified in advance (out-gassing at
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29 403 K, > 24 hours) in an organic molecular beam evaporator system (OMBE, base
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31 pressure < 10-9 mbar). Subsequently, TPT molecules were deposited onto Cu(111) and
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34 Ag(111), respectively, at a sublimation temperature of 413 K. During evaporation, the
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37 substrates were kept at 297 K (room temperature, RT) and the deposition rates of TPT
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39 were 0.28 ML min-1 on Cu(111) and 0.25 ML min-1 on Ag(111), respectively. The
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42 deposition rate was monitored by a quartz crystal microbalance (QCM), and was further
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44 calibrated by counting the adsorbed-molecule coverage in a large-scale STM at 106 K
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47 below one monolayer (one ML = one full monolayer of TPT nano-porous network at
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50 106 K with molecule conjugated π-planes oriented parallel to the substrate surface).
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52 The prepared samples were in-situ transferred to a surface analysis chamber (base
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55 pressure < 510-10 mbar), where the LEED experiments were performed in a
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57 temperature range from RT to 170 K (cooling by a cold N2 gas stream). In an in-situ
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60 STM chamber (VT-AFM XA, Omicron GmbH, base pressure < 210-11 mbar), all STM
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4 images were recorded in constant current mode with the tunneling current It usually in
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7 the range of 70-90 pA. The bias voltage (Vtip) is applied to the STM tip, hence the
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9 positive (negative) values correspond to tunneling into the occupied (unoccupied) states
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12 of the sample. During the STM measuring, the sample temperature can be tuned
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between RT and 106 K by a liquid N2 cooling system (LakeShore 335 Temperature
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17 controller). Here, the used electrochemically etched tungsten tips were cleaned by
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20 sputtering (1.5- 4.0 kV) and direct current heating. The STM images were processed
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22 with the Nanotec Eletrnica WSxM software19 and the Gwyddion software.20
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26 RESULTS AND DISCUSSION
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29 Formation of 2D Nano-Porous Networks on Metal Surfaces. We start our discussion
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31 with the structure of a TPT monolayer film on Cu(111). After deposition at room
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34 temperature (RT, 297 K) using organic molecular beam epitaxy (see Figure 1b), the
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37 TPT molecules form an ordered superstructure without further thermal activation. The
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39 corresponding LEED pattern and STM data are shown in Figure 2. Both remain
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42 unchanged between RT and 170 K indicating the absence of any structural phase
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44 transition in this temperature range. The LEED pattern in Figure 2a displays well-
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47 resolved sharp diffraction spots corresponding to a high structural quality and large
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50 molecular islands of the TPT film. The first-order spots of the diffraction pattern are
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52 more clearly visible in the LEED data in the inset of Figure 2a which were obtained for
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55 a smaller electron energy of 12 eV. The diffraction spots are arranged in a hexagonal
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3 0
[ ]
pattern which can be modeled by a superstructure matrix 0 3 (or wood’s notation
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60 of p(3 × 3)). The simulated diffraction pattern of the p(3 × 3) superstructure is
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4 superimposed onto the experimental diffraction pattern as yellow dashed circles and
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7 fully describe the experimental result.
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37 Figure 2. Close-packing (RT phase) of one ML TPT on Cu(111) at 170 K. (a) LEED pattern obtained
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at a beam energy 70 eV. The lower part of the LEED data is superimposed with a simulated diffraction
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pattern of a [30 03] superstructure (yellow dashed circles). The red circles indicate the substrate spots.
42 A LEED pattern of the same structure recorded at 12 eV is shown in the inset. (b) Large scale STM image
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44 (Vtip = 0.20 V) and corresponding FFT pattern (lower-left inset, direct comparison with 12 eV LEED
45 pattern of (a)) indicating an ordered structure. (c) Intra-molecular resolved STM image (Vtip = 0.63 V)
46 recorded at the white square in panel (b). The unit cell of the superstructure determined is indicated by
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48 red vectors A and B. (d) Proposed structural model of TPT on Cu(111), showing the long range order of
49 the TPT phenyl groups (highlighted by the yellow filling spots). The black dash arrows indicate the
50 nearest-neighbor TPT molecules.
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55 The local arrangement of the TPT molecules can be deduced from the STM data
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57 in Figure 2b and c. The TPT molecules appear as triangular shaped objects with three
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60 bright loops by the three peripheral phenyl groups of the molecule, which generally
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4 indicate a higher adsorption height to the TPT central triazine group.21 We find a
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7 homogeneous, close-packed arrangement of lying TPT molecules with low defect
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9 density (RT phase). The fast Fourier Transform (FFT) of the STM image in the inset of
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12 Figure 2b reveals an identical pattern as observed by LEED in the inset of Figure 2a.
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Interestingly, the periodicity of the p(3 × 3) superstructure of 7.65 Å is much smaller
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17 than the distance between the centers of two neighboring TPT molecules which was
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20 determined by STM (dTPT-TPT = 11.69 Å, marked by the black dash arrows in Figure
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22 2d). On close inspection of the STM data, we find that the phenyl groups of the TPT
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25 molecules arrange in a long range ordered pattern with a periodicity of approximately
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7 Å (intra- and inter-molecules distance between the phenyl groups, see yellow spots
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30 in Figure 2c, d). We hence propose that the diffraction pattern of the p(3 × 3)
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33 superstructure is caused by an intra- and inter-molecular scattering of electrons at the
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35 periodically ordered phenyl groups of the TPT film.
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38 The more detailed view onto the molecular structure can be obtained in the sub-
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molecular resolution STM image shown in Figure 2c. The TPT molecules do not form
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43 a periodic superstructure, but arrange in a short-range ordered pattern exhibiting two
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46 well defined azimuth orientations of the nearest-neighbor TPT molecules (Y-up and Y-
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48 down) which are rotated by 60° with respect to each other. The specific azimuthal
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51 orientations of the molecules enable the formation of a periodic lattice of neighboring
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peripheral phenyl groups with a hexagonal unit cell, as marked by red arrows in Figure
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56 2c. The unit cell vectors are aligned along the <110> high symmetry directions of the
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59 Cu(111) surface with A = 7.1 ± 0.2 Å and B = 6.8 ± 0.2 Å. These structural parameters
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4 are in good agreement with the commensurate p(3 × 3) superstructure determined by
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7 LEED, which allows us to propose a structural model for the adsorption configuration
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9 of TPT on Cu(111). The commensurate registry between the peripheral phenyl groups
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12 and the Cu(111) atoms points clearly to the existence of a single well-defined
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adsorption site of the peripheral phenyl groups for both Y-up and Y-down orientated
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17 molecules. This is only possible for an adsorption configuration with an on-top
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20 adsorption site of the peripheral phenyl groups as well as of the central triazine ring as
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22 illustrated in Figure 2d. Noticeably, the N-atoms of trazine ring are located on a highly
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25 symmetric hollow adsorption sites and not on a top adsorption sites as previously
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observed for TPT on highly reactive Fe and Co metal surfaces.21, 22 This is particularly
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30 interesting, since this top adsorption site was identified as the energetically favored
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33 adsorption configuration for TPT on metal surface. Our result clearly suggests that the
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35 structure formation of TPT on Cu(111) at RT is not only determined by the molecule-
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38 surface interaction, but also strongly influenced by intermolecular interactions between
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TPT molecules.
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58 Figure 3. Nano-porous network of one ML TPT on Cu(111) at 106 K (NPN-Cu). (a) Large scale
59 STM topography image (Vtip = 0.87 V) and the corresponding FFT pattern (upper-left corner, 4 × 4 nm-
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3 2). (b) Sub-molecularly resolved STM image (Vtip = 0.87 V). The unit cell is indicated by the green unit
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5 cell vectors A and B, the superimposed TPT models show a clockwise nanopore (green arrow) due to
6 the rotated TPT phenyl groups (blue filling spots). (c) Structural model of the NPN-Cu phase on Cu(111).
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9 Cooling down the sample temperature to 106 K leads to a reversible structural
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12 phase transition of the TPT structure and to the formation of a nano-porous network
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phase (NPN-Cu phase). The lateral order of the NPN-Cu phase is illustrated in the STM
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17 image in Figure 3a. In contrast to the RT phase, the NPN-Cu phase reveals a long-range
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20 order with a well-defined nano-pores pattern. Each pore is surrounded by six flat lying
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22 TPT molecules and the diameter of each nano-pores is 8.5 ± 0.2 Å. The periodicity of
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25 the NPN-Cu is shown in Figure 3b and can be described by a hexagonal unit cell A =
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17.9 ± 0.2 Å, and B = 17.7 ± 0.2 Å. The vectors A and B are both rotated by 21.5 ± 0.5°
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30 with respect to the <110> high symmetry directions of the Cu(111) and the
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33 superstructure can be described by the matrix [53 ―3
]
8 . The reliability of this
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35 commensurate superstructure matrix is confirmed by the FFT of our large scale STM
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38 data and simulated diffraction pattern based on this matrix as superimposed in the inset
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of Figure 3a.
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43 The molecular orientations in the NPN-Cu structure can be deduced from the sub-
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46 molecularly resolved STM image in Figure 3b. The TPT molecules adapt the same
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48 azimuthal orientations as in the RT phase, and the orientations of two nearest-neighbor
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51 molecules (Y-up and Y-down) are rotated by 60°, which indicate a highly symmetric
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adsorption site of the TPT molecules on Cu(111). Furthermore, the peripheral phenyl
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56 groups form a clockwise rotated chirality in nano-porous lattice (shown by the green
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59 rotation arrow in Figure 3b). It is interesting to note that a mirror domain structure of
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4 this NPT-Cu (see Figure S1) exhibits the corresponding porous structures with
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7 counterclockwise rotated chirality, similar to recent studies conducted on other
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9 supramolecular systems, such as NC-Ph5-CN and DBBA on Ag(111) surface.23-25
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12 Based on these results, we can propose an unambiguous structure model for the
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NPN-Cu phase which is shown in Figure 3c. At low temperature, the N atoms of the
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17 triazine ring adsorb on top sites of the Cu surface atoms while the center of the
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20 peripheral phenyl groups are located on hollow adsorption sites. Similar adsorption
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22 sites of TPT molecules have recently been reported on other metal surfaces at extremely
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25 low temperature (< 5 K)21, 22, 26 which is the energetically most favourable adsorption
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configuration for this metal-organic system. Most interestingly, this adsorption
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30 configuration clearly differs from the one of the RT phase in which the N atoms of the
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33 triazine ring are located on hollow adsorption sites and the center of the peripheral
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35 phenyl groups on top sites. This suggests that the molecule-surface interaction plays a
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38 more dominant role for the TPT structure on Cu(111) at low temperatures. Hence, our
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experimental findings clearly show that the formation of the 2D TPT nano-porous
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43 network on Cu(111) is driven by a temperature dependent modification of the TPT
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46 favourable adsorption site.
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48 Further insights into the role of the different interfacial interaction mechanisms
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51 for the structural order of 2D nano-porous structures can be obtained by turning to a
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TPT monolayer structure on another noble metal surface with similar symmetry but
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56 lower surface reactivity. As prototypical example we have selected the Ag(111) surface
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59 which has been extensively studied as surface with an intermediate molecule-surface
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4 interaction strength for hydrocarbon molecules compared to Cu(111) and Au(111).27-29
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19 Figure 4. Structural phase transition of one ML TPT on Ag(111) as a function of sample
20 temperatures of (a)170, (b) 130 and (c) 120 K. (a) A perfect close-packing (Vtip = 0.60 V) at 170 K and
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22 the corresponding FFT insert in upper-left corner. (b) A closed-packed TPT pattern (Vtip = 0.40 V) with
23 small amount of nano-porous defects at 130 K. (c) STM image (Vtip = 0.41 V) showing the coexistence
24 of two phases: close-packing (Phase I) and nano-porous network (Phase II), the boundary as highlighted
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by the black dash curve. (d) LEED pattern (beam energy 12 eV) of one ML TPT on Ag(111) at 170 K,
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27 indicating two types of domains, as marked by yellow (Domain I) and light blue (Domain II) dashed
28 circles, respectively, in which the respective light blue diffraction circles are consistent with the FFT in
29 panel (a).
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34 In analogy to TPT/Cu(111), we find a continuously reversible phase transition of
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37 TPT/Ag(111) between a close-packed structure at RT and a 2D nano-porous network
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39 structure at low temperatures as shown in the STM images at selected temperatures in
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42 Figure 4. In the temperature range between RT and 170 K, the TPT molecules on
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44 Ag(111) show a similar short-range order as on Cu(111) (see Figure 4a) with an almost
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47 identical packing density. Interestingly, the LEED pattern of RT structure in Figure 4d
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50 reveals two mirror domains suggesting a different alignment of the TPT molecules with
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52 respect to the Ag(111) high symmetry direction (the diffraction data of the RT phase is
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55 shown in Figure S2 of the supplementary material) . Reducing the sample temperature
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57 to 130 K (Figure 4b) results in the formation of a few isolated nano-pores embedded in
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60 the close-packed phase. The pores density increases continuously with temperature
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4 decreasing (e.g. at 120 K in Figure 4c) until a homogeneous 2D nano-porous network
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7 structure is formed at 106 K (Figure 5).
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23 Figure 5. Nano-porous network of one ML TPT on Ag(111) at 106 K (NPN-Ag). (a) STM image
24 (Vtip = 0.30 V) and the FFT pattern (upper-left corner, 2 × 2 nm-2) showing a well-ordered network
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structure. (b) High resolution STM image (Vtip = 0.30 V), and the unit cell vectors A and B are marked
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27 in green rhombus. The superimposed TPT models show a clockwise nanopore (green arrow) due to the
28 rotated TPT phenyl groups (blue filling spots). (c) Structural model of the NPN-Ag phase on Ag(111).
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31 A detailed view of TPT/Ag(111) at 106 K can be obtained in Figure 5a, b. Even at
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34 first glance, it is clear that TPT forms a well-defined nano-pores network, with a
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37 different periodicity and the molecular arrangement than on Cu(111). We hence refer
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39 to this phase as NPN-Ag phase. In this phase, the porous diameter is 9.5 ± 0.5 Å. The
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42 unit cell parameters of the hexagonal NPN-Ag structure are A = 31.2 ± 0.3 Å, B = 30.7
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44 ± 0.3 Å and the superstructure lattice is rotated by 45.9 ± 0.3° with respect to the <11
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47 0> high symmetry directions of Ag(111). The superstructure lattice corresponds to a
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commensurate superstructure matrix [39 ―9
]
12 . Hence, the distance between
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52 neighboring nano-pores in the NPN-Ag structure is almost twice as large as in the NPN-
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55 Cu phase on Cu(111). The larger distance between the nano-pores is the result of the
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57 double ring structure of TPT molecules surrounding the nano-pores on Ag (see in
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60 superimposed TPT models of Figure 5b), which is in contrast to the single ring structure
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4 of NPN-Cu phase. This leads to a larger barrier width between the nano-pores on Ag
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7 surface. The inner ring (marked by light green region) of TPT molecules reveals a
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9 clockwise chiral arrangement while the molecules of the outer ring (light blue region)
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12 attach themselves to the inner ring to form a close-packed structure. Despite this more
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complex molecular arrangement on Ag(111), we again only find two molecular
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17 orientations (Y-up and Y-down) in the NPN-Ag which are rotated by 60° with respect
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20 to each other. Furthermore, NPN-Ag also has a mirrored domain. Figure S3 shows the
21
22 two mirrored domains are deviated from the high symmetry direction <112> of Ag(111)
23
24
25 by 15.9 ± 0.3°, their nanoporous lattices keep the opposite chirality.
26
27
Based on the STM data, we can build a structural model for the NPN-Ag which is
28
29
30 shown in Figure 5c. In analogy to TPT on Cu(111), we expect that both molecular
31
32
33 orientations of Y-up and Y-down are located on equal adsorption sites. This conclusion
34
35 is supported by the homogeneous contrast of both types of molecules in STM as well
36
37
38 as by the commensurate registry between the NPN-Ag structure and the Ag(111)
39
40
surface. From the structural model in Figure 5c, we find that the center of TPT triazine
41
42
43 ring is located on top adsorption site of the surface Ag atom while the N atoms are
44
45
46 located at bridge sites. We have to point out that this bridge adsorption site of the N
47
48 atoms is the only adsorption configuration in which all N atoms of the Y-up and Y-
49
50
51 down molecules can be located on identical adsorption sites. This is due to the large
52
53
mismatch (13%) between the lattice constant of Ag(111) (2.88 Å) and the distance
54
55
56 between neighboring N atoms of TPT triazine ring ( 2.50 Å) (marked in Figure 1a), i.e.,
57
58
59 the distance of two equivalent three-fold symmetric adsorption sites of Ag (top-site,
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4 hollow-site) is too large for the intramolecular distance of the equivalent N atoms. On
5
6
7 the other hand, the distance between neighboring bridge adsorption sites of Ag(111) is
8
9 2.49 Å, and hence matches perfectly with the distance of the N atoms in TPT molecule.
10
11
12 The adsorption of the N atoms of TPT on a bridge site on Ag(111) compared to
13
14
the top site on Cu(111) is a clear indication for a weaker molecule-surface interaction
15
16
17 between TPT and Ag(111) compared to Cu(111). This observation is fully in line with
18
19
20 previous studies of other prototypical molecules such as PTCDA and F16CuPc30, 31 on
21
22 Ag(111), which generally exhibit a smaller adsorption energy and a larger adsorption
23
24
25 height compared to more reactive metal surfaces.
26
27
28
29 Table 1. Summary of temperature-dependent phases parameters of one ML TPT on Cu(111) and
30
31 Ag(111). The unit cell calibrated by the corresponding matrix excluding the reasonable errors. The TPT
32 molecular density and porous density calculated from the corresponding matrix.
33
34
35 Adsorption sites Molecular Porous
36 1 ML TPT Unit Cell Matrix
of triazine N atom density nm-2 density nm-2
37
38
39 / Cu(111) @ 170 K |A| = |B| = 7.65 Å
40
[30 03] Hollow 0.66 0
Close-packing γ = 120°
41
42
43 / Cu(111) @ 106 K |A| = |B| = 17.85 Å
44
NPN-Cu γ = 120°
[53 ] [ ―3
―3
8 &
8 3
5] Top 0.72 0.36
45
46
47
/ Ag(111) @ 106 K |A| = |B| = 31.15 Å
48 [39 ] [
―9 9
12 & 3
―3
12 ] Bridge 0.71 0.12
49 NPN-Ag γ = 120°
50
51
52
53
54
The comprehensive characterization of the structures of TPT on Cu(111) and
55
56
57 Ag(111) revealed the formation of (i) two different of nano-porous networks (NPN
58
59
60 phase) with different periodicities at low sample temperature, and (ii) short-range
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4 ordered close-packed structures (RT phase) at RT with almost identical packing density.
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7 The superstructure parameters of these phases are summarized in Table 1. The
8
9 structural phase transition can be understood by the temperature-dependent competition
10
11
12 between intermolecular interactions and molecule-substrate interactions, which is a
13
14
ubiquitous principle at organic-metal interfaces.32-35 In the following, we will discuss
15
16
17 the role of the intermolecular interactions for the structure formation of TPT on surfaces
18
19
20 using pair potential simulations.
21
22
23
24
25 Intermolecular Interactions and Pair Potential Simulations. Our experimental
26
27
results indicate that the molecule-surface interaction seems to dominate the adsorption
28
29
30 site of the TPT on the Cu(111) and Ag(111) surfaces. However, it does not directly
31
32
33 explain the formation of TPT dimers and the relative azimuthal orientations of the TPT
34
35 molecules with respect to each other, i.e., the formation of the Y-up and Y-down
36
37
38 configuration. To gain a qualitative understanding of the role of these different
39
40
interaction mechanisms on the structure formation, we quantified the intermolecular
41
42
43 interactions between neighboring TPT molecules using pair potential simulations. This
44
45
46 theoretical framework allows us to calculate the intermolecular interaction energy
47
48 between two rigid molecules TPTA and TPTB considering electrostatic as well as van
49
50
51 der Waals interactions between the molecules.36-38 The results of these simulations are
52
53
presented in so called pair potential maps showing the interaction energy for different
54
55
56 relative positions ΔR = (Δx, Δy) and one relative azimuthal orientation Δθ of the TPT
57
58
59 molecules. In Figure 6. the TPTA molecule is fixed in the center at ΔR = (0,0) while the
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4 position of the second TPTB molecule is varied in the map for a fixed azimuthal
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7 orientation Δθ. In all simulations, we assume a flat adsorption geometry and an equal
8
9 adsorption height of both TPT molecules.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45 Figure 6. (a)-(c) Pair potential maps for different relative azimuthal orientations Δθ of two TPT
46 molecules. The orientations of the molecules are illustrated by the structural models of TPT. The red
47
48 areas indicate regions of intermolecular repulsion, the green and blue areas indicate attraction (negative
49 pair potential energy). The insets highlight the pair potential landscape in the vicinity of the potential
50 minima. Panel (d) shows radial line profiles through the global minima of pair potential maps for different
51
Δθ. The direction of the radial line profile is indicated by the colored arrow in the first quadrant of the
52
53 corresponding pair potential map.
54
55
56
57 The results of the pair-potential simulations are shown in Figure 6a-c for three
58
59
60 different relative TPTB molecule orientations of Δθ = 0°, 30° and 60°, the corresponding
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4 orientations of TPTA and TPTB molecules are shown in the structural models. The pair
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6
7 potential energy is encoded in the color code of these maps depending on the relative
8
9 distance ΔR = (Δx, Δy) between two TPT molecules. Regions of attractive
10
11
12 intermolecular interaction reveal a negative pair potential energy which are shown in
13
14
green and blue while regions with a repulsive intermolecular interaction (positive pair
15
16
17 potential energy) are shown in red. All potential maps reveal an overall repulsive
18
19
20 intermolecular interaction between TPTA and TPTB molecules for all orientation Δθ
21
22 which can be attributed to the electrostatic repulsion between the positively charged
23
24
25 hydrogen atoms of TPT. In addition, each potential map also exhibits very sharp and
26
27
distinct potential minima in the pair potential landscape which can lead to an attractive
28
29
30 intermolecular interaction and to well define intermolecular distances in the TPT
31
32
33 molecules on surface. These potential minima are mainly caused by an attractive
34
35 Coulomb interaction between N-atom of the TPT triazine ring with the hydrogen atoms
36
37
38 of another TPT molecule, as well as by the Van der Waals interaction.
39
40
To quantify the attractive potential between two TPT molecules, we extracted
41
42
43 radial line profiles in Figure 6d through the map potential minima in different relative
44
45
46 molecule orientations. Each one dimensional potential curve reveals a well-defined
47
48 global potential minimum with a potential barrier having its maximum in the range
49
50
51 between ΔR = 12.8 Å (Δθ = 60°) and ΔR = 14.0 Å (Δθ = 0°). The largest energy gain
52
53
of ~ 25 meV is found for a relative orientation of Δθ = 60° at a distance of ~ 10.6 Å
54
55
56 (energy minimum of the green profile), which indicated that with a relative azimuthal
57
58
59 orientation of Δθ = 60° is the most stable configuration of two TPT molecules. This
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4 intermolecular arrangement reflects very well the relative alignment of neighboring
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6
7 TPT molecules on Cu(111) and Ag(111). In particular, it can fully explain the existence
8
9 of only two molecular orientations (Y-up and Y-down) which a form dimer-like
10
11
12 structure with two TPT molecules rotated by 60°. These dimers are the essential
13
14
building blocks of experimentally observed TPT structures discussed above.
15
16
17 Moreover, the intermolecular interactions also play a crucial role for the structural
18
19
20 order of the nano-porous networks. In Figure 7a and b, the structural models of the
21
22 NPN-Cu and NPN-Ag phase are superimposed onto the pair potential map for a relative
23
24
25 orientation of Δθ = 60°. We place one molecule TPTA in the center position of the pair
26
27
potential map and only show the directly neighboring molecules for clarity. Most
28
29
30 interestingly, the lateral position and the adsorption sites of the TPT molecules in both
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48 Figure 7. Superimposed experimentally obtained structural models on simulated potential landscape: (a)
49 NPN-Cu on Cu(111) (b) NPN-Ag on Ag(111) and (c) close-packed RT phase on Cu(111), plotted onto
50
51 the pair potential map for Δθ=60°. The lateral position of one TPT molecules coincides with the center
52 of the pair potential map, i.e., with the position of the center TPTA molecule in the calculation. For clarity,
53
54 we only show some neighboring TPT molecules for different structures. The arrows mark the position
55 of the potential minima of the pair potential simulations. A close up of the potential landscape in the
56
57 vicinity of the potential minima are shown as insets.
58
59
60 networks coincide almost exactly with the map potential minima. This clearly suggests
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4 that the intermolecular interaction is some responsible for the selection of an adsorption
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7 position of TPT on the respective surface at low temperatures while the appearance of
8
9 well-defined adsorption sites can be attributed to the molecule-surface interaction. At
10
11
12 the RT phase of TPT/Cu(111) in Figure 7c, the TPT molecules are not located in a
13
14
potential minimum of the pair potential landscape but are found at a slightly larger
15
16
17 intermolecular distance (11.6 Å between the neighboring triazine centers of two TPT
18
19
20 molecules) compared to NPN-Cu phase of Figure 7a (10.3 Å). The distance of 11.6 Å
21
22 is still in the attractive regime of the pair potential landscape from the one dimension
23
24
25 pair potential curve in Figure 6d (green curve). This attractive force between TPT
26
27
molecules at RT is responsible for the formation of short-range ordered molecular
28
29
30 islands. Finally, we attribute the larger intermolecular distance and the correspondingly
31
32
33 smaller packing density of TPT molecules in the RT phase (see in Table 1) to the higher
34
35 molecule kinetic energy (thermal energy) and the larger entropy of the molecular
36
37
38 system at RT.
39
40
Hence, the thermal energy can modulate the balance of molecule-surface
41
42
43 interactions and intermolecular interactions. we can identify the major mechanisms
44
45
46 determining the structure formation and the structural phase transitions of TPT on noble
47
48 metal surfaces. In the ground state of the adsorbate system, i.e., at LT, the structural
49
50
51 ordering is mainly determined by the substrate-dependent molecule-surface interaction
52
53
as well as by intermolecular interactions. While the molecule-surface interaction leads
54
55
56 to a site-specific interaction between TPT and the noble metals and to well-defined
57
58
59 adsorption sites, the global potential minima in the pair potential map determine the
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4 relative orientation and position of neighboring TPT molecules at LT. At RT, the
5
6
7 additional thermal energy of the adsorbate system results in an entropy driven loss of
8
9 long-range order and in a larger intermolecular distance compared to the LT phase. The
10
11
12 TPT molecules in the RT phase are still located on well-defined adsorption sites leading
13
14
overall to a close packed arrangement with a smaller packing density. Therefore, during
15
16
17 the temperature dependent phase transition, the intermolecular distance reduces from
18
19
20 RT to LT, i.e., the TPT molecules hop to different adsorption sites matching the global
21
22 potential minima. This adsorption configuration can only be realized by creating nano-
23
24
25 pores and forming a 2D network structure at LT. These nanoscale porous networks are
26
27
promising templates for surface quantum states.9, 12 Furthermore, the differences of
28
29
30 lattice and surface reactivity of Cu(111) and Ag(111) lead to two nano-porous networks
31
32
33 of TPT molecules, in which porous density on Cu(111) is three times that on Ag(111)
34
35 (see in Table 1). It is well known the porosity will have a great influence on the chemical
36
37
38 properties of porous materials.2 On the other hand, the different barrier widths of NPN-
39
40
Cu and NPN-Ag provide a possible to modulate the coupling between quantum states
41
42
43 trapped in the porous quantum dot array.10
44
45
46
47
48 Conclusions
49
50
51 In conclusion, we have investigated the structure formation and phase transition
52
53
of TPT monolayer film on Cu(111) and Ag(111) at various sample temperatures. At
54
55
56 low temperature of 106 K, the TPT molecules form well-ordered chiral nano-porous
57
58
59 networks on both surfaces with different periodicity and porous barrier widths. Both
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4 networks exhibit a commensurate registry with the corresponding surface grid and the
5
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7 TPT molecules adsorb on different well-defined adsorption sites. Increasing the sample
8
9 temperature to RT results in a continuously structural phase transition from the 2D
10
11
12 nano-porous network into a close-packed TPT phase. In conjunction with pair potential
13
14
simulations, we are able to understand the mechanisms of molecular interactions for
15
16
17 TPT films on noble metal surfaces. At LT, the TPT structure is determined by two
18
19
20 major contributions: (i) the molecule-surface interactions result in local, well-defined
21
22 adsorption sites of TPT in networks phases on noble metal surfaces. In addition, (ii) the
23
24
25 global minima in the potential landscape of the intermolecular interaction selects the
26
27
relative orientation and position of neighboring TPT molecules. The temperature driven
28
29
30 phase transition from 2D porous network (at LT) to a close-packed structure (at RT) is
31
32
33 due to the larger thermal energy resulting an entropy driven loss of long-range ordering.
34
35 Our findings clearly demonstrate that temperature or thermal energy in general can be
36
37
38 employed to actively manipulate the balance between intermolecular- and molecule-
39
40
surface interactions in 2D organic porous networks and thereby to tune and design the
41
42
43 periodicity, size, and barrier width between network pores on surfaces. This provides a
44
45
46 highly exciting platform to design the surface quantum confinements or to tailor the
47
48 functionalities of nano-porous materials for surface chemistry. 3, 9, 10
49
50
51
52
53
54
Acknowledgments
55
56 The research leading to these results was financially supported by the German Science
57
58
59 foundation (DFG) via SFB/TRR 173 Spin+X: spin in its collective environment
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4 (Project B05). B.S. thankfully acknowledge financial support from the Graduate School
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6
7 of Excellence MAINZ (Excellence Initiative DFG/GSC 266). H.H. acknowledges the
8
9 financial support from the National Natural Science Foundation (NSF) of China (Grant
10
11
12 No. 11874427).
13
14
15
16
17 Author Contributions
18
19
20 B.S., M.A. and L.L conceived and design the research project. L.L, M.M., S.M, and
21
22 S.B. conducted the STM and LEED study, B.S. performed the pair potential simulations.
23
24
25 The data were analysis by L.L and discussed with all authors. The manuscript was
26
27
written by L.L, H.H, M.A. and B.S, with support and input from all authors.
28
29
30
31
32
33 Supporting Information
34
35 Detailed description of the mirrored nano-porous Domain II of one ML TPT on Cu(111)
36
37
38 and Ag(111) at 106 K, respectively, and the close-packed Domain I of one ML TPT on
39
40
Ag(111) at 170 K.
41
42
43
44
45
46 References
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48 1. Côté, A. P.; Benin, A. I.; Ockwig, N. W.; Keeffe, M.; Matzger, A. J.; Yaghi, O. M. Porous,
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