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(a–d) The STM images of four types of melem hexamers which have been observed within the porous nanostructure, where the brighter ones are indicated by the green dashed ellipses. (e–h) The STM images (a)–(d) and their corresponding DFT-optimized structural models are superimposed. (i–l) The charge density difference maps of the corresponding STM images, where red and blue isosurfaces indicate charge accumulation and depletion, respectively (an isosurface value of 0.016 e Å À3 ).
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We studied the self-assembly of melem on the Au(111) and Ag(111) surface. By scanning tunneling microscopy imaging, we have observed two different STM appearances of the melem molecule within the self-assembled nanostructure on Au(111), which are resulted from the different intermolecular bonding configurations. Moreover, further DFT details includ...
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... calculations of the two motifs on the Au(111) surface (Fig. S1, ESI †), and the two models stand out as shown in Fig. 1c and d. Both of the melem molecules in the two motifs adopt a flat-lying geometry with a similar height in Fig. 1e and f. In addition, we have not reproduced the different contrasts from their Tersoff-Hamann STM simulations in Fig. S2 (ESI †), where the top figure shows experimental data and the bottom figure (showing no difference in contrast) shows simulated data. Their flat geometries imply that hydrogen-bonded inter- molecular interaction among melem molecules determines the formation of the observed melem network, whereas the surface serves primarily as a ...
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... addition, we could divide the porous structure into hexamers which consist of six melem molecules to further analyze the porous structure. As shown in the STM images of Fig. 2, we find four different hexamer structures. One is formed exclusively by the dim melem, i.e., Motif-1 ( Fig. 2a and e). One is composed of four dim molecules and a pair of bright dimers (Fig. 2b and f). One is composed of two dim molecules and two pairs of bright dimers at the opposite position ( Fig. 2c and g). And the other is ...
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... addition, we could divide the porous structure into hexamers which consist of six melem molecules to further analyze the porous structure. As shown in the STM images of Fig. 2, we find four different hexamer structures. One is formed exclusively by the dim melem, i.e., Motif-1 ( Fig. 2a and e). One is composed of four dim molecules and a pair of bright dimers (Fig. 2b and f). One is composed of two dim molecules and two pairs of bright dimers at the opposite position ( Fig. 2c and g). And the other is composed of two dim molecules and two pairs of bright dimers at the neighboring position ( Fig. 2d and h). In the last case, ...
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... could divide the porous structure into hexamers which consist of six melem molecules to further analyze the porous structure. As shown in the STM images of Fig. 2, we find four different hexamer structures. One is formed exclusively by the dim melem, i.e., Motif-1 ( Fig. 2a and e). One is composed of four dim molecules and a pair of bright dimers (Fig. 2b and f). One is composed of two dim molecules and two pairs of bright dimers at the opposite position ( Fig. 2c and g). And the other is composed of two dim molecules and two pairs of bright dimers at the neighboring position ( Fig. 2d and h). In the last case, the interaction between two Motif-2 units is in a staggered form as that of ...
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... structure. As shown in the STM images of Fig. 2, we find four different hexamer structures. One is formed exclusively by the dim melem, i.e., Motif-1 ( Fig. 2a and e). One is composed of four dim molecules and a pair of bright dimers (Fig. 2b and f). One is composed of two dim molecules and two pairs of bright dimers at the opposite position ( Fig. 2c and g). And the other is composed of two dim molecules and two pairs of bright dimers at the neighboring position ( Fig. 2d and h). In the last case, the interaction between two Motif-2 units is in a staggered form as that of Motif-1. Based on these STM images, we performed the corresponding DFT calculations for the four hexamer structures ...
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... the dim melem, i.e., Motif-1 ( Fig. 2a and e). One is composed of four dim molecules and a pair of bright dimers (Fig. 2b and f). One is composed of two dim molecules and two pairs of bright dimers at the opposite position ( Fig. 2c and g). And the other is composed of two dim molecules and two pairs of bright dimers at the neighboring position ( Fig. 2d and h). In the last case, the interaction between two Motif-2 units is in a staggered form as that of Motif-1. Based on these STM images, we performed the corresponding DFT calculations for the four hexamer structures in the gas phase. The optimized structural models are superimposed on the relative STM images in Fig. 2e-h, and a good ...
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... neighboring position ( Fig. 2d and h). In the last case, the interaction between two Motif-2 units is in a staggered form as that of Motif-1. Based on these STM images, we performed the corresponding DFT calculations for the four hexamer structures in the gas phase. The optimized structural models are superimposed on the relative STM images in Fig. 2e-h, and a good agreement is achieved in all cases. Moreover, we also performed DFT calculations of the charge density difference maps of the four hexamer structures in the gas phase as shown in Fig. 2i-l, which clearly illustrate the cyclic double hydrogen bonds between the amino groups and heptazine ...
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... DFT calculations for the four hexamer structures in the gas phase. The optimized structural models are superimposed on the relative STM images in Fig. 2e-h, and a good agreement is achieved in all cases. Moreover, we also performed DFT calculations of the charge density difference maps of the four hexamer structures in the gas phase as shown in Fig. 2i-l, which clearly illustrate the cyclic double hydrogen bonds between the amino groups and heptazine ...
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Citations
... Unsupported H-Bonded Networks. As shown in previous STM studies, [20][21][22]39 melem molecules adsorb planar on surfaces like Au(111) and Ag(111) forming a variety of different, mostly coexisting, hydrogen-bonded networks. All structures feature two basic intermolecular H-bond patterns, which are usually denoted as side-by-side (SS) and head-to-tail (HT) (see Figure 1 panels a and b, respectively). ...
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... As with melamine, melem is stable in two different forms on the surface as well as forming chiral rings on the surface. 34 The structure of the three stable forms are shown in ...
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... [10][11][12][13] By contrast, H-bonded structures formed by melaminea nd/or melem on different kinds of surfaces have been the subject of extensive STM and theoretical investigations. [14][15][16][17][18][19][20] Both compoundsc an be sublimed under ultrahigh vacuum to grow highly pure and defect-free epitaxial films. In the monolayerr egime, surfaces such as Au(111)a nd Ag(111)p romote flat adsorption and the formationo fp eculiar honeycombs tructures, in which the nitrogen functional groups are involved in intermolecular H-bonds that are very similart ot hose existing in the crystalline structures of melon and PHI. ...
... An amendatory way is to introduce more proper anchoring sites into the molecular building blocks which has considerable binding strength with these Au adatoms. Therefore in this work, we have considered the trimerization product of melamine, melem (triamino-s-heptazine), which can also form similar honeycomb self-assembly structure but with larger pores concomitantly decorated with heterocyclic N (h-N) atoms at the sides [35][36][37][38][39] . Our scanning tunneling microscopy (STM) experiments clearly show that exposing the melem film to CO atmosphere can efficiently generate quite a number of Au adatoms inside the nanopores of melem, whose concentration and stability are greatly enhanced in comparison with those on melamine films. ...
... It possesses three amino endgroups (denoted as D) and six on-side heterocyclic nitrogen atoms (denoted as A), through which the intermolecular hydrogen bonding (HB) pairs can be formed and the corresponding assembly structures can be constructed on inert surfaces. Honestly, all the predictable HB patterns (Scheme 1b-d) have been observed in the assemblies of melem 35,39 . We want to point out that in the cases of Scheme 1c, As already addressed in our previous report, a nearly pure phase of honeycomb structure of melem (termed as H-melem) can be achieved by controlling the melem coverage during deposition in combination with subsequent thermal annealing 35 . ...
The controllability of metal adatoms has been attracting ever-growing attention because the metal species in particular single-atom metals can play an important role in various surface processes, including heterogeneous catalytic reactions. On the other hand, organic self-assembly films have been regarded as an efficient and versatile bottom-up method to fabricate surface nanostructures, whose functionality and periodicity can be highly designable. In this work, we have developed a novel strategy to steer the generation and distribution of metal adatoms by combining the surface self-assemblies with exposure to small inorganic gaseous molecules. More specifically, we have prepared a honeycomb structure of melem (triamino-s-heptazine) on the Au(111) surface based on a well-structured hydrogen bonding network. The achieved melem self-assembly contains periodic hexagonal pores having diameters as large as around 1 nm. More importantly, the peripheries of the nanopores are decorated with heterocyclic N atoms that can probably form strong interactions with the metal species. Upon exposing the melem self-assembly to a CO atmosphere at room temperature, a fair number of Au adatoms were produced and trapped inside the nanopores encircled by the melem molecules. Single or clustered Au vacancies were concomitantly formed that were also trapped by the melem pores and stabilized by the surrounding molecules, as confirmed by high-resolution scanning tunneling microscopy (STM) images. Both types of added species showed positive correlations with the CO exposure and saturated at around 0.01 monolayer. In addition, owing to the large pore size, as well as the presence of multiple docking sites inside the nanopores, more than one Au adatom can reside in a melem nanopore; they can be distributed in a variety of configurations for bi-Au (two Au adatoms) and tri-Au (three Au adatoms) species, whose population can be manipulated with the CO exposure. Moreover, control experiments demonstrated that these CO-induced Au species, including the adatoms and vacancies, can survive annealing treatments up to the temperature at which the melem molecules start to desorb, indicating a substantial thermal stability. The formed Au species may hold great potential for serving as active sites for surface reactions. More interestingly, the bi-Au and tri-Au species have moderate Au-Au intervals, and can be potentially active for certain structurally sensitive bimolecular reactions. Considering all these aspects, we believe that this work presents a fresh approach to utilizing organic self-assembly films and has demonstrated a rather novel strategy for preparing various single-atom metal species on substrate surfaces.
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