Molecular self-assemblies (SAM) on metals have retained large research interests for decades due to its spontaneity, designability and functionality, which constitute a fascinating strategy for constructing variously functionalized nanostructures [1-3]. Generally, on the active metals the ordering of the molecules is governed by the strong adsorbate-substrate interactions, whereas on the inert metals by various intermolecular interactions including van der Waals interaction, hydrogen bonding [4-6], metal coordination [7, 8], halogen bonding [9, 10], as well as indirect intermolecular interactions mediated by the strong surface states of the substrate . The metal adatoms, either intrinsically existing on the substrate or dosed intentionally, are frequently found participating and tailoring the self-assembly structures [8, 12, 13]. On these occasions, metal adatoms are strongly involved in the chemical reactions of the molecules by initiating the covalent bond breaking as well as the coordinating bond formation [14-19]. In other cases, they may also be trapped by the assembly frameworks, leaving no apparent influence on the assembly structures [5, 20]. Here we report another example wherein the dosed metal shows relatively weak interaction with the assembled molecules, but can significantly change the assembly pattern by tailoring the existing intermolecular interactions.
Melamine (1, 3, 5-triazine-2, 4, 6-triamine) contains a triazine core with three terminal amino groups, and is widely applied as building block of hydrogen bonding networks [5, 21]. On coinage metals such as Au(111) [22, 23] and Ag(111) , melamine has been reported to form highly ordered honeycomb structures, mediated by the ideal hydrogen bonding networks between the flat-lying molecules. Whereas on more active transition metals such as Ni(111)  and Pd(111) , dehydrogenation occurs upon depositing melamine at room temperature thus the assembly structure is governed by the
In this work, we intentionally dosed Cu onto the Au(111) surface which was pre-covered with melamine assembly, and evidenced the changes of the molecular arrangements as functions of the amount of the dosed Cu. We found the interactions between Cu adatom and melamine molecule were moderate and could not lead to coordinative assembly on the Au(111) surface, but their incorporation already caused the changes of the hydrogen bonding patterns of melamine. These findings are expected to shed new lights on the role of metal adatoms in organic self-assemblies.Ⅱ. EXPERIMENTS
All experiments were carried out on a Createc low-temperature scanning tunneling microscope (LT-STM) housed in an ultra-high vacuum (UHV) system with base pressure of 1×10-10 mbar. The Au(111) single crystal was cleaned by repeated Ar+ sputtering and subsequent radiative heating at 800 K. Melamine (
The self-assembly of melamine on Au(111) at room temperature has been extensively studied previously [15, 16]. It is revisited as a reference for revealing the doping effect of the Cu adatom. FIG. 1(a) shows the clean Au(111) surface with herringbone structure far before doping with any metals. A clear and periodic (22×
On the basis of the clean Au(111) substrate, we prepared a Cu-doped surface by depositing small amount of Cu followed by annealing to ~400
With these fundamental knowledge in mind, we start to consider the effect of dosing Cu atoms on the assembled melamine film on Au(111). As shown in FIG. 3(a), upon depositing about 0.005 ML Cu onto the 0.7 ML honeycomb film of melamine at room temperature, a new track-like structure (termed as TK structure) immediately emerged coexisting with the remained honeycomb structure. Continuing dosing Cu to about 0.01 ML, the honeycomb network completely disappeared and only the TK structure was left on the surface, as shown in FIG. 3(b). Meantime coexisting were the scattered large holes that are imbedded in the assembled film. The primary axes (the black lines in FIG. 3(b) of the TK structure) are basically parallel to three equivalent
Increasing the Cu dosage to about 0.05 ML, the assembly structure changes again. As shown in FIG. 4(a), a new structure with hexagonal symmetry was formed and aggregated into large islands. Coexisting with these islands are the small Cu patches formed by the excessive Cu atoms. Their existence also implies that no other new assembly structures would be formed with even more Cu deposited, which is exactly what we found at higher Cu dosages. FIG. 4(b) shows the high resolution STM image of this new assembly structure, wherein dim triangular species together with brighter protrusions can be clearly identified. The triangles can be assigned as flat-lying melamine molecules as in other assembly structures while the brighter protrusions as complexes of melamine and Cu adatoms. Obviously here the bright protrusions have arranged into a hexagonal pattern which naturally define the periodic unit cell, as shown by the black rhombus in FIG. 4(b). The measured unit cell (marked by black rhombus) is superimposed on the high resolution STM image and the lattice parameters are measured as a=b=1.92
The above all experimental results have clearly demonstrated that upon dosing Cu onto the half monolayer melamine on Au(111) the assembly structure has drastically changed and presented an obvious dependence on the dosing amount of Cu. The newly emerged structures, i.e. the TK and TA types of structures are therefore proposed to be the co-assemblies of melamine with different number of Cu adatoms. Such proposition was based on the consideration of the potential interactions between melamine and the Cu adatoms. It also found concreate supports from our experimental observations of the same assembly structures of melamine on a Cu/Au(111) film . On the basis of these models, we may briefly discuss the formation mechanism of these assembly structures as well as the connections between them.
Firstly, we look at the interaction between Cu adatom and melamine molecule. Previous studies of melamine on Cu(111) and Cu(100) have demonstrated a strong interaction between melamine and bulk Cu surfaces. Upon depositing at room temperature, melamine would dehydrogenate and form strong chemical bonds with the Cu surfaces. Such chemisorption is so strong that the adsorbed molecules would not desorb even after heated to above 700 K. However, for the singly dispersed Cu atoms, the situation may be different. The STM images in FIG. 2 clearly demonstrate that the melamine molecules maintain their honeycomb structure on an Au(111) surface doped with diluted Cu atoms. This fact manifests that melamine has little tendency to drag Cu atoms from the subsurface to the top surface. In addition, our thermal treatments of the Cu-melamine co-assemblies (TK and TA structures) lead to the same desorption behavior upon mild heating, indicating that the interactions between Cu adatom and the melamine cannot be that strong. Both arguments would support our assumption of the complex of Cu adatom with the melamine molecules, among which the latter keeps undissociated state.
Concerning the phase transition of the assembly films, in most cases it is driven by formation of a thermodynamically more stable structure. The principle may be applied to the co-assembly of melamine and Cu adatom in the present study as well. Table I lists the structural parameters of the three types of assembly structures of melamine with and without Cu adatoms. It can be found that upon forming the co-assembled structure the melamine density increased from 1.63 nm-2 in the honeycomb structure to 2.04 nm-2 in the TK structure and 1.87 nm-2 in the TA structure, respectively. The density of hydrogen bonds is also increased correspondingly since in all three assemblies each melamine molecule forms six hydrogen bonds with three neighboring molecules. Such increment demonstrates that both TK and TA structures have lower energy than the honeycomb structure, thus being the favored phases with the presence of Cu adatoms. It is noticed that the TA structure was formed on the basis of TK structure with more Cu adatoms deposited, yet it has slightly lower densities of both melamine and hydrogen bond compared to the latter. We assume such disadvantage may be compensated by the number of density of Cu adatoms in the assembly structure considering that the incorporation of each Cu adatom with the amino group would induce an energy drop. In FIG. 4 we present a model with only one Cu adatom in a melamine triangular cluster. As a matter of fact, we frequently found that two or three bright protrusions aggregate together close to the middle of the triangular cluster (see FIG. S2 in supplementary materials), suggesting more than one Cu adatoms can be co-assembled in the structure. As a result, the averaged Cu adatom density in the TA structure can be estimated around 0.62 nm-2 (corresponding to two Cu adatoms per triangular molecular cluster), becoming slightly larger than 0.51 nm-2 in the TK structure. And the induced energy gain should be able to compensate the loss caused by the dilution of the melamine-substrate interactions and the hydrogen bonds.
Finally, let us briefly discuss the formation mechanism of the Cu-melamine co-assembly. As shown in FIG. 4, the TA structure is composed of many identical triangular melamine clusters. Actually, the repeating unit of the TK structure can also be divided into similar triangular cluster each containing one Cu adatom, as shown in FIG. 3(d). In this regard, in TK structure the triangles have to share their sides with neighboring triangles, making large difference from the TA structures. In addition, the detailed structure of the triangular cluster of TK is not identical with that in TA. Nevertheless, both TK and TA structures contain similar hydrogen bonding network, particularly around the melamine molecule with one Cu adatom underneath, as shown in FIG. 5(b). Such common substructure of Cu-melamine co-assemblies presents drastic difference from that in the honeycomb structure of solely melamine.
As shown in FIG. 5, in the honeycomb structure the hydrogen bonds around each melamine form a pin-wheel pattern with three-fold rotational symmetry. Upon depositing Cu onto the honeycomb film, the Cu adatoms may attach to the N atom of either the triazine cycle or the amino group. But the former would only take place at the periphery melamine molecule of the assembly domain, as the tiazine-N is fully occupied by the hydrogen bonds. While the latter becomes more feasible since the bonded melamine can just shift by one N atom and form new hydrogen bonds, as shown in FIG. 5(b). By doing so, the molecular interdistances become reduced while keeping the number of hydrogen bonds. As a result, the density of the melamine molecule as well as the hydrogen bonds is increased, in combination with the added Cu-melamine interactions. In this way, the honeycomb structure can be gradually transformed into the track-like or triangular structures, depending on the number of the incorporated Cu adatoms as well as the spreading pattern of the triangular melamine clusters.Ⅴ. CONCLUSION
We have researched the regulation effect of Cu adatoms on the self-assembly structures of melamine on an Au(111) surface with STM. It is found that the evaporated Cu adatoms incorporate into the melamine assembly by accommodating the underneath positions of the amino groups. The interaction between the Cu adatom and melamine molecule is moderate, yet it significantly modulates the hydrogen bond patterns from a three-fold rotationally symmetrical pattern to an unsymmetrical pattern. Such metal-incorporated self-assembly may potentially serve as a reservoir for metal atoms, the latter may get involved and thus play important roles in various surface reactions.
Supplementary materials: Additional STM images of the Cu-doped bare Au(111) surface (FIG. S1), the TA structure incorporating two or three Cu adatoms (FIG. S2), and the coexistence of TK and TA structures (FIG. S3) are given.Ⅵ. ACKNOWLEDGMENTS
This work was supported by the National Natural Science Foundation of China (No.91545128, No.21333001, No.91227117) and Ministry of Science and Technology of China (No.2011CB808702), the Fundamental Research Funds for the Central Universities and the Thousand Talent Program for Young Outstanding Scientists of the Chinese Government, and the "Strategic Priority Research Program" of the Chinese Academy of Sciences (XDB01020100).
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