李若寧，張雪，薛娜，李杰，吳天昊，徐榛，王一帆，李娜，唐浩，侯士敏,,＊，王永鋒 ,5,＊

1北京大學(xué)電子學(xué)系，納米器件物理與化學(xué)教育部重點實驗室，碳基電子學(xué)中心，北京 100871

2天津第五中心醫(yī)院，天津市早產(chǎn)兒器官發(fā)育表觀遺傳學(xué)重點實驗室中心實驗室，天津 300450

3北京大學(xué)(天津濱海)新一代信息技術(shù)研究院，天津 300450

4法國國家科學(xué)研究中心，材料與結(jié)構(gòu)研究所，表界面納米研究組，圖盧茲 31055，法國

5北京量子信息科學(xué)研究院，北京 100193

1 Introduction

Surface-assisted metal-organic nanostructures have been investigated extensively because of their structural stability and potential applications in catalysis, sensor systems, gas storage and electron confinement1–5. Metal-organic nanostructures are built on single-crystalline surfaces using externally deposited metals6–10or native surface atoms11–13. Externally deposited metals are of rich diversity, depending on the targeted nanostructures. Native surface atoms, on the other hand are only obtained from a few kinds of single-crystalline metal surfaces of gold, silver and copper usually used in surface science4.Scanning tunneling microscopy (STM) is an effective tool to study surface nanostructures14–17. Most of the metal-organic nanostructures are constructed by Au-coordinated or Cucoordinated motifs, while only a few are synthesized using surface Ag atoms18. A further investigation into the interactions between molecules and surface Ag atoms, is helpful to the controlled fabrication of desired nanostructure.

As building blocks of metal-organic nanostructures, organic molecules coordinate with native surface atoms by M―C,M―N and M―O bonds19. The C―M―C linkages of coordinated structures are realized by reactions of terminal alkynes20,21or Ullmann couplings22,23. Molecules with terminal cyano or pyridyl groups, could coordinate with Cu and Au atoms and form N―M―N bonds24–26. Moreover, surface Ag adatoms could coordinate with phthalocyanine through Ag―N bonds27.For M―O coordination bonds, a comprehensive study is still lacking. Thus, we planned to use hydroxyl-terminated molecules to coordinate with Ag adatoms to form metal-organic coordination nanostructures.

Here, a series of ordered two-dimensional structures of H3PH optimized molecular model shown in Fig. 1 inset on Ag(111)were studied by low-temperature STM. H3PH molecules with 120° backbone could easily form 2D crystals and Sierpiński triangle fractals18,28,29. Upon deposition of molecules on Ag(111), densely packed patterns appeared which were stabilized by cyclic hydrogen bonds. When the annealing temperature increased to 330 K, an ordered nanostructure formed by O―Ag bonds and hydrogen bonds. Further raising the annealing temperature to 420 K resulted in an ordered hierarchical nanoarchitecture. Density functional theory (DFT)calculations revealed a low energy barrier of the O―Ag coordination bond and a large coordination energy of the O―Ag―O linkage were the reasons for the formation of the hierarchical metal-organic nanostructure.

Fig. 1 A close-packed ordered structure of H3PH molecules at room temperature. (a) STM image of such nanostructures are formed by cyclic hydrogen bonds on Ag(111). Imaging conditions: constant current, VB = 2 V, It = 10 pA. Inset: optimized model structure of an H3PH molecule. (b) High-resolution STM image of the ordered structure superimposed with optimized molecular model, The black dashed rectangles highlight two types of different bonding nodes denoted as I and II, respectively. Imaging conditions: constant current,VB = 1 V, It = 10 pA. (c), (d) The two enlarged bonding nodes depict different arrangements of cyclic hydrogen bonds (dashed orange lines).

2 Experimental and computational section

2.1 STM

All STM experiments were carried out with an ultrahigh vacuum LT-STM system (Unisoku-1500). Clean Ag(111)surfaces were prepared by repetitive cycles of Ar+sputtering and annealing at about 500 °C. H3PH molecules were thermally deposited from a homemade tantalum boat onto the clean substrate at room temperature. Then, the sample was annealed at different temperatures in the preparation chamber and transferred to the STM scanner. All STM images were acquired at liquid helium temperature with a sharpened Pt/Ir tip and processed by WSxM software30. The bias voltages were applied to the sample.

2.2 DFT calculations

DFT calculations of the electronic structures and the transition states of molecules obtained by Climbing Image Nudged-Elastic Band (CI-NEB)31method were carried out by using the 5.3.3 version of the ViennaAb initioSimulations Package (VASP)32,33.The ion-electron interaction was handled by the projector augmented wave (PAW) method34,35. To take into account the van der Waals (vdW) dispersive interaction, the exchangecorrelation energy was described by the opt-B88 functional36–38.Geometries were fully relaxed in a box with 12 ? (1 ? = 0.1 nm)vacuum in all directions to distinguish the interactions among the periodic images. The force convergence threshold at each atom was set at 0.05 eV·??1.

3 Results and discussion

Fig. 1b show the STM images of H3PH molecules deposited on Ag(111) at room temperature. Similar to previous works14,H3PH molecules aggregate into a close-packed ordered 2D structure by cyclic hydrogen bonds. As shown in the optimized molecular model, the unit cell consists of eight H3PH molecules,bonded by different bonding nodes (black dashed tectangles)denoted as I and II, respectively. The two enlarged bonding nodes are formed by cyclic hydrogen bonds in different arrangement (dashed orange lines) in Fig. 1c,d. The symmetrical nodes I are trimeric cyclic hydrogen bonds (O―H···O). Node II consists of two trimeric hydrogen bonds. In contrast, the ordered self-assembled structure on Au(111) is slightly different14,whose unit cell is stabilized by four H3PH molecules through cyclic hydrogen bonds. The discrepancy between the assembly patterns of H3PH on Au(111) and on Ag(111) was due to the difference of molecule-substrate interaction strength of the two surfaces. For Ag(111), the interaction between molecules and substrate is strong enough to affect the arrangement of H3PH molecules. The ordered structure reflects the subtle balance between the molecule-molecule and molecule-substrate interactions.

Then, upon annealing of the sample at 330 K for 20 min, a new ordered structure emerged (Fig. 2a). From the highresolution STM image in Fig. 2b, we can see that molecules arrange back-to-back in the repeat unit (black dashed parallelogram). In the annealing process, H3PH molecules formed coordination bonds with Ag adatoms (O―Ag―O), since no other metal atoms were introduced into the system. In addition, hydrogen bonds formed between the oxygen atoms of the hydroxyl groups of one molecule and the hydrogen atoms of adjacent molecules. Thus, the formation of the new ordered structure were ascribed to the coexistence of coordination and hydrogen bonds.

Fig. 2 An ordered nanostructure formed at 330 K. (a) Large-scale STM image of the ordered structures formed by H3PH molecules through hydrogen bonds and coordination bonds. Imaging conditions:constant height, VB = 20 mV, It = 60 pA. (b) High-resolution STM image of the ordered structure. A repeat unit of the structure marked by black dashed parallelogram, contains ten H3PH molecules.Imaging conditions: constant current, VB = 10 mV, It = 100 pA.(c) Optimized molecular model of a unit cell consisting of coordination bonds and possible hydrogen bonds.

Further increasing the annealing temperature to 420 K for 20 min, a honeycomb structure and a coexisting close-packed structure appeared. The hexagonal-ring-like building unit of the honeycomb structure (Fig. 3b) consists of six H3PH molecules and six Ag adatoms connected by coordination bonds(O―Ag―O). As a porous network, the host honeycomb structure can accommodate one or three guest molecules. These hexagonal rings packed into large-area honeycomb structuresviamolecule-molecule interactions, similar to the FePc networks grown on single-layer graphene39. Therefore, the ordered honeycomb nanoarchitecture can be seen as a hierarchical assembly based on its assembling process. A close-packed structure composed of two-fold coordination chains (Fig. 3d)were also observed. Obviously, the type of coordination node(windmill) is different from that in the hexagonal ring. So the pattern of the ordered structure depends on the type of the coordination nodes. Increasing the annealing temperature further, most molecules desorbed and the ordered structure were destroyed.

Fig. 3 Emergence of a honeycomb structure and a close-packed structure after 20 min annealing at 420 K. (a) Large-scale STM image of the honeycomb structure. Imaging conditions: constant current,VB = 30 mV, It = 80 pA. (b) High-resolution STM image of the hexagonal rings (white dotted hexagon) associated by coordination bonds.Imaging conditions: constant height, VB = 1 mV, It = 1 nA.(c) Optimized molecular model of the hexagonal ring constructed by O―Ag―O coordination bonds. (d) High-resolution STM image of the close-packed structure linked by coordination bonds. The repeat unit is shown in white dotted rectangle. Imaging conditions: constant height, VB = 10 mV, It = 100 pA. (e) Optimized molecular model of the close-packed structure with 2-fold nodes (windmill).

To explore the formation mechanism of the ordered structures, we calculated the energy barriers (Fig.4) to form coordination bonds and the coordination energies of the molecule-Ag-molecule system. The energy calculations of a H3PH-Ag system along the reaction coordinate of atom distance during the coordination process gave an energy barrier of 1.41 eV. The initial state (IS), transition state (TS) and finial state (FS)represent different states of the system during the O―Ag coordination bond formation. The coordination energy (ΔE) of the system is evaluated using the following equation, ΔEn=Esys?EAn?EBn(n= 1, 2), whereEsys,EAnandEBnrepresent the energies of the Ag coordination system, molecules and molecule-Ag, respectively. Considering the two different ways to form coordination bonds, the coordination energy of the system by the two ways were calculated respectively. As a result,the coordination energy of the system ΔE1is ?1.86 eV close to ΔE2, with negligible difference. The energy barrier of the O―Ag―O linkage is large than that one of O―Ag bond. A low energy barrier of the O―Ag bond and a large coordination energy of the O―Ag―O linkage were the reasons for the formation of the hierarchical metal-organic nanostructure.Moreover, the large coordination energy also confirms the stability of the two-fold coordinate system.

Fig. 4 Metal-molecule reaction pathways and energy barriers of the O―Ag―O bonds are calculated. (a) Calculated energies of all atoms of H3PH and an Ag atom in the initial state (IS), transition state (TS) and final state (FS) along the reaction coordinate during the O―Ag bond formation. H3PH coordinates with a Ag atom by overcoming an energy barrier of 1.41 eV. Optimized molecular models of different states during the coordination process were added on the plot.(b) Coordination energies of the molecule-Ag-molecule coordination system.

4 Conclusions

In conclusion, we have successfully constructed a series of ordered structures by depositing H3PH molecules on Ag(111).At room temperature, a close-packed ordered 2D structure was formed by H3PH molecules through cyclic hydrogen bonds. The formation of O―Ag bond led to changing of the assembly pattern upon increasing the annealing temperature. H3PH molecules and Ag adatoms formed hierarchical nanoarchitectures consisting of close-packed structures at 330 K,and large honeycomb structures based on hexagonal rings at 420 K. DFT calculations were carried out to further understand the formation of these structures. The coordination energy of the two-fold coordination system and the energy barrier of O―Ag bond formation both proved a facile construction of ordered structures by H3PH molecules and Ag adatoms. Our results demonstrate an approach of designing and building 2D hierarchical structures with organic small molecules and metal adatoms.

Acknowledgment:The calculations were carried out at National Supercomputer Center in Tianjin on TianHe-1 and High-performance Computing Platform of Peking University.