Halogen-Adatom Mediated Phase Transition of Two-Dimensional

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Halogen-adatom Mediated Phase Transition of Twodimensional Molecular Self-assembly on Metal Surface Tianchao Niu, Jinge Wu, Faling Ling, Shuo Jin, Guang-Hong Lu, and Miao Zhou Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.7b03796 • Publication Date (Web): 21 Dec 2017 Downloaded from http://pubs.acs.org on December 21, 2017

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Halogen-adatom Mediated Phase Transition of Twodimensional Molecular Self-assembly on Metal Surface Tianchao Niu†, Jinge Wu‡, Faling Ling§, Shuo Jin‡, Guanghong Lu‡, and Miao Zhou‡* †

College of Materials Science & Engineering, Nanjing University of Science and Technology,

Nanjing 210094, China. ‡

School of Physics, Beihang University, Beijing 100191, China.

§

Key Laboratory of Optoelectronic Technology & Systems (Ministry of Education), College of

Optoelectronic Engineering, Chongqing University, Chongqing 400044, China.

ABSTRACT

Construction of tunable and robust two-dimensional (2D) molecular arrays with desirable lattices and functionalities over macroscopic scale relies on spontaneous and reversible noncovalent interaction between suitable molecules as building blocks. Halogen bonding, with active tunability of direction, strength and length, is ideal for tailoring supramolecular structures. Herein, by combining low-temperature scanning tunneling microscopy and systematic first-principles calculations, we demonstrate novel halogen bonding involving single halogen

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atoms and phase engineering in 2D molecular self-assembly. On Au(111) surface, we observed catalyzed dehalogenation of hexabromobenzene (HBB) molecules, during which negatively charged bromine adatoms (Brδ-) were generated and participated in assembly via unique CBrδ+···Brδ- interaction, drastically different from HBB assembly on chemically inert graphene substrate. We successfully mapped out different phases of the assembled superstructure, including densely packed hexagonal, tetragonal, dimer chain and expanded hexagonal lattices at room temperature, 60°C, 90°C and 110°C respectively, and the critical role of Brδ- in regulating lattice characteristics was highlighted. Our results show promise for manipulating the interplay between noncovalent interaction and catalytic reaction for future development of molecular nanoelectronics and 2D crystal engineering.

Two-dimensional (2D) molecular self-assembly, which relies on spontaneous and reversible intermolecular noncovalent interaction, is governed by the physical/chemical properties of molecules as building blocks1,2. A variety of well-established noncovalent intermolecular interactions, including hydrogen bonding 1 , π-π stacking 2 , donor-acceptor 3 , dipole-dipole 4 and halogen bonding 5 , have been widely exploited and extensively studied in supramolecular chemistry and materials science6,7. In order to tailor the shape and functionalities of the assembled architecture, one has to tune the intermolecular interaction by selecting suitable molecular components in the bottom-up processing 8,9 , which has also become one key issue for further development of 2D molecular crystals and nanoelectronics.

The above-mentioned noncovalent interactions are often accompanied by electrostatic attraction between molecules. As depicted in Scheme 1, intermolecular interaction is dominantly


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governed by edge atoms of molecules, which tends to be mediated by substrate simultaneously. A net attractive interaction between an electrophilic region (X: δ+ polarized) in one molecule and a nucleophilic region (Y: δ- polarized) in another stabilizes the assembled structure by minimizing the total energy (Scheme 1, Left). Many noncovalent interactions belong to this category. For instance, hydrogen bonding involves an attractive interaction between a hydrogen atom in a fragment X-H (X=C, N, O etc.) and an electronegative atom nearby, with typical examples including water, DNA, proteins and polymers. π-π stacking often occurs in aromatic nucleus, for which an off-center or slipped geometry instead of a face-to-face stacking is favoured to maximize the electrostatic interaction10. In a donor-acceptor system, such as tetrathiafulvalene (TTF) and tetracyanoquinodimethane (TCNQ) co-deposited on substrate, S atoms in TTF are positively charged while N atoms of C≡N groups in TCNQ are negatively charged, forming strongly bonded donor-acceptor complexes 11 . Dipole-dipole interaction is induced by inequivalent electron distribution of a molecule with partially negative charge on one side and positive charge on the other side, which can be either attractive or repulsive that depends on the molecular orientation12. Halogen bonding is quite special in a sense that covalently bonded halogen element is positively polarized along the covalent bond axis and negatively polarized in the perpendicular direction, so that net attraction can happen between an electrophilic region in one molecule and a nucleophilic region in another, or in the same molecule13 (Fig. S1 in Supporting Information). Consequently, recent years have witnessed a surge of research pertaining to supramolecular structures assembled via halogen bonding14,15.


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Scheme 1. Illustration of electrostatic interactions in molecular self-assembly. Interactions between positively polarized region (δ+) and negatively polarized region (δ-) within molecules (Left), molecules and single adatoms (Right) are illustrated. They are mediated by substrate and can be effectively applied for 2D self-assembly on metal surface. This scheme is inspired by ref. 16

. In general, molecules experience minimal structural change with noncovalent interaction,

especially on inert substrate like graphene or boron nitride17. However, it remains an open question if these molecules endure severe distortion or even chemical reaction, how the assembled superstructure will be influenced. This is highly possible for halogenated molecules on metal substrate, because molecules with halogen elements (particularly heavier atoms like Br and I) tend to be dissociated at finite temperature18 and metal surface can function as catalyst for chemical reaction

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. However, insightful understanding with careful theoretical and experimental

investigation is still lacking20,21,22,23. In present study, we aim to address this by exploring the catalytic dissociation of molecule on metal surface and its impact on the phase engineering of self-assembled 2D superstructures. The smallest halogenated molecule with π-conjugation, hexabromobenzene (HBB, C6Br6), was


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used as model molecule to reveal the critical role of generated individual Br adatoms in mediating the phase transition of HBB on Au(111) surface. It should be noted that although Au(111) is chemically inert, previous reports showed that it could break C-Br bond at elevated temperature24. Via joint low-temperature scanning tunnelling microscopy (STM) and systematic density functional theory (DFT) based ab initio calculations, we mapped out different phases of the assembled 2D molecular superstructure, including densely packed hexagonal, tetragonal, dimer chain and expanded hexagonal phases at room temperature, 60°C, 90°C and 110°C, respectively. We found that Br adatoms gain charge from Au due to the large electronegative difference, leading to negatively charged Brδ- adatoms. For the first time, we demonstrate a new form of halogen bonding involving HBB molecule and individual Brδ- adatom, i.e. C-Brδ+···Brδ-, which determines the structural phase transition (Scheme 1, Right). For comparison, we also investigated assembly of HBB molecules on graphene, which shows drastically different behaviours due to the chemically inertness of substrate. Careful utilization and manipulation of halogen adatoms for molecular self-assembly and phase engineering may provide an interesting way to design and fabricate large-scale 2D superstructures in a controlled manner.

Results and Discussion Deposition of HBB on Au(111) surface at room temperature leads to a highly ordered selfassembled monolayer, as shown in the large scale STM image in Fig. 1a. Periodic bright lines are the typical herringbone reconstruction of Au(111), which is useful to define the surface lattice direction. Molecularly resolved STM images reveal a hexagonal pattern with a unit cell of a1=b1=9.5 Å, 1=60°(Fig. 1b and c). Each molecule is featured by six bright dots indicating the six peripheral Br atoms and a relatively dark centre. There observations are in excellent agreement with previous report25.


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Figure 1. Structural and electronic properties of HBB on Au(111) with a densely packed hexagonal pattern. (a) Large area STM image of monolayer HBB deposited on Au (111) surface at room temperature (-1.5 V, 200 pA). (b) Zoom-in scan showing a densely packed hexagonal phase (-0.5 V, 200 pA). (c) Experimental (Left) and DFT simulated (Right) STM image. (d) Top and side view of a single HBB molecule adsorbed on Au(111), with adsorption distance indicated. (e) Charge density redistribution plot for the adsorbed system, with an isovalue of 0.0002 e/Å3. Yellow and blue isosurfaces indicate charge accumulation and depletion, respectively. (f) DOS for gas-phase HBB (Left) and partial DOS projected to C6Br6, 5d and 6s orbitals of Au for the adsorbed system (Right). Fermi level is set to zero. Real-space molecular orbitals including highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) are also shown, with dashed lines indicating orbital shift after adsorption. (g) DFT optimized structure of the densely packed hexagonal phase, with white dashed lines indicating the unit cell. Six halogen bonds within


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nearest-neighbour HBB are marked by red dashed lines. Inset shows a side view of the structure with adsorption distance indicated.

To understand the detailed packing geometry and intermolecular interaction, we performed DFT calculations including van der Waals corrections. We first studied single HBB molecule adsorbed on Au(111) surface. Various adsorption configurations were considered to determine the ground-state geometry with the lowest energy, and then adsorption energies and electronic structures were analysed. As shown in Fig. 1d, single HBB adsorbs on bridge site of Au(111) with molecular axis parallel to [1-10] direction. The adsorption distance was calculated to be 3.56 Å with adsorption energy of -1.75 eV. Charge density redistribution plot in Fig. 1e clearly shows that HBB loses charge to substrate, suggesting electrostatic interaction between HBB and Au(111). To quantitatively see the charge transfer, we used the integration of differential charge density above the plane cutting through the middle point between HBB and Au(111), in together with Bader charge analysis method26. It was found that about 0.1 e is transferred from molecule to substrate, making HBB positively charged. To gain deeper insight into the electronic structure, we calculated the partial density of states (DOS) of the system. We found that although there is an energy overlap between molecular orbitals of HBB and 5d, 6s orbitals of Au, essentially no orbital hybridization can be seen (Fig. 1f). Interestingly, all molecular orbitals have a rigid shift to lower energies upon adsorption, with a little yet noticeable broadening feature of HOMO and LUMO, which further confirms the electrostatic interaction between HBB molecule and Au(111) surface. Figure 1g shows the optimized structure for a densely packed hexagonal monolayer of HBB on Au(111). We found that similar to single molecule adsorption, HBB molecules prefer to reside at the bridge site with an adsorption distance of 3.66 Å, but the molecular axis rotates by 45°


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respect to [1-10] direction. With this orientation, the nearest Br···Br distance of a Br3 synthon is 3.8 Å, which is in the range of typical halogen bonds16. Our simulated STM images agree perfectly well with the experimental result (Fig. 1c), suggesting the validity of our theoretical models. The adsorption energy was calculated to be -1.76 eV, comparable to the case of single molecule adsorption.

Figure 2. Brδ- adatom formation and phase transition to tetragonal phase. (a) Carbon clusters formed from dissociated HBB molecules and tetragonal phase of HBB on Au(111) after annealing at 60 °C (1 V, 200 pA). (b) Enlarged view of a domain boundary between hexagonal and tetragonal pattern (200 mV, 500 pA). (c) Molecularly resolved STM image showing the presence of an adatom at the center of each unit cell (-200 mV, 500 pA). (d) Top and (e) side view of the DFT


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optimized structure. White dashed lines indicate the unit cell. Halogen bonding between HBB molecules, HBB and Brδ- adatoms are marked by red and blue dashed lines, respectively. Adsorption distances of HBB and Br adatoms are indicated. (f) Charge density difference plot (isovalue=0.0002 e/Å3), with yellow and blue denoting charge accumulation and depletion, respectively. (g) Experimental (Left) and DFT simulated (Right) STM image. Brδ- adatoms are clearly visible as grey white dots.

Next, we annealed the system at 60°C and studied the chemical dissociation of HBB molecules and possible phase transition. As shown in Fig. 2a, a tetragonal phase appears preferentially near step edges of the surface. These edges become rough and decorated with random bright spots that can be assigned to be carbon clusters formed from catalysed chemical reaction, as we will discuss below. It is well known that uncoordinated atoms along step edges of Au(111) are relatively reactive27, so that HBB molecules can be dissociated when the substrate temperature is elevated, which generates single Br atoms and carbon radicals. Carbon radicals are generally active, so they react with each other to form complex carbon clusters near step edges, appearing as bright spots in STM (Fig. 2a). Single Br atoms tend to be adsorbed on Au(111) and diffuse on the surface. Figure 2b shows a domain boundary between the densely packed hexagonal and tetragonal phase. As guided by the white dashed arrow, all molecules in the tetragonal phase assembled in the same orientation, which rotates by 30°with respect to [11-2] direction of Au(111), exhibiting a square unit cell around 10 Å with an azimuth angle of 45°. Remarkably, as shown in the enlarged STM image in Fig. 2c, a single grey white dot appeared at the centre of each unit cell, surrounded by four HBB molecules, which is the diffused Br adatom.


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Figure 2e depicts the DFT optimized structure of the tetragonal phase. It has a unit cell lattice of a2=10.09 Å, b2=9.99 Å that matches well with experimental observations. Our calculations show that Br adatoms sit at the hollow site of the Au(111) surface with an adsorption distance of 2.44 Å, much shorter than that of HHB molecule (3.53 Å), thus appearing as grey white dots (Fig. 2c). By plotting out the charge density difference (Fig. 2f), we found that significant charge was transferred from Au to Br adatoms (~ 0.32 e per atom) due to the high electronegativity of Br, resulting in negatively charged Br adatoms (Brδ-), while HBB molecules remain positively charged. Therefore, there are two forms of halogen bonding in the system, intermolecular C-Brδ+···Brδ--C interaction with a bond distance of 4.0 Å (red dashed lines in Fig. 2d) and C-Brδ+···Brδ- interaction between HBB and Brδ- adatoms (3.8 Å, blue dash lines). The different bonding environment of peripheral Br atoms in each HBB molecule, particularly with strong C-Brδ+···Brδ- interaction, leads to structural deformation of HBB molecules in the tetragonal phase, which is shown in Fig. S2. As a result, adsorption of HBB molecule became much stronger than the densely packed hexagonal phase, with adsorption energy increased from -1.76 eV to -1.92 eV. Interestingly, due to the strong adsorption, we found that the herringbone reconstruction of Au(111) disappeared underneath the tetragonal phase. Annealing the assembled structure at 90°C for 20 mins, we found that both hexagonal and tetragonal phases evolved to another new phase comprising HBB dimer chain arrays. As shown in Fig. 3a, terraces of Au(111) were populated with ordered dimer arrays. It could also be found that plenty of disordered carbon clusters appeared around steps and domain boundaries, which were produced by the higher degree of HBB molecule dehalogenation under an elevated substrate temperature. As a result, more Brδ- adatoms were generated and diffused into the self-assembled HBB molecular arrays. As shown in the zoom-in scan (Fig. 3b), these dimer pairs were linked by


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many grey white dots with an asymmetric feature, suggesting the participation of diffused Brδadatoms. The dimer array has a unit cell of a3=2.56 nm, b3=1.26 nm with 3=69°. To obtain the detailed atomic structure, especially the number of Brδ- adatoms and their location, we constructed a series of initial configuration and used DFT calculations to find the most likely structure as compared to the STM image. As shown in Fig. 3c, we found that there were 14 Brδ- adatoms surrounding two HBB molecules within one dimer pair, and these adatoms were asymmetrically distributed on the surface. With this structure, simulated STM image agrees perfectly well with the experimental observations (Fig. 3d). Furthermore, surrounded by Brδ-, HBB molecules were adsorbed much closer to the substrate (3.4 Å) and the adsorption energy was calculated to be -1.82 eV, much larger than the densely packed hexagonal phase, which can be ascribed to the larger number of C-Brδ+···Brδ- bonding.


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Figure 3. Phase transition to dimer array pattern. (a) Large area STM image showing HBB dimer chain arrays on terraces, with step edges decorated by bright carbon clusters (1.5 V, 100 pA); (b) Zoom in scan of the dimer array and Brδ- adatoms connecting dimer pairs (0.05 V, 200 pA). (c) Top and side views of the DFT optimized structure, with unit cell and the number of surrounded Brδ- labelled (1~14). Red and blue dashed lines indicate nearest halogen bonding between HBB molecules, HBB and Brδ- adatoms, respectively. (d) Experimental and simulated STM image of the dimer array.

Increasing the annealing temperature to 110°C, dimer array phase vanished and disordered regions increased (Fig. 4a). Meanwhile, a hexagonal phase with an expanded unit cell appeared, and a clear domain boundary between the expanded hexagonal pattern and disordered phase can be seen (Fig. 4b). More carbon clusters were formed due to the more dissociated molecules at this temperature. Each HBB molecule was surrounded by more Brδ- adatoms, and the structure exhibited an expanded unit lattice of a4=b4=1.31 nm, 4=60°(Fig. 4c). We carefully studied the atomic structure by using DFT, and found that there were 9 Brδ- adatoms surrounding each HHB molecule, with the simulated STM image agreeing well with the experimental observations (Fig. 4d). In particular, the asymmetric bonding environment of HBB with Brδ- can be distinctly identified (Fig. 4e). As a result, the expanded hexagonal phase is purely stabilized by C- Brδ+···Brδbonding. It is noteworthy that molecular orientation of HBB in this expanded hexagonal phase can be varied from each other, as highlighted by coloured arrows in Fig. 4f. This feature is dramatically different from previous three phases, in which HBB molecules retain identical conformation in the same domain. The physical origin can be traced back to the bonding environment of HBB


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molecules, particularly the number of surrounding Brδ- adatoms. For the densely packed hexagonal phase, each HBB was surrounded by six HBB molecules. Varying the orientation of one molecule changes the orientation of all molecules, demanding for high energy. HBB molecule is effectively linked by two Brδ- adatoms in the tetragonal phase, and two HBB in each dimer pair is surrounded by nine Brδ- adatoms in the dimer chain phase. In both cases, varying orientation of one molecule will influence the orientation of the molecule nearby, which also needs energy. However, for the expanded hexagonal phase, each HBB is encircled by nine Brδ- adatoms without neighbouring molecules, so that HBB can change its orientation easily, because Brδ- adatoms are highly diffusible that allow enough space for molecule reorientation. As shown in Fig. S3, two metastable structures with different orientations deviated from Fig. 4c are shown, which have adsorption energies smaller by only ~0.02 eV, suggesting that HBB may flexibly change orientation on Au(111) surface.


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Figure 4. Expanded hexagonal phase after further annealing at 110°C. (a) Large area STM image showing the coexistence of ordered and disordered phase (1.2 V, 220 pA). (b) Boundary between ordered and disordered phase (-500 mV, 500 pA). (c) High resolution STM image, with HBB molecules surrounded by Brδ- adatoms, giving rise to an expanded unit cell. (d) DFT optimized structure, with unit cell (white dashed lines) and halogen bonding (blue dashed lines) between HBB and Brδ- indicated. (e) Experimental and simulated STM image. (f) Representative STM image showing the flexible orientation of HBB within ordered and disordered domains (white and black dashed arrows).

We also calculated the bonding strength for C-Br+... Br- interaction of our systems, for which the bonding energy (Ebond) can be calculated by 𝐸bond = [𝐸total − (𝐸HBB@Au + 𝐸Br@Au ) + 𝐸Au ]/ 𝑛, where Etotal, EHBB@Au, EBr@Au and EAu are the energies of the total adsorbed system, HBB molecules on Au(111), Br adatoms on Au(111) and pure Au(111) surface, respectively. n is the number of C-Br+... Br- bonding in the supercell. It should be noted that structures of HBB molecules on Au(111), Br adatoms on Au(111) and pure Au(111) surface are kept unchanged with respect to the total adsorbed system in order to rule out other interactions so that we can obtain pure C-Br+... Br- bonding. Our calculations show that bonding strength of C-Br+... Br- ranges from -0.35 eV to -0.11 eV and -0.07 eV for the tetragonal, dimer array and expanded hexagonal phases, respectively. Such energy difference is closely related to the distance of C-Br+... Br- bonds and the charging state of Br atoms that determine the strength of electrostatic interaction. The calculated averaged bonding distances of C-Br+... Br- are 3.93 Å, 3.97 Å and 4.48 Å and charge transfers from Br adatom to Au are 0.36 e, 0.31 e and 0.29 e for the three systems respectively, which agree well with their bonding strength.


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In literature, it has been shown that chemical bonding of molecules on metal surface (such as thiolate-metal coordination) may cause metal atoms being pulled out of substrate, particularly for noble metal surfaces28, which may further participate in the molecular self-assembly29,30. However, in present study we show that the small bright dots involved in different phases were negatively charged Brδ- adatoms that were generated from the catalysed dehalogenation of HBB molecules. This originates from the very different bonding behaviour of halogenated molecules and metal surface. Our DFT calculations show that with Au adatoms on Au(111) surface, adsorption of HBB molecules becomes much weaker compared to the case of Brδ- (over 0.6 eV for the tetragonal phase), suggesting the favourable C-Brδ+···Brδ- bonding as our STM demonstrates. Previous studies reported that C-Br bond can be dissociated by heating or contacting with a proper catalyst18, and Br islands on Cu(111) can be created by heating the substrate to 600 K after exposure to C6H5Br31. Room-temperature deposition of HBB on Cu(111) led to decomposition of molecules to carbon clusters and Br adatoms 32 , and dehalogenation reactions of brominated organic precursors such as tribromobenzene and 1,4,5,8-tetrabromonaphthalene were also reported on Cu(111)33 and Au(111) 34 surfaces. Our experiments at 60°C, 90°C and 110°C clearly show that step edges of Au(111) surface acted as an efficient catalyst for the dehalogenation dissociation, with carbon clusters and diffusible Brδ- adatoms as products. We further performed experiments by heating the system to higher temperatures of 150°C, 240°C and 420°C, and the corresponding STM images are shown in Fig. S4 in Supporting Information. We found that with increased temperature, a majority of HBB molecules were dehalogenated to carbon clusters and Br atoms that cover the whole Au(111) surface. Interestingly, Br atoms aggregated into a hexagonal pattern, while carbon clusters form large patches with a disordered feature.


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It should be emphasized that Au(111) surface not only offers a robust physical support but also an efficient catalyst for the dissociation of HBB molecules. To gain more physical insight on this, we conducted control experiments on deposition of HBB molecule on monolayer graphene, which itself was grown on Ge(110) wafer by using chemical vapour deposition (CVD). As shown in Fig. S5, adsorption of HBB on CVD graphene leads to a well-ordered monolayer with large domains that span over the entire surface, through steps and wrinkles of graphene. Molecularly resolved STM images show that HBB molecules are packed in a hexagonal lattice with a measured unit cell of a=b= 0.95nm, = 60°. DFT calculations suggest that benzene ring of HBB prefers to sit on top of hexagon in graphene to maximize interfacial π-π interaction (Fig. S6). Annealing the asprepared monolayer structure on graphene leads to desorption instead of dehalogenation, drastically different from the results on Au(111). During the whole process, we do not see any Br adatoms in the structure, indicating that graphene is not a proper substrate to tune the molecular superstructure via simultaneous on-surface reaction. Finally, we would also like to highlight the importance of chemical dehalogenation of molecules during growth of other types of low-dimensional nanostructures on metal surface. Due to the relatively weak C-Br bond, there has been a veritable explosion in constructing functional covalent organic architectures by using brominated organic molecules as precursors. In particular, one of the most widely employed approaches to synthesize atomically precise graphene nanoribbon (GNR)35, relies on using delicately designed brominated monomers, such as 10,10’dibromo-9,9’-bianthryl monomers to grow armchair GNRs

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and halogenated U-shape

dibenzo[a,j]anthracene monomers to synthesize zigzag GNRs 37 . Also, step-by-step covalent connection of halogenated porphyrins in a hierarchical manner on Au(111) demonstrates a promising approach of fabricating heterogeneous architectures with well-defined arrangement


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based on the selective and sequential activation of the carbon-halogen bonds 38 . Despite these studies, little attention has been paid to the halogen adatoms that are generated during the dehalogenation process39. Understanding the behaviour of halogen atoms on surface can provide valuable information on the reaction mechanism of dehalogenation process, which may shed light on molecular polymerization towards functional nanomaterials 40 . As a typical example, Giovannatonio et al. recently reported that generated halogen atoms during Ullman coupling on Cu surface stabilize organometallic intermediates41. In present study, we show that Br adatoms play a critical role in manipulating the structural phase transition in the simultaneous molecular self-assembly and surface reaction process 42 . Therefore, careful investigations on the physical properties of molecular building blocks and precise interpretation of the possible chemical reactions are essential for construction of self-assembled molecular networks and on-surface polymerization towards large area 2D covalently bonded architectures.

Conclusions In summary, by combining high-resolution STM and systematic DFT calculations, we have demonstrated one novel form of halogen bonding involving individual halogen atoms generated during dehalogenation reaction on metal surface. For the first time, we show that negatively charged Brδ- adatoms participate in 2D molecular self-assembly via electrostatic interaction with a unique feature of C-Brδ+⋯ Brδ- bonding. Controllable phase transition of the assembled molecular superstructure highlights the critical role of Brδ- adatoms in mediating the detailed geometry and packing feature. This work provides a prototype for exploring the interplay of noncovalent interaction and catalytic reaction for molecular crystal engineering, which may also provide guidance for fabrication of large area covalent organic frameworks and 2D nanostructures beyond graphene with minimum vacancies and regular motifs.


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EXPERIMENTAL SECTION STM measurements. All STM experiments were conducted using a multichamber STM system with a base pressure better than 1×10-10 mbar, housing a SPECS Joule-Thomson STM interfaced to a Nanonis controller. STM measurements were acquired in a constant-current mode at 4.5 K. Bias voltages were referred to the sample. An electrochemically etched tungsten tip was used for topographic measurements. All the STM images were processed using the commercial SPIP software. Sample preparation. Au(111) single crystal (Mateck) was cleaned by repeated cycles of Ar+ sputtering (0.9 kV) followed by annealing at 650˚C. HBB molecules were evaporated from a commercial Knudsen cell (CreaPhys GmbH) with a sublimation temperature of 85˚C. Au(111) substrate was kept at room temperature during evaporation. HBB molecules were purchased from Sigma-Aldrich and used without further purification. BN crucible was thoroughly degassed prior to deposition. Theoretical calculations. First-principles calculations were performed by using the projector augmented wave formalism of DFT in the Vienna ab initio simulation package (VASP). 43,44 For exchange and correlation functional, we have used the generalized gradient approximation (GGA) in Perdew-Burke-Ernzerhof (PBE) with van der Waals corrections (optPBE-vdW)45,46,47, which has been shown to be able to properly treat the noncovalent electrostatic interaction. A 500 eV was used for the plane-wave basic set and a dense enough k-point mesh was used for structural, energy and electronic property calculation. Au(111) surface was modelled by a slab of five atomic layers with a vacuum region of over 20 Å, with bottom three layers ﬁxed and all other atoms fully relaxed until forces on each atom were smaller than 0.02 eV/Å. STM images were simulated within the


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Tersoff-Hamann approximation48. Adsorption energies (Ea) of molecules were calculated by 𝐸a = [𝐸total − (𝐸Au + 𝑛 ∗ 𝐸HBB )]/𝑛 for adsorbed systems without Br adatoms, and 𝐸a = [𝐸total − (𝐸Br@Au + 𝑛 ∗ 𝐸HBB )]/𝑛 for systems with Br adatoms, respectively, where Etotal, EAu, EHBB and EBr@Au are the energies of the adsorbed system, pure Au(111) surface, HBB molecule in the gas phase, and Au(111) with Br adatoms, respectively. n is the number of HBB molecule. Charge density difference is calculated by ∆𝜌 = 𝜌total − (𝜌Au + 𝜌HBB ) and 𝜌 = 𝜌total − (𝜌Au + 𝜌Br + 𝜌HBB ) for the two cases, where ρtotal, ρAu, ρBr and ρHBB represent charge density of the adsorbed system, Au(111), Br adatoms and HBB molecules, respectively. ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website. (1) Schematic model showing the three types of attractive halogen bonding; (2) Molecularly resolved STM image of the hexagonal and tetragonal phase of HBB on Au(111); (3) DFT-optimized metastable structures of HBB in the expanded hexagonal phase; (4) STM images showing the fully decomposition of HBB to clusters and bromine adatoms; (5) Monolayer HBB on graphene/Ge(110); (6) DFT simulated most stable conﬁgurations for the adsorption of individual HBB on graphene AUTHOR INFORMATION Corresponding author * E-mail: [email protected].


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ORCID: Miao Zhou: 0000-00031390-372X Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS This work is supported by National Natural Science Foundation of China (Grant Nos. 11227902, 21403282, 11674042), Fundamental Research Funds for the Central Universities (Grant Nos. 011900/KG12018801, ZG226S1791, ZG216S1783) and Thousand Young Talents Program of China (Grant No. 0210002102026). REFERENCES

. N. A. Wasio, R. C. Quardokus, R. P. Forrest, C. S. Lent, S. A. Corcelli, J. A. Christie, K. W. Henderson and S. A. Kandel, Self-assembly of hydrogen-bonded two-dimensional quasicrystals. Nature 2014, 507, 86–89 2 . A. Ciesielski and P. Samorì, Supramolecular Approaches to Graphene: From Self-Assembly to Molecule-Assisted Liquid-Phase Exfoliation. Adv. Mater. 2016, 28, 6030-6051. 3 . J. L. Zhang, S. Zhong, J. Q. Zhong, T. C. Niu, W. P. Hu, A. Wee and W. Chen. Rational design of two-dimensional molecular donor–acceptor nanostructure arrays. Nanoscale 2015, 7, 4306–4324. 4 . D. A. Kunkel, J. Hooper, S. Simpson, D. P. Miller, L Routaboul, P.Braunstein, B. Doudin, S. Beniwal1, P. Dowben, R. Skomski, E Zurek and A. Enders. Self-assembly of strongly dipolar molecules on metal surfaces. J. Chem. Phys. 2015, 142, 101921. 5 . Q. N. Zheng, X.-H. Liu, T. Chen, H.-J. Yan, T. Cook, D. Wang, P. J. Stang, and L.-J. Wan. Formation of halogen bond-based 2D supramolecular assemblies by electric manipulation. J. Am. Chem. Soc. 2015, 137, 6128−6131. 6 . E. Goiri, P. Borghetti, A. El-Sayed, J. E. Ortega and D. G. de Oteyza, Multi-component organic layers on metal substrates. Adv. Mater. 2016, 28, 1340–1368 7 . Q. Chen, J. R. Cramer, J. Liu, X. Jin, P. Liao, X. Shao, K. V. Gothelf and K. Wu, Steering On‐Surface Reactions by a Self-Assembly Approach. Angew. Chem. 2017, 129, 5108−5112. 8 . J. W. Colson and W. R. Dichtel, Rationally synthesized two-dimensional polymers. Nat. Chem. 2013, 5, 453−465. 9 . L. Bartels, Tailoring molecular layers at metal surfaces. Nat. Chem. 2010, 2, 87−95. 10 .C. R. Martinez and B. L. Iverson, Rethinking the term “pi-stacking”. Chem. Sci., 2012, 3, 2191−2201. 11 . N. Gonzalez-Lakunza, I. Fernández-Torrente, K. J. Franke, N. Lorente, A. Arnau, and J. I. Pascual, Formation of dispersive hybrid bands at an organic-metal interface. Phys. Rev. Lett. 2008, 100, 156805. 12 . C. J. Murphy, D. P. Miller, S. Simpson, A. Baggett, A. Pronschinske, M. L. Liriano, A. J. Therrien, A. Enders, S. Y. Liu, E. Zurek and E. C. H. Sykes, Charge-transfer-induced magic cluster formation of azaborine heterocycles on noble metal surfaces. J. Phys. Chem. C 2016, 120, 6020–6030. 13 . L. C. Gilday, S. W. Robinson, T. A. Barendt, M. J. Langton, B. R. Mullaney and P. D. Beer. Halogen bonding in supramolecular chemistry. Chem. Rev. 2015, 115, 7118–7195. 14 J. C. Lu, D. L. Bao, H. L. Dong, K. Qian, S. Zhang, J. Liu, Y. F. Zhang, X. Lin, S. X. Du, W. P. Hu and H. J. Gao. 1


20

Page 21 of 22 1 2 3 4 5 6 7 8 9 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 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

Construction of Two-dimensional Chiral Networks Through Atomic Bromine on Surfaces. J. Phys. Chem. Lett., 2017, 8, 326–331. 15 G. Vasseur, Y. Fagot-Revurat, M. Sicot, B. Kierren, L. Moreau, D. Malterre, L. Cardenas, G. Galeotti, J. LiptonDuffin, F. Rosei, M. D. Giovannantonio, G. Contini, P. Le Fèvre, F. Bertran, L. Liang, V. Meunier and D. F. Perepichka. Quasi one-Dimensional Band Dispersion and Surface Metallization in Long Range Ordered Polymeric Wires. Nat. Comm. 2016, 7, 10235. 16 G. P. Cavallo, Metrangolo, R. Milani, T. Pilati, A. Priimagi, G. Resnati and G. Terraneo, The halogen bond. Chem. Rev. 2016, 116, 2478–2601. 17 H.-Z. Tsai, A. A. Omrani, S. Coh, H. Oh, S. Wickenburg, Y.-Woo Son, D. Wong, A. Riss, H. S. Jung, G D. Nguyen, G. F. Rodgers, A. S. Aikawa, T. Taniguchi, K. Watanabe, A. Zettl, S. G. Louie, J Lu, M. L. Cohen, and M. F. Crommie, Molecular self-assembly in a poorly screened environment: F4TCNQ on graphene/BN. ACS Nano. 2015, 9, 12168–12173. 18 . J. Björk, F. Hanke and S. Stafström, Mechanisms of halogen-based covalent self-assembly on metal surfaces. J. Am. Chem. Soc. 2013, 135, 5768−5775. 19 . A. Basagni, L. Ferrighi, M. Cattelan, L. Nicolas, K. Handrup, L. Vaghi, A. Papagni, F. Sedona, C. Di Valentin, S. Agnoli and M. Sambi, On-surface photo-dissociation of C–Br bonds: towards room temperature Ullmann coupling. Chem. Comm. 2015, 51, 12593−12596. 20 T. Wang, H. F. Lv, Q. Fan, L. Feng, X. Wu and J. F. Zhu. Highly Selective Synthesis of cis-Enediynes on a Ag(111) Surface. Angew. Chem. Int. Ed. 2017, 129, 4840-4844. 21 J. A. Lipton-Duffin, O. Ivasenko, D. F. Perepichka and F. Rosei, Synthesis of Polyphenylene Molecular Wires by Surface-Confined Polymerization. Small 2009, 5, 592–597. 22 M. Bieri, M.-T. Nguyen, O. Gröning, J. Cai, M. Treier, K. AïtMansour, P. Ruffieux, C. A. Pignedoli, D. Passerone, M. Kastler, K. Müllen and Roman Fasel, Two-dimensional polymer formation on surfaces: insight into the roles of precursor mobility and reactivity. J. Am. Chem. Soc., 2010, 132, 16669–16676 23 Z. M. Han, G. Czap, C. L. Chiang, C. Xu, P. J. Wagner, X. Y. Wei, Y. X. Zhang, R. Q. Wu and W. Ho. Imaging the Halogen Bond in Self-assembled Halogenbenzenes on Silver. Science 2017, 358, 206–210. 24 J. Björk, F. Hanke and S. Stafström, Mechanisms of halogen-based covalent self-assembly on metal surfaces. J. Am. Chem. Soc. 2013, 135, 5768−5775. 25 H. Huang, Z. Y. Tan, Y. W. He, J. Liu, J. T. Sun, K. Zhao, Z. H. Zhou, G. Tian, S. L. Wong and A. T. S. Wee. Competition between Hexagonal and Tetragonal Hexabromobenzene Packing on Au(111). ACS Nano, 2016, 10, 3198–3205. 26 R. F. W. Bader, Atoms in Molecules: A Quantum Theory, Oxford University Press, New York, 1990. 27 J. Wang, M. McEntee, W. J Tang, M. Neurock, A. P. Baddorf, P. Maksymovych, and J. T. Yates, Jr. Formation, migration, and reactivity of Au–CO complexes on gold surfaces. J. Am. Chem. Soc. 2016, 138, 1518–1526. 28 H. J. Walch, Dienstmaier, G. Eder, R. Gutzler, S. Schlögl, T. Sirtl, K. Das, M. Schmittel and M. Lackinger. Extended Two-Dimensional Metal–Organic Frameworks Based on Thiolate–Copper Coordination Bonds. J. Am. Chem. Soc. 2011, 133, 7909–7915. 29 Wang, G. A. Rühling, S. Amirjalayer, M. Knor, J. B. Ernst, C. Richter, H. J. Gao, A. Timmer, H. Y. Gao, N. L. Doltsinis, F. Glorius and H, Fuchs, Ballbot-type motion of N-heterocyclic carbenes on gold surfaces. Nat. Chem. 2017, 9, 152–156. 30 P. Maksymovych and J. T. Yates, J. Am. Chem. Soc. 2008, 130, 7518–7519. 31 S. U. Nanayakkara, E. C. H. Sykes, L. C. Fernández-Torres, M. M. Blake and P. S. Weiss, Au adatoms in selfassembly of benzenethiol on the Au(111) surface. Phys. Rev. Lett. 2007, 98, 206108. 32 L. Jiang, T. Niu, X. Lu, H. Dong, W. Chen, Y. Liu, W. Hu and D. Zhu, Low-temperature, bottom-up synthesis of graphene via a radical-coupling reaction. J. Am. Chem. Soc. 2013, 135, 9050−9054. 33 Q. Fan, T. Wang, L. Liu, J. Zhao, J. Zhu and J. M. Gottfried, Tribromobenzene on Cu(111): temperaturedependent formation of halogen-bonded, organometallic, and covalent nanostructures. J. Chem. Phys. 2015, 142, 101906. 34 H. M. Zhang, H. P. Lin, K. W. Sun, L. Chen, Y. L Zagranyarski, N. Aghdassi, S. Duhm, Q. Li, D. Zhong, Y. Y. Li, K. Müllen, H. Fuchs and L. Chi, On-surface synthesis of rylene-type graphene nanoribbons. J. Am. Chem. Soc. 2015, 137, 4022–4025. 35 L. Talirz, P. Ruffieux, R. Fasel, On-surface synthesis of atomically precise graphene nanoribbons. Adv. Mater. 2016, 28, 6222–6231. 36 J. M. Cai, P. Ruffieux, R. Jaafar, M. Bieri, T. Braun, S. Blankenburg, M. Muoth, A. P. Seitsonen, M. Saleh, X. Feng, K. Müllen and R. Fasel, Atomically precise bottom-up fabrication of graphene nanoribbons. Nature 2010, 466, 470–473. ACS Paragon Plus Environment

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Page 22 of 22

P. Ruffieux, S. Y. Wang, B. Yang, C. Sánchez-Sánchez, J. Liu, T. Dienel, L. Talirz, P. Shinde, C. A. Pignedoli, D. Passerone, T. Dumslaff, X. Feng, K. Müllen and R. Fasel. On-surface synthesis of graphene nanoribbons with zigzag edge topology. Nature 2016, 531, 489–492. 38 L. Lafferentz, V. Eberhardt, C. Dri, C. Africh, G. Comelli, F. Esch, S. Hecht and L. Grill, Controlling on-surface polymerization by hierarchical and substrate-directed growth. Nat. Chem. 2012, 4, 215–220. 39 Q. Shen, H. Y. Gao and H. Fuchs, Frontiers of on-surface synthesis: From principles to applications. Nano Today 2017, 13, 77–96. 40 T. A. Pham, F. Song, M.-T. Nguyen, Z. Li, F. Studener and M. Stöhr. Comparing Ullmann Coupling on Noble Metal Surfaces: On-Surface Polymerization of 1,3,6,8-Tetrabromopyrene on Cu(111) and Au(111). Chem. Eur. J. 2016, 22, 5937-5944. 41 M. Di Giovannantonio, M. E. Garah, J. Lipton-Duffin, V. Meunier, L. Cardenas, Y. F. Revurat, A. Cossaro, A. Verdini, D. F. Perepichka, F. Rosei and G. Contini. Insight into organometallic intermediate and its evolution to covalent bonding in surface-confined Ullmann polymerization. ACS Nano 2013, 7, 8190–8198. 42 L. Xie, C. Zhang, Y. Ding and W. Xu, Structural Transformation and Stabilization of Metal–Organic Motifs Induced by Halogen Doping. Angew. Chem. Int. Ed 2017, 56, 5077-5081. 43 G. Kresse and J. Hafner, Ab initio molecular dynamics for liquid metals. Phys. Rev. B 1993, 47, 558. 44 G. Kresse and J. Furthmüller, Efficient iterative schemes for ab initio total-energy calculations using a plane-wave basis set. Phys. Rev. B 1996, 54, 11169. 45 J. P. Perdew and K. Burke, M. Ernzerhof, Generalized gradient approximation made simple. Phys. Rev. Lett. 1996, 77, 3865. 46 G. Kresse and D. Joubert, From ultrasoft pseudopotentials to the projector augmented-wave method. Phys. Rev. B 1999, 59, 1758. 47 J. Klimeš, D. R. Bowler and A. Michaelides, Van der Waals density functionals applied to solids. Phys. Rev. B. 2011, 83, 195131. 48 J. Tersoff and D. R. Hamann, Theory and application for the scanning tunneling microscope. Phys. Rev. Lett. 1983, 50, 1998. 37


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