Competition between Hexagonal and Tetragonal Hexabromobenzene

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Competition Between Hexagonal and Tetragonal Hexabromobenzene Packing on Au(111) Han Huang, Zhiyu Tan, Yanwei He, Jian Liu, Jiatao Sun, Kang Zhao, Zhenhong Zhou, Guo Tian, Swee Liang Wong, and Andrew Thye Shen Wee ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.5b04970 • Publication Date (Web): 23 Feb 2016 Downloaded from http://pubs.acs.org on February 27, 2016

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Competition Between Hexagonal and Tetragonal Hexabromobenzene Packing on Au(111) Han Huang1,2,4,5*, Zhiyu Tan1,2# , Yanwei He1# , Jian Liu3, Jiatao Sun3, Kang Zhao1, Zhenhong Zhou1, Guo Tian1,2, Swee Liang Wong6, Andrew Thye Shen Wee4,5 1

Institute of Super-microstructure and Ultrafast Process in Advanced Materials, School of Physics and Electronics, Central South University, Changsha, Hunan 410083, P. R. China

2

Hunan Key Laboratory for Super-microstructure and Ultrafast Process, Central South University, Changsha, Hunan 410083, P. R. China 3

4

Institute of Physics, Chinese Academy of Sciences, Beijing 100190, P. R. China

Department of Physics, National University of Singapore, 2 Science Drive 3, 117542, Singapore. 5

Centre for Advanced 2D Materials, National University of Singapore, Block S14, Level 6, 6 Science Drive 2, 117546, Singapore

6

Institute of Materials Research and Engineering, Agency for Science Technology and Research, 3, Research Link, Singapore 117602, Singapore

Corresponding author: [email protected] (Dr. H. Huang) #

These authors contributed equally to this work

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Abstract Low temperature scanning tunneling microscope investigations reveal that Hexabromobenzene (HBB) molecules arrange in either hexagonally closely packed (hcp) ቂ 2

2 7 0 ቃ or tetragonal ቂ ቃ structure on Au(111) dependent on a small −2 4 −2 4

substrate temperature difference around 300 K. The underlying mechanism is investigated

by

density

functional

theory

calculations

which

reveal

that

substrate-mediated intermolecular noncovalent C-Br···Br-C attractions induce hcp HBB islands, keeping the well-known Au(111)-22×√3 reconstruction intact. Upon deposition at 330 K, HBB molecules trap freely diffusing Au adatoms to form tetragonal islands. This enhances the attraction between HBB and Au(111) but partially reduces the intermolecular C-Br···Br-C attractions, altering the Au(111)-22× √3 reconstruction. In both cases, the HBB molecule adsorbs on a bridge site forming a ~15º angle between the C-Br direction and [112ത ]Au, indicating the site-specific molecule-substrate interactions. We show that the competition between intermolecular and

molecule-substrate

interactions

determines

molecule

packing

at

the

sub-nanometer scale, which will be helpful for crystal engineering, functional materials, and organic electronics.

Keyword: halogen bonding, molecular arrangement, Au(111)-22×√3, LT-STM, DFT calculations


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The adsorption, diffusion, nucleation and growth of organic molecules on inorganic substrates are dominated by the competition between molecule-substrate and intermolecular interactions.1, 2 Supramolecular structures at the nanometer scale typically form on solid surfaces via weak noncovalent intermolecular interactions such as van der Waals (vdW) forces,3 dipole-dipole interactions,4 hydrogen bonding,5 and metal-ligand interactions.6 Halogen bonding is an electrostatic interaction similar to hydrogen bonding, which has been extensively investigated. Generally, halogen bonding has two typical bonding modes, the CC-CC interaction (CC refers to charge concentration) and the CD-CC interaction (CD refers to charge depletion).7 While halogen bonding is fairly well known in three dimensional (3D) organic systems in crystal

engineering,8-9

molecular

recognition,10-12

functional

materials,13-18

biomolecular systems and biomedical applications,19-21 etc., it is only in recent years that its importance in 2D self assembly has been demonstrated.22-27 The so-called sigma-hole derived from the non-spherical atomic charge distribution on halogen substituent can attract the nucleophilic site of adjacent molecules,24, 28, 29 resulting organic molecules in distinct patterns, for instance, 1,3,5-tris(4-bromophenyl)benzene (TBB) on Ag(111).27 Recently, extended halogen bonding is visualized in fully ﬂuoro-substituted phenyleneethynylene (BPEPE-F18) networks by high resolution atomic force microscopy.30 Hexabromobenzene (HBB), a derivative of benzene in which all hydrogen atoms are replaced by bromine atoms, is a model system to investigate the role of intermolecular and interfacial interactions on the growth. To date, there is only one


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report on the self-assembly of HBB on HOPG or MoS2.31 Recently, it is reported that high quality graphene can be synthesized using HBB molecules on Cu foil in a two-temperature furnace at a temperature as low as ~500 K.32 Therefore, understanding the underlying mechanism of HBB molecule packing may also give further insight into graphene formation at low temperature. In this article, we used low temperature scanning tunneling microscopy (LT-STM) to investigate the growth behaviors of HBB molecules on Au(111) around room temperature (RT) and observed the phase transition from hexagonal to tetragonal arrangements.

Density

functional

theory

(DFT)

calculations

indicate

that

substrate-mediated intermolecular noncovalent C-Br···Br-C interactions play key roles in HBB self-assembly. Results and discussion

Figure1: A representative STM image (VT=-1.89 V) of ~0.5 ML HBB on Au(111) (a) and the corresponding line-profile (b) along the red line in panel a showing the


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Au(111) monoatomic step height of 0.23±0.01 nm and momolayer HBB island height of 0.32±0.01 nm. Both the herringbone-like reconstruction on monolayer HBB and that on clean Au(111) show the identical periodicity of 22×√3. The atomic model of reconstructed Au(111) is inserted at the lower-left corner. Upon ~0.5 ML (ML corresponds to a closely packed layer of flat-lying molecules) HBB deposition at RT, Au(111) are partially covered with large single-layered molecular islands, as shown in Figure 1a. This suggests a rather long diffusion length (low diffusion barriers) for HBB on Au(111). While debromination of HBB molecules on Cu (111) under UHV conditions occurs at RT and bromine adatoms aggregate in hcp islands distributed along Cu step edges and terraces,32 no similar phenomena were observed on Au(111), indicating intact HBB molecules on Au(111). Polycyclic aromatic hydrocarbon (PAH) molecules such as, benzene,33 perylene,34 perfluoropentacene,35 CuPc,36

2H-porphine,37

usually

form

two

dimensional gas at low coverage and ordered structures at a coverage close to 1 ML on surfaces due to the intermolecular repulsive interactions. Thus, it can be concluded that intermolecular attraction exists in between and drives HBB molecules into the

hcp arrangement. Usually, clean Au (111) surface reconstructs into a 22×√3 herringbone-like structure due to the anisotropic surface stress,38-42 whose corresponding atomic model is inserted at the lower-left corner of panel a. This herringbone-like reconstruction appears on Au(111) terraces and goes continuously across the edges of HBB islands, indicating rather weak molecule-substrate interactions.39-42 Figure 1b shows the line-profile taken along the red line in Figure 1a over a single monoatomic Au(111) step with bare Au(111) on the lower step (dark part) and


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HBB islands on both steps. The measured height of monoatomic Au(111) step is 0.23±0.01 nm, in good agreement with the theoretical value of 0.235 nm. The apparent height of single-layered HBB islands is measured to be 0.32±0.01 nm. Taking its vdW diameter of ~1 nm into consideration, we can conclude that HBB molecular plane is parallel to the Au(111) surface, which allows HBB molecules to have maximum exposure of their extended conjugated π-plane of electrons to the substrate surface for electronic coupling between the π-orbital of the molecules and the Au d-bands. It is consistent with previous reports of flat-lying HBB adsorbed on either HOPG or MoS2.31


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Figure 2: (a) molecularly resolved STM image (VT= 0.3 V) taken from the black square in Figure 1a and its corresponding FFT pattern (inset at upper-left corner), showing HBB in a hexagonal arrangement. The inserted line-profile shows a lattice constant of 0.98±0.05 nm. (b) Intramolecularly resolved STM image (VT= 2.13 V) with HBB molecules superposed, showing an angle of ~15ºbetween one diagonal of HBB and [112ത]Au. (c) a proposed HBB packing model on Au(111) based on DFT calculation. Four molecular adsorption sites (hcp hollow, bridge, fcc hollow, and top) represented by the upper triangle, rectangle, lower triangle, and circle respectively, were considered in the DFT calculation. Three molecular orientations (0°, 15°, 30°) are chosen here with respect to [112ത]Au. The nearest neighbor Br atoms of the adjacent two HBB molecules is linked by green dashed line with distance of 3.8 Å and the next nearest neighbor distance of 4.4 Å is shown with red line. (d) model of single HBB molecule on Au(111).(e) A possible HBB packing model without considering substrate. (f) The slice of charge density difference Δρ is drawn along the black dotted line in Panel c. The carbon, gold and bromine atoms are marked by cyan, yellow, green balls respectively.

Figure 2a presents a molecularly resolved STM image zoomed-in from the black square in Figure 1a, showing long range ordering of HBB. Each protrusion corresponds to one HBB molecule. The corresponding fast Fourier transform (FFT) pattern inserted at the upper left corner consists of six sharp spots in hexagon, indicating HBB in an hcp arrangement. The line profile taken along [112ത]Au reveals an average intermolecular distance of 0.98±0.05 nm, which is about twice the lattice constant along that direction. We propose that the HBB molecules are commensurate

ܾ 2 with the Au(111) substrate with an overlayer matrix ൤ ଵ ൨ = ቂ ܾଶ −2

2 ܽଵ ቃ ቂ ቃ, where ܽ௜ , 4 ܽଶ

ܾ௜ are the lattice vectors of the Au(111) substrate and the adsorption structures respectively, implying site-specific interfacial interactions on Au(111). Apart from the herringbone structures, the larger dark spots at elbow sites are attributed to the missing HBB molecules; similarly, dimmer features at other sites can be attributed to missing or dissociated HBB molecules. The higher resolution STM image in Figure 2b displays the intramolecular


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structure of HBB, a pronounced minimum in the center surrounded by a few protrusions, which is tip-bias dependent.31 Due to the larger electronegativity of Br with respective to C, the protrusions are assigned to Br atoms and the depression center to the benzene ring. Since the hexagonal appearance of HBB is maintained, we superpose HBB molecule structures onto the STM image. It reveals an angle of ~15º between one C-Br direction and the molecular row direction, that is [112ത]Au, as labeled in Figure 2b. To further understand the underlying mechanism of HBB packing on Au(111), we have performed first principles calculations in the framework of density functional theory where the effect of vdW correction to the standard DFT calculations were taken into account using the Grimme's empirical scheme (DFT+D/PBE)) for periodic systems.43,44 The adsorption configurations were taken into account in terms of orientation and sites of the molecules. Taking the hexagonal center of HBB molecule as reference, four typical adsorption sites, HCP hollow (▲ for HCP), bridge ( ▌for BDG), FCC hollow (▼ for FCC) and Top (● for TOP) are shown at the upper right corner of Figure2c. The six-fold symmetry of the HBB molecule imposes only three preferred orientations (white arrows correspond to 0°, 15°, 30°). The above consideration leads to twelve adsorption configurations for the hexagonal structure (like BDG00, BDG15, BDG30, FCC00, FCC15, FCC30, HCP00, HCP15, HCP30, TOP00, TOP15, TOP30, where letters and numbers stand for adsorption site and orientation). After geometrical relaxation, all the structures remain at the local energy minimum with 3.2 Å separation between HBB molecule and the Au(111) substrate,


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consistent with the experimental results. The total energy results favor BDG15 as the most stable configuration. For details of the adsorption energy difference of each configuration relative to BDG15, please refer to table S1 in supporting information. The adsorption energy difference of few meV between BDG15 and HCP15, the next closet configuration, witnesses the physical adsorption nature. However, BDG00 is favored for isolated HBB on Au(111) among the three BDG configurations, as shown in Figure 2d, similar to the case of isolated benzene45 or coronene46 on Au(111), indicating the important role of intermolecular interactions. The model proposed in Figure 2c is based on both experiment and DFT calculations. The yellow rhombus shows one supercell. The charge density difference Δߩ due to molecule-substrate adsorptive interaction along the black dotted line in panel c is shown as a slice in Figure 2f. Charge redistribution occurs not only in the carbon ring periphery of the HBB molecule but also mostly at the interface between the HBB molecule and the substrate. The interface has large mirror symmetrical charge redistribution, indicating the obvious interactions. The charge redistribution in the carbon ring periphery of the HBB molecule also shows the mirror symmetry, representing the equivalent interaction of both bromine atoms with the neighboring HBB molecules. According

to

previous

reports,

halogenated

molecules

such

as

trihalomesitylenes,24 hexachlorobenzene7 and TBB27 are able to form X3 synthons (X refers to Cl, Br, and I) via halogen bonding, which are attributed to Coulombic “donor-acceptor” attraction between the σ-hole and nucleophilic site of halogen


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atoms.24, 28, 29 However, since long range molecular orbitals usually affect the STM contrast, a detailed chemical structure can be hard to achieve. According to the proposed model, such Br3 synthons among HBB molecules on Au(111) can be ruled out, because the corresponding Br···Br distance (4.4 Å) labeled by red line in Figure 2c is too large, about 15.8% larger than double the vdW radii of bromine atoms (3.8±0.1 Å). The distance between the nearest neighbor Br atoms of adjacent HBB molecules is 3.8 Å, labeled by the green dashed line in Figure 2c, suggesting the existence of C-Br···Br-C bonding in the CC-CC mode.7, 8 Such halogen bonding gives rise to HBB molecules to rotate by ~15º with respect to [112ത]Au. The green dashed lines in Figure 2c indicate the 6 noncovalent C-Br···Br-C bonds around each HBB molecule.

Figure 3 (a) STM image (VT=-1.89 V) taken from the green square in Figure 1a, showing a second, tetragonal, molecular arrangement of HBB on Au(111) attaching to hcp HBB monolayer island. The black dash-dotted line indicates the boundary, which is highlighted in the inserted derivative STM image (VT=2.13 V). (b) STM image from pure tetragonal HBB monolayer islands on Au(111) recorded after deposition at 330 K. The Au(111) reconstruction is not visible on the molecular island but surrounding it. The inserted line profiles show the periodicity of ~0.98 nm for the tetragonally packed HBB island.


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Closer inspection of the boundaries of HBB hcp islands reveals the relative weakness of the intermolecular C-Br···Br-C bonds. Figure 3a, the corresponding high resolution image of the green square in Figure 1a, reveals a second HBB arrangement in tetragonal geometry attached to the HBB hcp island near the step edges. (Please refer to Figure S1 for the overview of the distribution of HBB in tetragonal arrangement.)

A dash-dotted

line

highlights

the

boundary.

The

inserted

intramolecularly resolved STM (in derivative mode) image reveals that HBB molecules in both arrangements running along [112ത ]Au have identical molecular in-plane orientation. Yet, the tetragonal arrangement shows a slightly lower apparent height, indicating an increased interfacial interaction. Upon ~0.5 ML HBB deposition on Au(111) at 330 K, monolayer islands of HBB molecules in tetragonal geometry are observed as shown in Figure 3b. It is obvious that the herringbone-like 22×√3 reconstruction of Au(111) disappears under the HBB island and reshapes into curving soliton walls surrounding the island instead, indicating the existence of centripetal stress towards the island.38 This must be caused by the strongly enhanced interfacial interaction.39-42


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Figure 4: (a) intramolecularly resolved STM image (VT=1.0 V) and the corresponding FFT pattern (inset) from tetragonally packed HBB island. (b) at lower scanning bias (VT=0.3 V), an additional protrusion, highlighted by a white arrow, appears among neighboring HBB molecules in the STM image. A DFT calculated model is superposed. The black rectangle indicates a unit cell containing two HBB molecules. (c) and (d): the charge density difference Δρ along the red and green lines in panel (b) respectively. Figure 4a exhibits the intramolecularly resolved STM image (VT=1.0 V) of tetragonally packed HBB molecules and the corresponding FFT pattern with four sharp spots in square. These HBB molecules appear in the same full six fold symmetry as on MoS2 with a depressed center and six outer maxima, indicating no debromination. The nearest neighboring intermolecular distance is measured to be 0.98±0.05 nm, indicating vdw intermolecular interaction between HBB. Therefore, this tetragonal HBB phase is commensurate with the Au(111) substrate with an 7 0 overlayer matrix of ቂ ቃ, with two HBB molecules in the unit cell. According to −2 4 above discussions, we propose that the two HBB molecules in the supercell are adsorbed at two equivalent bridge sites (rotated by 60 º ) respectively.2,37 The molecular surface density (0.071 molecule per Au atom) is lower than hcp phase (0.083 molecule per Au atom). When the scanning bias is reduced to 0.5 V, an additional protrusion is detected in the center of the tetragonal vacancy as highlighted by a white arrow in Figure 4b. The bias dependent STM images in Figure S2 give a clearer picture of the protrusions. As shown in the proposed model in Figure 4b, there are 2 (2) halogen (-like) bonds for each HBB molecule in tetragonal packing. The green dashed lines highlight two C-Br···Br-C bonds, which are the same as those in hcp phase. The blue dashed lines


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highlight two similar C-Br···Br-C bonds with ~3.7% larger distance. The latter ones may be slightly weaker due to the larger distance. The nature of the interactions between HBB and Au(111) should be the same in both phases, which results in the tetragonal phase being energetically unstable. Futhermore, such weak vdW interfacial interactions have no reason to induce local DOS redistribution on Au(111). Therefore, we attribute these protrusions to Au adatoms. With the computationally expensive vdW-DF functional,47 the binding energies for hexagonal and tetragonal phases are calculated to be 1.387 and 1.405 eV/molecule respectively, which agrees well with our experimental observations. Adatom stabilized molecular arrangements have been widely reported both experimentally and theoretically.26, 48-51 The underlying mechanism is proposed to be that positively charged adatoms, which donate some charge to the nearest neighbor surface atoms, could easily interact with negatively charged molecules or active sites.48 For example, the G-quartet-Na complex with a Na atom at the center of four Guanine (G) molecules is able to form via the cooperative eﬀect of hydrogen bonding and electrostatic ionic bonding, where the Na atoms appear as protrusions at the center.26 Higher substrate temperatures (330 K) cause greater adatom diffusion, and facilitate the coordination of Au adatoms with HBB. Such thermally induced adatom facilitated phase transition has been reported for tetrapyridyl-porphyrin (TPyP) molecules on Cu(111).51 Upon deposition at room temperature, TPyP molecules undergo a distinct conformational adaptation allowing anchoring to the substrate. Annealing up to 500 K drives assembly processes to the formation of chain structures


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that originate from lateral coordination bonds mediated by the presence of a Cu adatom gas interacting with the pyridyl mesogroups. The adatoms here also appear as small medium-bright features at the corners of molecules in the STM image. DFT calculation reveals that the charge redistribution along the red line (the C-Br···Br-C bond direction) in panel b shows mirror symmetry, as shown in Figure 4c, which is similar to the case of the hexagonal structure. Figure 4d displays the charge redistribution between the HBB molecule and the Au adatom, along the green line in panel b, indicating stronger interaction. These experimental findings illustrate that the formation of the tetragonal HBB network requires thermal activation and temperature threshold is close to 330 K. Figure S3 displays the top view of the proposed model of coexistence of hcp and tetragonally packed HBB on Au(111). Although HBB molecules arrange in a (quasi-)square unit cell, the appearance of the corresponding islands are not square but elongated along [112ത]Au, as shown in Figure 3b, indicating the anisotropic growth with preference along [11 2ത ]Au. C-Br···Br-C bonds between two neighboring HBB molecules are the driving force. The additional Au atoms, which coordinate with the existing HBB molecules in the row, prevent other HBB molecules attaching in the hcp arrangement. The attraction between HBB molecules drives the approaching HBB molecule to seek a stable position close to the existing molecular row. Two additional C-Br···Br-C like bonds (labeled by blue dashed lines in Figure 4b) will form when the approaching HBB molecule attaches side-on to the molecular row (along [11ത0]Au). Therefore, there are four C-Br···Br-C bonds for one HBB molecule with its nearest neighbors in tetragonal


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arrangement, two less compared to the HBB molecule in hcp arrangement. Tetracyano-p-quinodimethane (TCNQ) has been reported to interact with Cu(100) surface through charge transfer, so as to render the Cu atoms diverging from their equilibrium position.52 The Au(111) surface has various reconstructions depending on surface stress anisotropy induced by different amounts of charge transfer with adsorbed molecules.39-42 Therefore, it is proposed that the interfacial interaction between HBB molecules and Au(111) can be enhanced via coordinating with Au adatoms, resulting in Au(111) surface stress relief and lifting of the 22×√3 reconstruction.

Figure 5: STM images of annealed ~0.5 ML HBB on Au(111). (a) annealed at ~400 K for 2 min: a quasi-hexagonal structure formed on Au(111) terraces (b) annealing at ~430 K for 3 min: randomly desorption occurs accompanied by residual islands in uniform size on Au(111) terrace The weakness of intermolecular and interfacial interactions in both arrangements can be witnessed by the poor thermal stability. Figure 5a is a representative STM image of a 0.5 ML HBB covered Au(111) annealed at ~400 K for 2 min, displaying quasi-hexagonal networks on Au(111) terrace. The corresponding


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FFT pattern is inserted at the lower left corner. It consists of six bright spots equally separated by ~4.0 nm-1, corresponding to a period of ~1.8 nm for the network in real space. Increasing the annealing temperature to ~430 K with a duration of 3 min results in random desorption and a partial clean Au(111) surface with reconstructed soliton curves surrounding the residual islands of such networks appears on the terraces, as shown in Figure 5b. Both HBB structures could not be observed any more, indicating the breaking of intermolecular interaction. Due to the poor resolution, we are not able to determine the atomic structures of the networks. We assume that they might be Au atoms coordinated (debrominated) HBB molecules.27 Desorption of adsorbates at ~430 K prevents the formation of graphene at higher temperature, indicating the lesser catalytic activity compared to Cu(111).32

Conclusion Self-assembly of HBB on Au(111) has been investigated using LT-STM and DFT calculations. Upon deposition at RT, HBB molecules self-assemble in hcp arrangement governed by substrate-mediated halogen bonding. A new tetragonal packing phase occurs at ~330 K, where HBB-Au networks are formed by trapping Au adatoms. Thermal annealing experiments demonstrate poor catalytic activity of Au(111) for graphene preparation from HBB. Our findings provide insight into molecule-substrate and intermolecular interactions, and will be helpful for future investigations on molecular self-assembly based crystal engineering, functional materials, and organic electronics.


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Method All the experiments were carried out in a custom-built multichamber ultrahigh vacuum system housing an Omicron LT-STM with base pressure better than 6.0 × 10-11 mbar. The clean Au(111) surface was achieved in situ by several cycles of Ar+ sputtering and subsequent annealing at ~800 K. The sample’s cleanliness and surface structure were verified by LT-STM and low-energy electron diffraction measurements. Sublimation purified HBB molecules (Aldrich, 98+%) (80 °C) evaporated in situ from Knudsen cells (MBE Komponenten, Germany) onto Au(111) in the growth chamber.32 The nominal deposition rate of HBB (0.01 nm/min) was pre-calibrated by a quartz crystal microbalance. All STM images were recorded in constant current mode using chemically etched tungsten (W) tips at 77 K. The low temperature used minimized thermal noise to give atomically resolved STM images, which were analyzed using WSxM software.53 First-principles calculations54,55 were carried out in the framework of density functional theory (DFT) with the pseudopotential approach and plane wave basis set as implemented in the Vienna ab initio simulation package (VASP) code.56,57 We use generalized gradient approximation (GGA) for the exchange-correlation energy functional,56 with the functional proposed by Perdew-Burke-Ernzerhof (PBE).58,59 The electron-ion interaction is described by the projector augmented wave (PAW).The cutoff energy of the plane wave basis for wave function expansion is 400 eV. All the calculations were performed with the Au lattice constant of 4.08 Å. A slab model of four layer Au substrate with the vacuum gap of 15 Å is adopted to simulate the


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adsorption systems. The molecules and top two metal layers were allowed to relax in structural optimization until the atomic forces were smaller than 0.01 eV/Å. The effect of vdW correction to the standard DFT calculations was taken into account by using the empirical scheme of Grimme (DFT+D/PBE)) for periodic systems.43 The vdW-DF functional was performed to obtained the binding energy.47 The interfacial interaction between the HBB molecule and the substrate is studied by calculating the charge density difference, which is defined as Δߩ = ߩ௠௢௟ା௦௨௕ − ߩ௠௢௟ − ߩ௦௨௕ . ߩ௠௢௟ା௦௨௕ , ߩ௠௢௟ and ߩ௦௨௕ is the charge density of total HBB molecule and the substrate, the charge density of the isolated HBB molecule and the substrate. In squared structure, the ߩ௦௨௕ is calculated with the gold adatom.

Supporting Information: The adsorption energy difference of each adsorption configuration relative to BDG15 for HBB on Au(111) in hexagonal phase. Bias dependent STM images of tetragonally packed HBB on Au(111). The proposed model of coexistence of hcp and tetragonally packed HBB on Au(111). This material is available free of charge via the Internet at http://pubs.acs.org.

Acknowledgements

We acknowledge the financial support from NRF-CRP grants R-143-000-360-281: “Graphene and Related Materials and Devices” and R-144-000-295-281: “Novel 2D materials with tailored properties – beyond graphene”. Dr. Han Huang acknowledges


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the “Shenghua Professorship” startup funding from Central South University. Dr. Jia-Tao Sun acknowledges "Strategic Priority Research Program（B）" of the Chinese Academy of Sciences (Grant No. XDB07030100). The support from the NSF of China (Grant No.11304398, 61306114, 11334014, 51173205) is acknowledged.

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TOC 85x40mm (300 x 300 DPI)


Competition between Hexagonal and Tetragonal Hexabromobenzene

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