Disorder–Order Transformation of Trithiocyanuric Acid Adlayer on a

ARTICLE pubs.acs.org/JPCC

Disorder
Order Transformation of Trithiocyanuric Acid Adlayer on a Au(111) Surface Induced by Electrode Potential Ting Chen, Pei-Xia Dai, Jing-Yi Wu, Dong Wang,* and Li-Jun Wan* Institute of Chemistry, Chinese Academy of Sciences, and Beijing National Laboratory for Molecular Sciences, Beijing 100190, People’s Republic of China ABSTRACT: Self-assembled monolayers (SAMs) of trithiocyanuric acid (TTCA) on Au(111) surfaces are investigated by scanning tunneling microscopy (STM), cyclic voltammetry, and density functional theory calculation (DFT). The SAM exhibits a wormlike disordered structure from 1.0 V to 260 mV vs RHE (reversible hydrogen electrode). At 260 mV, an irreversible disorder
order phase transformation occurs and results in an ordered striped phase. High resolution STM images and DFT calculations suggest that TTCA molecules are parallel to each other within the stripes, which prompts the π
π stacking interactions between the molecules. Each molecule adsorbs vertically on the substrate via two S atoms and one heterocyclic N atom anchoring on the surface at the bridge sites and the top site, respectively. This adsorption geometry promises the SAM with good stability (binding energy 3.27 eV) and free reactive S terminals. The high stability and free S terminals of the TTCA SAM makes it a promising scaffold for applications in molecular electronics and chem/bio sensors.

’ INTRODUCTION Due to the ubiquitous existence of surfaces and interfaces, it is of great importance to tailor the structures and properties of surfaces and interfaces, especially for nano-objects.1 Surface modification with molecular self-assembled monolayers (SAMs) offers a simple, versatile, and effective way to tune the surface composition, structure, and physical/chemical properties. SAMs of organothiols on metal surfaces are among the most extensively studied systems and have found applications in numerous fields including corrosion inhibition, electrocatalysis, chem/bio sensors, and molecular electronic devices (see refs 1
5 and references therein). Initial studies mostly addressed the SAMs from relative simple thiols, like linear alkanethiols. Recently, increasing research efforts have been devoted to thiol SAMs containing aromatic backbones.6
15 The rigid and π-conjugated nature of aromatic thiols enable much more efficient charge transportation between the SAMs and the electrodes,13
17 which makes aromatic thiol SAMs ideal test platforms for molecular electronics and excellent scaffolds to promote the charge transfer between metal surfaces and functional systems such as enzymes or proteins.18
24 SAMs of aromatic thiols show some distinct features when compared with those of normal alkanethiols.17,25 In the case of alkanethiols, the van der Waals interactions between alkyl chains facilitate the formation of ordered SAMs with well-defined adsorption geometry for individual molecules. On the other hand, the involvement of π-electrons, and heteroatoms if heterocyclic thiols are employed, makes the SAMs of aromatic thiols have diverse adsorption geometries and structures. In addition, the structural fluctuation of S-metal bond may raise a reliability issue in the molecular electronics based on S-metal linker for practical r 2011 American Chemical Society

applications.26 Recently, organothiols with di- or multi-S headgroups, such as cyclic disulfides,16,27 spiroalkanedithiols,28
31 conjugated dithiols,32
35 and chelating aromatic dithiols27,36
38 are increasingly studied. These di- or multithiols can attach to metal surfaces via multiple S
metal bonds and thus enhance the stability of the SAMs.28
31,36 Lee et al. reported that both the electric potential stability and the thermal stability of chelating alkanethiol SAMs on gold are highly correlated to the degree of chelation.30,31 Another benefit of the di- or multithiol SAMs is that the large footprint of anchoring groups allows incorporating bulky SAM backbones or different terminal functional groups and adjusting the composition, distribution, and density/ coverage of functional moieties efficiently.28,39,40 However, the SAMs from dithiols or trithiols usually are less well ordered compared to the SAMs of monothiols due to the loosely packed alkyl chains as well as the poor mobility resulting from the multiple S-metal bonds. So far, only few results demonstrate the formation of densely packed and highly orientated SAMs from di- or multithiols with aromatic backbones.36,37 A thermo-annealing step is reported to be crucial for the formation of ordered SAM for 2-mercaptomethylbenzenethiol.37 Trithiocyanuric acid (TTCA) is an aromatic trithiol which has been used as a corrosion resist for metals or alloys.41 It contains a N-heterocyclic backbone and three thiol headgroups. Therefore, it is expected that TTCA forms stable thiol-terminated SAMs on metal surfaces. Herein, we studied the adsorption and assembly Received: May 31, 2011 Revised: July 21, 2011 Published: July 25, 2011 16583

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Figure 1. (a) CVs of Au(111) electrode in 0.1 M HClO4 before (black line) and after (red line) the addition of 4.0  10
6 M TTCA. (b) CVs of TTCA/ Au(111) electrode in 0.1 M HClO4. Red line and black line: the first and second cycles of CVs of TTCA/Au(111) electrode in 0.1 M HClO4. Blue line and green line: continual cycles of CVs of TTCA/Au(111) electrode in 0.1 M HClO4 after holding at 100 mV for 30 min. Scan rate v = 50 mV s
1.

of TTCA on a Au(111) surface by combining experimental characterizations and theoretical simulations. A potential induced irreversible disorder
order phase transformation was observed, which results in the formation of stable thiol-terminated striped SAMs on Au(111) surfaces. The molecular orientation, adsorption site, and packing structure in the striped SAMs are characterized by using electrochemical scanning tunneling microscopy (STM), cyclic voltammetry, and theoretical calculations.

’ EXPERIMENTAL SECTION Materials. The electrolyte used in the STM and cyclic voltammetric measurements is 0.1 M HClO4 solution prepared with ultrapure HClO4 (Kanto Chemical Co.) and Milli-Q water (18.2 MΩ, TOC e 4 ppb). TTCA was purchased from Aldrich and dissolved in 0.1 M HClO4 to obtain saturated TTCA solution (about 8.0  10
3 M). To prepare TTCA SAMs on Au(111), some TTCA stocking solution was dropped into the electrochemical cell. Then a flame-annealed fresh Au(111) surface was used to record the cyclic voltammograms(CVs) in the resulted electrolyte. Cyclic Voltammetric Measurements. Cyclic voltammetric measurements were performed on a Parstat 2273 Advanced Electrochemical System (Princeton Applied Research) using the hanging meniscus method under a nitrogen atmosphere. A three-electrode system was used with a reversible hydrogen electrode (RHE) as reference electrode and a Pt wire as counter electrode. STM Measurements. STM experiments were carried out with a NanoScope E microscope (Veeco Inc.). The Au(111) surfaces were prepared by the Clavilier method. Before each measurement, the Au(111) surface was annealed in a hydrogen
oxygen flame and quenched in ultrapure water saturated with hydrogen. The STM tips were prepared by electrochemically etching W wire in 0.6 M KOH. To minimize Faradic current, the tips were sealed with nail polish. All of the STM images were recorded in the constant-current mode. All the potentials were reported relative to RHE. Computational Details. Periodic density functional theory (DFT) calculations were performed using the Cambridge sequential total energy package (CASTEP). The generalized gradient approximation (GGA) in Perdew
Burke
Ernzehof (PBE) formula was applied to describe the exchange-correlation effects.42 The plane wave basis set with a cutoff of 360 eV is used for the expansion of wave functions. The Au(111) surface was modeled

as a repeated slab geometry consisting of three atomic layers√ that were separated by a vacuum region of 20 Å. Since the (22  3) herringbone reconstruction of the Au(111) surface is reported to be lifted upon thiolate adsorption, a Au(111) surface with (1  1) structure was used in the calculations.43 To reduce the calculation √ cost, the adsorption of the TTCA was modeled by a (4  3) unit cell of the overlayer structure. The calculation of isolated TTCA molecule employed a larger (3  3) unit cell to avoid possible intermolecular interactions. For the adsorption geometry of a single TTCA molecule on the Au(111) surface, a (2  5  1) Monkhorst
Pack k-point mesh with a Methfessel
Paxton smearing of 0.1 eV was applied for the integration over the first Brillouin zone. Extensive convergence tests with higher energy cutoff and larger k point meshes were performed to validate our choices. The results demonstrate that our choice of energy cutoff and k-point mesh results in an energy error of 20
30 eV. This is well within the error tolerance of the present study. During the geometry optimization, two bottom layers of the Au(111) surface were kept fixed at their optimized bulk truncated geometry, while the one upmost layer and all atoms of TTCA molecule were relaxed until the forces were less than 0.01 eV/Å. The energy needed to break the covalent bond between TTCA and Au(111) is defined as binding energy Ebind ¼ EAuð111Þ þ ETTCA
radical
ETTCA=Auð111Þ in which EAu(111) is the energy of a bare Au(111) surface, ETTCA-radical is the energy of the isolated TTCA radical, and ETTCA/Au(111) is the total energy of TTCA adsorbed on Au(111).

’ RESULTS AND DISCUSSION 1. Electrochemical Behavior of TTCA on Au(111) Surface. Figure 1a gives the CVs of a Au(111) electrode in 0.1 M HClO4 before and after the addition of TTCA molecules. In 0.1 M HClO4, a typical CV of bare Au(111) is obtained (black line), √ in which a pair of current peaks corresponding to the (22  3) reconstruction and dereconstruction of the Au(111) surface can be observed clearly. This suggests that a well-defined and clean Au(111) surface was used. Then, some TTCA stocking solution was added into the electrolyte to 4.0  10
6 M. The red line is the CV of the Au(111) electrode in the resulted electrolyte. An anodic peak and a cathodic peak appear at ∼650 and ∼260 mV, respectively, which are nearly unchanged upon continual scans. 16584

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Figure 2. STM images of bare Au(111) surface (a) and TTCA SAM modified Au(111) surface (b) at 550 mV. Tunneling conditions: (a) Scan area = 200  200 nm2, Ebias =
194.1 mV, I = 1.000 nA. (b) Scan area = 33  33 nm2, Ebias =
151.6 mV, I = 1.108 nA.

CVs of a TTCA modified Au(111) electrode in 0.1 M HClO4 give similar anodic and cathodic peaks, as shown in Figure 1b. The red curve and the black curve are the first and the second cycles of the CVs. It can be seen that the peak currents decrease with continual scanning. The result implies that the processes corresponding to the current peaks are irreversible in the potential range. Thus, the electrode was held at 100 mV for 30 min. The blue line and the green line are the continual cycles of CVs after holding at 100 mV for 30 min. The cathodic peak disappears in the first cycle. Moreover, both the anodic and the cathodic peaks in the following cycles decrease significantly compared to CVs of fresh prepared TTCA modified Au(111) electrode. This confirms the irreversibility of the cathodic and anodic processes. As reported, an anodic peak appears at 0.6 V vs SCE in the CV of triazine dithiol derivatives on Pt plates. This oxidative peak was attributed to the oxidation of anions of the triazine dithiol derivatives to their thiyl radicals.44 Morin et al. investigated the electrodeposition of 1,4-benzenedimethanethiol on Au(111) surface and observed two oxidative current peaks corresponding to the oxidative adsorption of vertically oriented dithiols and the dimerization of the unreacted thiolate groups. They found that the oxidative deposition occurs at more positive potential in 0.1 M NaF because the SH bond must be broken for chemisorptions.45 Similar oxidative adsorption has been observed for BDT on Pt(111) in 0.1 M HClO4 and for butanethiol and octanethiol on gold in 0.1 M LiOH/KOH.46,47 Thus, the anodic peak at ∼650 mV in the CV of TTCA is ascribed to the oxidative adsorption of the TTCA molecules on the Au(111) surface. With respect to the cathodic peak at ∼260 mV, it may be originated from the reductive desorption of TTCA SAMs, just as observed for other organothiols,8,48
50 or from a structural or orientational transformation of the TTCA SAMs. To disclose the molecular level information of the potential dependent SAM structures, in situ STM measurements are carried out. 2. Structures of the TTCA SAMs on Au(111) Surface. Based on the cyclic voltammetric results, the Au(111) surface was initially imaged at 550 mV, a potential within the double layer potentials of TTCA adlayer. Figure 2a is a typical STM image of the bare Au(111) surface obtained in 0.1√ M HClO4 at 550 mV. A flat and reconstructed Au(111)
(22  3) terrace is observed. After that, 1.5 μL saturated TTCA solution was dropped into the STM cell and the final concentration of TTCA in the electrolyte is about 4.0  10
6 M. Figure 2b is a large-scale STM image of the TTCA SAMs. The flat and clean Au(111) terrace is now covered with randomly packed wormlike structure. Single bright

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spot can be distinguished within the disordered adlayer. The diameter of these spots is about 0.3 nm, consistent with the dimension of a TTCA molecule. The result suggests that TTCA molecules adsorb on the Au(111) surface and form a wormlike disordered adlayer. Similar SAMs with poor packing order have been observed in the SAMs of bi- and tridentate alkanethiols. It is believed that the increased binding energy between the adsorbate and the substrate should responsible for these less ordered SAMs.37,39,51
53 For the present system, it is inferred that multiple bonding might form between the adsorbed TTCA and the Au(111) surface. This strong binding limits necessary diffusion of the adsorbate to form a well-ordered arrangement. Compared with the bare Au(111) surface shown in Figure√2a, the characteristic herringbone structure for the (22  3) reconstruction of the Au(111) surface disappears. This is consistent with the lifting of the Au(111) reconstruction with the adsorption of thiols. However, no obvious gold island supplied by lifting the herringbone reconstruction of the Au(111) surface is observed. This may be due to that the gold adatoms have been incorporated with TTCA molecules in the disordered adlayer. Besides, etching pits, which usually exist in alkanethiol SAMs,54 were not observed in the TTCA SAMs. The absence of etching pits has been observed in many other aromatic thiol SAMs.55 Cyclic voltammetric results suggest that either a reductive desorption or a structural/orientational transformation occurs at ∼260 mV for TTCA SAM on Au(111) surface. After determining the structure of the SAM at 550 mV, the substrate potential was shifted negatively step by step to monitor the evolution of the TTCA adlayer. From 550 to 260 mV, no obvious structure change is observed. When the substrate potential is negative of 260 mV, the wormlike structure disappears gradually. Meanwhile, some short straight sticks consisted of several TTCA molecules form on the surface. Figure 3a gives a STM image acquired at 170 mV. The wormlike structure nearly disappears. Instead, short sticks composed of bright spots cover the whole terrace. These sticks cross each other with an angle of 60
or 120
. Figure 3b
e is a series of STM images recording the phase transformation process at 170 mV. The change of the adlayer structure is very quick in the initial several minutes. Figure 3b is obtained after keeping the substrate potential at 170 mV for 2 min. The straight sticks have increased a lot. Several sticks arrange parallel to each other and appear in pair. In some region, small ordered domains consisted of the pairing sticks can be observed. When the substrate potential is kept at 170 mV for 5 min, several bigger domains with well-ordered striped structure are observed. The ordered domains can be correlated with each other by 60
rotation operation. Short molecular sticks crossing with 60
or 120
still can be seen among the ordered domains. The size of the ordered domains further increases with holding at 170 mV. Meanwhile, some randomly arranged short sticks among the ordered domains transfer to parallel stripes, as outlined in Figure 3d. After that, the transformation of the adlayer structure becomes slower. Figure 3e is the resulted adlayer after keeping the substrate at 170 mV for 28 min. Compared with figure 3d it is found that both the short molecular sticks locating between the domains and some small ordered domains disappear. The terrace is now covered with a few big ordered domains. Several islands are observed at the domain boundaries or structural defects of the adlayer. These islands may attribute to Au adatoms or molecular clusters. Images with higher resolution such as that in Figure 3f are analyzed to determine the internal structure of the stripes. It can 16585

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Figure 3. Series of STM images of TTCA SAMs on Au(111) surface at 170 mV with a function of time. Tunneling conditions: (a
e) Scan area = 33  33 nm2, Ebias =
135.6 mV, I = 1.206 nA. (f) Scan area = 10  10 nm2, Ebias =
201.2 mV, I = 1.797 nA.

Figure 4. (a) Top view of the upmost layer of the Au(111) surface. The colored circles denote the initial adsorption sites of the S atoms of TTCA during the calculations: green, top
top; red, hollow
hollow; black, bridge
bridge. (b) Top view of the optimized bridge
bridge configuration for TTCA on Au(111). (c) Side-view of the optimized bridge
bridge configuration for TTCA on Au(111).

be seen that two neighboring √ molecular stripes appear in pair. The stripes are along the 3 direction of the Au(111) surface. The intermolecular distance within the stripes is about 0.49 nm, which coincides with the lattice distance of underlying Au(111) substrate. This intermolecular distance is too close to accommodate TTCA molecules lying flat on the surface without overlapping. Another important feature of the STM image is that each bright spot shows a 2-fold symmetric elliptical contour, as illustrated by the white ellipses in the figure. As reported by Wan, TTCA molecules with flat-lying orientation on Cu(111) surface show a distinctly characteristic, propeller-like feature in STM image.56 The elliptical STM contour of TTCA molecules on Au(111) surface further excludes the flat-on adsorption configuration. Thus, it is inferred that TTCA molecules should adsorb on the Au(111) surface with a tilted or vertical mode, which prompt the π
π stacking interactions between the heterocyclic rings of TTCA molecules. The long axis of the TTCA molecules have a cross angle θ = √ 60
to the molecular stripes, suggesting that they are along the 3 direction of the Au(111) lattice too. A unit cell is outlined in the image, in which the parameters are a = 0.49 √ ( 0.02 nm, b = 1.72 ( 0.02 nm, and R = 90 ( 1
, giving a (6  3) superstructure for the ordered double-striped phase. √ The (6  3) superstructure exists stably until 0 mV. At potential negative of 0 mV, desorption of the TTCA adlayer

occurs. All the adsorbed species disappear from the STM image. After the formation of the well-ordered double-striped structure, the substrate potential was set back to 550 mV or even 1.0 V. Intriguingly, identical double-striped structure is observed. The result demonstrates that the structural transformation from the disordered wormlike structure to the well-ordered double-striped structure is irreversible. Combining the in situ STM results with the cyclic voltammetric results, it is reasonable to conclude that the irreversible cathodic peak at ∼260 mV corresponds to the disorder
ordered phase transformation of the TTCA SAMs. This potential-induced structural transformation is supposed to be a result of variation of the adsorbate
substrate interaction at different substrate potential.12 At positive potentials, TTCA molecules are deprotonated.57,58 The binding between the TTCA molecule and the Au(111) substrate is strong. The mobility of the adsorbate is constrained and necessary diffusion of the molecules to form effective intermolecular interactions and ordered packing is unfavored. With the negative shift of the potential, the adsorbate
substrate interaction decreases and the adsorbate, either the gold thiolate species or the thiol molecules, can diffuse on the surface and adopt energetically more favored arrangement. This potential-induced structural transition of TTCA SAMs is quite similar to that of the SAMs of 4-methylbenzenthiol and 4-mercaptopyridine on Au(111).12,59 16586

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To further determine the energetically favored adsorption configuration of the TTCA molecules on Au(111) surface, structural optimizations with different initial configurations were performed by using periodic DFT calculations. As revealed by the STM results, TTCA molecules adsorb on Au(111) surface in a tilted or vertical model with its ring plane along the Æ121æ direction of the Au(111) surface. Moreover, each TTCA molecule shows a 2-fold symmetric ellipse contour in STM image. It is supposed that two S headgroups anchor on the Au(111) surface at the same sites, whereas the remaining S headgroup points away from the substrate. Thus, four possible adsorption sites, that is, top-top, hollow
hollow (fcc and hcp), and bridge
bridge (Figure 4a), are chosen as starting geometries, which are subsequently relaxed to find the local energy minimum and optimized structures. According to previous report, TTCA adsorbs on evaporated silver films in trithiol form though trithione form is stable in KBr disk.60 1,3,5-triazine-2,4-dithione derivatives are also demonstrated to adsorb on Au, Ag, and Cu substrates in the thiol form to facilitate chemisorption onto the metal surfaces.61
63 Therefore, TTCA in the trithiol form is adopted in the theoretical calculations. Intriguingly, whatever initial adsorption geometry with different adsorption sites and tilted angle is set, the TTCA molecule relaxes to the bridge sites after structural optimization. Figure 4, panels b and c, illustrate the optimized structure of TTCA molecule adsorbed on Au(111) surface with both S headgroups locating at the bridge sites and the molecular backbone normal to the surface. In this configuration, two S atoms adsorb at the bridge site slightly shifted to hollow site, and the N atom locates at the on-top site. This site preference of S atom at bridge site has also been found for other aromatic thiols such as benzene-1, 4-dithiol and 4-mercaptopyridine.10,35 Table 1 lists the binding energy and the geometry parameters of the optimized structure. As reported, the binding energy for

4-mercaptopyridine adsorbed at the fcc-bridge site of Au(111) as monothiolate is about 1.415 eV.10 The binding energy for 6-mercaptopurine on Au(111) surface with the S and N atoms adsorbed at the near bridge and top sites is about 1.96 eV.55 The binding energy for TTCA adsorbed at the bridge
bridge site of Au(111) is about 3.27 eV, indicating the formation of multibinding sites between TTCA molecule and the Au(111) substrate. This result is further supported by the electron density difference analysis for the optimized bridge
bridge geometry, as shown in Figure 5a. Obvious increases in the charge density are clearly seen not only between the S atoms and the surface Au atoms but also between the N atom and the Au atom. This result confirms the strong interactions between the S/N atoms and the substrate.43,64 Hirshfeld charge analysis reveals that both S and N atoms anchoring on the Au(111) surface are negatively charged (
0.05 and
0.14 eV, respectively), and the Au atoms bound to S and N are positively charged (0.03 and 0.06 eV, respectively). The Au atom bound to N atom has the largest Hirshfeld charge. Figure 5b shows the simulated STM image of the optimized geometry of TTCA on Au(111) surface. The local density of state focuses on the thiolate bonds. Thus, S atoms bonded to the substrate are the brightest parts in the STM images. The symmetry of the simulated STM image of TTCA agrees well with the 2-fold symmetry of single TTCA observed in the experimental STM image. Based on above analysis, Figure 6 proposed a structural model for the double-striped phase, in which the hydrogen atoms in the free S headgroups are not shown. TTCA molecules adsorb on Au(111) surface vertically with two S atoms and one N atom anchoring at the bridge sites and the top site respectively.

Table 1. Optimized Binding Energy and Geometry Parameters for TTCA on Au(111)a Ebind (eV) adsorption sites

3.27 S

bridge

N

top

dS
Au (Å)

2.56/2.58/3.20

dN
Au (Å)

2.29

θ (deg)

0

a

dS
Au is the distance from the bonded S atom to the nearest Au atoms; dN
Au is the distance from the bonded N atom to the nearest Au atom; θ is the angle between the aromatic plane and the Au(111) surface normal.

Figure 6. Proposed structural model for the double-striped phase.

Figure 5. (a) Electron density difference of the optimized adsorption geometry of TTCA on Au(111) surface. (b) Simulated STM image of the optimized adsorption geometry of TTCA on Au(111) surface, Vsample = 1.0 V, isosurface value = 0.002 electron Å
3. 16587

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Figure 7. STM images (a, c, and d) and cross-sectional profile (b) of the TTCA bilayer on Au(111) surface. Tunneling conditions: (a) Scan area = 112  112 nm2, Ebias =
220.1 mV, I = 1.536 nA. (b) Scan area = 36  36 nm2, Ebias =
275.9 mV, I = 1.956 nA. (c) Scan area = 12  12 nm2, Ebias =
136.9 mV, I = 1.547 nA.

This vertical adsorption geometry implies that the remaining S headgroup is free and can serve as reactive site to engineer other functional molecules or materials with the SAMs. The free S headgroups can be readily handled to interface with other metal atoms, metal particles, or functional species, which makes the TTCA SAMs a promising scaffold for applications in molecular electronics and chem/bio sensors. In addition, the triple bonding interaction between TTCA molecule and Au substrate is so strong that we expect that other N-heteraromatic dithiols with similar chemical fragment as TTCA may form similar SAMs on Au substrate, as exemplified by recent report of 1,3,5-triazine-2, 4-dithione derivatives on Au.63 The large footprint of the TTCA molecules on Au(111) surface enables the possibility to fabricate ordered arrays of some bulky functional groups by simply attaching TTCA-like moieties as a “foot”. When the concentration of the TTCA molecule in the electrochemical cell is higher than 5  10
5 M, islands can be observed on the Au(111) surface, as shown in Figure 7a. To demonstrate whether the islands originate from TTCA bilayer or from the TTCA monolayer on Au(111) islands, a cross section of Figure 7a was analyzed, as shown in Figure 7b. The height of the island is measured to be ∼0.18 nm. This value is smaller than the height of a Au(111) atom step, suggesting that the island observed in Figure 7a is a bilayer of TTCA. The formation of the bilayer may be related to the S
S bonds45,65
68 or hydrogen-bonding between the reactive S or N headgroups in different layers. Similar to the molecular arrangement in the wormlike disordered monolayer, both the bottom layer and the second layer are disordered (the images are not shown here) at 550 mV. In addition, some brighter clusters with a height nearly equal to that of the bilayer can be observed. These clusters may be at the initial stage for the formation of the 2D bilayer. When the substrate potential shifts to negative of 260 mV, a transformation of the molecular packing in the SAMs occurs again. This structural transformation does not affect the location and the dimension of the bilayer structure.

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Figure 7b shows the detailed information about the bilayer obtained at 170 mV. Except for the small double-striped domain in the lower right side of the image, both the first and the second layer possess another short-range ordered structure. The domains of the other ordered structure are quite small, usually smaller than 10 nm  10 nm. This may be related to the higher concentration of the molecule. The high resolution STM image suggested that the network structure is also composed of molecular stripes, as shown in Figure 7c. The molecules within the stripes are imaged as symmetrical ellipses with intermolecular distance of about 0.49 nm. Compared with the substrate lattice, it is found that both the long axis of molecular ellipses and the direction of the molecular stripes are parallel to the Æ121æ direction of the Au(111) surface. These features are consistent with those of the well-ordered striped phase. However, rather than appearing in pair, a line of single TTCA molecules are dispersed between the two nearest stripes with a periodicity of 3 times of that of molecular stripes. These inserted molecules appear with a round-shape in the STM image (represented with blue circles in Figure 7b), implying that they might adopt a different adsorption configuration from the molecules in the stripes. Moreover, intersection angles of θ1 = 60
and θ2 = 100
are found in the second layer, and an offset angle j = 170
between the stripes in the first layer and the second layer is measured in the image. Though the interlayer interaction for the formation of such an offset epitaxial bilayer structure is unclear, it may imply that the interaction between the first layer and the second layer is not as strong as that between the Au(111) substrate and the first layer. Consequently, the orientation of the molecules in the second layer is not correlated with respect to the first layer.

’ CONCLUSIONS The SAMs of TTCA molecule on Au(111) electrode surface were investigated by combining experimental characterizations and theoretical calculations. Negative shift of the substrate potential induces a structural transformation from wormlike disordered phase to striped ordered SAMs. High resolution STM images and DFT calculations prove that TTCA molecules adsorb on the Au(111) surface vertically in the thiolate form via two S headgroups and one N atom binding to the bridge and top sites of the substrate respectively. The formation of triple bonding between TTCA and the Au surface results in a high binding energy of 3.27 eV. The high stability and free S termination of the TTCA SAM makes it a promising scaffold for applications in molecular electronics and chem/bio sensors.

’ AUTHOR INFORMATION Corresponding Authors

*E-mail: [email protected]; [email protected].

’ ACKNOWLEDGMENT The authors thank financial support form National Natural Science Foundation of China (Grants 20733004, 20821003, 20821120291, and 21003131), National Key Project on Basic Research (Grants 2011CB808701 and 2011CB932304), and the Chinese Academy of Sciences. 16588

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