Modulated Intermolecular Interactions in Ferrocenylalkanethiolate Self

Article pubs.acs.org/JPCC

Modulated Intermolecular Interactions in Ferrocenylalkanethiolate Self-Assembled Monolayers on Gold Huihui Tian,†,‡ Yun Dai,† Huibo Shao,*,‡ and Hua-Zhong Yu*,† †

Department of Chemistry, Simon Fraser University, Burnaby, British Columbia V5A 1S6, Canada Key Laboratory of Cluster Science (Ministry of Education of China) and School of Chemistry, Beijing Institute of Technology, Beijing 100081, P. R. China

‡

ABSTRACT: Redox-labeled self-assembled monolayers (SAMs) on gold are excellent model systems for the study of long-range electron transfer processes at electrolyte−electrode interfaces, particularly the distance and reorganization energy dependences. In this work, we have shown that the intermolecular interaction among redox centers is in fact a crucial factor in the overall, nonideal electrochemical response of ferrocenylalkanethiolate SAMs on gold. In both single-component and high-ratio binary monolayers of 11-ferrocenyl-1-undecanethiol (FcC11SH), the two distinct pairs of redox peaks are corresponding to rather moderate differences in the packing densities of the two structural domains. We have discovered that the redox peak at lower potential becomes narrower and higher when organic solvents (nitrobenzene or octanol) are added to the aqueous electrolyte, while the peak at the higher potentials is barely influenced. On the basis of the Frumkin isotherm, we have obtained the intermolecular interaction parameters in the different structural domains of the monolayers by fitting the experimental data. The results showed that the intermolecular interaction in the FcC11S−Au SAMs can change from repulsion to attraction upon adding organic solvent in the aqueous electrolyte. It is suggested that the solvent perturbation to the SAM structure at monolayer/electrolyte interface induces remarkable change in the intermolecular interactions and therefore modulates the observed electrochemical responses from nonideal to nearly ideal.

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INTRODUCTION Self-assembled monolayers (SAMs) of alkanethiols and their derivatives on noble metals have been studied widely in the past two decades.1−4 By attaching different functional groups at the ends of the alkyl chains, one can easily control the surface and structural properties of these organized molecular assemblies. SAMs have been adapted to many applications including the modulation of nanomaterial properties and scanning probe lithography.5,6 Others have explored the application of alkanethiolate SAMs on mercury to change the interfacial nature of molecular junctions and to design functional nanoelectronic devices.7−10 Redox-active SAMs on gold are ideal model systems to study long-range electron transfer kinetics at electrolyte−electrode interfaces11 and, recently, to investigate the charge transport properties of electrode-bound DNA strands.12 Because of their reversible electrochemical responses and ease of preparation, ferrocenylalkanethiolate SAMs on gold are the most popular molecular probes to study the kinetics and thermodynamics of surface redox reactions.11−21 In order to achieve ideal redox properties, binary SAMs of ferrocenylalkanethiols and n-alkanethiols are typically prepared, in which the ferrocene centers are assumed to be isolated from each other by the unsubstituted alkyl chains. Nevertheless, such binary SAMs often have nonideal voltammetric properties (peak splitting, narrowing, or shifting), which are typical for single-component ferrocenylalkanethiols.11,14,15,21 Besides con© XXXX American Chemical Society

ventional electrochemical methods, SPR (surface plasmon resonance), QCM (quartz crystal microbalance), STM (scanning tunneling microscopy), and AFM (atomic force microscopy) have been employed to study the formation and structure of ferrocenylalkanethiolate SAMs in order to reveal the origin of the structural/redox heterogeneity; it has been demonstrated that the redox behavior of ferrocenylalkanethiolate SAMs are strongly affected by their microenvironments as determined by ion-pairing22−25 and double-layer effects.21,26−29 The only option to achieve ideal redox behavior is to construct a binary monolayer on gold with a very low proportion of ferrocenylalkanethiols (χ < 0.1), which minimizes the intermolecular interactions among the redox centers.11,15,30 In this work, we demonstrate an alternative approach to modulate intermolecular interactions and to obtain ideal redox behavior from single-component ferrocenylalkanethiolate SAMs on gold. Our combined experimental and theoretical investigation of the structural origin of split redox peaks has revealed that the redox properties of ferrocenylalkanethiolate SAMs can be modulated by adding minute amounts of organic molecules to the electrolyte. We confirmed that the aggregated organic molecules at the monolayer/electrolyte interface effectively perturb the film structure, which leads to significant Received: October 10, 2012 Revised: December 18, 2012

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wire was used as the counter-electrode and a Ag|AgCl |3 M NaCl electrode as the reference. All experiments were carried out at room temperature.

variation of the intermolecular interactions among end-tethered ferrocence centers.

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EXPERIMENTAL SECTION Reagents and Materials. 11-Ferrocenyl-1-undecanethiol (FcC11SH) (98%) was purchased from Dojindo Laboratories (Japan), perchloric acid (98%) and 1-octanol (C8OH) from Fisher Scientific, ethanol (95%) from Commercial Alcohols, and nitrobenzene (NB) from Allied Chemistry. NB was purified by washing with 0.1 M HCl, 0.1 M NaOH, and deionized water successively. All other reagents were used as received. NB- and 1-octanol-saturated HClO4 solutions were prepared by adding NB and 1-octanol, respectively, to aqueous 1.0 M HClO4, shaking, and then equilibration for at least 24 h; their solubilities in water at room temperature are as low as 0.19% and 0.59%, respectively.31,32 Gold slides (regular microscope glass slides coated with 5 nm Cr and then 100 nm Au) were purchased from Evaporated Metal Films (EMF) Inc. Prior to use, the gold slides were cleaned by immersion in a Piranha solution (3:1 mixture of concentrated H2SO4 and 30% H2O2) for 8−10 min at 90 °C (CAUTION: Piranha solution reacts violently with organic solvents; it must be handled with extreme care). They were subsequently rinsed with copious amounts of deionized water and dried under N2. The surface roughness factor (1.2) was determined by measuring cyclic voltammograms (CVs) in 1.0 mM aqueous K3Fe(CN)6 at varied scan rates.33 The gold bead electrode was prepared by melting one end of a 3 cm Au wire (0.5 mm in diameter, 99.999%, Cedar Lane Laboratories) into a spherical shape (the diameter is ∼2.0 mm) and then attach another end to a copper wire. The Au bead electrode was flame-annealed several times and washed with deionized water before the formation of SAMs on it. Polycrystalline Au disk electrode with a diameter of 2 mm sealed in Teflon was purchased from CH Instruments Inc. (Austin, TX). It was first polished with alumina (0.05 μm), rinsed with deionized water, and then electrochemically cleaned by cycling between the potential of −0.2 and +1.6 V (vs Ag| AgCl) with a scan rate of 50 mV s−1 in 1 M H2SO4. The ferrocenylalkanethiolate SAMs were prepared by immersing freshly cleaned gold slides (or other two types of gold electrodes for comparison) in 1.0 mM FcC11SH (in 95% ethanol) solution at room temperature (23 ± 1 °C). The binary SAMs were prepared by (1) immersing in 1.0 mM C11SH/ ethanol solution for 24 h, then the modified electrode was washed with 95% ethanol, and then soaked in 1.0 mM FcC11SH/ethanol for 24 h; (2) coadsorption of FcC11SH and C11SH from their ethanol solution with a total concentration of 1.0 mM for 24 h, the mole fraction of FcC11SH was 0.5. The SAM-modified gold electrodes were all cleaned by sonication in 95% ethanol for 3−4 min, then washed with copious deionized water, and dried under N2 prior to characterization. Electrochemical Measurements. For the gold slide electrode, electrochemical measurements were performed in a three-electrode, single chamber Teflon cell with a CHI 1040A Electrochemical Analyzer in a Faraday cage. The cell was constructed with an opening at the bottom, where the working electrode (SAM modified-gold slide) was attached. The area of the gold electrode exposed to the electrolyte was defined by an O-ring seal. In the case of gold disk electrode or gold bead electrode, a conventional glass cell from Princeton Applied Research (Oak Ridge, TN) was used. For both cells, a platinum

RESULTS AND DISCUSSION Redox/Structural Heterogeneity of FcC11S−Au SAMs. Extensive studies of binary ferrocenylalkanethiolate/n-alkanethiolate SAMs on gold have been reported in the past two decades.15−21 However, single-component ferrocenylalkanethiolate SAMs have lacked general interest because of their redox/structural nonideality.23,24 In fact, these systems should be investigated carefully because of their function and fundamental role in the formation of more complex molecular assemblies (vide infra). Figure 1 shows representative cyclic voltammograms (CVs) of FcC11S−Au SAMs formed on three types of conventional

Figure 1. Representative CVs of FcC11SH SAMs on (a) mechanically polished gold disk electrode; (b) annealed Au bead electrode; and (c) evaporated gold film on glass. The supporting electrolyte is 1.0 M HClO4 solution, and the scan rate is 50 mV/s.

gold electrodes: a mechanically polished gold disk, a flameannealed gold bead, and an evaporated gold film on glass, respectively. As shown in Figure 1a, a pair of broad peaks appear at E° = 350 mV (vs Ag|AgCl) in the CV of the monolayer prepared on the gold disk electrode, corresponding to the oxidation and reduction of the end-tethered ferrocene moiety. In Figure 1b, a pair of much sharper peaks with a very similar formal potential E° = 325 mV are observed on the gold B

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bead electrode. On an evaporated gold film on glass (Figure 1c), in contrast, we always observe two pairs of redox peaks in the same potential range. The formal potential of one pair of these peaks is not much different from those of Figure 1a,b, E° = 330 mV vs Ag|AgCl, but the other pair appears at the much lower potential of 220 mV. It is interesting to investigate the molecular structure of the monolayer that gives rise to these shoulder peaks, which are not observed on either much flatter beads (Figure 1b) or on macroscopically rough disk electrodes (Figure 1a).15,22,25,34 In retrospect, the phenomenon of peak splitting has been previously observed for binary monolayers as well;11,15,30 it is generally accepted that different structural domains form on a polycrystalline surface because of differences in the adsorption affinities of alkanethiols vs. ferrocenylalkanethiols on gold. Figure 2 shows three typical examples of peak splitting behavior

particularly in single-component ferrocenylalkanethiolate SAMs, we need to accurately determine the individual amplitudes of the peaks by deconvoluting the overlapped CV peaks. A Gaussian−Lorentzian fitting protocol, developed by Lee et al.,15,35 was adapted to simulate the anodic waves shown in Figure 2 for obtaining the individual peak areas (here, we attributed the peak at 220 mV to peak I and the one at 330 mV to peak II) and, in turn, the integrated charges. With this approach, we were able to obtain the surface concentration of ferrocene groups (ΓFc) with the help of eq 1 ΓFc = Q Fc/nFA

(1)

where QFc is the integrated charge of the redox peak, n is the number of electrons involved in the redox reaction (here, n = 1 for Fc/Fc+), F is the Faraday constant, and A is the real surface area of the electrode (0.79 cm2 considering the roughness factor of the electrode). The total surface concentration was obtained by integration of the overlapped anodic peaks of ferrocene (Fc) with correction for the charging current contribution, and the individual surface densities of ferrocenylalkanethiols were from the deconvoluted peaks (shown in Figure 2). For a single-component FcC11S−Au SAM, the saturated surface density is 4.3 ± 0.2 × 10−10 mol/cm2, which is consistent with the literature values. 13,15−17,21,22,37 The contributions of peaks at 220 and 330 mV to the total surface density are 33 ± 2% and 67 ± 2%, respectively. It has been proposed that the closely packed, standing-up phase of alkanethiolate SAMs is formed from a lying-down phase via a stacked lying down phase,38 which should be applicable to ferrocenylalkanethiolate SAMs as well (Scheme 1). Considering that there is only one component in these Scheme 1. Three Possible Orientations of FcC11SH on Gold and the Respective Footprint Areas

Figure 2. Anodic traces and their respective deconvolution peaks of the CVs for (a) single-component FcC11S−Au SAMs. (b) Binary FcC11SH/C11SH SAMs prepared by immersion in 1.0 mM C11SH/ ethanol solution for 24 h and exchange in 1.0 mM FcC11SH/ethanol for 24 h. (c) Mixed SAMs of FcC11S/C11S−Au formed by coadsorption of FcC11SH and C11SH (1:1) in ethanol (1.0 mM in total) for 24 h.

SAMs, the peak splitting phenomenon should be directly related to the heterogeneity of the surface density of ferrocenylalkanethiols on gold. The key question is how different are the packing densities of FcC11SH in the two structural domains, i.e., are the lying-down vs standing-up phases coexisting? Because the diameter of ferrocene (0.66 nm) is bigger than the cross-section of alkyl chains (diameter of 0.46 nm if modeled as cylinders), the maximum surface density of FcC11SH will be dictated by the terminal ferrocene groups and cannot completely fill the √3 × √3R 30° lattice.13,38 Without considering how the missing sites are distributed, the maximum packing of FcC11S−Au SAM should be 4.65 × 10−10 mol·cm−2, which represents a closely packed, standing-up FcC11S−Au monolayer.15 As mentioned above, the experimentally

of SAMs all containing FcC11SH but prepared differently. The anodic CV waves of two binary SAMs prepared by exchange and coadsorption methods are shown in Figure 2a,b, respectively; in both cases, two pairs of redox peaks are evident but with different relative amplitudes. In comparison, Figure 2c shows the anodic trace of a representative CV for a singlecomponent FcC11S−Au SAM on gold (after incubating the gold electrode for 18 h). The most significant finding here is that the two pairs of peaks in the single-component system are in fact quite similar to those CVs of binary SAMs formed on the same type of gold substrates (in terms of peak potentials). In order to understand the origin of these two peaks, C

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determined surface density for a complete FcC11SH monolayer is 4.3 × 10−10 mol·cm−2, in which case each FcC11SH would occupy an area of 0.38 nm2. If we relate peak II (at higher potential) to the closely packed standing-up domain in a complete FcC11SH SAM (0.36 nm2/molecule, calculated with the ferrocene diameter of 0.66 nm), the area of each molecule in the loosely packed domain (A1) can be calculated from eq 2.

ΓFc(A1R1 + A 2 R 2) = 1

(2) 2

The estimated A1 is 0.44 nm /molecule with an intermolecular distance of 0.76 nm, which is about 20% larger than that in the closely packed domain (0.36 nm2). Scheme 2 illustrates Scheme 2. Illustration of the Different Orientations and Footprint Areas in the Two Different Structural Domains of FcC11S−Au Monolayers

Figure 3. CVs of FcC11SH SAMs performed in 1.0 M aqueous HClO4 solution with different concentrations of (a) nitrobenzene (NB), from left to right, 0%, 40%, 70%, 80%, and 100% of saturation; and (b) 1-octanol (C8OH), from left to right, 0%, 20%, 40%, 70%, and 100% of saturation, respectively.

when g > 0, attraction forces exist among molecules and the peak becomes narrower and higher. The next task was to deduce the interaction parameter among the Fc centers related to the two CV peaks of the FcC11S−Au SAMs. Figure 4 shows a comparison of experimental CVs with theoretical i−E curves obtained in aqueous HClO4 electrolyte at different saturation percentages of NB and C8OH, individually. The theoretical i−E curves were generated from eqs 3 and 433,39

the two different structural domains (closely packed and relatively loosely packed structures) in the FcC11S−Au monolayer, which we believe correspond to the two pairs of distinct redox peaks. It is reasonable to conclude that the molecular packing density and orientation in the two structural domains may not be as remarkable as previously proposed.38 Modulation of Intermolecular Interactions in FcC11S−Au SAMs. Creager et al. have previously noted that the redox properties of ferrocenylalkanethiolate SAMs change when small amounts of organic molecules are added to the aqueous electrolyte .28 To investigate whether these molecules influence the two redox peaks differently in the singlecomponent FcC11S−Au monolayer, we added nitrobenzene (NB) and 1-octanol (C8OH) at different concentrations up to saturation. Figure 3 shows that, as the concentration of NB or C8OH is raised, peak I becomes narrower and higher, and its formal potential shifts positively, while the shape and formal potential of peak II does not change. For a reversible electrochemical reaction (O + ne− ⇄ R) in a surface-bound monolayer and when the adsorption obeys a Langmuir isotherm (i.e., no lateral interactions), the peak width at half-height (ΔEfwhm) should be 90.6 mV/n.33 When the adsorbed molecules interact, the shape, width, and height of the peak depend on the interactions between O with O (aO), R with R (aR), and O with R (aOR), and the overall interactions can be expressed by the interaction parameter νgθT, where g = aO + aR − 2aOR, v is the number of water molecules displaced from the surface by adsorption of one O or R, and θT is the total coverage of both O and R. When g = 0, the monolayer can be regarded as following the Langmuir isotherm, i.e., the attraction and repulsion forces are equal; when g < 0, the repulsion forces predominate, and the peak is wider and flatter than the peak in the Langmuir adsorption case; in contrast,

i=

⎤ n2F 2Aυ Γ*0 ⎡ f (1 − f ) ⎥ ⎢ RT ⎣ 1 − 2νgθTf (1 − f ) ⎦

(3)

n(E − Ep) = (RT /F )[ln{f /(1 − f )} + νgθR (1 − 2f )] (4) 2 2

n F Aυ Γ*0 (4 − 2νgθT)−1 (5) RT where Γ*0 is the total surface density of FcC11SH on gold (Γ*0 = ΓO + ΓR, ΓO and ΓR are the surface density of oxidized and reduced FcC11SH molecules, individually), υ is the scan rate, θO and θR are the surface coverage values (θi = Γi/Γ0*), and θO + θR = θT; f is defined as the fraction of the oxidized FcC11SH molecules at a certain potential, and θO/θT = f, while θR/θT = 1 − f. The initial value of νgθT for the fitting process was derived from eq 5 using the CV data shown in Figure 3, then optimized using eqs 3 and 4 until the best fit was achieved. In Figure 4, six representative fitting results of the CVs of the FcC11S−Au monolayers at different saturation percentages of NB (a−c) and C8OH (d−f), respectively. In each case, the two overlap peaks in the anodic trace of the CV can be nicely matched with theoretical i−E curves upon optimizing the intermolecular interaction parameter (νgθT). As the concentration of NB or C8OH in the aqueous HClO4 solution increases, the ip =

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Figure 4. Experimental (open circles) and simulated (solid lines) i−E curves of the CV anodic traces of the FcC11S−Au SAMs in HClO4 aqueous solution at various saturation percentages of C8OH (a,b,c) and NB (d,e,f). The saturation percentages of C8OH/NB and the fitted interaction parameters for peak I (wave at lower potential) are indicated.

interaction parameter related to peak I changes from negative to positive, which means the intermolecular force among molecules goes from repulsion to attraction. We have shown the transition point where the peak width at the half-height is 90.6 mV/n, and νgθT = 0; the saturation percentages of NB and C8OH are 70% (Figure 4e) and 20% (Figure 4b), respectively. In contrast to peak I, as the amount of the NB or C8OH added to the electrolyte increases, the shape and potential of peak II do not change as much, and the fitted interaction parameter remains close to −1.0, which means that there is always repulsion force existing in the monolayer domain related to peak II. In order to shed light on the structural perturbation of the organic molecules on the FcC11S−Au SAMs, we have obtained the fitted νgθT values and the formal potentials of peak I for the CVs of the monolayers in electrolytes of 1 M HClO4 containing various amounts of NB or C8OH. As illustrated in Figure 5a, when the amount of C8OH increases, the νgθT value rises quickly and changes from negative to positive at 20% saturation (+1.75 at 100% saturation); for NB, it rises gradually and only reaches +1.1 at saturation. At the same time, in C8OHsaturated electrolyte, the formal potential of peak I shifts from 220 to 290 mV and does not change upon adding NB (Figure 5b). We can conclude from these results that the addition of C8OH or NB to the electrolyte induces both variations of

intermolecular interaction and a positive shift of the peak potential. First, to explain how these organic additives modulate the interactions among redox centers in the monolayer, we take C8OH as an example (Scheme 3). When the loosely packed domains of the ferrocenylalkanethiolate monolayer on gold are tested in aqueous solution (dielectric constant, ε = 78.0), the nonpolar ferrocene groups form clusters because of the hydrophobic nature of the monolayer surface, and repulsion forces dominate the intermolecular interactions (Scheme 3a). C8OH (or NB) molecules added to the bulk electrolyte solution will then aggregate atop the monolayer driven by the hydrophobic interaction between the monolayer and the alkyl chains of C8OH (or the benzene ring of NB) and distribute around the ferrocene groups. Accordingly, the nonpolar ferrocene groups would disperse from the clustered to a more uniform phase in which they are separated and even buried in a thin layer of C8OH (or NB) molecules (Scheme 3b). In this situation, the surface structural domains related to peak I becomes similar to that of a mixed monolayer of ferrocenylalkanethiol/alkanethiol with dominant attraction forces among ferrocene groups located in the alkane-like environment. Second, we need to understand the positive formal potential shift of peak I when small organic molecules are added to the aqueous electrolyte. When the concentration of C8OH is raised above 20% of saturation, a double-layer effect is induced on the E

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Scheme 3. Hypothetic Structures of a FcC11S−Au SAM When the Electrolyte Is (a) Aqueous HClO4 Solution; (b) Aqueous HClO4 Solution Containing a Low Concentration of C8OH (< 20% of saturation); or (c) Aqueous HClO4 Solution Containing a High Concentration (>20% of Saturation) of C8OH

Figure 5. Dependence of (a) the interaction parameter and (b) the formal potential of peak I in the CV of FcC11S−Au electrodes (ΓFc= 4.3 × 10−10 mol/cm2) on the concentration of NB (open circles) and C8OH (closed circles) added to the electrolyte.

monolayer surface.21,28,36 As illustrated in Scheme 3c, the amount of C8OH (or NB) adsorbed on the monolayer surface increases, and it eventually forms a nanometer thick thin film. Compared with the dielectric constant of water (ε = 78.0), the thin film of NB (ε = 39.4) or C8OH (ε = 10.3) aggregated near the monolayer surface will decrease the polarity of the microenvironment of the monolayer, which favors ferrocene relative to the ferrocinium ion. The less polar environment requires a higher polarization energy, which leads to the positive shift of the formal potential. Another explanation is that, as the aggregation of small organic molecules occurs, the concentration of ClO4− near the SAM surface decreases, and the formal potential shifts to a higher value because of the inhibited ion-pairing equilibrium.34,40,41 In Figure 4b, we have shown that the formal potential does not change as much on the addition of NB, although the peak shape changes drastically. We believe that this is due to the low solubility and large dielectric constant of NB in the aqueous electrolyte, in comparison to C8OH.31,32 Another important aspect of the redox behavior of FcC11S− Au SAMs is that peak II showing up at higher potential in the CVs does not change when the organic compounds are introduced to the electrolyte. It is suggested that the wellordered FcC11SH molecules in this case are already in a nonpolar environment and that the organic molecules unlikely aggregate on or penetrate into the closely packed monolayer domains. We noticed that the area of peak I decreased slightly, whereas that of peak II increased upon adding less polar molecules; this may indicate that some of the initially loosely packed FcC11SH molecules (at the domain boundaries) stand up and join the closely packed structural domain. While the present study provides a better understanding of the structural/redox heterogeneity of single-component ferrocenylalkanethiolate SAMs on gold by investigating the

effects of the C8OH or NB thin-film aggregated on the monolayer, the above-noted structure−property correlation remains hypothetical. To overcome the limitations of electrochemical methods, a combined in situ microscopic and spectroscopic study of the monolayers would be desirable. It is also important to have a more precise control of the surface morphology of the gold substrates; here, we have tested the most popularly used evaporated Au/Cr/glass slides and two other conventional gold electrodes as starting points; a study of these SAMs on single crystals would help to further illustrate the origin of peak splitting. We are currently in the process of designing and executing new experiments to tackle the above questions, which are beyond the scope of this report.

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CONCLUSIONS The peak splitting in the CVs of single-component FcC11S− Au SAMs on evaporated gold electrodes was investigated by varying the polarity of the supporting electrolyte for the electrochemical measurements and by fitting the anodic wave to the Frumkin adsorption isotherm. It is evident that FcC11S−Au SAMs form two structurally different domains: one is less organized, and the other is closely packed and highly ordered. The intermolecular interactions among adsorbed molecules were demonstrated to be a crucial factor in the overall electrochemical response of ferrocenylalkanethiolate F

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self-assembled monolayers. We have shown in this study that they can be modulated by adding minute amounts of organic molecules to the electrolyte, which can tune the redox behavior from nonideal (sharper or broader peak) to nearly ideal.

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AUTHOR INFORMATION

Corresponding Author

*(H.Y.) Tel: +1-778-782-8062. Fax: +1-778-782-3765. E-mail: [email protected]. (H.S.) Tel: +86-10-68912667. Fax: +86-1068912667. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS

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REFERENCES

We gratefully acknowledge the financial support from the Natural Science and Engineering Research Council of Canada, the National Science Foundation of China (Project 21173023), and the National 111 Project (B07012) of China. H.T. thanks the State Scholarship Fund of China for supporting her stay as a visiting student at Simon Fraser University.

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