How does an external electrical field affect adsorption patterns of thiol and thiolate on the gold substrate ?
aa r X i v : . [ c ond - m a t . m t r l - s c i ] J a n How does an external electrical field affect adsorptionpatterns of thiol and thiolate on the gold substrate ?
Jian-Ge Zhou a,b , Quinton L. Williams b a Center for Molecular Structure and Interactions, Jackson State University, Jackson, MS39217, USA b Department of Physics, Atmospheric Sciences, and Geoscience, Jackson State University,Jackson, MS 39217, USA
ABSTRACT
The responsive behavior of methanethiol and methylthiolate molecules on the Au(111) sur-face with an applied electrical potential is studied, and it is shown how the sulfur adsorptionsite, the S-H bond orientation and the interacting energy change with an external electricfield strength. The electron charge density corresponding to an electric field minus that ob-tained in zero field, with zero-field optimal geometry, is calculated to explain the responsivebehavior. The interacting energy for the intact methanethiol adsorption is larger than thatfor the dissociative one, showing that an external electric field can not make the hydrogendissociate from the sulfur.PACS: 36.40.Cg, 73.20.Hb, 68.43.Bc, 61.46.-w1
Introduction
Materials and devices that change properties and functions in response to external stimuli arethe focus of research in fields of physics, chemistry, biology, material science and engineering[1]-[6]. Physical effects such as external fields are advantageous in the process of controllingsurface adsorption and growth. As we know, the surface morphology can be easily affected byan external electric field. The potential-induced surface morphological changes are observedin metal/electrolyte interface [1]. An excess surface charge can induce a reconstruction ona silver surface [3]. Adsorbates on the surface are stabilized by the presence of the scanningtunneling microscopy (STM) tip [4]. An electric field or surface charging changes the metalbcc(100) surface configurations [5]. The electric field effects on surface diffusion has beenstudied by the field ion microscopy (FIM) technique, and it was found that an electric fieldcan inhibit or promote surface self-diffusion on Pt(001) surface [6].On the other hand, alkanethiols form self-assembled monolyers (SAM) on the Au(111)surface, which has wide applications in molecular electronics [7], lubrication [8], lithogra-phy [9], and bio-chemical surface functionalization [10]. Its highly ordered structures andchemical stability make these systems ideal for study with a variety of techniques includingatomic-force microscopy [11], infrared spectroscopy [12, 13], high-resolution electron-energy-loss spectroscopy [14], grazing X-ray diffraction [15], scanning probe microscopy [16], low-energy electron diffraction [17], STM [18]-[22] and others [23]-[28]. Recently, Maksymovychand coworkers exploited the STM tip to manipulate the formation and decomposition ofthe methanethiol dimer on the Au(111) surface [29], which shows that an external electricfield does affect the adsorption pattern of a thiol molecule. Then, the question of whetheran external electrical field induces conformal reorientation of thiol molecule on the Au(111)surface at the low coverage arises. Because of its importance for a wide variety of surfacephenomena (i.e., STM, FIM and electrochemical), understanding the influence of an exter-nal electric field on surface adsorption is essential for explaining some experimental results.Besides these, the defect on the substrate can catalyze the S-H bond breaking in the processof the methanethiol adsorption on the Au(111) surface [30, 31], however, it is unclear ifan external electric field can trigger such a dissociation. Heretofore, the mechanism of theresponsive behavior and the dissociation of thiol molecule on the Au(111) surface under anexternal electrical field is still a mystery.This prompted us to investigate the interacting behavior of methanethiol and methylth-iolate molecules with the Au(111) surface under an external electrical field by the densityfunctional theory. We will present the interacting energies and geometries for methanethioland methylthiolate adsorbates on the Au(111) surface in the presence of an external electric2eld. We show how the sulfur adsorption site, the S-H bond orientation, and the interactingenergy of the methanethiol and methylthiolate molecules with the Au(111) substrate are af-fected by an external electric field applied to the surface. We have calculated the z-directionelectron charge density difference between the charge density obtained with an electric fieldand that without a field at the zero-field optimized geometry to interpret these responsivebehaviors. To see if an external electric field can trigger the dissociation of the S-H bondin the methanethiol adsorbed on the Au(111) surface, we compare the interacting energiesbetween the intact adsorption and dissociative one. We find that the interacting energy forthe intact methanethiol adsorption is larger than that for the dissociative adsorption, whichshows that even in the presence of an external electric field, the intact adsorption is stillstable.
The calculations were done in the slab model by density functional theory (DFT) [32].The electron-ion interaction has been described using the projector augmented wave (PAW)method. All calculations have been performed by Perdew-Wang 91 (PW91) generalizedgradient approximation. The wave functions are expanded in a plane wave basis with anenergy cutoff of 400 eV. The k points were obtained from Monkhorst-Pack scheme, and3 × × k point mesh was for the geometry optimization. The supercell consisted of 4 layersand each layer with 12 Au atoms. The Au atoms in the top three atomic layers are allowedto relax, while those in the bottom layer are fixed to simulate bulk-like termination [33].The vacuum region comprises seven atomic layers, which exceeds substantially the extensionof the methanethiol molecule. To apply an external electrical field, a planar dipole layer isplaced in the middle of the vacuum region [32, 34]. In the presence of an external electricalfield, the eight Au layers slab resulted in charge sloshing. We also compared the six layersslab with the four layers slab, and found that the differences of the interacting energy arewith 5.3%. However, the computing time for the six layers slab is much longer than that forthe four layers slab. In our work, we computed more than 150 configurations, so the bestchoice for us is the four layers slab. We calculated the gold lattice constant and found it toagree with the experimental value [35] to 2.1%.3able 1: The geometries and interacting energies for the stable methanethiol configurationson the Au(111) surface (0.25 ML) at various external electric field strengths. The entries E ext ,S site, θ , tilt, d S − Au (˚ A ) and E int (eV) refer to an external electric field (V/˚ A ) perpendicularto the Au(111) surface, the S atom adsorption site, the angle between the S-C bond directionand the normal to the Au(111) surface, the region of the S-C bond tilted, the shortest S-Aubond length and the interacting energy, respectively.E ext S site θ tilt d S − Au E int We begin with the geometries and interacting energies of the optimized structures for themethanethiol (CH SH) on the Au(111) surface at the coverage of 0.25 ML (1.00 ML means1 sulfur per 3 gold atoms, and 0.25ML stand for 1 methanethiol on a gold surface with 12gold atoms) with various external electric field strengths [34], as displayed in Table 1 (ateach value of the external electric field, 15 different structures have been optimized, the moststable structure is listed on table 1). The interacting energy is defined as E int = E CH SH + E Au (111)+ field - E CH SH + Au (111)+ field . The symbol top-fcc (or top-hcp) in Table 1 representsthat the S atom is at the atop site of the gold atom, but leaned toward the fcc (or hcp) hollowcenter. Some stable configurations on the Au(111) surface in the presence of an externalelectric field are illustrated in Fig. 1, where only the methanethiol (or methylthiolate)adsorbate and the top layer of the Au(111) surface are displayed.Table 1 shows that when the strength of an applied negative electric field increases, theinteracting energy E int rises, the sulfur adsorption site shows little variation (on the atopsite of the gold atom, but leaned to fcc center), the angle between the S-C bond directionand the normal to the Au(111) surface decreases, and the bond length between S atom andsubstrate d S − Au becomes shorter. Thus, when the strength of an applied negative electricfield increases, the interaction between the methanethiol adsorbate and gold substrate getsstronger and stronger. If the negative electric field is in the range of 0 to -0.5 V / ˚ A , thegeometry changes slightly, which is in accord with the experimental observation [36, 37].4igure 1: (a) The methanethiol (CH SH) on the Au(111) surface without an external electricfield. (b) CH SH on the surface with a negative external electric field (-1.0
V / ˚ A ). (c) CH SHon the surface with a positive external electric field (0.5
V / ˚ A ). (d) Methylthiolate (CH S) onthe Au(111) surface without external electric field. (e) CH S on the surface with a negativeexternal electric field (-1.0
V / ˚ A ). (f) CH S on the surface with a positive external electricfield (1.0
V / ˚ A ).When a positive electric field (0.5 V / ˚ A ) applied, the methanethiol molecule starts to desorbfrom the Au(111) surface. Fig. 1c depicts this desorption structure in which the distancebetween S and Au is 3.95 ˚ A (longer than the zero-field S-Au bond length 2.73 ˚ A ). Thus inthe low coverage, the orientation of the methanethiol molecule on the Au(111) surface canbe tuned by an applied negative electrical field in a certain range (-0.5 V / ˚ A to -1.5 V / ˚ A ).To see how an external electric field influences the interaction between the methylthiolatemolecule (CH S) and the substrate, we calculated the geometries and interacting energies forthe optimized structures of the methylthiolate on the Au(111) surface (0.25 ML) at variousexternal electric field strengths, as shown in Table 2. Table 2 displays that if the strengthof an applied negative electric field becomes stronger, the interacting energy increases, thesulfur adsorption site is sliding from fcc-bri to fcc, the angle θ decreases, but the bond length d S − Au shows little variation. When the electric field goes to -1.0 V / ˚ A , the previous tiltedmethylthiolate molecule begins to stand up, i.e., the angle θ jumps from 55 ◦ to 1 ◦ . Whenthe strength of a negative electric field increases, the interaction between the methanethioladsorbate and gold substrate gets stronger, but the bond length d S − Au remains unchanged.5able 2: The geometries and interacting energies for the stable methylthiolate configurationson the Au(111) surface (0.25 ML) at various external electric field strengths.E ext S site θ tilt d S − Au E int V / ˚ A , the angle of the S-C bond is around55 ◦ , but within -1.0 to -1.5 V / ˚ A , the methylthiolate is nearly vertical to the surface. Whenthe negative electric field is in the range of 0 to -0.5 V / ˚ A , even when the interacting en-ergy varies, the orientation of the methylthiolate molecule almost does not change, which isconsistent with the experimental results [38].Let us calculate the electron charge density difference along the surface normal to inter-pret the responsive behavior. The charge density subtraction is between the charge densityobtained with an electric field and that without an electric field at the zero-field optimizedgeometry. We have plotted the plane-integrated charge density difference as a function of thez-coordinate (Fig. 2), which shows how the charges rearrange on application of an externalelectric field. In the case of the methanethiol adsorption, the positive electric field pulls theelectrons back to the gold surface. Troughs 1 and 2 in Fig. 2a indicate the removal of theelectrons from the region between the gold surface and sulfur (trough 1) and that betweensulfur and CH methyl group (trough 2). The corresponding S-Au bond becomes weakerand more electrons have accumulated on the other side of the slab (peak 3 in Fig. 2a) thanin cases without an electric field. The peaks 1 and 2 in Fig. 2b display that the negativefield pushes more electrons into the region between S and Au (peak 1) and that around CH I n t e r a c t i ng E ne r g y ( e V ) Electric Eield Strength (V/Angstrom)
Intact CH SH adsorption Dissociative adsorption
Figure 3: The interacting energies for the intact methanethiol adsorption and dissociativeadsorption.methyl group (peak 2). The S-Au bond gets stronger than that without an electric field. Inthe presence of a negative electric field, the negatively charged methyl group tends to moveaway from the surface. However, table 1 shows that the S-C bond length changes slightly in anegative electrical field. Thus, the net effect is that when the strength of an applied negativeelectrical field increases (Fig. 2b - Fig. 2d), the angle between the S-C bond and the surfacenormal decreases, which explains the responsive behavior of the angle θ . The methylthiolateadsorption is similar to the methanethiol case. In an applied negative electrical field, thereare more electrons accumulated around the methyl group (CH ) in the methylthiolate thanin the methanethiol (Fig. 2b - Fig. 2d and Fig. 2e - Fig. 2g). When the amount of theelectron accumulation exceeds a certain level, the methylthiolate becomes nearly verticalto the surface. In a positive potential, some electrons flow back to the gold surface (Fig.2h - Fig. 2j); the S-Au bond in methylthiolate gets weaker than that without an electricfield. Thus, we have shown how the system responds geometrically to the rearrangement ofcharges in the presence of an applied field.When the methanethiol is adsorbed on the Au(111) surface, the S-H bond remains intact[30]. If the temperature rises, the methanethiol will desorb from the surface. To see if anapplied electric field can break the S-H bond of the methanethiol adsorbate, we calculatedthe interacting energies for the stable structures of the intact (CH SH) and dissociativeadsorption (CH S + H-Au) on the Au(111) surface. The interacting energies for the intactand dissociative adsorption versus the electric field are plotted in Fig. 3. Fig. 3 reveals that8rom -1.5 to 0.5
V / ˚ A , the interacting energy for intact methanethiol adsorption decreases andfrom 0.5 to 1.0 V / ˚ A , it increases. In the case of dissociative adsorption, from -1.5 to 0.0 V / ˚ A ,the interacting energy decreases; however, above 0.0 V / ˚ A , the interacting energy increases.Fig. 3 displays that in the whole region, the interacting energy for the intact methanethioladsorption is larger than that for the dissociative one, i.e., the intact adsorption is morestable than the dissociative one. This shows that an external electric field cannot make thehydrogen dissociate from the sulfur. Based on ab initio calculations, we have shown for the first time how the methanethiol andmethanethiolate molecules on the Au(111) surface respond to an applied electrical potential.The sulfur adsorption site, the S-H bond orientation, and the interacting energy vary with thestrength of the external electric field. In the low coverage, the orientation of the methanethiolmolecule on the Au(111) surface can be tuned by the application of a negative electricalfield through a certain range and the methanethiol desorbs from the gold substrate witha positive electrical field. However, the orientation of the methylthiolate on the Au(111)surface cannot be adjusted continuously. The electron charge density (along the surfacenormal) corresponding to the external field minus that obtained in zero field, with zero-field optimal geometry, has been calculated to interpret these responsive behaviors. Theinteracting energies between the intact and dissociative adsorption with an applied electricalpotential have been compared. It has been found that the interacting energy for the intactmethanethiol adsorption is larger than that for the dissociative adsorption, showing that anexternal electric field cannot make the hydrogen dissociate from the sulfur.
Acknowledgments
This work is funded in part by Department of Defense through Contract