Article pubs.acs.org/JPCC
Molecular Nanocorrals on Si(111)-(7×7): Temperature-Dependent Site Selectivity Wei Mao,†,‡,▽ Jing Hui He,§,▽ Yong Jie Xi,† Wei Chen,†,∥,‡ Kai Wu,⊥,‡ Chun Zhang,∥ Eng Soon Tok,∥ and Guo Qin Xu*,†,‡,# †
Department of Chemistry, National University of Singapore, 3 Science Drive 3, Lower Kent Ridge Road, Singapore 117543, Singapore ‡ Singapore-Peking University Research Centre for a Sustainable Low-Carbon Future, 1 CREATE Way, #15-01, CREATE Tower, Singapore 138602, Singapore § College of Chemistry, Chemical Engineering and Materials Science, Collaborative Innovation Center of Suzhou Nano Science and Technology, Soochow University, Suzhou 215123, P. R. China ∥ Department of Physics, National University of Singapore, 2 Science Drive 3, Lower Kent Ridge Road, Singapore 117542, Singapore ⊥ BNLMS, College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, P. R. China # National University of Singapore (Suzhou) Research Institute, 377 Lin Quan Street, Suzhou Industrial Park, Jiang Su 215123, P. R. China S Supporting Information *
ABSTRACT: Chemisorbed molecular nanocorrals on semiconductor surfaces are of both fundamental and technical importance. 1-Propanethiol molecules adsorbed on Si(111)-(7×7) were investigated using high-resolution energy loss spectroscopy (HREELS), scanning tunneling microscopy (STM), and periodic density functional theory (DFT) calculations. HREELS spectra show that the 1-propanethiol molecules undergo S−H bond dissociative adsorption on Si(111)-(7×7). STM images reveal the temperature-dependent site selectivity for the binding of C3H7S- fragments. At room temperature, C3H7Sprefers to bind to faulted subunits compared to unfaulted subunits. At 110 K, C3H7S- binding on center adatoms over corner adatoms is dominant, resulting in an ordered array of molecular nanocorrals. DFT calculations were performed on a periodic slab including the entire 7×7 reconstruction rather than on a cluster model. The theoretical studies suggest that the temperaturedependent site selectivity originates from the thermal-plus-electron-induced diffusion of dissociative products and the sitepreferential accommodation of the mobile physisorbed precursors. Our results provide a fundamental understanding on the origin of site selectivity of molecular binding on Si(111)-(7×7).
1. INTRODUCTION The fabrication of ordered atomic/molecular nanostructures on surfaces has been extensively studied due to both fundamental and technical importance.1,2 Fundamentally, new physical and chemical phenomena have been revealed on individual nanostructures, such as nanocorrals3,4 and atomic chains.5 Practically, the ordered assembly of nanostructures is of longterm vision for lithography6 with atomic resolution, which is an essential step toward the fabrication of molecular devices, bimolecular recognition chips, and information storages.7,8 Both fields require the nanostructural entities to be fabricated © 2016 American Chemical Society
with a high quality and ordering. This needs to optimize the fabrication processes by interplaying between the adsorbate− adsorbate and the adsorbate−substrate interactions.9 Selfassembled patterns via molecular interaction on the metal substrates are limited by the thermal stability, which prevents their potential applications. In contrast, the molecular assembly on semiconductors via molecule−substrate covalent interaction Received: July 26, 2016 Revised: October 15, 2016 Published: October 17, 2016 24780
DOI: 10.1021/acs.jpcc.6b07477 J. Phys. Chem. C 2016, 120, 24780−24788
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The Journal of Physical Chemistry C is of sufficient thermal stability and mechanical robustness.9,10 Besides, the semiconductors, particularly Si, are superior to the metal substrates because of the facile incorporation of modern semiconductor-based microelectronic techniques. However, the semiconductor surfaces have unevenly distributed and highly reactive dangling bonds, which may react with molecules in multiple sites and binding manners. For example, one of the structurally well-studied surfaces, Si(111)(7×7), has complicated surface reconstruction as described by the dimer−adatom-stacking (DAS) fault model.11 One unit cell of Si(111)-(7×7) has two triangular subunits (faulted and unfaulted) and contains 19 dangling bonds associated with 12 adatoms, six rest atoms, and one corner hole. During reaction, each adjacent adatom−rest atom pair on Si(111)-(7×7) serves as a reactive biradical to react with unsaturated organic functionalities through various reactions, including addition reaction, dissociative adsorption, and dative bonding.12 The Si(111)-(7×7) surface also shows different reactivities among the adatom−rest atom pairs due to the uneven charge distribution of adatoms. Generally, the 12 adatoms fall into four categories: unfaulted center (UCe), unfaulted corner (UCo), faulted center (FCe), and faulted corner (FCo) ones. Many molecules exhibit site-selective adsorption behaviors, such as alcohols, thiols, and amines.13−18 Using this site selectivity, Polanyi’s group fabricated ordered arrays of nanopatterns and molecular switches on Si(111)-(7×7).19−21 Our group prepared ordered nanocorrals by preferential reaction of pyrrole on the center adatom−rest atom pairs.22 The proposed mechanisms for the site selectivity on Si(111)(7×7) were attributed to the topological discrepancies,23 the dissociation barriers difference,14,15 and the intermolecular steric interaction,13,15,16,24−26 which were mostly based on the STM investigations without in-depth analysis or theoretical supports. Tanaka’s and Xie’s groups performed the systematic statistical analyses and dynamic modeling, which successfully revealed the importance of the topological discrepancies.13,15−17,27−29 However, the theoretical calculations were performed on the cluster calculation or a periodic unit cell adsorbing a single molecule.26,30 Some mechanisms, such as the dissociation barrier difference and the intermolecular steric interaction, remain unclear due to the flaw in the theoretical supports, which will severely hinder the future design and fabrication of the molecular devices on Si(111). In this work, 1-propanethiol molecules adsorbed on Si(111)(7×7) were investigated using HREELS, STM, and periodic DFT calculations. It is shown that the site selectivity of C3H7S− H dissociation on Si(111)-(7×7) is temperature-dependent. The preferential residence of the C3H7S- species at the faulted halves was experimentally observed at room temperature, which was supported by the periodic DFT calculations of energetic consideration of simultaneous adsorption of multiple molecules per unit cell. Besides, an ordered array of molecular nanocorrals of 1-propanethiol on Si(111)-(7×7) was fabricated at a low temperature of 110 K. The periodic DFT calculations of dissociation barrier searching based on one molecule per unit cell clearly show that the site selectivity leading to the formation of the surface-ordered nanocorrals originates from the site-preferential accommodation of the physisorbed precursors.
chamber was equipped with an Omicron variable-temperature STM and a HREELS (EELS-3000, LK Technologies). The sample was cut from a p-doped mirror-polished Si(111) wafer with a resistivity of 1−2 Ω·cm and a size of 12 mm × 2 mm × 0.5 mm. The clean Si(111)-(7×7) surfaces were obtained by flashing the sample to 1200 K several times. 1-Propanethiol (99%, Sigmal-Aldrich) was purified by several freeze−pump− thaw cycles before being dosed into the chamber. In STM experiments, the sample was scanned at 110 or 300 K during dosing. The dosing tube aperture was about 0.5 m away from the scanning sample and did not aim at the sample face. The coverage was directly monitored by STM. For HREELS experiments, the dosing tube was aimed at the surface with a distance of 5.0 cm. The dosing amount was measured by the pressure and time in Langmuir units (1 Langmuir = 1 × 10−6 Torr·s). The vibrational spectra were collected in a specular mode with an electron beam of ∼6.23 eV impinging on the sample surface at an incident angel of 60° with respect to the surface normal. An energy resolution of ∼58 cm−1 in the full width at half-maximum (fwhm) of the elastic peak was routinely achieved during the measurements. The periodic DFT calculations were performed using the Vienna ab initio simulation package (VASP).31,32 A 7×7 slab model of four layers of Si was used to simulate the Si(111)(7×7) surface. The coordinates of all Si atoms were obtained from the LEED experiments,33 and the bottom layer of Si was saturated by H. The generalized gradient approximation (GGA) with Perdew−Burke−Ernzerhof (PBE) parametrizations was employed to estimate the exchange-correlation effect.34 The projector augmented wave (PAW)32,35 pseudopotentials method with a kinetic energy cutoff 400 eV was used. Γ point was used for k-point sampling with a SCF convergence criterion of 1 × 10−5 eV. A Gaussian type of electronic smearing with a width of 0.01 eV was used to minimize the errors in the Hellmann−Feynman forces. The force criterion in the geometric optimizations was 0.02 eV Å−1. We estimated the reaction barriers using the climbing image-nudged elastic band (CI-NEB) method with a force criterion of 0.05 eV Å−1.36 To predict the vibrational frequencies, we also used a cluster model of Si16H18 extracted from the four-layer slab to mimic an adjacent adatom−rest atom pair. This model has been proven to be successful in predicting the adsorption energies and binding configurations of the organic molecules on Si(111)(7×7).22,37 We performed DFT calculations using DMol3 6.1 software of Accelarys.38,39 B3LYP hybrid functional was employed to treat the exchange-correlation effect. The values of 1 × 10−5 and 0.05 eV Å−1 were used as the energy and force convergence criteria of the geometric optimization, respectively. To theoretically calculate the vibrational frequencies, the dissociative product model was employed to represent the chemisorption, whereas a free molecule was used to simulate the physisorbed state.
3. RESULTS AND DISCUSSION 3.1. High-Resolution Electron Energy Loss Spectroscopy. We first studied the binding configuration of 1propanethiol on Si(111)-(7×7) using HREELS. Figure 1a shows the EELS spectrum of multilayered physisorbed 1propanethiol after dosing 8.0 L of molecules onto Si(111)(7×7) at 110 K. Figures 1b,c show the vibrational features of chemisorbed molecules obtained after exposing 0.2 L of 1propanethiol onto Si(111)-(7×7) at 110 K and annealing the multilayered physisorbed molecule-covered sample (8.0 L) to
2. EXPERIMENTAL AND COMPUTATIONAL DETAILS The experiments were carried out in an ultrahigh vacuum (UHV) chamber with a base pressure of 1 × 10−10 Torr. The 24781
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Figure 1. HREELS spectra of 8 L of 1-propanethiol-covered Si(111)(7×7) at 110 K (a), 0.2 L of 1-propanethiol-covered Si(111)-(7×7) at 110 K (b), and the saturated chemisorption monolayer prepared after annealing the surface in part a to 300 K (c).
300 K, respectively. Comparing the physisorption/chemisorption curves with the infrared spectra of liquid 1-propanethiol as well as the theoretical calculations (Figure S1 in the Supporting Information) in Table 1,40,41 several distinct features can be resolved and assigned. Visually, in the chemisorbed state, there are two distinct features in contrast to the physisorbed curve. The peak at 2076 cm−1 can be assigned to the Si−H stretching mode, which clearly indicates the saturation of Si dangling bonds and cleavage of S−H bonds. The other strong intensity at 537 cm−1 is related to Si−S stretching mode, indicating the bond formation of Si−S. The features between 700 and 1500 cm−1 and ∼2950 cm−1 can be assigned to the CH2- and CH3related vibrations. Despite slight shifts in these curves, the analogous appearance of these peaks in both physical and chemical adsorption suggests that no C−H cleavage occurs. The vibrational spectral evidence indicates that in the chemisorbed state, 1-propanethiol breaks its S−H bond and attaches to the Si dangling bonds with C3H7S- and H- species, consistent with most of the RX−H (X = O, N, S) molecules on Si(111)-(7×7).16,42,43 Because the adatoms on Si(111)-(7×7) carry positive charges, they are more thermodynamically and kinetically favorable for the attachment of C3H7S- fragments, and the electronegative rest atoms bind to H- fragments. 3.2. Scanning Tunneling Microscopy. A clean Si(111)(7×7) surface with a defect density less than 1.3% was first prepared in our experiments. Figure 2 shows STM images of Si(111)-(7×7) after the exposure of 1-propanethiol at different coverages and temperatures. 1-Propanethiol adsorption causes the darkening of the binding adatoms at empty states. This is
Figure 2. Empty-state STM images (Vs = +1.5 V, It = 0.1 nA) obtained after 1-propanethiol exposure at different temperatures and coverages. (a) 300 K, 0.03 ML (1 monolayer (ML) = 12 sites/unit cell), (b) 300 K, 0.47 ML, (c) 110 K, 0.37 ML, (d) 110 K, 0.50 ML. Some reacted (7×7) half unit cells are outlined by triangles. The letter “Ce” refers to center adatoms, and “Co” refers to corner adatoms. All scale bars: 9.4 nm.
because that the cleavage of 1-propanethiol saturates the adatoms dangling bonds according to our HREELS results. Meanwhile, 1-propanethiol is composed of saturated σ bonds. Their related anti-σ bonds are far away from the Fermi level and out of the STM bias range and hence do not significantly contribute to STM tunneling electrons from the tip. At both 300 K and 110 K, the saturated coverage for chemisorption upon repeated over-exposure to 1-propanethiol is equal to 50(±0.5)% (Table 2). Because the dissociation of one molecule needs to consume one adatom−rest atom pair, the maximum number of adsorbed molecules per unit cell is six, corresponding to a coverage of 50%. Therefore, the value of 50% confirms that (1) all undissociated molecules desorb from the surface or only the dissociated molecules were detected by STM;15,16 (2) the dissociated H- solely binds to the rest atoms without any desorption, which inhibits further adsorption and
Table 1. Vibrational Frequencies (cm−1) and Their Assignments for 1-Propanethiol Physisorbed and Chemisorbed on the Si(111)-(7×7) Surface HREELS
calculation
vibrational modes
8 L @ 110 K
8 L @ 300 K
free molecule
dissociative product
IR of liquid 1-propanthiol40,41
CH3/CH2 stretch S−H stretch Si−H stretch CH2 scissor CH2 wag CH2 twist CH3 C−H bend CH2 rock Si−S stretch
2951
2938
2941 2683
2958
2961/2930/2872 2558
1460 1230 1061 887 722
2076 1443 1222 1034 860 743 537
1472 1224 1038 918 734
24782
2076 1479 1251 1073 900 740 558
1456 1290 1085 893/878 731
DOI: 10.1021/acs.jpcc.6b07477 J. Phys. Chem. C 2016, 120, 24780−24788
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The Journal of Physical Chemistry C Table 2. Binding Sites Statistics of 1-Propanethiol Chemisorbed on Si(111)-(7×7)a number of darkened adatoms Ce+Co
0
Ce, Co
0,0
1 1,0
2 0,1
2,0
1,1
3 0,2
3,0
2,1
4 1,2
0,3
3,1
2,2
A: 300 K, coverage (F) = 6.4%, coverage (U) = 2.4%, total coverage = 4.4%, F/U = 2.7, center/corner = 0.8 F 89 9 14 0 10 0 0 1 0 0 0 0 U 101 6 7 0 2 0 0 0 0 0 0 0 B: 300 K, coverage (F) = 13.3%, coverage (U) = 4.6%, total coverage = 9.2%, F/U = 3.2, center/corner = 0.8 F 61 9 26 4 21 1 0 2 2 0 0 0 U 86 10 13 1 3 0 0 0 0 0 0 0 C: 300 K, coverage (F) = 63.6%, coverage(U) = 37.0%, total coverage = 50.0%, F/U = 1.6, center/corner = 1.0 F 2 3 8 2 11 8 1 13 9 2 2 20 U 24 14 16 4 20 4 0 20 2 0 4 10 D: 110 K, coverage (F) = 9.1%, coverage (U) = 5.9%, total coverage = 7.3%, F/U = 1.5, center/corner = 4.2 F 99 21 10 8 1 0 7 4 0 0 0 0 U 107 26 8 4 3 0 2 0 0 0 0 0 E: 110 K, coverage (F) = 42.1%, coverage (U) = 27.2%, total coverage = 34.7%, F/U = 1.5, center/corner = 3.2 F 5 5 0 21 6 0 18 29 4 0 11 7 U 32 20 14 5 12 0 12 6 0 0 9 7 F: 110 K, coverage (F) = 52.1%, coverage (U) = 46.9%, total coverage = 49.5%, F/U = 1.1, center/corner = 3.4 F 0 0 1 5 0 0 43 76 4 0 6 8 U 0 13 1 23 5 0 45 33 2 0 10 3
5
6
total
1,3
3,2
2,3
3,3
Ce/Co
0 0
0 0
0 0
0 0
21/25 8/9
0 0
0 0
0 0
0 0
44/55 15/16
3 2
14 8
11 0
20 5
229/244 157/129
0 0
0 0
0 0
0 0
67/15 43/11
0 0
6 1
1 0
0 0
236/57 132/57
0 0
6 8
0 0
0 2
347/119 333/81
a
(Ce, Co) represents the distribution of reacted adatoms within each half unit cell obtained from STM images, where Ce refers to the reacted center adatom and Co is for corner adatom. F and U are faulted and unfaulted halves, respectively.
toward the further adsorption and reaction, possibly serving as a potential template for future nanolithography.44 The darkened corrals trapping surface electrons may also be employed as building blocks for molecular electronics or a model system to study the size-confinement of electrons.3 3.3. Density Functional Theory Calculations. To reveal the site selectivity of Si(111)-(7×7), many theoretical calculations have been performed.30,45−48 However, restrained by the ultra-large unit cell and the complex reconstruction of this surface, most of these calculations were conducted on a hydrogenated-Si cluster model, which is not site-specific.49,50 Although some recent studies attempted the use of periodic models, only geometric optimization was performed to find the stable products47,48 or the transition state on a single site.46 Here we conducted extensive periodic DFT calculations to search intermediates, transition states, and products involving all types of adatom (UCe, FCe, UCo, and FCo)−rest atom pairs. As shown in Figure 3, 1-propanethiol initially adsorbs on Si(111)-(7×7) in a physisorbed precursor state, which gains about 0.20 eV compared to the interaction free system. Because 1-propanethiol is polarized as indicated by its electrostatic potential surface and the Si(111)-(7×7) adatom−rest atom pairs carry opposite charges, the energy gain mainly stems from the dipole−dipole interaction and the dispersion force between the molecules and the substrates. This physisorbed precursor is generally mobile on the surface before being chemisorbed. The transition from physisorption to chemisorption is found to be barrierless, as indicated by the failure to find any transition state in our current energetic analysis. Due to the low binding energy, the physisorbed molecule diffuses on the surface and accommodates itself until achieving a proper orientation, such as the S atom approaching the Si adatom. Then the chemisorption starts with a dative-bonded state, where the S atom offers electron pairs to the electron-deficient adatom, stabilizing the system by a binding energy of ∼1.00 eV. There are slight discrepancies in adsorption energy between the dative-bonded states on different sites with UCe ≈ FCe > UCo
dissociation of over dosed molecules. This unique binding manner simplifies our analysis of site selectivity. At 300 K and a low coverage of 0.03 ML, 1-propanethiol is able to react with both the center and corner adatoms as shown in Figure 2a. Each half unit cell can accommodate one, two, or three molecules. At a higher coverage of 0.47 ML in Figure 2b, the number of darkened adatoms per half unit cell varies from one to six. Since there are only three adjacent adatom−rest atom pairs in each half unit cell, which can only dissociate three molecules at most. This indicates that the extra C3H7S- species may diffuse from other halves and accumulate into this half unit cell. Indeed, the triangle arrays in Figure 2b clearly show the preference of 1-propanethiol on the faulted halves alternatively isolated by the intact unfaulted halves. Statistics of the STM scanning runs A−C (300 K) in Table 2 clearly show that the coverage of the faulted halves at room temperature can be larger than 50%, with a F/U ratio varying from 2.7 → 3.2 → 1.6. The site selectivity was studied by Tanaka et al. in the case of methanol adsorbed on Si(111)-(7×7).15,16 They attributed the selectivity to the local conformation of the adsorption site. A methanol molecule occupies a particular Ce/Co site, thus changing the possible dissociative paths for the next molecules. Nevertheless, it indicated that the conformational hindrance factor only affects the Ce/Co ratios but not the F/U preference. On the other hand, there is no significant discrepancy of the site preference between the center and corner adatoms with Ce/Co ratios of runs A−C ranging between 0.8 and 1.0. At 110 K, the ratio of F/U at a saturated coverage decreases to 1.1, the preferential occupation of the faulted halves over the unfaulted ones is no longer dominant. In contrast, the Ce/Co preference changes from an initial ratio of 4.2 at the coverage of 7.3% to a final value of 3.4 at the saturation coverage of ∼50%. With most center adatoms being reacted and darkened, the unreacted corner adatoms are isolated and form hexagonal circle arrays surrounding the corner holes (Figure 2d). These highly ordered and isolated atomic circles remain reactive 24783
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2.70 eV, and the energy discrepancies between the four types of sites are insignificant. This large adsorption energy guarantees the sufficient stability of the attached C3H7S- species, which should be difficult to desorb from the surfaces. The above calculations only considered the adsorption of one molecule per unit cell, which was not the case for the experiments with 50% coverage of the surface. To evaluate the F/U selectivity for the diffusing C3H7S- species, a periodic Si(111)-(7×7) slab consisting of six 1-propanethiol molecules (P1−P6) bonded to the corresponding adsorption sites was constructed. Two initial configurations were considered. For the first configuration, three 1-propanethiol molecules (P1− P3) were positioned on the center adatom−rest atom pairs in the unfaulted half, while the other three 1-propanthiol molecules (P4−P6) on the corner adatom−rest atom pairs in the faulted half. Figure 4a presents the optimized adsorption configuration of the six 1-propanethiol molecules on Si(111)(7×7), with an adsorption energy of 13.06 eV, which corresponds to ∼2.17 eV per molecule. It is lower than the adsorption energy (∼2.70 eV) of one single 1-propanethiol molecule adsorbed in a 7×7 unit cell. This variation in adsorption energy could be attributed to the lateral repulsive interaction between the adsorbed 1-propanethiol molecules.15,16 Simulation of the diffusion of P2 from the initial position to its neighboring center adatom site in the faulted half resulted in a more stable binding configuration with an adsorption energy of 13.16 eV, as shown in Figure 4b. In contrast, the diffusion of P4 in the reverse direction to its adjacent corner adatom in the unfaulted half is endothermic by 0.04 eV (Figure 4c). This indicates that the surface diffusion of C3H7S- species from the unfaulted half to the faulted one is thermodynamically favorable. For the second configuration, as shown in Figure 5, P1, P2, and P3 were situated on the corner adatom−rest atom pairs in the unfaulted half and P4−P6 on the center adatom−rest atom pairs in the faulted half. The calculated results also show the preferential surface diffusion of the C3H7S- species from the unfaulted half to the faulted one. Compared to the pathway where P3 diffused from the unfaulted half to the faulted one by decreasing the system
Figure 3. Periodic DFT calculations for the dissociative adsorption of one 1-propanethiol molecule on a Si(111)-(7×7) slab. Five states are involved in the dissociation reactions. The electrostatic potential distribution of 1-propanethiol is rendered on the molecular surface (electron density 1 × 10−4 e/Å3). Blue refers to negative, and red means positive. The cluster models are cut from the slab in our own results. Gold: Si; white: H; brown: S; gray: C. The energies of all the systems except for the transition states are in the unit of electron volts (eV) and relative to the interaction free system.
≈ FCo. This means that the stability of the dative-bonded states is more sensitive toward the center/corner than to faulted/unfaulted. The next step for the dative-bonded states is the cleavage of S−H bonds. Our transition state search shows that it should be quite facile at room temperature if the reaction rates are roughly estimated from the barriers, which are very low (≤0.12 eV).51 Although all barriers are very small, the site selectivity may still be influenced by the barrier sequence: UCe > UCo > FCe > FCo, which we will discuss later. After dissociation, the systems were greatly stabilized with adsorption energies around
Figure 4. Periodic DFT calculations for optimized structure of six 1-propanethiol molecules (P1−P6) on a Si(111)-(7×7) slab. (a) P1−P3 on the center adatom−rest atom pairs in the unfaulted half, P4−P6 on the corner adatom−rest atom pairs in the faulted half, (b) the surface diffusion of P2 from the unfaulted half to the faulted half, and (c) the surface diffusion of P4 from the faulted half to the unfaulted half. Each structure has the following notation: red, sulfur atoms; gray, carbon atoms; white, hydrogen atoms; yellow, silicon atoms. The schematic diagram on the left of each slab shows the attachment position of the 1-propanethiol molecule within a 7×7 unit cell. The blue circles mean the adatoms binding with adsorbate. U, unfaulted half; F, faulted half. The energy of each configuration compared to the free system is shown below each slab. 24784
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Figure 5. Periodic DFT calculations for the optimized structure of six 1-propanethiol molecules (P1−P6) on a Si(111)-(7×7) slab. (a) P1−P3 on the corner adatom−rest atom pairs in the unfaulted half, P4−P6 on the center adatom−rest atom pairs in the faulted half, (b) the surface diffusion of P3 from the unfaulted half to the faulted half, and (c) the surface diffusion of P6 from the faulted half to the unfaulted half. Each structure has the following notation: red, sulfur atoms; gray, carbon atoms; white, hydrogen atoms; yellow, silicon atoms. The schematic diagram on the left of each slab shows the attachment position of the 1-propanethiol molecule within a 7×7 unit cell. The blue circles mean the adatoms binding with adsorbate. U, unfaulted half; F, faulted half. The energy of each configuration compared to the free system is shown below each slab.
energy of 0.17 eV, the opposite surface diffusion process of P6 from the faulted half to the unfaulted side is thermodynamically less favorable, lowering the system energy by only 0.05 eV. This is in accordance with the STM results in which the C3H7Sspecies prefer to accumulate in the faulted halves. Our calculations only consider the surface diffusion at the saturated coverage and the specific adsorption sites, as this system is computationally too costly with more than 300 atoms. We hope that the present work will motivate the advanced theoretical study of the molecular diffusion on Si(111)-(7×7). 3.4. Reaction Mechanism and Intrinsic Site Selectivity. According to HREELS, STM, and DFT calculations, 1propanethiol is found to dissociate on Si(111)-(7×7) via the cleavage of its S−H bond. The C3H7S- fragments bind to adatoms, whereas the H- radicals saturate the rest atoms. The adsorption of C3H7S- species shows the temperature-dependent site selectivity: at 300 K, F > U; at 110 K, Ce > Co. At 300 K, the STM statistics indicate that the C3H7S- species are able to diffuse on the surface and reside on the faulted halves. The site selectivity is only influenced by the mobility of the dissociated products, rather than any precursor state. The binding energies of the final products for one 1-propanethiol molecule adsorbed in a 7×7 unit cell are ∼2.70 eV. If the lateral repulsive interaction between the six adsorbed 1-propanethiol molecules is considered, our calculation suggests that the average adsorption energy of individual 1-propanethiol molecule on surface is as high as ∼2.17 eV. This prevents desorption of the C3H7S- species at room temperature without other external stimuli. However, the STM-induced molecular desorption on Si(111)-(7×7) via thermally activated electron attachment52 or concerted thermal-plus-electronic53 at room temperature may promote the initially chemisorbed state to a transient state, which could move laterally and cause a desorption−diffusion−adsorption event. The fact that the C3H7S- species can diffuse at room temperature suggests that the activation barrier for diffusion may not be high. A reasonable guess of the diffusion barrier might be Ediff ≈ 0.5Edesorb, as recently reported for other adsorbate−semiconductor cases.54,55
Experimentally, the preferential accommodation of the C3H7S- species at the faulted halves was observed, which is supported by the theoretical calculation that indicates the surface diffusion of the C3H7S- species from the unfaulted half to the faulted one is thermodynamically favorable. Such results may be attributed to the difference of density of states (DOS) between the faulted and unfaulted subunits.56,57 The faulted halves with the higher electrophilicity30,58 were reported as the preferential binding sites for the free incoming molecules42,59 or the surface mobile species.19 At 110 K, the preferential binding sites for the 1-propanethiol dissociation has changed to the center adatom−rest atoms pairs. In the previous study, the dissociation barrier discrepancy14 was employed to explain the site selectivity. It was assumed that the dissociation on the center adatoms was of a lower reaction barrier than that on the corner adatoms. This is obviously not the case as listed in Figure 3, where the barriers of dissociation on the corner adatoms are smaller than those on the center adatoms. At low temperature, the diffusion of dissociated species cross unit halves is significantly retarded. The dissociative process can be described as a localized atomic reaction (LAR) as proposed by Polanyi et al.10,60 The requirement of LAR is that the dissociated atoms/fragments chemically bind to the position displaced laterally from the precursor binding sites with a minimum distance. The site selectivity of dissociated products in LAR solely depends on the site distribution of the starting physisorbed precursors, which further irreversibly datively bind to the adatoms and dissociate. When the 1-propanethiol molecules are physisorbed on surfaces, their large polarity induces them to reorient and locate above the electronically positive adatoms. On Si(111)(7×7), each center adatom is adjacent to two rest atoms, whereas each corner adatom only has one nearest rest atom. Thus, the center adatoms transfer more electrons to the rest atoms and carry more positive charges. This can be understood from the calculated local density of states (LDOS) curves of center and corner adatoms.30 Therefore, the more positive center adatom sites are more attractive for the donor-type molecules, most of which contain Cl, Br, O, S, or N with lone24785
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The Journal of Physical Chemistry C pair electrons.13−18,23,61,62 The preferability of the center over the corner adatoms is also in agreement with the binding strength trend of the dative-bonded products in Figure 3, because the dative bond strength also benefits from a more positive acceptor. The existence of a mobile physisorbed state of organic molecules before dissociation was already identified by STM.63 The ratio of center/corner trapping molecules should follow the Boltzmann distribution:
ACKNOWLEDGMENTS
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REFERENCES
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where N, Eb, kb, and T are the population of center or corner adatoms, binding energies, Boltzmann constant, and temperature, respectively. To prepare perfect nanocorral, one needs to increase the Ce/Co dissociative selectivity via tune in the precursor population ratio. According to the above equation, the substrate temperature should be maintained as low as possible.64 A low temperature also freezes the diffusion of final product from F/U, which will smear the high Ce/Co ratio. This temperature-dependent site selectivity well explains the formation of nanocorrals in our experiments, and it may also be applied to adsorption of the molecules of CH3OH, phenol, pyrrole, R2NH.
4. CONCLUSIONS The temperature-dependent site-selective adsorption of 1propanethiol on Si(111)-(7×7) was investigated by HREELS, STM, and periodic DFT calculations. 1-Propanethiol first binds to the surface in a physisorbed precursor state and is gradually converted to a dative-bonded state. Then the S−H bond is cleaved, eventually forming Si−C3H7S and Si−H products. At room temperature, the dissociated C3H7S- species under the thermal-plus-electronic stimuli can diffuse on surfaces and accumulate on the faulted halves. At low temperature, the dissociative process adopts the localized atomic reaction. The starting physisorbed precursors prefer to reside on the center adatoms, leading to the dissociation preferentially on the center adatom−rest atoms pairs. The high site-preference results in the formation of a nanocorral composed of a cycle of 12 reacted center adatoms and 6 intact corner adatoms surrounding a corner hole. This temperature-dependent site selectivity might be applied to other molecular adsorption processes, prompting the preparation of the ordered molecular patterns on Si(111)(7×7). ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.6b07477. The optimized structures of intermediate and product of 1-propanethiol on Si(111)-(7×7) modeled by a Si16H18 cluster (PDF)
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The authors acknowledge the financial support from Singapore National Research Foundation CREATE-SPURc program R143-001-205-592.
⎛ E (Ce) − E b(Co) ⎞ NCe = exp⎜ − b ⎟ NCo k bT ⎠ ⎝
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Tel: (65)-65163595. Author Contributions ▽
These authors contributed equally to this work
Notes
The authors declare no competing financial interest. 24786
DOI: 10.1021/acs.jpcc.6b07477 J. Phys. Chem. C 2016, 120, 24780−24788
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DOI: 10.1021/acs.jpcc.6b07477 J. Phys. Chem. C 2016, 120, 24780−24788
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The Journal of Physical Chemistry C (64) Dobrin, S.; Rajamma Harikumar, K.; Polanyi, J. C. An STM study of the localized atomic reaction of 1,2- and 1,4-dibromobenzene at Si(1 1 1)-7×7. Surf. Sci. 2004, 561, 11.
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DOI: 10.1021/acs.jpcc.6b07477 J. Phys. Chem. C 2016, 120, 24780−24788
