Formation and Structure of Highly Ordered Self-Assembled

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C: Surfaces, Interfaces, Porous Materials, and Catalysis

Formation and Structure of Highly Ordered Self-Assembled Monolayers by Adsorption of Acetyl-Protected Conjugated Thiols on Au(111) in Tetrabutylammonium Cyanide Solution Taesung Park, Hungu Kang, Sicheon Seong, Seulki Han, Young Ji Son, Eisuke Ito, Tomohiro Hayashi, Masahiko Hara, and Jaegeun Noh J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.9b00521 • Publication Date (Web): 13 Mar 2019 Downloaded from http://pubs.acs.org on March 14, 2019

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The Journal of Physical Chemistry

Formation and Structure of Highly Ordered Self-Assembled Monolayers by Adsorption of Acetyl-Protected Conjugated Thiols on Au(111) in Tetrabutylammonium Cyanide Solution

Taesung Park,† Hungu Kang,† Sicheon Seong,† Seulki Han,† Young Ji Son,† Eisuke Ito,‡ Tomohiro Hayashi,ǁ Masahiko Hara,§ and Jaegeun Noh†,,*

†Department

of Chemistry, Hanyang University, 222 Wangsimni-ro, Seongdong-gu, Seoul

04763, Korea ‡Chemical

Materials Evaluation and Research Base (CEREBA), Higashi 1-1-1, AIST Central

5-2, Tsukuba, Ibaraki 305-8565, Japan ǁDepartment

of Materials Science and Engineering, Tokyo Institute of Technology, 4259

Nagatsuta-cho, Midori-ku, Yokohama, Kanagawa 226-8503, Japan §Department

of Chemical Science and Engineering, Tokyo Institute of Technology, 4259

Nagatsuta-cho, Midori-ku, Yokohama, Kanagawa 226-8503, Japan Institute

of Nano Science and Technology, Hanyang University, 222 Wangsimni-ro,

Seongdong-gu, Seoul 04763, Korea

1 ACS Paragon Plus Environment

The Journal of Physical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ABSTRACT

The surface structure and binding conditions of self-assembled monolayers (SAMs) on Au(111) derived from 1-acetylthio-4-[(phenyl)ethynyl]benzene (OPE2-SAc) without and with tetrabutylammonium cyanide (TBACN) as a deprotecting reagent were examined using scanning tunneling microscopy (STM) and X-ray photoelectron spectroscopy (XPS). STM observation revealed that OPE2-S SAMs on Au(111) formed from direct adsorption of OPE2SAc in 1mM methanol solution at RT for 24 h were composed of short-range, ordered phase separated by a disordered phase. In contrast, adsorption of OPE2-SAc SAMs on Au(111) at a higher solution temperature of 50 °C for 24 h led to formation of a fully ordered phase with slightly increased domain size. The structural quality of OPE2-S SAMs on Au(111) was remarkably enhanced when TBACN was used. OPE2-S SAMs at RT had a well-ordered (√3 × √ 7)R30 ° structure with a domain size larger than 80 nm. The SAMs deposited at 50 °C contained very uniform and highly ordered domains with a size exceeding 100 nm, which can be assigned to a (2 × 3√3)rect structure. XPS measurements showed that OPE2-S SAMs were mainly formed via chemical interactions between sulfur and the Au(111) surface regardless of the use of TBACN deprotection reagent. In this study, we reported the first molecular-scale features of OPE2-S SAMs on Au(111) with highly ordered domains derived from OPE2-SAc and clearly demonstrated that TBACN can be used as an effective deprotecting reagent for formation of uniform and well-ordered OPE2-S SAMs with long-range domains derived from thioacetyl-protected OPE2-S molecules on Au(111).


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1. INTRODUCTION The physical and chemical properties of metal surfaces can be modified easily using selfassembled monolayers (SAMs) formed by spontaneous adsorption of sulfur-containing organic molecules.1-30 SAMs provide a simple and powerful route for a wide range of technological applications in corrosion inhibition,1 nanopatterning,1,2 chemical sensors,1,3 biosensors,1,3 molecular electronic devices,1,2,4-7,15 and organic thin film transistors (OTFTs).16,17,21 The performance and stability of OTFT devices can be greatly enhanced after modification of metal electrodes with SAMs formed by organic molecules with electron-withdrawing character.16,17 Recently, SAMs of -conjugated organic molecules on gold have drawn much attention because of their interesting electronic properties and technological applications in molecular electronics.1,4-7,16-20,29-39 Organic thiols are the most popular molecules for SAM formation on gold, but they can easily be oxidized to disulfides or other oxidized compounds in solution containing a trace amount of oxygen.40,41 This undesirable oxidation of thiols often generates an inhomogeneous interface structure and structural defects in the SAMs. To overcome this problem, organic thioacetates have been used as a primary alternative for SAM formation due to their robust chemical stability in solution or ambient conditions.4,6,42-48 However, it has been suggested that direct adsorption of these molecules onto gold surfaces results in formation of less ordered SAMs and shows slower adsorption kinetics compared to the corresponding thiols.42-48 For instance, scanning tunneling microscopy (STM)41,44 and infrared reflection adsorption spectroscopy (IRRAS)42-44 demonstrated that direct adsorption of simple n-alkanethioacetates on gold mainly led to formation of liquid-like disordered phases. To improve the structural order and quality of SAMs prepared by organic thioacetates, in situ deprotection of the acetyl group is necessary. This step is associated with structural transformation from thioacetate to more active free thiol in solution. Molecular-scale STM measurements revealed that adsorption of 3 ACS Paragon Plus Environment


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decanethioacetates in a deprotection solution containing 1,8-diazabicyclo[5.4.0]undec-7-ene or KOH created relatively well-ordered SAMs with a (3 ×3)R30° structure, as with SAMs derived from decanethiol. In contrast, disordered phases containing partially ordered small domains were mainly formed in solutions containing other deprotection reagents such as HCl, propylamine, NH4OH, and K2CO3.41 On the other hand, densely packed SAMs were formed by acetyl-protected diphenyl-derived dithiols on gold and silver surfaces in the presence of triethylamine (TEA) deprotection reagent, but multilayered biphenyl dithiol films were formed using NH4OH as a deprotection reagent.49 The SAMs of thioacetyl-terminated oligo(phenylene-ethynylene)s (OPEs-SAc) derivatives on gold have been often used for fabrication of molecular electronic devices4,6,17,31,35,38,50,51 and have been extensively studied to understand the formation and structure of SAMs in acid- or base-catalyzed solutions for deprotection.31-38 It was reported that the chemical structure of nitro-substituted OPE molecules (which are regarded as strong candidates for electronics applications) decomposed under deprotection conditions using NH4OH, preventing reliable application

of

thioacetyl-protected

molecules.40,49

OPE

Cyclic

voltammetry

(CV)

measurements suggested that OPE SAMs formed in the presence of NaOH deprotection reagent had a loosely packed and poorly ordered structure.52 Ellipsometry and X-ray photoelectron spectroscopy (XPS) measurements suggest that densely packed SAMs derived from acetylprotected dithiols of OPEs on gold formed in the presence of TEA but not in the presence of tetrabutyl ammonium hydroxide (Bu4NOH).51


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A new methodology for obtaining high-quality OPE SAMs with a long-range, ordered phase is required to fabricate reproducible and reliable OPE SAM-based molecular devices. It was reported that aliphatic or aromatic thiols were synthesized with a high yield (> 80%) via deprotection of acetyl groups from the corresponding thioacetates in methanol containing tetrabutylammonium cyanide (TBACN) at room temperature, implying that TBACN can be an effective deprotection reagent for use in organic solvents.51 Hence, we expect that TBACN may be a very useful deprotection reagent for formation of SAMs from aliphatic or aromatic thioacetates. To the best of our knowledge, there have been no scanning tunneling microscopy (STM) reports on the effects of TBACN deprotection reagents on formation and surface structure of OPE SAMs on Au(111) derived from thioacetyl-terminated OPE molecules or their molecular-scale SAM features. Therefore, we investigated the surface structure and binding condition of OPE2-S SAMs on Au(111) prepared by 1-acetylthio-4-[(phenyl)ethynyl)benzene (OPE2-SAc) having a highly -conjugated molecular backbone (Figure 1) without and with a TBACN deprotection reagent using STM and XPS. In this study, we report the first molecular-scale STM images of OPE2S SAMs on Au(111) with uniform and well-ordered domains derived from OPE2-SAc molecules using a TBACN deprotection reagent. We clearly demonstrate that TBACN is an

Figure 1. (a) Structural formula and (b) space-fill model structure of OPE2-SAc. 5 ACS Paragon Plus Environment


effective deprotecting reagent for achieving highly ordered SAMs formed by thioacetylprotected aromatic molecules.

2. EXPERIMENTAL SECTION Chemicals and Au(111) Substrate. OPE2-SAc molecules used in this study were synthesized according to a previously reported method.51 TBACN was purchased from Tokyo Chemical Industry (TCI, Japan) and used without further purification. The Au(111) substrates were prepared by thermal evaporation of gold onto freshly cleaved mica sheets pre-heated at 623 K with a base pressure of 10-5–10-6 Pa, as described in the literature.9,53 The Au(111) substrates with atomically flat and large terraces ranging from 100 to 400 nm were observed by STM measurements.53 Preparation of OPE2-S SAMs Derived from OPE2-SAc. OPE2-S SAMs were formed by dipping the Au(111) substrates in a 1 mM OPE2-SAc methanol solution (direct adsorption) or in a 1 mM OPE2-SAc methanol solution containing a TBACN deprotection reagent (0.25 mol equivalent per OPE2-SAc) at room temperature (RT) or 50 °C for 24 h. Note that an in situ deprotection process of acetyl groups in OPE2-SAc was performed by incubating a 1 mM OPE2-SAc methanol solution containing TBACN at 50 °C for 3 h before immersion of the Au(111) substrates. The deprotection mechanism of aliphatic or aromatic thioacetates by TBACB was described in the literature.54 The prepared SAM samples were then rinsed thoroughly with pure methanol to remove physisorbed molecules before surface characterization. STM and XPS Measurements. STM measurements were carried out with a NanoScope E (Veeco Instruments, USA) using a commercially available Pt/Ir (80:20) tip. All STM images were corrected in ambient conditions using a constant current mode at room temperature. Bias voltages ranging from 350 to 550 mV and tunneling currents ranging from 300 to 500 pA were 6 ACS Paragon Plus Environment

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applied between the tip and sample for STM imaging. XPS measurements were performed using a Theta Probe (Thermo Fisher Scientific, U.K.) with an Al K X-ray source (1486.6 eV). The emitted electrons were collected at angles from 23° to 83° with a multi-channel plate. The spectra were calibrated based on the Au 4f7/2 peak at 84.0 eV, and the energy positions of the observed peaks were determined using a curve-fitting analysis.

3. RESULTS AND DISCUSION 3.1. Surface Structure of OPE2-S SAMs on Au(111) Derived from Direct Adsorption of OPE2-SAc Molecules. It has been suggested that deprotection of acetyl groups in thioacetates could occur at the gold surface without deprotection reagents, resulting in formation of SAMs.40,42,49,52,55,56 XPS and surface plasmon resonance spectroscopy (SPR) measurements showed that direct adsorption of ,-organic dithioacetates on gold after 24 immersion at RT yielded fully covered SAMs.55 In contrast to this result, XPS, ellipsometer, and IRRAS measurements suggested that SAMs derived from direct adsorption of alkanethioacetates have loosely packed and poorly ordered structures, whereas SAMs formed by the corresponding alkanethiols have closely packed and well-ordered structures.43 STM imaging with molecular-scale spatial resolution also clearly showed that direct adsorption of alkanethioacetates on Au(111) surfaces led to formation of mainly disordered SAMs containing a few striped domains (with a flat-lying adsorption geometry) that have been usually observed for low-coverage alkanethiol SAMs.41,44 On the other hand, Bardin et al. reported that adsorption of dodecanethioacetates usually generated loosely-packed ordered SAMs with a striped phase and a p× 23 structure.56 There have been no molecular-scale STM results describing the effect of TBACN deprotecting reagent on formation and structure of conjugated OPE2-S SAMs on Au(111) from adsorption of OPE2-SAc molecules. To address this, we compared the surface structures of OPE2-S SAMs on Au(111) formed by OPE2-SAc 7 ACS Paragon Plus Environment


molecules without and with TBACN deprotection reagent. STM images in Figure 2 show the surface features of OPE SAMs on Au(111) formed after direct adsorption of OPE2-SAc molecules in a 1 mM methanol solution at RT for 24 h. STM observations revealed formation of a two-dimensional (2D) ordered phase (region A) containing short-range, ordered domains with typical sizes of 5-15 nm separated by a disordered phase (region B), as shown in Figure 2. The domain formation of OPE2-S SAMs on Au(111) differs markedly from that of alkanethiol or aromatic thiol SAMs containing the long-range, ordered domains and sharp domain boundaries.8,10,14,18,57,58 The ordered domains of OPE2-S SAMs on Au(111) have a row structure with an inter-row spacing of 0.900.02 nm, which is smaller than an entire molecular length of OPE2-S, which is 1.23 nm. Therefore, we assumed that the ordered row domain is not the striped phase, but the loosely packed standing phase that is usually observed as an intermediate phase prior to formation of closely packed alkanethiol SAMs.8,27,59-62 The ordered domains have three orientations with angles of 60 or 120 between them, as marked by white arrows in Figure 2b. This suggests that growth of OPE2-S SAMs was strongly influenced by the three-fold symmetry of the underlying Au(111) lattice. The ordered domain (region A) in the STM image was brighter than that of the disordered domain (region B), which implies that molecules in the

Figure 2. STM images of OPE2-S SAMs on Au(111) showing two mixed phases containing small 8 immersion of the Au(111) substrates in a 1 mM ordered domains and disordered phases formed after

methanol solution at RT for 24 h. ACS ScanParagon sizes were 120 nm × 120 nm and (b) 60 nm × 60 nm, Plus (a) Environment respectively.

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ordered phase protruded more from the surface compared to those in the disordered domain. This can be ascribed to the difference in molecular packing density of OPE2-S SAMs, as observed from alkanethiol SAMs.59 On the other hand, the first STM observation revealed that OPE2-S SAMs on Au(111) formed in a 0.1 mM dichloromethane solution of OPE2-SH contained partially ordered domains around step edges, disordered domains as a dominant phase, and many bright patches protruding to a height of ~ 2 Å.63 These features are significantly different from the surface features of OPE2-S SAMs on Au(111) formed by direct adsorption of OPE2-SAc in a 1 mM methanol solution, as shown in Figure 2. Moreover, alkanethiol SAMs were composed of many vacancy islands with the Au monatomic step of 2.5 Å resulting from the extraction of gold atoms during adsorption of thiols on gold surfaces. 1,8,18,21,25,27

In contrast, very few vacancy islands for OPE2-S SAMs on Au(111) were observed

as indicated by white circles in Figure 2. Similar results were also observed from the SAMs formed by adsorption of OPE3-SH or HS-OPE3-SH molecules, suggesting that the OPE group is closely involved in the formation of vacancy islands in the SAMs.18,63,64 A different formation mechanism of vacancy islands for alkanethiol and OPE SAMs on Au(111) was proposed by the previous literature.18 It was reported that the structural quality of SAMs prepared by alkanethiols65-69 and aromatic thiols11,30 on Au(111) was greatly enhanced when adsorption processes were conducted at an elevated temperature. The SAMs formed at an elevated temperature contained larger ordered domains and a smaller number of vacancy islands compared to those formed at RT. STM images in Figure 3 show the surface structures of OPE2-S SAMs on Au(111) formed after direct adsorption of OPE2-SAc molecules in a 1 mM methanol solution at 50 °C for 24 h. The degree of structural order of OPE2-S SAMs formed at 50 °C was greatly enhanced compared to that formed at RT, as shown in Figure 3. The ordered domains for OPE2-S SAMs on Au(111) with three domain orientations, as indicated by white arrows, fully covered the 9 ACS Paragon Plus Environment


entire Au(111) surface (Figure 3a). Disordered regions almost disappeared, and domain boundaries (indicated by white lines in Figure 3b) newly formed as a result of growth of ordered domains. STM images clearly showed formation of larger ordered domains with lateral dimensions from 5 to 30 nm at a higher solution temperature of 50 °C. This result can be ascribed to an increase in diffusion rate of molecules on the Au(111) surface during selfassembly.11,39,65-69 Interestingly, OPE2-S SAMs on Au(111) formed by direct adsorption of OPE2-SAc molecules in a 1 mM methanol solution at RT or 50 C were mainly composed of ordered domains with various shapes and sizes, as shown in Figures 2 and 3. Direct adsorption of OPE2-SAc was also observed in a 0.5 mM N,N-dimethyl formamide (DMF) solution at RT, and this led to formation of OPE SAMs with small aggregated or ordered islands a few nm in size.46,70 From these results, it was suggested that formation of such unique domains by direct adsorption of OPE2-SAc on Au(111) is primarily ascribed to slower growth kinetics of SAMs. These slower kinetics resulted from (i) the synergetic effects of lower adsorption affinity of acetyl-protected sulfur due to the electron-withdrawing character of the acetyl group and (ii) increased steric hindrance of the acetyl group attached to sulfur in the OPE2-SAc during

Figure 3. STM images of OPE2-S SAMs on Au(111) showing ordered domains and domain boundaries formed after immersion of the Au(111) substrates in a 1 mM methanol solution at 50 °C for 24 h. Scan sizes were (a) 120 nm × 120 nm and (b) 60 nm × 60 nm, respectively. 10 ACS Paragon Plus Environment

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adsorption compared to the corresponding thiols, as demonstrated by previous kinetic studies.40,42,45 The formation and growth of OPE2-S SAMs may also be influenced by removal of acetyl groups formed as a result of adsorption of OPE2-SAc molecules on a gold surface because these groups can hinder diffusion of OPE2-S molecules and inhibit SAM growth on Au(111). On the other hand, direct adsorption of terphenylethanethioacetates on Au(111) at 50 C generated SAMs with long-range order and very unique bright and dark stripes, which were markedly different from the corresponding thiol SAMs.71 In this study, we clearly revealed that OPE2-S SAMs on Au(111) formed after direct adsorption of OPE2-SAc at RT had very small ordered domains with an inter-row spacing of 0.900.02 nm, whereas OPE2-S SAMs formed at 50 C contained relatively larger and unique ordered domains with various sizes and shapes. However, despite many efforts, it was very hard to resolve individual molecular features in the ordered rows for OPE2-S SAMs on Au(111) formed by direct adsorption of OPE2-SAc molecules. 3.2. Surface Structure of OPE2-S SAMs on Au(111) Formed in an OPE2-SAc Solution Containing a TBACN Deprotection Reagent. To understand the effect of TBACN deprotection reagent on formation of OPE2-S SAMs on Au(111), we examined the surface structure of OPE2-S SAMs formed in a 1 mM OPE2-SAc methanol solution containing TBACN at RT for 24 h. A mixed phase with ordered phase containing small ordered domains (5-15 nm) and a disordered phase at RT were formed in a 1 mM methanol solution without a TBACN deprotection reagent (Figure 2). However, ordered phases of OPE2-S SAMs were formed on the entire Au(111) surface in a 1 mM methanol solution with TBACN (Figure 4). Ordered domains with various sizes and three directional orientations were observed from short-range, ordered domains of approximately 15 nm to long-range, ordered domains exceeding 80 nm, as shown in Figure 4a. Several structural defects (indicated by blue arrows) were observed in the ordered domains. The magnified STM image (60 nm × 60 nm) in Figure 11 ACS Paragon Plus Environment


4b clearly shows that OPE2-S SAMs on Au(111) had well-ordered domains separated by domain boundaries. Our STM observation clearly revealed that the structural quality of OPE2S SAMs on Au(111) was remarkably enhanced by using a TBACN deprotection reagent, as shown in the STM images of Figure 4. We believe that this enhanced structural quality of OPE2-S SAMs is due to direct adsorption of the corresponding thiolate (OPE2-S-) resulting from an in situ deprotection reaction of OPE2-SAc with TBACN in methanol solution at 50 °C for 3 h, as described in a previous paper on the deprotection mechanism.54 Our results are well supported by recent work showing that, although direct adsorption of decanethioacetate (C10SAc) on Au(111) generated poorly ordered decanethiolate SAMs, the degree of structural order for decanethiolate SAMs increased with increasing in situ cleavage of decanethioacetate depending on the deprotection reagent used.41


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The STM image (8 nm × 8 nm) in Figure 5a shows individual molecules in the ordered



rows for OPE2-S SAMs on Au(111) formed after immersion in a TBACN deprotection reagent at RT for 24 h. The cross-sectional profiles along lines a´ and b´ on the STM image of Figure 5a show the superperiodicities of packing arrangements for OPE2-S SAMs on Au(111). Based on this STM observation, we extracted the lattice constants of an oblique unit cell containing

Figure 4. STM images of OPE2-S SAMs on Au(111) showing well-ordered domains and domain boundaries formed after immersion of the Au(111) substrates in a 1 mM methanol solution containing a TBACN deprotection reagent at RT for 24 h. Scan sizes were (a) 120 nm × 120 nm and (b) 60 nm × 60 nm, respectively.

Figure 5. (a) Molecularly resolved STM image (7.3 nm  7.3 nm) and (b) proposed structural model of OPE2-S SAMs on Au(111) formed after immersion of the Au(111) substrates in a 1 mM methanol solution containing a TBACN deprotection reagent at RT for 24 h. Yellow balls represent the gold atoms of the Au(111) surface. (a and b) The height profiles along lines a and b on the STM image show the superperiodicities of OPE2-S SAMs on Au(111).


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one molecule: a = 5.0  0.2 Å = √3ah, b = 7.5  0.2 Å = √7ah,  = 71, and  = 30. Here, ah = 2.89 Å corresponds to the interatomic distance of the Au(111) lattice. Figure 5b shows a schematic structural model of the packing arrangement of OPE2-S SAMs on Au(111). The molecular packing structure of OPE2-S SAMs on Au(111) is a (√3 × √7)R30° structure, which is comparable to the ( √3 × √3)R30 ° structure for biphenylthiol (BPT) SAMs72 or the (2 √3 × √3)R30 ° structure for 4-methyl-4-biphenylthiol (MBPT) SAMs57 with two phenyl groups in the molecular backbone. From this structural model, we suggest that all sulfur atoms in OPE2-S SAMs adsorb on the three-fold hollow sites of the Au(111) lattice. The area per molecule for this structure was calculated to be ~35.5 Å2/molecule, which is 1.64 times lower than for BPT or MBPT SAMs, which are 21.6 Å2/molecule. Recently, STM observation showed that OPE3S SAMs formed after annealing in pure ethanol at 40 C for 5 h for pre-covered OPE3-S SAMs on Au(111) prepared via in situ deprotection of 1-[(4-acetylthiophenyl)ethynyl]-4[(phenyl)ethynyl]benzene (OPE3-SAc) using ammonium hydroxide had a unique striped pattern with 5 Å spacing between stripes and 34.6 Å spacing between vacancy rows.73 The large differences in surface structures of OPE2-S and OPE3-S SAMs on Au(111) derived from OPE2-SAc or OPE3-SAc are mainly due to the different SAM preparation conditions and backbone structures, as demonstrated for other SAM systems.1 Our STM study demonstrated that high-quality OPE2-S SAMs could be formed on Au(111) with a ( √3 × √7)R30° packing structure derived from OPE2-SAc at RT in a 1mM methanol solution containing a TBACN deprotection reagent.



On the other hand, STM images in Figure 6 clearly show that adsorption of OPE2-SAc molecules on Au(111) in a 1 mM methanol solution containing a TBACN deprotection reagent at 50 °C for 24 h generated highly ordered OPE2-S SAMs on Au(111) with larger ordered domains and fewer domain boundaries compared to those prepared at 50 °C without TBACN

Figure 6. STM images of OPE2-S SAMs on Au(111) showing long-range and well-ordered domains and domain boundaries formed after immersion of the Au(111) substrates in a 1 mM methanol solution containing a TBACN deprotection reagent at 50 °C for 24 h. Scan sizes were (a) 120 nm × 120 nm and (b) 60 nm × 60 nm, respectively.

(see Figure 3) or with TBACN at RT (see Figure 4). Very uniform and long-range, ordered domains with three domain orientations and domain angles of 60 were seen on the STM image of Figure 6a. Interestingly, a few small ordered domains less than 20 nm with different domain orientations existed in large ordered single domains, suggesting that these small domains were kinetically trapped during growth of 2D ordered domains with the same domain orientation. The magnified STM image (60 nm × 60 nm) in Figure 6b clearly showed that OPE2-S SAMs on Au(111) had uniform and long-range, ordered domains. The high-resolution STM image (8 nm × 8 nm) in Figure 7a shows the paired row packing structure of OPE2-S SAMs on Au(111) formed after immersion in a TBACN deprotection reagent at 50 °C for 24 h. The cross-sectional profiles along lines a´ and b´ (corresponding to 16 ACS Paragon Plus Environment

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the unit cell in Figure 7a) show a high degree of structural order of OPE2-S SAMs on Au(111). From this high-resolution image, the lattice constants of a rectangular unit cell containing four molecules were found: a = 5.8  0.2 Å = 2ah, b = 15.2  0.2 Å = 3 √3ah. Here, ah = 2.89 Å corresponds to the interatomic distance of the Au(111) lattice. Figure 7b shows a proposed structural model of OPE2-S SAMs on Au(111) with a (2 × 3 √3)rect structure. This model suggests that all sulfur atoms in OPE2-S SAMs sit on bridge sites of the Au(111) lattice. The areal density of the adsorbed molecule for this packing structure was calculated to be ~22.04 Å2/molecule, which is nearly the same as that of closely packed MBPT SAMs, i.e., 21.6 Å2/molecule.57 Surface coverage of OPE2-S SAMs formed at 50C increased almost 1.76 times compared to those at RT. Our STM study clearly revealed that the phase of OPE2-S SAMs on Au(111) changes from a ( √3 × √7)R30 ° structure to the (2 × 3 √3)rect structure when solution temperature increases from RT to 50C. This is a result of the increase in surface coverage that is mainly driven by the faster diffusion rate of OPE2-S molecules at a higher solution temperature during molecular self-assembly. We reported herein the first highresolution STM images showing detailed molecular features of OPE2-S SAMs on Au(111) derived from OPE2-SAc using a TBACN deprotection reagent. We also demonstrated that TBACN could be used as an effective deprotecting reagent for formation of well-ordered OPE2-S SAMs with large domains derived from thioacetyl-protected OPE2-S molecules on Au(111).



3. 3. Binding Conditions of OPE2-S SAMs on Au(111) Formed in an OPE2-SAc

Figure 7. (a) Molecularly resolved STM image (8 nm  8 nm) and (b) proposed structural model of OPE2-S SAMs on Au(111) formed after immersion of the Au(111) substrates in a 1 mM methanol solution containing a TBACN deprotection reagent at 50 ºC for 24 h. Yellow balls represent the gold atoms of the Au(111) surface. (a and b) The height profiles along lines a and b on the STM image show the superperiodicities of OPE2-S SAMs on Au(111).


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Solution without and with a TBACN Deprotection Reagent. XPS measurements were performed to understand the formation of OPE2-S SAMs on Au(111) prepared in a 1 mM OPE2-SAc methanol solution without and with a TBACN deprotection reagent at 50C for 24 h by monitoring the sulfur (S) 2p XPS spectra of these SAMs. Note that the S 2p peaks for SAMs on gold derived from organic thiols appeared as a doublet originating from the S 2p3/2 and S 2p1/2 peaks with an intensity ratio of 2:1.8-11,51,74-76 Figure 8 shows the S 2p XPS spectra of both OPE2-S SAMs on Au(111) having two doublet peaks (S1 and S2), implying the presence of two different sulfur binding states. The most prominent S1 peak (green) appeared as a doublet at 162.0 eV (S 2p3/2) and 163.2 eV (S 2p1/2), which is characteristic of the chemisorbed state of sulfur on a gold surface.8-11,74-76 This means that both OPE2-S SAMs were formed via chemical interactions between sulfur and the Au(111) surface during molecular self-assembly regardless of the presence of TBACN deprotection reagent. Similar XPS results showed that most sulfur atoms in alkanethiolate SAMs on gold observed after direct adsorption of alkanethioacetates such as decanethioacetate (C10-SAc) or octadaccanethioacetate (C18SAc) existed in a chemisorbed state.43 The additional S2 peaks (pink) with relatively weak intensity for OPE2-S SAMs in methanol (Figure 8a) and in TBACN solution (Figure 8b) were

Figure 8. XPS spectra in the S 2p region of OPE2-S SAMs on Au(111) formed (a) in a 1 mM methanol solution at 50 ºC for 24 h and (b) in a 1 mM methanol solution containing a TBACN deprotection reagent at 50 ºC for 24 h. 19 ACS Paragon Plus Environment


also observed as a doublet at 163.6 eV (S 2p3/2) and 164.8 eV (S 2p1/2) and as a doublet at 163.9 eV (S 2p3/2) and 165.1 eV (S 2p1/2), respectively. These peaks are characteristic of the physisorbed state of sulfur,8,51,74,75,77 suggesting the existence of some unbound sulfurs in the OPE2-S SAMs on Au(111). These kinds of unbound sulfurs appeared ranging from 163.4 eV to 164.2 eV have been often observed for other SAM systems on gold surfaces derived from organic thiols with complicated backbone structures,8,51,75,78 octythiocyanates,79 or loosely packed thiol SAMs.74,75 On the other hand, the relative intensities of chemisorbed sulfur (S 2p, S1 peak) against Au 4f (S 2p/Au 4f) for OPE2-S SAMs on Au(111) formed without and with a TBACN deprotection reagent were 0.0074 and 0.0085, respectively. The adsorption of chemisorbed sulfur for OPE2-S SAMs formed using TBACN increased by 14.7% compared to that of OPE2-S SAMs formed without TBACN. This means that more densely packed OPE2S SAMs could be formed in solution containing TBACN deprotection reagent. The relative intensities of physisorbed sulfur (S 2p, S2 peak) against Au 4f (S 2p/Au 4f) for OPE2-S SAMs on Au(111) formed without and with a TBACN deprotection reagent were 0.00109 and 0.00062, respectively. In contrast, the adsorption of physisorbed sulfurs for OPE2-S SAMs formed by direct adsorption of OPE-SAc increased by 75.8% compared to that of OPE2-S SAMs formed in a solution containing TBACN reagent. It was reported that the structural order of SAMs decreased when the amount of physisorbed species increased in the SAMs.75 Our XPS measurements revealed that more densely packed OPE2-S SAMs could be formed in solution containing TBACN deprotection reagent. These XPS results are consistent with the obtained STM results showing that OPE2-S SAMs formed using TBACN have a uniform and highly ordered domain structure compared to those prepared by direct adsorption of OPE2SAc molecules.


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4. CONCLUSIONS The surface structure and binding conditions of OPE2-S SAMs on Au(111) derived from OPE2-SAc molecules without and with a TBACN deprotection reagent were examined using STM and XPS. STM observation clearly revealed that OPE2-S SAMs on Au(111) formed by direct adsorption of OPE2-SAc in a 1mM methanol solution at RT for 24 h were composed of two coexisting phases: a short-ranged, ordered phase with domain sizes of 5-15 nm and a disordered phase. In contrast, OPE2-S SAMs on Au(111) formed at a higher solution temperature of 50 °C had short-range and fully ordered phases with slightly increased domain sizes of 5-30 nm. We clearly showed that the structural quality of OPE2-S SAMs on Au(111) was markedly enhanced when OPE2-S SAMs on Au(111) were formed using TBACN deprotection reagent. OPE2-S SAMs at RT had a well-ordered (√3 × √7)R30° structure with large domains. Moreover, OPE2-S SAMs at 50 °C had uniform and highly-ordered domains with a (2 × 3 √ 3)rect packing structure containing ordered domains over 100 nm. XPS measurements showed that formation of OPE2-S SAMs is mainly driven by chemical interactions between sulfur and the Au(111) surface regardless of the use of TBACN deprotection reagent. However, the amount of chemisorbed sulfur for OPE2-S SAMs using TBACN increased by 14.7%, suggesting formation of more densely packed SAMs. In this study, we clearly demonstrated that highly ordered OPE2-S SAMs with long-range domains over 100 nm can be fabricated using a TBACN deprotection reagent.

AUTHOR INFORMATION Corresponding Author E-mail: [email protected], phone: +82-2-2220-0938.



ACKNOWLEDGMENTS This work was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (NRF2012R1A6A1029029, NRF-2015R1D1A1A01058769, NRF-2015K2A2A4000124, and NRF2018R1D1A1B07048063).


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TOC Graphic


Formation and Structure of Highly Ordered Self-Assembled

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