High-Temperature Adsorption of p-Terphenylthiol on Au(111) Surfaces

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High-Temperature Adsorption of p-Terphenylthiol on Au(111) Surfaces Vladimir V. Korolkov, Stephanie Allen, Clive J. Roberts, and Saul J.B. Tendler* Laboratory of Biophysics and Surface Analysis, School of Pharmacy, The University of Nottingham, Nottingham, NG7 2RD, United Kingdom ABSTRACT: We have investigated p-terphenylthiol (TPT) adsorption on the Au(111) surfaces from dimethylformamide solution at 373 K. Scanning tunneling microscopy (STM), X-ray photoelectron spectroscopy, and spectroscopic ellipsometry were used to follow the dynamics and explore the structure of the selfassembled monolayer on Au(111). This combined approach revealed that TPT molecules form self-assembled domains within the first seconds of Au surface exposure to thiol solution. The complete monolayer structure forms within ∼1 min. It presents itself as a well-ordered defect-free self-assembled monolayer. Further exposure does not introduce observable changes to the monolayer structure. Two competing phases (R and β) of differently packed molecules were observed within the first seconds of adsorption, with the β-phases dominating from 1 min. Close inspection of adsorbed TPT molecules with STM demonstrated their almost upright conformation on the surface with a tilt angle of 0 ( 5° in the β-phase and tilted (10 20°) conformation in the R-phase.

’ INTRODUCTION Long-chain alkanethiols were historically the first organic thiols considered and widely studied for self-assembly on Au.1 3 These studies were further extended to different aromatic thiols.4 7 Among the many different organic thiols used for self-assembly, aromatic thiols occupy a unique niche of possible candidates for molecular electronics and nanoelectronic devices. For example, some of them (e.g., conjugated arenethiol derivatives) are considered as promising molecular wires.8 This possibility arises from their conjugated aromatic structure with extended π-systems. The latter is responsible for strong π π interactions between aromatic moieties of single molecules within the layer. As a result selfassembled monolayers (SAMs) composed of aromatic thiols usually exhibit different molecular packing within the monolayer.9 11 The monolayer phase composition for aromatic SAMs is largely dependent on the molecule structure and preparation conditions, with the temperature being the most important one.11 Another property that distinguishes them from alkanethiols and related compounds with flexible hydrocarbon motifs is a rigid, rodlike aromatic backbone. This makes aromatic thiols similar to cage thiols, which have a rigid but spherical structure. Previously we suggested that the lack of flexibility should significantly reduce the time required for self-assembly.15 We found that the simplest representative of cage thiols, 1-adamantanethiol,12 14 self-assembles on Au within ∼1 s at 373 K forming a defect-free and almost ideally packed monolayer.15 Assuming the above-mentioned structural similarity, we may expect the same dynamics of selfassembly for aromatic thiols, at least for simplest representatives like biphenylthiol (BPT) or terphenylthiol (TPT). We should note that SAMs made of BPT and TPT on Au have been previously investigated;9,16,17 although most of these studies focused on SAM structure and spectroscopic properties rather r 2011 American Chemical Society

than on the dynamics of self-assembly within the first seconds and minutes of adsorption. Of the studies that did investigate adsorption kinetics, it is noteworthy, that they were limited to just one experimental technique to follow the process, e.g., quartz crystal microbalance (QCM)18 or STM.19 As a result many of the surface features were not studied and some conclusions are discrepant. For example a QCM study on 4-mercaptobiphenyls adsorption on Au(111) demonstrated that the monolayer saturates within 2 3 min of exposure;18 however, it did not provide information either on the monolayer structure nor its chemical composition. Thus in this work several complementary techniques (spectroscopic ellipsometry, X-ray photoslectron spectroscopy (XPS), and scanning tunneling microscopy (STM)) have been predominantly utilized to focus on the very early stages of terphenylthiol adsorption on Au(111) at 373 K and provide a thorough understanding of this process.

’ EXPERIMENTAL SECTION TPT (Figure 1b) was purchased from Sigma (Poole, UK) and was used without further purification. Evaporated gold of 99.99% purity (300 nm) on mica substrates were purchased from Georg Albert PVD-Beschichtungen (Silz, Germany). Prior to adsorption Au substrates were UV-ozone cleaned for 20 30 min (BioForce Nanosciences, Ames, IA) and rinsed with analytical grade EtOH to remove physisorbed ozone and oxygen. They were then immediately used for thiol adsorption. All SAMs were prepared via a high-temperature adsorption carried out at 373 K from 1 mM TPT solution in dimethylformamide (DMF) Received: April 15, 2011 Revised: June 20, 2011 Published: July 02, 2011 14899

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Figure 1. (a) Film thickness vs adsorption time. A shadowed area represents possible range for the theoretical film thickness. (b) A molecular structure of TPT with estimated dimensions from ab initio calculations at the 6-31G level. A shadowed ellipse shows molecule projection on the Au surface with certain size l.

Figure 2. HR XPS: C 1s, S 2p, N 1s, and O 1s regions for samples prepared at different adsorption times. An inset showing S2p and C1s normalized areas vs time.

(high-performance liquid chromatography grade, 99.9%). The solvent was sonicated for ∼10 min in nitrogen atmosphere to reduce the amount of dissolved oxygen and thus prevent possible thiol oxidation. This procedure was proved to be of crucial importance for preparing samples via high-temperature adsorption. Adsorption times investigated were ∼1, 10, 60, and 600 s. All prepared samples were thoroughly rinsed with fresh DMF and dried in a stream of nitrogen and were kept in a nitrogen ambient environment between measurements. The TPT/Au surface was imaged using an Agilent Technologies STM (Santa Clara, CA). STM tips were made by mechanically cutting Pt/Ir (20% Ir) wire (ADVENT, Research Materials Ltd., Oxford, UK). Imaging was performed in a constant current mode with a tunneling current (I) ranging between 50 and 100 pA

and bias voltage (V) 150 mV. All presented STM images were extracted from raw data files using WSxM 5.0 program20 and have not been manipulated. High-resolution XPS spectra were obtained using a Kratos system with a monochromatic Al KR X-ray source (1486.6 eV) (Kratos Ltd., Manchester, UK). All high-resolution spectra were acquired with pass energy of 20 eV. The binding energy scale was calibrated according to the Au 4f7/2 line position (84.00 eV). For all measurements samples were put in electrical contact with the sample holder and no other charge compensation techniques were applied. Film thicknesses for SAM samples were estimated using an R-SE spectroscopic ellipsometer (J. A. Woollam Co., Inc.). The same Au substrate was used to measure substrate’s optical 14900

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Figure 3. A set of large scale STM images for samples prepared at 1 (a), 10 (b), 60 (c), and 600 s (d).

constants. A Cauchy layer with default parameters was used to model SAM. Au substrate and Cauchy layer parameters were fixed, whereas film thickness was turned on as a fit parameter.

’ RESULTS AND DISCUSSIONS Ellipsometry. Film thickness is a macroscopic parameter directly linked to molecular conformation/orientation and packing on a surface. Hence it provides an important relationship between the macroscopic and microscopic properties of a selfassembled monolayer. For an upright orientation for TPT molecules the film thickness should not significantly exceed ∼17 Å (the Au S bond length was assumed to be 2.5 Å,21 and molecule length was estimated with the 6-31G method22 to be 14.3 Å (excluding S H bond length)). The exact value for the Au S bond length in SAMs is not well-defined experimentally. For many crystalline Au-containing organic compounds it is reported to be around 2.3 Å.23 25 Whereas computer simulations for adsorbed thiols on Au predict it to be close to 2.521,26 28 or even 2.88 Å:29 hence the uncertainty in the Au S bond length value reaches almost 0.5 Å.

All measured thicknesses are presented in Figure 1a together with a theoretical estimate. All results are grouped closely around 17 Å and are slightly above the estimated value. Our measured value matches a previously reported one.17 It is clear that the thickness reaches ∼17 Å within the first second of adsorption and remains at almost the same level for the next 600 s. The thickness deviation between samples does not exceed 0.4 Å. This value of thickness clearly indicates that according to ellipsometry the surface coverage saturates almost instantly and that molecules should presumably exhibit a predominantly upright orientation. Although one should keep in mind that measured thicknesses represents an average value over a large area of sample (approximately a few of millimeters squared) and do not reveal the fine organization of a monolayer at a molecular level. It was not possible to measure thickness for the sample prepared for 18.5 h due to increased gold surface roughness, presumably a result of the long exposure to a high temperature medium. XPS. XPS was used to assess thiol binding to Au surfaces and overall film quality (presence of contaminations and oxidized species). The 168 158 eV region in an X-ray spectrum is key in understanding chemical species in SAMs since it provides the fundamental information on interactions between the sulfur 14901

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The Journal of Physical Chemistry C group of a thiol and Au surface. High resolution scans for the S 2p region are presented in Figure 2. For all samples a clear S 2p doublet with a binding energy of ∼161.9 eV (S 2p3/2) was observed. The presence of only one S 2p doublet indicates that thiolate species dominate on Au surfaces even for samples prepared at extremely short adsorption times (1 10 s). This fact supports a very rapid formation of S Au bonds. A close inspection of the 168 158 eV region reveals no other significant peaks indicating an absence of oxidized sulfur species (SOx), formed disulfides, elemental sulfur, or unbound thiols. It is worth noting that none of these species were observed even for the sample prepared for 18.5 h. The absence of oxidized sulfur species (SOx) also indirectly suggests an oxygen free environment since thiols are likely to oxidize fairly quickly at elevated temperatures. For all samples the precise

Figure 4. 500 nm  500 nm STM images for 1-min (top image) and 10-s (bottom image) samples.

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position for S 2p 3/2 is in a good agreement with the previously published data.30 A normalized S 2p surface area vs time was plotted for 5 samples prepared at different times (Figure 2) to follow the dynamics of SAM formation. As the plot suggests, the monolayer coverage saturates within the first 60 s of adsorption and remains on the same level for the next 18.5 h. A small but noticeable decrease in S 2p intensity at the end is most likely due to the competitive process of thiol desorption. Thus, both S 2p peak position (S 2p 3/2) and its time dependency clearly suggests a saturated monolayer formation of thiolate species within ∼1 min over the Au surface. A further insight into dynamics and structure of SAM can be gathered via examining C 1s, O 1s, and N 1s regions. We need to observe the latter region since DMF ((CH3)2NCHO) was used as a solvent. High-resolution scans for the C 1s, O 1s, and N 1s regions are presented in Figure 2. The C 1s binding energy was observed at a typical position for aromatic SAMs on Au to be 284.4 eV.17 For all samples, the C 1s region lacks any noticeable peaks related to oxygen-containing groups or any other possible contaminations. A normalized C 1s area vs time is presented together with corresponding S 2p dependency. It shows the same trend—a monolayer saturates within the first minute of adsorption. Another parameter which reflects the SAM integrity is the full width at half-maximum (fwhm) of the C 1s peak. A noticeable decrease in fwhm was observed toward longer adsorption times. The fwhm of 1.46 eV was measured for a sample prepared within 1 s, and for subsequent values it remained virtually unchanged at ∼1.1 eV. Barely noticeable O 1s and N 1s peaks were observed for the 1-s sample (Figure 2). Neither of these was observed for the rest (10, 60, and 600 s) of the samples. As above, the presence of nitrogen and oxygen in the 1 s sample is more likely to be due to adsorption of DMF. The latter observation once again shows a unique self-cleaning character of SAM formation since adsorbing thiols remove most of the contaminants from the surface. Although both ellipsometry and XPS suggest that monolayer formation finishes within one minute, they provide only a macroscopic examination and do not reveal structural changes happening at a single molecular level. STM. A set of molecular resolved STM images for four samples (prepared at 1, 10, 60, and 600 s) are presented in Figure 3. They

Figure 5. STM images for 1-min (a) and 10-min (b) samples showing the nature of bright areas. 14902

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Figure 6. High-resolution STM images showing two different molecular phases.

illustrate the evolution of both surface coverage and SAM structure within the first 10 min of adsorption at 373 K. The STM reveals that the Au surface is almost fully covered with TPT molecules right from the very first second of adsorption; large domains of well-packed molecules dominate the Au surface. The domain size varies from 5 to 40 nm, but the monolayer does not form a longrange ordered structure within this period as numerous defects and small rotational domains are observed. The most common defects are areas with disordered or missing molecules (dark areas) and single molecule defects (which appear as dark dots within the wellpacked domain structure). The size of the latter type of defect does not exceed 10 nm for the 1 s sample, and it reduces to a few nanometres for the 10 s image. After 1 min of adsorption one can hardly observe any of these defects in the STM image. A striking difference that appears within 1 min of adsorption is a combination of bright and dark domains which is very distinctive. It is reasonable to assume that the difference in the contrast comes from some structural differences in the monolayer itself, since both the tip and imaging parameters (bias voltage, tunneling current) were kept constant within each image. Thus we suggest that domains with different contrast represent two phases of differently packed molecules. The further increase of adsorption time to 10 min slightly improves the monolayer structure and leads to an almost defect free layer. Thus the overview of large scan images unambiguously indicates that the monolayer structure was almost complete within the very first minute of adsorption at 373 K. STM identifies that the growth of brighter domains is the most noticeable change occurring within the next minutes and hours of adsorption (Figure 3). A clear overview of this is presented on a combined 500 nm  500 nm image for samples prepared for 10 and 60 s

(Figure 4). The lower inset corresponding to 10 s sample is much more populated with protruding defects whereas the upper one barely shows any of those. The above large scale images strongly support our XPS and ellipsometer findings that a complete monolayer forms within ∼1 min of Au(111) surface exposure to TPT solution in DMF at 100 °C. A closer inspection of the high-resolution STM images provides a further insight into SAM structure, as well as a possible assessment of molecule orientation. A molecular resolved STM image for the 1 min sample is shown in Figure 5. This image brings further insight on the nature of the observed bright regions. The image identifies that the Au surface is covered with various rotational domains of TPT molecules. Domain boundaries are highlighted with black lines. The smallest domain size is about 10 nm in length. Figure 5a shows that all bright areas are located on both sides of domain boundaries, being more intense right on the line separating them. Since the SAM microstructure remains the same on both sides of domain boundary this noticeable contrast difference would be hard to interpret unambiguously. Thus our previous assumption that bright domains represent a different phase may not be valid. At the same time, few observable defects are again located on the domain boundaries. These defects might well be imaging artifacts since both the resolution and hence image quality usually significantly deteriorates when scanning close to domain boundaries. Figure 5b demonstrates this proposal. However, while the image quality in the boundary region is inferior compared to the rest of the image, we still can observe single molecules there. These defects, or at least part of them, might therefore be areas with disordered or missing thiol molecules, which would be difficult to resolve with STM. In either case the size of these possible defects does not exceed ∼10 15 Å for the 1-min 14903

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Figure 7. STM images showing molecule shape dependence vs scan angle. A white arrow indicates the scan direction.

sample which corresponds to 4 6 chemisorbed TPT molecules. Hence they do not have a profound effect on SAM properties on a macroscopical scale. Close inspection of the high-resolution images identifies molecule arrangements for two different phases for the sample prepared within 1 s. High-resolution images for both phases are presented in Figure 6. The R-phase is the one that exists only throughout the first few seconds of adsorption, and the β-phase is clearly observed for the 1-min sample and onward. We should note that we observed characteristic phase separation on large scan images for 1 s samples only; we were unable to identify any traces of the R-phase when inspecting large scan images for 10-, 60-, and 600-s samples. STM reveals that the R-phase has a rectangular unit cell with lattice constants of a = 4.9 ( 0.1 Å and b = 6.2 ( 0.1 Å. The β-phase has an oblique unit cell with the following lattice constants a = 5.2 ( 0.1 Å, b = 5.7 ( 0.1 Å, and angle of 52°. Close inspection of the large scale images (Figure 3) allows us to note that within 1 s the Au surface is almost equally covered with R- and β-phases. Both phases have distinctive domain boundaries. This fact might suggest an almost instant and independent formation of each phase since we did not clearly

observe any transitional phases. An almost equal amount of the two phases also might tell us that both of them have very similar kinetic parameters of self-assembly; as adsorption time increases the relative thermodynamic phase stability takes over the process. No traces of the R-phase remain after 10 s of adsorption, and the β-phase dominates on the Au surface onward. A further inspection of single molecules within various rotational domains reveals that the molecular shape significantly depends on the relative angle between the scan direction and domain orientation. The molecular shape varies from almost round to elliptical. Figure 7 provides further insight. It shows four 5 nm  5 nm molecular resolved scans of the same area. Three of them (a, c, and d) were taken at different scan angles. The scan direction is indicated by the white arrow (Figure 7a). The observed dependence on the scan angle exclusively comes from the elliptical shape of p-terphenyl backbones as viewed from the top, as in STM. For parts a and b of Figure 7 the scan angle is 0° and results in round shaped molecule profiles. Part b was acquired with reversed tip polarity compared to part a. Once the angle is changed to 90° elliptically shaped molecule profiles are observed (part c). Part c is a combination of 10 nm  10 nm (top inset) and 5 nm  5 nm images taken as one image by 14904

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The Journal of Physical Chemistry C changing scan size during acquisition (other scanning parameters were kept the same). The top 10 nm  10 nm inset presents two rotational domains and the boundary between them. The domain on the left has the “wrong” orientation and a round shape molecular profile as a result. A precise elliptical shape is observed for only half of molecules for which the top phenyl ring plane is exactly orthogonal to the scanning direction. The top left rotational domain appears much brighter than the one on the right on the top inset. It is clear that this difference is due to different domain orientation toward the scanning direction. This observation explains the location of bright areas around domain boundaries. Although it remains unclear why there are distinctive borders between dark and bright areas on the large (>100 nm  100 nm) images. We should note here that in fact the angle was changed gradually until the right molecule profile shape was achieved. The further sample rotation by 35° (the resulting scan angle is 125°) results in increasingly rounded distorted molecule profiles and some loss of resolution (Figure 7d). This dependence illustrates the vital importance of careful selection of the scan angle when carrying out high-resolution imaging of nonround features on surfaces. It also reveals the TPT molecule orientation on the surface, since the molecule shape in the STM image strongly depends on its surface conformation. It is clear that for any tilted conformation TPT molecules will appear as either almost round or a slightly oval shape set of bright features in STM images. Moreover there will not be any noticeable dependence of molecule shape upon scanning direction. Thus both the observed angular dependence and precise elliptical shape of top phenyl rings in adsorbed molecules suggests an upright or almost upright conformation with a tilt angle of ∼0 ( 5° within the β-phase (Figure 7c). This estimation agrees with previously reported STM data on similar aromatic systems on Au.8 In the case of the R-phase, we neither observed a dependency on the scanning direction nor elliptically shaped molecules. In fact all molecules appeared to have a triangular shape suggesting some tilted conformation. By assumption of a tilted conformation, we can still estimate the tilt angle for TPT molecule arranged in the R-phase to be within 10 20°. This estimation comes from the following relation: arcsin(l, molecule size on STM image (3 5 Å)/molecule length (14.3 Å)). The above relation is valid if we assume that the size of triangle shaped features observed on the STM image for the R-phase (Figure 6, R-phase) corresponds to the size of molecule plane projection (Figure 1b). Given the rodlike molecule structure and knowing the molecule length we can use the above simple equation to estimate the tilt. Hence with an average projection size for a single molecule of ∼3.5 Å (Figure 6, R-phase), an average tilt angle is ∼15°.

’ CONCLUSIONS We have observed that TPT self-assembles on Au(111) surface at 373 K within ∼1 min forming an almost defect-free monolayer. Approximately ∼90% of the monolayer forms within the first ∼10 s of adsorption, and then it takes ∼50 s to develop into a well-ordered structure. Molecular resolved STM images clearly demonstrate that the β-phase (with primitive unit cell 0.43  0.56 nm at 52°) dominates on the Au surface from ∼10 s of exposure and onward. A closer inspection of the β phase on a single molecule level allowed us to propose an upright conformation for TPT molecule on Au with the tilt angle of 0 ( 5°. We also estimated the tilt angle for the R-phase to be within 10 20°. Both STM and XPS analysis suggest a high chemical and structural

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quality of the monolayer. Overall we have proposed a simple and straightforward protocol for the fabrication of TPT SAM on Au surface, which we believe can be readily extended for other similar molecular systems.

’ AUTHOR INFORMATION Corresponding Author

*Phone: +44115 8232480. Fax: +44115 9515110. E-mail: saul. [email protected].

’ ACKNOWLEDGMENT The authors would like to thank EPSRC for funding (EP/ D048761/1). We thank Prof Peter Beton, School of Physics and Astronomy, The University of Nottingham for access to the Agilent scanning tunneling microscope. We thank Adrian Boatright, School of Chemistry, The University of Nottingham for assistance with the XPS measurements. ’ REFERENCES (1) Nuzzo, R. G.; Allara, D. L. J. Am. Chem. Soc. 1983, 105, 4481– 4483. (2) Ulman, A. Chem. Rev. 1996, 96, 1533–1554. (3) Love, J. Ch.; Estroff, L. A.; Kriebel, J. K.; Nuzzo, R. G.; Whitesides, G. M. Chem. Rev. 2005, 105, 1103–1170. (4) Leung, T. Y. B.; Schwartz, P.; Scoles, G.; Schreiber, F.; Ulman, A. Surf. Sci. 2000, 458, 34–52. (5) Stapleton, J. J.; Harder, Ph.; Daniel, T. A.; Reinard, M. D.; Yao, Y.; Price, D. W.; Tour, J. M.; Allara, D. L. Langmuir 2003, 19, 8245–8255. (6) Kafer, D.; Witte, G.; Cyganik, P.; Terfort, A.; Woll, Ch. J. Am. Chem. Soc. 2006, 128, 1723–1732. (7) Azzam, W.; Wehner, B. I.; Fischer, R. A.; Terfort, A.; Woll, Ch. Langmuir. 2002, 18, 7766–7769. (8) Yang, G.; Qian, Y.; Engtrakul, Ch.; Sita, L. R.; Liu, G-y. J. Phys. Chem. B 2000, 104, 9059–9062. (9) Cyganik, P.; Buck, M. J. Am. Chem. Soc. 2004, 126, 5960–5961. (10) Azzam, W.; Cyganik, P.; Witte, G.; Buck, M.; Woll, Ch. Langmuir 2003, 19, 8262–8270. (11) Cyganik, P.; Buck, M.; Strunskus, Th.; Shaporenko, A.; WiltonEly, J. D. E. T.; Zharnikov, M.; Woll, Ch. J. Am. Chem. Soc. 2006, 128, 13868–13878. (12) Dameron, A. A.; Charles, L. F.; Weiss, P. S. J. Am. Chem. Soc. 2005, 127, 8697–8704. (13) Dameron, A. A.; Mullen, T. J.; Hengstebeck, R. W.; Saavedra, H. M.; Weiss, P. S. J. Phys. Chem. C. 2007, 111, 6747–6752. (14) Mullen, T. J.; Zhang, P.; Srinivasan, Ch.; Horn, M. W.; Weiss, P. S. J. Electroanal. Chem. 2008, 621, 229–237. (15) Korolkov, V.; Allen, S.; Roberts, C. J.; Tendler, S. J. B. J. Phys. Chem. C. 2010, 114, 19373–19377. (16) Ishida, T.; Mizutani, W.; Azehara, H.; Sato, F.; Choi, N.; Akiba, U.; Fujihira, M.; Tokumoto, H. Langmuir 2001, 17, 7459–7463. (17) Himmel, H.-J.; Terfort, A.; Woll, Ch. J. Am. Chem. Soc. 1998, 120, 12069–12074. (18) Liao, S.; Shnidman, Y.; Ulman, A. J. Am. Chem. Soc. 2000, 122, 3688–3694. (19) Ishida, T.; Mizutani, W.; Azehara, H.; Sato, F.; Choi, N.; Akiba, U.; Fujihira, M.; Tokumoto, H. Langmuir 2001, 17, 7459–7463. (20) Horcas, I.; Fernandez, R.; Gomez-Rodríguez, J. M.; Colchero, J.; Gomez-Herrero, J.; Baro, A. M. Rev. Sci. Instrum. 2007, 78, 01370578. (21) Srinivasan, V.; Cicero, G.; Grossman, J. C. Phys. Rev. Lett. 2008, 101, 185504 1–185504 4. (22) Schmidt, M. W.; Baldridge, K. K.; Boatz, J. A.; Elbert, S. T.; Gordon, M. S.; Jensen, J. H.; Koseki, S.; Matsunaga, N.; Nguyen, K. A.; Su, S. J.; Windus, T. L.; Dupuis, M.; Montgomery, J. A. J. Comput. Chem. 1993, 14, 1347–1363. 14905

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