6056
Langmuir 2003, 19, 6056-6065
Structures of Annealed Decanethiol Self-Assembled Monolayers on Au(111): an Ultrahigh Vacuum Scanning Tunneling Microscopy Study Yile Qian,† Guohua Yang,‡ Jingjiang Yu,‡ Thomas A. Jung,§ and Gang-yu Liu*,‡ Department of Chemistry, Wayne State University, Detroit, Michigan 48202, Department of Chemistry, University of California, Davis, California 95616, and Laboratory for Micro- and Nanotechnology, ODRA/107, Paul Scherrer Institute, CH-5232 Villigen PSI, Switzerland Received October 30, 2002. In Final Form: May 13, 2003 While the structures of self-assembled monolayers (SAMs) of alkanethiols on Au(111) are extensively studied and well-known, new structures and complex phase behavior have been progressively discovered when coverage of these layers falls below saturation. Structures and phase transitions of annealed decanethiol monolayers on Au(111) surfaces were systematically investigated using scanning tunneling microscopy (STM) under ultrahigh vacuum (UHV) conditions. Rich structures were revealed as a result of annealing in UHV. At temperatures below 345 K, no significant changes in coverage were observed, although the size of two-dimensional crystalline c(4x3 × 2x3)R30° domains increases as annealing progresses. A twodimensional melting occurs at 345 ( 5 K and was captured in situ from time-dependent STM studies. Above 400 K, significant desorption takes place. In the temperature range of 345-400 K, within which desorption progresses to gradually decrease the surface coverage, a variety of striped phases have been observed, each having distinct molecular-level packing and unit cells. Well-known striped phases have been confirmed: (p × x3), with p values (integer or half-integer multiples of the Au(111) periodicity) of 7.5, 9, and 11. In addition, new structures such as mixed striped phases and mesh-like structures are revealed, which are often found to coexist with the regions of pure striped phases. The systematic investigations of the structural and phase evolution shed light on the SAM desorption process at the molecular level.
Introduction Self-assembled monolayers (SAMs) have attracted much attention among researchers because of promising applications in biosensing,1-5 corrosion inhibition,6-8 lubrication,9-12 surface patterning,13-15 and molecular device fabrication.16,17 The molecular-level packing significantly influences the physical and chemical properties of SAMs, such as the passivation efficiency, lubricity, and frictional * To whom correspondence should be addressed. Telephone: 530754-9678. Fax: 530-752-8995. E-mail:
[email protected]. † Wayne State University. Present address: RHK Technology, Inc., 1050 E. Maple Road, Troy, MI 48083. ‡ University of California. § Paul Scherrer Institute. (1) Dubois, L. H.; Nuzzo, R. G. Annu. Rev. Phys. Chem. 1992, 43, 437. (2) Dimilla, P. A.; Folkers, J. P.; Biebuyck, H. A.; Harter, R.; Lopez, G. P.; Whitesides, G. M. J. Am. Chem. Soc. 1994, 116, 2225. (3) Crooks, R. M.; Ricco, A. J. Acc. Chem. Res. 1998, 31, 219. (4) Flink, S.; van Veggel, F.; Reinhoudt, D. N. Adv. Mater. 2000, 12, 1315. (5) Willner, I.; Katz, E. Angew. Chem., Int. Ed. 2000, 39, 1180. (6) Whitesides, G. M.; Laibinis, P. E. Langmuir 1990, 6, 87. (7) Laibinis, P. E.; Whitesides, G. M. J. Am. Chem. Soc. 1992, 114, 9022. (8) Chailapakul, O.; Sun, L.; Xu, C.; Crooks, R. M. J. Am. Chem. Soc. 1993, 115, 12459. (9) Porter, M. D.; Bright, T. B.; Allara, D. L.; Chidsey, C. E. D. J. Am. Chem. Soc. 1987, 109, 3559. (10) Bain, C. D.; Troughton, E. B.; Yao, T.-T.; Evall, J.; Whitesides, G. M. J. Am. Chem. Soc. 1989, 111, 321. (11) Troughton, E. B.; Bain, C. D.; Whitesides, G. M.; Nuzzo, R. G.; Allara, D. L.; Porter, M. D. Langmuir 1988, 4, 365. (12) Chidsey, C. E. D.; Loiacono, D. N. Langmuir 1990, 6, 682. (13) Xia, Y. N.; Whitesides, G. M. Annu. Rev. Mater. Sci. 1998, 28, 153. (14) Xia, Y. N.; Rogers, J. A.; Paul, K. E.; Whitesides, G. M. Chem. Rev. 1999, 99, 1823. (15) Liu, G. Y.; Xu, S.; Qian, Y. L. Acc. Chem. Res. 2000, 33, 457. (16) Tour, J. M. Acc. Chem. Res. 2000, 33, 791. (17) Joachim, C.; Gimzewski, J. K.; Aviram, A. Nature 2000, 408, 541.
coefficient. A better understanding of the structure and stability of various phases at room and elevated temperatures is essential for potential applications of organic thin films. Decanethiol adsorbed on Au(111) [CH3(CH2)9S/Au(111) or abbreviated as C10S/Au] is a representative SAM that has been widely investigated. Previous studies, using microscopy, diffraction, and spectroscopy,18-21 have provided detailed information regarding the surface structure and packing of decanethiol SAMs. Among these investigations, scanning tunneling microscopy (STM) provides a real-space picture at the molecular level.22-34 At saturation coverage, alkanethiols adopt a two-dimensional, close(18) Ulman, A. Chem. Rev. 1996, 96, 1533. (19) Delamarche, E.; Michel, B.; Biebuyck, H. A.; Gerber, C. Adv. Mater. 1996, 8, 719. (20) Poirier, G. E. Chem. Rev. 1997, 97, 1117. (21) Schreiber, F. Prog. Surf. Sci. 2000, 65, 151. (22) Fitts, W. P.; White, J. M.; Poirier, G. E. Langmuir 2002, 18, 2096. (23) Fitts, W. P.; White, J. M.; Poirier, G. E. Langmuir 2002, 18, 1561. (24) Poirier, G. E.; Fitts, W. P.; White, J. M. Langmuir 2001, 17, 1176. (25) Camillone, N.; Eisenberger, P.; Leung, T. Y. B.; Schwartz, P.; Scoles, G.; Poirier, G. E.; Tarlov, M. J. J. Chem. Phys. 1994, 101, 11031. (26) Bucher, J. P.; Santesson, L.; Kern, K. Langmuir 1994, 10, 979. (27) Poirier, G. E.; Tarlov, M. J. Langmuir 1994, 10, 2853. (28) Poirier, G. E.; Tarlov, M. J.; Rushmeier, H. E. Langmuir 1994, 10, 3383. (29) Delamarche, E.; Michel, B.; Gerber, C.; Anselmetti, D.; Guntherodt, H. J.; Wolf, H.; Ringsdorf, H. Langmuir 1994, 10, 2869. (30) Yamada, R.; Uosaki, K. Langmuir 1998, 14, 855. (31) Staub, R.; Toerker, M.; Fritz, T.; Schmitz-Husch, T.; Sellam, F.; Leo, L. Langmuir 1998, 14, 6693. (32) Poirier, G. E. Langmuir 1999, 15, 1167. (33) Toerker, M.; Staub, R.; Fritz, T.; Schmitz-Hubsch, T.; Sellam, F.; Leo, K. Surf. Sci. 2000, 445, 100. (34) Pflaum, J.; Bracco, G.; Schreiber, F.; Colorado, R.; Shmakova, O. E.; Lee, T. R.; Scoles, G.; Kahn, A. Surf. Sci. 2002, 498, 89.
10.1021/la0267701 CCC: $25.00 © 2003 American Chemical Society Published on Web 06/21/2003
Decanethiol Self-Assembled Monolayers on Au(111)
packed structure on Au(111), with a hexagonal lattice and a periodicity of 0.499 nm. The packing arrangement follows a (x3 × x3)R30° lattice referenced to Au(111).25-34 Each hydrocarbon chain has an all-trans, zigzag configuration, and the backbone tilts 30° with respect to the surface normal.18-21 A superlattice, expressed as c(4x3 × 2x3)R30° with respect to the Au(111) lattice, was revealed by diffraction35-38 as well as by high-resolution STM studies.27,29 The lattice of the Au(111) surface (a ) 0.289 nm) is used as a reference to describe these commensurate structures. STM is well-known for its high spatial resolution and sensitivity to local structures. Thus, STM is able to reveal new information for adsorbed molecules on solid surfaces.32,39-41 Further investigations of SAMs using STM under ultrahigh vacuum (UHV) conditions reveal that alkanethiol SAMs exhibit a rich phase diagram of various structures at packing densities less than saturation.18-20,25,30-33,42-45 One class of structures is the so-called striped phase, expressed as (p × x3), where p is a multiple of the Au(111) lattice constant, a ) 0.289 nm.28,31 In striped structures, the interstripe distance is p × 0.289 nm, while the nearest neighbor distance within each molecular row (stripe) is x3 × 0.289 ) 0.500 nm. These striped structures have been revealed by STM,25,30-32 low-energy electron diffraction,46,47 and helium diffraction.25,48 In most of the striped phases observed, each stripe contains two rows of decanethiol molecules, and the molecules are close-packed within the row.31,32 The most frequently observed striped structures of decanethiols include (7.5 × x3) (δ phase), (11.5 × x3) (β phase), and (9.5 × x3) (χ phase).32 Furthermore, a mixed (7.5, 11.5 × x3) striped phase (χ* phase)33 and three mesh phases were reported, with characteristics of large unit cells and mesh-like lattices.30,32,33 Among them, the δ and β phases are stable and the χ phase is reported to be metastable.32 These phases at low coverages have also been observed during the growth of SAMs in gas-phase deposition as well as in solution-phase adsorption. In this article, we report a systematic structural characterization of decanethiol SAMs at room and elevated temperatures. Using freshly prepared SAMs and annealing under UHV conditions, STM imaging provides direct information on the structure and phase evolution of decanethiol SAMs. Well-known structures of (x3 × x3)R30° and c(4x3 × 2x3)R30° at saturation coverage have been confirmed. As the coverage progressively (35) Camillone, N.; Chidsey, C. E. D.; Liu, G.-Y.; Scoles, G. J. Chem. Phys. 1993, 98, 3503. (36) Fenter, P.; Eberhardt, A.; Eisenberger, P. Science 1994, 266, 1216. (37) Fenter, P.; Schreiber, F.; Berman, L.; Scoles, G.; Eisenberger, P.; Bedzyk, M. J. Surf. Sci. 1998, 413, 213. (38) Fenter, P.; Schreiber, F.; Berman, L.; Scoles, G.; Eisenberger, P.; Bedzyk, M. J. Surf. Sci. 1999, 425, 138. (39) Yoon, H. A.; Salmeron, M.; Somorjai, G. A. Surf. Sci. 1998, 395, 268. (40) Speller, S.; Rauch, T.; Bomermann, J.; Borrmann, P.; Heiland, W. Surf. Sci. 1999, 441, 107. (41) Rose, M. K.; Mitsui, T.; Dunphy, J.; Borg, A.; Ogletree, D. F.; Salmeron, M.; Sautet, P. Surf. Sci. 2002, 512, 48. (42) Schreiber, F.; Eberhardt, A.; Leung, T. Y. B.; Schwartz, P.; Wetterer, S. M.; Lavrich, D. J.; Berman, L.; Fenter, P.; Eisenberger, P.; Scoles, G. Phys. Rev. B 1998, 57, 12476. (43) Schwartz, P.; Schreiber, F.; Eisenberger, P.; Scoles, G. Surf. Sci. 1999, 423, 208. (44) Poirier, G. E.; Tarlov, M. J. J. Phys. Chem. 1995, 99, 10966. (45) Poirier, G. E.; Pylant, E. D. Science 1996, 272, 1145. (46) Dubois, L. H.; Zegarski, B. R.; Nuzzo, R. G. J. Chem. Phys. 1993, 98, 678. (47) Balzer, F.; Gerlach, R.; Polanski, G.; Rubahn, H. G. Chem. Phys. Lett. 1997, 274, 145. (48) Camillone, N.; Leung, T. Y. B.; Schwartz, P.; Eisenberger, P.; Scoles, G. Langmuir 1996, 12, 2737.
Langmuir, Vol. 19, No. 15, 2003 6057
Figure 1. Schematic diagram of a walker-type scanning head and sample holder assembly.
decreases as a result of thermal desorption processes, a variety of less densely packed structures have been observed in our studies. New structures, including one striped lattice, two mesh-like structures, and four intermediately mixed stripes, have been revealed in these systematic annealing studies of decanethiol SAMs on Au(111). In addition, systematic I-Z measurements help to determine molecular orientations. Coverage-dependent structural characterization at the molecular level deepens our understanding of intermolecular and moleculesurface interactions, involving relatively large molecules such as long-chain alkanethiols. Experimental Section Commercially available decanethiol (Aldrich, 98% purity) was used without further purification. Au(111) thin films were prepared in a high vacuum evaporator (Denton 502A). The evaporation rate was 2.2 ( 0.2 Å/s, and the mica substrate was maintained at 600 K during Au deposition. After evaporation, the films were annealed at 600 ( 5 K to yield relatively large Au(111) terraces, 100-200 nm in lateral dimensions according to our STM and atomic force microscopy measurements.49,50 The terraces exhibit the well-known (23 × x3) Au reconstruction.51,52 SAMs on Au(111) thin films were prepared by soaking freshly evaporated gold thin films in a 1 mM thiol solution in ethanol for at least 24 h. For STM studies, the SAMs were taken from the solution, washed sequentially with ethanol and hexane, and then immediately transferred to the UHV STM chamber. A variable-temperature STM microscope (STM100, RHK Technology, Inc.) was employed for imaging. The STM microscope has a “walker-type” scanner (as is shown in Figure 1) and is operated under UHV conditions. A filament heater is mounted underneath the sample stage for annealing, and the final temperatures (300-420 K) were adjusted by varying the current. Temperatures were monitored using two thermocouples, one installed directly underneath the sample to measure the surface (49) Xu, S.; Cruchon-Dupeyrat, S. J. N.; Garno, J. C.; Liu, G. Y.; Jennings, G. K.; Yong, T. H.; Laibinis, P. E. J. Chem. Phys. 2000, 113, 9357. (50) Yang, G. H.; Qian, Y. L.; Engtrakul, C.; Sita, L. R.; Liu, G. Y. J. Phys. Chem. B 2000, 104, 9059. (51) Woll, C.; Chiang, S.; Wilson, R. J.; Lippel, P. H. Phys. Rev. B 1989, 39, 7988. (52) Chambliss, D. D.; Wilson, R. J.; Chiang, S. J. Vac. Sci. Technol., B 1991, 9, 933.
6058
Langmuir, Vol. 19, No. 15, 2003
Qian et al.
Figure 2. STM topographs of decanethiol SAMs on Au(111) at saturation coverage: (A) a 100 × 100 nm2 scan of a freshly prepared SAM, (B) a freshly prepared SAM annealed at 325 K for 6 h and imaged at room temperature, and (C) one after annealing at 340 K for over 6 h. Parts A′-C′ are zoomed-in images of parts A-C, respectively. The periodicity of (x3 × x3)R30° (Φ phase) is visible in part A′, and the (4x3 × 2x3)R30° superstructure is shown in parts B′ and C′. Imaging conditions: -1.2 V, 75 pA for parts A and A′; 1.2 V, 20 pA for parts B-C′. temperature, as is indicated in Figure 1, and the other attached to the sample stage to monitor heat transfer (not included in Figure 1). For the investigations reported in this paper, the SAMs were heated from room temperature to the desired annealing temperature, at a rate of about 1 K/min. After reaching the chosen temperature and annealing for a desired time period, the samples were cooled for 2-5 h to room temperature for STM imaging. The STM tips used for this study were prepared by mechanically cutting tungsten wires, followed by electrochemical etching.53,54 A home-constructed electrochemical potentiostat was used to control the redox reaction time and current to avoid overetching. The potentiostat monitors the current and automatically halts the etching process at the moment when the current drops suddenly as a result of the breakage of the wire. Typical etching conditions were 2.1 V in a 3 M KOH solution. During imaging, the background pressure in the UHV chamber was maintained at 3 × 10-10 Torr. The conditions for imaging decanethiol SAMs ranged from (1 to (8 V bias voltage and 1-100 pA tunneling current. All images reported in this work were acquired in a high-impedance, constant-current mode. The relative tip-sample separation (Z) was determined from current-distance [I-Z or ln(I)-Z] measurements. The tipmethyl termini contact point is identified easily from I-Z curves because the slopes change when contact occurs. The actual separation can be calculated according to the approaching distance from the designated position to the contact point, prior to contact. We denote the Z excursion between the first contact point and the final position as the penetration distance. Such spectroscopic measurements facilitate the determination of the molecular orientation for various phases of SAMs.
Results and Discussions On the basis of experimental observations, the evolution of structures can be classified into three stages. In the (53) Zhang, R.; Ivey, D. G. J. Vac. Sci. Technol., B 1996, 14, 1. (54) Lein, M.; Schwitzgebel, G. Rev. Sci. Instrum. 1997, 68, 3099.
first stage, when the annealing temperature is below 345 K, no significant changes in coverage are observed, even with annealing durations of 24 h in an UHV. A second stage refers to annealing temperatures from 345 to 400 K, under which decanethiol monolayers melt and molecules start to desorb from the surface. The surface packing density of decanethiols decreases, revealing diverse structures. A third stage becomes apparent above 400 K, when significant desorption exposes large portions of the Au(111) substrate. 1. Molecular Structure of Decanethiol SAMs at Saturation Coverage. An STM topograph of a freshly prepared C10S/Au(111) monolayer at room temperature is shown in Figure 2A. The surface consists of ordered domains (3-15 nm in size) punctuated by dark pits and domain boundaries. The dark pits are 0.23 ( 0.02 nm deep with lateral dimensions ranging from 1 to 5 nm. The depth corresponds well with the monatomic step height of Au(111) surfaces. Within ordered domains, decanethiol molecules are closely packed, forming the well-known commensurate (x3 × x3)R30° structure with respect to the Au(111) substrate. A zoomed-in view is shown in Figure 2A′. Molecular-level defects are present at the domain boundaries. The morphology observed is consistent with previous STM studies.20 Following the notations used by Poirier,32 φ is used to describe the (x3 × x3)R30° lattice with saturation coverage of C10S/Au(111). Figure 2B shows a decanethiol SAM annealed at 325 K for 6 h. Changes in coverage and periodicity are not observed for these mild annealing conditions. However, the surface morphology differs from that in Figure 2A because of the coarsening of domain boundary networks and the ripening of Au vacancy islands. The total number of vacancy islands clearly decreased, and the average
Decanethiol Self-Assembled Monolayers on Au(111)
Langmuir, Vol. 19, No. 15, 2003 6059
Figure 3. Conformation of the (7.5 × x3) striped phase (δ phase). (A) Coexistence of c(4x3 × 2x3)R30° (Φ phase) and the (7.5 × x3) striped phase (δ) in an 80 × 80 nm2 area, after annealing at 350 K for 6 h. (B) A high-resolution image (8 × 8 nm2) reveals the molecular packing and interstripe distance. (C) Top and side views of the model for the (7.5 × x3) striped phase. The open circles represent Au, and the filled circles represent S atoms in all models hereafter. The hexagonal h(5x3 × x3)R30° and the centered rectangular c(15 × x3) unit cells are indicated in parts B and C. All images were acquired at 1.2 V and 30 pA.
domain size increased at the expense of smaller domains. Domain boundaries, as are observed in Figure 2A, appear as dark or depression lines.19 The average width of the depression lines in Figure 2A,B is approximately 2 nm. The coverage of decanethiol adsorbates at domain boundaries is typically smaller compared with the neighboring densely packed domains. The previously reported c(4x3 × 2x3)R30° superlattice of the basic (x3 × x3)R30° periodicity (Figure 2B′) can be readily observed on these domains.27,29 True molecular resolution is demonstrated by simultaneously resolving the periodicity and molecular-level defects. These SAMs are considered clean and well-ordered for subsequent annealing experiments. Annealing of decanethiol SAMs just above the twodimensional melting temperature of 345 ( 5 K results in the formation of large domains with the c(4x3 × 2x3)R30° structure, which are almost free of pits and defects. Areas as large as 80 × 80 nm2 are shown in Figure 2C,C′ with a single c(4x3 × 2x3)R30° domain as large as the substrate terraces. We refer to this structure as phase φ′, which indicates the same structure as that in φ (Figure 2A) but with a different morphology due to the dramatic decrease in surface defects. The molecular surface density corresponds to 0.216 nm2 per molecule. The packing density in the defect-free areas represents the saturation coverage for alkanethiol SAMs (including decanethiol) on Au(111); therefore, this serves as a reference (packing density ) 1) for subsequently observed phases. 2. Striped Structures. At lower packing densities caused by thermal desorption processes, striped phases of alkanethiol SAMs on Au(111) appear. Low-density structures can be prepared by various methods, such as gas-phase deposition,32,48 brief immersion in dilute solutions (
