Nanoscale observation of dissociative adsorption during self ...

1 downloads 0 Views 1MB Size Report
Alternatively, if the dissociative adsorption of disul- fides on gold takes place, what is the adsorption behavior of thiol moieties after the S-S bond cleavage on the ...
RIKEN Review No. 37 (July, 2001): Focused on Nanotechnology in RIKEN I

Nanoscale observation of dissociative adsorption during self-assembly processes of dialkyl disulfides on Au(111) Jaegeun Noh and Masahiko Hara Local Spatio-Temporal Functions Laboratory, Frontier Research System, RIKEN

The formation of striped phases of dialkyl disulfides on Au(111) and graphite has been monitored by scanning tunneling microscopy (STM). STM imaging clearly exhibits the formation of striped phases having corrugation periodicities that are nearly consistent with the molecular length of alkanethiolate moieties formed after the S-S bond cleavage of dialkyl disulfide on a gold surface. This is the first direct observation of the dissociative adsorption of disulfides on the nanometer scale. Self-assembled monolayers (SAMs) of dialkyl disulfide on a graphite surface displayed longrange, well-ordered monolayers with one striped pattern that shows periodicity as a function of molecular length via nondissociative adsorption. In this study, we have demonstrated a new, simple and insightful method of comparing monolayers chemisorbed on Au(111) with those physisorbed on graphite.

Introduction During the past decade, organic self-assembled monolayers (SAMs) prepared from organosulfur compounds on metal surfaces have drawn great interest because of the possibility of various technical applications such as in nanolithography, molecular recognition, corrosion inhibition, and nanoparticles. 1,2) In particular, SAMs derived from alkanethiols and dialkyl disulfides on gold have been extensively studied because of the formation of highly ordered and densely packed monolayers and their high stability.3,4) It is generally believed that SAMs made from both precursors form identical species that are adsorbed as alkanethiolates at the threefold hollow sites of a Au(111) surface. 5,6) However, it has also been reported that SAMs formed on gold may exist as a dimerlike form of sulfur head groups rather than as gold-bound alkanethiolates.7,8) In spite of a number of such studies, there remain some questions concerning the adsorption processes and states of dialkyl disulfide SAMs on gold. For examples, does the S-S bond cleavage of disulfide really occur in the course of the formation of monolayers, and thereafter, do monolayers consist of gold-bound thiolates or a dimerlike form of sulfur head groups? Alternatively, if the dissociative adsorption of disulfides on gold takes place, what is the adsorption behavior of thiol moieties after the S-S bond cleavage on the gold surface? To date, the bond cleavage adsorption of disulfide on gold has usually been studied by macroscopic tools such as contact angle measurements, X-ray photoelectron spectroscopy (XPS), high-resolution electron energy loss spectroscopy (HREELS), time-of-flight secondary ion mass spectroscopy (TOF-SIMS), and Fourier transform infrared spectroscopy (FT-IR). On the other hand, although scanning probe microscopy (SPM) is a very powerful tool for obtaining information on molecular behavior and the surface structures of adsorbates on substrates, no clear evidence has been identified by SPM regarding this matter. The main object of our study is to clearly elucidate the adsorption processes of dialkyl disulfide from the nanoscopic

54

viewpoint using scanning tunneling microscopy (STM). A number of earlier studies have attempted to resolve this problem using dialkyl disulfides with STM and atomic force microscopy (AFM); these works were mainly performed on the fully covered monolayers derived from dialkyl disulfides on the gold surface.9 ) However this presents a limitations in clarifying this problem because the exchange process, in which the shorter alkyl chain thiolates to longer alkyl chain ones during the increasing surface coverage, the lateral diffusion rate of adsorbed molecules on the surface, and the interaction differences between adsorbing molecules and the solvent induced by two different alkyl parts, can affect the SAM formation. 10,11) Therefore the most important task is to overcome these experimental restraints. To eliminate them, we have attempted to observe striped phases of dialkyl disulfides formed during the initial growth stage of SAMs based on the following logic. It is well known that the periods of striped phases formed on gold depend strongly on the overall length of the molecule used for the preparation of the SAMs. 12–18) From investigations of the periods and patterns as well as the structures of striped phases, we can readily discuss various fundamental aspects of organic molecules on substrates. This is the main concept of this study. To interpret the striped phases of dialkyl disulfides formed on gold more precisely, we have also observed striped phases formed by physisorption on graphite.

Experiment We used an asymmetric 11-hydroxyundecyl octadecyl disulfide (CH3 (CH2 )17 SS(CH2 )11 OH, HUOD) and a symmetric dioctadecyl disulfide ((CH3 (CH2 )17 )2 S2 , DODS). These two compounds were recrystallized several times from diethyl ether for further purification, and the purity was confirmed by mass spectrometry. Au(111) substrates were prepared by vacuum deposition of approximately 100 nm of gold film onto freshly cleaved mica sheets prebaked at 320◦ C under a pressure of 10−7 –10−8 Torr prior to the deposition of the gold. After deposition, the substrates were annealed at 350◦ C in a vacuum chamber for 2 h to obtain large, flat, single-crystal

terraces. The STM images showed the herringbone reconstruction characteristic of clean Au(111) surfaces on 100– 300 nm single-crystal domains. The SAMs were formed by immersing the gold substrates in a freshly prepared 0.25 µM ethanol solution of HUOD and a diethyl ether solution of DODS. All STM images were obtained at room temperature with a Pt/Ir tip using the constant current mode in air. Bias voltages (Vb ) between 1.5 and −1.5 V and tunneling currents (It ) ranging from 0.12 to 0.5 nA were applied between the tips and samples. STM imaging of the HUOD and DODS molecules physisorbed at the liquid/graphite interface was performed in the same manner as described in previous reports.19–21)

domains. Striped phases similar to Fig. 1(c) were reported in SAMs of alkanethiols on graphite surfaces.21,22) In the case of akanethiols on gold, it was often observed that striped phases exhibit corrugation periods with twice the overall length of the molecules because of the head-to-head orientation. 13,14) These results indicate that the molecules may be adsorbed as disulfides rather than as gold-bound thiolates.7,8) On the other hand, high-resolution STM imaging for such a striped phase suggests that alkanethiol molecules are bound as thiolates rather than adsorbed as disulfides.23) However, problems concerning adsorption states on gold have been continually encountered until now. These problems must be clarified for both technical applications and an understanding of the fundamental aspects of surface science, which have been strongly associated with the stability of SAMs.

Results and discussion Striped phases, in which the molecular axis is lying flat on the surface, were observed in the self-assembly processes of sulfurcontaining organic molecules such as alkanethiols on gold. To monitor the formation of striped phases on gold, we must investigate the initial process of SAM growth because these phases usually form during the initial self-assembly. In addition, such phases can be observed in SAM samples obtained in extremely dilute solutions containing target molecules. Therefore, to observe striped phases on the gold surface, we used a 0.25 µM solution and a short dipping time of less than 3 min. Figure 1 depicts three possible patterns of striped phases of dialkyl disulfides on a substrate. The striped phase shown in Fig. 1(a) can only be observed in the case of the nondissociative adsorption of these molecules. In this case, a corrugation period corresponding to the overall length of a molecule can be observed in this striped phase. This value was readily measured by STM imaging due to the markedly enhanced contrast of the sulfur atoms relative to that of the methyl groups. 19–21) The final molecular features derived from the dissociative adsorption of dialkyl disulfides on gold are identical to those of alkanethiols, as previously reported. 3) The R-S moieties produced by the S-S bond cleavage of dialkyl disulfides on gold can give rise to striped phases where the corrugation periods are one-half of the original molecular length due to the head-to-tail orientation, as delineated in Fig. 1(b). Additionally, dissociative adsorption can be observed in the form of a mixed striped phase, as shown in Fig. 1(c) as well as in Fig. 1(b), resulting in phase-separated

Fig. 1. Three possible patterns of striped phases formed during the selfassembly of dialkyl disulfides on a substrate: (a) a striped phase after the nondissociative adsorption of dialkyl disulfides, and (b) and (c) striped phases after the dissociative adsorption of dialkyl disulfides.

The STM image in Fig. 2 clearly exhibits a disordered phase and two types of striped-phase domains with phase separation from the HUOD SAM sample on gold after 3 min deposition. The corrugation periodicities of the striped phase in regions A and B are 1.80 and 2.53 nm, which shows good agreement with the length of HO(CH2 )11 S and CH3 (CH2 )17 S molecules, respectively. In this case, the striped pattern and period are consistent with the model in Fig. 1(c). As already mentioned using this model, this result strongly supports the dissociative adsorption of HUOD molecules. To begin with, in order to interpret the origin of these two striped phases more carefully, the orientation of the two alkyl chains attached to the disulfide group on the surface must be considered. It was revealed that the molecular axis of didococyl disulfide physisorbed on the graphite surface is linear and that the periodicity of the striped phase corresponds to the length of the molecules. 22) This orientation is energetically much more favorable than the face-to-face orientation between two alkyl chains in a molecule, because the face-to-face orientation between two alkyl chains can induce mutual steric repulsion and large strain in a C-S-S bond angle. In fact, as shown in Fig. 3, observation of paired lines in molecular arrangements of HUOD molecules on the graphite surface reveals that the molecular axis is nearly linear, as in the case of didococyl disulfide. From these viewpoints, two striped phases, showing that the corrugation periodicity is nearly consistent with the length of HO(CH2 )11 S and CH3 (CH2 )17 S molecules, observed on Au(111) could not be found without

Fig. 2. STM images of 11-hydroxyundecyl octadecyl disulfide (HUOD) SAMs on Au(111) obtained after 3 min deposition in 0.25 µM ethanol solutions of HUOD. Two phase-separated domains (A and B) formed as the result of the S-S bond cleavage of disulfide were clearly observed in this image; 106 × 106 nm2 , It = 0.21 nA, and Vb = 0.50 V.

55

Fig. 3. STM images exhibiting the striped phase of HUOD physisorbed on graphite. Bright rows correspond to the disulfide groups. Molecular arrangements were strongly controlled by the hydrogen bond derived from the hydroxyl group positioned at the end of the shorter alkyl chain among the two different alkyl chain lengths attached to the disulfide group. (a) 150 × 150 nm2 , It = 0.12 nA, and Vb = 1.5 V. (b) 15 × 15 nm2 , It = 0.12 nA, and Vb = 1.20 V.

the bond cleavage of the disulfide group. This result strongly implies that the S-S bond cleavage of asymmetric disulfide occurs when the HUOD molecules adsorb on the surface during the reaction with gold at room temperature, and that the direct adsorption of the original disulfide cannot take place on the surface during the SAM formation. To confirm our result of S-S bond breaking on Au(111), we have examined molecular arrangements of HUOD molecules physisorbed on graphite. The main difference in the selfassembly process of HUOD molecules on gold and graphite involves the interactions (i.e., chemisorption or physisorption) between molecules and substrate. Therefore, organic molecules on a graphite surface usually lie flat on the surface similar to striped phases observed on gold, and mainly follow the striped pattern of model a in Fig. 1. The STM image of a large scan area in Fig. 3(a) shows well-ordered molecular arrangements of HUOD molecules that orient parallel to the graphite surface. The high-resolution STM image in Fig. 3(b) displays individual disulfide groups indicated by spots in bright lines and an approxiamtely 23◦ tilted orientation of alkyl chains when compared to the linear molecular axis of didococyl disulfide. The bright lines corresponding to the disulfide group in this STM image may be due to the large molecular polarizability and the increase of the local density of states (LDOS) near the Fermi level of the surface. 22,24) Paired bright lines in this image are formed by a hydrogen bond with a hydroxyl group facing another hydroxyl group of adjacent molecules. Here, two longer alkyl moieties are positioned in the large dark area (region A) between two bright

56

Fig. 4. STM images showing striped phases of dioctadecyl disulfide (DODS) SAMs on Au(111) obtained after 3 min deposition. (a) 110 × 110 nm2 , It = 0.18 nA, and Vb = 0.48 V. (b) 30 × 30 nm2 , It = 0.15 nA, and Vb = 0.42 V.

lines, and the two shorter hydroxylated alkyl moieties are located in the small dark area (region B). From this molecular arrangement, it is clear that the self-assembly process of HUOD molecules physisorbed on graphite is mainly governed by a hydrogen bond without any S-S bond cleavage of the disulfide group, unlike the adsorption process of HUOD molecules on gold. The STM images in Fig. 4 were obtained from SAM samples adsorbed on Au(111) after 3 min deposition in a 0.25 µM diethyl ether solution of DODS. Figures 4(a) and (b) show striped phases (region A) obtained from 110 × 110 nm2 and 30 × 30 nm2 scan areas, respectively. The corrugation period in these striped phases is 2.55 nm, which is nearly consistent with the molecular length of octadecanthiolate molecules adsorbed on a gold surface after the S-S bond cleavage of dioctadecyl disulfide. The striped pattern and period are consistent with the model in Fig. 1(b). As already explained using this model, this result strongly suggests the dissociative adsorption of dioctadecyl disulfides. If there was no bond rupture of the disulfide, such striped phases would not be formed on the gold surface. In most cases, we observed mainly such striped phases. The disordered phase in region B was usually observed during the self-assembly of alkanethiols and dialkyl disulfides on gold and was regarded as a phase transition from a striped phase to a two-dimensionally grown upright phase as surface coverage increased. Infrequently, two phaseseparated stripes were observed, showing the adsorption patterns of model c in Fig. 1. From the investigation of striped phases in dioctadcyl disulfide SAMs, we confirmed the dissociative adsorption of dialkyl disulfides on gold surfaces from the nanoscopic viewpoint. In fact, our STM results are in

Conclusions We have observed striped phases formed during self-assembly processes of 11-hydroxyundecyl octadecyl disulfide (HUOD) and dioctadecyl disulfide (DODS) on both gold and graphite surfaces using scanning tunneling microscopy (STM). From our STM results, we confirmed for the first time from the nanoscopic viewpoint that the formation of dialkyl disulfide SAMs on gold proceeds via a dissociative adsorption process without direct adsorption of the disulfide group on the gold surface, as revealed mainly by macroscopic techniques. We believe that observation of striped phases with a welldesigned model compound will provide new guidelines to examine and interpret new fundamental aspects such as surface reactions, interactions and orientations of molecules, as well as the monitoring of target molecules on a surface, as demonstrated in our study.

References

Fig. 5. STM image showing a short- and long-range ordering and three distinct domain orientations of DODS SAMs physisorbed on graphite. (a) 150 × 150 nm2 , It = 0.15 nA, and Vb = −1.50 V. (b) 20 × 20 nm2 , It = 0.14 nA, and Vb = −1.25 V.

accord with previous studies performed by macroscopic techniques.3,5,25) Again, to confirm the dissociative adsorption of dialkyl disulfides on gold, we have attempted to observe striped patterns of DODS molecules adsorbed on graphite surfaces. The STM images in Fig. 5 exhibit well-ordered monolayers formed by DODS molecules physisorbed on graphite. Figure 5(a) shows three distinct domain orientations of 60 or 120◦ respective to each domain with a short- and long-range ordering. The molecular axis is oriented parallel to the graphite surface and the corrugation period is 4.57 nm, which is slightly smaller than the length of DODS molecules because the axis of the molecule lying flat on the graphite surface is somehow bent at a disulfide bridge, as shown in Fig. 5(b). As we would expect, disordered phases and striped phases whose period is one-half the length of a DODS molecule were not observed in this monolayer which originated primarily from physisorption interactions. Actually, the STM image of DODS SAMs on graphite is identical to the model in Fig. 1(a) because the SAMs formed through nondissociative adsorption as opposed to the dissociative adsorption of disulfide on gold. From the nanoscopic viewpoint, we have successfully elucidated the adsorption processes of dialkyl disulfides during SAM formation on gold and graphite surfaces.

1) K. Motesharei and D. C. Myles: J. Am. Chem. Soc. 120, 7328 (1998). 2) H. X. He, H. Zhang, Q. G. Li, T. Zhu, S. F. Y. Li, and Z. F. Liu: Langmuir 16, 3846 (2000). 3) R. G. Nuzzo, B. R. Zegarski, and L. H. Dubois: J. Am. Chem. Soc. 109, 733 (1987). 4) G. E. Poirier and M. J. Tarlov: Langmuir 10, 2853 (1994). 5) C. D. Bain, H. A. Biebuyck, and G. M. Whitesides: Langmuir 5, 723 (1989). 6) B. Hagenhof, A. Benninghoven, J. Spinke, M. Liley, and W. Knoll: Langmuir 9, 1622 (1993). 7) P. Fenter, A. Eberhardt, and P. Eiesenberger: Science 266, 1216 (1994). 8) N. Nishida, M. Hara, H. Sasabe, and W. Knoll: Jpn. J. Appl. Phys. 35, 5866 (1996). 9) T. Ishida et al.: Langmuir 13, 3261 (1997). 10) K. Kajikawa, M. Hara, H. Sasabe, and W. Knoll: Jpn. J. Appl. Phys. 36, L1116 (1997). 11) J. P. Folkers, P. E. Laibinis, G. M. Whitesides, and J. Deutch: J. Phys. Chem. 98, 563 (1994). 12) N. Camillone III, T. Y. B. Leung, P. Schwartz, P. Eisenberger, and G. Scoles: Langmuir 12, 2737 (1996). 13) G. E. Poirier and E. D. Pylant: Science 272, 1145 (1996). 14) R. Yamada and K. Uosaki: Langmuir 14, 855 (1998). 15) J. Noh and M. Hara: Langmuir 16, 2045 (2000). 16) J. Noh, T. Murase, K. Nakajima, H. Lee, and M. Hara: J. Phys. Chem. B 104, 7411 (2000). 17) J. Noh and M. Hara: Mol. Cryst. Liq. Cryst. 349, 223 (2000). 18) G. E. Poirier: Langmuir 15, 1167 (1999). 19) B. Venkataraman, J. J. Breen, and G. W. Flynm: J. Phys. Chem. 99, 6608 (1995). 20) J. Noh, D. Lee, M. Hara, H. Lee, H. Sasabe, and W. Knoll: Jpn. J. Appl. Phys. 38, 3897 (1999). 21) C. L. Claypool, F. Faglioni, W. A. Gorddard III, H. B. Gray, N. S. Lewis, and R. A. Marcus: J. Phys. Chem. B 101, 5978 (1997). 22) A. P. Gunning, A. R. Kirby, X. Mallard, and V. J. Morris: J. Chem. Soc., Faraday Trans. 90, 2551 (1994). 23) R. Staub, M. Toerker, T. Fritz, T. Schmitz-Hubsch, F. Sellam, and K. Leo: Langmuir 14, 6693 (1998). 24) L. Wan, Y. Hara, H. Noda, and M. Osawa: J. Phys. Chem. B 102, 5943 (1998). 25) H. A. Biebuyck and G. M. Whitesides: Langmuir 9, 1766 (1993).

57