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Structural evolution of dibutyldisulfide adsorbed on Au(1 1 1)
Applied Surface Science 169±170 (2001) 100±103
Structural evolution of dibutyldisul®de adsorbed on Au(1 1 1) Tomohiro Hayashia, Chitose Kodamaa, Hisakazu Nozoyea,b,* a
Institute of Materials Science, University of Tsukuba, 1-1 Higashi Tsukuba 305-8565, Japan National Institute of Materials and Chemical Research, 1-1 Higashi Tsukuba 305-8565, Japan
b
Received 2 August 1999; accepted 29 October 1999
Abstract Self-assembling behavior of dibutyldisul®de (DBDS: CH3(CH2)3SS(CH2)3CH3) molecules on Au(1 1 1) was investigated with a combined system of high-resolution electron energy loss spectroscopy (HREELS) and scanning tunneling microscopy (STM). We observed two stripe phases with different stripe periodicities with STM, depending on the density of DBDS on the surface for the ®rst time. At very low exposure of DBDS (0.5 L), STM showed that DBDS formed fragmentary chains and the periodicity of the stripe chains was about 2.1 nm. At exposure of 5 L, the periodicity of the stripe chains converged to 1.38 nm. In this phase, DBDS is lying down on the surface and the C±C±C plane of the alkyl isparallel to the surface. At pchain p the saturation coverage, the molecular structure changes to a densely packed c(4 3 2 3) structure, that is the p p superstructure of 3 3R30 , containing bright and dark spots. # 2001 Published by Elsevier Science B.V. Keywords: HREELS; STM; Gold; SAM; Thiol; Disul®de
1. Introduction Among many processes to form ultrathin organic ®lms, self-assembling of thiol or disul®de molecules on various single crystal surfaces gain considerable attention, because they form periodic superstructures spontaneously using the regular arrays of surface lattices as molds. Self-assembled monolayers (SAM) have been expected to be applied to new electric device materials, molecular recognition sensors, nonlinear optical materials and so on [1,2]. Many studies on SAM have been carried out with scanning tunneling microscopy (STM) [3], X-ray diffraction [4], He atom diffraction [5], and infrared
*
Corresponding author. Tel.: 81-298-54-4527; fax: 81-298-54-4504. E-mail address: [email protected] (H. Nozoye).
(IR) spectroscopy [6] and so on. However, the most fundamental problems about SAM remain unresolved, such as the location of sulfur atoms, the structure of SAM and, especially, the self-assembling processes. Many STM studies have been devoted to analyze the structure of SAM. Only with STM images, however, we cannot discuss the details of the structure of SAM such as an adsorption state of an individual molecule, because STM images are determined by the interaction of the electronic structures of a surface and an STM tip. Therefore, we have applied a combined system of STM and high-resolution electron energy loss spectroscopy (HREELS) to study SAM. There are two main processes in electron scattering, that is, the dipole scattering and the impact scattering. HREELS enables us to observe IR active as well as IR inactive vibration modes; the former is observed by the dipole scattering and the latter is observed by the impact scattering mechanism. The impact scattering
0169-4332/01/$ ± see front matter # 2001 Published by Elsevier Science B.V. PII: S 0 1 6 9 - 4 3 3 2 ( 0 0 ) 0 0 6 4 7 - 4
T. Hayashi et al. / Applied Surface Science 169±170 (2001) 100±103
gives relatively homogeneous distribution of scattered electrons, on the contrary, the dipole scattering gives specular distribution of scattered electrons. Therefore, in the specular geometry, we can observe exclusively the modes whose transition dipoles are perpendicular to the surface. In addition, the scanning range of HREELS covers low frequency range (<800 cmÿ1) which is usually not accessible to IR spectroscopy [7]. In this paper, we discuss the adsorption states and structural evolution of dibutyldisul®de (DBDS: CH3(CH2)3SS(CH2)3CH3) on Au(1 1 1) from the results of HREELS and STM. 2. Experimental All experiments were carried out in an ultrahigh vacuum chamber (base pressure < 2:0 10ÿ8 Pa). This chamber consists of an HREELS (Vacuum Science Instruments) chamber, a preparation chamber, and a load-lock chamber (base pressure < 3:0 10ÿ7 Pa) for rapid sample introduction. The preparation chamber is equipped with low-energy electron diffraction (LEED) facilities, a CMA analyzer for Auger electron spectroscopy (AES), an ion gun, quadrupole mass spectrometer (Q-Mass), and STM (JEOL, JSTM-4500XT) A single crystal gold substrate (7 mm 5 mm and thickness: 0.7 mm) was cleaned in situ by cycles of Ar sputtering at 700 K for 20 min and annealing at 800 K for 15 min followed by slow cooling. Cleanliness of the specimen p was checked by AES and the Au(1 1 1) 23 3 herringbone reconstruction feature in STM images. DBDS (95% purity TCI Co., Ltd.) was degassed by several freeze-pump-thaw cycles prior to use. Gas exposure was conducted in the load-lock chamber through a variable leak valve at room temperature. The amount of exposure which is expressed in Langmuir units (1 L 1:3 10ÿ4 Pa s) controlled by the pressure of DBDS in the load-lock chamber and exposure time. The pressure was monitored by an ion gauge without normalizing for the ion sensitivity of DBDS. All measurements of STM and HREELS were carried out at room temperature. An STM tip was made of polycrystalline tungsten. STM images were collected in the constant height mode.
101
For comparative purpose, similar measurements for diamyldisul®de (DADS: CH3(CH2)4SS(CH2)4CH3) were conducted. 3. Results and discussion Fig. 1 shows STM images of DBDS on Au(1 1 1) at different exposures: (a) 0.5 L; (b) 5 L; and (c) 5000 L. In Fig. 1(a), DBDS started self-assembling with forming a stripe structure in the direction of h1ÿ21i. Distances between stripes varied from 2.0 to 2.5 nm and bright spots were observed along the stripes every 0.5 nm. The stripes were fragmentary with length of 5.8±6.5 nm. This periodicity was equal to the pnearly unit cell of a clean surface: the 3 23 herringbone structure. In the region, where the stripes were not observed, a two-dimensional disordered phase and imperfect herringbone reconstruction structures were observed in the STM images. Above 5 L, distances between each stripe were ®xed to 1.38 nm. In the case of DADS, the nascent periodicity of stripes was 2.1 nm and they converged to 1.65 nm at 10 l (data are not shown.). This result is similar to the report by Camillone et al. in which they observed different periodicities of stripes depending on the density of hexanethiol molecules on Au(1 1 1) from the results of He scattering measurements (the periodicities of stripe are 2.16 and 2.3 nm) [8]. When an additional 50 L of was exposed, the stripe structures turned to a disordered phase. At 100 L, p p c(4 3 2 3) islands began to grow from the disordered phase. Above p2000pL, the surface was fully covered with the c(4 3 ) structure. p2 3p p This structure is the superlattice of 3 3 2 3, containing twopbright spots p and two dark spots in a unit cell. The c(4 3 2 3) structure of DBDS is very similar to that of thiols on Au(1 1 1) [9,10]. It has been suggested that difference of the contrast of the spots is due to the different twisting angles of the C±C±C plane of an alkyl chain [5]. Fig. 2 shows HREEL spectra in the specular scattering geometry at different exposures of DBDS. The primary energy of an electron beam was ®xed at 4 eV. In order to give assignments to each peak, the results of IR measurements of n-hexadecane thiol on Au(1 1 1) were referred [6]. The low, vibrational frequency modes of a butanethiol molecule were
102
T. Hayashi et al. / Applied Surface Science 169±170 (2001) 100±103
Fig. 1. (a) An STM image measured at 0.5 L (image size: 88 nm 88 nm, sample voltage V s 2:0 V, and tunneling current I t 0:3 nA). A magni®ed image of the stripe structure (image size: 12 nm 12 nm, V s 1:0 V and I t 0:3 nA) is shown at right bottom of the STM image. The periodicity of the stripes is about 2.1 nm; (b) an STM image at 5 L (image size: 11 nm 11 nm, V s 2:0 V, and I t 0:3 nA). The periodicity of stripe is 1.38 nm; (c) an STM image at 5000 L (image size: 14 nm 4 nm, V s 2:0 V, and It 0:5 nA). A unit cell of the c(4 2) structure is indicated by a rectangular box.
calculated by using Gaussian 94 (method: density functional theory (BLYP), basis set: 6±31G). The assignments for each peak is as follows: the C±S stretch (653 cmÿ1), the CH2 rock (722 and 776 cmÿ1 (the transition dipole of the CH2 rock at
776 cmÿ1 is very small compared with that of the mode at 722 cmÿ1)), the CH2 wag (893 and 1285 cmÿ1), the CH2 twist (1052 cmÿ1), the CH3 rock (1180 cmÿ1), the CH3 symmetric deform (1370 cmÿ1), and the CH2 scissors (1479 cmÿ1).
T. Hayashi et al. / Applied Surface Science 169±170 (2001) 100±103
Fig. 2. HREEL spectra measured at different exposures ((a) 0.5 L; (b) 5 L; and (c) 5000 L) in the specular geometry (qi qs 55 with respect to the surface normal). Each spectrum is normalized with the elastic peak. Full widths of half-maximum of elastic peaks are (a) 36 cmÿ1; (b) 24 cmÿ1; and (c) 24 cmÿ1.
Fig. 2(a) was measured at exposure of 0.5 L. At this exposure, the re¯ectivity of the electron beam from the surface was very low. The resolution of the electron beam was degraded in energy in order to get an adequate signal to noise ratio. When an additional 5 L of DBDS was exposed, the surface was ordered with forming the stripe phase (periodicity: 1.38 nm) and the re¯ectivity recovered. The intensity of the CH2 rocking mode (722 cmÿ1) became relatively strong compared with other vibration modes. At 5000 L, the relative intensity of the CH2 rocking mode (722 cmÿ1) was weakened, and those of the CH2 twisting mode (1052 cmÿ1) and the CH3 symmetric deformation mode (1370 cmÿ1) became strong. From these results, we suggest a following model: At low exposure (0.5 L), DBDS forms the stripe structure. Although the disordered phase coexists,
103
DBDS is considered to adsorb in a ¯at-lying con®guration from the STM observation. The periodicity of this stripe structure (2.1 nm) is independent of the length of the alkyl chains in the case of DBDS. The relative ratio of the peaks in Fig. 2(a) is different from Fig. 2(b). It is speculated that the orientations of the molecules are random in the disordered phase. At 5 L, the distances between the stripes converge to 1.38 nm. DBDS is ¯at lying on the surface and the C±C±C plane of the alkyl chain is parallel to the surface. This orientation of the alkyl chain is very similar to nalkanes adsorbed on Pt(1 1 1) [11]. At 5000 L, the DBDS stands up on the surface. The model presented here is very consistent with the results of DADS.
References [1] J.D. Swalen, D.L. Allara, J.D. Andrade, E.A. Chandross, S. Garoff, J. Israelachvili, T.J. McCarthy, R. Murray, R.F. Pease, J.F. Rabolt, K.J. Wynne, H. Yu, Langmuir 3 (1987) 932. [2] D.J. Revell, J.R. Knight, D.J. Blyth, A.H. Haines, D.A. Russell, Langmuir 14 (1998) 4517. [3] G.E. Poirier, Chem. Rev. 97 (1997) 1117. [4] P. Fenter, A. Eberhant, P. Eisenberger, Science 266 (1994) 1216. [5] N. Camillone, C.E.D. Chidsey, G.L.G. Scoles, J. Chem. Phys. 98 (1993) 3503. [6] R.G. Nuzzo, L.H. Dudois, D.L. Allara, J. Am. Chem. Soc. 112 (1990) 558. [7] H. Ibach, D.L. Mills, Electron Energy Loss Spectroscopy and Surface Vibrations, Academic Press, New York, 1982. [8] N. Camillone, T.Y.B. Leung, P. Schwartz, P. Eisenberger, G. Scoles, Langmuir 12 (1996) 2737. [9] G.E. Poirier, M.J. Tarlov, Langmuir 10 (1994) 2853. [10] H. Kondoh, C. Kodama, H. Sumida, H. Nozoye, J. Chem. Phys. 111 (1999) 1175. [11] M.A. Chesters, P. Gardner, E.M. McCash, Surf. Sci. 209 (1989) 89.
Structural evolution of dibutyldisul®de adsorbed on Au(1 1 1) Tomohiro Hayashia, Chitose Kodamaa, Hisakazu Nozoyea,b,* a
Institute of Materials Science, University of Tsukuba, 1-1 Higashi Tsukuba 305-8565, Japan National Institute of Materials and Chemical Research, 1-1 Higashi Tsukuba 305-8565, Japan
b
Received 2 August 1999; accepted 29 October 1999
Abstract Self-assembling behavior of dibutyldisul®de (DBDS: CH3(CH2)3SS(CH2)3CH3) molecules on Au(1 1 1) was investigated with a combined system of high-resolution electron energy loss spectroscopy (HREELS) and scanning tunneling microscopy (STM). We observed two stripe phases with different stripe periodicities with STM, depending on the density of DBDS on the surface for the ®rst time. At very low exposure of DBDS (0.5 L), STM showed that DBDS formed fragmentary chains and the periodicity of the stripe chains was about 2.1 nm. At exposure of 5 L, the periodicity of the stripe chains converged to 1.38 nm. In this phase, DBDS is lying down on the surface and the C±C±C plane of the alkyl isparallel to the surface. At pchain p the saturation coverage, the molecular structure changes to a densely packed c(4 3 2 3) structure, that is the p p superstructure of 3 3R30 , containing bright and dark spots. # 2001 Published by Elsevier Science B.V. Keywords: HREELS; STM; Gold; SAM; Thiol; Disul®de
1. Introduction Among many processes to form ultrathin organic ®lms, self-assembling of thiol or disul®de molecules on various single crystal surfaces gain considerable attention, because they form periodic superstructures spontaneously using the regular arrays of surface lattices as molds. Self-assembled monolayers (SAM) have been expected to be applied to new electric device materials, molecular recognition sensors, nonlinear optical materials and so on [1,2]. Many studies on SAM have been carried out with scanning tunneling microscopy (STM) [3], X-ray diffraction [4], He atom diffraction [5], and infrared
*
Corresponding author. Tel.: 81-298-54-4527; fax: 81-298-54-4504. E-mail address: [email protected] (H. Nozoye).
(IR) spectroscopy [6] and so on. However, the most fundamental problems about SAM remain unresolved, such as the location of sulfur atoms, the structure of SAM and, especially, the self-assembling processes. Many STM studies have been devoted to analyze the structure of SAM. Only with STM images, however, we cannot discuss the details of the structure of SAM such as an adsorption state of an individual molecule, because STM images are determined by the interaction of the electronic structures of a surface and an STM tip. Therefore, we have applied a combined system of STM and high-resolution electron energy loss spectroscopy (HREELS) to study SAM. There are two main processes in electron scattering, that is, the dipole scattering and the impact scattering. HREELS enables us to observe IR active as well as IR inactive vibration modes; the former is observed by the dipole scattering and the latter is observed by the impact scattering mechanism. The impact scattering
0169-4332/01/$ ± see front matter # 2001 Published by Elsevier Science B.V. PII: S 0 1 6 9 - 4 3 3 2 ( 0 0 ) 0 0 6 4 7 - 4
T. Hayashi et al. / Applied Surface Science 169±170 (2001) 100±103
gives relatively homogeneous distribution of scattered electrons, on the contrary, the dipole scattering gives specular distribution of scattered electrons. Therefore, in the specular geometry, we can observe exclusively the modes whose transition dipoles are perpendicular to the surface. In addition, the scanning range of HREELS covers low frequency range (<800 cmÿ1) which is usually not accessible to IR spectroscopy [7]. In this paper, we discuss the adsorption states and structural evolution of dibutyldisul®de (DBDS: CH3(CH2)3SS(CH2)3CH3) on Au(1 1 1) from the results of HREELS and STM. 2. Experimental All experiments were carried out in an ultrahigh vacuum chamber (base pressure < 2:0 10ÿ8 Pa). This chamber consists of an HREELS (Vacuum Science Instruments) chamber, a preparation chamber, and a load-lock chamber (base pressure < 3:0 10ÿ7 Pa) for rapid sample introduction. The preparation chamber is equipped with low-energy electron diffraction (LEED) facilities, a CMA analyzer for Auger electron spectroscopy (AES), an ion gun, quadrupole mass spectrometer (Q-Mass), and STM (JEOL, JSTM-4500XT) A single crystal gold substrate (7 mm 5 mm and thickness: 0.7 mm) was cleaned in situ by cycles of Ar sputtering at 700 K for 20 min and annealing at 800 K for 15 min followed by slow cooling. Cleanliness of the specimen p was checked by AES and the Au(1 1 1) 23 3 herringbone reconstruction feature in STM images. DBDS (95% purity TCI Co., Ltd.) was degassed by several freeze-pump-thaw cycles prior to use. Gas exposure was conducted in the load-lock chamber through a variable leak valve at room temperature. The amount of exposure which is expressed in Langmuir units (1 L 1:3 10ÿ4 Pa s) controlled by the pressure of DBDS in the load-lock chamber and exposure time. The pressure was monitored by an ion gauge without normalizing for the ion sensitivity of DBDS. All measurements of STM and HREELS were carried out at room temperature. An STM tip was made of polycrystalline tungsten. STM images were collected in the constant height mode.
101
For comparative purpose, similar measurements for diamyldisul®de (DADS: CH3(CH2)4SS(CH2)4CH3) were conducted. 3. Results and discussion Fig. 1 shows STM images of DBDS on Au(1 1 1) at different exposures: (a) 0.5 L; (b) 5 L; and (c) 5000 L. In Fig. 1(a), DBDS started self-assembling with forming a stripe structure in the direction of h1ÿ21i. Distances between stripes varied from 2.0 to 2.5 nm and bright spots were observed along the stripes every 0.5 nm. The stripes were fragmentary with length of 5.8±6.5 nm. This periodicity was equal to the pnearly unit cell of a clean surface: the 3 23 herringbone structure. In the region, where the stripes were not observed, a two-dimensional disordered phase and imperfect herringbone reconstruction structures were observed in the STM images. Above 5 L, distances between each stripe were ®xed to 1.38 nm. In the case of DADS, the nascent periodicity of stripes was 2.1 nm and they converged to 1.65 nm at 10 l (data are not shown.). This result is similar to the report by Camillone et al. in which they observed different periodicities of stripes depending on the density of hexanethiol molecules on Au(1 1 1) from the results of He scattering measurements (the periodicities of stripe are 2.16 and 2.3 nm) [8]. When an additional 50 L of was exposed, the stripe structures turned to a disordered phase. At 100 L, p p c(4 3 2 3) islands began to grow from the disordered phase. Above p2000pL, the surface was fully covered with the c(4 3 ) structure. p2 3p p This structure is the superlattice of 3 3 2 3, containing twopbright spots p and two dark spots in a unit cell. The c(4 3 2 3) structure of DBDS is very similar to that of thiols on Au(1 1 1) [9,10]. It has been suggested that difference of the contrast of the spots is due to the different twisting angles of the C±C±C plane of an alkyl chain [5]. Fig. 2 shows HREEL spectra in the specular scattering geometry at different exposures of DBDS. The primary energy of an electron beam was ®xed at 4 eV. In order to give assignments to each peak, the results of IR measurements of n-hexadecane thiol on Au(1 1 1) were referred [6]. The low, vibrational frequency modes of a butanethiol molecule were
102
T. Hayashi et al. / Applied Surface Science 169±170 (2001) 100±103
Fig. 1. (a) An STM image measured at 0.5 L (image size: 88 nm 88 nm, sample voltage V s 2:0 V, and tunneling current I t 0:3 nA). A magni®ed image of the stripe structure (image size: 12 nm 12 nm, V s 1:0 V and I t 0:3 nA) is shown at right bottom of the STM image. The periodicity of the stripes is about 2.1 nm; (b) an STM image at 5 L (image size: 11 nm 11 nm, V s 2:0 V, and I t 0:3 nA). The periodicity of stripe is 1.38 nm; (c) an STM image at 5000 L (image size: 14 nm 4 nm, V s 2:0 V, and It 0:5 nA). A unit cell of the c(4 2) structure is indicated by a rectangular box.
calculated by using Gaussian 94 (method: density functional theory (BLYP), basis set: 6±31G). The assignments for each peak is as follows: the C±S stretch (653 cmÿ1), the CH2 rock (722 and 776 cmÿ1 (the transition dipole of the CH2 rock at
776 cmÿ1 is very small compared with that of the mode at 722 cmÿ1)), the CH2 wag (893 and 1285 cmÿ1), the CH2 twist (1052 cmÿ1), the CH3 rock (1180 cmÿ1), the CH3 symmetric deform (1370 cmÿ1), and the CH2 scissors (1479 cmÿ1).
T. Hayashi et al. / Applied Surface Science 169±170 (2001) 100±103
Fig. 2. HREEL spectra measured at different exposures ((a) 0.5 L; (b) 5 L; and (c) 5000 L) in the specular geometry (qi qs 55 with respect to the surface normal). Each spectrum is normalized with the elastic peak. Full widths of half-maximum of elastic peaks are (a) 36 cmÿ1; (b) 24 cmÿ1; and (c) 24 cmÿ1.
Fig. 2(a) was measured at exposure of 0.5 L. At this exposure, the re¯ectivity of the electron beam from the surface was very low. The resolution of the electron beam was degraded in energy in order to get an adequate signal to noise ratio. When an additional 5 L of DBDS was exposed, the surface was ordered with forming the stripe phase (periodicity: 1.38 nm) and the re¯ectivity recovered. The intensity of the CH2 rocking mode (722 cmÿ1) became relatively strong compared with other vibration modes. At 5000 L, the relative intensity of the CH2 rocking mode (722 cmÿ1) was weakened, and those of the CH2 twisting mode (1052 cmÿ1) and the CH3 symmetric deformation mode (1370 cmÿ1) became strong. From these results, we suggest a following model: At low exposure (0.5 L), DBDS forms the stripe structure. Although the disordered phase coexists,
103
DBDS is considered to adsorb in a ¯at-lying con®guration from the STM observation. The periodicity of this stripe structure (2.1 nm) is independent of the length of the alkyl chains in the case of DBDS. The relative ratio of the peaks in Fig. 2(a) is different from Fig. 2(b). It is speculated that the orientations of the molecules are random in the disordered phase. At 5 L, the distances between the stripes converge to 1.38 nm. DBDS is ¯at lying on the surface and the C±C±C plane of the alkyl chain is parallel to the surface. This orientation of the alkyl chain is very similar to nalkanes adsorbed on Pt(1 1 1) [11]. At 5000 L, the DBDS stands up on the surface. The model presented here is very consistent with the results of DADS.
References [1] J.D. Swalen, D.L. Allara, J.D. Andrade, E.A. Chandross, S. Garoff, J. Israelachvili, T.J. McCarthy, R. Murray, R.F. Pease, J.F. Rabolt, K.J. Wynne, H. Yu, Langmuir 3 (1987) 932. [2] D.J. Revell, J.R. Knight, D.J. Blyth, A.H. Haines, D.A. Russell, Langmuir 14 (1998) 4517. [3] G.E. Poirier, Chem. Rev. 97 (1997) 1117. [4] P. Fenter, A. Eberhant, P. Eisenberger, Science 266 (1994) 1216. [5] N. Camillone, C.E.D. Chidsey, G.L.G. Scoles, J. Chem. Phys. 98 (1993) 3503. [6] R.G. Nuzzo, L.H. Dudois, D.L. Allara, J. Am. Chem. Soc. 112 (1990) 558. [7] H. Ibach, D.L. Mills, Electron Energy Loss Spectroscopy and Surface Vibrations, Academic Press, New York, 1982. [8] N. Camillone, T.Y.B. Leung, P. Schwartz, P. Eisenberger, G. Scoles, Langmuir 12 (1996) 2737. [9] G.E. Poirier, M.J. Tarlov, Langmuir 10 (1994) 2853. [10] H. Kondoh, C. Kodama, H. Sumida, H. Nozoye, J. Chem. Phys. 111 (1999) 1175. [11] M.A. Chesters, P. Gardner, E.M. McCash, Surf. Sci. 209 (1989) 89.