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A spectroscopic study of thiol layers prepared by contact printing
Applied Surface Science 141 Ž1999. 237–243
A spectroscopic study of thiol layers prepared by contact printing I. Bohm, A. Lampert, M. Buck ) , F. Eisert, M. Grunze ¨ Lehrstuhl fur ¨ Angewandte Physikalische Chemie, INF 253, 69120 Heidelberg, Germany Received 25 July 1998; accepted 8 August 1998
Abstract Films of hexadecane thiol on gold prepared by contact printing with a stamp of polydimethylsiloxane were investigated by IR–VIS sum frequency generation ŽSFG., IR spectroscopy, X-ray photoelectron spectroscopy ŽXPS., and contact angle measurements. The contact time was up to 60 s and the pressure ranged from 1 to 6 bar. SFG, as a method directly sensitive to molecular conformation, reveals a significantly higher degree of disorder in stamped films compared to monolayers prepared by immersion. Correspondingly, contact angle measurements yield a lower advancing contact angle and a higher hysteresis. XPS analysis shows that residues of the stamping material are left on the surface. q 1999 Elsevier Science B.V. All rights reserved. PACS: 42.65; 68.; 81.15 Keywords: Self-assembled monolayers; Thiols; Microcontact printing; Sum frequency generation
1. Introduction Organic monolayers prepared by self-assembly of thiols on metal substrates have attracted a steadily increasing interest throughout the past few years w1–3x. The combination of easy preparation by immersion of a substrate into a solution containing the thiol and the possibility to taylor surface properties via the alteration of the molecular structure makes this type of self-assembled monolayers ŽSAM. particularly interesting for a variety of applications, e.g., in biosensorics w4x or electrochemistry w5x. In addition, SAMs can serve as ultrathin photoresists since their passivating properties can easily be modified by radiation. Photo w6x, electron w7x or ion w8x beam )
Corresponding author. Tel.: q49-6221-545378; Fax: q496221-546199; E-mail: [email protected]
lithography, proximity printing w9x, or excited, metastable atoms w10x have been applied and can be used to produce structures down to the 20-nm range w11x. An additional, fascinatingly easy way of patterning SAMs was introduced in 1993 by Kumar and Whitesides w12,13x. Using an elastomer stamp they succeeded to generate patterns of thiols in the micrometer or even sub-micrometer range w14x. Fig. 1 illustrates the process of microcontact printing ŽmCP.. A stamp of polydimethylsiloxane ŽPDMS. is inked by the thiol Žstep 1. and pressed onto the substrate Žstep 2.. The thiol is transferred to the substrate such that after removing the stamp Žstep 3. thiols are adsorbed exclusively within the contact area. Since the stamp is saturated with thiol in the surface near region, the time required for the stamp being in contact with the substrate is much shorter compared to the time necessary to form films of good quality
0169-4332r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 4 3 3 2 Ž 9 8 . 0 0 5 1 0 - 8
238
I. Bohm ¨ et al.r Applied Surface Science 141 (1999) 237–243
Fig. 1. Scheme of the stamping process. A stamp of polydimethylsiloxane is wet either by a thiol solution or by the pure thiol Ž1.. Transfer of the pattern is accomplished by pressing the stamp onto the substrate Ž2.. After removal of the stamp the patterned SAM is left Ž3.. The magnified section illustrates the structure of a SAM formed by mCP and includes structurally well defined domains as well as defects.
by adsorption from solution. Typically, the contact time lasts from a few seconds up to a minute, whereas typical immersion times range from a few hours up to days. A comprehensive review of soft lithography is found in a recent publication by Xia and Whitesides w15x. From the technology point of view the contact printing ŽCP. has two important advantages over the immersion procedure ŽIP.. First, it is time saving and, second, it is a dry process, i.e., pure thiol solutions can be used as ink and, thus, less waste is produced. As known from numerous investigations, films of alkane thiols ŽCH 3 ŽCH 2 . my1 SH, MC m, 1 - m - 23. prepared by immersion form dense films in which the molecules are well-ordered w2,16,17x. For gold surfaces, which serve as substrates in the vast majority of experiments with thiol SAMs, the molecules adopt an all-trans conformation and are tilted by 30–358 with respect to the surface normal. However, despite the ease of preparation, film formation is a rather complex process involving several steps which proceed on rather different time scales w18x. Whereas the thiol adsorption up to a coverage between 0.8
and 0.9 occurs rather fast, saturation of the monolayer, which is characterized by the organization of the film with a laterally well-ordered structure of all-trans alkanethiolate chains, takes much longer. The short contact times in the mCP process, hence, raise the question if the films prepared by mCP have a structural quality comparable to SAMs prepared by immersion. To date only a few studies on the details of contact printing exist which are mainly relying on scanning probe techniques w19–22x, although spectroscopic techniques such as X-ray photoelectron spectroscopy ŽXPS. w19x and near edge X-ray absorption fine structure spectroscopy ŽNEXAFS. w21x were also applied. The present paper focuses on the spectroscopic characterization of alkanethiolate films stamped with short contact times and at pressures significantly higher than those reported before. The reasons to investigate stamping under these conditions are 2fold. First, with respect to technological applications, contact printing should be as fast as possible and a large tolerance in the contact pressure is desired. Second, in a recent work of Xia et al. w14x sub-micron patterns were generated by mechanical deformation of the stamp and in the case of small structures the quality of the films becomes more and more important. Hence, the emphasis is on conformational defects in the films which are probed by sum frequency generation, a nonlinear vibrational spectroscopy, and on impurities introduced by the printing process.
2. Experimental The substrates were prepared by evaporating 100 nm gold onto a SiŽ100. wafer with a 5-nm titanium adhesion layer. The stamp was prepared by pouring a 10:1 mixture of SYLGARD184 and the respective curing agent into a petri dish of polystyrene. The mixture was stored for at least 12 h at about 608C to ensure complete polymerization. Air bubbles present in the beginning disappeared in the initial stage of the polymerization process. A stamp of 10 = 10 mm2 was cut from the cured PDMS and the side facing the bottom of the dish served as printing side. Prior to use the stamp was thoroughly rinsed with ethanol
I. Bohm ¨ et al.r Applied Surface Science 141 (1999) 237–243
and dried under a stream of nitrogen. For each experiment a new stamp was used. Hexadecane thiol ŽMC16. from Fluka was used as received in all experiments. Analysis by gas chromatographyrmass spectrometry did no show any impurities. Adsorption from solution was accomplished by immersion of the substrates in a 1-mM solution of MC16 in ethanol for more than 24 h. Printed films were prepared under mechanically controlled conditions. A home-built apparatus allowed to approach the PDMS stamp in a defined way along the substrate normal. The stamping pressure could be adjusted by springs. For the inking process a glass plate was first immersed in pure MC16, then placed in front of the stamp, which was then pressed against the plate for 5 s. After the stamping process the sample was analyzed by SFG, XPS, IR reflection absorption spectroscopy, or contact angle measurements. For the SFG experiments, a home-built spectrometer was used w23x. For maximum SFG intensity the incoming beams and the analyzed SFG signal were p-polarized. This polarization combination yields the highest SFG signal. XP spectra were acquired with a Leybold Max 200 system using an Al K a source. Contact angles were determined with a Kruss ¨ Goniometer ŽModel G1..
239
condition of centro-symmetry w24x. This is the reason for the interface sensitivity of SFG. The signal originating from an interface where centrosymmetry is broken is not disturbed by a background signal from the amorphous or inversion-symmetric environment. If one of the frequencies is tunable in the range of the molecular vibrations, e.g., v 1 s v IR , the susceptibility is given by w25x
x s x NR q Ý n
C
vn y v IR y i Gn
Ž 2.
which reflects the fact that the SFG signal is a coherent superposition of non-resonant contributions from the interface between the metal substrate and the SAM Ž x NR ., and resonant contributions from the molecular vibrations which are described by the sum over all possible vibrations n . Gn is the vibrationspecific damping constant and C contains the Raman and IR cross-sections and is, in general, a complex quantity. Since non-resonant and resonant contributions superimpose coherently, an SFG spectrum can exhibit positive and negative peaks and, thus, can
3. Results and discussion Starting with the SFG spectra, we briefly summarize some basic aspects of nonlinear vibrational spectroscopy, necessary to understand the spectra presented here. In matter, strong electromagnetic fields at frequencies v 1 and v 2 produce a nonlinear polarization P which has contributions oscillating at the sum of the two frequencies. This polarization itself acts as the source of the sum frequency signal. Within the electric dipole approximation the intensity of the SFG signal can be written as w24x 2
ISFG A P Ž v SFG s v 1 q v 2 . A x E Ž v 1 . E Ž v 2 .
2
Ž 1. where x is the second order susceptibility and reflects the non-linear polarizability of the material. A crucial property of x is that it must vanish under the
Fig. 2. SFG spectrum of a thiol film on gold prepared by immersion of the substrate in a 1-mM solution of hexadecane thiol in EtOH and by stamping. The immersion time was 24 h. A pressure of 5 bar was applied for 5 s in the stamping process. The solid lines are fits to data points using Eq. Ž1. and Eq. Ž2.. The spectrum for the stamped sample is displaced for clarity. Both spectra are normalized to the non-resonant background at the low frequency side. The assignment of the prominent peaks of the CP ŽIP. film is: sym-CH 3 2878 cmy1 Ž2879., Fermi resonance of sym-CH 3 2940 cmy1 Ž2937., asym CH 3 in plane 2967 cmy1 Ž2968..
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I. Bohm ¨ et al.r Applied Surface Science 141 (1999) 237–243
differ significantly from the corresponding IR spectrum. Another crucial difference to the linear vibrational spectroscopy is that in the case of centrosymmetry no SFG signal is generated. This means that an all-trans hydrocarbon chain does not yield an SFG signal from the methylene groups whereas the IR spectrum is dominated by the CH 2 signals in the case of long alkane chains. Conversely, if gauche conformations are present an SFG signal is generated. Thus, SFG provides qualitative information on the conformational state of alkane chains. Fig. 2 displays SFG spectra in the regions of the C–H stretching vibrations of films of hexadecane thiol on gold prepared from solution and by stamping. Both spectra are normalized to the value of the non-resonant signal on the low frequency side. The spectrum of the printed sample is displaced for clarity. The assignment and positions of the vibrational bands are listed in Table 1. Due to destructive interference between the non-resonant and resonant signals the vibrational bands cause a reduction of the total SFG signal. Even though both spectra are dominated by the CH 3 vibrations, there are distinct differences. First, the bands associated with the methyl end groups differ in their relative intensities, in particular the Fermi resonance of the sym-CH 3 Ž2938 cmy1 . and the asym-CH 3 Ž2965 cmy1 .. Second, the region between 2900 cmy1 and 2925 cmy1 which is characteristic for the methylene vibrations is altered significantly. Whereas this range has low intensity for SAMs prepared by immersion, i.e., the SFG signal is not much lower than the base line given by the non-resonant signal, this region has gained significantly in intensity for the stamped films. This shows that the thiol molecules of the printed film must have gauche defects, contrary to the immersed film which is highly ordered and, thus, has a low level of CH 2
Fig. 3. Comparison of the SFG intensities of the asymmetric in plane methyl vibration and the methylene vibrations of hexadecane thiol on gold. The intensity of the methylene vibrations encompasses the range between 2900 cmy1 and 2925 cmy1 and includes the asymmetric stretch and the Fermi resonance of the symmetric stretch vibration. The open and filled symbols represent the samples prepared from solution and by stamping, respectively.
signals. The more disordered alkyl chains in the stamped films are in agreement with the changes of the methyl intensities which indicate a change of the orientation of the terminal CH 3 groups. This is further evaluated in Fig. 3 where the band intensities of films prepared by stamping at different
Table 1 Assignment of the IR bands of a contact printed film of hexadecane thiol on gold Assignment
Position wcmy1 x
Assignment
Position wcmy1 x
Asym CH 3-stretch Sym CH 3 -stretch, FR Asym CH 2-stretch Sym CH 3 -stretch Sym CH 2 -stretch
2965.1 2936.4 2920.8 2878.4 2851.6
CH 2-scissors, asym CH 3-bending sym-CH 3 bending SiCH 3 , sym CH 3 -deformation Si–O–Si stretching SiCH 3 , CH 3 -rocking
1468.8 1382.8 1263 1105 814
FR s Fermi resonance.
I. Bohm ¨ et al.r Applied Surface Science 141 (1999) 237–243
pressures are plotted in comparison to the respective intensities of SAMs prepared by immersion. The contact time was kept constant Ž5 s. whereas the pressure was varied between 1 and 5 bar. Every data point corresponds to one sample. It is obvious that stamping yields films with increased intensity of both the methyl and methylene mode, i.e., the degree of gauche conformations is higher and the average orientation of the methyl group is changed in printed films as compared to SAMS formed by immersion. We did not observe any systematic variation of the spectral features in the pressure range investigated. However, the contact time has an influence as illustrated by wettability measurements ŽFig. 4.. At short contact times the advancing and receding contact angles are significantly lower compared to samples prepared by immersion. Only for times exceeding 30 s the contact angles approach the value for the SAMS prepared by immersion. The contact angle hysteresis exhibits an analogous behavior and decreases slightly with increasing contact time. Whereas measurements of contact angles are a sensitive method to qualitatively judge the film quality and SFG yields conformational and orientational information about the molecules, XPS provides complementary information on the purity and thickness of the films. Fig. 5 summarizes the XP spectra in the
Fig. 4. Advancing Župper part. and receding Žlower part. contact angles of water on hexadecane thiol films as a function of contact time. The pressure was 3.75 bar. The open symbols are reference data from a sample prepared by immersion in a solution of 1 mM hexadecane thiol in ethanol for 96 h. Error bars reflect the scattering from different measurements.
241
Fig. 5. XP-spectra in the C1s and Si2s region of a sample which had been in contact with a PDMS stamp without thiol Ža., a sample prepared with a thiol-inked stamp Žb., a native gold substrate Žc. and a film prepared from solution Žd.. For the Si2s region Žd. has been omitted due to the identity with Žc..
C1s and Si2s region from samples which were subject to different treatments. A bare gold substrate which was in contact with a non-inked PDMS stamp Ža. is compared with a CP-SAM Žb., a bare gold substrate Žc., and an IP-SAM Žd.. The C1s intensity of the CP film is not significantly different from that of the IP-SAM thus indicating that the coverage is the same. This holds as well for the S2p signal Žnot shown.. However, striking differences are seen in the Si2s region which reveal that the printing process does not only transfer thiols onto the gold substrate but also causes PDMS to be transferred. As can be estimated from the intensity the amount of residues of the stamp material is significant and can exceed 10%. The transfer of PDMS fragments is confirmed by IRRAS spectra ŽFig. 6 and Table 1. which clearly show the fingerprints of PDMS. Since the asymmetric methyl mode is disturbed by the methyl mode of the PDMS residues we did not use the intensity of the methyl bands to analyze the orientation of the molecules in the CP-SAMs. Instead we rely on the asym-stretch vibration of the methylene moieties whose position has been used to identify the state of the chain as liquid- or crystalline-like w26x. In the present case the band is located at 2920.5 cmy1 which is in between the values for alkane chains in the disordered and crystalline state. For crystalline-
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I. Bohm ¨ et al.r Applied Surface Science 141 (1999) 237–243
Fig. 6. IRRAS spectrum of a SAM of hexadecane thiol on gold prepared by stamping Ž5 s at 3.9 bar.. The band assignments are given in Table 1.
like SAMs prepared by immersion the asym-methylene band is found between 2917 cmy1 and 2919 cmy1 w26x. The value for liquid thiols is 2924 cmy1 w26x. Thus, the IR spectrum indicates that the structure of stamped thiols are not identical to IP-SAMs. This is in agreement with our SFG spectra which show significantly perturbed alkane chains. The IR result is somewhat different from the literature which reports no significant differences between stamped and solution-prepared films w20x. Beside the fact that our experimental conditions were different, e.g., the pressure is significantly higher, the data reported in the literature vary as well and the result seems to be dependent on the technique. Whereas lateral force microscopy ŽLFM. identifies differences between CP- and IP-films w20,21x identical film structures were found by scanning tunneling microscopy w22x. NEXAFS, a spectroscopic technique which yields information over a macroscopic surface area, did not see any noticeable differences w21x. From this observation, together with the LFM results, it was concluded that the amount of packing defects does not exceed 30% w21x and that the contrast seen between CP- and IP-films in LFM is due to differences in the density of domain boundaries. This interpretation is consistent with our experiments which produced much larger changes in the SFG spectra compared to IR which is a technique
averaging over all molecules. As long as changes of the ratio between ordered and disordered molecules are below a certain level, only minor changes are expected in the IR spectra. In contrast, changes between CP- and IP-films in the region of the methylene vibrations can clearly be seen by SFG. This is due to the sensitivity of SFG to conformational defects which can be detected without the contribution from ordered regions. An important result of the present experiments is that PDMS fragments are transferred by the printing process. The amount of PDMS found in our experiments is surprisingly high and should have been seen also in other work. Since this observation has not been reported before, it is likely that the higher pressures used in the present experiments accounts for this difference. The larger force imposes a significantly higher mechanical stress on the stamp than in the other experiments, where the stamp is only gently pressed onto the substrate. However, no explicit statement whether PDMS is left or not is given in the work published. That mechanical deformation can cause fragments of PDMS to adsorb on a substrate has been found in experiments where shear forces, e.g., rubbing, were applied w27x. At present the details of how the siloxane residues are incorporated in the film and how they might affect the structural quality of the thiol films are not known. A combination of spectroscopic and scanning probed studies is required to elucidate this further.
4. Conclusion Stamping at pressures of a few bars produces thiol films which have different structural quality compared to SAMs prepared by immersion. In the range investigated contact pressure is not a critical parameter whereas contact time improves the quality. The identification of PDMS residues on the stamped films has a significant implication with respect to preparation of the films by microcontact printing. In the case where pressure was applied to push the size of the structures to smaller dimensions by elastical deformation of the stamp w14x this becomes an important issue since the required structural perfection
I. Bohm ¨ et al.r Applied Surface Science 141 (1999) 237–243
and thus the acceptable level of impurities scales with the size of the patterns. Future experiments have to determine where the optimum between pressure, structure size, contact time, and impurity level is.
Acknowledgements Helpful discussions with G. Hahner are gratefully ¨ acknowledged. We are indebted to R. Kohring and M. Zolk for their support in the SFG experiments. We thank G. Albert for preparing the substrates and P. Harder for assistance in the acquisition of IR spectra. The work was supported by the Office of Naval Research and by the Fonds der Chemischen Industrie.
References w1x G.E. Poirier, Chem. Rev. 97 Ž1997. 1117. w2x A. Ulman, Chem. Rev. 96 Ž1996. 1533. w3x D.R. Jung, A.W. Czanderna, CRC Crit. Rev. Solid State Mater. Sci. 19 Ž1994. 1. w4x T. Wink, A. van Zuilen, A. Bult, W.P. van Bennekom, Analyst 122 Ž1997. 43R. w5x H.O. Finklea, Electroanal. Chem., Vol. 19, Marcel Dekker, New York, 1996. w6x J.M. Behm, K.R. Lykke, M.J. Pellin, J.C. Hemminger, Langmuir 12 Ž1996. 2121. w7x M.J. Lercel, G.F. Redinbo, F.D. Pardo, M. Rooks, R.C. Tiberio, P. Simpson, H.G. Craighead, C.W. Sheen, A.N. Parikh, D.L. Allara, J. Vac. Sci. Technol. B 12 Ž1994. 3663.
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w8x P.C. Rieke, B.J. Tarasevich, L.L. Wood, M.H. Engelhard, D.R. Baer, G.E. Fryxell, Langmuir 10 Ž1994. 619. w9x C. David, H.U. Muller, B. Volkel, M. Grunze, Microelectr. ¨ ¨ Eng. 30 Ž1996. 57. w10x J.H. Thywissen, K.S. Johnson, R. Younkin, N.H. Dekker, K.K. Berggren, A.P. Chu, M. Prentiss, S.A. Lee, J. Vac. Sci. Technol. B 15 Ž1997. 2093. w11x M.J. Lercel, H.G. Craighead, A.N. Parikh, K. Seshadri, D.L. Allara, J. Vac. Sci. Technol. A 14 Ž1996. 1844. w12x A. Kumar, G.M. Whitesides, Appl. Phys. Lett. 63 Ž1993. 2002. w13x A. Kumar, H.A. Biebuyck, G.M. Whitesides, Langmuir 10 Ž1994. 1498. w14x Y. Xia, G.M. Whitesides, Langmuir 13 Ž1997. 2059. w15x Y. Xia, G.M. Whitesides, Angew. Chem. Intl. Ed. 37 Ž1998. 551. w16x A. Ulman, An Introduction to Ultrathin Organic Films, Academic Press, Boston, 1991. w17x L.H. Dubois, R.G. Nuzzo, Annu. Rev. Phys. Chem. 43 Ž1992. 437. w18x G. Hahner, Ch. Woll, ¨ ¨ M. Buck, M. Grunze, Langmuir 9 Ž1993. 1955. w19x B. Bar, S. Rubin, T.N. Taylor, B.I. Swanson, T.A. Zawodzinski Jr., J. Vac. Sci. Technol. A 14 Ž1996. 1794. w20x G. Bar, S. Rubin, A.N. Parikh, B.I. Swanson, T.A. Zawodzinski Jr., M.-H. Whangbo, Langmuir 13 Ž1997. 373. w21x D. Fischer, A. Marti, G. Hahner, J. Vac. Sci. Technol. A 15 ¨ Ž1997. 2173. w22x N.B. Larsen, H. Biebuyck, E. Delamarche, B. Michel, J. Am. Chem. Soc. 119 Ž1997. 3017. w23x A. Lampert, PhD thesis, Heidelberg, 1997. w24x Y.R. Shen, Nature 337 Ž1989. 519. w25x R.B. Hall, J.N. Russell, J. Miragliotta, P.R. Rabinowitz, in: R. Vanselow, R. Howe ŽEds.., Chemistry and Physics of Solid Surfaces VIII, Vol. 22, Springer-Verlag, Berlin, 1991. w26x M.D. Porter, T.B. Bright, D.L. Allara, C.E.D. Chidsey, J. Am. Chem. Soc. 109 Ž1987. 3559. w27x G. Hahner, private communication. ¨
A spectroscopic study of thiol layers prepared by contact printing I. Bohm, A. Lampert, M. Buck ) , F. Eisert, M. Grunze ¨ Lehrstuhl fur ¨ Angewandte Physikalische Chemie, INF 253, 69120 Heidelberg, Germany Received 25 July 1998; accepted 8 August 1998
Abstract Films of hexadecane thiol on gold prepared by contact printing with a stamp of polydimethylsiloxane were investigated by IR–VIS sum frequency generation ŽSFG., IR spectroscopy, X-ray photoelectron spectroscopy ŽXPS., and contact angle measurements. The contact time was up to 60 s and the pressure ranged from 1 to 6 bar. SFG, as a method directly sensitive to molecular conformation, reveals a significantly higher degree of disorder in stamped films compared to monolayers prepared by immersion. Correspondingly, contact angle measurements yield a lower advancing contact angle and a higher hysteresis. XPS analysis shows that residues of the stamping material are left on the surface. q 1999 Elsevier Science B.V. All rights reserved. PACS: 42.65; 68.; 81.15 Keywords: Self-assembled monolayers; Thiols; Microcontact printing; Sum frequency generation
1. Introduction Organic monolayers prepared by self-assembly of thiols on metal substrates have attracted a steadily increasing interest throughout the past few years w1–3x. The combination of easy preparation by immersion of a substrate into a solution containing the thiol and the possibility to taylor surface properties via the alteration of the molecular structure makes this type of self-assembled monolayers ŽSAM. particularly interesting for a variety of applications, e.g., in biosensorics w4x or electrochemistry w5x. In addition, SAMs can serve as ultrathin photoresists since their passivating properties can easily be modified by radiation. Photo w6x, electron w7x or ion w8x beam )
Corresponding author. Tel.: q49-6221-545378; Fax: q496221-546199; E-mail: [email protected]
lithography, proximity printing w9x, or excited, metastable atoms w10x have been applied and can be used to produce structures down to the 20-nm range w11x. An additional, fascinatingly easy way of patterning SAMs was introduced in 1993 by Kumar and Whitesides w12,13x. Using an elastomer stamp they succeeded to generate patterns of thiols in the micrometer or even sub-micrometer range w14x. Fig. 1 illustrates the process of microcontact printing ŽmCP.. A stamp of polydimethylsiloxane ŽPDMS. is inked by the thiol Žstep 1. and pressed onto the substrate Žstep 2.. The thiol is transferred to the substrate such that after removing the stamp Žstep 3. thiols are adsorbed exclusively within the contact area. Since the stamp is saturated with thiol in the surface near region, the time required for the stamp being in contact with the substrate is much shorter compared to the time necessary to form films of good quality
0169-4332r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 4 3 3 2 Ž 9 8 . 0 0 5 1 0 - 8
238
I. Bohm ¨ et al.r Applied Surface Science 141 (1999) 237–243
Fig. 1. Scheme of the stamping process. A stamp of polydimethylsiloxane is wet either by a thiol solution or by the pure thiol Ž1.. Transfer of the pattern is accomplished by pressing the stamp onto the substrate Ž2.. After removal of the stamp the patterned SAM is left Ž3.. The magnified section illustrates the structure of a SAM formed by mCP and includes structurally well defined domains as well as defects.
by adsorption from solution. Typically, the contact time lasts from a few seconds up to a minute, whereas typical immersion times range from a few hours up to days. A comprehensive review of soft lithography is found in a recent publication by Xia and Whitesides w15x. From the technology point of view the contact printing ŽCP. has two important advantages over the immersion procedure ŽIP.. First, it is time saving and, second, it is a dry process, i.e., pure thiol solutions can be used as ink and, thus, less waste is produced. As known from numerous investigations, films of alkane thiols ŽCH 3 ŽCH 2 . my1 SH, MC m, 1 - m - 23. prepared by immersion form dense films in which the molecules are well-ordered w2,16,17x. For gold surfaces, which serve as substrates in the vast majority of experiments with thiol SAMs, the molecules adopt an all-trans conformation and are tilted by 30–358 with respect to the surface normal. However, despite the ease of preparation, film formation is a rather complex process involving several steps which proceed on rather different time scales w18x. Whereas the thiol adsorption up to a coverage between 0.8
and 0.9 occurs rather fast, saturation of the monolayer, which is characterized by the organization of the film with a laterally well-ordered structure of all-trans alkanethiolate chains, takes much longer. The short contact times in the mCP process, hence, raise the question if the films prepared by mCP have a structural quality comparable to SAMs prepared by immersion. To date only a few studies on the details of contact printing exist which are mainly relying on scanning probe techniques w19–22x, although spectroscopic techniques such as X-ray photoelectron spectroscopy ŽXPS. w19x and near edge X-ray absorption fine structure spectroscopy ŽNEXAFS. w21x were also applied. The present paper focuses on the spectroscopic characterization of alkanethiolate films stamped with short contact times and at pressures significantly higher than those reported before. The reasons to investigate stamping under these conditions are 2fold. First, with respect to technological applications, contact printing should be as fast as possible and a large tolerance in the contact pressure is desired. Second, in a recent work of Xia et al. w14x sub-micron patterns were generated by mechanical deformation of the stamp and in the case of small structures the quality of the films becomes more and more important. Hence, the emphasis is on conformational defects in the films which are probed by sum frequency generation, a nonlinear vibrational spectroscopy, and on impurities introduced by the printing process.
2. Experimental The substrates were prepared by evaporating 100 nm gold onto a SiŽ100. wafer with a 5-nm titanium adhesion layer. The stamp was prepared by pouring a 10:1 mixture of SYLGARD184 and the respective curing agent into a petri dish of polystyrene. The mixture was stored for at least 12 h at about 608C to ensure complete polymerization. Air bubbles present in the beginning disappeared in the initial stage of the polymerization process. A stamp of 10 = 10 mm2 was cut from the cured PDMS and the side facing the bottom of the dish served as printing side. Prior to use the stamp was thoroughly rinsed with ethanol
I. Bohm ¨ et al.r Applied Surface Science 141 (1999) 237–243
and dried under a stream of nitrogen. For each experiment a new stamp was used. Hexadecane thiol ŽMC16. from Fluka was used as received in all experiments. Analysis by gas chromatographyrmass spectrometry did no show any impurities. Adsorption from solution was accomplished by immersion of the substrates in a 1-mM solution of MC16 in ethanol for more than 24 h. Printed films were prepared under mechanically controlled conditions. A home-built apparatus allowed to approach the PDMS stamp in a defined way along the substrate normal. The stamping pressure could be adjusted by springs. For the inking process a glass plate was first immersed in pure MC16, then placed in front of the stamp, which was then pressed against the plate for 5 s. After the stamping process the sample was analyzed by SFG, XPS, IR reflection absorption spectroscopy, or contact angle measurements. For the SFG experiments, a home-built spectrometer was used w23x. For maximum SFG intensity the incoming beams and the analyzed SFG signal were p-polarized. This polarization combination yields the highest SFG signal. XP spectra were acquired with a Leybold Max 200 system using an Al K a source. Contact angles were determined with a Kruss ¨ Goniometer ŽModel G1..
239
condition of centro-symmetry w24x. This is the reason for the interface sensitivity of SFG. The signal originating from an interface where centrosymmetry is broken is not disturbed by a background signal from the amorphous or inversion-symmetric environment. If one of the frequencies is tunable in the range of the molecular vibrations, e.g., v 1 s v IR , the susceptibility is given by w25x
x s x NR q Ý n
C
vn y v IR y i Gn
Ž 2.
which reflects the fact that the SFG signal is a coherent superposition of non-resonant contributions from the interface between the metal substrate and the SAM Ž x NR ., and resonant contributions from the molecular vibrations which are described by the sum over all possible vibrations n . Gn is the vibrationspecific damping constant and C contains the Raman and IR cross-sections and is, in general, a complex quantity. Since non-resonant and resonant contributions superimpose coherently, an SFG spectrum can exhibit positive and negative peaks and, thus, can
3. Results and discussion Starting with the SFG spectra, we briefly summarize some basic aspects of nonlinear vibrational spectroscopy, necessary to understand the spectra presented here. In matter, strong electromagnetic fields at frequencies v 1 and v 2 produce a nonlinear polarization P which has contributions oscillating at the sum of the two frequencies. This polarization itself acts as the source of the sum frequency signal. Within the electric dipole approximation the intensity of the SFG signal can be written as w24x 2
ISFG A P Ž v SFG s v 1 q v 2 . A x E Ž v 1 . E Ž v 2 .
2
Ž 1. where x is the second order susceptibility and reflects the non-linear polarizability of the material. A crucial property of x is that it must vanish under the
Fig. 2. SFG spectrum of a thiol film on gold prepared by immersion of the substrate in a 1-mM solution of hexadecane thiol in EtOH and by stamping. The immersion time was 24 h. A pressure of 5 bar was applied for 5 s in the stamping process. The solid lines are fits to data points using Eq. Ž1. and Eq. Ž2.. The spectrum for the stamped sample is displaced for clarity. Both spectra are normalized to the non-resonant background at the low frequency side. The assignment of the prominent peaks of the CP ŽIP. film is: sym-CH 3 2878 cmy1 Ž2879., Fermi resonance of sym-CH 3 2940 cmy1 Ž2937., asym CH 3 in plane 2967 cmy1 Ž2968..
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differ significantly from the corresponding IR spectrum. Another crucial difference to the linear vibrational spectroscopy is that in the case of centrosymmetry no SFG signal is generated. This means that an all-trans hydrocarbon chain does not yield an SFG signal from the methylene groups whereas the IR spectrum is dominated by the CH 2 signals in the case of long alkane chains. Conversely, if gauche conformations are present an SFG signal is generated. Thus, SFG provides qualitative information on the conformational state of alkane chains. Fig. 2 displays SFG spectra in the regions of the C–H stretching vibrations of films of hexadecane thiol on gold prepared from solution and by stamping. Both spectra are normalized to the value of the non-resonant signal on the low frequency side. The spectrum of the printed sample is displaced for clarity. The assignment and positions of the vibrational bands are listed in Table 1. Due to destructive interference between the non-resonant and resonant signals the vibrational bands cause a reduction of the total SFG signal. Even though both spectra are dominated by the CH 3 vibrations, there are distinct differences. First, the bands associated with the methyl end groups differ in their relative intensities, in particular the Fermi resonance of the sym-CH 3 Ž2938 cmy1 . and the asym-CH 3 Ž2965 cmy1 .. Second, the region between 2900 cmy1 and 2925 cmy1 which is characteristic for the methylene vibrations is altered significantly. Whereas this range has low intensity for SAMs prepared by immersion, i.e., the SFG signal is not much lower than the base line given by the non-resonant signal, this region has gained significantly in intensity for the stamped films. This shows that the thiol molecules of the printed film must have gauche defects, contrary to the immersed film which is highly ordered and, thus, has a low level of CH 2
Fig. 3. Comparison of the SFG intensities of the asymmetric in plane methyl vibration and the methylene vibrations of hexadecane thiol on gold. The intensity of the methylene vibrations encompasses the range between 2900 cmy1 and 2925 cmy1 and includes the asymmetric stretch and the Fermi resonance of the symmetric stretch vibration. The open and filled symbols represent the samples prepared from solution and by stamping, respectively.
signals. The more disordered alkyl chains in the stamped films are in agreement with the changes of the methyl intensities which indicate a change of the orientation of the terminal CH 3 groups. This is further evaluated in Fig. 3 where the band intensities of films prepared by stamping at different
Table 1 Assignment of the IR bands of a contact printed film of hexadecane thiol on gold Assignment
Position wcmy1 x
Assignment
Position wcmy1 x
Asym CH 3-stretch Sym CH 3 -stretch, FR Asym CH 2-stretch Sym CH 3 -stretch Sym CH 2 -stretch
2965.1 2936.4 2920.8 2878.4 2851.6
CH 2-scissors, asym CH 3-bending sym-CH 3 bending SiCH 3 , sym CH 3 -deformation Si–O–Si stretching SiCH 3 , CH 3 -rocking
1468.8 1382.8 1263 1105 814
FR s Fermi resonance.
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pressures are plotted in comparison to the respective intensities of SAMs prepared by immersion. The contact time was kept constant Ž5 s. whereas the pressure was varied between 1 and 5 bar. Every data point corresponds to one sample. It is obvious that stamping yields films with increased intensity of both the methyl and methylene mode, i.e., the degree of gauche conformations is higher and the average orientation of the methyl group is changed in printed films as compared to SAMS formed by immersion. We did not observe any systematic variation of the spectral features in the pressure range investigated. However, the contact time has an influence as illustrated by wettability measurements ŽFig. 4.. At short contact times the advancing and receding contact angles are significantly lower compared to samples prepared by immersion. Only for times exceeding 30 s the contact angles approach the value for the SAMS prepared by immersion. The contact angle hysteresis exhibits an analogous behavior and decreases slightly with increasing contact time. Whereas measurements of contact angles are a sensitive method to qualitatively judge the film quality and SFG yields conformational and orientational information about the molecules, XPS provides complementary information on the purity and thickness of the films. Fig. 5 summarizes the XP spectra in the
Fig. 4. Advancing Župper part. and receding Žlower part. contact angles of water on hexadecane thiol films as a function of contact time. The pressure was 3.75 bar. The open symbols are reference data from a sample prepared by immersion in a solution of 1 mM hexadecane thiol in ethanol for 96 h. Error bars reflect the scattering from different measurements.
241
Fig. 5. XP-spectra in the C1s and Si2s region of a sample which had been in contact with a PDMS stamp without thiol Ža., a sample prepared with a thiol-inked stamp Žb., a native gold substrate Žc. and a film prepared from solution Žd.. For the Si2s region Žd. has been omitted due to the identity with Žc..
C1s and Si2s region from samples which were subject to different treatments. A bare gold substrate which was in contact with a non-inked PDMS stamp Ža. is compared with a CP-SAM Žb., a bare gold substrate Žc., and an IP-SAM Žd.. The C1s intensity of the CP film is not significantly different from that of the IP-SAM thus indicating that the coverage is the same. This holds as well for the S2p signal Žnot shown.. However, striking differences are seen in the Si2s region which reveal that the printing process does not only transfer thiols onto the gold substrate but also causes PDMS to be transferred. As can be estimated from the intensity the amount of residues of the stamp material is significant and can exceed 10%. The transfer of PDMS fragments is confirmed by IRRAS spectra ŽFig. 6 and Table 1. which clearly show the fingerprints of PDMS. Since the asymmetric methyl mode is disturbed by the methyl mode of the PDMS residues we did not use the intensity of the methyl bands to analyze the orientation of the molecules in the CP-SAMs. Instead we rely on the asym-stretch vibration of the methylene moieties whose position has been used to identify the state of the chain as liquid- or crystalline-like w26x. In the present case the band is located at 2920.5 cmy1 which is in between the values for alkane chains in the disordered and crystalline state. For crystalline-
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Fig. 6. IRRAS spectrum of a SAM of hexadecane thiol on gold prepared by stamping Ž5 s at 3.9 bar.. The band assignments are given in Table 1.
like SAMs prepared by immersion the asym-methylene band is found between 2917 cmy1 and 2919 cmy1 w26x. The value for liquid thiols is 2924 cmy1 w26x. Thus, the IR spectrum indicates that the structure of stamped thiols are not identical to IP-SAMs. This is in agreement with our SFG spectra which show significantly perturbed alkane chains. The IR result is somewhat different from the literature which reports no significant differences between stamped and solution-prepared films w20x. Beside the fact that our experimental conditions were different, e.g., the pressure is significantly higher, the data reported in the literature vary as well and the result seems to be dependent on the technique. Whereas lateral force microscopy ŽLFM. identifies differences between CP- and IP-films w20,21x identical film structures were found by scanning tunneling microscopy w22x. NEXAFS, a spectroscopic technique which yields information over a macroscopic surface area, did not see any noticeable differences w21x. From this observation, together with the LFM results, it was concluded that the amount of packing defects does not exceed 30% w21x and that the contrast seen between CP- and IP-films in LFM is due to differences in the density of domain boundaries. This interpretation is consistent with our experiments which produced much larger changes in the SFG spectra compared to IR which is a technique
averaging over all molecules. As long as changes of the ratio between ordered and disordered molecules are below a certain level, only minor changes are expected in the IR spectra. In contrast, changes between CP- and IP-films in the region of the methylene vibrations can clearly be seen by SFG. This is due to the sensitivity of SFG to conformational defects which can be detected without the contribution from ordered regions. An important result of the present experiments is that PDMS fragments are transferred by the printing process. The amount of PDMS found in our experiments is surprisingly high and should have been seen also in other work. Since this observation has not been reported before, it is likely that the higher pressures used in the present experiments accounts for this difference. The larger force imposes a significantly higher mechanical stress on the stamp than in the other experiments, where the stamp is only gently pressed onto the substrate. However, no explicit statement whether PDMS is left or not is given in the work published. That mechanical deformation can cause fragments of PDMS to adsorb on a substrate has been found in experiments where shear forces, e.g., rubbing, were applied w27x. At present the details of how the siloxane residues are incorporated in the film and how they might affect the structural quality of the thiol films are not known. A combination of spectroscopic and scanning probed studies is required to elucidate this further.
4. Conclusion Stamping at pressures of a few bars produces thiol films which have different structural quality compared to SAMs prepared by immersion. In the range investigated contact pressure is not a critical parameter whereas contact time improves the quality. The identification of PDMS residues on the stamped films has a significant implication with respect to preparation of the films by microcontact printing. In the case where pressure was applied to push the size of the structures to smaller dimensions by elastical deformation of the stamp w14x this becomes an important issue since the required structural perfection
I. Bohm ¨ et al.r Applied Surface Science 141 (1999) 237–243
and thus the acceptable level of impurities scales with the size of the patterns. Future experiments have to determine where the optimum between pressure, structure size, contact time, and impurity level is.
Acknowledgements Helpful discussions with G. Hahner are gratefully ¨ acknowledged. We are indebted to R. Kohring and M. Zolk for their support in the SFG experiments. We thank G. Albert for preparing the substrates and P. Harder for assistance in the acquisition of IR spectra. The work was supported by the Office of Naval Research and by the Fonds der Chemischen Industrie.
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