Thermal Stability of Self-Assembled Monolayers: Influence of Lateral

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J. Phys. Chem. B 2002, 106, 10401-10409

10401

Thermal Stability of Self-Assembled Monolayers: Influence of Lateral Hydrogen Bonding Ramu j nas Valiokas,† Mattias O 2 stblom,† Sofia Svedhem,‡ Stefan C. T. Svensson,‡ and ,† Bo Liedberg* DiVision of Applied Physics and DiVision of Chemistry, Department of Physics and Measurement Technology, Linko¨ pings uniVersitet, S-581 83 Linko¨ ping, Sweden ReceiVed: January 10, 2002; In Final Form: June 10, 2002

Temperature-programmed desorption (TPD) of self-assembled monolayers (SAMs) on gold is investigated by using in parallel mass spectrometry (MS) and infrared reflection-absorption spectroscopy (IRAS). Monolayers formed by HS(CH2)n-OH (n ) 18, 22) and HS(CH2)15-CONH-(CH2CH2O)-H (EG1) are compared to reveal the influence of specifically introduced hydrogen-bonding groups on their thermal stability. The overall desorption process of the above molecules is found to occur in two main steps; a disordering of the alkyl chains followed by a complex series of decomposition/desorption reactions. The final step of the process involves desorption of sulfur from different chemisorption states. The amide-group-containing SAM, which is stabilized by lateral hydrogen bonds, displays a substantial delay of the alkyl chain disordering by about 50 K, as compared to the linear chain alcohols HS(CH2)n-OH. Moreover, the decomposition of the alkyls and the onset of sulfur desorption occur at a temperature that is higher by approximately 25 K as compared to the HS(CH2)18-OH SAM. The desorption process is also studied for two oligo(ethylene glycol)terminated SAMs, HS(CH2)15-X-(CH2CH2O)4-H (EG4-SAMs), where X is -CONH- and -COO- linking groups. In addition to the molecular chain disordering, the decomposition/desorption process of the EG4SAMs occurs in two steps. The first is associated with the loss of the oligomer portion and the second with the desorption of the alkylthiolate part of the molecule. Our study points out that lateral hydrogen bonding, introduced via amide groups, is a convenient way to improve the thermal stability of alkanthiolate SAMs.

Introduction Self-assembled monolayers (SAMs) form spontaneously on noble metal and glass-type substrates1. The best-known system today consists of organosulfur compounds, most often alkylthiols, and gold in which the SAM formation is driven by the strong sulfur-gold bond.2-4 The packing and stability of the SAMs also depend on other critical factors including, for example, intermolecular interactions between the constituent molecules. Extensive studies of alkylthiols on gold and silver have created a rather complete picture regarding the structural properties of alkyl chains interacting through van der Waals forces.4 However, as soon as specific functional groups are introduced into the SAMs, the picture becomes more complicated. The lateral interactions generated in that way can be of the type dipole-dipole,5-7 π-π stacking,6,8 covalent bonding,9 and hydrogen bonding.10-17 So far, only a few systematic studies have been reported on the self-assembly and stability of compounds bearing strongly interacting groups. The hydrogen-bonding groups deserve special interest because they can be easily introduced into the SAMs, for example, through an amide bond. The amide bond is formed via the common reaction between -COOH and -NH2-containing molecules either when synthesizing compounds for selfassembly or upon postfunctionalization of the COOH-terminated SAM in situ.18,19 The formation of ordered SAMs from amide compounds was first reported by Lenk et al.10 Tam-Chang et * To whom correspondence should be addressed. Tel: +46 13 281877. Fax: +46 13 288969. E-mail: [email protected]. † Division of Applied Physics. ‡ Division of Chemistry.

al. observed an improved thermal stability of such SAMs, and they also reported an improved stability against exchange with analogous compounds in solution.11 The lateral hydrogen bonding between amide groups also has been studied for shortchain ferrocenylalkyl disulfides on gold.12 Further on, highly ordered hydrogen-bonded long alkyl chain SAMs were investigated by Clegg et al.13 These authors also reported an improved thermal stability of SAMs interacting via a 3D hydrogenbonding network.14 The influence of varying alkyl chain length in combination with hydrogen bonding between amide groups has also been addressed by other researchers.15 More recently, it was suggested that the presence of amide groups could influence the packing properties of the SAMs.16,17 We have previously studied the influence of lateral hydrogen bonding on the self-assembly of the oligo(ethylene glycol) (OEG)-terminated and amide-group-containing SAMs.20,21 The OEG SAMs were chosen because they have been frequently used as a model surface in studies of protein adsorption phenomena,22-24 for micropatterning of biospecific surfaces,25,26 and as spacer arms for efficient anchoring of bioactive groups27-29 or lipid membranes to solid surfaces.30-33 The incorporation of an amide group into the compounds led to new and interesting properties of the OEG SAMs. We observed, for example, that lateral hydrogen bonding could be employed to fine-tune the oligomer conformation in the SAMs21 and that the oligomer phase of EG6-SAMs, which adopt the helical conformation at room temperature, could be reversibly switched to the all-trans conformation at approximately 330 K.21,34 In the present study, we focus on the contribution of lateral hydrogen bonding to the thermal stability of SAMs. Therefore, the study is divided into two parts. First, we use mass

10.1021/jp0200526 CCC: $22.00 © 2002 American Chemical Society Published on Web 09/12/2002

10402 J. Phys. Chem. B, Vol. 106, No. 40, 2002 SCHEME 1

spectrometry (MS) and infrared reflection-absorption spectroscopy (IRAS) to compare the temperature-programmed desorption (TPD) of linear and hydrogen-bonded SAMs. The compounds under investigation are shown in Scheme 1. One methylene group in compound 1 is replaced by an amide group, -CONH-, in compound 3. Also, an analogue 2 is compared to verify the effect of a longer alkyl chain. Next, we increase the complexity of the SAMs, by introducing a tetra(ethylene glycol) (EG4) tail as the terminal group. In this case, the linking groups used to construct compounds 4 and 5 are an ester (-COO-) and an amide (-CONH-), respectively. Thus, the present study addresses the influence of hydrogen bonding on the thermal stability of simple and oligomer-terminated alkylthiolate SAMs in terms of temperature-induced disorder, decomposition, and desorption. Materials and Methods Compounds 1 and 2 were obtained as a generous gift from Biacore International AB, Uppsala, Sweden. The synthesis of compounds 3-5 is described in a study by Svedhem et al.35 The compounds were dissolved in ethanol at 1 mM concentration. SAMs 1-3 were prepared from such solutions, whereas 20 µM solutions were used for the formation of SAMs 4 and 5. The gold surfaces were prepared by the electron-beam evaporation in a Balzers UMS 500 P system. The base pressure was on the low 10-9 mbar scale. First, a 25 Å thick titanium adhesion layer was evaporated on 20 × 20 mm2 samples, cut from standard (100)-silicon wafers. Then, a 2000 Å thick gold film was deposited at a constant evaporation rate of 10 Å/s and at an evaporation pressure of about 10-7 mbar. The prepared gold substrates could be stored in room conditions up to several months. Prior the SAM adsorption, they were cleaned in a 5:1:1 mixture of deionized (MilliQ) water, 25% hydrogen peroxide, and 30% ammonia for 5 min at 80 °C, followed by extensive rinsing in deionized water. For the control of the efficiency of the washing procedure, a sample from the cleaned batch was checked by measuring the optical characteristics of the gold surface on an automatic Rudolph Research AutoEL III ellipsometer with a He-Ne laser light source (λ ) 632.8 nm), aligned at an angle of incidence of 70°. The criterion for a clean and smooth gold surface was an ellipsometric angle ∆ of at least 110.00°. If an acceptable ∆ value was achieved, the other substrates from the same cleaned batch were soaked into the respective solutions in plastic beakers and incubated for at least 48 h. Before the analysis, the samples were rinsed in ethanol, ultrasonicated for 3 min, and rinsed in ethanol again. The TPD analysis was carried out in a home-built ultrahighvacuum (UHV) system that has been described in detail elsewhere.36 The temperature was measured with a Pt100 element mounted in the solid copper block of the sample holder. A Eurotherm controller was used to control the resistive heating of the sample holder from 293 to 530 K. The temperature readings from the Pt100 element in the copper sample holder

Valiokas et al. were calibrated against a thermocouple that was glued on the sample surface. The ramp rate changed over the temperature range from 0.33 K/s at room temperature to 0.26 K/s around 450 K, and to 0.18 K/s at 500 K. The system allowed us to monitor TPD of the SAMs by MS and IRAS simultaneously. For MS, a positive ion counting mass spectrometer (Hiden HAL 3F/301) was used. For IRAS, we used a modified Bruker IFS PID 22 spectrometer. It was equipped with f/16 transfer optics to guide the beam onto the sample in the UHV chamber at an angle of incidence of 82° and further into a chamber with a liquid nitrogen cooled narrow band MCT detector. The spectral resolution of the system was 2 cm-1. The IRAS monitoring during the temperature ramp was done in short consecutive series, 50 scans each. The duration of such a single series was 49 s. At the end of this period, the system required approximately 5 s to process and save the data. Thus, the actual data collection time for one spectrum was 44 s. The stacked plots in Figures 2-5 are obtained from a representative set of IRAS data from each molecule. A three-term Blackmann-Harris apodization function was applied to the interferograms before Fourier transformation. After the temperature ramp was completed, the sample was sputtered with neon ions and a background spectrum was recorded. The spectra were further analyzed using Bruker OPUS software. Results General Remarks. The mass spectra and infrared RA spectra were recorded for most of the samples simultaneously. However, in some cases, additional mass spectra were collected without IRAS monitoring. The reason for that was a higher sample-tosample variation in the MS traces, as compared to IRAS. It is important to stress, however, that the observed variations in the MS signal were always in terms of absolute intensity and not in the position of the peaks. Mass Spectrometry. Representative MS traces of the investigated SAMs are shown in Figure 1. The temperatureprogrammed desorption of SAM 1 (Figure 1a) has been extensively studied in our laboratory previously37 and the TPD trace of SAM 1 was therefore used as standard helping us to judge about the reliability of the MS traces of the other SAMs. The masses for the TP MS were chosen in the following way. A bar spectrum of each sample in the range of m/e ) 2-200 was first recorded. The masses that gave the strongest peaks in the bar spectrum were then selected for further monitoring. The same series of masses was used for all samples, thus ensuring that the recorded traces of the different compounds are comparable. The sequence of the monitored masses was m/e ) 34, 41, 43, 55, 57, 59, 60, 62, 69, 87, and 88. The pronounced m/e ) 34 signal seen for all SAMs under investigation is assigned to SH2. The other masses suggest the presence of various cracking fragments from the remaining parts of the molecules. It can be mentioned, that attempts to trace desorption of intact molecular chains cleaved at the C-S bond (m/e ) 269 and 298 for SAMs 1 and 3, respectively) were not successful. Figure 1a shows the TPD traces of the strongest signals from SAMs 1-3. The traces reveal two main steps of the desorption process. The first is associated with an appearance of a sharp peak at 423 K for SAM 1 and at 427 K for SAM 2. This peak is due to the fragment m/e ) 41. Several other peaks, with a similar shape as m/e ) 41, are observed at this temperature (not shown). However, the sulfur signal, m/e ) 34, behaves differently. An onset is seen close to the maximum of the hydrocarbon peak. Also, the shape of the m/e ) 34 peak is

Thermal Stability of Self-Assembled Monolayers

Figure 1. Temperature-programmed desorption of SAMs 1-5, obtained by mass-spectroscopy. Only the strongest signals for the corresponding SAMs are shown. In panels a and b is m/e ) 34, the thick lines, and the traces due to the hydrocarbon species, the thin lines.

complicated, with a long front developing into at least two strong overlapping peaks, apparently corresponding to different chemisorption sites at the surface. At around 475 K, the m/e ) 34 signal rapidly drops to the background level. The TPD trace is somewhat different for SAM 3 (Figure 1a). Note that the first peak of m/e ) 41 is shifted toward higher temperatures and appears at 449 K. The relative intensity of the m/e ) 41 peak is significantly lower in the trace of SAM 3 as compared to those of SAMs 1 and 2, Figure 1a. Simultaneously, an onset of the sulfur (m/e ) 34) desorption is seen. The sulfur desorption is again detected as two well-resolved peaks, at around 453 and 472 K. The signal then rapidly drops, but a shoulder is found around 480 K, revealing, in fact, the third phase in the sulfur desorption process. At this very last stage, a weak increase of the masses 41, 43, and 60 is detected (not seen at the scale shown in Figure 1a). Figure 1b shows the TP traces for SAMs formed by the long, EG4-terminated compounds 4 and 5. One difference that is seen in the traces of these SAMs, as compared to those shown in Figure 1a, is that the first desorption phase has not such a clearly defined onset. It can be observed that the MS signal, which is strongest for m/e ) 43 (m/e ) 41 is also observed), gradually increases above the background level starting at around 415 and 420 K for SAMs 4 and 5, respectively. Further on, the first peak reaches its maximum intensity at around 443 K, the same

J. Phys. Chem. B, Vol. 106, No. 40, 2002 10403 temperature for both SAMs. This broad peak is associated with an onset of the sulfur desorption, in analogy with the observations for the SAMs 1-3. However, the sulfur desorption kinetics is different for the two EG4-SAMs. For SAM 4, it gradually increases into a single feature at 470 K. Interestingly, the 470 K peak for SAM 4 is paralleled by a pronounced desorption of hydrocarbon species, giving rise to several cracking fragments. The strongest contribution comes from m/e ) 60. Note that an elevation of the m/e ) 43 level is also seen as a broad feature, vanishing at around 490 K. In the process of sulfur desorption from SAM 5, two equally strong peaks are observed at temperatures 453 and 470 K. Infrared Spectroscopy. Infrared reflection-absorption spectra of the SAMs 1-5 are shown in Figures 2-5. We refer to our previous papers regarding mode assignments of the peaks in the spectra of SAMs 3-5.20,21,34 Note that the thick lines in each IRAS series indicate when the peaks characteristic for the hydrocarbon fragments (m/e ) 41, 43, and 60) appear in the MS traces, Figure 1. SAMs 1-3. The typical shapes and frequencies of the alkyl CH2 asymmetric (d-) and symmetric (d+) stretches confirm that highly ordered assemblies are formed at room temperature. The SAMs display qualitatively the same phase behavior upon increasing the temperature, and the general behavior of the CH2 stretches in SAMs 1-3 is similar to that previously observed for long-chain alkylthiolates on gold.38-40 First, a gradual decrease in the d- and d+ intensities is observed, which can be interpreted as a reduction of the tilt angle (untilting) of the alkyl chains. The intensity change is seen in parallel with an increase in the full width at half-maximum and a gradual peak shift toward higher frequencies. This corresponds to an increase in the population of gauche defects along the alkyl chains. However, at a certain temperature, the peaks increase significantly in intensity. This step reflects a phase transition in the SAMs, in which the alkyl chains undergo an irreversible disordering into a liquidlike state. For long-chain alkylthiolate SAMs on gold, the phase transition previously has been found to occur at around 350 K.39,40 After the transition, the methylenes are randomly oriented, a situation that gives a larger average projection of the transition dipole moments of the d- and d+ modes parallel to the surface normal and an increased spectral intensity. Further on, as can be seen in Figure 2, at higher temperatures the intensity of the alkyl peaks drops again. The correlation with the mass spectra therefore suggests that the hydrocarbon part of the SAMs 1and 2 rapidly decomposes and desorbs at this point. Again, a good agreement is obtained with earlier studies, in which the partial desorption of alkylthiolates was identified near 420 K.39 The next three spectra recorded after the thick line in Figure 2 display a reduced intensity of the d- and d+ peaks, but their frequencies are not changed. At 468 K, the d- peak starts to shift toward lower frequencies, and the position is stabilized in the subsequent spectra. A broad feature around 2918 cm-1 remains, in fact, up to 530 K, indicating that a certain amount of the hydrocarbon species are permanently stuck to the gold surface. These species could appear as a product of the reaction at the high temperatures, and they are seen for all investigated SAMs 1-5. Neon ion sputtering is required to remove them from the gold surface. The decomposition and desorption reactions of SAM 3 (Figure 3) are different as compared to SAMs 1 and 2. The CH2 intensities drop on going from 452 to 461 K. The spectrum obtained at 470 K is already red-shifted, and no subsequent changes are seen upon increasing the temperature above this point. This behavior indicates that the desorption is delayed as

10404 J. Phys. Chem. B, Vol. 106, No. 40, 2002

Figure 2. Temperature-programmed desorption of SAMs (a) 1 and (b) 2, obtained by TP IRAS in the CH stretch region.

compared to the SAMs 1 and 2. Also, the infrared spectra of SAM 3 display another important feature in the fingerprint region, which can provide additional information about the mechanisms behind the decomposition and desorption processes. This is the so-called amide II mode at 1562 cm-1, a combination of C-N stretching and C-N-H bending. The strength of this mode in the spectra suggests that its transition dipole moment is aligned almost parallel to the surface normal. Another mode of the amide group, the CdO stretch (amide I) is absent in this region,20 indicating a parallel alignment of the CdO bond with respect to the surface plane. Moreover, the position of the amide II mode at 1562 cm-1 indicates that the amide groups are laterally hydrogen-bonded in SAM 3.13,14,16,17 During the temperature ramp, the amide II peak gradually shifts toward lower frequencies, a behavior that is interpreted as a lowering of the degree of hydrogen bonding.21,41 From Figure 3b, it can be seen that the red shift accelerates at around 400 K and stabilizes near 1545 cm-1 at 442 K. The intensity of the amide II decreases sharply during the next step. Note also that the drop of the amide II intensity is paralleled with the appearance of the amide I mode as a broad feature at around 1670 cm-1. The behavior of the amide I and II peaks suggests that the amide groups lose their hydrogen-bonding capabilities and become substantially disordered. Furthermore, the next spectrum recorded between 452 and 461 K reveals that the amide groups

Valiokas et al.

Figure 3. Temperature-programmed desorption of SAM 3, obtained by TP IRAS in (a) the CH stretch region and (b) the amide I and II region.

are no longer present on the surface, an observation that correlates well with the reduction of the characteristic C-H peaks (Figure 3a). SAMs 4 and 5. The conformational properties of the EG4 tails in SAMs 4 and 5 have been extensively studied in our previous work.21 Infrared RA spectra on these SAMs in the range of 293-368 K revealed two different conformations of the EG4 tails. For the COO-containing SAM 4, the oligomer adopted an amorphous conformation consisting of a mixture of all-trans and gauche ethylene glycol conformers. Contrarily, the EG4 portion of hydrogen-bonded SAM 5 was found to adopt preferentially the all-trans conformation. The TP IRAS data of the two EG4-SAMs is shown in Figures 4 and 5, respectively. The TP IRAS results in the CH region of SAM 4, Figure 4a, are similar to those observed for the CH2 stretches of the longchain alkylthiols 1 and 2, Figure 2. Therefore, the same process including the untilting and disordering of the alkyl chains is suggested for SAM 4. Interestingly, the fingerprint region of the spectra, Figure 4b, provides an additional evidence of the alkyl chain disordering, as well as information regarding the conformational changes in the EG4 portion. The CH2 progression peaks between 1340 and 1220 cm-1 disappear at around 364 K. Also, the skeletal COC stretch peak that appears at 1137 cm-1 at room temperature starts to shift toward lower frequen-

Thermal Stability of Self-Assembled Monolayers

J. Phys. Chem. B, Vol. 106, No. 40, 2002 10405

Figure 4. Temperature-programmed desorption of SAM 4, obtained by TP IRAS in (a) the CH stretch region and (b) the fingerprint region.

Figure 5. Temperature-programmed desorption of SAM 5, obtained by TP IRAS in (a) the CH stretch region and (b) the fingerprint region.

cies and loses intensity. These findings suggest that the overall disordering of the EG4 portion takes place along with the disordering of the alkyl underlayer of the SAM. Notably, the COC peak disappears completely on going from 441 to 471 K. This observation is assigned to a decomposition reaction and a cleavage of the EG4 tail from the alkyl part in SAM 4. The cleavage occurs initially above the ester group as the intensity of the CdO stretch (1740 cm-1 at room temperature) is almost unchanged at 441 K as compared to room temperature. Moreover, the d- peak appears at 2926 cm-1 (fully disordered alkyl chains), and its intensity does not change in the next three spectra (between the thick lines), Figure 4a. However, the intensity of the CH2 and CdO peaks drops rapidly on going to 480 K, suggesting an overall desorption of the alkylthiol part of the SAMs. The CH2 stretch and amide II peaks of SAM 5, Figure 5a,b, follow the behavior of the corresponding peaks in the spectra of SAM 3 up to the decomposition step. Also, the gradual weakening of the hydrogen bond (the red shift of the amide II) is paralleled with a shift of the COC peak toward lower frequencies that is characteristic for an increase in the amount

gauche conformers in the EG4 portion. At 442 K, the SAM is completely disordered. The position of the COC peak is now at 1133 cm-1, and the intensity of this peak is reduced, while the amide II peak appears at 1545 cm-1. Moreover, the amide I mode shows up in the spectra as a broad feature around 1670 cm-1. The COC and amide peaks weaken in the next few spectra (between the thick lines) and finally disappear at 482 K. However, no such dramatic changes are observed for the CH2 peaks in the same regime. The CH2 peaks start to decrease in intensity and undergo a shift on going from 472 to 491 K. Thus, the EG4 and the amide groups appear to decompose and desorb before the alkyl part. Discussion Correlation between IRAS and MS. To give a detailed comparison of the disordering and decomposition/desorption reactions seen in the SAMs 1-5, their infrared spectra (Figures 2-5) are further analyzed, and the results are summarized in Figure 6. This analysis is used to relate the specific events revealed by IRAS to those seen from the MS traces. For this reason, temperatures corresponding to the peaks of the traces

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Valiokas et al.

Figure 6. Spectral analysis of the temperature-programmed IRAS results from Figures 2-5. Section a shows SAMs 1 and 2, and sections b-d show SAMs 3-5, respectively. The normalized relative intensities of all peaks (shown as thin and thick lines) were measured as peak heights. In sections a-d filled and open circles correspond to the frequency of the CH2 d- mode. In section a, thin line (d- intensity) and open circles (dfrequency) denote SAM 1; thick line (d- intensity) and filled circles (d- frequency) denote SAM 2. In sections b-d, thick line is d- intensity and thin line amide II intensity (b and d) or ester CdO stretch intensity (c); the diamonds in sections c and d correspond to the skeletal COC stretch intensity. Also, the vertical dashed lines in all of the sections indicate temperatures corresponding to the hydrocarbon desorption peaks (m/e ) 41, 43, and 60) seen in the respective mass spectra (Figure 1).

m/e ) 41, 43, and 60 from the respective MS series are also indicated in Figure 6. Figure 6a,b compare the TP IRAS events occurring in SAMs 1 and 2 with those in the amide-linked analogue, SAM 3. The spectra of SAMs 1 and 2 display an identical phase behavior of the alkyls. The untilting of the alkyl chains proceeds up to about 360 K and is paralleled by a small shift of the d- peak to 2919 cm-1. Next, the d- intensity change levels out, while its frequency starts to increase with increasing temperature. At around 400 K the d- intensity is rapidly restored, before it starts to drop sharply. Note that the turning point in the d- intensity profile appears several kelvin later for SAM 2 as compared to SAM 1. This observation is consistent with the small delay of the m/e ) 41 peak seen in the MS traces of SAM 2 (Figure 1a). SAM 3 displays a substantial delay in the phase transition of the alkyl chains, Figure 6b. The d- intensity weakens all the way up to 420 K, but the position of this peak never exceeds 2921 cm-1. Thus, the thermal stability of the hydrogen-bonded SAM 3 is improved as compared to the linear-chain analogues of similar length. Along with the weakening of the d- intensity, the amide II peak also decreases, indicating a gradual reorientation of the amide group or a lowering of the degree of hydrogen bonding or both. At 420 K, the d- peak displays a gain in intensity, and its frequency indicates a rapid increase in the amount of gauche defects during the next few steps. At the

turning point, the alkyl chains are fully disordered. As mentioned above, the amide II intensity drops somewhat earlier than that of the alkyl peaks, suggesting that the decomposition of the SAM starts with the loss of the terminal part of the molecules, that is, the amide group and the -CH2CH2OH moiety, followed by the alkyl chains and sulfur. This is consistent also with the MS observations. The change in frequency and intensity of the d- peak of the EG4-terminated SAM 4, Figure 6c, resembles the behavior of the alkyl chains in SAMs 1 and 2. The first indication for a phase transition (disordering of the alkyls) can be identified near 360 K. Note also that the COC intensity is falling above this temperature, suggesting that disordering occurs within the entire SAM, including the oligomer portion. The drop in the intensity of the COC peak occurs slowly over a fairly broad temperature span (∼100 K) until all traces of ethylene glycol species are lost in the infrared spectra. We believe that this is due to an initially slow disordering and decomposition of the EG4 tail that accelerates near 430 K and appears as a broad and ill-defined hydrocarbon peak (m/e ) 43) at 443 K in the MS traces, Figure 1b. The loss of the d- and CdO intensities is indeed delayed as compared to the COC peak intensity. The coincidence of this latter step with the second peak of the hydrocarbon species and the sulfur peaks in the MS trace is in favor of a two-step decomposition mechanism that is initiated by preferential cleavage of the molecules above the linking group, followed

Thermal Stability of Self-Assembled Monolayers by decomposition and desorption of -S(CH2)15-COO-. This process shows up in MS as a broad SH2 peak (m/e ) 34) and multiple hydrocarbon peaks (m/e ) 60, 59, 41, and 43). Figure 6d illustrates that hydrogen bonding in SAM 5 has exactly the same effect on the phase behavior as that in the shorter SAM 3. The rearrangement of the SAMs takes place at around 410-430 K. Up to this temperature, no significant changes are seen for the EG4 portion of the SAM. Then, the intensities of the COC and amide II peaks drop simultaneously, reaching 50% of their initial intensity values around 440 K. At the same temperature, the d- mode increases sharply in frequency. This simultaneous reduction of the amide and COC peaks is in favor of a cleavage of the molecules below the amide moiety. However, it should be stressed that the EG4 and alkyl portions decompose and desorb within a relatively narrow range of temperatures for the amide-linked SAM 5 as compared to that for the ester-linked analogue 4. For the latter, the decomposition and desorption processes of the EG4 and alkyl parts are found to occur over a broader range of temperatures (∼100 K). The discussed TP IRAS observations, as well as the separated peaks appearing in the MS traces, suggest that the TPD process for SAMs 1-5 occurs via a decomposition reaction of the constituent molecules on the surface rather than as a desorption of the intact molecules. A similar process, involving cleavage of the C-S bond, was observed for butanethiolate on Au(100).42 In the present study of SAMs 1-3, a large number of wellresolved peaks, originating from fragmentation of the alkyl chains, are seen in the TP MS traces followed by a complex pattern of peaks due to desorption of sulfur (m/e ) 34). The IRAS data is also in favor of the C-S cleavage mechanism, because the appearance of the first peak in the MS traces of SAMs 1-3 occurs in parallel with a significant reduction of the peak intensities for the CH stretching modes. However, the assignment of the observed masses to specific cracking fragments of the molecules is not straightforward because of the large number of possible reactions on the surface. Tentatively, the peak of m/e ) 41 can be attributed to C3H5, and the masses 55 and 69 (not shown) are most likely due its homologues. Likewise, m/e ) 43 (the next strongest peak for SAMs 1-3) and 57 could be due to fragment C3H7 and its homologue, respectively. The m/e ) 60 is assigned to C2H4O2 or perhaps C3H8O. A surprising result is that the most prominent peaks in the MS traces of the EG4-SAMs 4 and 5 are the same as those for SAMs 1-3. The only difference is that m/e ) 43 is somewhat stronger than m/e ) 41 for the EG4-SAMs. Moreover, the IRAS data clearly show that the broad 443 K peaks (m/e ) 43) in the MS traces of the EG4-SAMs are related to the loss of the EG4 tail. Thus, the m/e ) 43 peak is most likely due to the C2H3O fragment from the decomposing EG4 tail. All SAMs under investigation display a second feature in the MS signal at temperatures close to the final decrease of the m/e ) 34 signal. However, the intensities of these late features are often very small as compared to the first hydrocarbon peak (not seen on the scale used in Figure 1). This suggests that most of the alkyls have decomposed and desorbed before reaching this final step. The exception is SAM 4, which displays an increase in intensity of the masses m/e ) 60 and 59 at ∼470 K. The TP IRAS data also show that the intensity of the alkyl CH peaks are decreasing less steeply with temperature after the maximum at ∼420 K, Figure 6c, and that a significant amount of -COO- groups is present on the surface all the way up to 471 K. Therefore, the masses m/e ) 60 and 59 indicate a delayed

J. Phys. Chem. B, Vol. 106, No. 40, 2002 10407 TABLE 1: Desorption Enthalpies (kJ/mol) Calculated for the Peaks in the TP MS Traces Shown in Figure 1 hydrocarbon fragments 1 Au/SC18OH 2 Au/SC22OH 3 Au/SC15CON-EG1 4 Au/SC15COO-EG4 5 Au/SC15CON-EG4

a

b

119 120 126 124 124

135 135 136, 137 134 134

m/e)34 c

d

e

119 128 134 121 127 132 127 134,138f 134 127 134

a Main (first) hydrocarbon peak. b Desorption peak related to the final decrease in the m/e ) 34 signal, very weak except for SAM 4. c Appear as an onset close to the first hydrocarbon peak. d,e Strong sulfur peaks, often of comparable intensity. f Shoulder.

decomposition/desorption process of the alkyl chains (including the linking group) in the temperature range 471-490 K. Energetics of TPD. As mentioned, the TP MS traces reveal that the sulfur desorbs from at least two chemisorption states on the gold surface, Figure 1. To identify the exact peak positions, the m/e ) 34 traces in Figure 1 were smoothed, and the second derivatives were calculated. Further on, the Redhead method was applied to the obtained peak temperatures, assuming the first-order desoprtion process.43 A preexponential factor ν ) 1013 s-1 44 was used, and the ramp rate was 0.26 and 0.18 K/s for the peaks below and above 460 K, respectively. We are aware of the fact that a first-order model is an oversimplification of the desorption processes under investigation. We prefer, however, to utilize this approach to be able to make a comparison with the desorption enthalpies available from previous studies. The desorption enthalpies obtained using the above-mentioned method are shown in Table 1. The enthalpies related to the first decomposition peak (column a) are most likely due to cleavage of intramolecular bonds, C-S and C-C. The assignments of the different chemisorption states, seen from the m/e ) 34 peaks, are far more complicated. Previous studies have shown that the annealing and desorption process of alkylthiolates follows different reaction schemes, for example, formation of disulfides.45-47 Further on, a thermal reorganization of the gold surface has been proposed to affect the chemisorption states.48 Also, if a certain amount of the thiolates retain alkyl fragments of varying length after the first decomposition step, they will interact laterally as well with the gold and thereby contribute to the observed desorption enthalpy. It has been suggested that such a contribution can be as high as 6-8 kJ/mol per methylene unit.48,49 Thus, the hydrocarbon peaks that are seen together with the desorption of the last m/e ) 34 peak at ∼470 K are expected to be due to such species (column b, Table 1). It should also be remembered that the samples used in this study are prepared on polycrystalline gold substrates. It is therefore not unlikely that some of subtle features (shoulders) in the complex sulfur desorption pattern originate from chemisorption states at grain boundaries and other imperfections on the gold surface. The first of the two sulfur chemisorption states (column d, Table 1) gives an average desorption enthalpy of 127 kJ/mol that does not vary with the complexity (size) of the compounds. A similar and constant enthalpy of 126 kJ/mol was obtained by Lavrich et al. for a series of alkylthiols and disulfides on Au(111).48 Nuzzo et al. found that that the heat of adsorption for a methylthiolate on Au(111) was ∼117 kJ/mol.49 The same authors later concluded from another TPD study that the activation energy for the desorption of HS-(CH2)15-X SAMs, terminated by methyl, amide, or carboxylic acid groups, was ∼167 kJ/mol.50 They attributed the enhanced thermal stability

10408 J. Phys. Chem. B, Vol. 106, No. 40, 2002 of these SAMs to the van der Waals interactions between the alkyls. We have not been able to observe desorption enthalpies that high for any of our compounds. The highest values, ∼134 kJ/mol, are observed for the m/e ) 34 peak, the second chemisorption state (column e, Table 1) corresponding, most likely, to desorption of thiolates bearing short alkyl chains of varying length.48,49 It would also be interesting to employ the above desorption enthalpies in the estimation of the energetic contribution of the lateral hydrogen bonding to the overall thermal stability of the SAMs. As seen from Figure 1 and Table 1, the 26 K difference between the desorption peaks of SAM 1 and its analogue 3 yields a difference in the desorption enthalpies of 7 kJ/mol. In fact, this value corresponds to the difference between interacting amide groups and the van der Waals interaction between adjacent methylene groups. Because the lateral interaction per methylene has been estimated to be at least 5 kJ/mol,50 the overall contribution of the hydrogen bond is around 12 kJ/mol. This value is significantly lower than the energy of hydrogen bonding between the amide groups, 20-25 kJ/mol, obtained from other studies.51,52 The discrepancy between our results and the cited results may have following explanations. First of all, the Readhead method is, as mentioned before, poorly applicable in this case because the real desorption process is most likely not of first order, the preexponential factor used is not relevant, or both. However, a sensitivity test based on the Readhead equation with the preexponential factor varying over a couple orders of magnitude (1012 < ν < 1015) revealed that the choice of ν has a marginal effect on the calculated desorption enthalpy ((1 kJ/mol). The second explanation takes into account the considerations about structural differences among the SAMs 1, 2, and 3. The amide group may very well force the alkyl tail to adopt a conformation different from the all-trans one, especially close to the terminal OH group. Certain conformational differences in the terminal chain CH2-CH2-OH group have been observed in a previous IRAS and contact angle study.20 If the terminal group is more disordered, then its contribution to the overall desorption enthalpy is lower than that for the same number of methylene groups in a perfectly organized all-trans SAM. The third and perhaps most plausible explanation is that changes in the alignment of the hydrogen bonds or breakage of the hydrogen bonds occur close to the desorption temperature. Note that the IRAS data clearly shows that the amide II peak is red-shifted just before the desorption, a phenomenon characteristic for a reduction of the hydrogen-bonding strength.13,21,41 Thus, the present qualitative analysis appears not to provide an exact account for the strength of the hydrogen bond and its contribution to the overall thermal stability of the amidecontaining SAMs. Nevertheless, our simple analysis indicates that the hydrogen-bond interaction of a -CONH-CH2CH2OH moiety is larger than that of a (CH2)3OH moiety by ∼7kJ/ mol and thereby contributes to the improved thermal stability of the amide-containing SAMs. Although the calculated contribution of the introduced amide group to the overall desorption characteristics is smaller than expected, our TP IRAS data provide conclusive evidence for an improved conformational stability of the hydrogen-bonded SAMs. The EG1- and EG4-SAMs 3 and 5 display a considerable delay of the alkyl chain disordering as compared to the linearchain and ester-linked thiol analogues 1, 2, and 4. Thus, it appears that the alkyls are locked in a stable all-trans conformation by the amide groups, regardless of the complexity of the terminal group. This improvement in conformational robustness is of enormous practical importance when utilizing SAMs as

Valiokas et al. templates for further functionalization at elevated temperatures and in chemically harsh environments. Conclusions The present study shows that the thermal stability of hydroxyalkanethiolate SAMs on gold can be improved by introducing an amide group in the vicinity of the terminal group. The resulting lateral hydrogen bonding has a 2-fold effect. First, such SAMs retain a higher degree of order within a broader range of temperatures, around 50 K, as compared to the nonhydrogen-bonded analogues. Second, the desorption of the hydrogen-bonded SAMs formed by long-alkyl chain thiolates on gold is delayed by 26 K, corresponding approximately to an increase of the desorption enthalphy of 7 kJ/mol. When the complexity of the compounds is increased by attaching tail groups such as EG4, the alkyl underlayer generally follows the same phase behavior as the nonderivatized analogues, that is, the onset of the overall disordering is delayed by approximately 70 K for the hydrogen-bonded SAM. However, the temperature of the decomposition/desorption is independent of the linking group and the corresponding intermolecular interactions. Instead, our study shows that the onset of the decomposition is associated with the loss of the EG4 portion and that the mechanisms of the cleavage of the different molecules depend on the type of the linking group. Acknowledgment. R.V. and S.S. thank the Graduate School Forum Scientum, which is funded by the Swedish Foundation for Strategic Research (SSF). This work is also supported jointly by the Swedish Research Council for Engineering Sciences (TFR) and the Biomimetic Materials Science program (SSF). References and Notes (1) Ulman, A. An Introduction to Ultrathin Organic Films; Academic Press: San Diego, CA, 1991. (2) Nuzzo, R. G.; Allara, D. L. J. Am. Chem. Soc. 1983, 105, 44814483. (3) Porter, M. D.; Bright, T. B.; Allara, D. L.; Chidsey, C. E. D. J. Am. Chem. Soc. 1987, 109, 3559-3568. (4) Ulman, A. Chem. ReV. 1996, 96, 1533-1554. (5) Evans, S. D.; Goppert-Berarducci, K. E.; Urankar, E.; Gerenser, L. J.; Ulman, A.; Snyder, R. G. Langmuir 1991, 7, 2700-2709. (6) Evans, S. D.; Urankar, E.; Ulman, A.; Ferris, N. J. Am. Chem. Soc. 1991, 113, 4121-4131. (7) Kang, J. F.; Ulman, A.; Liao, S.; Jordan, R. Langmuir 1999, 15, 2095-2098. (8) Reese, S.; Fox, M. A. J. Phys. Chem. B 1998, 102, 9820-9824. (9) Mowery, M. D.; Menzel, H.; Cai, M.; Evans, C. E. Langmuir 1998, 14, 5594-5602. (10) Lenk, T. J.; Hallmark, V. M.; Hoffmann, C. L.; Rabolt, J. F.; Castner, D. G.; Erdelen, C.; Ringsdorf, H. Langmuir 1994, 10, 4610-4617. (11) Tam-Chang, S. W.; Biebuyck, H. A.; Whitesides, G. M.; Jeon, N.; Nuzzo, R. G. Langmuir 1995, 11, 4371-4382. (12) Sabapathy, R. C.; Bhattacharyya, S.; Leavy, M. C.; Cleland, W. E.; Hussey, C. L. Langmuir 1998, 14, 124-136. (13) Clegg, R. S.; Hutchison, J. E. Langmuir 1996, 12, 5239-5243. (14) Clegg, R. S.; Reed, S. M.; Hutchison, J. E. J. Am. Chem. Soc. 1998, 120, 2486-2487. (15) Zhang, J.; Zhang, H. L.; Chen, M.; Zhao, J.; Liu, Z. F.; Li, H. L. Phys. Chem. Chem. Phys. 1998, 102, 701-703. (16) Clegg, R. S.; Reed, S. M.; Smith, R. K.; Barron, B. L.; Rear, J. A.; Hutchison, J. E. Langmuir 1999, 15, 8876-8883. (17) Clegg, R. S.; Hutchison, J. E. J. Am. Chem. Soc. 1999, 121, 53195327. (18) Duevel, R. V.; Corn, R. M. Anal. Chem. 1992, 64, 337-342. (19) Yan, L.; Marzolin, C.; Terfort, A.; Whitesides, G. M. Langmuir 1997, 13, 6704-6712. (20) Valiokas, R.; Svedhem, S.; Svensson, S. C. T.; Liedberg, B. Langmuir 1999, 15, 3390-3394. (21) Valiokas, R.; Svedhem, S.; Ostblom, M.; Svensson, S. C. T.; Liedberg, B. J. Phys. Chem. B 2001, 105, 5459-5469. (22) Pale-Grosdemange, C.; Simon, E. S.; Prime, K. L.; Whitesides, G. M. J. Am. Chem. Soc. 1991, 113, 12-20.

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