Nanoconfinement Effects on the Glass–Liquid Transition of Vapor

Mar 15, 2012 - ... the interface of Ni(111) is thought to play a role in the propagation of solidity in the film interior, thereby quenching surface-i...
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Nanoconfinement Effects on the Glass−Liquid Transition of VaporDeposited 1-Pentene Ryutaro Souda International Center for Materials Nanoarchitectonics, National Institute for Materials Science, 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan

ABSTRACT: The glass-transition temperature, Tg, of supported thin films is modified by the presence of free surface and substrate interface. To clarify the properties of nanoconfined liquids, the interactions of 1-pentene films with various substrates were investigated as a function of temperature using time-of-flight secondary ion mass spectrometry (TOF-SIMS) and time-offlight ion scattering spectroscopy (TOF-ISS). A monolayer of 1-pentene wets a Si substrate; surface mobility occurs in the sub-Tg region (50−60 K), as revealed by the uptake of molecules into a porous Si substrate. The enhanced surface mobility induces subTg dewetting of monolayer and multilayer films formed on a perfluoroalkyl-modified substrate. When a multilayer of 1-pentene is deposited on Ni(111), the topmost layer exhibits a liquidlike nature in the sub-Tg region, but mobility in the film interior is not induced up to Tg = 70 K. The adhesive interaction at the interface of Ni(111) is thought to play a role in the propagation of solidity in the film interior, thereby quenching surface-induced processes in the glass−liquid transition of thin films. The enhanced surface mobility is also associated with the structural transformation of porous 1-pentene films in the sub-Tg region.

1. INTRODUCTION The glass transition is a phenomenon that has long been observed commonly for metals, simple molecules, polymers, and ionic liquids. However, there has been intense interest in the modification of Tg by nanoconfinement.1 The most commonly studied system in this respect is polystyrene films.2−10 A large decrease in Tg relative to the bulk value has been observed from thermal expansivity measurements3,4 and explained as resulting from the presence of a liquidlike layer at the polymer−air interface. On the other hand, an increase in Tg was observed for an ultrathin poly(methyl methacrylate) film supported on a SiO substrate because an immobilized layer (a “dead layer”) formed at the substrate interface through hydrogen bonds.2 To date, numerous studies have reported that the Tg values of thin polymer films are reduced relative to the bulk value because of nanoconfinement. However, this phenomenon might be related to segmental motion rather than center-of-mass diffusion of polymer molecules; a few studies have claimed that Tg is independent of nanoconfinement.11−13 Using low-molecular-weight glass formers such as 3-methylpentane, Cowin and co-workers14 found that near-surface regions of glassy films are about 6 orders of magnitude less viscous than the bulk, whereas stiffening occurs at the interface © 2012 American Chemical Society

with a Pt substrate. This study suggested that the molecular mobility might be different locally, depending on the distance from the free surface and substrate interface. I have investigated the glass−liquid transition of vapordeposited molecular solid films using time-of-flight secondary ion mass spectrometry (TOF-SIMS).15−24 Despite the high surface sensitivity of TOF-SIMS, bulklike glass-transition behaviors have been observed for thin films of water,15 ethanol,16 toluene,17 ethylbenzene,18 3-methylpentane,19 and ionic liquids20 deposited on a Ni(111) substrate. The role of the substrate in glass transitions in thin films has also been discussed for water using hydrophobic substrates such as graphite,21 a vitrified ionic liquid,22 and hydrogen-terminated silicon.23 A supported monolayer is of particular interest in terms of nanoconfinement because the interplay between the free surface and the substrate interface can be explored in a straightforward manner through modification of the experimental Tg value. On the other hand, thin glassy films deposited from the gas phase are characterized by a higher enthalpy and a Received: September 16, 2011 Revised: March 1, 2012 Published: March 15, 2012 7735

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lower density (i.e., a microporous structure25,26), so that they undergo structural relaxation before supercooled liquids are formed. The structures of liquid-quenched glasses might be different from those of vapor-deposited glasses, but little is known about structural relaxation processes toward the equilibrium of supercooled liquid. Recently, it was demonstrated that structural transformation processes of thin films can be explored using time-of-flight ion scattering spectroscopy (TOF-ISS).27 This article reports a study of the mobility, morphology, and local structural changes of 1-pentene films on the basis of the temperature evolutions of TOF-SIMS and TOF-ISS intensities, to gain more insight into structural-relaxation processes and nanoconfinement effects on the glass−liquid transition. 1Pentene was used because its glass transition and structural relaxation in the bulk have been studied calorimetrically using two different samples prepared by vapor deposition and liquid quenching:28 Although the enthalpy relaxation rate of the vapor-deposited sample was much higher than that of the liquid-quenched one, the two samples exhibited essentially the same Tg value of 70 K after aging of the former at temperatures lower than Tg. To shed light on the effects of substrates, the interactions of 1-pentene with Ni(111), perfluoroalkyl-modified Ni, and hydroxylated Si were investigated; the surface mobility was explored by monitoring the uptake behaviors of adsorbed 1-pentene molecules into porous Si, as well as the solvation of D2O adspecies on thicker 1-pentene films.

heating. Specifically, the cleaned Ni plate was immersed immediately in an ethanolic solution (0.04 mmol/mL) of tricosafluorododecanoic acid [CF3(CF2)10COOH] for 20 min. The initially hydrophilic Ni surface became water- and ethanolrepellent after this treatment. A hydroxylated Si substrate was prepared through a UV−ozone treatment of a native oxide layer using a low-pressure mercury-vapor lamp after the Si had been degreased ultrasonically in ethanol. A porous Si substrate was prepared according to the procedure described in the literature.29,30 Briefly, a Si(100) wafer (p-type, resistivity of 0−0.02 Ωcm) was anodized in an electrolyte of a 1:1 mixture (by volume) of HF (55 wt %) and ethanol (99.5%) at a current density of 100 mA/cm2 for 3 min. It is known that high-density mesopores with a preferred growth direction perpendicular to the surface are created by this procedure.30 The porous Si substrate thus prepared was terminated with hydrogen; it was hydroxylated through the UV−ozone treatment in air. The sample was introduced immediately into the UHV chamber through a load-lock chamber evacuated with oil-free pumps. The temperature was ramped at a rate of 5 K min−1 for both TOF-SIMS and TOF-ISS measurements.

3. EXPERIMENTAL RESULTS Coverage of molecular solid films was determined based on evolution curves of sputtered ion intensities using TOFSIMS.15−17 Figure 1 shows the intensities of typical secondary

2. EXPERIMENTS Primary beams of 2-keV He+ and H2+ ions were used for TOFSIMS and TOF-ISS measurements, respectively. They were generated in an electron-impact-type ion gun (Specs, IQE 12/ 38) and chopped into pulses using electrostatic deflection plates. Substrates were placed in an ultra-high-vacuum (UHV) chamber with a base pressure of less than 1 × 10−8 Pa. For TOF-SIMS experiments, an electric field of 125 V/mm was applied to the sample surface to extract low-energy secondary ions; positive ions ejected perpendicularly to the surface were detected using a microchannel plate after they had traveled through a field-free TOF tube. In TOF-ISS, H+ and total (H0, H+, and H−) yields scattered from grounded samples were measured independently under forward-scattering conditions (glancing and scattering angles were 12 and 20°, respectively). The scattered ions were separated from neutrals using deflection plates and a movable TOF tube connected to the UHV chamber through a bellows. The H− yield (not shown in this article) was almost an order of magnitude smaller than the H+ yield. The fluence of the primary beams was kept below 1 × 1012 ions cm−2 to reduce damage to the sample surface. The substrate was cooled to 20 K by means of a closed-cycle helium refrigerator. The temperature was monitored using Au(Fe)-chromel thermocouples and controlled by a digital temperature programmer. Samples of 1-pentene (99%, Aldrich) and heavy water (99.8%, Kanto Kagaku) were purified by freeze−pump−thaw treatments before use. The molecules were admitted through precision leak valves, and thin films were deposited onto substrates by backfilling the UHV chamber. A Ni(111) substrate was cleaned under UHV by electron-beam heating from behind (∼1200 K) and ion-beam bombardment of the sample surface. The surface cleanliness was checked in situ based on TOF-SIMS spectra. A perfluoroalkyl monolayer was grown on a mirror-finished polycrystalline Ni plate after it had been cleaned in the UHV chamber by electron-beam

Figure 1. Intensities of C3H5+ and Ni+ ions sputtered from a Ni(111) substrate held at 20 K as a function of 1-pentene exposure. Also shown are CF+ and SiOH+ intensities from perfluoroalkyl-modified Ni and hydroxylated Si substrates, respectively. Their initial intensities are normalized relative to the peak intensity of Ni+.

ions sputtered from Ni(111), perfluoroalkyl-modified Ni, and hydroxylated Si substrates as a function of 1-pentene exposure, together with the C3H5+ intensity from adspecies on Ni(111). Ni+ ions are not emitted from the clean Ni(111) surface; the Ni+ intensity increases almost linearly up to ca. 10 L (langmuir, 1 L = 1.3 × 10−4 Pa s) and then begins to decrease. The intensity of the fragment C3H5+ ion tends to saturate at around the Ni+ peak position. Therefore, this exposure was assigned as the formation of one monolayer (1 ML). The film thickness was estimated from this value by assuming that the sticking probability of the molecule is unity. On the other hand, the intensities of CF+ and SiOH+ ions decrease monotonically with exposure; their emission is suppressed considerably when a 1pentene monolayer is formed. Thus, (sub)monolayer sensitivity of adspecies is fundamentally attainable based on the CF+ and 7736

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SiOH+ ions, whereas the Ni+ ion signal provides information about multilayer films as well. Figure 2 shows the temperature evolutions of typical secondary ion intensities from a 10-ML 1-pentene film

Figure 3. Temperature-programmed TOF-SIMS intensities from D2O- (0.5 ML) adsorbed 1-pentene (40 ML). The 1-pentene film was deposited on Ni(111) at 70 K; the D2O molecules were adsorbed on the 1-pentene film at 20 K.

on the surface are diffusive in the sub-Tg region: The mobile molecules take part in solvation in part of the D2O adspecies. However, the uptake of D2O into the film interior occurs only after the supercooled liquid is formed at Tg = 70 K, as evidenced by a steep increase in the C3H5+ intensity (i.e., the evolution of free 1-pentene molecules on the surface). The film might crystallize by annealing at higher temperatures. In TOFSIMS measurements, the occurrence of crystallization has been identified as more significant dewetting than observed during the glass transition,17,18 as well as quenching of solvation at Tg.31 However, a result quite similar to that shown in Figure 3 was obtained when a 1-pentene film annealed to 120 K was used (not shown), indicating that 1-pentene is not crystallized in this temperature range. The interaction of the 1-pentene monolayer with the hydroxylated Si substrate was investigated as shown in Figure 4a. The C3H5+ and SiOH+ intensities are fundamentally invariant up to 110 K because of the formation of a wetting monolayer. The emission of SiOH+ from the substrate is suppressed; no indication of a film morphology change is observed at Tg in contrast to the multilayer film formed on Ni(111). This result might be explained as the formation of an immobilized monolayer by a strong adhesive interaction. The nature of such a wetting monolayer was further explored using a porous Si substrate as shown in Figure 4b. The 1-pentene molecules tend to be incorporated in pores at T > 55 K because of the occurrence of surface mobility. The result obtained using a 10-ML film on the porous Si substrate is shown in Figure 4c. A steep increase in the SiOH+ intensity at around Tg indicates that a multilayer is also incorporated into the pores. The increase in the C3H5+ intensity at T > 60 K might be associated with the occurrence of surface mobility on the multilayer film: Mobile molecules on the topmost layer can diffuse into pores sequentially prior to the occurrence of bulk mobility at Tg. This result indicates that the invariance in morphology of the 1pentene monolayer on the nonporous Si substrate does not necessarily mean the formation of a dead layer. Probably, the molecules can move laterally without forming three-dimensional droplets through the attractive interaction with the substrate.

Figure 2. Temperature-programmed TOF-SIMS intensities from a 1pentene- (10 ML) adsorbed Ni(111) substrate. The temperature was ramped at a rate of 5 K min−1.

deposited on Ni(111) at 20 K. The film is sufficiently thick that only the fragment ions from 1-pentene are detectable initially. Upon heating, Ni+ ions emerge at around 70 K because of the film morphology change. Note that this temperature agrees well with the bulk Tg value of 1-pentene.28 The intensity of C3H5+ does not change at around this temperature. This result can be interpreted based on the coverage dependence of the ion intensities shown in Figure 1: The C3H5+ intensity is almost invariant after the completion of a monolayer, whereas Ni+ ions can be emitted through multilayers (∼5 ML). Therefore, the film morphology change is thought to be moderate without the creation of completely dried patches, suggesting high wettability of supercooled liquid on the Ni(111) substrate. The film evaporates at around 125 K where the C3H5+ (Ni+) intensity drops (jumps). A peak occurs in the Ni+ intensity because a monolayer is formed transiently during film evaporation (see Figure 1). A small amount of 1pentene residues is present at higher temperatures, as manifested by the decay curves of the Ni+ and C3H5+ intensities. The Ni+ intensity remaining at T > 160 K arises from water contaminants as revealed from the emission of Ni+(H2O)n ions. The mobility of molecules on the surface and in the film interior can be explored through the solvation behaviors of adspecies. For this purpose, a 1-pentene film (40 ML) was formed on Ni(111) at 70 K, and then D2O adspecies (0.5 ML) were deposited on the film at 20 K. Figure 3 shows temperature-programmed TOF-SIMS intensities from this surface. The 1-pentene film used here was so thick (4 times thicker than that used in Figure 2) that no morphological change at temperatures higher than Tg is identifiable on the basis of the Ni+ intensity (not shown). The D+ and D3O+ ions decrease gradually in intensity with increasing temperature and disappear from the surface up to 80 K. The H+ intensity increases gradually across Tg in comparison with the C3H5+ intensity. These results indicate that the 1-pentene molecules 7737

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Therefore, the increase in the CF+ intensity at 50 K is attributable to droplet formation of the 1-pentene adspecies. Thus, surface mobility of the molecules apparently occurs at this temperature. The evolutions of the CF+ intensities from the same substrate covered with multilayers of 1-pentene are shown in Figure 5b. Although the onsets of CF+ emission shift to higher temperatures with increasing film thickness, sub-Tg dewetting is identifiable for films thinner than 4 ML. Here, the emission of CF+ means that completely dried holes open even for multilayer 1-pentene films. This behavior contrasts sharply with the dewetting of the 1-pentene film on Ni(111) (Figure 2), where the occurrence of a small modulation in film morphology can be probed by Ni+ without the opening of dried holes. The TOF-ISS intensity of the H+ ions from the 1-pentene film (20 ML) formed on Ni(111) is shown in Figure 6a as a

Figure 4. Temperature-programmed TOF-SIMS intensities from 1pentene deposited on hydroxylated Si substrates with and without mesoporous layers. The results for 1-pentene monolayers formed on (a) nonporous and (b) porous Si substrates are compared, together with those for (c) 10 ML of 1-pentene on porous Si.

The attractive interaction is expected to be reduced on substrates with lower surface energies. Figure 5a shows TOF-

Figure 6. Temperature-programmed TOF-ISS intensities of the scattered (a) H+ ions and (b) total species (H+, H−, and H0) from a 20-ML 1-pentene film deposited on Ni(111) at 20 K.

function of temperature. The H+ intensity just after film deposition at 20 K is enhanced considerably relative to that in the range of 60−120 K. Note that the onset of the H+ intensity decay agrees well with the onset of surface diffusion rather than Tg. On the other hand, the intensity of total scattered species (H+, H0, and H−) exhibits a relatively small but completely different temperature evolution, as seen in Figure 6b. The intensity jumps steeply at around Tg and then decays gradually until the multilayer evaporation temperature of 120 K; the subsequent increase at higher temperatures corresponds to the desorption of residues. The difference between the H+ and total yields can be attributed to ionization/neutralization processes. The positive ion fraction, estimated from the ratio of the H+ yield to the total yield, is around 1−5%.

Figure 5. (a) Temperature-programmed TOF-SIMS intensities from a monolayer of 1-pentene adsorbed on a perfluoroalkyl-modified Ni substrate. (b) CF+ intensities from the same substrate on which multilayer 1-pentene films (2−10 ML) were deposited at 20 K.

SIMS intensities from a monolayer of 1-pentene formed on the perfluoroalkyl-modified Ni surface as a function of temperature. The CF+ intensity from the substrate evolves at T > 50 K, where the C3H5+ intensity from 1-pentene decreases. As shown in Figure 1, the emission of CF+ from the substrate is quenched almost completely when a monolayer of 1-pentene is present.

4. DISCUSSION The charge exchange mechanism of energetic H+ ions in solids and at the surface has been discussed in previous articles.27,32,33 Primary H2+ ions can penetrate into the thin-film interior and 7738

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softened layer was determined as 3 nm, which corresponds to ca. 7 ML of 3-methylpentane. The softened layer appears to be thicker than that observed in the present study. This discrepancy might be attributable to the difference in microscopic film structure (i.e., porosity):34 Longer-range transport of molecules into the film is likely to occur through surface diffusion on mesopore walls during the structural relaxation process. The formation of either a two-dimensional liquid or a threedimensional droplet is determined by the interplay between the adhesive force (adsorbate−substrate interactions) and the cohesive force (adsorbate−adsorbate interactions). Cohesion leads to cooperativity for liquid flow, whereas adhesion concerns wettability. If the adhesive interaction prevails over the cohesive interaction, the molecules are expected to hop individually, especially at submonolayer coverage, forming much slower diffusers (corresponding to scattered residues observed in this study). This is, in fact, the case for alcohols on hydroxylated Si substrates23 because strong adsorbate− substrate hydrogen bonds weaken intermolecular interactions. In general, however, the wetting monolayer is not necessarily regarded as such a dead layer, because molecules diffuse on the surface without changing film morphology, as demonstrated here. Consequently, Tg of the 1-pentene monolayer is reduced by nanoconfinement; cooperative motion of physisorbed molecules is ensured by the intermolecular cohesive interaction without influence of the adhesive interaction with the substrates. With increasing film thickness, the mobility of molecules in the film interior decreases relative to that on the topmost surface layer. This is true for a substrate with a higher adhesive force because the immobilized layer at the interface might play a role in the propagation of solidity throughout the film except for the two-dimensional liquid formed on the topmost layer. The two-dimensional liquid appears to induce a morphological change of very thin layers, as observed for the substrate with smaller adhesive energy (see Figure 5b). On the other hand, the formation of a cooperatively rearranging region (CRR) has been recognized as a key idea for elucidating the glass transition in the bulk.35−38 The size of the CRR has been estimated as 1− 4 nm,39 so that Tg is expected to be depressed on length scales smaller than this value. In the present study, sub-Tg dewetting of 1-pentene occurs for films thinner than ca. 4 ML, which corresponds well to the size of the CRR when the nominal thickness of 1 ML (∼0.3−0.4 nm) is taken into account. Therefore, sub-Tg dewetting of 1-pentene on the perfluoroalkyl-modified substrate might be associated with a reduction in the size of the thin films’ CRR. The two-dimensional liquid arises as an extreme of nanoconfinement. Supported liquid films tend to form droplets above Tg, although no consensus has been reached regarding the underlying physics of dewetting. A relatively thick film dewets the substrate as a result of the formation of dried holes through thermal nucleation40 or heterogeneous nucleation initiated by defects.41 In contrast, thinner films are known to form uniformly distributed surface undulations that exhibit a clearly defined wavelength (spinodal dewetting).42,43 In any case, dewetting is thought to be initiated when valleys formed during structural fluctuations reach the substrate. This is the case for monolayers and multilayers of 1-pentene on the perfluoroalkylmodified substrate, where completely dried patches and droplets are finally created. On the Ni(111) substrate, however, the fact that the fragment-ion intensities from 1-pentene are

dissociate completely during violent collisions with target species. The neutralization probability of H+ depends on the nature of bonding of target species32,33 because the H 1s orbital (13.6 eV) satisfies a resonance condition with the valence orbitals of molecules. Energetic H0 atoms can be (re)ionized during violent collisions by the electron promotion mechanism. Consequently, the energetic hydrogen in solids undergoes capture and loss sequences of valence electrons;33 the charge state of scattered particles is fixed just before leaving the surface. The total yield of H+, H0, and H− species is determined by the scattering cross section of molecules in the film along the trajectory of the primary beam. Therefore, the increase in the total yield at Tg is attributable to the change in film morphology during the glass−liquid transition. In contrast, the decrease in the H+ intensity at T > 57 K arises from changes in the configurations of the molecules: The neutralization probability of H+ can be enhanced when the 1-pentene molecules are bound to their surroundings tightly because the H 1s hole becomes more delocalized. Therefore, this phenomenon is thought to be associated with film densification (i.e., structural relaxation). Molecular solid films deposited at temperatures lower than Tg are expected to be low in density, characterized by a microporous structure. The loosely bound molecules in the film interior are likely to become mobile in the sub-Tg region, as enhanced mobility on the free surface. The densification of porous films might result from direct transport of molecules through microscopic pores or surface diffusion of molecules on mesopore walls. The good agreement of the onset temperatures between surface diffusion and structural relaxation supports this assignment. On the other hand, the calorimetric study reveals no clear onsets of the structural relaxation:28 The heat capacities below Tg correspond to the vibrational degrees of freedom; the exothermic effect of the vapor-deposited 1pentene starts just above the deposition temperature and continues to Tg = 70 K. Thus, the structural relaxation observed in calorimetry has nothing to do with the translational surface diffusion, suggesting that the reorientation of molecules also plays a role in the structural transformation of vapor-deposited glassy films. The occurrence of a well-defined onset of surface diffusion has an analogy with the glass−liquid transition in the bulk. Therefore, the mobile layer formed on the topmost surface can be regarded as a two-dimensional liquid. That the onset of surface diffusion is independent of the substrate implies that adsorbate−adsorbate interactions play an important role, which ensures cooperativity for liquidlike flow. The two-dimensional liquid on the hydroxylated Si surface does not result in droplet formation because small adsorbate−substrate attractive interactions contribute to film spreading (i.e., wetting). The presence of a two-dimensional liquid on the multilayer film of 1-pentene is also identified as the sub-Tg solvation of D2O on the surface (Figure 3). The D2O adspecies tend to be agglomerated into clusters, as evidenced by the preferential emission of D3O+ rather than HD2O+. The D2O clusters can be solvated partially at around 50 K by the two-dimensional liquid, but they are incorporated into the thin-film interior at temperatures higher than Tg. This result strongly suggests that only the topmost layer is laterally mobile below Tg on nonporous (high-density) 1-pentene films deposited at 70 K. Enhanced surface mobility was reported for vapor-deposited 3methylpentane films by Cowin and co-workers.14 They observed that the D3O+ ions soft-landed on the surface tend to diffuse into the bulk in the sub-Tg region. The depth of the 7739

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(5) van Zanten, J. H.; Wallace, W. E.; Wu, W. L. Phys. Rev. E 1996, 53, R2053−R2056. (6) DeMaggio, G. B.; Frieze, W. E.; Gidley, D. W.; Zhu, M.; Hristov, H. A.; Yee, A. F. Phys. Rev. Lett. 1997, 78, 1524−1527. (7) Ellison, C. J.; Torkelson, J. M. Nat. Mater. 2003, 2, 695−700. (8) Forrest, J. A.; Dalnoki-Veress, K.; Stevens, J. R.; Dutcher, J. R. Phys. Rev. Lett. 1996, 77, 2002−2005. (9) Fukao, K.; Miyamoto, Y. Phys. Rev. E 2000, 61, 1743−1754. (10) Hall, D. B.; Hooker, J. C.; Torkelson, J. M. Macromolecules 1997, 30, 667−669. (11) Liu, Y.; Russel, T. P.; Samant, M. G.; Stohr, J.; Brown, H. R.; Cossy-Favre, A.; Diaz, J. Macromolecules 1997, 30, 7768−7771. (12) Ge, S.; Pu, Y.; Zhang, W.; Rafailovich, M.; Sokolov, J.; Buenviaje, C.; Buckmaster, R.; Overney, R. M. Phys. Rev. Lett. 2000, 85, 2340−2343. (13) Ellison, C. J.; Ruszkowski, R. L.; Fredin, N. J.; Torkelson, J. M. Phys. Rev. Lett. 2004, 92, 095702-1−095702-4. (14) Bell, R. C.; Huang, H.; Iedema, M. J.; Cowin, J. P. J. Am. Chem. Soc. 2003, 125, 5176−5185. (15) Souda, R. Phys. Rev. Let. 2004, 93, 235502-1−235502-4. (16) Souda, R. J. Phys. Chem. B 2008, 112, 2649−2654. (17) Souda, R. J. Phys. Chem. B 2010, 114, 10734−10739. (18) Souda, R. Surf. Sci. 2011, 605, 793−798. (19) Souda, R. J. Chem. Phys. 2010, 133, 214704-1−214704-7. (20) Souda, R. J. Phys. Chem. B 2008, 112, 15349−15354. (21) Souda, R. J. Phys. Chem. B 2006, 110, 17524−17530. (22) Souda, R. J. Chem. Phys. 2008, 129, 124707-1−124707-8. (23) Souda, R. J. Phys. Chem. C 2011, 115, 8136−8143. (24) Souda, R. J. Chem. Phys. 2011, 135, 164703-1−164703-8. (25) Stevenson, K. P.; Kimmel, G. A.; Smith, R. S.; Kay, B. D. Science 1999, 283, 1505−1507. (26) Kimmel, G. A.; Stevenson, K. P.; Dohnalek, Z.; Smith, R. S.; Kay, B. D. J. Chem. Phys. 2001, 114, 5284−5294. (27) Souda, R. J. Phys. Chem. B 2010, 114, 11127−11132. (28) Takeda, K.; Yamamuro, O.; Suga, H. J. Phys. Chem. 1995, 99, 1602−1607. (29) Herino, R.; Bomchil, G.; Barla, K.; Bertrand, C. J. Eelectrochem. Soc. 1987, 134, 1994−2000. (30) Lehmann, V.; Stengl, R.; Luigard, A. Mater. Sci. Eng. B 2000, 69, 11−22. (31) Souda, R. J. Phys. Chem. B 2010, 11127−11132. (32) Souda, R.; Hayami, W.; Aizawa, T.; Ishizawa, Y. Phys. Rev. B 1993, 48, 17255−17261. (33) Kato, M.; Souda, R. Nucl. Instrum. Methods Phys. Res. B 2007, 256, 71−75. (34) Souda, R. J. Chem. Phys. 2010, 133, 214704-1−214704-7. (35) Adam, G.; Gibbs, J. H. J. Chem. Phys. 1965, 43, 139−146. (36) Kob, W.; Donati, C.; Plimpton, S. J.; Poole, P. H.; Glotzer, S. C. Phys. Rev. Lett. 1997, 79, 2827−2830. (37) Yamamoto, R.; Onuki, A. Phys. Rev. Lett. 1998, 81, 4915−4918. (38) Riggleman, R. A.; Yoshimoto, K.; Douglas, J. F.; de Pablo, J. J. Phys. Rev. Lett. 2006, 97, 045502-1−045502-4. (39) Donth, E. J. Non-Cryst. Solids 1982, 53, 325−330. (40) Reiter, G. Langmuir 1993, 9, 1344−1351. (41) Jacobs, K.; Herminghaus, S.; Mecke, K. R. Langmuir 1998, 14, 965−969. (42) Herminghaus, S.; Jacobs, K.; Mecke, K.; Bischof, J.; Fery, A.; Ibn-Elhaj, M.; Schlagowski, S. Science 1998, 282, 916−919. (43) Xie, R.; Karim, A.; Douglas, J. F.; Han, C. C.; Weiss, R. A. Phys. Rev. Lett. 1998, 81, 1251−1254. (44) Hare, E. F.; Zisman, W. A. J. Phys. Chem. 1955, 59, 335−340. (45) Kim, H. I.; Mate, C. M.; Hannibal, K. A.; Perry, S. S. Phys. Rev. Lett. 1999, 82, 3496−3499.

almost invariant below the evaporation temperature of the multilayer film (∼120 K) strongly suggests that no dried holes are created. The persistence of a wetting monolayer also refutes the formation of dried holes. In this case, a simple picture of dewetting, that is, the formation of contact lines between free surface and substrate, fails. The nonspreading behavior of liquid droplets on their own monolayer might be explained as “autophobic dewetting”, which occurs provided that the free energy has a minimum at a particular film thickness.44,45 The long-range dispersive force, originating from the adhesive force at the substrate interface and the presence of CRR in the film interior, might be associated with incomplete dewetting of thin supported liquid films in the deeply supercooled region.

5. CONCLUSIONS The roles of the free surface and substrate interface in the mobility of physisorbed molecules have been investigated using TOF-SIMS and TOF-ISS to gain more insight into the effects of nanoconfinement on the modification of Tg. Using a monolayer of 1-pentene, it was found that surface mobility occurs in the sub-Tg region (50−60 K), as evidenced by molecular uptake into porous Si layers and the formation of droplets on a perfluoroalkyl-modified Ni substrate. The presence of a well-defined onset temperature, resembling the glass−liquid transition in the bulk, strongly suggests that a twodimensional liquid is formed. The dominance of cohesive interactions between physisorbed molecules over a weak van der Waals force with the substrate ensures cooperativity for liquidlike flow on the surface. A two-dimensional liquid is also formed on the surface of multilayer 1-pentene films, as revealed from the partial solvation of the D2O adspecies in the sub-Tg region. Moreover, the two-dimensional liquid plays a role in the structural relaxation of glassy films: Both the transport of molecules through microscopic pores and the surface diffusion of molecules on mesopore walls result in film densification in the sub-Tg region. The morphology of the 1-pentene monolayer is invariant on the nonporous Si substrate, which can be explained as wettability of the two-dimensional liquid rather than formation of a dead layer. In addition to the presence of a two-dimensional liquid, the properties of supported supercooled liquid films are influenced by the long-range dispersive force from the substrate interface, which might be associated with the formation of CRR in the film interior.

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AUTHOR INFORMATION

Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS This research was partly supported by the Japan Society for the Promotion of Science through a Grant-in-Aid for Scientific Research (C), No. 22540339.

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REFERENCES

(1) Alcoutlabi, M; McKenna, G. B. J. Phys.: Condens. Matter 2005, 17, R461−R524. (2) Keddie, J. L.; Jones, R. A. L.; Cory, R. A. Faraday Discuss. 1994, 98, 219−230. (3) Keddie, J. L.; Jones, R. A. L.; Cory, R. A. Europhys. Lett. 1994, 27, 59−64. (4) Kawana, S.; Jones, R. A. L. Phys. Rev. E 2001, 63, 021501-1− 021501-6. 7740

dx.doi.org/10.1021/jp2089627 | J. Phys. Chem. C 2012, 116, 7735−7740