Article pubs.acs.org/JPCC
Glass Transition, Crystallization, Melting, and Decomposition of Nanoconfined Propane Films at Cryogenic Temperatures Ryutaro Souda* International Center for Materials Nanoarchitectonics, National Institute for Materials Science, 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan ABSTRACT: This study investigated translational diffusion of propane in thin films at cryogenic temperatures to elucidate crystallization and premelting of nanoconfined systems. The propane crystallizes on a Ni(111) substrate at around 48 K, but mobile molecules evolve on the surface immediately after crystallization, as revealed from their permeation through pores and gradual change in film morphology up to 80 K. The liquidlike behavior is attributable to surface premelting of crystallites. The crystallites become larger via surface diffusion, but the premelting layer thickness does not change over a wide temperature range. The crystal cores start to melt at 5 K below the bulk melting point, at which temperature the melting layer thickness increases dramatically. This behavior is consistent with the surface melting of macroscopic systems. Liquidlike propane molecules decompose at the interface of a deoxygenated V substrate during crystallization and premelting, as revealed from methane desorption via C−C bond truncation.
1. INTRODUCTION Melting of small systems is distinct from that of macroscopic systems. The size dependence of melting behaviors of a surface and a nanoparticle has attracted considerable attention because of their fundamental interest and technological importance. Theories1−9 and simulations10−19 have elucidated depression of the melting temperature, Tm, of nanoparticles based on various models, in which ensembles of clusters probably represent a mixture of solid and liquid phases. Weakly bound surface atoms are less constrained in their thermal motion than atoms in the interior. Consequently, the solid−liquid phase change of nanoparticles is initiated by surface premelting. Experimentally, the size dependence of Tm reduction has been studied extensively using free and supported clusters, as well as nanoconfined materials, based on transmission electron microscopy20−25 and various types of calorimetric methods.26−34 The heat capacity measurements revealed that melting transitions of small clusters are broad,31−34 but premelting is not identified clearly except for specific clusters.33 Thus, it remains unclear whether or not surface premelting of nanoparticles is a first-order phase transition. On the other hand, surface melting of macroscopic systems has been studied extensively,35−39 where dramatically increased thickness of the liquid layer is observed as the temperature approaches Tm. The glass transition is another type of solid−liquid transition below the crystallization temperature, Tc, without accompanying any local structural transformations. The glass transition temperature, Tg, is also depressed for small systems, but the reduction of Tg is much less than that of Tm for simple molecules confined in nanoporous media.40 In contrast, a large depression of Tg was reported for supported thin films of polymers41−44 and simple glass-formers45,46 because of the contribution of the free surface. Thin film’s Tg is also influenced © 2016 American Chemical Society
by substrates; no glass transition is observed when a dead layer is formed at the interface.46 The formation of a 2D liquid on the surface has been discussed recently along with its roles in reduction of thin films’ Tg.47,48 To date, no report of the relevant literature has described a systematic study of premelting of simple molecular solids in nanoconfined geometry or its mutual correlation with the bulk phase transition. In this paper, surface diffusion of thin propane films is investigated by using time-of-flight secondary ion mass spectrometry (TOF-SIMS) to reveal how mobile molecules formed on the free surface and at the interface of thin films play a role in the glass transition, crystallization, and (pre)melting. We specifically used propane for this purpose because its bulk Tg, Tc, and Tm are known;49 all are accessible under the ultrahigh vacuum (UHV) condition before multilayer films evaporate completely. The evolution of liquidlike propane is explored not only via film morphology change but also via molecular uptake into pores of amorphous solid water (ASW). The phase change of thin films has been discussed based on desorption kinetics of molecules,50 but very little is known about the roles of (pre)melting layers in thermal desorption. In this respect, recent studies have revealed that multilayer films of water,51 methanol,52 and hydrocarbons53 tend to decompose on a reactive V substrate during thermal desorption, where a liquidlike phase is expected to play a role. In this paper, therefore, the phase transition of glassy propane films and the nature of liquidlike propane are explored based on temperatureprogrammed desorption (TPD) and TOF-SIMS. Received: May 26, 2016 Published: July 22, 2016 17484
DOI: 10.1021/acs.jpcc.6b05299 J. Phys. Chem. C 2016, 120, 17484−17490
Article
The Journal of Physical Chemistry C
2. EXPERIMENT Details of experimental procedures are described in earlier papers.47,48 Briefly, the TOF-SIMS experiment was conducted in an UHV chamber with base pressure of less than 1 × 10−8 Pa. A primary beam of 2 keV He+ was generated in an electronimpact-type ion gun (Specs, IQE 12/38). It was chopped into pulses using electrostatic deflection plates and apertures. A bias voltage (125 V/mm) was applied to the sample, and positive secondary ions emitted perpendicularly to the surface were detected using a microchannel plate after passing through a field-free TOF tube. The fluence of the He+ ion was kept below 1 × 1012 ions cm−2 to minimize sample damage. TPD spectra were obtained using a quadrupole mass analyzer (Hiden, IDP300S) placed in a differentially pumped housing. Samples were mounted on a Cu cold finger extended from a closed-cycle helium refrigerator via a sapphire plate for electric insulation. The temperature was controlled using a cartridge heater by monitoring the temperature of the cold finger close to the sample position using Au (Fe)−chromel thermocouples. The temperature was ramped at a rate of ca. 5 K min−1 using a digital temperature programmer. This study examined effects of three chemically different substrates. A Ni(111) surface was cleaned by several cycles of electron-beam heating (∼1200 K) and ion-beam bombardment in UHV until no contaminants were detected in the TOF-SIMS measurements. To explore the surface diffusion of propane, a perfluoroalkyl layer was formed on the Ni substrate. The surface was first cleaned in the UHV chamber by heating and sputtering. It was immersed immediately in an ethanolic solution (0.04 mmol/mL) of tricosafluorododecanoic acid (TFDA: CF3(CF2)10COOH) for perfluoroalkyl modification of the surface. The possibility of propane decomposition was explored on a substrate of polycrystalline V (99.7%, 0.1 mm thick). The surface reactivity of V is known to depend strongly on the degree of deoxygenation because oxygen atoms tend to segregate from the bulk to the surface during heating treatments.51−53 Therefore, the V sheet was flash heated several times at ca. 1700 K not only for surface cleaning but also for deoxygenation from the bulk. Propane gas (99.99%) was used without further purification. Heavy water (99.5%) was purified by several cycles of freeze−pump−thaw treatments. The molecules that were introduced into the UHV chamber via precision leak valves were deposited onto a substrate that had been cooled to 20 K by backfilling the UHV chamber.
Figure 1. TOF-SIMS spectra from (a) Ni(111) and (b) TFDAmodified Ni surfaces before and after deposition of propane monolayer.
spectrum from the clean TFDA surface is characterized by intense fragment ions from the perfluoroalkyl chain. In contrast to the Ni+ ion, emission of CFn+ is suppressed considerably when the propane monolayer is formed on the substrate. Figure 2a shows temperature evolutions of typical secondary ion intensities from the Ni(111) surface on which 1 ML of propane is deposited at 20 K. The propane molecules tend to
3. EXPERIMENTAL RESULTS The film thickness was estimated based on evolution curves of the secondary ion intensities measured during the adsorption of molecules. Figure 1 shows TOF-SIMS spectra from the (a) Ni(111) and (b) TFDA-modified Ni substrates before and after deposition of 1 monolayer (ML) of propane at 20 K. The ion yield is small from the clean Ni(111) surface because sputtered Ni+ ions are neutralized almost completely via resonant electron transfer from the surface. Upon adsorption of propane, Ni+ and adducts are emitted from the surface together with fragment ions. The former is created during collisions of Ni and adspecies. The Ni+ ion peaks in intensity simultaneously with the saturation of fragment ion intensities. We have assigned 1 ML based on these behaviors. The Ni+ intensity disappears at coverage of ca. 5 ML. Therefore, the morphological change of multilayer propane films can be explored from evolution of the Ni+ intensity with increasing temperature. The TOF-SIMS
Figure 2. Temperature-programmed TOF-SIMS intensities from the propane monolayer deposited on (a) Ni(111) and (b) TFDAmodified Ni surfaces at 20 K. The temperature was ramped at a rate of 5 K min−1. 17485
DOI: 10.1021/acs.jpcc.6b05299 J. Phys. Chem. C 2016, 120, 17484−17490
Article
The Journal of Physical Chemistry C
spectrum shown in Figure 3b. Thus, the film morphology change is induced by mobile molecules, indicating that a liquidlike phase coexists with crystallites. The Ni+ intensity peaks at 80 K and then turns to decrease, forming a steep valley at around 92 K. This decrease results from (pre)melting of crystals although the experimental valley temperature is higher than Tm of propane (85.5 K). The Ni+ intensity decays because a liquid layer spreads over the substrate. The subsequent increase of Ni+ results from its evaporation. A small shoulder occurs in the TPD spectrum at around 85 K, indicating that the desorption rate of propane decreases during the solid−liquid phase transition at Tm. The presence of mobile molecules on the surface is explored further based on the uptake of propane into pores of ASW. Figure 4a shows temperature-programmed TOF-SIMS inten-
wet the Ni(111) surface, as evidenced by the absence of a drastic change in the C2H3+ intensity, whereas a decrease in the Ni+ intensity at T > 70 K implies that molecular desorption occurs (as described later based on TPD). The propane monolayer disappears at around 85 K, resulting in decay of the C2H3+ intensity. A few molecules remain on the surface up to 120 K, as revealed by both C2H3+ and Ni+ intensities. The experimental result for the propane monolayer deposited onto the TFDA modified surface is shown in Figure 2b. In this case, the C2H3+ (CF+) intensity decreases (increases) steeply at around 35 K. The emission of CF+ is quenched almost completely by the propane monolayer (see Figure 1). Therefore, increase of the CF+ intensity is attributable to propane uncovering of the TFDA surface. Thus, morphology of the propane monolayer changes at temperatures below 40 K as a result of surface diffusion. In contrast, no indication of the film morphology change is identified at this temperature on Ni(111). The difference between these experimental results is ascribable to surface wettability rather than diffusivity: The adhesive force of propane overcomes the cohesive force on the Ni(111) surface. Temperature evolutions of TOF-SIMS intensities from a 20 ML propane film deposited onto the Ni(111) substrate are shown in Figure 3a. The Ni+ ion cannot be emitted initially
Figure 4. Temperature-programmed TOF-SIMS intensities from binary films of heavy water and propane: (a) C3H8 (5 ML) on D2O (40 ML); (b) D2O (4 ML) on C3H8 (20 ML); (c) D2O (40 ML) on C3H8 (20 ML). The molecules were deposited at 20 K. The result obtained using a crystalline propane film (prepared by heating the glassy propane film to 80 K) instead of the glassy film is also shown in (c) as a blue solid line. Figure 3. (a) Temperature-programmed TOF-SIMS intensities and (b) TPD spectrum (m/e = 29) obtained using 20 ML propane films deposited onto the Ni(111) substrate.
sities from propane (5 ML) deposited onto ASW formed by exposure of Ni(111) to 40 ML of D2O at 20 K. The thusly prepared ASW film is characterized by its microporous structure.54 The ASW pores are expected to remain in some form until surface diffusion of water commences at 110−120 K.47 The propane tends to diffuse into pores at around 40 K and disappears almost completely from the surface up to 55 K. The topmost layer of the multilayer propane film is mobile, so that the uptake is expected to proceed in a layer-by-layer fashion,47 resulting in the delay of the complete uptake of the molecules relative to the onset of surface diffusivity at 35 K. The incorporated molecules desorb at around 90 K, at which temperature the C2H3+ intensity increases slightly. The experimental result for the 20 ML propane film embedded underneath 4 ML of ASW is displayed in Figure 4b. The surface
through the propane multilayer, but it emerges at 48 K because of the film morphology change. The onset temperature is close to calorimetric Tc (48.7 K) of propane rather than Tg (45.5 K).49 In any case, supercooled liquid formed at Tg is so shortlived that overall dewetting behaviors are attributable to crystallization of liquidlike propane. The Ni+ intensity increases continuously with increasing temperature after crystallization. This behavior is explainable as growth of crystal grains and the increase in the area of thinner propane patches ( 130 K.
segregation of propane molecules becomes evident at 45−48 K. After the occurrence of a plateau region, the C2H3+ intensity increases at temperatures higher than 80 K and subsequently decreases after formation of a peak at 92 K because of the evaporation of surface segregated propane. These temperatures correspond well to the peak and valley temperatures of the Ni+ intensity in Figure 3a. A similar result is obtained when a propane film (20 ML) is covered by a thicker ASW film (40 ML) as shown in Figure 4c. In this case, surface segregation occurs only after propane crystallizes. In fact, when an initially crystalline propane film (formed by heating of the glassy film to 80 K) is embedded underneath the same thick ASW film, a fundamentally equivalent result is obtained, as displayed by a blue solid line in the figure. Figure 5 displays (a) TOF-SIMS intensities and (b) TPD spectra obtained for a 5 ML propane film deposited onto the
4. DISCUSSION Surface diffusion of propane commences at 35 K, as revealed by droplet formation for the propane monolayer on the TFDAmodified surface and by the uptake of propane into pores of ASW. This onset is apparently lower than bulk Tg of propane (45.5 K). In contrast, no indication of the glass−liquid transition or crystallization is observed from the propane monolayer formed on the Ni(111) substrate, where the formation of droplets or grains is probably quenched because of the adhesive interaction with Ni. The interaction between propane and water molecules is also attractive because the propane monolayer wets a nonporous ASW surface without the droplet formation (not shown). Therefore, the physisorbed propane is incorporated in the porous ASW film (Figure 4a) under the presence of such an attractive force. However, diffusion of physisorbed propane through the capping ASW films tends to be quenched in the sub-Tg region (Figure 4b), indicating that sub-Tg mobility of propane is characteristic of the free surface. Such a 2D liquid plays a role in the depression of thin films’ Tg.47,48 The mobility of propane occurs in the film interior at temperatures higher than Tg = 45.5 K, which corresponds to the evolution onset of the C2H3+ intensity in Figure 4b. The mobility is expected to be quenched after crystallization at Tc = 48.7 K. In reality, however, liquidlike behavior is observed even after crystallization. This is also confirmed by the fact that propane segregates to the surface when a thicker ASW layer is deposited onto the crystalline propane film as seen in Figure 4c. The most likely scenario to elucidate this behavior is premelting. The nascent crystallites are small because only a few molecules supplied from the lateral direction of thin films take part in nucleation. The liquidlike propane formed on the crystallite surface or at the grain boundary is likely to segregate to the surface through the porous ASW film. The presence of the ASW film does not influence the interfacial mobility of such a liquid, providing a contrast to the 2D liquid that occurs only on the free surface in the sub-Tg region. The thickness of the premelting layer of propane remains unchanged at temperatures below 80 K, as revealed from saturation of segregated propane molecules on the ASW film surface (see Figure 4b). In general, the liquidlike phase (i.e., quasi-liquid) is expected to be correlated strongly with the underlying crystal. It can be regarded as a layer with large-amplitude vibrations. Thus, quasiliquid is fundamentally inseparable from the crystal core. However, the liquidlike propane is distinguishable from the crystal, as demonstrated using the porous ASW film as a “percolator”. In this respect, molecular dynamics (MD) simulations10 revealed that a few atoms have popped out from the surface of free clusters. They migrate as “floaters” during premelting. The diffusive motion and liquidlike cluster shape fluctuations are induced by such floaters and counter holes created in the surface layer. Such floater molecules might be separable from crystal cores and diffuse through pores of the ASW film to reach the surface.
Figure 5. (a) Temperature-programmed TOF-SIMS intensities and (b) TPD spectra of hydrogen (m/e = 2), methane (16), and propane (29) obtained using 5 ML propane films deposited onto the deoxygenated V substrate.
deoxygenated V substrate. The TOF-SIMS intensity of the V+ (C2H3+) ion increases (decreases) at ca. 45 K because the propane film dewets the V substrate. After the gradual change of the film morphology, the V+ intensity decays steeply because propane crystallites melt. The result resembles that obtained using the Ni(111) substrate although the tail of V+ tends to occur until higher temperatures. The TPD spectrum of propane (m/e = 29) exhibits the onset (70 K) and shoulder (85 K) similar to those observed for the multilayer film deposited onto Ni(111). A prolonged tail of the m/e = 29 species is conspicuous in the TPD spectrum, which is correlated with the SIMS V+ intensity. The most characteristic feature of the TPD spectra from V is desorption of methane (m/e = 16) that peaks in intensity during dewetting at 50 K and which continues until the multilayer film evaporates. The methane TPD peak is absent not only for the Ni(111) substrate but also for V when the deoxygenation is insufficient, indicating that propane is decomposed at the interface of the deoxygenated V substrate during film dewetting and after crystallization. 17487
DOI: 10.1021/acs.jpcc.6b05299 J. Phys. Chem. C 2016, 120, 17484−17490
Article
The Journal of Physical Chemistry C As regards the film morphology change (Figure 3a), it commences immediately after crystallization and continues over a wide temperature range. The smaller crystallites are expected to have smaller Tm. Therefore, the crystal grains can grow via coalescence of crystallites through contact boundaries (sintering) or viscous flow of molecules on the free surface (Ostwald ripening). The floater molecules separated from crystal cores might play a dominant role in both cases. The crystal gradually becomes larger up to 80 K. Then liquidlike molecules spread over the substrate. The thickness of the premelting layer increases above this temperature drastically, which is consistent with the increase in the amount of surface segregated propane molecules as seen in Figure 4b. The desorption rate of propane changes at bulk Tm = 85.5 K (see Figures 3b and 5b), but liquidlike species (i.e., floaters) preexisting on the crystallite surface at lower temperatures appear to have no effects on the desorption rate. Consequently, these behaviors are associated with the melting of crystal cores. The evolution of liquidlike layers near bulk Tm (approximately T > 0.9Tm) is observed during surface melting of macroscopic systems.35−39 However, premelting behaviors of nascent crystallites are apparently distinct from those of ripened ones in terms of the onset of mobility, thickness of the premelting layer, and the gradual nature of film morphology change. This behavior is elucidated by the premelting of nanoparticles as predicted by MD simulations.10−19 It is known that the melting transitions of clusters are broad;31−34 the melting temperatures also show large size-dependent fluctuations. To date, a bimodal heat capacity peak that is evidence of the different core and surface melting temperatures has not been reported except for specific size clusters like Al51+ and Al52+.33 Macroscopic tools such as calorimetry might not probe surface premelting efficiently because the melting of thin layers induces a very small latent heat like a second-order transition.10 On the basis of the microscopic molecular diffusivity measurements of thin crystalline films, we demonstrated that melting of the surface and crystalline cores is clearly distinguishable; two sorts of surface premelting, characteristic of microscopic and macroscopic systems, are also identified in the course of crystallite ripening. The surface segregation of liquidlike propane through the porous ASW film indicates that the interfacial interaction has no significant effects on premelting of crystallites. This behavior explains why the molecules confined in nanoporous media exhibits a greater reduction of Tm in calorimetric studies,40 in contrast to the small reduction of Tg. The interaction of multilayer propane films with V is fundamentally equivalent to that of Ni(111) except that a small amount of propane decomposes at the interface. The methane desorbs at T > 40 K as a result of the C−C bond scission. The desorption rate of methane peaks during the glass−liquid transition and crystallization, indicating that liquidlike propane that induces film morphology change reacts preferentially with V. Floater molecules formed during the crystallite ripening also react with V, as revealed from the high-temperature tail of the methane desorption. However, normal liquid of propane formed at ca. 90 K appears to undergo no decomposition on the V substrate. This behavior implies that the active sites of the V substrate for propane decomposition are used up before the crystal core melts although the decomposition rate is expected to increase with increasing temperature. In reality, however, the reactivity of deoxygenated V is so high (because of the contribution of subsurface sites) that almost complete decomposition is commonly observed for multilayer films
(10−20 ML) of other molecular species.51−53 Therefore, it is possible that chemical properties of the normal liquid are distinct from those of the liquidlike species formed during and after crystallization. The fact that no hydrogen is released during propane decomposition implies that the C−H bond scission is quenched or that hydrogen is absorbed by the V substrate. The low decomposition rate and the absence of the hydrogen desorption are characteristic of propane decomposition on V. In this respect, it has been revealed that n-hexane multilayer films also decompose to form methane at 105 K,53 but the hydrogen is released gradually after methane desorption. Longer chain hydrocarbons like n-hexane remain on the surface at higher temperatures, so that breaking of the C−H bond can be facilitated relative to propane. In any case, the reaction pathway of liquidlike propane on V differs significantly from that of (sub)monolayer alkane adspecies on other transition metal substrates55−57 because the C−H bond scission precedes the C−C bond truncation in the latter at high temperatures (∼300 K).
5. CONCLUSION Surface diffusion of thin propane films was investigated to gain insight into the nanoconfinement effects on reduction of Tg and Tm. The 2D liquid of propane occurs in the sub-Tg region, as confirmed from the onset of propane uptake into the porous ASW film and droplet formation on the TFDA-modified surface. However, surface segregation of embedded propane through the porous ASW film is quenched at temperatures below Tg because the 2D liquid is formed only on the free surface. The propane film morphology changes gradually after crystallization, together with the evolution of diffusive molecules that can penetrate through the porous ASW film. This behavior results from premelting of crystallites. The crystal grains can become larger via sintering or molecular transport through the physisorbed monolayer on Ni(111). The propane molecules spread over the substrate at temperatures higher than 80 K because of the melting of ripened crystallites, where thickness of the liquidlike surface layer increases dramatically, similar to surface melting of macroscopic systems. The surface premelting of nascent crystallites appears to be distinct from the melting of crystal cores at around Tm. Premelting commences immediately after crystallization and thickness of liquidlike layer is invariant until the crystal core melts. On the V substrate, the liquidlike propane species are decomposed in part during crystallization via C−C bond truncation, as evidenced by methane desorption. The C−H bond scission to yield hydrogen molecules is ineffective for propane because it requires higher temperatures, as inferred from comparison with the result of n-hexane. The floater molecules formed during crystallite ripening are decomposed as well, but the decomposition rate of the normal liquid propane formed at T > Tm is low.
■
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The author declares no competing financial interest.
■
REFERENCES
(1) Couchman, P. R.; Jesser, W. A. Thermodynamic Theory of Size Dependence of Melting Temperature in Metals. Nature (London, U. K.) 1977, 269, 481−483.
17488
DOI: 10.1021/acs.jpcc.6b05299 J. Phys. Chem. C 2016, 120, 17484−17490
Article
The Journal of Physical Chemistry C (2) Hoshino, K.; Shimamura, S. Simple-Model for the Melting of Fine Particles. Philos. Mag. A 1979, 40, 137−141. (3) Ross, J.; Andres, R. P. Melting Temperature of Small Clusters. Surf. Sci. 1981, 106, 11−17. (4) Berry, R. S.; Wales, D. J. Freezing, Melting, Spinodal, and Clusters. Phys. Rev. Lett. 1989, 63, 1156−1159. (5) Labastie, P.; Whetten, R. L. Statistical Thermodynamics of the Cluster Solid-Liquid Transition. Phys. Rev. Lett. 1990, 65, 1567−1570. (6) Wautelet, M. Estimation of the Vibration of the Melting Temperature with the Size of Small Particles, on the Basis of a SurfacePhonon Instability Model. J. Phys. D: Appl. Phys. 1991, 24, 343−346. (7) Vanfleet, R.; Mochel, J. M. Thermodynamics of Melting and Freezing in Small Particles. Surf. Sci. 1995, 341, 40−50. (8) Reiss, H.; Mirabel, P.; Whetten, R. L. Capillary Theory for the Coexistence of Liquid and Solid Clusters. J. Phys. Chem. 1988, 92, 7241−7246. (9) Nanda, K. K.; Sahu, S. N.; Behera, S. N. Liquid-Drop Model for the Size-Dependent Melting of Low-Dimensional Systems. Phys. Rev. A: At., Mol., Opt. Phys. 2002, 66, 013208. (10) Cheng, H. − P.; Berry, R. S. Surface Melting of Clusters and Implications for Bulk Matter. Phys. Rev. A: At., Mol., Opt. Phys. 1992, 45, 7969−7980. (11) Lewis, L. J.; Jensen, P.; Barrat, J. − L. Melting, Freezing, and Coalescence of Gold Nanoclusters. Phys. Rev. B: Condens. Matter Mater. Phys. 1997, 56, 2248−2257. (12) Sun, D. Y.; Gong, X. G. Structural Properties and Glass Transition in Aln Clusters. Phys. Rev. B: Condens. Matter Mater. Phys. 1998, 57, 4730−4735. (13) Cleveland, C. L.; Luedtke, W. D.; Landman, U. Melting of Gold Clusters. Phys. Rev. B: Condens. Matter Mater. Phys. 1999, 60, 5065− 5077. (14) Calvo, F.; Spiegelmann, F. Geometric Size Effects in the Melting of Sodium Clusters. Phys. Rev. Lett. 1999, 82, 2270−2273. (15) Aguado, A.; Lopez, J. M.; Alonso, J. A.; Stott, M. J. Orbital-Free Molecular Dynamics Simulations of Melting in Na8 and Na20: Melting in Steps. J. Chem. Phys. 1999, 111, 6026−6035. (16) Lee, Y. J.; Lee, E. − K.; Kim, S.; Nieminen, R. M. Effects of Potential Energy Distribution on the Melting of Clusters. Phys. Rev. Lett. 2001, 86, 999−1002. (17) Qi, Y.; Cagin, T.; Johnson, W. L.; Goddard, W. A. Melting and Crystallization in Ni Nanoclusters: The Mesoscale Regime. J. Chem. Phys. 2001, 115, 385−394. (18) Alavi, S.; Thompson, D. L. Molecular Dynamics Simulations of the Melting of Aluminium Nanoparticles. J. Phys. Chem. A 2006, 110, 1518−1523. (19) Wang, N.; Rokhlin, S. I.; Farson, D. F. Nonhomogeneous Surface Premelting of Au Nanoparticles. Nanotechnology 2008, 19, 415701. (20) Takagi, M. Electron Diffraction Study of Liquid-Dolid Transition of Thin Metal Films. J. Phys. Soc. Jpn. 1954, 9, 359−363. (21) Buffat, P.; Borel, J. − P. Size Effect on Melting Temperature of Gold Particles. Phys. Rev. A: At., Mol., Opt. Phys. 1976, 13, 2287−2298. (22) Allen, G. L.; Bayles, R. A.; Gile, W. W.; Jesser, W. A. Small Particle Melting of Pure Metals. Thin Solid Films 1986, 144, 297−308. (23) IIjima, S.; Ichihashi, T. Structural Instability of Ultrafine Particles of Metals. Phys. Rev. Lett. 1986, 56, 616−619. (24) Ajayan, P. M.; Marks, L. D. Experimental-Evidence for Quasimelting in Small Particles. Phys. Rev. Lett. 1989, 63, 279−282. (25) Krakow, W.; Joseyacaman, M.; Aragon, J. L. Observation of Quasi-Melting at the Atomic-Level in Au Nanoclusters. Phys. Rev. B: Condens. Matter Mater. Phys. 1994, 49, 10591−10596. (26) Unruh, K. M.; Huber, T. E.; Huber, C. A. Melting and Freezing Behavior of Indium Metal in Porous Glasses. Phys. Rev. B: Condens. Matter Mater. Phys. 1993, 48, 9021−9027. (27) Lai, S. L.; Guo, J. Y.; Petrova, V.; Ramanath, G.; Allen, L. H. Size-Dependent Melting Properties of Small Tin Particles: Nanocalorimetric Measurements. Phys. Rev. Lett. 1996, 77, 99−102.
(28) Fei, G. T.; Zheng, K.; Chu, Z. Q.; Qin, Y.; Zhang, L. D.; Liu, L. Thermal behavior of the Indium Particles Dispersed in Oil. Mater. Res. Bull. 1997, 32, 857−862. (29) Zhang, M.; Efremov, M. Y.; Schiettekatte, F.; Olson, E. A.; Kwan, A. T.; Lai, S. L.; Wisleder, T.; Greene, J. E.; Allen, L. H. Size Dependent Melting Point Depression of Nanostructures: Nanocalorimetric Measurements. Phys. Rev. B: Condens. Matter Mater. Phys. 2000, 62, 10548−10557. (30) Alcoutlabi, M.; McKenna, G. B. Effects of Confinement on Material Behavior at the Nanometer Size Scale. J. Phys.: Condens. Matter 2005, 17, R461−R524. (31) Schmidt, M.; Kusche, R.; Kronmuller, W.; von Issendorff, B.; Haberland, H. Experimental Determination of the Melting Point and Heat Capacity for a Free Cluster of 139 Sodium Atoms. Phys. Rev. Lett. 1997, 79, 99−102. (32) Schmidt, M.; Kusche, R.; von Issendorff, B.; Haberland, H. Irregular Vibrations in the Melting Point of Size-Selected Atomic Clusters. Nature (London, U. K.) 1998, 393, 238−240. (33) Breaux, G. A.; Neal, C. M.; Cao, B.; Jarrold, M. F. Melting, Premelting, and Structural Transitions in Size-Selected Aluminum Clusters with around 55 Atoms. Phys. Rev. Lett. 2005, 94, 173401. (34) Neal, C. M.; Starace, A. K.; Jarrold, M. F. Melting Transitions in Aluminum Clusters: The Role of Partially Melted Intermediates. Phys. Rev. B: Condens. Matter Mater. Phys. 2007, 76, 054113. (35) Frenken, J. W. M.; van der Veen, J. F. Observation of Surface Melting. Phys. Rev. Lett. 1985, 54, 134−137. (36) Zhu, D. − M.; Dash, J. G. Surface Melting and Roughening of Adsorbed Argon Films. Phys. Rev. Lett. 1986, 57, 2959−2962. (37) Stoltze, P.; Norskov, J. K.; Landman, U. Disordering and Melting of Aluminum Surfaces. Phys. Rev. Lett. 1988, 61, 440−443. (38) Chen, E. T.; Barnett, R. N.; Landman, U. Surface Melting of Ni(110). Phys. Rev. B: Condens. Matter Mater. Phys. 1990, 41, 439− 450. (39) Hakkinen, H.; Manninen, M. Computer-Simulation of Disordering and Premelting of Low-Index Faces of Copper. Phys. Rev. B: Condens. Matter Mater. Phys. 1992, 46, 1725−1742. (40) Jackson, C. L.; McKenna, G. B. The Glass-Transition of Organic Liquids Confined to Small Pores. J. Non-Cryst. Solids 1991, 131, 221− 224. (41) Keddie, J. L.; Jones, R. A. L.; Cory, R. A. Size-Dependent Depression of the Glass-Transition Temperature in Polymer Films. Europhys. Lett. 1994, 27, 59−64. (42) Kawana, S.; Jones, R. A. L. Character of the Glass Transition in Thin Supported Polymer films. Phys. Rev. E: Stat. Phys., Plasmas, Fluids, Relat. Interdiscip. Top. 2001, 63, 021501. (43) van Zanten, J. H.; Wallace, W. E.; Wu, W. L. Effects of Strongly Favorable Substrate Interactions on the Thermal Properties of Ultrathin Polymer Films. Phys. Rev. E: Stat. Phys., Plasmas, Fluids, Relat. Interdiscip. Top. 1996, 53, R2053−R2056. (44) Ellison, C. J.; Torkelson, J. M. The Distribution of GlassTransition Temperatures in Nanoscopically Confined Glass Formers. Nat. Mater. 2003, 2, 695−700. (45) Bell, R. C.; Wang, H. F.; Iedema, M. J.; Cowin, J. P. NanometerResolved Interfacial Fluidity. J. Am. Chem. Soc. 2003, 125, 5176−5185. (46) Keddie, J. L.; Jones, R. A. L.; Cory, R. A. Interface and Surface Effects on the Glass-Transition Temperature in Thin Polymer-Films. Faraday Discuss. 1994, 98, 219−230. (47) Souda, R. Roles of 2D Liquid in Reduction of the GlassTransition Temperature of Thin Molecular Solid Films. J. Phys. Chem. C 2011, 115, 8136−8143. (48) Souda, R. On sub-Tg Dewetting of Nanoconfined Liquids and Autophobic Dewetting of Crystallites. Phys. Chem. Chem. Phys. 2012, 14, 4118−4124. (49) Takeda, K.; Oguni, M.; Suga, H. Thermoanalytical Study of Vapor-Deposited Normal-Alkanes. J. Phys. Chem. Solids 1991, 52, 991−997. (50) Dohnalek, Z.; Ciolli, R. L.; Kimmel, G. A.; Stevenson, K. P.; Smith, R. S.; Kay, B. D. Substrate Induced Crystallization of 17489
DOI: 10.1021/acs.jpcc.6b05299 J. Phys. Chem. C 2016, 120, 17484−17490
Article
The Journal of Physical Chemistry C Amorphous Solid Water at Low Temperatures. J. Chem. Phys. 1999, 110, 5489−5492. (51) Souda, R. Interfacial Reaction of Water Ice on Polyscrystalline Vanadium and Its Effects on Thermal Desorption of Water. Phys. Chem. Chem. Phys. 2014, 16, 1095−1100. (52) Souda, R. Thermal Decomposition of Thin Methanol Films on Deoxygenated Vanadium. J. Phys. Chem. C 2014, 118, 11333−11339. (53) Souda, R. Decomposition of Multilayer Benzene and n-Hexane Films on Vanadium. Phys. Chem. Chem. Phys. 2015, 17, 22911−22916. (54) Kimmel, G. A.; Stevenson, K. P.; Dohnalek, Z.; Smith, R. S.; Kay, B. D. Control of Amorphous Solid Water Morphology Using Molecular Beams. I. Experimental Results. J. Chem. Phys. 2001, 114, 5284−5294. (55) Liu, A. C.; Friend, C. M. Alkane Activation-Adsorption and Reaction of Normal-Butane on Clean and Carbided W(100). J. Chem. Phys. 1987, 87, 4975−4985. (56) Kelly, D. G.; Salmeron, M.; Somorjai, G. A. The Adsorption and Reaction of Hydrocarbons on Molybdenum Single-Crystal SurfacesWhen Clean and in the Presence of Coadsorbed Sulfur or Carbon. Surf. Sci. 1986, 175, 465−486. (57) Zaera, F. An Organometallic Guide to the Chemistry of Hydrocarbon Moieties on Transition Metal Surfaces. Chem. Rev. 1995, 95, 2651−2693.
17490
DOI: 10.1021/acs.jpcc.6b05299 J. Phys. Chem. C 2016, 120, 17484−17490
