Effect of Confinement on Melting Behavior of Cadmium Arachidate

Feb 22, 2013 - UGC-DAE Consortium for Scientific Research, University Campus, Khandwa Road, Indore 452 017, India ... *E-mail: [email protected]. ... ...
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Effect of Confinement on Melting Behavior of Cadmium Arachidate Langmuir−Blodgett Multilayer Pallavi Pandit,† Ajay Gupta,*,‡ Dileep Kumar,‡ Mandira Banerjee,† and Sigrid Bernstorff§ †

School of Physics, Takshshila Campus, Khandwa Road, Indore 452 017, India UGC-DAE Consortium for Scientific Research, University Campus, Khandwa Road, Indore 452 017, India § Elettra-Sincrotrone Trieste, SS 14, Km 163.5, I-34149 Basovizza, Trieste, Italy ‡

ABSTRACT: The effect of confinement between two metallic layers on the melting behavior of a 13 monolayer cadmium arachidate (CdA) Langmuir−Blodgett (LB) multilayer has been studied. Temperature dependent diffraction measurements provide information about structural changes occurring in the film plane as well as in the out-ofplane direction. X-ray standing waves have been used to achieve depth selectivity in diffraction measurements. It is found that the difference in melting behavior of the surface and the bulk, which is observed in the film with free surface, disappears in the case of confined films; while the free surface transforms to hexaticlike phase via an intermediate smectic phase, confinement results in disappearance of this phase, and the sequence of transformations in the bulk and the interfacial regions becomes identical. Some anisotropy between (01 + 11̅) and (10) directions remains, with coherence along (10) direction decreasing at a faster rate. The confinement between metallic layers also significantly reduces the tilting of the chains observed at higher temperature. Further, both in the case of film with free surface and confined films, melting at the surface/interface occurs at a lower temperature as compared to the bulk.

I. INTRODUCTION The effect of confinement on the melting and freezing behavior of materials is of practical importance in applications like adhesion, lubrication, and so forth.1 From the point of view of basic understanding, effects of low dimensionality and/or confinement on the melting behavior of crystalline systems have been studied extensively.2−18 Most extensive studies of such effects have been done using colloidal crystals,2−9 since in such systems the length scale involved is in the range of micrometers and it is easy to get single particle resolution. Lowdimensional systems generally exhibit a two-step melting from crystal to hexatic phase and from hexatic to liquid phase.2,3,8,10,12−14 It is found that generally a crystal melts heterogeneously via surface/interface melting.2,3,19,20 Further, while films with thickness less than 4 layers are observed to melt homogeneously even for polycrystalline films, in films confined between two glass walls, melting starts from film−wall interfaces and grain boundaries.2,3 Experimental investigation of melting of thin films of atomic and molecular systems are challenging as the length scale involved is in the sub nanometer range. Very few studies on the melting behavior of such ultrathin films have been reported in the literature.18 Langmuir−Blodgett films of fatty acid salts like cadmium arachidate provide a system with intermediate length scale, with chain length of the molecules being a few nanometers. The effect of low dimensionality on the melting behavior of such LB films has also been studied in the literature.13−16 Existence of a hexaticlike phase has been clearly evidenced in such studies. Further, existence of an intermediate smectic or nematic phase © 2013 American Chemical Society

inserted between solid and hexatic phases in a system with a distorted hexagonal symmetry, as predicted by theory, has also been observed.16,21 Even in thick films of cadmium arachidate, low dimensionality effects have been observed in the surface region.13 In the present work, we investigate the effect of confinement between two metallic layers on the melting behavior of cadmium arachidate LB film. In situ in-plane as well as out-of-plane diffraction measurements have been used to follow the structural changes occurring as a function of temperature. Depth selectivity in the diffraction measurements has been obtained by making use of the X-ray standing wave/Xray waveguide structure. Such depth selectivity allows one to monitor the behavior of the interfacial region or the bulk of the film.22−26

II. EXPERIMENTAL SECTION Three different multilayers of cadmium arachidate (CdA) were deposited on Si (100) substrate coated with a buffer layer of 70 nm Ni, using a KSV 2000 LB trough. Structures of the films were as follows: Si substrate/Ni (70 nm)/CdA (13 monolayer)/ X, where X stands for (i) 2 nm Ni layer (film designated as CdA_Ni); (ii) 10 nm Al layer (film designated as CdA_Al); and (iii) no top layer (film designated as CdA). The LB films of CdA were prepared by the vertical deposition mode. Arachidate acid (purity >99.9%)) was dissolved in chloroform (HPCL grade) at a concentration of 1 g/L for use as the spreading solution. 70 μL of solution in chloroform was Received: November 15, 2012 Revised: February 20, 2013 Published: February 22, 2013 3950

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spread on the subphase surface. The subphase was prepared with CdCl2 (purity >99.8%) dissolved in Milli-Q water (resistivity >18 MΩ cm) to a concentration of 0.9966 g/L and adjusted to pH 6.7 using NaHCO3 (purity >99.9%). LB deposition was performed at a surface pressure of 30 mN m−1 with a dipping rate of 3 mm min−1 at 20 °C. The films were allowed to dry in air for 5 min after each upstroke. The buffer Ni film as well as the top Ni/Al films were deposited by ion beam sputtering of high-purity targets (99.99%) using 1000 eV Ar + ions. The ion source used was a 3 cm broad-beam Kaufman-type hotcathode gun. The base pressure was typically 5.2 × 10−8 mbar and the pressure with Ar gas flow was 5 × 10 −4 mbar. The purity of the Ar gas was 99.995%. The beam current was 18 mA. The target was kept at 45° with respect to the beam direction. The substrate was kept parallel to the target at a distance of about 15 cm. The chamber was flushed with pure Ar a few times before deposition, in order to reduce the oxygen and water vapor contamination. Thicknesses of the top Al or Ni layer were chosen so as to optimize the waveguide structure. Since the refractive index of Ni (1−2.4319 × 10−5-i 5.0980 × 10−7) is higher than that for Al (1−8.4737 × 10−6-i 1.5495 × 10−7), the thickness of the top Ni layer was taken to be smaller than that for Al. Depthresolved information about structural changes occurring with temperature is obtained using X-ray diffraction under standing wave conditions. X-ray standing waves are generated using total external reflection (TER) of X-rays by the Ni buffer layer. When the X-ray beam falls on the sample at an angle below the critical angle for TER from the Ni surface, the interference between the incident and reflected X-ray wave fronts produces an X-ray standing wave with planes of maximum intensity (antinodes) or minimum intensity (nodes) parallel to the surface.22,23 The sample acts as an asymmetric planar waveguide with the CdA film forming the waveguide cavity. Resonance enhancement of the X-ray field inside the cavity occurs whenever a waveguide mode is excited.24 The distribution of the X-ray field inside the multilayer changes significantly with the angle of incidence. As will be discussed later, by properly choosing the angles of incidence, preferential information about different regions of the film could be obtained. In the case of films CdA_Al and CdA_Ni, angles of incidence have been chosen so as to get information about the bulk and interfaces, while in the case of film CdA, the angles were chosen corresponding to the bulk and the free surface. X-rays of 8 keV were used for the simultaneous measurement of reflectivity (using a 1D detector kept in the forward direction) and diffraction (using a 2D Pilatus detector) at the SAXS beamline of Elettra synchrotron, Trieste. The Pilatus detector was kept in the film plane centered at 23° from the direction of the incident X-ray beam and covering an angular range of 5° in horizontal direction and 12° in vertical direction. The one-dimensional (1D) detector was kept at a distance of 1522 mm from the sample. The alignment of the sample was continuously controlled by monitoring the position of the specular spot on the 1D detector during temperature dependent measurements with an accuracy of ±0.003°. A miniature BN (boron nitride) furnace was used for controlled heating of the sample, which was kept in a protective atmosphere of nitrogen gas. The sample temperature was maintained with an accuracy of ±0.5 K. The incident beam had a cross section of 1.5 mm (h) × 100 μm (v). The intensity of the incident beam was attenuated with aluminum filter in the beam path, and an exposure time of 300 s was used for taking one diffraction pattern. The effect of the X-ray beam on the film was studied by doing diffraction measurements as a function of time at room temperature. It was found that up to a time of 1 h there was no change in the diffraction pattern. Beyond a time of 1 h, a small broadening of the diffraction lines and a decrease in the line intensities were observed. Accordingly, in order to avoid any effect of radiation damage during the measurements, after every 25−30 min of measurement the sample was shifted across the beam by 2 mm in order to expose a fresh area of the film to the X-rays.

Figure 1. (a) 2D diffraction pattern of CdA_Al LB film, taken at 300 K with the angle of incidence α2 = 0.19°, (b) in-plane diffraction pattern, and (c) out-of-plane diffraction pattern, as extracted from the 2D data. Inset in (b) shows the orientation of different diffraction planes in the distorted hexagonal 2D lattice.

The in-plane diffraction pattern (Figure 1b) exhibits two strong peaks around qxy = 15.7 nm−1 and 16.9 nm−1 corresponding to the (01 + 11)̅ and (10) reflections, respectively. This is in conformity with earlier works which show that the in-plane structure of CdA multilayer is a distorted hexagon.13,14 Figure 1c gives the out-of-plane diffraction profile. Diffraction peaks corresponding to vertical periodicity are clearly visible up to fourth order. In order to determine precisely the incidence angle corresponding to the excitation of waveguide modes in the film, the intensity of in-plane diffraction peaks was monitored as a function of the angle of incidence. Figure 2a,b gives the intensity of the (01 + 11̅) peak as a function of incidence angle in specimen CdA_Al and CdA, respectively. Because of the resonance enhancement of the X-ray intensity inside the waveguide cavity, one expects an enhancement of the intensity of the diffraction peaks whenever a waveguide mode is excited.24,25 Figure 2 does exhibit such behavior, and a resonance enhancement corresponding to the TE0 and TE1 modes can be clearly seen. The angles corresponding to these first two modes were precisely determined from the curve and were used for doing depth-dependent measurements. The inset

III. RESULTS AND DISCUSSION Figure 1 shows a typical 2D diffraction pattern of sample CdA_Al, taken at 300 K. The 2D pattern has been used to extract the in-plane as well as out-of-plane diffraction profiles. 3951

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Figure 2. Area of the qxy = 15.7 nm−1 (01 + 11̅) peak as a function of the angle of incidence of the X-rays. The inset shows the depth distribution of X-ray field intensity inside (a) CdA_Al at angles of incidence α1 = 0.25° and α2 = 0.19° and (b) α1 = 0.15° and α2 = 0.19° in the case of CdA.

Figure 3. (a) In-plane diffraction pattern, (b) out-of-plane diffraction patterns of the three samples, all taken at an angle of incidence α2 = 0.19°, and (c) X-ray reflectivity of the three samples in as-deposited state.

in Figure 2a shows the simulated X-ray intensity inside the LB film, for the angles of incidence α2 = 0.19° and α1 = 0.25° which correspond to the TE0 and TE1 modes, respectively. One can see that for the TE0 mode the intensity at the center of the LB film is about 3 times the intensity in the interfacial region. Thus, for the incidence angle corresponding to the TE0 mode information is preferentially obtained from the center of the LB film. On the other hand, for the TE1 mode the two antinodes lie close to the interfaces with the Ni and Al layers. Therefore, the information will be obtained from interfacial regions. All the temperature-dependent diffraction measurements on these as well as the CdA_Ni samples were done at these two angles so as to get information about the interfacial region and the center of the LB film, respectively. In the case of sample CdA, based on Figure 2b, the two angles of incidence for measurements were chosen as α1 = 0.15° and α2 = 0.19°. The simulated X-ray intensity for these two angles is shown in the inset of Figure 2b. The angle of 0.19° corresponds to the TE0 mode and gives information about the center of the film; on the other hand, the angle 0.15° is chosen such that evanescent wave is strongest just below the surface of the film, so that preferential information is obtained from the free surface region. Figure 3 shows the room-temperature in-plane and out-ofplane diffraction profiles of three samples CdA, CdA_Ni, and CdA_Al, taken at incidence angle α2. It may be noted that, for sample CdA_Ni, in-plane diffraction peaks appear to be asymmetric and shifted to higher q values, which may be attributed to some misalignment of the sample. However, it may be emphasized that the shift in peak position will not have any significant effect on the separation between the two peaks or the peak widths. Therefore, the information extracted from

the diffraction data will not be affected. The positions of inplane diffraction peaks have been used to determine the distortion of the lattice from a regular hexagon, in terms of parameter Δγ = 60 − γ,14 where ⎡q ⎤ γ = cos−1⎢ 2 ⎥ ⎢⎣ 2q1 ⎥⎦

(1)

with q1 and q2 being positions of the (01 + 11̅) and (10) peaks, respectively. The out-of-plane diffraction pattern has been used to get the periodicity in the vertical direction. It may be noted that, although the intensity of the first order Bragg peak in the vertical direction is highest, it rides over a strong nonlinear background, and therefore, it becomes difficult to get its position. Therefore, the second-order peak has been used to determine the vertical periodicity D, which in turn has been used to get vertical tilt angle β of the chains using the relation ⎛D⎞ β = cos−1⎜ ⎟ ⎝ D0 ⎠

(2) 15,27

where D0 = 55.4 Å is the length of the untilted chain. Figure 3c gives the X-ray reflectivity of the three pristine samples. Reflectivity data have been fitted to get the vertical periodicity of the multilayers. The results are summarized in Table 1. One can see that there is a small difference in the vertical periodicity as obtained from X-ray reflectivity and from the outof-plane diffraction data. However, the value obtained from Xray reflectivity is expected to be more accurate as the same has been obtained by fitting of the full reflectivity pattern. In the 3952

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Table 1. Results of the fitting of X-ray Reflectivity and X-ray Diffraction Data Corresponding to Angle α2 vertical periodicity sample

from XRR (nm)

from out of plane diffraction (nm)

tilt angle from XRR (deg)

Δγ (deg)

CdA CdA_Ni CdA_Al

5.525 ± 0.005 5.521 5.540

5.532 ± 0.005 5.291 5.532

4.2 ± 0.5 4.8 0

2.56 ± 0.06 2.79 2.90

peaks in order to extract the temperature-dependent peak positions as well as their widths. The peak positions have been used to extract the temperature-dependent distortion Δγ from the hexagonal lattice using eq 1. The line-widths of the two peaks have been used to get the temperature dependence of the coherence length along the (01 + 11̅) and (10) directions using the relation:

case of out-of-plane diffraction data, only the position of the second-order peak is used for calculating D, and errors may be introduced because of background or refraction effects. From Table 1, one can see that there is a small variation in the inplane distortion Δγ and the vertical tilt angle β from sample to sample. This may be either the effect of the capping layer or some variation in the preparation parameters. Figure 4 gives the temperature-dependent in-plane diffraction pattern for the sample CdA_Al taken at angles of incidence α1

ξ= with

2 Δ(qxy) Δ = (Δm − Δins)

(3)

where ξ is the coherence length, Δm is the experimental width of the peak at qx and Δins is instrumental width.13,28 The results of the analysis of the in-plane and out-of-plane diffraction data are shown in Figures 5−7. Figure 5a,b,c gives the temperature dependence of the distortion parameter Δγ for the samples CdA_Al, CdA_Ni, and CdA, respectively. The temperature-dependent line-widths for the three specimens are given in Figure 6. The out-of-plane diffraction data has been used to obtain the vertical tilt angle β and the results are shown

Figure 4. In-plane diffraction pattern of sample CdA_Al as a function of temperature for (a) α1 = 0.25° and (b) α2 = 0.19°.

and α2. One can see that with increasing temperature both peaks shift to lower q values, and the separation between the two peaks continues to decrease. Similar behavior was obtained for the diffraction pattern of CdA_Ni and CdA films (data not shown). From Figure 4, one can see that in specimen CdA_Al, for incidence angle α1, which corresponds to the interface region, the in-plane diffraction peaks completely disappear at a temperature of 370 K, indicating in-plane melting. On the other hand, for incidence angle α2, which corresponds to the bulk region of the film, the diffraction lines are still visible at 370 K and disappear only at 373 K. Thus, the melting transition occurs in the interface region at a lower temperature as compared to the bulk. Considering the fact that measurements were done with a temperature step of 3 K and the temperature stability during the measurement was ±0.5 K, the difference in the melting temperatures of interface and bulk region could be anywhere between 1 K to 6 K. In CdA_Ni and CdA films, the diffraction peaks for angles α1 and α2 disappear at 370 K and 373 K, respectively. Thus, the melting temperature does not depend upon whether the top layer is Al or Ni, or it is free surface. The diffraction data have been fitted with two Gaussian

Figure 5. Temperature dependence of distortion parameter Δγ for (a) CdA_Al, (b) CdA_Ni, and (c) CdA, as calculated from the fitting of in-plane diffraction patterns. For samples CdA_Al and CdA_Ni, α1 = 0.25° and α2 = 0.19°, while for CdA, α1 = 0.15° and α2 = 0.19. 3953

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Figure 6. Temperature dependence of the width of the (01 + 11̅) and (10) peaks of samples (a) CdA_Al, (b) CdA_Ni, and (c) CdA. For samples CdA_Al and CdA_Ni, α1 = 0.25° and α2 = 0.19°, while for CdA, α1 = 0.15° and α2 = 0.19.

Figure 7. Tilt angle of the CdA chain of specimens (a) CdA_Al and (b) CdA_Ni as obtained from the second-order Bragg peak at incidence angles α1 = 0.25° and α2 = 0.19° and (c) CdA at incidence angles α1 = 0.15° and α2 = 0.19°, as a function of temperature.

in Figure 7. From Figure 5, one can see that with increasing temperature the distortion of the lattice from regular hexagon decreases and goes to zero for sufficiently high temperature. Up to a temperature of about 348 K, there is a slow variation in the distortion. However, beyond this temperature distortion rapidly decreases to zero. This rapid decrease in Δγ signals the transformation to a hexaticlike phase.13,14 This is also confirmed from the tilting of the chains, as observed from out-of-plane diffraction pattern. From Figure 6, one can see that the temperature dependence of the width of diffraction peaks and hence that of the coherence length is significantly different along the (01 + 11̅) and (10) directions. However, in the case of confined films, there is no significant difference in the interface region and bulk. With increasing temperature, while the coherence length along the (01 + 11̅) direction remains almost constant, that along the (10) direction decreases in both the bulk and interfacial regions. Before complete melting of the LB film, at incidence angle α1 the coherence length along the (01 + 11)̅ direction is 9 and 7.5 nm, while along the (10) direction, it becomes 2.0 and 2.1 nm for the samples CdA_Al and CdA_Ni, respectively. The results of the CdA film are in agreement with those of an earlier study:13 (i) Around a temperature of 357 K, the bulk of the film transforms to a hexaticlike phase, as evidenced by a rapid decrease in the distortion parameter Δγ, accompanied by tilting of the chains. (ii) In the surface region, transition to the hexaticlike phase is preceded by a smectic phase in which coherence along (10) direction is almost completely lost while that along (01 + 11̅) still remains.

From a comparison of the results of CdA_Al, CdA_Ni, and CdA, the following points may be noted. 1. The most significant effect of confinement with the top metallic layer is observed on the temperature dependence of the widths of the in-plane diffraction peaks. In the film CdA, while the surface region exhibits a transition to the hexaticlike phase via an intermediate smectic phase, in the bulk of the multilayer the intermediate smectic phase is not observed. This difference in the melting behavior of the surface and bulk disappears in both CdA_Al and CdA_Ni films. The confinement results in disappearance of the intermediate smectic phase. However, one can see that an anisotropy exists between the (01 + 11̅) and (10) directions. With increasing temperature, while the coherence length along the (01 + 11̅) direction remains almost unchanged, that along direction (10) decreases with increasing temperature in both the bulk and the interfacial regions. The presence of an intermediate smectic phase has been seen in 3-monolayer-thick CdA LB film.16 This is in conformity with the theoretical prediction for two-dimensional systems with distorted hexagonal symmetry.21 Such low dimensionality effects are also observed in the surface layers of bulk films.13 Confinement with the top metallic layer makes the interface layers behave in a manner similar to that of the bulk. An interaction of the tail group at the surface with the top metallic layer should be responsible for the same. 2. Confinement of the film also significantly affects the vertical tilting of the chains. A comparison of Figure 7a, b, and c shows that rapid tilting of the chains which is observed in the CdA film beyond 353 K becomes obscure in the films confined with top Al and Ni layers. Further, one can see that in film CdA 3954

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the bulk of the film the smectic phase is absent. In the confined films, this difference in the behavior of the interface and of the bulk disappears. However, some anisotropy along (01 + 11̅) and (10) direction remains both in the bulk as well as in the interface region; coherence along (10) is lost at a faster rate as compared to that along the (01 + 11̅) direction. However, in both the film with free surface and those confined with top metallic layer, the melting temperature of surface/interface is found to be lower than that of the bulk. Stiffness of the chains against vertical tilting is also found to increase in the confined films.

the maximum tilting of the chains in hexaticlike phase is 12° in the surface region and 10° in the bulk. As compared to this, in films CdA_Al and CdA_Ni the maximum tilt angles are 8° and 5° in the interfacial and bulk regions, respectively. Thus, maximum tilting of the chains in both CdA_Al and CdA_Ni films is significantly lower as compared to CdA film. Further, even between CdA_Al and CdA_Ni there seems to be qualitative difference in the sense that in CdA_Ni film tilting of the chains with increasing temperature is significantly lower as compared to CdA_Al. This suggests that stiffness of the chains against tilting increases as one goes from CdA to CdA_Al and then to CdA_Ni. Some earlier studies have shown that stiffness of the molecular stack against tilting increases with the number of the monolayers.14,17 In the present case, the number of monolayers is the same for all three LB films. Therefore, the increased stiffness in films CdA_Al and CdA_Ni is a result of confinement by the top metallic layer. It has been suggested that stiffness against tilting of the molecular chains can be defined in terms of an internal field H, the temperature dependence of which can be written as14 ⎡ β (T )T ⎤ H = H 0 ⎢1 − ⎥ β(Tc)Tc ⎦ ⎣

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are thankful to DST, New Delhi for providing financial assistance to perform the experiment under the program Indo-Italian POC. Pallavi Pandit would like to acknowledge the Sr. Research Fellowship of CSIR, New Delhi.

(4)

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with β(T) being the in-plane expansion coefficient at temperature T, and Tc being the melting temperature. Variation of the stiffness of chains on LB film thickness has been attributed to variation in β(T).14 In the present case, the observed difference in the temperature dependence of tilting in the three samples may be related with possible difference in β(T) in the three samples due to different interfacial interactions with the top metallic layer. In all three samples, the maximum tilt angle in the bulk is lower than that in the surface/interface region. This difference can be attributed to higher free energy at the surface/interface as compared to the bulk. 3. The melting temperature at which the in-plane diffraction lines disappear does not get affected significantly by the top Al or Ni layer. In all three films, the diffraction peaks corresponding to incidence angle α1 (surface/Interface region) disappear at 370 K, while for incidence angle α2 (bulk region), the same disappear at 373 K. Thus, the maximum variations in the melting temperature among the three samples can be 3 K. In some earlier studies, it has been shown that a buffer layer can significantly affect the melting temperature of soft matter films like polyethylene and CdA.11,15 This change in the melting temperature has been attributed to a change in the interface energy with the substrate. An increase in the interface energy results in an increase in the melting temperature.11,15 Therefore, one expects that a variation in the energy of the top interface should also affect the melting temperature. In the present case, the maximum possible difference in the melting temperatures of the three films can be only 3 K. This suggests that the interface energy of LB film with Ni or Al is comparable to the free surface energy.

REFERENCES

(1) Alba-Simionesco, C.; Coasne, B.; Dosseh, G.; Dudziak, G.; Gubbinsand, K. E.; Radhakrishnan, R.; Sliwinska-Bartkowiak, M. Effects of confinement on freezing and melting. J. Phys.: Condens. Matter 2006, 18, R15−R68. (2) Peng, Y.; Wang, Z.; Alsayed, A. M.; Yodh, A. G.; Han, Y. Melting of Colloidal Crystal Films. Phys. Rev. Lett. 2010, 104, 205703−4. (3) Peng, Y.; Wang, Z. R.; Alsayed, A. M.; Yodh, A. G.; Han, Y. Melting of multilayer colloidal crystals confined between two walls. Phys. Rev. E. 2011, 83, 011404−12. (4) Pusey, Peter N. Freezing and Melting: Action at Grain Boundaries. Science 2005, 309, 1198−3. (5) Alsayed, A. M.; Islam, M. F.; Zhang, J.; Collings, P. J. Premelting at Defects Within Bulk Colloidal Crystals. Science 2005, 309, 1207−5. (6) Gasser, U.; Weeks; Eric, R.; Schoeld, A.; Pusey, P. N.; Weitz, D. A. Real-Space Imaging of Nucleation and Growth in colloidal Crystallization. Science 2001, 292, 258−262. (7) Gasser, U.; Eisenmann, C.; Maret, G.; Keim, P. Melting of Crystals in Two Dimensions. ChemPhysChem 2010, 11, 963−970. (8) Han, Y.; Ha, N. Y.; Alsayed, A. M.; Yodh, A. G. Melting of twodimensional tunable-diameter colloidal crystals. Phys. Rev. E 2008, 77, 041406−7. (9) Savage, J. R.; Dinsmore, A. D. Experimental Evidence for TwoStep Nucleation in Colloidal Crystallization. Phys. Rev. Lett. 2009, 102, 198302−4. (10) Angelescu, D. E.; Harrison, C. K.; Trawick, M. L.; Register, R. A.; Chaikin, P. M. Two-Dimensional Melting Transition Observed in a Block Copolymer. Phys. Rev. Lett. 2005, 95, 025702−4. (11) Wang, Y.; Rafailovich, M.; Sokolov, J.; Gersappe, D.; Araki, T.; Zou, Y.; Kilcoyne, A. D. L.; Ade, H.; Marom, G.; Lustiger, A. Substrate Effect on the Melting Temperature of Thin Polyethylene Films. Phys. Rev. Lett. 2006, 96, 028303−4. (12) Viswanathan, R.; Madsen, L. L.; Zasadzinski, J. A.; Schwartz, D. K. Liquid to hexatic to crystalline order in Langmuir-Blodgett films. Science 1995, 269, 51−4. (13) Gupta, A.; Rajput, P.; Bernstorff, S.; Amenitsch, H. Lowdimensionality Effects in the Melting of a Langmuir-Blodgett Multilayer. Langmuir 2008, 24, 7793−7796. (14) Mukhopadhyay, M. K.; Sanyal, M. K.; Datta, A.; Mukherjee, M.; Geue, Th.; Grenzer, J.; Pietsch, U. Transition from two-dimensional to three-dimensional melting in Langmuir-Blodgett films. Phys. Rev. B 2004, 70, 245408−9.

IV. CONCLUSION In conclusion, the effect of confinement of a LB film between two metallic layers on its melting behavior has been studied. The film with free surface exhibits a melting behavior similar to earlier studies, where the surface region exhibits a low dimensionality effect.13 While the surface region transforms to the hexaticlike phase via an intermediate smectic phase, in 3955

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dx.doi.org/10.1021/la304463q | Langmuir 2013, 29, 3950−3956