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Analytical Research Center, Central Research Center, Hyosung Corporation, Anyang-si, Gyeonggi-do 431-080, South Korea. Langmuir , 2012, 28 (28), ...
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Simple Synthesis of Palladium Nanoparticles, β-Phase Formation, and the Control of Chain and Dipole Orientations in PalladiumDoped Poly(vinylidene fluoride) Thin Films Dipankar Mandal,†,§ Kap Jin Kim,*,† and Jong Soon Lee‡ †

Department of Advanced Materials Engineering for Information and Electronics, College of Engineering, Kyung Hee University, Yongin-si, Gyeonggi-do 446-701, South Korea ‡ Analytical Research Center, Central Research Center, Hyosung Corporation, Anyang-si, Gyeonggi-do 431-080, South Korea S Supporting Information *

ABSTRACT: Palladium nanoparticles (Pd-NP's) are prepared by a simple one-step procedure when poly(vinylidene fluoride) (PVDF) is used as a polymer stabilizer. High-quality Pd-NP-doped PVDF thin films are fabricated where the heat-controlled spin-coating technique is adopted. The effect of Pd-NP's on the crystal modifications and lamellae orientation in PVDF films is investigated using Fourier transform infrared-grazing incidence reflection absorption spectroscopy. The electroactive β phase and edge-on crystalline lamellae are found to be formed preferentially in Pd-NP-doped PVDF films. As a result, Pd-NP-doped PVDF ultrathin films gave a very good discernible contrast between the written and erased data bits, which suggests that they can be used as a scanning-probe-microscopy-based ferroelectric memory device or a ferroelectric gate field-effect transistor memory device in the future.

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INTRODUCTION Poly(vinylidene fluoride) (PVDF) is widely used in a variety of electro-optical, electro-mechanical, and biomedical applications. It is a semicrystalline polymer consisting of at least four different crystalline polymorphs (α, β, γ, and δ phases).1 The most common polymorph, the α phase, is designated as an electrically inactive nonpolar phase, whereas others are electrically active polar phases.1,2 The α and γ phases can be obtained easily by either casting from a polar solution or from high-temperature melt crystallization.3 The δ phase is transformed from the α phase by employing a high electrical field.1 However, of all polymorphs, the all-trans (TTTT) pseudohexagonal β phase exhibits the largest spontaneous polarization. The presence of the β phase in PVDF shows superior ferroelectricity, piezoelectricity, and pyroelectricity in comparison to those of γ and δ phases.4 Therefore, many attempts have been made to induce the electroactive β phase in PVDF by various methods such as solution growth,5 melt quenching,6 mechanical stretching,7 application of high pressure,8 addition of metal salts,9 formation of a nanocomposite,10,11 blending with polymers consisting of carbonyl groups,12 polarization via an applied field,13 and electrospinning.14 However, it has been found that most of the research is mainly focused on nucleating the β phase in micrometer-thick PVDF films that are mostly applicable to designing the sensor and actuator types of devices. In nanoscale ferroelectric polymer-based device application, a very high quality (smooth and homogeneous surface) ultrathin film with a β phase is desirable.12 In this work, we explore the possibility of fully nucleating the β phase in PVDF thin films by palladium nanoparticles (Pd© 2012 American Chemical Society

NP's) doping as well as the preparation of Pd-NP's by a facile route. It is worth noting that the most of the available techniques such as wet-chemical synthesis, intercalation, pulsed-laser ablation, electrochemistry, sputtering, physical vapor deposition, microwave plasma, thermal decomposition, and so forth either involve multiple steps or need highly sophisticated equipment.15−17 In this respect, our methodology of Pd-NP preparation is not only simple but also cost-effective. It is well known that Pd has enormous capability for hydrogen storage (e.g., 1 volume unit of Pd holds 643.3 volume units of hydrogen).18 Therefore, apart from ferroelectric memory or piezoelectricity- or pyroelectricity-based device applications, a Pd-NP-doped PVDF film or membrane might be used for hydrogen storage applications that might have a potential impact on replacing the fossil-fuel-based economy. It has been found that hydrogen is dissolved and stored in the solid state, in the spaces between the Pd atoms of the crystal lattice of the host metal, because of the relatively high solubility and mobility of H in the face-centered-cubic (FCC) Pd lattice.18 Another idea behind Pd-NP doping in PVDF is to promote its use as a multiferroic polymer material (i.e., having the dual property of being ferroelectric and ferromagnetic) in the near future. Recently, the property of ferromagnetism was observed in PdNP's, which is superior to that of so-called 3d transition-metalbased magnetic materials.17 Therefore, it would be very interesting if Pd-NP's could give rise to the ferroelectric active Received: March 7, 2012 Revised: June 29, 2012 Published: July 2, 2012 10310

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β phase in PVDF. In this work, we have prepared 50−150-nmthick PVDF films by the heat-controlled spin-coating (HCSC) technique. This procedure is very efficient for achieving ultralow surface roughness and control over the chain orientation in ultrathin PVDF films because it prefers an edge-on orientation, where carbon−carbon backbone chains of PVDF are parallel to the substrate, which is most favorable for flipping the ferroelectric dipoles (CH2/CF2) when an external electric field is applied along the thickness of the films.19 The major difficulty in fabricating nanoscale electronic devices with PVDF lies in the choice of the selective solvents because all currently available solvents have high boiling points (>140 °C). Therefore, the slow evaporation rate in spin coating at ambient temperature favors a face-on orientation (i.e., where the carbon−carbon backbone chains of PVDF are perpendicular to the substrate that is energetically unfavorable in nanoscalebased ferroelectric memory device applications). In addition, it also gives rise to very rough, inhomogeneous film surfaces, which is in general undesirable for reliable device performance.

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RESULTS AND DISCUSSION The Pd-NP formation is confirmed from the TEM image (Figure 1a), and the NBD (nanobeam diffraction) pattern

EXPERIMENTAL SECTION

The Pd-NP's were prepared by a simple one-step procedure. A 2 wt % (w/v) PVDF (Mw = 275 000, Aldrich, USA) solution was prepared by dissolving PVDF in N,N-dimethylformamide (DMF, Aldrich, USA) at 60 °C with stirring for 4 h. The 0.1 and 0.2 wt % palladium chloride (PdCl2, Kojima Chemicals, Japan) was mixed with a PVDF−DMF solution and stirred for 7 days at room temperature (30 °C). We allowed 7 days of stirring to ensure that a desirable number of Pd NP's formed from PdCl2 because we did not use any external reducing agent except the known good solvent of PVDF, namely, DMF. Finally, the formation of nanoparticles was confirmed by a drop of solution placed on a copper grid coated with amorphous carbon film and analyzed under a transmission electron microscope (TEM, JEM2100F, JEOL, Japan). The neat PVDF- and Pd-NP-doped PVDF thin films were prepared by spin coating on ITO-coated glass substrates at 80 °C at a spinning speed of 2000 rpm for 60 s. The details of such an HCSC setup has been described elsewhere.20 Finally, the thin films were annealed at 130 °C for 4 days under vacuum. Fourier transform infrared grazing incident reflection absorption spectra (FTIR-GIRAS) of thin films were recorded using a Bruker 66 V FTIR spectrophotometer at a resolution of 4 cm−1 with 1000 scans at an angle of incidence of ca. 85°. The film thickness was measured by using a surface profilometer (Surfcorder, ET-3000, Japan), and it was found to be around 80 nm. The surface morphology and nanoparticle distribution were analyzed by field emission scanning electron microscopy (FE-SEM, Supra 50/50 VP), operated by an accelerating voltage of 10 kV. The X-ray photoelectron study was performed by a thermo-electron system, where Al Kα was used as the excitation source and an ion flood gun was used for charge neutralization. The ferroelectric response imaging studies of the 80 nm thin PdNP-doped PVDF sample was carried out using dynamic contact electrostatic force microscopy (DC-EFM, XE-100, PSIA, Korea) equipped with a lock-in amplifier (SR850, Stanford Research System Inc.), and the associated topographic image was also recorded. In this report, unless otherwise mentioned, the Pd-doped PVDF samples are prepared from 0.2 wt % PdCl2 mixed in 2 wt % PVDF−DMF solutions. The samples were poled from top to bottom (Figure SI 4 in the Supporting Information) by applying −10 V of dc bias voltage to the cantilever tip over a scanning area of 4.5 × 4.2 μm2 (writing a bit), and then a smaller area of 2.4 × 2.2 μm2 was oriented in the opposite direction (erasing a bit) by applying +10 V. The reading process was then performed over a scanning area of 6 × 6 μm2, where alternately poled areas (writing and erasing) are located in the middle by applying an ac-modulating voltage having a frequency and amplitude of 17 kHz and 2 V, respectively.

Figure 1. (a) TEM image of the Pd-NP's. The inset displays the NBD pattern associated with Pd-NP's. (b) Histogram of the size distribution of the Pd-NP's. (c) High resolution TEM image of Pd-NP's.

obtained by focusing the beam on the nanoparticles (inset of Figure 1a) reveals the crystalline nature. The histogram of the particle size distribution and the average Gaussian curve are shown in Figure 1b, which shows that the average diameter of the particles is around 3.0 nm. The high-resolution TEM image (Figure 1c) represents the arrangement of the Pd lattice with an interplanar distance of 0.27 nm, which is in good agreement with an FCC structure for Pd metal.21 Therefore, the yield of Pd-NP production in this process indicates that DMF is a good reducing agent of PdCl2, similar to the formation of silver and gold nanoparticles from silver nitrate and chloroauric acid, respectively.10,11 The reduction process involved in the reaction is proposed as follows: HCONMe2 + PdCl 2 + H 2O → Pdo + Me2NCOOH + 2HCl

The water required for the reaction is supplied from the nonanhydrous DMF. In order to understand the compositional change, which might be related to conformational ordering of the PVDF chains, XPS was undertaken for neat PVDF and Pd-NP-doped PVDF thin films. The high resolution C1s spectra is shown in Figure 2a, where two main carbon species, CH2 and CF2, are evident at the binding energies of 286.4 and 290.9 eV, respectively, from neat PVDF thin films22 as expected from the structure of PVDF. From curve fitting analysis, a small tail at around 285 eV is found to be due to a small amount of carbon contamination, which is inevitable in an organic based fabrication. Thereby, it is detected in both neat and Pd-NPdoped PVDF thin films. The main differences found in the PdNP's doped PVDF thin film are: (i) a peak is traced around 288.1 eV by deconvolution, which is in between the binding energies of CH2 and CF2 species and (ii) asymmetry with 10311

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Figure 2. High resolution XPS spectra of (a) C1s, (b) F1s, (c) Pd3d, (d) Pd3p/O1s and (e) Cl2p.

(Figure 2d). The main line at 532.6 eV is due to metallic Pd species, and PdO is detected at around 534.4 eV. The peak area analyses from both Figures 2c and 2d show that exhibits the yield of Pd-NP's formation is more than 75%. Therefore, it can be emphasized that the Pd-NP's observed in the TEM exhibit is mainly due to metallic behavior. It is well-known that the polar single crystalline phase, namely the β phase in PVDF thin films, is one of the major challenges. On the other hand, in order to reduce the operating voltage of the ferroelectric memory devices, only the ultrathin film is desirable.12 In this work, HCSC technique is adopted to improve the thin film quality (homogeneous and better surface roughness), since the boiling point of DMF (ca. 150 °C) is quite high with respect to room temperature. Therefore during spin-coating, rapid solvent evaporation plays a very important role in achieving uniform thickness and establishing control over the chain orientations of PVDF. Figure 3 shows the FTIRGIRAS of neat and Pd-NP-doped PVDF (comprised from 0.1 wt-% and 0.2 wt-% PdCl2) ultrathin films (thickness ∼80 nm), spin-coated on the ITO coated glass substrates at 80 °C. The neat PVDF thin film is composed of mainly α- and β-crystalline phases. The IR absorption peaks that appear at 796, 764, and 615 cm−1 represent the characteristic of the α phase, whereas

broadening is observed in both CH2 and CF2 lines (see Figure SI 1 of the Supporting Information). This observation is further confirmed by the high resolution F1s spectra (Figure 2b), where Pd-NP-doped PVDF shows a slight asymmetry at the higher binding energy side from the center at 688.1 eV.22 These results give rise to a valuable conclusion that there must be some interfacial interaction present between Pd-NP's and CH2/ CF2 species of PVDF.23,24 Hence, XPS studies confirmed the adequate interaction between nanoparticles and polymer. Recently a similar observation was reported by Tiwari et al.24 In a few cases a distinguished interface between nanoparticle and polymer (γ-PVDF) is also observed by high resolution TEM images.25 The reduction of the PdCl2 is evident from the deconvolution of the Pd3d and Pd3p lines shown in Figures 2c and 2d, respectively. The spin−orbit coupling split between the 3d5/2 and 3d3/2 line is 5.3 eV and the curve area ratio is 1.12, which is in good agreement with the theoretical value of 1.5.26 Some contribution of Pd2+ is evident from the curve fitting analysis (shown in Figure 2c), which is solely due to some oxidized species (Figure 2d) of palladium (PdO),26 as a very poor signal of chlorine is detected (Figure 2e). Further supporting evidence of metallic Pd (0) is confirmed by the high resolution of Pd3p spectra where O1s region is also merged 10312

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of γ and β phases, whereas the as-cast Pd-NP-doped PVDF showed the preferential formation of the β phase (Figure SI 2 in the Supporting Information), as in the case of the Pd-NPdoped PVDF ultrathin film obtained by HCSC. These facts hint that the enhanced β-phase formation in Pd-NP-doped thin films is due not only to the shearing strain but also to a much greater contribution of the Pd-NP's themselves. The reason behind β-phase nucleation by Pd-NP doping is solely governed by the electrostatic interaction between the surface charge of the nanoparticles and CH2/CF2 dipoles of the PVDF chains. A schematic has been presented in Figure 4 based on the surface

Figure 3. FTIR-GIRAS measured from (i) neat PVDF and (ii) Pd-NPdoped PVDF thin films comprising (ii) 0.1 wt % PdCl2 and (iii) 0.2 wt % PdCl2. (a) 1600−400 cm−1 region; (b) 3100−2900 cm−1 region.

the 1283 and 445 cm−1 bands are the evidence of the β phase.19,27 It is noteworthy that the absence of an isolated γ phase characteristic peak (Pd-NP-doped PVDF film comprised from 0.2 wt-% PdCl2) around 1236 cm−1 implies that another two bands at 845 and 510 cm−1 are the signature of the β phase, as these two bands are the dual characteristic of the β- and γ phases.27,28 However, another intense band at around 1225 cm−1 (observed in neat PVDF and a Pd-NP-doped PVDF film comprising 0.1 wt % PdCl2) might be due to the large content of the α phase, which is shifted 6 cm−1 from the reported results.19,29 In the case of Pd-NP-doped PVDF thin films, the intensity of the 1283 and 445 cm−1 bands are significantly improved (spectra ii and iii in Figure 3a), but a considerable amount of α phase is sustained in the case of the 0.1 wt % PdCl2 added film, whereas it completely diminishes when PdCl2 is at 0.2 wt %. The less-intense band at 1236 cm−1 in 0.2 wt % PdCl2 added films suggests the presence of a negligible amount of the semipolar γ phase. It should be noted that in our previous attempt at organic modified silicate−PVDF nanocomposite films prepared by the HCSC technique the intensity of the γ-phase characteristic bands (∼1236 cm−1) is noticeably higher in comparison with that in the present study.19 This fact indicates that a desirable amount of Pd-NP-doping may fully nucleate and improve the β-phase content. Therefore, the formation of the β phase in the neat PVDF films in the HCSC technique described herein is mainly due to shearing strain and the subsequent stabilization of the molecular chains as described in our earlier work.20 It is important to note that Pd-NP's play a significant role in enhancing the β-phase content as well as diminishing the γ phase and completely removing the α phase. For a better understanding, an attempt has been made to prepare solvent casting films, where the shearing strain effect can be neglected. The as-cast neat PVDF had a predominant α phase with a trace

Figure 4. Schematic representation of the electrostatic interaction between the surface charges of nanoparticles and the CH2/CF2 dipoles of PVDF.

charge and dipoles interaction model. Depending on the type of surface charge density (σeff) of the nanoparticles, mainly two types of interaction may take place (i.e., when it is dominated by negative charges, CH2 dipoles (δ+eff) may interact, whereas CF2 dipoles (δ−eff) also interact when the surfaces of the nanoparticles are surround by a majority of positive charges (σ+eff)). This model is quite consistent with the prerequisite observation made in the XPS results where the interfacial interaction between Pd-NP's and CH2/CF2 dipoles was confirmed (Figure SI 1 of the Supporting Information). However, it is further clarified by FTIR results (Figure 3b) as well. A clear shifting of the νas-CH2 and νs-CH2 vibrational bands toward the lower-frequency side is evident in Pd-NPdoped PVDF films with respect to the neat PVDF film, which affirms the electrostatic interaction model. A similar observation is also found in solution-cast Pd-NP's (Figure SI 2 in the Supporting Information) and Au-NP-doped films.11 FTIR-GIRAS is a popular technique because it provides simple, nondestructive, and adequate sensitivity for recognizing molecular orientations (Figure SI 3 in the Supporting Information and associated discussion).30 In PVDF crystalline lamella, mainly two types of molecular orientations are expected: one is the edge-on orientation where long molecular chains are parallel to the substrates, and the other is the face-on orientation where long molecular chains are perpendicular to the substrates.19 Because of the property of easy dipole switching, edge-on orientations are the main focus of our attention.19,20 In these orientations, molecular CF2 dipoles are 10313

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Figure 5. FE-SEM images of (a) neat PVDF and (b) Pd-NP-doped PVDF thin films. AFM topographic images (scan area 6 μm × 6 μm) of (c) neat PVDF and (d) corresponding line (height) profiles and (e) a Pd-NP-doped PVDF thin film and (f) the corresponding line (height) profile.

dipoles making a right angle to it must be increased. Therefore, the enhanced intensity of the A1 bands (at 1283 cm−1, νsCF2 coupled with νsCC and δsCCC; at 845 cm−1, νsCF2 coupled with νsCC; and at 510 cm−1, νsCF2) and the reduced intensity of the B1 bands (at 1409 cm−1, ωCH2 coupled with νasCC; at 487 cm−1, ωCF2) are the main indication of the edge-on orientations. In addition, the B2 bands (at 1194 and 887 cm−1, νasCF2 coupled with ρCF2 and ρCH2) are less sensitive to the crystallinity of the β phase, but the adequately high sensitivity of the CF2 dipole orientations enhances the absorption intensity. However, in the case of face-on orientations, all B1 bands must show a higher intensity than the A1 bands, which is found in the case of neat PVDF films (spectra i in Figure 3). In contrast, Pd-NP-doped PVDF films show (both spectra ii and iii) that the band at 1283 cm−1 (A1) is more intense than that at 1409 cm−1 (B1); likewise, the absorption at 510 cm−1 (A1) is enhanced compared to that at 487 cm−1 (B1), which is a clear indication of the edge-on orientations. It is noteworthy that the intensity of the band at 845 cm−1 (A1) should not be compared with the B1 bands because Pd-NP-doped PVDF films still do not have a single crystalline polymorph as in the case of P(VDF-TrFE) thin films.30 The improvement in the B2-band intensity in Pd-NP-doped PVDF films is further supporting

statistically distributed almost perpendicular to the substrates, and as a result, it is energetically favorable for them to align along the electrical field direction (i.e., the thickness direction). In contrast, the flipping of the dipoles in face-on orientations requires a very high electrical field that is expected to be higher than the electrical breakdown voltage. As mentioned in our previous publication,30 FTIR-GIRAS with the unpolarized IR incident angle closer to the grazing incident angle (ca. 80−88° from the surface normal) exhibits an enhanced p-polarized component (Ep) where the electric vector is normal to the reflective surface and corresponds to a decrease in the s-polarized component (Es) whose electric vector is parallel to the surface. At the 85° angle of incidence used in this study, GIRAS predominantly detects vibrational modes with the transition dipoles normal to the substrate surface and hence can be useful in clearly identifying the changes in the dipole and chain orientation. The relationship between the vibrational transition moment (μ) and the unit cell axis of the β crystal and each vibrational symmetry species (A1, B1, and B2 shown in Figure 3a) is as follows: μ∥b at A1, μ∥c at B1, and μ∥a at B2. Hence, per the principles of FTIR-GIRAS and the structure of the β-phase unit cell, when the molecular chains are oriented parallel to the plane of the substrate surface, the percentage of 10314

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Figure 6. DC-EFM images ((a, b) amplitude, (c, d) phase) of neat PVDF (left panel) and Pd-NP-doped PVDF (right panel) thin films. Scan area: 6 μm × 6 μm. The corresponding line (marked with red lines) profiles are shown at the bottom (a, b) and in the middle (c, d) of the each image. (Bottom of c and d) Schematic illustrations of the orientation of the dipoles (cross-sectional view) along the marked lines shown in the DC-EFM images.

also refers to the height profile, shown in Figure 5d) with surface defects, whereas the Pd-NP-doped PVDF films show several spherulitic structures with specific shapes (average diameter ∼3.2 μm) and boundaries (depth below ∼1.0 nm) resulting from the impingement of spherulites and the growth originating from the nucleus (Figure 5e). Each spherulite has a smooth surface (Rq ≈ 7 nm) composed of edge-on crystalline lamellae as illustrated in the FE-SEM image (Figure 5b). The spherulite structure in Pd-NP-doped PVDF films indicates that the adhesive force plays a more important role than the centrifugal force; otherwise, it might show a surface topography similar to that of neat PVDF. From the analogy of the FTIRGIRAS results and these topographical images, one concluding

evidence of the edge-on-type orientations. Therefore, in addition to the enhancement of β-phase content, Pd-NP's can also control the chain and dipole orientations in PVDF thin films. The FE-SEM image of a neat PVDF film (Figure 5a) exhibits the α-spherulitic growth of fibrils splaying over the surface. A small region (marked with a dotted line) of β-crystallite is also apparent, which is quite consistent with the FTIR results. However, the surface of the PVDF film is fully covered with βcrystallite (Figure 5b), and more promisingly, controlled edgeon-type crystalline lamellae (needlelike) are also observed. The AFM topographic image of the neat PVDF (Figure 5c) films also exhibits long twisted fibril-like structures and an inhomogeneous surface (rms roughness, Rq ≈ 25 nm, which 10315

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remark should be made that the β-phase-rich PVDF film may also exhibit the spherulitic structure (a β-spherulite).11 Because of an ultrathin film thickness of ∼80 nm, a ferroelectric response imaging study is selected to avoid the frequent electric breakdown from the metal−ferroelectric− metal (MFM)-type structure. Recently, ferroelectric imaging has been found to be an effective method of evaluating the ferroelectric properties of the thin films.19,20 The amplitude (A) images of neat and Pd-NP-doped PVDF films are shown in Figure 6a,b, respectively. There is no discernible color contrast of the poled area (4.5 μm × 4.2 μm2) over the unpoled region, whereas Pd-NP-doped films shows a clear contrast (Figure SI 5-i,ii and in the Supporting Information). Consequently, a constant surface potential (Vs) persists in neat PVDF films (refer to the line profile of Figure 6a), and a significant change in the surface potential (ΔVs) is evident (refer to the line profile of Figure 6b) in the Pd-NP-doped film. ΔVs is associated with the difference in the alignment of the CH2/ CF2 dipoles along the electric field direction in the poled region and the unpoled region where dipoles are statistically distributed perpendicular to the plane of the substrates in edge-on crystalline lamellae. Face-on-type crystalline lamellae where dipoles are statistically distributed parallel to the plane of the substrates are not aligned because of the insufficient strength of the filed poling (voltage) employed in the present study. It should be noted that no contrast between the writing area and the erasing area (Figure SI 5-ii in the Supporting Information) is detected in Pd-NP-doped films, which indicates that the absolute value of the upward polarization closely matches the downward polarization. An unchanged surface potential magnitude (evident from the line profile in Figure 6b, shown by the blue dotted line) is mentioned over the poled area (writing and erasing area) and is further evidence of the polarization reversal phenomena in the ferroelectric-based materials. The sharp fall of the surface potential (evident from the line profile) throughout the boundary region (see the 3D image, shown in Figure SI 5-ii in the Supporting Information) of the erasing area (illustrated by the dark square boundary in the middle) is due to polarization neutralization over the boundary. Therefore, the effective electrical poling does not take place in the neat PVDF film, whereas it happens in the Pd-NP-doped film preferentially containing the electroactive β phase. The phase (φ) images should illustrate a better contrast between the writing and erasing areas if the ferroelectric dipoles are flipped perfectly (i.e., close to ±180°). The phase image of Pd-NP-doped films (Figure 6d) exhibits a significant contrast between the dark area (written) and the bright area (erased), whereas very faint writing and erasing areas with negligible contrast are seen in neat PVDF films (Figure 6c). There is no average phase shift (ΔΦ ≈ 0°) between the writing and erasing areas apparent from the line profile of the phase image of neat PVDF, indicating that the dipoles are not effectively poled and thereby are statistically distributed in the plane of the substrate, as shown at the bottom of Figure 6c. This indicates that even the presence of the electroactive β phase in neat PVDF is not suitable for ferroelectric imaging because of the face-on crystalline lamellae type of orientations, as evaluated from the FTIR-GIRAS results. Because the b axis of the β phase in face-on crystalline lamellae is nearly perpendicular to the direction of the electric field (Figure SI 4 in the Supporting Information), in order to align the electric dipoles along the electric field a higher-strength electric field is needed, which is undesirable in nanoscale-based

devices. In contrast, because of edge-on crystalline lamellae stacking (the b axis of the β phase is almost parallel to the electric field) in Pd-NP-doped PVDF, a 180° average phase shift (ΔΦ) is observed between the writing and erasing area (line profile in Figure 6d), which indicates that ferroelectric dipole switching is favorable. Thus, the poled dipoles are perfectly aligned along the electric field direction (perpendicular to the plane of the substrate) in Pd-NP-doped films, as seen at the bottom of Figure 6d showing the cross-sectional schematic. Therefore, the indication of the feasibility of using Pd-NP-doped PVDF as the main component of ferroelectricbased nanoscale memory devices is exclusively demonstrated.

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CONCLUSIONS In this work, Pd-NP-doped PVDF films could be obtained by a simple technique where no external reducing agent of palladium chloride was used other than DMF, which is one of the best known solvents for PVDF. From an XPS analysis, the yield of metallic Pd-NP's is found to be around 75%. More interestingly, we have found that Pd-NP's can induce the polar β phase in solution-cast films as well as in spin-coated ultrathin films. From the FTIR-GIRAS analyses, it is confirmed that the HCSC technique can also promote the β phase in neat PVDF films, but dominant face-on-type crystalline lamella can render it unsuitable for nanoscale-based ferroelectric memory device fabrication. In contrast, Pd-NP-doped PVDF films show several merits: (a) they can enhance the β-phase content and diminish the other phases (i.e., α and γ phases); (b) they favor edge-ontype crystalline lamellae; and (c) desirable surface morphology can be achieved. The reason behind the β-phase promotion is described on the basis of the dipole−surface charge interaction model. From the DC-EFM study, the perfect ferroelectric dipole switching image indicates that Pd-NP-doped PVDF might be a very promising material to use as a nonvolatile ferroelectric polymer memory and SPM-based memory devices.

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ASSOCIATED CONTENT

S Supporting Information *

Evidence of the interfacial interaction between the surface charge of Pd-NP's and CH2/CF2 dipoles of PVDF and the electroactive β phase in Pd-NP/PVDF composite samples. Effective tool in molecular orientation studies using FTIRGIRAS. Schematic of the electrical poling procedure in a DCEFM study. Three-dimensional view of amplitude and phase images of DC-EFM. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Present Address §

Department of Physics, Jadavpur University, Kolkata 700032, India. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by a grant from Kyung Hee University in 2010 (KHU 20100153). 10316

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dx.doi.org/10.1021/la300983x | Langmuir 2012, 28, 10310−10317