Spontaneous Assembly of Carbon-Based Chains in Polymer Matrixes

Nov 29, 2013 - ABSTRACT: Stable chains of carbon-based nanoparticles were formed directly in polymer matrixes through an electrode-free approach...
0 downloads 0 Views 1MB Size
Download PDF
Article pubs.acs.org/Langmuir

Spontaneous Assembly of Carbon-Based Chains in Polymer Matrixes through Surface Charge Templates O. Gennari,† S. Grilli,*,† S. Coppola,† V. Pagliarulo,† V. Vespini,† G. Coppola,‡ S. Bhowmick,‡ M. A. Gioffré,‡ G. Gentile,§ V. Ambrogi,∥ P. Cerruti,§ C. Carfagna,§,∥ and P. Ferraro† †

National Institute of Optics, National Council of Research of Italy, Via Campi Flegrei 34, 80078 Pozzuoli, Italy Institute of Microelectronics and Microsystems, National Council of Research of Italy, Via Pietro Castellino, Napoli, Italy § Institute of Polymer Chemistry and Technology, National Council of Research of Italy, Via Campi Flegrei 34, 80078 Pozzuoli, Italy ∥ Department of Chemical, Materials and Production Engineering, University of Naples, Piazzale Tecchio, 80125 Naples, Italy ‡

S Supporting Information *

ABSTRACT: Stable chains of carbon-based nanoparticles were formed directly in polymer matrixes through an electrode-free approach. Spontaneous surface charges were generated pyroelectrically onto functionalized ferroelectric crystals, enabling the formation of electric field gradients that triggered the dipole−dipole interactions responsible for the alignment of the particles, while embedded in the polymer solution. The phenomenon is similar to the dielectrophoretic alignment of carbon nanotubes reported in the literature. However, here the electric fields are generated spontaneously by a simple heat treatment that, simultaneously, aligns the particles and provides the energy necessary for curing the host polymer. The result is a polymer sheet reinforced with well-aligned chains of carbon-based particles, avoiding the invasive implementation of appropriate electrodes and circuits. Because polymers with anisotropic features are of great interest for enhancing the thermal and/or the electrical conductivity, the electrode-free nature of this technique would improve the scaling down and the versatility of those interconnections that find applications in many fields, such as electronics, sensors, and biomedicine. Theoretical simulations of the interactions between the particles and the charge templates were implemented and appear in good agreement with the experimental results. The chain formation was characterized by controlling different parameters, including surface charge configuration, particle concentration, and polymer viscosity, thus demonstrating the reliability of the technique. Moreover, micro-Raman spectroscopy and scanning electron microscopy were used for a thorough inspection of the assembled chains.

■

shear flows,14 mechanical forces,15 virus and DNA templating,16,17 dewetting,18 and blown bubble films.19 Recently, thermocapillary flows have been used for creating arrays of purely semiconducting SWCNTs.20 Alignment of CNTs through dielectrophoresis (DEP21) is very appealing because it can be controlled easily by the application of electric fields. Dielectric NPs suspended in a liquid medium can be arranged in an orderly fashion under the influence of a nonuniform electric field. Different DEP-based techniques have been reported in the literature for manipulating NPs. An electric field has been used for aligning CNTs in glass fiber-reinforced thermosetting composites.22 CNT−FETs have been attained by DEP trapping of semiconductive CNTs.23 Selective deposition of SWCNTs has been proposed to align nanotubes in the form of thin films.24 Separation of CNTs and polystyrene microparticles has been

INTRODUCTION Among the wide variety of materials that are used in optics and biomedicine, carbon-based nanoparticles (NPs) are the best candidates1 (e.g., graphene, single- and multiwalled carbon nanotubes SWCNT/MWCNT, and fullerenes). Their perfect structure can enhance specific properties such as mechanical, thermal, and electrical conductivity.2 They are used in a wide range of fields including nanoelectronics and photonics,3−6 biocompatible materials,7 and biomedicine.1 Moreover, CNTs are widely used as additives to various structural materials, and it is largely demonstrated that they can modify drastically the properties of the matrix in which they are included.8 The final properties of the resulting polymer composite depend strongly on the CNT orientation.9 For this purpose, it is highly desirable to control their orientation for producing functionalized films and composites. In past years, it has been widely demonstrated that although carbon NPs are difficult to disperse in liquid suspensions, they can be oriented by various techniques.10 These techniques make use of electrical fields,11,12 magnetic fields,13 © 2013 American Chemical Society

Received: September 17, 2013 Revised: November 20, 2013 Published: November 29, 2013 15503

dx.doi.org/10.1021/la403603d | Langmuir 2013, 29, 15503−15510

Langmuir

Article

(purity: 50−70%, D × L 1.2−1.5 nm × 2−5 μm); graphite (platelet nanofibers, purity = 99%, D × L 50−250 nm × 0.5−5 μm); fullerene (purity: 99.5%). Simple magnetic stirring at room temperature for 30 min mixed the NPs with a concentration of 0.03% (p/V). All of the NPs were purchased from Sigma Aldrich, St. Louis, MO. The dispersion stability is shown in Figure 1a,

achieved by DEP.25 MWCNTs have also been deposited by surfactant-free DEP,26 and electric fields have been used for aligning functionalized MWCNTs into PMMA matrixes.27 Recently, self-assembly of NPs into chains has been achieved by DEP.28 The orientation mechanism is attributed to dipole− dipole interaction, and the attractive force between two polarized NPs varies inversely to the fourth power of the interparticle distance. CNTs dispersed in polymers are reported to be assembled into quasi one-dimensional chainlike structures by DEP approaches.29 However, most of these mentioned techniques often require external electrodes and voltages, and usually the chain formation varies with the electric potential and with the electrode spacing.9,30 Recently, we published results concerning the use of periodically poled ferroelectric crystals for DEP-based applications such as particle trapping,31 surface charge lithography,32 or liquid nanodispensing.33 Self-assembling of liquid polymer by electro-hydro-dynamic effects was also achieved on periodical poled substrates.34 Nevertheless, it is important to note that recently periodically poled ferroelectric crystals are emerging as smart functionalized substrates for nanoassembling particles by photochemical methods for realizing plasmonic nanostructures.35 However, to the best of our knowledge, the use of mere surface charges that arise from onedomain pyroelectric crystals (i.e., crystals without reversed domains) has never been reported for DEP manipulation of carbon NPs in liquid suspensions as well as in polymer matrixes. In this paper, we present an electrode- and circuit-free method for the alignment of carbon-based NPs along well-defined chains, embedded into polymer matrixes. Unlike the conventional techniques, the chains are formed through the electric fields arising spontaneously over the surface of a pyroelectric crystal under thermal gradient conditions. The observed effect is very rapid because the chains appear soon after warming the crystal, and they extend over relatively large areas with high uniformity and reproducibility. Thanks to the spontaneous nature of the driving electric fields, the chains are formed in a very versatile fashion. In fact, the chains can be generated free from constraints and even floating and reconfigurable, as well as anchored to some desired geometrical configurations. In this way, the method does not require the pretreatment of samples, as usually done in the case of traditional DEP techniques. In fact, lithographic and/or etching processes are required usually for fabricating the electrodes that ensure the desired differential potential. Moreover, appropriate wire systems are usually needed for assuring the electrical connection of the electrodes with the external circuit and voltage generator. Therefore, a sort of on-demand carbon chain formation is provided by the proposed method that, compared to traditional electrode-based DEP techniques,28 would favor the development of a more compact interconnection technology with a significant impact in many fields such as electronics and sensors. We demonstrate the reliability of the technique first by showing that the chains are obtainable for a wide variety of carbon-based NPs and second by presenting a deep characterization of the phenomenon in terms of different process parameters that regulate the dynamics.

Figure 1. (a) Dispersion stability of the suspensions of MWCNTs (A), SWCNTs (B), graphite (C), and fullerene (D). (b) Initial random state of MWCNTs. (c) Schematic view of the process (not to scale) showing the initial random state of the CNTs (left) that, after thermal treatment onto a conventional hot plate, is converted into an ordered distribution of carbon-based chains (right); the metallic layer represents the conductive coupling edge where the chains are anchored. Optical microscope view of typical chains of MWCNTs (d), SWCNTs (e), graphite (f), and fullerene (g).

while Figure 1b shows the typical random state of MWCNTs in PMMA prior to DEP manipulation. The charge template, responsible for the DEP mechanism, was generated by appropriate thermal gradients applied to the z surface of z-cut lithium niobate (LiNbO3) or lithium tantalate (LiTaO3) crystals (both sides polished, 500 μm thick, from Crystal Technology, Inc.). In fact, such crystals, hereafter called “driving crystals”, exhibit pyroelectricity. Figure 1c shows the schematic view of the experimental procedure. The metallic layer into the scheme represents the conductive coupling edge to which the chains are anchored. Each of the suspension samples was casted onto the zface of the driving crystal and sandwiched by a conventional coverslip to reduce the evaporation rate of the solvent. The sandwich was heated onto a conventional hot plate for about 1 min at 150 °C. After heating, the sample was allowed to cool at room temperature on the stage of a conventional upright optical microscope. Figure 1d−g shows the optical microscope images

■

CHAINS OF CARBON-BASED PARTICLES Because carbon-based building blocks are of great interest for a wide variety of applications, a number of PMMA solutions (950 000 molecular weight (MW) at 9% (p/V) in anisole; η = 1 cP, εr = 3.6, Microchem Corp., Newton, MA) were prepared with different kinds of carbon NPs dispersed therein: MWCNTs (purity >90%, D × L 110−170 nm × 5−9 μm); SWCNTs 15504

dx.doi.org/10.1021/la403603d | Langmuir 2013, 29, 15503−15510

Langmuir

Article

function) in the case of MWCNTs and PMMA medium. The conductivity and the dielectric constant of MWCNTs and of PMMA are reported to be 104 S/m and 10−3 S/m, and 2.5 and 3, respectively (Sigma Aldrich and MicroChem datasheets). In this case, however, a steady electric field gradient occurs, leading to a DEP phenomenon in DC regime where the polarizability is governed by the conductivities:39 σp − σm ω → 0: Re(fCM ) → σm

of the typical chains driven by the pyroelectric effect. The NPs are first randomly dispersed into the polymer matrix (see Figure 1a), and, after a short thermal treatment, they appear well chained. The images clearly show that all of the carbon allotropes investigated in this work chained after the application of the thermal gradient. Typically the longest chains were obtained in the case of MWCNTs, while the shortest ones were obtained in the case of fullerene (see Figure 1g). The phenomenon is due to the nonuniform electric field generated onto the surface of the driving crystal during the warming and cooling processes, which are regulated simply by a heat exchange of the crystal under atmospheric conditions. In fact, one of the powerful features of the technique is the usage of very simple equipment. No external atmospheric-controlled conditions are required, and the spontaneity of the surface charges is enough to form welldefined and stable chains of MWCNTs embedded into PMMA. Moreover, the free-constraint formation of the chains occurs without the use of any electrode and/or external circuit. In fact, despite the conventional DEP-based techniques, the chains assemble spontaneously along the electric field lines arising from the thermal gradient incurred by the crystal. According to DEP theory, the electric field of the driving crystal induces electric dipoles to the CNTs, leading to a particle−particle interaction. This interaction induces the particles to move into the polymer solution with the main effect of forming particle clusters mostly in the shape of chains aligned along the electric field lines.36,37 It is well-known that, under ellipsoid approximation, the DEP force FDEP incurred by the particle is proportional to the gradient of the square of the electric field38−40 FDEP =

Considering the conductivity values mentioned above, the CNTs are subject to a positive DEP force. A common feature of the various carbon allotropes is the electro-orientation along the electric field lines.41 The oriented NPs interact with each other as dipoles,22 and, driven by the opposite charges of their extremities, NPs move closer gradually, forming a network of head-to-head connections that leads to the aggregation of chainlike structures. The rotational force originates from the torque on the dipole moment. In fact, the dipole moment P is proportional to the applied electric field E as follows:42

P = αE where α is the static polarizability tensor that, in the case of NPs, is highly anisotropic.42 The polarizability along the tube axis α|| is much higher than that perpendicular to the tube axis (α⊥). Therefore, for a nanotube oriented at an angle θ with respect to E, unless for θ close to 90°, the dipole moment of the nanotube is along the tube axis with P = α E cos θ

2πabc εm Re{fCM }∇(E2) 3

Due to the real part of the induced polarization, a torque is exerted on the dipole moment: t = P × E = α E2 sin θ cos θ

where a, b, c (a > b = c) are the half lengths of the major ellipsoid axes, εm the permittivity of the medium, and E the electric field. The DEP force depends on the frequency ω of the applied field by means of the Clausius−Mossotti factor:39 fCM =

The resultant torque causes the CNTs to rotate and align in the direction of the field.22,43,44 The spatial inhomogeneity of the electric fields bring about nanotube migration toward the high field regions, thus attracting CNTs to one another and enabling the formation of chains. Because higher electric fields induce higher field-induced forces on the CNTs, a corresponding higher density of oriented tubes is obtained. In the case of fullerene, the pentagonal sheet structure of the lattice deviates from planar and thus prevents the delocalization of a net charge. Therefore, the spherical fullerene NPs are less influenced by the electric field distribution, compared to graphite and CNTs. Hence, the fullerene exhibits the shorter chains as shown in Figure 1g.

εp* − εm* εm*

where εp* and εm * are the complex permittivities of the particle and medium, respectively. A general complex permittivity is given by ε* = ε − j(σ/ω). Figure 2 shows the typical dispersion curve of the particle polarizability (real part of the Clausius−Mossotti

■

CONTROLLING THE CHAIN FORMATION The self-assembling effect was investigated more fully in the case of MWCNTs dispersed into PMMA, with particular attention to specific features that appeared to control the chain formation: the coupling edges; the CNT concentration; the polymer viscosity. Coupling Edges. The results shown in Figure 1d−g refer to chains grown under constraint-free configurations; that is, the NPs aggregate under the simple action of the spontaneous electric field generated by the thermal gradient of the driving crystal. However, the charge template can be configured easily to control the spatial distribution of the chains. What we call “coupling edges” here can be configured appropriately to redistribute the charges and configure the chains as desired. Different configurations were investigated to demonstrate the

Figure 2. Typical dispersion curve of the particle polarizability in the case of MWCNTs mixed with the PMMA solution. The values of permittivity and conductivity are reported in the main text. 15505

dx.doi.org/10.1021/la403603d | Langmuir 2013, 29, 15503−15510

Langmuir

Article

to the pressure gradient applied to the sandwich sample. Moreover, this movie shows clearly the possibility of restoring chains partially broken by additional heat pulses, namely by inducing supplementary heat gradients after thermal stabilization. The variety of chain configurations is demonstrated by a series of optical microscope images as shown in Figure 3, where the

reliability of the procedure, i.e., electrical conductive layers, crystal grooves, domain walls, and confined liquid layers. The conductive layers were of two different types. In one configuration, aluminum foils cut by a conventional scalpel and made adherent to the crystal surface by gentle pressure were adopted. In a second configuration, titanium tips deposited by ebeam evaporation and patterned by a lift-off process were used. The grooves resulted from gentle scratching of the crystal surface by a conventional diamond tip or by spontaneous stress-related cracks. The domain walls were obtained by standard electric field poling,32 and the confined liquid layer was deposited onto the crystal surface by a conventional swab tip soaked by the appropriate suspension. Movies 1 and 2 in the Supporting Information show the typical formation of chains in correspondence to the tip of an aluminum foil (dark region) adherent to the crystal surface. These movies have been recorded under a conventional inverted optical microscope. The sandwich sample (CNT suspension squeezed between a glass coverslip and LN wafer) was placed on the transparent stage of the microscope, while a hot glass slide (at about 170 °C) was placed on the top, in contact with the LN wafer, thus inducing a sort of “heat pulse” responsible for the pyroelectric effect. At the beginning, the CNTs clearly appear distributed randomly into the PMMA solution, and, immediately after placing the hot slide (made visible by the slight turbulence), the CNTs assemble into well-defined chains that follow the electric field lines emerging from the edges of the aluminum tip. The entire self-assembling process takes typically from about 5 to 15 s. Stable chains remain embedded into the polymer matrix while it becomes rigid due to solvent evaporation. In fact, the great advantage of the technique is two-fold. On one hand, the pyroelectric effect is very quick (i.e., charges appear on the crystal surface immediately after contact with the hot plate) in contrast to other works where tens of minutes are required for the formation of the chains. On the other hand, because one single heat pulse simultaneously induces the appearance of the charges and accelerates the solvent evaporation, a rigid polymer with embedded chains can be obtained easily. Hence, the operating principle of the technique is very simple, and the self-assembling of the chains is very rapid and repeatable. The only critical feature is the stability of the chains that depends significantly on the viscosity grade of the polymer solution. In fact, low viscosity is required at the very beginning of the self-assembling to have CNTs sufficiently free to move under the action of the electric field. However, viscosity must increase just after the selfassembling effect when stable and permanent chains are desired. The most stable chains were achieved typically by allowing the PMMA film to evaporate partially for about 5 min just after casting onto the crystal surface. Certainly, such preventive evaporation must be moderate so that the successive heat pulses do not make the polymer too hard and thus prevent NP assembly. This evaporation-related method can be used for other intriguing manipulations of the NP chains. For example, when using raw PMMA (without preventive evaporation), the chains are equally well formed but they can fluctuate well under the action of standard microfluidic instabilities, driven by pressure differences and/or air bubbles. Movie 3 in the Supporting Information shows a magnified view of the typical chain formation under no preventive evaporation. The CNT chains assemble well along the field lines, which are perpendicular to the straight edge of the aluminum foil. After completion, they clearly fluctuate and bend along the vertical direction that corresponds

Figure 3. Microscope view of typical MWCNT chains in PMMA formed across different kinds of conductive layer edges: parallel straight edges of titanium layers (a); far from a straight edge of a titanium layer (b); tip of a titanium (c) and of an aluminum foil layer (d); dots of a titanium layer (e); facing tips of aluminum foil layers (f). (g) COMSOL simulation of the electric field lines corresponding to the geometry in panel c. (h) COMSOL simulation of the electric field lines corresponding to the geometry in panel f.

CNT chains appear well anchored to the edges of conductive layers under different geometries. Parallel chains can be obtained by using straight edges (a, b), radial chains are generated by tiplike edges (c, d), and chains connecting two facing dots or tips are also obtainable (e, f). The connection between the facing layers was obtained by grounding one of them. The DEP interaction of neighboring particles is regulated by both DEP attractive chaining force and repelling force.45 In particular, the repulsion force affects the neighboring particles with their connecting line perpendicular to the local field lines. This repulsion force is a decreasing function of particle center-tocenter distance, thus always inducing a gap between neighboring chains aligned along parallel field lines (see Figure 3a−f). The inset in panel f of Figure 3 shows the instant during the heat pulse when the CNTs move frantically prior to attaining the final stable assessment. COMSOL simulations were performed for investigating the distribution of the electric field across the pyroelectric crystal (see Supporting Information for further details). Figure 3g,h shows the results in the case of two facing 15506

dx.doi.org/10.1021/la403603d | Langmuir 2013, 29, 15503−15510

Langmuir

Article

aturized electrical networks, useful for the future generation of flexible electronics. Concentration of Carbon Nanotubes. The tiplike configuration in Figure 3c was used for investigating the behavior of the chaining mechanism by varying the CNT concentration. Five different suspensions of MWCNTs in PMMA were prepared with the following values of particle/weight content: 0.0025%, 0.0049%, 0.0082%, 0.0098%, 0.049%. Figure 5a shows the trend of the chain length with increasing concentration of the CNTs, up to a maximum value of about 4 mm. In fact, the higher density of CNTs favors the particle−particle interaction over longer distances. The integrity of the MWCNTs after the pyroelectric treatment was examined by micro-Raman inspection before and after the chain formation (see Supporting Information for details about the equipment). Figure 5b shows the Raman spectra in the range 1200−1800 cm−1 of pristine CNTs (curve a), randomly dispersed in PMMA (curve b), and aligned in PMMA matrix (curve c). As an example, the result of the spectral deconvolution carried out on pristine CNTs was also reported (curves a′). Each spectrum collected on the area containing MWCNTs shows a band at ∼1345 cm−1 (D-band) and a complex band whose components are centered at ∼1575 cm−1 (G-band) and ∼1605 cm−1 (D′-band). Moreover, the spectra of MWCNTs embedded in PMMA show the presence of additional bands attributed to the acrylic matrix.47 These bands are centered at about 1448 cm−1 and 1729 cm−1. Concerning the bands characteristic of CNTs, the D band is usually attributed to amorphous or disordered carbon in the CNTs. The G band originates from the in-plane tangential stretching of carbon− carbon bonds in graphenelike sheets. Finally, the D′ band, evidenced in the Raman spectra as a shoulder of the G-band at higher frequencies, is another feature induced by disorder and defects in the CNTs. By performing the spectral deconvolution, the intensity ratio of the bands G and D (IG/ID) was calculated and can be considered as a parameter that indicates the degree of order in the nanotube structure. The results show that for pristine MWCNTs the value of IG/ID is about 13, thus indicating a low amount of defective sites in their structure. A similar value of IG/ID was recorded for the aligned MWCNTs, whereas a slight reduction of this ratio was observed for MWCNTs randomly dispersed in PMMA, for which IG/ID was ∼9. This reduction can be attributed to the insurgence of defects during the dispersion of

titanium tips and demonstrates a perfect agreement with the CNT alignment observed experimentally. The self-assembly across other coupling edges is shown in Figure 4, where the chains are generated across different kinds of crystal grooves (a), across a domain wall (b), and along a confined liquid layer (c).

Figure 4. Microscope view of typical MWCNT chains in PMMA formed across different kinds of grooves (a), across a domain wall (b), and along a confined strip (c).

A domain-reversed LN crystal46 was used for the second configuration (Figure 4b). All of these results demonstrate that, because of the constraint-free nature of the driving charges, this technique could open the way to a revolutionary platform for the fabrication of high versatile, cost-effective, and highly mini-

Figure 5. (a) Variation of the chain maximum length by varying the CNT concentration; (b) Raman spectra in the range 1200−1800 cm−1 of pristine CNTs (spectrum a), randomly dispersed in PMMA (spectrum b) and aligned in the PMMA matrix (spectrum c). 15507

dx.doi.org/10.1021/la403603d | Langmuir 2013, 29, 15503−15510

Langmuir

Article

relatively long CNT chains were generated, similarly to PMMAbased experiments. Figure 6c,d shows the chains obtained in the case of styrene (purity ≥99%, Sigma Aldrich) and glycerin (purity ≥99.5%, Carlo Erba Reagents, Italy), where the slightly higher viscosities made the process more difficult, leading to shorter chains. Figure 7a,b shows the optical microscope images of CNT chains embedded permanently into a PMMA sheet, after

the CNTs within the polymer solution and the solvent casting. In the case of MWCNT in PMMA, either randomly dispersed or aligned, a slight upshift was observed for the G band, from 1573 cm−1 (pristine MWCNT) to 1579 cm−1. This small up-shifting phenomenon, already reported for MWCNTs embedded in PMMA, can be attributed to a certain degree of interfacial interaction between the polymer and the CNTs that limits the nanotube−nanotube interactions.48,49 Polymer Viscosity. It is well-known that the viscosity can oppose particle motion,50 thus impacting the dynamics of chain assembly. However, in this case, the viscosity of the host polymer varies rapidly during chain formation because of the fast solvent evaporation induced by the thermal treatment. Therefore, both the early viscosity and the bulk fluid motion induced thermally are of crucial importance for the fabrication of stable chains. The particle mobility has to be sufficiently high at the very beginning of the process for favoring particle interaction and has to decrease rapidly to freeze the final configuration. The initial high mobility is ensured by the early low viscosity and by the increase of kinetic energy induced by the heat pulse. Successively, the viscosity increases rapidly by solvent evaporation. Additional experiments were performed for testing the technique in the case of various dynamic viscosities η by dispersing 0.049% MWCNTs in different matrixes: raw PDMS (Sylgard 184 Silicone Elastomer Kit, Dow Corning; η = 3900 cP, εp = 2.65 from datasheet); PDMS diluted (mix ratio 1:1) in toluene (purity ≥99.9%, Sigma Aldrich) (η = 0.07 Pa s) and in dichloromethane (DCM, purity ≥99.9%, Sigma Aldrich) (η = 0.12 Pa s); styrene (purity ≥99%, Sigma Aldrich); glycerine (purity ≥99.5%, Carlo Erba Reagents, Italy) (η = 1.57 Pa s). The viscosities were evaluated with the rheological measurements reported in the Supporting Information. No chains were observed in the case of raw PDMS because of the relatively high viscosity (see the inset in Figure 6a) that caused the friction to exceed the DEP force of the pyroelectric field, thus preventing the CNTs to move and redistribute along the field lines. The significant role of viscosity in chain formation was demonstrated by the results obtained in the case of PDMS diluted in toluene (Figure 6a) and DCM (Figure 6b), where

Figure 7. (a, b) Microscope images of MWCNT chains embedded permanently into a PMMA sheet after complete solvent evaporation. (c, d) SEM images of typical CNT chains. (e) Sample images of the typical stages leading to the formation of the CNT chains. The circles highlight the regions where the CNTs assemble according to the corresponding stage.

complete solvent evaporation. The alignment of the CNTs in the polymer matrix was investigated further by scanning electron microscopy (SEM), as shown in Figure 7c,d under two different magnifications. The SEM analysis was performed on a FEI Quanta 200 FEG SEM (Eindhoven, The Netherlands) at 10 kV acceleration voltage and with a secondary electron detector. Before the analysis, the samples were mounted onto SEM stubs by means of carbon adhesive disks and sputter-coated with a gold−palladium alloy. The images show the typical head-to-head alignment.

■

THEORETICAL INTERPRETATION The magnified view of chain formation in movie 3 in Supporting Information gives insight into what happens during the selfassembling effect. The development of chains can be broken down into five fundamental stages: comb; orientation; early stage chains; addition; stretching and tightening. Images of sample corresponding to these stages are shown in Figure 7e. First, the CNTs close to the conductive edge are strongly attracted and form a stable comblike structure with teeth along the electric field

Figure 6. Microscope images of MWCNT chains dispersed into matrixes with different viscosities: PDMS diluted in toluene (a), PDMS diluted in DCM (b), styrene (c), and glycerin (d). The inset in panel a shows the lack of chains in the case of undiluted PDMS. 15508

dx.doi.org/10.1021/la403603d | Langmuir 2013, 29, 15503−15510

Langmuir

Article

Programme (Protocol RBFR10FKZH) and EFOR-CABIR CNR project.

lines. The remaining CNTs, away from the coupling edge, move turbulently under the action of thermal convection, electrical field, and viscous drag. The DEP force polarizes and orients the particles along the electric field lines. In this way, widespread early stage chains form through the particle−particle interaction. Because the DEP force is proportional to the square of the electric field strength, the early stage chains appear denser in the regions closer to the aluminum edge, where the electric field is stronger. Moreover, the field E represents the field intensity at the particle center, and the center of individual tubes is much closer to the conductive edges than the center of early stage chains. Therefore, the individual tubes feel stronger DEP force over the viscous drag and, consequently, move faster. Indeed, individual CNTs, rather than in short chains, move most rapidly toward broken connections in subchains and provide the most commonly observed means of restoring the broken connections. Additional unchained particles progressively interact with the early stage chains that operate as catalysts for the formation of longer chains. At the end, turbulence decreases significantly due to thermal steadying and long chains stretch and tighten each other to the comb, thus leading to the stable final configuration. Other interesting considerations can be made in the case of the sharp edges in movie 1 in Supporting Information, where the formation stages exhibit an additional amazing effect. The early stage chains appear to form first across the two corners of the layer where the electric field gradient is in fact stronger and also being able to break relatively large bundles of CNTs. Even the branching effect is clearly visible across the corners, again according to the field lines.

■

(1) Ryoo, S.-R.; Kim, Y.-K.; Kim, M.-H.; Min, D.-H. Behaviors of NIH3T3 fibroblasts on graphene/carbon nanotubes: Proliferation, focal adhesion, and gene transfection studies. ACS Nano 2010, 4, 6587−98. (2) Sundaray, B.; Subramanian, V. Electrical conductivity of a single electrospun fiber of poly (methyl methacrylate) and multiwalled carbon nanotube nanocomposite. Appl. Phys. Lett. 2006, 88, 143114. (3) Huang, Y.; Duan, X.; Wei, Q.; Lieber, C. M. Directed assembly of one-dimensional nanostructures into functional networks. Science 2001, 291, 630−3. (4) Burg, B. R.; Poulikakos, D. Large-scale integration of single-walled carbon nanotubes and graphene into sensors and devices using dielectrophoresis: A review. J. Mater. Res. 2011, 26, 1561−1571. (5) Lipomi, D. J.; et al. Skin-like pressure and strain sensors based on transparent elastic films of carbon nanotubes. Nat. Nanotechnol. 2011, 6, 788−92. (6) Yamada, T.; et al. A stretchable carbon nanotube strain sensor for human-motion detection. Nat. Nanotechnol. 2011, 6, 296−301. (7) Pinto, A. M.; et al. Biocompatibility of poly(lactic acid) with incorporated graphene-based materials. Colloids Surf., B 2013, 104, 229−38. (8) Monti, M.; Natali, M.; Petrucci, R.; Kenny, J. M.; Torre, L. Impact damage sensing in glass fiber reinforced composites based on carbon nanotubes by electrical resistance measurements. J. Appl. Polym. Sci. 2011, 122, 2829−2836. (9) Duchamp, M.; et al. Controlled positioning of carbon nanotubes by dielectrophoresis: Insights into the solvent and substrate role. ACS Nano 2010, 4, 279−84. (10) Yan, Y.; Chan-Park, M. B.; Zhang, Q. Advances in carbonnanotube assembly. Small 2007, 3, 24−42. (11) Chen, X. Q.; Saito, T.; Yamada, H.; Matsushige, K. Aligning single-wall carbon nanotubes with an alternating-current electric field. Appl. Phys. Lett. 2001, 78, 3714. (12) Bubke, K.; Gnewuch, H.; Hempstead, M.; Hammer, J.; Green, M. L. H. Optical anisotropy of dispersed carbon nanotubes induced by an electric field. Appl. Phys. Lett. 1997, 71, 1906. (13) Hone, J.; et al. Electrical and thermal transport properties of magnetically aligned single wall carbon nanotube films. Appl. Phys. Lett. 2000, 77, 666. (14) Hobbie, E. K.; et al. Optical measurements of structure and orientation in sheared carbon-nanotube suspensions. Rev. Sci. Instrum. 2003, 74, 1244. (15) Lim, J. K.; et al. Alignment strategies for the assembly of nanochains with submicron diameters. Small 2010, 6, 1736−40. (16) Dang, X.; et al. Virus-templated self-assembled single-walled carbon nanotubes for highly efficient electron collection in photovoltaic devices. Nat. Nanotechnol. 2011, 6, 377−84. (17) Maune, H. T.; et al. Self-assembly of carbon nanotubes into twodimensional geometries using DNA origami templates. Nat. Nanotechnol. 2010, 5, 61−6. (18) Huang, J.; Kim, F.; Tao, A. R.; Connor, S.; Yang, P. Spontaneous formation of nanoparticle stripe patterns through dewetting. Nat. Mater. 2005, 4, 896−900. (19) Yu, G.; Cao, A.; Lieber, C. M. Large-area blown bubble films of aligned nanochains and carbon nanotubes. Nat. Nanotechnol. 2007, 2, 372−7. (20) Jin, S. H.; et al. Using nanoscale thermocapillary flows to create arrays of purely semiconducting single-walled carbon nanotubes. Nat. Nanotechnol. 2013, 8, 347−55. (21) Mokrý, P.; Marvan, M.; Fousek, J. Patterning of dielectric nanoparticles using dielectrophoretic forces generated by ferroelectric polydomain films. J. Appl. Phys. 2010, 107, 094104. (22) Monti, M.; Natali, M.; Torre, L.; Kenny, J. M. The alignment of single walled carbon nanotubes in an epoxy resin by applying a DC electric field. Carbon 2012, 50, 2453−2464.

■

CONCLUSIONS We developed here an electrode-free DEP approach for promoting a spontaneous formation of carbon-based chains embedded into polymer matrixes. The chaining effect is remarkably reliable and exhibits high repeatability. CNT chains up to 4 mm long were produced, also embedded into polymer layers. The usage of spontaneous pyroelectric charges provides the approach with unprecedented simplicity and compactness. These features would be of great interest for developing highly integrated circuits in those applications where the scaling down of the interconnection technology is highly desired, including electronic devices, sensors, and thermally conductive polymers, as well as biomedicine, where CNTs may operate as nanovectors for drug delivery purposes.

■

ASSOCIATED CONTENT

S Supporting Information *

Rheological measurements. Simulations of the pyroelectric fields. Micro-Raman equipment. Movies of NPs during the spontaneous assembly. This material is available free of charge via the Internet at http://pubs.acs.org.

■

REFERENCES

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS The authors acknowledge the Italian Ministry of Research for financial support, under the “Futuro in Ricerca 2010” 15509

dx.doi.org/10.1021/la403603d | Langmuir 2013, 29, 15503−15510

Langmuir

Article

(23) Cicoria, R.; Sun, Y. Dielectrophoretically trapping semiconductive carbon nanotube networks. Nanotechnology 2008, 19, 485303. (24) Li, P.; Xue, W. Selective deposition and alignment of single-walled carbon nanotubes assisted by dielectrophoresis: From thin films to individual nanotubes. Nanoscale Res. Lett. 2010, 5, 1072−8. (25) Zhang, C.; et al. Dielectrophoretic separation of carbon nanotubes and polystyrene microparticles. Microfluid. Nanofluid. 2009, 7, 633−645. (26) Moscatello, J.; et al. Surfactant-free dielectrophoretic deposition of multi-walled carbon nanotubes with tunable deposition density. Carbon 2010, 48, 3559−3569. (27) Chen, M.; et al. Alignment and dispersion of functionalized carbon nanotubes in polymer composites induced by an electric field. Authors’ reply. Carbon 2008, 46, 706−710. (28) Slopek, R. P.; Gilchrist, J. F. Self-assembly of chains in acrylate monomer via nanoparticle dielectrophoresis. J. Phys. D: Appl. Phys. 2010, 43, 045402. (29) Dimaki, M.; Bøggild, P. Frequency dependence of the structure and electrical behaviour of carbon nanotube networks assembled by dielectrophoresis. Nanotechnology 2005, 16, 759−763. (30) Liu, H.; Takagi, D.; Chiashi, S.; Homma, Y. Transfer and alignment of random single-walled carbon nanotube films by contact printing. ACS Nano 2010, 4, 933−8. (31) Grilli, S.; Ferraro, P. Dielectrophoretic trapping of suspended particles by selective pyroelectric effect in lithium niobate crystals. Appl. Phys. Lett. 2008, 92, 232902. (32) Grilli, S.; Vespini, V.; Ferraro, P. Surface-charge lithography for direct PDMS micro-patterning. Langmuir 2008, 24, 13262−5. (33) Ferraro, P.; et al. Dispensing nano-pico droplets and liquid patterning by pyroelectrohydrodynamic shooting. Nat. Nanotechnol. 2010, 5, 429−435. (34) Xi, X.; Zhao, D.; Tong, F.; Cao, T. The self-assembly and patterning of thin polymer films on pyroelectric substrates driven by electrohydrodynamic instability. Soft Matter 2012, 8, 298. (35) Yraola, E.; Molina, P.; Plaza, J. L.; Ramírez, M. O.; Bausá, L. E. Spontaneous emission and nonlinear response enhancement by silver nanoparticles in a Nd3+-doped periodically poled LiNbO3 laser crystal. Adv. Mater. 2013, 25, 910−915. (36) Hermanson, K. D.; Lumsdon, S. O.; Williams, J. P.; Kaler, E. W.; Velev, O. D. Dielectrophoretic assembly of electrically functional microchains from nanoparticle suspensions. Science 2001, 294, 1082−6. (37) Nicotra, O. E.; La Magna, A.; Coffa, S. Particle-chain formation in a dc dielectrophoretic trap; a reaction-diffusion approach. Appl. Phys. Lett. 2009, 95, 073702. (38) Jones, T. B. Electromechanics of Particles; Cambridge University Press: New York, 2005; Ch. 2. (39) Morgan, H.; Green, N. G. Dielectrophoretic manipulation of rodshaped viral particles. J. Electrostatics 1997, 42, 279−293. (40) Burg, B. R.; Bianco, V.; Schneider, J.; Poulikakos, D. Electrokinetic framework of dielectrophoretic deposition devices. J. Appl. Phys. 2010, 107, 124308-JAP107−124308−11. (41) Gimsa, J. A comprehensive approach to electro-orientation, electrodeformation, dielectrophoresis, and electrorotation of ellipsoidal particles and biological cells. Bioelectrochemistry 2001, 54, 23−31. (42) Benedict, L. X.; Louie, S. G.; Cohen, M. L. Static polarizabilities of single-wall carbon nanotubes. Phys. Rev. B 1995, 52, 8541−8549. (43) Liu, Y.; Chung, J.-H.; Liu, W. K.; Ruoff, R. S. Dielectrophoretic assembly of nanowires. J. Phys. Chem. B 2006, 110, 14098−14106. (44) Hsu, H.-Y.; Sharma, N.; Ruoff, R. S.; Patankar, N. A. Electroorientation in particle light valves. Nanotechnology 2005, 16, 312−319. (45) Ding, H.; Liu, W.; Shao, J.; Ding, Y.; Zhang, L.; Niu, J. Influence of induced-charge electrokinetic phenomena on the dielectrophoretic assembly of gold nanoparticles in a conductive-island-based microelectrode system. Langmuir 2013, 29, 12093−12103. (46) Grilli, S.; Paturzo, M.; Miccio, L.; Ferraro, P. In situ investigation of periodic poling in congruent LiNbO3 by quantitative interference microscopy. Meas. Sci. Technol. 2008, 19, 074008. (47) Mueller, A.; Vigolo, B.; McRae, E.; Soldatov, A. V. Raman study of inhomogeneities in carbon nanotube distribution in CNT−PMMA composites. Phys. Status Solidi B 2010, 247, 2810−13.

(48) Bokobza, L.; Zhang, J. Raman spectroscopic characterization of multiwall carbon nanotubes and of composites. eXPRESS Polym. Lett. 2012, 6, 601−8. (49) McClory, C.; McNally, T.; Baxendale, M.; Pötschke, P.; Blau, W.; Ruether, M. Electrical and rheological percolation of PMMA/MWCNT nanocomposites as a function of CNT geometry and functionality. Eur. Polym. J. 2010, 46, 854−68. (50) Li, Z.; Wang, H. Drag force, diffusion coefficient, and electric mobility of small particles. I. Theory applicable to the free-molecule regime. Phys. Rev. B 2003, 68, 061206−9.

15510

dx.doi.org/10.1021/la403603d | Langmuir 2013, 29, 15503−15510