Single-step self-assembly of multilayer graphene based dielectric nanostructures

FlatChem xxx (2017) xxx–xxx

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Single-step self-assembly of multilayer graphene based dielectric nanostructures Jeong Eun Baek, Ju Young Kim, Hyeong Min Jin, Bong Hoon Kim, Kyung Eun Lee, Sang Ouk Kim ⇑ National Creative Research Initiative (CRI) Center for Multi-Dimensional Directed Nanoscale Assembly, Department of Materials Science & Engineering, KAIST, Daejeon 34141, Republic of Korea

a r t i c l e

i n f o

Article history: Received 29 April 2017 Revised 11 June 2017 Accepted 12 June 2017 Available online xxxx Keywords: Nanocomposites Graphene Block copolymer Dielectric Self-assembly

a b s t r a c t We present high performance multilayer organic/inorganic nanostructured dielectric films including well dispersed chemically modified graphene (CMG) charge polarization layers based on single step solution self-assembly. Multilayer film structures of alternating aluminosilicate and amphiphilic block copolymer (BCP) lamellar layers intercalated with CMG in the hydrophilic aluminosilicate domains are prepared by simple solution casting process. The resultant composite dielectric film attains a high dielectric constant (30) along with low dielectric loss (<0.3). The increase of dielectric constant originates from the space charge polarization effect by high aspect ratio CMG, which is also precisely adjustable with the CMG composition in the composite films. Significantly, the dielectric loss rarely increased even with the injection of a large amount of conductive CMG fillers, as CMG plates are exclusively distributed in the hydrophilic lamellar domains without forming conductive connection in the film normal direction across hydrophobic domains. The composite dielectric film also demonstrates mechanically flexible and highly stable features while deposited on conventional flexible PEN substrates. Ó 2017 Published by Elsevier B.V.

Introduction Complex material design based on nanoscale composite structures may offer efficient routes to the synergistic improvement of mechanical [1,2], electrical [2–4] and optical properties [5,6], which are hardly obtainable by single component materials. Various approaches to control the spatial arrangement of nanoscale components and adjust the chemical or physical interactions among the mixed components have been proposed so far [7]. One of the interesting features is ordered nanoscale composite system with regular and periodic spatial arrangements of different components [8,9]. This material design is generally considered to be able to offer the delicate control of diverse material properties over a wide range. In this context, the block copolymers (BCPs) are interesting structure directing self-assembly materials for the advanced nanocomposite structures, as the microphase separation of BCPs may provide highly periodic ordered nanostructures consisting of nanoscale spheres, cylinders, lamellae [10–12], which have been widely exploited for photonic [13,14] and electronic [15–17] applications. ⇑ Corresponding author. E-mail addresses: [email protected] (J.E. Baek), [email protected] (J.Y. Kim), [email protected] (H.M. Jin), [email protected] (B.H. Kim), [email protected] (K.E. Lee), [email protected] (S.O. Kim).

Chemically modified graphene (CMG), which is commonly generated from graphene oxide (GO), is an intriguing material possessing widely controllable chemical and physical properties particularly depending on its reduction process. A notable advantage of CMG for composite process stems from the good dispersibility of GO in various solvents [18]. To date, CMG nanocomposites based GO have been extensively investigated for the enhancement of mechanical, electrical and optical properties [19–22]. Recently, CMG has also been employed as a conductive filler in dielectric materials. Dielectric constant, which is crucial index for the regulation of electrical interaction between the electric components, can be effectively increased by incorporating conductive filler. When a voltage is applied to dielectric/conductive filler composite structure, dielectric charge carriers can be electrically driven and some of charges adjacent to conductive filler are accumulated in the conduction band of the filler. Meanwhile, the adjacent dielectric can be turned into the opposite space charge type to retain electroneutrality. Consequently, local polarization is built up in the composite structure and contributes to the increase of dielectric constant. In this regard, the dielectric/conductive filler composites with various structural designs have been researched, such as randomly mixed bulk heterostructures [23–27], solutionprocessed stacking layer structures [28,29] and conductive filler interlayered dielectric structures based on graphene transfer [30].

http://dx.doi.org/10.1016/j.flatc.2017.06.006 2452-2627/Ó 2017 Published by Elsevier B.V.

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In this work, ordered nanoscale composites of aluminosilicate, BCPs and CMG are introduced for the realization of flexible dielectric composite materials with high dielectric constant as well as low dielectric loss. The dielectric constant is increased by the highly selective intercalation of CMG platelets in the ordered aluminosilicate/BCPs films. The amount of intercalated CMG enables a wide range control of dielectric constant, which is dominated by the space charge polarization by CMG. Also, lamellae BCP selfassembly with surface parallel orientation hinders the out-ofplane arrangement of CMG platelets. This efficiently blocks the formation of conduction path through the film normal direction [23] and thereby minimizes the dielectric loss. Concurrently, mechanical flexibility of composite films is offered by organic part of BCPs, which enables structural robustness while maintaining a stable dielectric constant during the bending test over 500 times and repetitive measurements over 10,000 times.

weight ratio of 1 to 7 were firstly mixed, and then GO solution with various concentration was added. After thorough stirring of the mixture solution, composite film was spin casted on a target substrate. Similar with aluminosilicate/CMG composite film, CMG was obtained from the thermal reduction of GO, whose concentration in the solution determined the composition in the film. Device fabrication For the measurement of dielectric constant and loss of each composite film, parallel electrode system was used. A composite solution was spin casted on ITO bottom electrode to form a film of 200–300 nm thickness. After thermal annealing of each sample at 250 °C for 12 h, top electrode with Cr (3 nm)/Au (20 nm) was deposited on the composite films by e-beam evaporation. Characterization

Materials and methods Materials Aluminosilicate was synthesized by sol–gel reaction. GPTMS (Sigma-Aldrich) and Al(OBus)3 (Sigma-Aldrich) were mixed with molar ratio of 4.28:1 and stirred for 1 h in ice bath. Then, 20 ml of 3% Hydrogen chloride solution (37% Hydrogen chloride purchased by Sigma-Aldrich and diluted with deionized water) was added in the mixture. After 15 min, identical solution of 90 ml was additionally blended and kept the solution with strong stirring at room temperature for 1 h. The resultant aluminosilicate solution consisted of small gel particles. CMG was formed from GO aqueous solution, following a modified Hummers method [31,32]. GO was dissolved in the mixture solvent of ethanol (anhydrous, SigmaAldrich) and deionized water with 9:1 vol ratio. As a structure directing agent, polystyrene-block-poly(ethylene oxide) (PS-bPEO) (19 kg mol 1-b-6.5 kg mol 1, Polymer Source Inc.) was used and dissolved in toluene (anhydrous, 99.8%, Sigma-Aldrich) with 5 wt%. Synthesis of composite solutions Aluminosilicate solution Aluminosilicate synthesized by sol–gel reaction was diluted in ethanol for the uniform thin film formation. Typically, 15 wt% aluminosilicate solution was used to form thin films with 200 nm thickness. Aluminosilicate/BCP composite solution For ordered organic/inorganic lamellar nanostructure formation, BCPs were blended with aluminosilicate. PS-b-PEO BCPs were employed because the hydrophilic PEO block having a high compatibility with aluminosilicate enables this hybrid assembly. Blending ratio of aluminosilicate to BCPs ranged from 1:3 to 1:10. Aluminosilicate/CMG composite solution For the formation of CMG intercalated aluminosilicate film, aluminosilicate was first mixed with GO aqueous solution. For the reduction of CMG from GO, thermal annealing was performed after the solution casting of thin films. Based on the control of GO concentration in the mixture solution, composition of CMG in the composite film was determined. Aluminosilicate/BCP/CMG composite solution The multicomponent solution consisting of aluminosilicate/ BCP/CMG was prepared by mixing the three components in the controlled sequence (Fig. S1). Aluminosilicate and BCP with the

Nanoscale morphology of dielectric composite films was characterized using scanning electron microscopy (SEM, Hitachi S4800) and atomic force microscopy (AFM, Bruker Multimode 8). For the quantitative analysis of CMG composition in a composite film, optical transmittance was measured by UV–vis spectrophotometer (Shimadzu, UV-2600). Dielectric properties were measured by electrochemical station (Bio-Logic SP-200). For the measurement of dielectric constant and loss, AC impedance measurement was performed in the frequency range from 10 Hz to 7 MHz with an applied voltage of 0.1 V. Dielectric properties were calculated from the impedance spectroscopy results. Results and discussion Self-assembly of multilayered dielectric nanocomposite films Fig. 1(a) schematically presents the fabrication process for multilayered dielectric composite film consisting of aluminosilicate/ BCP/CMG by simple single-step solution process. We firstly tested the composite solution of BCPs and aluminosilicate. Considering the effective swelling of PEO block by aluminosilicate mediated by hydrogen bonding, cylindrical PS-b-PEO (19 kg mol 1-b6.5 kg mol 1) was selected for the lamellar film formation. After testing various blending ratios, the solution with the blending (weight) ratio (aluminosilicate: BCPs) from 1:6 to 1:8 was confirmed to form the lamellar structures (Fig. S3). In particular, at 1:7 blending ratio, the multilayered lamellar film was stably formed with a lamellar period of 20 nm. It is quite noteworthy that such a well-defined hybrid self-assembled nanostructure with alternating layers of organic and inorganic material can be readily formed by single-step solution casting. For the selective intercalation of CMG flakes in the hydrophilic lamellar nanodomains of multilayered film, finely dispersed GO aqueous solution was mixed with BCP/aluminosilicate solution. In this step, the blending sequence of three components is highly important to avoid the unnecessary GO segregation. Due to the solubility difference of three components, the composite film blended in different mixing sequence revealed entire different morphology. In Fig. S4, mixing order of blue arrow leads to a well-dispersed composite film without any aggregation, while red arrow leads to a poorly defined morphology. In the order of blue arrow (Fig. S4(a)), aluminosilicate, which is well miscible with both BCP and GO solutions plays a role as a surfactant in the composite solution. As shown in Fig. 1(b), aluminosilicates chemically bonded with PEO blocks of BCPs preferentially interact with the oxygen group of GO and thus stabilizes GO flakes, showing a uniform dispersibility in the ethanol/water mixture. In addition, surface

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Fig. 1. (a) Fabrication scheme of hybrid dielectric composite film with aluminosilicate, CMG and BCPs by a simple solution process. (b) Formation mechanism of the dielectric film multilayer structure. (c) 60° Tilted SEM image of the hybrid dielectric film.

functionality of GO flakes might control the electrostatic interactions between GO and aluminosilicate for enhanced solution dispersibility [33]. Therefore, the composite film casted from this highly dispersed solution also shows highly selectively intercalated GO platelets between aluminosilicate layers. However, in the order of red arrow (Fig. S4(a)), GO platelets were strongly aggregated in the hydrophobic BCP solvent (toluene) during the mixing process. Even when aluminosilicate that acted as a surfactant in the order of blue arrow (Fig. S4(a)) was added immediately, the aggregated GO platelets could not be redispersed. As a consequence, welldispersed solution was hardly obtained (Fig. S4(b)). Interestingly, CMG formed from our procedure is highly aligned parallel to the substrate as shown in Fig. 1(c). This clearly contrasts to the composite film from the blends of GO and aluminosilicate (without BCP), where CMG platelets show a poor degree of alignment. Basically, shear force induced by the substrate spinning process enables the parallel positioning of GO platelets. However, this is insufficient to fully understand the high alignment behavior, considering the result of poorly oriented CMG/aluminosilicate composite films (Fig. 2(a) and Fig. S2). BCP self-assembly is believed to additionally guide the parallel ordering of CMGs. Also,

in-plane aligned CMGs having high aspect ratio serve to support the lamellar structure of surrounding BCP. Therefore, the composite film has well defined nanostructure by complementing CMG and BCP. Dielectric properties of the composite films Multilayer composite films with uniformly dispersed CMG flakes were exploited for high performance dielectric materials, taking advantage of its synergistic combination of different components and novel structure (Fig. 2). Preliminarily, single component films consisting of aluminosilicate or BCPs were tested. Pure aluminosilicate film, which is one of the materials with large molecular polarization, shows a high dielectric constant, while organic BCPs revealed a low dielectric constant. Thus, multilayer aluminosilicate/BCP films showed a lower dielectric constant than pure aluminosilicate film. Such a decrease of dielectric constant could be overcome by intercalating the conductive CMG flakes. As presented in Fig. 2(b), CMG flakes uniformly intercalated in the aluminosilicate/BCPs multilayer film increased the dielectric constant up to 25 at 100 Hz. It is considered that when the voltage is applied to

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Fig. 2. (a) SEM images of various dielectric films. (b) The dielectric constant and (c) dielectric loss tangent of composite dielectric films: pristine aluminosilicate, aluminosilicate/BCPs multilayer, randomly mixed aluminosilicate/CMG, and aluminosilicate/BCPs/CMG novel structure composite, as a function of applied frequencies ranging from 10 Hz to 1 MHz.

this dielectric composite film, electrically driven charge carriers should accumulate at CMG surface and the opposite space charges should be induced at the nearby dielectric area for the electroneutrality. This additionally generated polarization would contribute to the entire dielectric performance of composite materials. Notably, in contrast to the significant increase of dielectric constant, dielectric loss of the aluminosilicate/BCPs/CMG multilayer composite revealed a minimal increment. This is believed to be associated with the orientation of CMG platelets. Poorly aligned aluminosilicate/CMG composite films without BCP also showed the increase of dielectric constant driven by space charge polarization, but their dielectric loss tangent values were excessively increased up to 2 at 100 Hz. In the aluminosilicate/CMG composite, the randomly aligned conductive component of CMG may cause dielectric loss (Fig. 2(a) and S2). Dielectric loss mainly consists of the conduction loss (dominating at low frequency) and the polarization loss of space charges (dominating at 102-106 Hz) [28]. In this regard, the high dielectric loss of the aluminosilicate/CMG composite appears to be affected by conduction loss mainly, because it appears in low frequency (103 Hz). Conduction loss

is the energy loss by which the passage of an electric current through a conductor in dielectric releases heat called as a joule heating or ohmic heating [34]. In the composite material, the applied voltage induces 1) the polarization of the dielectric matrix, 2) facilitates that charges flow vertically along the vertical conductive CMG components and 3) generates space charge polarization by accumulating charges in parallel CMG components. In this case, even though vertical components in randomly aligned CMG do not cause direct vertical percolation, they provide some conducting pathway where charges pass through. Therefore, the high dielectric loss of the composite with randomly aligned conductive CMG is caused by the joule heating of an electric current through conductive CMG pathway. Vertically aligned CMG flasks are spontaneously realigned to film plane direction with the assistance of BCP self-assembly during thermal annealing. This phenomenon was confirmed by SEM images of the composite films with different annealing conditions (Fig. S5). Sufficient thermal energy enables BCP films to form uniform multilayer structures with a high degree of alignment. By contrast, parallel self-assembly of BCP/aluminosilicate multilayer

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film prevents the out-of-plane alignment of CMG platelets and inhibits the vertical electric flow and eventually minimizes the increase of dielectric loss[24]. The resultant loss tangent value of aluminosilicate/BCP/CMG multilayer film is remarkable (Fig. 2 (c)). The formation of conductive pathway in the film normal direction could be also detected by AC conductivity (Fig. S6). Since conductivity contribution from the CMG connected pathway is frequency-independent resistor component, the plateau state at low frequency regime, which has been reported as leakage path through conductive filler[35,36], was shown in the aluminosilicate/CMG composite film. By contrast, such a plateau state is not found in aluminosilicate/BCP/CMG composite. From these results, it is evident that parallel and uniformly intercalated CMG flakes not only increases the dielectric constant but also minimizes the increase of dielectric loss. Interestingly, the dielectric properties of composite film can be readily modified by the composition of CMG. Along with the concentration of initial GO solution from 0.06 to 0.15 wt% (Fig. S7 (a)), multilayer dielectric films with various amounts of CMG was fabricated, where the exact amount of CMG in each composite film was evaluated by the optical transmittance (Fig. S7(b)). As shown in Figs. 3(a) and (b), the dielectric constant was widely tunable from 6 to 30, depending on the amount of CMG. It is noteworthy again that despite the increased amount of CMG fillers, the composite film with novel structural merits illustrates a linear

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AC conductivity without plateau state and low dielectric loss. (Fig. 3(c) and (d)). Influence from interfacial area between conductive fillers and dielectric material on dielectric constant Space charge polarization phenomenally occurs at the interface between conductive fillers and dielectric materials. The degree of polarization should depend on the total interfacial area of a material, and accordingly a dielectric film with well-dispersed conductive fillers should have a high dielectric constant. In this respect, our multilayer composite film of aluminosilicate/BCP/CMG has remarkable advantages for large dielectric constant. We investigated on the interfacial area effect by comparing two different structural films with controlled CMG compositions (Fig. 4 (a) and (b)). For a quantitative comparison, a sandwich-type structure where CMG film layer is inserted between two aluminosilicate/BCPs multilayer films was prepared by layer-by-layer coating. The amount of CMG component was adjusted by repetitive spin coating from GO solution (Fig. 4(a)). AFM observation (Fig. S8) reveals that surface coverage of the spin coated CMG film increased up to the 3 times of coating, over which additional coating increased the thickness of CMG layer without change of interfacial area. By contrast, in our multilayer composite structure, the interfacial area between CMG and dielectric material steadily increased

Fig. 3. (a) Dielectric constant of novel composite dielectric films as a function of the concentration of CMG ranging from 0 to 0.15 wt%. (b) The dielectric constant, (c) AC conductivity and (d) dielectric loss tangent along the concentration of CMG as a function of applied frequencies ranging from 10 Hz to 1 MHz.

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Fig. 4. (a) Structures of the sandwich-type structure with CMG layer between two aluminosilicate/BCPs multilayer film and (b) Dielectric constant of (a) and novel composite film at 100 Hz frequency as a function of optical transmittance of films ranging from 100 to 60% at 450 nm.

with the composition of CMG. As anticipated, dielectric constants of the two films are closely related with the interfacial area. In the multilayer composite film, dielectric constant consistently increased without saturation. Dielectric constant of the sandwich-type composite film gradually increased up to 94% transmittance but saturates thereafter. Noticeably, the saturation of both dielectric constant and optical transmittance begins at the 3 times coating of GO layer, quantitatively confirming that the interfacial area of conductive filler with dielectric material is directly related to the dielectric constant. In addition, our composite dielectric film demonstrates mechanical flexibility by virtue of organic lamellar layers consisting of hydrophobic organic PS blocks. Dielectric constant value is stable during 500 cycles of bending tests with 40 mm bending radius. Owing to the high electrical and structural robustness, stable dielectric performance was observed during 10,000 cycles of repeated measurements at the surface of commercial PEN substrates (Fig. S9). Conclusion We have demonstrated multilayered composite dielectric films with spatially and orientationally ordered CMG flakes via straightforward single-step self-assembly. The fine solvent dispersing effect of GO by hydrophilic aluminosilicate and nanoscale structure directing effect from amphiphilic BCP self-assembly enabled highly ordered multilayer in-plane lamellar nanostructure consisting of alternating hydrophobic polymer, aluminosilicate and CMG layers. Owing to the unique structural and processing advantages, multilayered dielectric composite with high dielectric constant and low loss was realized. Furthermore, based on the quantitative comparison with sandwich-type dielectric film, the significant role of interfacial area between conductive fillers and dielectric media for dielectric property has been confirmed. We believe that relevant material design for complex composite structures would be useful for various applications. Moreover, this self-assembly assisted efficient process would open a new opportunity for wellordered nanocomposite systems consisting of various lowdimensional nanoscale components with intriguing chemical and physical functionalities. Acknowledgements This work was supported by the Global Frontier Hybrid Interface Materials (GFHIM) (2013M3A6B1078874) and Nano Material Technology Development Program (2016M3A7B4905613) through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT and Future Planning.

Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.flatc.2017.06.006.

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