Ps-b-PEO Inorganic

Jan 4, 2011 - Europa 1, 20018 Donostia-San Sebastián, Spain. J. Phys. Chem. C , 2011, 115 (5), pp 1643–1648. DOI: 10.1021/jp109474s. Publication Da...
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Conductive Properties of Photoluminescent Au/Ps-b-PEO Inorganic/Organic Hybrids Containing Nematic Liquid Crystals Agnieszka Tercjak,* Junkal Gutierrez, and I~naki Mondragon* Materials þ Technologies Group, Departamento Ingeniería Química y M. Ambiente, Escuela Politecnica, Universidad País Vasco/Euskal Herriko Unibertsitatea, Pza. Europa 1, 20018 Donostia-San Sebastian, Spain ABSTRACT: Novel inorganic/organic materials based on poly(styrene-b-ethylene oxide) block copolymers (PS-b-PEO) modified with synthesized Au nanoparticles and low molecular weight liquid crystals, 40 -(hexyloxy)-4-biphenylcarbonitrile (HOBC), have been investigated. The designed materials show a good dispersion of synthesized Au nanoparticles in one block of PS-bPEO block copolymers. The conductive properties of the designed materials were measured using tunneling atomic force microscopy (TUNA). Au nanoparticles maintain conductive properties in investigated inorganic/organic materials. Simultaneously, the addition of even small amounts of nematic HOBC liquid crystals multiplied the TUNA-current passing throughout the investigated ternary systems. Moreover, the Au-(HOBC/PS-b-PEO) systems pose photoluminescent behavior of the low molecular HOBC liquid crystals allowing the design of nanostructured multiphase materials with the conductive properties of Au nanoparticles multiplied by HOBC and the photoluminescent properties of HOBC, which can lead to the generation of novel materials with interesting specific properties.

’ INTRODUCTION Inorganic/organic nanostructured hybrids are an interesting class of materials from the point of view of their potential application as active electronic devices in different electronic components such as light emitting diodes,1,2 transistors,3,4 solar cells,5-7 optical shutters,8,9 and so on. In this field, one can find many different ways of preparing this class of materials. One of them is the in situ preparation of metallic or semiconducting nanoparticles confined in one block of block copolymers,10-17 which leads to particle size control, good dispersion, and stabilization. This kind of inorganic/organic hybrid material allows for the control of the distribution and the size of generated nanoparticles playing with reaction time, concentration of nanoparticles, type of matrix used as a template, and so forth.10-12 Frequently, they are based on self-assembled block copolymers, which possess the unique ability to form well-defined nanostructures making them interesting for the design of organic semiconductors. On the other hand, low molecular nematic liquid crystals (LC) patterned in block copolymers lead to interesting materials showing a continuous optical switching behavior response to external fields such as an electric field or temperature gradients due to the birefringence feature of LC, when embedded in the polymer matrix.8,9,18,19 This behavior makes polymer dispersed liquid crystals attractive materials for application in the field of thermo- and electro-optical devices, such as optic shutters, smart windows, optical sensors, memories and flexible display devices.20-22 In the present work, the conductive and photoluminescent properties of nanocomposites based on Au nanoparticles, generated using HAuCl4, confined in the PEO-block domains of microphase separated poly(styrene-b-ethylene oxide) (PS-b-PEO) diblock r 2011 American Chemical Society

copolymers were investigated using tunneling atomic force microscopy (TUNA) and spectrophotometer techniques. Low molecular weight 40 -(hexyloxy)-4-biphenylcarbonitrile (HOBC) liquid crystals were incorporated into prepared nanocomposites with the goal of developing inorganic/organic nanostructured hybrid materials with good photoluminescent and conductive properties.

’ EXPERIMENTAL SECTION 1. Materials. Diblock copolymers of poly(styrene-b-ethylene oxide), Polymer Source Inc., were used as self-assembling agents as well as polymer dispersed liquid crystals (PDLC). The number-average molecular weights for PS and PEO blocks were 58 600 and 31 000 g 3 mol-1, respectively, the polydispersity index for this copolymer being 1.03. The gold precursor used was chloroauric acid (gold(III)) chloride hydrate (HAuCl4), purum ∼46% as Au), purchased from Sigma-Aldrich and used as received. The low molecular weight nematic liquid crystal used was 40 -(hexyloxy)-4-biphenylcarbonitrile (HOBC) (supplied by Sigma-Aldrich), and was used without further purification. This liquid crystal exhibits a nematic-isotropic (TN-TI) transition at about 70 °C and a crystal-nematic (TC-TN) transition at about 49 °C. 2. Protocol for Blending. The hybrid inorganic/organic 10 or 20 wt % HAuCl4-(HOBC/PS-b-PEO) ternary systems Received: October 2, 2010 Revised: November 19, 2010 Published: January 4, 2011 1643

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The Journal of Physical Chemistry C with different HOBC liquid crystal/PS-b-PEO block copolymers ratios (10:90, 20:80) were prepared following the same procedure. First, a 10 or 20 wt % HAuCl4 precursor was sonicated in toluene at room temperature by using a microprocessor sonicator 750 W (Vibracell 75043 from Bioblock Scientific) with an amplitude range between 20 and 25%. After 5 h, the required contents of the PS-b-PEO block copolymers and/or HOBC liquid crystals were added, and these mixtures were again sonicated for an additional 30 min period. Finally, the HAuCl4(HOBC/PS-b-PEO) ternary systems and HAuCl4/PS-b-PEO binary systems were cast on ITO by using a spin-coater (2000 rpm). Though not shown, a thermogravimetric analysis (TGA) confirmed the incorporation of the 10 or 20 wt % of the synthesized Au nanoparticles into inorganic/organic materials. Consequently, the investigated nanocomposites were named Au-(HOBC/PS-b-PEO) systems. 3. Techniques. The morphological features of the inorganic/ organic hybrid materials were investigated by atomic force microscopy (AFM). Local electric properties of the investigated materials were measured using tunneling atomic force microscopy (TUNA) under ambient conditions using a Veeco Dimension 3100 scanning probe microscope equipped with a TUNAextended extension module. All the TUNA measurements were carried out in contact mode using a conductive Co/Cr coated MESP tip having a resonance frequency of approximately 75 kHz and with a cantilever spring constant of about 40 N/m. To obtain repeatable results, the local electrical properties of the investigated samples were measured in different regions of the specimens. Similar TUNA images were obtained, thus demonstrating the reproducibility of the results. The photoluminescent properties of the investigated materials were determined using a Felix32 spectrophotometer of Photon Technology International (PTI). The excitation wavelengths ranged from 220 to 280 nm depending on the investigated inorganic/organic materials. A Semiconductor Characterization System (Keithley model 4200-SCS) was used to study the conductive properties of the materials. Two-point probe experiments were carried out applying current from -100 to þ100 μA and from þ100 to -100 μA to verify the response of the investigated systems. Contact angle measurements were performed using Dataphysics OCA 20 contact angle system.

’ RESULTS AND DISCUSSION A representative AFM phase image of neat PS-b-PEO block copolymers is shown in the Figure 1a. As expected, taking into account our previous work,18,19 the sample shows a typical hexagonal structure, where well-ordered hexagonally packed cylinders are oriented parallel to the air-sample surface interface. Taking into account that the modulus of PS is higher than that for PEO, the darker microphase separated areas (Figure 1a) correspond to the segregated PEO block domains and the brightest areas correspond to the microphase separated PS block domains.15,16,19 The AFM phase image of the 10 wt % Au/PS-bPEO binary system shows well-dispersed Au nanoparticles, with an average size between 25 and 35 nm in diameter (Figure 1b). As can be clearly observed, the Au nanoparticles were located in one of the microphase separated domains (darker areas in Figure 1b) near the interface between the segregated phases of both the PEO and PS blocks. Additionally, as well-known from the literature, in the investigated systems Au nanoparticles

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precursors can interact with the PEO block of the PS-bPEO.23,24 Consequently, as can be easily visualized, Au nanoparticles are located in the microphase separated PEO block domains near the interface with the separated PS block domains. Moreover, under the same preparation conditions, the wellordered PS-b-PEO block copolymer morphology was destroyed by the incorporation of synthesized Au nanoparticles. An addition of a small amount of HOBC into the 10 wt % Au/ PS-b-PEO system (Figure 1c), 10:90 with respect to PS-b-PEO block copolymers, leads to a nanostructured system with Au nanoparticles selectively located in the microphase separated PEO rich domains. The morphology of the 10 wt % Au-(HOBC/ PS-b-PEO 10:90) ternary system seems to be much more defined than the morphology of the adequate secondary system without HOBC (see Figure 1b,c). Thus, under the same preparation and measurement conditions, the ternary system shows a more regular nanostructure. To better understand this phenomenon, it should be pointed out that on the one hand the HOBC, as published by us,19 show a higher affinity with the PS-blocks than with the PEO-blocks. However, on the other hand, as is wellknown from the literature,26,27 HOBC can also have high affinity with Au nanoparticles. Taking the above into account and the fact that synthesized Au nanoparticles generated using HAuCl4 precursor interact with PEO-blocks, as published by Mendoza et al.,23,24 it seems that in our investigated system synthesized Au nanoparticles located in the PEO block provoke the separation of the HOBC with them. Moreover, as can be clearly observed in Figure 1d, the addition of more HOBC into the 10 wt % Au/PS-b-PEO system, 20:80 with respect to the PS-b-PEO block copolymers, leads to a ternary system with still selectively dispersed Au nanoparticles in the microphase separated PEO block domains. These Au nanoparticles selectively dispersed in one of the microphase separated blocks also show a good dispersion of the synthesized Au nanoparticles in the whole self-organized block copolymer matrix. Representative AFM phase images of a 20 wt % Au/PS-b-PEO binary system modified with different amounts of HOBC (10:90 and 20:80 with respect to PS-b-PEO block copolymers) are shown in Figure 1e-g. The morphology generated in the 20 wt % Au/PS-b-PEO system confirms that even the addition of 20 wt % of Au precursor leads to good dispersion of the synthesized Au nanoparticles in the self-assembled block copolymer matrix. Moreover, Au nanoparticles are selectively located in the microphase separated PEO block. Addition of HOBC into the 20 wt % Au/PS-b-PEO binary system changed the morphology generated in the inorganic/organic ternary materials. However, in the 20 wt % Au-(HOBC/PS-b-PEO 10:90) and the 20 wt % Au-(HOBC/PS-bPEO 20:80) systems, the PS-b-PEO block copolymers maintain their capability for self-organization on the nanoscale. The conductive properties of the binary and ternary inorganic/organic Au-(HOBC/PS-b-PEO 0:100, 10:90, 20:80) systems were investigated using TUNA measurements. First, it has to be pointed out that the following results refer to the thin film behavior of these systems since a very small amount of the samples were spin-coated on the ITO plate. The thickness of the samples was always the same (∼80 nm), as confirmed by AFM measurement, to avoid confusion in results interpretation since the response of the samples is proportional to the applied bias and inversely proportional to the thickness. As was expected, the PS-b-PEO block copolymers did not show any TUNA currents passing throughout the sample when 1644

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Figure 1. TM-AFM phase images (5 μm  5 μm) of (a) neat PS-b-PEO, (b) 10 wt % Au/PS-b-PEO, (c) 10 wt % Au-(HOBC/PS-b-PEO 10:90), (d) 10 wt % Au-(HOBC/PS-b-PEO 20:80), (e) 20 wt % Au/PS-b-PEO, (f) 20 wt % Au-(HOBC/PS-b-PEO 10:90), and (g) 20 wt % Au-(HOBC/PS-b-PEO 20:80). The insets correspond to higher magnification AFM images.

a 4 V voltage was applied to the conductive AFM tip (Figure 2a). As shown in the typical TUNA-current profile (bottom left inset in the Figure 2a) corresponding to the neat block copolymers, the sample response was on the level of the noise of the TUNA (40 fA).25 The TUNA-current images and typical profiles corresponding to each image of the 10 wt % Au/PS-b-PEO system and the

analogous ternary 10 wt % Au-(HOBC/PS-b-PEO 10:90, 20:80) systems are shown in Figure 2b-d. The addition of 10 wt % Au into the PS-b-PEO block copolymers leads to detection of the TUNA current on the level of ∼24 nA at a voltage of 4 V, as clearly shown in Figure 2b. A TUNA-current profile has been added to the left bottom inset of the Figure 2b to better visualize 1645

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Figure 2. TUNA-current maps (5 μm  5 μm) of (a) neat PS-b-PEO, (b) 10 wt % Au/PS-b-PEO, (c) 10 wt % Au-(HOBC/PS-b-PEO 10:90), and (d) 10 wt % Au-(HOBC/PS-b-PEO 20:80) systems obtained applying 4 V. Insets: top right, scale color bar corresponding to the positive voltage; bottom right, typical vertical cross-section profile. (e) Voltage-current curves of investigated systems obtained by Keithley.

the current passing throughout the samples. As shown in the inset, in this case, the TUNA current was stable and reached almost the same local current level (∼20 nA). Taking into account both the lack of current passed throughout the neat PS-b-PEO block copolymer sample and the fact that synthesized Au nanoparticles pose conductive properties, one can easily conclude that in the designed 10 wt % Au/PS-b-PEO system Au nanoparticles were able to respond to the applied voltage.

Additionally, the good distribution of the local current on the whole sample surface confirmed the fine dispersion of the synthesized Au nanoparticles in this system since only these nanoparticles responded to the current transported throughout the designed materials. After the introduction of a small amount of HOBC liquid crystals, 10:90 with respect to PS-b-PEO block copolymer content, the system showed a more than 12 times higher local 1646

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The Journal of Physical Chemistry C TUNA current (compare Figure 2b,c) if compared to the local TUNA current of the 10 wt % Au/PS-b-PEO system. As clearly shown in the profile (the bottom left inset of Figure 2c), under the same preparation conditions, the local TUNA current reached the level of ∼300 nA at a voltage of 10 V. As expected, the addition of HOBC liquid crystals strongly affected the current passed throughout the sample while applying voltage to the conductive AFM tip. The obtained results confirmed that both the Au nanoparticles and the HOBC liquid crystals maintained conductive properties in the designed 10 wt % Au-(HOBC/PS-b-PEO 10:90) ternary system. In the same preparation and measurement conditions, the addition of a higher content of HOBC liquid crystals (20:80 with respect to PS-b-PEO block copolymers) led to a local TUNA current level 5 times higher passing throughout the sample if compared to the TUNA current for the 10 wt % Au-(HOBC/PS-b-PEO 10:90) systems and 62 times higher if compared to the current for 10 wt % Au/PS-b-PEO. In the 10 wt % Au-(HOBC/PS-b-PEO 20:80) sample, the local TUNA-current level reached 1.5 μA at a voltage of 4 V. Obtained results indicated that the increase of the HOBC content highly amplified the local TUNA current passing throughout the sample. Additionally, to confirm the conductive properties of this system, a conventional measurement was performed using the Semiconductor Characterization System. As shown in Figure 2e, the investigated systems responded to the applied voltage almost without hysteresis, independent of the voltage range applied to the samples; the curves are almost equal for the measurements performed for the range from -10 to þ10 V as for the range from þ10 to -10 V. The 10 wt % Au/PS-b-PEO system reached the lowest current level under the same measurement conditions. Moreover, the current level was amplified by increasing the amount of HOBC liquid crystals added to the 10 wt % Au/PS-bPEO system. This once more confirms that the conductive properties of the investigated systems strongly depend on the amount of both Au nanoparticles and HOBC liquid crystals in the systems. Here it should be pointed out that the molecular weight of the HOBC is almost 300 times lower than the molecular weight of the PS-b-PEO block copolymers. Consequently, the increase of the liquid crystal content in the Au-(HOBC/PS-bPEO) inorganic/organic nanocomposites produces a drastic decrease of the organic part which results in increase of the ratio between inorganic/organic parts thus leading to higher conductivity. Obtained results suggested that the current tendency detected at the nanoscale level using TUNA is maintained at the macroscale level since the system responded in the same way to the conventional Keithley measurement. As is well-known, PEO as a highly water sensitive component can have strong effect on the conductive properties of designed inorganic/organic Au-(HOBC/PS-b-PEO) hybrid composites. To study the effect of the addition of conductive Au nanoparticles on the hydrophilic behavior of PS-b-PEO matrix, static contact angle measurement were performed. Typical results obtained for each investigated system are shown in Figure 3. As expected, addition of Au nanoparticles in the binary Au/PS-bPEO systems led to higher hydrophobicity if compared to the hydrophobicity for the neat PS-b-PEO block copolymer (the static contact angle changed from 81° for PS-b-PEO block copolymer to 85° for 10 wt % Au/PS-b-PEO hybrid materials). However, here it should be worthwhile to note that these systems lost very quickly (even in less than 5 min) their hydrophobic character. The Au-(HOBC/PS-b-PEO) ternary systems showed

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Figure 3. Images of a water droplet in contact with (a) neat PS-b-PEO, (b) 10 wt % Au/PS-b-PEO, (c) 10 wt % Au-(HOBC/PS-b-PEO 10:90), and (d) 10 wt % Au-(HOBC/PS-b-PEO 20:80). Droplets were in contact with the surface for 30 s.

a similar tendency as that detected for Au/PS-b-PEO binary systems. The addition of both Au nanoparticles and HOBC liquid crystals led to higher water repellence if compared with the water repellence for neat PS-b-PEO and Au/PS-b-PEO binary systems. The ternary systems reached a static contact angle higher than 99° for 10 wt % Au-(HOBC/PS-b-PEO 10:90) and 101° for 10 wt % Au-(HOBC/PS-b-PEO 20:80). Moreover, ternary systems maintained hydrophobicity even after 24 h. This phenomenon is related to the fact that the Au nanoparticles precursor interacted with PEO-block; therefore, the materials became more hydrophobic. Simultaneously, the well dispersed low molecular weight HOBC liquid crystals phase also had its own contribution to the high water repellence of the ternary systems. As expected, taking into account the fact that Au nanoparticles were dispersed in microphase separated HOBC/ PEO-block domains, ternary Au-(HOBC/PS-b-PEO) systems showed higher water repellence, which led to the conclusion that in this particular case hydrophilic character of the PEOblock did not effect on the conductive properties of these materials. As is well-known, both nematic HOBC liquid crystals and Au nanoparticles show photoluminescent behavior. Taking this fact into account, the photoluminescent properties of the investigated systems in solutions were studied. The photoluminescent emission spectra of the binary and ternary inorganic/organic Au-(HOBC/PS-b-PEO 0:100, 10:90, 20:80) systems and the neat nematic HOBC liquid crystals are shown in Figure 4. The 10 wt % Au/PS-b-PEO system showed a narrow emission peak at around 345 nm. The addition of HOBC liquid crystals, 10:90 or 20:90 with respect to the PS-b-PEO block copolymers, into the 10 wt %Au/PS-B-PEO system provoked a shift of the emission peaks to higher wavelengths. The photoluminescent emission spectra related to the inorganic/organic ternary systems had a higher emission intensity if compared with the intensity of the emission spectrum of the 10 wt % Au/PS-b-PEO binary system and appeared almost in the same range as the photoluminescent emission peak of the neat HOBC, with the maximum intensity around 360 nm. This once more confirms that the addition of low molecular weight HOBC liquid crystals strongly affected the properties of the final inorganic/organic ternary systems. The addition of a small amount of HOBC, 10:90 with respect to 1647

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’ ACKNOWLEDGMENT Financial support from Basque Country Governments in the frame of ETORTEK iNANOGUNE (IE08-225 and IE09-243) and Grupos Consolidados (IT-365-07) is gratefully acknowledged. We also thank the Ministry of Education and Innovation for the project MAT-2009-06331. Additionally, J.G. thanks Eusko Jaurlaritza/Gobierno Vasco (Programas de becas para formacion y perfeccionamiento de personal investigador) and A.T. acknowledges MICINN for the Ramon y Cajal program. Moreover, we are grateful to the SGIker units of the UPV/EHU. ’ REFERENCES

Figure 4. Photoluminescent emission spectra in toluene solution of 10 wt % Au-(HOBC/PS-b-PEO) system without and with different nematic HOBC liquid crystal contents taken at room temperature. The excitation wavelength was 220 and 280 nm, respectively, for the systems without and with nematic HOBC liquid crystal.

PS-b-PEO block copolymers, amplified the TUNA-current passing throughout the sample and changed the photoluminescent behavior of the systems. Increasing the HOBC content, 20:80 with respect to PS-b-PEO block copolymers, multiplied the TUNA current 62 times if compared to the TUNA current for the system without HOBC. The ternary inorganic/organic 10 wt % Au-(HOBC/PS-b-PEO 20:80) system had the photoluminescent behavior of the neat HOBC. This phenomenon is in good agreement with literature, since In et al.26 and Kumar et al.27 published that inorganic nanoparticles can provoke the fast switching of liquid crystals while switching from an opaque to a transparent state. To confirm this behavior, further examination of these and related systems is currently underway to quantify the ratio between Au nanoparticles, low molecular weight nematic liquid crystals, and amphiphilic block copolymers adequate to generate novel inorganic/organic multifunctional thermally reversible nanostructured materials with fast switching from an opaque (off-state) to a transparent (on-state).

’ CONCLUSIONS The addition of small amounts of nematic HOBC liquid crystals as the third component of inorganic/organic hybrid materials based on poly(styrene-b-ethylene oxide) block copolymers (PS-b-PEO) modified with synthesized Au nanoparticles leads to a new family of multiphase nanostructured materials in which the conductive properties of well-dispersed Au nanoparticles are multiplied by the addition of HOBC liquid crystals. The TUNA-current level of the 10 wt % Au-(HOBC/PS-b-PEO 10:90 or 20:80) ternary systems was 5 or even 62 times higher if compared to the TUNA-current level of the 10 wt % Au-(PS-bPEO) binary system. Furthermore, these nanostructured inorganic/organic hybrids also maintained the photoluminescent properties of the low molecular weight HOBC liquid crystals; thus the generated hybrids are promising materials for nanooptoelectronic applications.

’ AUTHOR INFORMATION Corresponding Author

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*E-mail: A.T., [email protected]; I.M., inaki.mondragon@ ehu.es. 1648

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