Quantitative Nanoelectrical and Nanomechanical Properties of ...

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Quantitative Nanoelectrical and Nanomechanical Properties of Nanostructured Hybrid Composites by PeakForce Tunneling Atomic Force Microscopy Junkal Gutierrez, Iñaki Mondragon, and Agnieszka Tercjak* Group ‘Materials + Technologies’, Department of Chemical and Environmental Engineering, Polytechnic School, University of the Basque Country (UPV/EHU), Pza Europa 1, 20018 Donostia-San Sebastián, Spain ABSTRACT: Hybrid nanocomposites based on poly(styreneb-ethylene oxide) (PS-b-PEO) block copolymer modified with a mixture of both vanadium and titanium nanoparticles were synthesized via a sol−gel process. With the aim of studying the influence of the addition of V:Ti nanoparticles on the selfassembled PS-b-PEO block copolymer, two different V:Ti mixtures were prepared. Addition of even 60 vol % sol−gel to fabrication of hybrid nanocomposites allows us to reach welldispersed, uniform in size V:Ti nanoparticles on the substrate surface due to hydrogen-bond formation between the sol−gel network and PEO block. Atomic force microscopy results indicate that increasing of the sol−gel content to 40 vol % leads to change the matrix from an organic (PS-block-rich phase) to inorganic (V:Ti nanoparticles/PEO-block-rich) one without losing high nanometric order due to confinement of the V:Ti nanoparticles in the PEO block. Quantitative nanoelectrical and nanomechanical properties were studied that employed novel powerful PeakForce tunneling atomic force microscopy (TUNA) technology confirming conductive properties of the nanoparticles in designed advanced materials.

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INTRODUCTION Hybrid inorganic/organic materials based on block copolymer templates have become a topic of intense research during the past decade.1,2 The self-assembly capacity of the block copolymers provides an interesting route, denominated as a bottom-up approach, for designing and controlling assembly of nanoparticles into highly ordered structures, which offer possible applications in the design of chemical and biological sensors, energy storage and convertors, catalytic systems, optical devices, and others.3−7 Several methods have been developed for the incorporation of single-type nanoparticles selectively located inside one block of the block copolymer microdomains.8−19 Moreover, functional properties of these kinds of materials are still a challenge and only few works have been published in this field.20−27 Atomic force microscopy (AFM) is one of the most widely used techniques to study topography and morphology of different materials at the nanoscale level. The progressive development of this technique in the last years moves AFM measurements beyond just imaging contrast to quantitative nanomechanical and nanoelectrical properties.28,29 This novel technology called PeakForce tunneling atomic force microscopy (TUNA) allows one to simultaneously map the topography, modulus, adhesion, and conductivity of advanced materials on delicate samples that cannot be imaged with conventional conductive AFM. This is achieved by applying controlled, low forces on the tip during imaging, which allows a © 2013 American Chemical Society

direct comparison between the morphology and the electrical properties at the nanoscale. The other challenge of this novel PeakForce TUNA technique is the capability to operate also in spectroscopy mode which leads to the achievement of current current−voltage (I−V) curves. On the basis of our knowledge still only a few investigation works have been published employing this novel technique.30 In the present work, a mixture of vanadium and titanium nanoparticles was added to the poly(styrene-b-ethylene oxide) (PS-b-PEO) block copolymer matrix. The synthesized hybrid nanocomposite combines the antibacterial, self-cleaning, and photocatalytic properties of TiO2 nanoparticles with the photochromic and optical properties of V2O5 nanoparticles.31−35 Additionally, numerous studies have attempted to develop and characterize the vanadium/titanium nanoparticles system as a viable photofunctional material, which can find applications in the field of solar energy conversion and storage.34,36−38 The sol−gel technique offers the possibility to combine two different precursors to synthesize a mixture of two types of inorganic nanoparticles. However, an inherent problem in the preparation of the mixture of inorganic nanoparticles via the sol−gel process is that the phase separation can take place. In Received: August 1, 2013 Revised: November 13, 2013 Published: December 23, 2013 1206

dx.doi.org/10.1021/jp407690s | J. Phys. Chem. C 2014, 118, 1206−1212

The Journal of Physical Chemistry C

Article

Figure 1. AFM phase images (3 μm × 3 μm) of SEO block copolymer and hybrid 1:1-V:Ti/SEO nanocomposites with different vol % of sol−gel.

IIIa, Multimode from Digital Instruments equipped with an integrated silicon tip/cantilever having a resonance frequency of 300 kHz. Scan rates ranged from 0.7 to 1.2 Hz s−1. In order to obtain repeatable results, different regions of the specimens were scanned. Additionally, AFM was used to measure the thickness of the films after scratching. In this case, the film thickness was 80 ± 5 nm for all investigated samples. Quantitative nanomechanical and nanoelectrical properties were investigated using a Dimension Icon microscope equipped with a Nanoscope V controller from Bruker. Measurements were carried out using PeakForce TUNA (PF-TUNA) mode under ambient conditions. The PF-TUNA probes have a platinum−iridium coating tip with spring constant of 0.4 N/m having a resonance frequency of 45−95 kHz. The measurements have been performed with a calibrated optical sensitivity, the exact spring constant was calculated using the thermal tune option, and a defined tip radius was adjusted using PS as standard. In addition to the imaging mode, PF-TUNA allows one to measure local current−voltage (I−V). The I−V measurements were performed using the point and shoot feature at 10 different manually selected points on the investigated sample surface. The I−V curves were acquired while the tip was held at a fixed position with a constant force, and the applied voltage ramp was from 10 to −10 V with a scan rate of 0.05 Hz. Here it should be pointed out that in PeakForce mode the selection of the tip is an essential factor to discuss the quantitative value of nanoelectrical and nanomechanical measurements.

order to avoid this behavior (especially on the macroscopic level), in the present work, different mixtures were synthesized, varying the ratio between vanadium and titanium (V:Ti) sol− gel precursors. The novel PeakForce TUNA technique was employed to characterize the designed hybrid nanocomposites with the aim to study nanomechanical and nanoelectrical properties.

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EXPERIMENTAL METHODS Materials. The block copolymer used in this study was poly(styrene-b-ethylene oxide) (SEO) (MnPS = 58 600 g mol−1, MnPEO = 31 000 g mol−1, Mw/Mn = 1.03) from Polymer Source, Inc. Vanadium(V) oxytriisopropoxide (OV(OCH(CH3)2)3) (VOIT) and titanium isopropoxide (Ti(OCH(CH3)2)4) (TTIP), purchased from Sigma-Aldrich, were used as precursors. Analytical grade toluene, isopropyl alcohol, and aqueous hydrochloric acid (HCl, 37%) were purchased from Sigma-Aldrich. Synthesis of V:Ti/SEO Nanocomposites. Vanadium and titanium sol−gel solution was prepared by mixing VOIT or TTIP with isopropyl alcohol and toluene in 1:20:20 molar proportion under vigorous stirring in strong acid medium (pH ∼ 2 adequate by the addition of HCl) during 1 h at room temperature. Two different mixtures of titanium and vanadium nanoparticles were prepared, 1:1-V:Ti and 1:3-V:Ti, varying the molar ratio between vanadium and titanium sol−gel precursors. Thus, for the preparation of V:Ti sol−gel solutions the desired amounts of vanadium and titanium sol−gel solutions were mixed together with constant stirring at room temperature. In both cases, 1:1-V:Ti and 1:3-V:Ti sol−gel solutions were left for aging 12 h at room temperature under vigorous stirring.38,39 Finally, 5, 10, 20, 30, 40, 50, and 60 vol % sol−gel solution, 1:1-V:Ti or 1:3-V:Ti, was added to 10 wt % toluene solution of the SEO block copolymer and stirred for 30 min. Then, the mixed solutions were filtered using a 0.2 μm PTFE filter. SEO block copolymer and hybrid inorganic/organic nanocomposites thin films were prepared by spin-coating onto indium tin oxide (ITO)-coated glass substrates (from SigmaAldrich) at 2000 rpm for 120 s. Characterization Techniques. Morphological characterization was performed using atomic force microscopy (AFM). Corresponding AFM images were obtained by operating in tapping mode with a scanning probe microscope Nanoscope

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RESULTS AND DISCUSSION The final morphology of the nanocomposites was determined by the microphase separation, which took place between the V:Ti/PEO-block and PS-block domains. Figure 1 shows AFM phase images of SEO block copolymer and hybrid 1:1-V:Ti/ SEO nanocomposites with different vol % of sol−gel. In the corresponding AFM phase image of the SEO block copolymer the PS-block domains appear as light regions and PEO-block domains as dark regions.12,40 Addition, even 5 vol % 1:1-V:Ti completely changed the final morphology of the nanocomposite if compared with the morphology of the SEO block copolymer. Under the same preparation conditions, the morphology changed from the almost lamellar structure with the stripe spacing of ∼65 nm to a cylindrical structure due to 1207



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Figure 2. AFM phase images (2 μm × 2 μm) of hybrid 1:3-V:Ti/SEO nanocomposites with different vol % of sol−gel.

the introduction of 1:1-V:Ti nanoparticles. In the case of 5 vol % 1:1-V:Ti/SEO nanocomposite one can observe dark PEOblock cylindrical domains merged into an interconnected wormlike structure. For 10 vol % 1:1-V:Ti/SEO nanocomposite, hexagonally packed cylinder morphology was observed, where some percolated microdomains coexist with this cylindrical structure in some areas. Increasing the nanoparticle content, 20 vol % 1:1-V:Ti/SEO nanocomposite, all cylinders were located perpendicularly to the free surface with a regular size and space. Moreover, one can clearly observe that cylindrical morphology was destroyed by separated disordered areas. These macrophase-separated areas were attributed to aggregates of inorganic particles. The size of these inorganic macrophase domains was approximately 500− 900 nm and appeared on the surface of all nanocomposites with less than 30 vol % content. In the case of 30 vol % 1:1-V:Ti/SEO nanocomposite some bright aggregates of the 1:1-V:Ti nanoparticles were detected in the cylindrical morphology. For 40 vol % 1:1-V:Ti/SEO nanocomposite the size of aggregates decreased and a higher quantity of well-dispersed 1:1-V:Ti bright domains was distinguished. Hybrid nanocomposites synthesized using 50 and 60 vol % sol−gel showed well-ordered structure consisting of 1:1-V:Ti nanoparticle arrays. In both cases, high inorganic contents did not allow us to distinguish PS-block domains since V:Ti/PEOblock domains completely covered the surface of the substrate. With these sol−gel contents, phase inversion takes place, and 1:1-V:Ti nanoparticles became a matrix instead of the PS block of the SEO block copolymer. Here it should be mentioned that for 50 vol % 1:1-V:Ti/SEO nanocomposite individual spherical nanoparticles with larger diameters were detected being ∼90 nm for this sample and ∼30 nm for the 60 vol % 1:1-V:Ti/SEO nanocomposite. The addition of V:Ti nanoparticles with the ratio 1:3 results in hybrid inorganic/organic nanocomposites with different final morphologies. This ratio provokes different effects on the selfassembly of the SEO block copolymer. Changes in morphology of the nanocomposites with low and high sol−gel content are shown in Figure 2. Hybrid 1:3-V:Ti/SEO nanocomposites showed well-defined morphology of the SEO matrix and welldispersed nanoparticles located in the PEO block, especially if compared with hybrid 1:1-V:Ti/SEO nanocomposites. The pseudolamellar morphology of the SEO block copolymer was

distorted with increasing 1:3-V:Ti nanoparticle content in the PEO-block phase. 1:3-V:Ti nanoparticles located in dark PEOblock domains of the SEO block copolymer resulted in the changes of the morphology, from a lamellar to cylindrical one. For 5 vol % 1:3-V:Ti/SEO nanocomposite, the small bright spherical domains selectively located in PEO-block domains dispersed in the PS-block matrix can be clearly observed in the corresponding AFM phase image. These brighter spherical areas were attributed to 1:3-V:Ti nanoparticles confined in PEO-block domains of the SEO block copolymer. The diameter of the bright 1:3-V:Ti nanoparticles was ∼35 nm. A higher quantity of bright spherical domains was detected with the increasing of the vol % sol−gel. In the case of 20 vol % sol− gel/SEO, perpendicularly oriented hexagonally packed cylinders were detected along the whole surface. Bright 1:3-V:Tifilled PEO-block domains and dark unfilled PEO-block domains were clearly distinguished, thus indicating that still higher content of sol−gel can be added to the SEO matrix to fill in all PEO-block domains. The size of the 1:3-V:Ti-filled PEOblock domains was varied between small domains with a diameter ∼50 nm and aggregates with 90−110 nm. The addition of high amounts of 1:3-V:Ti nanoparticles in SEO block copolymer resulted in significant changes in final morphology of the nanocomposites. For the 30 vol % 1:3-V:Ti/ SEO nanocomposite, spherical nanoparticle aggregates, between 110 and 170 nm in diameter, were detected in the hexagonal morphology. With increasing 1:3-V:Ti nanoparticle content almost all PEO-block domains were filled. In the case of 40 vol % 1:3-V:Ti/SEO nanocomposite small well-dispersed bright nanoparticles were detected with a diameter of ∼40 nm, and only some domains exhibited larger diameter of ∼160 nm. Increase of the sol−gel content to 50 vol % led to the phase inversion between the organic and inorganic matrix. Thus, 1:3-V:Ti nanoparticles became the matrix. Nanocomposites synthesized using 50 and 60 vol % sol−gel showed very similar morphologies. In both cases, the PS-block phase is almost undistinguished since the 1:3-V:Ti/PEO-block phase completely covered the substrate surface. For these compositions generated thin-film surfaces consist of highly dense arrays of uniformly sized 1:3-V:Ti nanoparticles. The diameter of individual spherical nanoparticles was ∼35 nm. In order to visualize and verify the location of 1:3-V:Ti nanoparticles, AFM high-magnification images and corresponding profiles of 10, 20, and 60 vol % 1:3-V:Ti/SEO 1208



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Figure 3. AFM images (left/center/right: phase (1 μm × 1 μm)/profile/phase 3D (250 nm × 250 nm) of hybrid 1:3-V:Ti/SEO nanocomposites with different vol % of sol−gel. The white line on each image indicates the position where the profile was taken.

Figure 4. PF-TUNA images (2 μm × 2 μm) of 60 vol % 1:3-V:Ti/SEO nanocomposite: height, adhesion, and modulus.

nanocomposites are shown in Figure 3. Comparing the phase profiles, significant changes in the hardness of the PEO block can be observed as the addition of the sol−gel into SEO makes the 1:3-V:Ti/PEO-block domains harder (the contrast evolves from dark to bright) with respect to the PS-block matrix (light regions). Moreover, the phase profiles confirm the location of the inorganic nanoparticles in the PEO block. In the case of 10 and 20 vol % 1:3-V:Ti/SEO nanocomposites, bright 1:3-V:Ti-filled PEO-block domains and dark unfilled PEO-block domains were distinguished, and the corresponding profiles clearly indicate that for each nanoparticle a PEO-block interphase exists between them and the light PS-block matrix. Three-dimensional (3D) AFM phase images confirm that the 1:3-V:Ti nanoparticles domains were very sharp. This fact was related to employment of the mixture of nanoparticles, which confirmed the affinity of 1:3-V:Ti nanoparticles with the PEO

block since this phase was sharper than in the SEO block copolymer. In the case of 60 vol % 1:3-V:Ti/SEO nanocomposite, the corresponding profile and 3D AFM phase image confirm the phase inversion mentioned above. Quantitative nanomechanical and nanoelectrical properties of hybrid 60 vol % 1:3-V:Ti/SEO nanocomposite were characterized by the PF-TUNA technique. Simultaneously generated maps of height, adhesion, and elastic modulus are shown Figure 4. Three images show homogeneous densely packed nanoparticles. The roughness of the sample was measured from the height image. The average roughness (Ra) was equal to 3.8 nm. The adhesion image reveals the existence of two phases with different adhesion forces, the dark (low-adhesion) spherical nanoparticles separated by a thin continuous net of a brighter (high-adhesion) PS phase. This PS phase appeared as a nanoparticle boundary with an adhesion force values of ∼6 nN. 1209



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Figure 5. PF-TUNA current images (2 μm × 2 μm) and corresponding profiles of 60 vol % 1:3-V:Ti/SEO nanocomposite.

The modulus map detects bright (stiffer) areas in the whole sample surface. The modulus for these brightest nanoparticles was ∼700 MPa. Figure 5 shows PF-TUNA current images (2 μm × 2 μm) and corresponding profiles of 60 vol % 1:3-V:Ti/SEO nanocomposite when bias voltages of 10 and −10 V are applied. Current maps were recorded at the same time as the height, adhesion, and modulus. Positive and negative bias voltages were applied to investigate the sample in order to analyze the behavior of the sample as a function of the sign of the applied voltage. Taking into account the conductive behavior of these kinds of nanoparticles, in both PF-TUNA current images (for 10 and −10 V) individual spherical nanoparticles can be clearly detected covering completely the substrate surface. Moreover, a clear correlation between the topography and the regions of high current was found whenever a measurable current can be recorded. In the case of the PF-TUNA current image obtained at a 10 V bias voltage, conducting domains show as bright, and domains of lower conductivity as dark, areas. As can be seen in the corresponding current profile, domains with different brightnesses present differences in the current value. From the profile one can also confirm that each individual nanoparticle had a current level ∼25 pA. When negative bias voltage was applied a similar PF-TUNA current image was obtained; however, in this case, the color contrast scale changed. Negative bias voltages result in negative current flow, and in PF-TUNA current images high current was represented by a dark contrast and areas where the current was low were bright. Additionally, as can be observed in the corresponding current profile, for each nanoparticle a current level of −25 pA was achieved. Thus, the obtained results indicated that 60 vol % 1:3-V:Ti/SEO nanocomposite possessed the same conductive behavior regardless the sign of the applied voltage. Moreover, it should be further noted that, as is visible in both PF-TUNA current images and the corresponding profiles, the nanoparticles boundaries were associated with low-current areas. As confirmed by the AFM profile (Figure 3, bottom) this area corresponds to the PS-block interphase, and the PFTUNA current image demonstrated the nonconductive behavior of this material under these measurement conditions. In addition to the PF-TUNA current images, local I−V curves were recorded to evaluate the electrical properties of the 60 vol % 1:3-V:Ti/SEO nanocomposite. I−V measurements were performed in 10 points of the sample surface labeled in the 3D PF-TUNA height image (Figure 6, left), and a representative I−V curve was plotted (Figure 6, right). The shape of this curve is associated with a non-ohmic I−V behavior. As can be seen from the I−V curve, measurable currents appear only at high values of applied bias voltage.

Figure 6. Representative I−V curve recorded from points labeled in the 3D PF-TUNA height image (2 μm × 2 μm).

Thus, the TUNA current rapidly increased above a certain threshold voltage of ∼± 7 V.

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CONCLUSIONS SEO block copolymer was used as a template for selective arrangement of a mixture of vanadium and titanium (V:Ti) nanoparticles. Confinement of the synthesized V:Ti nanoparticles in the PEO block of the SEO block copolymer was controlled, playing with the ratio between titanium and vanadium sol−gel precursors. The nanocomposite fabricated with 1:3 ratio between V and Ti allowed us to successfully develop nanostructured hybrid inorganic/organic nanocomposites with a well-dispersed mixture of both vanadium and titanium oxide nanoparticles. PeakForce TUNA technology was successfully applied to study the nanomechanical and nanoelectrical properties of 60 vol % 1:3-V:Ti/SEO nanocomposite. Designed hybrid nanocomposites are able to respond to applied bias voltages regardless of the sign with almost the same current value (±25 pA). Consequently, we proved that employment of PeakForce TUNA is a useful innovative tool to achieve a quantitative characterization of both mechanical and electrical properties of advanced materials at the nanoscale level.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Fax: 0034 943017140. Phone: 0034 943017169. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support from the Basque Country Government in the frame of Grupos Consolidados (IT-776-13) and NABLOTOP (S-PE12UN106), and from MINECO in the frame of DECAION (MAT2012-31675), are gratefully acknowledged. J.G. thanks the Basque Government for “Programas de becas 1210



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