Elastic and Viscoelastic Properties of Cross-Linked Gold

Jan 30, 2014 - Creep tests performed at a pressure of 2 kPa revealed both viscoelastic retardation (time constant: 3.3 × 10–3 s–1) and nonrecover...
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Elastic and Viscoelastic Properties of Cross-Linked Gold Nanoparticles Probed by AFM Bulge Tests Hendrik Schlicke, Elisabeth W. Leib, Alexey Petrov, Jan H. Schröder, and Tobias Vossmeyer* Institute of Physical Chemistry, University of Hamburg, Grindelallee 117, 20146 Hamburg, Germany S Supporting Information *

ABSTRACT: To enable applications of nanoparticle films in flexible electronics, actuators, and sensors, their mechanical properties are of critical concern. Here, we demonstrate that the elastic and viscoelastic properties of covalently cross-linked gold nanoparticles (GNPs) can be probed using AFM bulge tests. For this purpose 30−60 nm thick films consisting of 1,9nonanedithiol (NDT) cross-linked GNPs (3.8 nm core diameter) were transferred onto substrates with ∼100 μm circular apertures. The resulting freestanding membranes were bulged by applying pressure differences of up to 10 kPa, and the deflection was measured by intermittent contact atomic force microscopy (AFM). Analyzing the pressure-deflection data using the spherical cap model, either by taking into account the peak deflection values or the measured arc profiles of the bulge, yielded 2.3 ± 0.3 and 2.7 ± 0.4 GPa for Young’s modulus, respectively. When cycling the stress−strain measurements at overpressures up to 2.4 kPa, hysteresis was observed and assigned to viscoelastic effects. Creep tests performed at a pressure of 2 kPa revealed both viscoelastic retardation (time constant: 3.3 × 10−3 s−1) and nonrecoverable relaxation (creep rate: 9.0 × 10−8 s−1). Several membranes resisted pressures up to 10 kPa without fracturing, indicating that the ultimate biaxial tensile strength of the films was above ∼30 MPa.

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the material’s hardness and reduced elastic modulus.13 Recently, the micromechanical properties of colloidal crystals and films of ligand-stabilized PbS, CdSe, and CoPt3 nanoparticles14 as well as dodecanethiol-capped GNPs15 have been measured by AFM nanoindentation. However, in nanoindentation measurements the thickness of the sample should be at least ∼10 times the indentation depth, in order to avoid interferences by the underlying substrate. With nanoparticle films, which often have thicknesses below ∼100 nm, this requirement is difficult to achieve. To prevent substrate-related interferences, Pileni and co-workers15 prepared freestanding, multilayer GNP films on transmission electron microscopy (TEM) grids with micrometer-sized circular apertures. The indentation experiments were then performed by positioning an AFM cantilever tip as point load in the center of the aperture, and the elastic properties of the films were evaluated using a plate model. These experiments resemble previous work of Jaeger and co-workers10,11 and Cheng et al.,12 who investigated freestanding monolayers prepared from nanoparticles of different core−ligand combinations. The membranes were deflected using an AFM cantilever, and the elastic moduli were extracted from resulting force−displacement data. These investigations showed that the membranes’ elastic

INTRODUCTION Thin films consisting of ligand-stabilized or cross-linked gold nanoparticles (GNPs) have received considerable scientific attention during the past two decades, and various applications have been demonstrated. For example, transduction elements based on thin GNP films have enabled the fabrication of novel resistive strain gauges,1−4 touch sensors,5,6 and chemiresistors.7 In these sensors the transduction mechanism is based on changes in the interparticle distances, due to either forceinduced strain or sorption-induced swelling. Because the tunneling current between neighboring nanoparticles is exponentially related to their distance, these sensors can afford extremely high sensitivities. Further, nanoparticle networks have great potential for the implementation in next-generation flexible electronics. Very recently, Kotov and co-workers8 reported on stretchable conductors made from GNP−polyurethane composites enabling electrical tunability of mechanical properties by dynamic self-organization of the nanoparticles under stress. Obviously, the performance of sensors and flexible electronics based on nanoparticle composites critically depends on their specific mechanical properties, e.g., elasticity, viscoelasticity, and ultimate strength. To some extent, these properties have been studied by nanoindentation9 and forcedeflection measurements employing atomic force microscopes (AFMs).10−12 In conventional nanoindentation experiments the indenter tip is pressed into the substrate-supported specimen, and the force−distance data are analyzed to extract © 2014 American Chemical Society

Received: September 13, 2013 Revised: December 22, 2013 Published: January 30, 2014 4386

dx.doi.org/10.1021/jp4091969 | J. Phys. Chem. C 2014, 118, 4386−4395

The Journal of Physical Chemistry C

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cantilever (MikroMasch, k ∼ 3.5 Nm−1, f 0 ≈ 75 kHz) was used. Microscope cover slides (borosilicate glass, 22 × 22 mm2, thickness: 170 ± 5 μm) for deposition of substrate-supported films were purchased from Carl Roth GmbH. Dodecylaminestabilized GNPs (∼3.8 nm core size) were prepared according to the method of Leff et al.29 A representative transmission electron micrograph and the UV/vis absorption spectrum of the particles can be found in the Supporting Information. Deposition of Substrate-Supported GNP Films. 1,9Nonanedithiol (NDT) interlinked gold-nanoparticle films were deposited onto glass slides as described earlier24 with slight modifications. After cleaning the glass cover slides with acetone in an ultrasonic bath, they were rinsed with demineralized water, dried under nitrogen flow, and treated in an oxygen plasma (SPI Supplies Plasma Prep II). During the following spin-coating procedure the substrates were rotated constantly at 3000 rpm. First, a solution of NDT (100 μL, ∼7.4 mM) in toluene was spin-coated two times. Afterward, 10 μL of GNP stock solution in n-heptane (75 μM, determined by the method of Haiss et al.30) was spin-coated, followed by two deposition steps of NDT solution in methanol (10 μL, ∼7.4 mM). The latter three steps, representing one deposition cycle, were repeated 2−4 times. Between each step a delay of 20−30 s was kept. After finishing the film deposition by spin-coating, the coated substrates were immersed into the NDT/methanol solution overnight, cleaned by rinsing with acetone, and dried under ambient atmosphere. Lift-Off and Transfer of Membranes to Holey Substrates. Following deposition, the substrate-supported films were floated on 0.2 M sodium hydroxide solution to achieve alkaline underetching, similar as described by Grzybowski and co-workers. 31 After several hours, the membranes detached from the substrates and remained floating at the liquid/air interface. The solution was carefully exchanged with demineralized water (Millipore, 18.2 MΩ) 3−4 times. The floating membranes could afterward be transferred onto goldcoated transmission electron microscopy (TEM) grids with circular ∼100 μm apertures to form freestanding membranes as schematically depicted in Figure 1a. For each sample the actual aperture diameter was determined by optical microscopy. TEM Analysis of GNP/NDT Membranes. For TEM analysis, a GNP/NDT membrane was prepared following the standard procedure and transferred to carbon-coated TEM grids. Samples were analyzed using a JEOL JEM-1011 microscope, equipped with a LaB6 cathode and operated at 100 kV. Film Thickness Measurements. Thicknesses of the substrate-supported GNP-films were determined by atomic force microscopy as described earlier24 using a DI Multimode AFM equipped with a Nanoscope IV controller and a 100 μm scanner. Bulge Test Experiments. Acquisition of topographic data in the bulge test experiments was conducted using a JPK Nanowizard atomic force microscope (AFM) equipped with a NSC18 cantilever (MikroMasch). Measurements were conducted in intermittent contact mode. For applying overpressure to the freestanding membranes, the holey substrates were fixed to a sample holder (see Figure 1b) using double-sided sticky tape (Tesa 05338). The sample holder was mounted on the AFM’s sample stage, and the cantilever was positioned next to the membrane’s dome. First a topographic full scan was conducted (100 × 100 μm2, resolution 512 × 512 pixel2) at an applied pressure of ∼700

responses are set by how tightly the ligands are bound to the particle cores and by the noncovalent ligand interactions.10 Bulge tests are an interesting alternative to indentation experiments. Again, the composite films are prepared as freestanding membranes. In contrast to conventional indentation experiments, in which a point load displaces the film, the membrane is uniformly bulged by applying a pressure difference. Thus, this technique samples the mechanical properties over significantly lager areas than locally confined indentation experiments. In a series of papers, Tsukruk and coworkers16−19 reported on the micromechanical properties of polyelectrolyte membranes enclosing a central GNP layer. To determine stress−strain curves, they used an interferometric bulge test setup. The deflection of the membrane was measured by counting the number of Newton rings, which appeared on the bulge while applying the overpressure. Thus, the resolution of the deflection measurements was limited by the wavelength of the laser used in the interferometer.18 Recently, a few reports described the use of an AFM bulge test combination to probe the mechanical properties of freestanding films from various materials.20−22 For example, Gölzhäuser and co-workers22,23 used this method to probe the mechanical properties of nanometer thin carbon nanosheets. The method combines the advantages of bulge tests with the high-resolution height profiling and imaging capabilities of the AFM. In contrast to interferometric measurements, the AFM readout of the bulge data is independent of the films’ optical properties. In previous work, we developed a layer-by-layer spin-coating method allowing for the preparation of freestanding membranes of dithiol-cross-linked GNPs.24 Taking into account their mechanical robustness as well as their electric conductivity,24−27 these membranes are promising candidates for the development of highly responsive electromechanical sensing elements. Here, we study their micromechanical properties and report on three major findings: First, we demonstrate that disordered GNP membranes, cross-linked by low molecular weight dithiols, are robust enough to be characterized by bulge tests. To the best of our knowledge, this is the first example demonstrating that the AFM bulge method is well suited to study the mechanical properties of such nanocomposites. The method affords well-reproducible results and, thus, is an interesting alternative to previously reported nanoindentation experiments. Second, comparing our results to those from earlier nanoindentation studies14,15,28 suggests that covalent cross-linking of the nanoparticles significantly affects the elastic properties of these nanocomposites. Third, we show that the AFM bulge method allows for studying the viscoelastic properties of nanoparticlebased membranes. To the best of our knowledge, this is the first report on flow and retardation effects in films consisting of nanoparticles stabilized or cross-linked by low molecular weight compounds. For enabling applications of these films, e.g. as electromechanical sensing elements or as components in flexible electronics, it will be necessary to take these properties into account.

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EXPERIMENTAL SECTION Materials. Chemicals were purchased from Aldrich, Fluka, Grüssing, and VWR and used as received. Transmission electron microscopy grids (ATHENE G225G1) with ∼100 μm circular apertures were obtained from Plano GmbH, Germany. For AFM bulge test measurements a NSC18 4387

dx.doi.org/10.1021/jp4091969 | J. Phys. Chem. C 2014, 118, 4386−4395

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recorded using two digital pressure sensors (Sensortechnics HDIM100DUF8P5 and HDIM010DUF8P5 for pressures ranging from 1−10 kPa and 0−1 kPa, respectively); see Figure S1 in the Supporting Information. After complete acquisition of the full scan, the height data were leveled on the basis of the four areas around the dome to locate its peak. The cantilever was aligned with the peak (central point) of the dome in x (slow scan) direction and subsequent profile scans (0.977 × 100 μm2, 5 × 512 pixel2) were acquired traversing the central point in y (fast scan) direction. A detailed summary of the data processing is provided in the Supporting Information. For creep test experiments, the freestanding membranes were loaded with a constant pressure of 2 kPa after acquisition of pressure/deflection data from 0 to 1600 Pa. While keeping the pressure constant, AFM profile scans were conducted repeatedly every ∼30 s. All experiments were conducted at room temperature and ambient humidity.

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RESULTS AND DISCUSSION Covalently cross-linked GNP-films were prepared via layer-bylayer spin-coating on glass substrates, as described previously.24 This method requires the use of amine-stabilized GNPs because amine ligands are efficiently being replaced by bi- or polyfunctional thiols during the spin-coating process. Here, we used dodecylamine-stabilized GNPs with a core diameter of 3.8 ± 0.6 nm and 1,9-nonanedithiol (NDT) as cross-linker. The optical and charge transport properties of similar NDT crosslinked GNP films have already been studied extensively,25−27 and applications such as chemiresistors25,32 and strain gauges2,4,33 have been demonstrated. Thus, GNP/NDT films have become a well-established standard system for covalently cross-linked GNP-assemblies. The conductivity of (4.2 ± 0.4) × 10−3 S cm−1 and the surface plasmon resonance (SPR) absorption band of as-prepared films (see Supporting Information Figure S5) were in reasonable agreement with previously published data,24,25 indicating that the vast majority of particles did not destabilize and fuse during or after film deposition. Figure 2 shows transmission electron micrographs

Figure 1. (a) Freestanding GNP/NDT membranes were prepared by skimming the cross-linked GNP films floating on water. Gold-coated TEM grids with a circular aperture of 100 μm were used as substrates. (b) Schematic depiction of the AFM sample holder.

Pa, ensuring that sections of the substrate were imaged at four points around the dome and along the slow scan direction (x) at one edge of the image (an exemplary scan is depicted in the Supporting Information, Figure S6). For all scans, the line rate was set to 0.2 Hz. To avoid errors due to nonlinearities of the zpiezo, the JPK Nanowizard microscope is equipped with a strain gauge to accurately measure piezo displacements in the micrometer range. In the bulge test experiments the pressure was controlled by using the backpressure of nitrogen passing two needle valves. The applied pressure was continuously

Figure 2. Transmission electron micrographs of a GNP/NDT membrane (thickness 41 nm). Scale bars: (a) 200, (b) 50, and (c) 20 nm. 4388

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of a GNP/NDT membrane transferred onto a TEM grid. A quite homogeneous coverage of the substrate is observed on the micrometer scale. On the nanometer scale the images clearly reveal the disordered granular structure of the interlinked particle assembly. Covalent cross-linking of the GNP/NDT membranes is inherently evidenced by the method used for film preparation. With each deposition cycle an increase in film thickness was observed due to cross-linking of GNPs by NDT.24 In a control experiment we replaced the NDT cross-linker by monodentate nonanethiol and did not observe any significant additional deposition of particles after the first spin-coating cycle. In addition, because of successful cross-linking, it was impossible to redissolve the as-prepared GNP/NDT films in toluene. A previously published XPS analysis on layer-by-layer self-assembled GNP/NDT films revealed that about 20% of the NDT thiol groups exist as free thiol groups, suggesting that about 60% of the NDT molecules are attached on both ends to GNP surfaces.25 However, this analysis cannot provide information on the ratio of NDT molecules serving as linker to those being attached with both ends to the same GNP. Thus, quantitative information about the degree of cross-linking is currently not available. The thicknesses of the films, which were prepared by applying 2−4 spin-coating cycles, ranged from 30 to 60 nm. These films could easily be lifted off the substrates by alkaline underetching and transferred onto holey substrates with circular ∼100 μm apertures to form freestanding membranes, suitable for bulge test experiments. In Figure 1a, the transfer of films onto holey TEM grids is depicted, and Figure 1b shows the placement of the holey substrate onto the AFM sample holder. Figure 3 depicts optical micrographs of three different freestanding GNP/NDT membranes. Most of the as-prepared membranes were slightly slack and wrinkled, in contrast to freestanding, self-assembled monolayers of nanoparticles10,11 or polymer/GNP membranes,16 for which significant residual stress was reported. Optical micrographs of all membranes investigated in this study are provided in the Supporting Information (Figure S8). Their thicknesses, which are listed in Table 1, were determined by AFM measurements as described earlier.24 A typical AFM scan and height profile used for determining the film thickness is povided in the Supporting Information (Figure S4). The average roughness of GNP/NDT films prepared following the layer-by-layer spin-coating method was investigated in an earlier study, yielding a value of Ra = 0.47 nm.24 After transferring the GNP/NDT films onto TEM grids, they were allowed to dry and then glued onto an AFM sample holder (Figure 1). In some preliminary tests the membranes were bulged using a syringe coupled via a T-connector to a Utube filled with water. These experiments revealed that the membranes were sufficiently gastight and resisted pressures up to several kPa without damage. In the actual bulge tests we controlled the applied pressure using the backpressure of nitrogen passing two needle valves in series. Details of the AFM bulge test setup are provided in the Supporting Information (Figure S1). Figure 4 shows a series of topographic AFM images of the GNP/NDT membrane depicted in Figure 3a, deflected at overpressures in the range 0.4−2.4 kPa. As expected, the measurements show a well-developed spherical bulge, which increased in deflection with increasing overpressure. Typically, the deflection was within the range of a few micrometers.

Figure 3. Optical micrographs of samples (a) 44B, (b) 31A, and (c) 41B. Residual stress values σ0 of the samples, extracted by fitting eq 1 to the data, were −2.4, 1.8, and −6.3 MPa, respectively. These values are qualitatively in good agreement with the wrinkles observed in the freestanding membrane section.

Table 1. Membrane Thicknesses and Biaxial Moduli Determined for the GNP Membranes Analyzed in This Study via the Peak-Deflection Method (YD) and the CircularFit Method (YC) sample 31A 41A 41B 41C 44A 44B 60A 60B

thickness t (nm)

YD (GPa)

YC (GPa)

± ± ± ± ± ± ± ±

4.0 3.5 2.7 3.9 3.0 3.2 (4.3)a 3.7

3.9 3.6 3.2 4.9 3.4 4.0 4.4 5.0

31 41 41 41 44 44 60 60