Temperature-Induced Structural Transitions on Hybrid Nanothick

Article pubs.acs.org/JPCC

Temperature-Induced Structural Transitions on Hybrid Nanothick Metal/Polymer Assemblies D. Siniscalco, J.-F. Bardeau, M. Edely, A. Gourbil, and N. Delorme* Institut des Molécules et Matériaux du Mans (IMMM), UMR CNRS 6283, Université du Maine, Avenue Olivier Messiaen, 72000 Le Mans, France ABSTRACT: Performance of nanothick metal/polymer assemblies crucially depends on the internal structure of the metal−polymer assemblies. As only a combination of high sensitive techniques is able to access the fine structure changes, combined X-ray reflectivity (XRR) and electrical AFM analysis are proposed. This combination of techniques is used to follow the fine evolution of the nanostructure of an ultrathin gold layer (10 nm) deposited on a polystyrene thin film (30 nm) as a function of the metal deposition temperature (from Td = −40 to 220 °C). Nanothick metal/polymer assemblies are prepared by evaporating ultrathin gold layer ( 120 °C), whereas for gold deposited on silica the roughness remains constant at ∼2.4 nm, the RMS roughness of gold deposited on polystyrene reaches a minimum plateau value at 0.3 nm. The decrease of the roughness can be attributed to a progressive embedment of the gold grains into the polymer layer. As the minimum roughness value is equal to freshly prepared polystyrene thin films, we can thus suppose a total embedding of the gold particles into the polystyrene. This assumption is in good agreement with previous studies reporting a full embedment of gold nanoparticles (i.e., 20 nm in diameter) in polystyrene film upon annealing to 130 °C.26 Indeed, when polystyrene is heated above the glass transition temperature, freely moving macromolecules can lead to a more viscous material, allowing the diffusion of metal particle into the polymer.13,22 In order to confirm the temperature-induced structural transitions on hybrid gold/PS assemblies, X-ray reflectivity (XRR) measurements were performed on samples elaborated at different deposition temperatures. Cross-section TEM could have been a more direct way to study the structure evolution.22 However, because of the thinness of the layers and the presence of the hard silicon substrate, we were not able to cut the

Figure 2. Circularly averaged 2D-PSD curves as a function of the deposition temperature for SiPSAu samples.

Figure 2 illustrates two behavior between Td = −20 °C and Td = 180 °C. Below 64 °C, PSD curves are similar and characteristic of a mounded surface.20 The local maximum is found at k = 0.209 nm−1 and corresponds to a characteristic length of 30 nm, which is consistent with the averaged value determined for the grain size on the AFM images. For Td = 100 °C a strong local maximum located at k = 0.08 nm−1 appeared on the 2D-PSD curve corresponding to a distance of 78 nm. This value can be associated with the averaged intergrain distances distinguished in the AFM image (Figure 1). For Td = 120 °C the local maximum intensity slightly decreases, indicating the loss of the 2D surface periodic arrangement. For deposition temperatures above 160 °C, the PSD curves follow an exponential decrease shape, characteristic of a selfaffine surface.19 The drastic change on surface morphology for samples with Td > 100 °C may be linked to the thermal properties the polystyrene and more specifically to the glass temperature which is reduced by almost 5 °C (compared to the bulk glass temperature measured by DSC) as expected for thin film.11,21 The surface morphology is significantly modified with the temperature treatment and the RMS roughness changes have been investigated as a function of the deposition temperature and compared in Figure 3 to the surface RMS roughness of gold deposited directly on silica (under the same condition). At low temperature (far below Tg), the polymer layer can be considered as a glassy material (hard and brittle). Therefore, during deposition, gold film growth and roughness will depend on the value of the sticking coefficient (dimensionless number), defined as the ratio of the metal amount (number of atoms/ clusters) that becomes trapped on the surface to the total amount of metal hitting the surface.22 For gold deposited on polystyrene and silica, these sticking coefficients were measured to be less than 1;23 thus, the roughness measured values are expected to be of the same order of magnitude for both samples. These values are less than 1 nm between Td = −40 °C and Td = 0 °C for gold deposited on silica and polystyrene. 7393

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Figure 4. (a) X-ray reflectivity curves obtained for different deposition temperatures (curves were vertically shifted for comparison) and (b) electronic density profiles obtained from the fits to the experimental data.

From 24 to 64 °C, the electronic density of the gold layer decreases (shift of the first minimum) to 2.6 e−/Å3, and the fringes related to the PS and gold layers are both less pronounced, indicating a modification of the metal/polymer assembly and thus a degradation of the stratified layers. Our model shows a slight increase of the electron density of the surface layer and a decrease of the polystyrene thickness from 44 nm (at Td = 24 °C) to 35 nm (at Td = 64 °C). Accordingly, we can suppose that with increasing the deposition temperature the gold layer is less compact and the empty grain boundary of the surface layer is filled with polystyrene. This behavior is in good agreement with previous works31 which recently demonstrated that the embedment process for gold nanoparticles in polystyrene films starts at T = 35 °C and that the higher the temperature the fastest the embedment.32 For deposition temperature Td higher than the glass transition of the polystyrene (Tg = 103 °C), significant changes are observed on the reflectivity curves. At Td = 110 °C, the low-frequency oscillation essentially disappears from the reflectivity profiles, indicating a broadening of the metal−polymer interfaces due to the motion of both gold clusters and polymer molecules along the interfaces. The analysis of the curves between Td = 110 °C and Td = 220 °C shows that the roughnesses of PS, gold, and interfacial layers could increase from about 1 nm to more than 5 nm and that the well-defined Kiessig fringes arise from the thickness of the entire layered. The evolution of the electronic density (Figure 4b) confirms clearly the broadening of the metal−polymer interfaces above 110 °C. For Td > 110 °C, we also observed that the wave-vector transfer Q values of the surface layer (estimated at about Qc = 0.025 Å−1) were systematically close to the Q value of polystyrene (i.e., 0.23 Å−1).27 This result is not so surprising since it has been demonstrated that when the deposition temperature reaches a value close to Tg, supported PS films show evidence of enhanced mobility at about 7 nm into the film from the free surface which should strongly increases the embedment of the gold particles.33 In addition, Deshmukh et al. also indicate that, close to Tg, 20 nm gold NP were rapidly covered by a thin PS wetting layer with a thickness close to 1.5 nm.26 In addition, if we take into account that our AFM results showed a decrease of the surface roughness above 110 °C to 0.3 nm, similar to a freshly prepared polystyrene thin film, we thus demonstrate the presence of a thin polystyrene layer at the top surface for Td > Tg. The thickness of this layer was estimated to

samples for an effective analysis of the diffusion profile. The advantage of the X-ray reflectivity profiles is that in the case homogeneous stratified layers the XRR curves are very sensitive to film thicknesses, layer compositions, and interface roughnesses.27−30 The data were recorded as X-ray reflectivity, R, as a function of scalar momentum transfer qz = (4π/λ) sin θ, where λ is the X-ray wavelength (1.542 Å) and θ the specular reflectance angle. A quantitative analysis of the X-ray reflectivity curves has been carried out using the matrix method including a set of multilayers characterized by thickness, electron density, and roughness. The silicon substrate was supposed to have an infinite thickness and a critical wave-vector transfer of 0.0316 Å−1; for silicon oxide it was fixed to 0.029 Å−1.27 Figure 4a shows both the X-ray reflectivity curves and the best fits after refinement, and Figure 4b illustrates the electronic density profiles of the samples as a function of deposition temperature. From Td = −20 °C to Td = 64 °C, evidence of a double-layered structure was found in the X-ray reflectivity curves. The profiles show a low-frequency beating corresponding to the presence of a gold cluster layer and rapid oscillations corresponding to the thick PS layer. The Kiessig fringes spacing (i.e., distances between two local minima) is inversely proportional to the layer thickness and the wave-vector transfer Q value of the first minimum (Qmin) is linked to the electron density of the top layer of the film (i.e., ρ = 711 × Q2).27 From Td = −20 °C to Td = 24 °C, the electronic density of the gold layer increases in accordance with the observed shift of the first minimum (Figure 4a) from Qc = 0.062 Å−1 (2.7 e−/Å3) to the expected value for bulk gold (i.e., Qc = 0.079 Å−1, 4.4 e−/ Å3)), suggesting that at low temperature the deposited gold layer is not structurally a homogeneous and condensed film. Moreover, the electron density of the surface layer (related to the layer at the interface with air) is about 1.7 e−/Å3 (∼0.049 Å−1), which is in agreement with the mounded morphology of the surface observed by AFM. From the AFM height profiles, we determined that the top layer gold mounds represent around 62% of the total volume. A simple calculation allows to estimate the averaged wave-vector transfer of the surface layer to 0.62 × Qbulk Au = 0.048 Å−1, which is similar to the value obtained from the model. We also found that the estimated total gold thickness is less than 10 nm (experimental deposit thickness), confirming the expected penetration of the gold particles (about 2 nm) onto the polystyrene film during the deposition. 7394

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be 2 nm at Td = 120 °C, and for deposition temperature higher than Tg, gold particles diffusion into the polymer is expected to be faster.34 It was reported that the depth of embedment is inversely related to the polymer viscosity,26 and since the polymer viscosity decreases with temperature,35 the embedment depth should increase. This was confirmed by both our model since at Td = 220 °C, the top layer possesses the Q value of polystyrene and a thickness of 5 nm and more clearly by the experimental reflectivity curves since the first minimum (Qmin) shifts progressively to a lower Qz value. In conclusion, the full analysis of the XRR curves, associated with AFM measurements, allows us to propose a physical mechanism to explain the temperature-induced structural transitions on hybrid metal/polymer assembly (Figure 5). For

a continuous contact between the tip and the surface independently of the surface roughness. Finally, the length of the deposited Ag electrode and the distance between the AFM tip and the electrode were kept constant. With this operating mode, direct surface resistance can be accessed.36 Figure 7 shows AFM topography images taken close to a scratch performed on the film and the surface resistance mapping of the same region. The Z scale of the surface resistance mapping images has to be converted to electrical resistance (RS) using the following formula: RS = 10(V+2), where V is the value in volts on the image. As it can be seen on the left side of the images, the scratch region (where the silicon substrate is visible) appears dark on the topography image because of its lower height compared to the film and light (i.e., high electrical resistance) on the surface resistance mapping image for Td = 24 °C because the current flow is on the surface and the silicon surface should possesses an infinite resistance. At room temperature, the gold film is conductive with an average surface resistance of R = 2.5 kΩ. The real resistance may be smaller, but such values are not accessible because there are in the range of the AFM tip resistance. For Td = 220 °C, the gold film is no more conductive since the measured resistance is RS = 2012 Ω (i.e., upper value accessible with our apparatus). Therefore, the performed electrical measurements support our assumption of an insulating polymer layer on the top surface for high deposition temperatures. In order to link the electrical properties of the assemblies and the expected structure of the film (determined by XRR), we measured the averaged film surface resistance values as a function of the deposition temperature. In Figure 8 four regimes can be identified. For Td < 24 °C, highly conductive film surfaces are revealed in accordance with gold’s very high electrical conductivity. For thin metallic film, the conductivity is made by grain boundary,37 and the measured surface resistance is in the range of the results recently obtained by Yajadda et al. (i.e., for 8 nm thick gold deposited on glass)38 and by Sun (for 20 nm thick gold deposited on silicon).39 The second regime (24 °C < Td < 110 °C) is characterized by a linear increases of log(RS) with a slope of 57.5 × 10−3 °C−1. Previous works had studied the variation of the electrical resistivity as a function of the temperature.39 They observed also a linear increase in the log(R) = f(log(T)); the extracted slope is associated with a temperature coefficient of resistance (TCR). The sign of the TCR is then used to explain the conduction mode inside the film. Here we want to emphasize that contrary to their works, our measurements were made at room temperature, and thus the obtained slope cannot be associated with a TCR. Indeed, evolution of surface morphology cannot be the reason to explain the increase of the surface resistance in this region since no evolution of RMS roughness was observed between −20 and 90 °C. One possible explanation is that with increasing temperature the polymer starts to fill the grain boundary of the gold film, leading to an increase of the electrical resistance. In the third regime (100 °C < Td < 160 °C), the resistance is high (RS ∼ 1010 Ω) and remains constant. From the AFM and XRR analysis we have concluded that a very thin PS layer (estimated to less than 2 nm) was present on the top surface; however, we believe that this top layer is sufficiently thin to allow the electrical conduction when the AFM tip is in contact with the surface. In the last regime (Td > 160 °C) no surface current can be measured with our apparatus. As expected from the XRR measurements, the gold film is fully embedded into the polymer and the top polymer layer thickness (estimated to

Figure 5. Schematic representation of the evolution of hybrid gold/PS assemblies as a function of the deposition temperature.

Td < 24 °C, the polystyrene film is hard and the penetration of gold during deposition is low. The metal/polymer assembly is thus composed of nanostructured layers. For 24 °C < Td < Tg, the polymer material softens because of the increasing temperature, leading to a slight embedment of the gold film. Despite this embedment, the metal/polymer assembly consists of a multilayer composite structure. When Td is close to Tg, a major change of the mechanical properties of the polymer film leads to a large diffusion of the polymer material through the gold film. The organization of the gold clusters into the film is modified as well of the roughness of the surface. For higher temperature, the embedment of the gold particles goes on, and the gold clusters penetrate more deeply in the polymer film, leading to a top surface with similar polymer film properties. As polymer−gold interpenetration is believed to play an important role in the modification of electrical properties in integrated devices, we have specifically studied its influence on the surface electrical conductivity. To perform surface resistance measurements, we used an AFM equipped with a Resiscope apparatus, as illustrated in Figure 6. In this experiment, the applied force on the tip is sufficient to allow

Figure 6. Schematic representation of the surface resistance measurement. 7395

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Figure 7. AFM height image and AFM surface resistance map obtained for Td = 24 and 220 °C.

metal/polymer thin layers and thus to be of prior importance for all polymer metallization applications.

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AUTHOR INFORMATION

Corresponding Author

*Fax 33-2 43 83 24 44; e-mail [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are grateful to Pascal Chrétien (LGEP - Paris) for his useful discussions on electrical measurements using the Resiscope.

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Figure 8. Surface resistance (in log scale) measured at different deposition temperatures. Dashed lines indicate the range of accessible surface resistance with our apparatus.

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CONCLUSIONS We have demonstrated the strong influence of the thermal properties of the polymer on the structure of nanothick metal/ polymer assemblies. We have shown that RMS roughness is characterized by a sharp maximum when deposition temperature is close to the glass temperature of the polymer substrate. Combined XRR and AFM studies were also used to reveal the modification of the metal/polymer assembly structure as a function of the deposition temperature. Clearly, the structure of nanometric metallic layers deposited on polymer films is complex, and the representation of polymers with coatings as two layered systems with a well-pronounced interface depends clearly on the deposition temperature. For the first time to our knowledge surface electrical conductivity measurements were analyzed in regards of the metal/polymer film structure. We believe that the exposed method can be applied to all kinds of 7396

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