Monolayered Wires of Gold Colloidal Nanoparticles for High

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Monolayered Wires of Gold Colloidal Nanoparticles for High-Sensitivity Strain Sensing Cosmin Farcau,† Helena Moreira,† Benoit Viallet,† Jeremie Grisolia,† Diana Ciuculescu-Pradines,‡ Catherine Amiens,‡ and Laurence Ressier*,† † ‡

Universite de Toulouse, LPCNO, INSA-CNRS-UPS, 135 avenue de Rangueil, Toulouse 31077, France Universite de Toulouse, Laboratoire de Chimie de Coordination (CNRS), 205 route de Narbonne, Toulouse 31077, France ABSTRACT: High-sensitivity resistive strain gauges based on electron tunneling in assemblies of gold colloidal nanoparticles are fabricated and characterized. The active area of these strain gauges consists in welldefined arrays of parallel, few micrometer wide wires of close-packed 18 nm gold nanoparticles. These nanoparticle wires are obtained by convective self-assembly (CSA) on flexible polyethylene terephtalate substrates, without any lithographic prepatterning. The fine control over the thickness and the width of the nanoparticle wires through the substrate temperature and the meniscus speed during the CSA process allows demonstrating the strong impact of the dimensionality (2D or 3D) of the nanoparticle assembly on the strain gauge sensitivity. Wires made of a single monolayer of nanoparticles turn out to give strain gauges about three times more sensitive than those made of multilayers. This work shows that the simplicity and versatility of convective self-assembly over other alternative methods make this technique very suitable for the reliable and low-cost fabrication of miniaturized, highly sensitive nanoparticle-based strain gauges.

’ INTRODUCTION High-sensitivity strain gauges able to detect localized deformations are of the utmost importance for efficient structural health monitoring in many technological fields.1,2 At the same time, down-scaling, low fabrication costs, and simple postacquisition signal treatment are desirable. The conventional metal foil resistive strain gauges cannot fulfill all these demands with their usual gauge factor G = (ΔR/R0)/ε (where ΔR/R0 is the relative variation of electrical resistance and ε is the strain) of about 2 4 and their millimetric dimensions. Semiconductor strain gauges in turn are by far more sensitive, with a gauge factor G reaching 100 200, and can be miniaturized. Alas, they exhibit many disadvantages: their sensitivity decreases with the applied strain, their functioning is limited to the typical strain regime ε < 0.6%, they require important temperature compensation and are more expensive.3 Recently, metal colloidal nanoparticle assemblies on flexible polymer substrates have been proposed as highly sensitive resistive strain gauges.4,5 The reported sensitivity of such nanoparticle-based strain gauges reaches that of semiconductor ones for tensile strains. This high sensitivity relies on the exponential dependence of the tunneling resistance on the separation between adjacent nanoparticles.6,7 The use of chemically synthesized colloids as building blocks for such strain sensors is very interesting due to their availability in different sizes, shapes, and surface functionalities. This provides opportunities to further optimize the sensor performance.8 For example, it was indeed r 2011 American Chemical Society

recently demonstrated that the variation of nanoparticle film resistance under bending has a strong dependence on the molecular species capping the nanoparticles.9 Moreover, the nanometric size of these colloidal building blocks offers the opportunity to fabricate miniaturized strain sensors. Different techniques have been used to assemble colloidal nanoparticles on flexible substrates for making resistive strain gauges: airbrush spraying,4 layer-by-layer deposition,5 and self-assembly at air water interface followed by transfer via microcontact printing.9 Another approach for making nanoparticle-based resistive strain gauges consists in embedding nanoparticles in a polymer matrix through dielectrophoresis10 or using magnetic fields.11 Nevertheless, none of these techniques allows controlling the morphology of the nanoparticle assembly in both the lateral and vertical directions. Here we propose the simple, reliable, and low-cost fabrication of high-sensitivity nanoparticle-based strain gauges by horizontal convective self-assembly (CSA) of gold nanoparticles from a colloidal suspension. The fine control of the nanoparticle assembly offered by this process allowed studying a key point for the successful development of such innovative colloid-based sensors: the impact of the morphology of the strain gauge active area on their sensitivity. Received: March 7, 2011 Revised: May 11, 2011 Published: June 07, 2011 14494

dx.doi.org/10.1021/jp202166s | J. Phys. Chem. C 2011, 115, 14494–14499

The Journal of Physical Chemistry C

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’ MATERIAL AND METHODS Synthesis and Functionalization of Gold Colloidal Nanoparticles. The metallic building blocks used to fabricate the

nanoparticle-based strain gauges are gold colloidal nanoparticles, chosen for their air stability and ease of synthesis in water. These gold colloids were synthesized in the aqueous phase by the standard Turkevich method of reduction of tetrachloroauric acid by sodium citrate.12 They were then functionalized with bis(p-sulfonatophenyl) phenylphosphine dihydrate dipotassium (BSPP)13 to increase the tunnel barrier surrounding them and at the same time to make the colloids stable for months at the relatively high concentrations used in this study. An average diameter of 18 nm with a standard deviation of 15% was determined by transmission electron microscopy. The gold colloids were concentrated by centrifugation to 26 nmol L 1 (corresponding to 0.0045% vol), determined by UV visible extinction spectroscopy. Convective Self-Assembly. Such gold colloidal nanoparticles were deposited on 125 μm thick polyethylene terephtalate (PET) flexible substrates by convective self-assembly. PET from Putz Gmbh. was chosen for its high electrical resistivity (425 GΩ 3 m), high elastic deformation limit (∼6%), and very low surface roughness (Ra ∼ 3.5 nm). CSA is a well-documented, simple, and low-cost technique widely used to deposit micro- and nanoparticles from a suspension onto rigid substrates like glass, silica, or silicon.14,15 Briefly, a constrained drop of colloidal suspension is translated across a substrate, and particles are deposited along the straight meniscus of the drop. Recently, we demonstrated that tunable arrays of nanoparticle conductive wires can be made by CSA on silicon substrates16,17 thanks to the so-called “stick slip” mechanism,16 18 which involves the periodic pinning and depinning of the moving meniscus. In this work, a homemade CSA setup was used to fabricate similar wire arrays of 18 nm gold nanoparticles on PET flexible substrates. A glass deposition plate was placed in the vicinity of the PET substrate, at a 30° angle, and a 20 μL droplet of gold colloids was injected into the formed wedge. The PET substrate was fixed on a copper plate and its temperature Ts was regulated by water circulation. By translating the substrate, the meniscus formed by the colloidal suspension with the substrate was dragged over the substrate at a constant speed v. The ambient temperature and the relative humidity were kept constant at 24 ( 1 °C and 42 ( 1% during all the experiments. Electrical Addressing. The nanoparticle wires obtained by CSA were finally electrically connected by two 50 nm thick gold electrodes fabricated by stencil lithography, which avoids any contamination of the nanoparticle assembly by the use of resists, solvents, or etching. Thirty to forty nanoparticle wires were simultaneously connected by electrodes, to reduce the electrical resistance of the devices and ensure their robustness. Large gold electrodes were designed to facilitate further external electrical connections and mechanical tests. A scheme and a photograph of the typical resulting nanoparticle-based strain gauge, connected to copper wires, are presented in Figure 1. Morphology Characterization. The morphology of the fabricated nanoparticle-based strain gauges was observed by optical microscopy (Olympus BXFM), atomic force microscopy in tapping mode (Multimode Nanoscope IIIA from Veeco Instruments), and scanning electron microscopy (Hitachi S-4800).

Figure 1. (a) Scheme and (b) photograph of a typical nanoparticlebased resistive strain gauge.

Electromechanical Tests. A homemade four-point bending setup was employed to induce mechanical strain in the nanoparticle-based strain gauges. A high-precision piezoelectric inchworm motor (4 nm minimum step) was used to control the movement of two lateral parallel rods, while two inner ones were fixed. During four-point bending, a dc voltage of 0.5 V was applied to the strain gauges, and their electrical resistance was monitored with a Keithley 6430 sourcemeter. The strain ε induced in the nanoparticle wires was estimated by ε = h/(d + h), where h is the thickness of the PET substrate and d is the diameter of the circle followed by it. To avoid any errors related to humidityinduced changes of the resistivity of the nanoparticle assembly, the ambient temperature and the relative humidity were kept constant at 22 ( 1 °C and 39 ( 1% during the measurements on the different samples.

’ RESULTS AND DISCUSSION Morphology Characterization. Atomic force microscopy (AFM) analyses were performed on a series of samples prepared by varying the substrate temperature Ts between 19 and 25 °C and the meniscus speed v from 0.6 to 1.8 μm/s. Figure 2 presents the dependence of the thickness (a) and the width (b) of the nanoparticle wires on Ts and v. It reveals that the wire width w varies between 4 and 12 μm, while the wire thickness varies from one single layer to four layers of nanoparticles. Moreover, two general tendencies can be identified: (i) by increasing the substrate temperature, both the width and the thickness of the nanoparticle wires increase; and (ii) by increasing the meniscus speed, both the width and the thickness of the nanoparticle wires decrease. Occasionally, an inverse local variation of the wire width is observed on the graphs of Figure 2b due to an increase of the number of nanoparticle layers. Finally, in the case of a meniscus speed of 1.8 μm/s and substrate temperatures of 19 and 20 °C, it is to be noted that no values are reported on the graphs of Figure 2 because nanoparticle wires are interrupted in these cases and form island-like assemblies. By increasing the substrate temperature, and thus also the temperature of the colloidal suspension, the water evaporation rate is increased which causes a rising particle flux toward the meniscus. This means that for a given meniscus speed a rise of temperature causes an increase of the volume of the fabricated nanoparticle wires which therefore become wider and/or thicker. By increasing the meniscus speed at a fixed temperature, i.e., a fixed particle flux, a larger substrate surface is wiped per time unit, thus a lower number of nanoparticles is deposited per surface unit of the substrate, causing the formation of narrower and/or thinner nanoparticle wires. Island-like assemblies of nanoparticles are obtained when high meniscus speeds and low 14495

dx.doi.org/10.1021/jp202166s |J. Phys. Chem. C 2011, 115, 14494–14499


Figure 2. Dependence of the number of layers (a) and width (b) of nanoparticle wires on the substrate temperature and the meniscus speed during the CSA process.

substrate temperatures work in synergy to reduce the amount of deposited particles. This is in agreement with our previous works showing that the stick slip mechanism does not operate anymore when decreasing the substrate temperature.17 The elaboration of monolayered assemblies of metallic nanoparticles is rather a rare result, especially by such a simple and direct technique as CSA. Some examples of nanoparticle monolayers were reported in the literature using other techniques, including kinetically induced assembly at the surface of an evaporating drop of colloidal suspension19,20 or more conventional Langmuir Blodgett deposition.21 However, each of these techniques is efficient with small-sized nanoparticles (∼5 6 nm) dispersed in a volatile organic solvent. On the contrary, most often CSA studies dealt with aqueous suspensions of colloids larger than a few tens of nanometers.22,23 As shown here, CSA can also be successfully used for the elaboration of monolayered wires of 18 nm nanoparticles from an aqueous suspension. However, this study reveals that the experimental conditions for obtaining such monolayered nanoparticle assemblies by CSA are restricted to narrow intervals of substrate temperatures and meniscus speeds, for which the meniscus speed matches the sufficiently decreased evaporation rates. Due to the nanometric size of the particles, a low contact angle (lowest attainable value, which still avoids the spreading of a drop of colloidal suspension across the substrates) is also

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required, to ensure particle-to-substrate pinning and to avoid mechanisms of deposition other than stick slip.17 It is to be noted that the quantitative values of thicknesses and widths of nanoparticle wires presented in Figure 2 were obtained in specific experimental conditions. These values strongly depend on the colloid concentration, the contact angle of the colloidal suspension on the substrates, the ambient temperature, and the relative humidity. However, we anticipate that monolayered nanoparticle assemblies could also be obtained in other conditions when the right balance between the evaporation rate and the meniscus speed is found. Moreover, the morphology study revealed that the distance between adjacent nanoparticle wires roughly varies between 3.5 and 7 μm across the whole range of substrate temperature and meniscus speed values explored. Nevertheless, the distance between fabricated nanoparticle wires cannot be controlled independently of their width and thickness with this CSA process based on the stick slip motion. For instance, indeed, a thick wire is more ‘sticky’ for the meniscus than a thin one, allowing the film of colloidal suspension to elongate more before its depinning. This causes a longer "slip" stage toward its next equilibrium position where a new nanoparticle wire is formed. Further, three specific morphologies of nanoparticle wires (called M1, M2, and M3) were selected to make strain sensors. The geometrical parameters of these nanoparticle wires are reported in Table 1, together with the substrate temperature Ts and the meniscus speed v used during the CSA process. Figure 3 presents typical optical microscopy images (a, c, e) and AFM 3D images with corresponding cross sections (b, d, f) of the three types of strain gauges made with nanoparticle wires of morphologies M1, M2, and M3, respectively. As seen on the optical microscopy images, the active areas of these strain gauges are composed of parallel wires of nanoparticles, spanning over a length L0 of 150 160 μm between the two gold electrodes (note, however, that the total length of the nanoparticle wires is 1 cm, the rest of their length being covered by the gold electrodes). Each strain gauge contains approximately thirty to forty nanoparticle wires, extending over a lateral distance of 500 μm. AFM observations show that the wires constituting the strain gauges with morphologies M1 and M2 are composed of multilayers of particles, with a stairlike nanoparticle arrangement observed on both sides of the wires (Figures 3b and 3d). Contrarily, the wires with morphology M3 consist of a single monolayer of nanoparticles (Figure 3f). Scanning electron microscopy (SEM) analyses were conducted to check the degree of organization in the fabricated nanoparticle wires. A well compact assembly of gold nanoparticles was observed in the wires with morphologies M1, M2, Table 1. Experimental Conditions of CSA (Meniscus Speed v and Substrate Temperature Ts) and Geometry (Width w, Thickness t, and Cross-Section Area A) of the Resulting Nanoparticle Wires Measured by AFM, in Three Specific Cases, M1, M2, and M3 CSA parameters v label [μm/s]

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geometry of the resulting nanoparticle wires

Ts

w

t

number of

A

[°C]

[μm]

[nm]

nanoparticle layers

[μm2]

M1

0.6

25

11

60

4

0.49

M2

1

25

7

45

3

0.22

M3

1

21

12

18

1

0.16



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Figure 3. Optical microscopy images (upper row) of nanoparticle-based strain gauges with three different morphologies and associated AFM observations of a typical nanoparticle wire forming their active area (lower row): (a) and (b) M1; (c) and (d) M2; (e) and (f) M3.

Figure 4. SEM micrographs of nanoparticle assemblies in wires with morphology (a) M1 and (b) M3.

and M3. Figures 4a and 4b present typical SEM micrographs of the nanoparticle assembly in the case of the two extreme wire morphologies M1 and M3, respectively. In both cases, the images were taken on one edge of a nanoparticle wire. By moving from the bottom right to the top left corner in Figure 4a, the transition from the monolayer border area toward the multilayer one can be observed (as seen also in Figure 3b, in this case from left to right). Despite the difficulty to obtain high-quality SEM images (due to the insulating character of the PET substrate), a close-packed nanoparticle assembly can be observed. Contrarily to our previous work17 on similar gold nanoparticles, synthesized by the standard citrate reduction but without any further chemical functionalization and assembled on silicon substrates, no cracks are observed all across the wire width in the present case. We checked that both the cracks observed in assemblies of nonfunctionalized gold nanoparticles and their absence in the assemblies of BSPP-modified gold nanoparticles were not substrate dependent. It means that the BSPP functionalization of the gold nanoparticles favors a high degree of organization of nanoparticles and allows obtaining a very compact nanoparticle

arrangement by CSA. This result can be attributed to the better protection of the nanoparticles by BSPP than citrate, which prevents the formation of aggregates at the meniscus during the CSA process. This is in agreement with the higher long-term stability, at high concentrations, of colloidal suspensions of BSPPfunctionalized gold nanoparticles in comparison with those of nonfunctionalized particles. Electromechanical Characterization. Prior to testing the response of the nanoparticle-based gauges to applied strain, their initial electrical resistances R0 were measured. The current versus voltage curves were linear at room temperature for the three morphologies of nanoparticle wires. The extracted R0 values were 1.5 106 Ω, 5.5 106 Ω, and 7 108 Ω for M1, M2, and M3, respectively. By taking into account the geometry of the nanoparticle wire arrays, the corresponding resitivities of nanoparticle assemblies were estimated to 0.12 Ωm for M1, 0.26 Ωm for M2, and 21 Ωm for M3. These resistivity values are quite similar for multilayered nanoparticle assemblies (M1 and M2) and are in stark contrast with that obtained for monolayered assembly (M3). To record the performance of the fabricated nanoparticlebased strain gauges, the resistance variation ΔR of the nanoparticle wire arrays was monitored while bending the gauge. The ΔR/R0 versus strain ε = ΔL/L0 curves of the gauges with morphology M1 (black circles), M2 (red triangles), and M3 (blue squares) are displayed in Figure 5 compared to the response of a metal foil gauge with a typical gauge factor G = 2 (green dashed line). These experimental results demonstrate that the nanoparticle-based strain gauges are largely more sensitive than the conventional metal foil gauges and reach the performances of conventional semiconductor ones. For comparison, ΔR/R0 of the nanoparticle-based strain gauge with morphology M3 takes a value 100 times higher than the conventional metal foil at 0.6% tensile strain. With respect to the previous works on nanoparticle-based resistive strain gauges,4,5,10 the sensitivity 14497



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gauges with M1 and M2 morphologies: although the nanoparticle wires have quite distinct cross sections (Figures 3b and 3d), they both consist of multilayers of nanoparticles, so they support the same type of 3D conduction. The higher sensitivity of the strain gauge with M3 morphology compared to the ones with M1 and M2 morphologies can also be tentatively corroborated with its 2 orders of magnitude higher resistivity. Note that the four-point bending configuration used in this study allowed testing the nanoparticle strain gauges under strains up to ∼0.6%, which is approximately the limit deformation for semiconductor gauges. Some preliminary results from our current experiments, involving mechanical tests under uniaxial stretching of the nanoparticle-based strain gauges, show that these devices can function efficiently even at strains of several percent. Due to the discretized nature of their active element, the nanoparticle-based gauges seem to be able to withstand high stresses without damage. Figure 5. Relative resistance variation as a function of induced strain for nanoparticle-based gauges with different morphologies: M1, four layered nanoparticle wires; M2, three layered nanoparticle wires; M3, monolayered nanoparticle wires. The solid lines are fits with the exponential law ΔR/R0 = exp(g 3 ε) 1. The dashed green line represents the response of a conventional metal foil gauge with a gauge factor G = 2. The inset shows a scheme of the nanoparticle-based strain gauge characterization setup. For simplicity, only a single nanoparticle layer is represented.

reached by the nanoparticle-based strain gauge with morphology M3 places it among the most efficient reported so far. As expected, the exponential dependence of the interparticle tunnel resistance on the particle separation actually leads to an exponential dependence of ΔR/R0 on ε. All experimental ΔR/R0 versus strain ε curves can be well fitted with the equation ΔR/R0 = exp(g 3 ε) 1, where g is a constant characterizing the sensitivity of the nanoparticle-based strain gauges. Examples of such fits are given by the solid lines in Figure 5, yielding g factor values of 59, 72, and 135 for strain gauges with morphology M1, M2, and M3, respectively. A key result revealed by this study is the strong impact of nanoparticle wire morphology on the sensitivity of the strain gauge. Strain gauges with M1 and M2 morphologies, made of multilayered wires of nanoparticles, exhibit a similar dependence of relative resistance variation on applied strain, while the strain gauge with M3 morphology, made of monolayered wires of nanoparticles, provides a much higher sensitivity. The strain gauge with M3 morphology exhibits indeed a relative resistance variation about three times higher than the strain gauge with M1 morphology in the studied domain. In monolayered wires of nanoparticles, the applied strain effectively modifies interparticle gaps24 and therefore modifies the resistance of the nanoparticle assembly. In multilayered nanoparticle wires, it is easy to conceive that nanoparticles in the second layer form bridges between particles in the first layer. These interlayer gaps are less affected by the applied strain, so the overall variation of resistance is smaller. We suggest that the improved sensitivity of the strain gauge with M3 morphology over the gauges with M1 and M2 morphologies is in fact determined by the confinement of conduction paths25 in two dimensions for the former and three dimensions (3D) in the latter. This can also explain the almost similar responses of strain

’ CONCLUSIONS Convective self-assembly was used to fabricate arrays of parallel wires of close-packed 18 nm gold colloidal nanoparticles on PET flexible substrates, without any lithographic prepatterning. It was shown that a fine control over the thickness (one to four layers of nanoparticles) and the width (4 12 μm) of these nanoparticle wires can be achieved by tuning the substrate temperature and the meniscus speed during the process. Highly sensitive resistive nanoparticle-based strain gauges were elaborated by connecting such nanoparticle wire arrays between electrodes. Their sensitivity to applied strain is indeed almost 2 orders of magnitude larger than that of conventional metal foil gauges and reaches that of semiconductor ones. By fabricating wires with different cross sections, we demonstrated the strong impact of the dimensionality of nanoparticle assembly on the strain gauge sensitivity. The morphology which optimizes the performance of nanoparticle-based strain gauges is a monolayered assembly of nanoparticles. This work reveals that the simplicity and versatility of convective self-assembly make this technique very suitable for the reliable and low-cost fabrication of miniaturized, highly sensitive nanoparticle-based strain gauges. Moreover, the conductive nanoparticle wires obtained on flexible substrates by this approach could potentially be used for developing other types of sensors, like chemiresistors or humidity sensors. ’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].

’ ACKNOWLEDGMENT This work was supported by the French National Agency (ANR) in the framework of its program “Recherche technologique Nano-INNOV/RT” (NANOCOMM project n°ANR-09NIRT-004). ’ REFERENCES (1) Kenny, T. In Sensor Technology Handbook; Wilson, J. S., Ed.; Elsevier: Oxford, 2005; pp501 529. (2) Sahin, M.; Shenoi, R. A. Eng. Struct. 2003, 25, 1785. (3) Starr, J. E. In Strain Gauge Users’ Handbook; Hannah, R. L., Reed, S. E., Eds.; Chapman & Hall: London, 1994; pp 1 78. 14498



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Monolayered Wires of Gold Colloidal Nanoparticles for High

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