Dense Brushes of Tilted Metallic Nanorods Grown onto Stretchable

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Dense Brushes of Tilted Metallic Nanorods Grown onto Stretchable Substrates for Optical Strain Sensing Joseph Marae-Djouda, Arthur Gontier, Roberto Caputo, Gaetan Leveque, Bogdan Bercu, Yazid Madi, Guillaume Montay, Pierre-Michel Adam, Michael Molinari, Stephen Stagon, and Thomas Maurer ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.8b00441 • Publication Date (Web): 20 Apr 2018 Downloaded from http://pubs.acs.org on April 20, 2018

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ACS Applied Nano Materials

Dense Brushes of Tilted Metallic Nanorods Grown onto Stretchable Substrates for Optical Strain Sensing *Joseph Marae-Djouda1,2,3, Arthur Gontier1, Roberto Caputo5, Gaëtan Lévêque6, Bogdan Bercu4, Yazid Madi3, Guillaume Montay2, Pierre-Michel Adam1, Michael Molinari4, Stephen Stagon7, and *Thomas Maurer1 1. Laboratoire de Nanotechnologie et d’Instrumentation Optique, ICD CNRS UMR 6281, Université de Technologie de Troyes, CS 42060, 10004 Troyes, France 2. Laboratoire des Systèmes Mécaniques et d’Ingénierie Simultanée, Institut Charles Delaunay, ICD CNRS UMR 6281, Université de Technologie de Troyes, CS 42060, 10004 Troyes, France 3. Ermess, EPF- Ecole d’ingénieurs, 3 bis rue Lakanal 92 330 Sceaux, France 4. Laboratoire de Recherche en Nanosciences, Université de Reims Champagne-Ardenne, UFR Sciences Exactes et Naturelles, Moulin de la Housse, BP 1039, 51687 Reims Cedex 2, France 5. Department of Physics and CNR-NANOTEC, University of Calabria, 87036 Arcavacata di Rende (CS), Italy

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6. Institut d'Electronique, de Microélectronique et de Nanotechnologie (IEMN, CNRS8520), Cité Scientifique, Avenue Poincaré, 59652 Villeneuve d'Ascq, France 7. Mechanical Engineering Department, University of North Florida, Jacksonville, FL 32224, USA

KEYWORDS

Plasmonics; nanorods; smart skin; sensor; plasmomechanics; physical vapor deposition ABSTRACT

This study aims to investigate the potential of small densely packed tilted Au nanorods grown on a flexible substrate by physical vapor deposition for strain sensing. By exciting the rods with linearly polarized white light, perpendicularly impinging onto the sample substrate, interesting plasmonic properties emerge. Electron microscopy characterization shows that the rods are grown at a shallow angle relative to the substrate, as expected for glancing angle deposition conditions. Due to their non-orthogonal orientation, specific coupled multi-rod plasmon modes are detected for both longitudinal and transverse illumination under illumination normal to the substrate. In a second step, we have performed in-situ mechanical tests and showed higher sensitivity to the applied strain for longitudinal E-field directions, which more strongly affected by changes in inter-rod gaps than for transverse illumination. What is remarkable is that, despite the inherent disorder to this self-assembled system, clear features like polarization dependency and localized surface plasmon resonance (LSPR) wavelength shift with applied strains, may be observed due to local changes of the nanorods environment. These nanorod coated flexible


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substrates rank among the most sensitive plasmonic strain sensors in the literature and have potential to be embedded in real strain sensing devices.

Introduction The development of smart sensors has become a technological challenge which mobilizes substantial resources due to the huge promises of applications, especially for structural health monitoring1. In this technological rush, stretchable materials have been widely and intensively investigated for the past twenty years since they offer the possibility to be easily integrated into matter. In particular, elastomeric substrates appear as a material of choice to design efficient stretchable conductors which can act as strain sensors for large deformations monitoring, which is not achievable through commercial resistive strain gauges2. The latest developments in stretchable composite films focused on the integration of advanced materials, like carbon nanotubes or graphene sheets, with the aim to embed these strain sensors or electronic photodetectors into skin for human breathing monitoring3–9. Very recently, pioneering groups developed colored elastomeric substrates supporting metallic nanoparticles (NPs) as a disruptive alternative to design a novel generation of strain sensors. The working principle of the first prototype, developed by Liz-Marzan and co-workers, was based on a drop of the light absorption when the substrate is stretched and less NPs are illuminated10. Very quickly, other groups (including ours) investigated color-changing stretchable films exploiting the variation of the plasmonic coupling between NPs when strains, applied to the substrates, lead to NPs displacements11–16. Finally, by embedding free-standing nanorod arrays into an elastomeric thin film, it has been demonstrated that suitable excitation conditions (off-normal incidence) can induce both long and short axis plasmon oscillations and can also be exploited for strain sensing applications17. These pioneering works lead to the emergence of a new field which


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can be identified as “plasmomechanics” and whose essence is based on both the use of mechanical stress to investigate plasmonic coupling and on plasmonic coupling monitoring to detect strains applied to the substrate12. In this research article, the latest advances in metallic nanostructure synthesis techniques are exploited to realize a novel large-area elastomeric tunable plasmonic device that can be efficiently involved

in

sensing

applications.

The system

is

made of

a

flexible

polydimethylsiloxane (PDMS) substrate18, supporting out-of-plane and tilted Au nanorods. These nanorods are closely-packed and among the smallest ever grown onto stretchable substrates with a length of about 100nm and a diameter of about 20nm. As shown in the following from comparison between measurements and numerical simulations, this unique geometry results in the possible excitation, at normal incidence, of several coupled plasmon modes simultaneously, conferring the system’s high potential for sensing. Considering the fundamental side of the research, the nanorods anisotropy induces a nanoscale coupling between individual plasmon modes, resulting in gap-sensitive modes between nearest neighboring nanorods, whose response to local environment variations is evidenced by in-situ tensile tests of the sample. This induced sample deformation can be controlled in a continuous and accurate way offering thus the possibility to measure the slightest change in plasmon coupling between the rods.

Experimental section


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Figure 1. a) Fabrication setup. b) Modeled result of the grown gold nanorods on PDMS with glancing angle physical vapor deposition.

As depicted on figure 1, aligned Au nanorods samples on PDMS substrates were fabricated using glancing angle physical vapor deposition. First, 5 cm x 5 cm x 1 mm PDMS substrates (McMaster-Carr Co.) were ultrasonically cleaned in isopropanol and di-ionized water and then allowed to dry in air. The substrates were then placed onto a precision glancing angle holder 20


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cm from the source and positioned at an angle of 87° relative to the source normal. Au source material (99.99% Kurt J. Lesker Co.) was placed into a tungsten boat at the base of the vacuum chamber, which is a cylinder with a diameter of 40 cm and a height of 50 cm. The chamber was pumped down to a base pressure of 1 x 10-4 Pa and the working pressure during deposition was ~ 1 x 10-3 Pa. Au was then deposited using the thermal evaporation source to a nominal thickness of 1000 nm, measured normal to the source via quartz crystal microbalance, and the deposition rate was controlled at 0.5 nm/s. Images of the sample were acquired at different magnification using a scanning electron microscope (SEM JEOL 7900F Prime) in low vacuum mode and with a beam energy of 1.5 keV to avoid charging effects due to the substrate. Statistics were performed on different area of the samples to get reliable sizes for the nanorod diameters, spacing tilting angle and lengths. The core of this study is the optical characterization of the above detailed sample executed by performing extinction spectroscopy measurements with in-situ tensile tests. Considering the specificity of the experiment, a homemade setup has been exploited for performing the extinction cross-section measurements (Figure 2). The setup is constituted by an illumination part, where a white light source, polarized and connected to an objective lens (50x, 0.42 NA), represents the excitation source. The sample is mounted on a traction micro-machine, mounted itself on the sample holder and thus enabling optical measurements during the tensile test. The sample holder has been exactly fabricated to the scope and as well as embedding the traction micro-machine, it can also allow angle-resolved measurements19. The light is collected after the sample by an objective (20x, 0.28 NA); a beam splitter is used to direct 8% of the collected light to a CCD camera for the identification of the area under investigation; the remaining part of collected light (92%) is transmitted to the spectrophotometer via a multimode optical fiber.


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Figure 2. a) Schematics of the optical extinction spectroscopy experimental set-up; b) Photograph of the optical extinction spectroscopy set-up; c) A high-precision traction machine controls the elongation of the sample during the tensile tests. Results and discussion A typical SEM top view image of the sample (fig. 2a) at low magnification shows the homogeneity of the nanorods at large scale while the inset shows a magnified image of the


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nanorods grown on the PDMS substrate. The nanorod diameters are between 20 and 25nm with a mean diameter of 23nm while the spacing between nanorods are between 33 and 38 nm with a mean spacing of 36nm as seen of Figure 3b. While looking at tilted pictures of the nanorods as in Figure 3c, the general morphological structure of the sample appears like a “brush” made of tilted nanorods exhibiting an angle ψ different from 0° when measured with respect to the sample normal and a measured length in the order of 100nm. Moreover, the closely-packed nanorods show an average in-plane component at an angle φ=45° with respect to the sample main directions (xy, being respectively the directions parallel and perpendicular to the applied elongation). The shallow angle relative to the substrate is expected for glancing angle deposition conditions20. In the inset of the Figure 3c, two photographs of the considered large-area sample are reported, respectively representing its appearance in reflection (R) and transmission (T); the plasmonic behavior of the sample emerges from its color under different illumination conditions. In theory it would be very useful to have a visual proof of the gap change while stretching the substrate. However this is experimentally not feasible since we are dealing with gap changes of few nanometers. It is unfortunately too challenging to perform this analysis due to the nonconductive substrate that is not an ideal condition for an accurate extreme-resolution SEM analysis. However, it must be highlighted that this is also the interest of such a study : putting into evidence that use of polarized-dependent optical measurements allows deducing information about the tilt of the nanorods but also about the gap change in-between the rods when strains are applied to the substrate. Therefore, optical measurements provide information for smaller nanorods which is not achievable using SEM or AFM. That is why optical extinction spectroscopy with the sample at rest has been performed by illuminating it at normal incidence and by exploiting linear polarization with five different angles


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of the exciting electric field (ψ = 0°, 30°, 45°, 60°, 90°, inset of Figure 3d) measured with respect to the x axis (elongation direction).

Figure 3. a) Top view SEM image of the Au nanorods array grown onto a PDMS substrate using PVD; b) Statistical values of the nanorod diameter and spacing; c) side view SEM picture of the nanorod array; Inset: R and T respectively represent the reflection and transmission photographs of the considered sample; d) Extinction spectra obtained with the sample at rest for different polarization directions of the exciting field; Inset: excitation geometry of the Au nanorods array. Several polarization directions of the exciting electric field have been indicated with different colors.


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By varying the polarization angle of the exciting field vector, considerable differences result in the measured extinction spectra (Figure. 3d). This would not be the case for perfectly aligned and homogeneously distributed vertical nanorods (θ=0°) that would be ideally polarization independent. Therefore, the presence of an average tilt (θ angle, inset of Figure 3b) of the nanorods with respect to the normal to the sample substrate (z axis) represents an interesting opportunity because it allows the excitation of a longitudinal plasmon mode (e.g. along the nanorod long axis), which is observed in the spectra at larger wavelengths and detailed as in the following. The features of the obtained spectra deserve some consideration. Similar behavior is observed in case of 0° and 90° linear E-field directions. Common features (but different than in 0° and 90° cases) are also obtained for intermediate directions (30°, 45° and 60°). This result confirms the average orientation φ=45° of the nanorods deducted from the SEM analysis. The prominent peak observed at about 560nm in both 0° and 90° curves (respectively red and black curves in Figure 3b) evidences the excitation of a transverse mode of the rods grating, excited by the component of 0° and 90° fields in the direction perpendicular to the rods axis. In the assumption φ=45°, these two polarization directions are symmetric compared to the average nanorods direction, which is most probably the reason of the similarity in the spectral response of the sample observed for these excitations. In the case of intermediate polarizations, a peak at about 670nm is quite pronounced while the transverse peak is less evident. The larger value of the spectral position of this peak (670nm) and the polarization directions close to 45° support the excitation of a longitudinal plasmon mode of the nanorods distribution. Finally, in almost all excitation directions considered, a third peak with lower amplitude is observed, as a shoulder in the curves, at about 830nm, which nature is to be elucidated. Notice that due to the fact that the grating is


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very dense, plasmon modes of single nanorods are strongly modified by the interaction with the other particles, which is why it is preferable to talk about longitudinal and transverse plasmon modes of the grating as a whole. In order to further investigate the plasmonic properties of that system, finite element method (Comsol Multiphysics) was used to perform numerical simulations (Figure 4). The structural parameters were taken from the SEM characterization i.e. the dense brush is modeled as a triangular lattice of period L=36 nm composed of gold nanorods with length of 100nm and diameter of 23nm. The nanorods orientation is characterized in spherical coordinates by the polar angle θ=53° and the azimuthal angle φ’=30° measured from the grating principal direction x’ (see Figure 4a). For convenience, we introduce the plane Π, normal to the substrate interface and containing the nanorod axis. Due to the inclination of the nanorods and the symmetry of the system, each nanorod is separated from four closest neighbors by a gap of 3nm. The system is illuminated from the air by a normally incident planewave with polarization directions indicated by colored lines on Figure 4b, varying from longitudinal illumination (solid magenta) to transverse illumination (dashed orange) by steps of 15°, where solid lines correspond to the direction of polarization used in the experiment. Notice that the simulation x’y’z frame is rotated by 15° around the z axis compared to the experimental xyz frame (see Figure 4b). Despite the idealized geometry compared to the real system, we obtain a good agreement between experimental and numerical extinction spectra, which follow similar evolution with the polarization direction ψ, see Figure 4c. Notice that the maximum in absorption is slightly redshifted from the extinction one in longitudinal illumination, indicating a Fano-type interference between the field scattered by the nanorods and the transmitted incident field. However, both


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longitudinal and transverse maxima appear blue shifted compared to the experiment, and the shoulders observed in the experiments around 800-850nm are not reproduced here.

Figure 4. a) View of one nanorod in the simulation frame, x’y’z. Axes x’ and y’ are rotated by 15° from the experiment axes x and y of Fig. 2(a); b) Top view of the periodic system of tilted gold nanorods. The experimental frame xy is indicated by gray dashed arrows. The colored lines indicate the polarization of the normally incident electric field: solid magenta is longitudinal, dashed orange is transverse, the angle step is 15°. Solid arrows show the experimental angles with corresponding colors; c) Extinction spectra, 1-T/T0, as a function of the polarization (colors correspond to (b)), where T is the transmission through the whole structure and T0 the transmission through the PDMS substrate only. The absorption spectra for longitudinal (resp. transverse) illumination is plotted in black (resp. gray) line; d) Top views of distributions of the


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surface charges and real part of the electric field (green arrows) along one nanorod: fundamental longitudinal mode (1), longitudinal mode first higher harmonic (2), and transverse mode (3).

The surface charges distributions plotted on Figure 4d bring useful information on the nature of the excited modes. Two modes, (1) and (2), appear in longitudinal illumination, with charges distribution symmetric compared to the Π plane. The light is essentially confined in the overlapping part between nearest nanorods, of extent about 50nm, the mode (1) being characterized by a more rapid oscillation of the surface charge along the Π plane than mode (2). Hence, the mode (2) is the fundamental longitudinal mode of the system while the mode (1) is the first higher harmonic. For transverse illumination (orange dashed line in Figure 4c), charges are antisymmetric compared to the Π plane, and are again enhanced in the overlapping area between nanorods, without any change of sign along the Π plane. In order to verify how the flexibility of the substrate could be exploited for sensing applications, the sample was mechanically stretched along the x direction up to an elongation of about 30%. Figure 5 shows the spectra recorded at the initial step (Figure 5a) and after 30% of substrate elongation in the x direction (Figure 5b) for the previous electric field excitations. The unique plasmon modes of the system, whose natures have been identified above through the simulation, progressively vary with the mechanical manipulation. It is clearly evidenced in Figures 5a,b that the wavelengths of the longitudinal and transverse modes change, respectively. These results suggest that the mechanical manipulation leads to a change of the geometrical nanoscale configuration of the Au nanorods deposited on the PDMS substrate. Unfortunately, the insulating nature of the PDMS substrate and the nanometric size of the rods and the gaps between them prevent from accessing to such information from SEM or AFM images. However, as discussed above, number and kind of excited modes, observed in the measured spectra,


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depend on the polarization direction of the exciting field. Useful information can thus be obtained by analyzing the change of spectral position and concavities of the modes in the optical spectra upon a 30% elongation of the substrate. In the following, the evolution in each polarization will be considered in detail.

Figure 5. a) Extinction spectra obtained with the sample at rest for different polarization directions of the exciting field (as reported in Fig. 2b); b) Extinction spectra obtained after 30% of the substrate elongation in the x direction for the same polarization directions of the exciting field. In the insets, pictorial views are reported of the PDMS substrate containing the array of Au nanorods, at rest and under stretching, and the involved excitation geometry with different polarization directions.

Figure 6 presents the evolution of the plasmon modes as a function of the sample elongation in case of 0° (Figure 6a) and 90° (Figure 6b) exciting field directions. In both cases, the most noticeable variation is related to the transverse mode. The spectral variation of this mode is weak and mainly takes place for the 0° linear polarization direction as a blueshift (∆λ=-16nm) between the condition of the sample at rest and after 30% of elongation of the substrate (Figure 6c, red


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curve). For the 90° linear polarization direction, the spectral variation of this mode upon stretching is almost negligible. In both (0° and 90°) configurations, the extinction intensity decreases with the elongation. This decrease is consistent with the variation of the Au nanorods density that diminishes while elongating the substrate. This intensity evolution has been used in past as a means for following the sample deformation4. In the present study, the quantification of the plasmon mode evolutions and the sensitivity evaluation are the main goals. The different spectral sensitivity to the applied strains (Figure 6c) may be related to the close packing of the nanorods on the flexible substrate and will be carefully considered in the following. The evolution of the extinction response, with applied elongation, is also reported for the three intermediate polarization directions: 30°, 45° and 60° (Figures 7a,b,c). When the electric field of the exciting light is closely aligned to the average nanorod axis direction, an evident excitation of the longitudinal mode takes place while the transverse mode is partially involved. This reflects in Figure 7d with a spectral shift of the longitudinal mode estimated in about ∆λ=60nm, for the case of 30° and 60° intermediate polarization directions, and ∆λ=50nm for the 45° curve. From Figures 6a,b,c, no visible shift of the transverse mode can be appreciated. A comparison of curves in Figures 7 and 6 evidences instead a much more sensitive response of the longitudinal mode (excited by intermediate polarization directions) than that of the transverse mode (excited by 0° and 90° electric fields). The key question remains to understand why the longitudinal mode is much more sensitive to the applied strains than the transverse mode. In order to answer this question, the effect of the elongation has been simulated numerically using the following method.


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Figure 6. Evolution of the extinction spectra as a function of the applied elongation at 0° (a) and 90° (b) polarization directions of the exciting field. (c) Spectral dependency of the LSPR wavelength as a function of the macroscopic elongation for both the 0° and 90° linear polarization directions.

First, the locations of the contact points between nanorods and the substrate have been modified supposing the deformation is applied along the x axis (rotated from the x’ axis by an angle of -15°) in order to match the experimental configuration. The corresponding deformation of the PDMS interface is given by the following linear transformation expressed in the x’y’ plane:

′ ′ ′ ′

where δ is the elongation along the x’ direction, ν the Poisson’s ratio (0.5 for PDMS), and (x’0, y’0) and (x’δ, y’δ) are the coordinates of one point of the x’y’ plane before and after deformation.


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Figure 7. Spectra evolution in intermediate polarization directions (a) 30°, (b) 45° and (c) 60° with applied elongation; (d) Evolution of the resonance wavelength with the elongation for the three polarization directions.

Second, we suppose that the Π plane, which contains the nanorod axis, follows the deformation of the substrate interface. The deformation of the system is illustrated in top-view on Figure 8 for an elongation of 30%, the red arrow showing the direction of the elongation.


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Figure 8. Top view of the shape of the array corresponding to the position and orientation of the nanorods, for δ=0%, (a), and δ=30%, (b), elongation. The red arrow indicates the direction of the applied elongation.

Figure 9 shows the evolution of the extinction spectra with the deformation (δ=0% to 30% by step of 6%, bottom to top) for polarization ψ=0° (a), ψ=45° (b), and ψ=90° (c), which correspond to the experimental configuration of Figures 6a,b and Figure 7b. Notice that when the elongation increases, the nanorods are not aligned with the ψ=45° polarization anymore, as the ψ angle is measured from the direction of the applied deformation and that the Π plane rotates. For ψ=0°, we observe, similarly to the experiment (Fig.5b), a slight blue shift (∆λ=-10nm) of the transverse extinction maximum, which dominate the spectrum whatever the elongation. For the ψ=90° polarization, symmetric of the ψ=0° polarization compared to the average orientation of the nondeformed nanorods array, the transverse mode shift is barely noticeable, mostly hidden by the signature of the longitudinal modes whose amplitude grow with larger deformations. Again, those different behaviors are similar to the experimental results, see Fig.5b. For ψ=45°, we find a clear red shift (from 650nm to 700nm) with the elongation of the extinction maximum associated to the fundamental longitudinal mode, similarly to the experimental results, while the absorption maximum shifts from 670 to 730nm. The surface charges distributions bring useful information


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on the origin of those shifts. Notice first that, when the elongation increases from 0% to 30%, the gap between the closest neighbors decreases from 3nm to 1nm. As a consequence, the field is more closely confined close to the gap area compared to the non-deformed system. Second, while for δ=0%, each nanorod is surrounded by four closest neighbors, when the elongation increases two of them gets closer while the two others move further away (with a corresponding gap of 8.2nm for δ=30%). Then, with increasing elongation, the interaction changes from a fourneighbor interaction with gap of 3nm to a two-neighbor interaction with a gap of 1nm. The decrease of gap is clearly the origin of the red-shift of the longitudinal modes, while the blueshift of the transverse mode might be attributed to the decrease of the number of nearest neighbors. Notice finally that gaps are all above 1nm, which implies that quantum tunneling effects, which takes place for gap smaller than 0.5nm21, does not need to be taken into consideration in those simulations.


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Figure 9. Simulated extinction spectra of the nanorods array as a function of the deformation δ=0%, 6%, 12%, 18%, 24%, 30% (bottom to top) for an incident polarization angle (measured from the deformation direction, similarly to the experimental configuration) of: ψ=0° (a), 45° (b), and 90°(c); absorption spectra are plotted for maximal elongation in dashed line on top; d) Top views of distributions of the surface charges and real part of the electric field along one nanorod for maximal elongation (δ=30%): fundamental longitudinal mode (1), longitudinal mode first higher harmonic (2), and transverse mode (3).


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Hence, we believe that our numerical simulations capture the basic physics of our system and adequately capture the modifications it undergoes under elongation. The closely coupled nanorods support two types of modes, characterized by enhanced electric field close to the narrow gaps between closest neighbors. Longitudinal modes are excited by the component of the incident electric field parallel to the Π plane, containing the nanorod axis. Those modes are mostly symmetric compared to that plane and are composed of a fundamental mode of longer wavelength and harmonics of shorter wavelengths. When the interparticle gap decreases, the longitudinal modes shift to the red and the number of excitable harmonics is larger. Transverse modes are mostly antisymmetric compared to the Π plane. The blue shift observed for increased elongation cannot be attributed to the smaller gap as it should induce a red-shift. We suspect that it results from the fact that the number of interacting neighbors decreases with the elongation, but this should be investigated further, which is beyond the scope of this article. The main discrepancies between experimental and numerical results are the shape of the extinction curve (the minima due do the Fano interferences are not visible in the experiment) and the location of the modes which are red-shifted compared to the simulation. Those discrepancies most likely originate from the disorder characterizing the sample: the nanorods are distributed around average values in size, orientation and position. Hence the experimental curves might be the result of the superposition of curves similar to the numerical ones but averaged on geometrical parameters, thus smoothing the Fano minima. Besides, one of the key parameters in the formation of the modes is the gap between closest nanorods, which is on the nanometer scale. A small variation in gap induces an appreciable variation in the longitudinal mode resonance wavelength. Hence, it is reasonable to suppose that the average gap between two nanorods is shorter than 3 nm, in such a way that the fundamental longitudinal mode resonates around 800-


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850 nm, the first higher harmonic should at 630 nm and the transverse mode around 575 nm. Owing to the large number of degrees of freedom that the idealized system possesses, it is very difficult to find optimum geometrical parameters which behave consistently with the deformation, while a simulation taking into account disorder would be very demanding in terms of computing resources. Another possible question is the role of very small gaps of few Angstrom which might probably exist as well in the real system, but are subject to quantum tunneling between nanorods, and what is their contribution to the optical spectra. These preliminary results indicate that such an elastomeric film could be an efficient system to monitor large applied strains. For resistive strain gauges, it is relatively common to introduce the Gauge Factor K defined as:

(1)

where R and ∆R respectively are the sensor resistance and its variation when a deformation rate ε is applied. Commercial resistive gauges usually provide gauge factors K values around 2. In order to make an analogy with the resistive gauges efficiency, we introduce here a similar plasmonic gauge factor K’ defined as:

FWHM

(2)

where ∆λ and FWHM are respectively the LSPR wavelength shift and Full-Width HalfMaximum of a single nanorod when a deformation rate ε is applied. Notice that the Au nanorods are initially coupled and the initial values of the plasmon modes wavelengths do not correspond to the ones of single nanorods. Values up to 2 are obtained which seems to be particularly reasonable for a sensor which is ideally suited for large strain monitoring. Besides, Figure 7d reveals a spectral shift for the longitudinal mode of 2nm per 1% change in strain which is higher than in the study recently presented by Y. Wang et al.22, where the author stated that 1.62nm per


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1% change in deformation in the visible range was ranked among the most sensitive straintunable plasmonic devices. Therefore, these brushes of tilted nanorods offer promising perspectives for strain sensing. This is indeed a real step forward in comparison with the pioneering work of L. Liz-Marzan and coworkers(Correa-Duarte et al. 2007) ten years ago since such strain sensors are not based on absorption drop but on LSPR shifts which are measurable via portative spectrometer and can be linked to applied strains.

Conclusions The present study represents a first characterization of dense brushes of tilted Au nanorods supported by a flexible substrate considered for sensing applications. The performed experiments demonstrated a variety of intriguing plasmonic properties of the system. Indeed, as revealed by exploiting a linear polarization excitation with several E-field directions, these nanorods are not vertical (as initially expected) but instead tilted to the substrate normal, which allows for the formation of both transverse and longitudinal grating modes, excitable under normal light incidence. Moreover, due to the high density of the Au nanorods brush, these modes are both coupled and their wavelength is thus dependent on the distance between the rods. Furthermore, the optical measurements were coupled with mechanical tests performed in-situ. These measurements clearly evidenced that the most sensitive mode for strain sensing is precisely the longitudinal mode. Despite the high geometric complexity of the system, our numerical simulations confirmed and gave insight to trends observed in the experiments. The key feature for explaining such high sensitivity is (1) the tilt of the rods which allows the excitation of the longitudinal modes and (2) the high density of the rods which leads, through plasmonic coupling, to a strong inter-rods distances dependency. Therefore, this emphasizes the high potential for


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strain sensing of out-of-the substrate grown nanorods. The sensitivity was indeed assessed to about 2nm per 1% change in strain which ranks such a sensor among the most sensitive in the literature, opening the way for further investigation on Au nano-objects standing out-of-the substrate. It must be emphasized that the unique plasmonic signatures observed for different exciting polarization directions represents a valuable possibility for technological outcomes in new generation multi-functional plasmonic sensors. Indeed, such N-particle systems, in spite of the inherently disordered nanorods organization, exhibit ultimate nanogaps that, under applied strains, result in extremely sensitive polarization dependency and LSPR wavelength shift thus conferring on the macroscopic device functionalities beyond the state-of-the-art. One valuable research axis in the next future will be the investigation of longer nanorods in order to probe if the coupling between longitudinal modes and thus the sensitivity to the applied strains could be reinforced. Finally, it must be underlined that such optical nanosensors may offer a real added-value for strain sensing in comparison with other types of strain sensors(Lacour et al. 2003), in particular resistive stretchable sensors among which those made of graphene or carbon nanotubes (Darabi et al. 2015; Chiang et al. 2015; Tang et al. 2015) which belong to the state-of-the-art strain gauges. As a matter of fact, the technology presented here exhibit several key advantages : (i) this is a non-contact technology and does not require any connection to an acquisition system, thus giving the possibility to monitor strains suffered from pieces into rotation for instance, (ii) it provides access to large strains (>1%) monitoring contrary to most conventional technologies and (iii) it gives access to the strain directions through polarized measurements. For these three reasons, it appears that such an innovative technology will give solutions to a niche market for which there is no satisfactory technological solutions for the moment.


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* Corresponding authors: Joseph Marae-Djouda email: [email protected], Thomas Maurer email: [email protected] Funding Sources ACKNOWLEDGMENT The authors thank the “Conseil régional Champagne-Ardenne” and the “Fonds Européen de Développement Régional (FEDER)” for funding the MecaOpt essaimage program. Financial support of NanoMat (www.nanomat.eu) by the “Ministère de l’enseignement supérieur et de la recherche” and the “Conseil général de l’Aube” is also acknowledged. T. M thanks the Labex ACTION project (contract ANR-11-LABX-01-01) and the CNRS via the chaire « optical nanosensors » for financial support. ABBREVIATIONS LSPR, localized surface plasmon resonance; NP, nanoparticle; PDMS, polydimethylsiloxane; SEM, scanning electron microscope; PVD, physical vapor deposition; AFM, atomic force microscope; FWHM, full width half maximum. REFERENCES (1) Spencer, B. F.; Ruiz-Sandoval, M. E.; Kurata, N. Smart Sensing Technology: Opportunities and Challenges. Struct. Control Health Monit. 2004, 11, 349–368. (2) Lacour, S. P.; Wagner, S.; Huang, Z.; Suo, Z. Stretchable Gold Conductors on Elastomeric Substrates. Appl. Phys. Lett. 2003, 82, 2404–2406.


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(3) Darabi, M. A.; Khosrozadeh, A.; Wang, Q.; Xing, M. Gum Sensor: A Stretchable, Wearable, and Foldable Sensor Based on Carbon Nanotube/Chewing Gum Membrane. ACS Appl. Mater. Interfaces 2015, 7, 26195–26205. (4) Chiang, C.-W.; Haider, G.; Tan, W.-C.; Liou, Y.-R.; Lai, Y.-C.; Ravindranath, R.; Chang, H.-T.; Chen, Y.-F. Highly Stretchable and Sensitive Photodetectors Based on Hybrid Graphene and Graphene Quantum Dots. ACS Appl. Mater. Interfaces 2016, 8, 466–471. (5) Yoon, S. G.; Koo, H.-J.; Chang, S. T. Highly Stretchable and Transparent Microfluidic Strain Sensors for Monitoring Human Body Motions. ACS Appl. Mater. Interfaces 2015, 7, 27562–27570. (6) Tang, Y.; Zhao, Z.; Hu, H.; Liu, Y.; Wang, X.; Zhou, S.; Qiu, J. Highly Stretchable and Ultrasensitive Strain Sensor Based on Reduced Graphene Oxide Microtubes–Elastomer Composite. ACS Appl. Mater. Interfaces 2015, 7, 27432–27439. (7) Son, D.; Lee, J.; Qiao, S.; Ghaffari, R.; Kim, J.; Lee, J. E.; Song, C.; Kim, S. J.; Lee, D. J.; Jun, S. W.; Yang, S.; Park, M.; Shin, J.; Do, K.; Lee, M.; Kang, K.; Hwang, C. S.; Lu, N.; Hyeon, T.; Kim, D. H. Multifunctional Wearable Devices for Diagnosis and Therapy of Movement Disorders. Nature Nanotechnology 2014, 9, 397. (8) Benight, S. J.; Wang, C.; Tok, J. B. H.; Bao, Z. Stretchable and Self-Healing Polymers and Devices for Electronic Skin. Progress in Polymer Science 2013, 38, 1961–1977. (9) Yamada, T.; Hayamizu, Y.; Yamamoto, Y.; Yomogida, Y.; Izadi-Najafabadi, A.; Futaba, D. N.; Hata, K. A Stretchable Carbon Nanotube Strain Sensor for Human-Motion Detection. Nature Nanotechnology 2011, 6, 296.


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(10) Correa-Duarte, M. A.; Salgueiriño-Maceira, V.; Rinaldi, A.; Sieradzki, K.; Giersig, M.; Liz-Marzán, L. M. Optical Strain Detectors Based on Gold/Elastomer Nanoparticulated Films. Gold Bull 2007, 40, 6–14. (11) Zhu, X.; Shi, L.; Liu, X.; Zi, J.; Wang, Z. A Mechanically Tunable Plasmonic Structure Composed of a Monolayer Array of Metal-Capped Colloidal Spheres on an Elastomeric Substrate. Nano Res. 2010, 3, 807–812. (12) Maurer, T.; Marae-Djouda, J.; Cataldi, U.; Gontier, A.; Montay, G.; Madi, Y.; Panicaud, B.; Macias, D.; Adam, P.-M.; Lévêque, G.; Bürgi, T.; Caputo, R. The Beginnings of Plasmomechanics: Towards Plasmonic Strain Sensors. Front. Mater. Sci. 2015, 9, 170–177. (13) Cataldi, U.; Caputo, R.; Kurylyak, Y.; Klein, G.; Chekini, M.; Umeton, C.; Bürgi, T. Growing Gold Nanoparticles on a Flexible Substrate to Enable Simple Mechanical Control of Their Plasmonic Coupling. J. Mater. Chem. C 2014, 2, 7927–7933. (14) Sannomiya, T.; Hafner, C.; Vörös, J. Strain Mapping with Optically Coupled Plasmonic Particles Embedded in a Flexible Substrate. Opt. Lett., OL 2009, 34, 2009–2011. (15) Sarrazin, A.; Gontier, A.; Plaud, A.; Béal, J.; Yockell-Lelièvre, H.; Bijeon, J.-L.; Plain, J.; Adam, P.-M.; Maurer, T. Single Step Synthesis and Organization of Gold Colloids Assisted by Copolymer Templates. Nanotechnology 2014, 25, 225603. (16) Caputo, R.; Cataldi, U.; Bürgi, T.; Umeton, C. Plasmomechanics: A Colour-Changing Device Based on the Plasmonic Coupling of Gold Nanoparticles. Molecular Crystals and Liquid Crystals 2015, 614, 20–29.


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(17) Böhm, M.; Uhlig, T.; Derenko, S.; Eng, L. M. Mechanical Tuning of Plasmon Resonances in Elastic, Two-Dimensional Gold-Nanorod Arrays. Opt. Mater. Express, OME 2017, 7, 1882– 1897. (18) Niu, X.; Stagon, S. P.; Huang, H.; Baldwin, J. K.; Misra, A. Smallest Metallic Nanorods Using Physical Vapor Deposition. Phys. Rev. Lett. 2013, 110, 136102. (19) Marae-Djouda, J.; Caputo, R.; Mahi, N.; Lévêque, G.; Akjouj, A.; Adam, P.-M.; Maurer, T. Angular Plasmon Response of Gold Nanoparticles Arrays: Approaching the Rayleigh Limit. Nanophotonics 2017, 6, 279–288. (20) Barranco, A.; Borras, A.; Gonzalez-Elipe, A. R.; Palmero, A. Perspectives on Oblique Angle Deposition of Thin Films: From Fundamentals to Devices. Progress in Materials Science 2016, 76, 59–153. (21) Scholl, J. A.; García-Etxarri, A.; Koh, A. L.; Dionne, J. A. Observation of Quantum Tunneling between Two Plasmonic Nanoparticles. Nano Lett. 2013, 13, 564–569. (22) Wang, Y.; Liu, L.; Wang, Q.; Han, W.; Lu, M.; Dong, L. Strain-Tunable Plasmonic Crystal Using Elevated Nanodisks with Polarization-Dependent Characteristics. Applied Physics Letters 20160215.

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(d) ACS Paragon Plus Environment

Dense Brushes of Tilted Metallic Nanorods Grown onto Stretchable

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