Controlling the Nanocontact Nature and the Mechanical Properties of

Article Cite This: J. Phys. Chem. C 2017, 121, 23769-23776

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Controlling the Nanocontact Nature and the Mechanical Properties of a Silica Nanoparticle Assembly J. Avice,†,‡ C. Boscher,‡ G. Vaudel,† G. Brotons,† V. Juvé,† M. Edely,† C. Méthivier,§ V. E. Gusev,∥ P. Belleville,‡ H. Piombini,‡ and P. Ruello*,† †

Institut des Molécules et Matériaux du Mans, UMR CNRS 6283, Le Mans Université, 72085 Le Mans, France Commissariat ł’Energie Atomique et aux Energies Alternatives, Centre du Ripault, Monts, France § Laboratoire de Réactivité de Surface, UMR CNRS 7609, LRC-CEA/UPMC/CNRS no 1, 4 Pl. Jussieu, Université Pierre et Marie Curie, 75252 Paris, France ∥ Laboratoire d’Acoustique, UMR CNRS 6613, Le Mans Université, 72085 Le Mans, France ‡

S Supporting Information *

ABSTRACT: Elaborating advanced nanomaterials based on the assembly of nanoparticles (NPs) is a versatile route for targeting and tuning a wide variety of properties like optical, magnetic, and electrical properties or sensing. This route usually employs a so-called soft chemistry which has the advantage of being quite cheap and transferable to an industrial level. However, getting quantitative information on the quality of the mechanical consolidation of the nanoparticle assembly (ordered or disordered) in a nondestructive manner is not often achieved, although it is crucial for applications and integration of materials in devices. Therefore, we present in this Article a complete method where we evaluate the elasticity of weakly (van der Waals nanocontacts) to strongly (covalent-hydrogen nanocontacts) interacting nanoparticle assemblies. This complete work is realized on a disordered silica nanoparticle network obtained by the sol−gel method. A precise control of the chemical and physical properties of the nanoparticle surface molecular landscape is achieved thanks to infrared, visible, and ultraviolet spectroscopies as well as surface tension measurements and atomic force microscopy, while the nanoparticle assembly elastic stiffness is evaluated by ultrafast nanoacoustics based on an optical pump−probe method.

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spectroscopic measurements,3−5 but the strength of the nanocontacts and the overall elasticity of the NP network are not evaluated usually.3−5 Besides the importance of the mechanical integrity of the NP assemblies,10,11 the quality of the nanocontact governs many other properties such as electronic12,13 or thermal transport.14 The nanocontact elastic stiffness can be studied with spectroscopic methods such as Brillouin15 or Raman spectroscopies.16,17 While the spectroscopic methods are based on the analysis of thermally excited vibrations in the solids, the ultrafast optic methods offer the possibility to excite and detect with light in an efficient way some mechanical eigenmodes of nanomaterials and nanostructures. This so-called ultrafast nanoacoustics is known indeed as a powerful method to evaluate the elasticity at nanoscale and has already been used to probe nanostructured materials11,18,19 and colloidal nanostructures.11,20−23 However, to get a full understanding of the

INTRODUCTION Nanomaterial engineering based on the assembly of nanoparticles (colloidal films) permits many properties of advanced functionalized film and smart coatings to be tailored.1−5 By adjusting the nanoparticle (NP) volume fraction and the nature of the nanoparticle interconnection, the physical and chemical properties of colloidal films can be designed for specific applications in optics, plasmonics, and photovoltaics.3 These properties have also been studied as a function of the geometry of the packing, and the role of the nanoparticle/nanocrystals shape has been previously deeply studied.3,6,7 While continuous efforts have been made to optimize the growth and the control of the properties of these nanoparticle assemblies,3−5 including the recent programmable CND-coated assemblies,8,9 it is admitted that nanoparticle assemblies suffer from a poor mechanical reliability and durability. Despite this situation, the control of the physical and chemical nature of the nanocontacts on the collective mechanical integrity has been poorly evaluated3 although crucial for future integration in any devices and smart coatings. The nature of the chemical bonds involved in the nanocontact is well identified with traditional © 2017 American Chemical Society

Received: August 23, 2017 Revised: September 29, 2017 Published: September 29, 2017 23769

DOI: 10.1021/acs.jpcc.7b08404 J. Phys. Chem. C 2017, 121, 23769−23776

Article

The Journal of Physical Chemistry C

Figure 1. Nanoparticle assembly prepared as a thin film. Formation of a disordered packing of silica nanoparticles with either (a) van der Waals (VdW) or (b) covalent-hydrogen bonds (CV-H). The process shown in part b is called the hardening process and is achieved thanks to the NH3 catalyser (see Methods).

medium favors the nucleation of hydrolyzed species, and the silica sol consists of monodispersed roughly spherical particules of 10 nm diameter. The pH of the final silica is 6 after distillation of NH3, its content is 4% in weight, and the sol viscosity is 1.2 cP. The NP assembly films are obtained by dip coating. The NP packing has been realized either with van der Waals nanocontacts or with covalent-hydrogen bonds. This chemical/physical modification of the nanoparticle surface was achieved thanks to a postprocessing by an ammonia catalyser based treatment allowing a modification of nanocontact bonds from van der Waals to hydrogen/covalent bonds.26,27 This process is called the hardening process in the following. The chemical route and the colloidal film formation are sketched in Figure 1. The assembly of NPs was prepared on various substrates depending on the techniques of characterization used. For IR vibrational spectroscopy, the assembly was realized on a transparent silicon substrate, for near UV−visible-near IR optical transmission interferometry, we used a silica substrate, and for the picosecond acoustic methods (see Picosecond Acoustic Methods for more details), the assembly of NPs was obtained on a thin chromium film (20 nm) that is deposited on a silicon substrate. This thin chromium film is used as a local optoacoustic nanotransducer required for realizing ultrafast nanoacoustic experiments.11,19,28 Film Thickness Measurements. The colloidal film thickness was determined from a spectral measurement in transmission (T) of a silica substrate coated by dip-coating (both silica surfaces were coated with the same colloidal film

relation between the macroscopic elastic modulus of thin colloidal films and the nanoparticle surface chemical landscape, simultaneous investigations are needed including spectroscopic, thermodynamic, and nanomechanical methods. In this Article, we unravel the complex mechanisms of consolidation of a silica NP network. By using a combination of techniques, we quantitatively evaluate the elastic modulus of the NP network and we show how it evolves as a function of the colloidal film surface tension (contact angle) in line with the evolution of the nature of the interparticle chemical bonds (IR spectroscopy). Finally, we also evaluate the role of defects and we evidence a crossover in the nanomechanical properties from a regime where nanocontacts control the network elasticity to a situation where submicrometer cracks significantly influence the colloidal network elastic stiffness. The association of several experimental methods demonstrates that it is possible to fully characterize and to control the chemical, physical, and mechanical properties of silica colloid based thin films. This method could be extended to any colloidal film.

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METHODS Sample Preparation. The silica colloids have been prepared following first a sol−gel process as described in previous studies,24 and the thin films are obtained with a dipcoating method.25 The NP silica suspension we have used is a sol of amorphous silica prepared by the base catalyzed (NH3) hydrolysis of distilled tetraethylorthosilicate in pure ethanol according to the well-known Stober method.26,27 The basic 23770


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Figure 2. Spectroscopic, optical, and contact angle analysis of the crossover from the van der Waals to the covalent-hydrogen bonded nanocontacts. (a) IR vibrational spectroscopy revealing the chemical change during the hardening process. (b) Histogram of the silica nanoparticles in solution (obtained by the dynamic light scattering method) showing that the distribution is centered on an average diameter of around 8−9 nm. Bottom images are typical contact angles observed for van der Waals (left image) and covalent-hydrogen (right) nanocontacts. (c) Contact angle as a function of the nanoparticle contact hardening (blue circle and red square symbols for the film of thickness H = 210 and 70 nm, respectively). A comparison of the contact angle obtained on various bare silica substrates submitted to the same hardening treatment is given (light blue and purple crosses and green stars). (d) Shrinkage of the hardened colloidal film evaluated by visible optical transmission interferometry (film thickness of H = 210 nm). The inset shows the evolution of the optical transmission spectrum with the blue shift of the interferometric fringes when the film shrinks.

amplifier scheme. Since the silica nanoparticle network is transparent to both the pump (830 nm) and probe (580 nm) beams, the femtosecond laser does not directly excite the NP assembly film. For this reason, the colloidal film was deposited on a thin chromium layer acting as a thermoelastic transducer to create acoustic nanowaves (this multilayer is sketched in Figure 3a). The use of such a thermoelastic nanotransducer is commonly employed to probe the elasticity of transparent nanomaterials.11,30 A typical signal of transient optical reflectivity is shown in Figure 3b. This signal is made of a sharp increase and decrease of the optical reflectivity in the time scale of 1 ps which corresponds to the optical excitation of the electrons of the chromium film, followed by a fast decay characteristic of the relaxation of the hot electrons. Then, oscillatory signals evidenced over more than 1 ns are the signature of the nanoacoustic waves in the thin NP assembly. In our particular case, we generate compression/dilatation acoustic waves in the chromium layer (thermoelastic optoacoustic transducer) that are transferred within the NP assembly film (Figure 3a). After propagating and reflecting at the different interfaces (air/colloid and colloidal film/ chromium layer), these acoustic waves induce some mechanical resonances of the NP assembly film ringing. The analysis of the mechanical resonances provides direct information on the NP assembly elastic properties as detailed later on.

thickness, i.e., side 1 and side 2). The transmission measurements of silica were made by a Perkin 900 spectrophotometer over 200−1500 nm. Following the standard approach,29 a full numerical fitting of the spectrum was performed to extract both the thickness and the refractive index values with a Cauchy model for the wavelength dependence of the refractive index. The fitting algorithm is given in detail in the Supporting Information with some examples of fits shown in Figures S1, S2, and S3 and Table S1. Picosecond Acoustic Methods. The ultrafast nanoacoustic method is based on the generation and detection of a short acoustic pulse having an acoustic wavelength of tens of nanometers suitable for probing nanomaterials. This is based on a 80 MHz repetition rate Ti:sapphire femtosecond laser. The beam is split with a polarizing beam splitter into pump and probe beams. The pump beam excites the system (generation of acoustic pulses), and the probe beam can follow in time how the acoustic pulse propagates within the nanostructure. It is somehow a nanoechography. All of the information is contained in the modification of the optical reflectivity (ΔR(t)) induced by the initial pump excitation. The timeresolved measurement signals (ΔR(t)/R) are obtained thanks to a mechanical delay stage (delay line) which enables a controlled arrival time of the probe pulse regarding the arrival of the pump pulse. The experiments were conducted with incident pump and probe beams perpendicular to the surface of the nanoparticle film, as shown in Figure 3a with a typical pump and probe spot diameter of 10 μm. The pump beam is modulated in intensity which permits a small transient optical signal (ΔR/R ∼ 10−5/10−4) to be extracted with a lock-in

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RESULTS AND DISCUSSION The modification of the nature of the chemical bonds on the nanoparticle surface during the VdW−CV-H crossover has been first evaluated by IR spectroscopy, as shown in Figure 2a. 23771


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Figure 3. Ultrafast nanoacoustics. (a) Principle of the pump−probe optoacoustic method. The pump laser energy is absorbed in the chromium layer from which acoustic nanowaves are emitted and induce a mechanical resonance vibration of the NP assembly. This resonance modifies the optical reflectivity (ΔR/R) that is assessed by the delayed probe laser pulse. (b) Typical transient optical reflectivity signal (ΔR/R) revealing the fast electronic response coming from the chromium transducer followed by the oscillatory components coming form the collective mechanical resonance of the NP assembly. (c) Evolution of the coherent acoustic phonon signals when the nanocontacts are changed from van der Waals bonds (no hardening process) to covalent and hydrogen bonds (long hardening time). (d) A typical fast Fourier transform (FFT) done on the oscillatory component of the transient optical reflectivity signal revealing the two first mechanical resonance eigenmodes f 0 and f1.

The evolution of the surface energy of nanoparticles during the sample hardening process (transition from VdW to CV-H bonds) has been also assessed by the contact angle measurements (Young−Dupré angle) and ultrafast nanoacoustics as described below. First of all, some typical contact angle profiles are shown in Figure 2b where we can see clearly the change of the surface thermodynamics. Figure 2c shows how this thermodynamic parameter evolves as a function of the hardening time. A drastic evolution from a hydrophobic (θ > 90°) to hydrophilic (θ < 90°) behavior (Figure 2c) is evidenced within the first 30 min of the hardening process. The contact angle measurement was performed for two films (thickness of H = 210 nm and H = 70 nm), and both films exhibit a similar evolution of the surface energy (see blue and red circles, respectively). The rapid evolution of the contact angle versus hardening time (i.e., versus the amount of H-CV/VdW bonds ratio) is in correspondence with the chemical modification of the nanoparticle surface probed with IR spectroscopy. All of these contact angles were measured around 1 h after the sample was prepared. It is worth emphasizing that a long hardening time leads to a continuous increase of the contact angle, showing the hydrophilicity decreases. A similar phenomenon was checked on a bare silica surface submitted to the NH3 hardening process (black circles in Figure 2c), while a stable hydrophilic contact angle is preserved for silica in air or wet silica (see light blue circles in Figure 2c). This phenomenon is likely due to a nitrogen compound adsorption coming from the NH3 catalyst evidenced by X-ray photoemission spectroscopy (XPS), as shown in Figure S4. However, as we will show in the following, this does not impact the nanoparticle elastic stiffness network. Indeed, as ammonia molecule adsorption increases onto the

Figure 2a shows a comparison between the infrared absorption response of standard and NH3 post-treated silica layers revealing obvious chemical changes due to the ammonia treatment. Due to the hydrolysis−condensation process during the ammonia treatment, the methyl bonds (CH3) disappear, while, on the other hand, the hydrogen bonds (H−OH and Si− OH) are created and the Si−O−Si bonds evolve due to the appearance of a covalent nanocontact. All IR bands can be assigned either to Si−O, Si−OH, or OH bond vibrations or to remaining ethoxy groups (bands appearing in the 1400 cm−1 region) in agreement with the literature.31 After treatment, a strong modification occurred in the relative intensity of the Si− O−Si stretching broad band. The increase of the Si−O−Si LO asymmetric stretching vibration band at 1220 cm−1 and that of the Si−OH vibration band (950 cm−1) is interpreted to result from a strengthening of the silicate gel network through crosslinking. As already mentioned, the ammonia-treatment effect on colloidal silica layers is consistent with the base-catalyzed condensation mechanism proposed by Iler.32 With ammoniacuring, the particle-to-particle linking is enhanced via Hbonding of neighbor particles through vicinal silanols and condensation reactions via siloxane bridging. The first main important information from IR vibrational spectroscopy is that around 30 min of post-treatment is enough to make the methyl bonds disappear. After typically 30 min, we do not see indeed the IR transmission spectra evolution, indicating, from this spectroscopic point of view, that the chemical transformation is complete (saturation effect). We have integrated the intensity of some selected IR bands and showed that saturation appears within less than 1 h of duration of the hardening process (not shown). Whatever the NP assembly film thickness (70−210 nm), the transformation of the IR spectrum is rapid. 23772


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Figure 4. Elastic properties of the nanoparticle network and the role of the submicrometer cracks. (a) Elastic modulus of the silica colloid film as a function of the hardening time (i.e., from van der Waals (VdW)- to hydrogen-covalent (CV-H)-connected nanoparticles) and as a function of the colloidal film thickness. (b) Thickness dependence of the elastic modulus for two hardening times. The pink curve shown in the bottom figure is a calculation performed according to an effective medium approach including the contribution of cracks. (c) Evidence of the increase of the diffuse scattering contribution in the optical reflectivity signal measured as a function of the colloidal film thickness and for a hardening time fixed at 17 h. (d) Comparison of scattering defect volume versus collidal film thickness for a NH3-hardened film (blue) and nonhardened film (red). (e) AFM image of typical cracks for thick films (215 nm). The profiles of some typical cracks, labeled 1, 2, and 3, are shown in part f. These profiles reveal a characteristic depth of around 40 nm.

surface with hardening time, the hydrophilicity of the film decreases (contact angle increases above 50°) because of the covering of OH surface groups with NH3 molecules having a quite low dipole moment value compared to absorbed H2O (1.47 D versus 1.86 D). Furthermore, while modifying the nature of the silica nanoparticle nanocontact, we observe (Figure 2d) a saturation behavior of the geometrical shrinkage of the film (ΔH/H < 0) for a long time of hardening process (the example is shown for H = 210 nm, but similar behaviors are observed whatever the sample). The shrinkage is due to the consolidation of the nanoparticle assemblies whose interparticle interactions are strengthened by the hydrogen and covalent bonds. Some solvents (alcohol, water) are also removed during the hardening post-treatment of the NP assembly. The

saturation of the thickness evolution demonstrates that the film reaches a stable packing and, compared to the contact angle, no continuous drift is observed when increasing the hardening time. This thickness measurement was realized thanks to a spectrophotometry measurement (Figure 2d), as detailed in the Methods section and the Supporting Information. The consequence of this transformation of the nanocontact on the elastic stiffness of the 3D assembly is assessed by ultrafast acoustic methods. The laser-induced acoustic wave signals are shown for various films in Figure 3c after the baseline subtraction (electronic decay and thermal relaxation). The signals clearly evidence the existence of two characteristic modes f 0 and f1. The Fourier transforms shown in Figure 3d 23773


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plotted, for a given hardening time, the thickness dependence of the elastic modulus, as shown in Figure 4b. There is a tendency of elastic modulus softening with increasing thickness that we possibly attribute to the appearance of some submicrometer defects that we have revealed by different methods, as shown below. First of all, while scrutinizing the near-UV optical reflectivity signals (Figure 4c), we have observed that, for thick colloidal films, an increase of the UV light scattering clearly appears and is evidenced by the development of a tail in the optical reflectivity spectrum in the range 250−450 nm. Dark-field imaging also allows the observation of the diffuse light scattering induced by surface defects (Figure 4d). A proper model that is fully described in an upcoming paper34 permits one to extract the diffuse scattering intensity (ID) which is ascribed to a volume of the defects. As shown in Figure 4d, this defect volume clearly increases with the thickness whatever the nature of the nanocontacts but with a variable threshold effect. This threshold is indeed dependent on the nature of the nanocontacts: the defects appear above a critical thickness which is larger when the nanocontacts are soft (VdW bonds) compared to the case of a network consolidated with CV-H bonds. This clearly indicates that, while strengthening the network, the hardening process leads to a larger defect volume and that hardening somehow makes the lattice more brittle. These optical measurements are consistent with Figure 4b where, within the thickness range 70−300 nm, only the elastic modulus of CV-H colloidal films is affected by defects, while the elastic modulus of VdW films does not evolve with thickness in this range. The morphology of these defects has been well evidenced by atomic force microscopy (AFM) images shown in Figure 4e where characteristic cracks appear on the surface with a regular spatial distribution. The in-plane extension of the cracks is around 300 nm (Figure 4f). There are craters with a typical width and depth of around 50 nm. We remind that ultrafast nanoacoustic experiments were conducted with a laser focusing area of around πr2 with r ∼ 5 μm. This means that the extracted modulus is an average measurement that includes the contribution of tens of cracks. The cracks dimension is of the order of the optical wavelengths (250−450 nm) where near-UV optical scattering has been identified (Figure 4c). The effect of cracks on elastic properties of macroscopic solids is a long-standing research area, and different effective models exist to describe it.35 For noninteracting cracks and with anisotropic distribution, an effective Young modulus E was proposed as a function of the cracks volume fraction pcracks:35

indicate there is a ratio of 3 between these two frequencies, indicating these modes are eigenmodes with f 0 = V/4H and f1 = 3V/4H, where V and H are the sound velocity and the thickness of the colloidal film, respectively.11,18 With a known thickness H determined by spectrophotometry, the sound velocity is straightforwardly deduced with the above eigenmodes frequency equation. Because the optical interferometry also provides an evaluation of the optical refractive index n, then, by applying a mixing law that is relevant for a large porosity p medium such as our NP assembly, we can extract the mass fraction (1 − p) with the relation ϵ = pϵair + (1 − p)ϵSiO2, with n = √ϵ being the refractive index of the NP assembly. We thus deduce the mass density ρfilm = (1 − p)ρSiO2. Finally, the elastic modulus is determined following M = ρfilmV2 and its values are given in Figure 4a versus the hardening time and the NP assembly film thickness. As expected, the nanocontact hardening leads to an increase of the elastic modulus by a factor of 5−6. The larger elastic modulus remains around 10 times smaller than in the bare silica materials (∼60−90 GPa). Consistently with the shrinkage measurements (Figure 2d), an asymptotic behavior is also observed which confirms that, after a given time, the NP assembly is entirely consolidated. Besides the consolidation mechanism, we observe some fluctuations of the values of the elastic modulus of so-called thick films (>210 nm) that may come from submicrometer cracks that we will discuss in the following. It is important to remember that the thickness and the sound velocity are physical properties related to the bulk of the NP assembly and both of them show an asymptotic behavior indicating that the hardening process efficiency (i.e., transformation of VdW into H-CV bonds) saturates in the NP network after a critical time. The analysis of the IR-vibrational bands also confirms this saturation with an even faster saturation effect as already mentioned. This is in contrast with the contact angle that continuously evolves with the hardening time after the transformation from VdW to H-CV bonds. The long exposure to H2O−NH3 vapors appears to modify the film surface energy due very likely to NH3 adsorption, as suspected by XPS measurements (Figure S4). Such a presence of NH3 on the silica surface, already observed previously on silica based materials,33 might explain why the contact angle evolves with the hardening time (i.e., upon the NH3 exposure) while the nanocontact stiffness (hydrogencovalent bonds) is unaffected, as demonstrated by nanoacoustic experiments. This first discussion is clear evidence of the necessity to combine experiments to really disentangle the surface energy of NPs from the relevant nanocontacts participating in the effective elastic stiffness of the assembly of these NPs. In this particular case, we show that contact angle alone cannot provide relevant information to the mechanical network established between nanoparticles because it does not provide information on the nature and robustness of NP nanocontacts. Moreover, it is remarkable to notice that the contact angle evolves in the same way for thin (H = 70 nm) and thick (H = 210 nm) films (see Figure 2c), indicating the thermodynamic properties of the surface does not depend on the film thickness. On the other hand, with the same set of materials, the ultrafast nanoacoustic measurements reveal that some variations of the elastic modulus appear when comparing the different samples (Figure 4a). We observe that the systematic largest elastic modulus is achieved for the thinner samples (70 nm) whatever the hardening time. We have

E = E0(1 + π × pcracks )−1

(1)

The definition of pcracks is not straightforward, since it can depend on the shape of the defects. We will assume here the 1 simplest situation where pcracks = V ∑ αϕ3, where V is a representative volume of the material within which a volume of defects Vcracks = ∑αϕ3 is present (ϕ is the characteristic radius of the defect). α is a parameter which depends on the geometry of the crack. If the cracks volume fraction pcracks is small, then the effective elastic modulus becomes E ≈ E0(1 − π × pcracks), revealing a linear decrease of the elastic modulus with defect concentration. This behavior is in line with our observations even if more data need to be accumulated to verify this model. Moreover, to establish a more definitive relationship between the measured longitudinal elastic modulus (M) and the above model that uses the Young modulus E, the Poisson coefficient ν 23774


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of the silica NP assembly is needed, since 1−ν M=E , but is unknown up to now. ((1 − 2ν)(1 + ν)2 )

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CONCLUSIONS This complete study shows that it is possible to follow and to control the transformation of the nanocontact physical and chemical properties in a disordered assembly of silica nanoparticles from weak coupling (van der Waals bonds) to a strong coupling regime (covalent-hydrogen bonds). Moreover, we have established a correlation between the nanocontact properties (chemical nature of the molecular landscape, surface energy of the NPs) and the collective mechanical response with the ultrafast nanoacoustic method which is a nondestructive/noncontact method. This nanomechanical analysis provides some new insight on the long-range mechanical interaction within the NP network, and in particular has permitted one to reveal the role of the cracks appearing when the NP assembly film thickness increases or when the elasticity is strengthened from a VdW to a CV-H bonded nanocontact. We have discussed the effective elastic modulus of this NP assembly in the presence of a given volume fraction of cracks resulting from the strengthening of the nanocontact (CV-H bonds). This work shows that it is possible to quantify the mechanical quality of a NP assembly, and this approach could be extended to any kind of NP network a priori.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.7b08404. Algorithm giving the calculation of the refractive index and the thickness of the colloidal layers and XPS measurements on colloidal layers before and after the amonia curing (hardening process) (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +33 (0)2 43 83 32 68. ORCID

P. Ruello: 0000-0002-5398-1610 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS J.A. thanks the CEA and Region Pays de la Loire for his Ph.D. grant. REFERENCES

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