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Enhancing the Elasticity of Ultrathin Single-Wall Carbon Nanotube Films with Colloidal Nanocrystals Meshal Alzaid, Joseph Roth, Yuezhou Wang, Eid Almutairi, Samuel L. Brown, Traian Dumitrica, and Erik K. Hobbie Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.7b01988 • Publication Date (Web): 25 Jul 2017 Downloaded from http://pubs.acs.org on July 27, 2017

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Enhancing the Elasticity of Ultrathin Single-Wall Carbon Nanotube Films with Colloidal Nanocrystals Meshal Alzaid,1,† Joseph Roth,1,† Yuezhou Wang,2 Eid Almutairi,1 Samuel L. Brown,1 Traian Dumitrică,2,3 Erik K. Hobbie*1 1

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North Dakota State University, Fargo, North Dakota 58108 Department of Chemical Engineering and Materials Science, University of Minnesota Twin Cities, Minnesota 55455 3 Department of Mechanical Engineering, University of Minnesota, Twin Cities Minnesota 55455

RECEIVED DATE () ABSTRACT: Thin bilayers of contrasting nanomaterials are ubiquitous in solutionprocessed electronic devices and have potential relevance to a number of applications in flexible electronics. Motivated by recent mesoscopic simulations demonstrating synergistic mechanical interactions between thin films of single-wall carbon nanotubes (SWCNTs) and spherical nanocrystal (NC) inclusions, we use a thin-film wrinkling approach to query the compressive mechanics of hybrid nanotube/nanocrystal coatings adhered to soft polymer substrates. Our results show an almost two-fold enhancement in the Young modulus of a sufficiently thin SWCNT film associated with the presence of a thin interpenetrating overlayer of semiconductor NCs. Mesoscopic distinct-element method simulations further support the experimental findings by showing that the additional noncovalent interfaces introduced by nanocrystals enhance the modulus of the SWCNT network and hinder network wrinkling.

KEYWORDS: single-wall carbon nanotubes, thin films, semiconductor nanocrystals, mechanics, excluded volume

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INTRODUCTION Thin films and coatings are ubiquitous in everyday life, from consumer products to microelectronics. For most applications, solution deposition processes are valued for their ease and scalability, while the rigidity and durability of the films are critical to their function and performance. For some applications, such as touch-screen displays, solar cells and light-emitting diodes, the electrical characteristics of the film are critical. Within the rapidly evolving field of nanomaterials, single-wall carbon nanotubes (SWCNTs) continue to show considerable promise and potential for a wide range of applications. In very simple terms, SWCNTs are rolled-up graphene sheets roughly 1 nm in diameter and 100 nm to millimeters in length.1 Depending on the molecular symmetry of the tube, they can be either metallic or semiconducting, portending numerous potential applications in microelectronics2-4 and flexible electronics.5-10 Their polymer-like shape and 1.2 TPa modulus1 also make them well-suited to nanocomposite applications,11-13 although they have yet to realize their full mechanical potential. While recent new approaches have led to improvements in mechanical performance,14,15 the response still falls well short of the behavior one would anticipate based on the modulus of an individual nanotube, which currently limits the use of SWCNTs in mechanically-robust multifunctional thin films. In the context of durable multifunctional SWCNT films and coatings, our recent experimental work on the mechanics of thin SWCNT films adhered to elastic polymer substrates examines some of these issues in great detail.16,17 The films in question represent the thinnest SWCNT networks ever studied in this regard, and ‘wrinkling’ approaches borrowed from polymer science were thus utilized to query the mechanics.1821

The results provide a glimpse of the remarkable TPa mechanics of SWCNTs, with

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extrapolated zero-strain moduli on the order of hundreds of GPa, but with significant plasticity (i.e., strain softening) under small strains.16,17 Mechanical failure occurs because the applied deformation (in this case, compression) induces SWCNT bundling through strong van der Waals (vdW) forces between contacted nanotubes,22,23 degrading the connectivity of the network. In general, vdW interactions play a critical role in determining SWCNT solubility and self-assembly24-29 and they also dictate the mechanical response of SWCNT networks and films.30,31 Although vdW forces are not considered particularly strong, the small diameter, long length, and high aspect ratio characteristic of SWCNTs create a relatively deep attractive potential well between parallel nanotubes, which favors the formation of bundles or ropes. In addition, the lack of any significant friction between pristine graphitic surfaces leaves little resistance to the coarsening of such bundles in response to large-scale mechanical perturbation, e.g. strain. An obvious solution to this is chemical crosslinking, which has been shown to be effective for improving the mechanical behavior of SWCNT fibers and bundles.32-35 However, chemical crosslinking can have an unacceptable level of impact on SWCNT electronic structure,36 and less-invasive methods of mechanical stabilization are thus desirable for applications that require the electronic structure of electronically pristine SWCNTs. In response to this need, we recently used the mesoscopic distinct-element method (mDEM) to explore an excluded-volume based approach for stabilizing SWCNT networks against strain-induced plastic change.37 The central idea behind the hypothesis is that the ability of the mesoporous SWCNT network to restructure in response to strain is reduced if the voids in the network are ‘filled’ with a second nanomaterial, such as

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nanocrystals (NCs) of sufficiently small size. In the view we are espousing, the SWCNTs are analogous to ‘rebar’ and the NC phase (the filler) is analogous to concrete. In a much broader view, the topic of hybrid organic-inorganic nanomaterials is of considerable current interest,38 while nanoparticle ‘fillers’ have already been used to enhance the mechanical performance of porous polymer nanocomposites and SWCNT aerogels.39-43 Indeed, it is now well appreciated that multiphase nanoscale structures exhibiting periodic soft-hard domains,44-46 and even periodic soft-soft domains,47 can exhibit improved mechanical performance. However, none of these studies address the problem of enhancing thin SWCNT films for multifunctional applications in flexible electronics. In this paper, we use a thin-film wrinkling technique to experimentally test the effectiveness of the excluded-volume approach, which we consider in the context of hybrid SWCNT-nanocrystal films on soft, elastic polymer substrates. In agreement with predictions, our results demonstrate a nearly two-fold enhancement in the Young modulus of sufficiently thin SWCNT films, which we associate with the presence of a thin interpenetrating overlayer of semiconductor NCs. We use mDEM to simulate the compressive/tensile mechanical response of pristine and hybrid films and to confirm the role of excluded-volume interactions in enhancing thin SWCNT film stability. Beyond flexible electronics, our results also have potential implications for engineering durable multifunctional nanocomposites.

RESULTS AND DISCUSSION Experiments. Details related to the experiments can be found in the Materials and Methods section. Ligand stabilized CdSe nanocrystals were purified by size using density-gradient ultracentrifugation (DGU) in mixtures of chloroform and m-xylene. The

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parent suspension exhibited PL emission in the vicinity of 560 nm and the PL spectra of the resulting fractions are shown in Fig. 1a. As detailed in the Supporting Information, two fractions in close proximity (PL peak near 560 nm, mean NC diameter near 4 nm)48 were use for the measurements described here. While NC size uniformity is not essential, it provides a cleaner point of reference. However, a critical benefit of DGU is the removal of excess ligand from the NC suspensions. The NC diameter distribution based on transmission electron microscopy (TEM) is provided in the Supporting Information, along with additional spectral data. The nanotubes were mixed-type CoMoCat SWCNTs in aqueous dispersions of 2 % sodium deoxycholate (DOC). These SWCNTs have a natural enrichment in the (6,5) semiconducting species and a mean length near 700 nm. A comparison of the NC PL spectrum and the UV-Vis-NIR absorption spectrum of the SWCNTs is shown in Fig. 1b, while Fig. 1c-d show TEM images of the NCs and SWCNT network, respectively.

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Figure 1. (a) PL spectra of the colloidal NC fractions, where the fraction denoted in red was used in the present study. (b) UV-Vis absorption spectrum of the SWCNTs (purple) with the PL spectrum of the NC (red). The solid and dashed purple curves depict the SWCNTs in colloidal and thin-film (40 nm thickness) form, respectively. (c) TEM image of a NC superlattice (10 nm scale) where the inset shows a TEM image of a single NC (3.9 nm diameter). (d) TEM image of a SWCNT film (50 nm scale). (e) Schematic of the LB deposition scheme and the wrinkling experiment used to interrogate the bilayers. Typically, colloidal nanorod/nanosphere mixtures exhibit phase separation and phase ordering upon drying.49 Additionally, the SWCNTs are suspended in water while the NCs are suspended in organic solvents. It was therefore not possible to cast mixed particle films from a common solvent in a single step, and we instead used a Langmuir-Blodgett (LB) method to deposit the nanocrystals on top of the SWCNTs. The deposition process is summarized in Fig. 1e. The SWCNT films were collected in a pipette as freestanding macroscopic nanosheets suspended in ethanol and deposited on prestretched polydimethylsiloxane (PDMS) substrates.50 Our approach for making CdSe monolayers was a slightly modified version of the LB method described by Wang, et. al.51 First, 10 µL of distilled water was deposited as a small droplet on the target area. Due to the hydrophobic nature of the SWCNT substrate, the water remains in a sharply curved droplet. Next, 8 µL of a chloroform/CdSe solution was placed on top of the water droplet. Initially, the NCs settle around the edges of the drop, but a monolayer eventually forms over the entire air-water interface. As the water evaporates, the NC monolayer is deposited on top of the SWCNTs. Depositing the two types of nanoparticles in two separate steps yields what closely resembles a NC/SWCNT bilayer and gives us a degree

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of control over the final morphology of the hybrid film. The use of water or a comparable polar solvent is critical because of the hydrophobic nature of both the SWCNTs and the NCs. The interaction energy between the bare SWCNTs and the hexadecylamine-coated CdSe NCs will be comparable to or better than that between SWCNTs and PDMS,24 for example, which is reasonably favorable. In the absence of additional doping,52 the sheet resistance of the SWCNT films on the order of 500 Ω/sq, while the NCs are insulating. Although the creation of functional devices would thus requires chemical modification of both phases, our immediate interest relates to excluded-volume effects at the simplest level in a bilayer system. To query film mechanics, we used the thin-film wrinkling approach that is now well established for thin polymer films.18-21,53 Systematically releasing the strain in the PDMS substrate introduces a compressive stress on the SWCNT or SWCNT/NC film, which exhibits periodic buckling in response. In the simplest representation, the modulus of the adhered film is related to the wavelength of sinusoidal wrinkles (λ) by = 3 /2ℎ ,

(1)

where htot is the total film thickness, = /1 − is the plane-strain modulus, the subscript i denotes either the film (f ) or substrate (s), and ν is Poisson ratio. For a purely elastic film, nonlinear effects typically emerge at around 10 % strain.53 Below this strain, the characteristic length-scale exhibits bifurcation triggered by inhomogeneities in film structure and thickness.54 We determine λ predominantly from strain (x) projections of the two-point correlation function, g(r), digitally computed from gray-scale reflection optical

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micrographs using ImageJ, as shown in Fig. 2a-b. In select instances, these were supplemented with direct in situ AFM measurements on PDMS substrates. The position of the first nearest-neighbor peak in g(x) defines the mean spacing of ridges or wrinkles. To calibrate this to the true wrinkling wavelength λ, we use low strain images to identify the ‘harmonic’ wavelength for sinusoidal deformation, often evident as a faint periodic pattern in the background of much darker ridges, and often not explicitly present in g(r). A comparison of this length-scale with the position of the first nearest-neighbor peak in g(x) then informs as to whether the mean spacing of ridges is 2λ or 4λ. With increasing strain and decreasing thickness, the appropriate length-scale bifurcation factor can then be deduced from the requirement that the effective modulus is a continuous function of strain and thickness. A significant change in wrinkling length-scale associated with the presence of a CdSe NC layer clearly evident in composite reflection/PL optical micrographs (Fig. 2c) and in in situ AFM measurements at the same type of SWCNTSWCNT/NC interface (Fig. 2d). The effect is most pronounced for thinner SWCNT films, as shown in Fig. 2e-f.

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Figure 2. (a) Reflection optical micrograph of wrinkling in a SWCNT/NC bilayer (h = 30 nm, 4 % strain) and (b) the corresponding 2D pair correlation function, g(r), from an ensemble of such images (5 µm scale, both panels). (c) Reflection optical micrograph of wrinkling at a NC film edge (h = 21 nm, 10 % strain, 5 µm scale) where the right panel is a composite PL image of the same spot (3 µm scale, NC = green). (d) Measured wrinkling amplitude measured with AFM on either side of the bilayer edge (h = 40 nm, 10 % strain). (e) Correlation function g(r) for pure SWCNT (top, h = 13 nm) and SWCNT/NC bilayer (bottom, h = 14 nm, 5 % strain, 3 µm scale), and (f) projections of g(r) along the direction of strain, g(x). The question as to whether or not there is an enhancement in modulus associated with the NC layer thus comes down to a precise determination of how much the nanocrystals increase the mean total thickness of the hybrid film. To answer this, we utilized a

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combination of in situ PL spectroscopy/microscopy and in situ AFM, as depicted in Fig. 3a-f. Figure 3a shows a TEM image of the edge of a NC layer and the underlying SWCNT network. Figure 3b shows a similar interface imaged with PL microscopy (zero strain), and Fig. 3c shows a blow-up composite PL/AFM image of the yellow-enclosed region of Fig. 3b. Although not obvious in the AFM image of Fig. 3c, there is an AFM step-height associated with a jump in thickness at such an interface (Fig. 3d), with an overall average value of Δℎ = (2.65 ± 0.25) nm.

Figure 3. (a) TEM image of a NC film edge on an underlying SWCNT film (20 nm scale). (b) PL image of the bilayer edge on an underlying SWCNT film (10 µm scale, h = 40 nm). (c) Overlay of PL and AFM images for the region indicated by the yellow rectangle in panel (b) (5 µm scale). (d) Mean step height from SWCNT to NC based on the AFM data in panel (c). (e) Nanocrystal PL spectrum measured for the NC-SWCNT bilayer in (b)-(d). (f) Peak PL intensity for the dimmest resolvable region of the 10


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composite films as a function of SWCNT film thickness. A comparison of measured film moduli both with and without NC for (g) h = 13 nm, (h) h = 17 nm, and (i) h = 40 nm. In collecting such information, all measurements (strained and unstrained) were taken at spots where the NC PL intensity (EPI illumination) was the smallest detectable value for a given film, based on both local PL image intensity (Fig. 3b) and peak spectral emission strength (Fig. 3e). From TEM images obtained for samples processed in an identical manner (Fig. 3a), we associate this minimal PL signal with a ‘monolayer’ of NCs. This is consistent with TEM images collected for composite films processed in an identical manner (Supporting Information). Figure 3f shows a plot of the minimal detectable PL spectral intensity as a function of the local SWCNT thickness, h, where the fit is an exponential decay. The fit can be rationalized by noting that the NC emission under EPI illumination is isotropic, with half the PL prone to absorption by the SWCNT underlayer, which is an effect compounded by the overlap of the PL peak with the second inter-band absorption resonance of the semiconducting (6,5) SWCNT (Fig. 1b). The exponential trend is thus simply a reflection of Beer’s law associated with optical extinction of the NC PL in the SWCNT film, with the constant value at large h reflecting the limit where only a small fraction of the emitted PL (roughly 10 %) escapes from the thickest SWCNT films. We attribute the observation that this limit is significantly smaller than 50 % to the rough, porous nature of the underlying SWCNT film and the fact that the NC layer is interpenetrating (Δℎ < 2). Accounting for the measured Δℎ , Fig. 3g-i shows the resulting strain dependent effective modulus, both with and without the nanocrystal capping layer, for three SWCNT films over a varied range of thickness. For the thinner SWCNT films, the 11


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measured enhancement in modulus associated with the NC overlayer is almost a factor of two, although thicker films still suggest somewhat of an improvement in yield strain (Fig. 3i). The error bars in Fig. 3g-i represent two standard deviations in the experimental uncertainty associated with total film thickness, ℎ Δℎ , where Δℎ is zero for pure SWCNT. Trends similar to those in Fig. 3g-i, a one- to two-fold enhancement in modulus associated with the thinnest measurable NC layer, were reproduced several times in independent experiments. They are also reminiscent of somewhat similar enhancements in modulus recently measured for thin SWCNT films with a thin polymer overlayer.55

Figure 4. (a) mDEM-simulated type 1 (T1) SWCNT network, measuring 500 nm × 500 nm × 11 nm in size. Color reflects the magnitude of the bending moments stored by the parallel contact bonds. The callout details the entangled SWCNT bundles. (b) Type 2 (T2) SWCNT network filled with 2,000 NCs. The callout details the location of nanoparticles (shown as spheres) close to the entangled SWCNT bundles. (c) Wrinkles developed in the T1 SWCNT network under 15 % compression. Arrows indicate the 12


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compression direction. (d) Number of pores versus pore size in the stress-free T1 and T2 networks. (e) Engineering stress–strain curves in pristine and filled T1 and T2 networks.

Simulations. Next, we used mDEM simulations as a complementary approach to probe the impact of the NC phase. In order to check the robustness of the effect, we considered two SWCNT network structures with very different pore-size distributions and degrees of bundling. Figure 4a displays a freestanding mDEM-simulated network, labeled type 1 (T1), featuring a nanoporous structure of entangled SWCNT bundles. A second network, labeled type 2 (T2), was considered to probe the mechanical response in the regime of SWCNT networks with smaller pore sizes and less bundling. The difference in pore-size distribution is summarized in Fig. 4d. In comparison to the T2 network, the T1 network exhibits very large pores, with cross-sections larger than 300 nm2. Both networks were constructed from 350 (10,10) SWCNTs, each 475 nm in length. Their stability arises from the energy balance between the bending strain energy stored in the covalent bonds (also included in the color code of Fig. 4a) and long-range van der Waals attraction between SWCNTs. After relaxation, the T1 and T2 networks were subsequently filled with 2,000 spherical NCs and further relaxed (Fig. 4b). In general, it is expected that the mechanical response of the network will depend significantly on the precise network structure. We observe that during the relaxation process, the NCs fill the natural pores of the network without any practical change in structure of the initial SWCNT network. Thus, any potential differences in the mechanical response of the corresponding pristine and filled samples can be attributed to the presence of the second phase.

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We have also probed the mechanical stability of the mDEM networks to uniaxial compressive/tensile loading. To impose the desired deformation, two thin layers of distinct elements at the opposite edges of the film were designated as grips. Displacement-controlled loading was enabled by prescribing both grips to accelerate from 0 to a specified velocity over a time interval of 0.6 ns. This acceleration period is used to reduce the undesired dynamic response, which is significant in the case of instantaneous acceleration of the grips. Afterwards, the network was deformed at the constant strain rate (108 s-1). The computed compression/tension stress-strain curves presented in Fig. 4e indicate that the NC-filled T1 and T2 networks are stiffer than the pristine ones. For example, in compression at 1 % deformation, we obtain a Young modulus of 4.7 GPa for the pristine T2 network and 6.6 GPa for the network with 2,000 NCs. These values are comparable to the experimental results for the thinnest SWCNT films (Fig. 3g). An analogous conclusion can be drawn through an analysis of the elongation curves. The networks appear to be stiffer in tension than in compression; we measured at 1 % deformation, a Young’s modulus of 7.2 GPa in the pristine T2 network and 9.7 GPa for the T2 network with 2,000 NCs. On one hand, the compressed pristine networks both develop out-of-plane deformations in the form of wrinkles. Wrinkles emerge just after 1 % and their amplitude increases as the compression progresses. A wrinkled morphology for the T1 network under 15 % compression is shown in Figure 4c. On the other hand, the compressed filled network is able to resist the wrinkling deformation, indicating that the NC addition also increases the bending rigidity of the film. In our mDEM simulations, the film remained flat up to the 9 % considered compression.

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CONCLUSIONS In conclusion, we use a wrinkling-based experimental approach to query the mechanics of hybrid nanotube/nanocrystal thin films compressed on elastic PDMS substrates. Our measurements indicate a nearly two-fold enhancement in the Young modulus of a sufficiently thin SWCNT film associated with the presence of just a thin interpenetrating overlayer of semiconductor NCs. To support the measurements, we use mDEM to simulate the mechanical response of the compressed films, where the mechanical synergy of the nanocrystal interaction is rationalized through an excludedvolume stabilization scheme: The presence of NCs within the voids of the mesoporous network of SWCNT bundles inhibits strain-induced structural rearrangements and network wrinkling and increases the modulus of the network. Our mDEM simulations reveal that the NCs are typically located near SWCNT crossings, a location that most effectively delays the relaxation time of the SWCNTs into bundles. Based on this observation, we conjuncture that the effect reported here is robust with respect to important experimental parameters like NC size and the pore size of the network. Additional mDEM simulations will be needed to elucidate how other parameters like NC concentration, packing morphology and the presence/absence of NC crystalline order in the pores correlate with the mechanical performance of the hybrid film. The synergy could be further optimized through a design approach based on the relative Hamaker constants of SWCNT-NC interactions and through a more efficient overlayer deposition process that optimizes the interpenetration of the NCs and the underlying SWCNT film. MATERIALS AND METHODS

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PDMS substrates (Sylgard 184, Sigma-Aldrich) were prepared at a 10:1 monomer/cross-linker ratio, degassed under vacuum for 1 h, baked at 80 °C for 2 h and then cut into individual substrates (75 mm × 25 mm × 1.5 mm). For each PDMS substrate, we directly measured in an Instron 5545 Tensile Tester (1.4 ≤ ≤ 2.5 MPa) after all other measurements had been performed. CoMoCat SG65i SWCNTs were dispersed at 1 mg SWCNT/mL in a 2 % aqueous solution of sodium deoxycholate (DOC) through tip sonication (Thomas Scientific, 0.64 cm tip, 1 W/mL, 1 h, 0 °C) and the suspension was centrifuged for 2 h at 21000g to remove large bundles and impurities. The resulting supernatant contained primarily individual nanotubes with a small number of fewnanotube bundles. The SWCNT length distribution was measured with AFM (Agilent Technologies Model 5500) as detailed elsewhere50 and is approximately log normal with a mean length near 670 nm. Using a mean diameter of 0.75 nm, the aspect ratio is close to 103, implying a percolation threshold near 10 nm.50 Colloidal CdSe nanocrystals dispersed at 5 mg/mL in toluene and surface stabilized with hexadecylamine ligands were obtained from Sigma Aldrich (Lumidot©). These were purified through density-gradient ultracentrifugation (DGU) as detailed elsewhere.56,57 Both a 365 nm LED and an X-Cite 120Q (120 W) lamp were used for PL excitation. SWCNT films were prepared on cellulose-ester filter paper through vacuum filtration50 and freestanding SWCNT films were produced from the supported films by dissolving the paper backing in an acetone bath. This was followed by three successive baths in clean acetone, with three successive baths in ethanol to facilitate removal of residual DOC,50 which can adversely impact SWCNT-SWCNT contacts.58 The freestanding SWCNT films were handled, transported and manipulated in ethanol using a

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pipette-mediated transfer scheme as detailed elsewhere.50 The SWCNT contribution to the film thickness was determined from the optical extinction of the films at a wavelength of 583 nm as measured in transmission using a broadband lamp for excitation and a spectrometer (Ocean Optics QE65000) for light detection. Using an independently determined extinction coefficient, this approach allows us to optically measure the local SWCNT film thickness in situ.50 Tapping mode AFM was also used in situ to measure the nanocrystal contribution to film thickness, as detailed in the main paper. Transmission electron microscopy (TEM) images of pure and NC-decorated SWCNT films were taken on a JEOL JEM-2100 analytical TEM (200 kV) with a GATAN Orius SC1000 CCD. The

mesoscale-level

simulations

were

performed

using

the

mDEM

approach.30,31,35,37,59 In mDEM, the SWCNT network is represented by a collection of cylindrical elements interacting via bonded and non-bonded contacts, which represent the atomic-scale interactions; mDEM is a coarse-grained representation of the system in question as each element generally represents a segment of the detailed SWCNT containing many carbon atoms. The parallel-bond contacts30,31 are trained to capture the intra-tube covalent interactions responsible for the local linear elasticity of individual SWCNTs. In addition, we account for the microscopic vdW interactions, which are represented at the mesoscale by the non-bonded contacts acting between elements in close spatial proximity.30,31 We recently included a second NC phase,37 which is represented by a collection of spherical distinct elements interacting through non-bonded contacts with each other and with the mDEM-represented SWCNT network.

SUPPORTING INFORMATION AVAILABLE

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The Supporting Information contains additional spectral data, TEM size-distribution data, and additional TEM images of the SWCNT/NC hybrids. This information is available free of charge via the Internet at http://pubs.acs.org. †

These two authors contributed equally to this work

* Corresponding author: [email protected]

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