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Flexible light-emitting nanocomposite based on ZnO nanotetrapods Vinh Diep, and Andrea M. Armani Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.6b02887 • Publication Date (Web): 29 Nov 2016 Downloaded from http://pubs.acs.org on November 29, 2016

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Flexible light-emitting nanocomposite based on ZnO nanotetrapods Vinh M. Diep, Andrea M. Armani* Mork Family Department of Chemical Engineering and Materials Science, University of Southern California, Los Angeles, California, 90089 USA

ABSTRACT

Flexible, light-emitting materials have shown promise in a wide range of applications. Here, we develop an inverse soft-lithography process for embedding zinc oxide nanotetrapods (ZnO NTP) uniformly and non-destructively into a host matrix. The crystalline NTPs were synthesized using a catalyst-free, environmentally-friendly chemical vapor transport method. The fluorescent emission of the ZnO NTPs was measured before and after the embedding process. Cyclical mechanical bend tests (N>100) were performed. The emission of the nanomaterial remains throughout.

KEYWORDS: ZnO Nanotetrapods, Functional nanomaterials, Fluorescence nanomaterials, Softlithography

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Flexible luminescent materials have been widely used in numerous applications including sensors1-3, biomedical devices4, and optical displays5. While previous research on opticallyactive materials has focused on semiconducting polymers6 and small molecules7, recent research efforts have shifted

to luminescent inorganic nanomaterials8. These nanomaterials have a

number of distinct advantages including favorable quantum confinement effects, increased optical lifetime, and improved environmental stability9. The use of these nanomaterials in combination with flexible polymers has allowed for the development of flexible luminescent materials based on quantum dots10,

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, carbon nanotubes12, graphene13,

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, and silica

nanoparticles15. However, one of the main challenges in developing these composite materials is the incompatibility of the nanomaterial with the polymer matrix, leading to problems such as aggregation of the particles in the matrix and decreased fluorescence quantum yield. Zinc oxide (ZnO) has attracted significant interest due to its favorable optical and electrical properties, including a large band gap and a high exciton binding energy. Additionally, it can be grown in many different nanostructure morphologies to further enhance its optical and electrical properties16-20. Lastly, because ZnO is biocompatible, chemically stable, and environmentally friendly, it is a promising material for use in a wide range of applications including optoelectronics21-23, therapeutics24, 25, and energy harvesting26, 27. One particularly promising ZnO nanostructure is the ZnO nanotetrapod (ZnO NTP). This nanomaterial exhibits UV and green emission when exposed to UV light28. Bottom-up synthesis of ZnO NTPs based on chemical vapor transport (CVT) allows for a simple and low-cost method of manufacture that produces no harmful waste29. However, ZnO is highly brittle. Previous work has investigated solving this challenge by embedding the ZnO NTP in a polymer host matrix, forming a luminescent nanocomposite. The first method developed relied on mechanically


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mixing a highly entangled network of ZnO NTPs into an elastomeric host2. While the ZnO NTPs were distributed throughout the polymer, NTPs were also broken during the mixing process, creating ZnO nanowires. Additionally, the entanglement effectively created regions within the elastomer that behaved as brittle sheets, limiting the stability of the fluorescent signal upon deformation. The second strategy relied on two photon polymerization30. This extremely precise and nondestructive method is also very time-consuming and resulted in a rigid nanocomposite material. Therefore, the development of nondestructive method for fabricating a flexible ZnO NTP nanocomposite still remains an unsolved challenge. In this work, we synthesize crystalline ZnO NTPs using a catalyst-free approach. Additionally, we demonstrate an inverse soft lithography stamping technique for embedding ZnO NTPs uniformly and non-destructively in a flexible host matrix (Figure 1a) and characterize the photoluminescence properties of the nanocomposite before and after embedding and after repeated mechanical strain. There is minimal change in the emission from the ZnO NTPs, indicating that the fabrication process does not introduce defects and that the PDMS is able to protect the ZnO NTPs even under bending stress. The CVT synthesis of the ZnO NTPs is self-catalyzed. A small amount of purum-grade zinc powder (Sigma-Aldrich) is placed in a small quartz vial, which is then placed in an alumina boat. Several silicon wafers serve as growth substrates and are placed in a different boat at a distance of 5-12 cm downstream from the zinc source. Argon is flowed through the tube for 1 h at 370 mL/min. The furnace temperature is then increased to 750°C under 200 mL/min argon flow at a rate of 24°C/min. After 20 min at 750°C, a 0.5% O2 in argon gas mixture was flowed in at 50 mL/min for 15 min. The gas flow is then switched back to pure argon while the furnace cools down to room temperature at a rate of 6°C/min.


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Electron microscopy and spectroscopy are performed to confirm the morphology and crystallinity of the CVT-grown ZnO NTPs. Experimental details are located in Supporting Information. Figure 1b shows a scanning electron microscope image of ZnO NTPs grown on silicon wafers. The growth mechanism of zinc oxide tetrapods is believed to occur by growth of four wurtzitic arms from an octahedron zinc-blende embryo, each at a 109.5° angle from the adjacent one.31-35 The hexagonal (0001) basal plane of the wurtzite structure is visible in Figure 1b inset. The tapered ends of some of the tetrapod arms indicate continued growth of zinc oxide when the oxygen flow had been turned off but residual oxygen remains in the growth chamber. In the TEM images, the lattice fringes are clearly visible, indicating the single-crystalline nature of the nanostructures (Figure 1c). The lattice spacing is 2.6 Å, indicating growth in the [0001] direction, which is the fastest direction of crystal growth in ZnO structures and is consistent with previous studies.31, 33, 36, 37 X-ray diffraction (XRD) and energy-dispersive X-ray spectroscopy (EDX) analysis are also performed to confirm the crystal structure and elemental composition of the NTPs (Supporting Information, Figure S1 and S2).


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Figure 1. (a) Schematic of the process for fabricating the ZnO NTP-PDMS composite material. (b) Scanning electron microscope image of as-grown ZnO NTP structures on the silicon growth substrate, with hexagonal basal plane of the tetrapod legs visible (inset). (c) Transmission electron microscope image of a nanotetrapod leg shows lattice fringes with spacing 2.6 Å. (d) Photograph of composite material exhibiting transparency and fluorescence under UV light (inset). Based on an analysis of the TEM and SEM images, the ZnO NTP arm lengths ranged from 0.5 µm to 3.5 µm and the diameters varied from 120 nm to 350 nm. Previous studies have shown that the growth parameters including gas flow rate, growth temperature, position from the zinc source, and reaction time can affect the size, morphology, and density of the zinc oxide nanostructures.36-39 To create a flexible, transparent nanocomposite, the inverse soft lithography stamping approach shown in Figure 1a is used. Polydimethylsiloxane (PDMS, Sylgard 184) is used as the elastomer. The base is mixed with the curing agent at a ratio of 10:1. The mixture is degassed under vacuum for 15 min until all air bubbles disappeared. The PDMS is then poured into a petri dish that contained the ZnO NTPs grown on silicon wafers. The PDMS-ZnO NTP composite is


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cured in a gravity oven set at 75°C for 2 hours before being removed from the wafer. The thickness of the samples was ~2.5 mm after curing. PDMS-ZnO NTP samples were removed from the petri dish and cut to approximately 1.5 cm x 3 cm rectangles. The final structure is very flexible and transparent (Figure 1d). Additionally, the ZnO NTPs are confined at the surface of the PDMS. This localization maximizes the efficiency of the optical excitation and emission by minimizing the optical absorption of the elastomer (Figure 1d, inset). It is important to note that, after peeling off the PDMS, the underlying silicon growth handler substrate is almost entirely clear of any remaining ZnO NTPs. Fluorometry data (Figure S3) shows that the fluorescence spectrum of the silicon wafer after the PDMS is removed no longer contains the characteristic peaks in the UV and green region indicative of ZnO. This spectrum indicates that the handler substrate can be reused, reducing the cost and waste associated with manufacturing. This simple and catalyst-free growth process, which is limited only by the size of the furnace used for the vapor transport growth of ZnO, makes production scale-up feasible. The entire process is also environmentally friendly, without requiring any toxic chemicals or gases. Fluorometry data for the initial nanotetrapods on a silicon wafer and the embedded ZnO NTPs in PDMS are shown in Figure 2a, c. At room temperature, both spectra reveal two prominent peaks near 380 nm and 493 nm, in agreement with previous studies on zinc oxide tetrapod structures28, 40. These peaks have been attributed to exciton recombination and deep-level oxygen vacancy defect emission, respectively. Because the UV emission peak intensity is independent of mechanical damage to the sample and the 493nm peak intensity depends on the concentration of the various defects in the nanostructures, by measuring the ratio of these peaks, a self-normalized metric for nanocomposite performance can be calculated. It is notable that upon embedding the NTPs in


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PDMS, the peak positions and relative intensities do not change, indicating that the NTPs are being encapsulated, but not chemically modified or physically degraded by the process. It is known that elastomers such as PDMS can infiltrate small micrometer-sized features by capillary action.41 As can be observed in Fig 2b,d, there is minimal agglomeration during the encapsulation process, and the density of the NTPs does not change. The ability to maintain the NTP density without fracturing or breaking the NTPs is one of the main advantages of the inverse soft lithography fabrication process.

Figure 2. Fluorescence spectrum (λexc=325 nm) for ZnO NTPs (a) as-grown on a silicon substrate and (b) corresponding dark-field optical microscope image. (c) Fluorescence spectrum (λexc=325 nm) of ZnO NTP-PDMS composite with (d) corresponding dark field microscope image. No observable wavelength shift in either peak was observed after embedding the nanostructures in PDMS. To determine the mechanical robustness of the nanocomposite, the optical response of the ZnO NTP-PDMS devices is measured after bending the devices cyclically (N>100) at two different bend radius (Rb=6.42mm and 13.85mm). The ZnO NTPs are located near the outer portion of bending where they are subject to tensile stress.


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Figure a and 3b shows the fluorometry spectra for the ZnO NTP-PDMS composite during the bend tests. The ratio of the intensity of the UV fluorescence to the green fluorescence remained the same throughout the bending tests for both bend radii (Figure c and 3d). Additionally, no red or blue shift in either of the peaks was seen throughout the bending cycles. This finding is particularly notable given prior work in the field that demonstrated a pressure sensor based on a ZnO NTP nanocomposite. However, in this prior work, entangled networks of ZnO NTPs were used. Most likely, this strategy facilitated the mechanical energy transfer between the individual NTPs, amplifying the damage and the resulting signal. In contrast, in the present nanocomposite, the NTPs are moderately isolated, enabling the PDMS to provide a protective buffer and to act as a strain relief.

Figure 3. (a)-(b) Fluorometry spectra (λexc=325 nm) for ZnO NTPs in PDMS that were subject to flexural bending to radii of 13.85 mm and 6.42 mm, respectively. (c)-(d) The change in the ratio of the intensity of the UV fluorescence peak to the green fluorescence peak over l00 bending cycles for 13.85 mm and 6.42 mm bending radii, respectively. Control measurements were also performed with PDMS not containing NTPs (Supporting Information Figure S4).


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We also performed complementary finite element method (FEM) modeling of this bending scheme with COMSOL Multiphysics software in order to assess the primary regions in the PDMS that are subject to the highest stresses during the flexural bending. The FEM modeling was performed in COMSOL Multiphysics. In the FEM model, a PDMS block with dimensions 10 mm x 40 mm x 2.5 mm is flexurally bent across a half cylinder of steel with diameter 5 mm to exceed the bending done in the experiment. A gradient mesh was manually generated, and mesh tetrahedra for the PDMS were limited to maximum lengths of 200 µm. A total stress of 0.1 N/m2 was applied to two regions on the top surface of the PDMS section. The results of the finite element method model are summarized in Error! Reference source not found.. As expected, the top region of the PDMS, which is subject to tensile stress, displays a positive strain with a maximum value of 1.29×10-5. Conversely, the bottom region, which is under compressive stress, exhibits a negative strain value with a maximum value of -3.4×10-5. In between, there is neutral plane with zero strain. Because the ZnO NTPs are located at the top surface, they experience tensile stress and positive strain. The strain at this region is much lower than the fracture strain of 7.7% for zinc oxide nanowires with diameters of 130 nm.42


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Figure 3. COMSOL finite element method simulations of strain imparted upon (a) a PDMS slab of 10 mm x 40 mm x 2.5 mm when bent to a radius of 5 mm and (b) corresponding cross section at the center. The strain is positive at the top where it is subject to tensile stress and negative at the bottom where it is subject to compressive stress. In between these regions is the neutral strain plane. In summary, we have proposed and demonstrated an inverse soft lithography stamping approach for fabricating transparent, optically active and flexible nanocomposites based on ZnO NTPs. The ZnO NTPs are synthesized using a catalyst-free CVT method, and the emission behavior of the NTPs is unaffected by the embedding process. When the samples are subject to flexural bending, the optical response is constant over more than 100 cycles. This behavior shows that the device holds promise as a flexible UV-responsive material that can be utilized in many different applications where mechanical strain may occur. Furthermore, the ease of


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fabrication will allow for quick manufacture of these composite materials on a large scale with very little waste and no harmful chemicals or gas byproducts.

ASSOCIATED CONTENT Supporting Information. Experimental details and additional data

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Funding Sources V. Diep was supported by the NSF GRFP program. This work was supported by the Northrop Grumman-Institute for Optical Nanomaterials and Nanophotonics and by the Office of Naval Research [N000141410374, N000141110910]. ACKNOWLEDGMENT The authors would like to thank Mr. Soheil Soltani for help with COMSOL modeling work.


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ABBREVIATIONS ZnO NTP, zinc oxide nanotetrapod; CVT, chemical vapor transport; SEM, scanning electron microscopy; TEM, transmission electron microscopy; EDX, energy dispersive x-ray spectrometry; XRD, X-ray diffraction; PDMS, Polydimethylsiloxane; λexc, excitation wavelength; FEM, finite element method; REFERENCES 1. Wang, Z. L.; Zhang, R. L.; Ma, Y.; Peng, A. D.; Fu, H. B.; Yao, J. N. J Mater Chem 2010, 20, (2), 271-277. 2. Jin, X.; Gotz, M.; Wille, S.; Mishra, Y. K.; Adelung, R.; Zollfrank, C. Adv Mater 2013, 25, (9), 1342-1347. 3. Zhou, J. H.; Yan, H.; Zheng, Y. Z.; Wu, H. K. Adv Funct Mater 2009, 19, (2), 324-329. 4. Kim, D. H.; Lu, N. S.; Ma, R.; Kim, Y. S.; Kim, R. H.; Wang, S. D.; Wu, J.; Won, S. M.; Tao, H.; Islam, A.; Yu, K. J.; Kim, T. I.; Chowdhury, R.; Ying, M.; Xu, L. Z.; Li, M.; Chung, H. J.; Keum, H.; McCormick, M.; Liu, P.; Zhang, Y. W.; Omenetto, F. G.; Huang, Y. G.; Coleman, T.; Rogers, J. A. Science 2011, 333, (6044), 838-843. 5. Yin, S. N.; Wang, C. F.; Yu, Z. Y.; Wang, J.; Liu, S. S.; Chen, S. Adv Mater 2011, 23, (26), 2915-+. 6. Burroughes, J. H.; Bradley, D. D. C.; Brown, A. R.; Marks, R. N.; Mackay, K.; Friend, R. H.; Burn, P. L.; Holmes, A. B. Nature 1990, 347, (6293), 539-541. 7. Baldo, M. A.; O'Brien, D. F.; You, Y.; Shoustikov, A.; Sibley, S.; Thompson, M. E.; Forrest, S. R. Nature 1998, 395, (6698), 151-154. 8. Bruchez, M.; Moronne, M.; Gin, P.; Weiss, S.; Alivisatos, A. P. Science 1998, 281, (5385), 2013-2016. 9. Wang, F.; Tan, W. B.; Zhang, Y.; Fan, X. P.; Wang, M. Q. Nanotechnology 2006, 17, (1), R1-R13. 10. Woelfle, C.; Claus, R. O. Nanotechnology 2007, 18, (2). 11. Liang, R. Z.; Yan, D. P.; Tian, R.; Yu, X. J.; Shi, W. Y.; Li, C. Y.; Wei, M.; Evans, D. G.; Duan, X. Chem Mater 2014, 26, (8), 2595-2600. 12. Satishkumar, B. C.; Doorn, S. K.; Baker, G. A.; Dattellbaum, A. M. Acs Nano 2008, 2, (11), 2283-2290. 13. Kou, L. J.; Li, F. S.; Chen, W.; Guo, T. L. Org Electron 2013, 14, (6), 1447-1451. 14. Li, Y.; Hu, Y.; Zhao, Y.; Shi, G. Q.; Deng, L. E.; Hou, Y. B.; Qu, L. T. Adv Mater 2011, 23, (6), 776-+. 15. Sato, K.; Fukata, N.; Hirakuri, K.; Murakami, M.; Shimizu, T.; Yamauchi, Y. ChemAsian J 2010, 5, (1), 50-55. 16. Hsu, J. W. P.; Tian, Z. R.; Simmons, N. C.; Matzke, C. M.; Voigt, J. A.; Liu, J. Nano Lett 2005, 5, (1), 83-86. 17. Wang, Z. L. J Nanosci Nanotechno 2008, 8, (1), 27-55. 18. Tong, D. G.; Wu, P.; Su, P. K.; Wang, D. Q.; Tian, H. Y. Mater Lett 2012, 70, 94-97.


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Figure 1. (a) Schematic of the process for fabricating the ZnO NTP-PDMS composite material. (b) Scanning electron microscope image of as-grown ZnO NTP structures on the silicon growth substrate, with hexagonal basal plane of the tetrapod legs visible (inset). (c) Transmission electron microscope image of a nanotetrapod leg shows lattice fringes with spacing 2.6 Å. (d) Photograph of composite material exhibiting transparency and fluorescence under UV light (inset). Figure 1 75x31mm (600 x 600 DPI)


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Figure 2. Fluorescence spectrum (λexc=325 nm) for the lowest density ZnO NTPs (a) as-grown on a silicon substrate and (b) corresponding dark-field optical microscope image. (c) Fluorescence spectrum (λexc=325 nm) of ZnO NTP-PDMS composite with (d) corresponding dark field microscope image. No observable wavelength shift in either peak was observed after embedding the nanostructures in PDMS. Figure 2 51x31mm (600 x 600 DPI)


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Figure 3. (a)-(b) Fluorometry spectra (λexc=325 nm) for ZnO NTPs in PDMS that were subject to flexural bending to radii of 13.85 mm and 6.42 mm, respectively. (c)-(d) The change in the ratio of the area under the UV fluorescence peak to the area under the green fluorescence peak over l00 bending cycles for 13.85 mm and 6.42 mm bending radii, respectively. Similar analysis is performed for the ratio between the intensities of the two peaks (Supporting Information Figure S5, and control measurements were also performed with PDMS not containing NTPs (Supporting Information Figure S4). Figure 3 68x56mm (300 x 300 DPI)


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Figure 4. COMSOL finite element method simulations of strain imparted upon (a) a PDMS slab of 10 mm x 40 mm x 2.5 mm when bent to a radius of 5 mm and (b) corresponding cross section at the center. The strain is positive at the top where it is subject to tensile stress and negative at the bottom where it is subject to compressive stress. In between these regions is the neutral strain plane. Figure 4 71x107mm (300 x 300 DPI)


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