Patterning of ZnO Quantum Dot and PMMA Hybrids with a Solvent

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Patterning of ZnO Quantum Dot and PMMA Hybrids with a SolventAssisted Technique Yifeng Lin,† Kathy Lu,*,† and Richey Davis‡ Department of Materials Science and Engineering and ‡Department of Chemical Engineering, Virginia Polytechnic Institute and State University, 213 Holden Hall, 445 Old Turner Street, Blacksburg, Virginia 24061, United States

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ABSTRACT: Imprinting of nanoparticle−polymer hybrids has been a challenging task due to the agglomeration of nanoparticles, especially for metal oxides because of their highly hydrophilic and polar surfaces. We hereby report an effective submicron patterning process of ZnO quantum dot/poly(methyl methacrylate) hybrids with a solvent-assisted lithographic technique. Feature sizes down to 250 nm have been achieved with a ZnO content up to 50 vol %, about 10 times higher than the literature-reported inorganic contents. With higher ZnO contents, particles show a tendency to aggregate, and the samples have less flexibility as demonstrated by larger bending radii before failure. The higher ZnO content samples also produce stronger photoluminescence responses. This family of materials has a great potential to be used in flexible optical devices.



polymer matrix is a potential way to endow flexible polymer materials with useful functionalities. For example, flexible lightemitting diodes and displays have been made from quantum dots.13−16 Other devices, such as thin-film field-effect transistors17 and photodetectors,18 can also be fabricated from quantum dots. Such systems allow bottom-up fabrication while creating novel properties with excellent performance.19 However, there are several challenges that must be overcome to make flexible and functional hybrids. The quantum dots to be imbedded in the polymer matrix must have a narrow size distribution and be uniformly dispersed inside the matrix polymer. For nonoxide quantum dots, such as CdS, PbSe, this is less an issue as they can be synthesized with narrow size distributions; the synthesized quantum dots are also covered with capping agents to be compatible with organic solvents.20,21 However, metal oxide particles, such as ZnO and TiO2, are more difficult to synthesize with narrow size distributions and diameters smaller than 10 nm. They are mostly synthesized in aqueous systems without any capping agents and have very polar surface chemistries, which, without proper modifications, makes them incompatible with organic solvents and more likely to agglomerate in nonpolar environments.22−24 To avoid excessive/nonuniform particle growth during synthesis and potential agglomeration following that, ZnO nanoparticles have been synthesized by liquid combustion.25,26 However, the resulting nanoparticles usually have sizes of tens of nanometers. Particles with dimensions as large as this scale often need polymer stabilizers to provide enough steric

INTRODUCTION Micropatterning has been a key part of the fabrication of integrated circuits since the 1950s.1,2 Proven techniques include photolithography and extreme UV lithography, which are widely used to make computer chips and electronic devices. Although such techniques are mature and applicable at an industrial scale, their high cost and limited potential to continue to shrink feature sizes limit the advancements in the field. Imprint lithography, which mechanically deforms a polymeric substrate to form intricate patterns up to the nanoscale, promises low cost, short processing time, and high efficiency.3−5 Using the patterning approaches, micro- and nanoscale devices can be easily and quickly made. In addition, such devices can be made in parallel. Complicated features can be made in one step. In recent years, flexible or wearable devices have become very important for consumer electronics and optoelectronics. To fabricate a flexible device, a polymeric substrate is a logical choice due to its liquid processing nature and adaptability to thin-film forms. Sub-100 nm features were made by physically stamping a thermoplastic material.6 This kind of hybrids can be used in next-generation electronics, optoelectronics, coatings, shielding, etc. Different types of components and devices made from such organic and inorganic hybrids, such as flexible light harvesting devices, floating gate memory devices, and flexible displays, have been proposed and evaluated to take advantages of the desirable attributes such as simple fabrication, low cost in mass production, and flexibility. Active species can be introduced during the polymer chain crosslinking or consolidation step to enable novel functional properties.7−9 Quantum dots are another active research area due to their unique functionalities, such as photoluminescence,10 photocatalysis,11 and biosensing.12 Combining quantum dots with a © XXXX American Chemical Society

Received: January 27, 2019 Revised: March 26, 2019 Published: April 11, 2019 A

DOI: 10.1021/acs.langmuir.9b00256 Langmuir XXXX, XXX, XXX−XXX

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Figure 1. (a) Surface structures of the PDMS mold for rods and ridges and the imprint patterning setup. The green color represents the suspension, (b) a custom-made apparatus based on a digital caliper used to measure the bending radius of the hybrid film.

repulsion against the attractive van der Waals force.27−30 In addition, irreversible agglomeration is very difficult to avoid due to the formation of Zn−O−Zn bonds between particles.24 Consequently, agglomeration leads to functional loss, such as luminescence quenching, and sometimes causes nanoparticles to lose their functionalities altogether.31 In situ polymerization on particle surfaces has been attempted. However, it is complicated and difficult to control the colloidal stability of the particles before and in the early stages of the polymerization when the steric layer is not well developed.29,30 To simplify the complicated functionalization step, Meulenkamp et al.32 synthesized monodispersed ZnO quantum dots using a modified method proposed by Spanhel and Anderson.33 By reducing the nanoparticle sizes to