Comparing sintering and atomic layer deposition as methods to mechanically reinforce nanocolloidal crystals
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Yun-Ru Huangb) and Daeyeon Lee Department of Chemical and Biomolecular Engineering, University of Pennsylvania, Philadelphia, Pennsylvania 19104, USA (Received 25 May 2015; accepted 27 October 2015)
Nanocolloidal crystals (NCCs) have promising applications in optical and photonic devices. However, it is critical to mechanically reinforce NCCs for device reliability, since as-synthesized NCCs are fragile due to weak interparticle bonding. Thermal sintering is currently the most common reinforcement technique; however, this method could induce serious cracking and is not suitable for temperature-sensitive materials. In this study, by characterizing silica NCCs reinforced through sintering and alumina atomic layer deposition (ALD), we find that the ALD treatment is much more effective for hardening, stiffening, and more importantly toughening NCCs. Thermally sintered NCCs are prone to indentation-induced cracking due to large residual tensile stress, significantly impairing the toughness. In contrast, the ALD treatment toughens NCCs by much over 300%. Our finding provides insights for reinforcing and toughening various nanoparticle-based and nanoporous materials.
I. INTRODUCTION
Nanocolloidal crystals (NCCs) have been intensively studied due to their promising applications, such as optical devices,1 data storage,2,3 waveguides,4–6 and sensors.7 However, their poor mechanical properties significantly hinder these applications.8–12 Thermal sintering is the most common reinforcement technique for colloid assemblies.13–20 Moreover, to synthesize inverse opals which are another type of emerging advanced optical materials, it is typical to use NCCs as infiltration templates which are usually pre-reinforced to maintain their integrity during the template-infiltration process,11,21–32 for example, SiO2 NCC templates are commonly sintered at temperatures above 800 °C,21–26,28,30–32 and polystyrene NCC templates are commonly sintered at 80–85 °C.11,29 The initial stage of sintering-induced reinforcement of colloid assemblies involves the formation of interparticle neck through various mechanisms, such as surface diffusion, volume diffusion, evaporation-condensation, and plastic/viscous flow.20,33–37 The dynamics of neck growth depends strongly on sintering temperature Ts, particle size, and the associated neck-formation mechanism(s).20,33–37 For example, Poppe quantitatively studied the effects of Contributing Editor: Yang-T. Cheng a) Address all correspondence to this author. e-mail: [email protected] b) Author’s present affiliation is: Department of Chemical Engineering, Carnegie Mellon University, Pittsburgh, Pennsylvania 15213, USA. DOI: 10.1557/jmr.2015.347 J. Mater. Res., Vol. 30, No. 23, Dec 14, 2015
sintering temperate and duration on neck growth for monodisperse SiO2 micro-colloids (MCs).33 Poppe found that33 the SiO2 MC assembly has a melting temperature of ;1250 °C, and that significant sintering-induced neck formation occurs at Ts ≳ 1000 °C. Also, the dominant mechanism of neck transitions from surface diffusion to
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