Featured Letter Impact of microfabrication processing temperatures on the resonant wavelength of gold nanoplates J. Beharic a , K.T. James b , R.S. Keynton b,1 , M.G. O’Toole b , C.K. Harnett a,⇑ a Electrical and Computer Engineering Department, University of Louisville, Louisville, KY 40208, USA b Bioengineering Department, University of Louisville, Louisville, KY 40208, USA article info Article history: Received 19 June 2020 Received in revised form 9 October 2020 Accepted 2 November 2020 Available online 5 November 2020 Keywords: Gold nanoplates Melting Thermal properties Plasmonic resonance Nanoparticles abstract We show that low temperature processing (100–400 °C) of gold nanoplates (GNPs) permanently shifts their resonant wavelength peak from the infrared to visible (860–650 nm) after an hour at 350 °C. The mechanism is melting-induced shape change; as sharp triangular corners become round the smaller effective GNP diameter produces shorter resonant wavelengths. These wavelength shifts are relevant to GNP-driven microelectromechanical systems, where temperatures of 150–350 °C arise during thin film processing. Strategies to accommodate the observed thermal shifts include starting with particles that have a longer resonant wavelength, ensuring that nanoplates are well-dispersed, and using sub-200 °C processing methods. Ó 2020 Elsevier B.V. All rights reserved. 1. Introduction Gold nanoparticles are valued for their tunable resonant wave- lengths. Isotropic [1,2] and anisotropic [3] nanoshells, nanorods [4], and nanoplates [5-7] have size-controlled resonance peaks. Gold nanoshells with near-infrared (NIR) plasmonic resonances have been investigated for photothermal cancer therapy [1,2] and as light-responsive coatings [8] to drive microdevices [9]. Gold nanoplates (GNPs) absorb NIR wavelengths, yet are thin enough (~10 nm) not to disrupt oxide (70 nm) and polymer (100– 300 nm) films used in microelectromechanical systems (MEMS). Conversely, gold nanoshells need diameters exceeding 100 nm to absorb in the NIR [10]. Nanoparticles’ melting temperature is known to be significantly lower than bulk (1064 °C). For example, larger plate-shaped gold particles (70 nm thick, 30–50 mm side lengths vs. our ~10-nm thick, 100–200 nm side lengths) break into fragments at only 450 °C [11]. Mild thermal exposure modifies nanoparticle resonances because absorbance wavelength is a strong function of shape [12]. Gold nanorods in the 50–100 nm length range permanently shift even at 200 °C [13,14]. In microfabrication, 150–350 °C temperatures arise during thin film vacuum deposition and spin-casting when building thermally- driven MEMS around nanoparticles. This work investigates the impact of sub-400 °C temperatures on melting and resonant wave- lengths of GNPs so researchers can account for these issues. 2. Material and methods We measured heating-induced wavelength shifts using optical transmission spectroscopy between 650 and 1100 nm for triangular GNPs on glass surfaces. The GNPs, produced according to [8] and then purified to reduce the fraction of spherical particles [15], were dis- persed in an aqueous solution. Borosilicate glass slides (VWR, Inc) were soaked in a base bath overnight, then rinsed 3x with DI water and sonicated in 70% ethanol for 30 min. (3-aminopropyl) triethoxysilane (APTES) was added at 1% v/v. After an hour of sonica- tion, sides were rinsed 3x with 70% ethanol and sonicated for another hour, then dried before applying GNPs (Fig. 1b). Fig. 1c shows three sampling zones used in a Cary 100 UV–Vis spectrometer (Agilent Technologies, Santa Clara, CA) to inspect for variations. Single nanoparticles were also measured at 60 Â magnification in a darkfield À epifluorescence microscope with hyperspectral imaging system (CytoViva, Inc., Auburn, AL). This method has previously captured scattering profiles from sin- gle GNPs [16,17]. Individual particles were located on numeric grids etched into the slides for collection of before- and after- heating spectra from the same GNP; these solutions were diluted so the ~ 0.6 mm diameter hyperspectral collection region contained a single particle. https://doi.org/10.1016/j.matlet.2020.128967 0167-577X/Ó 2020 Elsevier B.V. All rights reserved. ⇑ Corresponding author. E-mail address: c0harn01@louisville.edu (C.K. Harnett). 1 Present address: UNC Charlotte MEES Dept. Materials Letters 284 (2021) 128967 Contents lists available at ScienceDirect Materials Letters journal homepage: www.elsevier.com/locate/mlblue