Article pubs.acs.org/Langmuir
Molecular Coatings for Stabilizing Silver and Gold Nanocubes under Electron Beam Irradiation Shu Fen Tan,† Michel Bosman,‡,§ and Christian A. Nijhuis*,†,∥ †
Department of Chemistry, National University of Singapore, 3 Science Drive 3, Singapore 117543, Singapore Institute of Materials Research and Engineering, A*STAR (Agency for Science, Technology and Research), 2 Fusionopolis Way, Singapore 138634, Singapore § Department of Materials Science and Engineering, National University of Singapore, 9 Engineering Drive 1, Singapore 117575, Singapore ∥ Centre for Advanced 2D Materials and Graphene Research Centre, National University of Singapore, 6 Science Drive 2, Singapore 117546, Singapore ‡
S Supporting Information *
ABSTRACT: We study the degradation process of closely spaced silver and gold nanocubes under high-energy electron beam irradiation using transmission electron microscopy (TEM). The high aspect ratio gaps between silver and gold nanocubes degraded in many cases as a result of protrusion and filament formation during electron beam irradiation. We demonstrate that the molecular coating of the nanoparticles can act as a protective barrier to minimize electron-beam-induced damage on passivated gold and silver nanoparticles.
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INTRODUCTION Noble metal nanoparticles (NPs) with tunable surface properties are used in many applications, ranging from nanostructured catalysis1 to (quantum) plasmonics,2−5 to surface-enhanced Raman spectroscopy.6−9 The characterization of NPs often involves the utilization of transmission electron microscopy (TEM) or scanning electron microscopy (SEM), but these techniques rely on high-energy electron-beams that can cause unwanted changes to the specimen such as degradation,10 the radiolysis of surface molecules,11 the buildup of contamination,12 electron beam-induced atomic displacements,13 or heating.14 NPs have limited stability under typical TEM and SEM conditions (high vacuum, 10−300 keV electron beam), which can complicate their structural characterization.14 Therefore, a good understanding of NP stability under typical TEM and SEM conditions is needed to avoid drawing erroneous conclusions from experimental observations. This article describes the stability of cube-shaped Ag and Au NPs under typical imaging conditions, the mechanism of NP degradation, and methods to significantly reduce electron beam radiation damage. Even though they are not in their most stable, spherical thermodynamic form, cuboidal noble metal NPs can be kinetically stabilized by an organic monolayer15 such as cetyltrimethylammonium bromide,16 self-assembled monolayers of alkanethiolates,17 or polymers including poly(vinylpyrrolidone) or ethylene glycol,18 but it is not clear how © 2017 American Chemical Society
stable these structures are under electron beam irradiation.11,19−22 Electron-beam-induced damage comes in many forms, such as electron−nucleus scattering,12 which has been identified to produce electrostatic charging, knock-on damage, sputtering,23 and specimen heating.24 On the other hand, inelastic scattering of the fast electrons with the sample core- and valence electrons causes ionization damage,25 desorption-induced electronic transitions (DIET),26 and hydrocarbon contamination12 or electron-beam-induced deposition (EBID).12,27,28 Among the effects mentioned above, the knock-on damage (where the atomic nuclei are displaced to interstitial positions and thereby degrading the crystalline order of metal NPs) and the ionization damage often affect organic monolayer-coated inorganic NPs. 14 Knock-on damage usually occurs in conducting inorganic specimens14 where the accelerating voltage of the incident electron beam is higher than the displacement energy of the material or when the electron dose is high (>1000 e/Å2·s). On the other hand, ionization damage is predominant in insulating materials, such as the organic molecular monolayers in this study. Received: October 12, 2016 Revised: December 4, 2016 Published: January 9, 2017 1189
DOI: 10.1021/acs.langmuir.6b03721 Langmuir 2017, 33, 1189−1196
Langmuir
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To minimize the radiation damage caused by the electron beam, many strategies have been proposed including lowenergy or low-dose imaging techniques,29 specimen cooling with liquid nitrogen,30 or specimen coating.31,32 Specifically, ionization damage could be reduced by lowering the specimen temperature by reducing the atomic mobility30,33,34 while lowering the incident beam energy, and the electron dose could minimize the knock-on effect.35,36 In addition, specimen coating with carbon31,32,37 has also been demonstrated to show a protective effect where the coating minimizes charging and acts as a diffusion barrier, reducing the escape rate for light gaseous elements. Muller and Silcox38 also reported that nonconducting electron-beam-induced carbon contamination can possibly act as both diffusion and sputtering barriers, thereby preventing structural damage. For inorganic materials, the conductive coatings do not reduce only the rate of DIET26 but also the electrostatic charging effect.31 Although this method is able to reduce the radiation damage, coatings may be a source of hydrocarbon contamination, 12 which may complicate the interpretation of the TEM measurements. Sintering is dominant when two NPs are separated by a narrow gap of 1 to 2 nm. Two kinds of sintering mechanisms have been reported:11 (1) Ostwald ripening, where the metal atoms leave a metal particle, diffuse over the sample support and attach to a nearby metal particle, and (2) surface diffusion,2,11,21 where the surface atoms diffuse and come into contact with a neighboring particle, leading to the formation of necklike structures bridging adjacent particles, further driven by the large surface tension resulting from the small particle size. The sintering behavior of bare metal particles is dependent on size, whereas for passivated particles, it is dependent on the stability of the organic ligands with respect to electron beam irradiation. Scholl et al.2 demonstrated the use of the electron beam to induce the motion of NPs along a substrate, following earlier work by Batson et al.,39 who demonstrated the controlled convergence and coalescence of NPs. They attributed the cause of the movement to electron-beamfacilitated surface diffusion of the atoms of the NPs and polarization of the NP in response to the electric field of a passing high-energy electron; these polarized NPs can attract each other via Coulombic interactions. The surface modification of noble metal NPs with organic ligands provides a direct and simple method to altering their surface and optical properties,40 thereby providing tunability and design flexibility in plasmonic materials fabrication.41 These thin (
