Mechanism of morphology transformation during annealing of nanostructured gold films on glass

Abstract

Nanostructured, just-percolated gold films were prepared by evaporation on bare glass. Annealing of the films at temperatures close to or higher than the softening temperature of the glass substrate induces morphological transformation to discrete Au islands and gradual embedding of the formed islands in the glass. The mechanism and kinetics of these processes are studied here using a combination of in situ high-temperature optical spectroscopy; ex situ characterization of the island shape by high-resolution scanning electron microscopy (HRSEM), atomic force microcopy (AFM) and cross-sectional transmission electron microscopy (TEM); and numerical simulations of transmission spectra using the Multiple Multipole Program (MMP) approach. It is shown that the morphological transformation of just-percolated, 10 nm (nominal thickness) Au films evaporated on glass and annealed at 600 °C, i.e., in the vicinity of the substrate glass transition temperature (Tg = 557 °C), proceeds via three processes exhibiting different time scales: (i) fast recrystallization and dewetting, leading to formation of single-crystalline islands (minutes); the initial spectrum characteristic of a continuous Au film is transformed to that of an island film, displaying a surface plasmon (SP) absorption band. (ii) Reshaping and faceting of the single-crystalline islands accompanied by formation of circumferential glass rims around them (first few hours); the overall optical response shows a blue shift of the SP band. (iii) Gradual island embedding in the glass substrate (tens of hours), seen as a characteristic red shift of the SP band. The influence of the annealing atmosphere (air, vacuum) on the embedding process is found to be minor. Numerical modeling of the extinction cross-section corresponding to the morphological transformations during island recrystallization and embedding is in qualitative agreement with the experimental data.

Encapsulated Annealing: Enhancing the Plasmon Quality Factor in Lithographically–Defined Nanostructures

“…solution of hydrofluoric acid (HF) for 15 s. The shape and surface roughness of the nanostructures were preserved due to the encapsulation. In contrast, control samples without encapsulation experienced changes in shape, size, and surface roughness16 as can be seen in Fig. S1 of the Supplementary Information (SI).…”

“…However, more detailed studies are needed, e.g. to anneal–out grain boundaries without shape change, and suppress Rayleigh instabilities that break up long and narrow nanostructures1617. As high Q–factor plasmonic resonators are useful in practical applications such as surface–enhanced Raman scattering (SERS)18, color printing19, and light extraction from quantum emitters20, efforts to improve Q–factors are relevant.…”

The process of solid-state dewetting of Au thin films studied by in situ scanning transmission electron microscopy

“…For example, measuring the laser-light transmittance through a Au thin film during heating allowed Jiran et al [42,43] following the kinetics of dewetting and developing a quantitative model described by a Johnson-Mehl-Avrami-Kolmogorov-like behavior. Further in situ methods applied are Auger electron spectroscopy [44], X-ray diffraction [6,[45][46][47], electrical probing [6,48,49] and optical characterization [6,[50][51][52]. All of these methods have in common that they are not capable of directly imaging the process of dewetting.…”

Quasi-coherent thermal emitter based on refractory plasmonic materials

“…On the other hand, metals are well known to support interface modes called surface plasmon-polaritons in a rather broad wavelength range. However, noble metals have thermal stability limits, so heating a metallic structure eventually leads to delamination, diffusion with aggregation into islands and eventually melting and evaporation [16,17]. According to Stefan-Boltzmann's law, intensity of thermal emission source is proportional to the fourth power of the temperature (T 4 ).…”