Abstract
The growth and solid-state dewetting behavior of Au thin films (0.7 to 8.4 nm) deposited on the formvar film (substrate) by sputtering technique have been studied using transmission electron microscopy. The size and number density of the Au nanoparticles (NPs) change with an increase in the film thickness (0.7 to 2.8 nm). Nearly spherical Au NPs are obtained for <3 nm thickness films whereas percolated nanostructures are observed for ≥3 nm thickness films as a consequence of the interfacial interaction of Au and formvar film. The covered area fraction (CAF) increases from ∼13 to 75 % with the change in film thickness from 0.7 to 8.4 nm. In-situ annealing of ≤3 nm film produces comparatively bigger size and better sphericity Au NPs along with their narrow distributions, whereas just percolated film produces broad distribution in size having spherical as well as elongated Au NPs. The films with thickness ≤3 nm show excellent thermal stability. The films having thickness >6 nm show capability to be used as an irreversible temperature sensor with a sensitivity of ∼0.1 CAF/°C. It is observed that annealing affects the crystallinity of the Au grains in the films. The electron diffraction measurement also shows annealing induced morphological evolution in the percolated Au thin films (≥3 nm) during solid-state dewetting and recrystallization of the grains.
Highlights
Studies on island, percolated, and continuous polycrystalline metal (Au, Ag, and Cu etc.) films are of great interest due to their potential applications in the field of catalysis,[1,2] magnetic memory,[3] solar cell[4] and optical response based chemical, biological and gas sensors.[5,6] The ultra-thin metal films of different morphologies are used to improve the sensitivity of surface-enhanced Raman spectroscopy (SERS)[7,8,9] as well as for tuning the color of an object in transmission and reflection mode.[10]
Various stages of growth are identified in term of real space parameters (RSP’s) like particle size, circularity, number density and the covered area using transmission electron microscopy (TEM), which fulfills the demand of controlled morphological growth of Au thin films
The just percolated film (4.2 nm thickness) shows transition threshold for percolation-to-island state after ∼120 min annealing that results in discrete spherical nanoparticles along with some dumb-bell, tri-bell and more complex geometries
Summary
Similar to other techniques, sputtering imposes a constraint on the size and number density of the particles because of the formation of a semicontinuous or continuous film for longer deposition time that results in a broad or no LSPR response. The transformation of such semicontinuous or continuous thin films into random island films is possible by post-deposition thermal annealing.[33,34] study of the growth mechanism of thin metal films and its morphological evolution during post-deposition annealing is important in order to produce the nanoparticles of appropriate sizes and shapes from different thickness films for efficient LSPR and sensor applications. The percolation-to-island transition state as well as thermal stability of the films has been discussed for the fabrication of irreversible temperature sensor along with recrystallization during the solid-state dewetting process
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