Abstract
Metallic nanostructures (NSs) have been widely adapted in various applications and their physical, chemical, optical and catalytic properties are strongly dependent on their surface morphologies. In this work, the morphological and optical evolution of self-assembled Pt nanostructures on c-plane sapphire (0001) is demonstrated by the control of annealing temperature and dwelling duration with the distinct thickness of Pt films. The formation of Pt NSs is led by the surface diffusion, agglomeration and surface and interface energy minimization of Pt thin films, which relies on the growth parameters such as system temperature, film thickness and annealing duration. The Pt layer of 10 nm shows the formation of overlaying NPs below 650°C and isolated Pt nanoparticles above 700°C based on the enhanced surface diffusion and Volmer-Weber growth model whereas larger wiggly nanostructures are formed with 20 nm thick Pt layers based on the coalescence growth model. The morphologies of Pt nanostructures demonstrate a sharp distinction depending on the growth parameters applied. By the control of dwelling duration, the gradual transition from dense Pt nanoparticles to networks-like and large clusters is observed as correlated to the Rayleigh instability and Ostwald ripening. The various Pt NSs show a significant distinction in the reflectance spectra depending on the morphology evolution: i.e. the enhancement in UV-visible and NIR regions and the related optical properties are discussed in conjunction with the Pt NSs morphology and the surface coverage.
Highlights
In last decade, the Pt nanostructures (NSs) have been adapted in widespread applications such as electro-catalytic systems [1,2,3], hydrogen storages [4, 5], light emitting diodes (LEDs) [6], solar cells [7] and photocatalytic applications [8, 9]
As indicated in Fig 2(C), as the diffusion length lD can be enhanced at an increased temperature [29, 30], the diffusing Pt adatoms can have high possibility to travel a further distance and be adsorbed by the nanoclusters driven by the surface energy minimization mechanism [31, 32]
When the annealing temperature (AT) was increased to 650 ̊C, the dimension of NSs was obviously increased with the enhanced surface diffusion, i.e. NSs height reached over 15 nm
Summary
The Pt nanostructures (NSs) have been adapted in widespread applications such as electro-catalytic systems [1,2,3], hydrogen storages [4, 5], light emitting diodes (LEDs) [6], solar cells [7] and photocatalytic applications [8, 9]. Due to the localized surface plasmon resonance (LSPR), metallic NSs are reported in various photochemical and optoelectronic applications and their size, density and configuration dependent light interactions can be utilized to tailor the optical properties, e.g. spectral absorption and scattering [10,11,12]. The controllable fabrication of various Pt NSs on sapphire can provide the essential reference on the pertinent modulation of surface physical, chemical, optical and catalytic properties necessary for the corresponding applications based upon. In this work, we study the self-assembled Pt NSs evolution on c-plane sapphire (0001) based on the control of annealing temperature and duration, in which the coherent effect of surface diffusion, surface energy minimization, Rayleigh instability and Ostwald ripening lead to the formation of various configuration, size and density of Pt NSs. Based on the morphological and elemental characterizations by AFM, SEM and EDS, the evolution of various Pt NSs are demonstrated and the growth models are discussed with the distinct deposition amount. The corresponding variation of intensity and peak positions are discussed along with the NS morphology as well as the surface coverage changes
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