Modulation of Morphology and Optical Property of Multi-Metallic PdAuAg and PdAg Alloy Nanostructures
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In this work, the evolution of PdAg and PdAuAg alloy nanostructures is demonstrated on sapphire (0001) via the solid-state dewetting of multi-metallic thin films. Various surface configurations, size, and arrangements of bi- and tri-metallic alloy nanostructures are fabricated as a function of annealing temperature, annealing duration, film thickness, and deposition arrangements such as bi-layers (Pd/Ag), tri-layers (Pd/Au/Ag), and multi-layers (Pd/Au/Ag × 5). Specifically, the tri-layers film shows the gradual evolution of over-grown NPs, voids, wiggly nanostructures, and isolated PdAuAg alloy nanoparticles (NPs) along with the increased annealing temperature. In contrast, the multi-layers film with same thickness show the enhanced dewetting rate, which results in the formation of voids at relatively lower temperature, wider spacing, and structural regularity of alloy NPs at higher temperature. The dewetting enhancement is attributed to the increased number of interfaces and reduced individual layer thickness, which aid the inter-diffusion process at the initial stage. In addition, the time evolution of the Pd150 nm/Ag80 nm bi-layer films at constant temperature show the wiggly-connected and isolated PdAg alloy NPs. The overall evolution of alloy NPs is discussed based on the solid-state dewetting mechanism in conjunction with the diffusion, inter-diffusion, alloying, sublimation, Rayleigh instability, and surface energy minimization. Depending upon their surface morphologies, the bi- and tri-metallic alloy nanostructures exhibit the dynamic reflectance spectra, which show the formation of dipolar (above 700 nm) and quadrupolar resonance peaks (~ 380 nm) and wide dips in the visible region as correlated to the localized surface plasmon resonance (LSPR) effect. An absorption dip is readily shifted from ~ 510 to ~ 475 nm along with the decreased average size of alloy nanostructures.
KeywordsPdAuAg nanostructures PdAg nanostructures Alloy nanostructures Multi-metallic nanostructures Solid-state dewetting Atomic diffusion process Plasmonic
Atomic force microscope
Energy-dispersive X-ray spectroscope
Localized surface plasmon resonance
Pulsed laser deposition
Surface area ratio
Scanning electron microscope
The recent growing interest in the development of nanodevice and applications is mainly focused on the technique to produce and design the multi-metallic nanostructures, semiconducting polymer as well as thermal transport of metal/semiconductor nanomembrane [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]. Multi-metallic nanostructures are essential components in various applications owing to their multi-functionality, electronic heterogeneity, and site-specific response. Multi-metallic nanostructures can add promising potentials to the development of various sensing, photovoltaic, biomedical, and catalysis applications due to the collective optical, electronic, and catalytic properties [1, 2, 3, 4, 5, 6]. In specific, the multi-metallic nanostructures can offer multi-functionality, specific site response, and electronic heterogeneity, which cannot be exhibited by the monometallic counterparts [11, 12, 13, 14]. For instance, an enhanced light absorption was demonstrated by the bimetallic Ag-Au alloy nanoclusters through the expansion of LSPR bandwidth, which resulted in the significantly improved power conversion efficiency of photovoltaics as compared to the monometallic Ag or Au nanoclusters [15, 16]. In addition, a much higher electro-catalytic activity in the electrochemical oxidation of ethanol was exhibited by the NiAuPt alloy NPs due to the synergetic effect of tri-metallic components of NPs, in which the Pt facilities the ethanol dehydrogenation while the Ni and Au remove the adsorbed intermediates simultaneously . Among the various metallic elements, the Au and Ag NPs have demonstrated promising plasmonic properties while the Pd NPs have exhibited enhanced catalytic properties and chemical stability [18, 19, 20]. Therefore, the controlled fabrication of multi-metallic PdAg and PdAuAg nanostructures by the physical deposition can find further opportunities in the related applications, which, however, has not been reported till now. In this paper, the systematic fabrication of PdAg and PdAuAg nanostructures is demonstrated through the solid-state dewetting on sapphire (0001). The growth dynamics was sharply and systematically compared by using the same thickness of 15 nm tri-layers (Pd/Au/Ag) and multi-layers (Pd/Au/Ag × 5). Various growth parameters such as annealing temperature, annealing duration, film thickness, and deposition order are systematically controlled to achieve diverse configuration, size, and density of PdAg and PdAuAg nanostructures. The evolution of nanostructures is mainly analyzed based on the inter-diffusion, alloying, and diffusion of constituent alloy atoms as well as Rayleigh instability and surface energy minimization mechanisms. The reflectance spectra of corresponding PdAuAg nanostructures exhibit the gradual evolution of absorption dip, quadrupolar, and dipolar resonance peaks at specific wavelength along with the morphology evolution. On the other hand, dynamic plasmonic behavior is observed in the reflectance spectra depending on the size evolution of nanostructures.
Materials Preparation and Thin Film Deposition
Fabrication of PdAuAg and PdAg Alloy Nanostructures
After the depositions, both the tri-layer and multi-layer samples showed smoother morphologies as shown in Additional file 1: Figure S1(c)–(d). Subsequently, the tri-layer and multi-layer PdAuAg samples were annealed at various temperature between 400 and 900 °C to investigate the evolution process along with temperature control in a PLD chamber under 1 × 10− 4 Torr. Each target temperature was reached at a ramping rate of 4 °C/s. For the Pd/Ag bi-layer samples, the annealing duration was systematically varied between 0 and 3600 s at 850 °C to see the time behavior. The temperature was chosen to ensure a sufficient dewetting of thick Pd/Ag bi-layers.
Characterization of PdAuAg and PdAg Alloy Nanostructures
The surface morphology was characterized by an atomic force microscope (AFM) (XE-70, Park Systems Corp., United States of America). The scanning was performed under a non-contact mode at an ambient condition. A scanning electron microscope (SEM) (COXEM, CX-200, South Korea) was utilized for the large-scale morphology characterization operated at 20 kV in vacuum. The elemental analysis and maps of samples were obtained by the energy-dispersive X-ray spectroscope (EDS) system (Thermo Fisher, Noran System 7, United States of America), operated under vacuum. The optical characterization (UV-VIS-NIR reflectance) of corresponding samples was performed using an UNIRAM II system (UniNanoTech Co. Ltd., South Korea).
Results and Discussion
The fabrication of bi- and tri-metallic alloy nanostructures of Pd, Ag, and Au has been successfully demonstrated on sapphire (0001) via the solid-state dewetting. The various surface morphology of the alloy nanostructures were obtained by controlling the annealing temperature, time, and deposition scheme such as bi-, tri-, and multi-layers arrangement. The gradual evolution of over-layered NPs, voids, wiggly nanostructures, and isolated PdAuAg alloy NPs was observed by the annealing of 15-nm-thick Pd/Au/Ag tri-layers. In contrast, the multi-layers films of same thickness (15 nm) demonstrated significantly enhanced overall dewetting at identical annealing temperature such that the voids were evolved at lower temperature and well-spaced regular alloy NPs obtained at higher temperature, which was attributed to the enhanced inter-diffusion and alloying with thinner layers. Furthermore, depending upon the control of annealing time with the Pd150 nm/Ag80 nm bi-layer, the configuration transition from the wiggly connected nanostructure geometry to the isolated PdAg alloy NPs was observed along with the enhanced diffusion of alloyed atoms. The overall growth of the alloy NPs was discussed based on the solid-state dewetting process in conjunction with surface diffusion, interdiffusion, alloy formation, Rayleigh-like instability, and energy minimization. The optical properties of such alloy NPs were investigated by the reflectance spectra, which revealed the formation of absorption dip, quadrupolar, and dipolar resonance peaks at specific wavelength based on the dynamic LSPR effect of different alloy NPs. Both the bi- and tri-metallic alloy NPs exhibited the strong absorption in the visible region and dipolar and quadrupolar resonance peaks in the NIR and UV region, respectively. The quadrupolar was seems to be insensitive with the morphological variation whereas the absorption dip and dipolar peaks were gradually blue shifted with the formation of isolated and smaller alloy NPs.
Financial support from the National Research Foundation of Korea (no. 2011-0030079 and 2016R1A1A1A05005009), and in part by the research grant of Kwangwoon University in 2018 is gratefully acknowledged.
Availability of Data and Materials
The datasets generated during and/or analyzed during the current study are available from the corresponding authors on reasonable request.
PP, SK, MS, SB, and JL participated in the experimental design and carried out the experiments. PP, SK, MS, SB, and JL participated in the analysis of data. PP, SK, and JL designed the experiments and testing methods. PP and JL carried out the writing. All authors helped in drafting and read and approved the final manuscript.
The authors declare that they have no competing interests.
Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
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