Various Silver Nanostructures on Sapphire Using Plasmon Self-Assembly and Dewetting of Thin Films
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Silver (Ag) nanostructures demonstrate outstanding optical, electrical, magnetic, and catalytic properties and are utilized in photonic, energy, sensors, and biomedical devices. The target application and the performance can be inherently tuned by control of configuration, shape, and size of Ag nanostructures. In this work, we demonstrate the systematical fabrication of various configurations of Ag nanostructures on sapphire (0001) by controlling the Ag deposition thickness at different annealing environments in a plasma ion coater. In particular, the evolution of Ag particles (between 2 and 20 nm), irregular nanoclusters (between 30 and 60 nm), and nanocluster networks (between 80 and 200 nm) are found be depended on the thickness of Ag thin film. The results were systematically analyzed and explained based on the solid-state dewetting, surface diffusion, Volmer–Weber growth model, coalescence, and surface energy minimization mechanism. The growth behavior of Ag nanostructures is remarkably differentiated at higher annealing temperature (750 °C) due to the sublimation and temperature-dependent characteristic of dewetting process. In addition, Raman and reflectance spectra analyses reveal that optical properties of Ag nanostructures depend on their morphology.
KeywordsAg nanostructures Surface plasmon Self-assembly Dewetting
Various configurations of Ag nanostructures were systematically fabricated on sapphire (0001) by controlling the deposition thickness and annealing environment in a plasma ion coater.
The results were systematically analyzed based on the solid-state dewetting, surface diffusion, Volmer-Weber growth model, coalescence and surface energy minimization mechanism.
Silver (Ag) nanostructures, the dimension range within nanoscale with definite geometric shape, size, and configuration such as nanoparticles (NPs), nanoclusters, and nanowires, have been widely used in optical, electronic, catalytic, sensing, and biomedical devices [1, 2, 3, 4, 5, 6]. Such devices utilize Ag nanostructures for the improved performances, i.e., enhanced scattering and absorption of light, high electrical conductivity, and enhanced sensitivity due to the strong surface plasmon resonance exhibited by the Ag nanostructures [7, 8, 9]. Furthermore, the large surface-to-volume ratio, selective binding, and detection with specific target enable Ag nanostructures to be applicable as the catalysis as well as the chemical and biological sensors [5, 10, 11, 12, 13]. In specific, the increased photocurrent density due to the high electromagnetic field strength in the vicinity of excited surface plasmons can enhance the power conversion efficiency of organic solar cells . The electro spun polymer nanofibers immobilized with Ag NPs can exhibit the superior catalytic reduction with high efficiency and reusability . Generally, metallic nanostructures show the controllable configuration, shape, size, and distribution. For instance, photoelectron lifetime of TiO2 nanotube arrays decorated by Ag NPs can be enhanced because of size-dependent localized surface plasmon resonance, which in turn can maximize the photo conversion efficiency . Although several methods for the Ag nanostructures synthesis have been practiced, thermal approach taking the advantage of solid-state dewetting of the thin film is still a relatively simple and cost-effective approach to control the configuration, shape, and size of the Ag nanostructures [17, 18, 19, 20, 21].
Sapphire has been successfully used in optical devices such as light-emitting diode, laser diodes, and IR-UV detector owing to its wide transparency window from 180 to 5500 nm, wide band-gap, and thermal, chemical, mechanical stability. Therefore, the systematic characterization of Ag nanostructures on sapphire (0001) may be very important for novel applications, which is rarely reported up to date. In this work, we demonstrate the configurational and dimensional transformation of Ag nanostructures on sapphire (0001) via the systematic control of deposition thickness in various annealing environments. The evolution begins with the self-assembly of diffused Ag adatoms at sufficient thermal energy. With the controlled deposition amount, the tiny to the enlarged dome-shaped NPs, the merged nanoclusters, Ag nanocluster networks were fabricated. On the other hand, for the identical deposition range of Ag thin films annealed at a higher temperature, Ag nanostructures show distinct evolution such as the formation of tiny round NPs, round and widely spaced large Ag NPs, and elongated Ag NPs due to the substantial sublimation and high diffusion rate. Furthermore, the optical characteristics of Ag nanostructures were studied by the Raman and reflectance spectra.
3 Experimental Section
3.1 Substrate Preparation
The systematic study of Ag nanostructures was performed on 430-micron-thick c-plane sapphire (0001) with off-axis ±0.1° (iNExus Inc, South Korea). Prior to the fabrication, the wafers were diced into small uniform pieces using mechanical saw and subjected to the degassing in a pulsed laser deposition (PLD) chamber to remove the gaseous and particle contaminants. The degassing was performed at 600 °C for 1800 s under 1 × 10−4 Torr. Figure S1 presents the surface morphology of sapphire after the degassing with smooth surface morphology and the Raman spectra depicts the five active phonon modes of the sapphire.
3.2 Fabrication of Ag Nanostructures
In order to investigate the evolution of Ag nanostructures, the deposition amount of Ag was systematically varied at a fixed and distinct annealing condition. Various amounts of Ag were deposited on substrates by sputtering in a plasma ion coater. The thickness of Ag films was controlled by the deposition time at a growth rate of 0.1 nm s−1 with ionization current of 5 mA under 1 × 10−1 Torr. After the deposition, the uniform distribution of Ag atoms on substrates was confirmed by the atomic force microscope (AFM) scanning before annealing. The surface morphology became rougher with higher deposition thickness as shown in Fig. S2. Two series of samples with identical thickness between 2 and 200 nm were prepared. Then, the as-deposited samples were subsequently annealed at distinct temperatures of 550 and 750 °C with the linear ramping rate of 4 °C s−1 in the PLD chamber under 1 × 10−4 Torr for 180 s. Then the system temperature was quenched down to the ambient.
The morphologies of the as-prepared Ag nanostructures were carried out by AFM (XE-70, Park Systems Corp., South Korea) in an ambient condition with a non-contact mode. For the consistency of measurement and minimal tip effect, all the samples were scanned using the NSC16/AIBS tips with a drive frequency of ~270 kHz. The surface morphologies and the evolution of Ag nanostructures were analyzed by XEI software in terms of top-views, side-views, cross-sectional line profiles, Fourier filter transform (FFT) spectra, surface area ratio (SAR), and RMS roughness (R q). Larger-scale surface morphology was investigated by a scanning electron microscope (SEM, CX-200, COXEM, South Korea). The elemental characterization of samples was performed by an energy-dispersive X-ray spectroscope (EDS, Noran System 7, Thermo Fisher Scientific, USA). The Raman spectra were measured using UNIRAM II (UniNanoTech Co. Ltd, South Korea) with a CW 532-nm laser at 220 mW excitation. Reflectance spectra were obtained using deuterium light for UV region and halogen light for visible and NIR region. The optical measurements were carried out at in an ambient condition in dark room.
4 Results and Discussions
From the above two relations, the diffusivity and the diffusion length l D can be constant for all the samples due to the constant annealing environment. Initially, at the deposition of 2 nm, the tiny and highly compact 3D Ag NPs were fabricated due to the stronger interatomic interaction between Ag adatoms than that between the adatoms and the sapphire atoms as shown in Fig. 1a. As a result, once the nuclei form at low energy sites, they absorb the diffusing Ag adatoms due to strong bonding energy . Therefore, the evolution of 3D Ag NPs can be explained on the basis of the Volmer–Weber growth model . For the 3D island growth, the surface and interface energy enforces the dewetting by the condition: γ sapphire < γ Ag + γ interface, where γ sapphire is the surface energy of substrate; γ Ag is the surface energy of the film; and γ interface is the interfacial energy. Similarly, the short-range intermolecular forces (van der Waals forces) correspondingly govern the self-assembly of the Ag NPs which in turn enhance the dewetting process . Consequently, the equilibrium morphology with reduced free energy can be obtained by the Ag NPs, and the overall energy of the thermodynamic system can be minimized .
After increasing the film thickness, the surface morphology developed into the enlarged semi-spherical (dome) NPs, and eventually increased the spacing between the neighboring NPs. As the thickness of Ag film increases, coalescence between adjacent Ag NPs occurs, resulting in the formation of enlarged NPs and the decreasing in NP density . During the coalescence, relatively smaller NPs are ripened, in other words, small NPs can be absorbed by the large ones due to the difference in the surface energy. As a result, NPs’ size keeps increasing with the increased initial film thickness that was driven by the total surface energy minimization mechanism . On the other hand, the weak intermolecular forces like Van der Waal forces between Ag NPs lead the self-assembly process more pronounced in order to attain the equilibrium with lowest energy configurations. Meanwhile, the Ag NPs exhibit dome-shaped structures with an isotropic energy distribution, which can be the natural adaption of total surface energy minimization by the nanostructures [33, 34].
The consequence of Ag NP evolution is more clearly depicted in the small-scale AFM side-views in Fig. 1a-3 to e-3. The vertical bars of 3D side-views represent the increased range of height with the color variation. For instance, with the deposition thickness of 2 nm, the color variation ranges from −2.5 to 5 nm, whereas for 20 nm the range extended from −20 to 40 nm. Furthermore, the overall morphological enhancement can be correspondingly observed in terms of the cross-sectional line profiles and 2D FFT power spectra as shown in Fig. 1a-1 to e-1 and a-2 to e-2. The cross-sectional line profiles represent the height profiles of surface in reference to the line drawn in corresponding AFM side-views. As suggested by the line profiles, the average height of the NPs is gradually elevated such as ~6 nm for 2–40 nm for 20 nm. Similarly, the dome-shaped cap of the line profiles is gradually expanded horizontally and vertically as the initial film thickness is increased, which shows the simultaneous growth of Ag NPs in both directions. The 2D FFT power spectra, that denote the height distribution of overall surface, uniformly reduce in size with the higher degree of film thickness.
Figure 7 represents the reflectance spectra of samples with respect to the film thickness. The reflectance spectra were analyzed within the range of 250–1100 nm including UV, visible, and NIR regions. For the bare sapphire (0001), the average reflectance is measured as 8.97%. Depending on the evolution of Ag NPs, merged NPs, and nanoclusters network, the reflectance curve shows distinct shape and average reflectance. As shown in Fig. 7a, correspondent to the Ag NPs phase between 2 and 20 nm, the average reflectance is gradually increased from 7.64% to 33.92% as the dome-shaped NPs size is increased with respect to the film thickness. At the deposition of 2 nm, highly compact small Ag NPs can cause the maximum forward scattering of photon, which cannot be detected. As a result, average reflectance can be lower than that of bare sapphire (0001). For 6–20 nm samples, the development of peaks in the reflectance signals was observed within the spectral range from 400 to 700 nm, whereas reflectance minima were observed in longer wavelength. Meanwhile, the peaks are gradually red-shifted from ~507.53 to ~536.15 nm as the average size of Ag NPs is gradually increased. The high reflection exhibited by the samples within visible spectral range can be attributed to the backscattering of the Ag NPs, which increases with the increased NPs’ size. At the same time, much lower reflectance was observed in the NIR (~650–1000 nm) region, which can be explained by the substrate influence on the angular scattering from the NPs that can randomize the direction of reflected light [42, 43]. Moreover, the plasmon resonance effect of the Ag NPs can significantly reduce the reflectance within the NIR region . With the growth of merged nanostructures, the reflectance is gradually decreased with the higher initial film thickness of Ag. As the density of Ag NPs is decreased and the spacing between the adjacent nanoclusters has been increased with the higher film thickness, less amount of backscattering can be expected as the surface coverage is reduced . Furthermore, the shape transformation from the dome to the irregular can influence the backscattering and hence lower the reflectivity . In the growth phase of Ag nanoclusters network evolution with the deposition thickness varied between 40 and 100 nm, the reflectance is again gradually increased. The increase in the average reflectivity can be attributed to the increased size of Ag nanoclusters. The specific average reflectance values with respect to the film thickness are summarized in Table S2.
At the 6 and 10 nm of initial film thickness, the surface developed with the formation of highly dense tiny NPs. For the further deposition variation between 14 and 20 nm, few relatively large Ag NPs are distinguished along with the tiny NPs on the background. With increased deposition between 30 and 80 nm, the fabrication of round dome-shaped Ag NPs is witnessed as shown in Fig. 8f–i. With the further increased initial film thickness to 100 and 200 nm, the Ag NPs merge and irregular Ag NPs are fabricated. More detailed analysis along with the enlarged views of AFM images and corresponding line profiles is presented in Fig. 9. At 2 nm of film thickness, the line profile shows almost flat surface profile, and for 6 nm few up and down are observed caused by the Ag NPs. The average height of cross-sectional line profiles is consistently increased with the initial film thickness, which represents the dimensional enhancement of surface morphology due to the formation of Ag NPs and merged nanostructures. The large-scale surface configuration at relatively higher Ag thickness can be witnessed on the SEM images in Fig. 10a–f. In addition, large-scale AFM top-views are presented in Figs. S7 and S8.
Surface enhancement can be equivalently discussed on the basis of surface parameters, such as RMS roughness (R q) and surface area ratio (SAR) as represented in Fig. 10g, h. As the surface consists of Ag NPs, whose dimensions are gradually increased with the added initial film thickness, the magnitude of R q also consistently increased. However, the SAR value increases up to 40 nm due to the formation of round dome NPs with the enhanced dimension and gradually decreased for higher thickness from 30 to 100 nm. The R q and SAR values are summarized in Table S1. The elemental characterization by EDS spectra as shown in Fig. S9, indicates the significantly low counts of Ag Lα1 due to the Ag sublimation at 750 °C. For a comparison with 200 nm sample, at 550 °C the Ag Lα1 count is ~14,494, whereas at 750 °C the count is ~2028. Similarly, for other corresponding samples at 750 °C, the Ag Lα1 counts are significantly lower than that of 550 °C due to sublimation.
In summary, we demonstrate various configurations of Ag nanostructures on sapphire (0001) including NPs, irregular nanoclusters, and nanoclusters network formed at different annealing conditions and dependent on the variable thickness of Ag thin film. Based on the control of initial film thickness between 2 and 200 nm at 500 °C of annealing, three distinctive growth regimes of Ag nanostructures were observed: Specifically, the tiny to the enlarged dome-shaped Ag NPs between 2 and 20 nm, the merged and irregular nanoclusters between 30 and 60 nm, and the Ag nanoclusters network between 80 and 200 nm. The evolution of Ag nanostructures is symmetrically analyzed on the basis of surface diffusion, Volmer–Weber growth model, coalescence, and surface energy minimization mechanisms. The identical range of film deposition range was studied at higher annealing temperature (750 °C), which shows distinctive evolution of Ag nanostructures as a result of thickness-dependent characteristics of dewetting phenomena as well as the significant sublimation of Ag. Furthermore, the morphology dependence optical properties of Ag nanostructures were probed by the Raman and reflectance spectra analysis.
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 2016 is gratefully acknowledged.
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