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Abstract
Here, we prepared silver (Ag) dendrite fractal nanostructures by a facile electrochemical deposition method. The silver dendrite fractal nanostructures exhibit multiple plasmon resonances, which leads to an enhanced second harmonic generation (SHG). Interestingly, the prepared thin film can be used as plasmonic substrates. As an example application, we selected surface-enhanced Raman scattering (SERS) spectroscopy with tunable sensitivity. Using 1,4-benzenedithiol (1,4-BDT) as a probe molecule, the SERS intensity of dendrite fractal nanostructures is approximately 77 times larger than that of the flower-like nanoplates (one of the structures that appears during the growth), and a low detection limit of 10−14 M 1,4-BDT can be achieved. Our results offer a low-cost and easy strategy in fabricating nonlinear photonic nanodevices and SERS chips.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Metallic nanostructures with largely enhanced local electric field near the metal surface and tunable optical response have attracted considerable research interests with various applications in the areas of nanoscience, biology and light–matter interactions [1–10].The local field enhancement and confinement in plasmonic nanostructures provide suitable conditions for various linear or nonlinear optical processes [11–18]. Since Xu et al. found that the “hot spots” in the nanogap could be used for efficient light harvesting and concentration, metallic nanostructures with strong electromagnetic enhancement have gained enormous attention in SERS for single molecule detection and biosensing during the past two decades [11]. To achieve higher plasmonic enhancement, the gap distance between adjacent nanostructures should be as narrow as possible. E-beam lithography and ion-beam lithography feature high resolution in tuning the gap distance in a plasmonic antenna arrays with arbitrary patterning [19–21]. However, it suffers from high cost and low throughput. Chemical synthesis methods could provide tailorable sizes, shapes and compositions of the prepared nanostructure with down to the atomic level, yet, it still lacks the versatile of patterning of particles on solid-state substrates, which is highly desirable in device applications.
Electrochemical deposition provides an easy and low-cost strategy for fabrication of large-area plasmonic substrates. Conditions like the deposition time, current density, and electrolyte solution concentration can be tuned, which make it relatively easy to control the morphology of electrodeposited products. In recent years, this electrochemical deposition method has been used extensively and successfully to fabricate metal micro/nanostructures with well-defined shapes, such as nanorod arrays [22,23], nanosheets [24], pyramids [14], flower-like particles [25] and dendrites [26]. For example, Chen et al. have reported a novel pyramid-shape silver microstructure for in situ Raman spectroscopy study using a two indium tin oxide (ITO) glass electrode system [14]. Among all these micro/nanostructures, hierarchical fractal structure is a promising candidate with many multi-level branches, edges, tips, and nanogaps. These hierarchical fractal structures with high surface area could generate a large amount of “hot spots” with highly concentrated electromagnetic field [13,26–31]. Additionally, the coupled plasmon resonances of hierarchical/fractal clusters span a broad region of the spectrum, allowing for strong resonance effects over a wide wavelength range [32,33]. Therefore, metallic dendrite fractal nanostructures are very useful for some nonlinear optical applications and SERS detection.
In this paper, we have successfully electrodeposited the Ag dendrite fractal nanostructures on ITO glass substrate by a facile electrochemical method. Interestingly, the multiple plasmon resonances induced by dendrite fractal nanostructures and the enhanced SHG were observed. Moreover, the Ag dendrite fractal nanostructures also exhibited large SERS enhancement for detecting 1,4-BDT molecules. FDTD simulations were also carried out to reveal the electric field distributions and large field enhancement in the dendrite fractal nanostructures.
2. Experimental
The Ag dendrite fractal nanostructures were prepared through an electrochemical deposition procedures that have been described in our previous work [25]. Electrochemical deposition was performed with a two-electrode system, where the ITO glass (1.5 × 1 cm2, 17 Ω/square) was used as the cathode and a platinum (Pt) plate as the anode, as shown in Fig. 1(a). The electrolyte solution was composed of an aqueous solution of 2 g/L silver nitrate (AgNO3) and 40 g/L citric acid. Electrochemical deposition was carried out under a constant current density of 1 mA·cm–2 at room temperature. After the electrodeposition process, all of the samples were rinsed with ultrapure water several times and dried with high-purity flowing nitrogen before analysis.
Fig. 1 (a) Schematic illustration for shape-controlled synthesis of Ag micro/nanostructures. (b)-(e) SEM images of the Ag micro/nanostructures electrodeposited at different concentration of AgNO3: 0.5, 1, 2 and 4 g/L, respectively. The citric acid concentration was 40 g/L and electrodeposition was carried out under a constant current density of 1 mA·cm–2 for 120 s.
The morphology of the Ag nanostructures was characterized by using a scanning electron microscope (SEM, S4800). The extinction spectra were measured at room temperature using an ultraviolet-visible-near infrared (UV-Vis-NIR) spectrophotometer (Varian Cary 5000). The samples were immersed in 3 mL of 10−6–10−14 M 1,4-BDT ethanol solutions for 6 h, and then taken out and high-purity flowing nitrogen for the SERS measurements. The SERS spectra were measured using a micro-Raman system (HORIBA Jobin Yvon LabRAM HR800) equipped with a thermoelectrically cooled CCD detector. The excitation source for SHG was a mode-locked Ti/Sapphire laser (Mira 900, Coherent) with a pulse width of 150 fs and a repetition rate of 76 MHz. The SHG signals were collected by using a spectrometer (Spectrapro 2500i, Acton) with a liquid-nitrogen-cooled CCD (SPEC-10, Princeton).
3. Results and discussion
The electrochemical deposition method introduced here is a facile and effective way for shape-controlled synthesis of metal micro/nanostructures. As shown in Fig. 1, we have obtained four morphologies of Ag micro/nanostructures by adjusting the concentration of AgNO3 and keeping all other parameters fixed. When AgNO3 concentration is varied from 0.5, 1, 2 to 4 g/L, the morphology of the Ag product is transformed from meatball-like nanoparticles (Fig. 1(b)), leaf-like rods (Fig. 1(c)), highly-branched dendrites (Fig. 1(d)) to micro-hemispheres (Fig. 1(e)). Here, the electrochemical deposition of Ag micro/nanostructures is a non-equilibrium growth process, the final morphology depends on the formation conditions departure from thermodynamic equilibrium. These results indicated that the proper concentration of AgNO3 was critical to the formation of Ag dendrite fractal nanostructures.
The growth processes of Ag dendrite fractal nanostructures were systematically investigated by collecting SEM images of Ag nanostructures electrodeposited at different deposition times, which are presented in Fig. 2. The other electrodeposition conditions are the same (i.e., the electrolyte containing 2 g/L AgNO3 and 40 g/L citric acid, and the current density is 1 mA·cm−2). A transformation stage from the flower-like Ag nanoplates to the highly-branched dendrite fractal nanostructures was found in the morphological evolution. For the short deposition time (t < 30 s), no Ag dendrite was found on the ITO glass substrate, and only some flower-like nanoplates grew and scattered on the substrate (Fig. 2(a)). As the deposition time increased to 30 s, some small Ag dendrites with short numerous branches begin to grow from the tips of flower-like nanoplates as depicted in Fig. 2(b). However, when the deposition is prolonged to 180 s, the Ag dendrites become larger and more complicated (Fig. 2(d)), including a long main trunk with secondary or multi-level branches. With longer deposition time (t ≥ 240 s), the dendrites extended laterally and vertically to form a multilayer dense “jungle” coating on the whole surface of the ITO glass as depicted in Fig. 2(e) (low magnification SEM image and the SEM image of the cross section of the sample are shown in Fig. 2(f)). Fractal growth is non-equilibrium, and the growth mechanism is explained as a diffusion-limited aggregation model [34,35]. In the formation process of Ag dendrite fractal nanostructures, a large number of nanoparticles were formed and evolved into the dendrites by self-assembly that was dominated by oriented attachment. Citric acid can serve as a capping agent for the formation of Ag hierarchical structures as it can selectively bind to Ag surfaces controlling the growth of Ag nanoparticles [36–38]. Here, citric acid selective adhesion to different crystallographic planes of the Ag nanoparticles was believed to play an important role in the formation of Ag dendrite fractal nanostructures.
Fig. 2 (a)-(e) SEM images of the Ag nanostructures electrodeposited on ITO glass substrates for 20, 30, 60, 180, and 240 s, respectively. (f) Low magnification SEM images of the Ag dendrite fractal nanostructures prepared by electrodeposition for 240 s. The inset shows the SEM image of the cross section of the sample.
The UV-Vis extinction spectra of the Ag nanostructures electrodeposited on ITO glass substrates at different deposition time was measured, as shown in Fig. 3(a). Multiple plasmon resonance peaks could be observed. Furthermore, the UV-Vis spectra peaks corresponding to higher-order multipole plasmon resonance modes at the shorter wavelength region (< 500 nm) were observed due to large size and complex morphology of the Ag nanostructures. The extinction strength at 400 and 800 nm increased with the increase of deposition time (Fig. 3(b)), which is ascribed to the increase in size, amount and electromagnetic coupling strength of the Ag nanostructures. When the deposition time is shorter than 240 s, the Ag nanostructures on the substrate are isolated, and thus the extinction strength at 400 nm is slightly larger than that at 800 nm for the localized surface plasmon resonances effect. While with the deposition time increases, i.e., t > 240 s, the dendrites connect with each other and form “pseudo-continuous film” structure on the whole surface of ITO glass (Fig. 2(f)), which leads to the percolation lineshape of spectrum in the near infrared regime.
Fig. 3 (a) UV-visible extinction spectra of the Ag micro/nanostructures electrodeposited at different deposition time. Curves are shifted vertically for clear presentation. (b) The plots of extinction intensities against the deposition time of Ag nanostructures.
Dendrites have a hierarchical fractal structure with a large amount of “hot spots”, which could enhance the sensitivity of SERS measurement [13,26–31]. The SERS performance of the Ag dendrite fractal nanostructures was investigated by using 1,4-BDT as a probe molecule. Figure 4(a) shows the SERS spectra of 10−6 M 1,4-BDT molecules adsorbed on the surface of Ag nanostructures with different deposition time. The excitation laser wavelength is 488 nm. The bands at 730, 1178, and 1563 cm−1 were assigned to the 7a, 9a, and 8a vibrational modes [39,40], respectively. The broad peak centered at 1067 cm−1 was assigned to the ν1 fundamental in Fermi resonance with a combination mode consisting of 6a and 7a vibrational modes [39,40], which may be due to the adsorption configuration of 1,4-BDT molecules on the Ag surface [40]. The corresponding SERS intensities (at 1563 cm−1) varying with the deposition time is shown in Fig. 4(b). It is clearly seen that the SERS intensity dramatically increased with the deposition time of Ag nanostructures and reached a maximum value at t = 240 s. The SERS intensity of 1,4-BDT adsorbed on the Ag dendrite fractal nanostructures (t = 240 s) is approximately 77 times larger than that of the flower-like nanoplates (t = 20 s). The Ag dendrite fractal nanostructures exhibited intense SERS signals may be ascribed to three reasons: (i) the Ag dendrite supports abundant “hot spots” with high local filed enhancement factors, which originate from the plasmon coupling formed in the nanogaps between the adjacent branches [27,28]; (ii) the Ag dendrite provides a larger surface area for the spatial loading pore molecules and longer pathway for light propagation in the multilayered Ag dendrite “jungle”; (iii) the laser depth of field could be efficiently used due to the 3D SERS substrate [41]. To further evaluate the SERS sensitivity of the Ag dendrite fractal nanostructures, a series of SERS spectra of 1,4-BDT with concentrations from 10−6 to 10−14 M adsorbed on the Ag dendrite fractal nanostructures were measured as shown in Fig. 4(c). The SERS signals of 1,4-BDT monotonically decrease with the decreased concentration. Additionally, the SERS spectrum of 1,4-BDT could be still detected clearly when the concentration of 1,4-BDT solution was as low as 10−14 M, which indicated that the prepared Ag dendrite fractal nanostructures can act as excellent SERS platforms for low concentration molecules detection.
Fig. 4 (a) SERS spectra of 10−6 M 1,4-BDT molecules adsorbed on the Ag micro/nanostructures electrodeposited at different deposition time. (b) SERS (peak at 1563 cm−1) intensities against the deposition time of Ag nanostructures. (c) SERS spectra of 1,4-BDT molecules with concentrations from 10−6 to 10−14 M adsorbed on the Ag dendrites prepared by electrodeposition for 240 s.
As is known to all, SHG of metal nanostructures is a second order nonlinear optical process, which strongly benefits from the large enhancement of the local electromagnetic fields of fundamental wavelength and second harmonic frequency [15,16,42–46]. In recent years, various metal nanostructures with plasmon resonances at both the excitation and emission wavelengths have been designed to enhance SHG, such as multiresonant log-periodic silver nanoantennas [44], asymmetric Au-core/Ag-shell nanorods [16], doubly-resonant arrays of nanoparticles [45] and bowtie apertures [46]. SHG response is also constrained by material symmetry. The symmetry breaking in a plasmonic nanostructure can introduce higher-ordered multipole plasmon resonance modes, which is accompanied by larger local field enhancements than symmetric nanostructures [16,47]. Recently, SHG response of noncentrosymmetric metal nanostructures such as, Au-core/Ag-shell nanorods [16], 3D nanocup [48], T-/L-shaped [42,49,50], and nanodolmens [51] have also been investigated.
Here, we also show that the prepared Ag structures could support strong SHG. Figure 5(a) presents the SHG spectra of Ag nanostructures electrodeposited at different deposition times. Note that the experimental conditions are the same for all the samples (laser wavelength, laser power and integration time during the spectra recording). The excitation wavelength used in our experiment is 800 nm, the peak positions and full width at half maximum (FWHM) were almost invariant in all SHG spectra. The central SHG wavelength is ~400 nm and the FWHM is ~3.7 nm. As the deposition time increases, the intensity of SHG signal at the wavelength of 400 nm is significantly enhanced. The quadratic power dependence of the SHG signal with the excitation power reveals the second-order nature of the nonlinear process (Fig. 5(b)). The corresponding varied SHG intensities as a function of deposition time are shown in Fig. 5(c). The SHG intensity gradually increased with the increase in the deposition time from 20 s to 300 s, and decreased when the duration was prolonged to 420 s. The SHG intensity of Ag dendrite fractal nanostructures (t = 300 s) was enhanced by 77 times compared to that of the flow-like nanoplates (t = 20 s). It should be noted that the SHG signal of ITO glass is negligible here (Fig. 5(a)). Both local field enhancement and accumulation of Ag dendrites contribute to the SHG enhancement. The SHG of the Ag dendrites deposited at 300 s is stronger than that of the Ag dendrites prepared for longer deposition time (360 and 420 s), which indicates that the SHG enhancement is not dominant by the accumulation of Ag dendrites. In our previous work [25], we also compared the SHG results of flower-like Ag mesoparticles and micro-hemispheres samples with comparable particle distribution density, and proved that multi-resonant response at the SHG emission wavelengths plays an important role in SHG enhancement. Therefore, we strongly believe that the collaborative enhancement of local fields at both laser excitation and second harmonic wavelengths via fractal-induced multiple plasmon resonances plays a more important role in SHG enhancement [25,44].
Fig. 5 (a) SHG spectra of the Ag micro/nanostructures electrodeposited at different deposition time. (b) Log−log plot of the intensity relationship between SHG emission and excitation power. (c) SHG intensities against the deposition time of Ag nanostructures.
In Figs. 4(b) and 5(c), it is clearly seen that the SHG and SERS intensity first increased with the deposition time of Ag nanostructures and reached a maximum value at t = 300 and 240 s, respectively. The enhancement of SHG and SERS result from both the large local field enhancement induced by the Ag dendrites and the accumulation of Ag products. Additionally, the electric field at the SHG wavelength is enhanced by the multipolar plasmon resonance of Ag fractal nanostructures. The morphology of the Ag nanostructures becomes more complicated and generates abundant “hot spots” as well as the stronger localized electromagnetic field, which was induced by the electromagnetic coupling between the adjacent branches. After reaching the maximum value, the SHG intensity decreases with the deposition time. The reason is that after long time deposition, Ag dendrites extend vertically and form a dense multilayered “jungle” (Fig. 2(f)), which can be treated as a quasi-continuous film (also see the percolation lineshape of extinction spectra in Fig. 3), leading to the decrease of local field enhancement. However, for the SERS, excess amount of Ag dendrites could not affect the signal intensity obviously because the spot range and depth of the laser beam on the samples are limited [30,52]. This is why they reach the maximum at different deposition times and exhibit different behaviors with the increase of deposition time.
Theoretical simulations were carried out by using the finite- difference time domain (FDTD) method with the software FDTD Solutions 8.6. The dielectric constant of silver was taken from Palik’s book [53]. A circularly polarized incident light is used to illuminate the dendrite fractal nanostructures. Figure 6(a) presents the calculated extinction spectrum of a single Ag dendrite fractal model. Multiple plasmon resonance peaks are observed. The calculated spatial distribution of the electric field intensity at 800 nm and 488 nm are shown in Figs. 6(b) and 6(c), respectively. It can be found that numerous hot spots exist in the nanogaps between the adjacent branches and the end of branches, which indicated strong plasmon coupling formed in the nanogaps. It should be noted that we only calculate the local field distribution of a single Ag dendrite fractal nanostructure, and the actual local field enhancement and distribution of the sample should be larger and more complex.
Fig. 6 (a) Calculated extinction spectrum of a single dendrite fractal nanostructure illuminated by a circularly polarized incident light. (b) and (c) FDTD simulations of local field distribution at 800 nm and 488 nm, respectively.
4. Summary
In conclusion, we have presented a simple electrochemical deposition method for the shape-controlled fabrication of Ag dendrite fractal nanostructures at a proper AgNO3 concentration and deposition time. The Ag dendrite fractal nanostructures exhibited large SERS enhancement, and the detection limit of 1,4-BDT is as low as 10−14 M. Additionally, both the multiple plasmon resonances induced by dendrite fractal nanostructures and significantly enhanced SHG were observed in the Ag dendrite fractal nanostructures, which was 85 times stronger than that in the flower-like nanoplates. Moreover, by using FDTD simulation, the significantly plasmon coupling is formed in the nanogaps, and large field enhancement effect is also observed in the dendrite fractal nanostructures. The Ag dendrite fractal in our experiment may have potential applications in nonlinear photonic nanodevices and SERS substrates.
Funding
National Natural Science Foundation of China (11674254, 11804093, 11565013); National Key R&D Program of China (2017YFA0303402).
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