1. Introduction

Surface-enhanced Raman scattering (SERS) signals in live cells can be efficiently obtained using single Ag nanowires without causing serious damage to the cells; this technique is thus widely used for cell endoscopy in biosensing and bioimaging applications [1–3]. The SERS signal from single Ag nanowire probes is mainly dependent on plasmon excitation at the nanowire tips [4]. Owing to the low plasmon excitation with the original tip morphology, the intrinsic SERS signals when using Ag nanowires are generally low and often require the assistance of the plasmon excitation of metal nanoparticles adsorbed onto the nanowire surface [5]. However, the adhesion of metal nanoparticles on Ag nanowires is probabilistic, which causes difficulties in the preparation of the SERS probes and the acquisition of steady signals. Meanwhile, the high workability of Ag nanowires (yield strength of only 35 MPa) could introduce defects in the Ag nanowires during the nanowire transfer process or during the SERS detection process [6, 7], which would have a significant influence on the resulting signal quality.

J. Y. Huang et al found that ionic liquids can flow along the electric field direction on the surface of the semiconductor nanowires [8]. J. Q. Hu further discovered that the electric field treatment is not only suitable for ionic liquids, but that molten Au nanoparticles inside Ge nanowires can also flow along the electric field direction [9]. With regard to this electric field driven flow, an accurate melting at the tip of Ag nanowires is needed to achieve controllable tip engineering. The existence of plasmonic phase transitions on metal nanoparticles was discovered by A. Plech et al [10]. They found that a liquid viscous shell with a thickness of several nanometers exists in the plasmon excitation region on the surface of metal nanoparticles under light illumination. Taking advantage of this feature, E. C. Garnett et al welded Ag nanowires via light illumination at temperatures far below the melting point of Ag nanowires [11]. W. Albrecht et al engineered the morphology of Au nanorods within a SiO₂ shell by controlling the excitation region [12]; the tip of Ag nanowires can readily be engineered via the photoelectric field.

Herein, we conducted photoelectricity-mediated tip engineering on a single Ag nanowire for SERS. In our study, an Ag nanowire was arranged between a nano-electrode and the applied light and electric field. In situ atomic force microscopy (AFM) revealed a significant sharpening of the tip and reduction of the surface fluctuations on the Ag nanowire tip under the action of the photoelectric field. After the photoelectric field treatment, the SERS signal of sodium tartrate on an Ag nanowire was greatly enhanced. By using a Finite-Difference Time-Domain (FDTD) simulation and via the deduction of the electrostatic force exerted on the Ag nanowire tip, we were able to attribute the photoelectricity-mediated tip engineering to light-induced melting and electrostatic force driven diffusion on the surface of the Ag nanowire. The sharpened tip of the Ag nanowire provides stronger plasmon excitation, which leads to the enhancement of the SERS signal. This method provides not only an easy and low-cost way to engineer to tip of a single Ag nanowire, but also has great potential applications for nano-device design and fabrication.

Ag nanowires were purchased from Nanjing XFNANO Materials Tech Co., Ltd. (lengths of ~50 µm and diameters of ~200 nm). As shown in Fig. 1, Ag nano-electrodes (thicknesses of ~100 nm and gap widths of ~55 µm) on silica substrates were patterned through a mask using magnetron sputtering. The Ag nanowires were transferred (using AFM) to the substrate such that one end of each nanowire was on the surface of the electrodes and the other within the gap between the electrodes. A schematic diagram illustrating the device design is shown in Fig. 1. A voltage of 5 V was applied between the electrodes to provide an electrostatic force on the Ag nanowire tip. Negative charges on the Ag nanowire with the end lapped on the negative electrode rather than the whole nanowire located in the electrode gap were used to avoid the air ionization oxidation. A laser (wavelength of 532 nm and power of 100 mW/cm² and without focus) illuminated the gap between the electrodes to excite plasmon on the Ag nanowire tip, which causes little light damage on the tip morphology of Ag nanowire. The whole process was performed under exposure to the atmospheric environment at room temperature.

 

Fig. 1 Schematic diagram of photoelectricity-mediated tip engineering on a single Ag nanowire. The panel in the upper right-hand corner of the image shows an SEM image of the Ag nanowires and a photograph of them suspended in ethanol (as used by us). The scale bar is 2 μm. The lower right-hand panels show a photograph of a single Ag nanowire arranged between the nano-electrodes.

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The Ag nano-electrodes were completed by magnetron sputtering (Quorum Q150TS). The morphology of the Ag nanowires was determined by field emission scanning electron microscopy (Jeol, model JSM-6700F). The morphology changes of the Ag nanowire tip were characterized by AFM (Agilent 5500). An optical photograph of a single Ag nanowire arranged between the nano-electrodes was acquired via optical microscopy (Yongxiang Corp.,model 10XB-PC).

The tip morphology and the surface fluctuation of the Ag nanowire can be clearly observed in Fig. 2 (in situ AFM measurement in non-contact mode). The original morphology of the Ag nanowire tip is presented in Fig. 2(a). The surface height profiles in Fig. 2(b) show significant height variations on the Ag nanowire's surface owing to the nanowires' pentagonal characteristics. Some small surface height changes can also be found at the Ag nanowire's tip; these changes may be generated during the nanowire transfer process. After applying 5 V for 40 min, the tip morphology and surface profile of the Ag nanowire shows few changes, as shown in Figs. 2(c) and 2(d). The slight difference between Figs. 2(b) and 2(d) is within the margin of error of the AFM measurement. The electrostatic force $F$provided by the parallel plate nano-electrodes is approximately $F=U^2\epsilon {\epsilon }_{0}S/d_1\times d_2$, where $U$ is the voltage applied between the nano-electrodes; $d_1$ and $d_2$are the distances between the nano-electrode and the center of the nano-electrode gap and the nanowire tip, respectively; $S$ is the radial cross-sectional area of a Ag nanowire; and $\epsilon$ and ${\epsilon }_0$ are the relative dielectric constant and vacuum permittivity, respectively. Considering the pattern of charge aggregation on a typical nano-cone [13], the electrostatic force on this Ag nanowire tip was found to be 2 to 3 times larger than the theoretically predicted results. As a result, the application of a 5 V voltage between the nano-electrodes may exert a pressure of 0.1 to 0.4 Pa on the Ag nanowire tip; this is, however, far below its yield pressure limit. In our study the tip morphology of the Ag nanowire could not be engineered only using an electric field. As shown in Figs. 2(e) and 2(f), a significant reduction of the surface height changes can be observed after a 10 min application of both light and an electric field. However, distinct morphology changes of the Ag nanowire tip were found after 40 min exposure to the photoelectric field, as shown in Figs. 2(g) and 2(h). The Ag nanowire tip was elongated and sharpened after the exposure. Meanwhile, the pentagonal edges of the nanowire had nearly disappeared, causing a significant reduction of the height of the surface features. The Ag nanowire showed a significant tip deformation on application of the photoelectric field; this is therefore an efficient method for tip engineering for single Ag nanowires.

 

Fig. 2 AFM images and surface height profiles of the Ag nanowires. (a) Original morphology and (c) morphology after 40 min of applied electric field. Panels (e) and (g) show the morphology after 10 and 40 min applications of the optoelectronic field, respectively. Panels (b), (d), (f), and (h) show the surface height profiles corresponding to panels (a), (c), (e), and (g). The surface height profile positions are marked in all panels with black, red, and blue lines.

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To better understand the photoelectricity-mediated tip engineering of single Ag nanowires, we conducted FDTD simulations to study the plasmon excitation. Owing to the ohmic losses of Ag nanowires in the visible spectral range, significant heat generation occurs around the light enhancement position [14–16], which causes a phase transition to occur on the surface of the Ag nanowires [17]. In our model, the length of the Ag nanowire placed onto the silica surface was 10 µm and its diameter was 100 nm. A plane wave light source with a wavelength of 538 nm was used to illuminate the Ag nanowire. Cross-sectional views of the plasmon excitation of the Ag nanowire tip are marked by dotted lines with different colors in Fig. 3. Compared with the original plasmon excitation presented in Fig. 3(b), the plasmon excitation is significantly enhanced by the sharping the tip presented in Fig. 3(c). Radial cross-sectional views (marked i, ii, and iii in panel (a)) of the light enhancement of the tip as the nanowire size gradually changes is shown in Figs. 3(d)–(f) and reveals that the plasmonic enhancement increases significantly as the size of the nanowire decreases toward its tip. Except for the top edge, the light enhancement also occurs at the edge of the pentagon, which could offer enough energy for a phase transition on the Ag nanowire's surface [10]. By applying an electrostatic force on the tip of the Ag nanowire, a liquid viscous shell layer is generated through a phase transition; this liquid layer flows along the direction of the electric field between the nano-electrodes, as shown in Fig. 3(a). Similar to the electrospinning process [18–20], the liquid viscous shell layer gathers at the nanowire tip and finally stretches forward to form a Taylor cone, as shown in Fig. 2(g). Owing to the continuous movement of the liquid viscous shell layer, the edges of the Ag nanowire surrounding the tip or the surface features (defects) near the edges become blurred, leading to a reduction in the height of the surface features.

 

Fig. 3 Plasmon excitation for the Ag nanowire with the sharpened tip. (a) Schematic diagram of the Ag nanowire with the sharp tip. Panels (b) and (c) show an axial cross-sectional view of the light enhancement of the Ag nanowires with the original and the sharped tip, while (d) to (f) show radial cross-sectional views for the positions labeled i, ii, and iii in panel (c), respectively, of the light enhancement of the tip as it gradually decreases in size.

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Including the blurred edges of the Ag nanowire near the tip, the elimination of the surface features (defects) near the edges of the Ag nanowire that were introduced mainly during the nanowire transfer process was also observed. A significant plasmon excitation could be observed in these square holes, which causes the plasmonic phase transition at these positions, as shown in Figs. 4(a) and (b). Driven by the electrostatic force, thermo-diffusion force, and capillary force [21–23], the liquid viscous shell layer flowed into the square holes and reduced their depths. As shown in Figs. 4(a)–(c), the surface plasmon excitation diminishes and fades away in these holes as their size decreases, which indicates that the elimination of the surface feature depths away from the edges of the Ag nanowire is self-limited. As a result, this method could potentially be applied to eliminate small defects on metal nanostructures.

 

Fig. 4 Panels (a) to (d) show the evolution of the light enhancements in the surface features of the Ag nanowire tip over time. Symmetric square holes were set on the Ag nanowire's top surface (100-nm-long wires), and the diameter gradually changed from a width and depth of about 20 and 30 nm, to 15 and 20 nm, and finally to 10 and 10 nm, respectively.

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A drop of sodium tartrate solution with a concentration of 10⁻³ M generated intense Raman signals on the Ag nanowire and displayed characteristic peaks at 2100 and 2950 cm⁻¹ presented in Fig. 5, after air drying. As reported, the heights of vibration peaks directly reflect the SERS enhancement of material with the same concentration [24]. In contrast, the signal from the Ag nanowire after the photoelectric field treatment remarkably increased to about 4.3 times that of the signal from the Ag nanowire with the original tip. The photoelectric field treatment on the Ag nanowire thus provides an enhanced SERS signal because of the enhanced plasmon excitation in the sharpened tip of the Ag nanowire, as shown in Figs. 3(b)–(f).

 

Fig. 5 SERS spectra of air-dried sodium tartrate (about 0.01 µL, 1 × 10⁻³ M) on a Ag nanowire; red line shows the spectra for the Ag nanowire with the original tip while the black line shows that for the Ag nanowire tip after the photoelectric field treatment. Dot lines are used to measure the heights of vibration peaks around 2950 cm⁻¹.

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In summary, an obvious sharpening on the tip of an Ag nanowire was observed after a photoelectric field treatment. The reduction of the height of the surface features on the Ag nanowire tip occurred both close and away from the nanowire edge. After the electricity-mediated plasmonic tip engineering, the cone-shaped Ag nanowire tip showed a much higher SERS signals than when using the original tip. Theoretical models based on FDTD were proposed and the relevant dynamic mechanisms were discussed for the phenomena. It was found that the tip sharpening and the reduction of the height of the surface features on the Ag nanowire tip in particular depended on the position of the plasmon excitation. This present work not only offers a precise and low-cost way for tip engineering for single Ag nanowire SERS probes, but also presents a new way to fabricate and engineer metal nanostructures.