1. Introduction

Network films consisting of nanomaterials such as single-walled nanotubes (SWNTs), graphenes, and silver nanowires (AgNWs) have emerged as a potential platform for optoelectronic devices [1–8]. Essential to these applications is their low sheet resistance (R_s) along with their ability to maintain good optical transparency. Alongside, solution-based processes can allow the use of the relatively low-cost methods of spin-coating, roll-to-roll processing, and printing. Therefore, we consider them as an alternative platform for transparent conductive electrodes, with the potential to replace the conventional indium tin oxide (ITO) films—which are costly and have poor physical properties. For example, figure of merits (FOMs) of as high as 24 and 70 have been reported for SWNT network films and single-layer graphenes, respectively [9, 10]. However, these are lower than the ~500 FOMs that ITO films yield. On the other hand, the FOM of AgNW network films has been reported to be as high as 360 [5], which represents a much higher performance than that of functional films based on low-dimensional carbon materials.

Plasmonic devices and metamaterials have attracted much interest due to their potential use in optoelectronic applications such as superlenses [11], active plasmonic devices [12, 13], sub-diffraction focusing [14, 15], and invisibility cloaks [16, 17] in the visible to microwave ranges. Recently, we demonstrated that SWNT network films can work as a platform for plasmonic devices and metamaterials in the terahertz (THz) frequency range [4]. The conductivity of SWNT films is not as good as that of metal films, and therefore, devices can suffer from the broadening of resonance due to the large scattering rates. Besides, the thickness of SWNT films—vital for the development of THz optoelectronic devices—does not permit for the transmission of visible light. There has been an increasing need for functional devices that operate in the mid-IR to the THz frequency range while preserving good visible range transparency. Since AgNW network films have superior electrical and optical properties, it is plausible that they will serve as an excellent platform for THz optoelectronic devices. AgNWs have recently been characterized in the THz frequency range [18, 19]; however, the fabrication of the related plasmonic devices has not been addressed.

In this study, we fabricated THz plasmonic devices based on AgNW network films. The high conductivity of the films allowed us to fabricate the devices with superior resonance characteristics while preserving good optical transmission. We achieved the plasmonic resonances with their resonance characteristics, varying with the R_s of the AgNW films.

2. Experimental results and discussion

The AgNW films were fabricated using a spin-coating method from the AgNW dispersion solution (with a weight percentage of 0.3–1.0 g/ml) on clean quartz substrates treated by (3-aminopropyl) triethoxysilane [5]. The thickness and the sheet resistance of the film did not vary noticeably with the use of different solution densities as long as the resultant amount of the AgNWs in the film remains the same. The spin-coating speed and time was 1000 rpm and 20 s. Using these films, we derived slot antenna array patterns on the AgNW films using a conventional photolithography technique, followed by a wet etching process with an Ag etchant (HNO₃: HCl: H₂O = 1: 1: 1 mixture solution) for 5 s at 90°C. The average diameter and length of individual AgNWs were about 20 nm and 20 µm, respectively.

Figure 1(a) shows a scanning electron microscope (SEM) image of AgNW network films spin-coated on the quartz substrate. The thickness of the films reaches 80 nm when we spin-coat four times with the solution of 0.3 g/ml. In addition, Fig. 1(b) shows a photograph of series of AgNW samples for different numbers of spin-coating times (N = 1, 2, 4). In order to confirm the transparency accurately, we measured the transmission spectra for different spin-coating times as shown in Fig. 1(c). The transmission decreases (from 93 to 73% at the wavelength of 550 nm) with the increasing spin-coating times, accompanied by the dramatic decrease in the R_s (from 17.8 to 4.97 Ω/sq). The FOM (Z), defined by the Tinkham formula [20], decreases with an increase in the number of spin-coating times as shown in Fig. 1(d). In the case of N = 1, it reaches Z = 294, which is almost as good as that of the commercialized ITO films we tested (Z = 304). Thus, AgNW network is an ideal candidate for THz applications—which could possibly replace conventional metals while preserving excellent optical transparency. Accordingly, they have been extensively used as transparent conducting electrodes in optoelectronic devices such as solar cells and light-emitting diodes [5, 21].

 

Fig. 1 (a) SEM image of AgNW network film. (b) Photograph of AgNW network film on quartz substrate with different thicknesses. (c) Optical transmission spectra for different spin-coating times. (d) FOM of an ITO film and AgNW network films with different R_s’s.

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Transmissions through the films and the devices were measured by THz time-domain spectroscopy (THz-TDS) using an electro-optic sampling technique [4, 22]. We used a photoconductive antenna as an emitter and a ZnTe crystal as a detector. A femtosecond pulse at λ = 800 nm was incident on the photoconductive antenna, which emits a linearly polarized THz pulse with a spectral bandwidth of ~2.5 THz. Subsequently, this THz pulse was focused on the AgNW film surface with ~2 mm spot diameter under ambient conditions. Both amplitude and phase time traces for the transmitted THz electrical field were recorded by varying the time delay between the 800 nm probe beam and the THz pulse. THz spectra were taken by applying a fast Fourier transform (FFT) to the time trace and normalized with respect to the reference.

Figure 2(a) shows a time trace of the THz transmission amplitudes through an AgNW film for N = 4, together with a reference measured through a quartz substrate. It is clearly observable that the transmitted THz amplitude through the AgNW film deceases significantly, reaching ~4% of the incident fields. The transmission spectra, normalized by the quartz references, are shown in Fig. 2(b) for N = 1 and N = 4. We have shown the THz transmission amplitude for the two pristine films (dash lines) with their different thicknesses and R_s, fabricated by varying the spin-coating times from the AgNW solutions with 1.0 g/ml. As expected, the transmission decreased as we increased the spin-coating times.

 

Fig. 2 (a) Time trace of THz transmission through a AgNW network film for N = 4 (red curve), shown together with free-space reference (black curve). (b) THz transmission spectra of AgNW network films with different spin-coating times (N), before (dash) and after (solid) GO treatment. (c) Plot of THz transmission amplitude at 1 THz as a function of N for pristine (black) and GO-treated (red) films.

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Importantly, we found that the AC conductivity of AgNW films can be engineered using post processes. For example, it has been recently reported that the conductivity of AgNWs can be manipulated by coating GO flakes to pristine AgNW networks [21]. GO flakes improve the electrical conductivity between the NWs by tightly binding them, resulting in an overall improvement in R_s. In our test with GO flakes, we found that a significant improvement in the THz response upon the deposition of flakes [23]. The transmission amplitude decreased after we apply the GO flakes as shown as solid lines in Fig. 2(b) for different spin-coating times, which reflects increased film conductivity. In Fig. 2(c), we plotted the THz transmission at 1 THz as a function of spin-coating times, for the pristine (black boxes) and GO-treated (red circles) films. The corresponding AC conductivity can be obtained from the transmission amplitude using the relation $T\approx {\left|(1+n_{sub})/(1+n_{sub}+Z_0\sigma d)\right|}^2$, where T is the THz transmission intensity, n_sub is the refractive index of substrate, Z₀ is the impedance of free space, σ is the conductivity, and d is the thickness of the film [24]. We increased the conductivity by about a factor of 1.7 (from 37.2 × 10³ to 62.4 × 10³ S/cm for N = 4) after the deposition of GO flakes, regardless of film thickness.

Using the highly conductive and transparent AgNW films, we fabricated a slot antenna array pattern on quartz substrates as schematically shown in Fig. 3(a) [4]. These rectangular apertures, fabricated on metallic film, attracted great interest due to their extraordinary transmission accompanied by large field enhancement assisted by shape resonance [15, 25]. Figure 3(b) shows an SEM image of the device with a slot antenna length (l) of 140 µm and a width (w) of 15 μm; the THz transmission amplitudes are shown in Fig. 3(c). The R_s and film thickness were 8.09 Ω/sq and 30 nm, respectively. The transmission shows a clear transmission resonance at 0.63 THz for TM polarization geometry. Conversely, the transmission is suppressed in the TE polarization geometry, as found previously with conventional metal films [4]. Figure 3(d) shows a series of THz transmissions for different l’s: 170, 140, 100, and 60 µm. The resonance frequencies are primarily determined by the length of the antenna, i.e., f_res = c/2n_effl, where n_eff is the effective index that can be determined by the refractive indexes of the air and the substrate, as extensively examined in the literature [26].

 

Fig. 3 (a) Schematic of slot antenna array fabricated on AgNW films. (b) SEM image of rectangular slot antenna arrays with lengths of l = 140 μm. (c) THz transmission spectra for a bare AgNW network film (black curve), a plasmonic device with TE polarization (blue curve, parallel to the long axis of the rectangle) and with TM polarization (red curve, perpendicular to the long axis of the rectangle: See red arrow in (b)). (d) THz transmission spectra for TM polarization at various l values from 60 μm to 170 μm.

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Our focus in this work is on the change of the slot antenna resonance as a function of R_s of the AgNW network films. We manipulated R_s (48.0, 17.8, 8.1, and 4.8 Ω/sq.) by changing the spin coating times (N = 0.5, 1, 2, 4) of the AgNW solution (0.3 g/ml). N = 0.5 indicates the single spin-coating process using a solution of half weight density. Figure 4(a) shows a series of THz transmission amplitudes for devices (l = 100 μm) fabricated on films with different R_s’s (as shown in the dotted lines). We have shown Lorentzian fitting curves as solid lines. The peak position as well as the spectral linewidth of the resonance change with samples. We achieved a relatively sharp peak at f_res = 0.95 THz for R_s = 4.8 Ω/sq. The peaks, however, are red-shifted and broadened as R_s increases: f_res = 0.78 THz for R_s = 48 Ω/sq. We emphasize that we were successfully able to fabricate THz plasmonic devices while preserving optical transparency of more than 75% in the case of film with R_s = 4.8 Ω/sq.

 

Fig. 4 (a) THz transmission amplitudes for the AgNW plasmonic device with the different R_s’s. (b) Plot of resonance frequency (red) and Q-factor (blue) for the AgNW plasmonic device as a function of the R_s. (c) THz transmission amplitudes for the plasmonic device based on Ag bulk films with different thickness.

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We showed in Fig. 4(b), the peak position and the Q-factor as a function of R_s, extracted from Fig. 4(a). The Q-factor as well as the peak position can be controlled by adjusting spin-coating times and the concentration of the solutions used for fabricating the AgNW films. This is not the case with conventional metal films as shown in Fig. 4(c); we need very accurate control over the percolation behavior possibly with the metal film as thin as 10 nm [24]. Conversely, AgNW films with the finite conductivity enables us to control the resonant characteristics simply by controlling the film thickness although the film conductivity (normalized by the thickness) did not change significantly for the different films.

Finally, we demonstrate that the resonance characteristics can be tuned (without changing the film thickness) by the post processes such as the chemical treatments as schematically shown in Fig. 5(a). For the chemical welding process, AgNW films (with a thickness of 80 nm) and plasmonic devices fabricated on the film were exposed to H₂O₂ vapor heated at 70 °C. The chemical welding occurs because of the capillary condensation of H₂O₂ vapor at the nanogaps between the NWs. This chemical treatment leads to strong contact among the AgNWs, and hence, it is expected to modify the THz optoelectronic performance of the devices fabricated on the films. We show in Fig. 5(b), the R_s as a function of welding time (i.e., time for H₂O₂ vapor exposure) on the AgNW film. Interestingly, R_s is reduced initially, indicating that the inter-nanowire connectivity has been improved, whereas it starts to increase, and after 5 min, R_s becomes higher than its initial value. This is due to the dissolution of metallic nanostructures, leading to the reduction in electrical conductivity [27].

 

Fig. 5 (a) Schematic of the AgNW plasmonic device with H₂O₂ treatments. (b) Plot of R_s for the AgNW film as a function of H₂O₂ exposure time. (c) THz transmission spectra of the AgNW plasmonic device for different exposure times. (d) Plot Q-factor for the AgNW plasmonic device as a function of the H₂O₂ exposure times.

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As shown in Fig. 5(c), the resonance characteristics of our slot antenna arrays (l = 100 μm, fabricated on a Si substrate) also change with R_s (and hence, the film conductivity). The resonance amplitude increased as R_s is reduced by the chemical treatment and showed its maximum value at 5 min. The peak amplitude decreases as conductivity decreases, accompanied by the reduction in Q-factors shown in Fig. 5(d). Therefore, it is obvious that the chemical treatments can work as one of the efficient post-processing techniques by which the resonant characteristics of optoelectronic device can be tuned even after the film fabrication process has been complete.

In general, a high Q-factor is required for optoelectronic and plasmonic applications. There are, however, devices that need a low Q-factor—for instance, those that conduct broadband operations and multiband coupling [28, 29]. The dielectric constants of conventional metals are very large in the THz and GHz frequency range [25]; and hence, the Q-factor control is highly restricted in THz optoelectronic applications. Besides, due to the extremely low penetration depth, it is believed that surface plasmons cannot be excited on conventional metal. Instead, the observed plasmonic behaviors such as the extraordinary large transmission in the subwavelength holes arrays have been attributed to “spoof” plasmons [30]. We have since strived to develop a novel metal platform with finite conductivity, because this will help us better understand the behavior of electromagnetic waves in subwavelength structures — as well as develop new applications in which the THz electromagnetic waves interact with the metal film with large penetration depth. Furthermore, as mentioned above, the AgNW network films can be an ideal platform for THz optoelectronic devices where visible-range transparency is required.

3. Conclusion

In conclusion, we fabricated plasmonic devices operating in the THz frequency range using AgNW network films that exhibit high conductivity and good optical transparency in the visible range. The THz conductivity of AgNW films can be engineered by post-treatment, including the welding technique assisted by GO flakes, reaching 60,000 S/cm. We fabricated plasmonic devices such as slot antenna arrays using photolithography with different R_s’s, in which we manipulated the peak positions and Q-factors. Importantly, the Q-factors could be engineered by the post-processing such as the H₂O₂ vapor treatment without the need to change the pattern design and the film thickness. Devices with low Q-factors could be useful for applications that need broadband operations, for instance, in sensing materials with spectral fingerprints. We proved that AgNW network films can be used in plasmonic devices operating effectively in the THz frequency range while preserving their good visible-range transparency.