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Abstract
The optical properties of aligned nickel nanowire arrays (NiNWAs) with different degrees of oxidation for terahertz (THz) polarizer applications have been investigated by using THz time-domain spectroscopy. In frequency-domain spectra, the full width at half maxima of transmitted peaks was broadened and the peak positions have a blue shift with increasing oxidation levels, besides the enhancement in peak intensity. It is indicated that the oxidation of Ni nanowires (NWs) has a significant influence on the interaction between Ni NWs and THz wave. The transmittance of the aligned NiNWAs increases with annealing temperature increasing. Conversely, the degree of polarization and extinction ratio (ER) decreases. A corresponding relationship between the change of ER and degree of oxidation is summarized by means of thermogravimetric analysis. The change of ER for the annealing sample with the degree of oxidation of 0.507% is 27.32%, which induced the polarization properties of aligned NiNWAs to be sensitive to the oxidation of Ni NWs. These findings can provide new positive features in the development of future polarization-based device applications for THz electronics and photonics.
© 2023 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
The tunable electromagnetic response to THz waves has been a significant challenge due to the lack of nature materials with suitable properties. To address this, one-dimensional nanomaterials including carbon nanotubes, semiconductor nanowires (NWs), and metal NWs have been widely employed in various fields such as imaging [1,2], wireless communication [3,4], and spectroscopy [5,6]. Among of them, nickel NWs are of particular interest in the development of THz devices with anisotropic electrical conductivity and high aspect ratios [7,8]. A static THz polarizer based on the stacks of aligned Ni NW arrays (NiNWAs) was reported with a degree of polarization (DOP) of 99.9% and an average extinction ratio (ER) of 46.6 dB in the frequency of 0.3-2.3 THz [9]. In addition, a mechanically tunable THz polarizer by using aligned NiNWAs has been fabricated with a modulation depth of 85% throughout a frequency range of 0.3-1.8 THz [10].
Though Ni NWs have higher oxidation energy compared to copper NWs, they suffer from natural oxidation and time-dependent oxidation under environmental conditions. Han et al. revealed the formation of Ni/NiO core-shell structures with a 4.5 nm amorphous NiO layer after exposing Ni NWs to ambient air for 3 days [11]. Furthermore, post-annealing at 300°C for 0.5 hours under atmospheric conditions resulted in the development of a 5 nm polycrystalline NiO layer on the surface of Ni NWs [12]. In our previous report, the evolution of morphological, structural, and magnetic properties of Ni NWs caused by post-annealing was investigated [13,14]. Based on these research, the oxidation problem of Ni NWs will significantly impact the performance of Ni NW-based devices. However, the variation of the interaction between THz wave and NiNWAs as a function of oxidation is still not well understood. This paper presents a characterization of the optical properties of aligned NiNWAs, examining their variations under different annealing temperatures. Moreover, the relationship between polarization characteristics and the oxidation of NiNWAs is analyzed, aiming to gain valuable insights into the influence of oxidation on the behavior of Ni NW-based devices in THz applications.
2. Experimental
Ni NWs were synthesized by chemical reduction with average lengths of 30 µm and diameters of 200-300 nm. Subsequently, we prepared the aligned NiNWAs with the density of 5 × 108 cm−2 by using a magnetic alignment technology on quartz substrates with the area of 1cm × 1 cm. Details of the synthesis processes of Ni NWs and of fabrication of aligned NiNWAs are reported elsewhere [10,15]. Then, the aligned NiNWAs samples were annealed in atmospheric conditions at 200 °C, 250 °C, and 300 °C for 1 hr. Meanwhile, the NiNWAs as a control sample were also prepared for comparison (denoted as S). In addition, in order to acquire the oxidization behavior of Ni NWs, two samples with annealing conditions of 100 °C and 325 °C for 1 hr were prepared.
Scanning electron microscopy (SEM) was used to characterize the size and alignment of Ni NWs. The oxidation processes of Ni NWs were in-situ monitored by the Pyris 1 thermogravimetric analyzer (TGA; PerkinElmer, USA). The optical properties of the samples were carried out by using transmissive THz time-domain spectroscopy (THz-TDS). The GaAs photoconductive antennas were used in the THz-TDS system to emit linearly polarized THz waves and detect THz pulses and employed a Ti: Sapphire Femtosecond Laser with a central wavelength of 808 nm, a pulse width of 100 fs, and a repetition rate of 80 MHz as the stimulus. The time-domain signal is determined by locking detection. A THz wave vertically incident on the surface of the samples and the transmittance of THz wave through the samples was measured. Then, fractional Fourier transform (FFT) is used to convert the time-domain waveform to the frequency-domain, and normalization is performed. The THz system chamber is constantly purified with dry nitrogen to avoid THz waves being affected by water vapor in the air.
3. Results and discussion
Figure 1(a) shows the SEM image of the sample S. It can be seen that Ni NWs were arranged in parallel with each other tightly, forming an aligned texture along the same direction. Figure 1(b) shows the time-domain waveform of THz transmission through the sample S with different rotation angles α. As shown in the inset, α is defined as the angle between the orientation of the Ni NWs and the polarization direction of the incident linear THz wave, where the direction of the THz electric field parallel/perpendicular to the orientation of the Ni NWs is α= 0°/90°. As shown in Fig. 1(b), the intensity of transmitted THz pulses is enhanced with the increase of α. It is indicated that a highly aligned NiNWAs was formed.
Fig. 1. (a) SEM image, and (b) THz TDS of the NiNWAs. The upper left inset indicates the experimental scheme for the polarization state measurements.
The oxidation process of Ni NWs at different annealing temperature was investigated by using TGA. The weight gain of the samples continuously increased with the increase of oxidation temperature and oxidation time, as shown in Fig. 2(a). For instance, the total weight gain is ∼ 0.045% when the annealing temperature is 100 °C, whereas it elevates from 0.507% to 3.153% as the annealing temperature rises from 200 °C to 325 °C. In order to analyze the oxidation process of Ni NWs, the degree of oxidation (DO) was calculated. The DO is defined as DO = (Ma-Mb)/Mb, where Mb and Ma represent the mass of Ni NWs before and after annealing, respectively. It can be seen from Fig. 2(b) that the DO increases exponentially with an increase in annealing temperature. In addition, it is interesting to note that the weight gain of Ni NWs increases linearly with annealing time when the annealing temperature was below 200 °C. It is indicated that the whole surface of Ni NWs has not been completely oxidized. The weight gain of Ni NWs exhibits a parabolic relationship when the annealing temperature is higher than 250 °C. It's attributed to oxidation, which is controlled by the progressively thicker oxide film formed on the nanosurface that hinders the diffusion of ions. In addition, Fig. S1 shows the SEM images of surface morphology of the as-prepared Ni NWs and the Ni NWs annealed at 200 °C and 300 °C. As shown in Fig. S1, the surface morphology of all samples has no obviously change. It is in agreement with the results in our previous report [14].
Figure 3(a) presents the THz time-domain transmission signals of all samples at α=90° showing almost consistent results. It confirms that the interaction between THz wave and the aligned NiNWAs can be omitted whether Ni NWs were oxidized or not [10]. This is to say, Ni NWs in all samples are highly aligned. Conversely, the transmission signals of the samples increase with the increase of annealing temperature when the α is 0°, as shown in Fig. 3(b). To better clarify the changes of the samples, the time-domain spectra in Fig. 3(b) are converted by FFT to obtain the frequency-domain spectra shown in Fig. 3(c). The peak intensities of the samples increase with annealing temperature increasing. Figure 3(d) shows the variation of the full width at half maxima (FWHM) and the peak position of transmission peaks with different DO. The sample S exhibits the FWHM and peak position of 0.476 THz and 0.234 THz, respectively. As the increase of DO, the peak positions has a blue shift and the frequency domain spectra were broadened. At a DO of 2.108%, the FWHM increases by ∼30%, and the peak position blue-shifts by 0.063 THz compared to the sample S. These results demonstrate that the oxidation of Ni NWs has a significant impact on the interaction between the NiNWAs and THz wave.
Fig. 3. THz transmitted time-domain spectra with α of 90° (a) and 0° (b), respectively; (c) THz frequency-domain spectra with α of 0°; (d) The variation of FWHM and peak position of the transmitted peaks from the data in Fig. 3(c) as a function of DO.
According to the antennalike plasmon resonance effect, the resonance frequency of a Ni NW has been calculated using the following expression [16]:
where m is a natural number, c is the speed of light in a vacuum, n is the refractive index of the surrounding medium, and l is the length of a Ni NW. It can be seen that the resonance frequency f decreases with the increase of l. In the aligned NiNWAs, Ni NWs are interconnected with each other. Moreover, the NW-to-NW junctions have a lower resistance. Therefore, a series of Ni NWs with different equivalent lengths are formed due to the interconnection of Ni NWs. It induced that THz transmission peaks will have a red shift and the peak intensity decreases with the increasing Ni NW density in NiNWAs, which has been confirmed in our previous report [17]. It is indicated that there are not only individual effects of Ni NWs, but also overall effect of the NiNWAs when THz waves interact with the NiNWAs. When Ni NWs were annealed, the Ni/NiO core-shell structures were formed. The oxidation of Ni NWs surfaces will increase the junction resistance and the transport of carriers through the NW-to-NW junctions is hampered. As the annealing temperature increases, the individual effect of Ni NWs gradually dominates the interaction between THz waves and the NiNWAs, while the overall effect gradually decreases. As a result, the THz transmission peaks have a blue shift and the peak intensity increases, as shown in Fig. 3(c).Figure 4 displays the transmittance spectra of the samples. It can be seen that the spectral shape of the annealing samples is similar to that of the sample S; i.e., the transmittance decreased with the upswing of frequency. The transmittance of the samples increases as the annealing temperature increases. Especially, the change of transmittance is more pronounced at low frequency compared to high frequency. For example, as the annealing temperature increases from 200 °C to 300 °C, the transmittance increases from 0.067 to 0.395 at 0.6 THz, whereas it only increases from 0.019 to 0.108 at 1.6 THz. This highlights that the effect of Ni NW oxidation on the interaction between Ni NWs and terahertz waves is larger in the low-frequency region than in the high-frequency region.
The parameters ER and DOP are utilized to assess the polarization properties of aligned NiNWAs with different annealing temperatures, which are defined as:
where T1, T2 are the transmittance of the samples with α of 0 ° and 90 ° respectively. Figure 5(a) and (b) depict the variation of DOP and ER of the samples with different annealing temperatures as the frequency increased from 0.2 THz to 2 THz. Both DOP and ER decrease with the increasing annealing temperature. The sample S exhibits an average DOP and ER of 93.168% and 18.057 dB, respectively. At the annealing temperatures of 200 °C and 300 °C, the average DOP and ER are 85.229%, 51.387%, 13.123 dB, and 5.576 dB, respectively. Moreover, the attenuation amplitude of DOP and ER of the annealing samples in the high-frequency region is smaller than that in the low-frequency region. Figure 5(c) illustrates the change rate of average ER (ΔER) of the annealing samples as a function of DO compared to the sample S. An exponential relationship is observed for the variation of ΔER regarding the DO. When the DO is 0.507%, the ΔER is 27.32%. The polarization properties of aligned NiNWAs are almost lost as the DO increases to 2.108%. At this juncture, the ΔER is 69.34%. In Wang’s report, they found the sheet resistance of NiNWAs annealed at 300 °C for 1 hr increased 1.28 times [18]. It just goes to show that the polarization properties of NiNWAs are more sensitive to the oxidation of Ni NWs compared to the change of their electrical conductivity.Fig. 5. (a) the DOP and (b) the ER of all samples; (c) the variation of ΔER of the annealing samples with different DO.
4. Conclusion
In conclusion, the optical properties of aligned NiNWAs with different annealing temperatures have been evaluated. We found that the FWHM increases and the peak position has a larger blue shift as the increase of annealing temperature. the DOP and ER of aligned NiNWAs decrease with annealing temperature increasing; However, the transmittance increases. Moreover, the interaction between Ni NWs and terahertz waves influenced by Ni NW oxidation in the low-frequency region is larger than that in the high-frequency region. An exponential function was found to express the corresponding relationship between ΔER and DO of aligned NiNWAs. The ER decreased by 27.32% while the DO is about 0.507%. These results showed that the oxidation problem of Ni NWs is an important issue to be concerned for Ni-NW based THz devices, and research on hindering the oxidation of Ni NWs is necessary.
Funding
National Natural Science Foundation of China (62075245).
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the corresponding authors upon reasonable request.
Supplemental document
See Supplement 1 for supporting content.
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