Improvement of optical transmittance and electrical conductivity of silver nanowires by Cu ion beam irradiation

In the last few decades, there is an escalating demand of flexible transparent conducting electrodes (TCEs) for flexible optoelectronic devices such as in solar cells, organic light-emitting diode [OLED]), touch screens, super capacitors [1, 2]. Indium tin oxide (ITO) are commonly used as transparent conductive electrodes in optoelectronic devices due to their low resistivity and high transparency in the visible ranges [1], but they have number of disadvantages such as scarcity of indium, high materials costs, brittleness and elevated temperature processing step required to fabricate it cause damaging of organic substrates [3]. Therefore, search for alternate transparent conductive flexible materials is imperative for flexible electronics devices and optoelectronic devices [4]. To achieve this goal, various alternatives materials have been scrutinized [5]. Carbon nanotubes films and graphene films have also been explored their use as TCEs in optoelectronic devices [6]. However, their optical and electrical performance is still inferior to ITO, thereby stimulating researchers to search for alternate materials. Recently, researchers have reported alternate materials and structures like conductive polymers [7], nanowires [8, 9], and metal mesh structures [10]. Among these alternatives, 1D metal nanowires thin films have been singled out as an astounding candidate for next generation transparent electrodes owing to their better optical transparency, high mechanical flexibility and improved electrical conductivity [11].

In order to exploit the exceptional properties of metal nanowires for application as a transparent electrode, improvement of optical transmission along with electrical conductivity is always remains indispensable. It is therefore important to find a suitable method that not only improve the electrical conductivity, but also enhance transparency simultaneously using post in situ treatment methods [12]. Different methods were adopted to fabricate highly transparent metal nanowires thin films resulting in an improved electrical conductivity, however it usually results in lower transparency and vice versa [12–15]. Moreover, to date, few articles are available in literature about improvement of both electrical and optical properties of metal nanowires thin film simultaneously [12, 16], namely using ion beam etching process and by pulse laser exposure techniques [16]. In addition, ion beam technology is a versatile and less explored method with an advantage that both optical and electrical properties of nanowires/nanotbes could be tuned concurrently as per requirement [17–23]. In our recent analogous study of irradiated Ag-NWs thin film with carbon ion beam irradiation, we have observed an improved electrical conductivity up to 3 times [19], but we could not be able to establish as to what happened in transparency of those thin film. Based on our previous results, we decided to repeat our experiment by irradiating heavy ions, to not only establish a relationship between transparency and conductivity of ion irradiated Ag-NWs thin film, but also to conduct a comparative study of the different ions irradiation effect on electrical and optical properties of Ag-NWs.

Therefore, in this study, an attempt is made to fabricate highly conductive and transparent Ag-NWs thin film by junction welding through ion beam irradiation induced welding. The possibility of modifying the optical and electrical properties of Ag-NWs thin film by ion beam irradiation technology anticipate that these kinds of observations will be useful for the design of transparent electrodes and Ag-NWs based spacecraft components.

Ag-NWs were purchased from American Chemical Society (ACS) material and employing spray atomizer Ag-NWs solution was sprayed on glass substrate placed on a hot plate at 180 °C. Transparent thin films formed on glass substrate were cut in to 1  ×  1 cm² dimensions to ensure that all samples having same physical properties. Whole Ag-NWs samples were irradiated with Cu ions at different fluencies (from 1  ×  10¹² to 1  ×  10¹⁵ ions cm⁻²) using a 5UDH-Pelletron tandem accelerator. The substrate temperature was initially set to room temperature at the time of ion irradiation. Cu ion beam energy was selected to be 5 MeV. Cu ions with 5 MeV were simulated using TRIM code [24] to ensure that ions pass through the Ag-NWs.

The refined structural features of as-grown Ag-NWs and Cu ion irradiated samples were probed by qualitative Rietveld refinement of the high quality x-ray powder diffraction data employing the TOPAS program (version 4.1) using the Rietveld refinement technique [25].

Scanning electron microscope was used for morphological study of un-irradiated and irradiated Ag-NWs. For optical measurements, Hitachi (U-2001) spectrophotometer was used. Four-point probe technique was utilized for conductivity measurement of Un-irradiated and Cu irradiated thin film sampled of Ag-NWs. Surface conductivity (G) of Ag-NWs thin films were measured using following formula:

$$\begin{eqnarray}&&V=I/2\pi SG.\end{eqnarray}$$

Where V is voltage potential, I is current passing the film and S is the distance between the current probe and voltage measurement.

The topographic surface morphology of un-irradiated and Cu ion irradiated Ag-NWs thin films are shown in figure 1. Un-irradiated Ag-NWs have diameter in the range of 120–200 nm as shown in figure 1(a). Moreover, un-irradiated Ag-NWs thin film shows that Ag-NWs are un-welded having cylindrical shape before irradiation. The Ag-NWs thin films were then subjected to Cu ion irradiation at low fluence of 1  ×  10¹⁴ ions cm⁻². It is noted in figure 1(b) that Ag-NWs are diffused to some extent at junction points.

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However, after increasing irradiation dose, randomly welded Ag-NWs thin films have been obtained as shown in figure 2(a). Two NWs at contact positions were fused with each other i.e coalescence occurs. Welding of these individual Ag-NWs might be occurred due to collision cascade effects at junction points and also ion beam induced heating. These results are well agreement with our previous research work. Only difference is that at comparatively low Cu ion irradiation dose of 5  ×  10¹⁴ ions cm⁻², the coalescence of Ag-NWs were occurred [19].

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Further increase in irradiation dose (1  ×  10¹⁵ ions cm⁻²), the diameters of the nanowires are greatly reduced due to ion beam induced sputtering as shown in figure 2(b). These fascinating results show that nanowires could be cut or slice through ion beam irradiation. Similar results were observed in our previous experiment. Only difference is that at comparatively low Cu-ion irradiation dose of 5  ×  10¹⁴ ions cm⁻², the evaporation of Ag-NWs were happened [19]. We first time are reporting ion beam induced evaporation of metal NWs instead of ion beam sputtering. Similarly, electron beam induced evaporation was reported previously in in situ TEM experiment on ZnO NWs [26].

To confirm the structural variation of silver nanowire thin films after Cu ion irradiation, XRD measurements were also performed at room temperature [27]. Figures 3(a)–(d) shows the Rietveld refined x-ray powder diffractograms for both the un-irradiated and Cu-ion irradiated Ag-NWs samples showing a good fit between the simulated model [28] and observed pattern as depicted from difference curve.

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It shows that lattice constant 'a' and unit cell volume 'V' are slightly changed after Cu ion beam irradiation as compared to unirradiated sample. The lattice parameter 'a' of Ag-NWs of irradiated samples slightly increase from 4.028 29(22) Å to 4.031 77(23) Å respectively. The Rietveld refinement fit of pristine un-irradiated Ag-NWs and the one irradiated at respective fluences depicts that for the whole powder diffraction patterns no impurities were detected ensuring the stability of structure at high doses. Moreover, with the increases in dose the full width half maximum (FWHM) slightly decrease indicating that the sample get more crystalline with increase in crystallite size from 64.5(66) nm to 71.4(45) nm, attributed to irradiation induced thermal heating of nanowires. In contrast to our previous study, wherein at high dose of proton beam irradiation the Ag-NWs crystallinity changed from single to polycrystalline orientation, while no such polycrystalline orientation was observed upon Cu-ion irradiation [29]. The XRD powder patterns of pristine Ag-NWs before and after Cu-ion irradiation were simulated employing the space group Fm$\bar{3}$m with Ag at 4a (0 0 0) Wyckoff position. During the refinement, all the atomic positions isotropic thermal vibration parameters (B_iso) and occupancies were simulated without any constraints. The weighted profile R_wp factor values show a good agreement between the experimental model and the observed x-ray diffraction data respectively.

In addition, the texture co-efficient value indicates the maximum preferred orientation of NWs along the corresponding diffraction plane. For all Ag-NWs thin films (pristine and irradiated at different doses), the plane (1 1 1) presents texture co-efficient greater than unity, indicating that the Ag-NWs thin films have strongly preferred orientation along (1 1 1) direction [30]. This strong preferred orientation in welded NWs thin film may be attributed to strain energy minimization and due to nano confinement introduced by nanopores. The particular effects of anisotropy (directionality) in polycrystalline thin films triggers pronounced texture effects. Accordingly, during the Rietveld refinement a correction for preferred orientation effect using a spherical harmonics function of fourth order was also employed to cater the small variation in intensity of the Ag (1 1 1) peak.

After morphological and structural observations, DC conductivity of Ag-NWs network of unirradiated and irradiated samples were measured using four point probe techniques and results are summarized in table 1. Whereas, G/G₀ is the relative conductivity (G) of irradiated sample over un-irradiated (G_o) as shown in figure 4(a). It shows that conductivity increases and then decreases slightly with increase in Cu ion fluence. The conductivity versus irradiation dose trend is same with Carbon ion irradiated Ag-NWs thin film as reported by Bushra et al [19], but only difference in our observation is that the conductivity rises with Cu ion irradiation even at low dose. Moreover, ion beam induced local heating is relatively much higher than Carbon ion irradiation at Ag-NWs [19]. This may be attributed to sudden rise local temperature and collision cascade diffusion of Ag atoms by Cu-ion irradiation and immediately start coalescence between Ag-NWs which increase the conducting path length cause increase conductivity. Second observation is that no decrease trend in conductivity is observed at low dose of Cu-ions irradiation. Whereas Bushra et al reported that initially conductivity of Carbon ion irradiated Ag-NWs thin film decreased at low dose and then increased at higher dose [19]. The increase trend of electrical conductivity of Ag-NWs after Cu-ions irradiation can be ascribed to the catenation and welding of Ag-NWs. Ag-NWs thin film have mainly junction resistance between Ag-NWs which need to be reduced by means of welding. It was reported elsewhere that un-welded Ag-NWs thin film have conductivity less than welded thin film [11]. Therefore, different researchers try different methods to increase conductivity of Ag-NWs to reduce junction resistance by means of welding. Our approach to weld Ag-NWs using ion beam technology is emerging technology for future to integrate nano-devices. In this work, we succeeded to weld Ag-NWs thin film and eliminate junction resistance cause reduction of resistivity.

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After measuring conductivity, we measured optical transmittance of Ag-NWs thin film in ultraviolet and visible ranges at room temperature and compared these results with electrical conductivity. Figure 4(b) shows the transmittance of Ag-NWs thin film irradiated at different doses (5  ×  10¹³–1  ×  10¹⁵ ions cm⁻²). It shows that transparency increases with increasing ion irradiation dose. Figure 5 show the transmittance in visible and ultraviolet region respectively. Figure 5(a) shows that transmittance in visible range is increased about 34% as compared with un-irradiated sample whereas transmittance in ultraviolet range is increased upto 19% as shown in figure 5(b). Both increased transmittance would be useful for visible and ultraviolet based solar cells. This remarkable increased transparency shows that transparency of any metal nanowires based transparent electrodes for solar cell applications could be enhanced as per controlled ion beam irradiation. Enhancement of transparency of Ag-NWs thin film can be ascribed to the reduction in the diameter of Ag-NWs by ion beam induced sputtering. Enhancement of simultaneously optical transparency and DC conductivity of Ag-NWs by ion beam irradiation is reported here for the very first time in our knowledge.

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Figure 6 shows the combine transmittance (ultraviolet and visible ranges) compared with relative electrical conductivity graph taken as a function of ion dose. It shows that transparency increased with increasing conductivity. At higher Cu-ion dose, conductivity decreased due to heavy ion induced thinning, cutting and slicing of Ag-NWs [19]. Increase of both transmittance and conductivity simultaneously is another remarkable breakthrough in transparent electrode technology. Ion beam irradiation technology is thus proved to be a facile approach to enhance electrical conductivity and transparency of metal nanowires based transparent electrodes for diverse technological applications.

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In conclusion, enhancement of relative DC conductivity and optical transmittance of Ag-NWs thin film investigated by Cu ion beam irradiation have been observed. Ion beam technology is a promising approach to selectively carry out the melting and subsequent catenation of Ag-NWs percolation networks resulting in enhanced electrical conductivity and optical transmittance with enormous potential for their use in fabrication of various flexible electronics and optoelectronic applications.

This study is very useful and needed as per current requirement of transparent electrodes for optoelectronic applications where high electrical conductivity and optical transmittance are required.

Higher education commission of Pakistan, iThemba LABS, UNESCO and TWAS are gratefully acknowledged. Plagiarism has been carried out via ID 619217590 (similarity index 11%) in Turnitin software.