Ion Beam Irradiation-Induced Effects in Metal Nanowires Used For Transparent Electrodes

Abstract

Metal Nanowires (MNWs) are promising as a kind of novel conducting materials for next generation of nanodevices for space applications either in form of interconnecting conducting nanowires to integrate nanodevices or for Transparent Electrodes (TEs) for solar cells. In this work, ions irradiation induced damage study of MNWs e.g., (Ag, Cu) at different energies, doses and ions species is presented. After irradiation, samples are characterized using scanning electron microscopy (SEM), x-ray diffraction (XRD) and transmission electron microscopy (TEM). The results of irradiated samples are then compared with un-irradiated samples. Finally, a database of effects of ions irradiation on MNWs is made. This database will be useful for future design of MNWs based devices to be used under harsh conditions such as upper space. Mechanism of damage creation in MNWs by ions irradiation is explained by collision cascade effect and thermal spike model. Moreover, MeV proton and argon beam irradiation-induced nanowelding technique to fabricate X-, Y-, II- and T-shaped molecular junctions between Ag-NWs is presented. These nanowires are irradiated by 2.5MeV protons at a dose of 5x1015ions/cm2 and 3.5 MeV argons at dose of 5x1016 at room temperature. Transmission electron microscopy (TEM), scanning electron microscopy (SEM) and x-ray diffraction (XRD) results reveal that nano-welding of Ag-NWs is achieved with stable crystal structure. Thereafter, a random two-dimensional large scale network of Ag-NWs is fabricated by 3 MeV proton ion beam irradiation induced welding of Ag-NWs at intersecting positions. Proton ion beam induced network fabrication on large scale is confirmed by transmission electron microscopy (TEM) and scanning electron microscopy (SEM). It is observed that at a beam fluence of 1x1015ions/cm2, perfect X-, II-, and V-shape molecular junctions between Ag-NWs are achieved and ultimately lead to an optimum welded network without distorting the morphology of nanowires. Structure of Ag-NWs remains stable under proton ion beam and networks are optically transparent. The results exhibit that the formation of Ag-NWs network proceed through three steps: ion beam induced thermal spikes lead to local heating of Ag-NWs, formation of simple junctions on small scale, and the formation of large scale network. Furthermore, an important consideration for space applications is that the material should be as radiation hard as possible in order for it to reliably operate for extended periods. Therefore, total dose radiation tolerance of Ag-NWs under proton environment is investigated. Ag-NWs are irradiated with 5 MeV proton ions at different doses ranging from 5x1015 to 8x1016 protons/cm2 and their effect on morphology and structure is studied. It is observed that Ag-NWs remain stable under proton beam irradiation at high doses. In addition, “amorphous Ag-NW have been fabricated from crystalline Ag-NWs using 5 MeV helium (He+) ions beam irradiation. At low beam fluence (5x1015 ion/cm2), few defects are created in Ag-NW with increase in density while increasing He+ ions beam. As dose increases, more damage of the crystalline structure of Ag-NWs is observed. Finally at high dose (5x1016 ions/cm2), the face-centered cubic (FCC) structure of Ag- NWs is transformed into amorphous structure with similar morphology as un-irradiated Ag-NWs. Phase transformation of crystalline Ag-NWs upon irradiation with 5 MeV He+ ions is observed through high resolution transmission electron microscopy (HRTEM).” Besides, the effect of -irradiations on the structural and morphological properties of copper nanowires (Cu-NWs) within the  fluencies varying from 6 to 25 kGy are also studied. At 9 kGy, the Cu-NWs start to join, forming perfect X-, V-, II-, and Y-shaped molecular junctions. Further increasing the  fluence up to 15 kGy cause the Cu-NWs to fuse and form larger diameter NWs. At the highest fluence of 25 kGy, Cu-NWs converted into a continuous Cu thin film. However, x-ray diffraction (XRD) results show that the structure of the Cu-NWs remains stable even after converting into a thin film. The formation of cuprite (CuO) phases is observed at higher fluencies. The mechanism of forming welded networks of Cu-NWs and Cu thin films is explained via the thermal spike model.