Align Ag Nanorods via Oxidation Reduction Growth Using RF-Sputtering

1 Institute of Microelectronics, Center for Micro/Nano Science and Technology, National Cheng Kung University, Tainan 701, Taiwan 2 Department of Materials Science and Engineering, Center for Micro/Nano Science and Engineering, National Cheng Kung University, Tainan 701, Taiwan 3 The Instrument Center, Institute of Nanotechnology and Microsystems Engineering, National Cheng Kung University, Tainan 701, Taiwan


Introduction
One-dimensional (1D) metal nanostructures, including rods, wires, and tubes, have recently attracted interest and been applied to optical-electronic and sensing nanodevices due to their unique optical and conductive properties and thermal conductivity [1][2][3][4][5].Among the various metals, silver exhibits effective thermal conductivity and the highest electrical conductivity and can be synthesized into Agbased compounds with different compositions.Silver is more reactive than gold or platinum; therefore, silver is the most favorable candidate for various applications [6,7].For the silver oxide system (Ag x O), silver can be reduced by a thermal decomposition reaction due to its low melting point and active energy [8].
One-dimensional (1D) silver nanostructures are used as the substrates to detect surface enhanced Raman scattering (SERS) and surface plasmon resonance (SPR) and for enhancing the photoluminescence of the ZnO system [9][10][11][12][13][14].Over the last decade, many methods have been used to fabricate Ag nanowires and nanorods.Xu et al. used porous alumina templates to carry out chemical deposition for the fabrication of silver nanorod arrays [15].Chen et al. fabricated Ag nanowires via a polybasic process [16].Murphy and Jana demonstrated a seed-mediated method for obtaining Ag nanowires [17].All the aforementioned methods require a chemical solution or template to fabricate the Ag 1D nanostructure.To obviate the chemical solutions and reduce complexity, Zhao et al. grew Ag nanorod arrays using oblique angle deposition [18].Mohanty et al. used vapor-solid phase synthesis to produce Ag nanowires [19].Besides the aforementioned fabrications, a few literatures have demonstrated the formation of the various nanostructures by sputtering [20][21][22][23], and oxide-assisted growth has been developed for one-dimensional nanostructures in previous studies [24][25][26].Notably, oxidation reduction growth (ORG) of 1D metal structures prepared using a sputtering method has never been reported.Additionally, when the bulk metal reduces to nanoscale, the luminescence will affect the electron-hole recombination due to the larger aspect ratio [27,28].
This study has demonstrated an oxidation reduction growth (ORG) technique with mixed-gas sputtering to create Ag nanorod arrays without any chemical solutions or contamination from aqueous solution via oxide-assisted growth [29].The ORG methodology is used to deposit an Ag buffer layer with silver oxide nanoclusters to obtain Ag nanorods arrays using a two-step mixed gaseous process.The success of the technique provides support for the oxidation reduction growth (ORG) mechanism and proves suitable for fabrication of Ag nanorods in the semiconductor industry.

Experiment
Prior to sputtering, a p-Si (100) substrate was first cleaned in acetone and then cleaned in isopropanol for 300 s.The p-Si substrate was then dipped into hydrofluoric acid to remove the native oxide and finally rinsed with deionized water.For RF sputtering, the cleaned p-Si was loaded into the sputter chamber to perform sputtering, and the distance between the sample surface and silver target was approximately 3 cm.The RF sputter system was evacuated to below 1.8 × 10 −4 Torr, and argon gas was introduced with the flow rate controlled at 15 sccm.The gas pressure was fixed at 15 × 10 −3 Torr during the sputtering process, and the RF power was set at 80 W. The sputtering times are 20 and 30 min for silver oxide clusters and silver nanorods, respectively.The deposition rates of silver, silver oxide, and nanorods are 3, 1, and 10 nm/min, respectively.The amount of the introduced oxygen is 1 sccm during reactive sputtering.
A schematic diagram for the oxidation reduction growth of silver nanorods is illustrated in Figure 1.The whole sputtering process was continuous without venting until Ag nanorods had grown.In Figure 1(a), at the beginning of sputtering, a thin silver film was predeposited on the Si surface with a constant flow of 15 sccm argon gas.Next, a trace of oxygen was introduced and mixed with argon as the source of plasma to form the silver oxide seed during sputtering, as shown in Figure 1(b).The silver oxide seed randomly dispersed over the Ag buffer layer and was able to form the contiguous thin film due to the trace amounts of oxygen.Notably, the melting point of silver oxide is normally approximately 300 • C and even lower at the nanoscale [8].When engaged in the sputtering process, the temperature of the sample will rise to between 200 • C and 300 • C allowing silver oxide to dissolve and release O 2 and return the silver to the original silver oxide site (Figure 1(c)).In Figure 1(d), sputtering is performed without oxygen, and the silver nanorods start to grow from the reduction metal silver site.The oxidation reduction growth (ORG) of silver nanorods is a novel and continual process, which uses sputtering.
The morphology of the Ag nanorods was investigated by scanning electron microscopy (SEM), and the crystallization of the silver nanorods and silver oxide seeds were examined using X-ray diffraction (XRD).The incident angle of Xray was fixed 1 • for the phase identification of the extreme surface.For the detailed structure, the orientation was detected by transmission electron microscopy (TEM), and the selected area electron diffraction (SAED) was observed on the body of the nanorods and the silver buffer layer.For PL measurements, a 405 nm laser was used as an excitation light, and detection was performed in a photomultiplier tube.

Results and Discussions
Figure 2 shows tilted view SEM images and glancing angle XRD for the silver oxide seeds under sputtering 20 min and Ag nanorods under sputtering 30 min.As shown in Figures 2(a) and 2(b), not only were the silver oxide seeds observed but also the Ag nanorods uniformly distributed over the sample surface.Notably, the density of the silver oxide seeds was approximately 150/um 2 , approaching that of Ag nanorods.The range for the diameters of the Ag nanorods was from 40 to 55 nm while the diameter of the silver oxide seeds was approximately 60 nm.The average diameter of the Ag nanorods was slightly smaller than that of the silver oxide seeds.
The energy dispersive spectrometer (EDS) data of the Ag nanorods is exhibited in Figure 2(c).A main peak of silver and a weak oxygen signal can be observed on the spectrum.According to the ratio of weight between Ag and oxygen, most of the peaks were Ag, and these were initially identified as pure Ag nanorods.The inset of Figure 2(c) is a cross section view of the Ag nanorods using a focused ion beam and most of the Ag nanorods aligned to the substrate.A silver buffer layer existed beneath the Ag nanorods, and this result is consistent with previous reports (not the present growth method) that one-dimensional aligned nanostructures also require the same material layer to grow due to the lattice match [30][31][32][33].The thickness of the present Ag buffer layer was about 45 nm, and the length of the Ag nanorods was from 250 to 300 nm.
The crystallization of both the silver oxide nanoseeds and Ag nanorods was investigated using glancing angle XRD. Figure 2(d) shows the main Ag peaks, including (111), ( 200), (220), and (311), which coincide with the thin silver film [34].In Ag nanorods case, all signals really came from Ag nanorods due to glancing angle XRD with 1 • incidence, but the peaks of Ag signal in silver oxide nanoseeds resulted from the Ag buffer layer.Notably, the signal of silver oxide (11 − 1) appears at 28 • in some of the silver oxide nanoclusters in addition to the Ag peaks.The silver oxide phase was Ag 3 O 4 and was produced during the mixed-gas sputtering process with the input of oxygen [35,36].After the thermal decomposition, the oxygen resulting from the reduction reaction of silver oxides was pumped out.In addition, the production of silver oxides reduced metallic silver seeds for the growth of Ag nanorods.However, when the Ag 3 O 4 phase is compared with the AgO phase, the Ag 3 O 4 phase is more thermodynamically unstable (lower free energy) [37,38]; therefore, the Ag 3 O 4 can easily react in the consequent sputtering process to induce the growth of Ag nanorods with a chemical reaction of Ag 3 O 4(s) → 3Ag (s) + 2O 2(g) [39].This two-step oxidation reduction process is responsible for the growth of the nanorods.Therefore, the present oxidation reduction growth (ORG) mechanism can also be applied to other metal systems with low melting point [40].Figure 3(a) shows TEM observations with Ag nanorods aligned to the Ag buffer layer, and the corresponding SAED pattern from the white circle zone is shown in Figure 3(b).According to the electron diffraction pattern, the Ag nanorods were ascertained to be the single-crystal structure; the high resolution TEM image is shown in Figure 3(c).Additionally, the Ag nanorods grow from the thermal-reduced Ag nucleus above the Ag buffer layer.A lattice image of the interfacial layer between the Ag nanorods and the grain is shown in the Figure 3(d), which demonstrates a perfect lattice arrangement.Figure 3(b) also shows that the Ag nanorods are crystalline and the interface zones between the Ag nanorods and the thermal-reduced Ag nucleus are coherent.From the cross-section observation, the Ag buffer layer was composed of many silver grains and this result was coincident with the phases of XRD spectra (Figure 2(d)).Notably, the Ag nanorods of the ORG structure also possessed excellent properties.
A PL spectrum of the Ag nanorod arrays under the irradiation of a 408 nm laser could be seen in Figure 4.The spectra ranges of all samples, which exhibit the strongest intensity near 2.17 eV, were from ∼1.9 to ∼2.45 eV.Basically, PL peaks in the visible wavelength can be regarded as a radiative recombination between Fermi level electrons and sp-or d-band holes [39,41].However, silver oxide forms easily in air and it may also provide a possible route of luminescence for photoreduced metallic Ag under visible illumination [27,42].Herein, PL spectrum of Ag nanorods was also measured under the vacuum to avoid the formation of silver oxide during the irradiation of laser in air, and there is no shift for the spectra of Ag nanorods both in air and in vacuum.

Conclusion
In summary, Ag nanorods were fabricated using an oxidation reduction growth (ORG) method by sputtering without catalysts or chemical solutions.The Ag nanorods grew in the original locations of reduced metal nuclei after thermal composition of silver oxide nanoclusters.The Ag nanorods stood vertically on the Ag buffer interlayer and grew from the interface between the Ag grains and the Ag buffer interlayer.PL spectra of Ag nanorods both in air and vacuum were observed at 2.17 eV due to the photoactivation caused by a radiative recombination of Fermi level electrons and d-band holes.

Figure 2 :
Figure 2: SEM images of (a) silver oxide nuclei, (b) Ag nanorod arrays, (c) EDS of Ag nanorods.The inset SEM image is a cross-section of Ag nanorods.(d) XRD spectra of Ag nanorods and silver oxide clusters.

SiFigure 3 :
Figure 3: (a) A typical TEM image of Ag nanorods.(b) SAED of the white circle in (a), (c) the HRTEM image of Ag nanorod, (d) the interface between Ag nanorod and reduced metal nucleus.

Figure 4 :
Figure 4: Room temperature PL spectra of Ag nanorods and silver oxide clusters.