Effect of annealing parameters and activation top layer on the growth of copper oxide nanowires

Journal Pre-proofs Full Length Article Effect of annealing parameters and activation top layer on the growth of copper oxide nanowires Vipin Chawla, Neha Sardana, Harshdeep Kaur, Arvind Kumar, Ramesh Chandra, Sunita Mishra PII: DOI: Reference:

S0169-4332(19)33185-X https://doi.org/10.1016/j.apsusc.2019.144369 APSUSC 144369

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Applied Surface Science

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24 June 2019 14 September 2019 10 October 2019

Please cite this article as: V. Chawla, N. Sardana, H. Kaur, A. Kumar, R. Chandra, S. Mishra, Effect of annealing parameters and activation top layer on the growth of copper oxide nanowires, Applied Surface Science (2019), doi: https://doi.org/10.1016/j.apsusc.2019.144369

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Effect of annealing parameters and activation top layer on the growth of copper oxide nanowires Vipin Chawla1, 3*, Neha Sardana2*, Harshdeep Kaur1, Arvind Kumar3, Ramesh Chandra3, Sunita Mishra1 1

CSIR-Central Scientific Instruments Organization, Sector-30C, Chandigarh, India 2 Indian Institute of Technology Ropar, Ropar, Punjab, India 3 Nano Science Laboratory, Institute Instrumentation Centre, IIT Roorkee, Roorkee, Uttarakhand, India *Corresponding author: [email protected], [email protected]

The present work focuses on the study of annealing parameters and activation top layer on the growth of copper oxide nanowires. The copper films of varying thickness (60-800 nm) were synthesized on SiO2/Si substrate by DC magnetron sputtering and the samples annealed in terms of different parameters, especially the optimized annealing temperatures from 300-700 oC for 2-8 hrs to understand the growth mechanism and to optimize parameters for the nanowire formation. Furthermore, a thin conductive gold film as an activation top layer enhances the density and aspect ratio of the copper oxide nanowires. The orientation of crystal planes was characterized by XRD and the nanowires growth after annealing was characterized by SEM. The changes in the film were analyzed by the Raman spectroscopy. Keywords: Sputtering, Copper films, nanowires, annealing, gold layer, SEM

 Copper films deposited on SiO2/Si substrates by magnetron sputtering  Growth of copper oxide nanowires by thermal annealing  Optimization of parameters to grow highly dense and high quality nanowires  Conductive gold layer (10 nm) on the top of copper film to enhance the nanowires density  Scanning Electron Microscopy used to observe the growth of the nanowires

1.0 Introduction One dimensional nanostructure such as nanowires, nanobelts and nanorods have been the subject of intense research activity in recent years, not only because of low dimensionality and their intrinsic properties, but also due to their unique properties arising from quantum confinement effects and their capability for direct nanosystem integration [1]. As inspired by their unique characteristics, nanowires, such as semiconductor and metal oxide nanowires have become the research forefront in nanoscience and nanotechnology [1]. Recently, more and more nanowires are being developed from various materials by variety of methods, and these nanowires have been found to possess excellent, optical, electrical, mechanical, and thermal properties in comparison to their bulk states. Nanowires have potential applications in many research fields especially solar cells, gas sensing etc. and therefore, expected to be the key elements of future technologies and further research and development are needed to fill up the wide gap that exists between research status and their availability for practical applications. In this regard, cuprous oxide and cupric oxide nanowires play an important role in the energy conversion devices and opto-electronic devices due to the low cost, ease of fabrication, excellent electrical & optical properties, high reactivity and non-toxic nature. These nanowires have many interesting applications in electronics, gas sensors, field emission electron sources, opto-electronic devices etc. [2-10]. The copper oxide nanowires can be fabricated by sol-gel route, wet chemical route and thermal oxidation. Among all known fabrication techniques, copper oxide nanowires grown by thermal oxidation are simple and efficient method and widely used. Many researchers successfully obtained copper oxide nanowires by using variety of methods. Zhang et al. [11] synthesized aligned copper oxide nanowires by thermal annealing the copper films deposited on silicon substrate by two methods, thermal evaporation and electroplating. It was observed that the films grown by electroplating method have a lower density of nanowires in comparison to the thermal 2

evaporation. Morphological study shows the long and uniform nanowires at the edges as compared to the centre in the temperature range of 400 to 500 oC. Lichen et al. [12] reported the two step electrochemical process for the growth of copper oxide nanowires for photo electrochemical applications. An excellent contact was observed between the copper oxide nanowires and the substrate with FTO–coating leading to good charge transfer. The applied potentials control the oxidation states CuO or Cu2O and morphology of the nanowires. Zappa et al. [13] synthesized copper oxide nanowires by thermal oxidation of copper films from 200 to 600 oC for 15 hours for chemical sensing applications. At 400 oC, vertical and aligned nanowires with diameter of 170 nm were formed, but at lower temperature, the nanowires were thin and randomly arranged. Park et al. [14] fabricated copper oxide thin films of varying thickness on CuO-buffered SiO2/Si substrates by dc magnetron sputtering at room temperature. Uniformly distributed and densely packed, long nanowires were formed at 400 oC with sample thickness of 1000 nm and 1200 nm films. But the sample with thickness of 500 nm showed low density of nanowires with average length of 2 µm and diameter of 45-50 nm. The 1500 nm thick copper films also showed nanowires but it was peeled off from the substrate. In another study, Cerqui et al. [15] used copper oxide nanowires prepared by thermal oxidation for surface ionization based gas sensors. Wang et al. [16] fabricated copper oxide nanowires by annealing the copper films of thicknesses 0.5 µm and 1 µm obtained by electron beam evaporation. The samples were annealed in furnace from 300-700 o

C for 2-6 hrs and observed uniformly distributed and densely packed nanowires with an average

diameter of 50 nm and length of 2 µm, at 400 oC. Further increase in temperature only increases the grain size instead of the density of the nanowires. They also found that annealing time does not affect the density of copper oxide nanowires but it only affects the length of the nanowires. The average diameter of the nanowires was 50 nm and the length varies with annealing time. However, 4 hrs was observed as an optimum annealing time for the growth of the nanowires with an average length of 2-3 µm. Jiang et al. [17] synthesized copper oxide nanowires by the oxidation of copper foil at varying temperatures from 400 to 600 oC and able to grow highly dense nanowires of different diameters i.e. 30, 50 and 100 nm but the length of the nanowires was same (15 µm) in all the cases. Xu et al. [18] heated copper foils in wet air with flow rate of 4.0 ml min-1 at atmospheric pressure. The temperature has been varied from 300 to 800 oC and treated for 4 hrs. The results showed that the nanowires could only be grown in the temperature range from 400 to 700 oC and at 300 oC both CuO and Cu2O phases were present on the copper surface. Some other research groups also obtained the copper oxide nanowires by 3

oxidizing the copper foils [19-23]. The growth of controlled copper oxide nanowires for the next generation electronic and optoelectronic devices were fabricated by Hansen et al. [24] using copper or copper containing substrates. They investigated the direct growth of the copper oxide nanowires by oxidizing the copper containing substrates at 500 oC for 150 minutes in presence of oxygen. The analysis also showed the presence of two forms of copper oxide layers i.e. the Cu2O as bottom layer and CuO as intermediate layer. Different researchers have reported variety of studies on the synthesis and characterization of copper oxide nanowires and discussed the results based on the structure, optical, morphology and electrical properties. But still literature lacks study on an optimum annealing time and temperature to grow uniform and highly dense copper oxide nanowires and no literature is available on the effect of thin conducting top activation layer of gold on the growth of nanowires. Therefore, in the present work, in addition to optimize the best parameter to grow nanowires in terms of film thickness, annealing temperature and time, we have studied the effect of thin conductive top activation layer of gold on the growth of nanowires in terms of the annealing temperature and time. The magnetron sputtering deposition technique has been used to fabricate the copper thin films of varying thickness. The copper oxide nanowires obtained by annealing the films were characterized by FE-SEM and Raman spectroscopy for the analysis.

2.0 Experimental Details Copper thin films have been deposited on single crystal <100> silicon substrate covered with ~ 200 nm thick SiO2 layer by DC magnetron sputtering. The deposition chamber was evacuated to base vacuum < 10-5 Pa with the help of turbo molecular pump backed by rotary pump. Cu (99.9 % and 2 inch diameter) and Ta (99.5 % and 2 inch diameter) targets were used for the deposition of the films. The substrate was first cleaned in an ultrasonic bath of acetone for 15 mins and dried under nitrogen gas before being placed in the deposition chamber. For the sputtering process, following parameters were used; the argon gas flow rate was kept constant at 20 sccm, sputtering pressure at 0.67 Pa and sputtering power at 50 W. The distance between the target and substrate was fixed at 5 cm. The depositions have been carried out at room temperature. Initially, interlayer of tantalum film (~ 2-30 nm) has been deposited on SiO2/Si substrate as an adhesion layer between copper films and silicon substrate to prevent the layer of copper film from cracking during thermal oxidation or annealing. Afterwards, copper films of varying thickness 4

from 60 to 800 nm have been deposited. The deposited samples were cut down into smaller chips for further studies. The growth of copper oxide nanowires has been carried out by thermal annealing/thermal oxidation of copper thin films by using muffle furnace with a rampage of 10 oC/min to achieve the require temperature and then the samples were cooled down overnight to room temperature as shown in Figure 1. The samples were annealed in the furnace with continuous supply of normal air. In order to observe the effect of different process parameters on the growth of nanowires following procedure was opted. Firstly, the annealing time was kept constant at 6 hrs and the annealing temperature has been varied from 300 to 700 oC for the copper films of varied thickness from 60 to 800 nm. From this study, an optimum temperature was obtained for the growth of highly dense and uniform copper oxide nanowires. In the second case, the temperature was kept constant at an optimum of 450 °C and the effect of annealing time have been studied from 4 hrs to 8 hrs for all copper films from 60 to 800 nm. Lastly, an activation layer of conducting gold layer of thickness 10 nm and 20 nm has deposited on the top of copper films and afterwards the samples were annealed at 400 oC and 450 oC for 6 hrs to study its effect on the growth of nanowires. For the identification of different phases of the as prepared copper films, X- ray diffraction (D8 Advance: Bruker diffractrometer) analysis have been used. This analysis was done with Copper Kα radiation (λ=1.54 x 10-10 m) and the scanning rate and range were 1o min-1 and 35o to 55o, respectively. The morphology of the copper thin films have been observed by (Hitachi) S-4300E/N FE-SEM (Field Emission Scanning Electron Microscopy) operated at 10 kV. The length and diameter of copper oxide nanowires were measured from the SEM micrographs. Confocal Raman Spectrometer (Witec 300R) have been used to record the Raman spectra of copper and copper oxide materials with laser wavelength of 532 nm. 3.0 Results and discussions: 3.1 X-ray diffraction studies: X-Ray diffraction measurements have been carried out for the investigation of the crystalline property of as prepared copper thin films [Figure 2 (a)]. The XRD pattern shows that all the copper films of varying thickness from 60 to 800 nm are polycrystalline in nature and peaks are observed at 43o and 50.3o which correspond to {111} and {200} planes of copper. The XRD peak at 43o is of higher intensity in 5

comparison to the other peak and the intensity of the {111} peak increases with increase in the film thickness. Scherrer’s formula was used to calculate the crystalline size, 𝐷=

0.9 λ β cosθ

Where, λ=1.54 x 10-10 m for CuKα, β is full width half maximum and θ is the angle. The deposited copper films have a varying crystallite size (14 to 26 nm) with an increase in film thickness (60 to 800 nm). The texture coefficients of the Cu films as a function of film thickness were calculated from their respective XRD peaks using the following formula [25]; the results are shown in Figure 2(b). 𝑇𝑒𝑥𝑡𝑢𝑟𝑒 𝐶𝑜𝑒𝑓𝑓𝑖𝑐𝑖𝑒𝑛𝑡 =

𝐼 {ℎ𝑘𝑙} [𝐼{111} + 𝐼{200}]

where, hkl represents the {111} or {200} plane orientations. Initially, for the 60 nm film thickness, the texture coefficient of the {111} orientation is 0.76 which is high compared to 0.24 value of the {200} orientation. With an increase in the film thickness from 60 to 800 nm, the texture coefficient of the {111} orientation increases to 0.85 with a substantial decrease in the texture coefficient of the {200} orientation to 0.15. 3.2 Thermal annealing: The growth of copper oxide nanowires have been carried out by thermal annealing of copper films (60 – 800 nm) in air. This work involves the study of annealing effect on the growth of copper oxide nanowires with respect to the annealing temperature (300 to 700 oC) and annealing time (2 to 8 hrs). The morphology and structural characterization of the surface of the samples after annealing has been studied with SEM analysis and discussed in details as follows: 3.2.1 Effect of Annealing Temperature: Figure 3(a)-(e) shows the SEM images of copper film of thickness 800 nm which was annealed at different temperatures varying from 350 to 550 oC for 6 hrs in air. When copper was oxidized in air, two major products were formed i.e. Cu2O and CuO [26-27]. Firstly, Cu2O phase was formed which has the ability to release the stress at low temperature i.e. when annealing temperature was kept constant at 300 6

o

C and 350 oC, the activation of surface was observed without any growth of the nanowires. This will

help the film to get ready for the growth of the nanowires at higher temperature. And with further increase in the annealing temperature to 400 oC for Cu films, Cu2O starts reacting with oxygen and gets converted into CuO as: 2Cu2O + O2  4CuO It results in the small growth of nanowires with average diameter of 0.58 µm but not on the entire surface of the film [Figure 3(b)]. This may be due to the roughness of the film or composition of the film/ substrate or grain size and its orientation. Furthermore, the growth of nanowire also depends on the diffusion of Cu atoms from the Cu-Cu2O interface to the nanowire, and depends on the diffusion of the grain boundaries through Cu2O and CuO layers [28-29]. Highly dense, high quality CuO nanowires were formed when the annealing temperature increased to 450 oC [Figure 3(c)]. This can be attributed to the higher surface energies at the grain boundaries which enhance the growth rate of nanowires at the grain boundaries. The nanowires observed at 450 oC, have average length and diameter of 1.57 µm and 0.86 µm, respectively but as the temperature increases to 500 oC, nanowires density decreases drastically [Figure 3(d)], with average length of 0.66 µm. Further increase in temperature up to 550 oC, resulted in almost negligible nanowires along with large coagulated grains. At very high annealing temperatures such as 600 oC and 700 oC, no growth of the nanowires has been observed because of the diffusion of Cu/O atoms which results in the formation of CuO crystallites without nanowires. These SEM results shows that the annealing temperature plays a very critical role in the growth of the copper oxide nanowires and 450 oC was the optimum temperature (annealing time kept constant at 6 hrs) to observe the highly dense and high quality copper oxide nanowires.

Growth mechanism of Copper oxide nanowires The growth of nanowires is controlled initially by the chemical reaction at Cu/Cu2O interface and after that the growth of nanowires at Cu2O/CuO interfaces. In the initial stage at low temperature, an oxidation of copper gives the unstable state CuxO but with increase in temperature, these compounds converted into Cu2O. Further, at very high temperature, the Cu2O gets converted into CuO under oxygen

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rich conditions. The Cu2O stage, which is initially formed, is porous and defective, because of large compressive stress. At intermediate heating temperature, Cu ions diffuses across the grain boundaries of the Cu2O layer and get to the Cu2O/CuO interface, at the same time oxygen ions diffuse from the grain boundaries into the CuO layer and reach the interface. The solid state transformation at the Cu 2O/CuO interface associated with the change in volume caused by the compressive stresses, which are the driving forces for the growth of copper oxide nanowires as shown in Figure 4 [30]. With further increase in temperature, the Cu atoms underneath the CuO layer have two ways to reach the reaction interface for the complete oxidation namely lattice diffusion and grain boundary diffusion. The lattice diffusion, results in the formation of CuO layer whereas the diffusion through grain boundaries, leads to the formation of nanowires [28]. The CuO layer is very thin which allows the Cu ions to diffuse through it and also the oxygen ions diffuse from the top layer to reach the Cu2O/CuO interface. When Cu ions reach the Cu2O/CuO interface by the grain boundaries, the reaction occurs as: 2Cu + O2  2CuO This reaction guides the increase in the thickness of CuO layer. The density of the grain boundaries present in the thin layer of copper becomes the ideal site for the diffusion of copper atoms and oxygen ions which results in the growth of nanowires. The same mechanism has been observed and discussed in the effect of annealing temperature section, by varying temperature from 300 oC to 700 oC. 3.2.2 Effect of Annealing Time: The effect of annealing time on the growth and density of copper oxide nanowires was investigated by thermal annealing of Cu films (800 nm thick) at different annealing time varied from 2 to 8 hrs and annealing temperature of 450 oC which was optimized from the previous results. Figure 5(a)-(d) shows the SEM images of copper film of thickness 800 nm annealed for 2 to 8 hrs at 450 oC in air. From Figure 5(a), it is obvious that the copper film annealed for 2 hrs is sufficient for the activation of the surface and also there is some growth of copper oxide nanowires with average length of 1.15 µm with diameter of 41 nm but not on the entire surface. As the annealing time increases to 4 hrs [Figure 5(b)], the growth of nanowires increased to large extent with average length and diameter of 1.5 µm and 65 nm, respectively. With further increase in the annealing time to 6 hrs [Figure 5(c)], copper oxide nanowires cover the entire surface with average 8

length of 1.57 µm as we have already discussed in the previous Section 3.2.1. With further increase in the annealing time up to 8 hrs [Figure 5(d)], the growth of nanowires reduces drastically as compared to the 6 hrs sample but the average length of the nanowire increases slightly to 1.62 µm. The increase in nanowire length was observed due to the longer annealing time which helps to accumulate higher stress. The longer annealing time do not have much effect on the diameter of the nanowires but it greatly affects the length and aspect ratio of nanowires. Peeling off of the film was also observed at the corner of the sample with the annealing time of 8 hrs due to the higher stresses which plays a crucial role at higher temperature and longer annealing time. This could be the main cause of the low density of nanowires. Thus, the highly dense and high quality copper oxide nanowires have been observed when the samples were annealed for 6 hrs. 3.2.3 Effect of Film Thickness: SEM images of copper thin films of different thickness varied from 60 to 800 nm were annealed at 450 o

C for 6 hrs as shown in Figure 6(a)-(d). Initially, annealing of 60 nm thick copper film [Figure 6(a)]

initiate the grain growth along with the morphological changes in the grains but no nanowire has been observed. It means that the surface only activates but does not regulate or control the annealing time and temperature to initiate the growth of the nanowires. In the same way, when the thickness was increased up to 200 nm [Figure 6(a)], large grains were formed and again there was no sign of nanowires or their growth. This implies that in both 60 nm and 200 nm thick samples, the driving force was not sufficient to initiate the growth of nanowires. With further increase in the film thickness up to 400 nm [Figure 6(c)], the small growth of nanowires with length upto 0.5 µm were observed. This implies that for initiating the growth of nanowires a minimum thick film is required which can accumulate stresses during annealing process and provide the minimum material required for the growth of nanowire. As the film thickness increase up to 800 nm [Figure 6(d)], highly dense and lengthy copper oxide nanowires covers entire surface of the substrate having average length of 1.57 µm which we have already discussed in the Section 3.2.1. These results show that the 800 nm thick film observed as an optimum thickness within the parameter range studied to grow highly dense and high quality copper oxide nanowires. Therefore, the data obtained from different experiments revealed that different growth parameters produced nanowires of different length and diameter. Overall, the copper film of 800 nm thick, 9

thermally annealed at 450 oC for 6 hrs in air was the most optimum parameter to grow highly dense and high quality copper oxide nanowires. These results also imply that the driving force to grow copper oxide nanowires were the accumulation and relaxation of stresses in the annealing process. Moreover, the accumulation of high stresses need higher temperature and longer annealing times with a thick film. After attaining an optimum temperature or time, the stresses at the Cu2O and CuO interface become very high which results in the reduction of the interface area and thereafter the oxide layer itself relaxes by spontaneous growth of nanowires and one observe highly dense and lengthy nanowires. But sometimes, these stresses become very high and cause the thin film to peel off. Furthermore, this thermal annealing method is much faster and more convenient way to grow high density nanowires in comparison to the other complex chemical methods for the synthesis of copper nanowires. 3.2.4 Effect of thin conducting gold layer on the growth of nanowires: a) Annealed at 400 oC: Figure 7(a)-(c) shows the copper films of thickness 800 nm annealed at 400 oC (temperature is 50 oC less than the optimized temperature as discussed in Section 3.2.1) for 6 hrs without and with gold top layer of varying thickness i.e. 10 nm and 20 nm. As observed from Figure 7(a), without gold layer, sample annealed at 400 oC, small growth of the nanowires with average diameter of 58 nm but not on the entire surface of the film. When a similar sample was prepared with 10 nm gold layer on top of the copper film [Figure 7(b)], and annealed at 400 o

C for 6 hrs, an increase in the density of nanowires was observed. The density of nanowire decreases

with further increase in the gold layer thickness up to 20 nm [Figure 7(c)] and it was comparable with the sample, which was annealed without gold layer. As we have already observed and discussed, for the growth of nanowire; an optimum temperature and time of interaction with oxygen atom plays a crucial role. Moreover, for the growth of nanowire, initiation sites are grain boundaries which are more reactive but as the whole surface is covered with gold layer, ideally, it should get more difficult for the oxygen atoms to react with copper film. But on the contrary, the growth of nanowire increases with the thickness of gold layer up to 10 nm which might be due to the combination of stress in the copper film and diffusion / trapping of oxygen through gold layer and reaction with copper film atoms at grain boundaries. It might also be due to the formation of CuAu metastable compound. Referring to the Cu-

10

Au phase diagram in Figure 8 [31], it can be observed that at 50 at% Cu-50 at% Au composition, a metastable compound of CuAu is stable just below 410 oC. Hence, considering our system, where the interface of Cu-Au was annealed at 400 oC; formation of CuAu compound at the grain boundaries acts as an activation site, which increases the availability of Cu at the grain boundaries. Due to the metastable nature of CuAu, the Cu in the compound is more prone to oxidation than to stay in its metastable state, therefore, as diffusion of copper increases towards grain boundaries; it leads to the formation of copper oxide nanowires especially in comparison to the copper sample annealed without gold layer. With a 20 nm gold layer, the growth of nanowire decreases due to the decrease in the porosity of the film which inhibits the diffusion of oxygen atoms to react with copper film and initiate the growth of nanowire. Therefore, one observes fewer number of nanowires in comparison to the copper film covered with a 10 nm gold layer. Furthermore, it can be postulated that thicker the Au layer, more is the friction faced by nanowires to grow by moving the Au atoms from the Au layer. Moreover, the Au layer acts like an activation layer for copper oxide nanowire initiation and they start to grow at a lower temperature. b) Annealed at 450 oC: Figure 9(a)-(c) shows the copper films of thickness 800 nm annealed at 450 oC for 6 hrs without and with gold top layer of varying thickness i.e. 10 nm and 20 nm. As we can observe from Figure 9(a), without gold layer sample annealed at 450 oC, highly dense, thick and high quality copper oxide nanowires were formed. On the other hand, when copper film was covered with 10 nm gold layer [Figure 9(b)], and annealed at 450 oC for 6 hrs, resulted in decrease in the density of nanowires but leads to the enhancement in the aspect ratio of nanowires. The reason of this decrease might be due to the non-formation of any CuAu metastable compound at 450 oC which is generally formed at below 410 o

C as shown in Cu-Au phase diagram (Figure 8) and discussed in the previous paragraph. This means

that diffusion of copper atoms towards grain boundaries should be normal but the presence of heavy gold atoms inhibits copper diffusion towards grain boundaries and decreases the formation of copper oxide nanowires. With further increase in the gold layer thickness up to 20 nm, the density of nanowires decrease as shown in Figure 9(c) which may be attributed to the decrease in the porosity of the film due

11

to increased Au content, leading to inhibition of oxygen atom diffusion towards copper film which is essential to initiate the growth of nanowire.

3.3 Raman Spectroscopy studies: Raman spectroscopy has been used for the identification of different oxides due to the change in polarizability and phase structure of the sample. Raman scattering experiments have been performed at room temperature in the spectral region of 200 cm-1 to 1400 cm-1. The Raman spectra of copper oxide nanowires samples annealed at different annealing temperatures are shown in Figure 10. Raman spectrometry shows that when 800 nm thick film was annealed at 350 oC, characteristic peaks of CuO were observed at 298, 346, 632, 1100 cm-1 and one characteristic peak of pure Cu was also observed at 200 cm-1. This implies that at this annealing temperature, the sample was not fully oxidized and some un-reacted copper atoms were still presented in the sample. It is known that CuO is the only form of copper oxide that is present above 300 °C [32] while other phases of Cu occur below this temperature, especially, single-phase Cu2O which is existent only upto 200 °C. Above this temperature, Cu2O phases starts to reduce and CuO starts to build. Furthermore, the Raman peaks were centered at 109, 148, 218, 416, 515 and 635 cm−1. One of Raman peak of Cu2O i.e. 635 cm−1 was closely spaced i.e. just after the peak of CuO around 600 cm-1. Hence, when the annealing temperature increases, a decrease of the Raman peak intensity of Cu2O was evident around 632 cm−1 which can be attributed to the removal of trace amount of Cu2O. When the annealing temperature was increased to 450 oC, the Raman peaks significantly increased and the peak of Cu disappears which simply gave us the confirmation that all the copper atoms converted into copper oxide. The Raman peak shifted to the higher wave number and becomes sharper with an increase of intensity, showing the increase in crystallinity and grain size with increase in annealing temperature [23]. Figure 11 shows the Raman spectra of different thickness samples annealed at 450 oC for 6 hrs. The graph shows the peaks of CuO at 298, 346, 632, 1100 cm-1 positions and no peak was observed which belongs to Cu and Cu2O in both the graphs. This signifies that 450 oC was sufficient to completely oxidized the copper atoms and also Cu2O into CuO form. Also, the graph shows that the intensity of the peak increases with increase in film thickness from 400 to 800 nm. 12

The peaks centered at 282 or 332 and 619 cm−1 were assigned to the Ag and Bg modes of CuO, respectively [22]. The absence of Cu2O modes in the corresponding Raman spectra indicates the purity of the material [21].

4.0 Conclusion The present work focuses on the growth of copper oxide nanowires and optimization of the parameters to grow highly dense and high quality nanowires by thermal annealing. The copper films have been fabricated on SiO2/Si substrate by sputtering and annealed to grow nanowires. The results show that annealing of copper films having thickness 800 nm at 450 oC for 6 hrs is the best optimized parameter for the growth of high density nanowires. At higher temperature and time, the growth of nanowires reduces and low density was observed. The minimum thickness should be 400 nm to initiate the growth of nanowires as no nanowires were observed in thin samples i.e. 200 nm. A thin conductive gold film (10 nm) on the top of copper film was able to enhance the density of the nanowires in comparison to the sample without gold film and also lower its annealing temperature to 400 oC for 6 hrs. The changes in the crystalline size and copper oxide formation were also confirmed by the Raman spectroscopy studies.

Acknowledgements VC and NS acknowledges the CSIR–SRA funding (8857-A) from CSIR, New Delhi and SERB, New Delhi for the ECR funding (SERB-ECR/2016/000150), respectively. The authors acknowledge the FESEM facility of IISER Mohali and the Raman facility of INST, Mohali.

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FIGURES

Figure 1: Schematic diagram of synthesizing Copper oxide nanowires 15

T e x tu re C o e ffic ie n t

0.9

{1 1 1 }

0.8

{ 200}

0.2

0.1

0

200

400

600

800

1 000

F ilm T h ic k n e s s (n m )

Figure 2: (a) XRD graph of copper thin films on SiO2/Si substrate with varying thickness from 60 to 800 nm and (b) Texture coefficient of copper thin films with varying thickness from 60 to 800 nm

16

(a)

(b)

(c)

(d)

(e)

Figure 3: SEM images of Copper oxide nanowires grown at different annealing temperatures (a) 350 oC, (b) 400 oC, (c) 450 oC, (d) 500 oC and (e) 550 oC 17

Figure 4: The schematic model of growth of Copper oxide nanowires by thermal annealing [30]

18

(a)

(b)

(c)

(d)

Figure 5: SEM images of 800 nm thick copper films annealed at 450 oC for different annealing time: (a) 2 hrs, (b) 4 hrs, (c) 6 hrs and (d) 8 hrs

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(a)

(b)

(c)

(d)

Figure 6: SEM images of the annealed copper thin films at 450 oC for 6 hrs having thickness (a) 60 nm, (b) 200 nm, (c) 400 nm and (d) 800 nm

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(a)

(b)

(c)

Figure 7: SEM images of the copper film of 800 nm thickness annealed at 400 oC for 6 hrs with varying gold layer on the top (a) without Au, (b) with 10 nm and (d) with 20 nm

21

Figure 8: Phase diagram of Cu-Au system [28]

22

(a)

(b)

(c)

Figure 9: SEM images of the copper film of 800 nm thickness annealed at 450 oC for 6 hrs with varying gold layer on the top (a) without Au, (b) with 10 nm and (c) with 20 nm

23

Figure 10: Raman spectra of samples annealed at 350 oC and 450 oC

24

Figure 11: Raman Spectra of different thickness samples annealed at 450 oC for 6 hrs

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Highlights

 Copper films deposited on SiO2/Si substrates by magnetron sputtering  Growth of copper oxide nanowires by thermal annealing  Optimization of parameters to grow highly dense and high quality nanowires  Conductive gold layer (10 nm) on the top of copper film to enhance the nanowires density  Scanning Electron Microscopy used to observe the growth of the nanowires

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