Fabrication and Wetting Characteristics of Copper Thin Film: An Active Layer for SPR-based Sensor Applications

Fabrication and Wetting Characteristics of Copper Thin Film: An Active Layer for SPR-based Sensor Applications | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Research Article Fabrication and Wetting Characteristics of Copper Thin Film: An Active Layer for SPR-based Sensor Applications Mohammad Kamal Hossain, Abdullah Aljishi, Firoz Khan, Anwar Ul-Hamid, and 1 more This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-4427071/v1 This work is licensed under a CC BY 4.0 License Status: Posted Version 1 posted You are reading this latest preprint version Abstract In this work, a simple and two-step process was demonstrated to develop multifunctional Cu-based thin films that would be suitable for thin film photoactive devices. Cu thin films on quartz glass substrates were prepared by sputtering technique followed by a thermal treatment. The samples were annealed at high temperatures such as 200, 400, and 600°C for 2 hrs in a tubular furnace. Surface topography was investigated by a high-resolution scanning electron microscope (FESEM) and SEM-aided energy dispersion spectroscopy (EDS). At high temperatures, the thin films were found to have clusters and voids. Detailed studies on optical properties such as UV-vis absorptions, energy band gaps and Urbach energies have been carried out. A red shift in absorption edges (from 464 to 616 nm), a decrease in energy band gaps (from 2.38 to 1.54 eV) and an increase in Urbach energies (from 193 to 272 meV) were observed for those samples annealed at higher temperatures. Sessile drop tests were carried out to find the wetting contact angle and demonstrate the hydrophobicity of the thin film of pristine Cu and of those treated at high temperatures. Sessile drop tests were carried out to find the wetting contact angle (WCA) and demonstrate the hydrophobicity of the thin film of pristine Cu and of those treated at high temperatures. An approximate WCA of 71.9° was determined for the Cu thin film. After the samples were treated at 200°C and 400°C, respectively, the surface became more hydrophobic by 92.4° and 85.2°. Nevertheless, the same thin film's WCA was decreased and its hydrophilicity increased during additional annealing. Cu-based thin films have been suggested as the active layer in an SPR sensor model, and the spectrum and angular resolved reflectance properties have been thoroughly investigated. At spectral wavelengths of 600, 700, and 800 nm, the optimum thickness of Cu thin film was determined to be 40 nm at SPR angles of 44.7°, 42.7°, and 42.15°. Copper thin film sputtering deposition optical properties wetting contact angle SPR sensor Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7 Figure 8 Figure 9 Figure 10 1. Introduction Copper (Cu) and Cu-based thin films are widely used in industry and studied in research due to their unique characteristics and great potential in diverse fields of applications. Cu is an abundant element that is easy to extract, thus rendering it an economical option [ 1 – 4 ]. Cu thin films are simple to manufacture in an eco-friendly manner [ 5 – 8 ]. With regards to properties, Cu-based thin films offer excellent anti-interference characteristics and radiation resistance [ 9 – 12 ]. Cu-based thin films play a crucial role in Surface Plasmon Resonance (SPR) sensors, offering several important advantages and impacts [ 13 – 14 ]. Cu thin films can modulate the intensity and response of surface plasmon resonance. The thickness of the Cu film affects the modulation capability, with thinner films showing more obvious regulation [ 13 , 15 – 16 ]. This modulation capability is essential for detecting changes in refractive index and achieving high sensitivity in SPR sensors. Cu thin films are a competitive alternative to gold and silver in SPR sensors [ 13 , 17 – 18 ]. They offer interesting properties for optics, nanotechnology, and SPR sensors [ 19 – 20 ]. Recent studies have demonstrated that the performance of Cu-based SPR sensors is quite similar to those using gold [ 13 , 17 , 21 ]. Cu oxide, which is often present in Cu thin films, is a semiconductor with a non-toxic nature. This makes Cu-based SPR sensors more environmentally friendly and safer for various applications [ 22 ]. There are several techniques for depositing thin films of Cu, including electrochemical deposition, physical vapor deposition (PVD), chemical vapor deposition and supercritical fluid chemical deposition [ 23 – 26 ]. The sputtering technique, a subset of PVD, is the simplest technique for metal deposition. It is commonly used in numerous industries, such as the semiconductor industry and the coating industry. Sputtering technique offers numerous advantages such as great flexibility in choice of coating materials, excellent adhesion, high repeatability, and the ability to deposit thin films on a variety of substrates. These advantages can justify the costs associated with this technique, depending on the specific needs of the application [ 27 ]. This allows manufacture on a large-scale for industrial applications, which is important for applications such as solar cells. DC sputtering also only requires low sputtering voltage and low sputtering gas pressures, compared to other sputtering techniques such as RF sputtering [ 28 – 29 ]. Previous work on the fabrication of Cu thin films by DC sputtering has shown that better adhesion to the substrate is correlated with a lower current value and a longer deposition time [ 30 ]. It was also found that grain size increases as the substrate gets closer to the target [ 30 ]. The deposition rate of Cu increases in proportion to the applied DC voltage and decreases with deposition pressure [ 29 , 31 ]. Research on the effects of annealing on Cu films demonstrated a noticeable increase in grain size as the annealing temperature increased [ 32 ]. Surface roughness decreases as temperature increases up until 400°C, where the lowest surface roughness is obtained [ 32 ]. Cu thin films annealed at 150 and 200°C demonstrated great photocatalytic performance [ 33 ]. Improvements in sensitivity, affordability, and compatibility have propelled developments in Cu-based thin film surface plasmon resonance (SPR) sensors [ 34 – 35 ]. To some extent, silver- and gold-based sensors have been competitively replaced by Cu-based thin film SPR sensors [ 35 ]. Naturally occurring on the Cu surface is a semiconductor called Cu-based oxide, which is non-toxic. Cu-based thin films are therefore preferred for the sensitive components of SPR sensors. Cu-based thin films and other plasmon-active nanostructured thin films have drawn interest due to their potential in biosensing applications [ 36 – 37 ]. These commercially accessible films for chemical and biological detection provide increased sensitivity. Cu-based thin film SPR sensors have been integrated and made smaller thanks to developments in nanofabrication processes. As a result, portable and small sensing devices appropriate for on-site and point-of-care applications have been developed [ 37 – 38 ]. Cu-based structured thin-film nanorods have been investigated as a potential means of enhancing sensor sensitivity and efficiency [ 39 ]. Furthermore, improvements in waveguide coupling optimization for integrated light production and detection have been achieved. Application of Cu-based thin films as a plasmonic material presents benefits in terms of compatibility, cost-effectiveness, and sensitivity, making it a desirable choice for more study and advancement in the SPR sensing sector. SPR sensors detect changes in the refractive index of the dielectric near the metal layer by using electromagnetic surface plasmon waves (SPs) traveling at the interface of the dielectric and the metallic layer [ 40 ]. Attenuated total reflection is usually the mechanism that produces surface plasmons when p-polarized light contacts a metal-coated glass prism. A minimum in metal reflectivity as a function of incident angle or wavelength is known as the SPR angle of such sensors. Therefore, changes in the refractive index of the medium close to the metal layer may therefore be detected by shifting the SPR location [ 41 ]. SPR sensors are ideal for a variety of applications because of their exceptional sensitivity to even minute changes in the refractive index of the probed medium. These applications include site-specific absorption at metal surfaces, real-time, label-free investigation of metal-analyte interactions, and non-invasive measurement of thin-film thickness and optical constants [ 42 – 44 ]. This effort attempts to use the sputtering process to produce Cu-based thin films on quartz glass substrates. The samples underwent further treatment in a tube furnace for two hours at temperatures as high as 200, 400, and 600°C. High resolution scanning electron microscopy (FESEM) and SEM-aided energy dispersion spectroscopy (EDS) have been used to topographically validate the thin layer of virgin Cu and those processed at high temperatures. The identical thin films' optical characteristics, including their UV-vis absorptions, energy band gaps, and Urbach energies, have all been calculated. Both the pristine Cu thin films and the annealed at higher temperatures exhibited visible-range photoluminescence emissions. Measurements of the wetting contact angle (WCA) were used to show how hydrophobic the pristine Cu thin film and those modified at high temperatures are. A moderate WCA for the thin film of Cu was estimated to be ca. 71.9 o . The surface became more hydrophobic, 92.4 o and 85.2 o when the samples were treated at 200 and 400°C respectively. However, further annealing of the same thin film reduced the WCA and made it more hydrophilic. An SPR sensor model using Cu-based thin films as active layer has been proposed followed by rigorous investigation on the spectral- and angular-resolved reflectance characteristics. Optimized thickness of Cu thin film was found to be 40 nm at SPR angles of 44.7°, 42.7° and 42.15° at 600, 700 and 800 nm of spectral wavelengths. 2. Materials and methods Thin films of Cu were fabricated in a simple two-step process as shown in Fig. 1 . A circular Cu target (5 cm in diameter and 3.18 mm in thickness and 99.99% purity) was purchased from ACI Alloy, Inc. and used as received. In the first step, a sputtering coater (Nanomaster NSC3000, USA) was used to sputter Cu on quartz glass substrate. The samples were immediately transferred to tubular furnace (MTI Corporation OTF-1200X, USA) for thermal treatment in the second step. A direct current (DC, 30W) gun was used to sputter the Cu from the target onto the quartz glass substrate in a downward configuration. The target substrate distance was maintained at 10 cm. The base pressure and working pressure in the sputtering chamber were maintained at approximately 1.7×10 − 5 torr and 2.8×10 − 3 torr respectively. The argon (Ar) flow rate in the chamber was kept at 30 SCCM during the deposition. A thickness-bound process was initiated and the in-built thickness monitor was configured to 50 nm. As mentioned above, as-fabricated thin films were annealed in a quartz tube furnace at 200, 400, and 600°C for 2 hrs each at a heating rate of 20 o C/min. Detailed surface morphology was investigated using high-resolution FESEM. Insets (i)-(ii) of Fig. 1 display high-resolution FESEM images of typical pristine and treated specimen respectively. The elemental composition of each sample was confirmed using SEM-aided EDS. UV-vis absorption and transmission measurements were carried out by a UV-Vis-NIR spectrometer (Jasco V-670, Japan). PL emission of as-fabricated films and of those annealed at high temperatures was recorded at room temperature (RT) using a Horiba Jobin-Yvon spectrofluorometer (Flurolog-3, Japan) equipped with a 150W Xe lamp as the excitation source. WCA measurements were carried out using a goniometer (DSA25, Denmark) through sessile drop tests. In sessile drop tests, the droplet volume of DI water was controlled with an automatic dispensing system. A CCD camera interfaced with computer-controlled software was used to capture the images of the droplets and calculate WCA on the samples. An SPR sensor model comprised of “Cu thin on glass” has been designed and optical simulation was performed by a commercial software Setfos (version 5.3, Fluxim AG, Switzerland). The active thin film thickness was varied from 30 to 70 nm to understand the best performance and optimize the SPR characteristics. Considering the inherent properties of plasmonic thin films of Cu, incident angles were varied from 41 o to 50 o in this current investigation. 3. Results and discussion Understanding the surface topography of the film is crucial, since the optical properties as well as wetting characteristics of the same depend a lot on nanostructures. Morphology and in-depth analysis of the pristine sample and the samples treated at high temperatures were investigated by FESEM. A representative FESEM image of the thin film of Cu as depicted in Fig. 2 a indicated that the surface was quite smooth. A zoom-in view, as marked by the white square (2 µm × 2 µm) in Fig. 2 a, was shown in Fig. 2 b. A line profile along the dashed blue line shown in Fig. 2 b revealed that the surface was relatively smooth, as presented in Fig. 2 d. Figure 2 e shows the three-dimensional (3D) hawk-eye view of the zoon-in area as shown in Fig. 2 a confirming again the smooth top surface of the pristine thin film of Cu. The elemental composition of the same film was confirmed by SEM-aided EDS. Figure 2 c represents the EDS spectrum of the pristine thin film of Cu whereas Cu peaks were evident at 0.93 keV (CuL α ), 8.04 keV (CuK α ), and 8.91 keV (CuK β ) along with a Si peak at 1.74 keV (SiK α ) due to underneath substrate, an O peak at 0.53 keV (OK α ) due to oxides and Au peaks at 2.12 keV (AuM) and 9.71 keV (AuK α ) due to Au coating. Further to elaborate on the distribution of constituent elements SEM-aided EDS mapping was carried out. Insets (i) and (ii) of Fig. 2 c represent such mappings of the two main elements Cu and O respectively. It was observed that the constituent elements were homogenously distributed. No clusters or voids were observed. The weight percentage of the constituent elements is shown in the inset of (iii) of Fig. 2 c. Thermal treatment of Cu thin films at elevated temperatures, such as 600 o C was found to modify the surface morphology as well as subsequent optical characteristics and hydrophobicity. The optical and hydrophobic characteristics have been elaborated in the later part of the text. Let us confirm the details on surface topography and constituent elements. Figure 3 a shows a representative SEM micrograph of thin film of Cu annealed at 600 o C for 2 hrs. With reference to a pristine thin film of Cu, the treated film surface was found to be rougher and consisted of clusters and voids. A zoom-in view, as marked by the white square (5 µm × 5 µm) in Fig. 3 a was shown in Fig. 3 b. It is noteworthy that the clusters and voids were of different sizes. A typical cluster and a void are mentioned as the black circles in Fig. 3 b respectively. A line profile along the dashed blue line shown in Fig. 3 b indicated the height and depth of the corresponding cluster and void respectively as shown in Fig. 3 d. The cluster and void are marked by black arrows in Fig. 3 b corresponding to the exact positions of a hill and dip in the line profile shown in Fig. 3 d. Although the origin of such clusters and voids was not known, it was speculated that such clusters and voids could be due to the desorption of Cu and O at defect sites. When the thin films of Cu were treated at high temperatures, there was a possibility of defect site movement, known as the “Ostwald ripening effect”, within the system. As a consequence, the surface morphology as well as the stoichiometric ratio of the material were altered. Such phenomena can affect the wetting and optical properties of Cu thin film as mentioned in the later part of the text. A 3D hawk-eye view of the zoon-in area as shown in Fig. 3 a is presented in Fig. 3 e. the elemental composition of the same film was confirmed by SEM-aided EDS. Figure 3 c represents the EDS spectrum of the pristine thin film of Cu whereas Cu peaks were evident at 0.93 keV (CuL α ), 8.04 keV (CuK α ), and 8.91 keV (CuK β ) along with Si peak at 1.74 keV (SiK α ) due to underneath substrate, an O peak at 0.53 keV (OK α ) due to oxides and Au peaks at 2.12 keV (AuM) and 9.71 keV (AuK α ) due to Au coating. EDS mapping was carried out to understand the distribution of constituent elements. Insets (i) and (ii) of Fig. 3 c represent such mappings of the two main elements Cu and O respectively. It was observed that constituent elements were less homogenously distributed, although distinct clusters and voids were recorded. The weight percentage of the constituent elements as shown in inset (iii) of Fig. 3 c revealed a higher percentage of O (ca. 24.3%) with reference to that obtained in the case of a pristine sample. Figure 4 shows UV-Vis absorption spectra of as-fabricated Cu thin film as well as of those treated at 200, 400 and 600 o C for 2 hrs. Absorption edges were observed to have a red-shift towards longer wavelengths with increasing annealing temperatures as shown in Fig. 4 . The dashed red lines guide the readers to follow the approximate absorption edges. The as-fabricated film of Cu was expected to be amorphous and therefore no absorption edge was observed. The absorption edges appeared at 464 nm (2.67 eV), 535 (2.31 eV) and 616 nm (2.01 eV) for the samples annealed at 200, 400 and 600 o C in ambient condition respectively as shown in Fig. 4 . The Fig. 4 a-d represent the UV-Vis spectra of pristine Cu thin film as well as of those annealed at 200, 400 and 600 o C in ambient condition respectively. It is well-acknowledged that the transition in the UV-vis absorption edge of a thin film at higher temperatures depends on various factors, such as the composition, thickness, and deposition method of the film. A plausible reason behind the scene could be related to the effect of temperature on the atomic structure of the film [ 45 – 46 ]. As the temperature increases, the amorphous structure of the film undergoes rearrangements or phase changes, particularly leading to a relaxation of the atomic conformation. Therefore, a red-shift is speculated at higher annealing temperatures, as observed in the case of Cu thin film and shown in Fig. 4 b-d. The UV-Vis spectra recorded for Cu thin films annealed at different temperatures were further utilized to calculate optical bandgaps of the corresponding films. The Tauc plot is an important tool used to determine the optical band gap of materials, particularly semiconductors. It provides a graphical representation of the relationship between the absorption coefficient and the photon energy. By analyzing the intercepts of the linear portion of the plot, one can estimate the band gaps of various thin films. The Tauc plot can be used as a quality control tool to assess the structure or phase of materials. As demonstrated in the Fig. 5 a-c, the energy band gaps were estimated using a Tauc plot by plotting (α h ν) 2 versus h ν, where α and h ν represent the absorption coefficient and photon energy respectively. Figure 5 a-c represent the Tauc plots of thin films of Cu annealed at 200, 400 and 600 o C for 2 hrs in ambient condition respectively. The band gaps of 2.38, 1.69 and 1.54 eV as mentioned therein were estimated for the thin films annealed at 200, 400 and 600 o C in ambient condition respectively. The sputtered thin film of Cu is commonly known to be amorphous and therefore no absorption peak was observed as shown in Fig. 4 a. However, at mild temperature such as 200 o C, Cu-based oxide islands on the surface of as-grown thin film started to show up. As shown in Fig. 5 a, optical bandgap of 2.38 eV was estimated for the specimen annealed at 200 o C for 2 hrs in ambient condition. It is speculated that the thin film annealed at 200 o C for 2 hrs in ambient condition remained in the early stage of nano-crystalline Cu-based oxide film, particular Cu 2 O at the surface of amorphous film. Interestingly, it was noted that the same thin film annealed at 400 o C for 2 hrs in ambient condition turned into nano-crystalline Cu 2 O thin film as indicated in the Tauc plot shown in Fig. 5 b. The optical bandgap of 1.69 eV as estimated for the thin film annealed at 400 o C in ambient condition coincided well with reported optical bandgap of Cu 2 O [ 45 ]. The same thin film annealed at higher temperature such as 600 o C for 2 hrs in ambient condition facilitated to complete oxidation of copper to Cu 2 O and turned the film as CuO [ 47 ]. The optical bandgap of 1.54 eV as estimated for the thin film annealed at 600 o C in ambient condition coincided well with reported optical bandgap of CuO. It is still under debate whether inward diffusion of oxygen or outward diffusion of copper is responsible for growth of CuO thin film from Cu 2 O thin film. The Urbach energy is an important parameter in the characterization of thin films. It provides valuable information about the energy distribution within the thin film and is used to understand the structural and optical properties of the film. The Urbach energy is related to the exponential tail in the absorption spectrum of a material as per the following equation (Eq. 1). \(\alpha ={\alpha }_{0}{e}^{\frac{hv}{{E}_{u}}}\) (Eq. 1) whereas, α 0 and E u are constants related to the low energy limit of the absorption coefficient and Urbach energy respectively. The Urbach energy represents a measure of disorder within thin films. The Urbach plot can be obtained by plotting the natural logarithm of α (i.e. ln α) as a function of photon energy ( h ν). The Urbach energy can be determined by taking the inverse of the gradient at the linear region of the curve. Figure 6 a-c represent the Urbach plots of thin films of Cu treated with 200, 400 and 600 o C in ambient condition respectively. A reduction of the Urbach energy from 272 meV to 205 meV after the treatments at 400 and 600 o C was observed, although the film at the treatment of 200 o C was found to be lower (193 meV). The trend of decreasing Urbach energy coincides well with reported values [ 48 ]. Such a scenario is speculated due to the diminution of the density of states, the increase in crystallinity, the decrease in the degree of disorderness and a relaxation of the distorted bonds. To the extent, the fact also indicates that the thin film of Cu turned into oxides after heat treatment where energy band gaps were found to decrease as well with increasing annealing temperatures as demonstrated in Fig. 6 . The refractive index is an important parameter in the design of SPR-based sensors. SPR sensors rely on the interaction between light and the surface plasmons, which are collective oscillations of electrons at the metal-dielectric interface. The refractive index of the surrounding medium affects the propagation of surface plasmons and can be used to detect changes in the surrounding. Refractive index and energy band gap are interrelated and Dimitrove and Sakka proposed the following formula (Eq. 2) to obtain the refractive index ( n ) of any thin film, \(\frac{{n}^{2}-1}{{n}^{2}+2}=1-\sqrt{\frac{{E}_{g}}{20}}\) (Eq. 2) where, n and E g represent the refractive index and energy band gap of corresponding thin film. By simplifying the Eq. 2, one can find the refractive index, n $$n=\sqrt{\left(\frac{3+2\sqrt{\frac{{E}_{g}}{10}}}{\sqrt{\frac{{E}_{g}}{10}}}\right)}$$ Based on this relation, the refractive indexes of thin films of Cu annealed at 200, 400 and 600 o C were estimated to be 2.85, 3.05 and 3.11 respectively which are in good agreement with the reported values [ 49 ]. Photoluminescence in thin films is closely related to the presence of defects. The emission of light from defects provides insights into the defect density, distribution, and type within the film. Photoluminescence properties of thin film support to understand the defects involved. Therefore, a detailed study was carried out for Cu thin films under this investigation. Figure 7 shows the photoluminescence spectra of thin films of pristine Cu as well as of those annealed at 200, 400 and 600 o C for 2 hrs in ambient condition. For the pristine thin film of Cu, two main PL bands were observed at photon energies of 1.62 eV (765 nm) and 1.58 eV (785 nm) along with three shallow shoulder peaks at 1.67 eV (744 nm) 1.55 eV (808 nm) and 1.51 eV (821 nm) as shown by dashed vertical black lines in Fig. 7 . When the thin film of Cu went under thermal treatment, it was observed that the PL band at 765 nm shifted to right a bit (i.e. 768) and that at 785 nm shifted to left (i.e. 778 nm) as shown by dashed vertical red lines in Fig. 7 . The two bands 744 nm (1.67 eV) and 820 (1.55 eV) nm are attributed to oxygen vacancies corresponding to doubly ionized vacancies and single ionized vacancies respectively [ 50 – 51 ]. As for the thin film of pristine Cu, two PL peaks at about 765 nm (1.62 eV) and 785 nm (1.58 eV) are possibly due to electronic levels Vo + 2 and V Cu respectively. The samples annealed at higher temperatures indicated a shift and higher PL emission at 768 nm (1.61 eV) due to higher oxygen vacancies, whereas a shifted and low PL emission at 778 nm (1.59 eV) could be the transformation of Cu to CuO at high temperature treatments [ 51 ]. Due to the great demand for surface hydrophobicity for various applications, particularly those used in harsh weather conditions, it is always expected to make the thing more hydrophobic. In this context, sessile drop tests are frequently used to find out the WCA of the surface and thus to define whether the surface is hydrophobic or hydrophilic. Like most metals, the thin film of Cu surface is hydrophilic with a moderate WCA. Figure 8 shows the average WCA of pristine thin films of Cu as well as of those treated at 200, 400 and 600°C for 2 hrs in ambient condition. A moderate WCA for the thin film of Cu was estimated to be ca. 71.9 o as shown in Fig. 8 a. The surface became more hydrophobic when the samples were treated at 200 and 400°C for 2 hrs in ambient condition. The WCA for samples treated at 200 and 400°C as shown in Fig. 8 b-c was noted to be 92.4 o and 85.2 o respectively. However, further annealing of the same thin film reduced the WCA and made it more hydrophilic [ 52 ]. One of the plausible reasons could be the effect of oxidation. At high temperatures, the surface morphology and chemical properties might have changed and therefore the surface contact angle has been reduced to 42.3 o as shown in Fig. 8 d. The reduction of the contact angle may also be explained by the Wenzel theory where surface roughness contributes to the improvement of wettability [ 53 ]. As observed in the FESEM observation and explained above in Fig. 3 , the formation of clusters and voids could be responsible for the hydrophilic characteristics of the samples treated at 600°C for 2 hrs in ambient condition. The sensitivity and performance of any SPR-based sensor are greatly influenced by the thickness of their active layer [ 40 , 42 ]. Both the resonance condition and the angle at which the greatest SPR response happens can vary with changes in layer thickness. Resonance angle shifts can be used to identify variations in the refractive index of surrounding medium and/or the presence of analytes. Since the thickness of thin film is a key factor in determining the performance of the SPR sensor, much consideration has been given to finding the ideal combination, as shown in the accompanying Figures. Figure 9 a-e represent spectral and angular-resolved p-polarized reflectance mappings of Glass-Cu-Air interfaces over a broad spectral range starting from 500 to 800 nm wavelengths. Cu thin film thicknesses were taken into consideration to be 30, 40, 50, 60, and 70 nm, respectively. It was clear that reduced reflectance was seen at particular incidence angles and spectral regions for the different thicknesses of Cu thin films. The lowest intensity was found at shorter wavelengths with a larger spread for Cu thicknesses of 30 and 70 nm, as indicated by the white dashed rectangle in Figs. 9 a and 9 e. Conversely, for Cu thin film thicknesses of 40, 50, and 60 nm, the lowest reflectance was found over the 600–800 nm spectral region, as indicated by the white dashed areas in Fig. 9 b–d, respectively. As seen in Fig. 10 a-c, additional quantitative analysis was performed for three common spectral wavelengths, such as 600, 700, and 800 nm. Figure 10 a-c represent angular-resolved p-polarized reflectivity of the model “Glass-Cu-Air interfaces” used in the simulation at 600, 700 and 800 nm wavelengths respectively. At the line of 600 nm spectral wavelength, Cu thin films of 30, 40, 50, 60 and 70 nm thickness exhibited the minimum reflectivity located at the incident angles of 45.3 o (0.0861 a.u.), 44.7 o (0.0002 a.u.), 44.5 o (0.0910 a.u.), 44.45 o (0.2784 a.u.) and 44.4 o (0.4595 a.u.) respectively as shown in Fig. 10 a. It can be observed that at a Cu thin film thickness of 40 nm, reflectivity approaches zero, which allows for the highest possible conversion of incident light energy into surface plasmons. Any deviation from the aforementioned reference value in thickness results in an increase in minimum reflectance, which in turn indicates a reduced rate of light utilization, as seen in Fig. 10 d. The "Glass-Cu-Air interfaces" model utilized in this investigation has a minimum reflectance and SPR incidence angle as shown in Fig. 10 d for 600 nm wavelengths Cu thin film thicknesses were kept at 30, 40, 50, 60, and 70 nm. At 45.3 o , the lowest reflectance rises to 0.0861 a.u. for a Cu thin film thickness of 30 nm. Cu thin film thickness of 40 nm with minimal reflectance of 0.0002 a.u. at incidence angle of 44.7° produced the maximum stimulation of SPR, which was represented by a deep and wide curve with FWHM of 3.3°. In this case, as thicknesses increased from 30 to 40 nm, the SPR angle blue-shifted by 0.6°. A thicker Cu thin film, say 50, 60, or 70 nm thick, results in an almost linear increase in minimum reflectance to 0.0910, 0.2784, and 0.4595 a.u., respectively. As seen in Fig. 10 d, the SPR incidence angles were discovered to be decreasing with a slight variation. Cu thin films with thicknesses of 30, 40, 50, 60, and 70 nm showed the lowest reflectivity for the longer wavelength spectrum, such as 700 nm, and were located at incident angles of 43 o (0.1779), 42.7 o (0.0097 a.u.), 42.6 o (0.0629 a.u.), 42.55 o (0.3189 a.u.), and 42.5 o (0.5512 a.u.), respectively, as shown in the inset of Fig. 10 b. A zoomed-in view of the minimum reflectance, indicated by the black dotted rectangle in Fig. 8 b, is shown in the inset. It is shown that reflectivity approaches zero at a Cu thin film thickness of 40 nm, allowing for the maximum conversion of incident light energy into surface plasmons. As can be observed in Fig. 10 e, any deviation in thickness from the previously specified reference value causes an increase in minimum reflectance, which in turn signifies a decreased rate of light utilization. The model "Glass-Cu-Air interfaces" at 700 nm wavelength, with Cu thin film thicknesses maintained at 30, 40, 50, 60, and 70 nm, is shown in Fig. 10 e along with its lowest reflectance and SPR angles. At an SPR angle of 43°, the lowest reflectance rises to 0.0861 a.u. for a Cu thin film thickness of 30 nm. When Cu thin film thickness was fixed at 40 nm with lowest reflectance of 0.0097 a.u. at SPR angle of 42.7°, the maximum excitation of SPR was observed, resulting in a deep and narrow curve with FWHM of 0.816°. In this case, as thicknesses increased from 30 to 40 nm, the SPR angle blue-shifted by 0.3°. There is a nearly linear increase in minimum reflectance to 0.0629, 0. 3189, and 0. 5512 a.u., respectively, with a thicker Cu thin film, say 50, 60, or 70 nm thick. The SPR angles were found to be decreasing with a small fluctuation, as shown in Fig. 10 e. As observed in Fig. 10 c, the lowest reflectance was observed at SPR angles of 42.3° (0.1024 a.u.), 42.15° (0.0143 a.u.), 42.1° (0.1859 a.u.), 42° (0.4338 a.u.), and 42° (0.6519 a.u.) for Cu thin films with thicknesses of 30, 40, 50, 60, and 70 nm at 800 nm spectral wavelength respectively. The inset displays a close-up of the lowest reflectance, which is represented by the black dotted rectangle in Fig. 10 c. It is demonstrated that at a Cu thin film thickness of 40 nm, reflectivity approaches zero, permitting the highest possible conversion of incident light energy into surface plasmons. Any thickness variation from the previously defined reference value results in an increase in minimum reflectance, which in turn indicates a lower rate of light utilization, as shown in Fig. 10 f. For 800 nm wavelengths, the model "Glass-Cu-Air interfaces" exhibits the lowest reflectance and SPR angles, as shown in 10f. For a Cu thin film thickness of 30 nm, the lowest reflectance increases to 0.1024 a.u. at an SPR angle of 42.3 o . The maximum excitation of SPR was observed at an SPR angle of 42.15° when the thickness of the Cu thin film was fixed at 40 nm, with the lowest reflectance of 0.0143 a.u. This resulted in a narrow and deep curve with an FWHM of 0.5°. In this instance, the SPR angle blue-shifted by 0.15° as thicknesses increased from 30 to 40 nm. An increase in thickness of the Cu thin film, approximately 50, 60, or 70 nm, causes the minimum reflectance to grow almost linearly to 0.1859, 0.4338, and 0.6519 a.u., respectively. The SPR incidence angles were seen to be quite constant with just minor variations, as shown in Fig. 10 f. 4. Conclusion Cu-based thin films of ca. 50 nm thickness were fabricated using the DC sputtering technique. The thin films were further treated at 200, 400 and 600°C for 2 hrs in ambient condition. FESEM observations confirmed that clusters and voids were observed at the surface of the thin films annealed at high temperatures. UV-vis absorption measurements revealed a red shift in absorption edges from 464 (2.67 eV) to 616 nm (2.01 eV). Tauc plots and Urbach plots were extracted for each of the annealed samples. A decrease in energy band gaps from 2.38 to 1.54 eV and an increase in Urbach energies from 193 to 272 meV with increasing annealing conditions were noted. The observation was correlated to changing of Cu to Cu 2 O and CuO at increasing annealing temperatures. PL measurements confirmed the emission bands at 765 nm (1.62 eV) and 785 nm (1.58 eV) corresponding to electronic levels Vo + 2 and V Cu respectively. At high temperatures, the PL emission band at 765 nm was found to shift a bit to the right (768 nm) whereas the other PL band at 785 nm shifted to 778 nm. The hydrophobicity of the treated sample was confirmed by WCA measurements. The hydrophobicity was found to increase at 200 and 400°C (WCA 92.4° and 85.2° respectively), however the surface became more hydrophilic at 600°C (WCA 42.3°). A plausible explanation was elaborated. A proposed model towards Cu-based thin film SPR sensor has been analyzed thoroughly using a commercial simulation software. A detailed investigation has been conducted into the spectrum and angular resolved reflectance properties of Cu-based thin films, which have been proposed as the active layer in an SPR sensor model. The ideal thickness of Cu thin film was found to be 40 nm at SPR angles of 44.7°, 42.7°, and 42.15° at spectral wavelengths of 600, 700, and 800 nm respectively. Declarations Conflict of interest: The author declares that there is no conflict of interest. Author Contribution M.K.H. developed the concept and methodology and prepare the original draft; M.K.H. and A.A. did the formal analysis and investigation and F.K., A.H. and M.R. reviewed the draft. Acknowledgement: The authors thank the Interdisciplinary Research Center for Sustainable Energy Systems (IRC-SES), the Research Institute, and King Fahd University of Petroleum & Minerals (KFUPM) for their support. MKH acknowledges the support received under IRC-SES Grant INRE2420. AA acknowledges the support received from the Undergraduate Research Office (URO) under the Uxplore program. Data availability: All data generated or analysed during this study are included in this published article. References Huang, X., Ao, D., Chen, T., Chen, Y., Li, F., Chen, S., Liang, G., Zhang, X., Zheng, Z., Fan, P.: High-performance copper selenide thermoelectric thin films for flexible thermoelectric application. (2022) Alami, A.H., Rajab, B., Abed, J., Faraj, M., Hawili, A.A., Alawadhi, H.: Investigating various copper oxides-based counter electrodes for dye sensitized solar cell applications. Energy. 174 , 526–533 (2019) Pawar, P., et al.: A low-cost copper oxide thin film memristive device based on successive ionic layer adsorption and reaction method. Mater. Sci. Semiconduct. Process. 71 , 102–108 (2017) Yang, et al.: Transparent flexible thermoelectric material based on non-toxic earth-abundant p-type copper iodide thin film. Nat. Commun., 8 , 1, (2017) Djatoubai, Su, J.: First spray pyrolysis thin film fabrication of environment-friendly Cu2BaSnS4 (CBTS) nanomaterials. Chem. Phys. Lett. 770 , 138406 (2021) Liang, W., et al.: A solution-processed ternary copper halide thin films for air-stable and deep-ultraviolet-sensitive photodetector, Nanoscale, 12 , 33, pp. 17213–17221, (2020) Lee, S., et al.: Fabrication of high-quality single-crystal Cu thin films using radio-frequency sputtering. Sci. Rep., 4 , 1, (2014) Hussain, R., Hussain, I.: Copper selenide thin films from growth to applications. Solid State Sci. 100 , 106101 (2020) Bas, S., Cummins, C., Selkirk, A., Borah, D., Ozmen, M., Morris, M.: A Novel Electrochemical Sensor Based on Metal Ion Infiltrated Block Copolymer Thin Films for Sensitive and Selective Determination of Dopamine. ACS Appl. Nano Mater. 2 (11), 7311–7318 (2019) Barbee, B., et al.: Cu and Ni Co-sputtered heteroatomic thin film for enhanced nonenzymatic glucose detection, Scientific Reports, vol. 12, no. 1, (2022) Zhao, Y., Bai, Q., Liao, P., Ding, X., Zuo, X., Huang, W., Li, Y.: Radiation hardness of Cu2ZnSn (S, Se) 4 thin film solar cells under 10 MeV proton irradiation. Phys. Lett. A. 472 , 128804 (2023) LaGrange, T., Arakawa, K., Yasuda, H., Kumar, M.: Preferential void formation at crystallographically ordered grain boundaries in nanotwinned copper thin films. Acta Mater. 96 , 284–291 (2015) Barchiesi, D., Gharbi, T., Cakir, D., Anglaret, E., Fréty, N., Kessentini, S., Maâlej, R.: Performance of surface plasmon resonance sensors using copper/copper oxide films: influence of thicknesses and optical properties. Photonics. 9 (2), 104 (2022, February) Stebunov, Y.V., Yakubovsky, D.I., Fedyanin, D.Y., Arsenin, A.V., Volkov, V.S.: Superior sensitivity of copper-based plasmonic biosensors. Langmuir. 34 (15), 4681–4687 (2018) Arul, C., Moulaee, K., Donato, N., Iannazzo, D., Lavanya, N., Neri, G., Sekar, C.: Temperature modulated Cu-MOF based gas sensor with dual selectivity to acetone and NO2 at low operating temperatures. Sens. Actuators B. 329 , 129053 (2021) Potočnik, J., Božinović, N., Novaković, M., Barudžija, T., Nenadović, M., Popović, M.: Optical properties of copper helical nanostructures: the effect of thickness on the SPR peak position. Nanotechnology. 33 (34), 345710 (2022) Muthumanikkam, M., Vibisha, A., Lordwin Prabhakar, M.C., Suresh, P., Rajesh, K.B., Jaroszewicz, Z., Jha, R.: Numerical investigation on high-performance Cu-based surface plasmon resonance sensor for biosensing application. Sensors. 23 (17), 7495 (2023) Rodrigues, M.S., Borges, J., Lopes, C., Pereira, R.M., Vasilevskiy, M.I., Vaz, F.: Gas sensors based on localized surface plasmon resonances: Synthesis of oxide films with embedded metal nanoparticles, theory and simulation, and sensitivity enhancement strategies. Appl. Sci. 11 (12), 5388 (2021) Shukor, A.H., Alhattab, H.A., Takano, I.: Electrical and optical properties of copper oxide thin films prepared by DC magnetron sputtering. J. Vacuum Sci. Technol. B, 38 (1). (2020) Pana, I., Parau, A.C., Dinu, M., Kiss, A.E., Constantin, L.R., Vitelaru, C.: Optical properties and stability of copper thin films for transparent thermal heat reflectors. Metals. 12 (2), 262 (2022) Saad, Y., Selmi, M., Gazzah, M.H., Bajahzar, A., Belmabrouk, H.: Performance enhancement of a copper-based optical fiber SPR sensor by the addition of an oxide layer. Optik. 190 , 1–9 (2019) Fendi, F.W.S., Mukhtar, W.M., Abdullah, M.: Surface Plasmon Resonance Sensor for Covid-19 Detection: A Review on Plasmonic Materials. Sensors and Actuators A: Physical, 114617. (2023) Ramírez, C., Bozzini, B., Calderon, J.A.: Electrodeposition of copper from triethanolamine as a complexing agent in alkaline solution. Electrochim. Acta. 425 , 140654 (2022) Kadhim, M.J., Sukkar, K.A., Abbas, A.S.: Copper Thin Film Deposited by PVD on Aluminum AA4015 substrate for thermal solar application. In IOP Conference Series: Materials Science and Engineering (Vol. 518, No. 3, p. 032048). IOP Publishing. (2019), May Prud'Homme, N., Constantoudis, V., Turgambaeva, A.E., Krisyuk, V.V., Samélor, D., Senocq, F., Vahlas, C.: Chemical vapor deposition of Cu films from copper (I) cyclopentadienyl triethylphophine: Precursor characteristics and interplay between growth parameters and films morphology. Thin Solid Films. 701 , 137967 (2020) Usami, N., Ota, E., Higo, A., Momose, T., Mita, Y.: Continuity assessment for supercritical-fluids-deposited (SCFD) Cu film as electroplating seed layer. In 2019 IEEE 32nd International Conference on Microelectronic Test Structures (ICMTS) (pp. 54–57). IEEE. (2019), March Lisco, F., et al.: The structural properties of CdS deposited by chemical bath deposition and pulsed direct current magnetron sputtering. Thin Solid Films. 582 , 323–327 (2015) Ghazal, H., Sohail, N.: Sputtering Deposition. In: Thin Film Deposition-Fundamentals, Processes, and Applications. IntechOpen (2022) Greene, J.E. Tracing the recorded history of thin-film sputter deposition: From the 1800s to 2017. Journal of Vacuum ScienceTechnology, Vacuum, A.: Surfaces, and Films, 35(5), 05C204. (2017) Mech, R., Kowalik, Żabiński, P.: Cu Thin Films Deposited by DC Magnetron Sputtering for Contact Surfaces on Electronic Components. Arch. Metall. Mater., 56 , 4, (2011) Shukor, A.H., Alhattab, H.A., Takano, I.: Electrical and optical properties of copper oxide thin films prepared by DC magnetron sputtering. Journal of Vacuum Science & Technology B, Nanotechnology and Microelectronics: Materials, Processing, Measurement, and Phenomena, 38(1), 012803. (2020) Du, S., Li, Y.: Effect of Annealing on Microstructure and Mechanical Properties of Magnetron Sputtered Cu Thin Films, Advances in Materials Science and Engineering, vol. pp. 1–8, 2015. (2015) Srinivasan, S.S.G., Govardhanan, B., Aabel, P., Ashok, M., Kumar, M.S.: Effect of oxygen partial pressure on the tuning of copper oxide thin films by reactive sputtering for solar light driven photocatalysis. Sol. Energy. 187 , 368–378 (2019) Rodrigues, E.P., Oliveira, L.C., Silva, M.L., Moreira, C.S., Lima, A.M.: Surface plasmon resonance sensing characteristics of thin copper and gold films in aqueous and gaseous interfaces. IEEE Sens. J. 20 (14), 7701–7710 (2020) Yesudasu, V., Pradhan, H.S., Pandya, R.J.: Recent progress in surface plasmon resonance based sensors: A comprehensive review. Heliyon, 7 (3). (2021) Stebunov, Y.V., Yakubovsky, D.I., Fedyanin, D.Y., Arsenin, A.V., Volkov, V.S.: Superior sensitivity of copper-based plasmonic biosensors. Langmuir. 34 (15), 4681–4687 (2018) Naikoo, G.A., Awan, T., Salim, H., Arshad, F., Hassan, I.U., Pedram, M.Z., Ahmed, W., Faruck, H.L., Aljabali, A.A., Mishra, V., Serrano-Aroca: Á. Fourth‐generation glucose sensors composed of copper nanostructures for diabetes management: A critical review. Bioengineering & Translational Medicine, 7(1), e10248 Şahin, B.: Flexible nanostructured CuO thin film: A promising candidate for wearable real-time sweat rate monitoring devices. Sens. Actuators A: Phys. 341 , 113604 (2022) Jasim, H.A., Dakhil, O.A.A.: Highly sensitive non-enzymatic glucose sensor based on copper oxide nanorods. J. Nanopart. Res. 24 (11), 212 (2022) Divya, J., Selvendran, S., Raja, A.S., Sivasubramanian, A.: Surface plasmon based plasmonic sensors: A review on their past, present and future, vol. 11, p. 100175. X, Biosensors and Bioelectronics (2022) Jain, S., Paliwal, A., Gupta, V., Tomar, M.: SPR based refractive index modulation of nanostructured SiO2 films grown using GLAD assisted RF sputtering technique. Surf. Interfaces. 34 , 102355 (2022) Tao, L., Deng, S., Gao, H., Lv, H., Wen, X., Li, M.: Experimental investigation of the dielectric constants of thin noble metallic films using a surface plasmon resonance sensor. Sensors. 20 (5), 1505 (2020) Topor, C.V., Puiu, M., Bala, C.: Strategies for Surface Design in Surface Plasmon Resonance (SPR) Sensing. Biosensors. 13 (4), 465 (2023) Acharya, B., Behera, A., Behera, S.: Optimizing drug discovery: Surface plasmon resonance techniques and their multifaceted applications. Chem. Phys. Impact. 8 , 100414 (2024) Aromaa, J., Kekkonen, M., Mousapour, M., Jokilaakso, A., Lundström, M.: The Oxidation of Copper in Air at Temperatures up to 100 C. Corros. Mater. Degrad. 2 (4), 625–640 (2021) Koshy, J., George, K.C.: Annealing effects on crystallite size and band gap of CuO nanoparticles. catalysis, 5, 11. (2015) Zhu, Y., Mimura, K., Isshiki, M.: Oxidation mechanism of Cu 2 O to CuO at 600–1050 C. Oxid. Met. 62 , 207–222 (2004) Rahman, M.M., Miran, H.A., Jiang, Z.T., Altarawneh, M., Chuah, L.S., Lee, H.L., Amun, H.-L.L., Nicholas, A. M., Dlugogorski, B.Z.: Investigation of the post-annealing electromagnetic response of Cu–Co oxide coatings via optical measurement and computational modelling. RSC Adv. 7 (27), 16826–16835 (2017) Pelegrini, S., Tumelero, M.A., Brandt, I.S., Pace, D., Faccio, R.D., R., Pasa, A.A.: Electrodeposited Cu2O doped with Cl: Electrical and optical properties. J. Appl. Phys. 123 (16), 161567 (2018) Ito, T., Masumi, T.: Detailed examination of relaxation processes of excitons in photoluminescence spectra of Cu 2 O. J. Phys. Soc. Jpn. 66 (7), 2185–2193 (1997) Soltanmohammadi, M., Spurio, E., Gloystein, A., Luches, P., Nilius, N.: Photoluminescence Spectroscopy of Cuprous Oxide: Bulk Crystal versus Crystalline Films. (2023). physica status solidi (a), 2200887 Mabrouki, M.: Effect of annealing temperature on the structural, physical, chemical, and wetting properties of copper oxide thin films. Materials Today: Proceedings, 13, 771–776. (2019) Li, C., Zhang, J., Han, J., Yao, B.: A numerical solution to the effects of surface roughness on water–coal contact angle. Sci. Rep. 11 (1), 459 (2021) Additional Declarations No competing interests reported. Supplementary Files floatimage1.png Graphical abstract Cite Share Download PDF Status: Posted Version 1 posted You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. As a division of Research Square Company, we’re committed to making research communication faster, fairer, and more useful. We do this by developing innovative software and high quality services for the global research community. Our growing team is made up of researchers and industry professionals working together to solve the most critical problems facing scientific publishing. Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-4427071","acceptedTermsAndConditions":true,"allowDirectSubmit":true,"archivedVersions":[],"articleType":"Research Article","associatedPublications":[],"authors":[{"id":305238423,"identity":"e81c973b-b3ae-4096-9be0-f3969e16a020","order_by":0,"name":"Mohammad Kamal Hossain","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAABEklEQVRIie3Sv0vDQBTA8RcCdYl0kwuHyb9wkqVD4P6VPARdmtU5ILwsQtdA9X+IBFq7RQouBl0PXAyCk4G4FXTwol3EBuvmcN/hDQcfjvsBYDL9w1g36qSbVgJt6X0tO78RXBMrK4M/EQDb2Ya46S22eBVKP13SU1gJnO+V1mNDIIfJZsKduGBYHWFeYRqMlcDFNLIPLggwKzcTD+KcIS0jAUh83L5j/hAN+C5BBH1k2BQrTaQ/qYmPWtGRnTdNpN9DOItn3S5WovQuoD7JwNbEynuImzWzEZI+i6rJPatEsJjiqXt+x/Cyh7D7uFCvpG9scvzMVjdif84Pr9uXk1B6PeRnovsG6wfbmphMJpPpex8db2QWwFAOjwAAAABJRU5ErkJggg==","orcid":"","institution":"King Fahd University of Petroleum \u0026 Minerals (KFUPM)","correspondingAuthor":true,"prefix":"","firstName":"Mohammad","middleName":"Kamal","lastName":"Hossain","suffix":""},{"id":305238424,"identity":"ad555eb8-253e-4fa4-b576-102eed7ce2ef","order_by":1,"name":"Abdullah Aljishi","email":"","orcid":"","institution":"King Fahd University of Petroleum \u0026 Minerals (KFUPM)","correspondingAuthor":false,"prefix":"","firstName":"Abdullah","middleName":"","lastName":"Aljishi","suffix":""},{"id":305238425,"identity":"f480c54c-379e-4bf9-adde-e10d69c45668","order_by":2,"name":"Firoz Khan","email":"","orcid":"","institution":"King Fahd University of Petroleum \u0026 Minerals (KFUPM)","correspondingAuthor":false,"prefix":"","firstName":"Firoz","middleName":"","lastName":"Khan","suffix":""},{"id":305238426,"identity":"3ec7a4b9-e753-4ae9-a14b-be62a93763ff","order_by":3,"name":"Anwar Ul-Hamid","email":"","orcid":"","institution":"King Fahd University of Petroleum \u0026 Minerals (KFUPM)","correspondingAuthor":false,"prefix":"","firstName":"Anwar","middleName":"","lastName":"Ul-Hamid","suffix":""},{"id":305238427,"identity":"24d7c498-4ad9-481f-967a-3ae867109d23","order_by":4,"name":"Md Mosaddequr Rahman","email":"","orcid":"","institution":"BRAC University","correspondingAuthor":false,"prefix":"","firstName":"Md","middleName":"Mosaddequr","lastName":"Rahman","suffix":""}],"badges":[],"createdAt":"2024-05-15 19:38:21","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-4427071/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-4427071/v1","draftVersion":[],"editorialEvents":[],"editorialNote":"","failedWorkflow":false,"files":[{"id":57187977,"identity":"a3ab52e3-f5dc-4b2b-93bf-8edbfb981b73","added_by":"auto","created_at":"2024-05-27 06:30:16","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":92592,"visible":true,"origin":"","legend":"\u003cp\u003eFree-hand schematic showing two-step process involved in fabrication of thin film of copper. Inset (i)-(ii) represents high-resolution FESEM images of pristine and treated specimen respectively.\u003c/p\u003e","description":"","filename":"1.png","url":"https://assets-eu.researchsquare.com/files/rs-4427071/v1/6c7b578043f7a3c5eadd5b98.png"},{"id":57187979,"identity":"c8caff1d-f823-442d-b0c8-c1ecc57b67bb","added_by":"auto","created_at":"2024-05-27 06:30:16","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":436242,"visible":true,"origin":"","legend":"\u003cp\u003e(a) SEM micrograph of the as-developed Cu thin film, (b) a zoom-in view of the area marked by white square (2 µm × 2 µm), (c) SEM-aided EDS of the same thin film; inset (i)-(ii) SEM mapping of the same film for Cu and O\u003csub\u003e2\u003c/sub\u003e elements respectively and (iii) weight percentages of constituent elements, (d) a line profile along line showed by dashed blue line in Fig. 2b and (e) 3D hawk-eye view of the zoom-in view as shown in Fig. 2b.\u003c/p\u003e","description":"","filename":"2.png","url":"https://assets-eu.researchsquare.com/files/rs-4427071/v1/2ee58dab6379fd7394995099.png"},{"id":57188608,"identity":"f053b465-273b-4b09-86ac-e1138ecc89e3","added_by":"auto","created_at":"2024-05-27 06:38:16","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":514073,"visible":true,"origin":"","legend":"\u003cp\u003e(a) SEM micrograph of the annealed Cu thin film, (b) a zoom-in view of the area marked by white square (5 µm × 5 µm), (c) SEM-aided EDS of the same annealed thin film; inset (i)-(ii): SEM mapping of the same film for Cu and O\u003csub\u003e2\u003c/sub\u003e elements respectively and (iii) weight percentages of constituent elements, (d) a line profile along line showed by dashed blue line in Fig. 3b and (e) 3D hawk-eye view of the zoom-in view as shown in Fig. 3b.\u003c/p\u003e","description":"","filename":"3.png","url":"https://assets-eu.researchsquare.com/files/rs-4427071/v1/dc3f59e04bdcd8339ed74c16.png"},{"id":57187987,"identity":"2160dfa1-066f-4f80-b72b-f87e3542949c","added_by":"auto","created_at":"2024-05-27 06:30:16","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":17938,"visible":true,"origin":"","legend":"\u003cp\u003e(a)-(d) UV-Vis absorption spectra recorded for pristine thin film of Cu and of those annealed at 200, 400 and 600 \u003csup\u003eo\u003c/sup\u003eC for 2 hrs in ambient condition respectively.\u003c/p\u003e","description":"","filename":"4.png","url":"https://assets-eu.researchsquare.com/files/rs-4427071/v1/9fe4eda4bf21d8bf9f8047ed.png"},{"id":57187981,"identity":"3220abc3-3619-4c59-a344-279bd3d705b0","added_by":"auto","created_at":"2024-05-27 06:30:16","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":22541,"visible":true,"origin":"","legend":"\u003cp\u003e(a)-(c) Tauc plots of thin films of Cu annelaed at 200, 400 and 600 \u003csup\u003eo\u003c/sup\u003eC for 2 hrs in ambient condition respectively.\u003c/p\u003e","description":"","filename":"5.png","url":"https://assets-eu.researchsquare.com/files/rs-4427071/v1/93cc4b1f22ec73a1d3917675.png"},{"id":57187978,"identity":"e04ef5d9-7f85-4d63-8ec6-909be6a9e905","added_by":"auto","created_at":"2024-05-27 06:30:16","extension":"png","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":22352,"visible":true,"origin":"","legend":"\u003cp\u003e(a)-(c) Urbach plots of the same thin films annelaed at 200, 400 and 600 \u003csup\u003eo\u003c/sup\u003eC for 2 hrs in ambient condition respectively.\u003c/p\u003e","description":"","filename":"6.png","url":"https://assets-eu.researchsquare.com/files/rs-4427071/v1/fb62d6faa48f2f109308b586.png"},{"id":57189024,"identity":"383cfd6f-4d64-4658-b3d7-82c3af846c96","added_by":"auto","created_at":"2024-05-27 06:46:16","extension":"png","order_by":7,"title":"Figure 7","display":"","copyAsset":false,"role":"figure","size":23396,"visible":true,"origin":"","legend":"\u003cp\u003e(a)-(d) Photoluminescence emission spectra recorded for pristine thin film of Cu and of those annealed at 200, 400 and 600 \u003csup\u003eo\u003c/sup\u003eC for 2 hrs in ambient condition respectively.\u003c/p\u003e","description":"","filename":"7.png","url":"https://assets-eu.researchsquare.com/files/rs-4427071/v1/262920c44dd5a4876d22142f.png"},{"id":57187982,"identity":"7bbffb81-b279-49bd-a402-0b1e59427309","added_by":"auto","created_at":"2024-05-27 06:30:16","extension":"png","order_by":8,"title":"Figure 8","display":"","copyAsset":false,"role":"figure","size":149777,"visible":true,"origin":"","legend":"\u003cp\u003e(a)-(d) Images of water droplet on the surface of the film without treatment, treated at 200, 400 and 600 °C for 2 hrs in ambient condition respectively.\u003c/p\u003e","description":"","filename":"8.png","url":"https://assets-eu.researchsquare.com/files/rs-4427071/v1/893e2adced7b0c4e618dfa48.png"},{"id":57188611,"identity":"bf5d10fa-e70d-45b2-8d60-e106afefdfbb","added_by":"auto","created_at":"2024-05-27 06:38:16","extension":"png","order_by":9,"title":"Figure 9","display":"","copyAsset":false,"role":"figure","size":311680,"visible":true,"origin":"","legend":"\u003cp\u003e(a)-(e) Spectral and angular-resolved p-polarized reflectance mappings of Glass-Cu-Air interfaces over a broad spectral range starting from 500 to 800 nm wavelengths with Cu thin film thicknesses kept at 30, 40, 50, 60 and 70 nm respectively. Inset of Fig.6a shows the model “Glass-Cu-Air interface” used in the simulation. Color bar represents the reflectance intensity.\u003c/p\u003e","description":"","filename":"9.png","url":"https://assets-eu.researchsquare.com/files/rs-4427071/v1/9645cc17bbeddf9ffc5aa7fa.png"},{"id":57187985,"identity":"e87b1ec3-6a31-4ba3-a145-8f300643f496","added_by":"auto","created_at":"2024-05-27 06:30:16","extension":"png","order_by":10,"title":"Figure 10","display":"","copyAsset":false,"role":"figure","size":67778,"visible":true,"origin":"","legend":"\u003cp\u003e(a)-(c) Angular-resolved p-polarized reflectivity of Glass-Cu-Air interfaces at 600, 700 and 800 nm wavelengths respectively with Cu thin film thicknesses were set at 30, 40, 50, 60 and 70 nm. Corresponding insets confirms lowest p-polarized reflectivity at specific incident angles and (d)-(f) Minimum reflectance and SPR angles of the model “Glass-Cu-Air interfaces” at 600, 700 and 800 nm wavelengths respectively.\u003c/p\u003e","description":"","filename":"10.png","url":"https://assets-eu.researchsquare.com/files/rs-4427071/v1/ea5c1b3fde5bf94a639cf3cd.png"},{"id":58630529,"identity":"00da2b61-d398-41df-9971-6b59b673aa89","added_by":"auto","created_at":"2024-06-19 05:32:30","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":2003510,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-4427071/v1/9f6aac3d-7787-48a8-ba3e-c82eb3529556.pdf"},{"id":57188609,"identity":"de8019d9-1fc0-4397-bbac-1af11377b06d","added_by":"auto","created_at":"2024-05-27 06:38:16","extension":"png","order_by":1,"title":"","display":"","copyAsset":false,"role":"supplement","size":211970,"visible":true,"origin":"","legend":"\u003cp\u003eGraphical abstract\u003c/p\u003e","description":"","filename":"floatimage1.png","url":"https://assets-eu.researchsquare.com/files/rs-4427071/v1/c09d1b0f0f44037249446856.png"}],"financialInterests":"No competing interests reported.","formattedTitle":"Fabrication and Wetting Characteristics of Copper Thin Film: An Active Layer for SPR-based Sensor Applications","fulltext":[{"header":"1. Introduction","content":"\u003cp\u003eCopper (Cu) and Cu-based thin films are widely used in industry and studied in research due to their unique characteristics and great potential in diverse fields of applications. Cu is an abundant element that is easy to extract, thus rendering it an economical option [\u003cspan additionalcitationids=\"CR2 CR3\" citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e]. Cu thin films are simple to manufacture in an eco-friendly manner [\u003cspan additionalcitationids=\"CR6 CR7\" citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e]. With regards to properties, Cu-based thin films offer excellent anti-interference characteristics and radiation resistance [\u003cspan additionalcitationids=\"CR10 CR11\" citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e]. Cu-based thin films play a crucial role in Surface Plasmon Resonance (SPR) sensors, offering several important advantages and impacts [\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e]. Cu thin films can modulate the intensity and response of surface plasmon resonance. The thickness of the Cu film affects the modulation capability, with thinner films showing more obvious regulation [\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e, \u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e]. This modulation capability is essential for detecting changes in refractive index and achieving high sensitivity in SPR sensors. Cu thin films are a competitive alternative to gold and silver in SPR sensors [\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e, \u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e]. They offer interesting properties for optics, nanotechnology, and SPR sensors [\u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e]. Recent studies have demonstrated that the performance of Cu-based SPR sensors is quite similar to those using gold [\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e, \u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e, \u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e]. Cu oxide, which is often present in Cu thin films, is a semiconductor with a non-toxic nature. This makes Cu-based SPR sensors more environmentally friendly and safer for various applications [\u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e22\u003c/span\u003e]. There are several techniques for depositing thin films of Cu, including electrochemical deposition, physical vapor deposition (PVD), chemical vapor deposition and supercritical fluid chemical deposition [\u003cspan additionalcitationids=\"CR24 CR25\" citationid=\"CR23\" class=\"CitationRef\"\u003e23\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e26\u003c/span\u003e]. The sputtering technique, a subset of PVD, is the simplest technique for metal deposition. It is commonly used in numerous industries, such as the semiconductor industry and the coating industry. Sputtering technique offers numerous advantages such as great flexibility in choice of coating materials, excellent adhesion, high repeatability, and the ability to deposit thin films on a variety of substrates. These advantages can justify the costs associated with this technique, depending on the specific needs of the application [\u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e27\u003c/span\u003e]. This allows manufacture on a large-scale for industrial applications, which is important for applications such as solar cells. DC sputtering also only requires low sputtering voltage and low sputtering gas pressures, compared to other sputtering techniques such as RF sputtering [\u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e28\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e29\u003c/span\u003e]. Previous work on the fabrication of Cu thin films by DC sputtering has shown that better adhesion to the substrate is correlated with a lower current value and a longer deposition time [\u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e30\u003c/span\u003e]. It was also found that grain size increases as the substrate gets closer to the target [\u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e30\u003c/span\u003e]. The deposition rate of Cu increases in proportion to the applied DC voltage and decreases with deposition pressure [\u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e29\u003c/span\u003e, \u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e31\u003c/span\u003e]. Research on the effects of annealing on Cu films demonstrated a noticeable increase in grain size as the annealing temperature increased [\u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e32\u003c/span\u003e]. Surface roughness decreases as temperature increases up until 400\u0026deg;C, where the lowest surface roughness is obtained [\u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e32\u003c/span\u003e]. Cu thin films annealed at 150 and 200\u0026deg;C demonstrated great photocatalytic performance [\u003cspan citationid=\"CR33\" class=\"CitationRef\"\u003e33\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eImprovements in sensitivity, affordability, and compatibility have propelled developments in Cu-based thin film surface plasmon resonance (SPR) sensors [\u003cspan citationid=\"CR34\" class=\"CitationRef\"\u003e34\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR35\" class=\"CitationRef\"\u003e35\u003c/span\u003e]. To some extent, silver- and gold-based sensors have been competitively replaced by Cu-based thin film SPR sensors [\u003cspan citationid=\"CR35\" class=\"CitationRef\"\u003e35\u003c/span\u003e]. Naturally occurring on the Cu surface is a semiconductor called Cu-based oxide, which is non-toxic. Cu-based thin films are therefore preferred for the sensitive components of SPR sensors. Cu-based thin films and other plasmon-active nanostructured thin films have drawn interest due to their potential in biosensing applications [\u003cspan citationid=\"CR36\" class=\"CitationRef\"\u003e36\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR37\" class=\"CitationRef\"\u003e37\u003c/span\u003e]. These commercially accessible films for chemical and biological detection provide increased sensitivity. Cu-based thin film SPR sensors have been integrated and made smaller thanks to developments in nanofabrication processes. As a result, portable and small sensing devices appropriate for on-site and point-of-care applications have been developed [\u003cspan citationid=\"CR37\" class=\"CitationRef\"\u003e37\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR38\" class=\"CitationRef\"\u003e38\u003c/span\u003e]. Cu-based structured thin-film nanorods have been investigated as a potential means of enhancing sensor sensitivity and efficiency [\u003cspan citationid=\"CR39\" class=\"CitationRef\"\u003e39\u003c/span\u003e]. Furthermore, improvements in waveguide coupling optimization for integrated light production and detection have been achieved. Application of Cu-based thin films as a plasmonic material presents benefits in terms of compatibility, cost-effectiveness, and sensitivity, making it a desirable choice for more study and advancement in the SPR sensing sector. SPR sensors detect changes in the refractive index of the dielectric near the metal layer by using electromagnetic surface plasmon waves (SPs) traveling at the interface of the dielectric and the metallic layer [\u003cspan citationid=\"CR40\" class=\"CitationRef\"\u003e40\u003c/span\u003e]. Attenuated total reflection is usually the mechanism that produces surface plasmons when p-polarized light contacts a metal-coated glass prism. A minimum in metal reflectivity as a function of incident angle or wavelength is known as the SPR angle of such sensors. Therefore, changes in the refractive index of the medium close to the metal layer may therefore be detected by shifting the SPR location [\u003cspan citationid=\"CR41\" class=\"CitationRef\"\u003e41\u003c/span\u003e]. SPR sensors are ideal for a variety of applications because of their exceptional sensitivity to even minute changes in the refractive index of the probed medium. These applications include site-specific absorption at metal surfaces, real-time, label-free investigation of metal-analyte interactions, and non-invasive measurement of thin-film thickness and optical constants [\u003cspan additionalcitationids=\"CR43\" citationid=\"CR42\" class=\"CitationRef\"\u003e42\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR44\" class=\"CitationRef\"\u003e44\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eThis effort attempts to use the sputtering process to produce Cu-based thin films on quartz glass substrates. The samples underwent further treatment in a tube furnace for two hours at temperatures as high as 200, 400, and 600\u0026deg;C. High resolution scanning electron microscopy (FESEM) and SEM-aided energy dispersion spectroscopy (EDS) have been used to topographically validate the thin layer of virgin Cu and those processed at high temperatures. The identical thin films' optical characteristics, including their UV-vis absorptions, energy band gaps, and Urbach energies, have all been calculated. Both the pristine Cu thin films and the annealed at higher temperatures exhibited visible-range photoluminescence emissions. Measurements of the wetting contact angle (WCA) were used to show how hydrophobic the pristine Cu thin film and those modified at high temperatures are. A moderate WCA for the thin film of Cu was estimated to be ca. 71.9\u003csup\u003eo\u003c/sup\u003e. The surface became more hydrophobic, 92.4\u003csup\u003eo\u003c/sup\u003e and 85.2\u003csup\u003eo\u003c/sup\u003e when the samples were treated at 200 and 400\u0026deg;C respectively. However, further annealing of the same thin film reduced the WCA and made it more hydrophilic. An SPR sensor model using Cu-based thin films as active layer has been proposed followed by rigorous investigation on the spectral- and angular-resolved reflectance characteristics. Optimized thickness of Cu thin film was found to be 40 nm at SPR angles of 44.7\u0026deg;, 42.7\u0026deg; and 42.15\u0026deg; at 600, 700 and 800 nm of spectral wavelengths.\u003c/p\u003e"},{"header":"2. Materials and methods","content":"\u003cp\u003eThin films of Cu were fabricated in a simple two-step process as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig14\" class=\"InternalRef\"\u003e1\u003c/span\u003e. A circular Cu target (5 cm in diameter and 3.18 mm in thickness and 99.99% purity) was purchased from ACI Alloy, Inc. and used as received. In the first step, a sputtering coater (Nanomaster NSC3000, USA) was used to sputter Cu on quartz glass substrate. The samples were immediately transferred to tubular furnace (MTI Corporation OTF-1200X, USA) for thermal treatment in the second step. A direct current (DC, 30W) gun was used to sputter the Cu from the target onto the quartz glass substrate in a downward configuration. The target substrate distance was maintained at 10 cm. The base pressure and working pressure in the sputtering chamber were maintained at approximately 1.7\u0026times;10\u003csup\u003e\u0026minus;\u0026thinsp;5\u003c/sup\u003e torr and 2.8\u0026times;10\u003csup\u003e\u0026minus;\u0026thinsp;3\u003c/sup\u003e torr respectively. The argon (Ar) flow rate in the chamber was kept at 30 SCCM during the deposition. A thickness-bound process was initiated and the in-built thickness monitor was configured to 50 nm. As mentioned above, as-fabricated thin films were annealed in a quartz tube furnace at 200, 400, and 600\u0026deg;C for 2 hrs each at a heating rate of 20 \u003csup\u003eo\u003c/sup\u003eC/min.\u003c/p\u003e \u003cp\u003eDetailed surface morphology was investigated using high-resolution FESEM. Insets (i)-(ii) of Fig.\u0026nbsp;\u003cspan refid=\"Fig14\" class=\"InternalRef\"\u003e1\u003c/span\u003e display high-resolution FESEM images of typical pristine and treated specimen respectively. The elemental composition of each sample was confirmed using SEM-aided EDS. UV-vis absorption and transmission measurements were carried out by a UV-Vis-NIR spectrometer (Jasco V-670, Japan). PL emission of as-fabricated films and of those annealed at high temperatures was recorded at room temperature (RT) using a Horiba Jobin-Yvon spectrofluorometer (Flurolog-3, Japan) equipped with a 150W Xe lamp as the excitation source. WCA measurements were carried out using a goniometer (DSA25, Denmark) through sessile drop tests. In sessile drop tests, the droplet volume of DI water was controlled with an automatic dispensing system. A CCD camera interfaced with computer-controlled software was used to capture the images of the droplets and calculate WCA on the samples.\u003c/p\u003e \u003cp\u003eAn SPR sensor model comprised of \u0026ldquo;Cu thin on glass\u0026rdquo; has been designed and optical simulation was performed by a commercial software Setfos (version 5.3, Fluxim AG, Switzerland). The active thin film thickness was varied from 30 to 70 nm to understand the best performance and optimize the SPR characteristics. Considering the inherent properties of plasmonic thin films of Cu, incident angles were varied from 41\u003csup\u003eo\u003c/sup\u003e to 50\u003csup\u003eo\u003c/sup\u003e in this current investigation.\u003c/p\u003e"},{"header":"3. Results and discussion","content":"\u003cp\u003eUnderstanding the surface topography of the film is crucial, since the optical properties as well as wetting characteristics of the same depend a lot on nanostructures. Morphology and in-depth analysis of the pristine sample and the samples treated at high temperatures were investigated by FESEM. A representative FESEM image of the thin film of Cu as depicted in Fig.\u0026nbsp;\u003cspan refid=\"Fig15\" class=\"InternalRef\"\u003e2\u003c/span\u003ea indicated that the surface was quite smooth. A zoom-in view, as marked by the white square (2 \u0026micro;m \u0026times; 2 \u0026micro;m) in Fig.\u0026nbsp;\u003cspan refid=\"Fig15\" class=\"InternalRef\"\u003e2\u003c/span\u003ea, was shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig15\" class=\"InternalRef\"\u003e2\u003c/span\u003eb. A line profile along the dashed blue line shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig15\" class=\"InternalRef\"\u003e2\u003c/span\u003eb revealed that the surface was relatively smooth, as presented in Fig.\u0026nbsp;\u003cspan refid=\"Fig15\" class=\"InternalRef\"\u003e2\u003c/span\u003ed. Figure\u0026nbsp;\u003cspan refid=\"Fig15\" class=\"InternalRef\"\u003e2\u003c/span\u003ee shows the three-dimensional (3D) hawk-eye view of the zoon-in area as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig15\" class=\"InternalRef\"\u003e2\u003c/span\u003ea confirming again the smooth top surface of the pristine thin film of Cu. The elemental composition of the same film was confirmed by SEM-aided EDS. Figure\u0026nbsp;\u003cspan refid=\"Fig15\" class=\"InternalRef\"\u003e2\u003c/span\u003ec represents the EDS spectrum of the pristine thin film of Cu whereas Cu peaks were evident at 0.93 keV (CuL\u003csub\u003eα\u003c/sub\u003e), 8.04 keV (CuK\u003csub\u003eα\u003c/sub\u003e), and 8.91 keV (CuK\u003csub\u003eβ\u003c/sub\u003e) along with a Si peak at 1.74 keV (SiK\u003csub\u003eα\u003c/sub\u003e) due to underneath substrate, an O peak at 0.53 keV (OK\u003csub\u003eα\u003c/sub\u003e) due to oxides and Au peaks at 2.12 keV (AuM) and 9.71 keV (AuK\u003csub\u003eα\u003c/sub\u003e) due to Au coating. Further to elaborate on the distribution of constituent elements SEM-aided EDS mapping was carried out. Insets (i) and (ii) of Fig.\u0026nbsp;\u003cspan refid=\"Fig15\" class=\"InternalRef\"\u003e2\u003c/span\u003ec represent such mappings of the two main elements Cu and O respectively. It was observed that the constituent elements were homogenously distributed. No clusters or voids were observed. The weight percentage of the constituent elements is shown in the inset of (iii) of Fig.\u0026nbsp;\u003cspan refid=\"Fig15\" class=\"InternalRef\"\u003e2\u003c/span\u003ec.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eThermal treatment of Cu thin films at elevated temperatures, such as 600 \u003csup\u003eo\u003c/sup\u003eC was found to modify the surface morphology as well as subsequent optical characteristics and hydrophobicity. The optical and hydrophobic characteristics have been elaborated in the later part of the text. Let us confirm the details on surface topography and constituent elements. Figure\u0026nbsp;\u003cspan refid=\"Fig16\" class=\"InternalRef\"\u003e3\u003c/span\u003ea shows a representative SEM micrograph of thin film of Cu annealed at 600 \u003csup\u003eo\u003c/sup\u003eC for 2 hrs. With reference to a pristine thin film of Cu, the treated film surface was found to be rougher and consisted of clusters and voids. A zoom-in view, as marked by the white square (5 \u0026micro;m \u0026times; 5 \u0026micro;m) in Fig.\u0026nbsp;\u003cspan refid=\"Fig16\" class=\"InternalRef\"\u003e3\u003c/span\u003ea was shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig16\" class=\"InternalRef\"\u003e3\u003c/span\u003eb. It is noteworthy that the clusters and voids were of different sizes. A typical cluster and a void are mentioned as the black circles in Fig.\u0026nbsp;\u003cspan refid=\"Fig16\" class=\"InternalRef\"\u003e3\u003c/span\u003eb respectively. A line profile along the dashed blue line shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig16\" class=\"InternalRef\"\u003e3\u003c/span\u003eb indicated the height and depth of the corresponding cluster and void respectively as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig16\" class=\"InternalRef\"\u003e3\u003c/span\u003ed. The cluster and void are marked by black arrows in Fig.\u0026nbsp;\u003cspan refid=\"Fig16\" class=\"InternalRef\"\u003e3\u003c/span\u003eb corresponding to the exact positions of a hill and dip in the line profile shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig16\" class=\"InternalRef\"\u003e3\u003c/span\u003ed. Although the origin of such clusters and voids was not known, it was speculated that such clusters and voids could be due to the desorption of Cu and O at defect sites. When the thin films of Cu were treated at high temperatures, there was a possibility of defect site movement, known as the \u0026ldquo;Ostwald ripening effect\u0026rdquo;, within the system. As a consequence, the surface morphology as well as the stoichiometric ratio of the material were altered. Such phenomena can affect the wetting and optical properties of Cu thin film as mentioned in the later part of the text.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eA 3D hawk-eye view of the zoon-in area as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig16\" class=\"InternalRef\"\u003e3\u003c/span\u003ea is presented in Fig.\u0026nbsp;\u003cspan refid=\"Fig16\" class=\"InternalRef\"\u003e3\u003c/span\u003ee. the elemental composition of the same film was confirmed by SEM-aided EDS. Figure\u0026nbsp;\u003cspan refid=\"Fig16\" class=\"InternalRef\"\u003e3\u003c/span\u003ec represents the EDS spectrum of the pristine thin film of Cu whereas Cu peaks were evident at 0.93 keV (CuL\u003csub\u003eα\u003c/sub\u003e), 8.04 keV (CuK\u003csub\u003eα\u003c/sub\u003e), and 8.91 keV (CuK\u003csub\u003eβ\u003c/sub\u003e) along with Si peak at 1.74 keV (SiK\u003csub\u003eα\u003c/sub\u003e) due to underneath substrate, an O peak at 0.53 keV (OK\u003csub\u003eα\u003c/sub\u003e) due to oxides and Au peaks at 2.12 keV (AuM) and 9.71 keV (AuK\u003csub\u003eα\u003c/sub\u003e) due to Au coating. EDS mapping was carried out to understand the distribution of constituent elements. Insets (i) and (ii) of Fig.\u0026nbsp;\u003cspan refid=\"Fig16\" class=\"InternalRef\"\u003e3\u003c/span\u003ec represent such mappings of the two main elements Cu and O respectively. It was observed that constituent elements were less homogenously distributed, although distinct clusters and voids were recorded. The weight percentage of the constituent elements as shown in inset (iii) of Fig.\u0026nbsp;\u003cspan refid=\"Fig16\" class=\"InternalRef\"\u003e3\u003c/span\u003ec revealed a higher percentage of O (ca. 24.3%) with reference to that obtained in the case of a pristine sample.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eFigure\u0026nbsp;\u003cspan refid=\"Fig17\" class=\"InternalRef\"\u003e4\u003c/span\u003e shows UV-Vis absorption spectra of as-fabricated Cu thin film as well as of those treated at 200, 400 and 600 \u003csup\u003eo\u003c/sup\u003eC for 2 hrs. Absorption edges were observed to have a red-shift towards longer wavelengths with increasing annealing temperatures as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig17\" class=\"InternalRef\"\u003e4\u003c/span\u003e. The dashed red lines guide the readers to follow the approximate absorption edges. The as-fabricated film of Cu was expected to be amorphous and therefore no absorption edge was observed. The absorption edges appeared at 464 nm (2.67 eV), 535 (2.31 eV) and 616 nm (2.01 eV) for the samples annealed at 200, 400 and 600 \u003csup\u003eo\u003c/sup\u003eC in ambient condition respectively as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig17\" class=\"InternalRef\"\u003e4\u003c/span\u003e. The Fig.\u0026nbsp;\u003cspan refid=\"Fig17\" class=\"InternalRef\"\u003e4\u003c/span\u003ea-d represent the UV-Vis spectra of pristine Cu thin film as well as of those annealed at 200, 400 and 600 \u003csup\u003eo\u003c/sup\u003eC in ambient condition respectively. It is well-acknowledged that the transition in the UV-vis absorption edge of a thin film at higher temperatures depends on various factors, such as the composition, thickness, and deposition method of the film. A plausible reason behind the scene could be related to the effect of temperature on the atomic structure of the film [\u003cspan citationid=\"CR45\" class=\"CitationRef\"\u003e45\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR46\" class=\"CitationRef\"\u003e46\u003c/span\u003e]. As the temperature increases, the amorphous structure of the film undergoes rearrangements or phase changes, particularly leading to a relaxation of the atomic conformation. Therefore, a red-shift is speculated at higher annealing temperatures, as observed in the case of Cu thin film and shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig17\" class=\"InternalRef\"\u003e4\u003c/span\u003eb-d. The UV-Vis spectra recorded for Cu thin films annealed at different temperatures were further utilized to calculate optical bandgaps of the corresponding films.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eThe Tauc plot is an important tool used to determine the optical band gap of materials, particularly semiconductors. It provides a graphical representation of the relationship between the absorption coefficient and the photon energy. By analyzing the intercepts of the linear portion of the plot, one can estimate the band gaps of various thin films. The Tauc plot can be used as a quality control tool to assess the structure or phase of materials. As demonstrated in the Fig.\u0026nbsp;\u003cspan refid=\"Fig18\" class=\"InternalRef\"\u003e5\u003c/span\u003ea-c, the energy band gaps were estimated using a Tauc plot by plotting (α\u003cem\u003eh\u003c/em\u003eν)\u003csup\u003e2\u003c/sup\u003e versus \u003cem\u003eh\u003c/em\u003eν, where α and \u003cem\u003eh\u003c/em\u003eν represent the absorption coefficient and photon energy respectively. Figure\u0026nbsp;\u003cspan refid=\"Fig18\" class=\"InternalRef\"\u003e5\u003c/span\u003ea-c represent the Tauc plots of thin films of Cu annealed at 200, 400 and 600 \u003csup\u003eo\u003c/sup\u003eC for 2 hrs in ambient condition respectively. The band gaps of 2.38, 1.69 and 1.54 eV as mentioned therein were estimated for the thin films annealed at 200, 400 and 600 \u003csup\u003eo\u003c/sup\u003eC in ambient condition respectively. The sputtered thin film of Cu is commonly known to be amorphous and therefore no absorption peak was observed as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig17\" class=\"InternalRef\"\u003e4\u003c/span\u003ea. However, at mild temperature such as 200 \u003csup\u003eo\u003c/sup\u003eC, Cu-based oxide islands on the surface of as-grown thin film started to show up. As shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig18\" class=\"InternalRef\"\u003e5\u003c/span\u003ea, optical bandgap of 2.38 eV was estimated for the specimen annealed at 200 \u003csup\u003eo\u003c/sup\u003eC for 2 hrs in ambient condition. It is speculated that the thin film annealed at 200 \u003csup\u003eo\u003c/sup\u003eC for 2 hrs in ambient condition remained in the early stage of nano-crystalline Cu-based oxide film, particular Cu\u003csub\u003e2\u003c/sub\u003eO at the surface of amorphous film. Interestingly, it was noted that the same thin film annealed at 400 \u003csup\u003eo\u003c/sup\u003eC for 2 hrs in ambient condition turned into nano-crystalline Cu\u003csub\u003e2\u003c/sub\u003eO thin film as indicated in the Tauc plot shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig18\" class=\"InternalRef\"\u003e5\u003c/span\u003eb. The optical bandgap of 1.69 eV as estimated for the thin film annealed at 400 \u003csup\u003eo\u003c/sup\u003eC in ambient condition coincided well with reported optical bandgap of Cu\u003csub\u003e2\u003c/sub\u003eO [\u003cspan citationid=\"CR45\" class=\"CitationRef\"\u003e45\u003c/span\u003e]. The same thin film annealed at higher temperature such as 600 \u003csup\u003eo\u003c/sup\u003eC for 2 hrs in ambient condition facilitated to complete oxidation of copper to Cu\u003csub\u003e2\u003c/sub\u003eO and turned the film as CuO [\u003cspan citationid=\"CR47\" class=\"CitationRef\"\u003e47\u003c/span\u003e]. The optical bandgap of 1.54 eV as estimated for the thin film annealed at 600 \u003csup\u003eo\u003c/sup\u003eC in ambient condition coincided well with reported optical bandgap of CuO. It is still under debate whether inward diffusion of oxygen or outward diffusion of copper is responsible for growth of CuO thin film from Cu\u003csub\u003e2\u003c/sub\u003eO thin film.\u003c/p\u003e \u003cp\u003eThe Urbach energy is an important parameter in the characterization of thin films. It provides valuable information about the energy distribution within the thin film and is used to understand the structural and optical properties of the film. The Urbach energy is related to the exponential tail in the absorption spectrum of a material as per the following equation (Eq.\u0026nbsp;1).\u003c/p\u003e \u003cp\u003e \u003cspan class=\"InlineEquation\"\u003e \u003cspan class=\"mathinline\"\u003e\\(\\alpha ={\\alpha }_{0}{e}^{\\frac{hv}{{E}_{u}}}\\)\u003c/span\u003e \u003c/span\u003e (Eq.\u0026nbsp;1)\u003c/p\u003e \u003cp\u003ewhereas, α\u003csub\u003e0\u003c/sub\u003e and E\u003csub\u003eu\u003c/sub\u003e are constants related to the low energy limit of the absorption coefficient and Urbach energy respectively.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eThe Urbach energy represents a measure of disorder within thin films. The Urbach plot can be obtained by plotting the natural logarithm of α (i.e. \u003cem\u003eln\u003c/em\u003eα) as a function of photon energy (\u003cem\u003eh\u003c/em\u003eν). The Urbach energy can be determined by taking the inverse of the gradient at the linear region of the curve. Figure\u0026nbsp;\u003cspan refid=\"Fig19\" class=\"InternalRef\"\u003e6\u003c/span\u003ea-c represent the Urbach plots of thin films of Cu treated with 200, 400 and 600 \u003csup\u003eo\u003c/sup\u003eC in ambient condition respectively. A reduction of the Urbach energy from 272 meV to 205 meV after the treatments at 400 and 600 \u003csup\u003eo\u003c/sup\u003eC was observed, although the film at the treatment of 200 \u003csup\u003eo\u003c/sup\u003eC was found to be lower (193 meV). The trend of decreasing Urbach energy coincides well with reported values [\u003cspan citationid=\"CR48\" class=\"CitationRef\"\u003e48\u003c/span\u003e]. Such a scenario is speculated due to the diminution of the density of states, the increase in crystallinity, the decrease in the degree of disorderness and a relaxation of the distorted bonds. To the extent, the fact also indicates that the thin film of Cu turned into oxides after heat treatment where energy band gaps were found to decrease as well with increasing annealing temperatures as demonstrated in Fig.\u0026nbsp;\u003cspan refid=\"Fig19\" class=\"InternalRef\"\u003e6\u003c/span\u003e.\u003c/p\u003e \u003cp\u003eThe refractive index is an important parameter in the design of SPR-based sensors. SPR sensors rely on the interaction between light and the surface plasmons, which are collective oscillations of electrons at the metal-dielectric interface. The refractive index of the surrounding medium affects the propagation of surface plasmons and can be used to detect changes in the surrounding. Refractive index and energy band gap are interrelated and Dimitrove and Sakka proposed the following formula (Eq.\u0026nbsp;2) to obtain the refractive index (\u003cem\u003en\u003c/em\u003e) of any thin film,\u003c/p\u003e \u003cp\u003e \u003cspan class=\"InlineEquation\"\u003e \u003cspan class=\"mathinline\"\u003e\\(\\frac{{n}^{2}-1}{{n}^{2}+2}=1-\\sqrt{\\frac{{E}_{g}}{20}}\\)\u003c/span\u003e \u003c/span\u003e (Eq.\u0026nbsp;2)\u003c/p\u003e \u003cp\u003ewhere, n and E\u003csub\u003eg\u003c/sub\u003e represent the refractive index and energy band gap of corresponding thin film. By simplifying the Eq.\u0026nbsp;2, one can find the refractive index, \u003cem\u003en\u003c/em\u003e\u003cdiv id=\"Equa\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equa\" name=\"EquationSource\"\u003e\n$$n=\\sqrt{\\left(\\frac{3+2\\sqrt{\\frac{{E}_{g}}{10}}}{\\sqrt{\\frac{{E}_{g}}{10}}}\\right)}$$\u003c/div\u003e\u003c/div\u003e\u003c/p\u003e \u003cp\u003eBased on this relation, the refractive indexes of thin films of Cu annealed at 200, 400 and 600 \u003csup\u003eo\u003c/sup\u003eC were estimated to be 2.85, 3.05 and 3.11 respectively which are in good agreement with the reported values [\u003cspan citationid=\"CR49\" class=\"CitationRef\"\u003e49\u003c/span\u003e].\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003ePhotoluminescence in thin films is closely related to the presence of defects. The emission of light from defects provides insights into the defect density, distribution, and type within the film. Photoluminescence properties of thin film support to understand the defects involved. Therefore, a detailed study was carried out for Cu thin films under this investigation. Figure\u0026nbsp;\u003cspan refid=\"Fig20\" class=\"InternalRef\"\u003e7\u003c/span\u003e shows the photoluminescence spectra of thin films of pristine Cu as well as of those annealed at 200, 400 and 600 \u003csup\u003eo\u003c/sup\u003eC for 2 hrs in ambient condition. For the pristine thin film of Cu, two main PL bands were observed at photon energies of 1.62 eV (765 nm) and 1.58 eV (785 nm) along with three shallow shoulder peaks at 1.67 eV (744 nm) 1.55 eV (808 nm) and 1.51 eV (821 nm) as shown by dashed vertical black lines in Fig.\u0026nbsp;\u003cspan refid=\"Fig20\" class=\"InternalRef\"\u003e7\u003c/span\u003e. When the thin film of Cu went under thermal treatment, it was observed that the PL band at 765 nm shifted to right a bit (i.e. 768) and that at 785 nm shifted to left (i.e. 778 nm) as shown by dashed vertical red lines in Fig.\u0026nbsp;\u003cspan refid=\"Fig20\" class=\"InternalRef\"\u003e7\u003c/span\u003e. The two bands 744 nm (1.67 eV) and 820 (1.55 eV) nm are attributed to oxygen vacancies corresponding to doubly ionized vacancies and single ionized vacancies respectively [\u003cspan citationid=\"CR50\" class=\"CitationRef\"\u003e50\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR51\" class=\"CitationRef\"\u003e51\u003c/span\u003e]. As for the thin film of pristine Cu, two PL peaks at about 765 nm (1.62 eV) and 785 nm (1.58 eV) are possibly due to electronic levels Vo\u003csup\u003e+\u0026thinsp;2\u003c/sup\u003e and V\u003csub\u003eCu\u003c/sub\u003e respectively. The samples annealed at higher temperatures indicated a shift and higher PL emission at 768 nm (1.61 eV) due to higher oxygen vacancies, whereas a shifted and low PL emission at 778 nm (1.59 eV) could be the transformation of Cu to CuO at high temperature treatments [\u003cspan citationid=\"CR51\" class=\"CitationRef\"\u003e51\u003c/span\u003e].\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eDue to the great demand for surface hydrophobicity for various applications, particularly those used in harsh weather conditions, it is always expected to make the thing more hydrophobic. In this context, sessile drop tests are frequently used to find out the WCA of the surface and thus to define whether the surface is hydrophobic or hydrophilic. Like most metals, the thin film of Cu surface is hydrophilic with a moderate WCA. Figure\u0026nbsp;\u003cspan refid=\"Fig21\" class=\"InternalRef\"\u003e8\u003c/span\u003e shows the average WCA of pristine thin films of Cu as well as of those treated at 200, 400 and 600\u0026deg;C for 2 hrs in ambient condition. A moderate WCA for the thin film of Cu was estimated to be ca. 71.9\u003csup\u003eo\u003c/sup\u003e as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig21\" class=\"InternalRef\"\u003e8\u003c/span\u003ea. The surface became more hydrophobic when the samples were treated at 200 and 400\u0026deg;C for 2 hrs in ambient condition. The WCA for samples treated at 200 and 400\u0026deg;C as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig21\" class=\"InternalRef\"\u003e8\u003c/span\u003eb-c was noted to be 92.4\u003csup\u003eo\u003c/sup\u003e and 85.2\u003csup\u003eo\u003c/sup\u003e respectively. However, further annealing of the same thin film reduced the WCA and made it more hydrophilic [\u003cspan citationid=\"CR52\" class=\"CitationRef\"\u003e52\u003c/span\u003e]. One of the plausible reasons could be the effect of oxidation. At high temperatures, the surface morphology and chemical properties might have changed and therefore the surface contact angle has been reduced to 42.3\u003csup\u003eo\u003c/sup\u003e as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig21\" class=\"InternalRef\"\u003e8\u003c/span\u003ed. The reduction of the contact angle may also be explained by the Wenzel theory where surface roughness contributes to the improvement of wettability [\u003cspan citationid=\"CR53\" class=\"CitationRef\"\u003e53\u003c/span\u003e]. As observed in the FESEM observation and explained above in Fig.\u0026nbsp;\u003cspan refid=\"Fig16\" class=\"InternalRef\"\u003e3\u003c/span\u003e, the formation of clusters and voids could be responsible for the hydrophilic characteristics of the samples treated at 600\u0026deg;C for 2 hrs in ambient condition.\u003c/p\u003e \u003cp\u003eThe sensitivity and performance of any SPR-based sensor are greatly influenced by the thickness of their active layer [\u003cspan citationid=\"CR40\" class=\"CitationRef\"\u003e40\u003c/span\u003e, \u003cspan citationid=\"CR42\" class=\"CitationRef\"\u003e42\u003c/span\u003e]. Both the resonance condition and the angle at which the greatest SPR response happens can vary with changes in layer thickness. Resonance angle shifts can be used to identify variations in the refractive index of surrounding medium and/or the presence of analytes. Since the thickness of thin film is a key factor in determining the performance of the SPR sensor, much consideration has been given to finding the ideal combination, as shown in the accompanying Figures. Figure\u0026nbsp;\u003cspan refid=\"Fig22\" class=\"InternalRef\"\u003e9\u003c/span\u003ea-e represent spectral and angular-resolved p-polarized reflectance mappings of Glass-Cu-Air interfaces over a broad spectral range starting from 500 to 800 nm wavelengths. Cu thin film thicknesses were taken into consideration to be 30, 40, 50, 60, and 70 nm, respectively. It was clear that reduced reflectance was seen at particular incidence angles and spectral regions for the different thicknesses of Cu thin films. The lowest intensity was found at shorter wavelengths with a larger spread for Cu thicknesses of 30 and 70 nm, as indicated by the white dashed rectangle in Figs.\u0026nbsp;\u003cspan refid=\"Fig22\" class=\"InternalRef\"\u003e9\u003c/span\u003ea and \u003cspan refid=\"Fig22\" class=\"InternalRef\"\u003e9\u003c/span\u003ee. Conversely, for Cu thin film thicknesses of 40, 50, and 60 nm, the lowest reflectance was found over the 600\u0026ndash;800 nm spectral region, as indicated by the white dashed areas in Fig.\u0026nbsp;\u003cspan refid=\"Fig22\" class=\"InternalRef\"\u003e9\u003c/span\u003eb\u0026ndash;d, respectively. As seen in Fig.\u0026nbsp;\u003cspan refid=\"Fig23\" class=\"InternalRef\"\u003e10\u003c/span\u003ea-c, additional quantitative analysis was performed for three common spectral wavelengths, such as 600, 700, and 800 nm. Figure\u0026nbsp;\u003cspan refid=\"Fig23\" class=\"InternalRef\"\u003e10\u003c/span\u003ea-c represent angular-resolved p-polarized reflectivity of the model \u0026ldquo;Glass-Cu-Air interfaces\u0026rdquo; used in the simulation at 600, 700 and 800 nm wavelengths respectively.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eAt the line of 600 nm spectral wavelength, Cu thin films of 30, 40, 50, 60 and 70 nm thickness exhibited the minimum reflectivity located at the incident angles of 45.3\u003csup\u003eo\u003c/sup\u003e (0.0861 a.u.), 44.7\u003csup\u003eo\u003c/sup\u003e (0.0002 a.u.), 44.5\u003csup\u003eo\u003c/sup\u003e (0.0910 a.u.), 44.45\u003csup\u003eo\u003c/sup\u003e (0.2784 a.u.) and 44.4\u003csup\u003eo\u003c/sup\u003e (0.4595 a.u.) respectively as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig23\" class=\"InternalRef\"\u003e10\u003c/span\u003ea. It can be observed that at a Cu thin film thickness of 40 nm, reflectivity approaches zero, which allows for the highest possible conversion of incident light energy into surface plasmons. Any deviation from the aforementioned reference value in thickness results in an increase in minimum reflectance, which in turn indicates a reduced rate of light utilization, as seen in Fig.\u0026nbsp;\u003cspan refid=\"Fig23\" class=\"InternalRef\"\u003e10\u003c/span\u003ed. The \"Glass-Cu-Air interfaces\" model utilized in this investigation has a minimum reflectance and SPR incidence angle as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig23\" class=\"InternalRef\"\u003e10\u003c/span\u003ed for 600 nm wavelengths Cu thin film thicknesses were kept at 30, 40, 50, 60, and 70 nm. At 45.3\u003csup\u003eo\u003c/sup\u003e, the lowest reflectance rises to 0.0861 a.u. for a Cu thin film thickness of 30 nm. Cu thin film thickness of 40 nm with minimal reflectance of 0.0002 a.u. at incidence angle of 44.7\u0026deg; produced the maximum stimulation of SPR, which was represented by a deep and wide curve with FWHM of 3.3\u0026deg;. In this case, as thicknesses increased from 30 to 40 nm, the SPR angle blue-shifted by 0.6\u0026deg;. A thicker Cu thin film, say 50, 60, or 70 nm thick, results in an almost linear increase in minimum reflectance to 0.0910, 0.2784, and 0.4595 a.u., respectively. As seen in Fig.\u0026nbsp;\u003cspan refid=\"Fig23\" class=\"InternalRef\"\u003e10\u003c/span\u003ed, the SPR incidence angles were discovered to be decreasing with a slight variation.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eCu thin films with thicknesses of 30, 40, 50, 60, and 70 nm showed the lowest reflectivity for the longer wavelength spectrum, such as 700 nm, and were located at incident angles of 43\u003csup\u003eo\u003c/sup\u003e (0.1779), 42.7\u003csup\u003eo\u003c/sup\u003e (0.0097 a.u.), 42.6\u003csup\u003eo\u003c/sup\u003e (0.0629 a.u.), 42.55\u003csup\u003eo\u003c/sup\u003e (0.3189 a.u.), and 42.5\u003csup\u003eo\u003c/sup\u003e (0.5512 a.u.), respectively, as shown in the inset of Fig.\u0026nbsp;\u003cspan refid=\"Fig23\" class=\"InternalRef\"\u003e10\u003c/span\u003eb. A zoomed-in view of the minimum reflectance, indicated by the black dotted rectangle in Fig.\u0026nbsp;\u003cspan refid=\"Fig21\" class=\"InternalRef\"\u003e8\u003c/span\u003eb, is shown in the inset. It is shown that reflectivity approaches zero at a Cu thin film thickness of 40 nm, allowing for the maximum conversion of incident light energy into surface plasmons. As can be observed in Fig.\u0026nbsp;\u003cspan refid=\"Fig23\" class=\"InternalRef\"\u003e10\u003c/span\u003ee, any deviation in thickness from the previously specified reference value causes an increase in minimum reflectance, which in turn signifies a decreased rate of light utilization. The model \"Glass-Cu-Air interfaces\" at 700 nm wavelength, with Cu thin film thicknesses maintained at 30, 40, 50, 60, and 70 nm, is shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig23\" class=\"InternalRef\"\u003e10\u003c/span\u003ee along with its lowest reflectance and SPR angles. At an SPR angle of 43\u0026deg;, the lowest reflectance rises to 0.0861 a.u. for a Cu thin film thickness of 30 nm. When Cu thin film thickness was fixed at 40 nm with lowest reflectance of 0.0097 a.u. at SPR angle of 42.7\u0026deg;, the maximum excitation of SPR was observed, resulting in a deep and narrow curve with FWHM of 0.816\u0026deg;. In this case, as thicknesses increased from 30 to 40 nm, the SPR angle blue-shifted by 0.3\u0026deg;. There is a nearly linear increase in minimum reflectance to 0.0629, 0. 3189, and 0. 5512 a.u., respectively, with a thicker Cu thin film, say 50, 60, or 70 nm thick. The SPR angles were found to be decreasing with a small fluctuation, as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig23\" class=\"InternalRef\"\u003e10\u003c/span\u003ee. As observed in Fig.\u0026nbsp;\u003cspan refid=\"Fig23\" class=\"InternalRef\"\u003e10\u003c/span\u003ec, the lowest reflectance was observed at SPR angles of 42.3\u0026deg; (0.1024 a.u.), 42.15\u0026deg; (0.0143 a.u.), 42.1\u0026deg; (0.1859 a.u.), 42\u0026deg; (0.4338 a.u.), and 42\u0026deg; (0.6519 a.u.) for Cu thin films with thicknesses of 30, 40, 50, 60, and 70 nm at 800 nm spectral wavelength respectively. The inset displays a close-up of the lowest reflectance, which is represented by the black dotted rectangle in Fig.\u0026nbsp;\u003cspan refid=\"Fig23\" class=\"InternalRef\"\u003e10\u003c/span\u003ec. It is demonstrated that at a Cu thin film thickness of 40 nm, reflectivity approaches zero, permitting the highest possible conversion of incident light energy into surface plasmons. Any thickness variation from the previously defined reference value results in an increase in minimum reflectance, which in turn indicates a lower rate of light utilization, as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig23\" class=\"InternalRef\"\u003e10\u003c/span\u003ef. For 800 nm wavelengths, the model \"Glass-Cu-Air interfaces\" exhibits the lowest reflectance and SPR angles, as shown in 10f. For a Cu thin film thickness of 30 nm, the lowest reflectance increases to 0.1024 a.u. at an SPR angle of 42.3\u003csup\u003eo\u003c/sup\u003e. The maximum excitation of SPR was observed at an SPR angle of 42.15\u0026deg; when the thickness of the Cu thin film was fixed at 40 nm, with the lowest reflectance of 0.0143 a.u. This resulted in a narrow and deep curve with an FWHM of 0.5\u0026deg;. In this instance, the SPR angle blue-shifted by 0.15\u0026deg; as thicknesses increased from 30 to 40 nm. An increase in thickness of the Cu thin film, approximately 50, 60, or 70 nm, causes the minimum reflectance to grow almost linearly to 0.1859, 0.4338, and 0.6519 a.u., respectively. The SPR incidence angles were seen to be quite constant with just minor variations, as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig23\" class=\"InternalRef\"\u003e10\u003c/span\u003ef.\u003c/p\u003e"},{"header":"4. Conclusion","content":"\u003cp\u003eCu-based thin films of ca. 50 nm thickness were fabricated using the DC sputtering technique. The thin films were further treated at 200, 400 and 600\u0026deg;C for 2 hrs in ambient condition. FESEM observations confirmed that clusters and voids were observed at the surface of the thin films annealed at high temperatures. UV-vis absorption measurements revealed a red shift in absorption edges from 464 (2.67 eV) to 616 nm (2.01 eV). Tauc plots and Urbach plots were extracted for each of the annealed samples. A decrease in energy band gaps from 2.38 to 1.54 eV and an increase in Urbach energies from 193 to 272 meV with increasing annealing conditions were noted. The observation was correlated to changing of Cu to Cu\u003csub\u003e2\u003c/sub\u003eO and CuO at increasing annealing temperatures. PL measurements confirmed the emission bands at 765 nm (1.62 eV) and 785 nm (1.58 eV) corresponding to electronic levels Vo\u003csup\u003e+\u0026thinsp;2\u003c/sup\u003e and V\u003csub\u003eCu\u003c/sub\u003e respectively. At high temperatures, the PL emission band at 765 nm was found to shift a bit to the right (768 nm) whereas the other PL band at 785 nm shifted to 778 nm. The hydrophobicity of the treated sample was confirmed by WCA measurements. The hydrophobicity was found to increase at 200 and 400\u0026deg;C (WCA 92.4\u0026deg; and 85.2\u0026deg; respectively), however the surface became more hydrophilic at 600\u0026deg;C (WCA 42.3\u0026deg;). A plausible explanation was elaborated. A proposed model towards Cu-based thin film SPR sensor has been analyzed thoroughly using a commercial simulation software. A detailed investigation has been conducted into the spectrum and angular resolved reflectance properties of Cu-based thin films, which have been proposed as the active layer in an SPR sensor model. The ideal thickness of Cu thin film was found to be 40 nm at SPR angles of 44.7\u0026deg;, 42.7\u0026deg;, and 42.15\u0026deg; at spectral wavelengths of 600, 700, and 800 nm respectively.\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003e \u003ch2\u003eConflict of interest:\u003c/h2\u003e \u003cp\u003eThe author declares that there is no conflict of interest.\u003c/p\u003e \u003c/p\u003e\u003ch2\u003eAuthor Contribution\u003c/h2\u003e\u003cp\u003eM.K.H. developed the concept and methodology and prepare the original draft; M.K.H. and A.A. did the formal analysis and investigation and F.K., A.H. and M.R. reviewed the draft.\u003c/p\u003e\u003ch2\u003eAcknowledgement:\u003c/h2\u003e \u003cp\u003eThe authors thank the Interdisciplinary Research Center for Sustainable Energy Systems (IRC-SES), the Research Institute, and King Fahd University of Petroleum \u0026amp; Minerals (KFUPM) for their support. MKH acknowledges the support received under IRC-SES Grant INRE2420. AA acknowledges the support received from the Undergraduate Research Office (URO) under the Uxplore program.\u003c/p\u003e\u003ch2\u003eData availability:\u003c/h2\u003e \u003cp\u003eAll data generated or analysed during this study are included in this published article.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\u003cli\u003e\u003cspan\u003eHuang, X., Ao, D., Chen, T., Chen, Y., Li, F., Chen, S., Liang, G., Zhang, X., Zheng, Z., Fan, P.: High-performance copper selenide thermoelectric thin films for flexible thermoelectric application. (2022)\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eAlami, A.H., Rajab, B., Abed, J., Faraj, M., Hawili, A.A., Alawadhi, H.: Investigating various copper oxides-based counter electrodes for dye sensitized solar cell applications. Energy. \u003cb\u003e174\u003c/b\u003e, 526\u0026ndash;533 (2019)\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003ePawar, P., et al.: A low-cost copper oxide thin film memristive device based on successive ionic layer adsorption and reaction method. Mater. Sci. Semiconduct. Process. \u003cb\u003e71\u003c/b\u003e, 102\u0026ndash;108 (2017)\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eYang, et al.: Transparent flexible thermoelectric material based on non-toxic earth-abundant p-type copper iodide thin film. Nat. Commun., \u003cb\u003e8\u003c/b\u003e, 1, (2017)\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eDjatoubai, Su, J.: First spray pyrolysis thin film fabrication of environment-friendly Cu2BaSnS4 (CBTS) nanomaterials. Chem. Phys. Lett. \u003cb\u003e770\u003c/b\u003e, 138406 (2021)\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eLiang, W., et al.: A solution-processed ternary copper halide thin films for air-stable and deep-ultraviolet-sensitive photodetector, Nanoscale, \u003cb\u003e12\u003c/b\u003e, 33, pp. 17213\u0026ndash;17221, (2020)\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eLee, S., et al.: Fabrication of high-quality single-crystal Cu thin films using radio-frequency sputtering. Sci. Rep., \u003cb\u003e4\u003c/b\u003e, 1, (2014)\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eHussain, R., Hussain, I.: Copper selenide thin films from growth to applications. Solid State Sci. \u003cb\u003e100\u003c/b\u003e, 106101 (2020)\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eBas, S., Cummins, C., Selkirk, A., Borah, D., Ozmen, M., Morris, M.: A Novel Electrochemical Sensor Based on Metal Ion Infiltrated Block Copolymer Thin Films for Sensitive and Selective Determination of Dopamine. ACS Appl. Nano Mater. \u003cb\u003e2\u003c/b\u003e(11), 7311\u0026ndash;7318 (2019)\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eBarbee, B., et al.: Cu and Ni Co-sputtered heteroatomic thin film for enhanced nonenzymatic glucose detection, Scientific Reports, vol. 12, no. 1, (2022)\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eZhao, Y., Bai, Q., Liao, P., Ding, X., Zuo, X., Huang, W., Li, Y.: Radiation hardness of Cu2ZnSn (S, Se) 4 thin film solar cells under 10 MeV proton irradiation. Phys. Lett. A. \u003cb\u003e472\u003c/b\u003e, 128804 (2023)\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eLaGrange, T., Arakawa, K., Yasuda, H., Kumar, M.: Preferential void formation at crystallographically ordered grain boundaries in nanotwinned copper thin films. Acta Mater. \u003cb\u003e96\u003c/b\u003e, 284\u0026ndash;291 (2015)\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eBarchiesi, D., Gharbi, T., Cakir, D., Anglaret, E., Fr\u0026eacute;ty, N., Kessentini, S., Ma\u0026acirc;lej, R.: Performance of surface plasmon resonance sensors using copper/copper oxide films: influence of thicknesses and optical properties. Photonics. \u003cb\u003e9\u003c/b\u003e(2), 104 (2022, February)\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eStebunov, Y.V., Yakubovsky, D.I., Fedyanin, D.Y., Arsenin, A.V., Volkov, V.S.: Superior sensitivity of copper-based plasmonic biosensors. Langmuir. \u003cb\u003e34\u003c/b\u003e(15), 4681\u0026ndash;4687 (2018)\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eArul, C., Moulaee, K., Donato, N., Iannazzo, D., Lavanya, N., Neri, G., Sekar, C.: Temperature modulated Cu-MOF based gas sensor with dual selectivity to acetone and NO2 at low operating temperatures. Sens. Actuators B. \u003cb\u003e329\u003c/b\u003e, 129053 (2021)\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003ePotočnik, J., Božinović, N., Novaković, M., Barudžija, T., Nenadović, M., Popović, M.: Optical properties of copper helical nanostructures: the effect of thickness on the SPR peak position. Nanotechnology. \u003cb\u003e33\u003c/b\u003e(34), 345710 (2022)\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eMuthumanikkam, M., Vibisha, A., Lordwin Prabhakar, M.C., Suresh, P., Rajesh, K.B., Jaroszewicz, Z., Jha, R.: Numerical investigation on high-performance Cu-based surface plasmon resonance sensor for biosensing application. Sensors. \u003cb\u003e23\u003c/b\u003e(17), 7495 (2023)\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eRodrigues, M.S., Borges, J., Lopes, C., Pereira, R.M., Vasilevskiy, M.I., Vaz, F.: Gas sensors based on localized surface plasmon resonances: Synthesis of oxide films with embedded metal nanoparticles, theory and simulation, and sensitivity enhancement strategies. Appl. Sci. \u003cb\u003e11\u003c/b\u003e(12), 5388 (2021)\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eShukor, A.H., Alhattab, H.A., Takano, I.: Electrical and optical properties of copper oxide thin films prepared by DC magnetron sputtering. J. Vacuum Sci. Technol. B, \u003cb\u003e38\u003c/b\u003e(1). (2020)\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003ePana, I., Parau, A.C., Dinu, M., Kiss, A.E., Constantin, L.R., Vitelaru, C.: Optical properties and stability of copper thin films for transparent thermal heat reflectors. Metals. \u003cb\u003e12\u003c/b\u003e(2), 262 (2022)\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eSaad, Y., Selmi, M., Gazzah, M.H., Bajahzar, A., Belmabrouk, H.: Performance enhancement of a copper-based optical fiber SPR sensor by the addition of an oxide layer. Optik. \u003cb\u003e190\u003c/b\u003e, 1\u0026ndash;9 (2019)\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eFendi, F.W.S., Mukhtar, W.M., Abdullah, M.: Surface Plasmon Resonance Sensor for Covid-19 Detection: A Review on Plasmonic Materials. Sensors and Actuators A: Physical, 114617. (2023)\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eRam\u0026iacute;rez, C., Bozzini, B., Calderon, J.A.: Electrodeposition of copper from triethanolamine as a complexing agent in alkaline solution. Electrochim. Acta. \u003cb\u003e425\u003c/b\u003e, 140654 (2022)\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eKadhim, M.J., Sukkar, K.A., Abbas, A.S.: Copper Thin Film Deposited by PVD on Aluminum AA4015 substrate for thermal solar application. In IOP Conference Series: Materials Science and Engineering (Vol. 518, No. 3, p. 032048). IOP Publishing. (2019), May\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003ePrud'Homme, N., Constantoudis, V., Turgambaeva, A.E., Krisyuk, V.V., Sam\u0026eacute;lor, D., Senocq, F., Vahlas, C.: Chemical vapor deposition of Cu films from copper (I) cyclopentadienyl triethylphophine: Precursor characteristics and interplay between growth parameters and films morphology. Thin Solid Films. \u003cb\u003e701\u003c/b\u003e, 137967 (2020)\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eUsami, N., Ota, E., Higo, A., Momose, T., Mita, Y.: Continuity assessment for supercritical-fluids-deposited (SCFD) Cu film as electroplating seed layer. In 2019 IEEE 32nd International Conference on Microelectronic Test Structures (ICMTS) (pp. 54\u0026ndash;57). IEEE. (2019), March\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eLisco, F., et al.: The structural properties of CdS deposited by chemical bath deposition and pulsed direct current magnetron sputtering. Thin Solid Films. \u003cb\u003e582\u003c/b\u003e, 323\u0026ndash;327 (2015)\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eGhazal, H., Sohail, N.: Sputtering Deposition. In: Thin Film Deposition-Fundamentals, Processes, and Applications. IntechOpen (2022)\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eGreene, J.E. Tracing the recorded history of thin-film sputter deposition: From the 1800s to 2017. Journal of Vacuum ScienceTechnology, Vacuum, A.: Surfaces, and Films, 35(5), 05C204. (2017)\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eMech, R., Kowalik, Żabiński, P.: Cu Thin Films Deposited by DC Magnetron Sputtering for Contact Surfaces on Electronic Components. Arch. Metall. Mater., \u003cb\u003e56\u003c/b\u003e, 4, (2011)\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eShukor, A.H., Alhattab, H.A., Takano, I.: Electrical and optical properties of copper oxide thin films prepared by DC magnetron sputtering. Journal of Vacuum Science \u0026amp; Technology B, Nanotechnology and Microelectronics: Materials, Processing, Measurement, and Phenomena, 38(1), 012803. (2020)\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eDu, S., Li, Y.: Effect of Annealing on Microstructure and Mechanical Properties of Magnetron Sputtered Cu Thin Films, Advances in Materials Science and Engineering, vol. pp. 1\u0026ndash;8, 2015. (2015)\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eSrinivasan, S.S.G., Govardhanan, B., Aabel, P., Ashok, M., Kumar, M.S.: Effect of oxygen partial pressure on the tuning of copper oxide thin films by reactive sputtering for solar light driven photocatalysis. Sol. Energy. \u003cb\u003e187\u003c/b\u003e, 368\u0026ndash;378 (2019)\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eRodrigues, E.P., Oliveira, L.C., Silva, M.L., Moreira, C.S., Lima, A.M.: Surface plasmon resonance sensing characteristics of thin copper and gold films in aqueous and gaseous interfaces. IEEE Sens. J. \u003cb\u003e20\u003c/b\u003e(14), 7701\u0026ndash;7710 (2020)\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eYesudasu, V., Pradhan, H.S., Pandya, R.J.: Recent progress in surface plasmon resonance based sensors: A comprehensive review. Heliyon, \u003cb\u003e7\u003c/b\u003e(3). (2021)\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eStebunov, Y.V., Yakubovsky, D.I., Fedyanin, D.Y., Arsenin, A.V., Volkov, V.S.: Superior sensitivity of copper-based plasmonic biosensors. Langmuir. \u003cb\u003e34\u003c/b\u003e(15), 4681\u0026ndash;4687 (2018)\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eNaikoo, G.A., Awan, T., Salim, H., Arshad, F., Hassan, I.U., Pedram, M.Z., Ahmed, W., Faruck, H.L., Aljabali, A.A., Mishra, V., Serrano-Aroca: \u0026Aacute;. Fourth‐generation glucose sensors composed of copper nanostructures for diabetes management: A critical review. Bioengineering \u0026amp; Translational Medicine, 7(1), e10248\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eŞahin, B.: Flexible nanostructured CuO thin film: A promising candidate for wearable real-time sweat rate monitoring devices. Sens. Actuators A: Phys. \u003cb\u003e341\u003c/b\u003e, 113604 (2022)\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eJasim, H.A., Dakhil, O.A.A.: Highly sensitive non-enzymatic glucose sensor based on copper oxide nanorods. J. Nanopart. Res. \u003cb\u003e24\u003c/b\u003e(11), 212 (2022)\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eDivya, J., Selvendran, S., Raja, A.S., Sivasubramanian, A.: Surface plasmon based plasmonic sensors: A review on their past, present and future, vol. 11, p. 100175. X, Biosensors and Bioelectronics (2022)\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eJain, S., Paliwal, A., Gupta, V., Tomar, M.: SPR based refractive index modulation of nanostructured SiO2 films grown using GLAD assisted RF sputtering technique. Surf. Interfaces. \u003cb\u003e34\u003c/b\u003e, 102355 (2022)\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eTao, L., Deng, S., Gao, H., Lv, H., Wen, X., Li, M.: Experimental investigation of the dielectric constants of thin noble metallic films using a surface plasmon resonance sensor. Sensors. \u003cb\u003e20\u003c/b\u003e(5), 1505 (2020)\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eTopor, C.V., Puiu, M., Bala, C.: Strategies for Surface Design in Surface Plasmon Resonance (SPR) Sensing. Biosensors. \u003cb\u003e13\u003c/b\u003e(4), 465 (2023)\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eAcharya, B., Behera, A., Behera, S.: Optimizing drug discovery: Surface plasmon resonance techniques and their multifaceted applications. Chem. Phys. Impact. \u003cb\u003e8\u003c/b\u003e, 100414 (2024)\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eAromaa, J., Kekkonen, M., Mousapour, M., Jokilaakso, A., Lundstr\u0026ouml;m, M.: The Oxidation of Copper in Air at Temperatures up to 100 C. Corros. Mater. Degrad. \u003cb\u003e2\u003c/b\u003e(4), 625\u0026ndash;640 (2021)\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eKoshy, J., George, K.C.: Annealing effects on crystallite size and band gap of CuO nanoparticles. catalysis, 5, 11. (2015)\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eZhu, Y., Mimura, K., Isshiki, M.: Oxidation mechanism of Cu 2 O to CuO at 600\u0026ndash;1050 C. Oxid. Met. \u003cb\u003e62\u003c/b\u003e, 207\u0026ndash;222 (2004)\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eRahman, M.M., Miran, H.A., Jiang, Z.T., Altarawneh, M., Chuah, L.S., Lee, H.L., Amun, H.-L.L., Nicholas, A. M., Dlugogorski, B.Z.: Investigation of the post-annealing electromagnetic response of Cu\u0026ndash;Co oxide coatings via optical measurement and computational modelling. RSC Adv. \u003cb\u003e7\u003c/b\u003e(27), 16826\u0026ndash;16835 (2017)\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003ePelegrini, S., Tumelero, M.A., Brandt, I.S., Pace, D., Faccio, R.D., R., Pasa, A.A.: Electrodeposited Cu2O doped with Cl: Electrical and optical properties. J. Appl. Phys. \u003cb\u003e123\u003c/b\u003e(16), 161567 (2018)\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eIto, T., Masumi, T.: Detailed examination of relaxation processes of excitons in photoluminescence spectra of Cu 2 O. J. Phys. Soc. Jpn. \u003cb\u003e66\u003c/b\u003e(7), 2185\u0026ndash;2193 (1997)\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eSoltanmohammadi, M., Spurio, E., Gloystein, A., Luches, P., Nilius, N.: Photoluminescence Spectroscopy of Cuprous Oxide: Bulk Crystal versus Crystalline Films. (2023). physica status solidi (a), 2200887\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eMabrouki, M.: Effect of annealing temperature on the structural, physical, chemical, and wetting properties of copper oxide thin films. Materials Today: Proceedings, 13, 771\u0026ndash;776. (2019)\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eLi, C., Zhang, J., Han, J., Yao, B.: A numerical solution to the effects of surface roughness on water\u0026ndash;coal contact angle. Sci. Rep. \u003cb\u003e11\u003c/b\u003e(1), 459 (2021)\u003c/span\u003e\u003c/li\u003e\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":true,"highlight":"","institution":"","isAcceptedByJournal":false,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true},"keywords":"Copper thin film, sputtering deposition, optical properties, wetting contact angle, SPR sensor","lastPublishedDoi":"10.21203/rs.3.rs-4427071/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-4427071/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eIn this work, a simple and two-step process was demonstrated to develop multifunctional Cu-based thin films that would be suitable for thin film photoactive devices. Cu thin films on quartz glass substrates were prepared by sputtering technique followed by a thermal treatment. The samples were annealed at high temperatures such as 200, 400, and 600\u0026deg;C for 2 hrs in a tubular furnace. Surface topography was investigated by a high-resolution scanning electron microscope (FESEM) and SEM-aided energy dispersion spectroscopy (EDS). At high temperatures, the thin films were found to have clusters and voids. Detailed studies on optical properties such as UV-vis absorptions, energy band gaps and Urbach energies have been carried out. A red shift in absorption edges (from 464 to 616 nm), a decrease in energy band gaps (from 2.38 to 1.54 eV) and an increase in Urbach energies (from 193 to 272 meV) were observed for those samples annealed at higher temperatures. Sessile drop tests were carried out to find the wetting contact angle and demonstrate the hydrophobicity of the thin film of pristine Cu and of those treated at high temperatures. Sessile drop tests were carried out to find the wetting contact angle (WCA) and demonstrate the hydrophobicity of the thin film of pristine Cu and of those treated at high temperatures. An approximate WCA of 71.9\u0026deg; was determined for the Cu thin film. After the samples were treated at 200\u0026deg;C and 400\u0026deg;C, respectively, the surface became more hydrophobic by 92.4\u0026deg; and 85.2\u0026deg;. Nevertheless, the same thin film's WCA was decreased and its hydrophilicity increased during additional annealing. Cu-based thin films have been suggested as the active layer in an SPR sensor model, and the spectrum and angular resolved reflectance properties have been thoroughly investigated. At spectral wavelengths of 600, 700, and 800 nm, the optimum thickness of Cu thin film was determined to be 40 nm at SPR angles of 44.7\u0026deg;, 42.7\u0026deg;, and 42.15\u0026deg;.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e","manuscriptTitle":"Fabrication and Wetting Characteristics of Copper Thin Film: An Active Layer for SPR-based Sensor Applications","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2024-05-27 06:30:11","doi":"10.21203/rs.3.rs-4427071/v1","editorialEvents":[{"type":"communityComments","content":0}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true}}],"origin":"","ownerIdentity":"323ce451-bdeb-4ad4-a65e-7e4b78a915fe","owner":[],"postedDate":"May 27th, 2024","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"posted","subjectAreas":[],"tags":[],"updatedAt":"2024-06-19T05:16:21+00:00","versionOfRecord":[],"versionCreatedAt":"2024-05-27 06:30:11","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-4427071","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-4427071","identity":"rs-4427071","version":["v1"]},"buildId":"qtupq5eGEP_6zYnWcrvyt","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}