Fabrication of silver and silver-copper bimetal thin films using co-sputtering for SERS applications

Optical Materials 97 (2019) 109381

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Fabrication of silver and silver-copper bimetal thin films using co-sputtering for SERS applications

T

P. Nandhagopala, Anil Kumar Palb, D. Bharathi Mohana,∗ a b

Department of Physics, Pondicherry University, Kalapet, Puducherry, 605014, India School of Chemistry, University of Birmingham, Birmingham, B15 2TT, United Kingdom

A R T I C LE I N FO

A B S T R A C T

Keywords: Bimetal thin films Co-sputtering Micro structure Surface plasmon resonance Surface enhanced Raman spectroscopy

The objective of this work is to tune the surface plasmon resonance (SPR) of thin silver and silver-copper bimetallic films into a strong and broad absorption in the red and near infrared regions for applications in Surface Enhanced Raman Spectroscopy. The SPR tuning was achieved by varying the composition of silver and copper in the films fabricated using magnetron co-sputtering method and also by annealing it under high vacuum condition. Atomic Force Microscope was used to study the film surface morphology and its growth with annealing temperatures. SPR exhibited by the films were directly studied using UV–Visible spectroscopy. Surface Enhanced Raman Scattering studies taken on Methylene Blue drop-casted on film surface shows a threefold increase in the enhancement factor of Ag–Cu films as compared to pure Ag sputtered films.

1. Introduction Human population is vulnerable to the diseases culminating from pathogenic, genetic, life style and environmental conditions. Diseases can have an adverse effect on the physical and mental health of the patients. The key to the recovery of the patients lies in early diagnosis and in undergoing proper treatments. The presence of abnormal levels or the existence of certain biomolecules in our body is indicative of a disease state and hence the detection and quantification of these vital biomolecules have become an indispensable step in disease diagnosis. Surface-enhanced Raman spectroscopy (SERS) is a versatile platform for detection of molecular species, organic and bio-molecules under very low concentration levels [1]. The noble metal nanoparticles possesses localised surface plasmon resonance (LSPR) modes which upon excited with light will exhibit immense absorption and scattering [2]. Any molecules that are close to the nanoparticles will get enhanced in their Raman scattering cross section giving rise to a detectable Raman signal. The use of functionalized biocompatible SERS nanoparticles rendered in vivo Raman imaging of living cells possible [3]. Combination of SERS with other imaging modalities such as positron emission tomography (PET) has recently highlighted the accessibility of this technique for practical purposes [4]. Since these methods uses labelling, tagging and functionalization, it restricts the detection to a specific compound or molecular species. On the other hand, patterned thin film nanostructures on dielectric, metal and flexible substrates provides a

∗

label-free, easy, drop or dip method to adsorb analyte molecules [5–7]. It has been shown that for efficient use of metal nanoparticles for Raman detection, a strong coupling between the excitation light and the metal nanoparticles must exist, which requires the excitation wavelength and the LSPR maxima to be nearly equal [8]. In addition to that, matching of excitation wavelength and electronic band gap of probe molecule will result in enormous signal from SERRS [9]. However, with narrow LSPR peaks reported, the tunability of excitation wavelength to match the probe molecules energy gap is restricted for a small range. Hence instead of preparing a traditional SERS substrates with a narrow LSPR peak, it is favourable to have a nanostructure with a wide SPR spanning visible to near-IR region. Tuning the SPR band of nanoparticles into a wide absorption spectra was realized by preparing bimetallic nanoparticles consisting of two noble metals. The Ag–Au, Ag–Al and Ag–Cu bimetal systems were mostly prepared to study the improvement in SERS properties with broad and red shifted SPR features [10,11]. Owing to the higher cost of gold, copper is advantageous to use as a cheap alternative for red shifting and broadening of the bimetal Ag–Cu films. A pulsed lased deposition method was incorporated to deposit Ag–Cu nanoparticles where a red shifted and broad SPR was observed [12]. Laser ablation was used to prepare Ag and Cu colloidal nanoparticles individually, followed by laser irradiation of mixed solution to fuse them into Ag–Cu alloy nanoparticles [13]. A stable SERS substrate was prepared using the melting and mixing of Ag and Cu bilayer films

Corresponding author. E-mail address: [email protected] (D. Bharathi Mohan).

https://doi.org/10.1016/j.optmat.2019.109381 Received 9 July 2019; Received in revised form 26 August 2019; Accepted 10 September 2019 0925-3467/ © 2019 Elsevier B.V. All rights reserved.

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UV–Visible spectrophotometer (Shimadzu 3600 plus, Japan) for optical absorbance at 1 nm interval and Atomic Force Microscope (AFM) (Veeco Nanoscope, Bruker) in ScanAsyst in air for study of surface morphology of films. For SERS studies, the Methylene Blue solution at 10−6 M concentration was drop-casted over the films in 6 μL aliquots and dried overnight. The Raman spectra were taken on three random spots and averaged over the dried area using a 50x confocal Raman microscope with spectrometer (Renishaw InVia, England) at an excitation wavelength of 785 nm and a laser power of 0.5 mW with 10 s exposure time. The atomic composition of the films in percentage were obtained using an Energy Dispersive X-ray Spectrometer (Element EDS system, AMETEK, New Jersey, U.S.A) mapping on the surface of asdeposited Ag–Cu films. The surface morphology data from AFM was processed using Gwyddion software.

deposited using thermal evaporation and subsequent CVD growth of graphene above it. The individual Ag and Cu nanoparticles showed SPR maximum at 400 and 630 nm respectively. After alloy formation a single broad SPR positioned at 550 nm was observed [14]. Ag–Cu nanoparticle embedded in PVA thin films showed a shifted SPR over the visible region [15]. Thermal evaporation of mixed Copper–Silver powder formed an improved SERS substrate as compared to pure Ag films [16]. We herein report the first time use of magnetron co-sputtering technique to deposit silver and copper simultaneously, to obtain an ultra-thin Ag–Cu bimetallic films of various compositions. The coating was done with different powers applied for the silver target to allow increased sputtering rate, thereby changing the composition of Ag–Cu in the films. The prepared films were annealed under high vacuum to modify its surface properties such as average particle size, interparticle gap and surface roughness. The bimetallic Ag–Cu films were compared with pure Ag films prepared under similar conditions to show the superior plasmonic quality of bimetallic films.

3. Results and discussions 3.1. Elemental analysis

2. Experimental method The EDS data for all Ag–Cu films deposited on Silicon wafers was recorded as point mapping of silver and copper constituents. The atomic percentages of copper and silver as found from EDS results is given in Fig. 1. With increasing DC voltage for sputtering silver target, the atomic percentage ratio of silver to copper increases which is agreement with the known fact that with the sputtering power increases the sputtering yield also increases.

The silver and copper targets (99.99% purity) were purchased from Testbourne Ltd, England. Glass slides were procured from Blue Star (Polar Industrial Corporation, Mumbai). The glass slides were cut into 1 cm square substrates for use in characterization and SERS studies and 0.5 cm square for AFM studies. Labolene soap solution and hydrofluoric acid (HF) were purchased from Qualigens (Fisher scientific, India). Nitric acid, 98% (HNO3) and acetone were purchased from Emplura (Merck, India).

3.2. Measurement of film thickness Table 2 shows the film thicknesses of all as-deposited films grown using different DC and RF powers. The film thickness increases proportionally with sputtering voltage, due to the increase in the sputtering yield of silver [17]. For each sputtering voltage, the co-sputtered (Ag–Cu) films have higher thickness than the pure (Ag) films. This is due to the combined deposition rate of both silver and copper onto the substrate. Fig. 2 shows the thicknesses of as-deposited films as measured using AFM. The films have been scratched using a clean blade edge to wipe away a portion of the coating. Then AFM measurements were carried out on the overlapping regions of the film and scratch portions. The first column in Fig. 2 shows the top-view of the films whereas the second column shows the 3-D view of the respective films. The third column gives the surface profile of the films taken across the intact and scratched portion of the films. The thickness were taken as a difference between the height profiles of the intact and scratched portions. As it is clearly seen the film deposition does not exhibit a smooth and uniform profile in nanometer scale. This is due to the Volmer-weber type of growth [18] that is common among metal depositions. So the films exhibit surface roughness which is taken to be the error in measuring the film thicknesses. The thickness of pure silver film ranges from 6 to 18 nm, while that of silver-copper bimetal films have a thickness of 8–26 nm. The films with distinct nanoparticles are lower thickness films and films with indistinguishable particles with continuous film nature are higher thickness films. In Ag films, Ag6-asd to Ag14-asd are lower thickness while in Ag–Cu films, AC8-asd to AC14-asd are lower thickness films. With increasing thickness (above 320 V for Ag and 300 V for Ag–Cu films), the as-deposited particles sizes increases inhomogeneously and distinct nanoparticles are hard to visualize in films, due to smooth coverage of overall substrate area.

2.1. Cleaning of substrates The glass substrates were then cleaned using soap solution and flushed with DI water. The substrates were then etched in dilute nitric acid (1 mL in 20 mL DI water) for 1 min followed by sonication in acetone for 10 min. It was dried in a hot air oven for further use. The silicon wafers were cut into 0.5 cm squares and were etched in diluted HF acid for 40 s. The dilution was prepared by adding 1 mL of concentrated HF acid with 20 mL of DI water. The etched wafers were then flushed with DI water and ultrasonicated in acetone for 10 min before use. 2.2. Fabrication of SERS substrates Ag–Cu and Ag ultrathin films were deposited in the cleaned glass substrates and silicon wafers using an upward dual magnetron sputtering system. Initially the chamber was evacuated to a base pressure of 5 × 10−6 m bar and maintained at a working pressure 8.5 × 10−3 m bar during deposition. The silver and copper targets were connected to a DC (Digitronics, Pune, India) and a RF (RF VII Inc, USA) power sources respectively. Keeping the RF power for Copper as constant, the DC power was varied from 260 V to 340 V in the interval of 20 V for different samples. The height of substrate holder was kept at 11.5 cm from cathode and the substrate was rotated at a constant speed of 12 rpm for obtaining a uniform coating. The co-sputtering coating of Silver and Copper was done for 30 s without any substrate biasing and at room temperature. For comparison purpose, pure Ag films were also deposited under same conditions. The as-deposited samples were then annealed under high vacuum (1 × 10−5 m bar) at temperatures of 100, 200, 300 and 400 °C. The details of the prepared films are given in Table 1. The thickness of the as-deposited samples as measured from AFM are given in Table 2. The samples names are assigned with the respective film thickness and annealing temperature.

3.3. Surface morphology The surface morphologies of the as-deposited, 200 and 400 °C annealed films are shown in Fig. 3 and the films corresponding particle size distribution is shown in Fig. 4. The plots of statistical parameters for the morphology of both the Ag and Ag–Cu films, such as average

2.3. Characterization techniques The deposited films were then characterised using a dual beam 2

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Table 1 Samples code of all prepared samples. DC Sputtering Voltage (V)

RF Sputtering Power (W)

Sample code (Annealing Temperature, °C)

260 280 300 320 340 260 280 300 320 340

0 0 0 0 0 10 10 10 10 10

As-deposited

100

200

300

400

Ag6-asd Ag9-asd Ag11-asd Ag14-asd Ag18-asd AC8-asd AC10-asd AC14-asd AC16-asd AC26-asd

Ag6-100 Ag9-100 Ag11-100 Ag14-100 Ag18-100 AC8-100 AC10-100 AC14-100 AC16-100 AC26-100

Ag6-200 Ag9-200 Ag11-200 Ag14-200 Ag18-200 AC8-200 AC10-200 AC14-200 AC16-200 AC26-200

Ag6-300 Ag9-300 Ag11-300 Ag14-300 Ag18-300 AC8-300 AC10-300 AC14-300 AC16-300 AC26-300

Ag6-400 Ag9-400 Ag11-400 Ag14-400 Ag18-400 AC8-400 AC10-400 AC14-400 AC16-400 AC26-400

more like holes (defects) in the continuous silver films. In Ag18-200 film, the gaps are not found among the (clustered) nanoparticles, while between these clusters the gaps run like mazes. For Ag6-asd, the interparticle distance is 4 nm which increases to 7 nm for Ag14-asd. The gap also increases during annealing process from 4 nm to 6 nm for Ag6400. The 400 °C annealed films has gaps of 6, 5, 9, 9 and 15 nm respectively Ag6-400 to Ag18-400. For 200 °C annealed films the interparticle distance is 5, 7, 7 and 8 nm respectively for Ag6-200 to Ag14200. The surface roughness for silver films are shown in the inset of Fig. 4. The RMS roughness value of the films also shows an increasing trend with increasing particle size. Its value is the lowest for Ag6-asd, the roughness is 1.7 nm which increases to 1.9 nm with annealing at 400. With increasing thickness the roughness value changes as 1.7, 2.8, 3.3, 3.1 and 2.3 nm for as-deposited films. The value initially increase up to 11 nm thickness and starts falling afterwards. It indicates a transition from island like morphology to continuous thin film formation. For 200 °C annealed films, the roughness values are 1.7, 4.5, 8.6, 8.8 and 8.1 nm with increasing thickness. For 400 °C annealed films, 1.9, 2.5, 10.6, 9.1 and 15.3 are the roughness values. The surface morphologies and particle size distributions of Ag–Cu thin films are shown in Fig. 6 and Fig. 7 and its formation mechanism is shown in Fig. 8. The lower thickness films AC8-asd and AC10-asd are deposited with both smaller and bigger nanoparticles. The particle size distribution shows a broad Gaussian distribution centered at 30 nm for AC8-asd and a narrow with long-edge tail centered at 20 nm for AC10asd. The separation of distribution into Gaussian and long-edge tail in AC10-asd is due to a greater difference in the sizes of smaller and bigger nanoparticles, whereas in AC8-asd this difference is much less and the distribution is almost a single broad curve. From AC14-asd to AC26-asd the particle sizes are highly homogeneous. From Fig. 6 we see that the average particle sizes decreases with increasing thickness. This tendency could be due to the combined effect of presence of copper in various proportions and different DC sputtering voltages applied for different films. Again the higher thickness (AC16-asd and AC26-asd) films ceases to be a discontinuous films yet discrete nanoparticles are formed (unlike Ag higher thickness films) because of the copper content in the films. Fig. 7 (j) shows an inset of the surface profile of AC16-asd, the surface is wavy in nature with four to five ups and downs. But in AC26-asd, the film is continuous with flat surface profile. Unlike pure Ag sputtered films, the 200 °C annealed Ag–Cu film,

particle size and its standard deviation are shown in Fig. 5. In lower thickness Ag films, the particle size is much lower (12, 18 and 26 nm) as compared to higher thickness films (31 and 33 nm), rendering a high particle number densities, so an average of 3.5/4 particles are used up to form a single particle in 200 and 400 °C annealed films. This number is valid for films with thickness up to 14 nm. The particle size distribution is narrow for lower thickness annealed films as compared to annealed films of higher thickness, which shows a more uniform size profile even after annealing has altered the surface properties of the films. Whereas the annealed films of higher thicknesses also has coalescence of smaller particles to form bigger particles, but with a broader size distribution, with range as much as 30–170 nm for 18 nm Ag film. The particle size distribution broadens and the distributions maximum increases progressively with increasing thickness and annealing temperature. The shift in maximum with increasing thickness is because of the increased mass of Ag deposited, so that the average particle size also increases. Whilst the broadening associated with annealing temperature is not easy to understand as a couple of factors involved such as initial size deviation (range of particle sizes) in as-deposited films and increased mass deposited in higher thickness films. In annealed films of Ag6-asd up to Ag14-asd, the particle size range increases gradually and almost linearly, but for annealed films of Ag18-asd film the range rapidly broadens. This may be due to the continuous (connected) nature of this film, which when applied heat energy favours formation of bigger (> 140 nm) nanoparticles. The 200 °C annealed films for Ag6-asd to Ag14-asd shows spherical to irregularly shaped distinct nanoparticles similar to 400 °C films. But the Ag18-200 film shows an indistinct clustering of nanoparticles. Due to higher thickness and lower thermal energy supplied the film is unable to form complete coalescence of the nanoparticles. Also as the annealing stops after 1 h and natural cooling takes place, the coalescing clusters are frozen. This is not the case in 400 °C annealing of Ag18-asd film. The clustering completes to form the biggest nanoparticles observed in this work. The inter-particle distance between the nearest nanoparticles increases with increasing annealing temperature, as it is directly affected by the nanoparticles volume (increases) and number density (decreases). This concept of interparticle distance cannot be applied to Ag14-asd, Ag18-asd and Ag18-200, since the interparticle distance/ gaps are not uniform. Ag14-asd and Ag18-asd has a smooth surface morphology with gaps occurring erratically in the films. These gaps are

Table 2 Film thickness of all as-deposited films with error (RMS surface roughness) measured using AFM. DC voltage

260 V

280 V

300 V

320 V

340 V

Thickness (RF power = 0 Watt) Thickness (RF power = 10 Watt)

6 ± 2 nm 8 ± 1 nm

9 ± 3 nm 10 ± 3 nm

11 ± 3 nm 14 ± 2 nm

14 ± 3 nm 16 ± 4 nm

18 ± 2 nm 26 ± 2 nm

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Fig. 1. Elemental mapping data of all as-deposited Ag–Cu films. Composition of silver and copper given in atomic percentages in bottom right.

3.4. Film growth

AC8-200 shows a decrease in the average particle size from 34 to 12 nm. The decrease in standard deviation of the sizes (11 nm–2 nm) also indicate a homogeneously formed particles. Although heat energy is supplied the nanoparticle sizes reduces as a result of the presence of large amount of copper (28%). Copper nanoparticles have much higher surface energy for a taken volume compared to silver nanoparticles. So when annealed, the initially stressed as-deposited films rearrange themselves to a more stable spherical and lower sized particles. In 400 °C annealing the particle size reduces but is larger (26 nm) compared to 200 °C annealed one. Here the film was given enough energy to rearrange as well as coalesce to form bigger particles. The trend is reversed after this film, as there is always an increase in average particle size with higher temperature annealing. However the standard deviation of the AC10-200 film shows lower average size of 7 nm than its asdeposited film (12 nm) indicating the absence of larger particles on this film. In general, the growth of Ag–Cu thin film can be grouped into lower (8–14 nm) and higher thickness (16 and 26 nm) films. As shown in Fig. 6, the as-deposited lower thickness films undergo coalescence during annealing process (200 °C) and forms a bigger and discrete nanoparticles. Whereas in higher thickness films, the film undergoes clustering of nanoparticles, and coalescence is partial, which makes the film appear to be cleaved. Since the coalescence is not complete the particle sizes does not improve much. The main reason is because of the continuity in the film, which requires higher energy to pull the particles apart completely to form separate bigger nanoislands. It happens in the case of 400 °C annealing where regardless of the film thicknesses, bigger nanoparticles are formed through coalescence. The average particle size of the 400 annealed films are 26, 39, 58, 50 and 78 nm with standard deviations much less compared with pure silver films (Fig. 5). The interparticle distance of the as-deposited Ag–Cu films varies as 7.6, 2.0, 4.8, 5.7 and 2.5 nm. The low value of average size (13 nm) and standard deviation (2 nm) along with less interparticle gap for AC26asd film results in the highest particle number density (4100 particles/ μm2) for the film. The gaps between 200 °C annealed films are 4.2, 4.9, 4.8, 5.1 and 4.6 nm. For 400 °C annealed films, the gap between particles are 6.5, 4.9, 10.6, 9.3 and 17 nm. The surface roughness of the Ag–Cu films are shown in the inset of Fig. 7. The RMS roughness of the as-deposited and 200 °C annealed films are oscillatory, with only 400 °C annealed films showing a consistent increase with increasing thickness of the films. The roughness values are 2.7, 4.8, 10.4, 11.0 and 13.2 nm respectively.

As discussed earlier, the film growth here is of Volmer-weber type [18], where the atoms in the film have higher cohesive force (metallic bond) among themselves than the adhesion between atom and substrate. This leads to the formation of nano-island [19] with discontinuity in the films. Fig. 8 shows the formation mechanism of Ag–Cu thin film under Volmer-weber growth condition and under post-annealing at 200 and 400 °C. The films sputtered at lower DC voltages (260 and 300 V) and RF (10 W) forms lower thickness films and the films formed at higher sputtering voltages (320 and 340 V) forms higher thickness films. For lower thickness Ag–Cu films (Fig. 6), the deposition happens with inhomogeneous particle sizes, due to relatively higher sputtering yield from copper target as compared to silver target. The lower deposition rate from silver and a relatively higher (and constant) rate from copper allows the formation of both smaller and bigger nanoparticles. Above 280 V, the deposition rate is comparable and gives a uniform film deposition with homogeneous particle sizes. In case of pure silver films, this scenario is averted with absence of copper atom flux. Since silver atom flux is the only contributor, the deposited film has particles that are smaller and uniform. As shown in bottom row of Fig. 8 the higher thickness films form stacked nanoparticles with less interparticle gaps. Whereas lower thickness films forms a single layer of nanoparticles as seen clearly from surface profiles of Fig. 2. The annealing effect on the as-deposited films were also studied using AFM (Figs. 3 and 6). By annealing the films for a fixed time, the surface morphologies of the films can be modified greatly. Here AFM images of 200 and 400 °C annealed films were taken to study the impact of applying external thermal energy on the surface morphologies of these films. In Figs. 3 and 6 the images in first column corresponds to as-deposited Ag and Ag–Cu films in increasing thickness and the second and third columns corresponds to the 200 and 400 °C annealed films. Figs. 4 and 7 gives the particle size distributions of the Ag and Ag–Cu films in the same order. For pure silver films, the formation mechanism is straight forward, the film with lowest thickness has the least average particle size and the narrowest size distribution. With increasing thickness the average particle size also increases proportionally, indicating a mass-limited growth in nanoparticle size. All as-deposited Ag films are spherical in shape, the geometry exhibiting lowest surface energy. When the films are annealed, the atoms in the nanoparticle gain mobility. Under attraction between neighbouring nanoparticles, two or 4

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Fig. 2. Film thickness of the as-deposited films from AFM measurements.

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Fig. 3. AFM images of Ag sputtered films. First column shows the as-deposited films sputtered at 260–340 V. The second and third column shows their 200 and 400 °C post-annealed counterparts.

formed between these two and eventually both the masses combine to form the final product. Thus the particles in as-deposited film forms the “building blocks” of the annealed films.

more particles coalesces to form a single bigger nanoparticle. The magnitude of mobility in atoms are dependent on the annealing temperatures. Annealing at 400 °C inflicts more mobility in atoms as compared to 200 °C annealing due to higher supply of thermal energy. In coalescence, the nanoparticles starts to reach out for the neighbouring particle as a whole by preferential bulging towards each other. As the bulged particles make contact with other particle, a “neck” is

3.5. Optical absorbance Light absorbance in thin metal films is due to the surface plasmon 6

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Fig. 4. Particle size distribution of the Ag sputtered films shown in histograms. The inset in each distribution shows the surface roughness profile of the films.

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Fig. 5. Average size and standard deviation of nanoparticles in (a) Ag and (b) Ag–Cu films.

vivid colours as shown in Fig. 10. Ag6-asd is purple and when annealed at 400 shows bright yellow colour. As the thickness increases, the colours varies as violet to blue. The change in the colour with annealing is sharp at 300 °C or more. At this temperature, the nanoparticles in the thin films are discrete and forms nano-islands structure. Further the nature of the sputtered asd films are highly disordered having smaller grain boundaries with large internal stress due to it. With annealing at sufficiently high temperature this stress is relieved and grain boundaries grow bigger. When the disorder is more, the nanoparticles offer higher resistance and more scattering sites for free electrons during the charge density oscillations, due to which the SPR peaks are red-shifted and broadened. When less scattering sites are prevalent, the charge density oscillation has an increased lifetime and strength. The Ag6-asd film shows a weak peak centered at 470 nm. With annealing at 100 and 200 °C there is no peak shifts or narrowing observed. When annealed at 300 and 400 °C, the peak is narrowed and blue-shifted by 20 nm. For the LSPR to be Gaussian and narrow (high quality), the ensemble of nanoparticles have to be homogeneous, highly crystalline and discrete, so that each nanoparticles act as an independent dipole oscillator exhibiting a narrow resonance line width. In case of as-deposited films, although the particles appear to be spherical and independent, it could be in physical contact with the neighbouring nanoparticles forbidding the individual dipole assumption. Whereas above 300 °C annealing (Fig. 9 (d) and (e)), the particles are clearly away from each other and SPR peaks are centered always at 435 nm except at 485 nm for AC16-400 and AC26-400 films. The higher limit on the SPR peak position for distinct spherical nanoparticles motivates us to associate the broadened SPR peak for as-deposited films with either large electron scattering sites or physical contact between neighbouring nanoparticles. With increasing thickness, the SPR peak position also increases as 520, 565 and 660 for 9, 11, 14 nm. For 18 nm, the film shows a flat absorption associated with quasi-continuous silver film. The peaks slowly blue shifts to 465, 470 and 435 nm for 400 °C annealing. Although Ag18-400 films have the largest nanoparticles in the film, its SPR is narrow and is at 435 nm, but the profile is slightly non-symmetric. This is because of the presence of more number of particles in 30–80 nm range and fewer particles above 80 nm, which adds up to the weightage of the non-symmetric Gaussian absorption profile. Fig. 11 shows the digital images of co-sputtered Ag–Cu bimetal films at different proportions and thicknesses. The as-deposited 8 nm film is light grey in colour which transfers to grey, blue and dark grey with thickness of 14, 16 and 26 nm. AC16-asd film shows blue colour which implies other colours are absorbed and after annealing at 200 °C becomes slightly grey and at 400 °C, the film colour changes into bright

resonance (SPR) of free electrons charge density oscillation [19]. Depending upon whether the film is continuous or discontinuous, the oscillations are classified as surface plasmon polaritons (SPP) or localised surface plasmon resonance (LSPR). In our case, the light incident on discontinuous thin films will result in a LSPR of free charge carriers. The absorption peak of each film is directly affected by the nanoparticle size, shape, material and refractive index of the surrounding medium [2]. With increase in the size of the nanoparticle and refractive index of surrounding medium, the LSPR peak red-shifts [20]. While an addition of a sharp feature to the geometry of the nanoparticle will also leads to the red-shifting of the LSPR peaks. Apart from peak shifting, broadening of the peak can also occur as a result of a number of phenomena such as bilayer of metal films fabricated with a thin dielectric spacer will have a wider LSPR as a result of the coupling between the oscillating electric fields in two metal films [21]. And through fabricating a bimetal matrix of two noble metals with different LSPR maximum, a broad LSPR enveloping both the regions can be obtained [16]. We have used silver with LSPR at blue region and copper with LSPR at red-orange region to prepare a bimetallic thin film to tune the surface and optical properties of the film. Fig. 9 shows the optical absorbance of pure Ag and Ag–Cu bimetal thin films coated on glass substrates. The absorbance study was done on as-deposited, 100, 200, 300 and 400 °C annealed films under vacuum, to study the changes associated with the thickness variation, the inclusion of copper at various proportions and the surface structure modification. From AFM studies the lower and higher thickness films were identified and their surface morphologies were found to vary greatly with increasing thickness of the films. For lower thickness sputtered silver films (AFM images in Fig. 3), the LSPR curves are Gaussian shaped with symmetrical tails about the maximum, except for Ag11-asd for which a slight distortion in symmetry is noted with its longer wavelength edge, it is taken as transition point from symmetrical Gaussian to non-symmetrical absorption profile. For higher thickness as-deposited films, the absorption is broad without any symmetry and it extends from blue to near-IR region. For Ag18 as-deposited and 100 °C annealed films the absorption starts at 350 nm and becomes flat at 800 and 700 nm respectively. This film does not exhibit any peak in the region of our measurement. Annealing of the sputtered films helps in coalescence of smaller nanoparticles into bigger uniform nanoparticles, and thus effects the LSPR profile. In general, annealing at 100 and 200 °C does not affect the absorption profile much, in some cases the asd and annealed profiles are almost equivalent. But annealing at 300 and 400 °C has a drastic effect on the LSPR properties of the sputtered films. The appearance of as-deposited and annealed films itself displays 8

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Fig. 6. AFM images of silver sputtered films. First column shows the as-deposited films sputtered at 260–340 V (DC) and 10 W (RF). The second and third column shows their 200 and 400 °C post-annealed counterparts.

the lower thickness films display a symmetric Gaussian profile whereas the higher thickness (16 and 26 nm) films show asymmetric broad curves with absorption still persisting in red to near-IR regions. After annealing at 400 °C, there is only a slight blue-shift in the peak positions and small narrowing in the SPR. The reason for the SPR peak at 485 nm for AC16-400 and AC26-400 is because of the large number of particles present in the size range of 25–90 nm and 50–120 nm respectively

yellow indicating absorbance in green region and below. But for AC26400 film, the colour is dark grey, because of the broader absorption peak, blocking most of the visible light. The absorption profiles of Ag–Cu films are shown in Fig. 9 (b). All the as-deposited films are broad and covers orange-red through near-IR regions. Again here, annealing at 100 and 200 °C does not alter the absorption profiles of the films. Annealing at 300 °C leads to a sudden blue shift to 450–520 nm. Here, 9

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Fig. 7. Particle size distribution of the Ag–Cu sputtered films shown in histograms. The inset in each distribution shows the corresponding surface roughness profile of the films.

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Fig. 8. Growth mechanism of Ag–Cu thin metal films deposited from co-sputtering and post annealing. The first row shows the growth of lower thickness Ag–Cu film as-deposited and after annealing at 200 and 400 °C. The second row shows the growth of higher thickness Ag–Cu films under same condition.

(Figs. 4 and 7). The asymmetry in the peak shape of these films are attributed to the anisotropy in the shape of the nanoparticles. Some particles are elongated and some are irregular in shape, thus leading to lower resonance frequencies in different orientations. 3.6. SERS studies Surface Enhanced Raman Scattering (SERS) studies were taken on all as-deposited, 200 and 400 °C annealed Ag and Ag–Cu films with methylene blue (MB) as the analyte molecule. The concentration of the analyte in DI water is varied from 10−4 M to 10−8 M and 6 μL aliquot of dissolved MB in solution is drop-casted onto the sample surfaces. The drops made surface contacts at different angles resulting in different contact areas for the same volume of solution taken. This is due to the different wettability of the sample surface (hydrophobic) with different surface roughness. The samples with high surface roughness exhibited higher hydrophobicity. The solution was allowed to dry overnight to allow the MB molecules to settle down. The Raman spectra was taken

Fig. 10. Digital images of pure Ag sputtered films on glass.

Fig. 9. Optical absorbance of Ag and Ag–Cu thin films coated on glass substrate. (a) UV–visible absorption curves of silver films and (b) silver-copper bimetal films. (c,d,e) are the comparison of absorption profiles for as-deposited, 300 and 400 °C annealed film. 11

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Fig. 11. Digital images of co-sputtered Ag–Cu bimetal films on glass.

the thickness increases, the SPR peak broadens as well as strengthens and the peak maximum red-shifts to 700 nm. The increase in absorption of higher thickness films is a direct evidence of the increase in oscillator strength and the ability of the nanoparticles in the substrate to confine the light strongly. This will result in a higher local electromagnetic field strength between the coupled nanoparticles and an increased enhancement in Raman scattering of the nearby analyte molecule. Apart from oscillator strength and SPR peak positions, the surface morphology features of the substrates such as the particle number density, average particle size, homogeneity and inter-particle distance also influences the outcome of the SERS measurement [23]. For as-deposited Ag films the EF increases from Ag6-asd (1.2 × 103) to Ag14-asd (1.7 × 104) and drops suddenly for Ag18-asd (1.1 × 103). Initially the increase in EF was due to the increase in film thickness and the drop in EF is because of the decrease in surface roughness of the film over a transition from discontinuous to quasi-continuous films. The enhancement factors of 200 and 400 °C annealed Ag films also follow a similar trend of as-deposited films except for the 18 nm film which shows an EF of 104, a higher value as compared to its as-deposited parent film. It is a result of coalescence of continuous film into clustered or individual nanoparticles, restoring the strong LSPR and thus local EM field enhancement. For the Ag–Cu films, the EF is low for AC8 (asd, 200 and 400), may be due to low oscillator strength, but with increasing thickness, the EF increases rapidly to 104. The as-deposited Ag–Cu films show a consistent increase in the EF from 102 to 7.5 × 104, the highest among all films. This increase and such a high value of

on three random spots on the sample surface and averaged. The peak intensities of the Raman mode appearing in 448 cm−1 (corresponding to skeletal bending mode of methylene blue molecule) [22] from the normal Raman on 10−1 M and Surface-enhanced Raman on 10−6 M solution were measured to find the enhancement factors (EF) of all samples. 3.7. Calculation of enhancement factor The EF is a parameter that allows us to experimentally quantify the SERS sensitivity of the substrates. To facilitate comparison between samples, all measurement parameters such as laser exposure time, power and wavelength were kept constant. The EF of our substrates were calculated based on the intensities of strong vibrational band of methylene blue at 448 cm−1 using the formula: ISERS

E. F =

IRaman

NSERS NRaman

where ISERS and IRaman are the intensities from Surface Enhanced Raman scattered signal and normal Raman signal and NSERS and NRaman are the number of molecules present under the laser spot. Fig. 12(b) and (c) shows the EF values for Ag and Ag–Cu substrates. For lower thickness silver films, the EF values are very low of the order of 102 -103. The lower EF values are mainly attributed to the weak SPR centered below 500 nm and the low oscillator strength of the smaller sized particles. As 12

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Fig. 12. (a) A comparison of Raman scattered signals obtained from Methylene blue powder, methylene blue solution drop-casted on bare glass substrate and Ag–Cu SERS substrate. Enhancement factors of (b) silver and (c) silver-copper substrates found from 10−6 M solution of methylene blue. The y-axis representing the enhancement factor increments in logarithmic scale. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)

Fig. 13. Raman mapping of the intensity from 448 cm−1 peak of methylene blue obtained from (a) Ag18-200 and (c) AC16-200 films. (b) and (d) are the histograms of intensities from each data acquisition spots, the average value marked by red dotted lines. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)

has a significant effect in the confining of incident laser light (785 nm used here) in gaps between particles where the analyte molecules are also present. In case of 200 and 400 °C annealed films the EF increases for lower thickness films as compared to its as-deposited ones. But in higher thickness films, the annealed films are having a low value for EF. The reason is because of the increased interparticle distance and lower

enhancement factor is because of the higher particle number densities (closely packed), which significantly improved electromagnetic field coupling between particles. The presence of copper in smaller amount (10–15 atomic %) favours the formation of discontinuous films (substrate surface covered with nano-islands) even in higher thickness films. The SPR peak is wide and stronger in the case of 14, 16 and 26 nm Ag–Cu films. The presence of absorbance in near-IR region of spectrum 13

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particle number density. While for pure Ag films, the 200 annealed films have better sensitivity and enhancement factor (2.7 × 104), but for Ag–Cu bimetallic films the as-deposited films have the highest enhancement factor (7.5 × 104) and lowest limit of detection for methylene blue concentration up to 10−7 M.

[3]

[4]

3.8. Raman mapping [5]

To study the homogeneity of SERS enhancement of the fabricated films, Raman mapping was done on Ag18-200 and AC16-200 films with methylene blue concentration of 10−6 M. The Raman signals were obtained from 121 (11 × 11) spots in a square array of points as shown in Fig. 13 (a) and (c). The horizontal and vertical points are separated by a distance of 1.5 μm whereas the laser spot size spot for 785 nm wavelength and 0.75 numerical aperture (N.A) is calculated to be 1.28 μm is centered on these points during signal acquisition. The signal collected from each of these points is processed to obtain the histograms given in Fig. 13 (b) and (d) for Ag18-200 and AC16-200 samples. The histograms represent the intensities from each spot and the average intensities are 22090 and 12162 counts for these films. The standard deviation in the intensity values from these films are 2283 and 872 counts for Ag18-200 and AC16-200. The relative errors are found to be 10.34% and 7.17% which suggest an excellent homogeneity in enhancement of SERS intensities.

[6]

[7]

[8]

[9]

[10]

4. Conclusion [11]

The silver and silver-copper bimetal thin films prepared using magnetron co-sputtering technique show a broad and strong SPR in the red and near IR regions. The Ag14-200 film show enhancement factor up to 2.5 × 104, while AC26-asd bimetal film shows the highest enhancement factor of 7.5 × 104 which is roughly three times more than the Ag14-200 film. The particle size of 30 ± 8 nm with average interparticle gap of 5.7 nm and unique plasmonic property of Ag–Cu bimetal film contribute to the high EF. The detection sensitivity for the lowest limit of detection of the homogeneous AC26-asd film is up to 10−7 M for methylene blue. The fabricated substrates can be used with a wide range of excitation wavelengths owing to its broad and uniform SPR.

[12]

[13]

[14]

[15]

[16]

Declaration of competing interest

[17]

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

[18]

[19]

Acknowledgements [20]

Authors thank SERB-DST, India for providing financial support (SR/ FTP/PS-131/2012 and EEQ/2016/000228). Authors thank Dr. D. Selvakumar, Scientist-F, DEBEL (DRDO), Bangalore for the EDS mapping. PNG thanks Pondicherry University for providing research fellowship.

[21]

[22]

References

[23]

[1] M. Moskovits, Surface-enhanced spectroscopy, Rev. Mod. Phys. 57 (3) (1985) 783–826 https://doi.org/10.1103/RevModPhys.57.783. [2] M. Rycenga, C.M. Cobley, J. Zeng, W. Li, C.H. Moran, Q. Zhang, D. Qin, Y. Xia,

14

Controlling the synthesis and assembly of silver nanostructures for plasmonic applications, Chem. Rev. 111 (2011) 3669–3712 https://doi.org/10.1021/cr100275d. Z. Song, Z. Chen, X. Bian, L. Zhou, D. Ding, H. Liang, Y. Zou, S. Wang, L. Chen, C. Yang, X. Zhang, W. Tan, Alkyne-functionalized superstable graphitic silver nanoparticles for Raman imaging, J. Am. Chem. Soc. 136 (2014) 13558–13561 https://doi.org/10.1021/ja507368z. M.A. Wall, T.M. Shaffer, S. Harmsen, D. Tschaharganeh, C. Huang, S.W. Lowe, C.M. Drain, M.F. Kircher, Chelator-free radiolabeling of SERRS nanoparticles for whole-body PET and intraoperative Raman imaging, Theranostics 7 (12) (2017) 3068–3077 https://doi.org/10.7150/thno.18019. A.K. Pal, S. Pagal, K. Prashanth, G.K. Chandra, S. Umapathy, D.B. Mohan, Ag/ZnO/ Au 3D hybrid structured reusable SERS substrate as highly sensitive platform for DNA detection, Sens. Actuators B Chem. 279 (2019) 157–169 https://doi.org/10. 1016/j.snb.2018.09.085. J. Zou, W. Song, W. Xie, B. Huang, H. Yang, Z. Luo, A simple way to synthesize large-scale Cu2O/Ag nanoflowers for ultrasensitive surface-enhanced Raman scattering detection, Nanotechnology 29 (2018) 115703https://doi.org/10.1088/13616528/aaa72b. P.X. Chen, S.B. Shang, L.T. Hu, X.Y. Liu, H.W. Qiu, C.H. Li, Y.Y. Huo, S.Z. Jiang, C. Yang, A suitable for large scale production, flexible and transparent surface enhanced Raman scattering substrate for in situ ultrasensitive analysis of chemistry reagents, Chem. Phys. Lett. 660 (2016) 169–175 https://doi.org/10.1016/j.cplett. 2016.08.027. A.C. Sant'Ana, T.C.R. Rocha, P.S. Santos, D. Zanchet, M.L.A. Temperini, Size-dependent SERS enhancement of colloidal silver nanoplates: the case of 2-amino-5nitropyridine, J. Raman Spectrosc. 40 (2009) 183–190 https://doi.org/10.1002/jrs. 2103. K. Kneipp, Y. Wang, R.R. Dasari, M.S. Feld, Approach to single molecule detection using surface-enhanced resonance Raman scattering (SERRS): a study using rhodamine 6G on colloidal silver, Appl. Spectrosc. 49 (1995) 780–784 https://doi.org/ 10.1366/0003702953964480. W. Sun, R. Hong, Q. Liu, Z. Li, J. Shi, C. Tao, D. Zhang, SERS-active Ag-Al alloy nanoparticles with tunable surface plasmon resonance induced by laser ablation, Opt. Mater. 96 (2019) 109298https://doi.org/10.1016/j.optmat.2019.109298. L. Wang, F. Wang, L. Shang, C. Zhu, W. Ren, S. Dong, AuAg bimetallic nanoparticles film fabricated based on H2O2-mediated silver reduction and its application, Talanta 82 (2010) 113–117 https://doi.org/10.1016/j.talanta.2010.04.009. M. Hirai, A. Kumar, Wavelength tuning of surface plasmon resonance by annealing silver-copper nanoparticles, J. Appl. Phys. 100 (2006) 014309https://doi.org/10. 1063/1.2214213. M.S.S. Bharati, B. Chandu, S.V. Rao, Explosives sensing using Ag–Cu alloy nanoparticles synthesized by femtosecond laser ablation and irradiation, RSC Adv. 9 (2019) 1517–1525 https://doi.org/10.1039/c8ra08462a. X. Zhang, S. Xu, S. Jiang, J. Wang, J. Wei, S. Xu, S. Gao, H. Liu, H. Qiu, Z. Li, H. Liu, Z. Li, H. Li, Growth graphene on silver–copper nanoparticles by chemical vapour deposition for high-performance surface-enhanced Raman scattering, Appl. Surf. Sci. 353 (2015) 63–70 https://doi.org/10.1016/j.apsusc.2015.06.084. V.K. Rao, P. Ghildiyal, T.P. Radhakrishnan, In situ fabricated Cu-Ag nanoparticleembedded polymer thin film as an efficient broad spectrum SERS substrate, J. Phys. Chem. C 121 (2017) 1339–1348 https://doi.org/10.1021/acs.jpcc.6b10238. A.K. Pal, D.B. Mohan, SERS enhancement, sensitivity and homogeneity studies on bi-metallic Ag-Cu films through tuning of broad band SPR towards red region, J. Alloy. Comp. 698 (2016) 460–468 https://doi.org/10.1016/j.jallcom.2016.12.246. N. Laegreid, G.K. Wehner, Sputtering yields of metals for Ar+ and Ne+ ions with energies from 50 to 600 ev, J. Appl. Phys. 32 (1961) 365–369 https://doi.org/10. 1063/1.1736012. S.C. Seel, C.V. Thompson, Tensile stress evolution during deposition of Volmer–Weber thin films, J. Appl. Phys. 88 (2000) 7079–7088 https://doi.org/10. 1063/1.1325379. M. Gnanavel, D.B. Mohan, C.S. Sunandana, Optics of quasi-particle phase transitions in nanostructured Ag thin films, Thin Solid Films 517 (2008) 1058–1062 https://doi.org/10.1016/j.tsf.2008.06.044. T.R. Jensen, M.D. Malinsky, C.L. Haynes, R.P.V. Duyne, Nanosphere lithography: tunable localized surface plasmon resonance spectra of silver nanoparticles, J. Phys. Chem. B 104 (2000) 10549–10556 https://doi.org/10.1021/jp002435e. O.A. Yeshchenko, V.V. Kozachenko, A.P. Naumenko, N.I. Berezovska, N.V. Kutsevol, V.A. Chumachenko, M. Haftel, A.O. Pinchuk, Gold nanoparticle plasmon resonance in near-field coupled Au NPs layer/Al film nanostructure: dependence on metal film thickness, Photonics Nanostruct.: Fundam. Appl. 29 (2018) 1–7 https://doi.org/10.1016/j.photonics.2017.12.005. G. Xiao, S. Man, Surface-enhanced Raman scattering of methylene blue adsorbed on cap-shaped silver nanoparticles Chem, Phys. Lett. 447 (2007) 305–309 https://doi. org/10.1016/j.cplett.2007.09.045. A.K. Pal, D.B. Mohan, Structural, morphological and optical properties of Ag–AgO thin films with the effect of increasing film thickness and annealing temperature, Opt. Mater. 48 (2015) 121–132 https://doi.org/10.1016/j.optmat.2015.07.029.