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Ag-Zr alloy films on flexible polyimide as SERS substrates
Applied Surface Science 487 (2019) 1341–1347
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Self-formation of Ag particles/Ag-Zr alloy films on flexible polyimide as SERS substrates ⁎
T
⁎
X.X. Huanga, H.L. Suna,b, , G.X. Wanga, , H.R. Stocka a b
School of Materials Science and Engineering, Henan University of Science and Technology, Luoyang 471003, China Collaborative Innovation Center of Nonferrous Metals Henan Province, Luoyang 471003, China
A R T I C LE I N FO
A B S T R A C T
Keywords: AgeZr alloy film Flexible polyimide Annealing Ag particles SERS substrate
AgeZr alloy films were deposited on flexible polyimide (PI) substrates by magnetron sputtering and subsequently subjected to vacuum annealing. Effects of annealing temperature, Zr content and film thickness on formation of Ag particle and the surface enhanced Raman scattering property of Ag particles/AgeZr alloy films have been investigated. Results show that microstructure of AgeZr alloy film changes significantly after annealing and a large number of regular polyhedral Ag particles spontaneously formed on the surface of annealed AgeZr alloy films. The Ag particles are single crystal and uniformly distributed on the AgeZr alloy films surface to form the Ag particles/AgeZr alloy films composite structure with large specific surface area. The detection limit of R6G concentration using Ag particles/AgeZr alloy films covered by 12 nm Ag film as SERS substrate is significantly lower than 5 × 10−8 M (1 M = 1 mol/L) due to its abundant active “hot spots”. The particle/film composite structure obtained in this paper provides a method to prepare the highly efficient SERS substrates.
1. Introduction Silver (Ag) particles and film have been widely applied in optoelectronics, catalysis, sensing, medicine, and manufacturing due to their unique electrical and optical properties [1–3]. Ag particles can be used as an antibacterial agent in various textiles, natural fabrics, polymers, industrial products to inhibit bacteria [4], and printed electronic materials to prepare electronic circuits [5]. Since the observation of a surface-enhanced Raman scattering (SERS) signal from roughened silver electrodes adsorbed pyridine, SERS phenomenon had been observed mainly in Cu, Ag, Au, Pt materials [6–12] and widely applied to biological environment detection. Binding kinetics of DNA hybridization were reliably detected by using a multi-channel graphene biosensor [13]. Graphene/Cu nanoparticle hybrids [14] and graphene isolated Au nanoparticle arrays [15] as SERS substrates were used for label-free detection of adenosine. 3D and flexible plasmonic structure [16] and the flexible gyrus-SERS substrates [17] were applied to detect melamine in milk and magnesium residue on prawn skin, respectively. Previous researches show that SERS property is closely related to the Ag particle size, morphology and distribution of different shapes [18,19]. Ag particles with different shapes such as cubic structures, prisms, nanorods, nanowires, triangles, decahedron, icosahedrons [20] can be prepared by various methods. Usually, Ag particles are free-state particles and easy to agglomerate, and it is difficult to fix the particles on ⁎
the surface of the substrates or the films. Raman activity of SERS substrates will be greatly improved by combing particle SERS substrates with high detection sensitivity and thin film SERS substrates with good repeatability. As far as I am concerned that it is difficult to make Ag particles uniformly distribute on the surface of the film to obtain effective SERS substrates without templates. In view of Ag materials is one of the best materials for making SERS active surface, our recent work is focused on preparing Ag particles/Ag-Zr alloy films composite materials with high specific surface area without template as SERS substrates. As well known, all the sputtered films are almost in a certain stress state. Stress is generally considered to be a detrimental factor affecting stability of thin film. If the stress in the film cannot be released and will lead to film crack or even peel off. In order to analyze the evolution behaviors of residual stress and inhibit its harmful effects, many researchers have developed a lot of research on causes of stress, and proposed many theoretical models such as heat shrinkage effect, lattice defect elimination, impurity effect, etc. [21]. However, our work intends to utilize the stress relaxation to drive the Ag atom diffusion in the AgeZr alloy film to form Ag particles on alloy film surface. In the present work, we will investigate effects of annealing temperature, Zr content and film thickness on formation of Ag particle and the surface enhanced Raman scattering property of Ag particles/AgeZr alloy films.
Corresponding authors. E-mail addresses: [email protected] (H.L. Sun), [email protected] (G.X. Wang).
https://doi.org/10.1016/j.apsusc.2019.05.181 Received 5 March 2019; Received in revised form 30 April 2019; Accepted 15 May 2019 Available online 17 May 2019 0169-4332/ © 2019 Elsevier B.V. All rights reserved.
Applied Surface Science 487 (2019) 1341–1347
X.X. Huang, et al.
Intensity a. u.
width at half maximum [FWHM] of the film diffraction peak at 2θ in radian and θ is the Bragg diffraction angle in degree.
Ag film Ag-Zr alloy film
Ag (111)
3. Results and discussion
PI Ag (200)
45 nm
Ag (220) Ag (311) Ag (222)
66 nm
PI
3.1. XRD patterns of AgeZr alloy films Fig. 1 shows typical XRD patterns of AgeZr alloy films deposited on PI substrates. All alloy films predominately consist of Ag phase without any detectable Zr or other phases after annealed at 360 °C. It can be seen that Ag film and Ag-15.41%Zr alloy film possess (111) preferred orientation, as shown in Fig. 1. With increase of film thickness, intensity of Ag (200) diffraction peak decreases and Ag (111) diffraction peak increases. Compared with pure Ag films, AgeZr alloy film has a weakly broadened Ag (111) diffraction peak which suggests that grains in the AgeZr alloy film are very fine due to the inhibition effect of Zr on the growth of Ag grains. In addition, the weak diffraction peak of 69.149 deg. in the XRD patterns of 13, 45 and 65 nm Ag-15.41%Zr alloy films is the diffraction peak of the PI substrate, as shown in Fig. 1. In order to further analyze the crystallization properties of films with different thickness, parameters such as full width at half maximum [FWHM], diffraction peak intensity were calculated. Table 1 shows that the crystal parameters of Ag-15.41%Zr alloy films with different thickness were calculated by using formula (1) and Bragg formula (2d sin θ = kλ). As shown in Table 1, with the increase of film thickness, Ag (111) diffraction peak intensity increased and peak widths at half height decreased gradually, which indicated that crystallization of Ag grains became more obvious. The calculation result shows the grain size of AgeZr alloy films increase with the increase of film thickness.
45 nm 13 nm PI 30
45
60
2
75
90
Degree
Fig. 1. XRD patterns of 360 °C annealed Ag film and Ag-15.41%Zr alloy films with different thickness and PI substrate.
2. Sample preparation and characterization methods AgeZr alloy films were prepared on Polyimide (PI, thickness 125 μm) substrates by DC-sputtering deposition using a silver (Ag) target (99.99%, purity, Ø50 mm × 4 mm) overlaid with zirconium (Zr) plates (99.99%, purity，10 mm × 10 mm × 1 mm) in a JCP-350 magnetron sputtering machine. Zr components were controlled by changing the number of Zr plates. Substrates were ultrasonically cleaned with anhydrous alcohol. Pre-sputtering was carried out for 10 min with 100 W power to remove contamination on the target surface. The base pressure of the vacuum chamber was 5 × 10−4 Pa and the sputtering pressure was maintained at 0.5 Pa. Sputtering power, substrate holder rotation and target to substrate distance were fixed at 120 W, 30 r/m and 70 mm, respectively. The different thickness films were obtained by varying deposition time (1 min, 2.5 min, 5 min). Samples were annealed at different temperatures (160 °C, 260 °C, and 360 °C) for 1 h in the vacuum chamber. And then some annealed AgeZr alloy films were deposited 12 nm thickness pure Ag films to prepare SERS substrates. Rhodamine 6G (R6G) dye was used as Raman probe for SERS measurements. For preparation of SERS substrates, samples were immersed in R6G solutions (100 m L) for 10 min. After solution was completely dried in air, substrates were analyzed using a Laser microRaman spectrometer (Renishaw InVia) with a laser wavelength of 632.18 nm. The laser excitation energy and spot respectively were 5 mW and 2 μm. The diffraction grid was 1200 g/mm. A 50 × objective (N.A. = 0.80) and the integration time of 1 s were used in the present work. X-ray diffractometer (XRD, D 8 advance) and transmission electron microscope (TEM, JEM-2100) were used to characterize phase and crystal structure of the alloy films and Ag particles. Surface morphology and composition of alloy films as well as self-formed Ag particles on film surface were observed and confirmed by field emission scanning electron microscope (FESEM, JSM-7800F) and energy dispersive spectroscopy (EDS), respectively. The mean grain sizes (D) of AgeZr alloy films were calculated by Scherrer formula [22].
D = 0.9λ β cos θ
3.2. Effect of film thickness on the surface morphology of Ag-15.41%Zr alloy films Fig. 2 shows surface morphology of the annealed Ag film and Ag15.41%Zr alloy films with different thickness. It can be seen that the surface of the annealed 360 °C Ag film is rough and the grain size of the film is about 60–500 nm, as shown in Fig. 2(a). The grains in the asdeposited AgeZr alloy films are very fine and uniform due to the inhibiting effect of Zr atoms on Ag grain growth, as shown in Fig. 2(b). Fig. 2(c), (d) and (e) are the surface morphology of 360 °C annealed AgeZr alloy films with 15.41% Zr contents proved by EDS result in Fig. 2(g). It is worth noting that a large number of faceted particles with different sizes spontaneously formed on the annealed AgeZr alloy films' surface, which hasn't been reported previously. The self-formed particles have feature sizes ranging from several tens of nanometers to about 1 μm. According to the EDS result, the compositions of the particles are Ag, as shown in Fig. 2(f). Microstructure evolution behaviors of the AgeZr alloy films on flexible PI substrates after annealing is obviously different from that of annealed AgeCo alloy films on rigid substrates reported in previous researches [23]. The microstructure characteristics of the Ag particles/AgeZr alloy films composite microstructure differ from the traditional magnetic granular films obtained in the annealed AgeCo alloy film [23]. As can be seen from Fig. 2(c), (d) and (e), the average size of Ag particles increases and particle number in the same area decreases monotonously with increasing film thickness, as shown
(1)
whereby λ is the X-ray wavelength (0.15406 nm), and β is the full Table 1 Crystal parameters of Ag-15.41%Zr alloy films with different thickness. Thickness (nm) 13 45 66
2-Theta (deg) 38.169 38.180 38.239
Interplanar spacing (Å)
FWHM (deg)
Diffraction peak intensity of Ag (111) (s−1)
2.3517 2.3552 2.3559
0.359 0.240 0.224
83 127 153
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(a)
(b)
(c)
(d)
(e)
(f)
1
2
Zr
Ag
600
(h)
Variation of Average Particle Number Mean Particle Size Variation
18 15 12
500
9 400
6 3
300
The Number of Particles
(g)
Atomic percentage
Mean Size of Ag particles nm
700
Element
0 200
-3 10
20
30
40
50
60
70
Thin Film Thickness nm
Fig. 2. Surface morphology of Ag films and Ag-15.41% Zr alloy films with different thickness: (a) Ag film of 45 nm thickness annealed at 360 °C, (b) as-deposited Ag15.41% Zr alloy films, (c) and (d) and (e) annealed 360 °C Ag-15.41% Zr alloy films with 13, 45, 66 nm thickness, respectively, (f) EDS of a particle, (g) EDS of AgeZr alloy films, (h) Variation of particle size and number with different film thickness.
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Fig. 3. Surface morphology of annealed 160 °C AgeZr alloy films with (a) 5.37%, (b) 15.41% and (c) 32.62%, respectively, (d) Variation of particle size and number with different Zr contents.
(a)
(b)
(c)
(d)
Fig. 4. FESEM images of 45 nm thickness Ag-15.41%Zr alloy films annealed at (a) 160 °C and (b) 360 °C, (c) Magnification of typical Ag particles obtained by annealing at 360 °C, (d) The SAED patterns of the faceted Ag particles. 1344
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Fig. 5. Schematic diagram of Ag particles/AgeZr alloy films formation.
films obtained in the present work can be used as a universal “template” to grow different kinds of nano-sized finer particles [30]. In order to obtain Ag particles/AgeZr alloy films with large specific surface area, a thin Ag film can be deposited on the surface of Ag particles/AgeZr alloy films, as shown in Fig. 5(c).
in Fig. 2(h). 3.3. Effect of Zr content on the size of Ag particles Surface morphology of annealed 160 °C AgeZr alloy films with different Zr content on the PI substrate is shown in Fig. 3. As can be seen from Fig. 3(a), (b), (c) and (d) that the Ag particle size gradually decreases and particle number increases with the increase of Zr content. The mean size of Ag particles counted by IPP software on the surface of Ag-5.37%Zr, Ag-15.41%Zr, and Ag-32.62%Zr alloy films are 440, 187 and 111 nm, respectively, which can be ascribed to increase of Zr content inhibiting the diffusion of silver atoms [24].
3.6. SERS activity of Ag/Ag-Zr alloy films Fig. 6(a) shows the Raman spectra of Ag and Ag-15.41%Zr alloy films (thickness: 45 nm) as SERS substrates with different treatment process and soaked by R6G dye 5 × 10−6 M. It can be seen in Fig. 6(a) that no SERS peak can be detected in the Ag films annealed at 360 °C due to its relatively smooth surface. Obviously, SERS peak intensity of annealed Ag-15.41%Zr alloy films is stronger than that of deposited AgeZr alloy film and pure Ag film, as shown in Fig. 6(a). The average sizes of Ag particles on the annealed 360 °C Ag-15.41% Zr alloy films surface are 315 nm, which is larger than the size of 20–100 nm of Ag particles with the most SERS performance [30,31]. In view of this, we systematically studied the effect of film thickness on SERS activity of 360 °C annealed Ag-15.41%Zr alloy films as shown in Fig. 6(b). It can be seen that Raman signal is strongest when the thickness is 13 nm. This is because with the increase of film thickness, the number of particles on the films surface decreases gradually, and the size increases gradually in Fig. 2(h), which leads to the gap between adjacent nanoparticles enlarges and the number in decreases, as a result, the number of R6G probe molecules adsorbed on SERS substrate decreased. Fig. 6(c) shows the effect of Zr content on SERS activity of 160 °C annealed AgeZr alloy films with the same film thickness of 13 nm. It was found that the alloy films as SERS substrates with higher Zr content had better SERS performance due to increase of Zr content inhibiting the diffusion of Ag atoms and resulting in the smaller Ag particles (approaching 100 nm). In general, 13 nm thickness AgeZr alloy films annealed at 360 °C with higher Zr content have better surface-enhanced Raman scattering effect. Combined with Fig. 6(a), (b) and (c), hence, we choose 360 °C annealed Ag-15.41% Zr alloy films (thickness: 13 nm) as optimized SERS substrates (Ag particles/AgeZr alloy films). Subsequently, aforementioned optimized SERS substrates were covered by 12 nm Ag films as studied SERS substrates soaked by R6G dye. Comparing Fig. 6(a) and (d) it can be seen that the SERS activity of Ag particles/AgeZr alloy films covered by 12 nm Ag films is significantly better than that of Ag film and AgeZr alloy films. R6G solutions with the concentration of 5 × 10−5 M, 5 × 10−6 M, 5 × 10−7 M, 5 × 10−8 M and 5 × 10−9 M were prepared to soak the samples in order to investigate the concentration-dependent variation tendency of the Raman signal. R6G solution with lower concentration produced weaker Raman scattering signal, as shown in Fig. 6(d). It is noted that the Raman peak of the samples soaked in the 5 × 10−9 R6G solution is very low and not enough to accurately identify the existence of R6G. Fig. 6(f) shows that the SERS intensities of peak at 613 cm−1 and 771 cm−1 gradually decrease with the decrease of R6G concentration. The Raman peak at 613 cm−1 belongs to the in-plane deformation vibration of the ring [32,33] and vibrations at 771 cm−1 are assigned to CeH in-plane bending. The Raman peaks at 1362, 1514 and 1647 cm−1 are caused by CeC bond stretching vibration of R6G aromatic rings. Previous studies show that SERS signal mainly comes from probe molecules adsorbed at the “hot spots” of substrates. These “hot spots” include surfaces of metal nanoparticles with appropriate size
3.4. Effect of annealing temperature on the size of Ag particles Fig. 4 shows morphology of Ag particles on the surface of 45 nm thick Ag-15.41%Zr alloy films with different annealing temperatures, and the selected area electron diffraction (SAED) patterns of the Ag particle. It can be seen from Fig. 4(a) and (b), the average size of Ag particles increases with improving annealing temperatures. The average size of Ag particles in Fig. 4(a) and (b) is 187, and 315 nm, respectively. As the annealing temperature is increased, the average size and particles number increase because of more active diffusion of Ag atoms with the higher energy. Meanwhile the stress in the annealed alloy films will be further relaxed and inevitably lead to the growth of Ag particles. It is interested to note that unlike the Ag hillocks in the annealed Ag films [25], most of Ag particles obtained in this work are very regular and polyhedral with clear edges and corners. Fig. 4(c) shows the high magnification of a typical icosahedral Ag particle obtained by annealing at 360 °C for 1 h in the vacuum chamber. The SAED patterns of the faceted Ag particles indicate that they are single crystals, as shown in Fig. 4(d). As can be seen in Fig. 4(c), the grains in the regions of the AgeZr alloy films without self-formed Ag particles indicated by the blue ring are very fine and uniform due to inhibiting effect of Zr on Ag grains growth. 3.5. The formation mechanism of polyhedral Ag particles Based on the results, it is demonstrated that the sizes of faceted Ag particles obtained in the present work range from dozens of nanometers to about 1 μm and can be adjusted by modifying the Zr content, film thickness and annealing temperature. It is demonstrated that the formation mechanism of the Ag particles on PI substrates is same as that of regular Cu particles on annealed CueZr alloy films [26] and different from that of hillocks observed in pure metal thin films on Si substrates [27–29]. It can be inferred that the faceted particles are indeed formed through mass transportation on surfaces and grain boundaries driven by the relaxation of residual stress, thermal stress and distortion energy in the alloy films. In this process, some atoms are aggregated to form clusters, and they grow into small Ag particles at trigeminal grain boundaries and voids on the AgeZr alloy films surface. Surface diffusion dominates mass transportation during the growth of Ag particles due to the grain boundary diffusion of Ag atoms in the AgeZr alloy films inhibiting by Zr atoms. When the particle size becomes large enough, faceting becomes the most favorable process, as shown in Fig. 5(b), in which the minimization of surface and strain energy is the thermodynamic driving force. Obviously, the Ag particles/AgeZr alloy 1345
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Fig. 6. SERS spectra and morphology of R6G absorbed on the Ag films and AgeZr alloy films and Ag particles/AgeZr alloy films: (a) SERS spectra of 5 × 10−6 M R6G absorbed on the annealed 360 °C Ag films and AgeZr alloy films, (b) SERS spectra of 5 × 10−6 M R6G absorbed on annealed 360 °C Ag-15.41%Zr alloy films with different thickness:13 nm, 45 nm, 66 nm, (c) annealed 160 °C AgeZr alloy films (film thickness:13 nm) with different Zr content: 5.37%, 15.41%, 32.62%, (d) SERS spectra of R6G absorbed on Ag particles/AgeZr alloy films covered by 12 nm Ag films: a with concentrations ranging from 5 × 10−5 to 5 × 10−9 M and (e) 5 × 10−9 M (magnified), (f) SERS intensities variation of peak at 613 cm−1 and 771 cm−1 with different R6G concentration, (g) morphology of Ag particles/AgeZr alloy films covered by 12 nm Ag films, (h) Magnification of the red box area of panel (g). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
[33,34] and interstices between nanoparticles. Fig. 6(g) shows that the morphology of Ag particles/AgeZr alloy films covered by 12 nm Ag films which consists of many tiny Ag particles. It can be found that some Ag particles formed on the surface of annealed AgeZr alloy films exhibit a highly symmetrical icosahedron with sharp edges and corners which are SERS “hot spots” and good for adsorbing more R6G molecules. Furthermore, the interstices (see the red arrows in Fig. 6(h)) between large polyhedral Ag particles and the gap (see the green arrows in Fig. 6(h)) of tiny Ag particles provide a large number of “hot spots” and contribute greatly to the SERS performance of the composite structures. Combining surface morphology of particles/films composite structure, it can be inferred that highly symmetrical polyhedral silver nanoparticles play a key role in SERS and can be used as a universal “template” to grow different kinds of nanosized finer particles.
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4. Conclusion Polyhedral Ag particles with different sizes self-formed on the surface of films are obtained annealed AgeZr alloy films on flexible PI. It was interesting to find that the Ag particles are regular and single crystal. Further analysis indicates that the formation of Ag particles results from the Ag atoms diffusion driven by the relaxation of residual stress, thermal stress and distortion energy in the annealed alloy films. The sizes of regular Ag particles range from dozens of nanometers to about 1 μm and can be adjusted by modifying the Zr content, film thickness and annealing temperature. The Ag particles/AgeZr alloy films obtained in the present work can be used as a universal “template” to grow different kinds of nano-sized finer particles and prepare alloy films with large specific surface area. The detection limit of R6G concentration using Ag particles/Ag-Zr alloy films covered by 12 nm Ag film as SERS substrates is significantly lower than 5 × 10−8 M due to its abundant active “hot spots”. Acknowledgements This work was financially supported by the National Natural Science Foundation of China (Grant No. U12041869), the Chinese 02 Special Fund (Grant No. 2017ZX02408003) and the Chinese 1000 Plan for High Level Foreign Experts (Grant No. WQ20154100278). References [1] M.A. Shenashen, S.A. El-Safty, E.A. Elshehy, Part. Part. Syst. Charact. 31 (2014)
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Full length article
Self-formation of Ag particles/Ag-Zr alloy films on flexible polyimide as SERS substrates ⁎
T
⁎
X.X. Huanga, H.L. Suna,b, , G.X. Wanga, , H.R. Stocka a b
School of Materials Science and Engineering, Henan University of Science and Technology, Luoyang 471003, China Collaborative Innovation Center of Nonferrous Metals Henan Province, Luoyang 471003, China
A R T I C LE I N FO
A B S T R A C T
Keywords: AgeZr alloy film Flexible polyimide Annealing Ag particles SERS substrate
AgeZr alloy films were deposited on flexible polyimide (PI) substrates by magnetron sputtering and subsequently subjected to vacuum annealing. Effects of annealing temperature, Zr content and film thickness on formation of Ag particle and the surface enhanced Raman scattering property of Ag particles/AgeZr alloy films have been investigated. Results show that microstructure of AgeZr alloy film changes significantly after annealing and a large number of regular polyhedral Ag particles spontaneously formed on the surface of annealed AgeZr alloy films. The Ag particles are single crystal and uniformly distributed on the AgeZr alloy films surface to form the Ag particles/AgeZr alloy films composite structure with large specific surface area. The detection limit of R6G concentration using Ag particles/AgeZr alloy films covered by 12 nm Ag film as SERS substrate is significantly lower than 5 × 10−8 M (1 M = 1 mol/L) due to its abundant active “hot spots”. The particle/film composite structure obtained in this paper provides a method to prepare the highly efficient SERS substrates.
1. Introduction Silver (Ag) particles and film have been widely applied in optoelectronics, catalysis, sensing, medicine, and manufacturing due to their unique electrical and optical properties [1–3]. Ag particles can be used as an antibacterial agent in various textiles, natural fabrics, polymers, industrial products to inhibit bacteria [4], and printed electronic materials to prepare electronic circuits [5]. Since the observation of a surface-enhanced Raman scattering (SERS) signal from roughened silver electrodes adsorbed pyridine, SERS phenomenon had been observed mainly in Cu, Ag, Au, Pt materials [6–12] and widely applied to biological environment detection. Binding kinetics of DNA hybridization were reliably detected by using a multi-channel graphene biosensor [13]. Graphene/Cu nanoparticle hybrids [14] and graphene isolated Au nanoparticle arrays [15] as SERS substrates were used for label-free detection of adenosine. 3D and flexible plasmonic structure [16] and the flexible gyrus-SERS substrates [17] were applied to detect melamine in milk and magnesium residue on prawn skin, respectively. Previous researches show that SERS property is closely related to the Ag particle size, morphology and distribution of different shapes [18,19]. Ag particles with different shapes such as cubic structures, prisms, nanorods, nanowires, triangles, decahedron, icosahedrons [20] can be prepared by various methods. Usually, Ag particles are free-state particles and easy to agglomerate, and it is difficult to fix the particles on ⁎
the surface of the substrates or the films. Raman activity of SERS substrates will be greatly improved by combing particle SERS substrates with high detection sensitivity and thin film SERS substrates with good repeatability. As far as I am concerned that it is difficult to make Ag particles uniformly distribute on the surface of the film to obtain effective SERS substrates without templates. In view of Ag materials is one of the best materials for making SERS active surface, our recent work is focused on preparing Ag particles/Ag-Zr alloy films composite materials with high specific surface area without template as SERS substrates. As well known, all the sputtered films are almost in a certain stress state. Stress is generally considered to be a detrimental factor affecting stability of thin film. If the stress in the film cannot be released and will lead to film crack or even peel off. In order to analyze the evolution behaviors of residual stress and inhibit its harmful effects, many researchers have developed a lot of research on causes of stress, and proposed many theoretical models such as heat shrinkage effect, lattice defect elimination, impurity effect, etc. [21]. However, our work intends to utilize the stress relaxation to drive the Ag atom diffusion in the AgeZr alloy film to form Ag particles on alloy film surface. In the present work, we will investigate effects of annealing temperature, Zr content and film thickness on formation of Ag particle and the surface enhanced Raman scattering property of Ag particles/AgeZr alloy films.
Corresponding authors. E-mail addresses: [email protected] (H.L. Sun), [email protected] (G.X. Wang).
https://doi.org/10.1016/j.apsusc.2019.05.181 Received 5 March 2019; Received in revised form 30 April 2019; Accepted 15 May 2019 Available online 17 May 2019 0169-4332/ © 2019 Elsevier B.V. All rights reserved.
Applied Surface Science 487 (2019) 1341–1347
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Intensity a. u.
width at half maximum [FWHM] of the film diffraction peak at 2θ in radian and θ is the Bragg diffraction angle in degree.
Ag film Ag-Zr alloy film
Ag (111)
3. Results and discussion
PI Ag (200)
45 nm
Ag (220) Ag (311) Ag (222)
66 nm
PI
3.1. XRD patterns of AgeZr alloy films Fig. 1 shows typical XRD patterns of AgeZr alloy films deposited on PI substrates. All alloy films predominately consist of Ag phase without any detectable Zr or other phases after annealed at 360 °C. It can be seen that Ag film and Ag-15.41%Zr alloy film possess (111) preferred orientation, as shown in Fig. 1. With increase of film thickness, intensity of Ag (200) diffraction peak decreases and Ag (111) diffraction peak increases. Compared with pure Ag films, AgeZr alloy film has a weakly broadened Ag (111) diffraction peak which suggests that grains in the AgeZr alloy film are very fine due to the inhibition effect of Zr on the growth of Ag grains. In addition, the weak diffraction peak of 69.149 deg. in the XRD patterns of 13, 45 and 65 nm Ag-15.41%Zr alloy films is the diffraction peak of the PI substrate, as shown in Fig. 1. In order to further analyze the crystallization properties of films with different thickness, parameters such as full width at half maximum [FWHM], diffraction peak intensity were calculated. Table 1 shows that the crystal parameters of Ag-15.41%Zr alloy films with different thickness were calculated by using formula (1) and Bragg formula (2d sin θ = kλ). As shown in Table 1, with the increase of film thickness, Ag (111) diffraction peak intensity increased and peak widths at half height decreased gradually, which indicated that crystallization of Ag grains became more obvious. The calculation result shows the grain size of AgeZr alloy films increase with the increase of film thickness.
45 nm 13 nm PI 30
45
60
2
75
90
Degree
Fig. 1. XRD patterns of 360 °C annealed Ag film and Ag-15.41%Zr alloy films with different thickness and PI substrate.
2. Sample preparation and characterization methods AgeZr alloy films were prepared on Polyimide (PI, thickness 125 μm) substrates by DC-sputtering deposition using a silver (Ag) target (99.99%, purity, Ø50 mm × 4 mm) overlaid with zirconium (Zr) plates (99.99%, purity，10 mm × 10 mm × 1 mm) in a JCP-350 magnetron sputtering machine. Zr components were controlled by changing the number of Zr plates. Substrates were ultrasonically cleaned with anhydrous alcohol. Pre-sputtering was carried out for 10 min with 100 W power to remove contamination on the target surface. The base pressure of the vacuum chamber was 5 × 10−4 Pa and the sputtering pressure was maintained at 0.5 Pa. Sputtering power, substrate holder rotation and target to substrate distance were fixed at 120 W, 30 r/m and 70 mm, respectively. The different thickness films were obtained by varying deposition time (1 min, 2.5 min, 5 min). Samples were annealed at different temperatures (160 °C, 260 °C, and 360 °C) for 1 h in the vacuum chamber. And then some annealed AgeZr alloy films were deposited 12 nm thickness pure Ag films to prepare SERS substrates. Rhodamine 6G (R6G) dye was used as Raman probe for SERS measurements. For preparation of SERS substrates, samples were immersed in R6G solutions (100 m L) for 10 min. After solution was completely dried in air, substrates were analyzed using a Laser microRaman spectrometer (Renishaw InVia) with a laser wavelength of 632.18 nm. The laser excitation energy and spot respectively were 5 mW and 2 μm. The diffraction grid was 1200 g/mm. A 50 × objective (N.A. = 0.80) and the integration time of 1 s were used in the present work. X-ray diffractometer (XRD, D 8 advance) and transmission electron microscope (TEM, JEM-2100) were used to characterize phase and crystal structure of the alloy films and Ag particles. Surface morphology and composition of alloy films as well as self-formed Ag particles on film surface were observed and confirmed by field emission scanning electron microscope (FESEM, JSM-7800F) and energy dispersive spectroscopy (EDS), respectively. The mean grain sizes (D) of AgeZr alloy films were calculated by Scherrer formula [22].
D = 0.9λ β cos θ
3.2. Effect of film thickness on the surface morphology of Ag-15.41%Zr alloy films Fig. 2 shows surface morphology of the annealed Ag film and Ag15.41%Zr alloy films with different thickness. It can be seen that the surface of the annealed 360 °C Ag film is rough and the grain size of the film is about 60–500 nm, as shown in Fig. 2(a). The grains in the asdeposited AgeZr alloy films are very fine and uniform due to the inhibiting effect of Zr atoms on Ag grain growth, as shown in Fig. 2(b). Fig. 2(c), (d) and (e) are the surface morphology of 360 °C annealed AgeZr alloy films with 15.41% Zr contents proved by EDS result in Fig. 2(g). It is worth noting that a large number of faceted particles with different sizes spontaneously formed on the annealed AgeZr alloy films' surface, which hasn't been reported previously. The self-formed particles have feature sizes ranging from several tens of nanometers to about 1 μm. According to the EDS result, the compositions of the particles are Ag, as shown in Fig. 2(f). Microstructure evolution behaviors of the AgeZr alloy films on flexible PI substrates after annealing is obviously different from that of annealed AgeCo alloy films on rigid substrates reported in previous researches [23]. The microstructure characteristics of the Ag particles/AgeZr alloy films composite microstructure differ from the traditional magnetic granular films obtained in the annealed AgeCo alloy film [23]. As can be seen from Fig. 2(c), (d) and (e), the average size of Ag particles increases and particle number in the same area decreases monotonously with increasing film thickness, as shown
(1)
whereby λ is the X-ray wavelength (0.15406 nm), and β is the full Table 1 Crystal parameters of Ag-15.41%Zr alloy films with different thickness. Thickness (nm) 13 45 66
2-Theta (deg) 38.169 38.180 38.239
Interplanar spacing (Å)
FWHM (deg)
Diffraction peak intensity of Ag (111) (s−1)
2.3517 2.3552 2.3559
0.359 0.240 0.224
83 127 153
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(a)
(b)
(c)
(d)
(e)
(f)
1
2
Zr
Ag
600
(h)
Variation of Average Particle Number Mean Particle Size Variation
18 15 12
500
9 400
6 3
300
The Number of Particles
(g)
Atomic percentage
Mean Size of Ag particles nm
700
Element
0 200
-3 10
20
30
40
50
60
70
Thin Film Thickness nm
Fig. 2. Surface morphology of Ag films and Ag-15.41% Zr alloy films with different thickness: (a) Ag film of 45 nm thickness annealed at 360 °C, (b) as-deposited Ag15.41% Zr alloy films, (c) and (d) and (e) annealed 360 °C Ag-15.41% Zr alloy films with 13, 45, 66 nm thickness, respectively, (f) EDS of a particle, (g) EDS of AgeZr alloy films, (h) Variation of particle size and number with different film thickness.
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Fig. 3. Surface morphology of annealed 160 °C AgeZr alloy films with (a) 5.37%, (b) 15.41% and (c) 32.62%, respectively, (d) Variation of particle size and number with different Zr contents.
(a)
(b)
(c)
(d)
Fig. 4. FESEM images of 45 nm thickness Ag-15.41%Zr alloy films annealed at (a) 160 °C and (b) 360 °C, (c) Magnification of typical Ag particles obtained by annealing at 360 °C, (d) The SAED patterns of the faceted Ag particles. 1344
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Fig. 5. Schematic diagram of Ag particles/AgeZr alloy films formation.
films obtained in the present work can be used as a universal “template” to grow different kinds of nano-sized finer particles [30]. In order to obtain Ag particles/AgeZr alloy films with large specific surface area, a thin Ag film can be deposited on the surface of Ag particles/AgeZr alloy films, as shown in Fig. 5(c).
in Fig. 2(h). 3.3. Effect of Zr content on the size of Ag particles Surface morphology of annealed 160 °C AgeZr alloy films with different Zr content on the PI substrate is shown in Fig. 3. As can be seen from Fig. 3(a), (b), (c) and (d) that the Ag particle size gradually decreases and particle number increases with the increase of Zr content. The mean size of Ag particles counted by IPP software on the surface of Ag-5.37%Zr, Ag-15.41%Zr, and Ag-32.62%Zr alloy films are 440, 187 and 111 nm, respectively, which can be ascribed to increase of Zr content inhibiting the diffusion of silver atoms [24].
3.6. SERS activity of Ag/Ag-Zr alloy films Fig. 6(a) shows the Raman spectra of Ag and Ag-15.41%Zr alloy films (thickness: 45 nm) as SERS substrates with different treatment process and soaked by R6G dye 5 × 10−6 M. It can be seen in Fig. 6(a) that no SERS peak can be detected in the Ag films annealed at 360 °C due to its relatively smooth surface. Obviously, SERS peak intensity of annealed Ag-15.41%Zr alloy films is stronger than that of deposited AgeZr alloy film and pure Ag film, as shown in Fig. 6(a). The average sizes of Ag particles on the annealed 360 °C Ag-15.41% Zr alloy films surface are 315 nm, which is larger than the size of 20–100 nm of Ag particles with the most SERS performance [30,31]. In view of this, we systematically studied the effect of film thickness on SERS activity of 360 °C annealed Ag-15.41%Zr alloy films as shown in Fig. 6(b). It can be seen that Raman signal is strongest when the thickness is 13 nm. This is because with the increase of film thickness, the number of particles on the films surface decreases gradually, and the size increases gradually in Fig. 2(h), which leads to the gap between adjacent nanoparticles enlarges and the number in decreases, as a result, the number of R6G probe molecules adsorbed on SERS substrate decreased. Fig. 6(c) shows the effect of Zr content on SERS activity of 160 °C annealed AgeZr alloy films with the same film thickness of 13 nm. It was found that the alloy films as SERS substrates with higher Zr content had better SERS performance due to increase of Zr content inhibiting the diffusion of Ag atoms and resulting in the smaller Ag particles (approaching 100 nm). In general, 13 nm thickness AgeZr alloy films annealed at 360 °C with higher Zr content have better surface-enhanced Raman scattering effect. Combined with Fig. 6(a), (b) and (c), hence, we choose 360 °C annealed Ag-15.41% Zr alloy films (thickness: 13 nm) as optimized SERS substrates (Ag particles/AgeZr alloy films). Subsequently, aforementioned optimized SERS substrates were covered by 12 nm Ag films as studied SERS substrates soaked by R6G dye. Comparing Fig. 6(a) and (d) it can be seen that the SERS activity of Ag particles/AgeZr alloy films covered by 12 nm Ag films is significantly better than that of Ag film and AgeZr alloy films. R6G solutions with the concentration of 5 × 10−5 M, 5 × 10−6 M, 5 × 10−7 M, 5 × 10−8 M and 5 × 10−9 M were prepared to soak the samples in order to investigate the concentration-dependent variation tendency of the Raman signal. R6G solution with lower concentration produced weaker Raman scattering signal, as shown in Fig. 6(d). It is noted that the Raman peak of the samples soaked in the 5 × 10−9 R6G solution is very low and not enough to accurately identify the existence of R6G. Fig. 6(f) shows that the SERS intensities of peak at 613 cm−1 and 771 cm−1 gradually decrease with the decrease of R6G concentration. The Raman peak at 613 cm−1 belongs to the in-plane deformation vibration of the ring [32,33] and vibrations at 771 cm−1 are assigned to CeH in-plane bending. The Raman peaks at 1362, 1514 and 1647 cm−1 are caused by CeC bond stretching vibration of R6G aromatic rings. Previous studies show that SERS signal mainly comes from probe molecules adsorbed at the “hot spots” of substrates. These “hot spots” include surfaces of metal nanoparticles with appropriate size
3.4. Effect of annealing temperature on the size of Ag particles Fig. 4 shows morphology of Ag particles on the surface of 45 nm thick Ag-15.41%Zr alloy films with different annealing temperatures, and the selected area electron diffraction (SAED) patterns of the Ag particle. It can be seen from Fig. 4(a) and (b), the average size of Ag particles increases with improving annealing temperatures. The average size of Ag particles in Fig. 4(a) and (b) is 187, and 315 nm, respectively. As the annealing temperature is increased, the average size and particles number increase because of more active diffusion of Ag atoms with the higher energy. Meanwhile the stress in the annealed alloy films will be further relaxed and inevitably lead to the growth of Ag particles. It is interested to note that unlike the Ag hillocks in the annealed Ag films [25], most of Ag particles obtained in this work are very regular and polyhedral with clear edges and corners. Fig. 4(c) shows the high magnification of a typical icosahedral Ag particle obtained by annealing at 360 °C for 1 h in the vacuum chamber. The SAED patterns of the faceted Ag particles indicate that they are single crystals, as shown in Fig. 4(d). As can be seen in Fig. 4(c), the grains in the regions of the AgeZr alloy films without self-formed Ag particles indicated by the blue ring are very fine and uniform due to inhibiting effect of Zr on Ag grains growth. 3.5. The formation mechanism of polyhedral Ag particles Based on the results, it is demonstrated that the sizes of faceted Ag particles obtained in the present work range from dozens of nanometers to about 1 μm and can be adjusted by modifying the Zr content, film thickness and annealing temperature. It is demonstrated that the formation mechanism of the Ag particles on PI substrates is same as that of regular Cu particles on annealed CueZr alloy films [26] and different from that of hillocks observed in pure metal thin films on Si substrates [27–29]. It can be inferred that the faceted particles are indeed formed through mass transportation on surfaces and grain boundaries driven by the relaxation of residual stress, thermal stress and distortion energy in the alloy films. In this process, some atoms are aggregated to form clusters, and they grow into small Ag particles at trigeminal grain boundaries and voids on the AgeZr alloy films surface. Surface diffusion dominates mass transportation during the growth of Ag particles due to the grain boundary diffusion of Ag atoms in the AgeZr alloy films inhibiting by Zr atoms. When the particle size becomes large enough, faceting becomes the most favorable process, as shown in Fig. 5(b), in which the minimization of surface and strain energy is the thermodynamic driving force. Obviously, the Ag particles/AgeZr alloy 1345
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Fig. 6. SERS spectra and morphology of R6G absorbed on the Ag films and AgeZr alloy films and Ag particles/AgeZr alloy films: (a) SERS spectra of 5 × 10−6 M R6G absorbed on the annealed 360 °C Ag films and AgeZr alloy films, (b) SERS spectra of 5 × 10−6 M R6G absorbed on annealed 360 °C Ag-15.41%Zr alloy films with different thickness:13 nm, 45 nm, 66 nm, (c) annealed 160 °C AgeZr alloy films (film thickness:13 nm) with different Zr content: 5.37%, 15.41%, 32.62%, (d) SERS spectra of R6G absorbed on Ag particles/AgeZr alloy films covered by 12 nm Ag films: a with concentrations ranging from 5 × 10−5 to 5 × 10−9 M and (e) 5 × 10−9 M (magnified), (f) SERS intensities variation of peak at 613 cm−1 and 771 cm−1 with different R6G concentration, (g) morphology of Ag particles/AgeZr alloy films covered by 12 nm Ag films, (h) Magnification of the red box area of panel (g). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
[33,34] and interstices between nanoparticles. Fig. 6(g) shows that the morphology of Ag particles/AgeZr alloy films covered by 12 nm Ag films which consists of many tiny Ag particles. It can be found that some Ag particles formed on the surface of annealed AgeZr alloy films exhibit a highly symmetrical icosahedron with sharp edges and corners which are SERS “hot spots” and good for adsorbing more R6G molecules. Furthermore, the interstices (see the red arrows in Fig. 6(h)) between large polyhedral Ag particles and the gap (see the green arrows in Fig. 6(h)) of tiny Ag particles provide a large number of “hot spots” and contribute greatly to the SERS performance of the composite structures. Combining surface morphology of particles/films composite structure, it can be inferred that highly symmetrical polyhedral silver nanoparticles play a key role in SERS and can be used as a universal “template” to grow different kinds of nanosized finer particles.
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4. Conclusion Polyhedral Ag particles with different sizes self-formed on the surface of films are obtained annealed AgeZr alloy films on flexible PI. It was interesting to find that the Ag particles are regular and single crystal. Further analysis indicates that the formation of Ag particles results from the Ag atoms diffusion driven by the relaxation of residual stress, thermal stress and distortion energy in the annealed alloy films. The sizes of regular Ag particles range from dozens of nanometers to about 1 μm and can be adjusted by modifying the Zr content, film thickness and annealing temperature. The Ag particles/AgeZr alloy films obtained in the present work can be used as a universal “template” to grow different kinds of nano-sized finer particles and prepare alloy films with large specific surface area. The detection limit of R6G concentration using Ag particles/Ag-Zr alloy films covered by 12 nm Ag film as SERS substrates is significantly lower than 5 × 10−8 M due to its abundant active “hot spots”. Acknowledgements This work was financially supported by the National Natural Science Foundation of China (Grant No. U12041869), the Chinese 02 Special Fund (Grant No. 2017ZX02408003) and the Chinese 1000 Plan for High Level Foreign Experts (Grant No. WQ20154100278). References [1] M.A. Shenashen, S.A. El-Safty, E.A. Elshehy, Part. Part. Syst. Charact. 31 (2014)
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