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Synthesis and characterization of Ag–Ni bimetallic nanoparticles by laser-induced plasma
Thin Solid Films 519 (2011) 7116–7119
Contents lists available at ScienceDirect
Thin Solid Films j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / t s f
Synthesis and characterization of Ag–Ni bimetallic nanoparticles by laser-induced plasma Qingmei Xiao a, Zhi Yao a, Jiahong Liu a, b, Ran Hai a, Hassan Yousefi Oderji a, Hongbin Ding a,⁎ a School of Physics and Optical Electronic Technology, Key Laboratory of Materials Modification by Laser, Ion and Electron Beams, Chinese Ministry of Education Dalian University of Technology, Dalian, 116024, PR China b School of Science, Dalian Nationalities University, Dalian, 116600, PR China
a r t i c l e
i n f o
Available online 4 May 2011 Keywords: Ag–Ni bimetallic nanoparticles Laser ablation Raman spectroscopy Time-of-flight mass spectroscopy Emission spectroscopy
a b s t r a c t We present an approach in which laser ablation deposition is used to synthesize silver–nickel bimetallic nanoparticles. A variety of techniques, including scanning electron microscopy, energy disperse spectroscopy and X-ray photoelectron spectroscopy have been used to characterize the morphology, composition and construction of synthesized bimetallic nanoparticles, respectively. The formation mechanism of bimetallic nanoparticles has been discussed. The Raman spectra of silver–nickel bimetallic nanoparticles have been analyzed. Time-of-flight mass spectrometry has been applied to directly measure intermediate species. The results indicate that diatomic AgNi is the most abundant species and suggest that the AgNi is the most stable intermediate which may play an important role in the synthesis process. Emission spectra demonstrate that the electron temperature is in the range of 6000–10000 K during the ablation process and increases with the laser power density. © 2011 Elsevier B.V. All rights reserved.
1. Introduction The synthesis of bimetallic nanoparticles has received growing interest in the last decades [1–10]. Bimetallic nanoparticles can exhibit properties different from bulk materials with the same composition because of size confinement effects and their large volume fraction of interfaces. Particularly, the bimetallic nanoparticles of silver and nickel can be used as catalyst, electrical contacts, switches, and conductor material plating with silver and nickel may have good performance in anti-interference. The traditional methods such as mechanical alloy method, reduction method and some other chemical approaches are employed to synthesize nanomaterials. However, they suffer from some limitations mainly concerning the selection of materials or the feasibility of the size distribution. Laser ablated deposition (LAD) is suitable for producing multicomponents and/or multilayered thin films with controlled stoichiometry and has relatively low substrate temperature and high deposition rates compared to other techniques. LAD was used to grow thin films or synthesize nanotubes since 1965 [11]. Recently, some groups have investigated the synthesis of bimetallic nanoparticles in laser–liquid– solid environment [12–14]. Vaningen et al. synthesized crystalline AgxNi1−x solid solutions by LAD [15,16]. They studied the residual
⁎ Corresponding author at: School of Physics and Optical Electronic Technology, Dalian University of Technology, Dalian, 116024, PR China. Tel./fax: + 86 411 84706730. E-mail address: [email protected] (H. Ding). 0040-6090/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2011.04.201
macrostress and analyzed the structural imperfection in terms of crystallite size and microstrain. However, there are limited researches on the formation mechanisms, especially the intermediate species in the ablation process. In this work, we used a Nd:YAG laser to ablate silver nickel samples and studied the properties of nanoparticles and characterized the reactive plasmas with in-situ measurements. In addition, scanning electron microscopy (SEM), Energy Disperse Spectroscopy (EDS) and X-ray photoelectron spectroscopy (XPS) were employed to study the morphology and composition of the synthesized nanoparticles, respectively. Furthermore, Raman spectroscopy was employed to characterize the structures. Time-of-flight mass spectrometry (TOF-MS) was developed to directly measure reactive intermediate species during the ablation process. Emission spectra were collected to study the properties of produced plasma. Based on these investigations, the better knowledge of the process has been obtained and one may propose appropriate technological parameters to obtain optimized products. 2. Experimental setup and procedure 2.1. Synthesis of silver/nickel bimetallic nanoparticles A scheme of the experimental setup is shown in Fig. 1. The LAD chamber, which contained a substrate-holder and a target-holder, was evacuated to 10 − 1 Pa. Bimetallic nanoparticles were synthesized by laser ablation of solid targets, which were prepared by compressing a mixture powder of silver (99.9%, average size ≤0.5 μm) and nickel (99.9%, average size 1–2 μm). The experimental goal was to
Q. Xiao et al. / Thin Solid Films 519 (2011) 7116–7119
CCD detector Quartz prism Si
Target
Lens
Laser
Vacuum chamber
Nd:YAG Laser
Pump
Fig. 1. The schematic diagram of the experimental setup.
characterize the feature and size of nanoparticles produced by LAD. In order to get more information about bimetallic nanoparticles, we performed the experiments on four kinds of targets, i.e. pure Ag, pure Ni, silver–nickel mixture with mole ratio nAg:nNi = 1:1 and nAg: nNi = 32:13. We used the latter ratio because Ag–Ni nanoparticles have been found as Ag32Ni13 with the anti-Mackay icosahedron structure. The laser was a pulsed Nd:YAG laser (NT342B-SH model from EKSPLA), at the wavelength of 355 nm with pulse-width of 6 ns and energy of 1 to 100 mJ/pulse at a repetition rate of 10 Hz. The laser beam was conducted by a quartz prism (see Fig. 1) and then focused into a lens (f = 40 cm) approximately 35 cm from the target with the area of beam spot typically around 3 mm 2. Samples were mounted on the sample holder and were ablated by hundreds of laser shots. Then, nanoparticles emitted from the target were collected by monocrystalline silicon pieces. The laser beam was perpendicular to sample surface and parallel to the silicon collector (see Fig. 1). The deposited nanoparticles were characterized by a Hitachi S4800 scanning electron microscope (SEM) with an energy dispersive spectrum (EDS) analysis facility. The XPS measurements were carried out in a VG ESCALAB250 system, with a base pressure of 7.7 × 10 − 10 Torr and using the AlKα (1486.6 eV) X-ray monochromatized radiation with pass energy of 50 eV (resolution 0.1 eV). Raman spectrum measurements were carried out using the third harmonic (355 nm) of Nd:YAG laser as the exciting laser. 2.2. Characterization of ablated plasma with time-of-flight mass spectrometry Time-of-Flight mass spectrometer (TOF-MS) is a powerful tool to investigate the intermediate species distribution in reactive plasma. This method has several advantages such as high sensitivity, high recording speed and high ion transmission with high mass resolution [17]. The experimental setup has been described elsewhere [17,18]. In this study, a pulsed Nd:YAG laser with λ = 532 nm was used to ablate silver and nickel target. Output power of the laser was 90 mJ/pulse. Emitted ions were extracted by pulsed electrical field and detected by a microchannel plate (MCP) ion-detector. The signals from MCP detector were then fed into a fast oscilloscope and then transferred to a computer for data acquisition by homemade Labview program software package.
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detected using a charge-coupled device (CCD) (see Fig. 1). The spectrometer is SpectraPro-2500i with CCD camera which can record a wide range of wavelength in a few seconds. The plasma features during the synthetic process have been studied by varying the laser energy densities in the range between 180 MW/cm 2 and 500 MW/ cm 2. 3. Results and discussions 3.1. Characterization of silver–nickel nanoparticles The SEM images of synthesized nanoparticles are presented in Fig. 2. The figures show that the composition of sample has a significant effect on the morphology of particles. It is possible to obtain some information about the formation mechanism of bimetallic nanoparticles by comparing their SEM images with the SEM images of pure silver or pure nickel. The particles synthesized from pure silver target are mainly in spherical shape. There are many fine nanoparticles with size about 50 nm on the surface of a much bigger particle. Material ejected from the target might be evaporated under such high laser density. The emitted vapor phase of Ag continuously experienced a nucleation and growth process. Atoms near the surface of the agglomerated liquid drops underwent the higher temperature gradient especially when the liquid drops were trapped by the collection substrate. The surface of liquid silver drops might condense preferentially and form many fine nanoparticles. The Ni-rich particles have irregular shape and are highly agglomerated. There are no fine particles in the surface of a large nickel particle like silver due to the lower surface energy of silver compared to that of nickel. Some particles exhibit polyhedron structures made up of small geometric shells, see Fig. 2(b). The size of nickel particles is in the range from 100 nm to 10 μm, while silver particles have smaller particles in addition. The porous and trace line on the surface of particles can be interpreted by assuming that silver in the shell is not sufficient to cover the corresponding surface of polyhedral nickel-core. This can be supported by the fact that silver–nickel bimetallic nanoparticles are theoretically and experimentally considered to be core–shell structure [8,19,20]. The chemical composition and bonding states of the silver–nickel bimetallic nanoparticles have been characterized by XPS (Fig. 3). XPS analysis of AgNi sample shows that the Ag 3d5/2 binding energy after etching for 10 s or 30 s is shifted to −0.1 eV with respect to initial surface of silver. Intensities of the peaks corresponding to silver and nickel increase with etching time while the ones corresponding to oxygen and carbon decrease. This implies that oxygen and carbon might be adsorbed on the surface during the exposure to air. The intensity ratio, IAg/INi, does not change after etching for 10 s. The binding energy of nickel shifts toward lower values, which suggests that the existence of nickel is changed from oxidation to pure state. EDS analysis also shows that there is no oxygen or carbon inside the deposited film. In addition, we studied the Raman spectra for Ag–Ni mixture with mole ratio of nAg:nNi = 1:1 and 32:13 (Fig. 4(a), (b)) using a laser beam with wavelength of 355 nm produced by Nd:YAG laser instrument. Peaks labeled 1, 2, 1′, 2′ located at −74, − 47, 41, 78 cm − 1 (76 cm − 1) with respect to center frequency of 355 nm are tentatively assigned to Raman signals of silver–nickel nanoparticles. The peaks labeled 2 and 1 in Fig. 4 are corresponded to the 1-order and 2-order anti-stokes lines while the peaks labeled 2′and 1′ represent the 1-order and 2-order stokes lines, respectively.
2.3. Emission spectroscopy during the synthesis process 3.2. Mass spectrum of the intermediate species Several experiments were carried out to determine the properties of the ablation plasma. The experimental setup was similar with the previous mentioned system except that the target was placed at 45° angle with respect to the incoming laser beam. The emission light was collected at 90° to the incident beam by optical fiber probe and
The mass spectrum of the intermediate species, which are released from silver nickel mixture target (with mole ratio nAg:nNi = 1:1) during laser ablation, is shown in Fig. 5. These results have been obtained by TOF mass spectrometry technique using a Nd:YAG laser
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Q. Xiao et al. / Thin Solid Films 519 (2011) 7116–7119
a)
c)
200nm
nAg:nNi=1:1
d)
b)
200nm
Pure Ni
Fig. 2. SEM images of nanoparticles from different materials a)–d) are pure silver, pure nickel, mole ratio of silver and nickel nAg:nNi = 1:1 and 32:13, respectively.
with a power of 90 mJ at 532 nm. The mass spectrum reveals several different species of silver and nickel clusters. Some of them are in the form of divalent ions. The observed AgNi + has the strongest peak which demonstrates that AgNi is the most abundant species in the plasma. This indicates that AgNi molecule is the most stable compound comparing to other particles in these conditions. The peak broadening is ascribed to the initial velocity distribution of ions in laser ablation process. Silver–nickel nanoparticles in Fig. 2 may be aggregated by AgNi molecules.
Survey 1 Scan, 1m 7.5s, 500µm,CAE 150.0 1.00eV
Intensity (105counts/s)
12
Ag3d
O1s
Ni2p
10 360
8
380
525
545 844
888
Ni2p 279
4. Conclusion
O1s
295
C1s 0s 10s 30s
2 0 0
The electron temperature in the plasma has a major influence on the gas phase chemistry between the target and the substrate as well as on the surface component of growing film. Fig. 6 shows the emission spectra recorded during the ablation process. The metal vapor was generated by pulsed laser vaporization using 355 nm (power density 500 MW/cm 2). In these conditions, the laser induced evaporation is strong and the plasma shielding is not too critical [21] and the deposition rate for silver and nickel is similar according to Ref. [22], so there is no restriction or discrimination for emission intensity of silver and nickel. Optical emission spectroscopy was employed to analyze the parameters in the ablated plasma using the Boltzmann multiline slope method [23]. The results show that the temperature is in the range of 6000–10,000 K during the ablation process. The temperature increases with the laser power density. This suggests that the efficiency of laser energy coupling to the material increases with the laser power density.
Ag3d
C1s
6 4
3.3. Emission spectroscopy results
200
400
600
800
1000
1200
Binding Energy (eV) Fig. 3. X-ray photoelectron spectroscopy of film collected from silver/nickel (nAg:nNi =32:13) sample.
Bimetallic nanoparticles of silver and nickel have been synthesized by laser ablation of silver/nickel samples and characterized by SEM, EDS, XPS and Raman spectroscopy. The produced silver and nickel nanoparticles have spherical and polyhedral structures, respectively. The size of bimetallic nanoparticles is in the range of tens of nanometers to several micrometers. The spectra excited by laser are tentatively assigned to Raman stokes and anti-stokes lines of silver– nickel nanoparticles. Plasma properties during ablation process were diagnosed by time-of-flight mass spectroscopy and emission spectroscopy. Time-of-flight mass spectrum reveals that AgNi is the
Q. Xiao et al. / Thin Solid Films 519 (2011) 7116–7119
7119
10000
Nd:YAG laser 355nm, 95mJ, 6ns
a) nAg:nNi=1:1 Intensity (arb. units)
nAg:nNi=1:1
wave number (cm-1)
1 2 2' 1'
-74.53 -47.32 41.31 78.06
6000
4000
10000
2'
nAg:nNi=1:1
Intensity (arb. units)
2
8000
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1
2000
10000 nAg:nNi=32:13 10000 3
4
Ag 5
12
2000
Ni
10 7 8 9 11
0
0 348
350
352
354
356
358
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15000
Intensity (arb. units)
2 10000
wave number (cm-1)
1 2 2' 1'
-74.53 -47.32 41.31 75.61
0 348
350
352
600
700
800
900
Acknowledgements
1' 1
5000
500
Fig. 6. Emission spectrum of four kinds of different materials (1. 328.1 nm 2S1/2–2P2/3; 2. 338.3 nm 2S1/2–2P1/3; 3. 520.9 nm 2P1/2–2D2/3; 4. 546.5 nm 2P3/2–2D5/3; 5. 768.8 nm 2 P1/2–2S1/3 and 6. 827.3 nm 2P3/2–2S1/3 are lines of silver, while 7. 440.2 nm 2S1/2–2P2/3; 8. 471.4 nm 2S1/2–2P2/3; 9. 501.8 nm 2S1/2–2P2/3; 10. 503.5 nm 2S1/2–2P2/3; 11. 514.3 nm 2 S1/2–2P2/3 are lines of nickel).
2'
nAg:nNi=32:13
400
Wavelength (nm)
Wavelength (nm)
b) nAg:nNi=32:13
6
354
356
358
This study was supported by the Scientific and Technical Foundation of Liaoning Province (no. 20082168), the Scientific and Technical Key Project of Educational Ministry of China (no. 108034), the Ph. D research program (no. 20080141140), the National Science Foundation of China (no.10875023) and the National Magnetic Confinement Fusion Science Program (nos. 2009GB106004, 2008CB717801).
Wavelength (nm) References Fig. 4. Raman spectrum of silver/nickel samples using 355 nm laser exciting, 1, 2 are antistokes lines and 1′, 2′ are stokes lines: a) nAg:nNi =1:1 sample, b) nAg:nNi =32:13 sample.
0.00 +
AgNi2
+
Intensity (arb. units)
Ni
Nd:YAG laser + AgNi target 532nm, 90mJ, 6ns
Ag2Ni+
-0.05
-0.10
Ag+ 2+
AgNi
+
2+
2+
Ag2Ni5 AgNi3 Ag4Ni3
-0.15
-0.20
AgNi+ 0
200
400
600
800
1000
Mass (m/z) Fig. 5. Time-of-flight mass spectrometry (TOF-MS) of nAg:nNi = 1:1 sample.
most abundant species. Emission spectrum demonstrates that the temperature during the ablation process is about several thousands Kelvin.
[1] R. Ferrando, J. Jellinek, R.L. Johnston, Chem. Rev. 108 (2008) 845. [2] E. Cottancin, J. Lermé, M. Gaudry, M. Pellarin, J.L. Vialle, M. Broyer, B. Prével, M. Treilleux, P. Mélinon, Phys. Rev. B. 62 (2000) 5179. [3] M. Haruta, Catal. Today 36 (1997) 153. [4] U. Kreibig, M. Vollmer, Optical Properties of Metal, Springer, Berlin, 1995. [5] S. Lee, C. Fan, T. Wu, S.L. Anderson, J. Chem. Phys. 123 (2005) 124710. [6] A.M. Molenbroek, S. Haukka, B.S. Clausen, J. Phys. Chem. B 102 (1998) 10680. [7] L.M. Molina, B. Hammer, Phys. Rev. Lett. 90 (2003) 206102. [8] H. Portales, L. Saviot, E. Duval, M. Gaudry, E. Cottancin, M. Pellarin, J. Lermé, M. Broyer, Phys. Rev. B. 65 (2002) 165422. [9] G. Rossi, A. Rapallo, C. Mottet, A. Fortunelli, F. Baletto, R. Ferrando, Phys. Rev. Lett. 93 (2004) 105503. [10] M. Valden, X. Lai, D.W. Goodman, Science 281 (1998) 1647. [11] H.M. Smith, A.F. Turner, Appl. Opt. 4 (1965) 147. [12] D. Poondi, J. Singh, J. Mater. Sci. 35 (2000) 2467. [13] Q.X. Liu, C.X. Wang, W. Zhang, G.W. Wang, Chem. Phys. Lett. 382 (2003) 1. [14] S. Gadag, M. Gupta, Lasers Engineering 17 (2007) 239. [15] R.P. Vaningen, R.H.J. Fastenau, E.J. Mittemeijer, J. Appl. Phys. 76 (1994) 1871. [16] R.P. Vaningen, R.H.J. Fastenau, E.J. Mittemeijer, Phys. Rev. Lett. 72 (1994) 3116. [17] J.H. Liu, Q.M. Xiao, L.P. Wang, Z. Yao, H.B. Ding, Plasma Science Technology 11 (2009) 171. [18] Q.M. Xiao, J.H. Liu, Z. Yao, D.Z. Wang, H.B. Ding, Fusion Eng. Des. 85 (2010) 2165. [19] M. Gaudry, E. Cottancin, M. Pellarin, J. Lerme, L. Arnaud, J.R. Huntzinger, J.L. Vialle, M. Broyer, J.L. Rousset, M. Treilleux, P. Melinon, Phys. Rev. B. 67 (2003) 155409. [20] F. Baletto, C. Mottet, A. Rapallo, G. Rossi, R. Ferrando, Surf. Sci. 566 (2004) 192. [21] R.W. Dreyfus, Appl. Surf. Sci. 86 (1995) 29. [22] W. Svendsen, O. Ellegaard, J. Schou, Appl. Phys. A. 63 (1996) 247. [23] J.A. Aguilera, C. Aragón, Spectrochim. Acta. B. 62 (2007) 378.
Contents lists available at ScienceDirect
Thin Solid Films j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / t s f
Synthesis and characterization of Ag–Ni bimetallic nanoparticles by laser-induced plasma Qingmei Xiao a, Zhi Yao a, Jiahong Liu a, b, Ran Hai a, Hassan Yousefi Oderji a, Hongbin Ding a,⁎ a School of Physics and Optical Electronic Technology, Key Laboratory of Materials Modification by Laser, Ion and Electron Beams, Chinese Ministry of Education Dalian University of Technology, Dalian, 116024, PR China b School of Science, Dalian Nationalities University, Dalian, 116600, PR China
a r t i c l e
i n f o
Available online 4 May 2011 Keywords: Ag–Ni bimetallic nanoparticles Laser ablation Raman spectroscopy Time-of-flight mass spectroscopy Emission spectroscopy
a b s t r a c t We present an approach in which laser ablation deposition is used to synthesize silver–nickel bimetallic nanoparticles. A variety of techniques, including scanning electron microscopy, energy disperse spectroscopy and X-ray photoelectron spectroscopy have been used to characterize the morphology, composition and construction of synthesized bimetallic nanoparticles, respectively. The formation mechanism of bimetallic nanoparticles has been discussed. The Raman spectra of silver–nickel bimetallic nanoparticles have been analyzed. Time-of-flight mass spectrometry has been applied to directly measure intermediate species. The results indicate that diatomic AgNi is the most abundant species and suggest that the AgNi is the most stable intermediate which may play an important role in the synthesis process. Emission spectra demonstrate that the electron temperature is in the range of 6000–10000 K during the ablation process and increases with the laser power density. © 2011 Elsevier B.V. All rights reserved.
1. Introduction The synthesis of bimetallic nanoparticles has received growing interest in the last decades [1–10]. Bimetallic nanoparticles can exhibit properties different from bulk materials with the same composition because of size confinement effects and their large volume fraction of interfaces. Particularly, the bimetallic nanoparticles of silver and nickel can be used as catalyst, electrical contacts, switches, and conductor material plating with silver and nickel may have good performance in anti-interference. The traditional methods such as mechanical alloy method, reduction method and some other chemical approaches are employed to synthesize nanomaterials. However, they suffer from some limitations mainly concerning the selection of materials or the feasibility of the size distribution. Laser ablated deposition (LAD) is suitable for producing multicomponents and/or multilayered thin films with controlled stoichiometry and has relatively low substrate temperature and high deposition rates compared to other techniques. LAD was used to grow thin films or synthesize nanotubes since 1965 [11]. Recently, some groups have investigated the synthesis of bimetallic nanoparticles in laser–liquid– solid environment [12–14]. Vaningen et al. synthesized crystalline AgxNi1−x solid solutions by LAD [15,16]. They studied the residual
⁎ Corresponding author at: School of Physics and Optical Electronic Technology, Dalian University of Technology, Dalian, 116024, PR China. Tel./fax: + 86 411 84706730. E-mail address: [email protected] (H. Ding). 0040-6090/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2011.04.201
macrostress and analyzed the structural imperfection in terms of crystallite size and microstrain. However, there are limited researches on the formation mechanisms, especially the intermediate species in the ablation process. In this work, we used a Nd:YAG laser to ablate silver nickel samples and studied the properties of nanoparticles and characterized the reactive plasmas with in-situ measurements. In addition, scanning electron microscopy (SEM), Energy Disperse Spectroscopy (EDS) and X-ray photoelectron spectroscopy (XPS) were employed to study the morphology and composition of the synthesized nanoparticles, respectively. Furthermore, Raman spectroscopy was employed to characterize the structures. Time-of-flight mass spectrometry (TOF-MS) was developed to directly measure reactive intermediate species during the ablation process. Emission spectra were collected to study the properties of produced plasma. Based on these investigations, the better knowledge of the process has been obtained and one may propose appropriate technological parameters to obtain optimized products. 2. Experimental setup and procedure 2.1. Synthesis of silver/nickel bimetallic nanoparticles A scheme of the experimental setup is shown in Fig. 1. The LAD chamber, which contained a substrate-holder and a target-holder, was evacuated to 10 − 1 Pa. Bimetallic nanoparticles were synthesized by laser ablation of solid targets, which were prepared by compressing a mixture powder of silver (99.9%, average size ≤0.5 μm) and nickel (99.9%, average size 1–2 μm). The experimental goal was to
Q. Xiao et al. / Thin Solid Films 519 (2011) 7116–7119
CCD detector Quartz prism Si
Target
Lens
Laser
Vacuum chamber
Nd:YAG Laser
Pump
Fig. 1. The schematic diagram of the experimental setup.
characterize the feature and size of nanoparticles produced by LAD. In order to get more information about bimetallic nanoparticles, we performed the experiments on four kinds of targets, i.e. pure Ag, pure Ni, silver–nickel mixture with mole ratio nAg:nNi = 1:1 and nAg: nNi = 32:13. We used the latter ratio because Ag–Ni nanoparticles have been found as Ag32Ni13 with the anti-Mackay icosahedron structure. The laser was a pulsed Nd:YAG laser (NT342B-SH model from EKSPLA), at the wavelength of 355 nm with pulse-width of 6 ns and energy of 1 to 100 mJ/pulse at a repetition rate of 10 Hz. The laser beam was conducted by a quartz prism (see Fig. 1) and then focused into a lens (f = 40 cm) approximately 35 cm from the target with the area of beam spot typically around 3 mm 2. Samples were mounted on the sample holder and were ablated by hundreds of laser shots. Then, nanoparticles emitted from the target were collected by monocrystalline silicon pieces. The laser beam was perpendicular to sample surface and parallel to the silicon collector (see Fig. 1). The deposited nanoparticles were characterized by a Hitachi S4800 scanning electron microscope (SEM) with an energy dispersive spectrum (EDS) analysis facility. The XPS measurements were carried out in a VG ESCALAB250 system, with a base pressure of 7.7 × 10 − 10 Torr and using the AlKα (1486.6 eV) X-ray monochromatized radiation with pass energy of 50 eV (resolution 0.1 eV). Raman spectrum measurements were carried out using the third harmonic (355 nm) of Nd:YAG laser as the exciting laser. 2.2. Characterization of ablated plasma with time-of-flight mass spectrometry Time-of-Flight mass spectrometer (TOF-MS) is a powerful tool to investigate the intermediate species distribution in reactive plasma. This method has several advantages such as high sensitivity, high recording speed and high ion transmission with high mass resolution [17]. The experimental setup has been described elsewhere [17,18]. In this study, a pulsed Nd:YAG laser with λ = 532 nm was used to ablate silver and nickel target. Output power of the laser was 90 mJ/pulse. Emitted ions were extracted by pulsed electrical field and detected by a microchannel plate (MCP) ion-detector. The signals from MCP detector were then fed into a fast oscilloscope and then transferred to a computer for data acquisition by homemade Labview program software package.
7117
detected using a charge-coupled device (CCD) (see Fig. 1). The spectrometer is SpectraPro-2500i with CCD camera which can record a wide range of wavelength in a few seconds. The plasma features during the synthetic process have been studied by varying the laser energy densities in the range between 180 MW/cm 2 and 500 MW/ cm 2. 3. Results and discussions 3.1. Characterization of silver–nickel nanoparticles The SEM images of synthesized nanoparticles are presented in Fig. 2. The figures show that the composition of sample has a significant effect on the morphology of particles. It is possible to obtain some information about the formation mechanism of bimetallic nanoparticles by comparing their SEM images with the SEM images of pure silver or pure nickel. The particles synthesized from pure silver target are mainly in spherical shape. There are many fine nanoparticles with size about 50 nm on the surface of a much bigger particle. Material ejected from the target might be evaporated under such high laser density. The emitted vapor phase of Ag continuously experienced a nucleation and growth process. Atoms near the surface of the agglomerated liquid drops underwent the higher temperature gradient especially when the liquid drops were trapped by the collection substrate. The surface of liquid silver drops might condense preferentially and form many fine nanoparticles. The Ni-rich particles have irregular shape and are highly agglomerated. There are no fine particles in the surface of a large nickel particle like silver due to the lower surface energy of silver compared to that of nickel. Some particles exhibit polyhedron structures made up of small geometric shells, see Fig. 2(b). The size of nickel particles is in the range from 100 nm to 10 μm, while silver particles have smaller particles in addition. The porous and trace line on the surface of particles can be interpreted by assuming that silver in the shell is not sufficient to cover the corresponding surface of polyhedral nickel-core. This can be supported by the fact that silver–nickel bimetallic nanoparticles are theoretically and experimentally considered to be core–shell structure [8,19,20]. The chemical composition and bonding states of the silver–nickel bimetallic nanoparticles have been characterized by XPS (Fig. 3). XPS analysis of AgNi sample shows that the Ag 3d5/2 binding energy after etching for 10 s or 30 s is shifted to −0.1 eV with respect to initial surface of silver. Intensities of the peaks corresponding to silver and nickel increase with etching time while the ones corresponding to oxygen and carbon decrease. This implies that oxygen and carbon might be adsorbed on the surface during the exposure to air. The intensity ratio, IAg/INi, does not change after etching for 10 s. The binding energy of nickel shifts toward lower values, which suggests that the existence of nickel is changed from oxidation to pure state. EDS analysis also shows that there is no oxygen or carbon inside the deposited film. In addition, we studied the Raman spectra for Ag–Ni mixture with mole ratio of nAg:nNi = 1:1 and 32:13 (Fig. 4(a), (b)) using a laser beam with wavelength of 355 nm produced by Nd:YAG laser instrument. Peaks labeled 1, 2, 1′, 2′ located at −74, − 47, 41, 78 cm − 1 (76 cm − 1) with respect to center frequency of 355 nm are tentatively assigned to Raman signals of silver–nickel nanoparticles. The peaks labeled 2 and 1 in Fig. 4 are corresponded to the 1-order and 2-order anti-stokes lines while the peaks labeled 2′and 1′ represent the 1-order and 2-order stokes lines, respectively.
2.3. Emission spectroscopy during the synthesis process 3.2. Mass spectrum of the intermediate species Several experiments were carried out to determine the properties of the ablation plasma. The experimental setup was similar with the previous mentioned system except that the target was placed at 45° angle with respect to the incoming laser beam. The emission light was collected at 90° to the incident beam by optical fiber probe and
The mass spectrum of the intermediate species, which are released from silver nickel mixture target (with mole ratio nAg:nNi = 1:1) during laser ablation, is shown in Fig. 5. These results have been obtained by TOF mass spectrometry technique using a Nd:YAG laser
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Q. Xiao et al. / Thin Solid Films 519 (2011) 7116–7119
a)
c)
200nm
nAg:nNi=1:1
d)
b)
200nm
Pure Ni
Fig. 2. SEM images of nanoparticles from different materials a)–d) are pure silver, pure nickel, mole ratio of silver and nickel nAg:nNi = 1:1 and 32:13, respectively.
with a power of 90 mJ at 532 nm. The mass spectrum reveals several different species of silver and nickel clusters. Some of them are in the form of divalent ions. The observed AgNi + has the strongest peak which demonstrates that AgNi is the most abundant species in the plasma. This indicates that AgNi molecule is the most stable compound comparing to other particles in these conditions. The peak broadening is ascribed to the initial velocity distribution of ions in laser ablation process. Silver–nickel nanoparticles in Fig. 2 may be aggregated by AgNi molecules.
Survey 1 Scan, 1m 7.5s, 500µm,CAE 150.0 1.00eV
Intensity (105counts/s)
12
Ag3d
O1s
Ni2p
10 360
8
380
525
545 844
888
Ni2p 279
4. Conclusion
O1s
295
C1s 0s 10s 30s
2 0 0
The electron temperature in the plasma has a major influence on the gas phase chemistry between the target and the substrate as well as on the surface component of growing film. Fig. 6 shows the emission spectra recorded during the ablation process. The metal vapor was generated by pulsed laser vaporization using 355 nm (power density 500 MW/cm 2). In these conditions, the laser induced evaporation is strong and the plasma shielding is not too critical [21] and the deposition rate for silver and nickel is similar according to Ref. [22], so there is no restriction or discrimination for emission intensity of silver and nickel. Optical emission spectroscopy was employed to analyze the parameters in the ablated plasma using the Boltzmann multiline slope method [23]. The results show that the temperature is in the range of 6000–10,000 K during the ablation process. The temperature increases with the laser power density. This suggests that the efficiency of laser energy coupling to the material increases with the laser power density.
Ag3d
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Binding Energy (eV) Fig. 3. X-ray photoelectron spectroscopy of film collected from silver/nickel (nAg:nNi =32:13) sample.
Bimetallic nanoparticles of silver and nickel have been synthesized by laser ablation of silver/nickel samples and characterized by SEM, EDS, XPS and Raman spectroscopy. The produced silver and nickel nanoparticles have spherical and polyhedral structures, respectively. The size of bimetallic nanoparticles is in the range of tens of nanometers to several micrometers. The spectra excited by laser are tentatively assigned to Raman stokes and anti-stokes lines of silver– nickel nanoparticles. Plasma properties during ablation process were diagnosed by time-of-flight mass spectroscopy and emission spectroscopy. Time-of-flight mass spectrum reveals that AgNi is the
Q. Xiao et al. / Thin Solid Films 519 (2011) 7116–7119
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Acknowledgements
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Fig. 6. Emission spectrum of four kinds of different materials (1. 328.1 nm 2S1/2–2P2/3; 2. 338.3 nm 2S1/2–2P1/3; 3. 520.9 nm 2P1/2–2D2/3; 4. 546.5 nm 2P3/2–2D5/3; 5. 768.8 nm 2 P1/2–2S1/3 and 6. 827.3 nm 2P3/2–2S1/3 are lines of silver, while 7. 440.2 nm 2S1/2–2P2/3; 8. 471.4 nm 2S1/2–2P2/3; 9. 501.8 nm 2S1/2–2P2/3; 10. 503.5 nm 2S1/2–2P2/3; 11. 514.3 nm 2 S1/2–2P2/3 are lines of nickel).
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This study was supported by the Scientific and Technical Foundation of Liaoning Province (no. 20082168), the Scientific and Technical Key Project of Educational Ministry of China (no. 108034), the Ph. D research program (no. 20080141140), the National Science Foundation of China (no.10875023) and the National Magnetic Confinement Fusion Science Program (nos. 2009GB106004, 2008CB717801).
Wavelength (nm) References Fig. 4. Raman spectrum of silver/nickel samples using 355 nm laser exciting, 1, 2 are antistokes lines and 1′, 2′ are stokes lines: a) nAg:nNi =1:1 sample, b) nAg:nNi =32:13 sample.
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Mass (m/z) Fig. 5. Time-of-flight mass spectrometry (TOF-MS) of nAg:nNi = 1:1 sample.
most abundant species. Emission spectrum demonstrates that the temperature during the ablation process is about several thousands Kelvin.
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