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First nucleation steps of nickel nanoparticle growth on Al2O3 (0 0 0 1) studied by XPS inelastic peak shape analysis
Applied Surface Science 255 (2008) 3000–3003
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
First nucleation steps of nickel nanoparticle growth on Al2O3 (0 0 0 1) studied by XPS inelastic peak shape analysis C. Gallardo-Vega a,*, W. De La Cruz b, S. Tougaard c, L. Cota-Araiza b a
Centro de Investigacio´n Cientı´fica y de Educacio´n Superior de Ensenada (CICESE), Km 107 Carretera Tijuana-Ensenada, C.P. 22860, Ensenada, B.C., Mexico Centro de Nanociencias y Nanotecnologı´a, Universidad Nacional Auto´noma de Me´xico, Km 107 Carretera Tijuana-Ensenada, C.P. 22860, Ensenada, B.C., Mexico c Department of Physics and Chemistry, University of Southern Denmark, DK-5230, Odense M, Denmark b
A R T I C L E I N F O
A B S T R A C T
Article history: Received 1 February 2008 Received in revised form 11 August 2008 Accepted 24 August 2008 Available online 29 August 2008
A series of Ni nanoparticles has been deposited on sapphire (Al2O3 (0 0 0 1)) substrates using the pulsed laser deposition technique. The amount of material deposited has been controlled by means of the number of laser pulses utilized. The substrate temperature was varied from room temperature to 500 8C. The nanoparticles deposited were characterized in situ by X-ray photoelectron spectroscopy. The inelastic peak shape of O 1s was analyzed to obtain the mode of growth of the Ni nanoparticles. The results show the height of the Ni nanoparticles increases with deposition from 1 to 9 nm and the surface coverage increased simultaneously from 0.1 to 0.85. For 200 or more laser pulses, as the substrate temperature increased (300–500 8C) the height of the nanoparticles increased. On the other hand, the coverage always decreased as a function of substrate temperature. This implies that the mobility of the deposited Ni increases with substrate temperature thus forming taller islands with corresponding smaller coverage. ß 2008 Elsevier B.V. All rights reserved.
Keywords: Nanoparticles Ni XPS Pulsed laser deposition
1. Introduction Nano-structured ferromagnetic (Fe, Co and Ni) particles dispersed in dielectric matrices (SiO2, Al2O3, TiO2 etc.) have been intensively studied in recent years, because of their remarkable magnetic and catalytic properties, when they are produced on the nanometer scale. The most notable magnetic properties are: giant magnetoresistance, enhanced coercivity, superparamagnetism and low saturation magnetization [1,2]. These outstanding properties take place due to quantum size effects and the large surface area of the nanoparticles compared with their bulk counterpart. With respect to catalytic properties, determination of the surface area and the nanoparticle size are very important in the development of new highly efficient catalytic systems. Nickelbased supported catalysts have been widely employed in various important industrial chemical processes, including hydrogenation, reforming, methanation, desulphurization and dechlorination processes. These catalysts are generally synthesized by chemical methods and the major problem encountered in such methods is the formation of mixed oxides of nickel and the support. As a consequence, high temperatures (>600 8C) are required to reduce
* Corresponding author at: CNyN-UNAM, P.O. Box 439036, San Ysidro, CA 921439036, USA. Tel.: +1 52 646 1744602; fax: +1 52 646 1744603. E-mail address: [email protected] (C. Gallardo-Vega). 0169-4332/$ – see front matter ß 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2008.08.061
the Ni (II) cations to metallic nickel particles and this leads to sintering of the metal [3]. The initial stages in the growth of thin films (i.e., first nucleation steps) are critical for the evolution of the microstructure of the film as the thickness increases [4]. Typically, transmission electron microscopy (TEM) or scanning electron microscopy (SEM) [5] have been applied to study the initial stages of growth of thin films and more recently, atomic force microscopy (AFM) and similar probes were used [6,7]. However, depending on the system, the use of those techniques may have important limitations. This is particularly evident when the materials of the substrate and deposited layer are not clearly differentiated. For instance, TEM images of two materials with similar electron density or AFM when the substrate is relatively rough relative to the size of the deposited particles. For flat substrates and single crystals, the analysis of the initial stages of growth of thin films up to a few monolayers can be carried out by X-ray photoemission spectroscopy (XPS), by varying the detection angle and comparing the relative intensities of the peaks of the substrate and the deposited material [8,9]. While this angle resolved XPS (ARXPS) method works well for flat surfaces it gives erroneous results for nanoparticles grown on a flat surface because of shadowing effects that are inevitable for the larger emission angles needed in this technique [10] and its applicability is therefore quite limited since a priory knowledge (which is typically not available because it is actually the purpose of the analysis to find that) on the nanostructure is therefore needed
C. Gallardo-Vega et al. / Applied Surface Science 255 (2008) 3000–3003
[10,11]. Much more quantitative and detailed information on the nanostructure of surface films can be obtained by analysis of the energy distribution in an XPS spectrum over a wide energy range i.e. the peak intensity and the background signal of inelastically scattered electrons below the peak energy. This method which was developed by Tougaard et al. [12,13] gives a quite detailed and accurate characterization of the surface and it works for both flat and rough surface structures because all information is extracted from analysis of a single spectrum recorded at a fixed emission angle. In addition, unlike ARXPS, peak shape analysis is quite insensitive to the effect of elastic electron scattering and diffraction effects. The influence of these is only to slightly modify the apparent depths by an amount which is negligible for 1 nm depth increasing to 10–30% for depths of 5 nm [13,20]. With the XPS peak shape analysis method, it is thus easy to determine whether the film formation mechanism is a layer by layer, island growth or layer plus island growth [13]. In the present paper we show how pulsed laser deposition (PLD) can be applied successfully to study the initial growth of Ni nanoparticles. We apply the above mentioned peak shape analysis method to determine the height of the first nuclei of the deposited nanoparticles. Thus, the height and coverage were studied as a function of the amount of deposited material (that is, the number of laser pulses) and as a function of substrate temperature in the range from room temperature to 500 8C. 2. Experimental The Ni nanoparticles were deposited in a commercially available Riber LDM-32 Laser ablation system, equipped with Cameca Mac3 electron energy analyzer. The system allows for in situ characterization by Auger electron spectroscopy (AES) and XPS. Technical details of the system are given elsewhere [14]. The deposits were produced with a Kr–F excimer laser (l = 248 nm, with 20 ns pulse width) at a base pressure of 5 10 8 Torr during the deposition. The target used was 99.99%, high purity nickel, obtained from Target Materials, Inc. All nanoparticle deposits were made under the same laser condition, namely 400 mJ per pulse, a pulsed frequency of 2 Hz and a laser fluence of 5 J/cm2 at the target surface. All Ni nanoparticles were deposited on the (0 0 0 1) sapphire substrate at three substrate temperatures: room temperature (RT), 300 and 500 8C. The numbers of laser pulses used in the experiments were 50, 100, 200, 300, 400, 500 and 600. After each deposition, the sample was analyzed in situ by XPS in order to check the quality of the deposit and the amount of material
3001
deposited. The substrates were cleaned in situ by laser ablation before Ni nanoparticles were deposited. XPS spectra were recorded after each deposition step using Al Ka radiation to excite the core electrons. For the XPS peak shape analysis, the O 1s photoemission peak of the sapphire substrate/Ni nanoparticles system was used. Each spectrum was analyzed using the methodology and software known as QUASES [15] which implements the XPS peak shape analysis method developed by Tougaard et al. 3. Results and discussions When any material is deposited directly on as-received sapphire substrates, the presence of carbon impurities and of native oxide on the substrate surface can influence the growth mechanism. Traditionally, wet chemical techniques and the annealing process are used to remove any impurities on the substrate surface. In this work, a quick and relatively simple technique for stripping off all impurities was used, which consisted in ablating the substrate surface with the pulsed laser (before the metal deposition). This method was proposed previously by Cruz et al. [16] as a useful tool to clean the surface of silicon wafers. The laser cleaning process consisted in scanning the laser beam on the substrate surface. Fig. 1(a) shows an XPS spectrum of the asreceived sapphire substrate surface, with the carbon signal being the main contaminant. After the cleaning process, the carbon signal practically disappeared (Fig. 1(b)), so the substrate is ready for nanoparticle deposition. After the cleaning process, the substrate surface was checked by Reflection High Energy Electron Diffraction and the diffraction pattern (not shown here) indicates that the crystallinity of the surface is preserved. Fig. 2 shows the Ni 2p, Ni 2s and O KLL photoemission spectra for nickel nanoparticles deposited by PLD on the (0 0 0 1) sapphire substrate at 500 8C, for different numbers of pulses: (a) 0, (b) 100, (c) 300, (d) 500, (e) 800 and (f) 1000 pulses. These spectra clearly show that the intensity of the Ni 2p and Ni 2s peaks increase and the O KLL signal peak decreases when the number of laser pulses is increased and at the same time the background intensity of inelastically scattered electrons on the low energy side of the peaks also change. Similar effects are observed for the O 1s peak from the sapphire substrate shown in Fig. 3a. Here the O 1s peak intensity decreases while the shape of the spectrum below the peak changes strongly when nickel particles were deposited using: (a) 0, (b) 100, (c) 300, (d) 500, (e) 800 and (f) 1000 pulses. This change in peak shape is due to the increased fraction of photon excited O 1s
Fig. 1. XPS spectra of sapphire substrates (Al2O3). (a) Before laser ablation cleaning treatment. (b) After laser ablation cleaning treatment.
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C. Gallardo-Vega et al. / Applied Surface Science 255 (2008) 3000–3003
Fig. 2. Signals of Ni 2p3/2 and O KLL in XPS spectra for Ni particles deposited at a substrate temperature of 500 8C, from an over/layer experiment for an increasing number of laser pulses: (a) 0, (b) 100, (c) 300, (d) 500, (e) 800 and (f) 1000 pulses.
electrons from the substrate that have undergone inelastic scattering events as the Ni overlayer increases. The detailed change in the O 1s peak shape is determined by the size distribution of the Ni-nanoparticles which can thus be extracted with the peak shape analysis procedure. The position of the Ni 2p3/ 2 peak was fixed at 613.37 eV for all the deposits. The photoelectron peak shape analysis, including the background behind the photoemission peaks, was done according to Tougaard’s method [12,13] using the QUASES software [15]. With this analysis, it is possible to determine, for each deposit, the height of the nanoparticles and the fraction of the substrate surface that is covered (coverage). It is important to remark that these analyses
Fig. 4. Island height vs. surface coverage of Ni nanoparticles deposited at different number of laser pulses and substrate temperatures. The series of dashed lines represent curves of constant amount of deposited material.
depend on whether there is a clear change in the height and shape of the background intensity and the uncertainty in the determined nanostructure parameters is larger for small deposits [13]. The calculations to obtain the nanoparticles height and coverage have been made using the O 1s peak from the sapphire substrate (see Fig. 3b) and under the assumption that all the deposits have a homogeneous distribution of island heights [17]. The results of these analyses are shown in Fig. 4. The series of dashed lines in Fig. 4 represent curves with fixed amount of deposited material (ADM). ADM for any deposit is calculated as the product of the height of the nanoparticles times its respective coverage. The result of this product is used to predict the height of nanoparticles for any
Fig. 3. (a) O 1s spectra used to perform XPS inelastic peak shape analysis, applying the QUASES software and methodology. The different spectra correspond to the deposition of Ni particles for a different number of laser pulses: (a) 0, (b) 100, (c) 300, (d) 500, (e) 800 and (f) 1000 pulses. (b) Curves corresponding to the experimental spectra, calculated background and primary excited spectra are reported.
C. Gallardo-Vega et al. / Applied Surface Science 255 (2008) 3000–3003
coverage (or conversely). With this procedure, the ADM shown in Fig. 4 was obtained using the deposits at 500 8C as a guide. ADM can be associated to the number of pulses used during the over/ layer experiments. As the number of pulses is increased the ADM also increases. In Fig. 4 we show the height and coverage of the Ni-particles corresponding to three different series of nickel nanoparticles. Each series corresponds to a constant substrate temperature: room temperature (RT), 300 and 500 8C. Each point in Fig. 4 corresponds to nanoparticles produced under specific deposition conditions. The first point of each series corresponds to a deposit made with the first 50 laser pulses, after the substrate was cleaned. The second point of each series represents the deposit with 100 laser pulses and successively up to 600 pulses. For the first 50 pulses, the coverage was the same (10%) and the height of nanoparticles change from 2 (deposit at RT) to 1 nm (deposit at 300 or 500 8C). For deposits corresponding to more than 200 pulses, the nanoparticles deposited at RT showed a larger coverage, and smaller height than nanoparticles deposited at substrate temperatures of 300 or 500 8C. Deposits at RT showed an island shape, with heights in the 2–7 nm range, while nanoparticles deposited on a heated substrate showed heights in the 1–9 nm range (see Fig. 4). For those deposits, both the height and the coverage of the nanoparticles increased as the number of laser pulses was increased. Fig. 4 also shows that for a given number of pulses >200, the nanoparticle height increases with substrate temperature while the coverage decreases. This implies that the deposited Ni is more mobile on the surface at elevated temperatures which allows for coalescence of the metal after deposition, and therefore, the material will form nanoparticles of greater height for the same amount of material, thus causing the coverage to decrease. The heights of the Ni nanoparticles reported in the present work show that PLD is a good technique to grow Ni nanoparticles. Similar values of particle height were reported for Fe nanoparticles produced by PLD [18]. The heights obtained are very important in catalysis, since this parameter is associated with a higher catalytic activity. Similar values of Ni nanoparticle height have been reported by Z. Chu et al. [19] for nanoparticles deposited on MgO showing super-paramagnetic behavior. 4. Conclusions In this work Ni particles were produced by PLD, demonstrating that this is a powerful method to grow nanoparticles in a controlled manner. These results may provide opportunities to design structures having physical properties with potential
3003
technological applications. The heights of the nanoparticles were in the 1–9 nm range and the coverage 0.1–0.85. The results also show that for 200 laser pulses or more (ADM), the height of Ni nanoparticles increases, as the substrate temperature increases, while at the same time the coverage decreases, in comparison with deposits performed with the substrate at room temperature. At room temperature, the height was lower and the coverage was larger. Thus, for substrate temperatures 300–500 8C, the deposited Ni is more mobile and coalesce to form taller islands while the coverage is correspondingly decreased. Finally, these results show how XPS background analysis can be a useful tool to assess and understand the growth mechanism of nanoparticles and the parameters involved in the deposition techniques employed. Acknowledgments The authors are grateful to J.A. Diaz, E. Aparicio, D. Domı´nguez, F. Ruiz, I. Gradilla, V. Garcı´a, P. Casillas, M. Saenz, and J. Peralta for technical assistance. This work was partially supported by CONACYT, projects no. 52486 and 50203, DGAPA IN120206-02 and the Danish Agency for Science, Technology and Innovation, project no. 95-305-23359. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20]
W.D. Wang, F.W. Zhu, J. Weng, J.M. Xiao, W.Y. Lai, Appl. Phys. Lett. 72 (1998) 1118. D.L. Leslie-Pelecky, R.D. Rieke, Chem. Mater. 8 (1996) 1770. S. Velu, S.K. Gangwal, Solid State Ionics 177 (2006) 803–811. L. Pranevicius, Thin Solid Films 63 (1979) 77. J.C. Arnault, J. Delafond, C. Templier, J. Chaumont, O. Enea, Nucl. Instrum. Methods Phys. Res. Sect. B 81 (1993) 1384. P.H. Bolt, E. Ten Grotenhuis, J.W. Geus, F.H.P.M. Habraken, Surf. Sci. 329 (1995) 227. H. Brune, Surf. Sci. Rep. 31 (1998) 121. V.M. Jime´nez, J.P. Espino´s, A.R. Gonza´lez-Elipe, Surf. Interface Anal. 26 (1998) 62. Ch.T. Campbell, Surf. Sci. Rep. 27 (1997) 1. A.I. Martin-Concepcio´n, F. Yubero, J.P. Espinos, J. Garcia, S. Tougaard, Surf. Interface Anal. 35 (2003) 984–990. S. Oswald, F. Oswald, J. Appl. Phys. 100 (2006) 104504. S. Tougaard, J. Vac. Sci. Technol. A 14 (1996) 1415. S. Tougaard, Surf. Interface Anal. 26 (1998) 249. E.C. Samano, G. Soto, J. Valenzuela, L. Cota, J. Vac. Sci. Technol. A 15 (1997) 2585. S. Tougaard, QUASES, Software package for quantitative XPS/AES of surface nanostructures by inelastic peak shape analysis, http://www.QUASES.com. M.P. Cruz, J.A. Dı´az, J.M. Siqueiros, Int. J. Mod. Phys. B 18 (23 & 24) (2004) 3169– 3176. S. Tougaard, Surf. Interface Anal. 11 (1988) 453. W. De La Cruz, L. Cota Araiza, Phys. Stat. Sol. (b) 220 (2000) 569. S. Zhu, X. Xiang, X.T. Zu, L.M. Wang, Nucl. Instrum. Methods. Phys. Res. B 242 (2006) 114–117. A. Jablonski, S. Tougaard, Surf. Sci. 432 (1999) 211.
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
First nucleation steps of nickel nanoparticle growth on Al2O3 (0 0 0 1) studied by XPS inelastic peak shape analysis C. Gallardo-Vega a,*, W. De La Cruz b, S. Tougaard c, L. Cota-Araiza b a
Centro de Investigacio´n Cientı´fica y de Educacio´n Superior de Ensenada (CICESE), Km 107 Carretera Tijuana-Ensenada, C.P. 22860, Ensenada, B.C., Mexico Centro de Nanociencias y Nanotecnologı´a, Universidad Nacional Auto´noma de Me´xico, Km 107 Carretera Tijuana-Ensenada, C.P. 22860, Ensenada, B.C., Mexico c Department of Physics and Chemistry, University of Southern Denmark, DK-5230, Odense M, Denmark b
A R T I C L E I N F O
A B S T R A C T
Article history: Received 1 February 2008 Received in revised form 11 August 2008 Accepted 24 August 2008 Available online 29 August 2008
A series of Ni nanoparticles has been deposited on sapphire (Al2O3 (0 0 0 1)) substrates using the pulsed laser deposition technique. The amount of material deposited has been controlled by means of the number of laser pulses utilized. The substrate temperature was varied from room temperature to 500 8C. The nanoparticles deposited were characterized in situ by X-ray photoelectron spectroscopy. The inelastic peak shape of O 1s was analyzed to obtain the mode of growth of the Ni nanoparticles. The results show the height of the Ni nanoparticles increases with deposition from 1 to 9 nm and the surface coverage increased simultaneously from 0.1 to 0.85. For 200 or more laser pulses, as the substrate temperature increased (300–500 8C) the height of the nanoparticles increased. On the other hand, the coverage always decreased as a function of substrate temperature. This implies that the mobility of the deposited Ni increases with substrate temperature thus forming taller islands with corresponding smaller coverage. ß 2008 Elsevier B.V. All rights reserved.
Keywords: Nanoparticles Ni XPS Pulsed laser deposition
1. Introduction Nano-structured ferromagnetic (Fe, Co and Ni) particles dispersed in dielectric matrices (SiO2, Al2O3, TiO2 etc.) have been intensively studied in recent years, because of their remarkable magnetic and catalytic properties, when they are produced on the nanometer scale. The most notable magnetic properties are: giant magnetoresistance, enhanced coercivity, superparamagnetism and low saturation magnetization [1,2]. These outstanding properties take place due to quantum size effects and the large surface area of the nanoparticles compared with their bulk counterpart. With respect to catalytic properties, determination of the surface area and the nanoparticle size are very important in the development of new highly efficient catalytic systems. Nickelbased supported catalysts have been widely employed in various important industrial chemical processes, including hydrogenation, reforming, methanation, desulphurization and dechlorination processes. These catalysts are generally synthesized by chemical methods and the major problem encountered in such methods is the formation of mixed oxides of nickel and the support. As a consequence, high temperatures (>600 8C) are required to reduce
* Corresponding author at: CNyN-UNAM, P.O. Box 439036, San Ysidro, CA 921439036, USA. Tel.: +1 52 646 1744602; fax: +1 52 646 1744603. E-mail address: [email protected] (C. Gallardo-Vega). 0169-4332/$ – see front matter ß 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2008.08.061
the Ni (II) cations to metallic nickel particles and this leads to sintering of the metal [3]. The initial stages in the growth of thin films (i.e., first nucleation steps) are critical for the evolution of the microstructure of the film as the thickness increases [4]. Typically, transmission electron microscopy (TEM) or scanning electron microscopy (SEM) [5] have been applied to study the initial stages of growth of thin films and more recently, atomic force microscopy (AFM) and similar probes were used [6,7]. However, depending on the system, the use of those techniques may have important limitations. This is particularly evident when the materials of the substrate and deposited layer are not clearly differentiated. For instance, TEM images of two materials with similar electron density or AFM when the substrate is relatively rough relative to the size of the deposited particles. For flat substrates and single crystals, the analysis of the initial stages of growth of thin films up to a few monolayers can be carried out by X-ray photoemission spectroscopy (XPS), by varying the detection angle and comparing the relative intensities of the peaks of the substrate and the deposited material [8,9]. While this angle resolved XPS (ARXPS) method works well for flat surfaces it gives erroneous results for nanoparticles grown on a flat surface because of shadowing effects that are inevitable for the larger emission angles needed in this technique [10] and its applicability is therefore quite limited since a priory knowledge (which is typically not available because it is actually the purpose of the analysis to find that) on the nanostructure is therefore needed
C. Gallardo-Vega et al. / Applied Surface Science 255 (2008) 3000–3003
[10,11]. Much more quantitative and detailed information on the nanostructure of surface films can be obtained by analysis of the energy distribution in an XPS spectrum over a wide energy range i.e. the peak intensity and the background signal of inelastically scattered electrons below the peak energy. This method which was developed by Tougaard et al. [12,13] gives a quite detailed and accurate characterization of the surface and it works for both flat and rough surface structures because all information is extracted from analysis of a single spectrum recorded at a fixed emission angle. In addition, unlike ARXPS, peak shape analysis is quite insensitive to the effect of elastic electron scattering and diffraction effects. The influence of these is only to slightly modify the apparent depths by an amount which is negligible for 1 nm depth increasing to 10–30% for depths of 5 nm [13,20]. With the XPS peak shape analysis method, it is thus easy to determine whether the film formation mechanism is a layer by layer, island growth or layer plus island growth [13]. In the present paper we show how pulsed laser deposition (PLD) can be applied successfully to study the initial growth of Ni nanoparticles. We apply the above mentioned peak shape analysis method to determine the height of the first nuclei of the deposited nanoparticles. Thus, the height and coverage were studied as a function of the amount of deposited material (that is, the number of laser pulses) and as a function of substrate temperature in the range from room temperature to 500 8C. 2. Experimental The Ni nanoparticles were deposited in a commercially available Riber LDM-32 Laser ablation system, equipped with Cameca Mac3 electron energy analyzer. The system allows for in situ characterization by Auger electron spectroscopy (AES) and XPS. Technical details of the system are given elsewhere [14]. The deposits were produced with a Kr–F excimer laser (l = 248 nm, with 20 ns pulse width) at a base pressure of 5 10 8 Torr during the deposition. The target used was 99.99%, high purity nickel, obtained from Target Materials, Inc. All nanoparticle deposits were made under the same laser condition, namely 400 mJ per pulse, a pulsed frequency of 2 Hz and a laser fluence of 5 J/cm2 at the target surface. All Ni nanoparticles were deposited on the (0 0 0 1) sapphire substrate at three substrate temperatures: room temperature (RT), 300 and 500 8C. The numbers of laser pulses used in the experiments were 50, 100, 200, 300, 400, 500 and 600. After each deposition, the sample was analyzed in situ by XPS in order to check the quality of the deposit and the amount of material
3001
deposited. The substrates were cleaned in situ by laser ablation before Ni nanoparticles were deposited. XPS spectra were recorded after each deposition step using Al Ka radiation to excite the core electrons. For the XPS peak shape analysis, the O 1s photoemission peak of the sapphire substrate/Ni nanoparticles system was used. Each spectrum was analyzed using the methodology and software known as QUASES [15] which implements the XPS peak shape analysis method developed by Tougaard et al. 3. Results and discussions When any material is deposited directly on as-received sapphire substrates, the presence of carbon impurities and of native oxide on the substrate surface can influence the growth mechanism. Traditionally, wet chemical techniques and the annealing process are used to remove any impurities on the substrate surface. In this work, a quick and relatively simple technique for stripping off all impurities was used, which consisted in ablating the substrate surface with the pulsed laser (before the metal deposition). This method was proposed previously by Cruz et al. [16] as a useful tool to clean the surface of silicon wafers. The laser cleaning process consisted in scanning the laser beam on the substrate surface. Fig. 1(a) shows an XPS spectrum of the asreceived sapphire substrate surface, with the carbon signal being the main contaminant. After the cleaning process, the carbon signal practically disappeared (Fig. 1(b)), so the substrate is ready for nanoparticle deposition. After the cleaning process, the substrate surface was checked by Reflection High Energy Electron Diffraction and the diffraction pattern (not shown here) indicates that the crystallinity of the surface is preserved. Fig. 2 shows the Ni 2p, Ni 2s and O KLL photoemission spectra for nickel nanoparticles deposited by PLD on the (0 0 0 1) sapphire substrate at 500 8C, for different numbers of pulses: (a) 0, (b) 100, (c) 300, (d) 500, (e) 800 and (f) 1000 pulses. These spectra clearly show that the intensity of the Ni 2p and Ni 2s peaks increase and the O KLL signal peak decreases when the number of laser pulses is increased and at the same time the background intensity of inelastically scattered electrons on the low energy side of the peaks also change. Similar effects are observed for the O 1s peak from the sapphire substrate shown in Fig. 3a. Here the O 1s peak intensity decreases while the shape of the spectrum below the peak changes strongly when nickel particles were deposited using: (a) 0, (b) 100, (c) 300, (d) 500, (e) 800 and (f) 1000 pulses. This change in peak shape is due to the increased fraction of photon excited O 1s
Fig. 1. XPS spectra of sapphire substrates (Al2O3). (a) Before laser ablation cleaning treatment. (b) After laser ablation cleaning treatment.
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C. Gallardo-Vega et al. / Applied Surface Science 255 (2008) 3000–3003
Fig. 2. Signals of Ni 2p3/2 and O KLL in XPS spectra for Ni particles deposited at a substrate temperature of 500 8C, from an over/layer experiment for an increasing number of laser pulses: (a) 0, (b) 100, (c) 300, (d) 500, (e) 800 and (f) 1000 pulses.
electrons from the substrate that have undergone inelastic scattering events as the Ni overlayer increases. The detailed change in the O 1s peak shape is determined by the size distribution of the Ni-nanoparticles which can thus be extracted with the peak shape analysis procedure. The position of the Ni 2p3/ 2 peak was fixed at 613.37 eV for all the deposits. The photoelectron peak shape analysis, including the background behind the photoemission peaks, was done according to Tougaard’s method [12,13] using the QUASES software [15]. With this analysis, it is possible to determine, for each deposit, the height of the nanoparticles and the fraction of the substrate surface that is covered (coverage). It is important to remark that these analyses
Fig. 4. Island height vs. surface coverage of Ni nanoparticles deposited at different number of laser pulses and substrate temperatures. The series of dashed lines represent curves of constant amount of deposited material.
depend on whether there is a clear change in the height and shape of the background intensity and the uncertainty in the determined nanostructure parameters is larger for small deposits [13]. The calculations to obtain the nanoparticles height and coverage have been made using the O 1s peak from the sapphire substrate (see Fig. 3b) and under the assumption that all the deposits have a homogeneous distribution of island heights [17]. The results of these analyses are shown in Fig. 4. The series of dashed lines in Fig. 4 represent curves with fixed amount of deposited material (ADM). ADM for any deposit is calculated as the product of the height of the nanoparticles times its respective coverage. The result of this product is used to predict the height of nanoparticles for any
Fig. 3. (a) O 1s spectra used to perform XPS inelastic peak shape analysis, applying the QUASES software and methodology. The different spectra correspond to the deposition of Ni particles for a different number of laser pulses: (a) 0, (b) 100, (c) 300, (d) 500, (e) 800 and (f) 1000 pulses. (b) Curves corresponding to the experimental spectra, calculated background and primary excited spectra are reported.
C. Gallardo-Vega et al. / Applied Surface Science 255 (2008) 3000–3003
coverage (or conversely). With this procedure, the ADM shown in Fig. 4 was obtained using the deposits at 500 8C as a guide. ADM can be associated to the number of pulses used during the over/ layer experiments. As the number of pulses is increased the ADM also increases. In Fig. 4 we show the height and coverage of the Ni-particles corresponding to three different series of nickel nanoparticles. Each series corresponds to a constant substrate temperature: room temperature (RT), 300 and 500 8C. Each point in Fig. 4 corresponds to nanoparticles produced under specific deposition conditions. The first point of each series corresponds to a deposit made with the first 50 laser pulses, after the substrate was cleaned. The second point of each series represents the deposit with 100 laser pulses and successively up to 600 pulses. For the first 50 pulses, the coverage was the same (10%) and the height of nanoparticles change from 2 (deposit at RT) to 1 nm (deposit at 300 or 500 8C). For deposits corresponding to more than 200 pulses, the nanoparticles deposited at RT showed a larger coverage, and smaller height than nanoparticles deposited at substrate temperatures of 300 or 500 8C. Deposits at RT showed an island shape, with heights in the 2–7 nm range, while nanoparticles deposited on a heated substrate showed heights in the 1–9 nm range (see Fig. 4). For those deposits, both the height and the coverage of the nanoparticles increased as the number of laser pulses was increased. Fig. 4 also shows that for a given number of pulses >200, the nanoparticle height increases with substrate temperature while the coverage decreases. This implies that the deposited Ni is more mobile on the surface at elevated temperatures which allows for coalescence of the metal after deposition, and therefore, the material will form nanoparticles of greater height for the same amount of material, thus causing the coverage to decrease. The heights of the Ni nanoparticles reported in the present work show that PLD is a good technique to grow Ni nanoparticles. Similar values of particle height were reported for Fe nanoparticles produced by PLD [18]. The heights obtained are very important in catalysis, since this parameter is associated with a higher catalytic activity. Similar values of Ni nanoparticle height have been reported by Z. Chu et al. [19] for nanoparticles deposited on MgO showing super-paramagnetic behavior. 4. Conclusions In this work Ni particles were produced by PLD, demonstrating that this is a powerful method to grow nanoparticles in a controlled manner. These results may provide opportunities to design structures having physical properties with potential
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technological applications. The heights of the nanoparticles were in the 1–9 nm range and the coverage 0.1–0.85. The results also show that for 200 laser pulses or more (ADM), the height of Ni nanoparticles increases, as the substrate temperature increases, while at the same time the coverage decreases, in comparison with deposits performed with the substrate at room temperature. At room temperature, the height was lower and the coverage was larger. Thus, for substrate temperatures 300–500 8C, the deposited Ni is more mobile and coalesce to form taller islands while the coverage is correspondingly decreased. Finally, these results show how XPS background analysis can be a useful tool to assess and understand the growth mechanism of nanoparticles and the parameters involved in the deposition techniques employed. Acknowledgments The authors are grateful to J.A. Diaz, E. Aparicio, D. Domı´nguez, F. Ruiz, I. Gradilla, V. Garcı´a, P. Casillas, M. Saenz, and J. Peralta for technical assistance. This work was partially supported by CONACYT, projects no. 52486 and 50203, DGAPA IN120206-02 and the Danish Agency for Science, Technology and Innovation, project no. 95-305-23359. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20]
W.D. Wang, F.W. Zhu, J. Weng, J.M. Xiao, W.Y. Lai, Appl. Phys. Lett. 72 (1998) 1118. D.L. Leslie-Pelecky, R.D. Rieke, Chem. Mater. 8 (1996) 1770. S. Velu, S.K. Gangwal, Solid State Ionics 177 (2006) 803–811. L. Pranevicius, Thin Solid Films 63 (1979) 77. J.C. Arnault, J. Delafond, C. Templier, J. Chaumont, O. Enea, Nucl. Instrum. Methods Phys. Res. Sect. B 81 (1993) 1384. P.H. Bolt, E. Ten Grotenhuis, J.W. Geus, F.H.P.M. Habraken, Surf. Sci. 329 (1995) 227. H. Brune, Surf. Sci. Rep. 31 (1998) 121. V.M. Jime´nez, J.P. Espino´s, A.R. Gonza´lez-Elipe, Surf. Interface Anal. 26 (1998) 62. Ch.T. Campbell, Surf. Sci. Rep. 27 (1997) 1. A.I. Martin-Concepcio´n, F. Yubero, J.P. Espinos, J. Garcia, S. Tougaard, Surf. Interface Anal. 35 (2003) 984–990. S. Oswald, F. Oswald, J. Appl. Phys. 100 (2006) 104504. S. Tougaard, J. Vac. Sci. Technol. A 14 (1996) 1415. S. Tougaard, Surf. Interface Anal. 26 (1998) 249. E.C. Samano, G. Soto, J. Valenzuela, L. Cota, J. Vac. Sci. Technol. A 15 (1997) 2585. S. Tougaard, QUASES, Software package for quantitative XPS/AES of surface nanostructures by inelastic peak shape analysis, http://www.QUASES.com. M.P. Cruz, J.A. Dı´az, J.M. Siqueiros, Int. J. Mod. Phys. B 18 (23 & 24) (2004) 3169– 3176. S. Tougaard, Surf. Interface Anal. 11 (1988) 453. W. De La Cruz, L. Cota Araiza, Phys. Stat. Sol. (b) 220 (2000) 569. S. Zhu, X. Xiang, X.T. Zu, L.M. Wang, Nucl. Instrum. Methods. Phys. Res. B 242 (2006) 114–117. A. Jablonski, S. Tougaard, Surf. Sci. 432 (1999) 211.