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XPS studies of pulsed laser induced surface modification of vanadium phosphate glass samples
Journal of Physics and Chemistry of Solids 74 (2013) 13–17
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XPS studies of pulsed laser induced surface modification of vanadium phosphate glass samples G.D. Khattak n, A. Mekki, M.A. Gondal Physics Department, King Fahd University of Petroleum & Minerals, Dhahran 31261, Saudi Arabia
a r t i c l e i n f o
abstract
Article history: Received 8 June 2011 Received in revised form 26 May 2012 Accepted 13 July 2012 Available online 17 August 2012
Vanadium phosphate glass with the nominal chemical composition [(V2O5)0.5(P2O5)0.5] was irradiated using three different wavelengths (IR, Visible and UV) generated from Nd:YAG laser. The effect of this laser irradiation on the local glass structure as well as on the valence state of V ions was investigated by X-ray photoelectron spectroscopy (XPS). The core level spectra V 2p, O 1s, P 2p and C 1s have been recorded under the same conditions and analyzed. A decrease in the intensity of the V 2p core level peaks was observed indicating a gradual loss of V ions from the surface of the sample with a change of the laser wavelength from UV(355 nm) to IR(1064 nm) region. The O 1s and P 2p core level peaks also showed a significant decrease in intensity for the sample irradiated with 1064 nm laser. Asymmetries found in the O 1s, P 2p, and V 2p core level spectra indicate the presence of primarily P–O–P, P–O–V and V–O–V structural bonds, a spin–orbit splitting of P 2p core level, and more than one valence state of V ions being present. The curve fitting of the V 2p spectra for the unirradiated sample showed that vanadium ions are in V3 þ , V4 þ and V5 þ states while for the irradiated glass samples vanadium ions are mainly in the V3 þ and V4 þ states. The O 1s spectra were all curved fitted with two contributions, one from the presence of oxygen atoms in the P–O–P, P–O–V, V–O–V environment (bridging oxygen BO) and the other from oxygen atoms in P¼O environment (non-bridging oxygen NBO). The ratio of NBO to total oxygen was found to decrease with an increase in incident laser energy. & 2012 Elsevier Ltd. All rights reserved.
Keywords: A. Amorphous materials A. Glasses A. Non-crystalline materials B. Laser annealing C. Photoelectron spectroscopy
1. Introduction Vanadate and phosphate glasses continue to be of great interest because of their unique properties and their potential suitability for various applications. For example, binary and ternary V2O5 glasses can exhibit a semiconducting behavior [1–4] which arises from an unpaired 3d1 electron hopping between the transition metal (TM) ions [5–6] when the TM ions exist in two or more valence states, i.e., an electron hopping from a V4 þ site to a V5 þ site. On the other hand, the low thermooptical coefficient and large emission characteristics found in phosphate glasses make them suitable materials for high power laser devices [7]. Furthermore, most biocompatible glasses are based on phosphate glasses. In either glass system, however, information on the structure of a glass is imperative for understanding their electronic, optical, magnetic and mechanical properties and assessing their suitability for technological applications such as memory switching and laser devices. It is well known that laser irradiation can modify the surface chemistry of transition metal oxide glasses [8–11]. There are many
n
Corresponding author. Tel.: þ966 38602260; fax: þ 966 38602293. E-mail address: [email protected] (G.D. Khattak).
0022-3697/$ - see front matter & 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.jpcs.2012.07.010
interesting applications of surface modification using high intensity laser beams such as hardening of materials, bonding of different materials or alloys, thin films for corrosion inhibition, etc. There exist two conditions in laser–matter interaction experiments that lead to many applications and motivate the researchers to apply this technique. The first is that intense lasers can locally heat matter above 1000 1C in very short time (nanoseconds) which induces different photochemical reactions at the surface only. The second technique is that upon high power laser irradiation, the optical properties of materials could be modified [8–11]. The effect of the laser irradiation could be up to few microns depth, while the time of the laser irradiation is few nanoseconds. Hence, this is a surface effect and therefore, we used XPS, which is a surface investigation technique, to study the effect of laser irradiation on the glass samples. Recently thermal oxidation or reduction in air has been reported on several soda-lime silicate glasses, each of which contained one of the following polyvalent metals: Fe, Mn, Cu, Ce, Ti, V and Cr [12–14]. In this work polyvalent metals were oxidized in air and reduced in H2/N2 at their respective glass transition temperature for some period. During the heat treatment, a crystalline oxide surface layer is created on the glasses under the oxidizing condition, and the metallic ions are oxidized from lower to higher valence state. These ions diffuse outward or
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inward depending on the type of the polyvalent ions under the same heat-treatment conditions. Most of these experiments have been carried out at high temperatures above 600 1C which require special setups including high vacuum systems and controlled environment such as H2 or N2. Since the incident Nd:YAG laser has a pulse width of 8 ns the laser irradiation could raise the temperature of the glass surface rapidly (in nanoseconds) to 1000 1C which is above the glass transition temperature (Tg). Therefore the laser post treatment of glasses for reduction of TM ions has the advantages of being cleaner, with shorter processing time and lower costs, as it does not require costly and sophisticated vacuum systems like conventional thermal treatment systems. Hence, laser treatment using a pulsed laser is beneficial for fast (at nanosecond scale) thermal processing of glasses. The free electrons absorb the laser light, transferring this energy into the atomic lattice [15–19]. Depending upon the incident laser energy, melting and some vaporization of the target material could occur during this process. Because of the difference in the latent heat of vaporization for different chemical materials (or elements present in glass material), a thermal mechanism may induce breaking of the glass structure due to thermal stress. Moreover, if the incident laser photon energy is higher than the bonding energy between neighboring atoms in the glass matrix, the electromagnetic laser radiation can directly break the atomic lattice, inducing ion and electron ejection without traditional heating effects. This happens mostly under high intensity laser beam influence to create breakdown (plasma) on the surface of glass by focusing the laser beam at a small focal point. In this experiment we did not use any focusing lens to reach the plasma state at the glass surface rather laser radiations were employed only for thermal treatment for the reduction of V states. The above mentioned facts and advantages are key factors for our motivation to use laser radiation instead of conventional heating setups for reduction of V sates. In this paper we investigated the effect of laser irradiation using different wavelengths (355, 532, 1064 nm) on the local glass structure as well as on the valence state of the vanadium ions of phosphate glass containing V2O5 by X-ray photoelectron spectroscopy (XPS). The XPS technique has proven to be powerful for investigating not only the electronic structure in solids, but also the bonding and hence the local structure [20–22]. More recently, this technique has been shown to be a useful quantitative probe of the short-range structure in the oxide glasses as well. In particular, the O 1s spectrum is typically resolved into separate contributions from bridging and non-bridging oxygen resulting from different structural units of oxygen atoms in the oxide glasses. The XPS technique has also been successfully used to investigate quantitatively the valence state of TM ions in the glass structure. The structural and electronic (as well as optical, magnetic and mechanical) properties of these glasses depend on the relative proportion of different valence states of the TM ions. In order to account for the effect of these valence states on the structure and properties of these glasses, it is important to control and measure the ratios of the ion concentration in the different valence states of these TM ions. This is a continuation of our previous work carried out on the diffusion of copper ions in phosphate glass [23]. In that study, we observed a change in the oxidation state of Cu ions upon laser irradiation. We also found a decrease in the intensity of the Cu 2p core level peak with an increase in laser irradiation wavelength. This reduction in intensity could be as a consequence of the evaporation of some copper ions from the surface or the diffusion of copper ions into the bulk or both but no conclusion can be reached based on the available data. In the present work we have the same glass family/matrix (phosphate glass) with CuO replaced by V2O5 in order to study the dependence of the effect of laser irradiation on the TM ions present in the glasses.
2. Experimental details 2.1. Glass sample preparation The glass with nominal composition 0.50(V2O5)–0.50(P2O5) was prepared by melting dry mixtures of reagent grade V2O5 and P2O5 in alumina crucibles. P2O5 was preheated at 150 1C before mixing it with V2O5 at the calculated ratio. Approximately 20 g of chemicals were thoroughly mixed manually to obtain a homogenized mixture. The crucible containing the batch mixture was then placed in an electrically heated melting furnace and maintained at 1100–1200 1C for about 1–2 h under atmospheric conditions during which the melt was occasionally stirred with an alumina rod. The homogenized melt was then cast onto a stainless steel plate mold to form glass buttons for laser irradiation and glass rods of approximately 5 mm diameter for XPS measurements. X-ray powder diffraction analysis indicated that the glass formed was completely amorphous. 2.2. X-ray photoelectron spectroscopy (XPS) measurements Core level photoelectron spectra were collected on a VG scientific ESCALAB MKII spectrometer equipped with a dual aluminum– magnesium anode X-ray gun and a 150 mm concentric hemispherical analyzer using Al Ka (hn ¼1486.6 eV) radiation from an anode operated at 130 W. Photoelectron spectra of V 2p, O 1s, P 2p as well as C 1s core levels were recorded using a computer-controlled data collection system with the electron analyzer set at a pass-energy of 20 eV for the high resolution scans. For self-consistency, the C 1s transition at 284.6 eV was used as a reference for all charge shift corrections as this peak arises from hydrocarbon contamination and its binding energy is generally accepted as remaining constant, irrespective of the chemical state of the sample. For XPS measurements, a glass rod was cleaved in the preparation chamber at a base pressure of 2 10 9 mbar before being transferred to the analysis chamber where the pressure was maintained at o2 10 10 mbar. A non-linear, least-squares algorithm was employed to determine the best fit to each of the O 1s core level spectra with two Gaussian–Lorentzian curves in order to represent bridging and non-bridging oxygen. The fitting of the V 2p core level spectra was carried out to three Gaussian–Lorentzian peaks corresponding to three oxidation states (V3 þ , V4 þ , and V5 þ ) respectively. The fraction of non-bridging oxygen and V3 þ , V4 þ , and V5 þ were determined from the respective area ratios from these fits. Based on the reproducibility of similar quantitative spectral decompositions of spectra taken from other surfaces on the same glass samples, uncertainties of 75% and 710%, respectively, were estimated for these area ratios. A period of approximately2 h was required to collect the data set for each sample and there was no evidence of any X-ray induced reduction of the vanadium in the glasses during this period. 2.3. Laser irradiation Nd: YAG laser (Quanta-Ray, Model GCR-250) was applied for irradiation of glass samples at three different laser wavelengths. Since Nd:YAG laser emits a fundamental wavelength at 1064 nm, for the generation of second and third harmonic at 532 and 355 nm, frequency doubling and tripling was obtained by using a KDP crystal. The pulse width of the laser was about 8 ns and the rep rate was 10 Hz. In order to compare, the effect of different wavelengths (1064, 532,335 nm) of laser radiations on various glass samples, the laser intensity, laser irradiation time and laser beam diameter were kept constant. The laser energy was measured with a calibrated energy meter (ophir model 300). In this experiment, we placed a 5 mm diameter aperture inside the path
G.D. Khattak et al. / Journal of Physics and Chemistry of Solids 74 (2013) 13–17
of laser beam in order to get a uniform beam shape. The pulse energy at different wavelengths was 100 mJ and the laser fluence (peak power/area) estimated was 1.27 1011 W/ m2. In order to expose the sample homogeneously with the laser beam and to avoid the development of any crust on the surface of glass samples due to repeated exposure of laser on the same spot, the samples were mounted on a rotary table.
3. Results and discussion 3.1. V 2p spectra The V 2p spin–orbit doublet spectra for the unirradiated and irradiated glass samples are depicted in Fig. 1. It is observed that the intensities of the V 2p peaks decrease with increasing laser wavelength. It appears clearly from the figure that for the sample irradiated at 1064 nm, the decrease is quite significant. It is worth noting that all four samples were analyzed under similar conditions, i.e., X-ray energy, position between the X-ray source and samples as well as sampling time were kept the same. In addition, the V 2p peaks show significant asymmetry for both the irradiated and the unirradiated samples which indicates that V ions exist in more than one oxidation state. On the basis of our study performed on vanadium phosphate glasses [24], the fitting of the V 2p3/2 core
Intensity (arb. units)
V 2p
unirradiated 355 nm
15
level spectra for both the unirradiated and irradiated samples was carried out with three Gaussian-Lorentzian peaks corresponding to three oxidation states (V3 þ , V4 þ , and V5 þ ) respectively. This procedure was adopted because the initial fitting of the V 2p3/2 spectra to two Gaussian–Lorentzian peaks with the lower binding energy peak corresponding to V4 þ and the higher binding energy peaks to V5 þ lead to some quantitative inconsistencies with the magnetic analysis [24]. The satellite peak around 521 eV was fitted to a single Gaussian–Lorentzian peak and assumed to be associated with V5 þ since this satellite peak has been previously observed in the spectrum of pure V2O5 [25]. Hence the area under the satellite peak obtained from the curve fitting was added to the area of peak 3 (V5 þ ) to find the relative abundance of V5 þ . The relative abundance of these oxidation states (V3 þ , V4 þ and V5 þ ) is shown in Table 1. We do not expect to have metallic V1 in our samples and therefore its contribution was not included in the fitting. Such fitting is depicted in Fig. 2 for irradiated sample with the 355 nm laser wavelength. The decrease in intensity of the V 2p spectra for the irradiated samples is relatively smaller than that of Cu 2p spectra [23]. This could be as a consequence of the evaporation of vanadium ions from the surface due to the high temperature induced by the laser on the surface of the sample or the diffusion of vanadium into the bulk or both processes happening simultaneously. No definite conclusion could be reached based on the available data. However, the suggestion that it might be due to diffusion of vanadium from the surface toward the interior of the sample is supported by results reported in the literature [26,27]. In these studies, it is reported that thermal reduction of the transition metal can lead to inward diffusion of the mobile cations. The mechanism for irradiation could be the same. Similar behavior, as explained in the introduction, was observed in a study carried out on a copper phosphate glass as reported earlier by our group [23]. As clear from Table 1, V ions decrease with increasing incident radiation wavelength. V ions are mostly in the V4 þ and V5 þ states in the unirradiated sample. The reduction of V ions is though not very clear for the 355 nm laser radiation, it is quite obvious for the 532 nm and 1064 nm laser irradiated samples and the V ions mostly exist in V3 þ and V4 þ states.
532 nm 1064 nm 528
524
520
516
512
3.2. O 1s spectra
508 The O 1s core level spectra for the irradiated and unirradiated samples investigated in this study are shown in Fig. 3. The spectra were superimposed in order to clearly observe a decrease in the
Binding energy (eV) Fig. 1. Core level V 2p spectra for the unirradiated and irradiated glasses.
Table 1 Peak positions, FWHM, and relative abundances (relative areas) of the various V ions from the three-peak curve fitting of the V 2p core levelsþsatellite for the V2O5 phosphate glasses. Sample
Satellite FWHM area
V 2p3/2(1) FWHM area
V 2p3/2(2) FWHM area
V 2p3/2(3) FWHM area
V5þ / Vtotal
V3þ / Vtotal
V4þ / Vtotal
Unirradiated
520.54 1.90 90.09 520.40
515.71 2.00 1181 515.89
516.46 2.00 8809 516.66
517.74 2.00 1932 517.70
0.1683
0.0983
0.7333
0.1801
0.1255
0.6944
Irradiated (355 nm)
2.00 110.36
2.00 411.60
2.00 2278.3
2.00 480.56
Irradiated (532 nm)
520.48 2.00 78.64
515.96 2.00 1434.5
516.43 2.00 1909.0
517.78 2.00 215.878
0.0810
0.3943
0.5247
Irradiated (1064 nm)
–
515.60 2.00 471.89
516.40 2.00 471
517.70 2.00 14.456
0.0151 0.4929
0.4920
The uncertainty in the peak positions is 70.1 eV, FWHM 70.2 eV, and the relative areas 7 10%.
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G.D. Khattak et al. / Journal of Physics and Chemistry of Solids 74 (2013) 13–17
Fig. 2. Fitting of the V 2p3/2 core level spectra for the irradiated sample with laser wavelength 355 nm.
with the lower binding energy peak corresponding to NBO (P¼O and V¼O) and the higher energy peak to BO (P–O–P, P–O–V, V–O– V). The fitted spectrum is shown in Fig. 4 for the irradiated sample at 1064 nm laser radiation. The relative abundance of these oxygen sites determined from the respective area ratios from these fits is shown in Table 2. One notes from Table 2 that the measured area ratio, [NBO/TO], slightly decreases on laser radiation leading to an increase in bridging oxygen (BO) bonds and hence a compact structure [24].
O 1s
Intensity (arb. units)
Fig. 4. Fitting of the core level O 1s spectra for the irradiated sample at 1064 nm laser radiation. The resulting NBO and BO peaks from the least-squares fitting routine of two Gaussian–Lorentzian peaks are also shown.
unirradiatted
3.3. P 2p spectra
355 nm 532 nm 1064 nm 544
540
536
532
528
524
Binding energy (eV) Fig. 3. Core level O 1s spectra for the unirradiated and irradiated glasses.
peak intensities with an increase in the wavelength of the laser source. While the cation spectra provide some insight into the structure of these oxide glasses, the O 1s spectra are typically more informative as the binding energy of the O 1s electrons provides a measure of the extent to which electrons are localized on the oxygen or in the internuclear region. This is a direct consequence of the nature of the bonding between the oxygen and different cations. The appearance of an asymmetry in the O 1s spectra is not unexpected as it is well-known that the O 1s peak in glass can arise from different oxygen bonding sites. Correspondingly, the O 1s peaks for these V2O5–P2O5 glasses may arise from oxygen atoms existing in some or all of the following structural bonds: P–O–P, P–O–V, V–O–V, V¼O, and P¼O. Furthermore, the O1s peaks associated with these various structural bonds may be broaden by the vanadium ions existing in different valance states. Oxygen atoms that are more covalently bonded to glass former atoms on both sides are typically called bridging oxygen (e.g., P–O–P, V–O–V and P–O–V), while oxygen atoms that are more ionically bonded, at least on one side, or double bonded to a glass former atom are referred to as non-bridging oxygen atoms and have lower binding energies (e.g., P¼O, V¼O). The variation in binding energies of the different oxygen sites in different glass systems has been discussed previously in details [24]. Each O 1s spectrum was deconvoluted into two Gaussian–Lorentzian peaks
The P 2p core level spectra for all samples investigated are shown in Fig. 5. As one can notice in Fig. 5, the peak positions are independent of the laser irradiation wavelength. A decrease in intensity is observed which is quite significant for the 1064 nm laser irradiation. This decrease in intensity of the P 2p spectra was not observed for Cu-phosphate glasses [23]. This may be due to the fact that the bonding of P ions to oxygen and copper could be stronger as compared to their bonding in the vanadium phosphate glasses, but at present, we do not have any technique to confirm this hypothesis. However, the observed asymmetry in the peaks which resulted in the deconvolution of the P 2p spectra into two peaks could arise from the spin–orbit splitting of the P 2p core level resulting in the distinguishable P 2p3/2 and P 2p1/2 core levels with the lower binding energy peak corresponding to P 2p3/2. Since the predicted P 2p3/2/P 2p1/2 ratio of 2.0 [28] is within the uncertainty of the measured ratios of the relative areas of 2.170.1 eV, it is reasonable to conclude that the asymmetry in the P 2p spectra indeed arises from the spin–orbit splitting of the P 2p core level. This fitting is shown in Fig. 6 for the unirradiated sample and the relative abundances are shown in Table 2.
4. Conclusions Vanadium phosphate glass was irradiated using three different wavelengths (IR, Visible and UV) generated from Nd:YAG laser. The effect of the laser irradiation on the local glass structure as well as on the valence state of the vanadium ions has been investigated by X-ray photoelectron spectroscopy (XPS). The most apparent feature of the V 2p, O 1s and P 2p core level spectra is the decrease in their intensities with an increase of the laser wavelength. This decrease in intensity of V 2p spectra is probably
G.D. Khattak et al. / Journal of Physics and Chemistry of Solids 74 (2013) 13–17
17
Table 2 Peak positions, FWHM, areas, and relative abundance from the curve fitting of the O 1s and P 2p core levels. Sample
O 1s (I) FWHM area Abundance
O 1s (II) FWHM area Abundance
(NBO/Ototal)
P 2p3/2 FWHM area Abundance
P 2p1/2 FWHM area Abundance
P 2p3/2/P 2p1/2
Unirradiated
532.96 2.32 10210
531.34 2.03 31757
0.7567
133.69 2.00 3533
134.64 2.00 1712
2.063
Irradiated (355 nm)
532.65 2.32 7434.9
531.13 2.01 9983.4
0.5732
133.10 2.09 1547.8
134.49 2.09 779.89
1.985
Irradiated (532 nm)
532.83 2.30 7628.78
531.22 2.00 12919.5
0.6287
133.28 2.00 1901.59
134.50 2.00 885.924
2.146
Irradiated (1064 nm)
532.29 2.30 3543.5
530.79 2.00 4410.0
0.5545
133.40 2.00 4564.0
134.45 2.10 2176.0
2.097
The uncertainty in the peak positions is 7 0.1 eV, FWHM 7 0.2 eV, and areas 710%.
presence of primarily P–O–P, P–O–V, V–O–V, P¼O and V¼O structural bonds, a spin–orbit splitting of P 2p core level, and more than one valence state of V ions being present. The vanadium ions reduce to lower oxidation states with laser irradiation, the ratio [NBO/TO] decreases slightly with an increase in laser wavelength indicating the formation of a relatively compact structure and the P 2p3/2/P 2p1/2 ratio of 2.0 is within the uncertainty of the measured ratios.
Intensity (arb. units)
P 2p
unirradiated
Acknowledgments 355 nm
The support of the KFUPM Physics Department and Research Committee (Grant No. RG112-CS-02) is greatly acknowledged.
532 nm 1064 nm 120
125
130
135
140
145
Binding energy (eV) Fig. 5. Core level P 2p spectra for the unirradiated and irradiated glasses.
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20]
Fig. 6. Fitting of the P 2p spectra for the unirradiated sample.
due to diffusion of the V ions into the bulk of the sample which could be attributed to the thermal effect of the laser at longer wavelength as compared to shorter wavelengths. Asymmetries found in the O 1s, P 2p, and V 2p core level spectra indicate the
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XPS studies of pulsed laser induced surface modification of vanadium phosphate glass samples G.D. Khattak n, A. Mekki, M.A. Gondal Physics Department, King Fahd University of Petroleum & Minerals, Dhahran 31261, Saudi Arabia
a r t i c l e i n f o
abstract
Article history: Received 8 June 2011 Received in revised form 26 May 2012 Accepted 13 July 2012 Available online 17 August 2012
Vanadium phosphate glass with the nominal chemical composition [(V2O5)0.5(P2O5)0.5] was irradiated using three different wavelengths (IR, Visible and UV) generated from Nd:YAG laser. The effect of this laser irradiation on the local glass structure as well as on the valence state of V ions was investigated by X-ray photoelectron spectroscopy (XPS). The core level spectra V 2p, O 1s, P 2p and C 1s have been recorded under the same conditions and analyzed. A decrease in the intensity of the V 2p core level peaks was observed indicating a gradual loss of V ions from the surface of the sample with a change of the laser wavelength from UV(355 nm) to IR(1064 nm) region. The O 1s and P 2p core level peaks also showed a significant decrease in intensity for the sample irradiated with 1064 nm laser. Asymmetries found in the O 1s, P 2p, and V 2p core level spectra indicate the presence of primarily P–O–P, P–O–V and V–O–V structural bonds, a spin–orbit splitting of P 2p core level, and more than one valence state of V ions being present. The curve fitting of the V 2p spectra for the unirradiated sample showed that vanadium ions are in V3 þ , V4 þ and V5 þ states while for the irradiated glass samples vanadium ions are mainly in the V3 þ and V4 þ states. The O 1s spectra were all curved fitted with two contributions, one from the presence of oxygen atoms in the P–O–P, P–O–V, V–O–V environment (bridging oxygen BO) and the other from oxygen atoms in P¼O environment (non-bridging oxygen NBO). The ratio of NBO to total oxygen was found to decrease with an increase in incident laser energy. & 2012 Elsevier Ltd. All rights reserved.
Keywords: A. Amorphous materials A. Glasses A. Non-crystalline materials B. Laser annealing C. Photoelectron spectroscopy
1. Introduction Vanadate and phosphate glasses continue to be of great interest because of their unique properties and their potential suitability for various applications. For example, binary and ternary V2O5 glasses can exhibit a semiconducting behavior [1–4] which arises from an unpaired 3d1 electron hopping between the transition metal (TM) ions [5–6] when the TM ions exist in two or more valence states, i.e., an electron hopping from a V4 þ site to a V5 þ site. On the other hand, the low thermooptical coefficient and large emission characteristics found in phosphate glasses make them suitable materials for high power laser devices [7]. Furthermore, most biocompatible glasses are based on phosphate glasses. In either glass system, however, information on the structure of a glass is imperative for understanding their electronic, optical, magnetic and mechanical properties and assessing their suitability for technological applications such as memory switching and laser devices. It is well known that laser irradiation can modify the surface chemistry of transition metal oxide glasses [8–11]. There are many
n
Corresponding author. Tel.: þ966 38602260; fax: þ 966 38602293. E-mail address: [email protected] (G.D. Khattak).
0022-3697/$ - see front matter & 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.jpcs.2012.07.010
interesting applications of surface modification using high intensity laser beams such as hardening of materials, bonding of different materials or alloys, thin films for corrosion inhibition, etc. There exist two conditions in laser–matter interaction experiments that lead to many applications and motivate the researchers to apply this technique. The first is that intense lasers can locally heat matter above 1000 1C in very short time (nanoseconds) which induces different photochemical reactions at the surface only. The second technique is that upon high power laser irradiation, the optical properties of materials could be modified [8–11]. The effect of the laser irradiation could be up to few microns depth, while the time of the laser irradiation is few nanoseconds. Hence, this is a surface effect and therefore, we used XPS, which is a surface investigation technique, to study the effect of laser irradiation on the glass samples. Recently thermal oxidation or reduction in air has been reported on several soda-lime silicate glasses, each of which contained one of the following polyvalent metals: Fe, Mn, Cu, Ce, Ti, V and Cr [12–14]. In this work polyvalent metals were oxidized in air and reduced in H2/N2 at their respective glass transition temperature for some period. During the heat treatment, a crystalline oxide surface layer is created on the glasses under the oxidizing condition, and the metallic ions are oxidized from lower to higher valence state. These ions diffuse outward or
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inward depending on the type of the polyvalent ions under the same heat-treatment conditions. Most of these experiments have been carried out at high temperatures above 600 1C which require special setups including high vacuum systems and controlled environment such as H2 or N2. Since the incident Nd:YAG laser has a pulse width of 8 ns the laser irradiation could raise the temperature of the glass surface rapidly (in nanoseconds) to 1000 1C which is above the glass transition temperature (Tg). Therefore the laser post treatment of glasses for reduction of TM ions has the advantages of being cleaner, with shorter processing time and lower costs, as it does not require costly and sophisticated vacuum systems like conventional thermal treatment systems. Hence, laser treatment using a pulsed laser is beneficial for fast (at nanosecond scale) thermal processing of glasses. The free electrons absorb the laser light, transferring this energy into the atomic lattice [15–19]. Depending upon the incident laser energy, melting and some vaporization of the target material could occur during this process. Because of the difference in the latent heat of vaporization for different chemical materials (or elements present in glass material), a thermal mechanism may induce breaking of the glass structure due to thermal stress. Moreover, if the incident laser photon energy is higher than the bonding energy between neighboring atoms in the glass matrix, the electromagnetic laser radiation can directly break the atomic lattice, inducing ion and electron ejection without traditional heating effects. This happens mostly under high intensity laser beam influence to create breakdown (plasma) on the surface of glass by focusing the laser beam at a small focal point. In this experiment we did not use any focusing lens to reach the plasma state at the glass surface rather laser radiations were employed only for thermal treatment for the reduction of V states. The above mentioned facts and advantages are key factors for our motivation to use laser radiation instead of conventional heating setups for reduction of V sates. In this paper we investigated the effect of laser irradiation using different wavelengths (355, 532, 1064 nm) on the local glass structure as well as on the valence state of the vanadium ions of phosphate glass containing V2O5 by X-ray photoelectron spectroscopy (XPS). The XPS technique has proven to be powerful for investigating not only the electronic structure in solids, but also the bonding and hence the local structure [20–22]. More recently, this technique has been shown to be a useful quantitative probe of the short-range structure in the oxide glasses as well. In particular, the O 1s spectrum is typically resolved into separate contributions from bridging and non-bridging oxygen resulting from different structural units of oxygen atoms in the oxide glasses. The XPS technique has also been successfully used to investigate quantitatively the valence state of TM ions in the glass structure. The structural and electronic (as well as optical, magnetic and mechanical) properties of these glasses depend on the relative proportion of different valence states of the TM ions. In order to account for the effect of these valence states on the structure and properties of these glasses, it is important to control and measure the ratios of the ion concentration in the different valence states of these TM ions. This is a continuation of our previous work carried out on the diffusion of copper ions in phosphate glass [23]. In that study, we observed a change in the oxidation state of Cu ions upon laser irradiation. We also found a decrease in the intensity of the Cu 2p core level peak with an increase in laser irradiation wavelength. This reduction in intensity could be as a consequence of the evaporation of some copper ions from the surface or the diffusion of copper ions into the bulk or both but no conclusion can be reached based on the available data. In the present work we have the same glass family/matrix (phosphate glass) with CuO replaced by V2O5 in order to study the dependence of the effect of laser irradiation on the TM ions present in the glasses.
2. Experimental details 2.1. Glass sample preparation The glass with nominal composition 0.50(V2O5)–0.50(P2O5) was prepared by melting dry mixtures of reagent grade V2O5 and P2O5 in alumina crucibles. P2O5 was preheated at 150 1C before mixing it with V2O5 at the calculated ratio. Approximately 20 g of chemicals were thoroughly mixed manually to obtain a homogenized mixture. The crucible containing the batch mixture was then placed in an electrically heated melting furnace and maintained at 1100–1200 1C for about 1–2 h under atmospheric conditions during which the melt was occasionally stirred with an alumina rod. The homogenized melt was then cast onto a stainless steel plate mold to form glass buttons for laser irradiation and glass rods of approximately 5 mm diameter for XPS measurements. X-ray powder diffraction analysis indicated that the glass formed was completely amorphous. 2.2. X-ray photoelectron spectroscopy (XPS) measurements Core level photoelectron spectra were collected on a VG scientific ESCALAB MKII spectrometer equipped with a dual aluminum– magnesium anode X-ray gun and a 150 mm concentric hemispherical analyzer using Al Ka (hn ¼1486.6 eV) radiation from an anode operated at 130 W. Photoelectron spectra of V 2p, O 1s, P 2p as well as C 1s core levels were recorded using a computer-controlled data collection system with the electron analyzer set at a pass-energy of 20 eV for the high resolution scans. For self-consistency, the C 1s transition at 284.6 eV was used as a reference for all charge shift corrections as this peak arises from hydrocarbon contamination and its binding energy is generally accepted as remaining constant, irrespective of the chemical state of the sample. For XPS measurements, a glass rod was cleaved in the preparation chamber at a base pressure of 2 10 9 mbar before being transferred to the analysis chamber where the pressure was maintained at o2 10 10 mbar. A non-linear, least-squares algorithm was employed to determine the best fit to each of the O 1s core level spectra with two Gaussian–Lorentzian curves in order to represent bridging and non-bridging oxygen. The fitting of the V 2p core level spectra was carried out to three Gaussian–Lorentzian peaks corresponding to three oxidation states (V3 þ , V4 þ , and V5 þ ) respectively. The fraction of non-bridging oxygen and V3 þ , V4 þ , and V5 þ were determined from the respective area ratios from these fits. Based on the reproducibility of similar quantitative spectral decompositions of spectra taken from other surfaces on the same glass samples, uncertainties of 75% and 710%, respectively, were estimated for these area ratios. A period of approximately2 h was required to collect the data set for each sample and there was no evidence of any X-ray induced reduction of the vanadium in the glasses during this period. 2.3. Laser irradiation Nd: YAG laser (Quanta-Ray, Model GCR-250) was applied for irradiation of glass samples at three different laser wavelengths. Since Nd:YAG laser emits a fundamental wavelength at 1064 nm, for the generation of second and third harmonic at 532 and 355 nm, frequency doubling and tripling was obtained by using a KDP crystal. The pulse width of the laser was about 8 ns and the rep rate was 10 Hz. In order to compare, the effect of different wavelengths (1064, 532,335 nm) of laser radiations on various glass samples, the laser intensity, laser irradiation time and laser beam diameter were kept constant. The laser energy was measured with a calibrated energy meter (ophir model 300). In this experiment, we placed a 5 mm diameter aperture inside the path
G.D. Khattak et al. / Journal of Physics and Chemistry of Solids 74 (2013) 13–17
of laser beam in order to get a uniform beam shape. The pulse energy at different wavelengths was 100 mJ and the laser fluence (peak power/area) estimated was 1.27 1011 W/ m2. In order to expose the sample homogeneously with the laser beam and to avoid the development of any crust on the surface of glass samples due to repeated exposure of laser on the same spot, the samples were mounted on a rotary table.
3. Results and discussion 3.1. V 2p spectra The V 2p spin–orbit doublet spectra for the unirradiated and irradiated glass samples are depicted in Fig. 1. It is observed that the intensities of the V 2p peaks decrease with increasing laser wavelength. It appears clearly from the figure that for the sample irradiated at 1064 nm, the decrease is quite significant. It is worth noting that all four samples were analyzed under similar conditions, i.e., X-ray energy, position between the X-ray source and samples as well as sampling time were kept the same. In addition, the V 2p peaks show significant asymmetry for both the irradiated and the unirradiated samples which indicates that V ions exist in more than one oxidation state. On the basis of our study performed on vanadium phosphate glasses [24], the fitting of the V 2p3/2 core
Intensity (arb. units)
V 2p
unirradiated 355 nm
15
level spectra for both the unirradiated and irradiated samples was carried out with three Gaussian-Lorentzian peaks corresponding to three oxidation states (V3 þ , V4 þ , and V5 þ ) respectively. This procedure was adopted because the initial fitting of the V 2p3/2 spectra to two Gaussian–Lorentzian peaks with the lower binding energy peak corresponding to V4 þ and the higher binding energy peaks to V5 þ lead to some quantitative inconsistencies with the magnetic analysis [24]. The satellite peak around 521 eV was fitted to a single Gaussian–Lorentzian peak and assumed to be associated with V5 þ since this satellite peak has been previously observed in the spectrum of pure V2O5 [25]. Hence the area under the satellite peak obtained from the curve fitting was added to the area of peak 3 (V5 þ ) to find the relative abundance of V5 þ . The relative abundance of these oxidation states (V3 þ , V4 þ and V5 þ ) is shown in Table 1. We do not expect to have metallic V1 in our samples and therefore its contribution was not included in the fitting. Such fitting is depicted in Fig. 2 for irradiated sample with the 355 nm laser wavelength. The decrease in intensity of the V 2p spectra for the irradiated samples is relatively smaller than that of Cu 2p spectra [23]. This could be as a consequence of the evaporation of vanadium ions from the surface due to the high temperature induced by the laser on the surface of the sample or the diffusion of vanadium into the bulk or both processes happening simultaneously. No definite conclusion could be reached based on the available data. However, the suggestion that it might be due to diffusion of vanadium from the surface toward the interior of the sample is supported by results reported in the literature [26,27]. In these studies, it is reported that thermal reduction of the transition metal can lead to inward diffusion of the mobile cations. The mechanism for irradiation could be the same. Similar behavior, as explained in the introduction, was observed in a study carried out on a copper phosphate glass as reported earlier by our group [23]. As clear from Table 1, V ions decrease with increasing incident radiation wavelength. V ions are mostly in the V4 þ and V5 þ states in the unirradiated sample. The reduction of V ions is though not very clear for the 355 nm laser radiation, it is quite obvious for the 532 nm and 1064 nm laser irradiated samples and the V ions mostly exist in V3 þ and V4 þ states.
532 nm 1064 nm 528
524
520
516
512
3.2. O 1s spectra
508 The O 1s core level spectra for the irradiated and unirradiated samples investigated in this study are shown in Fig. 3. The spectra were superimposed in order to clearly observe a decrease in the
Binding energy (eV) Fig. 1. Core level V 2p spectra for the unirradiated and irradiated glasses.
Table 1 Peak positions, FWHM, and relative abundances (relative areas) of the various V ions from the three-peak curve fitting of the V 2p core levelsþsatellite for the V2O5 phosphate glasses. Sample
Satellite FWHM area
V 2p3/2(1) FWHM area
V 2p3/2(2) FWHM area
V 2p3/2(3) FWHM area
V5þ / Vtotal
V3þ / Vtotal
V4þ / Vtotal
Unirradiated
520.54 1.90 90.09 520.40
515.71 2.00 1181 515.89
516.46 2.00 8809 516.66
517.74 2.00 1932 517.70
0.1683
0.0983
0.7333
0.1801
0.1255
0.6944
Irradiated (355 nm)
2.00 110.36
2.00 411.60
2.00 2278.3
2.00 480.56
Irradiated (532 nm)
520.48 2.00 78.64
515.96 2.00 1434.5
516.43 2.00 1909.0
517.78 2.00 215.878
0.0810
0.3943
0.5247
Irradiated (1064 nm)
–
515.60 2.00 471.89
516.40 2.00 471
517.70 2.00 14.456
0.0151 0.4929
0.4920
The uncertainty in the peak positions is 70.1 eV, FWHM 70.2 eV, and the relative areas 7 10%.
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G.D. Khattak et al. / Journal of Physics and Chemistry of Solids 74 (2013) 13–17
Fig. 2. Fitting of the V 2p3/2 core level spectra for the irradiated sample with laser wavelength 355 nm.
with the lower binding energy peak corresponding to NBO (P¼O and V¼O) and the higher energy peak to BO (P–O–P, P–O–V, V–O– V). The fitted spectrum is shown in Fig. 4 for the irradiated sample at 1064 nm laser radiation. The relative abundance of these oxygen sites determined from the respective area ratios from these fits is shown in Table 2. One notes from Table 2 that the measured area ratio, [NBO/TO], slightly decreases on laser radiation leading to an increase in bridging oxygen (BO) bonds and hence a compact structure [24].
O 1s
Intensity (arb. units)
Fig. 4. Fitting of the core level O 1s spectra for the irradiated sample at 1064 nm laser radiation. The resulting NBO and BO peaks from the least-squares fitting routine of two Gaussian–Lorentzian peaks are also shown.
unirradiatted
3.3. P 2p spectra
355 nm 532 nm 1064 nm 544
540
536
532
528
524
Binding energy (eV) Fig. 3. Core level O 1s spectra for the unirradiated and irradiated glasses.
peak intensities with an increase in the wavelength of the laser source. While the cation spectra provide some insight into the structure of these oxide glasses, the O 1s spectra are typically more informative as the binding energy of the O 1s electrons provides a measure of the extent to which electrons are localized on the oxygen or in the internuclear region. This is a direct consequence of the nature of the bonding between the oxygen and different cations. The appearance of an asymmetry in the O 1s spectra is not unexpected as it is well-known that the O 1s peak in glass can arise from different oxygen bonding sites. Correspondingly, the O 1s peaks for these V2O5–P2O5 glasses may arise from oxygen atoms existing in some or all of the following structural bonds: P–O–P, P–O–V, V–O–V, V¼O, and P¼O. Furthermore, the O1s peaks associated with these various structural bonds may be broaden by the vanadium ions existing in different valance states. Oxygen atoms that are more covalently bonded to glass former atoms on both sides are typically called bridging oxygen (e.g., P–O–P, V–O–V and P–O–V), while oxygen atoms that are more ionically bonded, at least on one side, or double bonded to a glass former atom are referred to as non-bridging oxygen atoms and have lower binding energies (e.g., P¼O, V¼O). The variation in binding energies of the different oxygen sites in different glass systems has been discussed previously in details [24]. Each O 1s spectrum was deconvoluted into two Gaussian–Lorentzian peaks
The P 2p core level spectra for all samples investigated are shown in Fig. 5. As one can notice in Fig. 5, the peak positions are independent of the laser irradiation wavelength. A decrease in intensity is observed which is quite significant for the 1064 nm laser irradiation. This decrease in intensity of the P 2p spectra was not observed for Cu-phosphate glasses [23]. This may be due to the fact that the bonding of P ions to oxygen and copper could be stronger as compared to their bonding in the vanadium phosphate glasses, but at present, we do not have any technique to confirm this hypothesis. However, the observed asymmetry in the peaks which resulted in the deconvolution of the P 2p spectra into two peaks could arise from the spin–orbit splitting of the P 2p core level resulting in the distinguishable P 2p3/2 and P 2p1/2 core levels with the lower binding energy peak corresponding to P 2p3/2. Since the predicted P 2p3/2/P 2p1/2 ratio of 2.0 [28] is within the uncertainty of the measured ratios of the relative areas of 2.170.1 eV, it is reasonable to conclude that the asymmetry in the P 2p spectra indeed arises from the spin–orbit splitting of the P 2p core level. This fitting is shown in Fig. 6 for the unirradiated sample and the relative abundances are shown in Table 2.
4. Conclusions Vanadium phosphate glass was irradiated using three different wavelengths (IR, Visible and UV) generated from Nd:YAG laser. The effect of the laser irradiation on the local glass structure as well as on the valence state of the vanadium ions has been investigated by X-ray photoelectron spectroscopy (XPS). The most apparent feature of the V 2p, O 1s and P 2p core level spectra is the decrease in their intensities with an increase of the laser wavelength. This decrease in intensity of V 2p spectra is probably
G.D. Khattak et al. / Journal of Physics and Chemistry of Solids 74 (2013) 13–17
17
Table 2 Peak positions, FWHM, areas, and relative abundance from the curve fitting of the O 1s and P 2p core levels. Sample
O 1s (I) FWHM area Abundance
O 1s (II) FWHM area Abundance
(NBO/Ototal)
P 2p3/2 FWHM area Abundance
P 2p1/2 FWHM area Abundance
P 2p3/2/P 2p1/2
Unirradiated
532.96 2.32 10210
531.34 2.03 31757
0.7567
133.69 2.00 3533
134.64 2.00 1712
2.063
Irradiated (355 nm)
532.65 2.32 7434.9
531.13 2.01 9983.4
0.5732
133.10 2.09 1547.8
134.49 2.09 779.89
1.985
Irradiated (532 nm)
532.83 2.30 7628.78
531.22 2.00 12919.5
0.6287
133.28 2.00 1901.59
134.50 2.00 885.924
2.146
Irradiated (1064 nm)
532.29 2.30 3543.5
530.79 2.00 4410.0
0.5545
133.40 2.00 4564.0
134.45 2.10 2176.0
2.097
The uncertainty in the peak positions is 7 0.1 eV, FWHM 7 0.2 eV, and areas 710%.
presence of primarily P–O–P, P–O–V, V–O–V, P¼O and V¼O structural bonds, a spin–orbit splitting of P 2p core level, and more than one valence state of V ions being present. The vanadium ions reduce to lower oxidation states with laser irradiation, the ratio [NBO/TO] decreases slightly with an increase in laser wavelength indicating the formation of a relatively compact structure and the P 2p3/2/P 2p1/2 ratio of 2.0 is within the uncertainty of the measured ratios.
Intensity (arb. units)
P 2p
unirradiated
Acknowledgments 355 nm
The support of the KFUPM Physics Department and Research Committee (Grant No. RG112-CS-02) is greatly acknowledged.
532 nm 1064 nm 120
125
130
135
140
145
Binding energy (eV) Fig. 5. Core level P 2p spectra for the unirradiated and irradiated glasses.
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20]
Fig. 6. Fitting of the P 2p spectra for the unirradiated sample.
due to diffusion of the V ions into the bulk of the sample which could be attributed to the thermal effect of the laser at longer wavelength as compared to shorter wavelengths. Asymmetries found in the O 1s, P 2p, and V 2p core level spectra indicate the
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