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calcium phosphate films deposited by pulsed laser deposition
Applied Surface Science 256 (2009) 76–80
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
An investigation into the effects of high laser fluence on hydroxyapatite/calcium phosphate films deposited by pulsed laser deposition Le Quang Tri, Daniel H.C. Chua * Department of Materials Science & Engineering, Faculty of Engineering, National University of Singapore, Singapore 117574, Singapore
A R T I C L E I N F O
A B S T R A C T
Article history: Received 26 May 2009 Received in revised form 17 July 2009 Accepted 18 July 2009 Available online 25 July 2009
Pulsed laser deposited mixed hydroxyapatite (HA)/calcium phosphate thin films were prepared at room temperature using KrF laser source with different laser fluence varying between 2.4 J/cm2 and 29.2 J/ cm2. Samples deposited at 2.4 J/cm2 were partially amorphous and had rough surfaces with a lot of droplets while higher laser fluences showed higher level of crytallinity and lower roughness of surfaces of obtained samples. Higher laser fluences also decreased ratio Ca/P of as-deposited samples. X-ray photoelectron spectroscopy (XPS) revealed traces of carbonate groups in obtained samples, which were removed after thermal annealing. The decomposition of HA into TCP was observed to start at about 400 8C. The formation of new crystalline phase of HA was found after annealing as well. The cracks observed on surface of sample deposited at 29.2 J/cm2 after annealing indicated that the HA/ calcium phosphate films deposited at higher laser energy densities were probably more densed. ß 2009 Elsevier B.V. All rights reserved.
Keywords: Hydroxyapatite Pulsed laser deposition (PLD)
1. Introduction ‘‘Biomaterial’’ is the term used to describe either modified natural or synthetic materials, which can be used in medical and dental implants for repair or replacement of natural tissues [1]. According to the interaction with human body, biomaterials are divided into 2 types: bioinert and bioactive materials. ‘‘Bioinert’’ implies that the materials do not have any interaction with the immune system, therefore, will not cause any unexpected effect such as toxicity and inflammation [1]. In contrast, materials belonging to another type show ability to interact with human body to enhance basic cellular processes. Consequently, bioactive materials can support the growth of tissues and avoid unexpected effects from immune system [1]. Bioactive glass and calcium phosphate compounds, examples of second type biomaterials, are able to increase growth rate of bone due to the formation of apatite layer on the surface when they are in contact with human body [2,3]. Therefore, they are usually used for bone replacement. Among calcium phosphate compounds, Ca3(PO4)2 tricalcium phosphate (TCP), Ca4O(PO4)2 tetracalcium phosphate (TTCP) and Ca10(PO4)6(OH)2 hydroxyapatite (HA) are most commonly used due to their good biocompatibilities and high bone growth rates [3]. Due to the stability of crystalline HA in human body, further applications such as using HA as a protective
* Corresponding author. Tel.: +65 65168933; fax: +65 67763604. E-mail address: [email protected] (Daniel H.C. Chua). 0169-4332/$ – see front matter ß 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2009.07.062
layer for medical implants [3,4] or a matrix for different dopants to obtain desired functions in other biological applications [2,5]. Despite certain advantages of HA, applications of HA are still limited due to difficulties in fabricating HA films. HA films were found to have low adhesion to the substrate and bad mechanical properties, which make HA not suitable for long term load bearing applications [3]. In addition, it is also difficult to obtain exact composition of HA (Ca/P = 1.67– 1.7) due to the decomposition of HA. HA is decomposed into less biocompatible compounds at about 400 8C, which process might be described by following equation: 2Ca10 ðPO4 Þ6 ðOHÞ2 ¼ 4Ca3 ðPO4 Þ2 þ 4CaO þ Ca4 OðPO4 Þ2 þ P2 O5 þ 2H2 O
(1)
There are several ways to deposit HA films, such as sputtering, sol–gel, ion-beam deposition and plasma spray methods [4]. Pulsed laser deposition (PLD) has also been used to obtain films with desired compositions and good adhesion [3,4]. Many studies have been done on the effects of deposition parameters on films properties. Jelinek et al. (2002) reported that amorphous films were obtained if substrate’s temperature was maintained below 400 8C during deposition [4,6]. In their research, Dosta´lova´ et al. (2001) found that films deposited at temperature between 400 8C and 700 8C in gaseous environment which contained both inert gas and water vapor were composed of crystalline HA [3,7]. Effects of laser energy density were also studied by Lo et al. (2000) and Ball et al. (2001). It was reported that there was no discernable differences between films deposited at 3 J/cm2 and 9 J/cm2. However, after annealing,
L.Q. Tri, D.H.C. Chua / Applied Surface Science 256 (2009) 76–80 Table 1 Deposition conditions of HA/calcium phosphate films using PLD techniques. Sample
Duration (m)
Distance (cm)
Frequency (Hz)
Laser fluence (J/cm2)
Thickness (nm)
1 2 3 4 5 6
15 15 15 15 15 15
4.5 4.5 4.5 4.5 4.5 4.5
10 10 10 10 10 10
2.4 0.1 8.0 0.1 12.4 0.1 18.0 0.1 23.6 0.1 29.2 0.1
500 650 800 1100 1350 1500
sample deposited at 3 J/cm2 possessed 98% crystallinity whereas that portion for sample deposited at 9 J/cm2 was only 87% [8,9]. It is well known that deposition’s conditions can significantly affect the obtained films. Many researchers have been done on effects of gaseous environment and temperature, but there are only a few reports about the influence of laser energy density and the laser fluence used is almost lower than 9 J/cm2. In order to obtain better understanding about effects of laser fluence which might affect the crystallinity and compactness of films, in our research, wider range of laser energy densities was used. In addition, effects of annealing on surface’s morphologies and microstructure are also studied.
77
2. Experimental methods The HA target used for this research was directly provided by Analytical Technologies Pte Ltd at 99.01 purity. Depositions were performed using a PLD system with KrF excimer laser (l = 248 nm). Laser beam impinges at an incident angle of 458 onto the target with the beam spot area of 1 mm2. To avoid damaging the target due to interaction with laser for a long time, the target was rotated at a constant speed during depositions. In this study, commercially available n++ Si substrate with arsenic as dopant was used because the interface between film and substrate is not of a major concern. Furthermore, in order to study the morphology, the substrate has to be perfectly flat which silicon substrate is among the best choice. The substrates were placed parallel to the target at a constant distance. In order to fulfill the objective, laser fluence was varied by changing the laser energy and other parameters were kept constant. Deposition conditions are shown in Table 1. The crystallinity of deposited specimens was studied by Bruker D8 Advanced Thin Film XRD system utilizing Cu Ka X-ray source (l = 1.54 Angstrom). The scanning step of 0.028 and time of 1 s per step were used. The scanned region was fixed at 20–408. Effects of annealing were investigated in-situ by X’pert Pro high-temperature XRD system. Temperature was increased from room temperature (25 8C) to 1000 8C with steps of 100 8C for each. Annealing was done in vacuum (107 Torr) and XRD scanning was
Fig. 1. Top view SEM pictures of (a) as-deposited sample deposited at 29.2 J/cm2 (b) as-deposited sample deposited at 12.4 J/cm2 (c) sample deposited at 29.2 J/cm2 after annealing (d) sample deposited at 12.4 J/cm2 after annealing.
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conducted after each step. After reaching 1000 8C, the temperature was decrease with a rate of 40 8C/min to room temperature. Surface morphologies of samples were investigated using Jeol JSM–6700F field emission scanning electron microscopy instrument. Samples were irradiated by electron beam with energy of 5 kV and current of 10 mA. In our case, integration mode was used to obtain high quality images. Elemental composition analysis was conducted using Phillips XL 30 FEG–SEM instrument connected with EDAX EDX system. In order to obtain robust results, scanning was done on different areas on surface and average values were used. The energies of electron beam of 5 kV, 10 kV and 15 kV were used to ensure that there was no layer-like film. This information may be indirectly obtained due to different sampling depth at different electron beam voltages. A Kratos Axis Ultra DLD XPS instrument using Al Ka monochromatic X-ray source (1486.69 eV) was used to investigate bonding states. The analysis chamber was maintained at about 1010 mbar and core level spectra were collected at the take off angle of 908 with respect to samples’ surfaces. For survey scan and narrow scan, pass energies of 160 eV and 20 eV were used, respectively. In order to compensate the shift due to charging effect, C 1s peak of graphite at 284.5 eV was used as reference [10]. 3. Experimental results 3.1. As-deposited samples Fig. 1(a) and (b) shows representative SEM images of asdeposited HA/calcium phosphate films at 29.2 J/cm2 and 12.4 J/ cm2, respectively. Nearly pore-free surfaces were observed for all obtained samples. Droplets in spherical and irregular shapes are densely distributed on surfaces of as-deposited samples, which are typical features of films deposited by KrF excimer laser [3]. Droplets might result from target splashing during laser-target interaction or actual film nucleation. Comparing Fig. 1(a) and (b), it can be observed that films deposited at higher laser fluence shows smaller size of droplets on surfaces. The average size of the droplets is about 1 mm and 1.5–2 mm for samples deposited at 29.2 J/cm2 and 12.4 J/cm2, respectively. XRD spectra of as-deposited coatings are demonstrated in Fig. 2. Whereas sample 1 is mostly amorphous, samples deposited at higher energy densities are partially crystallized with a peak for (300) crystalline orientation of HA (2u = 32.88) [3,11–14] and the
Fig. 3. XPS spectra of as–deposited samples of (a) C 1s (b) O 1s. ‘‘*’’ is used to show the peak of pure carbon whereas ‘‘#’’ presents the peak of C1s in CO32. Table 2 Comparing Ca/P ratios of samples before and after annealing using EDX. Sample
Before annealing
After annealing
1 2 3 6
2.97 3.16 2.62 2.5
2.3 2.34
intensity of this peak increases at higher laser fluences. For samples 3 and 6, a peak at 37.48 is observed, which is attributed to the presence of CaO [13]. Likewise, the peak at 34.98 can be attributed to TCP. Fig. 3 shows XPS spectra of samples deposited at energy densities of 2.4 J/cm2, 12.4 J/cm2 and 29.2 J/cm2. It is obvious that XPS results for all samples are the same, except the small shift towards higher energy of sample deposited at 2.4 J/cm2 compared to others. However, the amount of the shift is insignificant compared to the resolution of XPS instrument. In addition to pure C, Ca, O and P, bonding state C 1s in CO32 is observed by the peak at 289 eV. XPS results correlate well with EDX results, where the existence of C, Ca, O and P is confirmed. EDX was further used to evaluate the stoichiometry of obtained films, ratio Ca/P was calculated and shown in Table 2. 3.2. Annealed samples Fig. 2. XRD spectra of as-deposited samples. Black, red and blue plots correspond to the spectra of samples deposited at 29.2 J/cm2, 12.4 J/cm2 and 2.4 J/cm2, respectively. ‘‘*’’, ‘‘#’’ and ‘‘o’’ present the peak of crystalline hydroxyapatite, Tricalcium phosphate and CaO, respectively.
Significant changes in the surface morphology of samples were observed after annealing as shown in Fig. 1(c) and (d). Sample deposited at 12.4 J/cm2 shows a highly porous surface with pores
L.Q. Tri, D.H.C. Chua / Applied Surface Science 256 (2009) 76–80
79
Fig. 4. In-situ XRD spectra during annealing of (a) Sample deposited at 12.4 J/cm2 (b) Sample deposited at 29.2 J/cm2. Spectra of samples annealed at 25 8C, 400 8C, 600 8C, 800 8C and 1000 8C are displayed different colors. In addition, peaks of crystalline hydroxyapatite, tri-calcium phosphate and CaO are presented using ‘*’’, ‘‘#’’ and ‘‘o’’, respectively.
distributed all over the surface. Surface of this sample also contains droplets of about 1.5 mm diameter. Open round pores are also found on surface of sample deposited at 29.2 J/cm2 but with much less surface density. Droplets of 1–2 mm average diameter and micro cracks are also observed for this sample, which might be due to the difference in thermal expansion coefficient between different compounds in films. In-situ XRD results of samples are displayed in Fig. 4. The intensity of CaO’s peak and (3 0 0) peak of HA gradually decrease and almost disappear at 1000 8C. In addition, the disappearing of (3 0 0) peak and the appearance of new peaks at 32.448, 31.768, 31.358 and 25.548 are also found. Whereas the peak at 31.768 represents (2 1 1) peak of HA, others show the existence of a and b phases of TCP in annealed samples [6,7,11,12,14]. XPS results of annealed samples also show the removal of carbonate bonds CO32 after annealing. Moreover, significant shifts of O, Ca and P peaks are shown in XPS spectra of sample 6 which was deposited at 29.2 J/cm2 after annealing (Fig. 5). 4. Discussion Previous research reports commonly use laser fluences of lower than 9 J/cm2 to study the deposition of HA/calcium phosphate by PLD [8,9] but our values are much higher, ranging from 2.4 J/cm2 to 29.2 J/cm2. At such fluence, energies of substances arriving on the surface are much higher so that they can penetrate deeper into the film and after that migrate to positions with highest bonding
Fig. 5. XPS results of (a) C1s (b) O1s of sample before and after annealing of coating deposited at 29.2 J/cm2. ‘‘*’’ is used to show the peak of pure carbon whereas ‘‘#’’ presents the peak of C1s in CO32.
energy leading to the formation of crystallized HA. The higher laser energy density is used, the higher level of crystallinity and higher density can be obtained SEM picture of sample deposited at 29.2 J/ cm2 after annealing shows a lot of cracks, which might be due to the thermal expansion of films. Comparing surface morphologies of annealed samples, we suspect that higher laser fluence leads to denser surface of obtained films. In addition, ultra high laser energy density is possibly the main reason for the formation of crystalline films, which is shown in XRD spectra of as-deposited samples. For deposition with high laser fluence, the temperature of plasma formed inside the chamber might be also very high. As the top layer of films was in contact with the plume during deposition, its temperature could be high enough to cause the decomposition of HA. However, Table 2 shows the ratio of Ca/P of the samples deposited at ultra high laser energy densities are significantly smaller than that deposited at 2.4 J/cm2. This fact can be explained as the high laser fluence causes a high ablation rate of the target, leading to high pressure plasma. This high pressure might help to suppress the evaporation of P and H2O from the top layer of films, hence to partially preserve the composition of HA.
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L.Q. Tri, D.H.C. Chua / Applied Surface Science 256 (2009) 76–80
XPS is a powerful method to investigate the composition of materials. However, in the case of HA depositions, XPS may not be an effective mean to study the deposited films as they possibly include TCP, TTCP, CaO and HA simultaneously. Boyd et al. show that XPS peaks of P 2p, O 1s, Ca 2p are almost the same in those compounds [15]. Furthermore, samples with significantly different compositions may still have very similar XPS spectra in term of peaks’ locations. This problem is also encountered in our research, in which XPS spectra of most of the as-deposited samples are almost nearly similar. However, by using XPS, CO32 is found as byproduct in obtained samples with bonding energy 289 eV. Insitu XRD and ex-situ XPS confirmed that after annealing, the concentration of CO32 was reduced. The decrease of CO32 might be the reason for the shift of XPS peak for O 1s of annealed sample. In-situ XRD results on the effect of annealing show the appearance of TCP peaks and one peak of HA at 31.38 at the expense of HA’s peak at 32.88 and CaO’s peak. The decrease of ratio Ca/P after annealing is also observed in EDX results. The appearance of a new peak of HA might be due to the crystallization of existing amorphous HA. However, the crystallization of HA cannot explain the reduction of the ration Ca/P of the surface. According to Eq. [1], when the temperature reached 300– 500 8C, the decomposition of HA took place, leading to the formation of P2O5, H2O and some other calcium phosphate compounds [4]. It is highly likely that P2O5 and H2O molecules were formed throughout the bulk of the films during the annealing and subsequently, there might be diffusion towards the surface and interface [16]. However, as the duration of the annealing conducted in this research was not sufficient for all compounds formed to escape, it was likely that some of them would still remain in the film after the annealing. Due to the addition of phosphorus from the bulk, phosphorus content of the surface layer increases as showed in EDX results of annealed coatings. 5. Conclusion In this research, crystalline HA films were successfully deposited at high laser energy densities in vacuum. Higher fluence caused the as-deposited films to be more densed and crystallized. In addition, laser energy density also causes a difference in the composition of
HA films. Higher energy densities lead to compositions which is closer to that of pure HA films. After annealing in vacuum, samples contained crystallized HA, TCP and TTCP. Lower value of Ca/P ratio is observed on the surface of samples after annealing. It might be due to the addition of phosphorus, product of the decomposition process of HA in the inner layer, diffusing outwards from the bulk to the surface of the coatings. Acknowledgments The authors acknowledge funding support from NUS YIA WBS R284000046123. References [1] European white book on Fundamental Research in Materials Science, 2001, www.planck.de/pdf/europeanWhiteBook/wb_materials_072_076.pdf. [2] S.A.M. Abdel Hameed et al., Preparation and characterization of some ferromagnetic glass ceramics contains high quantity of magnetite, Ceramics International, 2008, doi:10.1016/j.ceramint.2008.08.021. [3] Y. Zhao, C. Chen, D. Wang, Surface Review and Letters 12 (3) (2005) 401–408. [4] V. Nelea, C. Morosanu, M. Iliescu, I.N. Mihailescu, Surface and Coatings Technology 173 (2003) 315–322. [5] K. Funakoshi, T. Nonami, Journal of American Ceramic Society 89 (2006) 944–948. [6] M. Jelinek, T. Dosta´lova´, L. Himmlova´, C. Grivas, C. Fotakis, Molecular Crystal and Liquid Crystal 374 (2002) 599. [7] T. Dostta´lova´, L. Himmlova´, M. Jelinek, C. Grivas, Journal of Biomedical Optics 6 (2001) 239. [8] W.J. Lo, D.M. Grant, M.D. Ball, B.S. Welsh, S.M. Howdle, E.N. Antonov, V.N. Bgratashvili, V.K. Popov, Journal of Biomedical Materials Research 50 (2000) 536. [9] M.D. Ball, S. Downes, C.A. Scotchford, E.N. Antonov, V.N. Bagratashvili, V.K. Popov, W.J. Lo, D.M. Grant, S.M. Howdle, Biomaterials 22 (2001) 337. [10] C.D. Wagner, G.E. Muilenberg, Handbook of X-ray Photoelectron Spectroscopy: A Reference Book of Standard Data for Use in X-ray Photoelectron Spectroscopy, Perkin–Elmer Corp, Eden Prairie, 1979. [11] S. Johnson, M. Haluska, R.J. Narayan, R.L. Snyder, Materials Science and Engineering C 26 (2006) 1316–1323. [12] J.M. Ferna´ndez-Pradas, G. Sardin, L. Cle`ries, P. Serra, C. Ferrater, J.L. Morenza, Thin Solid Films 317 (1998) 393–396. [13] C.M. Lopatin, V. Pizziconi, T.L. Alford, T. Laursen, Thin Solid Films 326 (1998) 227– 232. [14] J.M. Ferna´ndez-Pradas, L. Cle`ries, E. Martı´nez, G. Sardin, J. Esteve, J.L. Morenza, Biomaterials 22 (2001) 2171–2175. [15] A.R. Boyd, H. Duffy, R. McCann, M.L. Cairns, B.J. Meenan, Nuclear Instruments and Methods in Physics Research B 258 (2007) 421–428. [16] P. Ducheyne, W.V. Raemdonck, J.C. Heughebaert, M. Heugheabert, Biomaterials 7 (1986) 97–103.
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
An investigation into the effects of high laser fluence on hydroxyapatite/calcium phosphate films deposited by pulsed laser deposition Le Quang Tri, Daniel H.C. Chua * Department of Materials Science & Engineering, Faculty of Engineering, National University of Singapore, Singapore 117574, Singapore
A R T I C L E I N F O
A B S T R A C T
Article history: Received 26 May 2009 Received in revised form 17 July 2009 Accepted 18 July 2009 Available online 25 July 2009
Pulsed laser deposited mixed hydroxyapatite (HA)/calcium phosphate thin films were prepared at room temperature using KrF laser source with different laser fluence varying between 2.4 J/cm2 and 29.2 J/ cm2. Samples deposited at 2.4 J/cm2 were partially amorphous and had rough surfaces with a lot of droplets while higher laser fluences showed higher level of crytallinity and lower roughness of surfaces of obtained samples. Higher laser fluences also decreased ratio Ca/P of as-deposited samples. X-ray photoelectron spectroscopy (XPS) revealed traces of carbonate groups in obtained samples, which were removed after thermal annealing. The decomposition of HA into TCP was observed to start at about 400 8C. The formation of new crystalline phase of HA was found after annealing as well. The cracks observed on surface of sample deposited at 29.2 J/cm2 after annealing indicated that the HA/ calcium phosphate films deposited at higher laser energy densities were probably more densed. ß 2009 Elsevier B.V. All rights reserved.
Keywords: Hydroxyapatite Pulsed laser deposition (PLD)
1. Introduction ‘‘Biomaterial’’ is the term used to describe either modified natural or synthetic materials, which can be used in medical and dental implants for repair or replacement of natural tissues [1]. According to the interaction with human body, biomaterials are divided into 2 types: bioinert and bioactive materials. ‘‘Bioinert’’ implies that the materials do not have any interaction with the immune system, therefore, will not cause any unexpected effect such as toxicity and inflammation [1]. In contrast, materials belonging to another type show ability to interact with human body to enhance basic cellular processes. Consequently, bioactive materials can support the growth of tissues and avoid unexpected effects from immune system [1]. Bioactive glass and calcium phosphate compounds, examples of second type biomaterials, are able to increase growth rate of bone due to the formation of apatite layer on the surface when they are in contact with human body [2,3]. Therefore, they are usually used for bone replacement. Among calcium phosphate compounds, Ca3(PO4)2 tricalcium phosphate (TCP), Ca4O(PO4)2 tetracalcium phosphate (TTCP) and Ca10(PO4)6(OH)2 hydroxyapatite (HA) are most commonly used due to their good biocompatibilities and high bone growth rates [3]. Due to the stability of crystalline HA in human body, further applications such as using HA as a protective
* Corresponding author. Tel.: +65 65168933; fax: +65 67763604. E-mail address: [email protected] (Daniel H.C. Chua). 0169-4332/$ – see front matter ß 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2009.07.062
layer for medical implants [3,4] or a matrix for different dopants to obtain desired functions in other biological applications [2,5]. Despite certain advantages of HA, applications of HA are still limited due to difficulties in fabricating HA films. HA films were found to have low adhesion to the substrate and bad mechanical properties, which make HA not suitable for long term load bearing applications [3]. In addition, it is also difficult to obtain exact composition of HA (Ca/P = 1.67– 1.7) due to the decomposition of HA. HA is decomposed into less biocompatible compounds at about 400 8C, which process might be described by following equation: 2Ca10 ðPO4 Þ6 ðOHÞ2 ¼ 4Ca3 ðPO4 Þ2 þ 4CaO þ Ca4 OðPO4 Þ2 þ P2 O5 þ 2H2 O
(1)
There are several ways to deposit HA films, such as sputtering, sol–gel, ion-beam deposition and plasma spray methods [4]. Pulsed laser deposition (PLD) has also been used to obtain films with desired compositions and good adhesion [3,4]. Many studies have been done on the effects of deposition parameters on films properties. Jelinek et al. (2002) reported that amorphous films were obtained if substrate’s temperature was maintained below 400 8C during deposition [4,6]. In their research, Dosta´lova´ et al. (2001) found that films deposited at temperature between 400 8C and 700 8C in gaseous environment which contained both inert gas and water vapor were composed of crystalline HA [3,7]. Effects of laser energy density were also studied by Lo et al. (2000) and Ball et al. (2001). It was reported that there was no discernable differences between films deposited at 3 J/cm2 and 9 J/cm2. However, after annealing,
L.Q. Tri, D.H.C. Chua / Applied Surface Science 256 (2009) 76–80 Table 1 Deposition conditions of HA/calcium phosphate films using PLD techniques. Sample
Duration (m)
Distance (cm)
Frequency (Hz)
Laser fluence (J/cm2)
Thickness (nm)
1 2 3 4 5 6
15 15 15 15 15 15
4.5 4.5 4.5 4.5 4.5 4.5
10 10 10 10 10 10
2.4 0.1 8.0 0.1 12.4 0.1 18.0 0.1 23.6 0.1 29.2 0.1
500 650 800 1100 1350 1500
sample deposited at 3 J/cm2 possessed 98% crystallinity whereas that portion for sample deposited at 9 J/cm2 was only 87% [8,9]. It is well known that deposition’s conditions can significantly affect the obtained films. Many researchers have been done on effects of gaseous environment and temperature, but there are only a few reports about the influence of laser energy density and the laser fluence used is almost lower than 9 J/cm2. In order to obtain better understanding about effects of laser fluence which might affect the crystallinity and compactness of films, in our research, wider range of laser energy densities was used. In addition, effects of annealing on surface’s morphologies and microstructure are also studied.
77
2. Experimental methods The HA target used for this research was directly provided by Analytical Technologies Pte Ltd at 99.01 purity. Depositions were performed using a PLD system with KrF excimer laser (l = 248 nm). Laser beam impinges at an incident angle of 458 onto the target with the beam spot area of 1 mm2. To avoid damaging the target due to interaction with laser for a long time, the target was rotated at a constant speed during depositions. In this study, commercially available n++ Si substrate with arsenic as dopant was used because the interface between film and substrate is not of a major concern. Furthermore, in order to study the morphology, the substrate has to be perfectly flat which silicon substrate is among the best choice. The substrates were placed parallel to the target at a constant distance. In order to fulfill the objective, laser fluence was varied by changing the laser energy and other parameters were kept constant. Deposition conditions are shown in Table 1. The crystallinity of deposited specimens was studied by Bruker D8 Advanced Thin Film XRD system utilizing Cu Ka X-ray source (l = 1.54 Angstrom). The scanning step of 0.028 and time of 1 s per step were used. The scanned region was fixed at 20–408. Effects of annealing were investigated in-situ by X’pert Pro high-temperature XRD system. Temperature was increased from room temperature (25 8C) to 1000 8C with steps of 100 8C for each. Annealing was done in vacuum (107 Torr) and XRD scanning was
Fig. 1. Top view SEM pictures of (a) as-deposited sample deposited at 29.2 J/cm2 (b) as-deposited sample deposited at 12.4 J/cm2 (c) sample deposited at 29.2 J/cm2 after annealing (d) sample deposited at 12.4 J/cm2 after annealing.
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L.Q. Tri, D.H.C. Chua / Applied Surface Science 256 (2009) 76–80
conducted after each step. After reaching 1000 8C, the temperature was decrease with a rate of 40 8C/min to room temperature. Surface morphologies of samples were investigated using Jeol JSM–6700F field emission scanning electron microscopy instrument. Samples were irradiated by electron beam with energy of 5 kV and current of 10 mA. In our case, integration mode was used to obtain high quality images. Elemental composition analysis was conducted using Phillips XL 30 FEG–SEM instrument connected with EDAX EDX system. In order to obtain robust results, scanning was done on different areas on surface and average values were used. The energies of electron beam of 5 kV, 10 kV and 15 kV were used to ensure that there was no layer-like film. This information may be indirectly obtained due to different sampling depth at different electron beam voltages. A Kratos Axis Ultra DLD XPS instrument using Al Ka monochromatic X-ray source (1486.69 eV) was used to investigate bonding states. The analysis chamber was maintained at about 1010 mbar and core level spectra were collected at the take off angle of 908 with respect to samples’ surfaces. For survey scan and narrow scan, pass energies of 160 eV and 20 eV were used, respectively. In order to compensate the shift due to charging effect, C 1s peak of graphite at 284.5 eV was used as reference [10]. 3. Experimental results 3.1. As-deposited samples Fig. 1(a) and (b) shows representative SEM images of asdeposited HA/calcium phosphate films at 29.2 J/cm2 and 12.4 J/ cm2, respectively. Nearly pore-free surfaces were observed for all obtained samples. Droplets in spherical and irregular shapes are densely distributed on surfaces of as-deposited samples, which are typical features of films deposited by KrF excimer laser [3]. Droplets might result from target splashing during laser-target interaction or actual film nucleation. Comparing Fig. 1(a) and (b), it can be observed that films deposited at higher laser fluence shows smaller size of droplets on surfaces. The average size of the droplets is about 1 mm and 1.5–2 mm for samples deposited at 29.2 J/cm2 and 12.4 J/cm2, respectively. XRD spectra of as-deposited coatings are demonstrated in Fig. 2. Whereas sample 1 is mostly amorphous, samples deposited at higher energy densities are partially crystallized with a peak for (300) crystalline orientation of HA (2u = 32.88) [3,11–14] and the
Fig. 3. XPS spectra of as–deposited samples of (a) C 1s (b) O 1s. ‘‘*’’ is used to show the peak of pure carbon whereas ‘‘#’’ presents the peak of C1s in CO32. Table 2 Comparing Ca/P ratios of samples before and after annealing using EDX. Sample
Before annealing
After annealing
1 2 3 6
2.97 3.16 2.62 2.5
2.3 2.34
intensity of this peak increases at higher laser fluences. For samples 3 and 6, a peak at 37.48 is observed, which is attributed to the presence of CaO [13]. Likewise, the peak at 34.98 can be attributed to TCP. Fig. 3 shows XPS spectra of samples deposited at energy densities of 2.4 J/cm2, 12.4 J/cm2 and 29.2 J/cm2. It is obvious that XPS results for all samples are the same, except the small shift towards higher energy of sample deposited at 2.4 J/cm2 compared to others. However, the amount of the shift is insignificant compared to the resolution of XPS instrument. In addition to pure C, Ca, O and P, bonding state C 1s in CO32 is observed by the peak at 289 eV. XPS results correlate well with EDX results, where the existence of C, Ca, O and P is confirmed. EDX was further used to evaluate the stoichiometry of obtained films, ratio Ca/P was calculated and shown in Table 2. 3.2. Annealed samples Fig. 2. XRD spectra of as-deposited samples. Black, red and blue plots correspond to the spectra of samples deposited at 29.2 J/cm2, 12.4 J/cm2 and 2.4 J/cm2, respectively. ‘‘*’’, ‘‘#’’ and ‘‘o’’ present the peak of crystalline hydroxyapatite, Tricalcium phosphate and CaO, respectively.
Significant changes in the surface morphology of samples were observed after annealing as shown in Fig. 1(c) and (d). Sample deposited at 12.4 J/cm2 shows a highly porous surface with pores
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Fig. 4. In-situ XRD spectra during annealing of (a) Sample deposited at 12.4 J/cm2 (b) Sample deposited at 29.2 J/cm2. Spectra of samples annealed at 25 8C, 400 8C, 600 8C, 800 8C and 1000 8C are displayed different colors. In addition, peaks of crystalline hydroxyapatite, tri-calcium phosphate and CaO are presented using ‘*’’, ‘‘#’’ and ‘‘o’’, respectively.
distributed all over the surface. Surface of this sample also contains droplets of about 1.5 mm diameter. Open round pores are also found on surface of sample deposited at 29.2 J/cm2 but with much less surface density. Droplets of 1–2 mm average diameter and micro cracks are also observed for this sample, which might be due to the difference in thermal expansion coefficient between different compounds in films. In-situ XRD results of samples are displayed in Fig. 4. The intensity of CaO’s peak and (3 0 0) peak of HA gradually decrease and almost disappear at 1000 8C. In addition, the disappearing of (3 0 0) peak and the appearance of new peaks at 32.448, 31.768, 31.358 and 25.548 are also found. Whereas the peak at 31.768 represents (2 1 1) peak of HA, others show the existence of a and b phases of TCP in annealed samples [6,7,11,12,14]. XPS results of annealed samples also show the removal of carbonate bonds CO32 after annealing. Moreover, significant shifts of O, Ca and P peaks are shown in XPS spectra of sample 6 which was deposited at 29.2 J/cm2 after annealing (Fig. 5). 4. Discussion Previous research reports commonly use laser fluences of lower than 9 J/cm2 to study the deposition of HA/calcium phosphate by PLD [8,9] but our values are much higher, ranging from 2.4 J/cm2 to 29.2 J/cm2. At such fluence, energies of substances arriving on the surface are much higher so that they can penetrate deeper into the film and after that migrate to positions with highest bonding
Fig. 5. XPS results of (a) C1s (b) O1s of sample before and after annealing of coating deposited at 29.2 J/cm2. ‘‘*’’ is used to show the peak of pure carbon whereas ‘‘#’’ presents the peak of C1s in CO32.
energy leading to the formation of crystallized HA. The higher laser energy density is used, the higher level of crystallinity and higher density can be obtained SEM picture of sample deposited at 29.2 J/ cm2 after annealing shows a lot of cracks, which might be due to the thermal expansion of films. Comparing surface morphologies of annealed samples, we suspect that higher laser fluence leads to denser surface of obtained films. In addition, ultra high laser energy density is possibly the main reason for the formation of crystalline films, which is shown in XRD spectra of as-deposited samples. For deposition with high laser fluence, the temperature of plasma formed inside the chamber might be also very high. As the top layer of films was in contact with the plume during deposition, its temperature could be high enough to cause the decomposition of HA. However, Table 2 shows the ratio of Ca/P of the samples deposited at ultra high laser energy densities are significantly smaller than that deposited at 2.4 J/cm2. This fact can be explained as the high laser fluence causes a high ablation rate of the target, leading to high pressure plasma. This high pressure might help to suppress the evaporation of P and H2O from the top layer of films, hence to partially preserve the composition of HA.
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XPS is a powerful method to investigate the composition of materials. However, in the case of HA depositions, XPS may not be an effective mean to study the deposited films as they possibly include TCP, TTCP, CaO and HA simultaneously. Boyd et al. show that XPS peaks of P 2p, O 1s, Ca 2p are almost the same in those compounds [15]. Furthermore, samples with significantly different compositions may still have very similar XPS spectra in term of peaks’ locations. This problem is also encountered in our research, in which XPS spectra of most of the as-deposited samples are almost nearly similar. However, by using XPS, CO32 is found as byproduct in obtained samples with bonding energy 289 eV. Insitu XRD and ex-situ XPS confirmed that after annealing, the concentration of CO32 was reduced. The decrease of CO32 might be the reason for the shift of XPS peak for O 1s of annealed sample. In-situ XRD results on the effect of annealing show the appearance of TCP peaks and one peak of HA at 31.38 at the expense of HA’s peak at 32.88 and CaO’s peak. The decrease of ratio Ca/P after annealing is also observed in EDX results. The appearance of a new peak of HA might be due to the crystallization of existing amorphous HA. However, the crystallization of HA cannot explain the reduction of the ration Ca/P of the surface. According to Eq. [1], when the temperature reached 300– 500 8C, the decomposition of HA took place, leading to the formation of P2O5, H2O and some other calcium phosphate compounds [4]. It is highly likely that P2O5 and H2O molecules were formed throughout the bulk of the films during the annealing and subsequently, there might be diffusion towards the surface and interface [16]. However, as the duration of the annealing conducted in this research was not sufficient for all compounds formed to escape, it was likely that some of them would still remain in the film after the annealing. Due to the addition of phosphorus from the bulk, phosphorus content of the surface layer increases as showed in EDX results of annealed coatings. 5. Conclusion In this research, crystalline HA films were successfully deposited at high laser energy densities in vacuum. Higher fluence caused the as-deposited films to be more densed and crystallized. In addition, laser energy density also causes a difference in the composition of
HA films. Higher energy densities lead to compositions which is closer to that of pure HA films. After annealing in vacuum, samples contained crystallized HA, TCP and TTCP. Lower value of Ca/P ratio is observed on the surface of samples after annealing. It might be due to the addition of phosphorus, product of the decomposition process of HA in the inner layer, diffusing outwards from the bulk to the surface of the coatings. Acknowledgments The authors acknowledge funding support from NUS YIA WBS R284000046123. References [1] European white book on Fundamental Research in Materials Science, 2001, www.planck.de/pdf/europeanWhiteBook/wb_materials_072_076.pdf. [2] S.A.M. Abdel Hameed et al., Preparation and characterization of some ferromagnetic glass ceramics contains high quantity of magnetite, Ceramics International, 2008, doi:10.1016/j.ceramint.2008.08.021. [3] Y. Zhao, C. Chen, D. Wang, Surface Review and Letters 12 (3) (2005) 401–408. [4] V. Nelea, C. Morosanu, M. Iliescu, I.N. Mihailescu, Surface and Coatings Technology 173 (2003) 315–322. [5] K. Funakoshi, T. Nonami, Journal of American Ceramic Society 89 (2006) 944–948. [6] M. Jelinek, T. Dosta´lova´, L. Himmlova´, C. Grivas, C. Fotakis, Molecular Crystal and Liquid Crystal 374 (2002) 599. [7] T. Dostta´lova´, L. Himmlova´, M. Jelinek, C. Grivas, Journal of Biomedical Optics 6 (2001) 239. [8] W.J. Lo, D.M. Grant, M.D. Ball, B.S. Welsh, S.M. Howdle, E.N. Antonov, V.N. Bgratashvili, V.K. Popov, Journal of Biomedical Materials Research 50 (2000) 536. [9] M.D. Ball, S. Downes, C.A. Scotchford, E.N. Antonov, V.N. Bagratashvili, V.K. Popov, W.J. Lo, D.M. Grant, S.M. Howdle, Biomaterials 22 (2001) 337. [10] C.D. Wagner, G.E. Muilenberg, Handbook of X-ray Photoelectron Spectroscopy: A Reference Book of Standard Data for Use in X-ray Photoelectron Spectroscopy, Perkin–Elmer Corp, Eden Prairie, 1979. [11] S. Johnson, M. Haluska, R.J. Narayan, R.L. Snyder, Materials Science and Engineering C 26 (2006) 1316–1323. [12] J.M. Ferna´ndez-Pradas, G. Sardin, L. Cle`ries, P. Serra, C. Ferrater, J.L. Morenza, Thin Solid Films 317 (1998) 393–396. [13] C.M. Lopatin, V. Pizziconi, T.L. Alford, T. Laursen, Thin Solid Films 326 (1998) 227– 232. [14] J.M. Ferna´ndez-Pradas, L. Cle`ries, E. Martı´nez, G. Sardin, J. Esteve, J.L. Morenza, Biomaterials 22 (2001) 2171–2175. [15] A.R. Boyd, H. Duffy, R. McCann, M.L. Cairns, B.J. Meenan, Nuclear Instruments and Methods in Physics Research B 258 (2007) 421–428. [16] P. Ducheyne, W.V. Raemdonck, J.C. Heughebaert, M. Heugheabert, Biomaterials 7 (1986) 97–103.