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Biomaterial thin film deposition and characterization by means of MAPLE technique
Materials Science and Engineering C 27 (2007) 1185 – 1190 www.elsevier.com/locate/msec
Biomaterial thin film deposition and characterization by means of MAPLE technique F. Bloisi a,⁎, L. Vicari a , R. Papa a , V. Califano b , R. Pedrazzani c , E. Bontempi c , L.E. Depero c a
CNR-INFM Coherentia — Napoli, Dip. Scienze Fisiche — Univ. Napoli “Federico II”, P.le V.Tecchio, 80 — 80125 Napoli, Italy b Dip. Scienze Fisiche — Univ. Napoli “Federico II”, P.le V.Tecchio, 80 — 80125 Napoli, Italy c Laboratorio di Chimica per le Tecnologie, Universita' degli Studi di Brescia, Via Branze, 38 — 25123 Brescia, Italy Received 5 May 2006; accepted 5 December 2006 Available online 10 January 2007
Abstract Polyethylene glycol (PEG) is a polymer with technologically important applications, especially as a biomaterial. Several biomedical applications (such as tissue engineering, spatial patterning of cells, anti-biofouling and biocompatible coatings) require the application of high quality PEG thin films. In order to have a good adhesion to substrate chemically modified polymer molecules have been used, but for some “in vivo” applications it is essential to deposit a film with the same chemical and structural properties of bulk PEG. Pulsed laser deposition (PLD) technique is generally able to produce high quality thin films but it is inadequate for polymer/organic molecules. MAPLE (Matrix Assisted Pulsed Laser Evaporation) is a recently developed PLD based thin film deposition technique, particularly well suited for organic/polymer thin film deposition. Up to now MAPLE depositions have been carried out mainly by means of modified PLD systems, using excimer lasers operating in UV, but the use of less energetic radiations can minimize the photochemical decomposition of the polymer molecules. We have used a deposition system explicitly designed for MAPLE technique connected to a Q-switched Ng:YAG pulsed laser which can be operated at different wavelength ranging from IR to UV in order to optimise the deposition parameters. The capability of MAPLE technique to deposit PEG has been confirmed and preliminary results show that visible (532 nm wavelength) radiation gives better results with respect to UV (355 nm) radiation. Despite usually UV wavelengths have been used and even if more systematic tests must be performed, it is important to underline that the choice of laser wavelength plays an important role in the application of MAPLE thin film deposition technique. © 2007 Published by Elsevier B.V. Keywords: MAPLE (Matrix Assisted Pulsed Laser Evaporation); Nd:YAG laser; PEG (polyethylene glycol); biomaterials; FTIR microscopy; X-ray reflectivity
1. Introduction Polyethylene glycol (PEG) is a polymer with technologically important applications, especially as a biomaterial (material interfacing with living tissues or biological fluids). Some of the biomedical applications of PEG are tissue engineering [1], spatial patterning of cells [2], anti-biofouling and biocompatible coatings [3], such as drug delivery coatings or orthopaedic implants. Biofouling is the cellular and proteinaceous adhesion on biomaterial surface and is commonly avoided through the immobilization of anti-fouling polymers on the surface to protect.
⁎ Corresponding author. Tel.: +39 081 76 82585; fax: +39 081 2391821. E-mail address: [email protected] (F. Bloisi). 0928-4931/$ - see front matter © 2007 Published by Elsevier B.V. doi:10.1016/j.msec.2006.12.005
For these applications, thin films of high quality are required. In order to have a good adhesion of PEG film to substrate chemically modified PEG has been used [4], but even this approach may be inadequate for some “in vivo” applications, such as anti-biofouling: in these situations it is essential to deposit a film with the same chemical and structural properties of bulk PEG. It is therefore required a technique allowing on one hand a control of the film morphology and thickness, and on the other hand the preservation of the chemical structure, the molecular weight distribution and the functionality of the bulk polymer. Matrix Assisted Pulsed Laser Evaporation (MAPLE) technique was shown to respond to both requirements [5–7]. MAPLE is a recently developed thin film deposition technique, slightly different from PLD (pulsed laser deposition), particularly well suited for organic/polymer thin film deposition.
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In PLD, largely applied for inorganic thin film deposition [8,9], a pulsed laser beam is focused onto a solid target whose material is ablated from target and deposited on a nearby substrate. Although some addition polymers [10] were successfully deposited, the PLD of addition polymers seems to proceed via a “depolymerization–ablation of monomers–repolymerization” mechanism. This is clearly not possible in general for condensation polymers or other organic molecules. In MAPLE deposition technique the pulsed laser beam is focused on a target obtained as a frozen solution in a relatively volatile solvent of the molecules to be deposited. The advantage of MAPLE with respect to PLD technique is that a great part of the laser beam energy is transferred to the solvent and molecules to be deposited are ejected from the target mainly due to solvent vaporization. Such technique is therefore especially well suited for deposition of large molecules as in organic or polymeric compounds, whose structure may be damaged by direct laser irradiation. It is therefore clear that great importance has the choice of laser beam wavelength, even if up to now most MAPLE depositions have been carried out operating with excimer lasers emitting either 248 nm [11,12] or 193 nm [12,13] wavelengths radiation. Using less energetic radiations can minimize the photochemical decomposition of the polymer molecules. MAPLE technique has already been successfully applied for PEG thin films deposition [5,7]. This work deals with the optimisation of the parameters (mainly laser wavelength) for PEG deposition. Moreover, often MAPLE systems are just a slight modification of an existing PLD system, while our system has been explicitly designed for MAPLE technique, allowing the introduction of some useful specific characteristics such as “in situ” target freezing (controlled atmosphere or vacuum), use of different (currently 355 nm, and 266 nm) laser wavelength, 2D target movement (and consequent full target surface scanning).
2. The MAPLE deposition system In MAPLE technique (Fig. 1) the target is a frozen matrix, composed by a dilute solution of the material to be deposited in the appropriate solvent. In this way most of the laser radiation energy is absorbed by the solvent, limiting the damage to the molecules of interest. The process leads to the formation of a plume, composed of solvent molecules in different phases (i.e. gas molecules and solid clusters [11,14]) which entrain the organic/ polymer molecules to deposit. During the target-to-substrate journey, the more volatile solvent molecules are pumped away by a vacuum system, while the organic/polymer molecules are deposited onto the substrate. The ideal solvent is the one that has no tendency to form films, has a relatively high vapour pressure, is optically absorbing and does not photochemically interact with the organic/polymer molecules. MAPLE provides, respect to PLD, a softer desorption mechanism since most of the incident radiation is absorbed by the frozen solvent. With MAPLE technique it is possible to obtain thin, homogeneous, well adherent coatings over large surfaces or selected areas with accurate thickness control, maintaining the chemical integrity and the physiochemical properties of the organic/polymer molecules deposited [5,11,12], though for some polymers molecular weight distribution variations towards lower molecular weights have been observed [11]. Our MAPLE deposition system (Fig. 1) consists of a pulsed laser deposition chamber with a target holder provided with a 2D computer controlled movement system. Using this system, it is possible to perform a full target surface scanning, avoiding excessive heating or erosion of a single spot on the target. The target holder (inside vacuum chamber) is in thermal contact with a liquid nitrogen tank (outside vacuum chamber). This set-up allows an “in situ” target freezing procedure: the target holder is
Fig. 1. Schematic of the MAPLE apparatus. The inset shows the MAPLE deposition process: the plume is composed by solvent and polymer molecules; the more volatile solvent is pumped away by the vacuum system while polymer molecules are deposited onto the substrate.
F. Bloisi et al. / Materials Science and Engineering C 27 (2007) 1185–1190
filled with about 2 ml of solution and the chamber is closed; the chamber is filled with a dry inert gas (e.g. nitrogen or helium) in order to avoid humidity condensation on target surface and to avoid solvent evaporation (the gas pressure can be set to the required value); the target temperature is then reduced by filling the tank with liquid nitrogen and, only after solidification, the chamber pressure is reduced. The deposition chamber is provided with a window for plume observation/analysis by optical (i.e. fast CCD with system upgrade) or spectroscopic (LIBS) techniques. The substrate is mounted parallel to the frozen target surface, on a device that allows a manual movement so that the target-tosubstrate distance can be opportunely varied as required. A Qswitched Nd:YAG pulsed (pulse duration 6 ns, pulse repetition rate up to 10 pulses per second) laser operating with either the first (1064 nm wavelength) or the successive (532 nm, 355 nm) harmonics is connected to the chamber by means of an articulated arm. The possibility of varying the laser wavelength offers the advantage of choosing the more appropriate wavelength for the solvent–organic/polymer pair. In this set of MAPLE depositions results obtained UV (355 nm wavelength) and visible (532 nm, green) laser radiation have been compared. 3. Experimental PEG (chemical formula is reported in Fig. 2) depositions were carried out at either 355 nm or 532 nm laser wavelength. About 2 ml of target solution were placed into the target holder within the vacuum chamber. PEG target was a solution of PEG (4.1 wt.%) and propanol (4.0 wt.%) in bidistilled water obtained starting from a Polyethylene glycol 3000 monodisperse solution 10% in H2O (PEG solution is from Fluka, propanol is from Romil, bidistilled water is from Carlo Erba). After target holder was filled, the vacuum chamber was closed and placed in a dry gas (helium) flow in order to reduce humidity. Target temperature was slowly reduced up to a value (∼−10 °C) below target melting point (∼0 °C), filling the liquid nitrogen tank. While the temperature still lowered, pressure was reduced up to about 5 10− 7 Torr. The final values, before starting pulsed laser deposition, were T = −187 °C, p = 5.3 10− 7 Torr. At this time target movement and laser emission started, while substrate was far (about 10 cm) apart from target in order to clean target surface. Substrate was then placed close (about 1 cm) to the target and deposition started. During deposition, the pressure inside vacuum chamber rose to about 2 10− 6 Torr. Deposition parameters used for different samples are summarized in Table 1. Deposed films were evaluated by means of FTIR microscopy and X-ray reflectivity, so as to control polymer chemical integrity and film thickness and homogeneity. A Hyperion 2000 infrared microscope (Bruker) was used; spectral measurements of surface were performed in reflectance mode, using a 15x Schwarzschild IR objective. X-ray reflectivity (XRR) spectra
Fig. 2. Chemical formula of polyethylene glycol (PEG).
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Table 1 MAPLE deposition parameters of polyethylene glycol Sample
S1
S2
S3
Target solution Wavelength (nm) Pulse repetition rate (pulse/s) Pulse duration (ns) Pulse energy (mJ) Beam spot size Total pulses Scanned target area (cm2)
PEG 355 10 6 70–94 ∼ 2 mm2 ∼ 20,000 ∼1
PEG 355 10 6 94 ∼ 2 mm2 ∼ 60,000 ∼1
PEG 532 10 6 68 ∼0.8 cm2 ∼60,000 ∼2
were collected by a Bruker “'D8 Advance” diffractometer equipped with a Goëbel mirror. 4. Results and discussion In Fig. 3 IR spectra of a drop of PEG dried on a microscopy glass are shown in comparison with the spectra of the glass substrate: the whole range and the fingerprint window 1600– 900 cm− 1 are shown. Figs. 4, 5 and 6 report spectra of samples S1, S2 and S3, respectively obtained by different measurements on the whole area. In Fig. 7 the X-ray reflectivity (XRR) spectra of the samples are shown. The spectral measurements were performed in reflectance mode, as previously stated; normally, reflectance spectra are of poorer quality than transmittance spectra, except for thin coatings on metallic substrates. In fact, a typical non-metallic material reflects only less than 5% of the light, while the remaining light is absorbed by the substrate. Therefore, measurements in reflectance mode are only suitable for samples with a very smooth surface such as chips, polymers, and thin coatings on metal (as in our specific case, although substrate is glass). For this reason, beside the studied molecule curve, CO2 peak at about 2400 cm− 1 can be clearly identified and the background curve of microscopy glass remarkably affects molecule typical pattern. Nevertheless, the main identification peaks of PEG can be evidenced (Fig. 3): CH2 antisymmetric stretching vibrations bands appear between 2940– 2855 cm− 1 and CH2 antisymmetric scissoring vibrations bands between 1475–1445 cm− 1; the C–OH deformation vibration yields an undefined absorption at 1400–1300 cm− 1; alcohols have the C–O single bond in common with ethers; thus, likewise, they show a strong band in the region of 1210–1000 cm− 1; moreover, C–O–C aliphatic ether antisymmetric stretching vibration bands (1150–1060 cm− 1) can be identified; additionally, in phase C–C–O stretching vibration of primary alcohols can be identified by 900–800 cm− 1 peak. IR analyses performed on sample S1 (Fig. 4) highlight its homogeneity, since all curves have the same shape. Nevertheless, the quantity of deposed PEG seems to be scarce, because the pattern of glass is prevailing. Anyhow, we can state that deposed PEG is unmodified with respect to the reference dried drop, in fact all the main peaks can be recognised. Sample S2 (Fig. 5) appears less homogeneous (measurements performed on different areas yield a different response), but, at the same time, PEG peaks are more evident with respect to sample S1. This phenomenon can be due to the higher amount of
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Fig. 3. IR spectra of microscopy glass substrate clean (black line) and with a dried drop of PEG (red line). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
PEG locally deposed (optical microscopy investigations confirmed this result). Maple deposition performed on sample S3 allowed to achieve a significant homogeneity and a higher amount of PEG with respect to samples S1 and S2 (Fig. 6).
X-ray reflectivity (XRR) is a surface-sensitive technique that provides information on mass density, thickness, and roughness of very thin films that are deposited on flat substrates. This measurement is based on the specular reflection of X-rays from planar surfaces. The reflected intensities show fringes that depend
Fig. 4. IR spectra of sample S1. The inset shows the fingerprint window 1600–900 cm− 1.
F. Bloisi et al. / Materials Science and Engineering C 27 (2007) 1185–1190
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Fig. 5. IR spectra of sample S2. The inset shows the fingerprint window 1600–900 cm− 1.
on the film thickness, and different modulation lengths correspond to the existence of different layers. The critical angle of total reflection is related to the mass density [15]. XRR spectra, reported in Fig. 7, show large modulations indicating that the films thickness is of the order of some nanometres. Because of the film roughness and the low difference between the substrate and the film density the quantification of the error in the thickness evaluation is very high. For all spectra the
critical angles result the same and are very near to the glass one; this is due to the low mass density of the PEG. 5. Conclusions The capability of MAPLE technique to deposit PEG (polyethylene glycol) has been confirmed using a deposition system explicitly designed for this technique. Preliminary results show
Fig. 6. IR spectra of sample S3. The inset shows the fingerprint window 1600–900 cm− 1.
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Fig. 7. X-ray reflectivity spectra of the samples.
that the use of visible (532 nm wavelength) light gives better results with respect to UV (355 nm) laser radiation. Despite usually UV wavelengths have been used and even if more systematic tests must be performed, it must be underlined that the choice of laser wavelength plays an important role in the application of MAPLE thin film deposition technique. Acknowledgment Work partially supported by FIRB 2001 research project “Study, realization and experiment of microsystems for controlled drug delivery in situ”. References [1] J.A. Burdick, K.S. Anseth, Biomaterials 23 (22) (2002) 4315. [2] S.O. Vansteenkiste, S.I. Corneillie, E.H. Schacht, X. Chen, M.C. Davies, M. Moens, L. Van Vaeck, Langmuir 16 (2000) 3330. [3] J.L. Dalsin, P.B. Messersmith, Mater. Today (2005) 38. [4] J.L. Dalsin, B.-H. Hu, B.P. Lee, P.B. Messersmith, J. Am. Chem. Soc. 125 (2003) 4253. [5] D.M. Bubb, P.K. Wu, J.S. Horowitz, J.H. Callahan, M. Galicia, A. Vertes, R.A. McGill, E.J. Houser, B.R. Ringeisen, D.B. Chrisey, J. Appl. Phys. 91 (4) (2002) 2055.
[6] D.B. Chrisey, A. Piqué, R.A. McGill, J.S. Horowitz, B.R. Ringeisen, D.M. Bubb, P.K. Wu, Chem. Rev. 103 (2003) 553. [7] P.K. Wu, B.R. Ringeisen, J. Callahan, M. Brooks, D.M. Bubb, H.D. Wu, A. Piqué, B. Spargo, R.A. McGill, D.B. Chrisey, Thin Solid Films 398–399 (2001) 607. [8] D.B. Chrisey, J.S. Horwitz, R.E. Leuchtner, Thin Solid Films 206 (1–2) (1991) 111. [9] D.H. Lowndes, D.B. Geohegan, A.A. Puretzky, D.P. Norton, C.M. Rouleau, Science 273 (1996) 898. [10] G.B. Blanchet, S.I. Shah, Appl. Phys. Lett. 62 (9) (1993) 1026. [11] A.L. Mercado, C.E. Allmond, J.G. Hoekstra, J.M. Fitz-Gerald, Appl. Phys. Lett. A 81 (2005) 591. [12] A. Piqué, R.A. McGill, D.B. Chrisey, D. Leonhardt, T.E. Mslna, B.J. Spargo, J.H. Callahan, R.W. Vachet, R. Chung, M.A. Bucaro, Thin Solid Films 355–356 (1999) 536. [13] A. Piqué, P. Wu, B.R. Ringeisen, D.M. Bubb, J.S. Melinger, R.A. McGill, D.B. Chrisey, Appl. Surf. Sci. 186 (2002) 408. [14] T.E. Itina, L.V. Zhigilei, B.J. Garrison, Nucl. Instrum. Methods Phys. Res. B Beam Interact. Mater. 180 (2001) 238. [15] E. Bontempi, X-ray reflectivity for the characterization of thin films, in: Transworld Research Network (Ed.), Recent Research Development in Chemical Physics, vol. 5, ISBN: 81-7895-155-X, 2004, p. 461.
Biomaterial thin film deposition and characterization by means of MAPLE technique F. Bloisi a,⁎, L. Vicari a , R. Papa a , V. Califano b , R. Pedrazzani c , E. Bontempi c , L.E. Depero c a
CNR-INFM Coherentia — Napoli, Dip. Scienze Fisiche — Univ. Napoli “Federico II”, P.le V.Tecchio, 80 — 80125 Napoli, Italy b Dip. Scienze Fisiche — Univ. Napoli “Federico II”, P.le V.Tecchio, 80 — 80125 Napoli, Italy c Laboratorio di Chimica per le Tecnologie, Universita' degli Studi di Brescia, Via Branze, 38 — 25123 Brescia, Italy Received 5 May 2006; accepted 5 December 2006 Available online 10 January 2007
Abstract Polyethylene glycol (PEG) is a polymer with technologically important applications, especially as a biomaterial. Several biomedical applications (such as tissue engineering, spatial patterning of cells, anti-biofouling and biocompatible coatings) require the application of high quality PEG thin films. In order to have a good adhesion to substrate chemically modified polymer molecules have been used, but for some “in vivo” applications it is essential to deposit a film with the same chemical and structural properties of bulk PEG. Pulsed laser deposition (PLD) technique is generally able to produce high quality thin films but it is inadequate for polymer/organic molecules. MAPLE (Matrix Assisted Pulsed Laser Evaporation) is a recently developed PLD based thin film deposition technique, particularly well suited for organic/polymer thin film deposition. Up to now MAPLE depositions have been carried out mainly by means of modified PLD systems, using excimer lasers operating in UV, but the use of less energetic radiations can minimize the photochemical decomposition of the polymer molecules. We have used a deposition system explicitly designed for MAPLE technique connected to a Q-switched Ng:YAG pulsed laser which can be operated at different wavelength ranging from IR to UV in order to optimise the deposition parameters. The capability of MAPLE technique to deposit PEG has been confirmed and preliminary results show that visible (532 nm wavelength) radiation gives better results with respect to UV (355 nm) radiation. Despite usually UV wavelengths have been used and even if more systematic tests must be performed, it is important to underline that the choice of laser wavelength plays an important role in the application of MAPLE thin film deposition technique. © 2007 Published by Elsevier B.V. Keywords: MAPLE (Matrix Assisted Pulsed Laser Evaporation); Nd:YAG laser; PEG (polyethylene glycol); biomaterials; FTIR microscopy; X-ray reflectivity
1. Introduction Polyethylene glycol (PEG) is a polymer with technologically important applications, especially as a biomaterial (material interfacing with living tissues or biological fluids). Some of the biomedical applications of PEG are tissue engineering [1], spatial patterning of cells [2], anti-biofouling and biocompatible coatings [3], such as drug delivery coatings or orthopaedic implants. Biofouling is the cellular and proteinaceous adhesion on biomaterial surface and is commonly avoided through the immobilization of anti-fouling polymers on the surface to protect.
⁎ Corresponding author. Tel.: +39 081 76 82585; fax: +39 081 2391821. E-mail address: [email protected] (F. Bloisi). 0928-4931/$ - see front matter © 2007 Published by Elsevier B.V. doi:10.1016/j.msec.2006.12.005
For these applications, thin films of high quality are required. In order to have a good adhesion of PEG film to substrate chemically modified PEG has been used [4], but even this approach may be inadequate for some “in vivo” applications, such as anti-biofouling: in these situations it is essential to deposit a film with the same chemical and structural properties of bulk PEG. It is therefore required a technique allowing on one hand a control of the film morphology and thickness, and on the other hand the preservation of the chemical structure, the molecular weight distribution and the functionality of the bulk polymer. Matrix Assisted Pulsed Laser Evaporation (MAPLE) technique was shown to respond to both requirements [5–7]. MAPLE is a recently developed thin film deposition technique, slightly different from PLD (pulsed laser deposition), particularly well suited for organic/polymer thin film deposition.
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F. Bloisi et al. / Materials Science and Engineering C 27 (2007) 1185–1190
In PLD, largely applied for inorganic thin film deposition [8,9], a pulsed laser beam is focused onto a solid target whose material is ablated from target and deposited on a nearby substrate. Although some addition polymers [10] were successfully deposited, the PLD of addition polymers seems to proceed via a “depolymerization–ablation of monomers–repolymerization” mechanism. This is clearly not possible in general for condensation polymers or other organic molecules. In MAPLE deposition technique the pulsed laser beam is focused on a target obtained as a frozen solution in a relatively volatile solvent of the molecules to be deposited. The advantage of MAPLE with respect to PLD technique is that a great part of the laser beam energy is transferred to the solvent and molecules to be deposited are ejected from the target mainly due to solvent vaporization. Such technique is therefore especially well suited for deposition of large molecules as in organic or polymeric compounds, whose structure may be damaged by direct laser irradiation. It is therefore clear that great importance has the choice of laser beam wavelength, even if up to now most MAPLE depositions have been carried out operating with excimer lasers emitting either 248 nm [11,12] or 193 nm [12,13] wavelengths radiation. Using less energetic radiations can minimize the photochemical decomposition of the polymer molecules. MAPLE technique has already been successfully applied for PEG thin films deposition [5,7]. This work deals with the optimisation of the parameters (mainly laser wavelength) for PEG deposition. Moreover, often MAPLE systems are just a slight modification of an existing PLD system, while our system has been explicitly designed for MAPLE technique, allowing the introduction of some useful specific characteristics such as “in situ” target freezing (controlled atmosphere or vacuum), use of different (currently 355 nm, and 266 nm) laser wavelength, 2D target movement (and consequent full target surface scanning).
2. The MAPLE deposition system In MAPLE technique (Fig. 1) the target is a frozen matrix, composed by a dilute solution of the material to be deposited in the appropriate solvent. In this way most of the laser radiation energy is absorbed by the solvent, limiting the damage to the molecules of interest. The process leads to the formation of a plume, composed of solvent molecules in different phases (i.e. gas molecules and solid clusters [11,14]) which entrain the organic/ polymer molecules to deposit. During the target-to-substrate journey, the more volatile solvent molecules are pumped away by a vacuum system, while the organic/polymer molecules are deposited onto the substrate. The ideal solvent is the one that has no tendency to form films, has a relatively high vapour pressure, is optically absorbing and does not photochemically interact with the organic/polymer molecules. MAPLE provides, respect to PLD, a softer desorption mechanism since most of the incident radiation is absorbed by the frozen solvent. With MAPLE technique it is possible to obtain thin, homogeneous, well adherent coatings over large surfaces or selected areas with accurate thickness control, maintaining the chemical integrity and the physiochemical properties of the organic/polymer molecules deposited [5,11,12], though for some polymers molecular weight distribution variations towards lower molecular weights have been observed [11]. Our MAPLE deposition system (Fig. 1) consists of a pulsed laser deposition chamber with a target holder provided with a 2D computer controlled movement system. Using this system, it is possible to perform a full target surface scanning, avoiding excessive heating or erosion of a single spot on the target. The target holder (inside vacuum chamber) is in thermal contact with a liquid nitrogen tank (outside vacuum chamber). This set-up allows an “in situ” target freezing procedure: the target holder is
Fig. 1. Schematic of the MAPLE apparatus. The inset shows the MAPLE deposition process: the plume is composed by solvent and polymer molecules; the more volatile solvent is pumped away by the vacuum system while polymer molecules are deposited onto the substrate.
F. Bloisi et al. / Materials Science and Engineering C 27 (2007) 1185–1190
filled with about 2 ml of solution and the chamber is closed; the chamber is filled with a dry inert gas (e.g. nitrogen or helium) in order to avoid humidity condensation on target surface and to avoid solvent evaporation (the gas pressure can be set to the required value); the target temperature is then reduced by filling the tank with liquid nitrogen and, only after solidification, the chamber pressure is reduced. The deposition chamber is provided with a window for plume observation/analysis by optical (i.e. fast CCD with system upgrade) or spectroscopic (LIBS) techniques. The substrate is mounted parallel to the frozen target surface, on a device that allows a manual movement so that the target-tosubstrate distance can be opportunely varied as required. A Qswitched Nd:YAG pulsed (pulse duration 6 ns, pulse repetition rate up to 10 pulses per second) laser operating with either the first (1064 nm wavelength) or the successive (532 nm, 355 nm) harmonics is connected to the chamber by means of an articulated arm. The possibility of varying the laser wavelength offers the advantage of choosing the more appropriate wavelength for the solvent–organic/polymer pair. In this set of MAPLE depositions results obtained UV (355 nm wavelength) and visible (532 nm, green) laser radiation have been compared. 3. Experimental PEG (chemical formula is reported in Fig. 2) depositions were carried out at either 355 nm or 532 nm laser wavelength. About 2 ml of target solution were placed into the target holder within the vacuum chamber. PEG target was a solution of PEG (4.1 wt.%) and propanol (4.0 wt.%) in bidistilled water obtained starting from a Polyethylene glycol 3000 monodisperse solution 10% in H2O (PEG solution is from Fluka, propanol is from Romil, bidistilled water is from Carlo Erba). After target holder was filled, the vacuum chamber was closed and placed in a dry gas (helium) flow in order to reduce humidity. Target temperature was slowly reduced up to a value (∼−10 °C) below target melting point (∼0 °C), filling the liquid nitrogen tank. While the temperature still lowered, pressure was reduced up to about 5 10− 7 Torr. The final values, before starting pulsed laser deposition, were T = −187 °C, p = 5.3 10− 7 Torr. At this time target movement and laser emission started, while substrate was far (about 10 cm) apart from target in order to clean target surface. Substrate was then placed close (about 1 cm) to the target and deposition started. During deposition, the pressure inside vacuum chamber rose to about 2 10− 6 Torr. Deposition parameters used for different samples are summarized in Table 1. Deposed films were evaluated by means of FTIR microscopy and X-ray reflectivity, so as to control polymer chemical integrity and film thickness and homogeneity. A Hyperion 2000 infrared microscope (Bruker) was used; spectral measurements of surface were performed in reflectance mode, using a 15x Schwarzschild IR objective. X-ray reflectivity (XRR) spectra
Fig. 2. Chemical formula of polyethylene glycol (PEG).
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Table 1 MAPLE deposition parameters of polyethylene glycol Sample
S1
S2
S3
Target solution Wavelength (nm) Pulse repetition rate (pulse/s) Pulse duration (ns) Pulse energy (mJ) Beam spot size Total pulses Scanned target area (cm2)
PEG 355 10 6 70–94 ∼ 2 mm2 ∼ 20,000 ∼1
PEG 355 10 6 94 ∼ 2 mm2 ∼ 60,000 ∼1
PEG 532 10 6 68 ∼0.8 cm2 ∼60,000 ∼2
were collected by a Bruker “'D8 Advance” diffractometer equipped with a Goëbel mirror. 4. Results and discussion In Fig. 3 IR spectra of a drop of PEG dried on a microscopy glass are shown in comparison with the spectra of the glass substrate: the whole range and the fingerprint window 1600– 900 cm− 1 are shown. Figs. 4, 5 and 6 report spectra of samples S1, S2 and S3, respectively obtained by different measurements on the whole area. In Fig. 7 the X-ray reflectivity (XRR) spectra of the samples are shown. The spectral measurements were performed in reflectance mode, as previously stated; normally, reflectance spectra are of poorer quality than transmittance spectra, except for thin coatings on metallic substrates. In fact, a typical non-metallic material reflects only less than 5% of the light, while the remaining light is absorbed by the substrate. Therefore, measurements in reflectance mode are only suitable for samples with a very smooth surface such as chips, polymers, and thin coatings on metal (as in our specific case, although substrate is glass). For this reason, beside the studied molecule curve, CO2 peak at about 2400 cm− 1 can be clearly identified and the background curve of microscopy glass remarkably affects molecule typical pattern. Nevertheless, the main identification peaks of PEG can be evidenced (Fig. 3): CH2 antisymmetric stretching vibrations bands appear between 2940– 2855 cm− 1 and CH2 antisymmetric scissoring vibrations bands between 1475–1445 cm− 1; the C–OH deformation vibration yields an undefined absorption at 1400–1300 cm− 1; alcohols have the C–O single bond in common with ethers; thus, likewise, they show a strong band in the region of 1210–1000 cm− 1; moreover, C–O–C aliphatic ether antisymmetric stretching vibration bands (1150–1060 cm− 1) can be identified; additionally, in phase C–C–O stretching vibration of primary alcohols can be identified by 900–800 cm− 1 peak. IR analyses performed on sample S1 (Fig. 4) highlight its homogeneity, since all curves have the same shape. Nevertheless, the quantity of deposed PEG seems to be scarce, because the pattern of glass is prevailing. Anyhow, we can state that deposed PEG is unmodified with respect to the reference dried drop, in fact all the main peaks can be recognised. Sample S2 (Fig. 5) appears less homogeneous (measurements performed on different areas yield a different response), but, at the same time, PEG peaks are more evident with respect to sample S1. This phenomenon can be due to the higher amount of
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Fig. 3. IR spectra of microscopy glass substrate clean (black line) and with a dried drop of PEG (red line). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
PEG locally deposed (optical microscopy investigations confirmed this result). Maple deposition performed on sample S3 allowed to achieve a significant homogeneity and a higher amount of PEG with respect to samples S1 and S2 (Fig. 6).
X-ray reflectivity (XRR) is a surface-sensitive technique that provides information on mass density, thickness, and roughness of very thin films that are deposited on flat substrates. This measurement is based on the specular reflection of X-rays from planar surfaces. The reflected intensities show fringes that depend
Fig. 4. IR spectra of sample S1. The inset shows the fingerprint window 1600–900 cm− 1.
F. Bloisi et al. / Materials Science and Engineering C 27 (2007) 1185–1190
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Fig. 5. IR spectra of sample S2. The inset shows the fingerprint window 1600–900 cm− 1.
on the film thickness, and different modulation lengths correspond to the existence of different layers. The critical angle of total reflection is related to the mass density [15]. XRR spectra, reported in Fig. 7, show large modulations indicating that the films thickness is of the order of some nanometres. Because of the film roughness and the low difference between the substrate and the film density the quantification of the error in the thickness evaluation is very high. For all spectra the
critical angles result the same and are very near to the glass one; this is due to the low mass density of the PEG. 5. Conclusions The capability of MAPLE technique to deposit PEG (polyethylene glycol) has been confirmed using a deposition system explicitly designed for this technique. Preliminary results show
Fig. 6. IR spectra of sample S3. The inset shows the fingerprint window 1600–900 cm− 1.
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Fig. 7. X-ray reflectivity spectra of the samples.
that the use of visible (532 nm wavelength) light gives better results with respect to UV (355 nm) laser radiation. Despite usually UV wavelengths have been used and even if more systematic tests must be performed, it must be underlined that the choice of laser wavelength plays an important role in the application of MAPLE thin film deposition technique. Acknowledgment Work partially supported by FIRB 2001 research project “Study, realization and experiment of microsystems for controlled drug delivery in situ”. References [1] J.A. Burdick, K.S. Anseth, Biomaterials 23 (22) (2002) 4315. [2] S.O. Vansteenkiste, S.I. Corneillie, E.H. Schacht, X. Chen, M.C. Davies, M. Moens, L. Van Vaeck, Langmuir 16 (2000) 3330. [3] J.L. Dalsin, P.B. Messersmith, Mater. Today (2005) 38. [4] J.L. Dalsin, B.-H. Hu, B.P. Lee, P.B. Messersmith, J. Am. Chem. Soc. 125 (2003) 4253. [5] D.M. Bubb, P.K. Wu, J.S. Horowitz, J.H. Callahan, M. Galicia, A. Vertes, R.A. McGill, E.J. Houser, B.R. Ringeisen, D.B. Chrisey, J. Appl. Phys. 91 (4) (2002) 2055.
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