Appl Phys A (2011) 105:651–659 DOI 10.1007/s00339-011-6624-5
Matrix-assisted pulsed laser evaporation of chemoselective polymers Alexandra Palla-Papavlu · Valentina Dinca · Maria Dinescu · Fabio Di Pietrantonio · Domenico Cannatà · Massimiliano Benetti · Enrico Verona
Received: 27 June 2011 / Accepted: 20 August 2011 / Published online: 19 October 2011 © Springer-Verlag 2011
Abstract In this work, matrix-assisted pulsed laser evaporation was applied to achieve gentle deposition of polymer thin films onto surface acoustic wave resonators. Polyepichlorhydrin, polyisobutylene and polyethylenimine were deposited both onto rigid substrates e.g. Si wafers as well as surface acoustic wave devices using a Nd-YAG laser (266 nm, 355 nm, 10 Hz repetition rate). Morphological investigations (atomic force microscopy and optical microscopy) reveal continuous deposited polymer thin films, and in the case of polyethylenimine a very low surface roughness of 1.2 nm (measured on a 40 × 40 µm2 area). It was found that only for a narrow range of laser fluences (i.e. 0.1–0.3 J/cm2 in the case of polyisobutylene) the chemical structure of the deposited polymer thin layers resembles to the native polymer. In addition, in the case of polyisobutylene it was shown that the irradiation at 355-nm wavelength produces deviations in the chemical structure of the deposited polymer, as compared to its bulk structure. Following the morphological and structural characterization, only a set of well established conditions was used for polymer deposition on the sensor structures. The surface acoustic wave resonators have been tested using the Network Analyzer before and after polymer deposition. The polymer coated surface acoustic wave resonator responses have been measured upon exposure to various A. Palla-Papavlu · V. Dinca · M. Dinescu () NILPRP, National Institute for Lasers, Plasma and Radiation Physics, 409 Atomistilor Street, PO Box MG-16, 077125 Magurele, Romania e-mail:
[email protected] F. Di Pietrantonio · D. Cannatà · M. Benetti · E. Verona “O.M. Corbino” Institute of Acoustics Italian National Research Council—CNR, Via del Fosso del Cavaliere 100, 00133 Rome, Italy
concentrations of dimethylmethylphosphonate analyte. All sensors coated with different polymer layers (polyethylenimine, polyisobutylene, and polyepichlorhydrin) show a clear response to the dimethylmethylphosphonate vapor. The strongest signal is obtained for polyisobutylene, followed by polyethylenimine and polyepichlorhydrin. The results obtained indicate that matrix-assisted pulsed laser evaporation is potentially useful for the fabrication of polymer thin films to be used in applications including microsensor industry.
1 Introduction A wide variety of naturally occurring substances (such as cells, enzymes, receptors, and antibodies) are commonly used as sensing elements in sensor technology. Besides these biological materials, polymers are also used as recognition elements in sensor structures, due to their wide range of capabilities [1]. Since the first reports on polymeric sensor materials (in the 1960s) [2–5], new knowledge of molecular interactions, micro/nanoscale fabrication, and analytical characterization tools has resulted in innovative chemical and biological sensing concepts that have been successfully demonstrated and validated for numerous applications [6]. Some of the most extensively used polymers as sensitive elements in sensor devices are conductive polymers (e.g. polypyrroles (PPy), and polyanilines (PANi)) [7, 8]. Other polymers such as poly(p-phenyleneethynylenes) (PPE), polymeric porphyrins, polysilanes, polyacetylenes, and poly(p-phenylenevinylenes) (PPV), present interesting properties for sensing applications due to their high permeability to small molecule analytes, such as nitroaromatics [9].
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In addition to the above mentioned polymers, polyisobutylene (PIB), polyepichlorhydrine (PECH), and polyethyleneimine (PEI) are also used in chemical sensing applications. PECH is an attractive material due to the presence of reactive chlorine groups on the polymer backbone which result in nucleophilic substitution for side chain modifications [10]. This polymer offers an excellent balance of properties having a high capability for absorption of dimethylmethylphosphonate (DMMP) and toluene [11, 12]. Another polymer of interest is PEI which has a variety of applications, i.e., ranging from chemical sensing, due to its reactivity with hydrazine based compounds, to the biomedical field, due to its capability as transfection enhancer [13, 14]. PIB, similarly to the above mentioned polymers (PEI and PECH), has been utilized in sensor applications, e.g. chemiresistors, chemicapacitors, microcantilever, and optical sensors. PIB is a strong hydrogen-bond acidic polymer, and presents a high sensitivity to organophosphorus nerve agents [11, 15, 16]. Of the systems already developed to detect toxic and chemical species in air, surface acoustic wave (SAW) resonators have emerged as a more convenient and cost effective alternative. However, the poor reliability caused by the lack of accuracy in placing the polymer membranes onto the SAW devices is an issue yet to be solved. The sensitivity of SAW resonators is dependent on the amount of vapor sorbed by the polymer overlayer and also by the SAW’s inherent ability to respond to the physical changes in the overlaying film. Therefore, it is very important to precisely place the polymer coating, and moreover for the coating to have the correct thickness and roughness. The simplest and most common methods for applying polymers to the surface of SAW devices are spin coating, spray coating, and ink jet printing. However, these techniques present some disadvantages, that is the lack of material efficiency, as in the case of spin coating [17], limited control over the amount of fluid flow [18], or as in the case of inkjet printing, clogging of the print heads. Moreover, spray coating is associated with macro-molecule “bubbles” at the polymer—SAW substrate interface [19, 20]. Due to the fact that the aforementioned approaches do not meet all requirements, e.g. precise positioning, repeatability, control of polymer layer thickness and roughness, novel techniques, which overcome these limitations are developed. Pulsed laser deposition (PLD) is a physical vapordeposition technique which involves the interaction of a laser beam with a target material (solid or liquid) resulting in a plume that is transporting the ablated species to a substrate, where a thin film is formed [21, 22]. PLD cannot be used for all types of polymer, biopolymer and protein, because the high laser fluences can result in photochemical
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or thermal decomposition. Even at relatively low fluences, some polymers are very photosensitive. To reduce the photochemical decomposition from the direct interaction of the UV laser light with organic and polymeric materials, a more gentle approach has to be used. As an alternative to conventional deposition techniques, matrix-assisted pulsed laser evaporation (MAPLE) has proved to be an attractive method for the deposition of organic thin films [22, 23]. MAPLE is similar to PLD, but has a different target preparation procedure. In MAPLE, a material, for example a polymer or a biomolecule, is dissolved in a solvent in concentrations of 0.1–5%, and the mixture is frozen, resulting in a solid target. Ideally, only the solvent will absorb the laser radiation. When the laser light irradiates the target, the solvent is evaporated and the dissolved material (the organic material or the polymer) is collected on a substrate, in the same way as for PLD. A successful deposition by MAPLE requires a high absorption of the laser light in the matrix, and as little absorption as possible by the guest (the polymer or the biomolecule) material, and that the matrix must not photochemically interact with the material to be deposited. Earlier work with MAPLE has demonstrated that with an appropriate choice of experimental parameters, such as laser wavelength, fluence and pulse duration, type of solvent, target and substrate temperature, and background gas pressure, MAPLE is capable of providing conditions for “soft” ejection and deposition of polymers and biological molecules without significant modifications of the chemical structure and functionality. A broad range of organic materials have been deposited as thin films by MAPLE, for example polymers (polyethylene glycol (PEG) [24, 25], polymer blends [26]), active proteins (lysozime [27], lactoferrin [28]), nanoparticles (TiO2 and SnO2 [29, 30]), polymer-carbon nanotube composites [31, 32], chemoselective polymers (fluoroalcoholpolysiloxane polymer (SXFA) [33]), polysiloxane with applications in chemical sensors [34], polyisobutylene [35]. In this work we discuss the use of MAPLE for the deposition of chemoselective polymers (PEI, PIB, and PECH) for chemical sensor applications. Surface acoustic wave resonators are used to demonstrate the capabilities of MAPLE when coated with chemoselective polymers. Finally, the response of these sensors when exposed to DMMP vapor is shown.
2 Experimental 2.1 Materials The guest materials are polyisobutylene (Mw = 600 kDa), polyethylenimine (Mw = 10 kDa), and polyepichlorhydrine
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Fig. 1 Chemical formulas of PECH, PIB, and PEI polymers further used for processing in this work
(Mw = 700 kDa), all obtained from Sigma-Aldrich. The chemical formulas of the three polymer used for processing in this work are given in Fig. 1. Toluene, acetone, and ethanol were obtained from SigmaAldrich and used as matrices without further purification. The polymers were dissolved in the proper solvent (PIB in toluene, PEI in ethanol, and PECH in acetone, three different experiments) to 1 and 2 wt% solutions and then flash frozen in liquid nitrogen resulting in solid targets which were used in the MAPLE experiments. 2.2 Deposition system A “Surelite II” pulsed Nd:YAG laser system (Continuum Company) with an emission wavelength of 355 nm, and 266 nm, 6 ns pulse duration, and 10 Hz repetition rate was used to irradiate the frozen polymer targets. The laser spot size was measured to be 0.02 cm2 , as measured by placing thermally sensitive paper in the plane of the target. The laser fluence was varied between 0.08– 0.7 J cm−2 . The target was rotated with a motion feedthrough driven by a motor resulting in the laser beam describing a circle onto the sample to achieve uniform evaporation. In order to control the temperature, two thermocouples were placed at two different positions of the target holder. The background pressures (air) ranging from 7 × 10−3 to 2 × 10−2 Pa, were obtained with a Pfeiffer-Balzers TPU 170 turbomolecular pump. During some depositions the pressure varied probably due to the vaporization caused by the laser pulses, or due to outgassing of the targets under vacuum. 2.3 Substrates For these experiments, two types of substrate were used: (i) silicon substrates Si(100) cut into 1 cm2 samples polished on both sides, transparent in the IR (for post-characterization), and (ii) surface acoustic wave resonators fabricated in twoport configuration. (i) The Si plates were cleaned prior to any deposition. The Si plates were ultra-sonicated in successive baths of acetone and ethanol, followed by rinsing them three times in the ultrasonic bath with ultrapure water (0.2 µm filtered). They were finally dried in a nitrogen flow.
(ii) SAW structures have been implemented on quartz substrates (ST-cut, x propagation), and the pattern has been transferred with a photolithography process using a poly(methyl metacrylate) (PMMA) resist and deep UV radiation, in order to obtain high resolution. The electrodes consisted of Al films grown by radio frequency reactive magnetron sputtering from a 99.999% pure Al target in an Ar atmosphere (thickness 100 nm). Finally, a lift-off process allowed obtaining the interdigital transducers with different features. 2-port SAW resonators operating at approximately 392 MHz have been used in this work. The interdigital transducers were shaped with a Gaussian apotization in order to minimize spurious transverse modes, with a wavelength of 8 µm and fingers overlap of 450 µm, while the cavity length was 1278 µm. The devices have been tested with an HP 8753A Network Analyzer and the S21 parameter is reported. During the polymer deposition by MAPLE, the SAW resonators were covered with a mask, in order to allow the deposition of the polymer chemically interactive membranes (pCIM) only onto the active area of the SAWs. The mask was placed on the interdigital transducers (IDTs) in order to allow the uniform polymer deposition on an area of 1 mm (length) × 400 µm (width). All substrates were placed at a distance of 4 cm from the frozen target and kept at ambient temperature during the deposition. No post-deposition annealing was carried out. 2.4 Morphological and structural characterization of the deposited polymer thin films The deposited features were investigated by optical microscopy. The images were acquired with an Axiovert 200 Microscope coupled to a Carl Zeiss AxioCam MRm camera. Atomic force microscopy (AFM) (XE 100 AFM setup from Park) measurements in non contact mode were carried out to analyze the film surface roughness on several different areas and dimensions (from 5 × 5 µm2 to 40 × 40 µm2 ). In addition, from the AFM measurements the thickness of the deposited thin films was determined. This was achieved by depositing the polymer thin films with a mask in order to create a sharp edge. Fourier Transform Infra Red Spectroscopy was applied to study the characteristic vibrations of functional groups in the deposited thin films. The infrared spectrum of the native
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molecule was measured and compared with the spectrum obtained from a thin film. The FTIR measurements were carried out with a Jasco FT/IR-6300 type A spectrometer in the range 400–7800 cm−1 . All spectra were obtained by absorption measurements, accumulating 128 scans, and CO2 /H2 O correction. 2.5 Dimethylmethylphosphonate measurements In order to test the sensor devices, a custom sealed chamber equipped with electronic oscillators, one for each SAW resonator, was used. In order to obtain different concentrations of DMMP in N2 , the sensor system was exposed to a total flux of 1000 sccm, controlled by two flow meters: the main for the gas carrier (N2 ) and the second for the analyte. The vapor concentrations were obtained fluxing N2 in the liquid analyte (e.g. DMMP) by using a bubbler. In addition, a flow switch together with a mixing chamber allowed obtaining the desired gas mixture. A frequency counter and a data acquisition system were used to monitor the frequency of the resonators during the measurements.
3 Results and discussion One of the biggest challenges in the field of chemical sensing is the development of microsensors with high selectivity and sensitivity toward a chemical environment. In designing SAW devices for chemical detection, interactive polymer membranes must be chosen that will collect and concentrate analyte molecules at the sensor’s interface [36]. PECH, PIB, and PEI were deposited by MAPLE as pCIM on SAW resonators due to the fact that they have the ability to specifically and selectively identify target gases immediately. Fig. 2 3D topographical images of PIB thin films deposited at 266 nm laser wavelength, on silicon substrates after 20k pulses at laser fluences of (a) 0.5 J/cm2 ; (b); 0.4 J/cm2 ; (c) 0.2 J/cm2 ; (d) 0.1 J/cm2
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3.1 Morphological analysis 3.1.1 Polyisobutylene For the applications envisioned in this work, i.e. the development of functional SAW resonators for the detection of chemical vapors, polymer film deposition with good morphology (low roughness and uniformity over all the deposited area), and thickness control is a fundamental requirement. The morphologies of the deposited polymer (PIB, PEI and PECH) layers were characterized by AFM and optical microscopy. The quality of the films surface is in general affected by the laser fluence. The AFM images of PIB layers obtained at different laser fluences (laser system operating at 266 nm and the target made out of 2% polymer and 98% toluene) are shown in Fig. 2. From these images it can be seen that lowering the fluence leads to a reduction of the density of droplets and of the number of cracks and holes on the film surface, consistent with an observation which has also been reported by Leveugle et al. 2005 [37]. The polymer film growth seems to follow a Stranski– Krastanov model, where a few monolayers are deposited on the bare substrate, and subsequently three-dimensional clusters are formed on these layers [38]. The AFM images of PIB films deposited on silicon substrates reveal a high roughness on all scanned areas. The best film smoothness is achieved by the deposition at fluences between 0.1–0.3 J/cm2 . It is noteworthy that at very low fluences,
