Matrix-assisted Pulsed Laser Evaporation Technique Research Articles

Thermal behavior and matrix-assisted pulsed laser evaporation deposition of functional polymeric materials thin films with potential use in optoelectronics

Thermal behavior studies of several polymers, i.e. polyvinyl chloride (PVC), polyacrylic acid (PAA), and polyaniline (PAni), together with thin film deposition by a MAPLE technique, are presented. Thermal stability studies are important before attempting any laser-assisted processing of such materials, as this provides a better understanding of their molecular behavior, to assess the evolution and dynamics of the deposition. Thin films were grown on silicon substrates by means of matrix-assisted pulsed laser evaporation (MAPLE) using a Nd:YAG laser (266 and 355nm, 7ns pulse duration). Above the fluence of ~0.7J/cm2 the polymeric films are highly non-uniform due to the transferring of matrix-droplets and/or clusters from the target. At low fluence, typically between 0.1 and 0.5J/cm2, the polymeric films are intact; the surface morphology is substantially improved and there are no measurable traces of the matrix molecules in the films. The important question of polymeric molecules integrity in the films with respect to that of the targets is discussed, in relation to their properties, and compared to data available in the literature.

Biosensor Applications of MAPLE Deposited Lipase.

Matrix Assisted Pulsed Laser Evaporation (MAPLE) is a thin film deposition technique derived from Pulsed Laser Deposition (PLD) for deposition of delicate (polymers, complex biological molecules, etc.) materials in undamaged form. The main difference of MAPLE technique with respect to PLD is the target: it is a frozen solution or suspension of the (guest) molecules to be deposited in a volatile substance (matrix). Since laser beam energy is mainly absorbed by the matrix, damages to the delicate guest molecules are avoided, or at least reduced. Lipase, an enzyme catalyzing reactions borne by triglycerides, has been used in biosensors for detection of β-hydroxyacid esters and triglycerides in blood serum. Enzymes immobilization on a substrate is therefore required. In this paper we show that it is possible, using MAPLE technique, to deposit lipase on a substrate, as shown by AFM observation, preserving its conformational structure, as shown by FTIR analysis.

Structural characterization of MAPLE deposited lipase biofilm

Lipases (triacylglycerol ester hydrolases) are enzymes used in several industrial applications. Enzymes immobilization can be used to address key issues limiting widespread application at industrial level. Immobilization efficiency is related to the ability to preserve the native conformation of the enzyme. MAPLE (Matrix Assisted Pulsed Laser Evaporation) technique, a laser deposition procedure for treating organic/polymeric/biomaterials, was applied for the deposition of lipase enzyme in an ice matrix, using near infrared laser radiation. Microscopy analysis showed that the deposition occurred in micrometric and submicrometric clusters with a wide size distribution. AFM imaging showed that inter-cluster regions are uniformly covered with smaller aggregates of nanometric size. Fourier transform infrared spectroscopy was used for both recognizing the deposited material and analyzing its secondary structure. Results showed that the protein underwent reversible self-association during the deposition process. Actually, preliminary tests of MAPLE deposited lipase used for soybean oil transesterification with isopropyl alcohol followed by gas chromatography–mass spectrometry gave results consistent with undamaged deposition of lipase.

Photoluminescence of Eu-doped LiYF4 thin films grown by pulsed laser deposition and matrix-assisted pulsed laser evaporation

Matrix-assisted pulsed laser evaporation (MAPLE) has been investigated as an alternative to the pulsed laser deposition (PLD) technique for Eu3+-doped crystalline LiYF4 thin-films deposition. MAPLE assumes laser ablation of a frozen target made of the material of interest diluted in a solvent, rather than that of a bulk target, of either pressed powder or single crystal, used in the case of PLD. Our approach stems from the assumption that laser ablation of a frozen dilute target would result in thin films with improved morphology, as compared to PLD. Indeed, we find that roughness values of samples obtained by the MAPLE technique are four times lower than in the case of PLD. A lower transmittance was noticed for PLD obtained layers with respect to those grown by MAPLE due to strong scattering of light by the morphological defects. Photoluminescence spectra are showing characteristic Eu3+-ion luminescence bands at 578, 591, 612, 650 and 698 nm (5D0 → 7FJ); crystal field splitting of the bands indicates dopant ions incorporation in the host material during transfer by either PLD or MAPLE.

Ferrocene carboxaldehyde thin films grown by matrix-assisted pulsed laser evaporation for non linear optical applications

Thin films of ferrocene carboxaldehyde, also known as cyclopentadienyl(formylcyclopentadienyl)iron, were grown on silicon and glass substrates by matrix-assisted pulsed laser evaporation technique, using a Nd:YAG device operating at 266nm(4ω). Spectroscopic-ellipsometry, Fourier transform infrared spectroscopy, scanning electron microscopy and atomic force microscopy investigations revealed that the films are homogeneous in thickness, with dense morphology and without cracks, low surface roughness (∼11nm), and no significant chemical damage. Second harmonic generation capabilities of the thin films were evidenced by using a femtosecond Ti:sapphire laser.

Study of the deposition of graphene oxide by matrix-assisted pulsed laser evaporation

Thin films composed of graphene-based materials exhibit promising functional properties for the development of high-performance devices in a wealth of applications. However, there are significant technological challenges which force one to search for alternative pathways of materials production and deposition. This paper reports the deposition of graphene oxide (GO) flakes on quartz substrates by using the ultraviolet matrix-assisted pulsed laser evaporation (MAPLE) technique in vacuum or controlled nitrogen gas environment. Water, which is highly transparent to UV radiation, was used as matrix solvent for the preparation of the MAPLE targets. The results reveal that GO platelets can be successfully transferred by MAPLE technique. Besides, the GO material experiences a significant deoxygenating process during deposition, leading to the formation of reduced GO. Numerical simulations also show that the thickness of GO platelets highly influences the deposition process and the structure of the immobilized material. Thick enough aggregates can reach temperatures of thousands of degrees and undergo a large degradation in their structure.

Matrix-Assisted Pulsed Laser Thin Film Deposition by Using Nd:YAG Laser

Matrix-Assisted Pulsed Laser Evaporation (MAPLE) is a deposition technique, developed from Pulsed Laser Deposition (PLD) especially well suited for producing organic/polymeric thin films, which can take advantage from using Nd:YAG laser. Depending on the relative values of light absorption coefficients of the solvent and of the molecules to be deposited, laser energy is directly absorbed by the solvent or is transferred to it, providing a softer desorption mechanism with respect to PLD. In PLD ultraviolet laser radiation is commonly used, but in MAPLE, since easily damaged molecules are usually involved, the use of Nd:YAG laser offers the advantage to allow selecting laser wavelength from ultraviolet (266 nm or 355 nm, corresponding to 4.66 eV or 3.49 eV photon energies, resp.) to visible (532 nm, 2.33 eV) to infrared (1064 nm, 1.17 eV). In this paper, the MAPLE technique is described in details, together with a survey of current and possible future applications for both organic and biomaterial deposition taking into account the advantages of using an Nd:YAG laser. Beside other results, we have experimental confirmation that MAPLE applications are not limited to transparent molecules highly soluble in light absorbing solvent, thus allowing deposition of poorly soluble light absorbing molecules suspended in a light transparent liquid.

Study of titania nanorod films deposited by matrix-assisted pulsed laser evaporation as a function of laser fluence

Chemically synthesized brookite titanium dioxide (TiO2) nanorods with average diameter and length dimensions of 3–4 nm and 35–50 nm, respectively, were deposited by the matrix-assisted pulsed laser evaporation technique. A toluene nanorod solution was frozen at the liquid-nitrogen temperature and irradiated with a KrF excimer laser (λ=248 nm, τ=20 ns) at the repetition rate of 10 Hz, at different fluences (25 to 350 mJ/cm2). The deposited films were structurally characterized by high-resolution scanning and transmission electron microscopy. 〈100〉 single-crystal Si wafers and carbon-coated Cu grids were used as substrates. Structural analyses evidenced the occurrence of brookite-phase crystalline nanospheres coexisting with individually distinguishable TiO2 nanorods in the films deposited at fluences varying from 50 to 350 mJ/cm2. Nanostructured TiO2 films comprising only nanorods were deposited by lowering the laser fluence to 25 mJ/cm2. The observed shape and phase transitions of the nanorods are discussed taking into account the laser-induced heating effects, reduced melting temperature and size-dependent thermodynamic stability of nanoscale TiO2.

The effect of the target structure and composition on the ejection and transport of polymer molecules and carbon nanotubes in matrix-assisted pulsed laser evaporation

Matrix-assisted pulsed laser evaporation (MAPLE) is a prominent member of a broad and expanding class of laser-driven deposition techniques where a matrix of volatile molecules absorbs laser irradiation and provides the driving force for the ejection and transport of the material to be deposited. The mechanisms of MAPLE are investigated in coarse-grained molecular dynamic simulations focused on establishing the physical regimes and limits of the molecular transfer from targets with different structures and compositions. The systems considered in the simulations include dilute solutions of polymer molecules and individual carbon nanotubes (CNTs), as well as continuous networks of carbon nanotubes impregnated with solvent. The polymer molecules and nanotubes are found to be ejected only in the ablation regime and are incorporated into matrix-polymer droplets generated in the process of the explosive disintegration of the overheated matrix. The ejection and deposition of droplets explain the experimental observations of complex surface morphologies in films deposited by MAPLE. In simulations performed for MAPLE targets loaded with CNTs, the ejection of individual nanotubes, CNT bundles, and tangles with sizes comparable or even exceeding the laser penetration depth is observed. The ejected CNTs align along the flow direction in the matrix plume and tend to agglomerate into bundles at the initial stage of the ablation plume expansion. In a large-scale simulation performed for a target containing a network of interconnected CNT bundles, a large tangle of CNT bundles with the total mass of 50 MDa is separated from the continuous network and entrained with the matrix plume. No significant splitting and thinning of CNT bundles in the ejection process is observed in the simulations, suggesting that fragile structural elements or molecular agglomerates with complex secondary structures may be transferred and deposited to the substrate with the MAPLE technique.

Effect of substrate temperature on MAPLE deposition of synthetic eumelanin films

Eumelanin is an important pigment almost ubiquitous in animals and plants exhibiting interesting charge transport capabilities. Its poor solubility in common solvents represents a severe limitation for preparing thin films. It was recently demonstrated that eumelanin films can be successfully deposited with the MAPLE (Matrix Assisted Pulsed Laser Evaporation) technique starting from a frozen water suspension, using infrared laser radiation. The low laser absorption of ice together with the high absorption of eumelanin suggests that the target ablation is due to laser energy absorbed by the eumelanin molecules, followed by thermal energy transfer, and ejection of ice/water/vapor containing undamaged eumelanin molecules and supramolecular structures. Here, we report on the deposition of eumelanin thin films on substrates at different temperatures eventually followed by in-situ annealing. Structural characterization (UV-VIS, FTIR, AP-MALDI) confirms that the deposited films maintain the characteristics of the eumelanin biopolymer. Morphological characterization (AFM) shows that surface roughness increases with increasing substrate temperature during MAPLE deposition, but is not influenced by annealing. Preliminary electrical characterization shows that eumelanin films seem to obey Ohm’s law without evidence that charge injection from gold electrodes is affected by the presence of significant energy barriers. Moreover, charge transport is drastically reduced in vacuum, even if the phenomenon is at least partially reversible.

The Matrix-Assisted Pulsed Laser Evaporation (MAPLE) process: origins and future directions

This review discusses the origins and evolution of the MAPLE technique as it started as an alternative to spray coating of thin films for chemical vapor sensors. It describes its numerous applications for the deposition of thin films of polymeric, organic and biomaterials for various applications. This is followed by an overview of several new variations of the MAPLE technique. This review concludes with an outlook on the future of this highly versatile and successful vapor deposition process.

Matrix-assisted pulsed laser deposition of polymer and nanoparticle films

Matrix-Assisted Pulsed Laser Evaporation (MAPLE) technique was used to deposit films of Poly(9,9-dioctylfluorene) – PFO and Methoxy Ge Triphenylcorrole [Ge(TPC)OCH3]. The PFO was dissolved in different matrices, like chloroform-CHCl3, tetrahydrofuran – THF and toluene with a 0.5 wt % concentration, while Ge(TPC)OCH3 was diluted in THF with a concentration of 0.01 wt %. The frozen targets were irradiated with a KrF excimer laser. The, films presented good emission properties to be exploited in light emitting devices and gas sensors based on luminescence quenching. The working principle of the MAPLE technique was used for the deposition of colloidal nanoparticles and nanorods, too. TiO2 colloidal nanoparticles (diameter: ∼10 nm) and nanorods (diameter: 5 nm; length: 50 nm) were diluted in deionised water (0.02 wt %) and toluene (0.016 wt %) respectively. The deposited nanostructures preserved dimensions and structural properties of the starting particles and the films showed very interesting electrical responses when exposed to oxidizing gases for sensing applications.

Matrix assisted pulsed laser evaporation: the surface cluster problem

The unexpected presence of agglomerates in polymer films deposited by the Matrix Assisted Pulsed Laser Evaporation (MAPLE) technique is discussed. Many experimental and theoretical works suggest that the simple model of individual molecule evaporation must be abandoned. Solute concentration, boiling temperature and vapor pressure of solvents, laser pulse number, and laser light penetration depth are important parameters to be considered to explain the morphology of the MAPLE deposited films. Nanorods films, which can be efficiently deposited on rough surfaces using the MAPLE technique, present more or less large surface droplets, also. Here, the reduced melting temperature of nanostructured materials can explain agglomeration even at low laser fluences.

Thin films of polymer blends for controlled drug delivery deposited by matrix-assisted pulsed laser evaporation

We demonstrate the use of matrix-assisted pulsed laser evaporation technique for incorporating drugs (indomethacin) within thin films of polymer blends, to be used as biodegradable implants that deliver drugs in a controlled manner. For irradiation at fluences up to 1 J/cm2, the films show excellent surface morphology and the chemical structure of all constituent polymers within the blend is well preserved. The optical characteristics of the films are assessed by spectroscopic ellipsometry. The presence of the drug within the polymeric films is confirmed by its specific absorption at 319 nm.

CdS thin films obtained by thermal treatment of cadmium(II) complex precursor deposited by MAPLE technique

Thin films of [Cd{SSi(O–Bu t ) 3}(S 2CNEt 2)] 2, precursor for semiconducting CdS layers, were deposited on silicon substrates by Matrix-Assisted Pulsed Laser Evaporation (MAPLE) technique. Structural analysis of the obtained films by Fourier transform infrared spectroscopy (FTIR) confirmed the viability of the procedure. After the deposition of the coordination complex, the layers are manufactured by appropriate thermal treatment of the system (thin film and substrate), according to the thermal analysis of the compound. Surface morphology of the thin films was investigated by atomic force microscopy (AFM) and spectroscopic-ellipsometry (SE) measurements.

Matrix-assisted pulsed laser evaporation of polyimide thin films and the XPS study

Compared with the traditional thin film techniques, the matrix-assisted pulsed laser evaporation (MAPLE) technique has many advantages in the deposition of polymer and organic thin films. It has a wide range of applications in many fields, such as non-linear optics, luminescent devices, electronics, various sensors. We have successfully deposited polyimide thin films by using the MAPLE technique. These films were characterized with XPS. The XPS spectra showed that the single-photon effect is obvious at low laser fluence and the chemical bonds will be broken, resulting in decomposition of the films. Contrarily, the single-photon effect will decrease and the multi-photon effect and the photothermal effect will increase at high laser fluence, resulting in the protection of the structure of the polyimide thin films and the obvious decrease in decomposition. High laser fluence is more suitable for the deposition of polymer and organic thin films than low laser fluence.

Multifunctional thin films of lactoferrin for biochemical use deposited by MAPLE technique

Lactoferrin (Lf) is an iron-binding glycoprotein present in almost all mammalian secretions which plays an important role in host defense against microbial and viral infections. The protein has been reported to also have anti-inflammatory activity and antitumoral effects in vitro and in vivo. Thin films of Lf were deposited on silicon, quartz and Thermanox plastic coverslip substrates by Matrix Assisted Pulsed Laser Evaporation (MAPLE) technique, using a Nd:YAG laser working at 266 nm, at different laser fluences (0.1–0.8 J cm −2). The deposited layers have been characterized by Fourier Transformed Infra-Red spectroscopy (FTIR), and the morphology of the various substrates was investigated by Atomic Force Microscopy (AFM). The biocompatibility of lactoferrin thin films was evaluated for each substrate, by in vitro biochemical tests.

MAPLE deposition of methoxy Ge triphenylcorrole thin films

Methoxy Ge Triphenylcorrole [Ge(TPC)OCH3] has been recently synthesized and deposited as thin film by the Matrix Assisted Pulsed Laser Evaporation (MAPLE) technique. In the last few years, corroles have been the object of an increasing number of studies and MAPLE technique seems to be a very promising deposition method for organic and polymeric films, producing good results for applications in chemical gas sensing layers production. In this work Ge(TPC)OCH3 thin films were deposited by both spin coating and MAPLE techniques for comparison. The morphology of the films was investigated by Atomic Force Microscopy (AFM), while their optical properties were analyzed by photoluminescence (PL) and UV-vis absorption measurements and were compared with the ones of the starting solution. The film absorption spectrum presented the same peaks with the same relative intensities of that recorded in solution. The luminescence spectra were acquired periodically to evaluate the aging effects and no detectable variations were recorded over a period of 1 month.

Matrix-assisted pulsed laser evaporation of poly(D,L-lactide) for biomedical applications: effect of near infrared radiation

The deposition of thin films of poly(D,L-lactide) (PDLLA) by using the matrix-assisted pulsed laser evaporation (MAPLE) technique is investigated. PDLLA is a highly biocompatible and biodegradable polymer, with wide applicability in the biomedical field. The laser wavelength used in the MAPLE process is optimized to obtain a good-quality deposition. The structure of the polymer film is analyzed by Fourier transform infrared spectroscopy (FTIR). It is found that the chemical structure of PDLLA undergoes little or no damage during deposition with near-infrared laser radiation (1064 nm). It is thus confirmed that at this wavelength, the MAPLE technique can be applied for fragile biopolymer molecules, which are easily damaged by other laser radiations (UV radiation). This method allows future development of tailored polymer coatings for biomedical applications.

Making Molecular Balloons in Laser-Induced Explosive Boiling of Polymer Solutions

The effect of the dynamic molecular rearrangements leading to compositional segregation is revealed in coarse-grained molecular dynamics simulations of short pulse laser interaction with a polymer solution in a volatile matrix. An internal release of matrix vapor at the onset of the explosive boiling of the overheated liquid is capable of pushing polymer molecules to the outskirts of a transient bubble, forming a polymer-rich surface layer enclosing the volatile matrix material. The results explain unexpected "deflated balloon" structures observed in films deposited by the matrix-assisted pulsed laser evaporation technique.