Applications of the matrix-assisted pulsed laser evaporation method ...

Appl Phys A (2011) 105:565–582 DOI 10.1007/s00339-011-6600-0

Applications of the matrix-assisted pulsed laser evaporation method for the deposition of organic, biological and nanoparticle thin films: a review Anna Paola Caricato · Armando Luches

Received: 9 June 2011 / Accepted: 20 August 2011 / Published online: 27 September 2011 © Springer-Verlag 2011

Abstract The matrix-assisted pulsed laser evaporation (MAPLE) technique offers an efficient mechanism to transfer soft materials from the condensed to the vapor phase, preserving the versatility, ease of use and high deposition rates of the pulsed laser deposition (PLD) technique. The materials of interest (polymers, biological cells, proteins, . . . ) are diluted in a volatile solvent. Then the solution is frozen and irradiated with a pulsed laser beam. Here, important results of MAPLE deposition of polymer, biomaterials and nanoparticle films are summarized. Finally, the MAPLE mechanism is discussed. A review of experimental and theoretical works points out that the simple model of individual molecule evaporation must be abandoned. Solute concentration, solubility, evaporation temperature of solvents, laser pulse power density and laser penetration depth emerge as important parameters to explain the morphology of the MAPLE-deposited films.

1 Introduction Due to the trend to miniaturization, many efforts are made to efficiently deposit inorganic, organic and bio-materials in form of thin, uniform and adherent films. The goal is the realization of controlled structures to be used, for instance, for drug delivery [1], tissue engineering [2], for gas and vapor detection [3], for light emitting devices [4, 5], etc. A second, not less important field is related to the synthesis and application of nanoparticles and nanostructures (dots, rods, A.P. Caricato · A. Luches () Department of Physics, University of Salento, Via Arnesano, 73100 Lecce, Italy e-mail: [email protected] Fax: +39-08-32297499

fullerenes, single- and multi-walled tubes, etc.). The choice of the deposition method depends above all on the particular physico-chemical properties of the material of interest, the requirements for film quality, the type of substrate and the production costs. For large-scale industrial applications, besides the purely thermal evaporation [6], thin films are commonly deposited by electron beam physical vapor deposition (EBPVD) [7], low-pressure chemical vapor deposition (LPCVD) [8], plasma impulse chemical vapor deposition (PICVD) [9], magnetron sputtering [10] and ion beam sputtering (IBS) [11]. The common feature to all the above techniques is the fact that the solid target material is decomposed to the atomic level by the deposition process. It means that these techniques cannot be used for complex, organic and biological materials. To deposit thin films of complex materials, simple methods, which involve liquid solutions of the material of interest in a volatile solvent, were introduced. They include aerosol, dip-coating and spin-coating techniques. In the first case [12], the control of the film thickness is poor and the use of propellants is being even more strictly regulated by safety and environment regulations (organic pollutant, ozone layer preservation, etc.). In the dip-coating method the substrate is slowly dipped into and withdrawn from a tank containing the sol, with a uniform velocity, in order to obtain a uniform coating. It was recently used to deposit Ag nanowire arrays on Si wafers [13]. In the spin-coating technique [14], the film-forming process is primarily driven by two independent parameters: viscosity and spin speed. The range of film thicknesses easily achieved is 1–200 µm. For organic thin films, Langmuir–Blodgett dip coating using self assembled monolayer is the common method to functionalize surfaces with a single bio-molecular layer [15]. Clearly, each of these deposition techniques presents its own merits and

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drawbacks. In general, each one allows the treatment of a limited class of compounds. The ability to deposit a wide class of materials using a single technique would provide a great advantage for the development and implementation of devices based on soft materials. The deposition of these kinds of material in the form of thin or ultra-thin films is not an easy task. There are several parameters which have to be controlled during and after the film deposition: first of all preservation of the chemical integrity and the required physical-chemical properties of the materials, then good thickness control, good adhesion to the substrate and film uniformity over an extended area, complete and homogeneous coverage of tridimensional substrates.

2 Laser-based deposition techniques The most commonly used laser-based deposition techniques are the laser-assisted chemical vapor deposition (LCVD) and pulsed laser deposition (PLD). LCVD is characterized, with respect to CVD, by the localized heating produced by a laser beam on a substrate. It is traditionally used to directly deposit complex geometries of different materials, including metals [16] and ceramics [17]. Novel applications, like nanowire structures, have also been approached [18]. Deposition is limited to materials with available gaseous precursors. A much wider range of applications is offered by the PLD technique. PLD has been applied to a wide range of materials, like semiconductors [19], metals [20], alloys [21] and compounds [22]. The PLD peculiarity of congruent ablation is of great importance for the deposition of complex compounds like high critical temperature superconductors [23], doped glasses [24] and magnetic alloys [25]. When the ablation is performed in a low-pressure atmosphere (1 Pa, or less) chemical reactions occur between the ablated material and the ambient gas. Nitrides [26], carbides [27] and oxides [28] of the ablated elements were deposited or the exact target stoichiometry eventually lost during the ablation process was restored [29]. PLD presents many other advantages: monolayer thickness control, good film-to-substrate adhesion, minimum material consumption, low substrate temperature, among others. However, it is not suited for the deposition of large delicate molecules like polymers and biomaterials, since the powerful laser pulse causes a strong heating at the laser beam spot, breaking molecular bonds in polymers and burning biological materials. There are few exceptions, since polymers like Teflon (PTFE) [30], polymethylmethacrylate (PMMA) [31], polypernaphthalene (PPN) [32] and maybe some other polymer films have been PLD deposited on appropriate substrates.

A.P. Caricato, A. Luches

To avoid photochemical damage and decomposition of the target materials caused by PLD, a new laser-based deposition technique was introduced: the matrix-assisted pulsed laser evaporation (MAPLE) [33–35]. The main difference with PLD is the target structure: the material of interest (solute) is diluted in a volatile solvent matrix to form a solution (solute concentrations typically of the order of 1 wt%). The solution is frozen at liquid nitrogen temperature (LNT, −196◦ C) and placed into a vacuum chamber to act as a target. The frozen target is irradiated with a pulsed laser beam, like in the PLD process. But now the laser pulse energy is mainly absorbed by the solvent and converted to thermal energy, allowing the solvent to vaporize. By collective collisions with the evaporating solvent, the molecules of the solute receive enough kinetic energy to be transferred in the gas phase. The highly volatile solvent is pumped away during the flight to the substrate and the deposited film should be composed only of the solute material. The ablation onset is defined by the thermodynamic parameters of the volatile solvent, rather than by the ones of the solute material. So, the deposition can proceed at much lower fluences (0.05–0.5 J/cm2 ), as compared to conventional PLD. Using low fluences, thermal damage or decomposition of the solute molecules is prevented or greatly reduced. Using shadow masks, i.e. lines, dots and arrays, pattern features with length scales as small as 20 µm can be deposited [36]. MAPLE could be considered as the opposite of MALDI (Matrix Assisted Laser Desorption/Ionization) [37]. Here, the solvent (e.g. glycerin) is selected to be almost transparent to the used laser light, which in contrast is strongly absorbed by dispersed particles (e.g., Co). The strong heating of the particles causes fast evaporation of the solvent and consequent evaporation/ionization of the molecules dispersed into the solvent. In other MALDI configurations, however, the solvent is the absorbing medium, like in MAPLE, but the application is very different: vaporization and ionization for mass spectroscopy investigation, not for film deposition. In the last years, MAPLE has been extensively used to deposit films of soft materials. Here, a short review is given of MAPLE deposition of polymers, organic materials and biomaterials. Moreover, another application, MAPLE deposition of nanostructured films, will be summarized. Finally, fundamental features of the MAPLE technique will be discussed.

3 MAPLE-deposition apparatus The MAPLE-deposition hardware does not substantially differ from the ones used in PLD. Excimer lasers (or Nd:YAG, third harmonic at 335 nm) are mostly used. In some particular cases, infrared laser sources are utilized [38]. The main difference with respect to PLD is the

Applications of the matrix-assisted pulsed laser evaporation method for the deposition of organic, biological

target holder, since it has to be kept at very low temperature during depositions. It means that a liquid nitrogen reservoir must be connected to the target holder. It is usually made of high-conductivity copper. The target must rotate (1–10 Hz), like in PLD, to allow smooth erosion of the frozen solution. Feedthroughs and connectors have to be accurately designed, with properly chosen gaskets, to allow rotation at low temperature without seizing problems.

4 MAPLE deposition of polymers and organic materials The compositional and structural complexity of polymers precludes their processing by conventional physical or chemical vapor-deposition methods. Laser techniques offer better opportunities. As mentioned above, some attempts to deposit thin films of polymeric materials were made by PLD. Several types of polymer (polyethylene, polycarbonate, polyimide and PMMA) were ablated using UV lasers at energies near the ablation threshold [39]. A decrease in the molecular weight of the polymers forming the films was always observed. PTFE (Teflon) films were also deposited [40]. Film formation was supposed to occur via pyrolytic decomposition, followed by repolymerization. Since repolymerization can be incomplete, frequently the properties of the PTFE films were found different from the ones of the target material [41]. Moreover, since the deposition seems to proceed via a “depolymerization–monomer ablation–repolymerization” mechanism, PLD clearly cannot be used in general for very complex polymers. MAPLE was expressly developed to overcome the above difficulties [35]. The first MAPLE studies were performed to deposit thin films of a hydrogen bond acid functionalized polysiloxane, known as SXFA, onto surface acoustic wave (SAW) devices. The MAPLE-deposited films (10–50 nm thick and highly uniform across the whole area, with a root mean square (rms) surface roughness of ∼3 nm) showed higher sensitivity and faster response times to various chemical vapors than analogous films deposited by spray coating [33, 42], due to the complete coverage of the substrate. After, many other polymers were MAPLE-deposited. For instance, GutiérrezLlorente et al. [43] grew films of regio-random and regioregular poly(3-hexylthiophene). The target was a frozen solution of the polymer in orthoxylene. MALDI mass spectroscopy proved that the deposition did not result in any degradation of the polymer. Houser et al. [44] demonstrated that MAPLE is well suited for deposition of high-quality thin films of especially synthesized fluoro-alcohol substituted carbo-polysiloxane polymers. Precise and accurate thickness control was achieved, film density was high and chemical integrity was fairly preserved. The quadrupled output of a Nd:YAG laser

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(266 nm) served as the laser source. Depositions were done in a background pressure of N2 , to reduce the speed of the ablated species by scattering. Using various solvents and varying the laser fluence, they optimized the deposition and fabricated high-quality thin films. The surface roughness was analyzed using atomic force microscopy (AFM) and found to be smoother than the one produced using conventional wet deposition techniques. Cristescu et al. [45] reported the successful MAPLEdeposition of polycaprolactone polymer using a KrF excimer laser (λ = 248 nm, τ = 7 ns). According to Fourier transform infrared (FTIR) spectra, the deposited films presented chemical structure close to the dropcast material. This work showed that laser fluence plays a key role in optimizing the performances of MAPLE-synthesized polycaprolactone structures. The authors claim that MAPLE allows for controlling the morphology of films to the level required in targeted drug delivery of pharmacologic agents. Since many authors (see below) claim chemical integrity of the MAPLE-deposited materials based on the overlapping of the film FTIR spectrum with the one of a reference solution, we stress here that this is a necessary, but not sufficient requisite. In fact depolimerization cannot be excluded even in the presence of identical FTIR spectra. Bloisi et al. [46] used a deposition system explicitly designed for MAPLE, based on a Q-switched Nd:YAG laser (λ = 1064 nm, 532 nm and 355 nm, τ = 6 s, 10 Hz), to fabricate poly [3-(4-octyloxyphenyl) thiophene] (POOPT) thin films. The frozen target was composed of 0.56 wt% of POOPT dissolved in chloroform. Laser fluences were F∼ = 136 mJ/cm2 at 532 nm and = 4.7 J/cm2 at 1064 nm, F ∼ 2 F∼ = 288 mJ/cm at 355 nm. Diffuse reflectance FTIR spectroscopy results demonstrated a highly regular disposition of the polymer backbone. In the film deposited at 532 nm the local chemical structure of the POOPT appeared well preserved, most probably due to the weaker fluence, while in the other two films decomposition phenomena occurred. Furthermore, the film prepared at 532 nm appeared more homogeneous. This work stresses the importance of using moderate laser fluences to avoid breaking of the chemical bonds of both solute and solvent. Socol et al. [47] studied some functionalized copolymers thin films prepared by MAPLE on silicon and quartz substrates. Two polymeric structures were synthesized by the copolymerization of maleic anhydride and methyl methacrylate, respectively, maleic anhydride and vinyl benzyl chloride, which were subsequently functionalized with 2,4dinitroaniline. UV–Vis, FTIR, Raman and photoluminescence (PL) spectroscopy were used to investigate the influence of substrate temperature (150 and 250◦ C), background N2 pressure (5–30 Pa) and polymer concentration into the target (2% and 3%) on the film properties. The authors evidenced that the MAPLE process does not damage the chem-

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ical structure of this kind of polymer. Scanning electron microscopy (SEM) investigations revealed the droplets type morphology of the polymeric films, with thickness between 41 nm and 105 nm. Recently, Paun et al. [48] demonstrated that MAPLE can be used for fabricating films of polymer blends, as well as for incorporating drugs in the polymeric films. The polymers [polyethylene glycol, PEG (1450 Da), poly (lactide-coglycolide), PLGA (40–75 kDa)] were separately dissolved in chloroform (1 wt%). Then, the two solutions were mixed together, frozen at LNT, placed in a vacuum chamber and submitted to laser irradiation (Nd:YAG laser, λ = 266 nm, F = 0.1–1.2 J/cm2 , 10 Hz). Films showed continuous surfaces, with uniform covering of the substrates. Irradiation at low fluences produced films with rms roughness of 20–100 nm. In contrast, irradiation at F ≥ 1 J/cm2 leads to increased roughness up to 200 nm. FTIR spectroscopy showed that both polymers were transferred preserving their chemical structure at F < 1 J/cm2 . Then indomethacin (INC), an antiinflammatory drug, was incorporated in the polymeric solution. Films of PEG:PLGA:INC blends were deposited. The presence of INC in the films was confirmed by the specific absorption of the drug at 319 nm. Laser deposition techniques were also used to deposit organic materials. Complex organic materials, like polynaphthalene and polyacrylonitrile were PLD deposited long ago [32, 49], generally with the same degradation problems presented by polymers. This is why MAPLE was used to deposit thin films of organic molecules, too. At first, thin films of various carbohydrates such as sucrose, glucose, and dextran were deposited [50]. Another organic polymer deposited by MAPLE in its early stage of application was poly(ethylene glycol-PEG). PEG has many biomedical applications; this is why a large number of papers continue to be published on PEG thin films. Thin films of PEG of average molecular weight 1400 amu were MAPLE-deposited by Bubb et al. [51]. The deposition was carried out in vacuum (∼10−4 Pa) with an ArF (λ = 193 nm) laser at F = 220–230 mJ/cm2 . Deionized water and chloroform (CHCl3 ) were used as matrices. The physiochemical properties of the films were studied using FTIR spectroscopy and electrospray ionization mass spectrometry. The results showed that the matrix used for MAPLE deposition of PEG can greatly affect the chemical structure and molecular weight distribution of the deposited film. The FTIR spectrum showed evidence for C–Cl bond formation when CHCl3 was used, while there was little evidence for photochemical modification when using H2 O (ice). Time-offlight analysis determined that neutral chlorine atoms were produced during CHCl3 target irradiation. The reduction in molecular weight and structural modification of the film deposited using CHCl3 was attributed to the presence of the


highly reactive chlorine atoms. MAPLE was also used by the same authors to grow polymer nanotube composite thin films using PEG as a polymer matrix [52] and to deposit thin films of various electrooptic organic polymers such as N-(4-nitrophenyl)-L-prolinol (NPP), polypyrrole, and aluminum tris-8-hydroxyquinline (Alq3), a luminescent organic compound [34]. The NPP films showed optical absorptions similar to that of their bulk counterpart and the polypyrrole films had electrical conductivities similar to polypyrrole films deposited by other techniques. In the case of the Alq3 films, the MAPLE-deposited samples exhibited optical absorption patterns different from those of bulk Alq3, most probably due to some decomposition occurred during the deposition process. PEG was also MAPLEdeposited by B. Toftmann et al. [53]. Water was used as a matrix for the guest material, at concentrations from 0.5 to 4 wt%. The target was irradiated with 6-ns laser pulses at 355 nm at high fluences (2.5–12 J/cm2 ). The authors claimed that, even at the highest fluence, FTIR spectra indicated a chemical structure of the deposit close to that of the un-irradiated PEG. The same authors investigated also the dependence of the surface morphology of the PEG films on the substrate temperature [54]. They found a clear difference between the films produced below and above the melting point of PEG. For temperatures above the melting point, the polymer material was non-uniformly distributed over the substrate. In a further paper [55] the laser wavelength was changed from Nd:YAG third harmonic (λ = 355 nm) to the fundamental one (λ = 1064 nm). Matrices of 1 wt% PEG frozen solutions were used and emission of PEG molecules and ions from PEG-doped water ice targets was investigated. Even though linear absorption in defect-free water ice is two orders of magnitude larger at 1064 nm than at 355 nm, the deposition rate and ion current density were found much smaller for IR than for UV laser light. The MAPLE technique was used again recently by Bloisi et al. [56] to deposit PEG films. Here a Q-switched Nd:YAG laser was used, which was operated at wavelengths ranging from IR to UV to optimize the deposition parameters. The results showed once more that visible (532 nm) radiation gives better results with respect to UV (355 nm) radiation in preserving the PEG integrity. Thin films of organic magnetic materials were also deposited: Meenakshi et al. [57] used both PLD and MAPLE (KrF laser with τ = 10 ns) to fabricate thin films of Mn12 acetate, a molecular nanomagnet. The deposited films were characterized by x-ray photoelectron spectroscopy (XPS), mass spectrometry and magnetic hysteresis. The results indicated that Mn12 -acetate chemical and magnetic integrity is preserved even using MAPLE at relatively high fluence (800 mJ/cm2 ), while it leads to fragmentation of the acetate. The MAPLE technique allowed for the fabrication of patterned thin films of molecular nanomagnets.


Purice et al. [58] used MAPLE to produce thin lysozyme films. Concentrations of the lysozyme in the range 0.1– 1 wt% in water were used. The matrix samples were irradiated with laser light at 355 nm, above the threshold for absorption of lysozyme (310 nm). The film deposition rate for 1 wt% lysozyme had a maximum (∼1 ng/cm2 per laser shot) at F = 2 J/cm2 . SEM images demonstrated that the silicon substrate was completely covered by films thicker than 100 nm. Analysis of the films demonstrated that the lysozyme molecules were not decomposed and preserved their activity. Deposition from a target with pressed (100%) solid lysozyme using PLD did not give good results. Myslik et al. [59] deposited thin layers of acetylacetonates and polypyrrole using a KrF laser at the repetition rate of 10 Hz in N2 at p = 3 Pa. Acetylacetonates (SnAcAc and InAcAc) were dissolved in acetone, while polypyrrole was dissolved in water matrix. For SnAcAc and InAcAc in acetone matrix the energy density of ablation threshold was found to be 0.07–0.1 J/cm2 , while for polypyrrole in water matrix it was 0.3 J/cm2 . The optimal energy density was ∼1.5 times the threshold fluence. FTIR analysis evidenced a good chemical composition similarity between the organic source and the deposited acetylacetonate films. The deposited layers were utilized for studying their gas detection characteristics. Further work on MAPLE deposition of thin layers of polypyrrole by the same authors [60] from water and dimethylsulphoxide matrices did not give appreciably better results. Cristescu et al. [61] reported the first successful deposition of triacetate-pullulan polysaccharide (a highly biocompatible and biodegradable polymer, with wide applicability in the biomedical field) thin films using a KrF laser (τ ≈ 20 ns, 10 Hz). FTIR spectra demonstrated that the thin films maintained the pullulan chemical structure. The same authors reported [62] the successful deposition of porous poly(D,L-lactide) using again a KrF laser (τ = 7 ns, 2 Hz). It was found that, here too, fluence played a key role in optimizing depositions. The authors claimed to have demonstrated that MAPLE was able to improve current approaches to grow high-quality thin films of poly(D,L-lactide), including porosity control, a characteristics which is highly required in targeted drug delivery. The MAPLE-deposition of thin films of poly(D,L-lactide) was investigated also by Califano et al. [63]. It was found that the chemical structure of the polymer underwent little or no damage during deposition with near-infrared laser radiation (1064 nm). The authors claimed that this method can allow future development of tailored polymer coatings for biomedical applications. In fact, recently, Cristescu et al. [64] demonstrated that MAPLE is a suitable technique to prepare composite thin films for controlled drug delivery. They deposited poly(1,3-bis-(p-carboxyphenoxy propane)-co-sebacic anhydride) 20:80 thin films containing several gentamicin concentrations, using a KrF laser. FTIR spectra demonstrated

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that the functional groups were not changed by the deposition process nor were new functional groups formed. SEM confirmed that the films were of good morphological quality. The activity of gentamicin-doped films against Escherichia coli and Staphylococcus aureus bacteria was demonstrated. Jelinek et al. [65] deposited thin films of various organic materials. The focus is on target preparation, solvents, materials and growth rate. Measured solvent optical transmissions and an overview of MAPLE applications are also presented. Tunno et al. and Caricato et al. [66, 67] used their customized MAPLE apparatus for a detailed study of the influence of solvents and laser fluence on the film characteristics. They deposited thin films of two compounds, poly(9,9-dioctylfluorene) (PFO) and a Ge-corrole derivative (Ge(TPC)OCH3 ), both of great technological interest, using a KrF laser (τ = 20 ns, 10 Hz). As regards PFO, the targets were prepared using chloroform (CHCl3 ), toluene (C7 H8 ) and tetrahydrofuran (THF–C4 H8 O) solvents at the PFO concentration of 0.5 wt%. To evaluate the influence of the solvents on the structural properties of the deposited films, FTIR spectra were acquired for films deposited at the same laser fluence (200 mJ/cm2 ). It resulted that chemical decomposition of PFO took place when chloroform was used. The chemical degradation of PFO films was likely related to the presence of high reactive Cl radicals released during laser irradiation [51]. Chemical reactions induced by chlorine were not detected by Bloisi et al. [46], most probably due to the very low fluence (136 mJ/cm2 ) and relatively large wavelength (532 nm) of their laser. In contrast, the FTIR spectra of the MAPLE-deposited PFO films using THF and toluene as solvents resulted very similar to the ones of the reference spin-coated films. It is important to notice that the peaks around 1716 and 1606 cm−1 , related to oxidized fluorene (fluorenone) [68], were not observed. The increasing of the laser fluence up to 500 mJ/cm2 did no influence the peak presence or position. The PL spectrum of the MAPLE film deposited from toluene solution showed glassy phase PFO emission, while the reference spin-coated film showed β-phase emission only. As the β-phase is formed in spin-coated films from toluene due to an interplay between aggregation in solution and solvent-induced chain planarization in the solid phase [69], the absence of β-phase emission in the MAPLE-deposited film suggests that molecular aggregation did not occur and that negligible interaction between PFO and toluene vapor took place during the deposition process. The PL spectrum of the MAPLE film from THF solution showed a superposition of the emission features of the glassy- and the β-phases. The absence of defect related emission in films deposited from toluene and THF solutions is strong evidence that no relevant chemical modification of the PFO emitting chromophors took place during MAPLE deposition.

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agglomerates and a non-uniform and non-complete coverage of the substrate. The UV-Vis spectra of the films deposited by MAPLE and spin-coating techniques presented a slight red-shift in the Soret and Q band positions with respect to the solution, possibly due to the formation of corrole aggregates. The higher red-shift and FWHM of the spectrum of the spin-coated film suggest a higher presence of corrole aggregates in the spin-coated film with respect to the MAPLE-deposited one. PL measurements were performed by using excitation radiation at 417 nm wavelength, since Ge(TPC)OCH3 has its maximum absorption at this wavelength. The MAPLE-deposited film presented well evident luminescence peaks around 600 nm and 650 nm, attributed to emission from Q bands, clearly indicating, together with the results in the UV-Vis absorption spectra, the successful transfer of the organic material on the substrate. Finally, we mention that recently Constantinescu et al. [74] deposited by MAPLE thin films based on the Cu(DAB)2 (a complex of o,o’-dihydroxy azobenzene with Cu2+ cation) organotransition metal compound using a Nd:YAG laser (λ = 355 nm, F = 0.4 J/cm2 ). This material is a new candidate for optical sensors chromophores because of its thermally robust chemical compounds. This work demonstrated that MAPLE could be a suitable technique for optical coating fabrication. Fig. 1 2D (a) and 3D (b) AFM micrographs of a typical MAPLE-deposited Ge(TPC)OCH3 thin film surface

Other materials of great interest are the porphyrin derivatives. In the porphyrinoid area, the unique features shown by corroles [70], make them very promising materials. Evidence of phosphorescence emission from corrole complexes was also reported [71]. Caricato et al. [67] deposited Ge(TPC)OCH3 films on silica substrates. Ge(TPC)OCH3 was prepared according to the method present in literature [71] and diluted in THF (0.01 wt%). Target irradiations were performed with a KrF laser (τ = 20 ns, 10 Hz, F = 500 mJ/cm2 ). For each film deposition 10,000 laser pulses were employed. For comparison, Ge(TPC)OCH3 thin films were deposited by the spin-coating method starting from a THF solution with concentration of 0.1 wt%, a 10 times higher value, in order to try to obtain homogeneous coverage of the silica substrate. The MAPLE-deposited Ge(TPC)OCH3 film showed (Fig. 1) the typical morphology of organic films [44, 53, 72] obtained with this deposition method. The surface presented circular and irregular regions and a high rms roughness value (48 ± 9 nm). The presence of these characteristic features was investigated, using molecular dynamic simulations, by Leveugle and Zhigilei [73], and will be discussed later. On the other end, the measurements performed on spin-coating deposited films showed a thinner layer with the presence of scattered

5 MAPLE deposition of biomaterials The MAPLE process was also successfully used for the growth of biomaterial films. Thin and uniform films of horseradish peroxidase (HRP) and insulin have been first deposited on a variety of substrates such as Si, NaCl and gold and platinum coated Si [75]. Analyses on insulin films demonstrated near-intact transfer of this protein with little or no photoinitiated decomposition. IR spectra and solvent-phase activity test of the HRP films indicated that most of the transferred proteins retained their chemical and physical structure as well as biological activity. These results represented the first demonstration that pure films of active biomolecules can be deposited using a vapordeposition technique. Additional studies were immediately after performed by the same authors on other biomaterials such as 50/50 Poly(D,L-lactide-co-glycolide), biotinylated bovine serum albumin (BSA), and phospholipid polymers, which also maintained function and chemical structure in the MAPLE-deposited thin films [76] A large number of studies have since been performed on many other biomaterials that, after deposition under well tailored MAPLE parameters, maintained their function and chemical structure. MAPLE deposition of biopolymers was also successfully tried, like mussels secrete adhesives known as mussel adhesive proteins, which allow attachment of the organisms to underwater marine environments. Doraiswamy et al. [77]


demonstrated the MAPLE deposition of Mytilus edulis foot protein-1 thin films. The FTIR spectrum suggested that the MAPLE process did not cause significant damage to the chemical structure of the protein. The motivation for depositing such thin films is the rising concerns over the environmental and health effects of solvents, monomers, and additives used in synthetic adhesives. The problem is that obtaining large quantities of naturally derived mussel adhesive proteins has proven rather problematic. Thus, synthetic analogs of mussel adhesive proteins have recently been developed. Cristescu et al. [78] demonstrated the successful thin film growth of 3,4-dihydroxyphenyl-l-alanine modified poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) (DOPA modified- PEO-PPO-PEO) block copolymer, a mussel adhesive protein analog, using MAPLE. It was shown that the main functional groups of the protein analog are present in the transferred film. In a further work on the same item, Cristescu et al. [79] deposited thin films of polymer mimics of cross-linking mussel adhesive proteins. The authors report the MAPLE-deposition of 1:100 and 1:1000 poly[(3,4-dihydroxystyrene)-co-styrene)] mussel adhesive protein analogs using an ArF laser. Films were homogeneous and adhesive, making the use of these materials in thin film form a viable option. In another paper Doraiswamy et al. [80] demonstrated that in MAPLE-deposited thin films of DOPA modified—PEG block copolymer mussel adhesive protein analogs the main functional groups were preserved. Successful MAPLE-deposition of thin films of another biopolymer, type I fibrilar collagen, was also reported by Cristescu et al. [81]. Deposition was performed in N2 ambient (20 Pa) using a KrF laser at 3 Hz. The incident laser energy was set in the range 20–35 mJ, with a laser spot area of 3.5–18.5 mm2 . The collagen films were characterized by Fourier transform infrared (FTIR) spectroscopy, AFM and high-resolution-transmission electron microscopy (HRTEM). It was demonstrated that they were composed of collagen, without impurities. Authors claim that film roughness can be controlled by the deposition conditions. MAPLE technique was also successfully applied for the deposition of films of calcium phosphates (CaPs), bioglasses and glass-ceramics, primary candidates for the manufacturing of medical implants. PLD proved to be a competitive method to grow high-quality biomaterial thin films (e.g. hydroxidapatite-HA) [82], but it cannot be used in the case of the very complex delicate biomolecules. Mihailescu et al. used MAPLE for the synthesis of hybrid organic-inorganic bionanocomposites synthesized from soluble CaPs, biopolymers for improving mechanical behavior of implants and alendronate doped HA for bone disease healing [83]. Results of SEM, TEM, HR-TEM, selected area electron diffraction (SAED,) glazing-incidence x-ray diffraction (GIXRD), XPS and FTIR diagnostics demonstrated that MAPLE deposition

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of simple or doped CaP films has the capacity of yielding pure, crystalline, stoichiometric nanofilms, allowing good control of their morphology, phase, crystallinity, and chemical composition. In vitro biocompatibility, bioactivity and biodegradability tests proved that human osteoblasts proliferate faster, reach a normal morphology and remain viable when cultured on MAPLE coatings. In vivo pull out tests of doped CaPs showed that these coatings strongly activated and enhanced bone repairing. Blood proteins were also MAPLE deposited, with excellent results. Stamatin et al. [84] reported the successful MAPLE deposition of fibrinogen blood protein thin films using a KrF laser (τ ≥ 20 ns, F = 440–770 mJ/cm2 , 2 Hz) in 15–20 Pa N2 atmosphere. FTIR spectroscopy demonstrated that the deposited thin films maintained their chemical structures. As frequently reported, the best results for fibrinogen deposition were obtained using low fluences. Another MAPLE-deposited blood component is bovine serum albumin (BSA), a protein with a molecular weight of 66 kDa. The first depositions were performed by Piqué et al. [34], who deposited discrete thin film micro-arrays of BSA with an ArF laser (F = 0.01–0.5 J/cm2 ). The deposited films, after washing with a blocking protein and fluorescently tagged streptavidin, fluoresced when exposed to UV radiation. According to the authors, this fluorescence indicates that the biochemical specificity of the transferred protein is unaffected by the MAPLE process. Later, Jelinek et al. [85] studied MAPLE deposition of thin films of BSA (ArF laser, F = 0.1–0.5 J/cm2 ). Films were deposited either in vacuum or in nitrogen ambient atmosphere. Targets were prepared from BSA solutions in phosphate buffered physiological saline, with an addition of UV absorbers as dimethylsulphoxide, phthalic acid and adenine. Film properties were studied with AFM and FTIR diagnostics. It was found that the deposition process changed the native conformation of albumin, resulting in the formation of waterinsoluble aggregates. Better results were obtained in a detailed study of BSA deposition made by Martino et al. [86]. Deionized water and phosphate buffered saline (PBS) were used as solvents. The BSA concentration (1 and 2 wt%) and the ArF (τ = 20 ns) laser fluence (75–500 mJ/cm2 ) were also varied, to find the best conditions. FTIR spectra showed that the major BSA absorption bands at 1653 cm−1 and 1550 cm−1 were present in the MAPLE-deposited BSA films, without red shifts. This result evidenced the absence of interaction of the protein molecules with the laser, thus avoiding their denaturation. Biological tests were performed to check the protein integrity after MAPLE deposition, using the sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE) [87]. The band relative to the entire BSA protein was detected in the film prepared at 500 mJ/cm2 , indicating that this fluence is high enough to produce a reasonable deposition rate, without damaging the solute.

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Smausz et al. [88] deposited urease thin films by MAPLE and PLD using frozen water solutions (1–10% m/v) for MAPLE and pure urease pellets for PLD irradiation. The fluence of their KrF laser (τ = 18 ns, 10 Hz) was varied between 300 and 2200 mJ/cm2 . The MAPLE-deposited films showed very strong dependence on the applied fluence. Significant activity was only measured at low fluence (F = 400 mJ/cm2 ) and 10% concentration target. The decrease of the urease concentration and the increase of the laser fluence resulted in a decrease of the activity. Morphologic studies proved that the films consisted of a smooth “base” layer with embedded micrometric droplets. The droplets were larger in films deposited by PLD. It must be finally stressed that, for the deposition of biopolymer and biological material thin films, the MAPLE technique is an efficient alternative to solution-based deposition techniques since MAPLE is a non-contact contamination-free process that can be integrated with other sterile processes. Moreover, MAPLE allows multiple-layer depositions without the risk of re-dissolving the first deposited layer. This characteristic offers interesting perspectives for multilayer deposition of composite coatings. 6 MAPLE deposition of nanoparticle films At present, the properties and possible applications of nanoparticles of different materials are most active research areas. The functional properties of the nanostructured films are largely determined by the size, composition, morphology and surface properties of the nanoparticles forming the film. High-quality nanoparticle films can be obtained by different methods like molecular beam epitaxy [89] and metalorganic chemical vapor deposition [90]. However, deposition processes are long, delicate and expensive. PLD is a much faster method to fabricate nanoparticles and nanoparticle films directly from bulk targets. Target ablation is performed in a noble gas or nitrogen atmosphere at pressures of the order of 100 Pa with nanosecond pulses [91], or in vacuum with femtosecond (fs) pulses [92], or in a liquid by high-power short (ps) laser systems [93]. Even if the results are of interest, the problem of the large size distribution poses strong limits to the extensive use of PLD for nanoparticle film fabrication. Recently, nanoparticles with well tailored size and low size dispersion have been obtained by chemical growth techniques, which are relatively easy and cheap [94]. Colloidal nanoparticles can be deposited as thin films by spin coating and drop casting. These deposition methods are quite simple, but they do not ensure a good control of the deposited film thickness and uniform coverage of the substrate, particularly on large areas or rough substrates. This is why a versatile, fast deposition technique, like MAPLE, was considered as a very attractive alternative. The colloidal nanosized particles can be diluted in a volatile


Fig. 2 SEM micrograph of TiO2 nanoparticle thin film MAPLE-deposited onto a rough alumina substrate

solvent and frozen at LNT, thus forming the target to be irradiated. Films of carbon nanotubes [95] were first deposited by this method. After, TiO2 and SnO2 colloidal nanoparticle films were prepared, preserving the size and crystalline phase of the starting particles [96]. Recently, TiO2 nanorod films were also deposited by the same authors [97]. A summary of the methodology and of the obtained results is given here. TiO2 colloidal nanoparticles (size 10 nm) in the anatase phase were prepared by using standard procedures [94], then diluted in deionized water with a concentration of 0.2 wt%. Afterwards, the solution was frozen at LNT and placed into a vacuum chamber on a rotating target holder, cooled with liquid nitrogen to guarantee a low and constant temperature (−160◦ C). The frozen target was irradiated in vacuum (5 × 10−4 Pa) with an ArF (τ = 20 ns, 10 Hz, F = 550 mJ/cm2 ) excimer laser. Films were deposited on silica, 100 Si and interdigital alumina (Al2 O3 ) slabs, for the different characterizations. High-resolution SEM images of the TiO2 nanoparticle films showed that a quite uniform film of nanoparticles was deposited also onto rough Al2 O3 substrates used for gas-sensing measurements, following the morphology of the alumina grains (Fig. 2). The optical energy gap resulted to be ∼3.6 eV, in agreement with the value reported in literature for TiO2 thin films [98]. Gas test measurements were carried out by recording the dynamic changes of the electrical resistance caused by the exposure to different concentrations of ethanol and acetone vapors (20–200 ppm in dry air). The relative variation of signal in electrical current was very high (up to about 1 order of magnitude) even at the lowest concentrations of both the considered vapors [96]. These very good gas-sensing properties may be attributed to the nanoscale dimensions of the TiO2 particles.


Fig. 3 SEM micrograph of the as-deposited (a) and annealed at 400◦ C (b) SnO2 samples on 100 Si substrate. The white marker is 100 nm

SnO2 colloidal nanoparticles in the cassiterite phase of 3.6 ± 0.6 nm diameter, with one monolayer of trioctylphosphine capping layer, were also prepared [94], then diluted in toluene (0.2 wt%), frozen at LNT and irradiated with a KrF laser (τ = 20 ns, 10 Hz, F = 350 mJ/cm2 ). SEM inspection showed that the as-deposited film consists of uniformly distributed elongated structures (Fig. 3). The FTIR spectrum showed different absorption bands, which are ascribed predominantly to the nanoparticle capping layer, which is necessary to avoid nanoparticle precipitation. Trioctylphosphine has a vapor pressure of 120 Pa at 20◦ C, which is much lower than that of toluene at the same temperature (2900 Pa). So, it is not effectively pumped out during the MAPLE process and consequently it reaches the substrate, contributing to the composition of the deposited film. No trace of the capping layer is present on the film after annealing at 400◦ C. The average nanoparticle dimension resulted to be of 4 ± 1 nm in accordance with the nanoparticle dimension of the starting solution [99]. Some bigger nanoparticles,

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Fig. 4 Bright field TEM image of the as-prepared TiO2 nanorods (a), HR SEM image of the film deposited on Si substrate (b) and histogram of the diameter distribution of the spherical structures (inset). The white marker is 100 nm

with dimensions of 10–20 nm, were also noted. The FTIR analysis, repeated on the annealed films, evidencedonly one peak at 667 cm−1 , due to the vibration of the antisymmetric O-Sn-O bridging bond [100]. SnO2 nanoparticle films were also deposited on interdigitated alumina substrates, for gas-sensing tests. As expected, no response toward low concentration of ethanol (200 ppm in dry air) were observed, due to the presence of the layer of trioctylphosphine, until the sensor working temperature reached ≈400◦ C [96]. Titanium dioxide nanorod thin films were also MAPLE deposited [97]. The starting materials were TiO2 nanorods in the brookite phase, having a mean size of 3–4 nm × 20–50 nm, prepared through a chemical road. They were covered with an oleate/oleyl amine capping layer. The nanorods were dissolved in pure toluene (0.016 wt% TiO2 ), then frozen at LNT and irradiated with a KrF excimer laser (τ = 20 ns, 10 Hz, F = 150, 250 and 350 mJ/cm2 ). 100 single-crystal Si wafers, silica slides, Cu carbon-

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coated grids and alumina interdigitated slabs were used as substrates to fully characterize the deposited layers. Highresolution SEM and TEM investigations evidenced the formation of quite rough films incorporating individually distinguishable TiO2 single nanorods and crystalline spheres with a mean diameter of ∼13 nm (Fig. 4). Very promising resistive sensing responses toward 1 ppm of NO2 mixed in dry air were obtained. Very recently, carbon nanopearls films were deposited onto silicon substrates using MAPLE [101, 102]. KrF laser pulses were directed onto a target consisting of ∼150 nmsized carbon nanopearls, synthesized using a chemical vapor-deposition process, diluted in a solvent and frozen. The morphology of deposited carbon nanopearl films was found to be influenced by matrix solvent, KrF laser energy, repetition rate, background pressure and substrate temperature. The optimal deposition conditions (dispersion in toluene at a concentration of 0.08 wt%, laser energy of 700 mJ/pulse focused onto a spot in a square pattern of 17.7 mm per side at the repetition rate of 1 Hz) were used for a hybrid process, where laser ablation from frozen dispersion solution targets was combined with magnetron sputtering from gold targets. A nanocomposite coating consisting of carbon nanopearls encapsulated in a gold matrix was synthesized. 7 Evolution of the MAPLE technique 7.1 MAPLE direct write The MAPLE direct write (MAPLE-DW) is a laser-based forward transfer direct writing technique that takes place in air. It was introduced by Chrisey et al. [103]. MAPLEDW is similar in experimental approach to laser induced forward transfer (LIFT) [104]. In LIFT, the laser beam ablates or vaporizes the donor coating into atoms, ions and small molecules, whereas in MAPLE-DW the laser beam softly transfers a coating of micron size composite materials, powders, nanoparticles, chemical precursors and various minor additives. Advantages of MAPLE-DW include highresolution (2 µm) patterns, high write speed (>200 mm/s), the ability to transfer a wide variety of materials (ceramics, metals, ferrites, polymers), CAD/CAM rapid prototyping, high-reproducibility, all in a dry process that is environmentally safe. Chrisey et al. used this technique for the fabrication of conformal electronic devices. They deposited Ag metal and BaTiO3 composite dielectrics with electrical quality comparable to the ones of films deposited with conventional techniques. MAPLE-DW was used by J.M. Fitz-Gerald et al. [105] to deposit phosphor powder screens for high-resolution display applications. Dense oxide phosphor powders of Y2 O3 :Eu (red) and Zn2 SiO4 :Mn (green) were deposited on alumina


and polymer substrates. Cathodoluminescent measurements showed that the luminous efficiency of the phosphor powders was not degraded by the deposition process. A 6 × 6 red and green matrix with pixel sizes of 100 µm (250 lines per inch) with a 100 µm spot size was demonstrated. The authors claim that this technique is easily scalable to pixel sizes 2500 lines per inch). As foreseen, MAPLE-DW was successfully used to transfer solutions or suspensions of several active proteins including a dopamine-sensitive enzyme (PPO) [36], biotinylated BSA, anti-BSA [106] and several antibodies specific to proteins involved in cell signaling pathways. Afterwards, efforts were focused toward the fabrication of a multiantibody microarray that could be used both for sensing of biological warfare agents and early detection of diseased states through protein screening and identification. Results indicated that the activity of MAPLE-DW deposited proteins is high, enabling antibody-protein binding and fluorescent imaging with high signal-to-noise ratios. Compared to traditional protein dispensing techniques such as pin arrayers, quill-pin arrayers, and ink jets, MAPLE-DW’s capabilities in terms of spot size, speed (≈1 m/s linear travel), and efficient use of material surpass current state-of-theart. MAPLE-DW was also used to form patterns and structures of living cells. This transfer effectively “punched out” a group of cells from a sustained culture, placing this small, dissected subculture over areas as small as 50 µm in a new environment or substrate. Both eukaryotic and prokaryotic cell patterns were formed with near 100% viability. Deposits of immobilized horseradish peroxidase, an enzyme, in the form of a polymer composite with a protective coating, i.e. polyurethane, retained their enzymatic functions. Moreover, a dopamine electrochemical sensor was fabricated by MAPLE-DW using a natural tissue/graphite composite [36, 106, 107]. MAPLE-DW was used to deposit patterns of a broad range of cell types including E. coli, Chinese hamster ovaries, human osteoblasts, mouse pluripotent cells, and mouse myoblasts. Rat cardiac cells and human osteoblasts were deposited adjacent to each other in a 2 × 2 microarray. MAPLE-DW was also used to transfer biomaterials from pathologic tissue [76]. Piquè et al. [108] successfully used both MAPLE and MAPLE-DW for depositing thin films of polymer materials for chemical sensor applications. Using MAPLE, highly uniform films of a fluoroalcoholpolysiloxane polymer (SXFA) [109] were deposited on the surface of SAW resonators. The performance of the MAPLE-coated sensors was comparable to that of spraycoated SAW devices. Using the MAPLE-DW technique, conductive pads of polyepichlorohydrin (PECH; [CH(CH2 Cl)CH2 O]n , average molecular weight 700,000 Da) were used to demonstrate a simple, conductimetric chemical-vapor sensor. All


the components for the chemiresistor sensors were fabricated by MAPLE-DW. The performance of these chemiresistive gas sensors was equivalent to that of similar sensors prepared by traditional coating techniques. But MAPLE and MAPLE-DW offer significant advantages, since they allow the deposition of solvent-free chemoselective polymers on a variety of substrates. Harris et al. [110] reported on recent progress in CAD/ CAM laser direct-writing of biomaterials. They showed that the MAPLE-DW process allows for computer-controlled deposition and machining of mesoscopic voxels of material with a high degree of spatial resolution. The above paper provides a review of current biomedical research activities that involve patterning of three-dimensional structures using the MAPLE-DW process. The authors conclude that the MAPLE-DW process may be used to fabricate free-standing cell-seeded networks with unique geometries for biomedical applications. 7.2 Resonant infrared MAPLE (RIR-MAPLE) Resonant infrared pulsed laser deposition (RIR-PLD) is a variant of conventional PLD where the laser wavelength is tuned to match vibrational modes in the target material. The intense laser irradiation is used to promote the solid phase material to highly vibrationally excited gas-phase species in the ground electronic state that can be collected on a nearby substrate as a thin film [35]. This approach has been used with polymers and organic materials in the mid-infrared wavelength range (2–10 µm). For a review see Bubb and Haglund [111]. RIR-MAPLE is based on the same mechanism. Here the laser wavelength is resonant with vibrational modes of the solute molecules. RIR-MAPLE was successfully demonstrated on an electroluminescent polymer: poly[2-methoxy-5-(2 -ethylhexyloxy)-1,4-phenylene vinylene] (MEH-PPV) with a molecular weight of 500,000 g/mole [112]. The MAPLE targets were frozen 0.3 wt% solutions of MEH-PPV in toluene, tetrahydrofuran and chloroform. A free-electron laser (FEL) was used as the light source. The FEL is tunable in the range from 2 to 10 µm; each macropulse has a width of 4 µs delivered at a repetition rate of 30 Hz. The macropulse comprises roughly 11,400 1-ps micropulses separated by 350 ps. The focal spot was approximately 1.3 mm2 , producing a macropulse fluence of ∼0.5 J/cm2 . It was shown that FTIR spectra of the films were similar to the spectrum from a solution cast film. This indicates that the polymer was deposited without significant changes in the local chemical structure. Using the same FEL system, thin films of the conducting polymer poly(3,4-ethylenedioxy-thiophene):poly(styrenesulfonate) (PEDOT:PSS) were deposited by Johnson et al. [113]. The PEDOT:PSS was frozen in various matrix solutions. The films exhibited morphologies and conductivities that were highly dependent on the solvent matrix and

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laser irradiation wavelength used. When deposited from a native solution (1.3 wt% in water), as in matrix-assisted pulsed laser evaporation (MAPLE), films were rough and electrically insulating. When the matrix included other organic “co-matrices” that were doped into the solution prior to freezing, however, the resulting films were smooth and exhibited good electrical conductivity (0.2 s/cm), but only when irradiated at certain wavelengths. These results highlight the importance of the matrix/solute and matrix/laser interactions in the ablation process. Bubb et al. [114] deposited fluoropolyol, a sorbent chemoselective oligomer, using both RIR-MAPLE and normal UV MAPLE. A comparison of films deposited with IR (2.94 µm) and UV (193 nm) laser radiation showed that photochemical and/or photothermal modification of the oligomer occurred for the UV-deposited films, while the IR-deposited ones appeared to be identical to the starting material. They conclude that, unless photochemical interactions are a desired outcome, the use of a UV laser for most matrix-assisted laser ablation and deposition techniques is unfavorable. Pate et al. [115] deposited CdSe colloidal quantum dot/poly[2-methoxy-5-(2 -ethylhexyloxy)-1,4-(1-cyano vinylene)phenylene (MEH-CN-PPV)] hybrid nanocomposite thin films. The interest about conjugated polymers is due to their potential application to light emitting diodes, solar cells and photodetectors. Pure polymer devices have been unable to demonstrate sufficient efficiency and stability to justify their use over conventional solid-state material systems. One route to improving efficiency, particularly in solar cell applications, has been to incorporate inorganic nanomaterials, such as colloidal quantum dots (CQDs) [116], into the conjugated polymer bulk film. The small molecule distribution inside the polymer bulk must form an interpenetrating network from electrode to electrode, and must also be separated by no more than one exciton diffusion length (5–20 nm) to effectively dissociate excitons and transport free carriers. Moreover, by controlling the material composition and size of CQDs, they can be sensitized to wavelengths in the solar spectrum that are not well absorbed by their conjugated polymer hosts. CQD/conjugated polymer nanocomposite films synthesized by spin-casting and dropcasting have not yielded the expected improvements due, in part, to the inability to control CQD distribution [117]. The RIR-MAPLE technique can be tuned to optimize the performance of different optoelectronic devices based on these materials. CdSe/MEH-CN-PPV hybrid nanocomposite thin films were deposited using a range of different solvents and CdSe:MEH-CN-PPV concentrations. Each target composition comprised an emulsion of MEH-CN-PPV and/or octadecylamine ligand-encapsulated CdSe CQDs. Emulsions of CdSe CQDs were prepared using a ratio of 1:10 benzyl alcohol:deionized water or 1:10 toluene:deionized water.

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The CdSe:MEH-CN-PPV wt% concentration ratios investigated in this study included 1:10, 2:10, 3:10, 4:10, 5:10, 6:10, 7:10, 8:10, 9:10, and 10:10. The CdSe CQD emulsions were deposited by RIR-MAPLE using two methods: (i) simultaneous RIR-MAPLE, wherein the two emulsions (benzyl alcohol:deionized water) were mixed together and co-deposited, and ii) sequential RIR-MAPLE, wherein a sectioned target was fabricated such that 50% of the target comprises the MEH-CN-PPV emulsion and 50% comprises the CdSe CQD emulsion (both materials deposited as target rotated). This study elucidated the extent to which hybrid nanocomposite films deposited by simultaneous RIRMAPLE preserve material domain segregation in the bulk of the film. Finally, in addition to the deposited nanocomposites, CdSe CQD thin films (without polymer) were synthesized by RIR-MAPLE and drop-casting as points of comparison to the CdSe/MEH-CN-PPV hybrid nanocomposites. All RIR-MAPLE depositions were conducted using a pulsed Er:YAG laser (λ = 2.9 µm, F = 2 J/cm2 , τ = 90 µs, 2 Hz). TEM and PL spectroscopy demonstrated that: (i) depending upon the deposition parameters used, the CdSe colloidal quantum dot distribution can be tuned between two morphology extremes, i.e. clustering or homogeneous dispersion; and (ii) the constituent materials of the nanocomposite are not damaged by the deposition process. The ability to control nanoparticle distribution within organic films was not achieved by any other deposition techniques. It has to be noticed that RIR-MAPLE depositions can be carried out at higher laser fluences (F = 3–5 J/cm2 ) and matrix concentrations (up to 20 wt%) than using UV-MAPLE procedures. This results in higher deposition rates. Very recently Torres et al. [118] published a review that critically examines the thermally induced ablation mechanisms resulting from irradiation of cryogenic solvent matrices by a tunable free-electron laser (FEL). A semi-empirical model is used to calculate temperatures as a function of time in the focal volume and determine heating rates for different resonant modes in two model solvents, based on the thermodynamics and kinetics of the phase transitions induced in the solvent matrices. Three principal ablation mechanisms are discussed, namely normal vaporization at the surface, normal boiling, and phase explosion. During high-power pulsed-FEL irradiation, phase explosion is shown to be the most significant contribution to polymer deposition in RIRMAPLE. Phase explosion occurs when the target is rapidly heated (108 to 1010 K/s) and the solvent matrix approaches its critical temperature. Spontaneous density stratification (spinodal decay) within the condensed metastable phase leads to rapid homogeneous nucleation of vapor bubbles. As these vapor bubbles interconnect, large pressures build up within the condensed phase, leading to target explosions and recoil-induced ejections of polymer to a near substrate. Phase explosion is a temperature (fluence) threshold-limited


process, while surface evaporation can occur even at very low fluences. The main obstacle to the large use of RIR-MAPLE is the lack of appropriate tunable IR laser sources, since FELs are not yet easy accessible facilities. 8 Discussion As summarized above, in the last ten years many papers demonstrated that with an appropriate choice of the laser wavelength, fluence, pulse duration and of the type of solvent, polymers, organic molecules and biomolecules can be MAPLE-deposited as thin films without significant modification of their chemical structure and functionality. However, despite the good results reported and the successful expansion of the technique to new applications, as for instance nanoparticle film deposition, the simple picture of the MAPLE process (the laser interaction vaporizes part of the solvent and the guest molecules receive enough kinetic energy by mechanical collisions to pass in the vapor phase and deposit uniformly on the substrate) is being questioned. In fact, except in very few cases (see, e.g. [44]), MAPLEdeposited films present high surface roughness with welldefined aggregates or clusters with typical dimensions from tens of nanometers to tens of microns (see, e.g. [54, 55, 82, 119]). The frequent formation of large polymer clusters is unexpected, especially when the original polymer concentration is low and the polymers are dissolved in the matrix down to the molecular level. Leveugle and Zhigilei [73] observed that the initial picture of the ejection and transport of individual polymer molecules in MAPLE introduced by Chrisey et al. [35] cannot explain the results of SEM and AFM imaging of most of MAPLE-deposited films, where significant surface roughness, with well-defined aggregates are observed. They formulated a computational model to get a better understanding of the relation between the basic mechanisms of laser interaction with the target material, nonequilibrium processes caused by the fast deposition of laser energy, parameters of the ejected plasma plume, and the resulting morphological characteristics of the growing film. They observed that so far simulations were focused on the analysis of matrix-assisted ejection of individual solute molecules, but at a concentration too low to allow any interaction among the molecules during the ejection process [120–122]. Since the concentrations of polymer molecules in MAPLE experiments are relatively high (typically 0.1–5 wt%), the collective behavior of multiple polymer molecules may play an important role in defining the mechanisms of molecular ejection and the morphological characteristics of the deposited films. To take into account this collective behavior, the laser-induced molecular ejection from a target was described by a coarse-grained molecular dynamics (MD) model. Simulations were performed for


MAPLE targets with concentrations of polymer molecules of 1, 3 and 6 wt%, as well as for pure matrix. Irradiation at a wavelength of 337 nm (3.68 eV), with pulse duration of 50 ps, was assumed. The laser fluences (3–9 mJ/cm2 ) were chosen to cover the range from below the ablation threshold (3.5 mJ/cm2 for pure matrix) up to more than twice the ablation threshold. In this irradiation regime the thermal energy is largely confined within the absorbing region. The conditions of thermal confinement are also characteristic for the majority of MAPLE experiments performed with nanosecond laser pulses. The fact that in the simulations and experiments the MAPLE process takes place under the same physical regime of thermal confinement suggests that the ejection mechanisms are similar. In the ablation of MAPLE target, the presence of polymer molecules does not radically alter the general picture of the ablation process: explosive disintegration and expansion of the overheated matrix driving the ejection process and entraining the polymer molecules along. While in the simulation for pure matrix the liquid emerging from the “phase explosion” quickly transforms into spherical droplets, the presence of polymer chains determines the formation of complex matrix-polymer liquid structures elongated in the direction of the ablation plume expansion. As the polymer concentration increases, the chains become more entangled, with the formation of intricate elongated structures, which can reach the substrate, resulting in the formation of complex surface morphology. In all the simulations, the ejected plume consists of a mixture of individual matrix molecules, small matrix clusters, and larger clusters/droplets composed of both matrix molecules and polymer chains. Evaporation of the volatile matrix in-flight and after the deposition to the substrate could also be responsible for the formation of the surface polymer features observed in the SEM images of MAPLE-deposited films. It is important that no photothermal bond scission events were detected in any of the simulations performed for polymer concentrations up to 6 wt% and laser fluences up to more than twice the ablation threshold. The ejection of molecular clusters and droplets seems to be inherently connected to the basic mechanism of laser ablation—explosive decomposition of a surface region of the target overheated up to the limit of its thermodynamic stability. In conclusion, these studies indicated that the MAPLE process is more complex than a simple evaporation process and outlighted intrinsic limitations for the deposition of uniform films. From the simulations, it clearly emerged that solute concentration strongly affects the surface morphology of the MAPLE-deposited polymer films. Another important parameter is the effect of the multipulse laser irradiation of the target. Leveugle and Zhigilei observed that significant structural, morphological, and compositional changes may accumulate in the surface region of a target irradiated by multiple

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laser pulses, increasing polymer concentration. The compositional and morphological changes in the surface region can change its optical and thermodynamic properties, and even heat transfer mechanisms in the heat-affected region of the target. Pate et al. [123] demonstrated experimentally the importance for the film morphology of the laser-target interaction volume, which is determined by the absorption length of the laser energy in the target. For large absorption depth, the laser energy is spread across that depth so that only a part of the irradiated volume is vaporized. The result is a significant solvent contamination of the grown polymer film. In contrast, for small absorption depth the laser energy density is high enough to vaporize completely the solvent, which is then evacuated from the vacuum chamber. Similar conclusions were also suggested by Mercado et al. [72]. They suggested that the geometric patterns present in their MAPLEdeposited films were solvent evaporation patterns, like the ones normally associated with the spin-casting process. This may be due to the ejection of large slices of the target surface layer, explosive evaporation, or spallation of layers onto the substrate, where they melt, leading to evaporation of the solvent. A high ablation rate supports this scenario, suggesting that large volumes of the target are being ejected, rather than entrainment transport of individual molecules. The final result is a non-uniform coverage of the substrate. The choice of the solvent plays an important role, too. Bubb et al. [124] reported that morphological properties of the films are related to the “goodness” of the solvent, as defined by the Hansen solubility parameters [125]. To accurately test relationships between the solvent properties (solubility and other physical properties) and the morphology of the MAPLE-deposited films, Caricato et al. [126] deposited poly(9,9-dioctylfluorene) (PFO) using different solvents. The PFO powder was diluted (0.5 wt% concentration) in tetrahydrofuran (THF–C4 H8 O), toluene (C7 H8 ) and toluene-hexane (C6 H14 ) mixtures. Two toluenehexane mixtures were chosen (85% toluene/15% hexane and 61% toluene/39% hexane by volume) to gradually change solubility and physical properties of the matrices. The physical properties, together with the solubility parameters of the pure solvents and mixtures relevant for this study are reported in Table 1. The solubility parameters have been determined using the Hoftyzer and Van Krevelen’s methods [127]. 10,000 KrF laser pulses (τ = 20 ns, 10 Hz, F = 250 mJ/cm2 ) were used to deposit PFO films on 100 Si substrates. From the AFM scans, quite uniform substrate coverage was observed. However, some circular aggregates are present on the films’ surface, whose dimensions decrease passing from the films deposited using pure toluene matrix to the films deposited using toluene-based mixtures with increasing amount of the hexane component. The film rms

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Table 1 Boiling temperature and solubility parameters of the considered matrices Solvent 1

Solent 2

Boiling temperature (◦ C)

Hansen solubility parameters (MPa)1/2 δd

δp

δh

δ

THF (C4 H8 O)

66

14.4

4.9

6.1

16.4

Toluene (C7 H8 )

110,6

17.3

1.03

0

17.4 14.6

Hexane (C6 H14 )

69

14.6

0

0

85% toluene/

15% hexane

103,8

16.9

0.88

0

16.92

61% toluene/

39% hexane

94

16.37

0.63

0

16.39

Fig. 5 Surface roughness of the deposited PFO films plotted as a function of the absolute value of the difference between the total solubility parameters

roughness values present an opposite behavior, passing from (22 ± 4) nm in the case of pure toluene matrix to (23 ± 2) nm for 85% toluene/15% hexane matrix, to (25 ± 2) nm for the 61% toluene/39% hexane matrix. The PFO film deposited using pure THF as matrix is characterized by the presence of polymer filaments, randomly distributed on the surface. Consequently, the film surface presents a quite high rms roughness (41±5) nm. As done by Bubb et al. [124], the rms roughness values were plotted against the solubility parameters (Fig. 5). The solubility parameters, which determine the effectiveness of the solvent in inducing dispersion forces (δd ), its polarity (δp ) and its tendency to hydrogen bond formation in solution(δh ) [125]. The total solubility parameter

is defined as δ = δd2 + δp2 + δh2 . The films prepared using the toluene-based matrices present a very small increase, if any, of the rms roughness values, even if the absolute value of the difference between the total solubility parameters (|δsol –δPFO |) increases, but the rms roughness of the PFO film deposited using the THF matrix is much higher (40 nm) than that of the sample prepared using the 61% toluene/39% hexane mixture

(25 nm), although the |δsol –δPFO | for the two solvents are equal (∼0.8 MPa1/2 ). It can be concluded that the solubility is not the only parameter to take into account. But what determines the difference? Most probably the different optical penetration depth at the laser energy and the different boiling temperature of the solvents [126] do. From molecular dynamic simulations, it results that the higher penetration depth is responsible for the ejection form the target surface of big clusters of polymer matrix. At the same time the matrix-polymer clusters broke and reduce their dimension with time because of solvent evaporation [73]. This process should be faster for solvents with lower boiling points, i.e. for THF with respect to toluene (66◦ C versus 110.6◦ C, respectively). This feature should accelerate polymer entanglement and induce a higher surface roughness. This assumption is confirmed by the plot of Fig. 5, where the rms roughness decreases as the boiling temperatures of the matrices increase (see Table 1). In any case, the solvent volatility cannot be too low. It was already mentioned that when trioctylphosphine was used [96] to avoid nanoparticle precipitation, it greatly influenced the MAPLE-deposited film surface morphology. Trioctylphosphine has a vapor pressure as low as 120 Pa at 20◦ C and it is not effectively pumped out during the time-of-flight. It reaches the substrate, affecting the composition of the film. A further parameter affecting surface morphology is the substrate temperature. A proper choice of the substrate temperature can reduce the surface features [71, 77]. To test temperature effects on the PFO film roughness, Caricato et al. [128] deposited PFO films from pure toluene matrix on 100 Si substrates at temperatures from −16 to 70◦ C. It can be seen in Figs. 6 and 7 that the density of polymer clusters decreases significantly with increasing substrate temperature. It can be also observed that, at the same substrate temperature (50◦ C), the rms roughness of the film prepared with 10,000 laser pulses is about half the value of the film prepared with 16,000 laser pulses (Fig. 6). This feature confirms that significant structural, morphological, and compositional changes may gradually accumulate in the surface region of a target irradiated by multiple laser pulses [73], leading to an increased roughness with increasing film thickness.


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Fig. 7 Surface roughness of the deposited PFO films plotted as a function of the substrate temperature (• films prepared with 10000 laser pulses; × film prepared with 10000 laser pulses)

Fig. 6 SEM micrographs of PFO MAPLE-deposited films on substrates at (a) T = 30◦ C and (b) T = 70◦ C, in otherwise identical experimental conditions

Possibly, the general picture of polymer film deposition can be extended to biomolecular and other complex material deposition by MAPLE technique, but not to nanoparticle and nanorod films. However, aggregates were observed in MAPLE-deposited TiO2 nanoparticle films [96]. To study this phenomenon, Luches et al. [128] deposited brookite (orthorhombic) TiO2 nanorod films. The nanorods were synthesized by a colloidal nonhydrolytic sol–gel route [129]. A nanorod solution in toluene (0,016 wt% TiO2 ) was frozen at LNT and irradiated with a KrF laser (τ = 20 ns, 10 Hz, F = 350 mJ/cm2 ). Single-crystal Si wafers, silica slides, carbon-coated Cu grids and alumina interdigitated slabs were used as substrates to allow performing different characterizations. Nanorod films, fabricated with 6,000 laser pulses, had an average thickness of ∼150 nm. A representative bright field (BF) TEM image of the used brookite TiO2 nanorods is shown in Fig. 4a. The diameter and length dimensions of nanorods are in the 3–4 nm and 35–

50 nm intervals, respectively. A HR-SEM image of a typical TiO2 MAPLE-deposited film onto a Si substrate is shown in Fig. 4b. The image evidences the formation of quite rough films incorporating individually distinguishable TiO2 nanorods and crystalline spherical nanoparticles with an average diameter of 13 ± 4 nm, with a broad size distribution (see inset in Fig. 4b). The unexpected observation of TiO2 spheres in films prepared starting from nanorods suggests occurrence of a laser-induced process, which induces a melting/coalescence of the nanorods, driving their transformation into the most thermodynamically stable spherical shapes. Under laser pulses, the energy released into the frozen matrix induces solution vaporization. However, the laser light heats both the nanorods contained in the irradiated target and the nanorods leaving the target in the vapor phase. Due to the low laser fluence (350 mJ/cm2 ), the nanorod temperature should be lower than the melting temperature of bulk TiO2 (∼1850◦ C). But a decrease of the actual melting temperature has been reported for low-dimensional solids, compared to their corresponding bulk materials [130–133]. Link et al. [134] reported a change in the size and shape of Au nanorods that were subjected to ns and fs laser pulses at F = 0.01–1 J/cm2 range. So, the formation of the spherical particles can be explained on the basis of combined effects involving the absorption of laser energy by the nanorods, their low thermal-loss rate and their decreased melting temperature due to the nanometric dimension effects.

9 Conclusions The MAPLE technique allows deposition of thin films of organic and biological materials with minimum modification of the chemical structure and functionality of the deposited molecules, with thickness control and surface coverage that cannot be achieved by solvent-based coating methods. But the fabrication of very smooth films appears difficult to be obtained. High-resolution imaging and computer

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simulations seem to contest the original simple model of molecule-by-molecule deposition. Aggregation of polymers and presence of residual matrix molecules result in local corrugations of the deposits. The roughness of the growing films can be, at least partially, controlled by limiting the solute concentration. Also the solvent properties can influence the surface morphology of polymer films. Together with solubility parameters, the boiling temperature of the solvent influences the polymer surface morphology, which depends also on the laser light fluence and absorption depth. These results confirm the validity of the molecular dynamic simulations, stating that polymers are always ejected as parts of matrix-polymer clusters, whose dimensions depend on the laser penetration depth. Moreover, also the different presence of entangled polymer filaments on the film surfaces can be ascribed to differences in the boiling temperatures of the solvents. This behavior was experimentally demonstrated for PFO dissolved in THF (boiling temperature 66.0◦ C), toluene 61%/hexane 39% (94.0◦ C), toluene 85%/hexane15% (103.8◦ C) and pure toluene (110.6◦ C) matrices. A higher rms roughness value, 40 nm, was measured in the THF-based film, against ∼23 nm for the toluenebased films. In fact, THF is characterized by a lower value of the boiling temperature with respect to toluene-based matrices. It was also shown that the moderate heating of the substrate reduces the polymer film roughness, which in contrast increases with increasing the laser pulse number. As regards the MAPLE-deposited nanoparticles and nanorods, formation of relatively large spherical particles can be explained on the basis of combined effects involving the absorption of laser energy by the nanorods, their low thermal-loss rates and their decreased melting temperature due to nanometric dimensions. In conclusion, MAPLE opened very perspective roads for the deposition of performing films of complex organic and biological molecules and the area of MAPLE applications is rapidly expanding.

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