Laser induced forward transfer of SnO2 for sensing applications using different precursors systems

This paper presents the transfer of SnO₂ by laser induced forward transfer (LIFT) for gas sensor applications. Different donor substrates of SnO₂ with and without triazene polymer (TP) as a dynamic release layer were prepared. Transferring these films under different conditions were evaluated by optical microscopy and functionality. Transfers of sputtered SnO₂ films do not lead to satisfactory results and transfers of SnO₂ nanoparticles are difficult. Transfers of SnO₂ nanoparticles can only be achieved when applying a second laser pulse to the already transferred material, which improves the adhesion resulting in a complete pixel. A new approach of decomposing the transfer material during LIFT transfer was developed. Donor films based on UV absorbing metal complex precursors namely, SnCl₂(acac)₂ were prepared and transferred using the LIFT technique. Transfer conditions were optimized for the different systems, which were deposited onto sensor-like microstructures. The conductivity of the transferred material at temperatures of about 400 ^°C are in a range usable for SnO₂ gas sensors. First sensing tests were carried out and the transferred material proved to change conductivity when exposed to ethanol, acetone, and methane.     

Laser printing of polythiophene for organic electronics

We report on the development of hybrid organic/inorganic thin-film transistors using regioregular poly-3-hexylthiophene (P3HT) semiconductor material deposited by means of the solid-phase Laser Induced Forward Transfer (LIFT) technique. P3HT pixels were LIFT-printed onto Au/Ti source and drain electrodes formed on silicon dioxide/p⁺-type Si substrate. Deposition of the P3HT pixels was investigated as a function of the laser fluence using donor substrates with and without a dynamic release layer. Device electrical characterization reveals efficient field-effect action of the bottom gate on the organic channel. The transfer I_DS-V_GS characteristics exhibit well-defined sub-threshold, linear and saturation regimes designating LIFT as a promising technique for hybrid organic/inorganic transistor technology.     

Polymer-coated compliant receivers for intact laser-induced forward transfer of thin films: experimental results and modelling

In this study, we investigate both experimentally and numerically laser-induced forward transfer (LIFT) of thin films to determine the role of a thin polymer layer coating the receiver with the aim of modifying the rate of deceleration and reduction of material stress preventing intact material transfer. A numerical model of the impact phase during LIFT shows that such a layer reduces the modelled stress. The evolution of stress within the transferred deposit and the substrate as a function of the thickness of the polymer layer, the transfer velocity and the elastic properties of the polymer are evaluated. The functionality of the polymer layer is verified experimentally by LIFT printing intact 1- \(\upmu \) m-thick bismuth telluride films and polymeric light-emitting diode pads onto a layer of 12- \(\upmu \) m-thick polydimethylsiloxane and 50-nm-thick poly(3,4-ethylenedioxythiophene) blended with poly(styrenesulfonate) (PEDOT:PSS), respectively. Furthermore, it is demonstrated experimentally that the introduction of such a compliant layer improves adhesion between the deposit and its substrate.     

A sensor for adenosine triphosphate fabricated by laser-induced forward transfer of luciferase onto a poly(dimethylsiloxane) microchip

Laser-induced forward transfer (LIFT) of the enzyme luciferase was explored as a potential technique to be used in the fabrication of a microchip adenosine triphosphate (ATP) sensor. Poly(dimethylsiloxane) (PDMS) was selected as the substrate for deposition of the luciferase. In comparison with other solid substrates, such as glass and polystyrene, it was found that the flexibility of PDMS made it a superior substrate for the immobilization of micro-spots of luciferase. LIFT of luciferase onto a PDMS substrate using a 355 nm laser was successfully carried out, while the bioactivity of the enzyme was maintained. Yellow luminescence ascribed to luciferase was observed from a transferred spot on the PDMS chip from the enzymatic reaction between luciferin and ATP. A microchip ATP sensor was also fabricated by attaching a small photodiode to the PDMS chip. On the basis of the fabricated microchip, the Michaelis-Menten relation between the luminescence intensity from the spot, and the ATP concentration was confirmed. The potential for fabricating biosensors using a combination of the LIFT technique with a PDMS substrate was shown to be very good.     

Study of the laser-induced forward transfer of liquids for laser bioprinting

Laser-induced forward transfer (LIFT) is a direct-writing technique that allows printing patterns of diverse materials with a high degree of spatial resolution. In conventional LIFT a small fraction of a solid thin film is vaporized by means of a laser pulse focused on the film through its transparent holder, and the resulting material recondenses on the receptor substrate. It has been recently shown that LIFT can also be used to transfer materials from liquid films. This widened its field of application to biosensors manufacturing, where small amounts of biomolecules-containing solutions have to be deposited with high precision on the sensing elements. However, there is still little knowledge on the physical processes and parameters determining the characteristics of the transfers.In this work, different parameters and their effects upon the transferred material were studied. It was found that the deposited material corresponds to liquid droplets which volume depends linearly on the laser pulse energy, and that a minimum threshold energy has to be overcome for transfer to occur. The liquid film thickness was varied and droplets as small as 10 μm in diameter were obtained. Finally, the effects of the variation of the film to substrate distance were also studied and it was found that there exists a wide range of distances where the morphology of the transferred droplets is independent of this parameter, what provides LIFT with a high degree of flexibility.     

Improved laser-induced forward transfer of organic semiconductor thin films by reducing the environmental pressure and controlling the substrate–substrate gap width

Laser-induced forward transfer (LIFT) has been investigated for bilayer transfer material systems: silver/organic film (Alq₃ or PFO). The LIFT process uses an intermediate dynamic release layer of a triazene polymer. This study focuses on the effect of introducing a controlled donor–receiver substrate gap distance and the effect of doing the transfer at reduced air pressures, whilst varying the fluence up to ∼200 mJ/cm². The gap between ‘in-contact’ substrates has been measured to be a minimum of 2–3 μm. A linear variation in the gap width from ‘in contact’ to 40 μm has been achieved by adding a spacer at one side of the substrate–substrate sandwich. At atmospheric pressure, very little transfer is achieved for Alq₃, although PFO shows some signs of successful doughnut transfer (with a large hole in the middle) in a narrow fluence range, at gaps greater than 20 μm. For the transfer of Ag/PFO bilayers at atmospheric pressure, the addition of a PFO layer onto the receiver substrate improved the transfer enormously at smaller gaps and higher fluences. However, the best transfer results were obtained at reduced pressures where a 100% transfer success rate is obtained within a certain fluence window. The quality of the pixel morphology at less than 100 mbar is much higher than at atmospheric pressure, particularly when the gap width is less than 20 μm. These results show the promise of LIFT for industrial deposition processes where a gap between the substrates will improve the throughput.     

Excimer laser forward transfer of mammalian cells using a novel triazene absorbing layer

We present a novel laser-based approach for developing tissue engineered constructs and other cell-based assembly's. We have deposited mesoscopic patterns of viable B35 neuroblasts using a soft direct approach of the matrix assisted pulsed laser evaporation direct write (MAPLE DW) process. As a development of the conventional direct write process, an intermediate layer of absorbing triazene polymer is used to provide gentler and efficient transfers. Transferred cells were examined for viability and proliferation and compared with that of as-seeded cells to determine the efficacy of the process. Results suggest that successful transfers can be achieved at lower fluences than usual by the incorporation of the intermediate absorbing layer thus avoiding any damage to cells and other delicate materials. MAPLE DW offers rapid computer-controlled deposition of mesoscopic voxels at high spatial resolutions, with extreme versatility in depositing combinations of natural/synthetic, living/non-living, organic/inorganic and hard/soft materials. Our approach offers a gentle and efficient transfer of viable cells which when combined with a variety of matrix materials allows development of constructs and bioactive systems in bioengineering.     

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.     

A study on the pulsed laser printing of liquid-phase exfoliated graphene for organic electronics

The aim of this work is the pulsed laser printing of liquid-phase exfoliated graphene in the nanosecond regime and the optimization of the printing process on Si/SiO₂ and flexible polymer substrates (polyethylene naphthalate) via the laser-induced forward transfer technique (LIFT). The laser printing conditions and the optimum energy fluence window for reproducible deposition have been investigated, while the deposited graphene features have been studied morphologically and structurally by means of optical microscopy, micro-Raman spectroscopy and electrical characterization. LIFT experiments were carried out using the fourth harmonic (266 nm) of a pulsed ns Nd:YAG laser combined with a high-power imaging micromachining system to monitor the printing process throughout the experiments. The irradiation of our graphene solution resulted in the deposition of well-resolved patterns on different surfaces, highlighting LIFT as an alternative technique for the printing and patterning of liquid-phase exfoliated graphene for organic electronics applications.     

Laser-induced forward transfer of intact chalcogenide thin films: resultant morphology and thermoelectric properties

We present a laser-based transfer method for the novel application of fabricating elements for planar thermoelectric devices. Thin films of thermoelectric chalcogenides (Bi₂Te₃, Bi₂Se₃ and Bi_0.5Sb_1.5Te₃) were printed via laser-induced forward transfer (LIFT) onto polymer-coated substrates over large areas of up to ～15 mm² in size. A morphological study showed that it was possible to partially preserve the polycrystalline structure of the transferred films. The films’ Seebeck coefficients after LIFT transfer were measured and resulted in ?49±1 μV/K, ?93±8 μV/K and 142±3 μV/K for Bi₂Te₃, Bi₂Se₃ and Bi_0.5Sb_1.5Te₃, respectively, which were found to be ～23±6 % lower compared to their initial values. This demonstration shows that LIFT is suitable to transfer sensitive, functional semiconductor materials over areas up to ～15 mm² with minimal damage onto a non-standard polymer-coated substrate.     

Ablation of transparent solids during self-limited deposition of diamond-like films from liquid hydrocarbons

A novel effect is studied of self-limitation of the diamond-like film thickness during laser irradiation of the interface of transparent substrates with liquid aromatic hydrocarbons. The interface is exposed through the transparent substrate to radiation of a copper vapor laser (wavelength of 510.6 nm, pulse duration of 20 ns). The thickness of diamond-like film increases linearly to 80-100 nm with the number of laser pulses and then saturates, while the substrate is ablated with nearly constant rate. This ablation rate depends on the thermal expansion coefficient of the substrate (glass, fused silica, sapphire, or CaF₂). The absorption of extinction coefficient of deposited films measured by ellipsometry is of order of 10⁴ cm^-1 and is sufficient to cause the significant heating of the interface. The ablation of the transparent substrates is due to their unequal thermal expansion compared to the diamond-like film having different thermal expansion coefficient. The measured ablation rates scale from 0.2 Å/pulse for glass to 4.5 Å/pulse for CaF₂. A 7m spatial resolution of the ablation process has been demonstrated for fused silica.     

2D spatially controlled polymer micro patterning for cellular behavior studies

A simple and effective method to functionalize glass surfaces that enable polymer micropatterning and subsequent spatially controlled adhesion of cells is reported in this paper. The method involves the application of laser induced forward transfer (LIFT) to achieve polymer patterning in a single step onto cell repellent substrates (i.e. polyethyleneglycol (PEG)). This approach was used to produce micron-size polyethyleneimine (PEI)-patterns alternating with cell-repellent areas. The focus of this work is the ability of SH-SY5Y human neuroblastoma cells to orient, migrate, and produce organized cellular arrangements on laser generated PEI patterns.     

Thin films of Cu(II)-o,o′-dihydroxy azobenzene nanoparticle-embedded polyacrylic acid (PAA) for nonlinear optical applications developed by matrix assisted pulsed laser evaporation (MAPLE)

Thin films based on two different metal-organic systems are developed by MAPLE and their nonlinear optical applications are explored. A complex of o,o′-dihydroxy azobenzene with Cu²⁺ cation is found to organize as a non-central symmetric crystallite. A simple protocol is developed for the in situ fabrication of highly monodisperse copper-complex nanoparticles in a polymer film matrix of polyacrylic acid. The thin films were deposited on quartz substrates by MAPLE (matrix assisted pulsed laser evaporation) using a Nd:YAG laser working at 355 nm. Atomic force microscopy (AFM), Fourier transform infrared (FTIR), X-ray photoelectron spectroscopy (XPS), and optical second harmonic generation (SHG) were performed on the samples. The optical limiting capability of the nanoparticle-embedded polymer film is investigated.     

Solvent-related effects in MAPLE mechanism

To study the role of the solvent and of the laser fluence in the matrix-assisted pulsed laser evaporation (MAPLE) process, we used a soft polymer (polydimethylsiloxane—PDMS) as “sensing surface” and toluene as solvent. Thin films of the PDMS polymer were placed in the position of the growing film, while a frozen toluene target was irradiated with an ArF laser at the conventional fluences used in MAPLE depositions (60–250 mJ/cm²). Apart the absence of solute, the MAPLE typical experimental conditions for the deposition of thin organic layers were tested. The effects on the PDMS films of the toluene target ablation, at different fluences, were studied using atomic force microscopy and contact angles measurements. The results were compared with the effects produced on similar PDMS films by four different treatments (exposure to a drop of the solvent, to saturated toluene vapors and to plasma sources of two different powers). From this comparative study, it appears that depending on the MAPLE experimental conditions: (1) the MAPLE process may be “semidry” rather than purely dry (namely the solvent is likely to be present in the deposition environment near the growing film), (2) the solvent, if sufficiently volatile, is in form of vapor molecules (neutral, ionized and probably dissociated) rather than in liquid phase near the substrate and (3) at relatively high laser fluences (>150 mJ/cm²), the formation of an intense plasma plume results which can damage/affect a soft substrate as well as a growing polymer film.     

Polymer pixel enhancement by laser-induced forward transfer for sensor applications

This paper presents polymer pixel printing for applications in chemoselective sensors where nanosecond laser direct transfer methods, with a triazene polymer (TP) acting as a Dynamic Release Layer (DRL), are used. A systematic study of laser fluence, donor film morphology and both single- and multiple-pixel deposition were optimized with the final goal to obtain continuous pixels of sensitive polymers, polyethylenimine (PEI) and polyisobutylene (PIB), on SAW surfaces. Morphology characterization after the laser transfer has been performed by Optical Microscopy and Scanning Electron Microscopy (SEM). The responses of the coated transducers were measured after deposition with different laser fluences and it was found that a fluence under 625 mJ/cm² was required in order to prevent damage of the interdigital transducers (IDT) of the sensor devices. The sensitivity of the polymer coated devices to acetone concentrations gives an indication that LIFT can be used for printing sensitive polymer pixels onto transducer devices.     

Electron‐transfer transparency of graphene: Fast reduction of metal ions on graphene‐covered donor surfaces

The mechanism of charge transfer through nanomaterials such as graphene remains unclear, and the amount of charge that can be transferred from/to graphene without damaging its structural integrity is unknown. In this communication, we show that metallic nanoparticles can be decorated onto graphene surfaces as a result of charge transfer from the supporting substrate to an adjoining solution containing metal ions. Au or Pt nanoparticles were formed with relatively high yield on graphene‐coated substrates that can reduce these metal ions, such as Ge, Si, GaAs, Al, and Cu. However, metal ions were not reduced on graphene surfaces coated onto non‐reducing substrates such as SiO₂ or ZnO. These results confirm that graphene can be doped by exploiting charge transfer from the underlying substrate; thus graphene is not only transparent with respect to visible light, but also with respect to the charge transfer. (© 2015 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)     

Matrix assisted growth of nanoparticles and nanoporous films

Since the inception of matrix assisted pulsed laser evaporation (MAPLE), a large body of research has focused on the structure and property preservation of soft materials. Departing from this precedent, a variation of MAPLE to grow complex inorganic nanoparticles and nanoporous thin films from acetate precursors is presented. While some aspects of MAPLE are retained, a weakly absorbing matrix solvent is used to promote absorption by the precursors, leading to photothermal decomposition. The diffusion of ions within the laser interaction volume results in the formation of nanoparticles, which are then ejected by subsequent pulses. The acetate precursors were processed into colloidal suspensions in deionized water and frozen to form solid targets, followed by irradiation with a pulsed excimer laser at fluences ranging from 0.25 to 0.75 J/cm². Nanoparticles and nanoporous films of unary, binary, and ternary metallic and oxide systems were deposited at room temperature onto substrates of Si and electron-transparent grids. Size distributions varied between different material systems with negligible pressure and energy effects, with distribution extrema ranging from 2 to 100 nm in diameter. Characterization of the nanoparticles was performed by high resolution scanning and transmission electron microscopy, and energy dispersive x-ray spectroscopy.     

Improvement in semiconductor laser printing using a sacrificial protecting layer for organic thin-film transistors fabrication

Laser-induced forward transfer (LIFT) has been used to deposit pixels of an organic semiconductor, distyryl-quaterthiophenes (DS4T). The dynamics of the process have been investigated by shadowgraphic imaging for the nanosecond (ns) and picosecond (ps) regime on a time-scale from the laser iradiation to 1.5 μs. The morphology of the deposit has been studied for different conditions. Intermediate sacrificial layer of gold or triazene polymer has been used to trap the incident radiation. Its role is to protect the layer to be transferred from direct irradiation and to provide a mechanical impulse strong enough to eject the material.     

Matrix assisted pulsed laser evaporation of cinnamate-pullulan and tosylate-pullulan polysaccharide derivative thin films for pharmaceutical applications

We have demonstrated the successful thin film growth of two pullulan derivatives (cinnamate-pullulan and tosylate-pullulan) using matrix assisted pulsed laser evaporation (MAPLE). Our MAPLE system consisted of a KrF* laser, a vacuum chamber, and a rotating target holder cooled with liquid nitrogen. Fused silica and silicon (1 1 1) wafers were used as substrates. The MAPLE-deposited thin films were characterized by transmission spectrometry, profilometry, atomic force microscopy (AFM), Fourier transform infrared (FTIR) spectroscopy and Raman spectroscopy. The deposited layers ranged from 250 nm to 16.5 μm in thickness, depending on the laser fluence (0.065-0.5 J cm⁻²) and number of pulses applied for the deposition of one structure (1500-13,300). Our results confirmed that MAPLE was well-suited for the transfer of cinnamate-pullulan and tosylate-pullulan.     

Printing technologies for fabrication of bioactive and regular microarrays of streptavidin

In this study, we report and compare two methods for fabricating patterns of streptavidin protein using soft litography microprinting technique (μCP) and laser-based method termed ‘matrix assisted pulsed laser evaporation direct write’ (MAPLE DW). The μCP approach is a parallel deposition technique capable of X depositions per stamper. The technique is limited in more sophisticated multicomponent deposition by the size of patterns that can be produced and the features obtained during the transfer process. The computer-aided design/computer-aided manufacturing (CAD/CAM) ability of MAPLE DW overcomes the limitations of the μCP approach. (i) We establish the science and engineering principles behind the effective transfer of microarrays and (ii) we explore issues regarding the direct immobilization, morphology and function of the deposited protein at the interface with an aqueous environment and in the precision of controlled ligand-receptor reactions. In summary, our objective was to develop simple, robust microfabrication techniques for the construction of model 2D and 3D bioscaffolds to be used in fundamental bioengineering studies.