Laser-induced forward transfer: fundamentals and applications

Laser-induced forward transfer (LIFT) is a digital printing technique that uses a pulsed laser beam as the driving force to project material from a donor thin film toward the receiving substrate whereon that material will be finally deposited as a voxel. This working principle allows LIFT to operate with both solid and liquid donor films, which provides the technique with an unprecedented broad spectrum of printable materials, and thus makes it very competitive over other digital technologies, like inkjet printing. It is not only that LIFT can access a much wider range of ink viscosities and loading particle sizes; the possibility of printing from solid films allows the single-step printing of multilayers and entire devices, and even makes possible 3D printing. This versatility translates, in turn, into a broad field of applications, from graphics production to printed electronics, from the fabrication of chemical sensors to tissue engineering. This monograph provides an extensive review of the LIFT technique, from its origins to the most recent achievements, focusing on the fundamental aspects of both its working principle and transfer dynamics, as well as on its broad range of applications

Laser-induced forward transfer: a method for printing functional inks

Laser-induced forward transfer (LIFT) is a direct-writing technique based in the action of a laser to print a small fraction of material from a thin donor layer onto a receiving substrate. Solid donor films have been used since its origins, but the same principle of operation works for ink liquid films, too. LIFT is a nozzle-free printing technique that has almost no restrictions in the particle size and the viscosity of the ink to be printed. Thus, LIFT is a versatile technique capable for printing any functional material with which an ink can be formulated. Although its principle of operation is valid for solid and liquid layers, in this review we put the focus in the LIFT works performed with inks or liquid suspensions. The main elements of a LIFT experimental setup are described before explaining the mechanisms of ink ejection. Then, the printing outcomes are related with the ejection mechanisms and the parameters that control their characteristics. Finally, the main achievements of the technique for printing biomolecules, cells, and materials for printed electronic applications are presented

Laser-induced forward transfer of conductive screen-printing inks

Laser-induced forward transfer (LIFT), unlike inkjet printing, presents few constrains concerning ink viscosity or loading particle size. This is clearly favorable for printed electronics applications, since high solid content inks, such those of screen printing, can be thus transferred in a digital fashion. In this work we propose a study of the transfer mechanisms during the LIFT of a commercially-available silver screen printing ink. The printing of single voxels on glass through the variation of pulse energy and donor-receiver gap reveals a linear dependence of voxel volume respect pulse energy for low energies and small gaps. The analysis of the transfer dynamics demonstrates that for the entire range of analyzed conditions the deposit takes place through bubble contact with the receiver. The printing of lines through variation of the overlap between successive voxels reveals that under none of the analyzed conditions we obtain uniform continuous lines through single scan: the lines always show scalloping, bulging, or discontinuities. These defects are a consequence of the modification of the donor film morphology induced by previous pulses in the line, which makes the transfer dynamics unstable. A final multiple scan approach proves the feasibility of the technique for printing uniform stable lines

Transparent and conductive silver nanowires networks printed by laser-induced forward transfer

Networks of silver nanowires (Ag-NWs) can be electrically conductive and optically transparent at the same time. Thus, Ag-NWs are promising candidates for substituting transparent and conductive oxides like indium-tin-oxide. Direct-write methods for printing patterns are suitable in order to reduce the amount of material used with respect to actual deposition methods on large areas that require post-processing steps. In this work, we study the laser induced forward transfer of Ag-NWs with the aim of printing conductive patterns that appear invisible at naked eye. A Nd:YAG laser system delivering 150 ns pulses at 1064 nm wavelength was coupled with a scan head for printing the Ag-NWs at different pulse energies (0.20-0.45 mJ). It has been found that the area coverage of Ag-NWs, which is directly related with the optical an electrical properties of the patterns, increases as the laser pulse energy increases. A sheet resistance of 140 Ω/sq is reached when printing at the highest pulse energy tested. As a proof-of-concept, we printed simple circuits with a pair of invisible electrodes connecting an LED on glass with a transmittance of 98.8 %, a haze of 0.5 %, a reflectance below 0.1 % and a sheet resistance of 340 Ω/sq

Time-resolved imaging of the laser forward transfer of liquids

Time-resolved imaging is carried out to study the dynamics of the laser-induced forward transfer of an aqueous solution at different laser fluences. The transfer mechanisms are elucidated, and directly correlated with the material deposited at the analyzed irradiation conditions. It is found that there exists a fluence range in which regular and well-defined droplets are deposited. In this case, laser pulse energy absorption results in the formation of a plasma, which expansion originates a cavitation bubble in the liquid. After the further expansion and collapse of the bubble, a long and uniform jet is developed, which advances at a constant velocity until it reaches the receptor substrate. On the other hand, for lower fluences no material is deposited. In this case, although a jet can be also generated, it recoils before reaching the substrate. For higher fluences, splashing is observed on the receptor substrate due to the bursting of the cavitation bubble. Finally, a discussion of the possible mechanisms which lead to such singular dynamics is also provided

Sub-wavelength laser nanopatterning using droplet lenses

When a drop of liquid falls onto a screen, e.g. a cell phone, the pixels lying underneath appear magnified. This lensing effect is a combination of the curvature and refractive index of the liquid droplet. Here, the spontaneous formation of such lenses is exploited to overcome the diffraction limit of a conventional laser direct-writing system. In particular, micro-droplets are first laser-printed at user-defined locations on a surface and they are later used as lenses to focus the same laser beam. Under conditions described herein, nanopatterns can be obtained with a reduction in spot size primarily limited by the refractive index of the liquid. This all-optics approach is demonstrated by writing arbitrary patterns with a feature size around 280 nm, about one fourth of the processing wavelength

Characterization of hydroxyapatite laser ablation plumes by fast intensified CCD-imaging

ArF excimer laser pulses (193 nm, 20 ns, 150 mJ) have been focused on a hydroxyapatite (HA) target in similar conditions to those normally used for thin film deposition. Fast intensified CCD images of HA laser ablation plumes have been taken in vacuum and under different water vapor pressures ranging from 0.01 mbar to 1 mbar. Images of HA ablation in vacuum have shown a plume freely expanding at a constant velocity of 2.3 106 cm/s. HA ablation under a water vapor pressure of 0.01 mbar has revealed an expansion behavior very similar to that of ablation in vacuum. Images taken under a water vapor pressure of 0.1 mbar have shown the formation of a shock structure in the plume. Finally, HA ablation under a water vapor pressure of 1 mbar has revealed the development of some irregularities in the shape of the plume

Superparamagnetic nanoparticles with efficient near-infrared photothermal effect at the second biological window

Superparamagnetic nanoparticles (iron oxide nanoparticles¿IONs) are suitable for hyperthermia after irradiating with radiofrequency radiation. Concerning the suitability for laser ablation, IONs present a low molar absorption coefficient in the near-infrared region close to 800 nm. For this reason, they are combined with other photothermal agents into a hybrid composite. Here, we show that IONs absorb and convert into heat the infrared radiation characteristic of the so-called second-biological window (1000-1350 nm) and, in consequence, they can be used for thermal ablation in such wavelengths. To the known excellent water solubility, colloidal stability and biocompatibility exhibited by IONs, an outstanding photothermal performance must be added. For instance, a temperature increase of 36 °C was obtained after irradiating at 8.7 W cm−2 for 10 min a suspension of IONs at iron concentration of 255 mg L−1. The photothermal conversion efficiency was ~72%. Furthermore, IONs showed high thermogenic stability during the whole process of heating/cooling. To sum up, while the use of IONs in the first bio-window (700-950 nm) presents some concerns, they appear to be good photothermal agents in the second biological window

3D features of modified photostructurable glass-ceramic with infrared femtosecond laser pulses

The exclusive ability of laser radiation to be focused inside transparent materials makes lasers a unique tool to process inner parts of them unreachable with other techniques. Hence, laser direct-write can be used to create 3D structures inside bulk materials. Infrared femtosecond lasers are especially indicated for this purpose because a multiphoton process is usually required for absorption and high resolution can be attained. This work studies the modifications produced by 450 fs laser pulses at 1027 nm wavelength focused inside a photostructurable glass-ceramic (Foturan®) at different depths. Irradiated samples were submitted to standard thermal treatment and subsequent soaking in HF solution to form the buried microchannels and thus unveil the modified material. The voxel dimensions of modified material depend on the laser pulse energy and the depth at which the laser is focused. Spherical aberration and selffocusing phenomena are required to explain the observed results

Laser-induced forward transfer: a digital approach for printing devices on regular paper

Inkjet printing (IJP) is the most widespread direct-write technique in paper electronics. However, its use is limited, since its low-viscosity nano-inks leak through the cellulose fibers. Thus, a planarization coating is frequently used as barrier, despite that this makes substrates more expensive and less ecofriendly. Alternatively, high solid content screen printing (SP) inks could allow printing on regular paper due to their high viscosity and their large particle size; however, they cannot be printed through IJP. Another digital technique is required: laser-induced forward transfer (LIFT). The aim of this work is to prove the feasibility of LIFT for printing devices on regular paper. The main transfer parameters are systematically varied to obtain uniform Ag-SP interconnects, which performance is improved by a multiple-printing approach, resulting in low resistances with a much better performance than those typical of IJP. After optimizing the printed lines functionality, a proof-of-concept consisting on a radio-frequency inductor is provided. The characterization of the device shows a substantially higher performance than that of the same device printed with IJP ink in similar conditions, which proves the potential of LIFT for digitally fabricating devices on regular paper