*2.3.1.2. Influence of the laser fluence*

controlled by selecting correctly the appropriate laser fluence, solute concentration and

18 Applications of Laser Ablation - Thin Film Deposition, Nanomaterial Synthesis and Surface Modification

MAPLE efficiency is determined by the correct choice of the solvent matrix, which has to absorb the laser energy during the deposition, thus protecting the complex organic compound.

**iii.** to have a high vapour pressure and a high volatility at room temperature, in order to

**Figure 15.** AFM micrographs of P(CPP:SA) 20:80 thin films deposited by MAPLE using ethyl acetate (a) and dimethyl chloride as matrix (b); typical FTIR spectra recorded for P(CPP:SA) 20:80 thin films obtained by MAPLE using ethyl

We present an example of MAPLE thin film synthesis of poly(1,3‐bis‐(p‐carboxyphenoxy, propane)‐co‐(sebacic anhydride)) (20:80) (P(CPP:SA)20:80) using two different solvents as a matrix to protect this hydrophobic anhydride copolymer [84]. One solvent was dimethyl chloride and the other, ethyl acetate. The copolymer was successfully transferred in both cases, but the morphology of the films was quite different. AFM images presented in **Figure 15a** for

(c). (Reproduced with permission from Ref. [84]).

be evacuated very fast from the deposition chamber;

**v.** to form a uniform solution with the complex organic material.

**iv.** not to be chemically active at laser beam exposure; and

substrate temperature.

*2.3.1.1. Influence of the matrix*

The solvent has to satisfy the following conditions: **i.** to absorb the laser beam, even when frozen;

**ii.** to have a high melting point;

acetate as matrix at a fluence of 0.3 J/cm2

McGill and Chrisey [82] proposed in their patent that the laser energy is absorbed majorly by the solvent matrix and is converted in thermic energy producing solvent evaporation. The complex organic compound will reach a kinetic energy high enough that will ensure the transfer and the immobilization onto the substrate.

Georgiou and Kokkinaki [85] advanced the hypothesis that the process takes place due to a photomechanical process (material expulsion). The complex organic compound will be ejected into the gaseous phase only if the laser irradiation takes place at a fluence that surpasses the ablation threshold. When the laser fluence is under the ablation threshold a thermic vapori‐ zation process, that produces the solvent desorption, occurs.

Itina et al. [86] suggested that after the laser irradiation upon the organic compound, kinetic energy is a result of both thermic and mechanic phenomena. They proposed a theoretic study that simulates the initial steps of molecule ejection. They observed that when the laser fluence surpasses the ablation threshold, clusters would be ejected from the target. The most important observation was that during the ablation process the organic compound molecules are not fragmented.

**Figure 16.** Top view AFM image of the RNase A thin films obtained from a frozen composite target containing 1% (w/v) biomaterial in buffer Hepes solvent, by irradiation with 15,000 subsequent laser pulses at 0.4 (a), 0.5 (b), 0.7 J/cm2 (c) laser fluence (Reproduced with permission from Ref. [87]).

In their support, we present the example of RNaseA enzyme thin films obtained by MAPLE [87]. When irradiation of targets was conducted with a laser fluence of 0.4 J/cm2 (**Figure 16a**) the films were constituted of tens of nanometre‐sized particles probably generated after surface evaporation and cluster formation in transit towards the substrate. By increasing the laser fluence to 0.5 J/cm2 (**Figure 16b**), both the mean diameter and the mean height of particles increase and double population could be identified: a majority population of 500 nm mean diameter and some large micronic particles. At 0.7 J/cm2 (**Figure 16c**), the film is covered by micrometric particles, caused by the droplet expulsion from the target surface as a result of explosive evaporation or spallation mechanisms [88, 89] termed cold laser ablation [90, 91].

### *2.3.2. Advantages of material deposition via MAPLE*

MAPLE was developed to surmount the difficulties in solvent‐based coating technologies such as inhomogeneous films, inaccurate placement of material, and intricate or erroneous thickness control. The process utilizes a low fluence pulsed UV laser and a frozen target consisting of a dilute mixture of the material to be deposited and a high vapour‐pressure solvent. The low fluence laser pulse interacts mainly with the volatile solvent, causing its evaporation. During the process, the solute desorbs intact, that is, without any significant decomposition, and is then uniformly deposited on the substrate.

‐ It enables the thin film deposition from a large amount of organic materials, such as polymers, proteins, enzymes and combination of organic‐inorganic materials.

‐ It is a non‐contact deposition technique free of pollution risks for the thin films. The molecular composition and structure of the material that is deposited by MAPLE are preserved during the transfer process.

**Figure 17.** FTIR images of (a) laser immobilized RNase A obtained by the irradiation of 1 wt% frozen composite RNase A target, and the drop‐cast samples of (b) initial and (c) final MAPLE target solutions of RNase A in buffer HEPES– NaOH, pH 7.5 (Reproduced with permission from Ref. [92]).

In support of this assertion, we give the FTIR spectra for a RNase A enzyme thin film (**Figure 17a**), for drop‐cast of the initial RNase A solution used to prepare MAPLE targets (**Figure 17b**) and for the final RNase A solution, collected from the target holder after the

laser irradiation experiments (**Figure 17c**) [92]. As can be seen in the **Figure 17** the spectrum of the transferred RNase A was comparable to that of the initial drop‐cast and to that of the target after irradiation. The bands of RNase A target material were also present in the spectra of deposited films: at 3400 cm−1 there was a band characteristic to N–H stretching vibrations from amide I groups; the one at 1654 cm−1 corresponded to C=O and C–N stretching vibrations of amide I, while the band at 1464 cm−1 was characteristic to in‐plane N–H bending as well as C–N stretching vibrations in the same functional group. At 1534 cm−1, the band was associated with tyrosine amino acid residues, while the band at 1400 cm−1 could belong to the amide III region [93–95]. The lower band intensity, in case of the MAPLE thin films, was due to the five times lesser amount of RNase A enzyme in the film as compared to the amount present in the drop‐cast samples.
