*2.1.1.1. Deposition conditions*

### *2.1.1.1.1. Influence of the ambient gas inside the deposition chamber*

Depending on the structure and composition of the thin films that one desire to achieve by PLD, in the deposition chamber a gas, which can be active or passive, can be introduced. In principle, the passive influence of the gas is necessary because it helps to compensate the eventual losses of the constituent elements. For example, the oxide thin films tend to be oxygen deficient.

We provide a relevant example of ZnO thin films synthesised in a vacuum (4 × 10−2 Pa) and in O2 (13 Pa) ambient. The aspect of both films was radically different: the films deposited in a vacuum were opaque, dark‐coloured (**Figure 2a**), while the films obtained in an oxygen flux were highly transparent (**Figure 2b**) [13].

**Figure 2.** Textile material partially coated with ZnO films: (a) dark‐coloured film deposited in vacuum and (b) trans‐ parent film deposited in a 13 Pa oxygen flux.

The explanation is that in oxygen ambient, due to the intense collisions with the environmental atoms, the ejected matter is confined to an elongated, 'cigar'‐ shaped plasma (**Figure 3a**). A thermal equilibrium is reached as a result of collisions during transfer and the substance condenses in large quantities forming compact thin films. In a vacuum, at much lower collision rates, the matter is ejected in all directions (**Figure 3b**), with high energies and speed. These high energetic species are bombarding the layers previously deposited and cause damage (by sputtering off atoms from the outer layers) or defects (dislocations, cracks, holes) on the deposited film. These bombardments occur for each pulse, resulting in a very disordered thin film that is full of defects. The defects are highly absorbent in the visible spectrum and hence the dark aspect.

**Figure 3.** Plasma plume in PLD recorded in 13 Pa O2 flux (a) and vacuum (b) (Reproduced with permission from Ref. [13]).

### *2.1.1.1.2. Influence of the target-substrate separation distance*

**Figure 1.** Experimental set‐up PLD/RPLD.

were highly transparent (**Figure 2b**) [13].

parent film deposited in a 13 Pa oxygen flux.

*2.1.1.1.1. Influence of the ambient gas inside the deposition chamber*

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

Depending on the structure and composition of the thin films that one desire to achieve by PLD, in the deposition chamber a gas, which can be active or passive, can be introduced. In principle, the passive influence of the gas is necessary because it helps to compensate the eventual losses of the constituent elements. For example, the oxide thin films tend to be oxygen

We provide a relevant example of ZnO thin films synthesised in a vacuum (4 × 10−2 Pa) and in O2 (13 Pa) ambient. The aspect of both films was radically different: the films deposited in a vacuum were opaque, dark‐coloured (**Figure 2a**), while the films obtained in an oxygen flux

**Figure 2.** Textile material partially coated with ZnO films: (a) dark‐coloured film deposited in vacuum and (b) trans‐

*2.1.1.1. Deposition conditions*

deficient.

The effect of the target‐substrate distance is reflected by the angular scattering of the ejected flux. Different features can occur depending on the position of the substrate. The optimal position of the substrate in order to obtain stoichiometric structures is determined by the plasma evolution. The best depositions (in terms of stoichiometry, uniformity and homoge‐ neity) are obtained when the plasma length is identical with the target‐substrate separation distance [13]. To support this assertion we provide an example of ZnO deposition using a low number of pulses and three separation distances: 3, 4, and 5 cm. The plasma plume was 4 cm in length (**Figure 4**).

As shown in **Figure 4**, the largest number of ZnO nanoparticles was present on the surface of the sample placed at 4 cm from the target, while smaller amounts of ZnO nanoparticles were observed for the samples positioned at 3 and 5 cm. As it is known [14], the quantity of deposited substance in PLD is inversely proportional to the square of the target‐substrate separation distance. However, this does not apparently apply in our case for the sample placed at 3 cm from the target. A possible explanation could be that the plasma plume deposited and removed ('washed') nanoparticles at the same time from the substrate because the separation distance, in this case, was too small (in any case, smaller than the plasma length).

**Figure 4.** SEM micrographs of ZnO nanoparticles deposited in a 13 Pa O2 flux on a Si substrate. Target‐substrate sepa‐ ration distance was of 3 cm (a), 4 cm (b) or 5 cm (c). Inset: water droplet in static mode and the measured CA.

For target‐substrate separation distances longer than plasma length, the species in plasma lost their kinetic energy by collisions with other species and gas molecules from the ambient and therefore the ablation rate was significantly lower than for 4 cm.

### *2.1.1.1.3. Influence of number of pulses*

A very low number of pulses (generally under 100) generate a deposition of nano/micro‐ particles on the substrate surface. Slightly increasing the number of pulses produces islands of material. Upon increasing the number of pulses, the substrate is covered by a continuous thin film [15].

**Figure 5.** Typical transmission spectra recorded in the case of PLD simple ZnO films (solid curve), and films covered with Au nanoclusters after ablation by 100 pulses from a Au target (dashed curve). (Reproduced with permission from Ref. [15]).

We present a case when ZnO thin films were synthesised by PLD after applying 50,000 pulses to a ZnO target. The target was further switched on with a gold one and by irradiating it with 100 laser pulses Au nanoclusters were generated on the ZnO thin film surface. In **Figure 5**, it can be found that the transmittance spectrum was shifted toward longer wavelengths follow‐ ing the thin film with Au nanoclusters. The infrared band‐gap energy was 3.26 eV for ZnO films covered with Au, which is slightly lower than that of simple ZnO films of 3.29 eV [15].
