3.1. Zinc Oxide
ZnO was the first simple oxide deposited using ultra-short PLD and the reason is probably to be found in the renewed interest, in the early 2000 s, on its optoelectronic applications as a semiconductor with a direct wide band gap [52
]. In 2000, Okoshi et al. deposited ZnO thin films in vacuum using PLD performed by a laser (λ = 790 nm) with a 130 fs pulse duration [53
] and with E
= 10 mJ. The substrates were Si (100) and quartz, and the results indicated that the films were crystalline in both cases with the best crystallinity obtained for the films deposited at a substrate temperature of 230 °C. In the same year, a paper on the same topic was published by Millon et al. [54
]. In this case a laser with a pulse duration of 90 fs (λ =620 nm, E
= 1 mJ) was used to grow heteroepitaxial ZnO thin films on (0001) sapphire substrates. The same group, in a paper published in 2002, compared ZnO films deposited using fs PLD with those obtained by ns PLD [55
], evidencing strong differences in the films deposited by the two techniques, differences justified by the higher energies of the atomic species present in the fs plume. Okoshi et al. published a paper in 2001 comparing the results of PLD performed on a ZnO target doped with Al2
(2 wt % Al2
) using lasers of different pulse durations and different wavelengths (λ = 690 nm with a pulse duration of 130 fs and λ = 395 nm with a pulse duration of 230 fs) [56
]. The results evidenced a dependence on the laser wavelength of the electrical properties of the films even if their real composition was not clearly reported. A brief summary and discussion of these first works on fs PLD of ZnO was later reported by Perriere et al. in a chapter of a book edited by Robert Eason [57
From these first works, it emerged that essentially by using fs PLD it was possible to deposit ZnO films in vacuum, while the presence of oxygen was necessary for the deposition using ns PLD. On the other hand, even if in some of these papers the analysis of the plasma plume was reported, the peculiar characteristics of the fs plasma were not yet evidenced, and also, the nanostructured film morphology was not clearly identified, even if some evidence of this morphology had been presented [54
]. In 2005, Klini et al. made a comparison between the deposition of ZnO films using ns and fs PLD on Si (100) and fused silica substrates at different temperatures [58
]. In particular, for the fs regime they used a laser with a pulse duration of 450 fs, a wavelength of 248 nm and a fluence of 1.7 J/cm2
. The results indicated that the films deposited using fs PLD were stochiometric, crystalline and presented a high surface roughness. The OES analysis of the plasmas confirmed the higher energy and ionization degree of the species produced in fs ablation in comparison with those produced using ns. The authors correctly associated the high surface roughness of the films to the presence of NPs ejected from the target, but their analysis did not reveal the presence of a secondary plume for the ZnO system.
A particular use of ultra-short PLD was carried out in 2005 by Zhang et al. [13
]. By irradiating a ZnO target using a Ti:sapphire laser (τ = 80 fs, λ = 800 nm, repetition rate = 10 Hz, E
= 10 mJ), they enabled the growth of ZnO nanowires on gold coated substrates, at T
= 600 °C and at different oxygen pressures. Of course, the result was not exactly a thin film but a collection of nanowires.
In all the works previously discussed the ultra-short pulse lasers used in PLD were fs lasers, but in 2015, Lansiart et al. used a ps laser source to perform PLD of ZnO [59
]. The laser was a frequency tripled Nd:YAG with a wavelength of 355 nm, a pulse duration of 42 ps, a fluence = 0.5–15 J/cm2
and a repetition rate of 10 Hz. The substrates used were (100) silicon, c-cut and r-cut sapphire mono-crystals, and the preferential orientations of the stoichiometric crystalline ZnO films strongly depended on the type of substrate. In general, the structural characteristics of the films were like those observed for ZnO films deposited using fs PLD, confirming the difference between the short and ultra-short pulse regimes.
In the period 2016–2019, four papers on ultra-short PLD of ZnO were published. In 2016, De Mesa et al. studied the deposition of ZnO ablated using a laser with a pulse duration of 100 fs, a power of 600 mW, a wavelength of 800 nm and a repetition rate of 80 MHz [60
]. Rather surprisingly, they were unable to obtain stoichiometric ZnO films both in vacuum and at a pressure of 10−2
mbar of oxygen. They attributed this failure to the relatively low temperature of the Si substrate (25 °C) and to the absence of a post annealing treatment, but another group, previously cited, were able to obtain stoichiometric ZnO films at room temperature and without post annealing. More probably, the reason for the failure can be found in the very high repetition rate of the laser source (80 MHz), which allowed an interaction between the expanding plasma produced by the ablation and the following laser pulses (with a repetition rate of 80 MHz the time delay among the pulses is 12.5 ns), inducing conditions very far from those of conventional fs PLD. The authors were able to deposit stoichiometric films only in an oxygen atmosphere, in this way losing one of the advantages of ultra-short PLD. Anyway, the confirmation of the problems that fs PLD meets when it is performed using a very high repetition rate laser source is interesting.
In the same year, Hashmi et al. also studied the deposition of ZnO by fs PLD [61
]. The laser source (Ti:sapphire) had a wavelength of 800 nm, a pulse duration of 100 fs and a repetition rate of 1 kHz. The substrates were made of borosilicate glass and the experiments were carried out at different target-substrate distances (60–80 mm), at different substrate temperatures (25–150 °C), with different laser energies (120–230 μJ) and both in vacuum and in oxygen pressure (0.7 mTorr). XRD spectra indicated that the films were formed by crystalline ZnO in all the deposition conditions, but the best results from the point of view of crystallinity and film orientation (c-axis) were obtained in oxygen pressure, with a substrate-target distance of 80 mm, an energy of 180 μJ and a substrate temperature of 150 °C. The morphology of the films’ surface is that typical of fs PLD, with NPs apparently becoming larger with the increase of the temperature. Since the films were produced to test their optical properties, UV-vis-NIR spectroscopy and photoluminescence were used for further characterization. Optical transmission increased with the increase of the crystalline quality of the films, and the luminescence spectra showed a strong UV emission close to the ZnO band gap. It is noteworthy that also in this case the best results were in an oxygen atmosphere.
In 2018, Kokaj et al. used a laser with a wavelength of 797 nm, a pulse duration of 117 fs, a power of 3 W and a repetition rate of 1 kHz to perform PLD on a ZnO target [62
]. The films were deposited on glass substrates, at room temperature and in low vacuum conditions. The results were not clearly reported, but they seemed to indicate that crystalline ZnO films were deposited. This was different to the approach of Hashmi et al., who studied the role of a dopant in the growth of ZnO films [63
]. The laser source was the same as already used by the same group [61
], while the dopant was samarium. The results indicated that with a Sm percentage of 1 wt %, the films retained the structure of crystalline ZnO with a preferential c-axis orientation, while for higher amounts of Sm, the formation of mixed oxides took place. In this case the deposition was carried out only in an oxygen atmosphere (1 mbar).
In conclusion, the deposition of ZnO films using ultra-short PLD produced crystalline stochiometric films, but their quality and their optical and electric properties were often not as good as in the of the films produced using short PLD. Furthermore, the nanostructure of the surface characteristics of the films produced using ultra-short PLD does not seem to improve those properties. The only possible advantage, i.e., the capability to deposit the films in vacuum, was not always appreciated.
3.2. Titanium Dioxide
In 2003, Albert et al. published a paper on the ablation of a titanium target using both ns and fs lasers [27
]. This paper is very important because it reports the first evidence of the presence of the secondary plume in fs ablation, correctly interprets it as formed by NPs and proposes these NPs as the origin of the nanostructured surface of the deposited films. The reason we cite this paper is related to the fact that Rutherford backscattering measurements evidenced that the films were formed by a defective titanium oxide (TiO0.55
) with the oxygen coming probably from residual gas in the vacuum chamber. Anyway, no further characterizations, except for SEM micrographs, were carried out on the films.
We must wait until 2009 for the first real study on fs PLD of TiO2
. In that year, Sanz et al. deposited TiO2
films on (100) Si using a Ti:sapphire laser (τ = 80 fs, λ = 266, 400, 800 nm, repetition rate = 10 Hz, fluences = 140, 124, 75 mJ/cm2
]. The films were deposited both in vacuum and in oxygen pressure with substrate temperatures ranging from room temperature to 700 °C. The best deposits were obtained using PLD performed at 266 nm, in vacuum and with the substrate at room temperature. The films were stoichiometric and nanostructured, with the change of the PLD parameters (laser wavelength, oxygen pressure, substrate temperature) influencing the NP size and distribution. Since the composition was performed using XPS, there was no evidence that the films were crystalline. The next year, Sanz et al. used a different laser source to deposit TiO2
films in vacuum [65
]. In that case they used a frequency doubled Nd:glass laser (τ = 300 fs, λ = 527 nm, repetition rate = 33 Hz), and the substrates were mica and (100) silicon. The authors studied both the plasma and the deposited films. The analysis of the plasma showed that in this case there was no clear separation between primary and secondary plumes, with the ejection of NPs, with the characteristic blackbody-like spectrum, observed a few hundreds of ns after the end of the laser pulse. The deposits were formed by NPs, and the stoichiometry corresponded to TiO2
with a small amount of Ti2
. Also in this case, there was no information about the films crystallinity.
In 2010, Gamez et al. deposited amorphous TiO2
films on (100) silicon using a laser with a pulse duration of 60 fs and emission wavelengths of 266, 400 and 800 nm [66
]. The repetition rate was 1 kHz, and the films were deposited at room temperature. The aim of the paper was to study the application of the nanostructured TiO2
films to laser-assisted Laser Desorption Ionization (LDI) of small peptides and synthetic polymers, and the results indicated good performances for the films deposited using fs PLD. It is interesting to note that the authors also used a ns laser to deposit crystalline TiO2
: depending on the presence or not of oxygen, different phases (rutile or anatase) were obtained, but in all cases, the films were deposited at a substrate temperature of 650 °C.
In 2013, Cavaliere et al. used a different approach to the fs PLD of TiO2
]. In that paper, they studied the first steps of the formation of the films, namely the aggregation processes among the NPs which took place on the substrate. They used a Ti:sapphire laser (λ = 800 nm, τ = 120 fs, repetition rate = 1 kHz, fluence = 6.43, 8, 9.6 J/cm2
) as ablation source, Si (100) wafers as substrates, and all the experiments were carried out in air and at room temperature. The authors observed that the NPs were organized in fractal structures whose characteristics depended on laser fluence and target-substrate distance. The NPs were formed by both anatase and rutile phases, as shown from Raman measurements, with a higher concentration of the former. Four years later, the same group returned on the topic (Celardo et al. 2017) [68
], studying the specific mechanisms leading to the formation of the fractal nanostructures they described in the previous work. The experimental conditions were the same as already described, but in this case, they used two different types of substrates (Si and quartz). By comparing the experimental results with a Monte Carlo model for fractals formation, where the NPs arriving on the substrate are considered hot and made of a large number of atoms, the authors deduced that the formation of the fractal structures, due to diffusion and aggregation of the NPs, took place on the substrate surface, with an important role played by the substrate thermal conductivity. In another paper of the same year (Cavaliere et al., 2017) the authors compared the results obtained for TiO2
NPs with those obtained by applying their model to the experimental results of fs PLD of gold [69
In the three works reported above, fs PLD was probably carried out from Ti targets, even if this detail was never clearly stated, so the results are interesting from the point of view of the general mechanism of film formation in fs PLD, but they are not immediately comparable with the majority of the other work which generally used TiO2
targets. Indeed, the peculiarities of the structure of the films reported in those papers, quite different from that found for films deposited using fs PLD in vacuum and in low oxygen pressure, is confirmed by a work published in 2017 by Gao et al. [70
]. In that paper the authors reported the fs PLD of a Ti target in air at room temperature and the laser source was a Ti:sapphire laser (λ = 800 nm, τ = 45 fs, repetition rate = 1 kHz, power = 0.368 W). The morphology of the films was like the fractal structures evidenced in [67
], finally resulting in a fluffy film (Figure 3
) composed of a mixture of anatase, rutile and amorphous TiO2
Coming back to fs PLD carried out in more conventional conditions, in 2013, Amoruso et al. studied the ablation of a TiO2
target using both fs and ns pulse lasers [71
]. The fs source was a Nd:glass laser, the same source as used by Sanz et al. in 2010 [65
], (λ = 527 nm, pulse duration = 300 fs, repetition rate = 33 Hz, fluence = 1.4 J/cm2
) and the films were deposited, at room temperature, on (100) Si in vacuum and in oxygen atmosphere. The paper reported both the plasma and deposit characterizations and showed that the films deposited at pressures <10−1
mbar were amorphous, but no information was reported about their composition, except that a post-annealing in air at 500 °C induced the formation of the anatase phase of TiO2
. On the contrary, the films deposited at higher oxygen pressure showed the presence of the crystalline anatase phase. In all cases, the films were formed by the coalescence of NPs but presented a more porous structure with the increase of the oxygen pressure (Figure 4
). The same laser source of the previous work was used, with the same experimental conditions, in the same year (2013) by Pallotti et al. to deposit TiO2
films to be utilized as gas sensors [72
]. The films were deposited at an oxygen pressure of 3 mbar and their photoluminescence was studied to explore a possible use for opto-chemical sensing. In a paper published the following year, the same group investigated the effect of O2
on photoluminescence of TiO2
, and the results showed variations in both NIR and Vis photoluminescence [73
]. Other applications of the nanostructured films described in [66
] were reported by Ni et al. in 2014 [74
] and by Sang et al. in 2015 [75
], this time deposited on Ti substrates. In the first case, the films were also doped with CdS NPs (5 wt %) to enhance their charge-transfer properties, and indeed, they presented a photocurrent enhancement and a higher photoelectron transmission rate. The measurements were carried out on films deposited in vacuum, which, after a post-annealing at 500 °C, showed the presence of anatase and rutile phases but that initially were probably amorphous. In the second case, the TiO2
films, again doped with CdS NPs, were tested for their photoelectrochemical properties.
In conclusion, fs PLD of TiO2 in vacuum produces films probably with the correct stoichiometry but amorphous in structure, while the addition of oxygen in the deposition chamber leads to crystalline phases. In all cases, the films are formed by the coalescence of a large number of NPs and the morphology and the amount of coalescence change slightly with the oxygen pressure. The characteristics of the films deposited using fs PLD of Ti targets in air are different. In this case, the NPs seem to aggregate in dendritic structures, which, with a deposition time long enough, evolve into films with a “fluffy” structure, composed probably of a mixture of anatase, rutile and amorphous phases.