Pulsed laser deposition technologies (PLD, MAPLE, LIFT): basic …vukovar2010.ifs.hr/Documents/Abstracts/4_Mihailescu... · 2011-06-07 · Pulsed laser deposition technologies (PLD,

Collaborative workshop „Charge density waves: small scales and ultrashort time“ Vukovar, Croatia, 28 – 31 October 2010.

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Pulsed laser deposition technologies (PLD, MAPLE, LIFT): basic phenomena and applications

Ion N. Mihailescu, Carmen Ristoscu National Institute for Lasers, Plasma and Radiation Physics, Lasers Department, 409 Atomistilor, PO

Box MG-54, RO-077125, Magurele, Ilfov, Romania

[email protected].

Since their introduction in 1960, pulsed lasers are recognized as a flexible and powerful tool for material processing applications. The plasma generated and supported by high intensity laser radiation was long time considered an important loss channel and so, a strong impediment in the development of efficient laser processing technologies of materials. In time however, it became clear that the plasma not only controls the complex interaction phenomena between the laser radiation and various media but can be exploited in view of improving laser radiation coupling and eventually the efficient processing of materials submitted to intense laser beams [1]. In materials science, laser plays a significant role either as a passive component for monitoring or as an active tool for coupling its radiation energy into material being processed. Pulsed laser action results in various applications such as localized melting, laser annealing, surface cleaning by desorption and ablation, surface hardening by rapid quench, and after 1988, pulsed laser deposition technologies for synthesizing high quality nanostructured thin films. We review herewith recent results we obtained by using pulsed laser deposition technologies: Pulsed laser Deposition (PLD), Matrix Assisted Pulsed Laser Evaporation (MAPLE), Laser Induced Forward Transfer (LIFT) and Combinatorial-PLD (C-PLD).

Figure 1. PLD experimental setup

The general PLD layout used in experiments of thin film synthesis can be described as follows (Fig. 1). Laser beam generated by a pulsed UV laser source enters the reaction chamber through a quartz window. We used a KrF* excimer laser source (COMPEXPro 205) generating at 248 nm pulses of 25 ns duration. The beam is focused onto target

surface by means of an AR cylindrical lens placed outside the deposition chamber. It hits the target surface at oblique incidence. The target is rotated and translated during multipulse irradiation to avoid piercing and improve film uniformity. A temperature controller is used to monitor substrate heating and cooling. The reaction chamber is initially evacuated down to a residual pressure of 10-4 Pa. The dynamic pressure during experiments is kept constant using a flow controller. PLD demonstrated to be a versatile technique for thin film processing with a high diversity of structural and morphological characteristics [2,3]. Many independent parameters can be changed under control in order to select the optimum deposition regimes of some specific structures and thin films. Growing thin films by PLD has numerous advantages, such as: - the laser source is placed outside the deposition chamber offering increased flexibility in handling the material, laying out the geometrical setup, and adjusting deposition parameters; - most of the solid and liquid materials can be ablated; - multistructures can be easy synthesized ; - laser pulses make possible to control the growth rate of the coatings very accurately (down to a few fractions of Å); - most of the ablated material is located inside the plasma generated under laser pulse action; - the stoichiometry of coating materials generally coincides with that of the target even for complex, highly unstable compounds (also described as congruent transfer); - the coatings adhere well due to the high plasma energy incident onto deposition substrate; and - species with metastable or nonequilibrium states and new phases can be synthesized. The presence of particulates of different geometrical shape and size was considered for long time one major handicap for the large scale implementation of PLD for the synthesis of surfaces with suitable finishing or applications in opto-electronics or lasing media, among other. It took several years till the origin of these particulates, which could be present either on surface or deeply embedded into the films, could be suitably understood and monitored. We know now that the major physical phenomena involved in particulates formation are : 1. explosive dislocation of the substance caused by the subsurface overheating of the target; 2. gas phase condensation of the evaporated material (clustering);


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3. liquid phase expulsion under the action of the recoil pressure of the ablated substance; 4. blast-wave explosion at the liquid (melt) – solid interface; and 5. hydrodinamic instabilities on target surface. It was nothing but a matter of time to elaborate protocols and rules to be applied for particulates limitation or even complete elimination. Among the most efficient methods to be applied we mention: - to avoid the presence of the liquid inside the crater; - proper choice of laser wavelength; - to set the incident laser fluence at a level high enough to vaporize all the melted substance; - application of electric and/or magnetic fields normally to expansion; or - intersection and elimination of particulates by a second laser beam. Nevertheless, in case of very complex delicate biomolecules, PLD fails because of the irreversible damage of the chemical bonds and consequent compositional modification of the deposited film. This disadvantage is eliminated by matrix assisted pulsed laser evaporation (MAPLE), capable to transfer compounds at low temperatures. MAPLE was developed after 1998 [4] as a complementary method to PLD, necessary for delicate (in respect with thermal and/or biological degradation and damage) materials transfer (Fig. 2). MAPLE essentially differs from PLD by target preparation, laser-material interaction and transfer mechanisms. It provides a more gentle mechanism for transferring many different compounds, including large molecular weight species, such as polymeric or organic molecules [5,6].

Figure 2. MAPLE experimental setup

Figure 3. Schematic of the MAPLE evaporation process

Specific to MAPLE is the use of a cryogenic composite target, a dilute mixture of the organic material to be deposited, and a light-absorbent, high vapor-pressure solvent matrix. The material to be deposited is dissolved or suspended in the solvent, usually 0.1–10 wt%, and cooled below the solvent freezing temperature. The solid composite target is evaporated by a pulsed UV laser and the material is collected on a nearby substrate as a thin film. The vaporized solvent does not form a film and is pumped away (Fig. 3).

Other new advanced transfer method is Laser Direct Writing (LDW), also known as Laser Induced Forward Transfer (LIFT). LDW technologies are used to produce or deposit materials on complex 2D or 3D structures. Even the concept of LDW was introduced at the end of ’60, this method concentrated a very large interest after 2000, when the technique progressed very much and fully demonstrated its possibilities. LDW has the potential to revolutionize manufacturing processes across a broad range of sectors including aerospace, pharmaceuticals, automotive, biotechnology, ceramics. A few approaches have been proposed to date to minimize heat induced damage of the transferred biomaterials. One of them is to initiate biomaterial transfer by laser ablation of a thin metal film (sacrificial layer) placed between the biomaterial layer and the transparent support. LIFT technique consists in the local transfer of material from a thin film (ribbon) covering a transparent support to a close receiving substrate under the action of a laser pulse (Fig. 4). The laser radiation is absorbed directly in the ribbon or by the thin metal film.

Figure 4. (a) LIFT schematic. (b) Detail of donor/receiver substrate showing thick sacrificial film covered with transfer material. Small shaded region indicates the volume in which the incident laser is absorbed [7]

A very recent development in the field is the Combinatorial-PLD technique (C-PLD). In C-PLD, the targets are located in two different positions and ablated. In the following, we will provide some recent examples illustrating the potential of the four mentioned techniques.

The Mg films obtained by PLD in UHV from Mg targets were characterized by Scanning electron microscopy (SEM) and X-ray diffraction (XRD). The surface of the films appeared smooth, with only few particulates of about 300 nm diameter (Fig. 5), with a density of only 0.01 /µm2. XRD spectrum evidenced the deposition of a pure Mg film,


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(002) textured (Fig. 6). The measured value of the rocking curve was 4.0683.

Figure 5. Typical SEM image of a Mg film obtained by PLD (F = 5.6 J/cm2)

30 35 40 45 50 55 60

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5

10

15

20

Inte

nsity

(a.u

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2theta (deg)

Mg

(100)

(002) (101)

(102)

(110)

Figure 6. XRD spectrum of a Mg film obtained by PLD

It is generally accepted that the synthesis of pure Mg films is very difficult, being jeopardized by the extremely high affinity of this metal for oxygen.

Extracellular matrix proteins (ECM) proved to be efficient in material bio-activation by adsorption and cell adhesion at the interface. We therefore tried to synthesize by PLD and MAPLE a double structure consisting of a hydroxyapatite (HA) “bed” and ECM proteins top-layer. The qualitative distribution of proteins during the MAPLE synthesis was monitored by Ponceau S Staining Solution on nitrocellulose membranes. The quantitative reproducibility of fibronectine (FN) and vitronectine (VN) was established by colorimetric protein assay using bicinchoninic acid (BCA) solution. The transfer of either FN or VN was demonstrated by FTIR studies where the peaks of protein dropcasts were coinciding with the ones of MAPLE deposited nanostructures. These results were supported by antibody staining studies using anti-human FN or VN rabbit polyclonal serum and FITC-conjugated anti-rabbit IgG when spot-like fluorescent regions were visualized by fluorescence microscopy (Figs. 7,8).

Human osteoblast (hOB) precursor cells were cultured on all structures for up to 14 days. The cells displayed a normal morphology, optimal viability and spread. The synergistic effect of the VN and FN coatings was evidenced by actin and vinculin staining, MTT assays and SEM studies. The long elongated actin filaments proved the excellent induced bioactivity, also showing a uniform distribution of the cells over the entire surface. The vinculin staining evidenced intimate contacts with material surfaces by focal adhesion patches for FN covered structures and small spots in case of VN ones. The MTT results sustained the higher potential for spreading and improved viability of protein covered structures versus Ti/HA and Ti/HA/BSA. SEM investigations revealed more flattened cell morphology with long developed filopodia when cultivated on FN and VN structures (Fig. 9).

Figure 7. Immunofluorescence detection of FN structures

Figure 8. Immunofluorescence detection of VN structures

Ti/HA Ti/HA/BSA


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Ti/HA/VN Ti/HA/FN

Figure 9. SEM images of hOB cells at 7 days from seeding

We have revealed that all porphyrin (TPP :SA) patterns produced by LIFT demonstrate comparable specific red fluorescence (Fig. 10) independently of thickness of the titanium film applied [8]. This points out that essential amount of the transferred porphyrin has retained its molecular structure in all cases.

Figure 10. Specific red fluorescence of TPP:SA pattern transferred from the ‘‘thin-film’’ target (dTi = 50 nm, F = 250 mJ/cm2).

We applied C-PLD to obtain ZnO –ITO mixed films [9]. The deposition sequence was 20 pulses from the ITO target followed by 20 pulses from the ZnO target to ensure a good atomic mixing of the components. The total number of pulses was varied from 1000 to 5000, which resulted in films thicknesses from around 50 to 250 nm across a 50 mm long substrate. The material was collected on Si or quartz substrates heated from 300 up to 500 C. Films grown separately from each target have their maximum thickness positioned at the intersection between the perpendiculars from the laser impact area on the targets with the substrate. For film mixtures the critical angle was found to be a function of the location where the measurement was performed. By inspecting Fig. 11, where spectra collected from a mixed film are displayed along those collected from pure films, one can see that the mixed film possesses a density that changes along with the location on the transversal axis. The calculated values were from 5.63 g/cm3 in the ZnO rich region up to 6.51 g/cm3 in the ITO

rich region, i.e. between those corresponding for pure ZnO and ITO films.

Figure 11. XRR spectra acquired from various locations on a ITO-ZnO mixed film

In conclusion, we foresee a bright avenue for these four pulsed laser techniques in synthesis of nanostructured thin films and multistructures of inorganic, organic or composite materials for applications in all science and technology top areas from optoelectronics, chemistry, and metallurgy to biology and medicine.

Acknowledgements: The authors acknowledge the financial support of the UEFISCSU under the contracts 511/2009 and 547/2009.

References

[1] I. N. Mihailescu, J. Hermann, Chapter 4 in “Laser Processing of Materials: Fundamentals, Applications, and Developments”, Ed. P. Schaaf, Springer Series in Materials Science, Springer Heidelberg (2010) pp. 51 – 90 [2] Pulsed Laser Deposition of Thin Films, edited by D. B. Chrisey and G. K. Hubler (Wiley, New York, 1994) [3] Pulsed Laser Deposition of thin films: applications-lead growth of functional materials, edited by R. Eason (Wiley & Sons, New York, USA 2007) [4] D. B. Chrisey, A. Pique‚ R. A. McGill, J. S. Horwitz, B. R. Ringeisen, D. M. Bubb, and P. K Wu, Chem. Rev. 103, 553 (2003) [5] Chapter 18 in Ref. 3 [6] I. N. Mihailescu, C. Ristoscu, A. Bigi, I. Mayer, Chapter 10 in “Laser-Surface Interactions for New Materials Production Tailoring Structure and Properties”, Series: Springer Series in Materials Science, Vol. 130, Miotello, Antonio; Ossi, Paolo M. (Eds.), 2010, pp. 235 – 260 [7] N.T. Kattamis, P. E. Purnick, R. Weiss, C. B. Arnold Applied Physics Letters 91, 171120 (2007) [8] T.V. Kononenko, I.A. Nagovitsyn, G.K. Chudinova, I.N. Mihailescu, Applied Surface Science 256, 2803 (2010) [9] D. Craciun, G. Socol, N.Stefan, M. Miroiu, I.N. Mihailescu, A.-C. Galca, V. Craciun, Applied Surface Science 255, 5288 (2009)