Femtosecond Pulsed Laser Deposition Research Articles

Fs-pulsed laser deposition of PbTe and PbTe/Ag thermoelectric thin films

For the first time, thermoelectric thin films were fabricated by femtosecond pulsed laser deposition (fs-PLD) that represents a challenging technological solution for this application since it provides a correct film stoichiometry compared to the starting target, capability of native nanostructuring and a high deposition rate. In particular, this paper shows a preliminary work on PbTe and PbTe/Ag thin films deposited at different substrate temperatures by fs-PLD from a microcrystalline PbTe target. Structural, morphological and compositional characterizations of the deposited films were performed to demonstrate the formation of films composed by crystalline nanograins (about 35 nm size) and characterized by a correct stoichiometry. A remarkable deposition rate of 1.5 nm/s was evaluated. The electrical conductivity and the Seebeck coefficient (thermopower) were measured as a function of operating temperature to derive the thermoelectric power factor that was found to be less than a factor 2 with respect to the bulk materials. Finally, a discussion about the influence of compositional and structural properties of the deposited films on the related thermoelectric performances was presented.

Femtosecond laser deposition of TiO2 nanoparticle-assembled films with embedded CdS nanoparticles

Based on the normal pulsed laser ablation method, femtosecond pulsed laser deposition (fs-PLD) is adopted in vacuum for the production of TiO2 nanoparticle-assembled films. We study the morphology and electronic characteristics of TiO2 nanoparticle-assembled films deposited at different oxygen background gas pressures from high vacuum (∼10−4 Pa) to 100 Pa and different deposition time. Our results show that TiO2 nanoparticle-assembled films obtained in high vacuum present both a mixture with rutile phase and anatase phase and a pure rutile phase. At the same time, there are more mesoporous structures in the film after annealing, which is beneficial for the enhancement of photocatalytic activity. In water splitting experiment, part of the TiO2 nanoparticle-assembled films embedded with a small mass fraction of CdS nanoparticles (∼5%) present an interesting photocurrent enhancement with a maximum value of ∼0.2 mA/cm2 under a solar simulator.

Highly Luminescent Thin Films of the Dense Framework 3∞[EuIm2] with Switchable Transparency Formed by Scanning Femtosecond‐Pulse Laser Deposition

Highly luminescent switchable thin films of the dense framework ∞(3)[EuIm2] were deposited by a scanning laser ablation technique using a femtosecond laser. The films can be controlled in terms of film thickness and amount of material deposited such that the material properties of the bulk material are retained on the nanometer scale. Polycrystalline films are formed that can be switched between transparent at visible light and nontransparent at UV light due to the intrinsic luminescence of the hybrid material, expanding the concept of smart films. The new femtosecond pulsed laser deposition method also provides a novel approach for coatings with framework compounds and coordination polymers.

Multi-band photoluminescence in TiO2 nanoparticles-assembled films produced by femtosecond pulsed laser deposition

We present an analysis of the photoluminescence (PL) properties of nanostructured titania (TiO2) thin films produced by femtosecond pulsed laser deposition. Up to four PL bands are evidenced and analyzed, corresponding to radiative transitions ranging from the blue/violet to the near-infrared. The PL analysis was carried out on as-grown and post-growth annealed samples in both above-bandgap and below-bandgap excitation conditions, and allowed evidencing definite correlations between surface area, crystal phase and PL efficiency. An interpretation of the various PL components is proposed in terms of various electron and/or hole states, according to the different structural characteristics and crystal phase of the nanoparticles-assembled films. In particular, similarities between the PL activity of as-grown amorphous nanoparticles and crystalline anatase nanoparticles highlight the role of specific surface states, offering interesting insights into the possibility of exploiting amorphous TiO2 nanoparticles for PL-based applications in which surface states play an active role.

Influence of film thickness on topology and related magnetic interactions in Fe nanoparticle films

Fe nanoparticle (NP)-assembled thin films with different thickness were prepared by femtosecond-pulsed laser deposition using different deposition times. The proper selection of the deposition time allows to control, to a certain degree, the morphology and topology of the deposited Fe nanoparticles (NPs) assembly, fostering non-uniform dense assemblies of NPs, with the consequent reduction of the influence of the exchange interactions on the macroscopic magnetic properties with decreasing thickness. The magnetic behavior of the Fe NP-assembled films with decreasing thickness is characterized by higher coercive field (H-c) values (a factor approximate to 4.5) and a good compromise between the hysteresis loops squareness and moderate exchange interactions, strongly correlated with the NPs topology.

Femtosecond pulsed laser deposition of silicon thin films

Optimisation of femtosecond pulsed laser deposition parameters for the fabrication of silicon thin films is discussed. Substrate temperature, gas pressure and gas type are used to better understand the deposition process and optimise it for the fabrication of high-quality thin films designed for optical and optoelectronic applications.

Annealing Effect on the Properties of Cu(In0.7Ga0.3)Se2 Thin Films Grown by Femtosecond Pulsed Laser Deposition

This work reports the crystallization, microstructure, and surface composition of CuIn0.7Ga0.3Se2 (CIGS) thin films grown by femtosecond pulsed laser deposition at different annealing temperatures. The structural and optical properties of the CIGS films were characterized by X‐ray diffraction, Raman scattering, UV‐visible spectroscopy, and Hall effect measurement. The results indicate that binary crystals of CuSe initially formed on the as‐deposited film, but then completely turned into a quaternary chalcopyrite structure after annealing at 400°C. Phase transformation significantly affects the surface morphology, Hall properties, and band gap. Transmission electron microscopy further revealed that an interface between the Mo substrate and CIGS crystallites contains an amorphous layer even at the high temperature of 500°C. For the application of photovoltaic devices, we also report on the photoresponse of both as‐deposited and annealed films as demonstrated by preliminary tests.

Magnetic Behavior of Ni Nanoparticle Films Produced by Two Laser Irradiations in Vacuum

Ni Nanoparticle assembled thin films were prepared using an unconventional approach based on the use of a secondary nanosecond (ns) ultraviolet (UV) laser irradiation interacting with the plume of ablated nanoparticles (NPs) during the femtosecond pulsed laser deposition (fs-PLD). The secondary laser beam determines the reduction of the NPs size and their dispersion by partial vaporization of the NPs during their flight from the target to the substrate. The proper selection of the time delay between fs and ns laser pulses allows the latter to interact selectively with different parts of the NPs plume, controlling, to a certain degree, the reduction of the NPs size and dispersion. Another original effect of the UV laser irradiation is the change in the deposited films topology, due to a reduction of the NP-aggregates density and size, fostering non-uniform dense assemblies of NPs with concentration well above the percolation threshold, with the consequent reduction of the influence of the exchange interactions on the macroscopic magnetic properties. The magnetic behavior of the films prepared using two laser beams with respect to that obtained in the case of fs-PLD only is characterized by higher H(c) values (up to approximately 70%) and a good compromise between the hysteresis loops squareness and moderate exchange interactions, strongly correlated with the NPs topology.

Structural and nanomechanical properties of InN films grown on Si(1 0 0) by femtosecond pulsed laser deposition

The structural and nanomechanical properties of InN films grown on Si(1 0 0) using femtosecond pulsed laser deposition were studied for different growth conditions. Atomic nitrogen was generated by either thermal cracking or laser-induced breakdown (LIB) of ammonia. Optical emission spectroscopy was conducted on the laser plasma and used to observe atomic nitrogen formation. An indium buffer layer was initially grown on the Si substrate at low temperature. The surface structure and morphology were investigated by in situ reflection high-energy electron diffraction, ex situ atomic force microscopy and x-ray diffraction (XRD). The results show that the initial buffer indium layers were terminated with the In(2 × 1) structure and had a smooth surface. With increased coverage, the growth mode developed from two-dimensional layers to three-dimensional islands. At room temperature (RT), formation of submicrometre islands resulted in mixed crystal structure of In and InN. As the substrate temperature was increased to 250–350 °C, the crystal structure was found to be dominated by fewer In and more InN, with only InN formed at 350 °C. The XRD patterns show that the grown InN films have wurtzite crystal structure. The film hardness near the surface was observed to increase from less than 1 GPa, characteristic of In for the sample grown at RT using the thermal cracker, to a hardness of 11 GPa at 30 nm from surface, characteristic of InN for samples grown at 350 °C by LIB. The hardness at deep indents reaches the hardness of the Si substrate of ∼12 GPa.

Femtosecond pulsed laser deposition of cobalt ferrite thin films

The insertion of different elements in the cobalt ferrite spinel structure can drastically change the electric and magnetic characteristics of CoFe2O4 bulks and thin films. Pulsed Laser Deposition (PLD) is a widely used technique that allows the growth of thin films with complex chemical formula. We present the results obtained for stoichiometric and Gadolinium-doped cobalt ferrite thin films deposited by PLD using a femtosecond laser with 1kHz repetition rate. The structural properties of the as obtained samples were compared with other thin films deposited by ns-PLD. The structural characteristics and chemical composition of the samples were investigated using profilometry, Raman spectroscopy, X-Ray diffraction measurements and ToF-SIMS analysis. Cobalt ferrite thin films with a single spinel structure and a preferential growth direction have been obtained. The structural analysis results indicated the presence of internal stress for all the studied samples. By fs-PLD, uniform thin films were obtained in a short deposition time.

Nanostructured molybdenum carbide thin films obtained by femtosecond pulsed laser deposition

AbstractThe films obtained by ultra‐short pulsed laser deposition of a target of molybdenum carbide, at different substrate temperatures, have been characterized by different techniques, including scanning electron microscopy, micro‐Raman spectroscopy and X‐ray photoelectron spectroscopy. The results indicate that the films are formed by a large number of particles with nanomentric dimensions and are composed mainly of Mo2C. The presence of an excess of carbon, found in the films, is probably due the presence of a certain amount of graphite in the target (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Low-temperature growth of Na doped CIGS films on flexible polymer substrates by pulsed laser ablation from a Na containing target

Femtosecond pulsed laser deposition (fs-PLD) was investigated to grow Cu(In,Ga)Se2 (CIGS) films at low temperature (≤300°C), which allowed the use of flexible polymer substrates such as polyimide (PI). X-ray diffraction showed that the films grown on the PI substrates had a pure chalcopyrite phase with a preferred (112) orientation. Because the CIGS films grown on the soda–lime glass substrates were known to have the improved photoelectrical properties owing to the Na diffusion from the substrates, in view of the absence of Na in the polymer substrates, we tried to dope the CIGS films intentionally by adding Na into the fs-PLD targets. The CIGS:Na targets were successfully synthesized from the Cu, In, Ga, Se and NaF powders by the solid state reaction. The results showed that the addition of 5at.% Na in the target did not change the fs-PLD growth parameters notably and well crystallized CIGS:Na films could be grown on the PI substrates, which showed an increased carrier concentration and conductivity.

Interplay between particle anisotropy and exchange interaction in Fe nanoparticle films

The magnetic properties of Fe nanoparticles films were investigated by magnetization measurements and Monte Carlo simulations. The films were produced by femtosecond pulsed laser deposition assisted by irradiation of nanoparticles with a nanosecond UV laser pulse, appropriately delayed, during their flight from the target to the substrate. The films consist of nanoparticles with a disklike shape. The initial magnetization curve exhibits a stepwise behavior, characterized by a reversible plateau followed by rapid irreversible increments. The observed behavior, induced by the peculiar nanoparticle film morphology, is the result of the competition between particle anisotropy, dipole-dipole interactions, and interparticle exchange coupling. This picture is supported by Monte Carlo simulations, satisfactorily reproducing the experimental observations.

Effect of deposition temperature on morphology and magnetic properties of Co 50Fe 50 thin films produced by femtosecond pulsed laser deposition

Deposition temperature effect on morphological, topological and magnetic characteristics of nanoparticle-assembled Co 50Fe 50 films produced by femtosecond pulsed laser deposition (fs-PLD) on Kapton substrate was investigated. For substrate temperature T s ≥ 550 K, a decrement of the nanoparticle's aggregates and an increment of the nanoparticle's density were observed with respect to room temperature deposition; this in association with a strong increase of the magnetoelastic anisotropy energy lead to a reduction of the remanence ratio, a significant rise of the saturation and coercive fields and an enhancement of the saturation magnetization. The results are discussed focusing on: i) the correlation between films structure and their magnetic behavior; ii) the role of the different anisotropy energies in determining the harder in-plane magnetic behavior for T s ≥ 550 K. The thermal strain of Kapton substrate holds an important role in generating morphological and topological characteristics of the fs-PLD films and corresponding magnetic properties.

Femtosecond pulsed laser deposition of Ge quantum dot growth on Si(1 0 0)-(2 × 1)

Ge quantum dots were grown on Si(100)-(2×1) by femtosecond pulsed laser deposition at various substrate temperatures using a femtosecond Ti:sapphire laser. In situ reflection high-energy electron diffraction and ex situ atomic force microscopy were used to analyze the film structure and morphology. The morphology of germanium islands on silicon was studied at different coverages. The results show that femtosecond pulsed laser deposition reduces the minimum temperature for epitaxial growth of Ge quantum dots to ∼280°C, which is 120°C lower than previously observed in nanosecond pulsed laser deposition and more than 200°C lower than that reported for molecular beam epitaxy and chemical vapor deposition.

Diamond-like carbon thin films produced by femtosecond pulsed laser deposition of fullerite

Diamond-like carbon films have been deposited from a fullerite target by ultra-short pulsed laser deposition technique. The results indicate that the films morphology and structure, determined by scanning electron microscopy, atomic force microscopy and energy dispersive X-ray diffraction, depend strongly on the substrate temperature. X-ray photoelectron, X-ray Auger electron, Raman and surface enhanced Raman scattering spectra indicate that the fs-DLC films composition involves a mixed sp, sp2 and sp3 carbon network consisting of aromatic rings and sp3 diamond-like structures linked by chains of different lengths and composition. The films deposited at room temperature, presenting the higher content of sp3 carbon (48%), also contain C60 crystalline phase and show a very high hardness of 49GPa.

Low temperature epitaxial growth of Ge quantum dot on Si(100)-(2×1) by femtosecond laser excitation

Low temperature epitaxy of Ge quantum dots on Si(100)-(2×1) by femtosecond pulsed laser deposition under femtosecond laser excitation was investigated. Reflection high-energy electron diffraction and atomic force microscopy were used to analyze the growth mode and morphology. Epitaxial growth was achieved at ∼70 °C by using femtosecond laser excitation of the substrate. A purely electronic mechanism of enhanced surface diffusion of the Ge adatoms is proposed.

Nanoparticle TiO2 Films Prepared by Pulsed Laser Deposition: Laser Desorption and Cationization of Model Adsorbates

Titanium dioxide is a low-cost photocatalytic material in the chemical industry. This work provides extended insights into the use of TiO2 nanostructures in laser-assisted laser desorption ionization (LDI) of small peptides and synthetic polymers. The investigation evaluates the interfacial morphologies in the nanoscale that enhance the LDI efficiency in nanoparticle-assembled TiO2 films prepared by femtosecond and nanosecond pulsed laser deposition (PLD) at different laser wavelengths. It is shown that PLD provides robust substrates yielding LDI pulse energy thresholds and signal/noise ratios in the mass spectra for detection of cationized adducts that can be comparable to those obtained with TiO2 nanoparticle suspensions and with conventional organic matrices. The best LDI performance is found for nanosecond PLD crystalline substrates of the anatase polymorph and for femtosecond PLD amorphous substrates, with average nanoparticle sizes in both cases in the range of 25−55 nm.

Fabrication of ZnO thin films by femtosecond pulsed laser deposition

The growth of ZnO thin films on sapphire substrate using the femtosecond PLD technique is reported. The effect of substrate temperature and oxygen pressure on the structural properties of the films was studied. Highly c-axis oriented ZnO films can be grown on sapphire substrates under vacuum conditions using the femtosecond PLD process. There is an optimum substrate temperature for the pulsed laser deposition of ZnO film that enhances the thermodynamic stability and allows the formation of well-crystallized thin films. The crystal quality of the films can be further improved by increasing the deposition time and introducing oxygen during the pulsed laser deposition process.

Elastomagnetic and Elastoresistive Effects in CoFe Films Produced by Femtosecond Pulsed Laser Deposition

Femtosecond pulsed laser deposition, carried out in high vacuum, leads the ablation of any target material, producing a plume of nano-drops which are deposited on a substrate as particles, preserving the parent material composition. The deposited nanoparticles (NPs) exhibit a characteristic disk-shape with a major diameter ranging from 5 nm up to 60 nm and a thickness ranging from 1 nm up to 10 nm. Also when the particles form a thick layer, they remain separated by an interface of free volume. This characteristic morphology of the deposited films gives an interplay of interparticle and intraparticle physical correlations which deeply differentiates their properties from those of similar nanogranular films deposited by other techniques. This investigation is focused on the influence of the novel structural conditions on the coupling between strain and magnetization, and/or strain and resistivity, in Co <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">50</sub> Fe <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">50</sub> NP films deposited on Kapton substrate. Due to the NP nature of the films, these couplings are substantially different from standard magnetostrictive and piezoresistive effects. Magnetization and resistivity versus applied strain were studied, thus evidencing novel elastoresistive and elastomagnetic functionalities and evaluating their relative sensitivities. The limits and potentiality for the application of the new NP magnetic films in microelectromechanical systems (MEMS) will be briefly discussed.