Femtosecond Pulsed Laser Deposition Research Articles

Versatile Synthesis of Nanofoams through Femtosecond Pulsed Laser Deposition

Nanofoam materials are gaining increasing interest in the scientific community, thanks to their unique properties such as ultralow density, complex nano‐ and microstructure, and high surface area. Nanofoams are attractive for multiple applications, ranging from advanced catalysis and energy storage to nuclear fusion and particle acceleration. The main issues hindering the widespread use of nanofoams are related to the choice of synthesis technique, highly dependent on the desired elemental composition and leading to a limited control over the main material properties. Herein, femtosecond pulsed laser deposition is proposed as a universal tool for the synthesis of nanofoams with tailored characteristics. Nanofoams made by elements with significantly different properties—namely, boron, silicon, copper, tungsten, and gold—can be produced by suitably tuning the deposition parameters. The effect of the background pressure is studied in detail, in relation to the morphological features and density of the resulting nanofoams and nanostructured films. This, together with the analysis of the specific features shown by nanofoams made of different elements, offers fresh insights into the aggregation process and its relation to the corresponding nanofoam properties down to the nanoscale, opening new perspectives toward the application of nanofoam‐based materials.

Improving the composition and multifunctional properties of amorphous boron nitride films prepared by post-annealing assisted femtosecond pulsed laser deposition method

Amorphous boron nitride (aBN) materials have similar density to crystalline phases and retain many unique electronic properties, valuable chemical inertness and high thermal stability characteristics. However, the current research on aBN materials has mainly focused on the synthesis and electrical properties of ultrathin aBN films. In this study, we developed a post-annealing assisted femtosecond laser deposition route towards stoichiometric, continuous, and multifunctional aBN films with thickness values of ∼1 μm. A series of boron nitride films were deposited on silicon wafers using a 1030 nm, 300 fs laser with a pulse energy of ∼1 mJ and a high repetition rate of 2 kHz to ablate a hexagonal boron nitride target. The deposited films were then annealed at 900 °C in a nitrogen atmosphere. The structures and chemical compositions of these obtained films were analysed by X-ray diffraction and X-ray photoelectron spectroscopy. Fourier transform infrared and nano-scratch tests were performed to measure the infrared optical and frictional properties of the adhered films. An infrared thermal imager was used to investigate the heat-dissipation performance of these films. The results indicate the components of the aBN film are further purified, the number of large heterogeneous particles is effectively reduced, and the surface becomes smooth after post-annealing treatment. This improvement promotes the transfer of heat flux and increases the transmittance in the mid-infrared light band. The significant effect mechanisms of the post-annealing treatment on the enhancement of the composition and multifunctional properties of aBN films prepared by the femtosecond pulsed laser were provided. The uniform coverage of the aBN films on the substrates, as well as the mid-infrared optical transparency and the protective performance are highly valuable and practical for infrared window protection applications.

A Two-Step Femtosecond Laser-Based Deposition of Robust Corrosion-Resistant Molybdenum Oxide Coating.

A two-step femtosecond-pulsed laser deposition (fs-PLD) process is reported for the rapid development of uniform, poreless, crack-free, and well-adhering amorphous coatings of source materials with a high melting point. The first step comprises a high-rate raw deposition of the source material via fs-PLD, followed by a second step of scanning the raw sample with fs laser pulses of optimized fluence and scan parameters. The technique is applied to develop substoichiometric molybdenum oxide (MoOx, x < 3) coatings on mild steel. The thickness of the layer was ~4.25 μm with roughness around 0.27 μm. Comprehensive surface characterization reveals highly uniform and relatively moderate roughness coatings, implying the potential of these films as robust corrosion-resistant coats. Corrosion measurements in an aqueous NaCl environment revealed that the coated mild steel samples possess an average corrosion inhibition efficiency of around 95% relative to polished mild steel.

The presence of superoxide ions and related dioxygen species in zinc oxide—A structural characterization by in situ Raman spectroscopy

AbstractZinc oxide exhibits unique properties that are reflected in a wide variety of applications, particularly in the field of transparent, conductive films. However, less attention has been paid to their color. Here, we present the synthesis of yellow‐gray ZnO films at room temperature by femtosecond pulsed laser deposition. In situ Raman investigations of these polycrystalline ZnO films reveal the existence of superoxide ions, O2−, in zinc oxide, which are responsible for the yellow color, and are also detected in ZnO powder and single crystals. In addition, further dioxygen species are identified in the samples, including the O2‐molecule. The negative charge excess caused by the dioxygen species creates metallic zinc as a byproduct. Structural analysis reveals an unforced realization of the dioxygen species in the ZnO lattice. Density functional theory (DFT) calculations support the assumed structural displacements as well as the observed, unexpected Raman bands. These results open up completely new insights into the behavior of ZnO.

Femtosecond Laser Deposition of Germanium Selenide onto Silicon Platform at Different Substrate Temperatures.

Germanium selenide (GeSe) thin films were fabricated by employing femtosecond pulsed-laser deposition (fs-PLD) on silicon (100) substrates at various substrate temperatures, ranging from 25 °C to 600 °C. The thin films’ surface morphology qualities and optical properties were studied by utilising transmission electron microscopy (TEM) and X-ray diffraction (XRD). The X-ray diffraction result signifies that the thin films deposited on the silicon at a substrate temperature below 400 °C were amorphous Ge-Se. In contrast, those grown at 400 °C and above exhibited crystallised peaks of Ge-Se orthorhombic and tetragonal structures. The deposition growth rate of the thin films was also found to decrease substantially with increasing substrate temperature. These results show that the fs-PLD process has great potential for fabricating good quality Ge-Se thin film. This technique could enable the manufacture of modern optoelectronic devices for applications in optical communication, sensing, and ovonic threshold switching for the high-density crossbar memory array.

Bottom-up scalable temporally-shaped femtosecond laser deposition of hierarchical porous carbon for ultrahigh-rate micro-supercapacitor

With the accelerated development of electronic devices, micro-supercapacitors (MSCs), as energy storage devices that can charge and discharge quickly, have attracted considerable attention. To improve the rate capability of MSCs with consideration of the energy density remains a challenge. We demonstrated a facile method for the preparation of thin films through bottom-up femtosecond pulsed laser deposition. The femtosecond laser irradiated the polyimide film through a transparent substrate to uniformly sputter the electrode material onto the lower surface of the substrate. We successfully deposited porous amorphous carbon, graphene, and carbon quantum dots with controllable properties by temporally shaping the femtosecond laser. The resulting MSC exhibited an ultrahigh frequency response and good performance at scan rates up to 10,000 V s−1. The characteristic frequency f0 of the MSC was as high as 42,000 Hz, and the relaxation time constant τ0 was 0.0238 ms. The MSC reached an impedance phase angle of −82.6° at a frequency of 120 Hz, an ultrahigh power density of more than 30 kW cm−3, and an energy density of 0.068 W h cm−3. This method provides a novel perspective for the preparation of ultrahigh frequency filters for future miniaturized portable electronic devices.

Low-energy femtosecond pulsed laser deposition of cerium (IV) oxide thin films on silicon substrates

Cerium (IV) oxide (CeO2, ceria, or cerium dioxide) has been investigated for various applications due to its excellent physical and chemical properties. Since high-precision processing of such functional material can be conceived through pulsed laser deposition (PLD) with a low-energy, ultrashort pulsed laser, we report the low-energy (∼ 8.0 nJ) femtosecond PLD (fs-PLD) of CeO2 thin films on silicon (Si) substrates. The CeO2 thin films are deposited for 2.0 to 5.0 h at room temperature with background oxygen (O2) gas and then annealed at 1000 °C for 0.5 h in ambient air. The as-deposited CeO2 film is amorphous and featureless due to the low-energy laser pulse and room-temperature deposition but exhibits good stoichiometry due to the background gas. The film properties likewise improve with post-deposition annealing coupled with a longer deposition time. Despite the change in stoichiometry, the annealed CeO2 films exhibit improved reflectivity and enhanced crystallinity. Although other deposition parameters need to be optimized further and the film stoichiometry entails additional examination, our results show the viability of low-energy fs-PLD for the fabrication of CeO2 thin films and similar functional materials.

Femtosecond Double-Pulse Laser Ablation and Deposition of Co-Doped ZnS Thin Films.

Nanostructured thin films of Co-doped zinc sulfide were synthesized through femtosecond pulsed laser deposition. The scheme involved ablation of physically mixed Co and ZnS with pairs of ultrashort pulses separated in time in the 0–300 ps range. In situ monitorization of the deposition process was carried out through a simultaneous reflectivity measurement. The crystallinity of generated nanoparticles and the inclusion of Co in the ZnS lattice is demonstrated by transmission electron microscopy and energy dispersive X-ray microanalysis (TEM-EDX) characterization. Surface morphology, Raman response, and photoluminescence of the films have also been assessed. The role of interpulse temporal separation is most visible in the thickness of the films obtained at the same total fluence, with much thicker films deposited with short delays than with individual uncoupled pulses. The proportion of Co in the synthesized doped ZnS nanoparticles is found to be substantially lower than the original proportion, and practically independent on interpulse delay.

The effect of molecular weight and deposition temperature on the formation of poly(ethylene oxide) films using the femtosecond pulsed laser deposition

Poly(ethylene oxide) (PEO) is a water-soluble, linear polymer with a high quantity of available molecular weights and is in many ways used in daily life. Coatings and films are an important issue for these applications. In this study, PEO films with average molecular weights of 1500, 2000, 4000, 20,000, and 200,000 g/mol were deposited by femtosecond pulsed laser deposition (PLD) on substrates held at 25 and −190°C. At room temperature, crystalline PEO films are formed with properties that coincide well with the precursor materials including the films of PEO-200,000, which contains chain lengths of around 1 μm. The deposition of PEO polymers at −190°C hinders the ordering in the films up to X-ray amorphous films for both PEO-20,000 and PEO-200,000. Upon warming to room temperature, the films crystallizes in the expected configuration. Furthermore, the analysis of the ejected molecules during the ablation process demonstrates the favorable light matter interaction of the ultrashort laser pulses. The results reveal the high potential of the femtosecond PLD as an easy and clean pathway for the preparation of intact polymer films.

High repetition rate deposition of boron nitride films using femtosecond pulsed laser

Cubic (c-BN), and hexagonal (h-BN) boron nitride thin films are of interest in many applications and industries because of their unique mechanical, thermal and chemical properties. In this work, we investigate high repetition rate deposition of BN films using femtosecond pulsed laser deposition. Boron nitride (BN) films were deposited on silicon wafers using 800 nm, 100 fs Ti:sapphire femtosecond laser with 2.4 mJ pulse energy and high repetition rate of 1 kHz using a c-BN target. The deposited films were analyzed using transmission electron microscopy (TEM), scanning electron microscope (SEM), and optical profilometer. Nano-indentation tests were performed to measure the hardness of the adhered film. The results indicate the influence of the high repetition rate on the film growth, crystalline arrangement and adhesion. The experimental work is utilized to identify the process parameters that can be used in pulsed laser deposition (PLD) process to grow thick and adherent BN films.

Phase evolution, morphological, optical and electrical properties of femtosecond pulsed laser deposited TiO2 thin films

In this paper, we report anatase and rutile titanium oxide (TiO2) nanoparticulate thin films fabricated on silica and Indium Tin Oxide (ITO) substrates using femtosecond pulsed laser deposition (fs-PLD). Depositions were carried-out at substrate temperatures of 25 °C, 400 °C and 600 °C from anatase and rutile phase target materials. Effect of substrate temperature on the surface morphology, microstructural, optical, and electrical properties of these films were systematically investigated by using various range of measurements such as scanning electron microscopy, (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), Raman spectroscopy, Ultraviolet–visible-near infrared (UV–Vis–NIR) spectroscopy, and Hall Effect measurements. It is observed that the TiO2 thin films surface are predominated with nanoparticulates of diameter less 35 nm, which constitute about ~ 70%; while the optical bandgaps and electrical resistivity decrease with increasing substrate temperature. A mixed-phase (anatase/rutile) TiO2 thin film was produced at a substrate temperature of 400 °C when samples are fabricated with anatase and rutile target materials. The results of this study indicate that the structural and crystallinity, optical, and electrical properties can be controlled by varying fs-PLD process parameters to prepare TiO2 thin films, which are suitable for applications in photovoltaics, solar cells, and photo-catalysis.

Phase changeable vanadium dioxide (VO2) thin films grown from vanadium pentoxide (V2O5) using femtosecond pulsed laser deposition

There are significant challenges accompanied by fabricating a pure crystalline VO2 (M1) thin film with an abrupt metal to insulator phase change properties. Most fabrication methods yield an amorphous VO2 thin film that requires a post-annealing process to be converted into crystalline VO2 (M1). Hence, the thickness of VO2 (M1) films produced is very limited. In this work, we report the growth of pure VO2 (M1) crystalline thin films onto a sapphire substrate in an oxygen atmosphere by the femtosecond pulsed laser deposition technique and using vanadium pentoxide (V2O5) as an ablation target. The thin films were deposited at substrate temperatures of 25 °C, 400 °C, and 600 °C, which reveal the crystallized structures of VO2 (M1) without post-annealing. The thin film deposited at a substrate temperature of 600 °C exhibits a sharp and an abrupt metal-to-insulator transition (MIT) at a temperature of 66.0 ± 2.5 °C with nearly four orders of magnitude of the resistivity change (3.5 decades) and a narrow MIT hysteresis width of 3.9 °C. Furthermore, the influence of the substrate temperature, nanoparticle or grain size, and film thickness on the MIT parameters such as sharpness of the transition temperature, hysteresis width, and amplitude are discussed for potential applications of tunable antennas, terahertz planar antennas, and RF-microwave switches.

Nanocrystalline lanthanum boride thin films by femtosecond pulsed laser deposition as efficient emitters in hybrid thermionic-photovoltaic energy converters

Lanthanum hexaboride (LaB6)-based thin films were successfully synthesized via femtosecond Pulsed Laser Deposition (fs-PLD) at room temperature and at high growth rate (>110 nm/min) for acting as electron and photon emitters in hybrid thermionic-photovoltaic devices applied in thermal-to-electrical energy conversion. The physical and chemical properties of the films were systematically investigated as a function of the most relevant deposition parameters (laser pulse repetition rate, pressure of the reactor, and deposition time) aimed at optimizing the material’s microstructure and functional optical and electronic properties. Specific growth conditions allowed for the achievement of thin films consisting of nanocrystalline LaB6 (10–20 nm grain-size) with enhanced spectral emissivity and low work function of (2.64 ± 0.03) eV, evaluated by thermionic emission measurements up to 1860 K, where a thermionic current density of 1.78 A cm−2 was measured. The obtained results demonstrate that fs-PLD represents a novel and rapid method for preparing efficient, and low-cost thermal electron emitters for innovative hybrid thermionic conversion devices operating at temperatures close to 2000 K.

Structural, Spectroscopic, and Excitonic Dynamic Characterization in Atomically Thin Yb3+‐Doped MoS2, Fabricated by Femtosecond Pulsed Laser Deposition

AbstractThe large area deposition and synthesis of 10 mm × 10 mm atomically thin Yb3+‐doped MoS2 films by femtosecond pulsed laser deposition on a silica glass optical platform for device applications are demonstrated for the first time. The presence of Yb3+‐ion doping is confirmed using photoluminescence (PL), X‐ray photoelectron spectroscopy (XPS), and Raman spectroscopy. The Yb3+‐doped MoS2 films, when excited with a 976 nm laser, exhibit room temperature PL with a peak at 1002 nm. The XPS and Raman spectroscopic analyses of the Yb3+‐doped and undoped films show that the deposited films are a mixture of 2H‐ and 1T‐MoS2 after postdeposition annealing at 500 °C. The density functional theory analysis shows that the 1T phase is metastable by +77 kJ (≈0.8 eV) mol‐1, when compared with the 2H state at 0 K. Ultrafast transient nonlinear optical spectroscopic measurements prove that the saturable absorption of undoped MoS2 is significantly modified after Yb3+‐ion doping, by displaying dopant‐host structure charge transfer. The complex transient absorption line shape shows a combination of bleach (negative) signals at the A (670 nm) and B (630 nm) exciton energies, and a strong induced absorption below the A exciton level. The results presented herein provide critical insight in designing novel rare‐earth‐ion doped 2D materials and devices.

The role of samarium incorporated structural defects in ZnO thin films prepared by femtosecond pulsed laser deposition

Samarium, a rare-earth element is used as an impurity in ZnO to study the effects it causes in structure thereby changing the optical and electrical characteristics of ZnO thin films. Samarium zinc oxide (SmxZn1-xO, SZO) films are deposited on glass substrates employing femtosecond pulsed laser deposition (fs-PLD). XRD results show that all films are crystalline and prefer c-axis growth. 1 wt% Sm is plausible for doping in ZnO and 2 wt% samarium addition results in reaction among impurity and host atoms forming a compound ZnSm2O4. Higher concentrations lead to defect generation and decline of crystalline quality. AFM results show that surface roughness is increased with samarium incorporation in addition to affecting particle size and shape. These microstructural variations significantly affect the optical transmittance, reflectance and band gap energies of thin films which are calculated using spectroscopic ellipsometry data. Presence of intrinsic defects and extrinsic defects caused by samarium is revealed by Photoluminescence Spectroscopy. Electrical resistivity of SZO films is significantly affected with Sm concentration in nanocrystalline ZnO. These types of films are suitable for use in optical devices for UV and blue emission.

Boron film laser deposition by ultrashort pulses for use as neutron converter material

This study investigated the production of boron films by femtosecond pulsed laser deposition (PLD) to be used as converters on bulk semiconductor neutron detectors. The ablation threshold of metallic boron was determined and the film growth was studied as a function of deposition time (5–90 min) and laser pulse energy (35–530 μJ). The films were characterized by scanning electron microscopy (SEM), revealing a flaky morphology, optical profilometry, which determined the films thicknesses (from 80 nm up to 4 μm), Ion Beam Analysis (IBA) that assessed their elemental composition and X-ray diffraction (XRD), which revealed an amorphous structure. In addition, a thermal load study was performed to evaluate the heat flux onto the substrate during deposition process. Stable boron films obtained show that the femtosecond PLD process is reliable and reproducible for the fabrication of thick boron coatings.

Erbium-doped chalcogenide glass thin film on silicon using femtosecond pulsed laser with different deposition temperatures

The properties of Er3+-doped gallium lanthanum sulphide thin films prepared on a silicon substrate by femtosecond pulsed laser deposition were studied as a function of process temperature. The films were characterised using transition electron microscopy imaging, X-ray diffractometry, Raman spectroscopy, fluorescence spectroscopy, and UV–Vis–NIR spectroscopy. The results show that by increasing the substrate temperature, the deposited layer thickness increases and the crystallinity of the films changes. The room temperature photoluminescence and lifetimes of the 4I13/2→4I15/2 transition of Er3+ are reported in the paper.

Thermal-stable void-free interface morphology and bonding mechanism of low-temperature Cu-Cu bonding using Ag nanostructure as intermediate

To lower the bonding temperature and save the time of the Cu-Cu bonding applied for 3D integration, Ag nanostructure was deposited on the Cu pads as intermediate by femtosecond pulsed laser deposition (PLD). The interface morphology after bonding including oxidation detection and microstructure evolution during high-temperature storage was investigated. The underlying bonding mechanisms were then analyzed and concluded. The element distribution of Ag nanostructures after PLD deposition is detected using the scanning transmission electron microscope equipped with energy dispersive X-ray spectroscopy mapping, and no oxygen is found inside the nanostructures. The electron energy loss spectroscopy shows that no oxidation happens across the Cu-Ag-Cu bonding interfaces during the bonding process by observing the same chemical state of Ag and Cu as the pure metal atom. The as-bonded Cu-Ag-Cu samples were stored at the temperature of 200 °C for 500 h and the microstructure at the bonding interface was observed. Based on the interface microstructure and crystal structure, the physical mechanisms of Cu-Cu bonding using Ag nanostructure as intermediate was explored and summarized in detail. The mechanical interlock between Ag nanostructures, large plastic deformation, surface melting and bulk diffusion are found to simultaneously contribute to the bonding.

Erbium-doped glass nanoparticle embedded polymer thin films using femtosecond pulsed laser deposition

Polymer materials doped with rare-earth ions are promising candidates for the formation of low-cost integrated optical amplifiers. However, there are significant technical challenges associated with the integration of these dissimilar materials and the formation of high-quality Er-doped polymer films. In this paper, therefore, we present for the first time the fabrication of polymer thin layers modified with erbium-doped sodium zinc tellurite (Er-TZN) glass nanoparticles (NPs) using femtosecond (fs) pulsed laser deposition (PLD) and the characterisation of their basic properties. The surface morphology and the compositional and structural characteristics of the samples produced with this method are evaluated using scanning and transmission electron microscopy (SEM and TEM) and X-ray diffraction (XRD), while photoluminescence (PL) measurements are carried out at room temperature under a 980 nm laser diode excitation. The studies indicate that the Er-doped TZN NPs are successfully integrated in the polymer layers. The obtained average NP size is measured to be in the range of 12 to 21 nm depending on the fabrication parameters, while broad PL emission at 1534 nm that corresponds to the 4I13/2 →4I15/2 transition of Er3+ is observed from the samples. The full-width at half-maximum (FWHM) of the PL spectra is found to be ~39 nm while the fluorescence lifetime is measured to be in the range of 3.52 to 4.18 ms. The obtained results clearly demonstrate the potential to efficiently dope polymer layers with glass NPs using fs-PLD and is a first step towards the successful formation of hybrid polymer-glass waveguide amplifiers.

Wettability modification of porous PET by atmospheric femtosecond PLD

In this study, porous structures were created on poly(ethylene terephthalate) (PET) by femtosecond (fs) laser micromachining. While such structures offer a texture that is desirable for several applications, their wettability does not always match the application in question. The aim of this investigation is to tune the wettability of such surfaces by incorporating a controlled amount of nanoparticles into the structure. The machined PET samples were thus used as substrates for fs pulsed laser deposition (PLD) of titanium under ambient conditions. The nanoparticles were deposited as nanochain clusters due to the formation of an oxide layer between individual nanoparticles. The stability of nanoparticle incorporation was tested by placing the samples in an ultrasonic ethanol bath. Results indicated that nanoparticles were still successfully incorporated into the microstructure after sonication. Nanoparticle surface coverage was observed to be controllable through the operating fluence. The dynamic contact angles of the resulting composite surface were observed to decrease with increasing titanium incorporation. Therefore, this work highlights atmospheric fs PLD as a method for wettability modification of high surface area microstructures without undermining their topology. In addition, this technique uses almost the same equipment as the machining process by which the microstructures are initially created, further highlighting its practicality.