Femtosecond Laser Deposition Research Articles

Airflow Triggered Water Film Self-Sculpturing on Femtosecond Laser-Induced Heterogeneously Wetted Micro/Nanostructured Surfaces.

Liquid manipulation is essential for daily life and modern industry, and it is widely used in various fields, including seawater desalination, microfluidic robots, and biomedical engineering. Nevertheless, the current research focuses on the manipulation of individual droplets. There are a few projects for water film management. Here, we proposed a facile method of wind-triggered water film self-sculpturing based on a heterogeneous wettability surface, which is achieved by the femtosecond laser direct writing technology and femtosecond laser deposition. Under the conditions of various airflow velocities and water film thicknesses, three distinct behaviors of the water film were analyzed. As a result, when the water film thickness is lower than 4.9 mm, the self-sculpture process will occur until the whole superhydrophobic surface dewetting. Four potential applications are demonstrated, including encryption, oil containers, reconfigurable patterning, and self-splitting devices. This work provides a new approach for manipulating a water film of fluid control engineering.

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.

Discrimination of different amorphous carbon by low fluence laser irradiation

Carbon thin film depositions were carried out with femtosecond laser ablation from two different sp 2 -bonded carbon targets, namely highly oriented pyrolytic graphite (HOPG) and glassy carbon (GC). Femtosecond laser deposition is generally known as a means of generating nanoparticles. To our knowledge, Raman spectroscopy is not able to directly discriminate the as-deposited amorphous films which may contain different sp 2 carbon nanoclusters while still showing similar Raman spectra. Moreover, these films are not easily distinguishable from each other by transmission electron microscopy observation due to their amorphous structure. In this work, we show that controlled laser irradiation of thin films at low fluence is an interesting approach to reveal the existence of structural differences for amorphous films deposited from sp 2 carbon targets (HOPG and GC).

Infrared optical properties modulation of VO2 thin film fabricated by ultrafast pulsed laser deposition for thermochromic smart window applications

Over the years, vanadium dioxide, (VO2(M1)), has been extensively utilised to fabricate thermochromic thin films with the focus on using external stimuli, such as heat, to modulate the visible through near-infrared transmittance for energy efficiency of buildings and indoor comfort. It is thus valuable to extend the study of thermochromic materials into the mid-infrared (MIR) wavelengths for applications such as smart radiative devices. On top of this, there are numerous challenges with synthesising pure VO2 (M1) thin films, as most fabrication techniques require the post-annealing of a deposited thin film to convert amorphous VO2 into a crystalline phase. Here, we present a direct method to fabricate thicker VO2(M1) thin films onto hot silica substrates (at substrate temperatures of 400 °C and 700 °C) from vanadium pentoxide (V2O5) precursor material. A high repetition rate (10 kHz) femtosecond laser is used to deposit the V2O5 leading to the formation of VO2 (M1) without any post-annealing steps. Surface morphology, structural properties, and UV–visible optical properties, including optical band gap and complex refractive index, as a function of the substrate temperature, were studied and reported below. The transmission electron microscopic (TEM) and X-ray diffraction studies confirm that VO2 (M1) thin films deposited at 700 °C are dominated by a highly texturized polycrystalline monoclinic crystalline structure. The thermochromic characteristics in the mid-infrared (MIR) at a wavelength range of 2.5–5.0 μm are presented using temperature-dependent transmittance measurements. The first-order phase transition from metal-to-semiconductor and the hysteresis bandwidth of the transition were confirmed to be 64.4 °C and 12.6 °C respectively, for a sample fabricated at 700 °C. Thermo-optical emissivity properties indicate that these VO2 (M1) thin films fabricated with femtosecond laser deposition have strong potential for both radiative thermal management or control via active energy-saving windows for buildings, and satellites and spacecraft.

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.

A femtosecond laser-assembled SnO2 microbridge on interdigitated Au electrodes for gas sensing

We unexpectedly found a femtosecond laser deposition (fs-LD) technique for single-step fabricating Tin-oxide gas sensors coupling with peripheral interdigitated electrodes. Photons hit the precursor for an ultrafast nucleation and crystal growth at a nanometric voxel to grow non-directional SnO2 nanocrystal. We utilized the tunable laser exposure dose for morphological control, and the SnO2 nanospheres were optically assembled into micro-structured bridges interconnecting metal electrodes. Au-based interdigital electrodes (thickness < 20 nm) contacted the laser-deposited SnO2 layer forming a hetero-material interfacial area. More quick passages to absorb and desorb gas were created by the gap during nanomaterials and bumpy surfaces. The nanostructured gas sensor exhibited abundant surface oxygen vacancies, desirable gas and ultraviolet light sensitivity, and the capability of detecting analytes (hydrogen sulfide, ethanol, and methanol) down to 20 ppb within 10 s at a footprint area of < 300 × 300 μm2. Results validated the reliability of the invented fs-LD technique, which promised more micro/nanoengineered, shape/function-programmable SnO2 sensors beyond demonstrations here.

Structural characterization of nanoparticles-assembled titanium dioxide films produced by ultrafast laser ablation and deposition in background oxygen

Ultrafast laser ablation of titanium dioxide and deposition of nanoparticles-assembled films in oxygen ambient gas at pressures going from high-vacuum up to several mbar is investigated. We identify various regimes of the plumes propagation into the background gas as well as of the material deposition rate. These reflect on the structural characteristics of the nanoparticles-assembled films: the film morphology changes from a structure with glue-like nanoparticulates, at low pressure, to a highly porous assembly of individual nanoparticles, at larger pressure. Our findings indicate that background gas pressure provides an interesting key for additional control on the structural characteristics of oxide nanostructures produced by femtosecond laser deposition.

Femtosecond laser deposition of TiO 2 by laser induced forward transfer

Femtosecond lasers have been used for laser induced forward transfer (LIFT) of TiO 2, a wide-band semiconductor with many industrial and research applications. TiO 2 polycrystalline thin films on quartz (obtained by pulsed laser deposition) were used as donors and both quartz and fluorine-doped tin dioxide coated glass substrates as acceptors. LIFT was performed at the laser wavelengths of 248 and 800 nm with pulses of 450 and 300 fs respectively. The transferred material was characterized by energy-dispersive X-ray spectroscopy, X-ray diffraction and micro-Raman spectroscopy to determine the composition and crystalline quality, and by scanning electron microscopy and atomic force microscopy to assess the surface morphology. The relation between these properties and the laser transfer conditions, including wavelength, pulse energy and acceptor substrate, are presented.

Femtosecond laser deposited zinc oxide film and its optical properties

Zinc oxide (ZnO) thin films have been grown on Si (100) substrates using a femto-second pulsed laser deposition (fsPLD) technique. The effects of substrate temperature and laser energy on the structural, surface morphological and optical properties of the films are discussed. The X-ray diffraction results show that the films are highly c-axis oriented when grown at 80 °C and (103)-oriented at 500 °C. In the laser energy range of 1.0 mJ–2.0 mJ, the c-axis orientation increases and the mean grain size decreases for the films deposited at 80 °C. The field emission scanning electron microscopy indicates that the films have a typical hexagonal structure. The optical transmissivity results show that the transmittance increases with the increasing substrate temperature. In addition, the photoluminescence spectra excited with 325 nm light at room temperature are studied. The structural properties of ZnO films grown using nanosecond (KrF) laser are also discussed.

Nanostructure and sp1/sp2 clustering in tetrahedral amorphous carbon thin films grown by femtosecond laser deposition

Diamondlike amorphous carbon films have been deposited on silicon and quartz substrates by laser ablation of graphite using 120 fs pulses from an amplified Ti:sapphire laser operating at 800 nm. Ultraviolet/visible (UV/VIS) and micro-Raman spectra of these materials have shown that the sp3-bonded carbon fraction in these films is ≈27%, 55%, and 20% when deposition occurs at substrate temperatures of 77, 300, and 573 K, respectively. The presence of sp1 chains in these films is indicated by the appearance of an excitation band at 2000–2100 cm−1 in UV-Raman spectra. We also find a remarkable increase in the Tauc energy gap calculated from in situ UV/VIS optical spectra immediately after exposure to air together with a 1 eV redshift of the C 1s core-level energy in x-ray photoelectron spectra of these samples. The properties of sp1-, sp2- and sp3-bonded components of these materials have also been studied using UV/VIS Raman spectroscopy. The enhanced stability of sp1 chains in tetrahedral carbon matrix is discussed. The present study sheds light on novel tetrahedral carbon materials embedded with both sp1 chains and sp2 clusters.

Particulate Filtering Upon Pulsed Femtosecond Laser Deposition

Pulsed femtosecond laser deposition has been shown to suffer from a severe particulate problem. However, in contrast to pulsed nanosecond laser deposition, particulates are not ejected as molten droplets, but as condensed clusters formed in the highly dense ablation plasma. In order to utilize the advantages of pulsed ultrashort laser deposition, three strategies for particulate avoidance are experimentally demonstrated: electrostatic filtering, scattering at a background gas, and magnetic filtering. Ultrathin layers prepared by these three techniques are characterized by scanning and transmission electron microscopy and promise for future developments are outlined.

Structural properties of ZnO thin films on Si substrate using femtosecond laser deposition

The effects of experimental parameters such as substrate temperature, laser intensity and pressure of oxygen on the structural properties of zinc oxide (ZnO) films using femtosecond laser deposition were studied in this paper. The c-axis-oriented ZnO films were obtained under the optimized conditions with the temperature of substrate and the pressure of oxygen being 80 °C and 1×10 −3 Pa, respectively. The structural properties were analyzed by X-ray diffraction and surface morphology of films was analyzed by field-emission scanning electron microscopy, and the results showed that the prepared films are of fine grains (30–50 nm) and of even density. Furthermore, the optical transmissivity of annealed ZnO films were also studied in the range from 200 to 900 nm.

Optical characterization of β-FeSi 2 thin films prepared on fused quartz by femtosecond laser ablation

Single-phase β-FeSi 2 thin films have been grown on quartz substrates using femtosecond laser deposition (800 nm, 50 fs, 1 KHz) under gas pressure of 3.0×10 −4 Pa. X-ray diffraction (XRD) and field-emission scanning electron microscopy (SEM) were used to determine the structural properties and surface images of the films. Typical XRD patterns of the film showed that no other diffraction peak except β-FeSi 2 was found. The SEM results indicated that the films were composed of well-distributed grains, in the range 50–150 nm in diameter. In addition, normal incidence spectral transmittance and reflectance data suggested that the β-FeSi 2 film has a direct energy gap of about 0.85 eV. The thickness of the layer and the refractive index of the film were determined by performed calculation in the wavelength range 1.9–2.7 μm. Furthermore, the Raman spectra of the films were also discussed.

Femtosecond pulsed laser deposition of thin films upon direct and inverse transfer of ablated particles of foam graphite in the nitrogen atmosphere

The comparative study of the preparation of carbon films by the methods of direct and inverse femtosecond laser deposition in the nitrogen atmosphere is performed for the first time. It is found that the surface of the film prepared by the method of inverse laser deposition does not virtually contain microparticles, whereas the surface of the directly deposited film is covered by many microparticles. The dependence of the film-surface roughness parameter on the radius (the distance to the centre of the deposition region) is measured. It is shown that the surface roughness of the film on the inverse collector is smaller than that on the direct collector. The ablation rates of foam graphite and volumes of the material ejected and deposited per laser shot are determined.

The influence of the silicon substrate temperature on structural and optical properties of thin-film cadmium sulfide formed with femtosecond laser deposition

CdS thin films have been grown on Si(1 1 1) substrates using femtosecond pulsed laser deposition (PLD). The crystalline quality and the surface morphology of the deposited CdS thin films have been characterized using X-ray diffraction (XRD) and atomic force microscopy (AFM). The photoluminescence (PL) of the CdS thin films was investigated by fluorescence spectra analysis. The influence of the substrates temperature in the range 100–600 °C on the structural and optical characterizations of CdS thin films have been systematically studied. The XRD and PL analysis show that the crystalline quality of the CdS thin films can be improved with increasing the substrates temperature to 450 °C, but further increase of the substrates temperature to 600 °C leads to the degradation of the crystalline quality. AFM results show that the surface roughness of CdS thin films can be increased with the substrates temperature increasing. In the corresponding fluorescence spectra, a blue shift of the CdS emission band is found, which may be a result of the quantum size effect.

Single-phase β-FeSi 2 thin films prepared on Si wafer by femtosecond laser ablation and its photoluminescence at room temperature

Single-phase β-FeSi 2 thin films were prepared on Si(100) and Si(111) wafers by using femtosecond laser deposition with a FeSi 2 alloy target for the first time. X-ray diffraction (XRD), field scanning electron microscopy (FSEM), scanning probe microscopy (SPM), electron backscattered diffraction pattern (EBSD), and Fourier-transform Raman infrared spectroscopy (FTRIS) were used to characterize the structure, composition, and properties of the β-FeSi 2/Si films. The orientation of β-FeSi 2 grains was found to depend on the orientation of the Si substrates, and photoluminescence at wavelength of 1.53 μm was observed from the single-phase β-FeSi 2/Si thin film at room temperature (20 °C).

Bonding configurations in amorphous carbon and nitrogenated carbon films synthesised by femtosecond laser deposition

The effect of nitrogen addition and laser fluence on the atomic structure of amorphous carbon films (a-C) synthesized by femtosecond pulsed laser deposition has been studied. The chemical bonding in the films was investigated by means of X-ray photoelectron (XPS) and Raman spectroscopies. XPS studies revealed a decrease in the sp3 bonded carbon sites and an associated increase in the N-sp2C bonding sites with increasing nitrogen content in the CNx films. An increase in laser fluence from 0.36 to 1.7 J/cm2 led to a rise in sp3C sites. These results were further confirmed by Raman spectroscopy. The ID/IG ratio increased monotonically and G line-width decreased with the increase of nitrogen content in the films indicating a rise in either the number or the size of the sp2 clusters. Furthermore a visible excitation wavelength dependence study established the resonant Raman process in a-C and CNx films.

High intensity femtosecond laser deposition of diamond-like carbon thin films

Hydrogen-free diamond-like carbon (DLC) films have been deposited with a 100 fs (FWHM) Ti:sapphire laser beam at intensities I in the 1014–1015 W/cm2 range. The films were studied with scanning probe microscopy, variable angle spectroscopic ellipsometry, Raman spectroscopy, and electron energy loss spectroscopy. DLC films with good scratch resistance, excellent chemical inertness, and high optical transparency in the visible and near infrared range were deposited at room temperature. As the laser intensity was increased from 3×1014 to 6×1015 W/cm2, the films showed an increased surface particle density, a decreased optical transparency (85%→60%), and Tauc band gap (1.4→0.8 eV), as well as a lower sp3 content (60%→50%). The time-of-flight spectra recorded from the laser plume exhibited a double-peak distribution, with a high energy suprathermal ion peak preceding a slower thermal component. The most probable ion kinetic energy showed an I0.55 dependence, increasing from 300 to 2000 eV, when the laser intensity was varied from 3×1014 to 6×1015 W/cm2, while the kinetic energy of suprathermal ions increased from 3 to over 20 keV and showed an I0.33 dependence. These high energy ions are believed to have originated from an electrostatic acceleration field established by suprathermal electrons which were formed by resonant absorption of the intense laser beams.