Stoichiometric controlling of pulsed laser deposited boron–carbon thin films

The stoichiometry of B–C thin films was controlled via pulsed laser deposition using a series of ceramic B–C targets (B/C ratio was 3.04–5.92). The effects of B/C ratio in target, laser power and substrate-to-target distance on deposition rate, microstructure, stoichiometry and chemical structure were investigated. The maximum deposition rate was obtained at laser power of 90 mJ and substrate-to-target distance of 50 mm. Boron rich B–C films were obtained and the stoichiometry in B–C thin films was controlled in the range 2.9–4.6. Carbon atoms were bonded with only sp³ hybridization when boron was rich,but with sp² and sp³ hybridizations when carbon was rich.     

Fabrication and performance of polymer–nanocomposite anti-reflective thin films deposited by RIR-MAPLE

Design of polymer anti-reflective (AR) optical coatings for plastic substrates is challenging because polymers exhibit a relatively narrow range of refractive indices. Here, we report synthesis of a four-layer AR stack using hybrid polymer:nanoparticle materials deposited by resonant infrared matrix-assisted pulsed laser evaporation. An Er:YAG laser ablated frozen solutions of a high-index composite containing TiO₂ nanoparticles and poly(methyl-methacrylate) (PMMA), alternating with a layer of PMMA. The optimized AR coatings, with thicknesses calculated using commercial software, yielded a coating for polycarbonate with transmission over 97 %, scattering <3 %, and a reflection coefficient below 0.5 % across the visible range, with a much smaller number of layers than would be predicted by a standard thin film calculation. The TiO₂ nanoparticles contribute more to the enhanced refractive index of the high-index layers than can be accounted for by an effective medium model of the nanocomposite.     

Growth of polymer–metal nanocomposites by pulsed laser deposition

Complex polymer–metal nanocomposites have a wide range of applications, e.g. as flexible displays and packaging materials. Pulsed laser deposition was applied to form nanostructured materials consisting of metal clusters (Ag, Au, Pd and Cu) embedded in a polymer (polycarbonate, PC) matrix. The size and amount of the metal clusters are controlled by the number of laser pulses hitting the respective targets. For Cu and Pd, smaller clusters and higher cluster densities are obtained as in the cases of Ag and Au due to a stronger reactivity with the polymers and thus a lower diffusivity. Implantation effects, differences in metal diffusivity and reactivity on the polymer surfaces, and the coalescence properties are discussed with respect to the observed microstructures on PC and compared to the metal growth on poly (methyl methacrylate), PMMA.     

Enhanced field emission from pulsed laser deposited nanocrystalline ZnO thin films on Re and W

Nanocrystalline ZnO thin films have been deposited on rhenium and tungsten pointed and flat substrates by pulsed laser deposition method. An emission current of 1 nA with an onset voltage of 120 V was observed repeatedly and maximum current density ∼1.3 A/cm² and 9.3 mA/cm² has been drawn from ZnO/Re and ZnO/W pointed emitters at an applied voltage of 12.8 and 14 kV, respectively. In case of planar emitters (ZnO deposited on flat substrates), the onset field required to draw 1 nA emission current is observed to be 0.87 and 1.2 V/μm for ZnO/Re and ZnO/W planar emitters, respectively. The Fowler–Nordheim plots of both the emitters show nonlinear behaviour, typical for a semiconducting field emitter. The field enhancement factor β is estimated to be ∼2.15×10⁵ cm⁻¹ and 2.16×10⁵ cm⁻¹ for pointed and 3.2×10⁴ and 1.74×10⁴ for planar ZnO/Re and ZnO/W emitters, respectively. The high value of β factor suggests that the emission is from the nanometric features of the emitter surface. The emission current–time plots exhibit good stability of emission current over a period of more than three hours. The post field emission surface morphology studies show no significant deterioration of the emitter surface indicating that the ZnO thin film has a very strong adherence to both the substrates and exhibits a remarkable structural stability against high-field-induced mechanical stresses and ion bombardment. The results reveal that PLD offers unprecedented advantages in fabricating the ZnO field emitters for practical applications in field-emission-based electron sources.     

Optical properties of PMN–PT thin films prepared using pulsed laser deposition

(1 ? x)Pb(Mg_1/3Nb_2/3)O₃–xPbTiO₃ (PMN–PT) thin films have been deposited on quartz substrates using pulsed laser deposition (PLD). Crystalline microstructure of the deposited PMN–PT thin films has been investigated with X-ray diffraction (XRD). Optical transmission spectroscopy and Raman spectroscopy are used to characterize optical properties of the deposited PMN–PT thin films. The results show that the PMN–PT thin films of perovskite structure have been formed, and the crystalline and optical properties of the PMN–PT thin films can be improved as increasing the annealing temperature to 750 °C, but further increasing the annealing temperature to 950 °C may lead to a degradation of the crystallinity and the optical properties of the PMN–PT thin films. In addition, a weak second harmonic intensity (SHG) has been observed for the PMN–PT thin film formed at the optimum annealing temperature of 750 °C according to Maker fringe method. All these suggest that the annealing temperature has significant effect on the structural and optical properties of the PMN–PT thin films.     

Hardening of smooth pulsed laser deposited PMMA films by heating

Smooth poly(methyl methacrylate) (PMMA) films without any droplets were pulsed laser deposited at a wavelength of 248 nm and a laser fluence of 125 mJ/cm². After deposition at room temperature, the films possess low universal hardness of only 3 N/mm². Thermal treatments up to 200°C, either during deposition or afterwards, lead to film hardening up to values of 200 N/mm². Using a combination of complementary methods, two main mechanisms could be made responsible for this temperature induced hardening effect well above the glass transition temperature of 102°C. The first process is induced by the evaporation of chain fragments and low molecular mass material, which are present in the film due to the ablation process, leading to an increase of the average molecular mass and thus to hardening. The second mechanism can be seen in partial cross-linking of the polymer film as soon as chain scission occurs at higher temperatures and the mobility and reactivity of the polymer material is high enough.     

Synthesis and characterization of Ti–Ni shape memory alloy thin films by pulsed laser deposition

Shape-memory alloys are crucial in various industrial fields. However, a high-quality film synthesis method has not been established yet. Here we examine optimum conditions for synthesis of thin films by pulsed laser deposition of Ti–Ni alloy target in vacuum. We investigated surface morphologies and chemical compositions of the films which were obtained under various conditions. We found that the suitable Ti/Ni ratio was obtained by adjusting the distance between the target and the substrate in vacuum. In parallel, we analyzed plasma plume by optical emission spectroscopy and time-of-flight mass spectrometry. We discuss the basic behavior of ablated particles in vacuum.     

Fabrication of compositionally graded thin films by gravity-assisted pulsed laser ablation

A compositionally graded thin film of FeSi₂ was fabricated by a gravity-assisted pulsed laser ablation (GAPLA) system. By this method, a compositionally graded structure was successfully produced under a gravity field of 5400 G. We demonstrate that the atomic fraction of Fe, the heavier component of the thin film measured by scanning electron microscope/energy dispersive X-ray (SEM-EDX), showed increasing spatial distribution with the direction of gravity. We found that optimal laser fluence exists to give a thin film having the largest possible spatial compositional gradient. We found that surface energy density on the substrate surface is the key parameter to control the composition distribution. Furthermore, the ratio of Fe/Si of the film did not match that of the target. This result shows that the Si component is selectively etched during the film-forming process. Relatively high laser fluence as well as a very narrow space between the target and the substrate are essential to etch the film once it is deposited, in order to re-ionize and etch Si selectively while gravity accelerates both Fe and Si particles to the direction of gravity. We hypothesize that this process accounts for both the change in the stoichiometry and the formation of composition distribution.     

Femtosecond laser interaction with pulsed-laser deposited carbon thin films of nanoscale thickness

The dependence of optical, electronic and thermal penetration zones on the thickness of nanoscale layers grown on silicon wafers is reported. Tetrahedral amorphous carbon (ta-C) and amorphous carbon nitride (a-C_xN_y) films were prepared by inverse pulsed laser deposition (IPLD). Single-pulse modification thresholds for femtosecond laser processing proved to be dependent on the actual film thickness below 60 nm for ta-C and 90 nm for a-C_xN_y. The modification behaviour was governed by multiphoton processes. An effective penetration depth of the laser radiation in a-C_xN_y was of ca. 110 nm in accordance with two-photon absorption. Both the emergence length of ballistic hot electrons and the heat diffusion length are negligible in these thin film materials. The lower bulk value of the threshold fluence of the a-C_xN_y films as compared to ta-C is mainly controlled by optical contributions due to nitrogen-related defects.     

Room temperature pulsed laser deposition of Si x C thin films in different compositions

Amorphous silicon–carbon alloy films in different compositions were prepared by pulsed laser deposition from two-component targets containing pure silicon and carbon parts. The silicon–carbon ratio in the films was varied by adjusting the number of laser shots on the constituent silicon and carbon targets. The composition, optical properties, thickness, and bonding structure of the films were determined by backscattering spectrometry, spectroscopic ellipsometry, and X-ray photoelectron spectroscopy, respectively. Backscattering spectrometry data were used to determine the deposition rate of silicon and carbon. This enabled the calculation of the number of the shots onto each target to reach a predefined composition. As the film composition changed from carbon to silicon, it was shown that the microscopic and macroscopic properties of the films also changed from a diamond-like carbon phase to an amorphous silicon phase via graphite- and silicon-carbide-like composite.     

Temperature dependent structural and optical properties of nanocrystallineCdO thin films deposited by sol–gel process

Nanocrystallites of cadmium oxide (CdO) thin films were deposited by sol–gel dip coating technique on glass and Si substrates. XRD and TEM diffraction patterns confirmed the nanocrystalline cubic CdO phase formation. TEM micrograph of the film revealed the manifestation of nano CdO phase with average particle size lying in the range 1.6–9.3 nm. UV–Vis spectrophotometric measurement showed high transparency (nearly 75% in the wavelength range 500–800 nm) of the film with a direct allowed bandgap lying in the range 2.86–3.69 eV. Particle size has also been calculated from the shift of bandgap with that of bulk value for the films for which the particles sizes are comparable to Bohr exitonic radius. The particle size increases with the increase in annealing temperature and also the intensity of XRD peaks increases which implies that better crystallinity takes place at higher temperature.This revised version was published online in August 2005 with a corrected issue number.     

Fabrication and characterization of pure and Sn/Sb-doped ZnO thin films deposited by sol–gel method

Undoped and doped ZnO thin films were prepared by sol–gel method and deposited on tin-doped indium oxides (ITO) substrate using spin coating technique. The effects of Sn and Sb dopants on structural and optical properties were investigated. The starting material was zinc acetate dihydrate, 2-methoxyethanol was used as solvent and monoethanolamine (MEA) as stabilizer. ZnO films were doped with 2% and 7% Sn and Sb concentrations. Optical measurements show an important effect of Sn and Sb dopants on optical band gap.     

Preparation and characterization of iron–molybdate thin films

Mixed Fe–Mo oxides are used in industrial catalytic processes of selective oxidation of methanol to formaldehyde. For better understanding of the structure-reactivity relationships of these catalysts we aim to prepare well-ordered iron–molybdate thin films as model catalysts. Here we have studied Mo deposition onto Fe₃O₄ (111) thin films produced on Pt(111) as a function of Mo coverage and annealing temperature using LEED, AES, STM and IRAS. At low temperatures, the iron oxide film is covered by Mo = O terminated molybdena nanoparticles. Upon oxidation at elevated temperatures (T > 900 K), Mo species migrate into the film and form new bonds with oxygen in the film. The resulting films maintain the crystal structure of Fe₃O₄, and the surface undergoes a (√3 × √3)R30° reconstruction. The structure is rationalized in terms of Fe substitution by Mo in the surface layers.     

Angle-sensitive and fast photovoltage of silver nanocluster embeded ZnO thin films induced by 1.064-μm pulsed laser

Silver nanocluster embedded ZnO composite thin film was observed to have an angle-sensitive and fast photovoltaic effect in the angle range from –90o to 90o, its peak value and the polarity varied regularly with the angle of incidence of the 1.064-μ m pulsed Nd:YAG laser radiation onto the ZnO surface. Meanwhile, for each photovoltaic signal, its rising time reached ～2 ns with an open-circuit photovoltage of ～2 ns full width at half-maximum. This angle-sensitive fast photovoltaic effect is expected to put this composite film a candidate for angle-sensitive and fast photodetector.     

Sol–gel synthesized powder and pulsed laser deposited film of amorphous indium zinc oxides doped with Fe

Chemical co-precipitation method was used to synthesize nano-structured α-Fe₂O₃-CeO₂ composite by calcination of the goethite–cerium hydroxide precursor. It was observed that the precursor contained goethite matrix doped with cerium. Calcination of the precursor at 400°C showed the formation of nanosize hematite. Mössbauer spectra show the presence of a paramagnetic component in the precursor but not in the samples calcined at 400°C to 800°C temperatures. Our study shows that Ce precipitated as CeO₂ and stuck on the surface of hematite particles. The precipitation of Ce as CeO₂ is independent of the concentration of Ce in the Ce–Fe–O composite.     

Effect of Mg doping and substrate temperature on the properties of pulsed laser deposited epitaxial Zn1?xMgxO thin films

In this paper, we report on the pulsed laser deposition of epitaxial (0002) oriented Zn_1−x Mg _x O thin films onto (0001) sapphire substrate in O₂ ambient at different deposition temperatures. Pulsed laser deposited Zn_1−x Mg _x O films showed (0002) oriented hexagonal wurtzite structure up to 34% of Mg concentration. The bandgap of Zn_1−x Mg _x O thin films is successfully tuned from 3.3 to 4.2 eV by adjusting the Mg concentration x=0.0 to x=0.34. Pulsed laser deposited Zn_1−x Mg _x O thin films were characterized by XRD, AFM, SEM, PL and UV–VIS spectrometer. We have also studied the effect of deposition temperature on to the structure, surface morphology and optical properties of Zn_1−x Mg _x O thin films.