Nanostructured silver thin films deposited by pulsed laser ablation

Nanostructured thin films were deposited by excimer laser ablation of silver targets in controlled atmospheres of He and Ar. The film structural properties were investigated by means of scanning electron microscope and transmission electron microscope imaging. The film growth mechanism was identified as the result of coalescence of nanometric clusters formed during plume flight. Cluster formation involves plume confinement as a consequence of the increased collisional rate among plasma species. Fast photography imaging of the laser-generated silver plasma allowed to identify plasma confinement, shock wave formation and plasma stopping.     

Synthesis by pulsed laser ablation in Ar and SERS activity of silver thin films with controlled nanostructure

Thin silver films were deposited by pulsed laser ablation in a controlled Ar atmosphere and their SERS activity was investigated. The samples were grown at Ar pressures between 10 and 70 Pa and at different laser pulse numbers. Other deposition parameters such as laser fluence, target to substrate distance and substrate temperature were kept fixed at 2.0 J/cm², 35 mm and 297 K. Film morphologies were investigated by scanning and transmission electron microscopies (SEM, TEM). Surface features range from isolated nearly spherical nanoparticles to larger islands with smoothed edges. Cluster growth is favored by plume confinement induced by background gas. After landing on the substrate clusters start to aggregate giving rise to larger structures as long as the deposition goes on. Such a path of film growth allows controlling the surface morphology as a function of laser pulse number and Ar pressure. These two easy-to-manage process parameters control the number density and the average size of the as-deposited nanoparticles. We investigated the influence of substrate morphologies on their surface enhanced Raman scattering properties. Raman measurements were performed after soaking the samples in rhodamine 6G aqueous solutions over the concentration range between 1.0 × 10⁻⁴ and 5.0 × 10⁻⁸ M. The sensitivity of the film SERS activity on the surface features is put into evidence.     

SERS activity of pulsed laser ablated silver thin films with controlled nanostructure

The surface‐enhanced Raman scattering (SERS) activity of silver thin films deposited by the pulsed laser ablation technique was investigated. The samples were grown in a controlled Ar atmosphere at pressures ranging between 10 and 70 Pa, and changing the number of laser pulses. Different surface morphologies, from isolated nearly spherical nanoparticles (NPs) to larger islands with smooth edges, were observed by means of scanning and transmission electron microscopies, as a function of the different deposition conditions adopted. SERS measurements were performed by soaking the samples in rhodamine 6G aqueous solutions over the concentration range between 1.0 × 10⁻⁴ and 5.0 × 10⁻⁸ M . Raman spectra were acquired using both the 632.8 and 514.5 nm excitation sources. The dependence of the SERS activity of the samples on the observed surface morphology is presented and discussed. The presence of the so called hot spots is envisaged. Copyright © 2011 John Wiley & Sons, Ltd.     

Electrical properties of boron-doped diamond-like carbon thin films deposited by femtosecond pulsed laser ablation

We report on electrical measurements and structural characterization performed on boron-doped diamond-like carbon thin films deposited by femtosecond pulsed laser deposition. The resistance has been measured between 77 and 300 K using four probe technique on platinum contacts for different boron doping. Different behaviours of the resistance versus temperature have been evidenced between pure DLC and boron-doped DLC. The a-C:B thin film resistances exhibit Mott variable range hopping signature with temperature. Potential applications of DLC thin films to highly sensitive resistive thermometry is going to be discussed.     

Deposition of NbC thin films by pulsed laser ablation

Niobium carbide thin films were prepared by pulsed laser ablation of a stoichiometric NbC target. XeCl (308 nm, 30 ns) and Nd:YAG (266 nm, 5 ns) lasers operating at a repetition rate of 10 Hz were used. Films were deposited on Si (100) substrates at room temperature either in vacuum or in an argon atmosphere (2᎒^-1 mbar). Different laser fluences (2, 4 and 6 J/cm²) and different numbers of pulses (1᎒⁴, 2᎒⁴ and 4᎒⁴) were tested. For the first time, NbC films were prepared through a clean procedure without the addition of a hydrocarbon atmosphere. The phase constitution of the films, unit cell size, mean crystallite dimensions and preferred orientation are determined as a function of deposition conditions by X-ray diffraction. Complementary morphological and structural analysis of the films were performed by scanning electron microscopy, atomic force microscopy and Rutherford backscattering spectroscopy.     

Thin films of silver nanoparticles deposited in vacuum by pulsed laser ablation using a YAG:Nd laser

We report the deposition of thin films of silver (Ag) nanoparticles by pulsed laser ablation in vacuum using the third line (355 nm) of a YAG:Nd laser. The nanostructure and/or morphology of the films was investigated as a function of the number of ablation pulses, by means of transmission electron microscopy and atomic force microscopy. Our results show that films deposited with a small number of ablation pulses (500 or less), are not continuous, but formed of isolated nearly spherical Ag nanoparticles with diameters in the range from 1 nm to 8 nm. The effect of increasing the number of pulses by one order of magnitude (5000) is to increase the mean diameter of the globular nanoparticles and also the Ag areal density. Further increase of the number of pulses, up to 10,000, produces the formation of larger and anisotropic nanoparticles, and for 15,000 pulses, quasi-percolated Ag films are obtained. The presence of Ag nanoparticles in the films was also evidenced from the appearance of a strong optical absorption band associated with surface plasmon resonance. This band was widened and its peak shifted from 425 nm to 700 nm as the number of laser pulses was increased from 500 to 15,000.     

Characterization of zirconium thin films deposited by pulsed laser deposition

Zirconium(Zr) thin films deposited on Si(100) by pulsed laser deposition(PLD) at different pulse repetition rates are investigated. The deposited Zr films exhibit a polycrystalline structure, and the X-ray diffraction(XRD) patterns of the films show the α Zr phase. Due to the morphology variation of the target and the laser–plasma interaction, the deposition rate significantly decreases from 0.0431 /pulse at 2 Hz to 0.0189 /pulse at 20 Hz. The presence of droplets on the surface of the deposited film, which is one of the main disadvantages of the PLD, is observed at various pulse repetition rates. Statistical results show that the dimension and the density of the droplets increase with an increasing pulse repetition rate. We find that the source of droplets is the liquid layer formed under the target surface. The dense nanoparticles covered on the film surface are observed through atomic force microscopy(AFM). The root mean square(RMS) roughness caused by valleys and islands on the film surface initially increases and then decreases with the increasing pulse repetition rate.The results of our investigation will be useful to optimize the synthesis conditions of the Zr films.     

Femtosecond pulsed laser ablation of thin gold film

Laser micromachining on 1000 nm-thick gold film using femtosecond laser has been studied. The laser pulses that are used for this study are 400 nm in central wavelength, 150 fs in pulse duration, and the repetition rate is 1 kHz. Plano-concave lens with a focal length of 19 mm focuses the laser beam into a spot of 3 μm (1/e² diameter). The sample was translated at a linear speed of 400 μm/s during machining. Grooves were cut on gold thin film with laser pulses of various energies. The ablation depths were measured and plotted. There are two ablation regimes. In the first regime, the cutting is very shallow and the edges are free of molten material. While in the second regime, molten material appears and the cutting edges are contaminated. The results suggest that clean and precise microstructuring can be achieved with femtosecond pulsed laser by controlling the pulse energy in the first ablation regime.     

Thermal stability of amorphous carbon films deposited by pulsed laser ablation

Diamond-like carbon (DLC) films have been grown on Si substrates at ambient temperature by the pulsed-laser ablation technique, using pulses of different durations both in the nano- and picosecond ranges and at various energy fluences. The stability of these films was investigated as a function of thermal anneals performed in UHV conditions up to 1273 K. Their physico-chemical properties have been characterized by different techniques including X-ray photo-emission, Auger electron and electron-energy-loss spectroscopies, Raman scattering, spectroscopic ellipsometry and atomic-force microscopy. The thermal stability of the films has been demonstrated to be related to their initial structural (sp³/sp² ratio) and chemical (contaminant) properties. DLC layers prepared under optimized conditions have been found to show a very good thermal stability up to 900 K. Received: 4 Jule 2000 / Accepted: 6 July 2000 / Published online: 6 September 2000     

Vibrational spectroscopy of SiC thin films deposited by excimer laser ablation

A large number of thin SiC films, prepared at different conditions by KrF excimer laser ablation of solid SiC targets and deposition onto Si substrates (some onto quartz glass (QG) and yttrium-stabilized zirconia (YSZ)) were characterized by infrared and Raman spectroscopy. The films consisted of nano- and microcrystalline SiC and contained nanocrystalline carbon in the case of QG or YSZ substrates. Raman spectra of nanocrystalline SiC (grains <30 nm) reflect the phonon density-of-state function of SiC by broad scattering effects at 220–600 and 650–950 cm⁻¹. Medium-size crystallites are represented by a relatively narrow asymmetric band at 790 cm⁻¹ and crystallites >200 nm by an additional asymmetric band at 960 cm⁻¹. Small satellite bands at 760 and 940 cm⁻¹, attributed to SiC surface layers, were resolved in some well-ordered samples. Optical modelling was needed to interpret the IR spectra. SiC films could be represented by an effective medium model containing a SiC host phase and embedded particles with free charge carriers. The crystalline order of SiC films can be estimated from the parameters of the SiC oscillators. Received: 5 October 1998 / Accepted: 8 January 1999 / Published online: 5 May 1999     

Growth and characterization of pulsed laser deposited ZnO thin films

One of the most important and promising materials from metal oxides is ZnO with specific properties for near UV emission and absorption optical devices. The properties of ZnO thin films strongly depend on the deposition method. Among them, pulsed laser deposition (PLD) plays an important role for preparing various kinds of ZnO films, e.g. doped, undoped, monocrystalline, and polycrystalline. Different approaches — ablation of sintered ZnO pellets or pure metallic Zn as target material are described. This contribution is comparing properties of ZnO thin films deposited from pure Zn target in oxygen atmosphere and those deposited from sintered ZnO target. There is a close connection between final thin film properties and PLD conditions. The surface properties of differently grown ZnO thin films are measured by secondary ion mass spectrometry (SIMS), atomic force microscopy (AFM) and scanning electron microscopy (SEM). Furthermore, different approaches — ablation of sintered ZnO pellet or pure metallic Zn as target materials are described. The main results characterize typical properties of ZnO films versus technological parameters are presented. Presented at 5-th International Conference Solid State Surfaces and Interfaces, November 19–24, 2006, Smolenice Castle, Slovakia     

Hardnes and resistivity of amorphous carbon thin films deposited by laser ablation

Amorphous carbon thin films were deposited by laser ablation of a graphite target, using the fundamental line of a 5 ns Nd:YAG laser. Deposition was carried out as a function of the plasma parameters (mean kinetic ion energy and plasma density), determined by means of a planar probe. In the selected working regimes the optical emission from the plasma is mainly due to atomic species, namely C⁺ (426.5 nm); however, there is also emission from other atomic species and molecular carbon. The hardness and resistivity could be varied in the range between 10 and 25 GPa, and 10⁸ and 10¹¹ Ω cm, respectively. The maximum values were obtained at a 200 eV ion energy and 6×10¹³ cm⁻³ plasma density, where the maximum quantity of C–C sp³ bonds was formed, as confirmed by Raman spectroscopy.     

Nano-ionic thin films of gadolinia-doped ceria prepared by pulsed laser ablation

Microstructural properties of nano-ionic thin films of gadolinia-doped ceria (GDC) prepared by pulsed laser ablation from sintered targets of gadolinia (5–20 mol%) doped ceria are investigated. The ionic conductivity measurements of the sintered pellets showed a decrease in the activation energy from 1.1 to 0.65 eV for 5 and 30 mol% gadolinia-doped ceria, respectively. The microstructural properties of the GDC films as a function of substrate temperature, oxygen partial pressure, and laser energy show that the films are polycrystalline in the entire range of substrate temperature. The grain size is found to increase with increasing temperature up to 873 K. Further improved crystallinity is noticed for the films grown with oxygen partial pressure of 0.1–0.2 mbar. X-ray diffraction and transmission electron microscopy (TEM) reveal nanocrystalline grains with textured growth along <111> orientation in these films at low substrate temperature and at lower oxygen partial pressure. TEM study shows a uniform distribution of nanocrystal of 8–10 nm for energies ≤200 mJ/pulse, and nanocrystals embedded in a large crystalline matrix of doped ceria for energies in the range 400–600 mJ/pulse. Raman spectroscopy also confirms the defects in these films. The study also reveals that the substrate temperature and oxygen partial pressure could influence preferred orientation, while the laser energy could significantly influence defect concentration in these films. Invited paper presented at the Third International Conference on Ionic Devices (ICID 2006), Chennai, Tamilnadu, India, Dec. 7–9, 2006.     

Deposition of zinc oxide thin films by reactive pulsed laser ablation

Thin films of zinc oxide have been deposited by reactive pulsed laser ablation of Zn and ZnO targets in presence of a radio frequency (RF) generated oxygen plasma. The gaseous species have been deposited at several substrate temperatures, using the on-axis configuration, on Si (1 0 0). Thin films have been characterized by scanning electron microscopy, atomic force microscopy, X-ray diffraction, X-ray photoelectron spectroscopy and infrared spectroscopy. A comparison among conventional PLD and reactive RF plasma-assisted PLD has been performed.     

Reactive pulsed laser deposition of gold nitride thin films

We report on the growth and characterization of gold nitride thin films on Si 〈1 0 0〉 substrates at room temperature by reactive pulsed laser ablation. A pure (99.95%) Au target was ablated with KrF excimer laser pulses in nitrogen containing atmosphere (N₂ or NH₃). The gas ambient pressure was varied in the range 0.1-100 Pa. The morphology of the films was studied by using optical, scanning electron and atomic force microscopy, evidencing compact films with RMS roughness in the range 3.6-35.1 nm, depending on the deposition pressure. Rutherford backscattering spectrometry and energy dispersion spectroscopy (EDS) were used to detect the nitrogen concentration into the films. The EDS nitrogen peak does not decrease in intensity after 2 h annealing at 250 °C. Film resistivity was measured using a four-point probe and resulted in the (4-20) × 10⁻⁸ Ω m range, depending on the ambient pressure, to be compared with the value 2.6 × 10⁻⁸ Ω m of a pure gold film. Indentation and scratch measurements gave microhardness values of 2-3 GPa and the Young's modulus close to 100 GPa. X-ray photoemission spectra clearly showed the N 1s peak around 400 eV and displaced with respect to N₂ phase. All these measurements point to the formation of the gold nitride phase.     

Optical characterization of thin silicon films deposited by CVD and annealed by pulsed laser

Polycrystalline Si layers 100 to 800 nm thick have been deposited on Si single crystal substrates by CVD and annealed with a Q-switched ruby laser at energies up to 1.5 J/cm². The optical characteristics of these layers have been measured by SEM and ellipsometry. The results can be attributed to a change in surface roughness with film thickness and laser energy.     

Structural and optical properties of pulsed laser deposited ZnO thin films

Nanocrystalline ZnO thin films were grown by means of pulsed laser deposition. The ablation process was carried out at relatively low background oxygen gas pressure (10 Pa) and by varying the substrate temperature up to 600 °C. Information on the structural and morphological properties of the deposited thin films have been obtained by means of X-ray photoelectron, Raman spectroscopies, X-ray diffraction (XRD) and atomic force microscopy (AFM). The results showed that all the deposited films are sub-stoichiometric in oxygen and with a hexagonal wurtzite crystalline structure, characterized by features of some tens of nanometers in size. An improvement of the films' crystalline quality was observed for the deposition temperature of 300 °C while the further increase of the deposition temperature up to 600 °C induces a worsening of the material's structural properties with the development of a large amount of nanoparticle's clusters. The analysis of the XRD patterns shows a growth crystallographic preferential direction as a function of the deposition temperature, in agreement with the appearance of the only E₂ optical phonon mode in the Raman spectra. Such findings are compatible with the changes observed in the photoluminescent (PL) optical response and was related to the modification of the ZnO thin film structural quality.     

Optical and structural properties of pulsed laser ablation deposited ZnO thin film

A limited number of reports exists in the literature concerning the systematic study of the structural and optical properties of ZnO thin films, produced by pulsed laser ablation, in correlation with the deposition parameters adopted. In this paper we present a characterization of a sample prepared by this technique and studied by photoelectron spectroscopy and X-ray diffraction. The dielectric function of both target and films has been deduced by reflection electron energy loss spectroscopy.