Photoluminescence properties of size-selected Si nanocluster films prepared by laser ablation

Size-dispersed Si nanocluster films have been synthesized by a method of crossing an Ar gas beam perpendicularly to a silicon cluster beam that is produced by a laser ablation technique. Flight directions of the nanoclusters are changed due to Ar gas collisions, and smaller nanoclusters are deflected further from the axis of the primary cluster beam. The size-dispersed nanocluster films exhibit strong red photoluminescence (PL) after exposure to the air. The PL peak energy changes between 1.42 and 1.72 eV depending on the sample position. The average diameter of the oxidized nanoclusters characterized by transmission electron microscopy is 10 nm at the position of the primary cluster beam axis and becomes smaller as deviated from the axis. The relation between the PL peak energy and the size of the oxidized Si nanoclusters is discussed. Received: 4 May 2000 / Accepted: 9 May 2000 / Published online: 13 July 2000     

Filamentary iron nanostructures from laser-induced pyrolysis of iron pentacarbonyl and ethylene mixtures

In this work, we present results of the synthesis and characterization of iron and iron oxide nanoparticles aggregated in filamentary, spider-web-like structures. The particles were produced in a flow reactor by CO₂ laser pyrolysis of gaseous mixtures of iron pentacarbonyl and ethylene. Low- and high-resolution electron microscopy reveals chain-like structures of particles, most of them being composed of an α-iron core and an iron oxide shell, identified as magnetite and, to a lesser extent, hematite. These results are in good agreement with a M?ssbauer analysis carried out for the same samples. The role of the reaction temperature on the synthesis of filamentary iron nanostructures by infrared laser pyrolysis of Fe(CO)₅/C₂H₄ mixtures is discussed. Received: 31 May 2000 / Accepted: 6 June 2000 / Published online: 2 August 2000     

Post-annealing effect upon optical properties of electron beam evaporated molybdenum oxide thin films

Molybdenum oxide (MoO₃) thin films were deposited by electron beam evaporation. The chemical composition, microstructure, optical and electrical properties of MoO₃ thin films depend on the annealing temperature and ambient atmosphere. X-ray diffraction (XRD) shows that crystalline MoO₃ films can be obtained at various post-annealing temperatures from 200 to 500 °C in N₂ and O₂. X-ray photoelectron spectroscopy (XPS) results reveal that the O-1s emission peak was shifted slightly toward lower binding energies as the annealing temperature in N₂ was increased. The oxygen vacancies and conductivity of MoO₃ film increased with the annealing temperature. However, when the MoO₃ films were annealed in an atmosphere of O₂, the optical transmission, the O/Mo ratio and the photon energy increased with the annealing temperature. The results differ from those for films annealed in a N₂ atmosphere.     

Size dependence of non-linear optical properties of SiO2 thin films containing InP nanocrystals

SiO₂ composite thin films containing InP nanocrystals were fabricated by radio-frequency magnetron co-sputtering technique. The microstructure of the composite thin films was characterized by X-ray diffraction and Raman spectrum. The optical absorption band edges exhibit marked blueshift with respect to bulk InP due to strong quantum confinement effect. Non-linear optical absorption and non-linear optical refraction were studied by a Z-scan technique using a single Gaussian beam of a He-Ne laser (632.8 nm). We observed the saturation absorption and two-photon absorption in the composite films. An enhanced third-order non-linear optical absorption coefficient and non-linear optical refractive index were achieved in the composite films. The nonlinear optical properties of the films display the dependence on InP nanocrystals size. Received: 27 June 2000 / Accepted: 27 June 2000 / Published online: 13 September 2000     

Refined interpretation of optical extinction spectra of nanoparticles in plasma polymer films

-1 to 50000 cm^-1, exhibiting an extinction maximum that could be assigned to the corresponding long-axis surface plasmon excitation in the spheroidal noble metal particles. For simulation of the measured spectra, the model of Gans [Ann. Phys. 37, 881 (1912)] was used to calculate the optical extinction spectrum for each analyzed particle in a sample. The various spectra of more than 500 particles were added up to get the total extinction spectrum of the sample, yielding very good agreement with the measured spectra. A remaining blueshift of the computed spectra compared to the measured spectra could be explained by considering electromagnetic interaction among the particles. Received: 22 July 1998 / Revised version: 22 October 1998 / Published online: 24 February 1999     

Selective suppression of extinction within the plasmon resonance of gold nanoparticles

We present extinction measurements on rectangular two-dimensional arrays of gold nanoparticles on a dielectric waveguide. The spectra exhibit spectrally narrow bands of suppressed extinction within the particle–plasmon resonance, resulting from destructive interference between the incident light field and the excited waveguide modes. The dependence of the spectral position of these high-transmission bands on different waveguide modes is investigated in detail. Received: 3 July 2001 / Published online: 10 October 2001     

Visible photoluminescence from Ge+-implanted SiO2 films thermally grown on crystalline Si

+ -implanted SiO₂ films is studied as a function of different fabricating conditions (implantation dose, annealing temperature and time). The SiO₂ films containing Ge nanocrystals exhibit two photoluminescence (PL) bands peaked at 600 nm and 780 nm. There are two excitation bands in the PL excitation (PLE) spectra. With variation in Ge nanocrystal size, the PL and PLE peak energies show no appreciable shift. The PL and PLE spectral analyses suggest that during the PL process, electron–hole pairs are generated by the E(l) and E(2) direct transitions inside Ge nanocrystals, which then radiatively recombine via luminescent centers in the matrix or at the interface between the nanocrystal/matrix. Received: 27 January 1998/Accepted: 18 March 1998     

Synthesis of LECBD grown cluster assembled SeO2 thin films

Cluster assembled selenium oxide (SeO₂) thin films, as a function of oxygen flow pressure (OFP) have been synthesized by a low energy cluster beam deposition (LECBD) technique. The OFP dependent surface morphology leading to well separated nanoclusters (size ranging from 50 to 200 nm) and fractal features are confirmed from transmission electron microscopic (TEM) measurements. A diffusion limited aggregation (DLA) mediated fractal growth with dimension as 1.71 ± 0.01 has been observed for high OFP (60 mbar). Structural analysis by glancing angle X-ray diffraction (GXRD) and selected area diffraction (SAD) studies identify the presence of tetragonal phase SeO₂ in the deposit. Micro-Raman studies indicate the shifts in bending and stretching vibrational phonon modes in cluster assembled SeO₂ as compared to their bulk counter part due to the phonon confinement effect.     

Anomalous dispersion in nanocomposite films at the surface plasmon resonance

Nanocomposite thin films formed by Cu nanocrystals (NCs) embedded in an amorphous aluminium oxide (Al₂O₃) host have been prepared by alternate pulsed laser deposition. Spectroscopic ellipsometry is used to determine the effective refractive index (n=n+ik). The extinction coefficient is non-negligible and shows a broad absorption band related to the surface plasmon resonance. In the neighbourhood of this wavelength, the real part of the refractive index undergoes an anomalous dispersion, leading to a significant increase of the n value of the composite compared to that of the host. When the Cu content is low enough, about 2 at. %, the use of an effective medium approach combined with a regression method allows us to determine the metal content and film thickness from the ellipsometric measurements. For larger concentrations this approach is no longer valid. Received: 31 July 2001 / Revised version: 21 September 2001 / Published online: 15 October 2001     

Structure and magnetic properties of Co nanoclusters fabricated by ion beam synthesis in SiO2 films

Co nanoparticles fabricated by ion beam synthesis in SiO₂ films were investigated with transmission electron microscopy and superconducting quantum interference device technique. Variation of the thermal treatment enables the formation of Co nanoclusters of different sizes ranging from 2 to 40 nm. Small nanoclusters of about 2–3 nm are amorphous, whereas clusters above 7 nm show the configuration of cubic Co nanocrystals. Measurements of magnetisation at temperatures between 2 K and 360 K reveal superparamagnetic behaviour for the small nanoclusters up to 3 nm and ferromagnetism for clusters above 7 nm. Received: 12 February 2001 / Accepted: 3 May 2001 / Published online: 27 June 2001     

Excimer-laser-assisted RF glow-discharge deposition of amorphous and microcrystalline silicon thin films

We combine the deposition of Hydrogenated amorphous Silicon (a-Si:H) by rf glow discharge with XeCl-excimer laser irradiation of the growing surface in order to obtain different kinds of silicon films in the same deposition system. In-situ UV-visible ellipsometry allows us to measure the optical properties of the films as the laser fluence is increased from 0 up to 180 mJ/cm² in separate depositions. For fixed glow-discharge conditions and a substrate temperature of 250° C we observe dramatic changes in the film structure as the laser fluence is increased. With respect to a reference a-Si:H film (no laser irradiation) we observe at low laser fluences (15–60 mJ/cm²) that the film remains amorphous but demonstrates enchanced surface roughness and bulk porosity. At intermediate fluences (80–165 m/Jcm²), we obtain an amorphous film with an enhanced density with respect to the reference film. Finally, at high fluences (165–180 mJ/cm²), we obtain microcrystalline films. The in-situ ellipsometry measurements are complemented by ex-situ measurements of the dark conductivity, X-ray diffraction, and Elastic Recoil Detection Analysis (ERDA). Simulation of the temperature profiles for different film thicknesses and for three laser fluences indicates that crystallization occurs if the surface temperature reaches the melting point of a-Si:H ( 1420 K). The effects of laser treatment on the film properties are discussed by taking into account the photonic and thermal effects of laser irradiation.Presented at LASERION 93, Munich, June 21–23, 1993     

Shaping nanoparticles and their optical spectra with photons

An experimental method to tailor the shape and the optical absorption spectra of metallic nanoparticles is presented. It exploits the influence of laser irradiation on particle growth by self-assembly of atoms deposited on a substrate surface. By applying nanosecond light pulses of appropriate fluence and three different wavelengths, oblate silver particles with three fixed axial ratios have been fabricated. Their optical extinction spectra were measured with s- and p-polarized light and are dominated by plasmon resonances at fixed photon energies determined by the axial ratio. Possible applications of such tailormade nanoclusters include catalytic converters and optical components with narrow-band extinction, the magnitude and center frequency of which can be specified in advance. Received: 9 September 1999 / Published online: 20 October 1999     

Room temperature photoluminescence from amorphous silicon nanoparticles in SiOx thin films

Amorphous SiO_x thin films with four different oxygen contents (x=1.15, 1.4, 1.5, and 1.7) have been prepared by thermal evaporation of SiO in vacuum and then annealed at 770 or 970 K in argon for various times ?40 min. The influence of annealing conditions and the initial film composition on photoluminescence (PL) from the annealed films has been explored. Intense room temperature PL has been observed from films with x?1.5, visible with a naked eye. It has been shown that PL spectra of most samples consists of two main bands: (i) a ‘green’ band centered at about 2.3 eV, whose position does not change with annealing conditions and (ii) an ‘orange-red’ band whose maximum moves from 2.1 to 1.7 eV with increasing annealing time and temperature and decreasing initial oxygen content. These observations have been explained assuming recombination via defect states in the SiO_x matrix for the first band and emission from amorphous Si nanoparticles for the second one.     

Synthesis and properties of In2(Se1-xTex)3 thin films: a new semiconductor compound

In₂(Se_1-xTe_x)₃ polycrystalline films were prepared by a dual-source thermal evaporation technique. The depositions onto glass and SnO₂-coated glass substrates were carried out in a vacuum chamber and followed by an annealing in neutral ambient (Ar or N₂). The structural, morphological and compositional studies of the films were made by X-ray diffraction, energy-dispersive X-ray analysis, X-ray photoelectron spectroscopy, scanning electron microscopy, Raman scattering and optical transmission. Optimum conditions are investigated for the formation of the ternary compound In₂(Se_1-xTe_x)₃ in order to tune the band gap by changing the Te concentration. The film properties as a function of Te amount are discussed. It is shown that single-phase, textured and homogeneous layers of In₂(Se_1-xTe_x)₃ can be grown with x≤0.2 at optimal deposition and heat treatment conditions. For x≅0.17 these films showed an energy band gap of about 1.45 eV and an electrical conductivity at room temperature six orders of magnitude higher than that of the binary γ-In₂Se₃ thin films. Received: 9 July 1999 / Accepted: 25 November 1999 / Published online: 13 July 2000     

Artificially nanostructured Cu:Al2O3 films produced by pulsed laser deposition

The processes leading to the formation of Cu:Al₂O₃ composite films on Si (001) with a well defined nanostructure by alternate pulsed laser deposition (a-PLD) in vacuum are investigated. Alternately amorphous Al₂O₃ layers and Cu nanocrystals nucleated on the Al₂O₃ surface are formed, according to the PLD sequence. The Al₂O₃ deposited on the Cu nanocrystals fills in the space between them until they are completely buried, and subsequently, a continuous dense layer with a very flat surface (within 1 nm) is developed. The nucleation process of the nanocrystals and their resulting oblate ellipsoidal shape are discussed in terms of the role of the energetic species involved in the PLD process and the metal–oxide interface energy. Received: 4 July 2000 / Accepted: 5 July 2000 / Published online: 13 September 2000     

Atomistic processes and the strain distribution in the early stages of thin film growth

Structural relaxations in small Co islands on the Cu(001) surface are investigated performing atomistic calculations. We demonstrate that the strain relief at the metal interface in the early stages of heteroepitaxy is more complicated than suggested by simple considerations based on the small mismatch between the Co and Cu bulk metals. We found that the strain distribution in the surface region near the islands varies strongly on an atomic scale. The effect of strain on the shape of the Co islands is revealed. Diffusion on the top of strained islands and edge diffusion are considered. Received: 10 April 2000 / Accepted: 15 May 2000 / Published online: 7 March 2001     

Infrared picosecond absorption spectroscopy of microcrystalline silicon: separation between carrier recombination in crystalline and amorphous fractions

We have studied ultra-fast carrier dynamics of photo-excited carriers in hydrogenated microcrystalline silicon prepared by a very high frequency glow-discharge technique. We report on direct observation of two types of dynamics using selective photo-excitation in picosecond pump and probe measurements. One type of the observed dynamics has been found to be independent of the sample preparation, while the other reflects the relative weights of crystalline and amorphous fractions. We propose a simple rate-equation model that describes the carrier dynamics in microcrystalline silicon in terms of the composition of those in Si microcrystallites and in the a-Si:H tissue which surrounds the microcrystallites. The model without any fitting parameters reproduces the experimental data very well when the dynamics are scaled with relative volume fractions as obtained from Raman spectra. Received: 23 November 2000 / Accepted: 17 March 2001 / Published online: 23 May 2001     

Ferroelectric SrxBa1-xNb2O6 optical waveguiding thin films on SiO2-coated Si(100) substrates

x Ba_1-xNb₂O₆ (x=0.5) films (abbreviated as SBN:0.5) on SiO₂-coated Si substrates are potential components for the application of integrated electro-optics devices. SBN:0.5 optical waveguiding thin films on SiO₂-coated Si substrates with a very thin MgO diffusion buffer have been successfully prepared by pulsed laser deposition. The as-grown films have a refractive index of 2.28, which is close to that of bulk SBN. X-ray analysis showed that the as-grown films have a single-phase tetragonal tungsten bronze structure. The SBN:0.5 thin films prepared by PLD exhibit favorable ferroelectric and optical waveguiding properties. The composition and the morphology of the films were also examined by XPS and by SEM, respectively. Ferroelectric SBN:0.5 optical waveguiding thin films on SiO₂-coated Si substrates are expected to be used in integrated electro-optic devices. Received: 27 February 1997/Accepted: 17 October 1997     

Photocatalytic activity of dc magnetron sputter deposited amorphous TiO2 thin films

For photocatalytic thin film applications TiO₂ is one of the most important materials. The most studied TiO₂ crystal phase is anatase, though also rutile and brookite show good photoactivity. Usually anatase or a mixture of rutile and anatase is applied for powder or thin film catalysts. It has been claimed that amorphous films do not exhibit any or only a very low photocatalytic activity.We have deposited amorphous thin films by dc magnetron sputtering from sub-stoichiometric TiO_2−x targets. The coatings are transparent and show a photocatalytic activity half of that of a thin layer of spin-coated reference photocatalyst powder. Annealing the thin films to yield anatase crystallization more than doubles their photocatalytic activity. At the same film thickness these thin films show the same activity as a commercially available photocatalytic coating.The dependence of the photocatalytic activity on deposition parameters like gas pressure and sputter power is discussed. A decrease in film density, as deduced from the refractive index and the microstructure, resulted in an increase in photocatalytic activity. Film thickness has a marked influence on the photocatalytic activity, showing a strong increase up to 300-400 nm, followed by a much shallower slope.     

The formation and thermostability of MgO and MgAl2O4 nanoparticles in oxidized SiC particle-reinforced Al-Mg composites

Interfacial reactions and their products in oxidized SiC particle-reinforced Al-Mg matrix composites were investigated using X-ray diffraction and Field EmissionScanning Electron Microscopy (FE-SEM). Observation of the interfacial reaction between oxidized SiC particles and aluminum alloys containing Mg showed that nanoparticles of MgO form initially and do not change form when more than 4 wt. % Mg is in the matrix. However, MgO transforms into octahedral MgAl₂O₄ crystals when less than 2 wt. % Mg is in the matrix .Comparison of the amounts and the sizes of the reaction products MgAl₂O₄ and MgO between the Al-Mg alloyswith different matrix compositions shows that fewer MgAl₂O₄ crystals form at the surface of the particles in the 2014Al matrix composite than in the Al-2 wt. % Mg (Al-2Mg) matrix composite. Also, the size of MgAl₂O₄ in the former composite is greater than that of the latter composite under the same conditions. However, the amount and the size of MgO crystals that form in the Al-4 wt. % Mg (Al-4Mg) matrix composite is almost the same as that of the Al-8 wt. % Mg (Al-8Mg) composite, and the size of MgO changes a little during heat-treatment at elevated temperatures. The amount of the reaction product (either MgO or MgAl₂O₄) depends on nucleation rates and density of nucleation sites on the oxidized SiC particles at the initial reaction. The more completely the nuclei cover the surface of the oxidized SiC particles, the smaller the resulting size. According to the results, an addition of Mg into the matrix can be used to control the interfacial characteristics in the oxidized SiC/Al composites. Received: 25 January 2001 / Accepted: 26 January 2001 / Published online: 23 May 2001