Optical properties of silver clusters formed by ion irradiation

Precipitation of silver clusters in silica is achieved by different methods: ion implantation, ion beam mixing of superimposed layers and ion irradiation of films deposited by means of co-sputtering or sol-gel technique. Main features of the nanoparticles depending on the preparation method are investigated by TEM. The optical extinction resonance of these clusters is analysed in terms of sizes and interaction between the clusters on the basis of calculations. We found that resonances in sputtered and gel films with low metal concentrations are well described by plasmon polaritons in isolated clusters and calculations based on Mie theory allow the study of their growth under irradiation. This theory is not appropriate to describe the optical response of silver clusters in silica implanted with Ag concentrations higher than 5 at.% or in ion beam mixed films, because of the interaction between clusters. Using an effective medium model, it is demonstrated that the random dispersion of clusters in implantation films causes fluctuations and, on average, an increase of the clusters polarization. On the contrary, the particular arrangement of the clusters with a bimodal size distribution in ion beam mixed films induces a screening effect between the clusters of largest size. Received 10 January 2002 / Received in final form 26 April 2002 Published online 19 July 2002     

Atomic mixing of Ni2O3/SiO2, NiO/SiO2, and Ni/SiO2 interfaces induced by swift heavy ion irradiation

We have investigated the interface mixing of Ni₂O₃/SiO₂, NiO/SiO₂, and Ni/SiO₂ induced by the irradiation with Ar, Kr and Xe ions of energies ranging from 90 MeV to 260 MeV. Since these energies are in the electronic stopping regime, atomic transport processes will not be directly initiated by elastic ion–target collisions, but need to be excited by secondary processes like electron–phonon coupling or Coulomb explosion. Nevertheless, we have observed a strong mixing effect in the ceramic systems if the electronic energy loss exceeds a certain threshold value. Estimation of an effective diffusion constant indicates that diffusion takes place in the molten ion track. In contrast to the ceramics, the metallic Ni layer is still insensitive even for the highest electronic stopping power used (S_e=28 keV/nm) and does not exhibit mixing with its SiO₂ substrate. In addition, NiO/SiO₂ and Ni/SiO₂ were irradiated in the nuclear stopping regime with 600 keV Kr and 900 keV Xe–ions. Here the intermixing effect is in good agreement with the assumption of ballistic atomic transport. Received: 5 February 2002 / Accepted: 11 February 2002 / Published online: 3 May 2002 RID="*" ID="*"Corresponding author. Fax: +49-711/685-3866, E-mail: bolse@ifs.physik.uni-stuttgart.de     

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     

Excimer laser ablation of gold-loaded inverse polystyrene-block-poly (2-vinylpyridine) micelles

Diblock-copolymers (PS(1700)-b-P2VP(450) or PS(1350)-b-P2VP(400)) forming spherical micelles, can be loaded with a Au-salt and deposited on top of various substrates. Such polymer films have been exposed to a pulsed ArF excimer laser in order to remove the polymer matrix and, in parallel, to chemically reduce the salt into metallic Au nanodots. To analyze this process in detail, it was subdivided into three steps: (a) laser ablation of thick and thin diblock-copolymer films; (b) laser irradiation of Au-salt loaded diblock-copolymer films; and (c) laser irradiation of arrays of metallic Au nanodots. In (a) it was found that a complete removal of the polymer by laser ablation is only possible in air under ambient conditions while identical laser irradiations under vacuum result in a residual layer of approximately 14 nm. Substep (b) revealed a nucleation process of the resulting metallic Au within the micellar core leading to clusters of small Au dots. Furthermore, this substep provided evidence for an asymmetric interplay between the macroscopic temperature of a polymer film during laser treatment and the energy density per laser pulse. In (c) it could be demonstrated that metallic Au nanodots on mica are stable against laser irradiation under conditions leading to a polymer removal. Received: 7 August 2000 / Accepted: 2 November 2000 / Published online: 3 April 2001     

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     

Optical response of aggregates of clusters with different dielectric functions

Received: 19 December 1997/Revised version: 24 March 1998     

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     

Deposition and diffusion of size-selected (Ag400+) clusters on a stepped graphite surface

400 clusters on a stepped graphite surface by a combination of scanning electron microscopy experiments and computer simulations (molecular dynamics and Monte Carlo methods). We find that the shape of the clusters is only partially deformed by the impact with the surface, moreover the clusters do not create surface defects upon landing, and so are able to diffuse freely over the surface. Many clusters are found to become trapped at surface steps, where their mobility is reduced by the higher binding energy. Exploring the 1-D diffusion of clusters along the steps reveals the low mobility for larger islands, as well as the importance of defects on the step (for example kinks), which trap the mobile clusters. Received: 9 April 1998/Accepted: 25 August 1998     

Plasmon resonances observed in light scattered by large alkali clusters

Received: 14 October 1996/Revised version: 15 January 1997     

Ion synthesis and laser annealing of Cu nanoparticles in Al2O3

Al₂O₃ samples with Cu nanoparticles, synthesised by ion implantation at 40 keV with a dose of 1×10¹⁷ ion/cm² and a current density from 2.5 to 12.5 μA/cm², were annealed using ten pulses from a KrF excimer laser with a single pulse fluence of 0.3 J/cm². The copper depth distribution, formation and modification of metal nanoparticles under the ion implantation and laser treatment were studied by Rutherford backscattering (RBS), energy dispersive X-ray (EDX) analysis, atomic force microscopy (AFM) and optical spectroscopy. It was found that laser annealing leads to a reduction in the nanoparticle size without diffusion of metal atoms into the bulk. The change in particle size and the possibility for oxidation of the copper particles are examined in the framework of Mie theory. Calculations presented show that under excimer laser treatment, Cu nanoparticles are more likely to be reduced in size than to undergo oxidation. Received: 19 April 2001 / Accepted: 7 November 2001 / Published online: 23 January 2002     

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     

Optical extinction of an assembly of spherical particles in an absorbing medium: Application to silver clusters in absorbing organic materials

The theory presented by Gerardy and Ausloos for the calculation of the linear optical response of aggregates of spherical particles is analytically continued for absorbing embedding media. The method is based on the calculation of the extinction rate by a single particle embedded in an absorbing matrix. Explicit expressions for the extinction and scattering cross-sections are given. The method is applied to calculate the energy losses in several organic matrices with embedded silver clusters. Comparison with experimental data shows a very good agreement. Received: 21 December 1998     

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     

Carbon nanotubes formed in graphite after mechanical grinding and thermal annealing

Multi-walled carbon nanotubes with cylindrical and bamboo-type structures are produced in a graphite sample after mechanical milling at ambient temperature and subsequent thermal annealing up to 1400 °C. The ball milling produces a precursor structure and the thermal annealing activates the nanotube growth. Different nanotubular structures indicate different formation mechanisms: multi-wall cylindrical carbon nanotubes are probably formed upon micropores and the bamboo tubes are produced because of the metal catalysts. A two-dimensional growth governed by surface diffusion is believed to be one important factor for the nanotube growth. A potential industrial production method is demonstrated with advantages of large production quantity and low cost. Received: 17 May 2002 / Accepted: 12 September 2002 / Published online: 4 December 2002 RID="*" ID="*"Corresponding author. Fax: +61-2/6125-8338, E-mail: ying.chen@anu.edu.au     

Erratum to Formation and control of Au and Ag nanoparticles inside borate glasses using femtosecond laser and heat treatment

We report the spectroscopic properties of femtosecond laser-irradiated sodium-alumino-borate glass doped with silver and gold ions. We precipitated gold and silver nanoparticles by laser irradiation and annealing at 400°C for 30 min. The irradiation and annealing treatment produced different absorption and emission characteristics in Au³⁺ doped and Au³⁺, Ag⁺ codoped glasses, and the possible mechanisms of the observed results are discussed. The size of the nanoparticles was estimated by TEM and absorption band analysis.     

Optical diffuse reflectance spectra of GaSb nanocrystals embedded in SiO2 matrix by radio-frequency magnetron co-sputtering

Nanocrystalline GaSb embedded in SiO₂ films was grown by radio-frequency magnetron co-sputtering. X-ray diffraction pattern and transmission electron microscopy (TEM) confirm the existence of GaSb nanocrystals in the SiO₂ matrix. The average size of GaSb nanoparticles is in the range of 3 to 11 nm. Diffuse reflectance spectra were used to characterize the small change of the band gap of the semiconductor. The diffuse reflectance spectra shows that the absorption peaks have a large blueshift of about 4.0 eV of the absorption relative to that of bulk GaSb. It has been explained by quantum confinement effects. Room temperature optical transmission spectra show that the absorption edge exhibits a very large blueshift of about 2.1 eV with respect to that of bulk GaSb in agreement with quantum confinement. Received: 28 July 1999 / Accepted: 27 October 1999 / Published online: 1 March 2000     

Nanotomographic examinations of ion tracks in cellulose nitrate by ion energy loss spectrometry

Received: 13 October 1997/Accepted: 25 August 1998     

Theory of spectral hole burning for the study of ultrafast electron dynamics in metal nanoparticles

In order to study the ultrafast relaxation dynamics of surface plasmon excitation in metal nanoparticles in the presence of inhomogeneous line broadening and investigate the influence of the reduced dimensions on the dephasing time T₂ in the size regime below about 10 nm, we have recently demonstrated a novel technique based on persistent spectral hole burning [1]. Here, we describe a theoretical model that has been developed for evaluation of the experimental data and precise determination of T₂ for particles of different size and shape. Comparison of the model to experimental data for Ag nanoparticles on sapphire shows that the theoretical treatment does not only reproduce the shape of the generated holes but also the dependence of their widths on the applied laser fluence. As a result, we have a reliable and versatile tool at hand making possible systematic studies of the ultrafast electron dynamics in small metal particles, and the dependence of the femtosecond dephasing time on their size, shape and surrounding dielectric. Received: 12 September 2001 / Published online: 15 October 2001     

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     

Light–induced charge transport processes in photorefractive barium titanate crystals doped with iron

3 ) are annealed in a hydrogen atmosphere at various temperatures. After these reducing treatments, absorption, light–induced absorption changes, two–beam coupling direction, photo electron paramagnetic resonance (photo EPR), dark and photoconductivity as well as bulk photovoltaic current density are investigated. The samples are electron conductive and the charge transport is governed by only one level, which is identified by photo EPR as Fe²⁺/Fe³⁺. The photoconductivity exceeds the dark conductivity for intensities above 1 kWm^-2. A relation between the absorption constant and the Fe²⁺ concentration is derived. From the known charge transport parameters the advantageous photorefractive properties of optimized reduced BaTiO₃:Fe are deduced; possible response times in the millisecond range at an intensity of 10 kWm^-2 are estimated. Received: 22 January 1997/Accepted: 23 January 1997