Broadening of resonances in yttrium nanoparticle optical spectra

The dielectric function of yttrium in the range between 0.2 μm and 2 μm is composed of a harmonic oscillator contribution due to a discrete interband transition and the contribution of free electrons. Hence, it is possible to discuss surface plasmon polaritons as well as other electronic resonances in the optical extinction spectra of yttrium nanoparticles. For the latter, we discuss the broadening of the resonance caused by the aggregation of particles. When particles are lumped into aggregates, the color of the particle system also changes. Aggregation also affects the surface plasmon resonance in yttrium nanoparticles in a way comparable to silver or gold nanoparticle aggregates. Comparison is made with the first experimental results on yttriumnanoparticles, showing that aggregation is the dominant effect for the broad resonance in the measured extinction spectra. Received: 2 July 2001 / Revised version: 10 September 2001 / Published online: 15 October 2001     

Local fields close to the surface of nanoparticles and aggregates of nanoparticles

A refined discussion of the near-field scattering of spherical nanoparticles and the electromagnetic fields close to the particle surface is given. New results for the dependence on the distance from the surface and the angular distribution of the scattered light in the near-field are given. It will be shown that the radial component of the electric field leads to striking differences in the phase functions in the near-field and the far-field. Exemplary computations are presented for Ag and Au particles with different size. In a second part the discussion is extended to assemblies of spherical Ag and Au nanoparticles. It will be shown that large near-fields at wavelengths commonly used in SERS experiments are obtained for aggregates. In the near-field scattering intensity “hot spots” mark regions between particles in the aggregate where the near-field is particularly high. Received: 4 May 2001 / Revised version: 20 July 2001 / Published online: 19 September 2001     

Characterization of laser-ablated and chemically reduced silver colloids in aqueous solution by UV/VIS spectroscopy and STM/SEM microscopy

Silver colloids in aqueous solution were studied by different scanning microscopy techniques and UV/VIS spectroscopy. The silver colloids were produced either by chemical reduction or by nanosecond laser ablation from a solid silver foil in water. Variation of laser power and ablation time leads to solutions of metal clusters of different sizes in water. We characterized the electronic absorption of the clusters by UV/VIS spectroscopy. STM (scanning tunneling microscope) imaging of the metal colloids shows atomic resolution of rod- or tenon-like silver clusters up to 10-nm length formed by laser ablation. Our scanning electron microscope measurements, however, show that much larger silver colloids up to 5-μm length are also formed, which are not visible in the STM due to their roughness. We correlate them with the long-wavelength tail of the multimodal UV/VIS spectrum. The silver colloids obtained by chemical reduction are generally larger and their electronic spectra are red-shifted compared to the laser-ablated clusters. Irradiation of the colloid solution with nanosecond laser pulses of appropriate fluence at 532 nm and 355 nm initially reduced the colloid size. Longer irradiation at 355 nm, however, leads to the formation of larger colloids again. There seems to be a critical lower particle size, where silver clusters in aqueous solution become unstable and start to coagulate. Received: 24 June 2002 / Revised version: 25 July 2002 / Published online: 25 October 2002 RID="*" ID="*"This work is part of the thesis of H. M?ltgen RID="**" ID="**"Corresponding author. Fax: +49-211/811-5195, E-mail: kleinermanns@uni-duesseldorf.de     

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 response of aggregates of clusters with different dielectric functions

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

Chromatographic size separation of single-wall carbon nanotubes

Received: 18 May 1998/Accepted: 22 May 1998     

Microgeometry and effective permittivity of discontinuous metal films

This work deals with the investigation of dielectric and optical properties of discontinuous metal films over a wide range of filling factors each side of the percolation threshold. A phenomenological two-parameter representation is used for the effective permittivity &εtilde; of such systems. In order to test the proposed approach, we have taken for gold films on a glass substrate the transmittance spectra in the visible and near-IR regions. It is shown that the representation used for the effective permittivity enables us to adequately describe the experimental transmittance spectra. The parameters entering the above representation are determined and the relationship between them and film microgeometry is discussed. Received: 6 November 2000 / Accepted: 30 May 2001 / Published online: 25 July 2001     

Synthesis and characterization of ZnS nanoparticles in water/AOT/n-heptane microemulsions

ZnS nanoparticles were synthetized using water-containing AOT reversed micelles as nanoreactors and characterized by UV-Vis spectrophotometry, HRTEM (high-resolution transmission electron microscopy), SAED (selected-area electron diffraction), and digital image processing. The experimental data evidence a slow growing process of fractal-like ZnS nanoparticles’ coupled with a change of their photophysical properties. Both these processes are well described by power laws. The nanoparticles size is mainly controlled by the micellar size. After evaporation of the organic solvent, it has been found that the deposit is constituted by smaller and more stable ZnS nanoparticles bathed in a surfactant matrix. Received: 20 April 1999 / Accepted: 23 April 1999 / Published online: 8 September 1999     

Scattering and extinction of evanescent waves by small particles

Received: 14 May 1998/Revised version: 31 July 1998     

Spectroscopy in our age

The development of modern spectroscopy is summarized from Bunsen’s detection of atoms as the beginning of spectral analysis to modern molecular spectroscopies including new high resolution techniques for molecular ions. Recent experiments involving long range charge migration in peptides and proteins are outlined. Received: 2 October 2000 / Published online: 11 October 2000     

Resonant absorption and scattering in evanescent fields

Received: 14 July 1998     

The use of rovibrationally excited NO photofragments as trace nitrocompounds indicators

A one-color laser performs photolysis of nitrocompounds and laser-induced fluorescence to monitor the generated NO photofragments and to sensitively detect vapor trace amounts of nitrocompounds. The NO is monitored via excitation and emission through A²Σ⁺(v^′=0)?X²Π(v^′′=2-0) and A²Σ⁺(v^′=0)?X²Π(v^′′=0,1) transitions, respectively. It is found that NO photofragments populate the vibrationless ground state and also the first two vibrationally excited states. The analytical performance of the method is demonstrated on 2,4 dinitrotoluene (DNT) via excitation through A²Σ⁺(v^′=0)?X²Π(v^′′=2). The achieved limits of detection are 3.7 and 2.7 parts per billion (ppb) by weight of gaseous DNT in 100 and 500 Torr of air, for 30 s integration time. The application of this scheme for trace nitrocompound detection has the advantage that no background of ambient ground-state NO interferes and that the fluorescence is collected at shorter wavelengths than the exciting radiation, precluding background fluorescence. Received: 18 August 1999 / Revised version: 22 November 1999 / Published online: 8 March 2000     

Frequency-modulation-enhanced remote sensing

6 to 10¹² molecules cm^-3. FMRS has the potential of being a general method for monitoring atmospheric trace gases at ambient levels. Received: 23 March 1998     

Scanning microscopy by mid-infrared near-field scattering

2 -laser radiation from an AFM tip. In the microscopic images we find and identify a new type of AFM-induced artifact (crosstalk via the tapping amplitude). Minimizing this by proper scan parameters we obtain evidence of true infrared contrast. The results demonstrate the material-sensitive potential of infrared-spectroscopic imaging and a spatial resolving power of better than 100 nm. Received: 18 November 1997/Accepted: 5 January 1998     

Prerequisites for high-quality magnetic tunnel junctions: XPS and NMR study of Co/Al bilayers

Aluminum films with thicknesses ranging from 1 nm to 12 nm have been sputtered on 20 nm thick Co layers. The properties of the Co/Al bilayers were studied by X-ray photoemission spectroscopy (XPS) and spin-echo nuclear magnetic resonance (NMR). Both methods show independently that a 1 nm Al film covers the Co surface completely. XPS and NMR also showed that layers thicker than 1 nm Al are not oxidized completely in ambient air. Similarities to and deviations from niobium with Al overlayers (Nb/Al) are described. Prerequisites for the fabrication of tunneling magnetoresistance devices based on Co or NiFe ferromagnets and an aluminum oxide barrier are discussed. Received: 7 July 1999 / Accepted: 11 November 1999 / Published online: 8 March 2000     

Surface relief gratings in photoaddressable polymers generated by cw holography

Surface relief gratings (SRGs) are a widely observed phenomenon in holographic experiments with photoaddressable polymers. This article proposes a model which is able to explain the observed effects in cw experiments. It is based on forces in an electrical field which are caused by the inhomogeneous spatial distribution of the refractive index and the permittivity in the illuminated sample. Received: 11 September 2000 / Revised version: 5 December 2000 / Published online: 21 February 2001     

Determination of size and concentration of copper nanoparticles dispersed in glasses using spectroscopic ellipsometry

For the generation of particular optical properties the melt of a commercially manufactured glass is doped with copper compounds. The glass obtained is opaque black at the usual thickness and looks dark red after making it into bulbs of incandescent lamps. It is generally assumed that copper particles cause this colouring. A proof in a spectrophotometric way fails due to the very high absorbance even for a sample thickness below 20 μm. It will be shown that in these cases spectroscopic ellipsometry is a suitable method of investigation. The pseudo-optical constants of this material were determined as a function of wavelength in the range from 350 nm to 700 nm by ellipsometric measurements. They can be reproduced very well by those of a model that consists of a roughness layer situated on a substrate of glass containing spherical copper particles with a Gaussian size distribution with =6.5 nm and σ=0.24 and a volume concentration of 2.4×10^-3. For this modelling the dielectric function of the roughness layer was approximated by Bruggeman effective-medium theory and that of the copper-containing glass substrate was calculated on the basis of the theory of Gans and Happel. The results were verified by transmission electron microscope investigations. Received: 1 July 2001 / Published online: 10 October 2001     

NO and PO photofragments as trace analyte indicators of nitrocompounds and organophosphonates

Laser photolysis that releases characteristic photofragments, combined with laser-induced fluorescence (LIF) that subsequently monitors them, facilitate detection of trace-vapor analytes in air. The feasibility of the technique is demonstrated for nitrocompounds and organophosphonates detection. Using one-color and two-color lasers, respectively, the representative NO and PO photofragments were tagged. Employment of longer wavelengths for photodissociation, and where possible also for LIF, than for fluorescence collection, obstructs background fluorescence and in the former compounds also prevents interference of ambient ground state NO. Received: 1 February 2000 / Revised version: 27 April 2000 / Published online: 20 September 2000     

A novel electrochemical approach for fabrication of photoluminescent erbium-doped porous silicon

21 /cm³. After this new ECD treatment, PS:Er was found to emit much more intense room-temperature visible photoluminescence than both the porous silicon control and the PS:Er prepared by constant-current ECD. Room-temperature IR photoluminescence around 1.54 μm was observed for the first time without any post-doping annealing. Received: 3 September 1998/Accepted: 9 September 1998     

Determination of the dielectric function of porous silicon by high-order laser-harmonic radiation

Porous-silicon reflectance has been determined over a large energy range, from 1 eV to 16 eV, by combining a NIR/visible/UV spectrometer with a new VUV light source as laser-harmonic radiation. The porous-silicon dielectric function was deduced from reflectance measurements by Kramers–Kronig analysis. We point out that, for the first time, laser harmonics have been applied in the optical characterization of materials as a new and suitable alternative to synchrotron radiation. Received: 9 January 2001 / Accepted: 28 April 2001 / Published online: 20 June 2001