Generation of Ag nanospikes via laser ablation in liquid environment and their activity in SERS of organic molecules

The formation of dense arrays of nanospikes occurs under laser ablation of bulk silver immersed in liquids such as water or ethanol. The average height of spikes is 50 nm and their density on the target amounts to 10¹⁰ cm^-2. This effect is observed with sufficiently short laser pulses. In particular, either a 350 ps or a 90 ps Nd:YAG lasers are used operating in their fundamental harmonics. These nanospikes are characterized by UV–visible reflection spectrometry and atomic force microscopy. The oscillations of electrons within Ag nanospikes results in a permanent coloration of the surface and a modification of the optical reflection spectra of the metal. Nanospikes show a peak of plasmon resonance around 380 nm, which shifts to the visible range upon oxidation in air. The initial spectrum may be restored by reduction of the oxidized Ag surface through processing in an ammonia aqueous solution. Scanning the laser beam along the metal surface allows its nanostructuring over extended areas (∼1 cm²). The nanostructured Ag surface shows enhanced Raman scattering of acridine molecules at a concentration of 10^-5 M/l, whereas initial Ag target do not show any signal within the accuracy of measurements. PACS 68.65.k; 64.70.Dv; 79.20.Ds; 42.62.Cf     

Synthesis and reactivity of 2-polyfluoroalkylchromene-4(4<Emphasis Type="Italic">H</Emphasis>)-thiones

2-Polyfluoroalkylchromene-4(4H)-thiones, synthesized from 2-polyfluoroalkylchromones and P₂S₅, react with aniline, phenylhydrazines, and hydroxylamine at the C(4) atom and afford corresponding anils, phenylhydrazones, and oximes of chromones. On heating in alcohol in the presence of concentrated HCl, chromone phenylhydrazones and oximes undergo ring closure to form 3-(2-hydroxyaryl)-1-phenyl-5-polyfluoroalkylpyrazoles and 5-hydroxy-3-(2-hydroxyaryl)-5-polyfluoroalkyl-Δ²-isoxazolines.__________Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 10, pp. 2188–2195, October, 2004.     

Low-threshold field electron emission of Si micro-tip arrays produced by laser ablation

Low-threshold field electron emission (FEE) is reported for periodic arrays of micro-tips produced by laser ablation of Si wafers. The best samples show emission at threshold fields as low as 4–5 V/μm for n-type Si substrates and of 1–2 V/μm for p-doped Si substrates, as measured with a flat-screen technique. Auger electron spectroscopy and X-ray electron spectroscopy reveal island-like deviation of the SiO₂ stoichiometry on the tip surfaces, with lateral dimensions of less than 100 nm. Microscopic studies using a special field-emission STM show that the emission originates from well-conducting regions of sub-micron size. The experimental data suggest FEE from the tip arrays by a geometric field enhancement of both the individual micro-tip and the narrow conducting channels in the tip body. Received: 3 May 2002 / Accepted: 1 July 2002 / Published online: 28 October 2002 RID="*" ID="*"Corresponding author. Fax: +7-095/135-82-34, E-mail: shafeev@kapella.gpi.ru     

Laser deposition of amorphous diamond films from liquid aromatic hydrocarbons

6 H₅CH₃, C₆H₆, and C₆H₅CH(CH₃)₂) to pulsed visible laser radiation of a copper vapor laser (λ=510.6 nm). The X-ray Auger electron spectroscopy (XAES), reflection high energy electron diffraction (RHEED), and Raman analysis are employed to characterize the deposited films. The sp³ fraction in deposited films amounts to 60–70% and depends on the precursor. The average film thickness on a glass substrate is about 100 nm. The films show excellent adherence, are transparent in the visible and have microhardness of 50–70 GPa, as measured by nanoindentor. Received: 28 September 1998 / Accepted: 13 January 1999     

Morphologies and phase compositions of copper–zinc coatings obtained by electrolysis

The influence of adding zinc nitrate(II) to a copper electrolyte on the morphologies, phase compositions, and specific surface areas of cupriferous coatings is studied. It is shown that copper–zinc coatings have high specific surface areas and exhibit good catalytic activities in the decomposition of formic acid.     

Femtosecond laser writing of nanostructures on bulk Al via its ablation in air and liquids

We report on the formation of self-organized nanostructures (NS) on bulk Al under its ablation in air and liquids with femtoseconds (fs) laser pulses. In case of exposure into liquids, NS are regularly formed on the Al surface with an average period of about 200 nm, independent of the laser polarization. A dispersion of Al nanoparticles (NPs) into the liquid additionally occurs. Irregular nano-bumps are produced when the irradiation is performed in air. NP dispersions as well as NS formed on Al surface show a characteristic absorption peak in the near UV which has been attributed to plasmon oscillation of electrons. The wings of this peak extending to the visible, lead to a distinct yellow coloration of the processed Al surface and the liquid dispersions. Ultrafast laser processing of bulk Al in liquids may be potentially a promising technique for efficient production of nanosized aluminum.     

Observation of laser-light effect on <Superscript>137</Superscript>Cs radioactivity in colloidal gold: first results

Laser-light effects on γ-activities of the ¹³⁷Cs and ¹³⁴Cs water solutions containing gold nanoparticles are investigated at laser-light intensity on the order of 10¹² W/cm². The irradiation is found to reduce the radioactivities of both nuclides. Over the period of irradiation, no enhancement of the γ lines of spontaneous decays is detected. Possible mechanisms of the observed effect are discussed.     

Influence of electric field on heterogeneous reactions stimulated by laser light

We present experimental data for vanadium and copper oxidation by cw CO₂ laser light in an external electric field. Direct influence of the external field on the crystallisation process has been demonstrated in both vanadium and copper cases; the oxidation rate and surface morphology depend sensitively on the sign and strength of the external field.     

Solar Cells Based on Laser-Modified Silicon

The fabrication of solar cells based on laser-modified silicon (LMS) has been experimentally investigated. Laser ablation of single-crystal Si in air results in the formation of a quasi-periodic array of microcones. After removing the oxide layer, this array is coated by a transparent conducting tin oxide layer. A comparison of the spectral dependence of the mirror reflection of laser-modified Si with that of the initial flat Si surface shows a 10 to 15% drop in reflectivity. The light-to-electricity conversion efficiency is shown to exceed that of the initial flat Si surface by more than 20%.