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     

Study of laser fragmentation process of silver nanoparticles in aqueous media

Laser fragmentation of Ag nanoparticles in Ag hydrosol was studied by simultaneous measurements of the transmitted fluence of the incident laser beam and the time evolution of the surface plasmon extinction (SPE) spectra. The experiments showed that the laser fragmentation in a small volume of hydrosol proceeds during first 20 pulses and then reaches saturation. The value of the transmitted fluence corresponding to saturation increases with incident pulse fluence, but the impact of the first pulse applied to the hydrosols shows an optical limitation. Fluences above 303 mJ/cm² cause the formation of less stable, aggregating nanoparticles, while fluences below 90 mJ/cm² do not provide sufficient energy for efficient fragmentation. The interval of fluences between 90–303 mJ/cm² is optimal for fragmentation, since stable hydrosols constituted by small, non-aggregated nanoparticles are formed.     

Tailor-made metal nanoparticles as SERS substrates

In this contribution we summarize recent experiments with the objective to generate optimized substrates for surface-enhanced Raman spectroscopy (SERS). For this purpose, the well-established laser-assisted growth technique has been applied, which relies on a precise control of the growth kinetics of supported metal nanoparticles. With this method reproducible and stable SERS substrates with tailor-made optical properties possing best field enhancements were produced for specific excitation wavelengths and detection ranges. Optimization of the SERS substrates has been achieved by stabilizing the localized surface plasmon polariton resonance (SPR) of gold nanoparticles in the vicinity of the laser wavelength of λ=647 nm and λ=785 nm used for SERS excitation. After nanoparticle preparation, SERS spectra of pyrene were obtained using naturally grown nanoparticles and nanoparticles prepared by laser-assisted growth. The most important result is that the optimized substrates prepared by laser-assisted growth exhibit a significantly higher signal-to-noise ratio as compared to naturally grown nanoparticles. They are even better than substrates whose SPR has been tuned to the excitation wavelength by an elevated temperature during preparation. Another important observation is that all SERS spectra exhibit excellent reproducibility and the substrates do not show degradation during the measurements. Finally, the SERS enhancement factors due to the optimized substrates have been estimated and are on the order of 10⁵ to 10⁶.     

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     

Ablation process of silica glass induced by laser plasma soft X-ray irradiation

Silica glass can be machined by irradiation with laser plasma soft X-rays on nano- and micrometer scale. We have investigated the ablation process of silica glass induced by laser plasma soft X-ray irradiation. We observed ionic and neutral species emitted from silica surfaces after irradiation. Dominant ions and neutrals are O⁺ and Si⁺ ions and Si, O, SiO and Si₂ neutrals, respectively. The ions have kinetic energies of 13 and 25 eV, which are much higher than those of particles emitted by evaporation. The energy of laser plasma soft X-rays absorbed to silica glass at a fluence of 1.4 J/cm² is estimated to be 380 kJ/cm³, which is higher than the binding energy of SiO₂ of 76 kJ/cm³. These results suggest that the most of the bonds in silica glass are broken by absorption of laser plasma soft X-rays, that several percent of the atoms are ionized, and that neutral atoms are emitted together with repulsive ions. The process possibly enables us to fabricate nano structures.     

Targeted destruction of murine macrophage cells with bioconjugated gold nanorods

Gold nanorods manifest a readily tunable longitudinal plasmon resonance with light and consequently have potential for use in photothermal therapeutics. Recent work by others has shown how gold nanoshells and rods can be used to target cancer cells, which can then be destroyed using relatively high power laser radiation (∼1×10⁵ to 1×10¹⁰ W/m²). Here we extend this concept to demonstrate how gold nanorods can be modified to bind to target macrophage cells, and show that high intensity laser radiation is not necessary, with even 5×10² W/m² being sufficient, provided that a total fluence of ∼30 J/cm² is delivered. We used the murine cell line RAW 264.7 and the monoclonal antibody CD11b, raised against murine macrophages, as our model system and a 5 mW solid state diode laser as our energy source. Exposure of the cells labeled with gold nanorods to a laser fluence of 30 J/cm² resulted in 81% cell death compared to only 0.9% in the control, non-labeled cells.     

Laser-assisted shape selective fragmentation of nanoparticles

Experimental results are presented on laser-assisted fragmentation of gold-containing nanoparticles suspended in liquids (either ethanol or water). Two kinds of nanoparticles are considered: (i) elongated Au nanorods synthesized by laser ablation of a gold target immersed in liquid phase; (ii) gold-covered NiCo nanorods with high aspect ratio (θ ∼ 10) synthesized by wet chemistry processes. The shape selectivity induced by laser fragmentation of these nanorods is gained via tuning the wavelength of laser radiation into different parts of the spectrum of their plasmon resonance corresponding to different aspect ratios θ. Fragmentation is performed using three laser wavelengths, involving a Cu vapour laser (510 and 578 nm) and a Nd:YAG (1064 nm). Nanoparticles are characterized by UV-vis spectrometry, Transmission Electron Microscopy (TEM). The effect of laser pulse duration (nanosecond against picosecond range) is also studied in the case of fragmentation with an IR laser radiation.     

Micromachining of a thin film by laser ablation using femtosecond laser with masks

A gold thin film was machined by laser ablation using a femtosecond laser with mask patterns in the shape of lines and numbers. The patterns were successfully transferred with proper focusing and laser fluence. The optimal femtosecond laser fluence to keep the line width was about 5.2 mJ/cm² on the mask, and 99 mJ/cm² on the film. The processing resolution was 13 μm, and the narrowest line width was about 4 μm.     

Laser-assisted etching of gallium arsenide in chlorine at 308 nm

Xenon chloride (308 nm) excimer laser-assisted etching of GaAs (100) in Cl₂ was demonstrated and characterized with respect to laser and gas parameters. The etch rate increased linearly with laser fluence from thresholds in the range of 50 to 75 mJ/cm² to the highest fluence studied, 650 mJ/cm². For a laser fluence of 370 mJ/cm², the etch rate varied with Cl₂ pressure reaching a maximum at a Cl₂ pressure of about 2 Torr. The etch rate decreased monotonically with Ar buffer gas pressure because of redeposition of GaCl₃ products into the etched channel. The redeposited GaCl₃ affected the etch rate and the etch morphology. The etch rate and morphology also varied with laser repetition rate. The mobility of chlorine on the surface also plays an important role in the etching mechanism.     

Features of plasma plume evolution and material removal efficiency during femtosecond laser ablation of nickel in high vacuum

We present an experimental characterization describing the characteristics features of the plasma plume dynamics and material removal efficiency during ultrashort, visible (527 nm, ≈300 fs) laser ablation of nickel in high vacuum. The spatio-temporal structure and expansion dynamics of the laser ablation plasma plume are investigated by using both time-gated fast imaging and optical emission spectroscopy. The spatio-temporal evolution of the ablation plume exhibits a layered structure which changes with the laser pulse fluence F. At low laser fluences (F<0.5 J/cm²) the plume consists of two main populations: fast Ni atoms and slower Ni nanoparticles, with average velocities of ≈10⁴ m/s for the atomic state and ≈10² m/s for the condensed state. At larger fluences (F>0.5 J/cm²), a third component of much faster atoms is observed to precede the main atomic plume component. These atoms can be ascribed to the recombination of faster ions with electrons in the early stages of the plume evolution. A particularly interesting feature of our analysis is that the study of the ablation efficiency as a function of the laser fluence indicates the existence of an optimal fluence range (a maximum) for nanoparticles generation, and an increase of atomization at larger fluences. PACS 52.50.-b; 52.38.Mf; 79.20.Ds; 81.07.-b     

Analyses of surface coloration on TiO2 film irradiated with excimer laser

TiO₂ film of around 850 nm in thickness was deposited on a soda-lime glass by PVD sputtering and irradiated using one pulse of krypton-fluorine (KrF) excimer laser (wavelength of 248 nm and pulse duration of 25 ns) with varying fluence. The color of the irradiated area became darker with increasing laser fluence. Irradiated surfaces were characterized using optical microscopy, scanning electron microscopy, Raman spectroscopy and atomic force microscopy. Surface undergoes thermal annealing at low laser fluence of 400 and 590 mJ/cm². Microcracks at medium laser fluence of 1000 mJ/cm² are attributed to surface melting and solidification. Hydrodynamic ablation is proposed to explain the formation of micropores and networks at higher laser fluence of 1100 and 1200 mJ/cm². The darkening effect is explained in terms of trapping of light in the surface defects formed rather than anatase to rutile phase transformation as reported by others. Controlled darkening of TiO₂ film might be used for adjustable filters.     

Influence of the laser wavelength on the epitaxial growth and electrical properties of La0.8Sr0.2MnO3 films grown by excimer laser-assisted MOD

Epitaxial La_1−xSr_xMnO₃ (LSMO) films were prepared by excimer laser-assisted metal organic deposition (ELAMOD) at a low temperature using ArF, KrF, and XeCl excimer lasers. Cross-section transmission electron microscopy (XTEM) observations confirmed the epitaxial growth and homogeneity of the LSMO film on a SrTiO₃ (STO) substrate, which was prepared using ArF, KrF, and XeCl excimer lasers. It was found that uniform epitaxial films could be grown at 500 °C by laser irradiation. When an XeCl laser was used, an epitaxial film was formed on the STO substrate at a fluence range from 80 to 140 mJ/cm² of the laser fluence for the epitaxial growth of LSMO film on STO substrate was changed. When the LaAlO₃ (LAO) substrate was used, an epitaxial film was only obtained by ArF laser irradiation, and no epitaxial film was obtained using the KrF and XeCl lasers. When the back of the amorphous LSMO film on an LAO substrate was irradiated using a KrF laser, no epitaxial film formed. Based on the effect of the wavelength and substrate material on the epitaxial growth, formation of the epitaxial film would be found to be photo thermal reaction and photochemical reaction. The maximum temperature coefficient of resistance (TCR) of the epitaxial La_0.8Sr_0.2MnO₃ film on an STO substrate grown using an XeCl laser is 4.0%/K at 275 K. XeCl lasers that deliver stabilized pulse energies can be used to prepare LSMO films with good a TCR.     

Adsorbate-induced transition between different mechanisms of laser-stimulated desorption

2 were measured as a function of laser fluence, number of laser pulses, and oxygen exposure. If the laser fluence exceeds 10 mJ/cm² desorption from clean particles occurs as a thermal reaction. Oxygen exposure as low as 1 L causes a strong decrease in the number of desorbed atoms and dimers. For larger oxygen coverages desorption of Na₂O molecules is observed and, surprisingly, the atom signal recovers. At this stage, the underlying mechanism is substantially different from that for clean particles. The results can be explained by a model that takes into account the formation of a Na₂O layer around a Na core and diffusion of Na atoms through the oxide layer prior to desorption. Received: 15 December 1997/Accepted: 16 December 1997     

Femtosecond laser ablation characteristics of nickel-based superalloy C263

Femtosecond laser (180 fs, 775 nm, 1 kHz) ablation characteristics of the nickel-based superalloy C263 are investigated. The single pulse ablation threshold is measured to be 0.26±0.03 J/cm² and the incubation parameter ξ=0.72±0.03 by also measuring the dependence of ablation threshold on the number of laser pulses. The ablation rate exhibits two logarithmic dependencies on fluence corresponding to ablation determined by the optical penetration depth at fluences below ∼5 J/cm² (for single pulse) and by the electron thermal diffusion length above that fluence. The central surface morphology of ablated craters (dimples) with laser fluence and number of laser pulses shows the development of several kinds of periodic structures (ripples) with different periodicities as well as the formation of resolidified material and holes at the centre of the ablated crater at high fluences. The debris produced during ablation consists of crystalline C263 oxidized nanoparticles with diameters of ∼2–20 nm (for F=9.6 J/cm²). The mechanisms involved in femtosecond laser microprocessing of the superalloy C263 as well as in the synthesis of C263 nanoparticles are elucidated and discussed in terms of the properties of the material.     

1064-nm laser fragmentation of thin Au and Ag flakes in acetone for highly productive pathway to stable metal nanoparticles

The laser-induced fragmentation of thin Au and Ag flakes in acetone by 1064-nm nanosecond laser (with the fluence typically ∼2 J/cm²) potentially offers a highly productive pathway to stable metal nanoparticles in liquid. Acetone serves as a superior liquid medium that keeps fine metal nanoparticles free from precipitation even in such concentrated nanoparticle solutions exceeding ∼0.1 M. Thin metal flakes have good capability to absorb the 1064-nm laser energy as efficiently as in the visible region. A part of the thus laser-heated molten flakes explosively split into submicroparticles, and some other significant part directly into fine nanoparticles. Both kinds of product particles have minor absorption cross-sections for subsequent laser pulses at 1064 nm, and thus no longer fragment further. One of the two kinds of Ag flakes studied in this work yielded fine Ag nanoparticles at a remarkable high production rate of 1.1 mg/min for the input laser power of only ∼0.65 W.     

Laser-induced growth and reformation of gold and silver nanoparticles

The sizes, shapes, and growth rates of gold and silver nanoparticles stabilized with polyvinylpyrrolidone in water can be controlled by using picosecond laser pulses. The nucleation of small metal clusters formed with NaBH₄ addition to produce nanoparticles takes two months with aging but 30 min with laser irradiation. Laser pulses can also induce nanoparticles to have narrow size and shape distribution or to undergo aggregation into much larger particles. The latter process is more likely found when the metal is silver or the irradiation wavelength is short. Laser-induced growth and shape transformation processes are explained in terms of BH₄ ⁻ depletion, metal fusion, and electron ejection followed by disintegration.     

XPS studies of short pulse laser interaction with copper

The effect of laser ablation on copper foil irradiated by a short 30 ns laser pulse was investigated by X-ray photoelectron spectroscopy. The laser fluence was varied from 8 to 16.5 J/cm² and the velocity of the laser beam from 10 to 100 mm/s. This range of laser fluence is characterized by a different intensity of laser ablation. The experiments were done in two kinds of ambient atmosphere: air and argon jet gas.The chemical state and composition of the irradiated copper surface were determined using the modified Auger parameter (α′) and O/Cu intensity ratio. The ablation atmosphere was found to influence the size and chemical state of the copper particles deposited from the vapor plume. During irradiation in air atmosphere the copper nanoparticles react with oxygen and water vapor from the air and are deposited in the form of a CuO and Cu(OH)₂ thin film. In argon atmosphere the processed copper surface is oxidized after exposure to air.     

Evolution of surface morphology of NiO thin films under swift heavy ion irradiation

NiO nanoparticle thin films grown on Si substrates were irradiated by 107 MeV Ag⁸⁺ ions. The films were characterized by glancing angle X-ray diffraction and atomic force microscopy. Ag ion irradiation was found to influence the shape and size of the nanoparticles. The pristine NiO film consisted of uniform size (∼100 nm along major axis and ∼55 nm along minor axis) elliptical particles, which changed to also of uniform size (∼63 nm) circular shape particles on irradiation at a fluence of 3 × 10¹³ ions cm⁻². Comparison of XRD line width analysis and AFM data revealed that the particles in the pristine films are single crystalline, which turn to polycrystalline on irradiation with 107 MeV Ag ions.     

CW laser-induced photothermal conversion and shape transformation of gold nanodogbones in hydrated chitosan films

We investigate the photothermal conversion and transformation of gold nanoparticles with an initial dogbone shape after dispersion in hydrated chitosan films, which is a representative model of biological tissue, and excitation by a CW diode laser for 1 min. Gold nanodogbones are observed to undergo a distinct modification above a sharp threshold of ~11 W cm⁻² and 110 °C. Surprisingly, the very same modification is achieved up to at least 36 W cm⁻² and 250 °C. We use an analytical model derived from Gans theory to associate the change in color of the films with the change in shape statistics of these gold nanoparticles. This model proves both convenient and dependable. We interpret the photothermal transformation as a rearrangement of particles with a dogbone shape and an aspect ratio of 4.1 into rods with an aspect ratio of 2.5, where material from the end lobes of the dogbones may relocate to the waists of the rods. In turn, additional transitions to stable gold nanospheres may exhibit fairly slower kinetics.     

Morphology and composition on Al surface irradiated by femtosecond laser pulses

We study the surface chemicals and structures of aluminum plates irradiated by scanning femtosecond laser pulses in air for a wide range of laser fluence from 0.38 to 33.6 J/cm². X-ray photoelectron spectroscopy and X-ray diffraction analyses indicate clearly that crystalline anorthic Al(OH)₃ is formed under femtosecond laser pulse irradiation. Besides aluminum hydroxide, crystalline Al₂O₃ is also found in the samples irradiated at high laser fluence. Field emission scanning electron microscopy demonstrates that the surfaces of the samples irradiated with low laser fluence are colloidal-like and that nanoparticles with a few nanometers in size are embedded in glue-like substances. For high laser fluence irradiated samples, the surfaces are highly porous and covered by nanoparticles with uniform size of less than 20 nm.