Influence of the liquid environment on the products formed from the laser ablation of tin

Tin targets immersed in ethanol and distilled water were ablated using a UV pulsed laser. The ablated products were investigated with transmission and scanning electron microscopy, X-ray photoelectron spectroscopy, and energy dispersive spectroscopy. For ablation in both liquids, the size distribution of the produced particles was bimodal, with particles having diameters of ～10 nm and ～1 μm. Formation mechanisms that caused the bimodal distribution are suggested. Ablation in ethanol resulted in nanoparticles that were found to be single crystals of tin coated with tin hydroxide (Sn(OH)₂) while ablation in water yielded nanoparticles that were polycrystalline tin dioxide (SnO₂) throughout.     

Generation and characterization of stable, highly concentrated titanium dioxide nanoparticle aerosols for rodent inhalation studies

The intensive use of nano-sized titanium dioxide (TiO₂) particles in many different applications necessitates studies on their risk assessment as there are still open questions on their safe handling and utilization. For reliable risk assessment, the interaction of TiO₂ nanoparticles (NP) with biological systems ideally needs to be investigated using physico-chemically uniform and well-characterized NP. In this article, we describe the reproducible production of TiO₂ NP aerosols using spark ignition technology. Because currently no data are available on inhaled NP in the 10?C50 nm diameter range, the emphasis was to generate NP as small as 20 nm for inhalation studies in rodents. For anticipated in vivo dosimetry analyses, TiO₂ NP were radiolabeled with ⁴⁸V by proton irradiation of the titanium electrodes of the spark generator. The dissolution rate of the ⁴⁸V label was about 1% within the first day. The highly concentrated, polydisperse TiO₂ NP aerosol (3?C6 × 10⁶ cm^?3) proved to be constant over several hours in terms of its count median mobility diameter, its geometric standard deviation, and number concentration. Extensive characterization of NP chemical composition, physical structure, morphology, and specific surface area was performed. The originally generated amorphous TiO₂ NP were converted into crystalline anatase TiO₂ NP by thermal annealing at 950 °C. Both crystalline and amorphous 20-nm TiO₂ NP were chain agglomerated/aggregated, consisting of primary particles in the range of 5 nm. Disintegration of the deposited TiO₂ NP in lung tissue was not detectable within 24 h.     

8.1 W/670.7 nm and 5.1 W/669.6 nm cw red light outputs by intracavity frequency doubling of a Nd:YAP laser with LBO

In this paper, the continuous-wave intracavity second harmonic generation of a laser diode side-pumped Nd:YAP laser operating in the 1.3-μm region is demonstrated. A type-I critical phase-matched LiB₃O₅ (LBO) crystal was used as the frequency doubler. 8.1 W/670.7 nm and 5.1 W/669.6 nm continuous-wave red light outputs were achieved from the 1341.4-nm laser beam polarized along the c crystalline axis and the 1339.2-nm laser beam polarized along the a crystalline axis, respectively. The stability of the 670.7-nm red laser is better than 3% at the output power of 7 W in an hour.     

Single particle size and fluorescence spectra from emissions of burning materials in a tube furnace to simulate burn pits

A single-particle fluorescence spectrometer (SPFS) and an aerodynamic particle sizer were used to measure the fluorescence spectra and particle size distribution from the particulate emissions of 12 different burning materials in a tube furnace to simulate open-air burning of garbage. Although the particulate emissions are likely dominated by particles <1 μm diameter, only the spectra of supermicron particles were measured here. The overall fluorescence spectral profiles exhibit either one or two broad bands peaked around 300–450 nm within the 280–650 nm spectral range, when the particles are illuminated with a 263-nm laser. Different burning materials have different profiles, some of them (cigarette, hair, uniform, paper, and plastics) show small changes during the burning process, and while others (beef, bread, carrot, Styrofoam, and wood) show big variations, which initially exhibit a single UV peak (around 310–340 nm) and a long shoulder in visible, and then gradually evolve into a bimodal spectrum with another visible peak (around 430–450 nm) having increasing intensity during the burning process. These spectral profiles could mainly derive from polycyclic aromatic hydrocarbons with the combinations of tyrosine-like, tryptophan-like, and other humic-like substances. About 68 % of these single-particle fluorescence spectra can be grouped into 10 clustered spectral templates that are derived from the spectra of millions of atmospheric aerosol particles observed in three locations; while the others, particularly these bimodal spectra, do not fall into any of the 10 templates. Therefore, the spectra from particulate emissions of burning materials can be easily discriminated from that of common atmospheric aerosol particles. The SFFS technology could be a good tool for monitoring burning pit emissions and possibly for distinguishing them from atmospheric aerosol particles.     

In-process ZnO thin films alloying during pulsed laser deposition

Alloyed ZnO:Al thin films were prepared by a pulsed laser deposition by the altering of two pure targets (ZnO and Al) during the deposition process. Two deposition temperatures (?197 °C and 400 °C) were applied and differences of diffusion dynamics were compared. As-grown layered films were annealed and aluminium distribution was homogenized. The results revealed that the amorphous structure (samples grown at ?197 °C) of ZnO provide more positive conditions for efficient Al diffusion in comparison with crystalline structure (samples grown at 400 °C). A detailed investigation by SIMS depth profiling confirmed a homogeneous chemical composition of annealed and recrystallized films which exhibited a porous nature and wurtzite crystalline structure.     

Sub-100-nm laser-induced periodic surface structures upon irradiation of titanium by Ti:sapphire femtosecond laser pulses in air

The formation of laser-induced periodic surface structures (LIPSS) on titanium upon irradiation with linearly polarized femtosecond (fs) laser pulses (τ=30 fs, λ=790 nm) in an air environment is studied experimentally and theoretically. In the experiments, the dependence on the laser fluence and the number of laser pulses per irradiation spot has been analyzed. For a moderate number of laser pulses (N<1000) and at fluences between ～0.09 and ～0.35 J/cm², predominantly low-spatial-frequency-LIPSS with periods between 400 nm and 800 nm are observed perpendicular to the polarization. In a narrow fluence range between 0.05 and 0.09 J/cm², high-spatial-frequency-LIPSS with sub-100-nm spatial periods (～λ/10) can be generated with an orientation parallel to the polarization (N=50). These experimental results are complemented by calculations based on a theoretical LIPSS model and compared to the present literature.     

Photoinduced darkening, crystallization and reversible emission in ZnTe thin films

Laser-induced darkening and crystallization of ZnTe-based thin films is reported. ZnTe thin films of 1500-nm thickness were deposited on bare and Zn buffer layered borosilicate glass substrates. The as-deposited films were subjected to laser irradiation at 532 nm. The as-deposited films were amorphous but transformed to the crystalline state under influence of the laser treatment. The X-ray diffraction patterns revealed that the ZnTe crystallized in the zinc blende structure. In addition, presence of peaks from Te was observed, signifying the dissociation of ZnTe. The spectral transmission of the films decreased by more than 15 % under the influence of the laser irradiation and this was accompanied by a red shift in the band gap. These results clearly point to the occurrence of laser-induced darkening and crystallization of the films. To understand the mechanisms of darkening and crystallization, all the films were annealed at 500 °C for 60 min. Similar to the laser-irradiated samples, the thermally annealed films showed an amorphous–crystalline transition, presence of Te in the X-ray diffraction patterns as well as a large decrease in spectral transmission (>70 %). Photoinduced emission analysis carried out as a function of laser intensities indicated a strong red shift of about 51 meV in emission energy with increase in laser intensity due to the photodarkening. The peak position of the emission spectrum can be tuned by increasing the laser intensity and is completely reversible with decrease in laser intensity. It is proposed that laser-induced darkening occurs due to the dissociation of ZnTe into ZnTe and Te and that crystallization is a consequence of laser annealing.     

Ni nanoparticles fabricated by laser ablation in water

Nickel nanoparticles were fabricated by ablating a bulk Ni target with pulsed 337-nm laser radiation in distilled water. Transmission electron microscope images of the removed material show spherical particles with two size scales: tens of nm and hundreds of nm. Phase explosion and Rayleigh–Plateau hydrodynamic instability are suggested as being responsible for this distribution. An X-ray diffraction pattern of the ablated material demonstrates the presence of both nickel and nickel oxide.     

The growth of Y2SiO5:Ce thin films with pulsed laser deposition

Y₂SiO₅:Ce phosphor thin films were grown onto Si(100) substrates with pulsed laser deposition (PLD) using a 248-nm KrF excimer laser. Process parameters were varied during the growth process and the effect on the surface morphology and cathodoluminescence (CL) was analysed. The process parameters that were changed included the following: gas pressure (vacuum (5×10⁻⁶ Torr), 1×1⁻² Torr and 1 Torr O₂), different gas species (O₂, Ar and N₂ at a pressure of 455 mTorr), laser fluence (1.6±0.1 J cm⁻² and 3.0±0.3 J cm⁻²) and substrate temperature (400 and 600°C). The surface morphology was investigated with atomic force microscopy (AFM). The morphology of the thin films ablated in vacuum and 10 mTorr ambient O₂ showed more or less the same trend. An increase in the pressure to 1 Torr O₂, however, showed a definite increase in deposited particle sizes. Ablation in N₂ gas resulted in small particles of 20 nm in diameter and ablation in O₂ gas produced bigger particles of 20, 30 and 40 nm as well as an agglomeration of these particles into bigger size clusters of 80 to 100 nm. Ablation in Ar gas led to particle sizes of 30 nm and the particles were much more spherically defined and evenly distributed on the surface. The higher fluence deposition led to bigger particle and grain sizes as well as thicker layers with respect to the lower fluence. The particle sizes of the higher fluence vary mainly between 130 and 140 nm and the lower fluence sizes vary between 50 and 60 nm. The higher fluence particles consist of smaller particles ranging from 5 to 30 nm as measured with AFM. The surface structure of the thin film ablated at 400°C substrate temperature is less compact (lesser agglomeration of particles than at 600°C). The increase in substrate temperature definitely resulted in a rougher surface layer. CL was measured to investigate the effect of the surface morphology on the luminescent intensities. The increased O₂ ambient (1 Torr) resulted in a higher CL intensity compared to the thin films ablated in vacuum. The thin film ablated in Ar gas showed a much higher CL intensity than the other thin films. Ablation at a high fluence resulted in a higher CL intensity. The higher substrate temperature resulted in better CL intensities. The more spherically shaped particles and rougher surface led to increase CL intensities.     

Workplace air measurements and likelihood of exposure to manufactured nano-objects,agglomerates, and aggregates

Workplace exposure to nanoparticles from gas metal arc welding (GMAW) process in an automobile manufacturing factory was investigated using a combination of multiple metrics and a comparison with background particles. The number concentration (NC), lung-deposited surface area concentration (SAC), estimated SAC and mass concentration (MC) of nanoparticles produced from the GMAW process were significantly higher than those of background particles before welding (P < 0.01). A bimodal size distribution by mass for welding particles with two peak values (i.e., 10,000–18,000 and 560–320 nm) and a unimodal size distribution by number with 190.7-nm mode size or 154.9-nm geometric size were observed. Nanoparticles by number comprised 60.7 % of particles, whereas nanoparticles by mass only accounted for 18.2 % of the total particles. The morphology of welding particles was dominated by the formation of chain-like agglomerates of primary particles. The metal composition of these welding particles consisted primarily of Fe, Mn, and Zn. The size distribution, morphology, and elemental compositions of welding particles were significantly different from background particles. Working activities, sampling distances from the source, air velocity, engineering control measures, and background particles in working places had significant influences on concentrations of airborne nanoparticle. In addition, SAC showed a high correlation with NC and a relatively low correlation with MC. These findings indicate that the GMAW process is able to generate significant levels of nanoparticles. It is recommended that a combination of multiple metrics is measured as part of a well-designed sampling strategy for airborne nanoparticles. Key exposure factors, such as particle agglomeration/aggregation, background particles, working activities, temporal and spatial distributions of the particles, air velocity, engineering control measures, should be investigated when measuring workplace exposure to nanoparticles.     

Synthesis,characterization, and thermal stability of SiO<Subscript>2</Subscript>/TiO<Subscript>2</Subscript>/CR-Ag multilayered nanostructures

The controllable synthesis and characterization of novel thermally stable silver-based particles are described. The experimental approach involves the design of thermally stable nanostructures by the deposition of an interfacial thick, active titania layer between the primary substrate (SiO₂ particles) and the metal nanoparticles (Ag NPs), as well as the doping of Ag nanoparticles with an organic molecule (Congo Red, CR). The nanostructured particles were composed of a 330-nm silica core capped by a granular titania layer (10 to 13 nm in thickness), along with monodisperse 5 to 30 nm CR-Ag NPs deposited on top. The titania-coated support (SiO₂/TiO₂ particles) was shown to be chemically and thermally stable and promoted the nucleation and anchoring of CR-Ag NPs, which prevented the sintering of CR-Ag NPs when the structure was exposed to high temperatures. The thermal stability of the silver composites was examined by scanning electron microscopy (SEM) and high-resolution transmission electron microscopy (HRTEM). Larger than 10 nm CR-Ag NPs were thermally stable up to 300 °C. Such temperature was high enough to destabilize the CR-Ag NPs due to the melting point of the CR. On the other hand, smaller than 10 nm Ag NPs were stable at temperatures up to 500 °C because of the strong metal-metal oxide binding energy. Energy dispersion X-ray spectroscopy (EDS) was carried out to qualitatively analyze the chemical stability of the structure at different temperatures which confirmed the stability of the structure and the existence of silver NPs at temperatures up to 500 °C.     

Enhanced broadband light absorption in silicon film by large-size lumpy silver particles

Broadband light absorption enhancement in crystalline silicon thin-film solar cells by rear-located 400 nm lumpy silver particles has been studied, based on the theoretical simulations of 3D finite-difference time-domain method. By simulations, we have investigated the light scattering properties of 400 nm lumpy Ag particles and put it to silicon thin-film solar cells. In addition, the varying rear-located Ag particles coverage and two surface situations of silicon films, which could influence on the light absorption of solar devices, have also been comprehensively considered. The results have shown that rear-located 400 nm lumpy Ag particles would enhance the absorption in silicon films in a broadband range. And it has been proved that 20 % coverage density of rear-located Ag particles is optimal for improving the light absorption of smooth silicon thin-film solar devices. When we create rough surface on one or both sides of silicon films, the absorbed light would further increase, and the theoretical maximum enhancement is 15.1 % compared with the smooth silicon thin-film solar cell without Ag particles.     

Applications of excimer laser in nanofabrication

This paper addresses novel applications of an excimer laser (308 nm wavelength, 20 ns pulse duration) in nanofabrication. Specifically, laser assisted nanoimprint lithography (LAN), self-perfection by liquefaction (SPEL), fabrication of metal nanoparticle arrays, and the fabrication of sub-10-nm nanofluidic channels are covered. In LAN, a polymeric resist is melted by the laser pulse, and then imprinted with a fused silica mold within 200 ns. LAN has been demonstrated in patterning various polymer nanostructures on different substrates with high fidelity and uniformity, and negligible heat effect on both the mold and the substrate. SPEL is a novel technology that uses selective melting to remove fabrication defects in nanostructures post fabrication. Depending on the boundary conditions, SPEL is categorized into three basic types: Open-SPEL that takes place with surface open, Capped-SPEL where a cap plate holds the top surface of the nanostructures and Guided-SPEL where a plate held a distance above the structure guides the molten materials to rise and form a new structure with better profile. Using SPEL (in less than 200 ns), we have achieved a reduction of line edge roughness (LER) of Cr lines to 1.5 nm (3σ) (560% improvement from the original), which is well below what the previous technologies permit, and a dramatic increase of the aspect ratio of a nanostructure. We have used SPEL to make sub-25-nm smooth cylindrical NIL pillar molds and smoothing Si waveguides. Excimer laser is also used to make metal nanoparticles. Monolayers of particles are fabricated on various substrates (silicon, fused silica and plastics) by exposing thin metal films to a single laser pulse. Periodic nanoparticle arrays have been fabricated by fragmentation of metal grating lines. The periodicity of these nanoparticles can be regulated by surface topography such as shallow trenches. Finally, an excimer laser pulse has been used to melt the top portion of 1D and 2D Si gratings to seal off the top surface, forming enclosed nanofluidic channel arrays. The channel width has been further reduced to 9 nm using self-limited thermal oxidation. DNA stretching using 20 nm wide self-sealed channels is also demonstrated.     

Laser cleaning of polymer surfaces

We have investigated the removal of small spherical particles from polymer surfaces by means of 193-nm ArF and 248-nm KrF laser light. Polystyrene (PS) particles with diameters in the range of 110 nm to 1700 nm and silica particles (SiO₂) with sizes of 400 nm and 800 nm are successfully removed from two different substrates, polyimide (PI) and polymethylmethacrylate (PMMA). Experiments were performed in air (23 °C, relative humidity 24–28%) and in an environment with a relative humidity (RH) of about 90%. Received: 13 July 2000 / Accepted: 14 July 2000 / Published online: 9 November 2000     

Amorphous-like Raman spectra of GaP microcrystals

Raman scattering from gas-evaporated GaP microcrystals smaller than about 400 Å has been investigated. As the crystalline size decreases from ～400 to ～170 Å, drastic changes in the Raman spectrum are observed; the TO and surface phonon peaks broaden and shift, and they strongly overlap with each other, finally transforming into a broad structure; broad bands located at around 80 and 200 cm^-1 appear and grow rapidly. The microcrystals smaller than about 250 Å show spectra very similar to those of amorphous GaP, even though the electron diffraction patterns prove that they are crystalline. The amorphous-like Raman signals seem to come from the surface layers of the microcrystals.     

Disorder and suppression of quantum confinement effects in Pd nanoparticles

Size-selectable ligand-passivated crystalline and amorphous Pd nanoparticles (<4 nm) are synthesized by a novel two-phase process and verified by high-resolution transmission electron microscopy. Scanning tunneling spectroscopy preformed at 5 K on these two types of nanoparticles exhibits clear Coulomb blockade and Coulomb staircases. Size dependent multipeak spectral features in the differential conductance curve are observed for the crystalline Pd particles but not for the amorphous particles. Theoretical analysis shows that these spectral features are related to the quantized electronic states in the crystalline Pd particle. The suppression of the quantum confinement effect in the amorphous particle arises from the reduction of the degeneracy of the eigenstates and the level broadening due to the reduced lifetime of the electronic states.     

Ion beam shaping of embedded metal nanoparticles by Si+ ion irradiation

Fine Co and Pt nanoparticles are nucleated when a silica sample is implanted with 400 keV Co⁺ and 1370 keV Pt⁺ ions. At the implanted range, Co and Pt react to form small Co _x Pt_(1?x) nanoparticles during Si⁺ ion irradiation at 300 °C. Thermal annealing of the pre-implanted silica substrate at 1000 °C results in the formation of spherical nanoparticles of various sizes. When irradiated with Si⁺ ions at 300 °C, particles in the size range of 5–17 nm undergo rod-like shape transformation with an elongation in the direction of the incident ion beam, while those particles in the size range of 17–26 nm turn into elliptical shape. Moreover, it is suspected that very big nanoparticles (size >26 nm) decrease in size, while small nanoparticles (size <5 nm) do not undergo any transformation. During Si⁺ ion irradiation, the crystalline nature of the nanoparticles is preserved. The results are discussed in the light of the thermal spike model.     

A study of oleic acid-based hydrothermal preparation of CoFe<Subscript>2</Subscript>O<Subscript>4</Subscript> nanoparticles

Nearly monodisperse, well crystalline, superparamagnetic CoFe₂O₄ nanoparticles with diameter of 6 nm were synthesized in oleic acid–water–pentanol system at 180 °C. Hydrothermal procedure, as an efficient and environment friendly alternative to organic decomposition methods, was investigated by variation of reaction conditions, and the particle formation mechanism was finally proposed (i.e., hydrolysis of metal oleates in organic phase, with size of the particles (5–8 nm) controlled by polarity-driven precipitation into water phase). As-prepared particles were hydrophobic due to coating by oleic acid. Further modification with dimercaptosuccinic acid led to water-dispersible particles with hydrodynamic diameter of 20 nm. Prepared particles were investigated by TEM, XRD, ICP-AES, light scattering, SQUID magnetometry, and Mössbauer spectroscopy.     

Characterization and electrical properties of high-<Emphasis Type="Italic">k</Emphasis> GdScO<Subscript>3</Subscript> thin films grown by atomic layer deposition

Gadolinium scandium oxide (Gd-scandate, GdScO₃) thin films were grown by atomic layer deposition (ALD) from β-diketonate precursors M(thd)₃ (M=Gd, Sc; thd=2,2,6,6-tetramethyl-3,5-heptanedionato) and ozone. The deposition parameters were optimized to produce films with the stoichiometric 1:1 metal ratio and a series of samples with nominal thicknesses of 5, 10, 15, and 20 nm were prepared. At 300 °C the metal precursor pulsing ratio Gd:Sc=5:6 yielded amorphous stoichiometric films and a growth rate of 0.21 Å/cycle. The films stayed amorphous up to 900 °C. The surface was probed with an AFM and the rms roughness was found to be 0.3 nm for the 5–20 nm thick films. The electrical properties of the as-deposited films proved to be very promising, with a dielectric constant of ～22 and leakage current density of 340 μA/cm², measured at -2 V.