Formation of silicon nanoparticles and web-like aggregates by femtosecond laser ablation in a background gas

We show that the mechanism of nanoparticle formation during femtosecond laser ablation of silicon is affected by the presence of a background gas. Femtosecond laser ablation of silicon in a H₂ or H₂S background gas yields a mixture of crystalline and amorphous nanoparticles. The crystalline nanoparticles form via a thermal mechanism of nucleation and growth. The amorphous material has smaller features and forms at a higher cooling rate than the crystalline nanoparticles. The background gas also results in the suspension of plume material in the gas for extended periods, resulting in the formation (on a thin film carbon substrate) of unusual aggregated structures including nanoscale webs that span tears in the film. The presence of a background gas provides additional control of the structure and composition of the nanoparticles during short pulse laser ablation. PACS 81.16.-c     

碳纳米粒子悬浮液光限幅性能数值模拟

碳纳米粒子悬浮液具有良好的光限幅性质,是一种优良的宽波段光限幅材料。通过热传导方程和米氏散射理论建立了微气泡半径与入射光能量、碳纳米粒子悬浮液散射系数和透过率的理论模型。采用Matlab数值模拟了散射系数随微气泡尺寸因子的变化关系,碳纳米粒子悬浮液光限幅性能随入射光能量的变化规律。研究了气泡尺寸因子、入射激光能量以及波长对碳纳米粒子悬浮液光限幅特性的影响。研究发现当激光能量达到一定值时,微气泡的半径保持恒定,不再随入射激光能量的增加而增加。微气泡尺寸的增大对碳纳米粒子悬浮液的透过率有着显著的影响。同时,碳纳米粒子悬浮液对不同入射光波长和光能表现出不同的光限幅性能。研究结果为实验研究提供了理论指导。     

Enhanced nonlinear optical response of linear carbon chain colloid mixed with silver nanoparticles

Nanostructured silver and linear carbon chain (LCC) particle water colloids were prepared by a pulsed laser ablation procedure. Sample's optical transmission response was investigated in the 190–900 nm range, the third-order nonlinear optical properties were studied using the z-scan method and a nanosecond laser. The silver nanoparticles induce a structural change in the LCC colloids: the mixed Ag and LCC optical absorption looses the signature of the short carbon chain maintaining the features attributable to the longer ones. The stability of LCC colloids and their nonlinear response are remarkably improved by the Ag nanoparticles addition to the carbon water colloids. The Ag nanoparticles induce a limiting threshold reduction, an increased nonlinear absorption coefficient β and a marked asymmetrical peak/valley profile of the (Ag:LCC)_mix when compared to the LCC. All these nonlinear contributions determine the increase of the third order susceptibility, while maintaining a significantly high linear transmission value (75%) at 532 nm and high photostability. The magnitude of the nonlinear optical response of these nanohybrids makes them promising candidates for potential optoelectronic applications.     

Time-resolved analysis of nonlinear optical limiting for laser synthesized carbon nanoparticles

Nonlinear optical limiting materials have attracted much research interest in recent years. Carbon nanoparticles suspended in liquids show a strong nonlinear optical limiting function. It is important to investigate the nonlinear optical limiting process of carbon nanoparticles for further improving their nonlinear optical limiting performance. In this study, carbon nanoparticles were prepared by laser ablation of a carbon target in tetrahydrofuran (THF). Optical limiting properties of the samples were studied with 532-nm laser light, which is in the most sensitive wavelength band for human eyes. The shape of the laser pulse plays an important role for initializing the nonlinear optical limiting effect. Time-resolved analysis of laser pulses discovered 3 fluence stages of optical limiting. Theoretical simulation indicates that the optical limiting is initialized by a near-field optical enhancement effect.     

Surface nanostructuring of metals by laser irradiation: effects of pulse duration, wavelength and gas atmosphere

Surface modifications by nanostructuring present a new laser application for improvement of surface properties such as adhesion, mechanical characteristics or corrosion protection. In this study, we report the formation of nanoparticles by laser irradiation of a steel surface. The influence of laser parameters such as pulse duration (25–30 ns, 500 fs), wavelength (248 nm, 308 nm), and the background gas pressure (10 mbar-1 bar) on the formation of this back deposition layer composed of aggregated iron oxide nanoparticles were investigated. Scanning electron microscopy and atomic force microscopy were used to characterise the irradiated steel surface and the particle morphology deposited by backward flux. In the nanosecond laser ablation regime, films are formed by aggregated nanoparticles with well developed cauliflower like structures, the size and the morphology depending on the nature and pressure of the background gas. In the femtosecond regime, we observed the formation of micrometer sized structures at the steel surface. In particular, a non-conventional mechanism of nanocluster condensation and growth is revealed since two different ablation rates corresponding to two different predominant processes are observed. These analyses demonstrate the possibility of controlling the distribution and the size of particles by varying the laser parameters and the background gas pressure and nature. PACS 52.38.Mf; 81.65.-b; 81.15.Gh.     

Structural and optical properties of silicon nanoparticles prepared by pulsed laser ablation in hydrogen background gas

We studied the structural and optical properties of silicon (Si) nanoparticles (np-Si) prepared by pulsed laser ablation (PLA) in hydrogen (H₂) background gas. The mean diameter of the np-Si was estimated to be approximately 5 nm. The infrared absorption corresponding to Si-H_n (n=1,2,3) bonds was observed at around 2100 cm^-1, and a Raman scattering peak corresponding to crystalline Si was observed at around 520 cm^-1. These results indicate that nanoparticles are not an alloy of Si and hydrogen but Si nanocrystal covered by hydrogen or hydrogenated silicon. This means that surface passivated Si nanoparticles can be prepared by PLA in H₂ gas. The band-gap energy of np-Si prepared in H₂ gas (1.9 eV) was larger than that of np-Si prepared in He gas (1.6 eV) even though they are almost the same diameter. After decreasing the hydrogen content in np-Si by thermal annealing, the band-gap energy decreased, and reached the same energy level as np-Si prepared in He gas. Thus, the optical properties of np-Si were affected by the hydrogenation of the surface of np-Si. PACS 81.15.Fg; 61.46.+W; 78.67.Bf     

Optical Kerr effect exhibited by carbon nanotubes and carbon/metal nanohybrid materials

Structural modification of carbon nanotubes in combination with metallic nanoparticles is reported. An enhancement in the nonlinear optical refraction of multi-wall carbon nanotubes by the incorporation of platinum nanoparticles was observed. Comparative results were analyzed taking into account the participation of single-wall carbon nanotubes that originate a decrease in the nonlinear optical response of the multi-wall carbon nanotubes integrating a thin film. A Nd:YAG laser system featuring 532 nm wavelength with 4 ns pulse duration in a two-wave mixing experiment was employed for exploring the studied optical nonlinearities of the samples. The contribution of optical processes to mechanical characteristics dependent on high optical irradiance in carbon nanotubes was described. A variation in the mass density associated to the optically irradiated tubes allowed us to calculate the change in Young's modulus in a thin film configuration. The estimation of an opto-mechanical phenomenon was based on the evaluation of the nonlinearity of index responsible for the optical Kerr effect. According to Raman and optical evaluations, the inclusion of metallic nanoparticles in carbon structures results in a modification of surface that also gives origin to noticeable optical Kerr nonlinearities. Potential applications for developing laser-induced controlled opto-mechanical nanohybrid systems can be contemplated.     

Single-walled carbon nanotube formation with double laser vaporization technique

Single-walled carbon nanotubes (SWNTs) were prepared with double laser vaporization of a graphite target and a metal/alloy target inside an electric furnace at 1200 ^°C ambient temperature with 500 torr Ar gas atmosphere. Each target was vaporized simultaneously with a different Nd:YAG laser. Several kinds of metal/alloy target (Ni, Co, Fe, and permalloy) were tested in order to see the difference in the resulting SWNT yield and the diameter distribution of them. The Raman spectra of SWNT-containing soot prepared by use of this technique with permalloy/carbon system indicated that permalloy gives almost the same yield as compared with Ni/Co carbon composite rod with single laser vaporization technique, though the diameter distribution of them is slightly different. Also, time-resolved images of the plume by carbon and permalloy nanoparticles after laser vaporization were collected using a high-speed video camera. These images suggest that the hot plumes due to carbon and permalloy nanoparticles do not mix together so extensively, at least in a few hundred microseconds after laser vaporization. The effect of time delay between two laser pulses on the yield and the diameter distribution of SWNTs was also presented and discussed.     

In situ STM of pulsed laser nanostructured deposits: First stages of film formation

In the synthesis of nanostructured thin films the characterization of the growth processes plays a fundamental role for the control of the film and surface properties. Moreover when the deposition technique is based on the production and the assembling of nanoparticles/clusters the characterization of the precursor size distribution is of fundamental importance.We have designed a pulsed laser deposition (PLD) apparatus for the production of nanostructured thin films and surfaces, connected to a UHV variable temperature scanning tunneling microscope (STM). The whole system is devoted to the synthesis and in situ study of nanostructured and nanoporous functional metal and metal oxide films and surfaces. We have deposited W nanoparticles produced by a few hundreds laser pulses in order to investigate the initial mechanisms of the film growth. Different deposition conditions have been explored by controlling the laser generated plasma expansion through a background gas in the PLD chamber. STM measurements have been performed on W thin films deposited on different substrates to study both the size distribution and the aggregation of the precursors on the surface. Although substrate effects must be taken into account, the control of the background gas pressure and of the target-to-substrate distance allows to produce surfaces with different morphologies. This opens the possibility to tailor the material properties through the control of the size and deposition energy of the building nano-units.     

多脉冲激光对碳纳米管悬浮液光限幅特性影响

利用实验的方法研究了碳纳米管悬浮液对脉宽8 ns,波长532 nm多脉冲激光的光限幅效应.分析了直径分布为10~20 nm的多壁碳纳米管悬浮液对重复频率分别为1 Hz、 3 Hz、 5 Hz、 10 Hz情况下532 nm激光的光限幅效应,分析计算了不同重复频率下碳纳米管悬浮液的限幅阈值,比较了不同焦距的透镜会聚入射光束情况下对碳纳米管悬浮液光限幅效果的影响.实验结果表明：碳纳米管悬浮液对不同重复频率的532 nm 激光都具有较强的光限幅特性;碳纳米管悬浮液对激光在不同重复频率入射情况下的光限幅阈值变化很大,当入射激光的重复频率为5 Hz时,碳纳米管悬浮液的光限幅阈值比单脉冲激光入射时的限幅阈值低了2倍,重复频率为10 Hz时的限幅阈值比单脉冲时的限幅阈值低了近3倍;碳纳米管在紧焦系统中的光限幅效果更好.     

Synthesis and characterization of carbon coated nanoparticles produced by a continuous low-pressure plasma process

Core–shell nanoparticles coated with carbon have been synthesized in a single chamber using a continuous and entirely low-pressure plasma-based process. Nanoparticles are formed in an argon plasma using iron pentacarbonyl Fe(CO)₅ as a precursor. These particles are trapped in a pure argon plasma by shutting off the precursor and then coated with carbon by passing acetylene along with argon as the main background gas. Characterization of the particles was carried out using TEM for morphology, XPS for elemental composition and PPMS for magnetic properties. Iron nanoparticles obtained were a mixture of FeO and Fe₃O₄. TEM analysis shows an average size of 7–14 nm for uncoated particles and 15–24 nm for coated particles. The effect of the carbon coating on magnetic properties of the nanoparticles is studied in detail.     

Molecular dynamics study of nanoparticle evolution in a background gas under laser ablation conditions

Long-time evolution of nanoparticles produced by short laser interactions is investigated for different materials. To better understand the mechanisms of the nanoparticle formation at a microscopic level, we use molecular dynamics (MD) simulations to analyse the evolution of a cluster in the presence of a background gas with different parameters (density and temperature). In particular, we compare the simulation results obtained for materials with different interaction potentials (Morse, Lennard-Jones, and Embedded Atom Model). Attention is focused on the evaporation and condensation processes of a cluster with different size and initial temperature. As a result of the MD calculations, we determinate the influence of both cluster properties and background gas parameters on the nanoparticle evolution. The role of the interaction potential is discussed based on the results of the simulations.     

Optical properties of nanocomposites based on polymers and metal nanoparticles

The optical properties of nanocomposites of metal nanoparticles and polymers of two types have been studied. Gold and silver nanoparticles were obtained by laser ablation of corresponding metal targets in acetone and chloroform. The thus formed colloidal solutions were used to prepare nanocomposites of these nanoparticles in polymer matrices of polymethylmethacrylate (PMMA) and fluorine-containing polymer LF-32. The polymer matrix is found to promote aggregation of the metal nanoparticles under study into elongated chains. In turn, metal nanoparticles affect the polymer matrix. In the case of PMMA, suppression of the vibrational peaks of polymer in the low-frequency region of its Raman spectrum occurs. In the case of LF-32, gold and silver nanoparticles amplify the Raman signal of the polymer matrix. In addition, the Raman spectra of nanocomposites indicate aggregation of disordered carbon around the nanoparticles obtained by laser ablation in organic solvents. The possibilities of studying the ultrafast (about 1 ps) optical response of the nanocomposites obtained in order to use it in photonics elements are discussed.     

Laser-induced modification of composite Cu-C nanosized particles synthesized using a pulsed electrical discharge in a liquid

Composite copper-containing carbon nanosized structures were synthesized in the plasma of a pulsed electrical discharge, initiated between two graphite electrodes in an aqueous copper chloride solution. We studied the effect of laser radiation on the morphology of the nanoparticles formed, whose properties we studied by optical absorption spectroscopy and transmission electron microscopy. We discuss the mechanisms for nanoparticle formation in a discharge submerged in a liquid, and the possibilities for laser-induced modification of the nanoparticles. We estimated the temperature of the nanoparticles when exposed to laser radiation pulses. __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 75, No. 3, pp. 372–378, May–June, 2008.     

Synthesis of GaN nanocrystallites by pulsed laser ablation in pure nitrogen background gases

GaN is a promising material not only for electronic devices but also for photocatalysts. Synthesis of GaN nanocrystal is a key issue to improve performance for these applications. In the present study, GaN nanocrystallites have been synthesized by pulsed laser ablation (PLA), where safe and inactive pure N₂ gases were used as reactive background gases. The third harmonics beam of a Q-switched Nd:YAG laser (355 nm, 10 mJ/pulse, 4 J/(cm² pulse)) was used to ablate a sintered high purity GaN target. The deposition substrates were not heated. It was clarified that the formed GaN nanoparticles contained a hexagonal system with the wurtzite structure. The diameter of the nanocrystallites was about 10 nm, and showed only little dependence on the background gas pressure, while the porosity of the assembly of nanocrystallites and content of GaN nanocrystallites in the assembly increased with background gas pressure. Highly porous nanometer-sized GaN film obtained at higher gas pressure is considered to be candidate structures for the photocatalysts.     

Sticking efficiency of nanoparticles in high-velocity collisions with various target materials

In order to find reliable collector surfaces for the Mesospheric Aerosol – Genesis, Interaction and Composition (MAGIC) sounding rocket experiment, intended to collect atmospheric nanoparticles, the sticking efficiency of nanoparticles was measured on several targets of different materials. The nanoparticles were generated by a molecular beam apparatus in Jena, Germany, by laser ablation (Al₂O₃ particles, diameter 5–50 nm) and by laser pyrolysis (carbon particles, diameter 10–20 nm). In a vacuum environment (>10⁻⁵ mbar) the particles condensed from the gas phase, formed a particle beam, and were accelerated to ∼ ∼1 km/s. The sticking efficiency on the target materials carbon, gold and grease was measured by a microbalance. Results demonstrate moderate to high sticking probabilities. Thus, the capture and retrieval of atmospheric nanoparticles was found to be quantitatively feasible.     

Silver nanoparticles produced by PLD in vacuum: role of the laser wavelength used

In this work we report the results of investigation of silver (Ag) nanoparticles prepared on a silica substrate by laser ablation. Our attention was focused on the mean diameter, size distribution and optical absorption properties of nanoparticles prepared in vacuum by using different laser wavelengths. The fundamental wavelength and the second, third, and fourth harmonics of a nanosecond Nd:YAG laser were used for nanoparticles fabrication. The corresponding values of the laser fluence for each wavelength were: 0.6 J/cm² at 266 nm, 0.8 J/cm² at 355 nm, 2.8 J/cm² at 532 nm, and 2 J/cm² at 1064 nm. The Ag nanoparticles produced have mean diameters in the range from 2 nm to 12 nm as the nanoparticles’ size decreases with the decrease of the wavelength used. The presence of the Ag nanoparticles was also evidenced by the appearance of a strong optical absorption band in the measured UV-VIS spectra associated with surface plasmon resonance (SPR). A redshift and widening of the absorption peak were observed as the laser wavelength was increased. Some additional investigations were performed in order to clarify the structure of the Ag nanoparticles.     

Laser induced forward transfer of SnO2 for sensing applications using different precursors systems

This paper presents the transfer of SnO₂ by laser induced forward transfer (LIFT) for gas sensor applications. Different donor substrates of SnO₂ with and without triazene polymer (TP) as a dynamic release layer were prepared. Transferring these films under different conditions were evaluated by optical microscopy and functionality. Transfers of sputtered SnO₂ films do not lead to satisfactory results and transfers of SnO₂ nanoparticles are difficult. Transfers of SnO₂ nanoparticles can only be achieved when applying a second laser pulse to the already transferred material, which improves the adhesion resulting in a complete pixel. A new approach of decomposing the transfer material during LIFT transfer was developed. Donor films based on UV absorbing metal complex precursors namely, SnCl₂(acac)₂ were prepared and transferred using the LIFT technique. Transfer conditions were optimized for the different systems, which were deposited onto sensor-like microstructures. The conductivity of the transferred material at temperatures of about 400 ^°C are in a range usable for SnO₂ gas sensors. First sensing tests were carried out and the transferred material proved to change conductivity when exposed to ethanol, acetone, and methane.     

Ligand-free gold–silver nanoparticle alloy polymer composites generated by picosecond laser ablation in liquid monomer

Polymer matrix nanocomposites filled with metallic and alloy nanoparticles add functionality in various applications such as optical devices and in the energy sector. However, matrix coupling agents or nanoparticle ligands may be unwanted additives, potentially inhibiting the resulting nanocomposite to be processed by injection molding. The generation of stabilizer-free Au, Ag, and AuAg alloy nanoparticle acrylate composites is achieved by picosecond-pulsed laser ablation of the respective metal target in the liquid monomer. Complementary to laser ablation of the solid alloy, we have alloyed nanoparticles by post-irradiation of Au and Ag colloids in the liquid monomer. The optical properties of the colloidal nanoparticles are successfully transferred to the solid poly(methyl methacrylate) matrix and characterized by their plasmon resonance that can be easily tuned between 400 and 600 nm by laser alloying in the liquid monomer.     

Characterization of Nano‐Particles Using Laser‐Induced Incandescence

Laser‐induced incandescence (LII) is introduced as a valuable tool for the characterization of nanoparticles. This optical measurement technique is based on the heating of the particles by a short laser pulse and the subsequent detection of the thermal radiation. It has been applied successfully for the investigation of soot in different fields of application, which is described here in the form of an overview with a focus on work done at the LTT‐Erlangen during the last 10 years. In laboratory flames the soot primary particle size, volume concentration, and relative aggregate size have been determined in combination with the number density of primary particles. Furthermore, the primary particle sizes of carbon blacks have been measured in situ and online under laboratory conditions and also in production reactors. Measurements with different types of commercially available carbon black powders, which were dispersed in a measurement chamber yielded a good correlation between LII results and the specified product properties. Particle diameters determined by LII in a furnace black reactor correlate very well with the CTAB‐absorption number, which is a measure for the specific surface area. It turned out that the LII method is not affected by variations of the aggregate structure of the investigated carbon blacks. The LII signal also contains information on the primary particle size distribution, which can be reconstructed by the evaluation of the signal decay time at, at least, two different time intervals. Additionally, soot mass concentrations have been determined inside diesel engines and online measurements were performed in the exhaust gas of such engines for various engine conditions simultaneously providing information about primary particle size, soot volume, and number concentration. The LII results exhibit good correlation with traditional measurement techniques, e.g., filter smoke number measurements. In addition to the soot measurements, primarily tests with other nanoparticles like TiO₂ or metal particles are encouraging regarding the applicability of the technique for the characterization of such different types of nanoparticles.