Particle size and chemical composition effects on elemental analysis with the nano aerosol mass spectrometer

In the Nano Aerosol Mass Spectrometer (NAMS), particles are irradiated with a high energy laser pulse to produce a plasma that quantitatively disintegrates each particle into positively charged atomic ions. Previous work with this method used electrodynamic focusing and trapping of particles 30 nm dia. and below. In the current work, an aerodynamic focusing inlet was used to study particles between 40 and 150 nm dia. The distribution of atomic ion charge states was found to be particle size dependent, shifting toward lower charges with increasing size. This shift also affected the calibration by which elemental composition was determined from atomic ion signal intensities. Size independent calibration could be achieved by restricting the analysis to particles that gave more than 90% of the total signal intensity as multiply charged ions. This approach worked best for particles smaller than about 100 nm dia. since most spectra met this criterion. For the nanoparticles studied, the elemental mole fractions of Group I and II metals, halogens, and low atomic mass nonmetals could be determined within 10% or less of the expected value when the mole fraction was at the 1% level or greater. Some transition and heavy metals could not be quantified, while others could. Quantification appeared to be dependent on the ability of the element to be vaporized. Elements with high melting and boiling points gave particle mass spectra similar to those obtained by laser desorption ionization—mostly singly charged ions with relative intensities strongly biased toward atoms with low ionization energies.

Copyright © 2017 American Association for Aerosol Research     

Laser fabrication and spectroscopy of organic nanoparticles

In working with nanoparticles, researchers still face two fundamental challenges: how to fabricate the nanoparticles with controlled size and shape and how to characterize them. In this Account, we describe recent advances in laser technology both for the synthesis of organic nanoparticles and for their analysis by single nanoparticle spectroscopy. Laser ablation of organic microcrystalline powders in a poor solvent has opened new horizons for the synthesis of nanoparticles because the powder sample is converted directly into a stable colloidal solution without additives and chemicals. By tuning laser wavelength, pulse width, laser fluence, and total shot number, we could control the size and phase of the nanoparticles. For example, we describe nanoparticle formation of quinacridone, a well-known red pigment, in water. By modifying the length of time that the sample is excited by the laser, we could control the particle size (30-120 nm) for nanosecond excitation down to 13 nm for femtosecond irradiation. We prepared beta- and gamma-phase nanoparticles from the microcrystal with beta-phase by changing laser wavelength and fluence. We present further results from nanoparticles produced from several dyes, C(60), and an anticancer drug. All the prepared colloidal solutions were transparent and highly dispersive. Such materials could be used for nanoscale device development and for biomedical and environmental applications. We also demonstrated the utility of single nanoparticle spectroscopic analysis in the characterization of organic nanoparticles. The optical properties of these organic nanoparticles depend on their size within the range from a few tens to a few hundred nanometers. We observed perylene nanoscrystals using single-particle spectroscopy coupled with atomic force microscopy. Based on these experiments, we proposed empirical equations explaining their size-dependent fluorescence spectra. We attribute the size effect to the change in elastic properties of the nanocrystal. Based on the results for nanoparticles of polymers and other molecules with flexible conformations, we assert that size-dependent optical properties are common for organic nanoparticles. While "electronic confinement" explains the size-dependent properties of inorganic nanoparticles, we propose "structural confinement" as an analogous paradigm for organic nanoparticles.     

In-situ characterization of ultrafine particles by laser-induced incandescence: sizing and particle structure determination

The laser-induced incandescence (LII) method is applied to the in situ size analysis of aerosol particles of different origin at room temperature. A detailed theoretical model of the particle heating and cooling for the different size fractions incorporating a solution of a Fredholm integral equation of the first kind is used to retrieve the particle size distribution from the time-dependent aerosol thermal emission detected after a ns laser pulse. The results are compared with TEM data of deposited aerosol particles along with online measurements employing a differential mobility analyzer (DMA). Besides the size distribution, the LII signal contains information on the internal structure of particle agglomerates, which can be obtained by analyzing the changes in the measured size distribution with the laser pulse energy. The objective of the paper is an evaluation of LII for its capability to measure the size distributions of various types of aerosols in the size range about 5–200 nm and to determine the primary particle sizes in the case of agglomerated particles.     

Nanoparticle removal from substrates with pulsed-laser induced plasma and shock waves

Removing particles with nanometer scale diameters from substrates is a challenging task with numerous critical applications. A novel removal method for nanoparticles is developed and tested. The technique, which is dry and non-contact, takes advantages of shock wavefronts initiated by plasma formation under a focused laser beam pulse and its interaction with the substrate. Experimental results indicate that silica particles down to 500 nm on silicon wafers can be removed without substrate damage. In the reported experiments, a Q-switched Nd : YAG pulsed laser with a 5-ns pulse width and 360-mJ pulse energy at 1064 nm wavelength is employed as a plasma generation source. It is reported that the traditional dry laser cleaning method based on the rapid thermal expansion under direct laser irradiation often results in surface damage in the nanometer scale due to light diffraction around nanoparticles and/or stress localization in the thermal skin. This occurs when the characteristic dimensions of the particles are comparable to the wavelength of incident beam. In the laser plasma method, various mechanisms are responsible for the removal effect. The strong shock wave in air generates complex pressure wavefields resulting in both drag/lift on the particle and acceleration of the substrate. However, shock waves are not transmitted to the solid substrate due to a large difference between the relevant wave phase speeds in the two media. The effects of the number of shots and the distance between the surface and the plasma boundary on the removal efficiency are reported.     

Influence of pulsed arc parameters on powder production in ethanol

Powder consisting nickel and carbon particles were synthesized using a pulsed arc between Ni electrodes submerged in pure ethanol. The ethanol was arc treated for 5 min with 20 and 40 μs duration pulses, at a repetition rate of 100 Hz. The pulse energy was varied in a range of 7.7-192 mJ. Powder samples were obtained by extracting liquid from the treatment vessel after a pre-determined sedimentation time, or by allowing the liquid to evaporate from the vessel, and collecting the residue. The samples were examined by HRSEM, EDX and XRD. Dependencies of the particle structure and size distribution, and the powder production rate and composition, on the pulse energy and duration were studied.The powder samples consisted of nickel and carbon particles. The surface of the nickel particle had a carbon coating. The Ni concentration increased from 32% to 46%, and the C concentration decreased from 68% to 54%, when the pulse energy was increased from 7.7 to 100 mJ with 20 μs pulses. For 40 μs pulses the same changes of Ni and C concentrations were obtained when the pulse energy increased from 60 to 100 mJ. The production rate of the Ni and C particles linearly increased with pulse energy.The particle quantity and size distribution width increased with pulse energy. The maximum particle diameter increased from 70 to 550 nm while its minimum diameter remained ∼ 50 nm when the pulse energy increased from 7.7 to 48 mJ with pulse duration of 20 μs.     

Field emission enhancement and microstructural changes of carbon films by single pulse laser irradiation

The effect of single nanosecond laser pulse irradiation on the microstructure and field emission (FE) properties of carbon films is studied. Amorphous carbon films were exposed to a single pulse of a 248 nm Excimer laser with pulse width of 23 ns. Microstructural changes of the films were investigated by Raman spectroscopy, transmission electron microscopy and electron energy loss spectroscopy. FE study was conducted in a parallel plate configuration. It was found that the landscape of the FE properties is not directly correlated to the laser energy in a simple way, whereas low energy laser irradiation (<117 mJ/cm²) leads to a lower emission threshold field due to the formation of sub-nanometer conductive sp² clusters within the insulating sp³ matrix. A medium energy range (117–362.5 mJ/cm²) would actually reduce field enhancement and increase the threshold field because of the increased size of the same sp² clusters. Interestingly, a much higher laser energy (>362.5 mJ/cm²) would reverse this effect by forming multiple continuous conductive sp² channels and thereby reduce the threshold field sharply again.     

Large-scale synthesis of CuS nanoparticles for photothermal materials using high-concentration Cu complex ion precursor

Copper sulfide (CuS), a copper-deficient p-type semiconductor material, has been widely utilized due to its unique optical properties, low toxicity, and cost-effectiveness. Although many studies have been conducted on synthesizing CuS nanoparticles, harsh synthetic conditions and low yield must be solved. This study presents a new methodology that can synthesize CuS nanoparticles in large quantities at room temperature and pressure using high-concentration Cu complex ion precursors. This methodology is based on the theory that the critical nucleus radius and the critical nucleation free energy decrease as the concentration of the precursor increases to synthesize a large number of nanoparticles by applying low energy. In addition, it is possible to minimize the aggregation of nanoparticles, which is a problem of nanoparticles synthesized at a high precursor concentration through complex ions in the solution. We synthesized nanoparticles by controlling the precursor concentration from 0.1 to 2.5 M to confirm the effect of the precursor concentration on the size, shape, and yield of nanoparticles. As the precursor concentration increased, the particle size decreased, and the yield improved. The CuS nanoparticles synthesized at the highest concentration had a size of about 17 nm without a strong agglomeration and a yield of about 213.9 g/L. Furthermore, the nanoparticles showed excellent photothermal performance due to their high near-infrared absorption. When about 0.1 g of the nanoparticles were irradiated with a Xenon lamp and an 808 nm laser, the maximum temperatures and rising rates were 53.7°C and 172.1°C and 13.8°C/mg and 33°C/mg, respectively. The excellent photothermal properties of CuS nanoparticles suggest the potential of this material for various applications.     

Preparation and adsorption properties of chitosan–poly(acrylic acid) nanoparticles for the removal of nickel ions

Chitosan (CS) nanoparticles with different mean sizes ranging from 100 to 195 nm were prepared by ionic gelation of CS and poly(acrylic acid) (PAA). Variations in the final solution pH value and CS : PAA volume ratio were examined systematically for their effects on nanoparticle size, intensity of surface charge, and tendency toward particle aggregation. The sorption capacity and sorption isotherms of the CS–PAA nanoparticles for nickel ions were evaluated. The parameters for the adsorption of nickel ions by the CS–PAA nanoparticles were also investigated. The CS–PAA nanoparticles could sorb nickel ions effectively. The sorption rate for nickel ions was affected significantly by the initial concentration of the solution, sorbent amount, particle size, and pH value of the solution. The samples of nanoparticles were well correlated with Langmuir's isotherm model, and the adsorption kinetics of nickel correlated well with the pseudo‐second‐order model. The maximum capacity for nickel sorption deduced from the use of the Langmuir isotherm equation was 435 mg/g, which was significantly higher than that of the micrometer‐sized CS. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 2008     

Laser-induced spark for measurements of the fuel-to-air ratio of a combustible mixture

This work investigates the use of the laser-induced gas breakdown for fuel-to-air ratio measurements. In essence, we examine the late time behavior of the line radiation at the wavelength of the H_α-lines and the O I triplet emitted from the laser-induced spark in CH₄-air mixtures. Sparks were produced using a single-mode, Q-switched Nd-YAG laser. The laser produced a beam of 6 mm in diameter at the wavelength of 1064 nm and a pulse duration of 5.5 ns. For the equivalence ratio from 0.1 to 5.0, the radiation intensity ratio of the H_α-lines to the O I triplet increased linearly with the equivalence ratio. For the laser energy from 10 to 50 mJ it was independent of the laser energy when the laser energy was higher than 20 mJ. The technique, therefore, has a potential for measuring the fuel-to-air ratio of a combustible flow environment.     

Temporal response of UV sensors made of highly oriented diamond films by 193 and 313 nm laser pulses

Ultraviolet sensors were fabricated on a Si substrate using an undoped, highly oriented diamond film with Pt and Al interdigited electrodes with a gap length of 5, 10 and 15 μm. Transient voltage outputs due to photocurrent of the sensors under a bias voltage of 20, 40 and 80 V were measured by the pulsed irradiation of an ArF excimer laser (λ=193 nm, pulse width=5 ns) or a dye laser (λ=313 nm, pulse width=7 ns). It was found that the transient voltage output by the ArF laser irradiation on a sensor (Pt electrodes with a 15-μm gap) consisted of main and second peaks at 1.1 and 3.8 ns, respectively, when the bias voltage was 20 V. The FWHM was approximately 4 ns. The second peak was observed in the output signals of most sensors. On the other hand, the voltage output was two orders of magnitude weaker when the sensors were irradiated by the dye laser.     

Nanoparticle decoration of carbon nanotubes by sputtering

Vapor phase growth of gold, nickel and titanium metal nanoparticles on multiwall carbon nanotube (MWCNT) buckypaper by sputtering was investigated. The size and distribution of nanoparticles was dependent on the intrinsic binding energy of the metal elements, but could be altered to mimic that of metals with different binding energies by in situ modification of the MWCNT surfaces by energetic metal ions or annealing of the buckypaper. A range of average gold particle diameters from approximately 5–30 nm could be produced depending on the intrinsic sputter process parameters (especially metal ion flux and kinetic energy) and defect density of the MWCNT surfaces, which could also be controlled by annealing prior to sputtering. The diameter of the MWCNTs had a significant influence on the geometry of the nanoparticles. Particles were elongated along the nanotube axis for tube diameters <30 nm. Remarkably strong alignment of the particles along the nanotube axis was observed, especially for MWCNTs with higher defect densities.     

Efficiency studies of particle removal with pulsed-laser induced plasma

The detachment and removal of micro- and nanoparticles from surfaces is of importance in many industries. The cleaning of silicon wafers is of great interest in the semiconductor, microelectronics, and optics industries. The development and adoption of dry, rapid, non-contact, and non-damaging particle removal technologies is a critical process. The proposed laser-induced plasma (LIP) removal technique is a novel method for detaching and removing fine particles from substrates. The current technique is a dry, non-contact method that takes advantage of the strong shock wavefront from expanding plasma, created by focusing a laser pulse in air above the substrate. The transient pressure field acts on the target particles to produce a rotational moment and a rolling mode of detachment from the substrate. In the current study, the LIP removal technique is employed repeatedly to remove particles over an area of a silicon wafer, and a systematic efficiency study for the removal effectiveness is performed. A Q-switched Nd:YAG pulsed laser operating at 1064 nm with a 370 mJ pulse energy and 5 ns pulse length is used in experiments. 0.99 μm diameter silica spheres and 3.063 μm diameter polystyrene spheres were successfully detached and removed with no substrate damage. The removal efficiency at various gap distances between plasma core and substrate is determined and reported. This work is the first laboratory demonstration of the LIP technique over an extended area. The reported results substantiate the LIP removal technique as a serious option for particle detachment and removal from extended areas.     

Fabrication of Monodispersed Barium Titanate Nanoparticles with Narrow Size Distribution

Pulsed laser ablation (PLA) was used to prepare nanoparticles of barium titanate (BaTiO₃). PLA using a titanium-rich Ba–Ti–O ceramic target in oxygen ambience at 400 Pa and following online annealing at 1073 K enabled the production of BaTiO₃ nanoparticles (BTNPs) with a perovskite structure having nearly stoichiometric composition. These nanoparticles were directly introduced into a differential mobility analyzer for size classification, leading to the production of monodispersed and nonagglomerated BTNPs with an average particle size of 13.5 nm. The size distribution of the size-classified BTNPs was found to be reasonably narrow with a geometric standard deviation of 1.07.     

Hydrodynamic modelling of binary mixture in a gas bubbling fluidized bed using the kinetic theory of granular flow

A multi-fluid Eularian CFD model with closure relationships according to the kinetic theory of granular flow has been applied to study the motions of particles in the gas bubbling fluidized bed with the binary mixtures. The mutual interactions between the gas and particles and the collisions among particles were taken into account. Simulated results shown that the hydrodynamics of gas bubbling fluidized bed related with the distribution of particle sizes and the amount of energy dissipated in particle-particle interaction. In order to obtain realistic bed dynamics from fundamental hydrodynamic models, it is important to correctly take the effect of particle size distribution and energy dissipation due to non-ideal particle-particle interactions into account.     

Influence of Dose on Particle Size and Optical Properties of Colloidal Platinum Nanoparticles

Attempts to produce colloidal platinum nanoparticles by using steady absorption spectra with various chemical-based reduction methods often resulted in the fast disappearance of the absorption maxima leaving reduced platinum nanoparticles with little information on their optical properties. We synthesized colloidal platinum nanoparticles in an aqueous solution of polyvinyl pyrrolidone by gamma radiolytic reduction method, which produced steady absorption spectra of fully reduced and highly pure platinum nanoparticles free from by-product impurities or reducing agent contamination. The average particle size was found to be in the range of 3.4–5.3 nm and decreased with increasing dose due to the domination of nucleation over ion association in the formation of metal nanoparticles by the gamma radiolytic reduction method. The platinum nanoparticles exhibit optical absorption spectra with two absorption peaks centered at about 216 and 264 nm and the peaks blue shifted to lower wavelengths with decreasing particle size. The absorption spectra of platinum nanoparticles were also calculated using quantum mechanical treatment and coincidently a good agreement was obtained between the calculated and measured absorption peaks at various particle sizes. This indicates that the 216 and 264-nm absorption peaks of platinum nanoparticles conceivably originated from the intra-band transitions of conduction electrons of (n = 5, l = 2) and (n = 6, l = 0) energy states respectively to higher energy states. The absorption energies, i.e., conduction band energies of platinum nanoparticles derived from the absorption peaks increased with increasing dose and decreased with increasing particle size.     

Frequency-doubled short nanosecond fiber lasers for glass drilling and grinding

We developed a fiber laser based 515 nm single-mode green laser with pulse width of 5 ns, peak power of greater than 20 kW, and repetition rates from 100 kHz to 500 kHz, which is suitable for glass drilling and grinding. The lithium triborate (LBO) crystal is used for second harmonic generation of 515 nm laser with efficiency of 68%. Glass drilling speed of 50 mm/s @ 0.5 mm thick glass was achieved for industrial environment continuous operation, which is equivalent to 1 s of time to drill a 4 mm diameter hole on a 2 mm thick glass. The typical chip sizes for the rear and front sides are 80 μm and 120 μm, respectively. Seven thousand holes are drilled in 24 hours in glass product line, and the success rate is larger than 99.7%. This laser glass drilling approach is rapidly accepted by glass industrial. Glass surface grinding was demonstrated by using 1 ns pulse width 515 nm lasers.     

Characterization of metal-bearing diesel nanoparticles using single-particle mass spectrometry

Acute and chronic health effects have been associated with diesel particulate matter (DPM). Since both ultrafine particles and metals have been implicated in this correlation, we are conducting investigations to characterize the metal content of diesel nanoparticles. For this study, DPM was generated by a 1.5 l engine and ferrocene was added to the fuel to raise the level of metal in the system. The exhaust particles were analyzed in real time using a recently developed single-particle mass spectrometer (SPMS) that has the capability of ablating each particle down to its elemental constituents, thereby yielding the relative mass of elements in each particle. Particle-size calibration of the instrument was achieved by correlating the SPMS signal intensity with measured DPM size. Using this approach, we present size- and composition-resolved elemental species distributions for both the nuclei mode and ultrafine portion of the accumulation mode of DPM. Results show that when the fuel is doped with ferrocene, iron-rich nanoparticles are formed and their number and size increase with level of doping. Larger iron-bearing particles are also formed, but it is observed that the metal to carbon ratios increase for smaller particle sizes. Hydrogen to carbon ratios were measured as a function of particle size, which allowed us to determine the relative amounts of organic carbon and elemental carbon in the particles and showed that the hydrogen to carbon ratios increase for smaller sized particles. The combined results are used to discuss the effects of metal doping level and engine load on particle nucleation and mechanisms of DPM formation.     

Influence of laser wavelength on the critical energy density for initiation of energetic materials

Critical densities of the energy of laser initiation of PETN containing nanoscale aluminum inclusions at radiation wavelengths of 1064 and 532 nm were measured experimentally. The critical initiation-energy density that corresponds to a 50%th probability of explosion was 1.15 J/cm² for the first harmonic of a neodymium laser and 0.7 J/cm² for the second. The dependence of the efficiency of radiation absorption by aluminum on the size of metal nanoparticles for the first and second harmonics of a neodymium laser is calculated. It is shown that the particle diameter corresponding to the absorption efficiency maximum and the amplitude of the maximum depend on the radiation wavelength. The absorption efficiency maximum for the first harmonic is observed in an inclusion 204 nm in diameter, and for the second, in an inclusion 96 nm in diameter. The amplitude of the maximum increases from 0.351 at a wavelength of 1064 nm to 0.490 at a wavelength of 532 nm. Dependences of the critical initiation energy density for energetic materials on the radius of metallic nanoparticles are calculated. Qualitative agreement between theoretical and experimental results is shown.     

One-step preparation of amorphous iron nanoparticles by laser ablation

Amorphous Fe nanoparticles are always difficult to prepare by physical gas-phase methods though rapid cooling rates are applied. Here we report a physical preparation of pure amorphous Fe nanoparticles by laser ablation of a 0.5-mm-diameter Fe wire and the investigation of their formation mechanism. Amorphous Fe nanoparticles with a shell of γ-Fe₂O₃ and the sizes of 1–3 nm are obtained at the laser power densities above the ablation threshold. Finally, the as-prepared nanoparticles are characterized by XRD, TEM, XPS and VSM to discover the structure, morphology, surface composition, crystallization and magnetic property in detail. We find that the holistic explosive evaporation induced by the small-size target not by the processing parameters determines the nature of the amorphous Fe nanoparticles. The as-prepared amorphous Fe nanoparticles are crystallized at 400 °C with an increase of particle size to about 10 nm.     

Effects of pulse duration in laser processing of diamond-like carbon films

Experimental studies of the interaction between amorphous hydrogenated carbon (a-C:H) film and short and ultrashort laser pulses in the near-infrared and visible spectral ranges (150 ns and 1064 nm, 10 ns and 1078 nm, 300 ps and 1078 nm, 220 ps and 539 nm, 100 fs and 800 nm.) are reported. The influence of the irradiation conditions (pulse duration, wavelength, laser intensity and the number of laser shots) on the structure and thickness of the laser-induced graphitized layer has been investigated. The effects of heat dissipation on the annealing duration and on the graphitized layer thickness are discussed for the case of laser processing with short pulses. It was found that the resulting morphology of the irradiated a-C:H film surface was determined by the concurrence of three processes: change of the mass density induced by structural transformations, multiple spallations of thin layers, and material evaporation. The laser-induced spallation is asserted to be the main factor limiting the laser microprocessing reproducibility for the examined a-C:H film; its effects were found to increase dramatically for longer (150 ns) laser pulses. The ablation (evaporation) rates of the a-C:H films and glassy carbon were revealed to be similar for femtosecond and picosecond pulses, but they essentially differed for nanosecond pulses. The ablation process demonstrated the same main features for both materials: (i) increase of the ablation rate with the pulse duration, and (ii) saturation of the ablation rate with fluence for picosecond and nanosecond pulses.