Low-pressure Differential Mobility Analyzer Research Articles

Determination of nanoparticle collision cross section distribution functions in low pressure plasma synthesis reactors via ion mobility spectrometry

Low pressure (<20 Torr) non-thermal plasma flow tube reactors are commonly used for the synthesis of high purity nanoparticles (NPs) via nucleation and subsequent growth from vapor phase precursors. In spite of their utility, process monitoring (i.e. output NP size characterization) in such reactors is difficult, as there is a dearth of techniques available for online NP size distribution function measurement. In this study, we developed and applied an ion mobility spectrometry (IMS) system consisting of a low pressure differential mobility analyzer (LPDMA) and electrical detector to determine the collision cross section distribution functions of Si NPs synthesized in a radio frequency non-thermal SiH4–Ar plasma operated at ∼2 Torr (266 Pa). The collision cross section, roughly proportional to projected area, is a parameter quantifying the size and structure of nanometer scale species in the vapor phase. We introduce the collision cross section distribution function as a metric for potential use in online process monitoring in NP synthesis. Proper inversion of the collision cross section distribution function requires a priori knowledge of the LPDMA transfer function. We utilized a tandem differential mobility analyzer approach, coupled with a Twomey–Markwoski based scheme to determine LPDMA transfer functions. Subsequent application of these transfer functions in collision cross section distribution function determination showed that at the outlet of the non-thermal plasma flow tube reactor, both negatively and positively charged Si NPs persisted with nearly identical collision cross section distribution functions. NPs were found to have mode ‘mobility equivalent’ diameters near 10 nm, and via TEM analysis were found to persist at the reactor outlet as small aggregates composed of ∼5 nm diameter primary particles. Size distribution functions inferred from collision cross section measurements were compared to size distribution functions inferred from TEM images; excellent agreements were found between IMS and TEM for both mean mobility equivalent diameter and distribution function width. In total, this study shows that (1) IMS is a viable approach for process monitoring in non-thermal plasma NP flow tube synthesis systems, (2) although NPs are modestly aggregated at low-pressure plasma reactor outlets, the extent of aggregation is considerably less than observed in most atmospheric pressure and equilibrium NP synthesis systems (e.g. flames), and (3) after exiting the plasma reactor, the decharging of NPs from highly negative charge levels to a bipolar charge distribution likely drives aggregation on the plasma boundary.

Fabrication and characterization of a granular film consisting of size-selected silver nanoparticles: application to a SERS substrate

Uniform-sized silver nanoparticles with average diameter of 13.7 nm have been prepared in the gas-phase by combining a pulsed laser ablation method with a low pressure-differential mobility analyzer (LP-DMA). By depositing the silver nanoparticles onto a silicon substrate, a granular film consisting of size-selected silver nanoparticles has been fabricated and its morphology and electronic properties have been examined using transmission electron microscopy (TEM) and UV-visible absorption spectroscopy. This granular silver film serves as a highly active substrate for surface-enhanced Raman scattering (SERS).

Designable formation of metal nanoparticle array with the deposition of negatively charged nanoparticles

Ultrafine gold and platinum nanoparticles (NPs) were fabricated by pulsed laser ablation in helium gas and the NPs 2–15nm in diameter were selectively classified by an electrostatic size-selection technique employing a low-pressure differential mobility analyzer (LP-DMA). The spherical NPs obtained showed a narrower distribution of diameters of anionic NPs over cationic NPs. With this knowledge, the anionic NPs were deposited onto silicon substrates designed by electron beam lithography processing, and designable patterns of arrayed NPs were produced by removing the photoresist layer.

Differential mobility analysis of nanoparticles generated by laser vaporization and controlled condensation (LVCC)

Silicon and iron aluminide (FeAl) nanoparticles were synthesized by a laser vaporization controlled condensation (LVCC) method. The particles generated by the laser ablation of solid targets were transported and deposited in the presence of well-defined thermal and electric field in a newly designed flow-type LVCC chamber. The deposition process of nanoparticles was controlled by the balance of the external forces; i.e., gas flow, thermophoretic and electrostatic forces. The size distributions of generated nanoparticles were analyzed using a low-pressure differential mobility analyzer (LP-DMA). The effect of synthesis condition on the size distribution was analyzed by changing the pressure of the carrier gas (20–200 Torr), the temperature gradient in the LVCC chamber (ΔT=0–190°C) and the electric field applied between the LVCC chamber plates (E=0–3000 V/m). It was found that electrostatic field was effective to selectively deposit small size nanoparticles (about 10 nm) with expelling large droplet-like particles.

Classification and characterization of gold and nickel nanoparticles with a differential mobility analyzer

Ultrafine gold and nickel nanoparticles (NPs) were fabricated by pulsed laser ablation in helium gas, and the NPs 2–15 nm in diameter were selectively classified by an electrostatic size-selection technique employing a low-pressure differential mobility analyzer (LP-DMA). In the classification of the NPs, a chain of several gold NPs or doubly charged NPs were produced as well as singly charged spherical NPs. The morphological selection was performed with the LP-DMA, using a relationship in which the cross section of the spherical NP chains was governed by the product of the diameter and the length, instead of by the square of the diameter. Below a blocking temperature of about 100 K, the Ni NPs that were 3.1 nm in diameter exhibited ferromagnetism.

Numerical Evaluation of the Transfer Function of a Low Pressure DMA by Using the Langevin Dynamic Equation

The effect of Brownian diffusive particle trajectory of nanoparticles on the transfer function of the low pressure Differential Mobility Analyzer (LPDMA) was evaluated by a numerical simulation of the Langevin dynamic equation. The results of the simulation were compared with previously reported experimental results (Seto et al. 1997; Seol et al. 2000) and Stolzenburg's transfer function (1988). As the operational pressure decreased, the peak and FWHM (full width of the transfer function at half of its maximum) values of transfer function, as calculated by numerical simulation, were increased, which was not evident from Stolzenburg's transfer function. In comparison with the experiments of Seto et al., discrepancies in the higher electrical mobility regime than from central mobility were found, which could be caused by the incomplete flow control of their LPDMA. However, the transfer function, as calculated by numerical simulation was in good agreement with experimental results reported by Seol et al., pe...

Ion Beam Charging of Aerosol Nanoparticles

In our ongoing efforts to achieve the high-efficiency charging of aerosol nanoparticles under low-pressure conditions, our group has recently developed an ion beam aerosol charger (IBAC) that ionizes aerosol nanoparticles using an He+ ion beam (Seto et al. 2003). In earlier studies we have observed both increases and decreases in the currents from the charged particles, depending on the pressure when the polydisperse particles were irradiated by the ion beam. None of our previous studies elucidated the mechanisms of the charging itself, however. In the present paper we evaluate the charging probability of monodisperse aerosol nanoparticles using a low-pressure differential mobility analyzer (LP-DMA) and aerosol electrical condenser (AEC). The particles were negatively charged by the attachment of free electrons generated via the ionization of carrier gas by ion beam irradiation under a pressure of more than 350 Pa. A charging probability of more than 60% was obtained experimentally for the particles of 10...

Magnetic Properties of Monodispersed Ni/NiO Core−Shell Nanoparticles

We have recently developed a method to fabricate monodispersed Ni/NiO core-shell nanoparticles by pulsed laser ablation. In this report, the size-dependent magnetic properties of monodispersed Ni/NiO core-shell nanoparticles were investigated. These nanoparticles were formed in two steps. The first was to fabricate a series of monodispersed Ni nanoparticles of 5 to 20 nm in diameter using a combination of laser ablation and size classification by a low-pressure differential mobility analyzer (DMA). The second step was to oxidize the surfaces of the Ni particles in situ to form core-shell structures. A superconducting quantum interference device (SQUID) magnetometer was used to measure the magnetic properties of nanostructured films prepared by depositing the nanoparticles at room temperature. Ferromagnetism was observed in the magnetic hysteresis loop of the nanostructured films composed of core-shell nanoparticles with core diameters smaller than the superparamagnetic limit, which suggests the spin of Ni core was weakly exchange coupled with antiferromagnetic NiO shell. In contrast, smaller nanoparticles with core diameters of 3.0 nm exhibited superparamagnetism. The drastic change in the hysteresis loops between field-deposited and zero-field-deposited samples was attributable to the strong anisotropy that developed during the magnetic-field-assisted nanostructuring process.

Characterization of Low Pressure DMA System for the size selection of magnetic nano-particles

For the size selection of magnetic nano particles produced in laser ablation, a low pressure differential mobility analyzer (LPDMA) was constructed. The LPDMA was characterized using the transmission electron microscopy (TEM) image of laser-ablated CoPt nano particles. Using TEM image, the geometrical standard deviation, σg, was measured to be around 1.13. The dependence of the performance of LPDMA on the gas path temperature and the pressure was measured using an electrode which measures the number of the selected particles. The size-selected magnetic nano-particles were deposited on Si substrates, whose magnetizations were measured by SQUID. It was found that nano-particles with a diameter of 20 nm have different temperature dependences from nano-particles with a diameter of 40 nm and the coercivity of 40 nm dia. nano-particles is smaller than that of 20 nm dia. nano-particles.

Laser synthesis and magnetic properties of monodispersed core–shell nanoparticles

This paper presents a method for fabricating size-selected nickel nanoparticles coated with oxide shells (shell thickness of about 2 nm). The size of the generated particles was controlled by a low-pressure differential mobility analyzer. The total mass of the deposited particles was estimated on the basis of their measured electric current. A high-resolution transmission electron microscope was used to observe the morphologies of the particles. We successfully synthesized a series of monodispersed (geometric standard deviation <1.2) core–shell particles with oxidized surface layers of 2 nm and analyzed their magnetic properties.

Crystallization of Monodisperse Lead Zirconate Titanate Nanoparticles Produced by Laser Ablation

Monodisperse lead zirconate titanate (PZT) nanoparticles with diameters of 9 nm were produced by laser ablation followed by a gas-phase thermal treatment in combination with a size-classification technique using a very low-pressure differential mobility analyzer. The particles prior to the thermal treatment are amorphous. When the laser power density for ablation increases from 0.5 GW/cm2 to 12 GW/cm2, the production yield of the particles increases regardless of the laser photon energy. When the laser photon energy is 3.5 eV, the nanoparticles generated with laser power densities as high as 7 GW/cm2 are crystallized to perovskite by thermal treatment at 900°C, while those generated with even higher power densities become pyrochlore. In contrast, when the laser photon energy is 2.3 eV, perovskite nanoparticles are obtained only at a power density of 0.5 GW/cm2. These results reveal that the crystalline structure of PZT nanoparticles can be adjusted to perovskite or pyrochlore by changing thermal treatment temperature, laser power density, and photon energy. Furthermore, X-ray photoelectron spectroscopy indicates that the crystallization to pyrochlore is caused by lead deficiency.

Formation of Size-Selected Ni/NiO Core−Shell Particles by Pulsed Laser Ablation

This paper presents the method for fabricating size-selected nickel nanoparticles (diameter: 5−20 nm) coated with nickel oxide shells (thickness: about 2 nm). The core−shell particles were synthesized by a series of sequential gas-phase processes including pulsed laser ablation, aerosol postannealing, size-classification, compulsory oxidation, and aerosol-jet deposition. The change in the size distribution of the generated particles due to postannealing was measured by a low-pressure differential mobility analyzer (LP-DMA) coupled with a Faraday cup electrometer. The peak size decreased as the postannealing temperature rose, indicating that a sintering and restructuring of the agglomerates was taking place. In the high-resolution TEM (HRTEM) observation, the Ni particles of less than 15 nm in diameter were found to be spherical and single crystal after the annealing at 1073 K. The 20 nm particles were also spherical, but they remained polycrystalline due to insufficient annealing treatment required for ...

Experimental transfer function for a low-pressure differential mobility analyzer by use of a monodisperse C 60 monomer

A C 60 monomer as an inherently monodisperse standard particle was produced by a C 60 vapor source equipped with an 241 Am neutralizer, and its mobility spectrum was measured by using a low-pressure differential mobility analyzer (LPDMA). The transfer function was derived from the mobility spectrum and was measured as dependences on aerodynamic and electrodynamic conditions. The experimental transfer function thus obtained was compared with the theoretical transfer function.

Gold nanoparticles and films produced by a laser ablation/gas deposition (LAGD) method

Gold nanoparticles have great potential for various nanoelectronic applications such as single electron transistors, an infrared absorption sensor and so on. It is very important to understand and control the size distribution of the particles for such a variety of applications. In this paper, we report the size distribution of gold nanoparticles and the relationship between the nanoparticle-films and the electrical property produced by a laser ablation method. Gold nanoparticle-films were prepared by a technique, which sprays nanoparticles on the substrate through a nozzle. We call it a gas deposition method. The nanoparticles were generated by the nanosecond pulsed Nd:YAG laser ablation of a gold substrate under a low-pressure inert gas atmosphere. The ambient pressure was changed to control the average size and their distribution. The particles produced in the generation chamber were transported by a helium carrier gas to the deposition chamber and deposited on a substrate to form the films composed of gold nanoparticles. The electrical resistivity of the generated gold nanoparticle-films on the glass substrates was measured using a four-probe method. The size distribution of the nanoparticles was examined using transmission electron microscopy (TEM) and a low-pressure differential mobility analyzer (LP-DMA). The relationship between the particle size and the electrical properties of each film made by the different synthesis conditions were analyzed. The electrical resistivity changed from the order of 10 −5 to 10 −1 Ω cm depending on the ambient pressure and the size distribution.

In-situ monitoring of UF 5 nanoparticle growth in a plug-flow reactor by a low-pressure differential mobility analyzer

The dependence of UF5 nanoparticle size distribution on growth time was investigated for various initial concentrations of UF5 monomers. UF5 nanoparticles prepared by photodissociation of UF6 in a mixed-flow reactor were allowed to pass through a plug-flow reactor (PFR), and their size distribution was measured by an in-situ size-monitoring system, namely a low-pressure differential mobility analyzer. By changing the length of the PFR, the growth time in the PFR was controlled from 0 to 30 s. An analysis using Lee’s coagulation model revealed that the growth mechanism of the UF5 nanoparticles involved a Brownian coagulation process and that the sticking probability of the nanoparticles was approximately unity.

C60 Monomer as an Inherently Monodisperse Standard Nanoparticle in the 1 nm Range

A C60 monomer was produced by evaporation of C60 powder under low-pressure conditions, and its mobility spectrum was observed using a low-pressure differential mobility analyzer (LPDMA). Pressure dependence for the C60 monomer in the spectrum was measured in order to examine the influence of C60 aggregates on the peak profile of the C60 monomer. Taking into account the diameter of the collision partner Ar atom, the diameter of the C60 monomer was estimated in the framework of Stokes' law.

IN SITU MEASUREMENT OF NANOPARTICLES FORMED IN AN RF PLASMA-ENHANCED CHEMICAL VAPOR DEPOSITION REACTOR USING A VERY LOW-PRESSURE DIFFERENTIAL MOBILITY ANALYZER

IN SITU MEASUREMENT OF NANOPARTICLES FORMED IN AN RF PLASMA-ENHANCED CHEMICAL VAPOR DEPOSITION REACTOR USING A VERY LOW-PRESSURE DIFFERENTIAL MOBILITY ANALYZER

Formation and size control of tungsten nano particles produced by Nd:YAG laser irradiation

Nanometer-size particles (nanoparticles) of tungsten (W) were synthesized by the laser irradiation of a substrate under low He gas pressure using a Q-switch Nd:YAG laser and measured with the TEM and a low-pressure differential mobility analyzer (LP-DMA). Two particle formation mechanisms were shown for the pressure dependency of the nanoparticles size distribution. The smaller size nanoparticles were mainly formed by an evaporation mechanism and the large crystalline nanoparticles were made by a kind of a splash induced by laser incidence. The two nanoparticle formation mechanisms were not observed clearly when using normal pulse Nd:YAG laser.

Classification Performance of a Low Pressure Differential Mobility Analyzer for Nanometer-sized Particles

Size distribution measurements and classification tests by a low pressure differential mobility analyzer (LPDMA) for nanometer-sized silver particles and cesium iodide particles under low pressure conditions (123–300 Torr) were performed using a transmission electron microscope (TEM) and the tandem DMA technique. When the ratio of the sheath gas flow rate and the aerosol gas flow rate was set at 5 : 1, the targeted sizes calculated from the classification voltage applied to the LPDMA at 160 Torr are found to be in good correlation with the count mean Feret diameter obtained from the TEM observation of the particles collected after a classification ranging from 6 to 25 nm, although the targeted sizes set by the LPDMA were approximately 15% greater than the count mean Feret diameter measured by the TEM after classification. The geometric standard deviations of DMA-classified particles measured by TEM and those obtained from the tandem DMA method ranged from 1.08 to 1.17 and from 1.05 to 1.13, respectively. They were slightly greater than the ideal geometric standard deviations (1.05) of the particles classified with the LPDMA, which was calculated by neglecting the broadening effects due to Brownian diffusion. We experimentally demonstrated the validity of our LPDMA system for size measurements and classification of the nanometer-sized particles under low pressure conditions.

Evaluation of morphology and size distribution of silicon and titanium oxide nanoparticles generated by laser ablation

Nanometer-sized particles of silicon and titanium oxide were generated by irradiating solid targets using a nanosecond pulsed-Nd : YAG laser in a low pressure atmosphere. A low pressure differential mobility analyzer (LP-DMA) was used to classify the size of the generated particles. The LP-DMA and electron microscopes (SEM and TEM) were used to measure the change in the size distribution and morphology of the generated particles with laser power density and system pressure. The size distribution of both silicon and titanium oxide ranged from two to one hundred nanometers in diameter depending on the laser power density and pressure. From the high resolution TEM observation and electron diffraction, it was found that the generated titanium oxide nanoparticles were composed of a ‘core’ of faceted metallic single crystals with an oxide layer 'shell’.