The aerosol impact spectrometer: a versatile platform for studying the velocity dependence of nanoparticle-surface impact phenomena
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A new apparatus designed to accelerate/decelerate and study the surface impact phenomena of charged aerosols and nanoparticles over a wide range of mass-to-charge (m/z) ratios and final velocities is described. A nanoparticle ion source coupled with a linear electrostatic trap configured as an image charge detection (ICD) mass spectrometer allows determination of the mass-to-charge ratio and the absolute charge and mass of single nanoparticles. A nine-stage linear accelerator/decelerator is used to fix the final velocity of the nanoparticles, and in the results reported here the coefficient of restitution for polystyrene latex spheres (PSLs) impacting on silicon is measured using ICD techniques. To enable this apparatus to study a wide range of m/z, the data acquisition system uses a transient digitizer interfaced to a field-programmable gate array module that allows real time calculation of m/z and determination of the pulse sequence for the linear accelerator/decelerator. Electrospray ionization of a colloidal suspension of PSL spheres of 510 and 990 nm has been used to demonstrate acceleration and deceleration of charged nanoparticles and the resolution of the apparatus. Measurements of the coefficient of restitution for PSLs on silicon over the range 10-400 m/s are consistent with previous studies.
KeywordsImage charge detection Nanoparticle acceleration Deceleration Coefficient of restitution
Coefficient of restitution
Field programmable gate array
Image charge detector
Nanoparticle electrostatic trap
As interest in the characterization of nanoparticles, aerosols and dusts increases, the need for the development of new tools for the manipulation and analysis of single particles continues to grow . An area of continued interest for an understanding of problems ranging from atmospheric chemistry to astrophysical phenomena to industrial applications pertains to the impact phenomena of nanoparticles. Theoretical and experimental studies of nanoparticle-surface collisions at both low and high velocity ranges continue to be reported [2, 3, 4, 5, 6, 7]. There is a large body of work on hypervelocity impact phenomena dating back to the early 1960’s motivated by the need to understand the effect of cosmic dust and micrometeoroid impacts on space vehicles. Larger objects are most conveniently accelerated with light-gas guns , however, for smaller particles that can be easily charged, electrostatic accelerators are a convenient approach [9, 10, 11, 12]. In early work in this area, a 2 MV van de Graaff dust accelerator was described and shown to accelerate 1 μm particles to ~ 6 km/s [10, 11]. One early example of a switched, multistage linear accelerator for large particles was described by Vedder , however it was not until advances in high-voltage switching circuitry that laboratory-scale linear accelerators, such as the one described by Hendell and Even , became more common. In the present work the development of a versatile new nanoparticle mass spectrometer/accelerator/decelerator, the Aerosol Impact Spectrometer (AIS), is described with a demonstration of its capabilities using polystyrene latex (PSL) spheres, including studies of collision inelasticity on silicon wafers by determination of incident and scattered velocities in measurements of the coefficient of restitution.
The key to carrying out mass spectrometric measurements on single charged nanoparticles is the use of charge detection mass spectrometry (CDMS) techniques (1). CDMS determines the absolute charge on a particle from the magnitude of the image charge induced on a pickup electrode when a charged particle passes through. The image charge waveform also yields the particle time-of-flight (TOF) and velocity through the pickup providing the mass-to-charge ratio for fixed energy particles . This method has been used in accelerator experiments since at least 1960 , and with the increasing interest in studies of massive biomolecules, cells and nanoparticles it has seen more recent applications in mass spectrometry measurements [16, 17, 18, 19, 20, 21, 22, 23, 24], with the state of the art recently described by Keifer and Jarrold . In our own laboratory, we make use of a variant of this approach, applying a charge-pickup electrode in an electrostatic fast-ion beam trap to monitor the ion density oscillating in the trap and carry out Fourier-transform (FT) mass spectrometry on ensembles of molecular ions [26, 27]. In the present apparatus the m/z ratio of a single particle provides the information required to accelerate or decelerate that particle for studies of particle impact.
As shown by Hendell and Even , and later applied by Hsu and co-workers , a linear accelerator for large molecular ions can be configured using modern high-voltage MOSFET switching techniques. In the present apparatus, the CDMS determines the m/z ratio for each particle in real time, providing the information required to accelerate or decelerate single particles over a wide range of m/z by generating the appropriate acceleration/deceleration pulsed waveform for a linear accelerator (LINAC) structure composed of a series of cylindrical electrostatic elements. Acceleration of ion ensembles by this technique is not hindered by space charge limitations since the effect of space charge is progressively reduced as the particles accelerate. Deceleration of ion ensembles does become problematic as a result of increasing space charge, and that is the benefit of working with single charged nanoparticles where this limitation is not present.
In the following sections, the initial results obtained with the AIS will be presented, showing the measurement of charge/mass distributions, acceleration and deceleration of single charged PSL spheres, and measurements of impact inelasticity of single particles with a silicon substrate. These results show that the AIS will have great utility for measuring the reflection of nanoparticles from surfaces, yielding quantitative information on the coefficient of restitution that describes the inelasticity of such collisions [29, 30].
Charged particles are formed by electrospray ionization and transferred into a low vacuum through a 150-μm Pt electron microscope aperture. A heater tube dries the particles before they enter the ADL, which collimates and focuses the particle beam. The particle beam passes through two stages of differential pumping and enters a chamber that houses the electrostatic quadrupole deflector. The QD selectively turns particles 90° based on their kinetic energy per charge into the next chamber that contains the NET. The NET is a linear electrostatic trap configured as an image charge mass spectrometer, as first described by Zajfman and co-workers on ion ensembles [31, 32] and in a single-particle application by Benner . Briefly, the NET is gated to trap one particle at a time, before measuring the mass-to-charge ratio and the absolute charge of the particle. These particles can be trapped in the NET with an efficiency ~70% for periods in excess of 5 seconds with a distribution of oscillation frequencies in the several hundred Hertz range. A Labview-based data acquisition code uses the m/z data to calculate the required switching times for the LINAC to accelerate the particle up to the required velocity. The particle is then released from the trap into the LINAC for acceleration to the desired velocity.
Electrospray ionization (ESI) source
The use of ESI for this type of application is based on prior work by Benner, Austin and others on polystyrene latex spheres, silica, mineral dusts and other systems [33, 34, 35, 36]. The ESI source consists of a 360 μm O.D., 75 μm I.D. fused silica capillary mounted on an isolated aluminum block, which is held at approximately +4–6 kV. To date the AIS instrument has only been run in positive mode, but no impediment is anticipated for using it to study negatively charged particles. The tip of the capillary is positioned ~10 mm in front of a 150 μm aperture and is enclosed within a glass tube. The aperture is mounted on a 6.4 mm OD, 3.86 mm ID stainless steel tube that passes through an Ultra-Torr fitting into the instrument. The portion of this tube that extends out of the vacuum chamber is surrounded by an enclosure, through which nitrogen, heated to ~85 °C, flows. The heated nitrogen helps to desolvate the particles, and also acts as a curtain gas. 510 nm (Polyscience #07307) and 990 nm (Polyscience #07310) PSL spheres were suspended in 1:1 mixtures of 25 mM aqueous ammonium acetate and methanol, then delivered to the electrospray tip by a syringe pump operating at a rate of 0.26 mL/h. The final number densities were 30 × 109 particles per mL for the 510 nm PSL suspension, and 4.1 × 108 particles per mL for the 990 nm suspension. After entering the vacuum chamber, the particles pass through a 120 mm long, 4.5 mm I.D. stainless steel tube, which is heated to ~185 °C to completely desolvate the particles, and to evaporate charged solvent droplets.
Aerodynamic lens (ADL)
After exiting the heater tube, the particles enter an ADL, which aerodynamically focuses and collimates the beam of particles [37, 38]. In this instance the ADL consists of 5 apertures separated by 59 mm long, 12.1 mm I.D. spacers. The diameters of the apertures are 5.9, 5.3, 4.6, 3.4 and 3.1 mm. The section after the 5th aperture is connected to a mechanical vacuum pump. The pressure at the start of the ADL is 2.6 Torr, and the pressure in the pumped region is typically in the range of 0.5-1 Torr. The precise pressure after the ADL is adjusted, by throttling the vacuum pump, in order to maximize transmission of particles in a specific size range. Calculations along the line of those described by Wang and McMurry indicate that this ADL should transmit over 95% of particles in the range of 75 – 1200 nm.
Quadrupole deflector (QD)
Following the ADL, the particles pass through a 3.1 mm diameter aperture into the first differential pumping stage, which is connected to a Roots blower backed by a rotary mechanical pump. This pumping stage also contains an image charge detector tube (ICD) to confirm transmission of particles through the aerodynamic lens. A 5 mm aperture leads to the second differential pumping stage, pumped by a Pfeiffer TMH 064 turbomolecular pump. A 3.2 mm aperture separates the second differential pumping stage from the quadrupole deflector chamber, which is pumped by an Osaka TG240 turbomolecular pump. The quadrupole deflector chamber is pumped to a vacuum of 3 × 10-5 Torr.
The QD assembly is in the center of the chamber and consists of 4 parallel, quarter-cylinder stainless steel rods (19 mm radius). The rods are mounted on 1/8″ precision ground glass spheres that locate the rods such that their curved faces are tangential to an inscribed circle of 33.7 mm diameter. Positive and negative potentials are applied to opposing pairs of rods, resulting in an electrostatic field that will turn particles that have kinetic energy, in eV per charge, equivalent to the potential applied to the rods. Particles with excess kinetic energy will overshoot the bend, whereas particles with less kinetic energy will impact the electrode on the inside of the curve.
There are two sets of ion optics in the chamber with the QD, each comprised of an einzel lens, an x-y deflector and an ICD, modeled after the detector presented by Fuerstenau and Benner . One is positioned before the entrance to the QD, the other after the exit from the QD. These ion optics serve to focus the selected particles and direct them to the next chamber, which contains the NET and LINAC. The first ICD (ICD-QD1) is used to confirm transmission of particles through the ADL. The second ICD (ICD-QD2) is used to determine the optimal deflector potential to direct the particle beam towards the next chamber.
Nanoparticle electrostatic trap
This data is measured and calculated on the fly for each particle and is used to create the timing sequence used by the HV switches for acceleration/deceleration.
The first three electrodes act as a lens with each element connected to external power supplies (two KIKUSUI PMC350-0.2A and one Canberra 3002). The remaining 9 electrodes are wired through two HV feedthroughs (30 kV) with every second element collectively wired to one feedthrough and every other element collectively wired to the other feedthrough. All elements in each set are connected together with copper rods. These two rods are connected to each HV feedthrough with a shielded HV cable. The elements are pulsed with two 30 kV HV switches (Behlke 301-03-GSM).
In Eq (4), t x is the time at position x, v x is the velocity at position x, E x is the electric field at position x, and dx is the step size of the calculation, in this case 0.01 mm. This timing calculation is performed while the particle is still trapped in the NET. Upon completion of the calculation, timing data is transferred to the FPGA. The FPGA then releases the particle from the trap by lowering the exit mirror and triggers the two high voltage (HV) switches connected to the LINAC at the times required to accelerate/decelerate the particle to a final energy that depends on the number of elements used and the potential applied to each element.
After acceleration, the FPGA digitizer captures a waveform from the output of ICD3. The waveform is transferred to the Labview program which calculates the accelerated velocity of the particle. The program also determines if the particle has rebounded from the collision target, and calculates the rebound velocity of the particle from the rebounding peak width. After completing this final acquisition the program saves all information to a data file and resets itself and the FPGA to accept a new particle. The entire run time for each particle in this experiment was approximately ~200 ms.
Results and discussion
Charge distributions in 510 and 990 nm PSL colloids
Variable acceleration and deceleration of charged nanoparticles
Impact dynamics and coefficient of restitution for PSL nanoparticles
Formation, trapping, acceleration/deceleration, and coefficient of restitution measurements of highly-charged submicron particles have been demonstrated. Single highly charged PSL spheres have been generated with an ESI source. Subsequent trapping and CDMS analysis of individual particles has been demonstrated. Individual particle acceleration/deceleration has been demonstrated, allowing the acquisition of quantitative coefficient of restitution data. The flexibility of single particle on-the-fly analysis allows the Aerosol Impact Spectrometer to function with a wide range of nanoparticle masses and charges from a given particle source. Additionally, the variable energy selection of the spectrometer allows a variety of particle sources to be implemented in addition to the demonstrated electrospray ionization, including liquid metal ion sources[54, 55, 56] and needle-charge dust sources [57, 58]. The wide range of final energies achievable with the variable linear accelerator/decelerator will allow for a variety of scattering experiments to be performed to examine both hypo- and hypervelocity impact phenomena
This work was supported by the NSF Major Research Instrumentation Instrument Development Program under grant number CHE-1229690 and the Center for Aerosol Impacts on Climate and the Environment (CAICE), a National Science Foundation Center for Chemical Innovation under grant number CHE-1305427. We acknowledge contributions to this project by R. Otto, J.J. Rivera, K.A. Nadler, C.K. Anderson and J. Taulane, as well as discussions with S. De Dea and partial support from Cymer, an ASML Company.
The authors declare that they have no competing interests.
BA and MM participated in the design of the apparatus including the data acquisition system, carried out the measurements and drafted the manuscript. RC conceived of the study, participated in design and coordination and helped draft the manuscript. All authors read and approved the final manuscript.
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