detecting nanoparticles using microplasmas jeff hopwood professor, ece department tufts university...
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Detecting Nanoparticles using Microplasmas
Jeff HopwoodProfessor, ECE Department
Tufts [email protected]
617-627-4358
Supported by NSF CCF-0403460 (in progress)
Problem Statement
• nanoparticles are too small to detect by scattered laser light (r<100nm).• nanoparticles may be too widely dispersed to sense and count accurately.• radio isotopes used for charging particles in DMA’s or IMS’s must be
tracked.• current systems are not portable.
Goals:To use low power, portable microplasma generators to charge particles.
To use ‘potential wells’ to trap and concentrate charged particles.To investigate novel modes of detecting particles by charge, mobility, or
chemical reactivity in microplasmas.
Charging Particles with PlasmasPlasma electrons will rapidly charge nanoparticles (negatively).
These particles may then be trapped within the potential well of the plasma.
(x)
x
+ +
ne = ni
ne~ 0(sheath)
x
V(x)
-qZ
-qZ
Table 1. Approximate charge on a nanoparticle with radius a (nm) trapped in a plasma with electron temperature Te (eV) -excludes photo-ionization
Te (eV) Approx. Number of Charges (Z)
1 1.9a
2 3.5a
3 4.9a
4 6.3a
5 7.6a
charging trapping
microplasma
Portable Microplasma System-- the Split Ring Resonator (SRR) --
VCO(900MHz)
Power Amp(GSM BandCell Phone,
4 watts, ~$1)
Split Ring Resonator: SRR
not shown: 6 v battery, power level control
Prototype SRR operating in air(3 watts)
Split Ring Resonator(SRR)Electric Field Intensity (@ 900 MHz)
Egap > 10 MV/m
25 m discharge gap
This device concentrates power from a cell phone into
a volume of ~ 1 nanoliter
Microplasma Particle Trap Experiment
200 m
20 mm
HeNe
Digital SLR
Microscope
632 nm filter
“shaker”
Argon microplasma (SRR)1 mmelamineparticles-
-
--
-- -
--
-coaxialline
Window
pump
particlecounter
37 mm
Microplasma Particle Trap Experiment
200 m
20 mm
HeNe
“shaker”
Argon microplasma1 mmelamineparticles-
-
--
-- -
--
-coaxialline
pump
particlecounter
Digital SLR
Microscope
632 nm filterWindow
Particle Trapping and Localization
Time (sec)
0 20 40 60 80 100
Par
ticle
Cou
nt
0
100
200
300
400
plasma "on"
shaker
lost particles
trapped particles
Time (sec)
2 cm tres = 2 s
microplasma
1 um - melamine formaldehyde
Particles Trapped by a Microplasma(observed through a 632nm filter to block plasma emissions)
200 um
/4 electrode
/4 electrode
SRRSRR
200 um
/4 electrode
/4 electrode
SRRSRR
Conceptualdetection and measurement of nanoparticles
O2
CF4
opticalspectrometer
1. Trap and concentrategas-borne nanoparticles
microplasmatrap
2. Pulse reactive gasesSiF
4. Detect emission of light from the etch reaction products
3. Etch the nanoparticles
Other concepts
• Use a voltage pulse to ‘push’ the charged nanoparticles from the trap, and detect particle size distribution using time-of-flight
• Use a miniature Ion Mobility Spectrometer to sort and detect charged nanoparticles– See sionex.com, for example
• Use a microplasma to charge the particles prior to entering a commercial DMA