Ion-Enhanced Field Emission for Control

of Atmospheric Pressure Discharges

(Grant FA9550-11-1-0020)

Prof. David B. Godgo@nd.edu

http://www.nd.edu/~sst

Aerospace and Mechanical Engineering

01/07/2013

2013 Plasma and Electro-energetic Physics Program Review

slide 2 D B. Go 01/07/2013

Outline• Overview / Motivation for Microdischarge Research

– The importance of field emission

• 2012 Accomplishments– Modeling and Theory– Experiments

• Ongoing work– Glow discharge simulations– Quantum modeling of ion-enhanced field emission– Electron/surface interactions

• Conclusions and Outlook

slide 3 D B. Go 01/07/2013

Effects of Confinement*• decreased electrode spacing

affects charge density distribution & Debye length

• increased surface-to-volume ratio affects energy balance and distribution

MicrodischargesMicrodischarges• gas discharges with a characteristic

dimension less than 1 mm• advantageous pd scaling enables stable

operation at high p (1 atm)• high pressure leads to new chemical

pathways new applications

Lighting

http://www.edenpark.com/ http://www.plasmainstitute.org/

Medical and DentalEnvironmental and Chemical Analysis

Nanomaterials

Harper et al., Anal. Chem., 2009 *Mariotti & Sankaran, J. Phys D: Appl. Phys., 2010

As surfaces begin to play a dominant role, it is necessary to establish a better understanding of plasma/surface interactions

slide 4 D B. Go 01/07/2013

Plasma/Surface Interactions• Plasma/surface interactions important for applications

– liquid/flesh/sputtering/cells/etc.– electrochemistry/biological/environmental

• Plasma/electrode damage important for device development– device lifetime/robustness/design …

• Plasma/electrode coupling important for fundamental understanding– emission processes (secondary/photo/thermionic/field)– charging processes (dielectric barriers)

At the microscale (< 10 μm), field emission becomes an important emission process that is not important at larger scales.

slide 5 D B. Go 01/07/2013

Summary of Research Program• At the microscale, the electric field can be very high (~10-100 V/μm) such

that electrons tunnel from the cathode– electron field emission acts as an additional charge source– mechanism responsible for deviation from Paschen’s curve in microscale gaps– at pressure, ions in the electrode gap affect the electric field ion-enhanced

field emission

• Prior work established theories for the modified Paschen’s curve that included field emission (Go and Pohlman, J. Appl. Phys., 2010; Tirumala and Go, Appl. Phys. Lett., 2010)

• Research Overview– explore relationship between field emission and discharge using combination

of theoretical modeling and computational simulations– experimentally determine impact of ions on field emission and establish

controllable field emission-driven Townsend discharges– new direction: explore how electrons produced in atmospheric-pressure

discharges interact with surfaces and liquids

slide 6 D B. Go 01/07/2013

Outline• Overview / Motivation for Microdischarge Research

– The importance of field emission

• 2012 Accomplishments– Modeling and Theory– Experiments

• Ongoing work– Glow discharge simulations– Quantum modeling of ion-enhanced field emission– Electron/surface interactions

• Conclusions and Outlook

slide 7 D B. Go 01/07/2013

PIC/MCC SimulationsPrior analytical models illustrate role of ion-enhanced field emission on breakdown, but do not articulate exact interaction. Can we dissect the discharge/field emission coupling and its role during breakdown?

particle-in-cell / Monte Carlo collision simulations (NDPIC1D)

anode

d ~ 2-10 μm

seco

ndar

y em

issi

on (

γ)io

n-en

hanc

ed F

-N b

ound

ary

cath

ode

PIC/MCC Parameters

• 1d/3v• argon• 100-760 torr• non-uniform grid to capture near cathode

effects• constant secondary emission coefficient γ

and field enhancement β• field emission modeled with Fowler-

Nordheim equation based on PIC-calculated surface field

Li, Tirumala, Rumbach, and Go, IEEE Trans. Plasma Sci., 2013

slide 8 D B. Go 01/07/2013

PIC/MCC: Microscale Breakdown

Li, Tirumala, Rumbach, and Go, IEEE Trans. Plasma Sci., 2013

below breakdown voltage

above breakdown voltage

inflection point = breakdown voltage

Defined PIC/MCC breakdown voltage (simulation method) that is consistent with theoretical basis for breakdown of gas gaps

4 μm gap, 760 torr

slide 9 D B. Go 01/07/2013

PIC/MCC: Microscale Breakdown

PIC/MCC simulations and a detailed comparison to theory confirm field emission-driven breakdown.

Li, Tirumala, Rumbach, and Go, IEEE Trans. Plasma Sci., 2013

slide 10 D B. Go 01/07/2013

PIC/MCC: Positive Space ChargeIn the pre-breakdown regime, transport is dominated by drift such that there is a net ion accumulation (103 – 104 cm-3 greater number density)

100 torr 760 torr

• Net positive space charge enhances the electric field in the domain, increasing field emission

• Appreciable build-up of positive ions (~1014 –1015 cm-3) suggests significant discharge below breakdown threshold

Li, Tirumala, Rumbach, and Go, IEEE Trans. Plasma Sci., 2013

5 μm gap, applied voltage is 98% of breakdown voltage

slide 11 D B. Go 01/07/2013

PIC/MCC: Ion Concentrations

emitted electron

ionization

ions enhance electric field

e–

e–

Li, Tirumala, Rumbach, and Go, IEEE Trans. Plasma Sci., 2013

100 torr 760 torr

5 μm gap, applied voltage is 98% of breakdown voltage

Because of the sensitivity to electric field, field emission closely tracks the ionic space charge creating a positive feedback mechanism leading to high ion densities

slide 12 D B. Go 01/07/2013

PIC/MCC: Cathode Processes

Li, Tirumala, Rumbach, and Go, IEEE Trans. Plasma Sci., 2013

10 μm gap, 760 torr applied voltage is breakdown voltage

3 μm gap, 760 torrapplied voltage is breakdown voltage

In larger gaps, secondary emission leads to breakdown before the electric field (or ionization) is sufficient to induce field emission. Secondary emission quickly becomes the dominant cathode process.

In small gaps, the electric field and ionization is sufficient to induce field emission and it grows comparably to secondary emission. Both phenomena are important cathode processes.

slide 13 D B. Go 01/07/2013

Fluid ModelPIC/MCC modeling useful for short transient effects and to get particle statistics, but not useful to parametrically explore impact of field emission and to get basic scaling laws. Can we understand the field emission-driven Townsend discharge regime?

1-D Steady State Fluid Model

Fluid model incorporates field emission into Townsend-type equations that describe steady state behavior below the breakdown threshold Townsend discharge regime

Fluid Model

1/α

e

e

e

e

e

+

+

+

+

+

Anode

e

e

+

+

Cathode

j+

positive ion current

j+

electron current

Rumbach, and Go, J. Appl. Phys., 2012

• system of coupled, non-linear ODEs• pseudo-analytical and numerical solution• constant secondary emission coefficient γ

and field enhancement β• field emission modeled with Fowler-

Nordheim equation based on self-consistently determined surface field

slide 14 D B. Go 01/07/2013

Fluid Model: Governing Equations

Maxwell’s equation:

electron conservation

ion conservation

incorporate field emission in the boundary condition

Boundary Conditions

Coupled by drift and

≈ 0

Governing ODEs

Pseudo-analytical assume E ≈ V/d to decouple

Rumbach, and Go, J. Appl. Phys., 2012

slide 15 D B. Go 01/07/2013

Fluid Model: Scaling Laws

where

total current density

electric field due to ions (space charge)

ion concentration

Rumbach, and Go, J. Appl. Phys., 2012

Pseudo-analytical solution leads to scaling laws• assumes space charge field is much smaller than applied field

All parameters scale directly with field emission current which implies

scaling with ~exp(d/V)

slide 16 D B. Go 01/07/2013

Fluid Model: Ion Concentrations

Field-emitted electrons create an abundance of ions in the gap consistent with PIC/MCC simulations

Rumbach, and Go, J. Appl. Phys., 2012

argon, 760 torr, applied voltages below the breakdown threshold

slide 17 D B. Go 01/07/2013

Fluid Model: Ion ConcentrationWhy are ion concentrations so large at the microscale?

non-dimensionalized Poisson’s equation:

implying

Abundantfield emitted electrons produce large number of ions sufficient to distort electric field and enhance ionization

ion density necessary to significantly distort applied electric field

Rumbach, and Go, J. Appl. Phys., 2012

φ = Φ/VA

X = x/d

space charge is negligible if

in smaller gaps, more space charge is needed to distort the applied field to cause breakdown

slide 18 D B. Go 01/07/2013

Fluid Model: Numeric ResultsNumerically solved fully coupled system

Rumbach, and Go, J. Appl. Phys., 2012

argon, 3 μm gap, 760 torr

divergence when space charge influences numerical solution

breakdown voltage

slide 19 D B. Go 01/07/2013

Fluid Model: Critical Space Charge

Assumption that space charge is negligible (ESC << V/d ) fails when n ~ 1013 cm-3 consistent with scaling!

Rumbach, and Go, J. Appl. Phys., 2012

argon, 3 μm gap, 760 torr

breakdown voltage

slide 20 D B. Go 01/07/2013

Fluid Model: BreakdownBoundary condition at the cathode produces a transcendental equation for the integration constant c1

Rumbach, and Go, J. Appl. Phys., 2012

A solution does not exist for large enough applied potentials model predicts breakdown voltage due to field emission

argon, 3 μm gap, 760 torr

LHS RHS

slide 21 D B. Go 01/07/2013

Fluid Model: Modified Paschen’s Curve

Rumbach, and Go, J. Appl. Phys., 2012

Fluid model produces multiple paths to predict the modified Paschen’s curve that are consistent with earlier theory, PIC/MCC simulations, and experiments

slide 22 D B. Go 01/07/2013

Outline• Overview / Motivation for Microdischarge Research

– The importance of field emission

• 2012 Accomplishments– Modeling and Theory– Experiments

• Ongoing work– Glow discharge simulations– Quantum modeling of ion-enhanced field emission– Electron/surface interactions

• Conclusions and Outlook

slide 23 D B. Go 01/07/2013

Experiments: Experimental Setup

• tungsten electrodes for durability and good field emission properties

• thin spacers made of photoresist enable varying the electrode gap

• transparent ITO anodes enable imaging of the discharge

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Experiments: I-V Measurements

Consistently observe field emission at pressure consistent with field emission-driven Townsend discharge

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Experiments: Pressure Scaling

Theory implies that current should scale with pressure and experiments support this conclusion (preliminarily).

j ≈ exp(αd)×jFE

N2, 4 μm gap, tungsten electrodes

fluid model predicts scaling of

where α scales with pressure

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Experiments: Exotic Cathode MaterialsSilver nanoparticles synthesized in our lab via microhollow cathode plasma jet-induced electrochemistry and dropcast on titanium substrates

Nanoparticles increase current 3 orders of magnitude at less than 50 V due to local field enhancement

slide 27 D B. Go 01/07/2013

Experiments: Exotic Cathodes Materials

Ar, 3 μm gap size

Carbon nanotubes (CNTs) synthesized at Georgia Tech via plasma-enhanced chemical vapor deposition

CNTs increase current 5 orders of magnitude at less than 100 V due to field enhancement and show similar pressure scaling

repeatability a challenge

slide 28 D B. Go 01/07/2013

Experiments: Post-breakdownWhat happens after breakdown? Depends on the active field emission area (sites)

Large Active Area → High current → Power supply current limitedbefore breakdown occurs

for this data, a stable glow was visually observed through the ITO anode.

Small Active Area → High current density j → Large flux j ~ 109 A/m2 causes explosive field emission and arcing

Moderate Active Area → If the area is just right, a transition from field emission to a glow regime can be observed → difficult to control active emission area

N2, 3 μm gap size, 630 torr, tungsten electrodes

slide 29 D B. Go 01/07/2013

Experiments: Practical ConsiderationsVarying surface properties make repeatability an issue

Active sitesLarge variability in βWork function and oxide layersGas adsorption

TEM image ofoxide layer oncobalt nanowire.

Xavier et al. Nanotechnology, 2008.

G.N. Fursey, App. Surf. Sci., 2003.

active sites

E. V. Nefyodtsev et al., IEEE Trans., 2011.

Field emission experiments at pressure especially challenging to due gas environment and ion bombardment continued area of study

slide 30 D B. Go 01/07/2013

Outline• Overview / Motivation for Microdischarge Research

– The importance of field emission

• 2012 Accomplishments– Modeling and Theory– Experiments

• Ongoing work– Glow discharge simulations– Quantum modeling of ion-enhanced field emission– Electron/surface interactions

• Conclusions and Outlook

slide 31 D B. Go 01/07/2013

Ongoing: Glow Discharge Simulations

total current potential distribution

ion distribution electron distribution

Glow discharge (PIC/MCC) simulations:• reveal thermodynamics of field emission-driven Townsend discharge (below

breakdown)• understand impact of field emission on DC glow discharges at microscale

(Macroscale) DC glow discharge using NDPIC1DElectron Energy Distribution (95% of breakdown voltage)

10 μm gap

3 μm gap

Note: high energy tail

slide 32 D B. Go 01/07/2013

Ongoing: Ion-Enhanced Field Emission

Emission current

1D time-independent Schrodinger equation

Preliminary calculations of pure Fowler-Nordheim field emission

Quantum modeling of ion-enhanced field emission:• understand quantum effects more completely• develop model that can be incorporated into PIC/MCC simulations for more

accurate discharge modeling

slide 33 D B. Go 01/07/2013

Ongoing: Electron/Surface Interactions

*Collaboration with R. Mohan Sankaran, Case Western Reserve University

Electrons produced by atmospheric discharges (either microhollow cathode or field emission-driven Townsend) can be used to drive electrochemical (reduction) reactions at both solid and liquid surfaces for applications such as materials synthesis

e-

100 μm

Microhollow cathode plasma jet impinging on liquid surface

Silver nanoparticles synthesized by reduction of Ag+

slide 34 D B. Go 01/07/2013

Ongoing: Electron/Surface Interactions

Cathode 2H+ + 2e- → H2(g)

Anode 2H2O → O2(g) + 4H+ + 4e-

Becomes more basic

Becomes more acidic

Witzke, Rumbach, Go, and Sankaran, J. Phys. D: Appl. Phys., 2012

Production and measurement of hydrogen gas and pH changes by plasma jet confirms gaseous electrons reduce protons in liquid

chromatograph of H2 production

*Collaboration with R. Mohan Sankaran, Case Western Reserve University

slide 35 D B. Go 01/07/2013

Outline• Overview / Motivation for Microdischarge Research

– The importance of field emission

• 2012 Accomplishments– Modeling and Theory– Experiments

• Ongoing work– Glow discharge simulations– Quantum modeling of ion-enhanced field emission– Electron/surface interactions

• Conclusions and Outlook

slide 36 D B. Go 01/07/2013

Conclusions• Clarified interaction between discharge and field emission that

leads to breakdown

• Established theoretical basis and basic scaling parameters for field emission-driven Townsend discharges

• Experimentally confirmed field emission-driven Townsend discharge regime and preliminarily confirmed pressure scaling– Showed nanomaterials can produce high field emission currents at

pressure under 100 V– Showed that it is possible to form a DC glow discharge in a microgap

slide 37 D B. Go 01/07/2013

Outlook• Areas of future work

– Clarifying the thermodynamics and properties of field emission-driven Townsend discharges and microscale glow discharges

– Understanding ion-enhanced field emission at a more fundamental level and incorporating more accurate physics into PIC/MCC

– Experimentally confirming the pressure scaling of field emission

– Exploring electron/surface interactions• Field emission-driven reduction reactions at solid surfaces• Microhollow cathode plasma jets at liquid surfaces

slide 38 D B. Go 01/07/2013

Relevant PublicationsJournal Articles

1. Y. Li, R. Tirumala, P. Rumbach, D. B. Go, “The coupling of ion-enhanced field emission and the discharge during microscale breakdown at moderately high pressures,” IEEE Transactions on Plasma Science – in press.

2. P. Rumbach, D. B. Go, “Fundamental properties of field emission-driven DC microdischarges,” Journal of Applied Physics, vol. 112, art. no. 103302, 2012.

3. M. Witzke, P. Rumbach, D. B. Go, R. M. Sankaran, “Evidence for the electrolysis of water by plasmas formed at the surface of aqueous solutions,” Journal of Physics D: Applied Physics, vol. 45, art. no. 442001, 2012.

Conferences (2012 only)

4. M. Witzke, P. Rumbach, D. B. Go, R. M. Sankaran, “Reactions at the Interface of Plasmas and Aqueous Electrodes: Identifying the Role of Electrons,” AVS International Symposium and Exhibition, Tampa Bay, FL, 2012.

5. P. Rumbach, D. B. Go, “Properties of a Field Emission-Driven Townsend Discharge” Gaseous Electronics Conference, Austin, TX, 2012.

6. P. Rumbach, J. Li, R. Tirumala, D. B. Go, “The Influence of Field Emission on Breakdown and Townsend Discharges in Microscale Gaps,” International Workshop on Mechanisms of Vacuum Arcs, Albuquerque, NM, 2012.

slide 39 D B. Go 01/07/2013

AcknowledgementsStudents

• Rakshit Tirumala• Jay Li• Paul Rumbach• Danny Taller• Michael Johnson • Sara Dale (ug)• Matt Goedke (ug)• Zack Woodruff (ug)• Adam Talbot (ug)

Air Force Young Investigator AwardGrant FA9550-11-1-0020