Dust formation : speculated mechanism

1. Dust formation : speculated mechanism Ar+/H3+sputtering/chemical sputtering/erosion Agglomeration Coagulation Gas phase Chemistry Surface growth nucleation C, C2, C3 Ni = density of particles with a size i R = nucleation rate (estimated from the chemical kinetics model) G = coagulation/agglomeration rate (two particles  larger particles) W = growth rate (surface growth - heterogeneous chemistry) T = particle losses due to transport : diffusion, thermophoresis, drag, ...

2. Model of nucleation, growth and transportof dust in DC discharges ignited in Ar/H2 (2) • Estimation of discharge main characteristics: flux and ion energy distribution or ion average energy on the cathode • Extraction of C1, C2 et C3 from the substrate surface • Chemistry and molecular growth  Formation of Cn=1,nl clusters, where nl is arbitrary chosen (nl=30 or 60) • Nucleation of carbon dusts from clusters: Assumption of ‘Largest Molecular Edifice’ • Growth, charging, transport and wall losses of dusts • Feed back on the gas phase chemistry  heterogeneous process • Size distribution of dusts

3. Molecular growth modelling of carbon clusters and dusts Molecular growth clusters Diffusion Mobility Gas phase chemistry and molecular growth Production rate of the Ci cluster Nucleation ni,z = density of the cluster Ci of charge z Dust Transport • Determination of the average diamater dp N = nucleation C= coagulation A= condensation

4. Carbon cluster growth reactions** Bernholc & Schweigert models (classical models) (**): • Growth = one single process (Cn + Cx Cn+x), but take into account the stability of the Cn clusters • First version of the model took into account neutral clusters • Molecular growth of clusters • Rates computed according to formation enthalpies • Clusters have configurational isomers (chains, rings, multi-cycles) distinguished by cyclization entropy (20 kcal/mol/cycle) • Extrapolation for unknown values according to cluster periodicities

5. Molecular growth modelling of neutral carbon clusters and dusts • Low pressure discharge : p=1-10 Pa • Diffusion characteristic time =1-10 ms very short as compared to the growth chemistry  no possibility for growth of neutral •  Need for species with higher residence time : • Negative clusters • And • Trapping electric field configuration •  Back to some basic discharge physics

6. Electric field reversal and molecular growth of negative clusters 2 V0/dc Energetic electrons (g) Passing electrons (j) ~ < 1 V Trapped electrons (ne) E ef NG FDS PC E0 x sheath x0 dc R x1 • Charging of dust particles only effective if electric field is confining! • Where is the confining electric field ?  Kolobov & Tsendin, Phys. Rev. A 46 7837, Boeuf &PitchFord, J. Phys. D, (1994) • Self-consistent electric field reversal: confinement • Three electron populations: energetic, passing, trapped and negative ions NG: Negative glow / FDS: Faraday Dark Space / PC: Positive Column

7. Negative carbon cluster growth reactions • Attachment Cn + e- Cn- • Rates computed according to electronic affinities • Charge exchange Cn- + Cx Cn + Cx- • Electronic affinities • Dust agglomeration (sticking) • Detachment Cn- + e- Cn + 2e- From Y. Achiba et al., J. Elect. Spect. Related Phen. 142, 231 (2005)

8. U=zV Z carbon particles aerosol dynamic in a DC dicharge Particle charging is a key point : ==> Enhanced particle charging insures a significant trapping and long residence time ==> Enhanced particle charging prevents coagulation and growth kcoag ====> Z+Z' Z Z' Kcoag(z,z’)

9. The only way to have growth ==> charge fluctuation and electron depletion Possible because particle charg ing is a discrete process  Dynamic fluctuation of small particles between positively and negatively charged states  Coagulation takes place between two particles that has opposite instantanous charges or no charge  involve small particles. tcoag<<tfluctuation<<ttrans Transport feels the average charge Coagulation feels the fluctuations Fluctuation

10. Molecular growth of negative clusters • Negative clusters have significant densities • Growth rate is a function of the electric field profile in the discharge • An accurate knowledge of the field profile is required

11. np DE Dust density Cathode np|max=1013 cm-3 Anode <ee>=1 eV <ee>=0.1 eV Field reversal np|max=5x1011 cm-3 Electric field reversal <=> electron average energy in the NG DE  <ee>

12. Dust average charge and diameter Cathode Cathode 25 -7 7.0x10 Anode 0.03 eV Anode 0.1 eV 20 -7 6.0x10 1 eV -7 5.0x10 15 <ee> 0.03 eV charge 0.1eV diamètre (cm) -7 1eV 4.0x10 10 -7 3.0x10 5 -7 2.0x10 0 0 6 12 -7 1.0x10 position (cm) 0 6 12 position (cm) It is indeed possible to explain particle formation through negative ion driven molecular growth  Discharge dynamic (field reversal) and sputtering kinetics are key-points Pbs : we need better description of the growth kinetics : Model  1 hour for dust formation (instead of few minutes) Take into account the size and charge distributions

13. CASIMIRDevice (Chemical Ablation, Sputtering, Ionization, Multi-wall Interaction, and Redeposition) 2nd module : Microwave plasma source "surfaguide" 1st module : Sputering/erosion of carbon susbtarte (H2/Ar plasmas) 3rd module : Redeposition chamber • Multipolar microwave discharge • - Gaz = H2/Ar, Pressure 10-2 mbar • carbon Substrate • (Controled temperature and voltage) - Collection of the deposit : filter and substrate) Decoupling gas phase and surface process

14. Measurement techniques • Mass spectrometer / ion energy analyzer • Detection of neutral and radivcalar species in the plasma (m/z 1-500 uma) • Detection of positive et négativeions • Measurement of IEDF (+/- 1000 eV) • Optical Emission Spectroscopy (H/D et carbonated species) (temperature and density measurements and characterization of plasma species in CASIMIR) • Analysis of the deposit microstructure by SEM and Raman

15. Results I. Mass spectrometry: Polarisation Sheath Polarisation Sheath graphite disc substrate Plane Substrat Photography of the negatively polarized disc substrate in Ar/H2 Photography of the plane polarized substrate in Ar/H2 plasma

16. Resultts I. Spectromètre de masse / analyseur d’énergie : b) Mass spectrometry and IEDF measurements : Ions in the discharge Ar2+, Ar+ mass spectra (0,60 kW, 10 sccm) H+, H2+, H3+ mass spectra (0,60 kW, 100 sccm) D+, D2+, D3+ mass spectra (0,60 kW, 100 sccm) D- mass spectrum (0,60 kW, 100 sccm)

17. Results c) IEDF D+ Ar+ D+ and Ar+ IEDF’s

18. Results I. Carbon detection : Detection of C, CH, CH3,CH4 et C2 I.2) deuxième études : sur la tête 1-500 uma • : E between 9,8 and 14,25 eV • CH3 + e- => CH3+ + 2 e- (in the plasma) • (2) : E > 14,25 eV • CH4 + e- => CH3+ + H + 2 e- (in the analyzer) • Hydrocarbon production through erosion/sputtering in CASIMIR Mass spectra in H2, Ar, et Ar/H2 plasma Threshold mode detection of CH3 radical CH3

19. Results I. Mass spectrometry: b) Effetc of the polarisation on the erosion yield Voltage contrôle  microarcs 600 V – 2 A Courant contrôle 300 mA – 1000 V Comparaison of masse spectra obtained with the two contrôle modes in Ar/H2 plasma Mass spectrum in H2 plasma With and without polarisation (Alim1)