Laser-induced particle formation in a methane discharge Eva Stoffels, Winfred Stoffels, Eindhoven University of Technology, PO Box 513, 5600 MB Eindhoven. Giacomo Ceccone, Francois Rossi, European Commission, Joint Research Center, Ispra (VA), Italy.

CH 4 radio-frequency plasma for diamond deposition pressure Torr 3rd harm. Nd:YAG laser ( = 355 nm), 8 ns pulse, 10 Hz repetition frequency detection at 90 o by photo- camera Experimental setup

Appearance of particles as a function of laser power Laser photon energy (ca. 3.5 eV) matches the bond energy of CH 4. High radical densities are created locally by photodissociation. Nanoparticles (<10 nm) are nucleated. Nanoparticles are detected by the same laser, due to their fluorescence upon laser irradiation Time needed to nucleate carbon nanoparticles during laser irradiation, as a function of laser power

500 nm SEM micrograph of particles After nucleation, nanoparticles grow in the plasma until they reach a size of 100 nm.

Coulomb crystal formation Formation of vertical strings First organised structures 100 nm particles have a permanent negative charge. Coulomb interactions lead to formation of organised structures.

0.2 mm SEM x2000Optical microscope x75 10  m Vertical strings In the plasma-sheath transition electric fields are present Charged particles gain kinetic energy of about 1 eV, enough to overcome Coulomb repulsion Particle coalescence takes place

V-shape structures floating in plasma 1 cm

Forces on a single particle and on V-shaped conglomerate Charge on a single particle of size a: Ze = 4  0 a V p,mass M = 4/3  a   Charge on a linear string of N particles with size a: Ze = 4  0 a V p N/ln(4N), mass M = 4/3  a  

1 cm Floating network above wafer 0.1 mm Coulomb repulsion Large structures in the plasma Deposited conglomerates Both charge and mass of a string increase with the string size N. Coulomb and gravitation forces remain balanced. Large strings are still floating.

SEM x50Light microscope x mm Structure of floating network

Growth on electrode 5 mm Eventually, floating network collapses and deposits on the electrode.

Overview of all phases