Carbon Nanotube Formation Detection of Ni atom and C 2 Gary DeBoer LeTourneau University Longview, TX NASA Johnson Space Center Thermal Branch Structures and Mechanics Division Engineering Directorate Summer, 2000 by Laser Induced Fluorescence

What are Carbon Nanotubes?

SEM of Nanotube Bundles

Why should we care? Strong light-weight materials Thermal and electrical properties Gas (hydrogen) storage

What’s the Problem? Nanotubes from Price = $1000/gram Minimum order = 250 milligrams Please order in 1/4-gram increments only. Carbon nanotubes, single-walled Sigma-Aldrich Package Sizes US $ 100MG MG Product Comments: CarboLex SE- grade, angstrom

Increase Production Understand the chemical mechanism (particularly the role of the catalyst) modify current methods or design new methods

Nanotube Formation Theories Atomic scooter Metal clusters (nm diameters) Melt (  m sized particles or droplets)

Laser Induced Fluorescence (LIF) LaserSample Optics Detector

Nanotube diagnostics

Laser Ablation target tube

Plume Emission Spectrum

Physical Principles for C 2 LIF Upper electronic state Intermediate state Detector Long wavelength filter Detector Fluorescence at 513 nm Absorbance at 473 nm Lower electronic state

C 2 LIF

C 2 Rotational Spectra

Rotational Temperature

DDG Boxcar Averager Laser 2 IR 1064 nm Laser 3 Dye Pump 355 nm Laser 4 Dye tunable Energy meter ICCD LeCroy or Digital Scope

C 2 Experiment and Synthetic

C 2 Rot Temperature and Intensity

C 2 Rot Temperature and Position

Summary of C 2 LIF results Lifetimes of more than 50  s Rotational temperatures K Rotational temperature is proportional to intensity Signal can be seen up to 5 mm from the target surface Signal propagates at 50 m/s

Physical Principles for Ni LIF filter detector Absorbance nm non radiative decay Fluorescence at 301 nm intermediate state Lower electronic state Upper electronic state

Nickel Transitions in LIF

Laser 1 Gr 532 nm DDG 1 DDG 2 Boxcar Averager 60 Hz - 10 Hz Laser 2 IR 1064 nm Laser 3 Dye Pump 355 nm Laser 4 Dye tunable Energy meter ICCD LeCroy or Digital Scope

Nickel LIF Spectra

Ni Experiment and Synthetic

Nickel Temperature Wavelength (nm) A B a. 0 b. 204 c. 879 c b b c a Pump-Probe Delay (  s) hot cold hot cold

Nickel Propagation

Summary of Ni LIF Results Lifetime of several milliseconds with a hot target, 20 microseconds with a room temperature target Electronic temperatures from K Electronic temperature is proportional to signal intensity Signal can be seen up to 3 mm from the target Signal propagates at about 10 m/s

Co results Laser Induced Luminescence (LIL) Lifetimes: Co atom milliseconds Carbon seconds Geohegan et al. Appl. Phys. Letts., 2000, 76 (3) p 182

Other Observations Hot emission and cooler LIF is not unique. Brinkman, Appl. Phys. B, p. 689 Pobst, IEPC, (28) p. 203 Raiche, Appl. Opt p Ablation: small molecules and atoms. Becker, Nanostructured Materials, (5) p. 853 Song, Applied Surface Science, p 111 Aguilera, Applied Surface Science, p. 309 Dillon, Advances in Laser Ablation of Materials (USA), 1998 p

Summary of Results ablation produces small molecules and atoms (lifetimes) C 2 - hot emission 50  s C 2 - cooler LIF/LIL 100  s Ni and Co LIF/LIL 3 ms C n LIL 3 s C 2 propagation 50 m/s Ni propagation 10 m/s

Conclusions Inconsistent with the melt theory Consistent with atomic catalyst theory Could be consistent with small metal cluster theory Need to know when and where nanotubes are formed

Future Work Analysis of three laser ablation experiments Analysis of DC arc spectra Further parametric studies C 2 LIF using two ablation lasers Computational modeling for –nanotube formation mechanisms –nanotube interactions with other materials

Acknowledgements Sivaram Arepalli William Holmes Pasha Nikolaev Carl Scott Brad Files SFF NASA-ASEE