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Digitale Dissertation

Catrinel Ruxandra Stanciu :
Thin films of C60, Li@C60 and carbon nanotubes: photopolymerisation, exciton dynamics and nonlinear optical properties
Dünne Schichten von C60, Li@C60 und Kohlenstoff Nanoröhren: Photopolymerisation, Exziton Dynamik und Nichtlineare Optische Eigenschaften

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Abstract

The C60 films are known to polymerise when they are irradiated with photons of more than 1.7 eV energy. We investigated the photopolymerisation induced with an Ar+ -laser (2.41 eV, 514 nm wavelength) by optical second-harmonic generation (SHG) using an Nd:YAG laser(1.16 eV, 1064 nm), under ultra-high vacuum conditions. The process of photopolymerisation was monitored in situ by SHG, which was measured in reflection geometry, with different input/output polarisation combinations and for different film thicknesses. We have found that this method is extremely sensitive to the early stages of the photopolymerisation process. A significant change in the SHG signal is observed after doses of the order of 020 photons/cm2,which is at least two orders of magnitude lower than the values previously reported in the literature from Raman spectroscopy. The reaction is suppressed for films thinner than 10 nm. This behaviour in the proximity of the substrate can be explained based on a reaction mechanism having the self-trapped state of the charge transfer excitons as promoter of the polymerisation. Li@C60, the first endohedral C60 molecule produced in macroscopic quantities and purified to a 95% nominal content, is expected to have interesting optical and electronic properties due to the almost complete charge transfer from the encapsulated Li atom to the C60 cage. The thermal stability of the molecule was studied with high performance liquid chromatography and double sublimation thermal desorption spectroscopy. First evidence for the intramolecular charge transfer in pristine Li@C60 was obtained from optical (UV-VIS) absorption spectra of thin films of Li@C60 prepared by sublimation using fast heating rates. Transient absorption measurements with 200 fs pulses at 1.25 µm probe wavelength show faster relaxation of the excitations induced by the 625 nm pump pulse (on a picosecond time scale) compared to C60. A higher mobility of the excitons in Li@C60 films can be deduced from these measurements. This effect indicates a strong intermolecular interaction in solid Li@C60, leading to a larger delocalisation of the electronic states than in solid C60. A first indication for phototransformation of endohedral fullerenes under Ar+ -laser irradiation was found for Li@C60 films from SHG measurements, and confirmed as photopolymerisation with infrared absorption spectroscopy and solubility tests. Optical harmonic generation from different not aligned and aligned carbon nanotubes (CNTs) samples was investigated. A wavelength range covering the fifth harmonic was measured in air using 200 fs pulses at 1.25 µm fundamental wavelength. Only third harmonic generation was observed emerging from a broad continuum. This broad spectrum is tentatively explained based on the production of free electrons. The electrons are exciting a non-destructive plasma in air, which produces the optical spectrum. The evident high efficiency of electron emission under the action of light is recommending CNTs as intense pulsed electron emitters.

Table of Contents

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Title and contents
1. Introduction 1
2. Photopolymerisation of C60 probed by Second Harmonic Generation 3
2.1. Introduction 3
2.2. The C60 photopolymerisation process 4
2.3. Second Harmonic Generation in C60 films 11
2.4. Experimental setup 19
2.5. Photopolymerisation of oxygen-free C60 films 23
2.6. The phototransformation of air-exposed C60 films 37
2.7. Conclusions 39
3. Thin films of Li@C60 41
3.1. Introduction 41
3.2. Endohedral fullerenes 42
3.3. Endohedral Li@C60 45
3.4. Thermal stability and sublimation of Li@C60 molecules 49
3.5. Optical properties of vapour-deposited Li@C60 films 58
3.6. Excitation dynamics 68
3.7. Li@C60 photopolymerisation 79
3.8. Conclusions 85
4. Carbon nanotubes 87
4.1. Introduction 87
4.2. Structure and properties of carbon nanotubes (CNTs) 88
4.3. Experimental studies 96
4.4. Conclusions 112
5. Summary 113
References 115
Publications, Acknowledgements, CV

More Information:

Online available: http://www.diss.fu-berlin.de/2002/121/indexe.html
Language of PhDThesis: english
Keywords: Endohedral fullerenes, Carbon nanotubes, Nonlinear optics, Exciton dynamics, Thin films
DNB-Sachgruppe: 29 Physik, Astronomie
Classification PACS: 78.66Tr, 78.67Ch, 78.47+p, 42.65-k
Date of disputation: 29-Apr-2002
PhDThesis from: Fachbereich Physik, Freie Universität Berlin
First Referee: Prof. Dr. Ingolf V. Hertel
Second Referee: Prof. Dr. Dr. hc. Günther Kaindl
Contact (Author): Catrinel.Stanciu@physik.uni-marburg.de
Contact (Advisor): hertel@mbi-berlin.de
Notes: Vollständig lesbar mit Acrobat Reader 5.0
Date created:12-Jul-2002
Date available:16-Jul-2002

 

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