The phase composition and morphology of nano-dispersed carbon phases obtained by nanosecond laser ablation of microcrystalline graphite target immersed in water were investigated. The second (λSHG= 532 nm) and the third (λTHG= 355 nm) harmonics of a Nd:YAG laser system were used to produce different water colloids of carbon. The values of the laser fluence applied for both wavelengths under the experimental conditions chosen were varied from several J/cm2 to tens of J/cm2 . Raman spectroscopy, X-ray photoelectron spectroscopy and SEM analyses were used to study the carbon target before and after laser treatment and the carbon colloids obtained. The study of the colloids is complemented through X-ray diffraction. A mix of different complex carbon phases and some hydrocarbon polymers (polystyrene predominantly) were found in the colloids.
Carbon exists in the form of many allotropes: zero-dimensional sp2 fullerenes, the two- dimensional (2D) sp2 honeycomb lattice of graphene (parent to graphite and carbon nanotubes), or three-dimensional (3D) sp3 crystals - diamond and lonsdaleite. Carbon can also exist in the form of carbine, a one dimensional (1D) infinite chain of sp1-hybridized carbon atoms. Each of them has notably different electronic and mechanical properties. Accordingly, it is highly important to correctly identify the hybridization state of carbon for the industrial application of these materials. It is well known that the phase ratio of sp2 and sp3 hybridized carbon is distinguishable by X-ray Photoelectron Spectroscopy (XPS) and especially by analysis of C1s lines. Here we report on a thorough XPS study of thin carbon films deposited by pulsed laser deposition (PLD). The studied films consist of a mix of carbon phases deposited on SiO2/(001)Si substrates. The obtained XPS results allows the optimization of PLD process parameters in order to synthesize single to few layered defected graphene/ graphene-like films.
The fabrication of nano-crystalline carbon films was implemented by the application of pulsed laser deposition (PLD) technology. The experiments were performed in a standard on-axis laser ablation (LA) configuration. The third harmonic of a Nd:YAG laser was used for ablation of a microcrystalline graphite target. All experiments were performed in vacuum at a pressure of 1×10-3 Pa for different deposition times. (001) Oriented silicon (Si) covered by either 350 or 450 nm silica (SiO2) layer was used as a substrate. The films have a thickness between 4 and 40 nm and are characterized by X-ray photoelectron spectroscopy (XPS), Raman spectroscopy, atomic force microscopy (AFM) and ellipsometry measurements. We established deposition of nano-sized graphene-like films on top of predominantly amorphous carbon films with a thickness of 1- 2 nm. The measured the (n and k) and determined the values for the forbidden gap of the films which are between 0.01 eV and about 1 eV with reference to the sp3 hybridized carbon content of the film.
Mixtures of micron to submicron complex carbon phases, namely, defective graphene, graphene-like (multi-layered graphene, graphene oxide etc.), graphite flakes etc. as fine suspensions were obtained by pulsed laser ablation of highly purified microcrystalline graphite targets immersed in double distilled water. The fundamental wavelength (λ = 1064 nm) and the fourth harmonic (λFHG = 266 nm) of a Nd:YAG laser system (15 ns pulse duration, 10 Hz pulse repetition rate) were used in the fabrication process. The laser fluence value corresponding to the onset of the ablation process, the one initiating optical breakdown in water and an intermediate value were used for each of the wavelengths mentioned above. The morphology of the particles dispersed in water was studied by scanning electron microscopy (SEM). Their phase composition and structure were explored by Raman spectroscopy. It showed the presence of some traces of polymerized hydrocarbons (polystyrene, polybutadiene etc.) in addition to the main carbon phases: defected graphene, reduced graphene oxide and graphite.