Morphology, structure and density evolution of carbon nano-structures deposited by N-IR pulsed laser ablation of graphite

Abstract

Carbon films were deposited by pulsed laser ablation on Si <100> substrates, heated at temperatures increasing from RT to 800 °C, from a pure graphite target, operating in vacuum (~ 10 − 4 Pa). The laser ablation was performed by an Nd:YAG laser, operating in the near IR wavelength ( λ = 1064 nm). Micro-Raman and grazing incidence X-ray diffraction analysis (GI-XRD) established the progressive formation of ordered nano-sized graphitic structures, increasing substrate temperature. The surface morphology is characterised by macroscopic roughness (SEM, AFM) while the low temperature samples are characterised by very smooth surface. The film density, evaluated by X-ray reflectivity measurements, is also affected by the substrate temperature. This structural property modification induces relevant variation on the emission properties of carbon films, as evidenced by Field Emission measurements. The film structure and texturing is also strongly related to laser wavelength: the low energy associated to the IR laser radiation (1.17 eV) causes an early aromatic cluster formation at T = 400 °C associated to a sensible increase in the aromatic plane stacking distance ( d 002 ~ 0.39 nm), compared to graphite. These density decrease shows a direct correlation with the electron emission properties. Roughness and presence of voids play a negative role both on the threshold electric field E th and enhancement factor ( β) The density decreasing and graphitic layer widening are notably to be ascribed to the very fast out-of-equilibrium growth and to the presence of large activated carbon species in the “plume”.