Orientation tendency of PLD carbon films as a function of substrate temperature: A NEXAFS study

Electronic properties of graphene nano-structures are highly anisotropic and correlated to their basal plane orientation. In this work, we tried to study and explain how deposition temperature can draw a definite plane growth direction. Carbon films have been deposited by Pulsed Laser Ablation (Nd:YAG, 2nd harmonic: λ=532 nm, hν=2.33 eV, τ=7 ns, ν=10 Hz, Φ ≈ 7 J/cm 2), from a pyrolytic graphite target on Si <100> substrates. PLD depositions were performed in vacuum, at increasing substrate temperature, ranging from room temperature (RT) to 900 °C. Near-edge X-ray absorption fine structure (NEXAFS), SEM and Raman spectroscopy have been used to characterise the films. Using linearly polarized Synchrotron radiation and HOPG (Highly Oriented Pyrolytic Graphite) as a reference system, we investigated, ex situ, the occurrence of an average orientation distribution of the nano-sized graphene clusters, grown at high temperature. The results of the angle-dependent study by NEXAFS of film grown at 900 °C provide evidence that at high temperature the graphene sheets show a tendency to grow with basal planes oriented perpendicularly to the substrate surface. At low temperature, the film is less structured. The features ascribable to graphitic character are less pronounced and broadened; the absence of an angle dependence of NEXAFS measurements suggest that C-structures are randomly oriented. The corresponding Raman spectrum substantiates the prevailing amorphous nature of the film