Thin Films & Materials Characterization

Growing Large-Area rGO Thin Films by Pulsed Laser Deposition and Annealing

Independently designed and executed deposition of large-area reduced graphene oxide films using a KrF excimer laser, followed by room-temperature pulsed laser annealing and full structural, chemical and electrical characterization.

By Musa Ibne Mannan · Erik Jonsson School of Engineering and Computer Science, The University of Texas at Dallas

Objective

Reduced graphene oxide is attractive because it offers much of graphene's electronic character through a route that scales. The difficulty is film quality: deposition conditions determine the degree of reduction, the defect density and the electrical transport behaviour, and those parameters interact. This project set out to deposit large-area rGO thin films by pulsed laser deposition (PLD) and then improve them further through pulsed laser annealing (PLA), mapping how oxygen partial pressure governs the result.

Deposition

Films were grown from a 99.99 percent pure graphite target onto SiO₂ (200 nm) on silicon substrates, ablated with a KrF excimer laser at 248 nm with a 25 ns pulse duration. Several parameters were deliberately fixed so that a single variable could be isolated: 5,000 laser shots at 5 Hz, a base vacuum pressure of 1×10^-6 Torr, and a substrate temperature held at 650 °C.

Active chamber pressure was maintained at 2×10^-2 Torr using a mixed oxygen and argon atmosphere. The oxygen partial pressure was the primary experimental variable, swept from 0 to 6 Torr in 0.5 Torr intervals, with the argon fraction adjusted to hold total pressure constant. That constraint is what makes the series interpretable — any change in film character can be attributed to oxygen chemistry rather than to a shift in overall deposition pressure.

After growth, films were treated by pulsed laser annealing at room temperature with an energy density of 0.8 J/cm², intended to drive off residual impurities and improve structural integrity without subjecting the substrate to a thermal budget.

Characterization

No single technique establishes that you have made good rGO, so the films were characterized across structure, chemistry and transport.

Structural and chemical

• Raman spectroscopy using a 532 nm green laser, confirming the characteristic D, G, 2D and D+G bands and tracking the I_D/I_G ratio as the defect metric. This was measured against laser shot count from 1,000 to 10,000, across the O₂/Ar ratio series, and before and after annealing.
• X-ray photoelectron spectroscopy for survey scans, C1s and O1s regions, with deconvolution of the C1s curve before and after laser annealing to estimate the sp²/sp³ ratio and the carbon-to-oxygen ratio.
• X-ray diffraction to track the shift in the 2θ peak across oxygen partial pressures, confirming the extent of graphene oxide reduction.
• Ellipsometry (Woollam M-2000) for precise film thickness.

Electrical transport

• Hall measurements by the Van der Pauw method with indium solder contacts, determining carrier type, concentration and mobility.
• Resistance versus temperature on a PPMS DynaCool from 1.8 K to 300 K, used to model charge carrier transport against Arrhenius behaviour and variable range hopping.

Transport behaviour

The low-temperature resistivity data proved the most interesting result. Plotting ln(ρ) against T^-1/2 and T^-1/3 from 2 K to 259 K allows the two competing variable range hopping models to be distinguished. As mobility increased, the relationship between resistivity and temperature moved closer to the Mott VRH model — but it did not depart from the Efros–Shklovskii VRH model either. The films sit in a regime where both descriptions retain explanatory power, which is itself informative about the disorder and Coulomb gap conditions in the material.

Context

This work was carried out in the Leem Research Group at UT Dallas, where I designed, assembled and operated the deposition and characterization equipment from the ground up, including high-vacuum CVD furnaces, a hydrogen generator, and integrated Raman spectroscopy instrumentation.