Surface Energy in Nanocrystalline Carbon Thin Films: Effect of Size Dependence and Atmospheric Exposure

Surface energy (SE) is the most sensitive and fundamental parameter for governing the interfacial interactions in nanoscale carbon materials. However, on account of the complexities involved of hybridization states and surface bonds, achieved SE values are often less in comparison with their theoretical counterparts and strongly influenced by stability aspects. Here, an advanced facing-target pulsed dc unbalanced magnetron-sputtering process is presented for the synthesis of undoped and H/N-doped nanocrystalline carbon thin films. The time-dependent surface properties of the undoped and H/N-doped nanocrystalline carbon thin films are systematically studied. The advanced plasma process induced the dominant deposition of high-energy neutral carbon species, consequently controlling the intercolumnar spacing of nanodomain morphology and surface anisotropy of electron density. As a result, significantly higher SE values (maximum = 79.24 mJ/m²) are achieved, with a possible window of 79.24–66.5 mJ/m² by controlling the experimental conditions. The intrinsic (size effects and functionality) and extrinsic factors (atmospheric exposure) are resolved and explained on the basis of size-dependent cohesive energy model and long-range van der Waals interactions between hydrocarbon molecules and the carbon surface. The findings anticipate the enhanced functionality of nanocrystalline carbon thin films in terms of selectivity, sensitivity, and stability

Thermoelectric Power Factor Enhancement by Pulsed Plasma Engineering in Magnetron Sputtering Induced Ge₂Sb₂Te₅ Thin Films

Precise control over microstructure and composition is desired prerequisite for the performance enhancement of thermoelectric materials. In conventional magnetron plasma sputtering synthesis, composition control is challenging when the sputtering-target is composed by different elements. Here, the potential of pulsed power utilization is demonstrated for compositional control of Ge₂Sb₂Te₅ thin films via pulse-reversal time and pulse-frequency engineering in pulsed DC-magnetron sputtering process. When annealed at 400 °C for 1 h in vacuum conditions, amorphous thin films (of 200 nm thickness, deposited on glass substrate) crystallize in to face centered cubic phase with average nanocrystallite size ∼10 nm. Power density enhancement to 5.56 W/cm² at low pulse reversal time induces maximum process throughput as 450 nm/min. Increase in either of pulse frequency or pulse reversal time decreases the discharge voltage and plasma density. As a consequence, kinetic energy of ions and ionization of plasma species are sequentially controlled to improve the stoichiometry of film and eventually; the electronic transport. The optimization of pulse plasma engineering yields maximum thermoelectric power factor value as 1.35 μW cm^–1 K^–2 with process throughput more than 300 nm/min. The obtained values are promising for applications in the automobile and microelectronics industry