Method for interfacial energy determination of gold nanoparticles on solid surfaces

Sammanfattning: The high surface-to-volume ratio of nanoparticles (NPs) makes them an interesting source for scientific research. How interfacial energies interplay between such a small structure and a solid bulk surface, are important because of the versatile applications NPs regard. In this thesis, a convenient and reliable method for interfacial energy determination was investigated. The interfacial energies were calculated using Young's equation, where the contact angle between particle and surface was determined using two different methods. Method 1 utilized imaging samples, tilted 85 degrees, in SEM. Method 2 utilized height measurements using AFM, and width measurements from top-view imaging in SEM. The NPs in this thesis were made of Au, and placed on Si wafers, on Al2O3 coated Si wafers, and on sapphire wafers. The Si samples were heated for 10 or 30 minutes at 600 degrees Celcius or for 3 minuter at 900 degrees celcius. The Al2O3 coated Si and sapphire samples were heated at 800 degrees Celcius for 75 hours. Regarding all substrates containing Si, the AuNPs became catalysts for nanowire (NW) growth and no energies could be calculated. This development was confirmed via TEM imaging on 60 nm AuNPs sintered for 30 min at 600 degrees Celcius. Using FFT of these particles, {1 1 1}- and {2 0 0}-planes were detected. These particles were concluded to be crystalline, but had no clear faceted shape. The interfacial energies of AuNPs on sapphire wafers were determined for 30, 60 and 90 nm NPs to be approximately 897.8, 1295.8 and 1546.4 ergs/cm2, respectively, by Method 1, and 1958.4, 1883.4 and 1830.8 ergs/cm2, respectively, by Method 2. The energies determined using Method 2 are closer to the value retrieved from literature, 1725 ergs/cm2, which regard particles below 100 nm in diameter. Due to lack of literature, the interfacial energies calculated in this thesis are only compared to R. M. Pilliar and J. Nutting, from 1967, and may not be the most recent nor accurate value. Method 1 was determined to be a method strongly dependent on how well the SEM imaging was performed, and resulted in unreliable and unstable energies. Method 2 eliminated the human factor that Method 1 implied, but did not consider particle shape and unexpected changes that may occur. Method 1 was easier to use but Method 2 was settled to be the most reliable method resulting in more stable energy-values. However, Method 2 was unable to detect the unexpected change of NW growth. More research is needed to decide whether Method 2 can be applied in every-day research practice.