Nicola Ferralis

Applied Materials and Surface Science Laboratory

Selected Research Interests

Mechanical, vibrational and structural investigations of epitaxial graphene layers on SiC crystals and films.

Graphene is considered by many as the "holy grail" as a material for the next generation of electronic and sensing devices. Graphene layers grown epitaxially on SiC substrate have shown to possess attractive and innovative properties, not all of them fully understood. In our effort, we are interested in relating the novel eletronic properties of epitaxial graphene (high carrier mobility, possible engineering of a band-gap) to its mechanical and vibrational properties. We recently discovered that epitaxial graphene possesses a tunable surface strain, with important consequences on the surface morphology. Surface stress might be responsible for some of the aforementioned properties.

Electron channeling contrast images of 1 ML epitaxial graphene layers synthesized on 6H-SiC(0001) substrates at different annealing times: (a) clean SiC surface (0 min); (b) 1 min; (c) 2 min; (d) 4 min; (e) 8 min; (f) 30 min. Scale bar: 1 um.

The role of gold as a metal catalyst during nanowire growth: Alloying, temperature self-induced pinning, nanolayering and diffusion. 

Gold is the most commonly used catalyst in the growth of Si nanowires. Recent research has focused on the interaction of Au with Si surfaces at high temperature, and on the initial stages of growth of Si nanowires under tightly controlled ultrahigh vacuum conditions. Gold diffusion during growth and the size and distribution of the catalyst droplets determine the length, shape, and sidewall properties of the nanowires. Gold from the catalyst droplets wets the nanowire sidewalls, eventually consuming the droplets and terminating VLS growth.

In my current research, I am interested in the structural and chemical evolution of the gold catalyst and gold silicide formation at annealing temperatures typical of nanowire growth. The outcomes of this research are currently used in understanding how the diffusion of gold from the catalyst on top of the silicon wires affects the growth and the structure of Si nanowires.

Schematic representation of the evolution in composition and morphology of gold on a silicon surface upon annealing to T_E= 363°C and T_F =827°C and subsequent quenching to room temperature, RT. Dewetting of a thick Au film on Si(111) (left) near the eutectic temperature (T_E) results in an alloyed droplet (with eutectic composition) in equilibrium with a highly structured thin film. The formation of depressions underneath the droplets ensues as they are annealed at higher temperatures (T_F), a consequence of the enrichment in the Si composition of the alloy. When starting from microscopic Au particles, depressions form already at the eutectic temperature, by extracting Si from the substrate to reach the eutectic composition. Whether starting with Au thick films or dispersed particles, after cooling to room temperature, the droplets exhibit a thin Si outer nanoshell (shown in the SEM image in the inset), and Si channels inside the droplets. The percentage X of Si in the alloyed droplet (Au_100-X Si_X)

varies from 19% at the eutectic to 37% at T_F=827°C.

Outreach

 I am very interested in the development of outreach programs that bring the science we currently develop in our lab to different levels of audiences. In this effort I frequently give talk to local nanotechnology clubs, and at special events (such as the NanoDay and NanoEnergy Forums) at science museums, such as the Exploratorium. 

Recent publications

N. Ferralis, J. Kawasaki, R. Maboudian, C. Carraro, "Evolution in Surface Morphology of Epitaxial Graphene Layers on SiC Induced by Controlled Structural Strain", Applied Physics Letters 93, 191916 (2008). ArXiv

N. Ferralis, R. Maboudian, C. Carraro, "Evidence of Structural Strain on Epitaxial Graphene Layers on 6H-SiC(0001)", Physical Review Letters 101, 156801 (2008). ArXiv

G.S. Doerk, N. Ferralis, C. Carraro, R. Maboudian, “Growth of Branching Si nanowires seeded by Au-Si Surface Migration”, to appear in the Journal of Materials Chemistry (2008).

N. Ferralis, K. Pussi, R.D. Diehl, "Debye temperature of the 10-fold surface of the decagonal Al-Ni-Co quasicrystal", Surface Science 603 (2008), 1223.

N. Ferralis, R. Maboudian, C. Carraro, "Temperature-Induced Self-Pinning and Nanolayering of AuSi Eutectic Droplets", J. Am. Chem. Soc. 130 (8)  (2008), 2681 -2685.

N. Ferralis, R. Maboudian, C. Carraro, , "Structure and Morphology of Annealed Gold Films Galvanically Displaced on the Si(111) Surface", J. Phys. Chem. C, 111, 20, 7508 - 7513 (2007).