Part 1 - Growth of nanowires and nanorods

Question 1

What is the difference between isotropy and anisotropy?

Answer 1

Isotropy: uniformity in all directions.
Anisotropy: directionally dependent

Question 2

What type of growth is needed for the formation of nanorods/nanowires?

Answer 2

Anisotropic growth. We need it to grow faster in one direction compared to the others.

Question 3

What are some factors that can influence the anisotropy of growth?

Answer 3

1. Different facets have different growth rates.
2. Presence of specific defects in specific crystal directions, such as screw dislocations.
3. Preferential accumulation or poisoning by impurities on specific facets.

Question 4

What are the growth limiting steps for 1D-growth?

Answer 4

Usually ad/desorption of growth species to the surface. When the concentration is higher, the growth is controlled by surface growth.

Question 5

If the growth of 1D-structures is rate-limited by the adsorption rate, what is the growth rate then determined by?

Answer 5

The condensation rate, J [atoms / m^2 s].

J = (alpha * sigma * P_0)/sqrt(2 pi m k T)

where alpha is the accommodation coefficient, the fraction of impigning growth species that becomes accommodated on the growing surface.

sigma is the relative supersaturation

P_0 is the equilibrium vapor pressure of the crystal at temperature T

m is the atomic weight of the growth species.

Question 6

What can be said of the accommodation coefficient for different adsorption sites?

Answer 6

Stepped and kinked surfaces will have unity accommodation coefficients, while for flat surfaces it depends on the availability of kink and ledge sites.

Question 7

How is the surface free energy related to the thermodynamical equilibrium crystal?

Answer 7

Only the facets with low surface free energy will survive. The ones with higher surface free energies will have more dangling bonds, and growth will more favorably occur at these sites so that they disappear.

Question 8

What is the VLS mechanism for growth of 1D-structures?

Answer 8

VLS = Vapor liquid solid. A catalyst droplet is placed on top of a substrate. Growth species is introduced in the gaseous phase, and is absorbed into the liquid catalyst. At saturation it is precipitated and grows towards the substrate and in only one direction. The diameter of the nanorod is dependent on the size of the droplet.

Question 9

Name some other mechanisms in which one can achieve 1D-growth, apart from VLS?

Answer 9

Templated methods, electrospinning and lithography.