  We study in this paper the atomic mechanisms of nanorod growth and propose
the way of diameter selection of nanorod. A characteristic radius is
demonstrated to be crucial in nanorod growth, which increases proportional to
one fifth power of the ratio of the interlayer hopping rate of adatoms across
the monolayer steps to the deposition rate. When the radius of the initial
island is larger than this characteristic radius, the growth morphology evolves
from a taper-like structure to a nanorod with radius equal to the
characteristic radius after some transient layers. Otherwise the nanorod
morphology can be maintained during the growth, with stable radius being
limited by both the radius of the initial island and the three-dimensional
Ehrlich-Schwoebel barrier. Therefore different growth modes and diameter of
nanorod can be selected by tuning the characteristic radius. The theoretical
predictions are in good agreement with experimental observations of ZnO growth.
