(557d) Design and Characterization of a Novel Chemical Vapor Deposition Reactor to Synthesize Nanoscale Structures

One-dimensional structures, such as nanowires, nanorods and
nanotubes, are being studied in an attempt to better understand the fundamental
properties of nanostructure materials for a variety of applications.  A common
synthesis method employed by numerous research groups are horizontal chemical
vapor deposition (CVD) tube reactors in which solid, liquid and gas precursors
are incorporated into a feed gas stream at elevated temperatures and reduced
pressures.  Nanostructures synthesized in these tube reactors generally grow by
the vapor liquid solid (VLS), vapor solid (VS) or metal-organic chemical vapor
deposition (MOCVD) mechanism.  While catalyzing the growth of the
nanostructures with well defined nanoparticles has resulted, in some cases, in narrow
radial size distributions, the length of the structures can vary over 2-3
orders of magnitude.  To gain insight into the large length discrepancies,
computation fluid dynamics (CFD) techniques were utilized to investigate
temperature and mass gradients as well as the velocity field of a typical horizontal
tube reactor with commonly used ?boat? substrate holders for varying operating
conditions.

The simulation results show large temperature, mass and
velocity gradients resulting in varying deposition rates as a function of axial
position.  To alleviate undesirable temperature gradients, mass transfer
limitations and detrimental stresses on the nanostructures during the growth
phase, a one-of-a-kind CVD reactor has been designed and fabricated.  Subsequent
CFD simulations show a significant enhancement in the consistency, with respect
to axial position, of the deposition rate when compared to the standard
horizontal system.  The prototype reactor is currently being used to repeat
well-established syntheses, such as silicon and germanium nanowire growth.  The
purpose is to investigate a possible enhancement over the control of variables
such as length, crystal growth direction and compositional homogeneity.  Upon
completion of the proof-of-principle experiments, the reactor will be
incorporated into the design of new reactions for compounds such as EuO.