Epitaxial films through spin coating
A simple way to coat a surface with a uniform film is by spin coating. The substrate is spun at high speed, and a droplet of solution containing the coating is added at the center, spreads out, and evaporates. This method is used to make polycrystalline inorganic coatings and amorphous films, such as polymers used in lithography. Kelso
et al.
performed spin coating with single-crystal substrates, carefully controlling the thickness of the spreading solution on the basis of its viscosity and the rotation rate. In this way, they achieved epitaxial growth—in which the crystallites are oriented by the substrate—for perovskites, zinc oxide, and sodium chloride.
Science
, this issue p.
166
Surfaces of achiral materials exhibit
two-dimensional chirality if they lack mirror symmetry. An example
is the (643) surface of face-centered-cubic metals such as Au. The
(643) and (6̅4̅3̅) surfaces are non-superimposable
mirror images of each other. Chiral surfaces offer the possibility
of serving as heterogeneous catalysts for chiral synthesis or providing
a platform for chiral separation or crystallization. Here, we show
the symmetry requirements for surface chirality, and we demonstrate
that chiral surfaces can be produced by electrochemically depositing
epitaxial films of Au onto commercially available Si(643) wafers.
Au(643) is deposited onto one side of the wafer, and its enantiomer
Au(6̅4̅3̅) is deposited on the other side of the
wafer. In addition to the (643) orientation, the (8 14 17) orientation
of Au is produced on the Si(643) wafers. The (8 14 17) orientation
has a similar kinked surface to the (643) surface, but it has staggered
kinks. Other metal films including Pt, Ni, Cu, and Ag that are electrodeposited
onto the Au films exhibit strong in-plane and out-of-plane order.
Hence, the method provides a pathway for producing chiral surfaces
of a wide range of materials, and it obviates the need to work with
expensive single crystals. The Ag/Au/Si(643) surface showed a preference
for the electrochemical oxidation of d-glucose, whereas the
Ag/Au/Si(6̅4̅3̅) surface showed preference for the
oxidation of l-glucose.
Lu and Tang claim that the spin-coated films in our study are not epitaxial. They assume that all of the background intensity in the x-ray pole figures of the spin-coated materials is due to randomly oriented grains. There is no evidence for randomly oriented grains in the 2θ x-ray patterns. The background intensity in the pole figures is also comparable to the background from the single-crystal substrates, which is inconsistent with their assumption.
In this paper, we will present an overview of metrology issues and some of the techniques currently under development in our group at NIST, aimed at understanding some of the potential performance limiting issues in such highly integrated systems. We will discuss our attempts to identify and characterize the various types of defects and relate them to where and why they form, without interrupting the responsible phenomena.
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