STEIN RESEARCH GROUP
POROUS SOLIDS — NANOCOMPOSITES — SELF-ASSEMBLED FRAMEWORKS
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..................Photonic
Crystals
As
light is used increasingly
in the flow and processing of information, it is necessary
to develop new
devices that can control photons.
An emerging field addressing
this issue involves
photonic
crystals, materials with a periodic
variation in refractive index.
Photonic crystals that
exhibit photonic band gaps (PBG,
a range
of wavelengths
that cannot
be transmitted through the
material) have foreseeable applications as
waveguides, microcavity
lasers, or inhibitors of light emission. Since
the seminal theoretical work
by John
and Yablonovitch on photonic
crystals, much research
effort has been dedicated
to fabricating
photonic crystal
structures. Lithographic and
reactive ion-etching techniques
have been used to prepare two-dimensional
photonic crystals, but the construction of three-
dimensional structures has been
more
difficult
and expensive. An
alternative, chemical route involves the use
of colloidal crystal templating
to
form three-dimensionally
ordered
macroporous (3DOM) dielectric structures.
Monodisperse spheres (polystyrene, poly(methylmethacrylate),
silica) are close-packed iinto ordered
arrays and infiltrated with a
fluid which
is solidified.
After
removal of the template, a solid skeleton
is obtained around an ordered
array of voids where
the original spheres were
located. We have pioneered methods
of preparing 3DOM structures with a wide
range
of compositions (metals, alloys,
oxides, hybrid
materials). 3DOM structures
meet several of the requirements
for PBG materials: they can be prepared
in desirable structures (face-centered
cubic, fcc) with periodicities on a length
scale overlapping with the wavelength of light, they have low volume fractions
of solid material, and a wide
range
of compositions
allows modification
of the refractive index.
3DOM
materials display a number of interesting optical properties with
less stringent structural requirements. We have been able to prepare
brightly colored 3DOM zirconia and titania samples in gram quantity,
covering the whole visible spectral range. These materials have potential
applications as non-toxic, chemically stable pigments. Furthermore
their colors (stop-band positions and intensities) can be modified
reversibly by filling the pores with fluids of varying refractive
index. The stop band position varies nearly linearly with refractive
index of the penetrating fluid, and can be altered by adjusting the
pore spacing, and the thickness, density, and composition of the
walls—all parameters that we can control through chemistry.
We have also altered photoemission properties of 3DOM silica materials
through structural modifications.
A critical factor in optimizing optical properties of these materials is the
ability to control the fine structure of the walls. The walls are composed
of fused nanocrystals whose sizes influence the optical response of the material.
For example, well-ordered 3DOM zirconia with 30 nm wall grains appears white,
while a material with 2 nm grains exhibits brilliant colors. One aspect of
our research involves optimizing nanograin sizes and phases by controlling
precursor chemistry, template–precursor interfacial interactions, and
processing conditions.
Fig.
1. Powders of 3DOM zirconia with increasing pore size (from
left: 201 nm, 252 nm, 283 nm). The two samples on the right are
the 252 and 283 nm samples filled with methanol.
Fig. 2. Transmission electron micrographs and photographs of two 3DOM
zirconia samples, showing the effect of wall-grain size on color. In spite
of its ordered macropore structure, the top sample appears white due to
secondary scattering at 30 nm wall grains. The walls in the bottom sample
are much smoother, eliminating secondary scattering. As a result, the sample
appears metallic green.