Executive Summary Final - the Center for Nanoscale Science - an ...

IRG 4: Electromagnetically-Coupled Nanostructures
Figure 1. Sub-wavelength nanoscale imaging is
important in the materials and biological sciences.
Our modeling shows that the high index contrast
possible in tapered, semiconductor filled MOFs
makes them very attractive for this application,
allowing for unprecedented light throughputs with
sub-wavelength resolution.
Development of the high-pressure materials chemistry
necessary for semiconductor growth in the
fiber geometry with accurately controlled, shallow
electronic dopant levels, sharply defined homo
and heterojunction layers and minimal material
Fig. 2. High speed (3 GHz) silicon pin photodiode
defects has allowed the IRG to fabricate p-i-n
fabricated in a silica optical fiber pore.
junctions in MOF templates by successive deposition
of layers, as shown in Figure 2 above. These
junctions exhibit well-defined rectifying curves, good fill-factor (55%), and have up to 3 GHz
bandwidth for infrared light, a major step forward towards all-fiber optoelectronics.
The high pressure deposition approach used to fill pores in MOFs must be understood before it
can be fully exploited. The IRG has modeled the flow of high pressure fluid/gas within the MOF
pores and its effect on the deposition reaction that produces well developed micro- or nanostructures.
This modeling has revealed how it is possible to completely fill optical fiber pores to
make low-optical-loss waveguides: hydrogen reaction byproducts and helium carrier gas can exit
the fiber through the silica walls, allowing for a sequential backfilling process. We find that
much higher precursor concentrations are possible in the confined geometry of the fiber pores
because nanoparticles that might nucleate and grow in larger reactors rapidly find the pore walls
in the confined geometry. The higher precursor concentrations possible in the confined geometry
allow for enhanced deposition rates and lower reaction temperatures compared with any conventional
CVD. The insights obtained from modeling have allowed for the fabrication of void-free
a-Si:H fibers with optical losses less than 3 dB/cm. It has not previously been possible to fabricated
a-Si:H microwires and nanowires because traditional techniques such as vapor-liquid solid
growth cannot be used for amorphous and hydrogenated materials. Now the superior nonlinear
optical properties of this material can be exploited over long lengths in fibers. Furthermore, we
are extending this high pressure deposition in a confined geometry approach enable ultraconformal
deposition in deep features on planar substrates. For example, it should be possible to
fabricate p-i-n silicon junctions in pores of polyimide sheets for pillar array solar cells. The resulting
periodic planar structure is more akin to that of the 2D meta-material work described below
and thus illustrates the connection between the two main efforts within this IRG; (note also
the quasi-2D structure of the sub-wavelength IR waveguide array described above). In exploratory
work, these techniques have also allowed us to deposit void-free amorphous and crystalline
sp 2 -bonded carbon fiber cores. We find that their strength thus far is as high as 4 GPa and they
can sustain strains of up to 4%. High strength carbon wires have never been made from precursors
such as methane before. We intend to extend these techniques in the coming year, with the